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Emissions of automobiles fueled with alternative fuels based on engine technology: A review
Accepted Manuscript Emissions of automobiles fueled with alternative fuels based on engine technology: a review Yisong Chen, Jinqiu Ma, Bin Han, Peng Zhang, Haining Hua, Hao Chen, Xin Su PII:
S2095-7564(18)30343-X
DOI:
10.1016/j.jtte.2018.05.001
Reference:
JTTE 185
To appear in:
Journal of Traffic and Transportation Engineering (English Edition)
Received Date: 20 February 2018 Revised Date:
18 May 2018
Accepted Date: 21 May 2018
Please cite this article as: Chen, Y., Ma, J., Han, B., Zhang, P., Hua, H., Chen, H., Su, X., Emissions of automobiles fueled with alternative fuels based on engine technology: a review, Journal of Traffic and Transportation Engineering (English Edition) (2018), doi: 10.1016/j.jtte.2018.05.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Reviews
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Emissions of automobiles fueled with
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alternative fuels based on engine
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technology: a review
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Yisong Chen, Jinqiu Ma, Bin Han, Peng Zhang, Haining Hua, Hao Chen*, Xin Su
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School of Automobile, Chang’an University, Xi’an 710064, China
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Received 20 Feb 2018
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Received in revised form 18 May 2018
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Accepted 21 May 2018
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Available online
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Highlights
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Emissions of automobiles fueled with main alternative fuels are reviewed.
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Emissions of NG/gasoline bi-fuel, NG and NG/diesel dual fuel engines are analyzed.
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Emissions of SI engines fueled with oxygenated fuels are analyzed.
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Emissions of CI engines fueled with oxygenated fuels are analyzed.
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Abstract
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Diversification of alternative fuels for automobiles is not only an actual situation, but also a
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development trend. Whether the alternative fuels are clean is an important issue. Emissions of
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automobiles fueled with natural gas (NG), methanol, ethanol, biodiesel, dimethyl ether (DME) and
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polyoxymethylene dimethyl ethers (PODEn) are investigated and reviewed based on engine
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technology and fuel properties. Compared to gasoline, NG/gasoline bi-fuel and NG automobiles have
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higher brake thermal efficiencies (BTE) and produce less HC, CO and PM emissions, while more NOx
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emission. Compared to diesel, NG/diesel dual fuel automobiles have lower BTE and emit lower soot
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and NOx emissions, but higher HC and CO emissions. Methanol and ethanol blending in gasoline can
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obviously reduce the HC, CO and PM emissions of spark ignition (SI) automobiles. Methanol or
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ethanol blending in diesel may prolong the ignition delay, shorten the combustion duration and improve
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the BTE, resulting in lower soot emissions. However, the HC, CO and NOx emissions of methanol or
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ethanol diesel blend fuels are uncertain due to low cetane number, high latent heat of vaporization.
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Most biodiesel has higher viscosity, distillation temperature, cetane number and oxygen content than
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diesel. Soot emission of biodiesel is lower than that of diesel, while NOx emission is higher. Both DME
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and PODEn do not contain C-C bonds and their blend with diesel can prohibit the formation of soot.
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PODEn has high cetane number and low viscosity, resulting in better ignitability and spray quality
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respectively. PODEn blending shortens both the ignition delay and the combustion duration, improves
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the BTE, and increases the temperature in the diffusion combustion phase, leading to a higher NOx
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emission.
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Keywords:
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Natural gas; Methanol; Ethanol; Biodiesel; DME; PODEn; emission.
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* Corresponding author. Tel.: +86 29 82334471.
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E-mail addresses: [email protected] (Y. Chen), [email protected] (H. Chen).
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1
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Since opening-up policy was implemented, China has experienced dramatic development, with
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averaged 9.8% annual growth rates of gross domestic product (GDP), in comparison with the world’s
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average of 3.3% (Xu et al., 2017). Problems of the world environment like global warming and energy
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resources will cause great constraint on the world economy and may affect profoundly the basic
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condition of human survival. As an important vehicle, automobile play an important role in people’s
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daily life and commercial activities. Besides, automobile industry is in an up road of the overall industry
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system (He et al.,2017).
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Introduction
Traditional automobile fuels derived from non-renewable fossil oil. Automobiles emit huge amounts
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of pollutants and greenhouse gases (GHG), such as NOx, PM, CO HC and CO2, and exert great
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influence on atmosphere environment and global warming. To protect the environment and ease the
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climate change, the automobile technology worldwide tends to develop in the directions of energy
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diversification and power electrification. Electrification of automobiles has been considered as the most
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effective way and the ultimate solution. However, even if the technologies of power battery, electric
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motor and electronic control continuously improve, two significant problems still exist. One is that the
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power structure is inadequate and the other is the power constraint if electric automobiles are widely
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applied. Diversification of clean alternative fuels for automobiles is regarded as the best choice and
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path in the transitional period between petroleum fuels and electrification.
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In general, alternative fuels include fossil and renewable fuels. They can also be classified into
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gaseous and liquid alternative fuels. Specifically, liquefied petroleum gas (LPG), natural gas (NG),
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alcohols mainly involving methanol and ethanol, ethers mainly including dimethyl ether (DME) and
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polyoxymethylene dimethyl ethers (PODEn), and biodiesel are in localized or industrial application. Gaseous fuels such as compressed natural gas (CNG) are promising alternative fuels which receive
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more attention all over the world. Natural gas is a very promising and highly attractive fuel because of
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its domestic availability, widespread distribution infrastructure, low cost, and clean-burning qualities to
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be used as a transportation fuel (Wei and Peng, 2016). In that case, CNG is considered to be a
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“cleaner” fuel compared to other fossil fuels. Therefore, it is used as an alternative fuel in motor
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vehicles to reduce emissions of air pollutants in transportation (Wang et al., 2016a). Besides, it is also
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applied in gasoline engine as a bi-fuel, added to diesel fuel and mixed with hydrogen to make better
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engine and emission performance. Generally, there are three application modes of natural gas, which
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are NG (CNG/LNG/HCNG), diesel/CNG dual fuel and CNG/gasoline bi-fuel.
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Among alternative fuels for gasoline, methanol (CH3OH) fuel has been considered to be one of the
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most favorable fuels for internal combustion (IC) engines due to the high octane number and the high
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intramolecular oxygen content (Avinash et al., 2014; Li, et al., 2010a; Zhen et al., 2013). A large
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number of domestic and foreign scholars have studied the application of methanol on internal
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combustion engine: it can be used with different application modes (mixed or pure methanol) in a
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spark-ignition (SI) engine (Agarwal et al., 2014; Gravalos et al., 2013; Lennox et al., 2014; Liu et al.,
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2007; Zhen and Wang, 2015) or with dual-fuel mode in a compression-ignition (CI) diesel engine (Pan
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et al., 2015; Park et al., 2017; Wang et al., 2008a; Wei et al., 2015). Ethanol and methanol have similar
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physical and chemical properties, is considered to replace fossil fuels of environmentally friendly fuel,
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which can be blended with other fuels at different proportions to improve engine emissions (Battal et al.,
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2017; Oh et al., 2010; Shi et al., 2015; Turner et al., 2011). The chemical formula of dimethyl ether (DME) fuel is CH3OCH3; it is the simplest ether compound.
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Among alternative fuels, the application of DME for diesel engines has been discussed by many
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investigators because it has no carbon-carbon bonds and excellent self-ignition characteristics
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compared to other fuels. Meanwhile, the cetane number of DME fuel is significantly higher than that of
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conventional diesel fuel (Park, 2012; Park and Lee, 2013; Roh et al., 2015; Semelsberger et al., 2006;
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Youn et al., 2011). Due to its fuel characteristics, DME is particularly suitable to be used as a complete
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substitute for diesel in compression ignition (CI) engines. It can be used as a blend with diesel to
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overcome limitations of using pure DME, such as poor viscosity, low density and its after-effects. DME
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could also be used as an additive or as an ignition promoter in conventional diesel combustion and for
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dual fuel operation (Lee et al., 2011; Su et al., 2016; Thomas et al., 2014; Wang et al., 2013, 2014). In
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recent years, it has been found that the mixture of polyoxymethylene dimethyl ethers (PODEn) is a
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promising engine fuel. Chemical expression of PODEn is CH3O(CH2O)nCH3. As an oligomer of ether,
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PODEn has a higher cetane number and oxygen content and does not contain C-C bonds, which has
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the potential to reduce diesel smoke and PM emissions (Lei et al., 2009; Liu et al., 2016b, 2017b;
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Pellegrini et al., 2012), and it can be mixed with diesel in any proportion (Burger et al., 2010; Härtl et al.,
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2015; Liu et al., 2016a; Pellegrini et al., 2013). In fact, n from 3 to 8 of PODEn has an excellent
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performance for diesel additives (Stroefer et al., 2010; Zhao et al., 2013). At room temperature, the
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PODEn-diesel mixture has excellent stability (Härtl et al., 2015; Pellegrini et al., 2012; Zhao et al.,
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2013). Generally, it can be used as a clean diesel blending components due to its diesel similar
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physical properties and do not need transformation of vehicle engine oil supply system (Liu et al., 2017;
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Xie et al., 2017b).
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Biodiesel, also called fatty acid methyl esters, is mainly made from vegetable oils or animal fats and
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is an ideal alternative fuel for diesel engines. Compared to petroleum diesel, biodiesel has higher
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cetane number, about 10% intramolecular oxygen, almost no aromatic hydrocarbon and sulfur. This
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leads to different fuel injection and spray properties, combustion characteristic, and exhaust emissions
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from petroleum diesel fuel (Hasan and Rahman, 2017; He et al., 2007; Lou and Tan, 2016; Miri et al.,
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2016; Xu et al., 2012). But there are some disadvantages of biodiesel which restrain its wide
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application and hinder its use as a complete replacement for diesel, which include higher kinematic
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viscosity, freezing temperature and density, as well as its low calorific value. Viscosity of biodiesel not
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only affects flow at all temperatures a fuel may be exposed to but also strongly influences the
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atomization of a fuel upon injection into the combustion chamber and ultimately the possible formation
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of engine deposits (Knothe and Gazon, 2017). That is why many scientists and investigators have
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studied blends of biodiesel with diesel by varying the proportions of biodiesel and diesel to investigate
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their suitability as a fuel in existing diesel engines. These problems associated with biodiesel can be
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overcome by using biodiesel-diesel blends (Nair et al., 2016; Sun et al., 2010; Yasin et al., 2015; Yusaf
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et al., 2011).
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Emissions of NG automobiles based on engine technology
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2.1
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In general, NG vehicles have lower emissions compared to gasoline or diesel engines (Kakaee and
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Paykani, 2013). Specifically, a decreasing trend is found for PAHs, SO2 and CO concentrations, while
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the NOx level is increased in comparison to those before the implementation of CNG (Ravindra et al.,
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2006). Due to the high octane number of NG, engines can be operated with a higher compression ratio
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(CR) for a better thermal efficiency (Liu et al., 2012). CRs of NG engines are commonly designed in the
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range of 11~13, which are higher than those of gasoline engines. CRs and application modes of NG
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(CNG/LNG/HCNG) sole fuel, diesel/CNG dual fuel and CNG/gasoline bi-fuel engines are shown in Fig.
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Pure NG (CNG/LNG)
Fig. 1 CRs and application modes for NG automobiles with different engine modes.
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Fig. 2 Typical natural gas composition by volume (Kakaee et al., 2014)
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It has been reported that the engine performance and emission are greatly affected by varying
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compositions of natural gas (Fig. 2). The most important NG fuel property is the Wobbe number (WN).
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Generally, it was agreed by researchers that the fuels with higher hydrocarbons, higher WN, and
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higher energy content exhibited better fuel economy and carbon dioxide (CO2) emissions. NOx
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emissions were also increased for gases with higher levels of higher WN, while total hydrocarbons
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(THCs) and CO showed some reductions (Kakaee et al., 2014). The results indicate that higher WN
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improves the combustion and the efficiency as well.
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Lean burn is an effective way to decrease the NOx emissions, while it results in high cyclic variation.
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Dilution is another method to achieve lean burn and low NOx emissions. Common dilution gases are N2,
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CO2 and Ar. The results show that the thermal efficiency first increases and then decreases as the
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dilution ratio (DR) of Ar increases and NOx emissions decrease significantly (Li et al., 2015). Ar dilution
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is superior in maintaining higher thermal efficiencies than CO2 and N2 for NG engines (Li et al., 2015).
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Lean burn as well as EGR successfully satisfied the legal emission regulation when the level of dilution
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was increased to the dilution limit, although there was a slight reduction in efficiency (Lee et al., 2014).
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2.2
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Natural gas/diesel dual fuel is an operation mode in which natural gas is introduced into the intake air
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of the inlet manifold and then ignited by the direct injected diesel in the cylinder. This mode has both
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economic and environmental benefits. Due to the high auto-ignition temperature, NG can hardly be
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burned through compression on diesel engines. In dual fuel mode, diesel acts as the ignition source
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and NG provides the main energy if needed for combustion. Generally, dual fuel engine exhibited
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longer ignition delay than diesel; had lower thermal efficiency than diesel at low and partial loads and
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higher at medium and high loads; emitted less NOx emissions than diesel engine, while more HC and
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CO emissions (Abdelaal et al., 2013; Abdelghaffar, 2011; Cheenkachorn et al., 2013).
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NG/diesel dual fuel
The application of dual fuel mode significantly decreases the NOx, CO2 and PM emissions
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(Cheenkachorn et al., 2013; Liu et al., 2013; Lounici et al., 2014; Meng et al., 2016), compared to
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diesel engines. The trade-off relationship between NOx and PM emission is solved. However, the
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hydrocarbon (HC) and carbon monoxide (CO) emissions may increase by several times in comparison
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to normal diesel combustion (Liu et al., 2013). The brake thermal efficiency (BTE) of dual fuel mode is
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lower at low and intermediate loads, while under high engine load conditions it is similar or a little
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higher when compared with normal diesel mode (Lounici et al., 2014). Dual fuel mode showed a
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simultaneous reduction of soot and NOx species over a large engine operating area (Lounici et al.,
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2014; Meng et al., 2016). In sum, trade-off relationship between soot and NOx emissions of diesel
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engines can be solved by using dual fuel mode, whereas the BTE of engine decreases and HC
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emission increases. Nithyanandan et al. (2016) found that the use of CNG affects the morphology and nanostructure of
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PM, and hence the oxidation reactivity of the soot. In Singh’s study (Singh et al., 2016), CMD (count
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mean diameter) graph showed that average size of particulate emitted CNG engines were much
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smaller compared to mineral diesel particulate. The addition of compressed natural gas lowers CO2
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emission and decreases opacity of exhaust gases in all load modes, the best positive impact has been
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achieved with the highest CNG portions (Vygintas et al., 2017). Liu et al. (2015) found that the
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developed dual fuel model is capable to predicate the flame propagation and emissions formation
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process in the dual fuel engine. Flame quench region of the fuel-lean mixture within the squish volume
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is the dominant source of CO emissions a low engine speed condition. However, bulk gas complete
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oxidation is impeded by the failure to transition into strong high temperature combustion in the cylinder
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center region, which accounts for the majority of CO emissions at high engine speed. NO formation
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region follows the development of the high temperature field for both low and high engine speed, which
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is generated by the combustion of the pilot diesel. Therefore, the injection strategy and quantity of the
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pilot fuel significantly determine the final exhaust NOx emissions during dual fuel operation conditions.
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On the whole, NG/diesel dual fuel automobiles simultaneously reduce CO2, NOx and PM emissions
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compared to diesel ones, while increase the HC and CO emissions.
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2.3
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Laminar burning velocity of NG is lower than that of methane, which is 48 cm/s (Korb et al., 2016). The
HCNG
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velocity of H2 is 290 cm/s and accordingly its addition in NG will surely accelerate the combustion
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speed, shorten the combustion duration and hence improve the thermal efficiency. Higher efficiency is
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generally correlated with higher NOx emission, lower HC and CO emissions, and lower brake specific
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fuel consumption (BSFC). Compared to NG, HNG present higher peak combustion pressure and
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combustion temperature, and more concentrated heat release as shown in Fig. 3 (Korb et al., 2016).
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Fig. 3 Comparison of combustion characteristics between NG and HNG (Korb et al., 2016). (a) In-cylinder pressure and mean
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temperature. (b) Mass fraction burned and ROHR.
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Mathai et al. ( 2012) made a comparative evaluation of performance, emission, lubricant and deposit
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characteristics of spark ignition engine fueled with CNG and 18% hydrogen-CNG and found that
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HCNG fueled engine decreased BSFC, CO and HC emissions with the increase of NOx emission.
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Another study showed the effects of 0, 5%, 10% and 15% blends of hydrogen by energy with CNG on
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bi-fuel NA SI engine using SPFIS (Nitnaware et al., 2016). MBT spark timing shown improvement in
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performance parameters with reduction in NOx emission. Carbon based emission reduced and NOx
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emission increased with increase in hydrogen addition. The optimum maximum brake torque (MBT)
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spark timing of 25°CA BTDC and injection pressure 2.6 bar is observed for 5% hydrogen addition at
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2500 rpm. Zareei et al. (2012) indicated that thermal efficiency, combustion performance, NOx
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emissions improved with the increase of hydrogen addition level. The HC and CO emissions first
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decrease with the increasing hydrogen enrichment level, but when hydrogen energy fraction exceeds
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12.44%, it begins to increase again at idle and stoichiometric conditions.
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Fig. 4 Comparison of emissions for NG vehicles.
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In sum, NG automobiles with different types of engines have different environmental effects,
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summarized in Fig. 4. As a whole, improvement of BTE brings high NOx emission and low HC, CO and
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PM emissions for SI engines. NG/diesel dual fuel engines are modified on diesel engines and the
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condition is complicated. Both NOx and PM emissions of dual fuel engines are reduced compared to
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diesel. The decrease of BTE of dual fuel engine causes high HC and CO emissions.
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The most common alcohols used on automobiles are methanol and ethanol. Although methanol can be
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produced from a wide variety of renewable sources and alternative fossil fuel based feedstocks, in
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practice methanol is mainly produced from natural gas and in China from coal (Chen et al., 2014;
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Sayah and Sayah, 2011; Vancoillie et al., 2013). Fuel ethanol can be produced from both edible
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feedstocks such as corn, wheat and stale grain and non-edible crops such as cassava and sweet
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sorghum.
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Emissions of automobiles fueled with alcohols based on engine technology
Methanol is a colorless, polar and flammable liquid which has higher octane number and heat of
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vaporization values as compared to gasoline (Zhen and Wang, 2015). It is known that ethanol and
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methanol have higher laminar flame speed, higher octane number and higher intramolecular oxygen
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contents than those of gasoline. Accordingly, the combustion duration will be surely shortened. Lennox
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et al. (2014) and Agarwal et al. (2014) studied the effects of methanol/gasoline blends on engine
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performance, combustion and emission characteristics, and compared with pure gasoline. The result
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showed that methanol/gasoline blends can reduce the combustion duration and exhaust gas
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temperature, increase the peak heat release rate (PHRR), and increase the BTE. Wu et al. (2016)
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experimentally investigated the effects of pure methanol on the combustion performance under idle
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condition based on a SI engine. The results showed that the SI engine fueled with methanol illustrated
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better lean burn performance than the engine fueled with gasoline. Compared with the engine fueled
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with gasoline, the indicated thermal efficiency (ITE) of engine fueled with methanol was increased; the
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flame development and propagation periods were shortened. On the whole, the combustion duration is
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shortened and the heat release is concentrated when alcohols are blended in gasoline. As a result, the
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combustion process is improved. Although the BTEs of alcohols/gasoline blends increase, engine
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power and torque decrease with the increase fraction of alcohol, due to the low heating value (Liu et al.,
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2007). Using methanol/gasoline blends on a spark-ignition engine can significantly reduce CO and HC
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emissions (Agarwal et al., 2014; Gravalos et al., 2013; Lennox et al., 2014; Liu et al., 2007; Wang et al.,
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2015). Battal et al. (2017), Turner et al. (2011) and Oh et al. (2010) investigated the effects of
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ethanol/gasoline mixtures on the performance and emissions at a spark-ignition engine. It is shown
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that the benefits of adding ethanol into gasoline are reduced combustion duration and increased
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in-cylinder pressure and combustion efficiency. Experiments and theoretical calculations showed that
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ethanol added fuels show reduction in CO, CO2 and NOx emissions without significant loss of power
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compared to gasoline. But it was measured that the reduction of the temperature inside the cylinder
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increases HC emission.
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N2+ O ↔ NO + N
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(1)
N2 + O2 ↔ NO + O
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N + OH ↔ NO + H
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As for NOx emission, the condition is extremely complicated. In general, thermal NOx, prompt NOx,
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and fuel NOx are the three formation processes. Thermal formation is representative when
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temperatures are high and the relative air to fuel ratio is close to 1. The reactions that take part on this
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mechanism were described firstly by Zeldovich (1946) and later extended by Bowman et al. (1975),
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described from Eq. (1) to Eq. (3). Agarwal et al. (2014) indicated that gasohol produced lower mass
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emissions of NO and smoke opacity. The result was attributed to the higher latent heat of vaporization
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of methanol compared to gasoline. Particulate size-number concentration was lower for gasohol
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blends in comparison to gasoline at all engine operating conditions. Wang et al. (2015) indicated that
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evident decrease in NOx emission was noticed with M15 and M25 fueling, but in the case of M40, NOx
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emissions were similar with gasoline. Mustafa et al. (2013) investigated the combustion and exhaust
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emissions characteristics of a SI engine fueled with the ethanol/gasoline (E5, E10) and
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methanol/gasoline (M5, M10) fuel blends. NOx emissions decreased for all wheel powers at the speed
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of 80 km/h. It has been also observed that the usage of alcohol fuel instead of gasoline caused to
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decrease the NOx, and to increase CO2 emission because of the improved and completed combustion
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(Balki et al., 2014; Mustafa and Balki, 2014; Wu et al., 2016).
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The increase of methanol increases the formaldehyde emissions and methanol emission increases
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with the increase of engine load under different speeds (Liu et al., 2007; Wang et al., 2015). Injection
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and ignition parameters also have significant influence on the combustion and emission of engines
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fueled with alcohol/gasoline blends. Advancing methanol injection timing decreased the HC and CO,
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while increased the NOx emission (Qu et al., 2015). Retarding ignition timing decreased the HC and
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NOx emissions and the effect of ignition timing changes on CO emission is small (Qu et al., 2015). The
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HC, CO, and NOx emissions of rich mixture are higher than those of lean mixture. Increasing intake air
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temperature decreased the HC and CO emissions. Retarding methanol injection timing, advancing
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ignition timing, using lean mixture and reducing intake air temperature can decrease the formaldehyde
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emission (Li et al., 2010b).
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Alcohols can also be applied on diesel engines through two methods: dual fuel mode or
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emulsification/micro-emulsification. For dual fuel mode, methanol is mainly used. Compared with
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conventional diesel, the methanol/diesel dual fuel combustion mode significantly reduces NOx and PM
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emissions (Pan et al., 2015; Park et al., 2017; Wang et al., 2008; Wei et al., 2015). Park et al. (2017).
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When the amount of methanol is increased, cylinder pressure and temperature decreased. The
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resulting decrease in the combustion efficiency lowered the NOx emission and BTE of the engine.
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Wang et al. (2008a) found that the increase in methanol mass fraction lowers the polytropic index of
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compression and the temperatures at BDC and TDC, as well as the oxygen concentration in the
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mixture. This prolongs the ignition delay under the same engine load and speed condition by
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comparison with diesel operation. The heat release rate changes from dual-peak mode to single-peak
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mode. The high methanol mass fraction will realize a simultaneous reduction in both smoke and NOx
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under all the operating conditions. Meanwhile, the NOx-smoke trade-off curve disappears in
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combustion of the dual fuel, but CO and HC increase. Wei et al. (2015) investigated the combustion
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and emission characteristics of a dual fuel diesel engine with high premixed ratio of methanol (PRM).
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High PRM prolonged the ignition delay but shortened the combustion duration and decreased the
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in-cylinder gas temperature. Both NOx and dry soot emissions were significantly reduced, while HC,
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CO, formaldehyde emissions and NO2 in NOx increased significantly with the increase of PRM (Wei et
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al., 2015). Pan et al. (2015) and Geng et al. (2014) found that there was a strong coupling between the
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intake air temperature and the methanol fraction to performance and emissions of the engine. At dual
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fuel operation mode, decreasing intake air temperature reduced the indicated thermal efficiency (ITE)
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and exhaust gas temperature and the trend was more evidently as methanol energy fraction increased.
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Decreasing of intake air temperature also prolonged the ignition delay, which caused a later
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combustion phasing and smaller peak cylinder pressure. By the induction of methanol, NOx, NO and
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smoke emissions decreased markedly, while NO2, CO, THC, formaldehyde and methanol emissions
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increased. However, increasing the intake air temperature would inhibit the NO2, THC, CO,
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formaldehyde and methanol emissions and increase NO, NOx and soot emissions. Overall,
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methanol/diesel dual-fuel combustion performs better in terms of engine performance and emissions
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reduction under rich mixture conditions (Amin et al., 2015). Chen et al. (2017b) and Li et al. (2016)
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investigate the effects of diesel injection parameters on the rapid combustion and emissions of the
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diesel-methanol dual-fuel engine. The experimental results show that the diesel injection parameters
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affect rapid combustion fraction (α) greatly, which increases as the diesel injection pressure rises while
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decreases as the diesel injection timing advanced or diesel injection quantity increases. Liu et al. (2015)
316
indicated that at low injection pressure, the IMEP of dual fuel mode is lower than that of pure diesel
317
combustion mode. COVIEMP of dual fuel mode firstly decreases and then increases with the increasing
318
of injection pressure. Their researches shows both of NOx and smoke emissions are reduced while CO
319
and HC emissions increased obviously in dual fuel mode (Liu et al., 2015). Smoke emission can be
320
further reduced by coupling the high diesel injection pressure and the advanced diesel injection timing.
321
However, NOx emission may be increased in this case (Park et al., 2017). The methanol
322
co-combustion ratio (CCR) is defined as the ratio of methanol energy to the total energy in the dual-fuel
323
mode. θin is the injection timing. Comparison between emissions of diesel/methanol dual fuel (DMDF)
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engine and normal diesel is presented in Fig. 5(a) and Fig. 5(b).
325
(a)
326
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Fig. 5 Comparison of emissions between diesel and DMDF (Li et al., 2016). (a) CO and HC emissions. (b) NOx and soot
328
emissions.
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Alcohols can hardly be mixed with diesel, because the alcohols are polar solvents and diesel is
330
non-polar fuel. Cosolvents and surfactants must be used to help the mixing of diesel and alcohols,
331
forming emulsified fuel or micro-emulsified fuel. Emulsified fuel is in the condition of milkiness and
332
opaque, and the mixing state is unstable. Micro-emulsified fuel is transparent and stable. Some studies
333
confirmed that diesel/biodiesel blend fuels can mixed with a low volume ratio alcohols, forming
334
micro-emulsified fuel. Further studies (Aydin et al., 2015, 2017; Shi et al., 2015) have found that
335
biodiesel/ethanol/diesel fuel blends can be directly used on a diesel engine for lower PM and THC
336
emissions. However, a major drawback is that ethanol is immiscible in diesel over a wide range of
337
temperatures and water content because of their difference in chemical structure and characteristic,
338
these can result in fuel instability (Prommes et al., 2007).
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Both methanol and ethanol can be applied on SI or CI engines. The ultimate aim is to improve the
340
oxygen content and thus the combustion efficiency. As a result, HC, CO and PM emissions are
341
reduced. Methanol/diesel dual fuel engine is an exceptional case, which is similar with NG/diesel dual
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fuel. The BTE of methanol/diesel dual fuel engine declines compared to diesel, producing lower NOx
343
and soot emissions and higher HC and CO emissions.
344
4
345
Most researchers reported that use biodiesel in a conventional diesel engine brings about a
346
considerable reduction in CO, CO2 and PM (Hasan and Rahman, 2017; He et al., 2007; Lou and Tan,
347
2016; Miri et al., 2016; Xu et al., 2012). Feedstocks of biodiesel are very abundant and they can be
348
classified into three generations. The first generation refers to the edible crops such as soybean,
349
rapeseed and palm oil, the second includes oil plants such as jatropha curcas, pistacia chinensis and
350
sapium sebiferum, and the third is microalgae such as Scenedesmus obliquus. Production principle
351
can be summarized into a common formula, shown in Fig. 6. Abundant feedstocks result in significant
352
difference in chemical composition of biodiesel and thus its properties. Table 1 lists the composition
353
and properties of some typical biodiesel (Jain and Sharma, 2011; Serrano et al., 2013; Wang et al.,
354
2012). Biodiesel mainly contained the same five components: methyl palmitate (C17H34O2), methyl
355
stearate (C19H38O2, C18:0), methyl oleate (C19H36O2, C18:1), methyl linoleate (C19H34O2, C18:2) and
356
methyl linolenate (C19H32O2, C18:3) (Shi et al., 2018). The molecular structure of the above
357
components is exhibited in Fig. 7. Cetane number of methyl palmitate, methyl stearate, methyl oleate,
358
methyl linoleate and methyl linolenate are respectively 86, 101, 59, 38 and 23. It can be concluded that
359
the higher the mass fraction of saturated fatty acid methyl esters (S-FAMEs) in biodiesel is, the higher
360
the cetane number is and the better the oxidative stability is, while the worse the low temperature
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Emissions of automobiles fueled with biodiesel based on engine technology
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fluidity is. To ensure the basic use on diesel engines, the low temperature fluidity of biodiesel must be
362
guaranteed and as a result the mass fraction of unsaturated FAMEs (U-FAMEs) is generally higher
363
than 70% by weight. Consequently, cetane number of biodiesel in common use is a little higher than
364
that of diesel, which contributes to a better ignitability. Fig. 8 presents the heat release and in-cylinder
365
temperature of a common rail diesel engine fueled with biodiesel and diesel. Biodiesel has higher
366
viscosity and distillation temperature, leading to a poor homogeneity of mixture gas and atomization
367
quality. Consequently, the HRR of biodiesel in the pre-mixed combustion phase is lower than that of
368
diesel, and so does the PHRR. With the proceeding of combustion, biodiesel produces more active
369
radicals and accelerates the speed of chemical reaction, resulting in higher HRR compared to diesel in
370
the diffusion combustion phase. As a result, the in-cylinder temperature of biodiesel is higher and thus
371
the NOx emission.
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Fig. 6 Chemical reaction formula for biodiesel production
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Fig. 7 Chemical structure of five major components in biodiesel
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Due to the high cetane number, SOC of biodiesel advances and the ignition delay shortens.
377
In-cylinder temperature of biodiesel is slightly higher than that of diesel (shown in phase Ⅰ from A to B,
378
Fig. 8). Lower LHV, higher viscosity and lower volatility commonly contribute to the slower combustion
379
and hence the lower PHRR and combustion temperature of biodiesel, compared to diesel (shown in
380
phase Ⅱ from B to C, Fig. 8). It can be concluded that the intensity of diffusion combustion for
381
biodiesel is obviously higher than that of diesel, leading to the higher PCT and combustion temperature
382
(shown in phase Ⅲ from C to D, Fig. 8). It is mainly because that the intramolecular oxygen increases
383
the OH intensity of biodiesel along the injector axial line and the active radicals surely accelerate the
384
combustion speed. Fast combustion promotes the complete combustion of biodiesel and the
385
combustion duration shortens. As a result, the in-cylinder temperature of biodiesel is lower than that of
386
diesel, as shown in the phase Ⅳ (Fig. 8).
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387 388 389 390
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(a)
(b)
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Fig. 8 Comparison of heat release and in-cylinder temperature between diesel and biodiesel. (a) Heat release. (b) Temperature.
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Table 1 Composition of biodiesel derived from three generation feedstocks. Vernicia
Idesia
Sapium
Scenedesmus
Xanthoceras
Armeniaca
(%)
fordii
polycarpa
sebiferum
obliquus
sorbifolia
sibirica
C14:0
/
/
/
/
/
0.03
C16:0
3.56
15.50
7.21
18.42
5.27
C16:1
/
6.65
/
2.31
/
C16:2
/
/
/
3.26
/
C18:0
2.62
1.39
2.28
3.43
1.92
C18:1
10.57
9.53
14.61
49.64
C18:2
14.64
64.81
31.72
11.30
C18:3
59.20
2.08
41.02
8.26
C20:0
/
/
/
/
C20:1
0.90
0.04
/
/
C20:3
/
/
/
Others
8.51
/
/
S-
6.18
16.89
9.49
93.61
83.11
90.51
Cetane number
37.0
45.0
CFPP
-11
-5
Oxidative
0.4
0.7
(Wang et al.,
(Wang et al.,
2012)
2012)
Ref.
Elaeis
chinensis
0.30
/
/
0.10
1.00
3.79
10.90
19.75
23.14
5.10
44.80
0.67
/
/
0.99
/
0.30
/
/
/
/
/
/
1.01
3.20
4.63
1.18
2.10
3.80
SC
curcas
M AN U
Rapeseed
guineensis
46.83
44.35
57.90
39.90
44.47
28.92
54.50
28.50
28.51
24.70
9.28
6.46
0.14
6.80
0.01
0.84
7.90
0.22
0.20
0.09
0.10
0.17
0.10
0.20
0.35
/
0.12
/
/
/
1
/
/
7.27
/
/
/
/
/
/
3.38
3.24
/
0.20
/
0.89
/
0.03
21.85
8.00
4.92
14.50
24.66
24.42
7.50
50.27
74.77
92.00
95.08
85.30
75.34
74.82
91.50
49.70
43.0
46.0
47.6
48.8
/
51.0
51.30
/
61.0
-11
-6
-8
-14
/
0
-3
/
12
0.8
1.2
1.7
2.7
2.9
3.3
4.2
4.6
7.7
(Wang et al.,
(Serrano et
(Jain and
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Pistacia
65.23
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U
Jatropha
31.17
FAMEs
FAMEs
Soybean
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Composition w
(Wang et al., (Serrano et al., (Wang et al., 2012)
2013)
2012)
2012)
(Wang et (Serrano et (Wang et al.,
al., 2013) Sharma, 2011) al., 2012)
al., 2013)
2012)
23
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There are many researches find that rapeseed biodiesel and its blends have higher cetane number,
397
increased oxygen content, higher density and viscosity, but inferior lower heating value, and
398
compressibility when compared to diesel fuel. Miri et al. (2016), Aldhaidhawi et al. (2017) and Ismet et
399
al. (2012) investigated the performance, combustion and emission characteristics of a diesel engine
400
fueled with rapeseed biodiesel and its blends and compared with pure petroleum diesel fuel.
401
Researches results reveal that rapeseed biodiesel, either pure or blended with diesel, has lower heat
402
release rate, reduced ignition delay and lower thermal efficiency. Meanwhile, effective power and
403
torque decrease at all engine loads. Regarding gaseous emissions, rapeseed biodiesel increase NOx
404
emissions, other emissions such as those of CO and particulate matter PM are usually found to
405
significantly decrease with rapeseed biodiesel content.
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Qi el al. (2009; 2010) and Özener et al. (2014) investigated the effect of biodiesel produced from
407
soybean crude oil on the combustion characteristics, performance and exhaust emissions of a diesel
408
engine. The results showed that biodiesel exhibited the similar combustion stages to that of diesel,
409
however, biodiesel showed an earlier start of combustion. At lower engine loads, the peak cylinder
410
pressure, the peak rate of pressure rise and the peak of heat release rate during premixed combustion
411
phase were higher for biodiesel than for diesel. At higher engine loads, the peak cylinder pressure of
412
biodiesel was almost similar to that of diesel, but the peak rate of pressure rise and the peak of heat
413
release rate were lower for biodiesel. The power output of biodiesel was almost identical with that of
414
diesel. It is also observed that there is a significant reduction in CO and smoke emissions at high
415
engine loads, while NOx emissions increased. Moreover, biodiesel provided significant reduction in CO,
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HC, NOx and smoke under speed characteristic at full engine load (Qi et al., 2009). Özer et al. (2016)
417
indicated that the maximum heat release rate and maximum in-cylinder pressure were mostly
418
increased with the combined effects of biodiesel fuel addition and EGR application. Improvements on
419
the THC emissions were obtained by the use of 20 vol.% soybean biodiesel fuel blend and increase of
420
EGR rate at the low and partial engine loads, while deterioration occurred at the high engine load when
421
the EGR was increased to over 5% rate.
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Wei et al. (2017) investigated the influence of waste cooking oil (WCO) biodiesel on the combustion,
423
emissions characteristics of a diesel engine. The increase of in-cylinder pressure is mainly due to the
424
advanced start of combustion and more complete combustion when using biodiesel. Lower maximum
425
heat release rate is due to the less intense combustion in the premixed combustion phase. Earlier start
426
of combustion is mainly attributed to the higher bulk modulus and higher viscosity of biodiesel.
427
Biodiesel reduces the weighted particle mass concentration and the weighted geometric mean
428
diameter of the particles. Enweremadu and Rutto (2010) found that the engine performance of the
429
WCO biodiesel and its blends were only marginally poorer compared to diesel. From the standpoint of
430
emissions, NOx emissions were slightly higher while HC emissions were lower for WCO biodiesel when
431
compares to diesel fuel. Compared with other vegetable oils and petroleum diesel fuels, palm oil is
432
associated with better engine performance and shorter ignition delay. Use of palm oil also reduces
433
exhaust emission of HC, CO and smoke and exhaust gas temperatures, while significantly improve
434
levels of NOx (Leevijit et al., 2017; Mosarof et al., 2015; Ndayishimiye and Tazerout, 2011).
435
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However, some researches showed that the addition of biodiesel reduces NOx emissions and
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increases in soot emissions (Nair et al., 2016; Yasin et al., 2015; Yusaf et al., 2011). Sun et al. (2010)
437
report that inconsistencies of NOx emission appear among studies due to different with engine type,
438
engine technology, and fuel feedstock.
439
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Emissions of automobiles fueled with ethers based on engine technology
DME, CH3-O-CH3, has a higher cetane rating and oxygen content than diesel and has good
441
evaporation characteristics in the combustion chamber. Meanwhile, it has no direct C-C bonds which
442
produces considerably less pollutants like HC, smoke and particulate matter (PM) than conventional
443
fuels. This property makes it very attractive as a clean fuel for transportation and domestic utilization.
444
Therefore, it is an excellent, efficient alternative fuel for diesel engines. At present, most of
445
investigations focused on the pure DME combustion or effects of DME quality on CI engine fuel
446
economy and emissions (Hou et al, 2014; Su et al., 2014; Su and Chang, 2014; Park and Lee, 2013;
447
Youn et al., 2011). DME has good solubility with diesel (Wang et al., 2008b) (Fig. 9(a)). It is also found
448
that compared with diesel, BSECs of blend fuels firstly decrease and then increase, as shown in Fig.
449
9(b). Consequently, the sequence of BTE is diesel < DME10 < DME15 > DME20. This condition is
450
similar with PODEn. Adequate addition of DME can surely promote the complete combustion and
451
increase the BTE. Both NOx and soot emissions of DME/diesel blend fuels are lower than those of
452
diesel (Fig. 10).
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Youn et al. (2011) investigated the combustions and emissions characteristics of DME compared to
454
conventional diesel fuels. In combustion characteristics, the peak combustion pressure and the ignition
26
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delay of DME fuel is higher and faster than those of ultra low sulfur diesel (ULSD), respectively. The
456
NOx emission of DME fuel shows slightly higher than that of diesel at the same engine load condition,
457
while HC and CO emissions were lower. Also, the soot emission of DME fuel is nearly zero level (Su
458
and Chang, 2014; Park and Lee, 2013). Hou et al. (2014) investigated combustion and emissions
459
characteristics of a turbocharged compression ignition engine fueled with DME and biodiesel blends.
460
The result shows that with the increase of DME proportion, ignition delay, the peak in-cylinder pressure,
461
peak heat-release rate, peak in-cylinder temperature decrease, and their phases retard. Compared to
462
biodiesel, NOx emissions of blends significantly decrease; HC emissions and CO emissions increase
463
slightly (Su et al., 2014). Zhao et al. (2014) investigated the effects of DME (dimethyl ether) premixing
464
ratio and cooled external EGR (exhaust gas recirculation) rate on combustion, performance and
465
emission characteristics of a DME-diesel dual fuel premixed charge compression ignition (PCCI)
466
engine. The result showed that HCCI combustion of the premixed gas promoted the in-cylinder
467
pressure and temperature, resulting in an earlier SOC and a shortened diesel ignition delay. The
468
decrease in the diesel fuel during the diffusion combustion improved the mixing uniformity between the
469
fuel and air. Thus, the combustion became more complete and the brake thermal efficiency improved.
470
A higher DME premixing ratio caused lower smoke and NOx emissions but higher HC and CO
471
emissions. PCCI engine with EGR exhibited an obvious postponed SOC and prolonged combustion
472
duration. Thus, the maximum values of in-cylinder pressure, mean charge temperature, heat release
473
rate and pressure rise rate all decreased. As the EGR rate increased, NOx emission decreased, but
474
smoke, CO and HC emission increased. Wang et al. (2013, 2014) found that both port DME quantity
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and injection timing remarkably influenced the combustion process and exhaust emission of engine.
476
They had little impact on the peak position of HRR during low temperature reaction (LTR) phase.
477
However, the peak value of HRR increased and the crank-angle corresponding to the HRR peak
478
advanced with an incremental DME quantity or an early injection during high-temperature reaction
479
(HTR) phase. The peak value of HRR dropped with an incremental DME quantity or a late injection
480
during the diffusion combustion phase. Peak values of in-cylinder pressure and temperature increased
481
with an incremental DME quantity or an early injection. For the fixed injection timing, NOx emissions
482
presented a decreasing trend with a rise of DME quantity but this decreasing trend ceased at a higher
483
DME quantity. Smoke emission reduced, but CO and HC emissions increased with a rise of DME
484
quantity. Su et al. (2016) also found that the ethanol fraction have a more obvious effect on the
485
indicated mean effective pressure (IMEP) for advanced in-cylinder injection timings than around the
486
top dead center (TDC) conditions. The application of the DME-ethanol dual-fuel combustion strategy
487
caused a significant reduction of indicated specific NOx without deterioration of indicated specific soot.
488
In addition, a high ethanol fraction led to a low NOx for the same premixed combustion duration, while
489
HC and CO emissions increased slightly.
490
(a)
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(b)
491 492
Fig. 9 Comparison of mutual solubility and BSEC between DME and diesel (Wang et al., 2008b). (a) Mutual solubility. (b) BSEC.
28
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(a)
(b)
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494
Fig. 10 Comparison of NOx and soot emissions between DME and diesel (Wang et al., 2008b). (a) NOx emission. (b) Soot
496
emission.
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Although DME is usually thought to be an alternative fuel for CI engines, the SI DME engine could be
498
started successfully and realize the stabile running. Shi et al. (2018) investigated the combustion and
499
emissions characteristics of a SI engine fueled with gasoline-DME blends. Test results showed that the
500
addition of dimethyl ether resulted in the raised indicated mean effective pressure for the gasoline
501
engine. Over increased and decreased spark timing tended to cause the dropped indicated mean
502
effective pressure. The coefficient of variation in indicated mean effective pressure was diminished
503
with the spark timing advances and dimethyl ether addition. NOx and HC emissions were dropped with
504
the spark timing decrease. NOx emissions from the dimethyl ether-mixed gasoline engine are
505
decreased with the decrease of spark angle. Ji et al. (2011) indicated that thermal efficiency, NOx and
506
HC emissions are improved with the increase of DME addition level. The combustion performance was
507
improved when DME addition fraction was less than 10%. CO emission first decreased and then
508
increased with the increase of DME enrichment level at stoichiometric condition.
509
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PODEn, with the structure CH3-O-(CH2O)n-CH3 and with no C-C bond, are promising blend fuels for
29
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diesel due to low viscosities and pour points, high oxygen contents and high CNs. The “n” of CH2O
511
group is from 1 to 8, and the main composition is from 2 to 6. Properties of PODEn components are
512
listed in Table 2. PODEn can also be soluble with diesel at any proportion. Added into the diesel
513
blending with 10% to 20%, PODEn can significantly reduce the diesel cold filter point, can improve the
514
diesel combustion in the engine quality, and improve thermal efficiency (Shi et al., 2012). Feng et al.
515
(2013) found that the ignition delay of PODE3-8-diesel mixed fuel is shortened, the fuel consumption is
516
increased, but the effective thermal efficiency is improved, compared with diesel fuel.
517
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Table 2 Properties of PODEn components (Chen et al., 2017a). Density (g/cm3)
Component
0.96
PODE3: CH3O(CH2O)3CH3
1.02
PODE4: CH3O(CH2O)4CH3 PODE5: CH3O(CH2O)5CH3
Oxygen content (%)
63
45.3
156
78
47.1
1.06
202
90
48.2
1.10
242
100
49.0
1.13
280
104
49.6
EP
PODE6: CH3O(CH2O)6CH3
Cetane number
105
TE D
PODE2: CH3O(CH2O)2CH3
Boiling point (℃)
Yang et al. (2015), investigated the performances of engine fueled with PODE2-4 blend fuel. The
519
result showed that with the increase of the mixing ratio of PODE2-4, the diesel engine power and torque
520
drop, but thermal efficiency increase. Xie et al. (2017) made a study about PODEn and its high
521
proportion of diesel blended fuel on the combustion and emission of the engine. It was found that the
522
engine was pulsed with pure PODEn, and its effective thermal efficiency was improved and discharged
523
relative to diesel oil. So PODEn can be used as an alternative fuel for diesel alone (Burger, 2010; Feng,
524
2013). The NOx-soot trade off relationship can be dramatically improved (Burger et al., 2010). The NOx
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and soot emissions can meet Euro Ⅴ standards at high load and Euro Ⅵ standards at medium load.
526
Oxygenated fuel is an important method to inhibit the formation of soot emissions and improving air
527
entrainment by blending high volatility fuel is another approach. PODEn has lower distillation
528
temperature than diesel and thereby higher volatility (Liu et al., 2016a, 2017b). Furthermore, PODEn
529
has lower viscosity than diesel. As a result, blending PODEn in diesel is helpful to improve the forming
530
quality of air-fuel mixture and the spray quality.
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Furthermore, Liu et al. (2017b) found the combustion efficiency can be dramatically improved which
532
leads to lower HC and CO emissions. In order to reduce emissions and improve thermal efficiency of
533
diesel engines, blends of GDP (gasoline/diesel/PODEn) were proposed and studied. GDP blends have
534
shorter ignition delay, lower max pressure rise rate and COVIMEP (coefficient of variation of indicated
535
mean effective pressure) than GD blends. GDP blends also have higher combustion efficiency and
536
thermal efficiency than GD blends, even slightly higher than diesel fuel. Pellegrini et al. (2013) studied
537
PODE3-5 on a diesel engine and the results showed that the use of 12.5% PODE3-5 mixture reduced
538
PM emissions; high mixing ratio PODE3-5/diesel could simultaneously optimize NOx, PM and noise, but
539
may cause problems with the engine hardware. Chen et al. (2017a) found that both soot and ultrafine
540
particles (UFP) emissions obviously decreased with the increase of PODEn ratio. At low engine loads,
541
the reduction effect for UFP is especially significant, as shown in Fig. 11.
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542 543 544 545
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(a)
(b)
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547
Fig. 11 Comparison of UFPs between diesel and its blends with PODEn (Chen et al., 2017a).
549
(a) Number concentration. (b) Volume concentration.
SC
548
BTEs of PODEn/diesel blend fuels firstly increase compared to diesel with the blending ratio of
551
PODEn and then decrease, which is similar with DME. This condition is attributed to the lower LHV of
552
PODEn. The above reviews concluded that with the blending of PODEn, HC, CO and smoke emissions
553
decrease, while NOx emission increases. Compared to P20, BTE of P30 decreases and thus reduce
554
the NOx emission. High oxygen content still has obvious effect in reducing smoke emission.
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Fig. 12 Schematic diagram of soot formation process (Dale and Kenth, 2007).
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557
Fig. 12 indicates the soot formation process of diesel engine (Dale and Kenth, 2007). Alkynes are
558
polymerized into aromatic hydrocarbons (PAHs) through fuel pyrolysis. The growth of PAHs leads to
559
the formation of soot (core formation). On the whole, blending oxygenated fuel in diesel may provide
560
oxygen in the fuel-rich area of the diesel jet, which can inhibit the soot formation. Common ethers used
561
as diesel additive were DME with the molecular formula of CH3OCH3 and PODEn of CH3O(CH2O)nCH3.
562
There are no C-C bonds in both DME and PODEn and these ethers have the best effects in soot 32
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reduction. Zhu et al. confirmed that the R-(C=O)O-R ′ group in biodiesel was less efficient in
564
suppressing the soot precursor’s formation than the R-OH group in n-pentanol (Zhu et al., 2016).
565
Further, it was confirmed that soot precursor generated in the biodiesel pyrolysis was proportional to
566
the concentration of unsaturated fatty acid methyl ester (the number of C=C double bonds) (Wang et
567
al., 2016b).
568
6.
569
Greenhouse gas (GHG) emissions derived from vehicles are the significant contributors to the global
570
warming and the climate change as shown in Fig.13. Life cycle assessment (LCA) methodology is
571
commonly used to evaluate the well-to-wheel greenhouse gas emissions of alternative fuels.
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Greenhouse gas emissions of alternative fuels based on life cycle assessment
Fig. 13 The transport sector as a major contributor to global energy-related CO2 emissions (Ashnani et al. 2015).
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Ou and Zhang (2013) found that CHG-powered and LNG-powered vehicles emit 10%-20% and
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5%-10% less GHGs than gasoline- and diesel-fueled vehicles respectively, which has the similar
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results with Rose et al. (2013) that a 24% reduction of GHG emissions (CO2-equivalent) realized by
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switching from diesel to CNG. Since NG has a lower carbon content than petroleum, gas to liquid
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(GTL)-powered vehicles emit approximately 30% more GHGs than conventional fuel vehicles. The
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carbon emission intensity of the LNG energy chain is highly sensitive to the efficiency of NG
580
liquefaction and the form of energy used in that process (Ou and Zhang, 2013). It was confirmed that biodiesel appears attractive since its use results in significant reductions of
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GHG emissions in comparison to gasoline and diesel (Nanaki and Koroneos, 2012). Nocker and Torfs
583
(1998) made a comparison of LCA and external-cost analysis for biodiesel and diesel, which found that
584
both approaches confirm that although biodiesel offers advantages in terms of greenhouse gas
585
emissions, and it has similar or higher impacts on public health and the environment. However, from a
586
LCA perspective, it is not an accurate way for only focusing on the use phase of the fuels. Carneiro et
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al. (2017) found that some biodiesel production pathways perform satisfactorily in terms of GHG
588
emissions compared to other biofuels, but some others can be even worst than fossil diesel, but
589
energy and GWP performances still can be improved if production pathways are carefully chosen and
590
optimized. Further, Collet et al., (2014) found that a large fraction of environmental impacts and
591
especially GHG emissions stem from the production of the electricity required for producing, harvesting
592
and transforming algae, in that case the source of electricity as well as algae production technology
593
may also play an important role in GHG reduction.
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As for ethanol, Blottnitz and Curran (2007) reported that bio-ethanol results in reductions in resource
595
use and global warming. A life cycle environmental impacts of selected U.S. ethanol production and
596
use pathways in 2022 was conducted and indicates that one kilometer traveled on E85 from the
597
feedstock-to-ethanol pathways evaluated has 43%-57% lower GHG emissions than a car operated on
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conventional U.S. gasoline (base year 2005) (Hsu, et al. 2010). Even though bio-ethanol production
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from sugarcane is considered to be a beneficial and cost-effective greenhouse gas (GHG) mitigation
600
strategy, it is still a matter of controversy due to insufficient information on the total GHG balance of this
601
system (Lisboa et al. 2011). In sum, the utilization of alternative fuels including CNG, biodiesel, and
602
ethanol is helpfuel to control the GHG emissions.
603
7.
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Diversification of fuels for automobiles is an inevitable strategy and trend of social and economic
605
sustainable development. Environmental pollution effects of automobiles fueled with alternative fuels
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are extremely complicated, determined by fuel properties, engine technology and application modes.
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In sum, improvement of BTE generally accompanies with the increase of NOx emission. High oxygen
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content will surely prohibit the formation of polycyclic aromatic hydrocarbons and soot.
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Conclusions and suggestions
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(1) Natural gas is the most important and successful alternative fuel for automobiles. NG/gasoline
610
bi-fuel automobiles have higher BTE than gasoline ones, producing less HC, CO, and PM emissions,
611
while more NOx emission. Pure NG automobiles have similar regulations with bi-fuel mode compared
612
to gasoline automobiles. NG/diesel dual fuel automobiles are commonly compared with diesel ones,
613
emitting higher HC and CO emissions and lower NOx and PM emissions with BTE decreasing. HCNG
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improves the BTE of NG engine and as a result the NOx emission increases significantly.
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(2) Methanol and ethanol are generally applied on SI automobiles, through mixing with gasoline in
616
certain volume proportion together with the additive. High intramolecular oxygen contents accelerate
617
the combustion speed, shorten the combustion duration and thus improve the BTE, promoting the
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complete combustion. Accordingly, HC and CO decrease obviously. Methanol and ethanol can also be
619
used on diesel engines, although their ignitability is poor due to the low cetane number. However, high
620
oxygen contents are helpful for inhibiting the formation of PM and their effects on the combustion
621
process are similar. The solubility of alcohols in diesel is very poor and many additives including
622
cosolvents and surfactants must be used, leading to the poor application.
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(3) Biodiesel is an ideal and renewable alternative fuel for diesel and its physical and chemical
624
properties are close to those of diesel. The most important advantage is that biodiesel can be mixed
625
with diesel in any volume ratio. Although the feedstocks for the production are extremely abundant,
626
most biodiesel has five major compositions and as a result its intramolecular oxygen content is 10% or
627
so by weight, accounting for more than 70% by weight. To balance the trade-off relationship between
628
properties including oxidation stability (induction period) and ignitability (cetane number) and low
629
temperature fluidity (freezing point), the mass fraction of U-FAMEs is controlled no less than 70%. High
630
viscosity and low volatility of biodiesel result in poor homogeneity of mixture gas and spray quality.
631
Biodiesel has lower BTE than diesel at low loads, and higher at medium and high loads. Biodiesel
632
produces higher NOx emission than diesel, while less HC, CO, and PM emissions.
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(4) DME and PODEn do not contain C-C bonds and have high oxygen contents. They are considered
634
as the most promising blend fuel in diesel. Both of them have higher BTE than diesel, due to the more
635
concentrated heat release and more completed combustion. DME blending in diesel can reduce both
636
NOx and soot emissions. PODEn blending can also decrease the soot or PM emission of diesel engine
637
significantly, while increase the NOx emission in general. In general, increasing the blending ratio of
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DME and PODEn reduce the soot or PM emissions, mainly due to the increasing of intramolecular
639
oxygen content. However, BTE does not increase throughout with the blending ratio and HC and CO
640
will increase when BTE decreases.
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(5) In most cases, high BTE means high NOx emission and low soot (or PM) emission for diesel
642
automobiles, exhibiting a trade-off relationship between NOx and soot (or PM). Automobiles equipped
643
with dual fuel engines, including both NG/diesel and methanol/diesel dual fuel, simultaneously reduce
644
the NOx and PM (or soot) emissions compared to diesel. The decline of BTE leads to the higher HC
645
and CO emissions.
646
Conflict of interest
647
The authors do not have any conflict of interest with other entities or researchers.
648
Acknowledgments
649
This study was supported by National Engineering Laboratory for Mobile Source Emission Control
650
Technology (NELMS2017B02), and the Special Fund for Basic Scientific Research of Central Colleges,
651
Chang’an University (310822172203).
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Dr. Yisong Chen is lecturer in Chang’an University, School of Automotive. He received his PhD degree in
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vehicle engineering from Hunan University in China at 2014, done postdoctoral research at Tsinghua
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University form 2014 to 2015. His research interests include alternative fuels of automobiles, life cycle
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assessment of automotive and strategic research into the automotive industry in China.
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Dr. Hao Chen is an associate professor in Chang’an University, School of Automotive. He obtained his
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PhD degree and Master degree in vehicle engineering from Chang’an University. He is interested in
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the fields of alternative fuels of automobiles, diesel engine, engine and fault diagnosis, life cycle
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assessment of automotive, etc.
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S2095-7564(18)30343-X
DOI:
10.1016/j.jtte.2018.05.001
Reference:
JTTE 185
To appear in:
Journal of Traffic and Transportation Engineering (English Edition)
Received Date: 20 February 2018 Revised Date:
18 May 2018
Accepted Date: 21 May 2018
Please cite this article as: Chen, Y., Ma, J., Han, B., Zhang, P., Hua, H., Chen, H., Su, X., Emissions of automobiles fueled with alternative fuels based on engine technology: a review, Journal of Traffic and Transportation Engineering (English Edition) (2018), doi: 10.1016/j.jtte.2018.05.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Reviews
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Emissions of automobiles fueled with
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alternative fuels based on engine
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technology: a review
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Yisong Chen, Jinqiu Ma, Bin Han, Peng Zhang, Haining Hua, Hao Chen*, Xin Su
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School of Automobile, Chang’an University, Xi’an 710064, China
10 Article history
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Received 20 Feb 2018
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Received in revised form 18 May 2018
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Accepted 21 May 2018
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Available online
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Emissions of automobiles fueled with main alternative fuels are reviewed.
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Emissions of NG/gasoline bi-fuel, NG and NG/diesel dual fuel engines are analyzed.
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Emissions of SI engines fueled with oxygenated fuels are analyzed.
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Emissions of CI engines fueled with oxygenated fuels are analyzed.
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Abstract
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Diversification of alternative fuels for automobiles is not only an actual situation, but also a
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development trend. Whether the alternative fuels are clean is an important issue. Emissions of
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automobiles fueled with natural gas (NG), methanol, ethanol, biodiesel, dimethyl ether (DME) and
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polyoxymethylene dimethyl ethers (PODEn) are investigated and reviewed based on engine
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technology and fuel properties. Compared to gasoline, NG/gasoline bi-fuel and NG automobiles have
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higher brake thermal efficiencies (BTE) and produce less HC, CO and PM emissions, while more NOx
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emission. Compared to diesel, NG/diesel dual fuel automobiles have lower BTE and emit lower soot
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and NOx emissions, but higher HC and CO emissions. Methanol and ethanol blending in gasoline can
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obviously reduce the HC, CO and PM emissions of spark ignition (SI) automobiles. Methanol or
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ethanol blending in diesel may prolong the ignition delay, shorten the combustion duration and improve
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the BTE, resulting in lower soot emissions. However, the HC, CO and NOx emissions of methanol or
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ethanol diesel blend fuels are uncertain due to low cetane number, high latent heat of vaporization.
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Most biodiesel has higher viscosity, distillation temperature, cetane number and oxygen content than
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diesel. Soot emission of biodiesel is lower than that of diesel, while NOx emission is higher. Both DME
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and PODEn do not contain C-C bonds and their blend with diesel can prohibit the formation of soot.
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PODEn has high cetane number and low viscosity, resulting in better ignitability and spray quality
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respectively. PODEn blending shortens both the ignition delay and the combustion duration, improves
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the BTE, and increases the temperature in the diffusion combustion phase, leading to a higher NOx
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emission.
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Keywords:
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Natural gas; Methanol; Ethanol; Biodiesel; DME; PODEn; emission.
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* Corresponding author. Tel.: +86 29 82334471.
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E-mail addresses: [email protected] (Y. Chen), [email protected] (H. Chen).
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1
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Since opening-up policy was implemented, China has experienced dramatic development, with
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averaged 9.8% annual growth rates of gross domestic product (GDP), in comparison with the world’s
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average of 3.3% (Xu et al., 2017). Problems of the world environment like global warming and energy
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resources will cause great constraint on the world economy and may affect profoundly the basic
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condition of human survival. As an important vehicle, automobile play an important role in people’s
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daily life and commercial activities. Besides, automobile industry is in an up road of the overall industry
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system (He et al.,2017).
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Introduction
Traditional automobile fuels derived from non-renewable fossil oil. Automobiles emit huge amounts
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of pollutants and greenhouse gases (GHG), such as NOx, PM, CO HC and CO2, and exert great
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influence on atmosphere environment and global warming. To protect the environment and ease the
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climate change, the automobile technology worldwide tends to develop in the directions of energy
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diversification and power electrification. Electrification of automobiles has been considered as the most
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effective way and the ultimate solution. However, even if the technologies of power battery, electric
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motor and electronic control continuously improve, two significant problems still exist. One is that the
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power structure is inadequate and the other is the power constraint if electric automobiles are widely
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applied. Diversification of clean alternative fuels for automobiles is regarded as the best choice and
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path in the transitional period between petroleum fuels and electrification.
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In general, alternative fuels include fossil and renewable fuels. They can also be classified into
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gaseous and liquid alternative fuels. Specifically, liquefied petroleum gas (LPG), natural gas (NG),
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alcohols mainly involving methanol and ethanol, ethers mainly including dimethyl ether (DME) and
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polyoxymethylene dimethyl ethers (PODEn), and biodiesel are in localized or industrial application. Gaseous fuels such as compressed natural gas (CNG) are promising alternative fuels which receive
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more attention all over the world. Natural gas is a very promising and highly attractive fuel because of
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its domestic availability, widespread distribution infrastructure, low cost, and clean-burning qualities to
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be used as a transportation fuel (Wei and Peng, 2016). In that case, CNG is considered to be a
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“cleaner” fuel compared to other fossil fuels. Therefore, it is used as an alternative fuel in motor
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vehicles to reduce emissions of air pollutants in transportation (Wang et al., 2016a). Besides, it is also
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applied in gasoline engine as a bi-fuel, added to diesel fuel and mixed with hydrogen to make better
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engine and emission performance. Generally, there are three application modes of natural gas, which
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are NG (CNG/LNG/HCNG), diesel/CNG dual fuel and CNG/gasoline bi-fuel.
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Among alternative fuels for gasoline, methanol (CH3OH) fuel has been considered to be one of the
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most favorable fuels for internal combustion (IC) engines due to the high octane number and the high
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intramolecular oxygen content (Avinash et al., 2014; Li, et al., 2010a; Zhen et al., 2013). A large
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number of domestic and foreign scholars have studied the application of methanol on internal
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combustion engine: it can be used with different application modes (mixed or pure methanol) in a
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spark-ignition (SI) engine (Agarwal et al., 2014; Gravalos et al., 2013; Lennox et al., 2014; Liu et al.,
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2007; Zhen and Wang, 2015) or with dual-fuel mode in a compression-ignition (CI) diesel engine (Pan
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et al., 2015; Park et al., 2017; Wang et al., 2008a; Wei et al., 2015). Ethanol and methanol have similar
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physical and chemical properties, is considered to replace fossil fuels of environmentally friendly fuel,
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which can be blended with other fuels at different proportions to improve engine emissions (Battal et al.,
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2017; Oh et al., 2010; Shi et al., 2015; Turner et al., 2011). The chemical formula of dimethyl ether (DME) fuel is CH3OCH3; it is the simplest ether compound.
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Among alternative fuels, the application of DME for diesel engines has been discussed by many
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investigators because it has no carbon-carbon bonds and excellent self-ignition characteristics
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compared to other fuels. Meanwhile, the cetane number of DME fuel is significantly higher than that of
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conventional diesel fuel (Park, 2012; Park and Lee, 2013; Roh et al., 2015; Semelsberger et al., 2006;
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Youn et al., 2011). Due to its fuel characteristics, DME is particularly suitable to be used as a complete
99
substitute for diesel in compression ignition (CI) engines. It can be used as a blend with diesel to
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overcome limitations of using pure DME, such as poor viscosity, low density and its after-effects. DME
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could also be used as an additive or as an ignition promoter in conventional diesel combustion and for
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dual fuel operation (Lee et al., 2011; Su et al., 2016; Thomas et al., 2014; Wang et al., 2013, 2014). In
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recent years, it has been found that the mixture of polyoxymethylene dimethyl ethers (PODEn) is a
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promising engine fuel. Chemical expression of PODEn is CH3O(CH2O)nCH3. As an oligomer of ether,
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PODEn has a higher cetane number and oxygen content and does not contain C-C bonds, which has
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the potential to reduce diesel smoke and PM emissions (Lei et al., 2009; Liu et al., 2016b, 2017b;
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Pellegrini et al., 2012), and it can be mixed with diesel in any proportion (Burger et al., 2010; Härtl et al.,
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2015; Liu et al., 2016a; Pellegrini et al., 2013). In fact, n from 3 to 8 of PODEn has an excellent
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performance for diesel additives (Stroefer et al., 2010; Zhao et al., 2013). At room temperature, the
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PODEn-diesel mixture has excellent stability (Härtl et al., 2015; Pellegrini et al., 2012; Zhao et al.,
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2013). Generally, it can be used as a clean diesel blending components due to its diesel similar
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physical properties and do not need transformation of vehicle engine oil supply system (Liu et al., 2017;
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Xie et al., 2017b).
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Biodiesel, also called fatty acid methyl esters, is mainly made from vegetable oils or animal fats and
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is an ideal alternative fuel for diesel engines. Compared to petroleum diesel, biodiesel has higher
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cetane number, about 10% intramolecular oxygen, almost no aromatic hydrocarbon and sulfur. This
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leads to different fuel injection and spray properties, combustion characteristic, and exhaust emissions
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from petroleum diesel fuel (Hasan and Rahman, 2017; He et al., 2007; Lou and Tan, 2016; Miri et al.,
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2016; Xu et al., 2012). But there are some disadvantages of biodiesel which restrain its wide
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application and hinder its use as a complete replacement for diesel, which include higher kinematic
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viscosity, freezing temperature and density, as well as its low calorific value. Viscosity of biodiesel not
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only affects flow at all temperatures a fuel may be exposed to but also strongly influences the
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atomization of a fuel upon injection into the combustion chamber and ultimately the possible formation
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of engine deposits (Knothe and Gazon, 2017). That is why many scientists and investigators have
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studied blends of biodiesel with diesel by varying the proportions of biodiesel and diesel to investigate
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their suitability as a fuel in existing diesel engines. These problems associated with biodiesel can be
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overcome by using biodiesel-diesel blends (Nair et al., 2016; Sun et al., 2010; Yasin et al., 2015; Yusaf
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et al., 2011).
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Emissions of NG automobiles based on engine technology
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2.1
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In general, NG vehicles have lower emissions compared to gasoline or diesel engines (Kakaee and
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Paykani, 2013). Specifically, a decreasing trend is found for PAHs, SO2 and CO concentrations, while
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the NOx level is increased in comparison to those before the implementation of CNG (Ravindra et al.,
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2006). Due to the high octane number of NG, engines can be operated with a higher compression ratio
135
(CR) for a better thermal efficiency (Liu et al., 2012). CRs of NG engines are commonly designed in the
136
range of 11~13, which are higher than those of gasoline engines. CRs and application modes of NG
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(CNG/LNG/HCNG) sole fuel, diesel/CNG dual fuel and CNG/gasoline bi-fuel engines are shown in Fig.
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1.
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Pure NG (CNG/LNG)
Fig. 1 CRs and application modes for NG automobiles with different engine modes.
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Fig. 2 Typical natural gas composition by volume (Kakaee et al., 2014)
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It has been reported that the engine performance and emission are greatly affected by varying
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compositions of natural gas (Fig. 2). The most important NG fuel property is the Wobbe number (WN).
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Generally, it was agreed by researchers that the fuels with higher hydrocarbons, higher WN, and
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higher energy content exhibited better fuel economy and carbon dioxide (CO2) emissions. NOx
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emissions were also increased for gases with higher levels of higher WN, while total hydrocarbons
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(THCs) and CO showed some reductions (Kakaee et al., 2014). The results indicate that higher WN
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improves the combustion and the efficiency as well.
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Lean burn is an effective way to decrease the NOx emissions, while it results in high cyclic variation.
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Dilution is another method to achieve lean burn and low NOx emissions. Common dilution gases are N2,
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CO2 and Ar. The results show that the thermal efficiency first increases and then decreases as the
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dilution ratio (DR) of Ar increases and NOx emissions decrease significantly (Li et al., 2015). Ar dilution
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is superior in maintaining higher thermal efficiencies than CO2 and N2 for NG engines (Li et al., 2015).
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Lean burn as well as EGR successfully satisfied the legal emission regulation when the level of dilution
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was increased to the dilution limit, although there was a slight reduction in efficiency (Lee et al., 2014).
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2.2
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Natural gas/diesel dual fuel is an operation mode in which natural gas is introduced into the intake air
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of the inlet manifold and then ignited by the direct injected diesel in the cylinder. This mode has both
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economic and environmental benefits. Due to the high auto-ignition temperature, NG can hardly be
161
burned through compression on diesel engines. In dual fuel mode, diesel acts as the ignition source
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and NG provides the main energy if needed for combustion. Generally, dual fuel engine exhibited
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longer ignition delay than diesel; had lower thermal efficiency than diesel at low and partial loads and
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higher at medium and high loads; emitted less NOx emissions than diesel engine, while more HC and
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CO emissions (Abdelaal et al., 2013; Abdelghaffar, 2011; Cheenkachorn et al., 2013).
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NG/diesel dual fuel
The application of dual fuel mode significantly decreases the NOx, CO2 and PM emissions
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(Cheenkachorn et al., 2013; Liu et al., 2013; Lounici et al., 2014; Meng et al., 2016), compared to
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diesel engines. The trade-off relationship between NOx and PM emission is solved. However, the
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hydrocarbon (HC) and carbon monoxide (CO) emissions may increase by several times in comparison
170
to normal diesel combustion (Liu et al., 2013). The brake thermal efficiency (BTE) of dual fuel mode is
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lower at low and intermediate loads, while under high engine load conditions it is similar or a little
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higher when compared with normal diesel mode (Lounici et al., 2014). Dual fuel mode showed a
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simultaneous reduction of soot and NOx species over a large engine operating area (Lounici et al.,
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2014; Meng et al., 2016). In sum, trade-off relationship between soot and NOx emissions of diesel
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engines can be solved by using dual fuel mode, whereas the BTE of engine decreases and HC
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emission increases. Nithyanandan et al. (2016) found that the use of CNG affects the morphology and nanostructure of
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PM, and hence the oxidation reactivity of the soot. In Singh’s study (Singh et al., 2016), CMD (count
179
mean diameter) graph showed that average size of particulate emitted CNG engines were much
180
smaller compared to mineral diesel particulate. The addition of compressed natural gas lowers CO2
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emission and decreases opacity of exhaust gases in all load modes, the best positive impact has been
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achieved with the highest CNG portions (Vygintas et al., 2017). Liu et al. (2015) found that the
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developed dual fuel model is capable to predicate the flame propagation and emissions formation
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process in the dual fuel engine. Flame quench region of the fuel-lean mixture within the squish volume
185
is the dominant source of CO emissions a low engine speed condition. However, bulk gas complete
186
oxidation is impeded by the failure to transition into strong high temperature combustion in the cylinder
187
center region, which accounts for the majority of CO emissions at high engine speed. NO formation
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region follows the development of the high temperature field for both low and high engine speed, which
189
is generated by the combustion of the pilot diesel. Therefore, the injection strategy and quantity of the
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pilot fuel significantly determine the final exhaust NOx emissions during dual fuel operation conditions.
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On the whole, NG/diesel dual fuel automobiles simultaneously reduce CO2, NOx and PM emissions
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compared to diesel ones, while increase the HC and CO emissions.
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2.3
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Laminar burning velocity of NG is lower than that of methane, which is 48 cm/s (Korb et al., 2016). The
HCNG
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velocity of H2 is 290 cm/s and accordingly its addition in NG will surely accelerate the combustion
196
speed, shorten the combustion duration and hence improve the thermal efficiency. Higher efficiency is
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generally correlated with higher NOx emission, lower HC and CO emissions, and lower brake specific
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fuel consumption (BSFC). Compared to NG, HNG present higher peak combustion pressure and
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combustion temperature, and more concentrated heat release as shown in Fig. 3 (Korb et al., 2016).
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Fig. 3 Comparison of combustion characteristics between NG and HNG (Korb et al., 2016). (a) In-cylinder pressure and mean
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temperature. (b) Mass fraction burned and ROHR.
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Mathai et al. ( 2012) made a comparative evaluation of performance, emission, lubricant and deposit
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characteristics of spark ignition engine fueled with CNG and 18% hydrogen-CNG and found that
207
HCNG fueled engine decreased BSFC, CO and HC emissions with the increase of NOx emission.
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Another study showed the effects of 0, 5%, 10% and 15% blends of hydrogen by energy with CNG on
209
bi-fuel NA SI engine using SPFIS (Nitnaware et al., 2016). MBT spark timing shown improvement in
210
performance parameters with reduction in NOx emission. Carbon based emission reduced and NOx
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emission increased with increase in hydrogen addition. The optimum maximum brake torque (MBT)
212
spark timing of 25°CA BTDC and injection pressure 2.6 bar is observed for 5% hydrogen addition at
213
2500 rpm. Zareei et al. (2012) indicated that thermal efficiency, combustion performance, NOx
214
emissions improved with the increase of hydrogen addition level. The HC and CO emissions first
215
decrease with the increasing hydrogen enrichment level, but when hydrogen energy fraction exceeds
216
12.44%, it begins to increase again at idle and stoichiometric conditions.
218
Fig. 4 Comparison of emissions for NG vehicles.
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In sum, NG automobiles with different types of engines have different environmental effects,
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summarized in Fig. 4. As a whole, improvement of BTE brings high NOx emission and low HC, CO and
221
PM emissions for SI engines. NG/diesel dual fuel engines are modified on diesel engines and the
222
condition is complicated. Both NOx and PM emissions of dual fuel engines are reduced compared to
223
diesel. The decrease of BTE of dual fuel engine causes high HC and CO emissions.
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3
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The most common alcohols used on automobiles are methanol and ethanol. Although methanol can be
226
produced from a wide variety of renewable sources and alternative fossil fuel based feedstocks, in
227
practice methanol is mainly produced from natural gas and in China from coal (Chen et al., 2014;
228
Sayah and Sayah, 2011; Vancoillie et al., 2013). Fuel ethanol can be produced from both edible
229
feedstocks such as corn, wheat and stale grain and non-edible crops such as cassava and sweet
230
sorghum.
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Methanol is a colorless, polar and flammable liquid which has higher octane number and heat of
232
vaporization values as compared to gasoline (Zhen and Wang, 2015). It is known that ethanol and
233
methanol have higher laminar flame speed, higher octane number and higher intramolecular oxygen
234
contents than those of gasoline. Accordingly, the combustion duration will be surely shortened. Lennox
235
et al. (2014) and Agarwal et al. (2014) studied the effects of methanol/gasoline blends on engine
236
performance, combustion and emission characteristics, and compared with pure gasoline. The result
237
showed that methanol/gasoline blends can reduce the combustion duration and exhaust gas
238
temperature, increase the peak heat release rate (PHRR), and increase the BTE. Wu et al. (2016)
239
experimentally investigated the effects of pure methanol on the combustion performance under idle
240
condition based on a SI engine. The results showed that the SI engine fueled with methanol illustrated
241
better lean burn performance than the engine fueled with gasoline. Compared with the engine fueled
242
with gasoline, the indicated thermal efficiency (ITE) of engine fueled with methanol was increased; the
243
flame development and propagation periods were shortened. On the whole, the combustion duration is
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shortened and the heat release is concentrated when alcohols are blended in gasoline. As a result, the
245
combustion process is improved. Although the BTEs of alcohols/gasoline blends increase, engine
246
power and torque decrease with the increase fraction of alcohol, due to the low heating value (Liu et al.,
247
2007). Using methanol/gasoline blends on a spark-ignition engine can significantly reduce CO and HC
248
emissions (Agarwal et al., 2014; Gravalos et al., 2013; Lennox et al., 2014; Liu et al., 2007; Wang et al.,
249
2015). Battal et al. (2017), Turner et al. (2011) and Oh et al. (2010) investigated the effects of
250
ethanol/gasoline mixtures on the performance and emissions at a spark-ignition engine. It is shown
251
that the benefits of adding ethanol into gasoline are reduced combustion duration and increased
252
in-cylinder pressure and combustion efficiency. Experiments and theoretical calculations showed that
253
ethanol added fuels show reduction in CO, CO2 and NOx emissions without significant loss of power
254
compared to gasoline. But it was measured that the reduction of the temperature inside the cylinder
255
increases HC emission.
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N2+ O ↔ NO + N
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(1)
N2 + O2 ↔ NO + O
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N + OH ↔ NO + H
(3)
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As for NOx emission, the condition is extremely complicated. In general, thermal NOx, prompt NOx,
260
and fuel NOx are the three formation processes. Thermal formation is representative when
261
temperatures are high and the relative air to fuel ratio is close to 1. The reactions that take part on this
262
mechanism were described firstly by Zeldovich (1946) and later extended by Bowman et al. (1975),
263
described from Eq. (1) to Eq. (3). Agarwal et al. (2014) indicated that gasohol produced lower mass
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emissions of NO and smoke opacity. The result was attributed to the higher latent heat of vaporization
265
of methanol compared to gasoline. Particulate size-number concentration was lower for gasohol
266
blends in comparison to gasoline at all engine operating conditions. Wang et al. (2015) indicated that
267
evident decrease in NOx emission was noticed with M15 and M25 fueling, but in the case of M40, NOx
268
emissions were similar with gasoline. Mustafa et al. (2013) investigated the combustion and exhaust
269
emissions characteristics of a SI engine fueled with the ethanol/gasoline (E5, E10) and
270
methanol/gasoline (M5, M10) fuel blends. NOx emissions decreased for all wheel powers at the speed
271
of 80 km/h. It has been also observed that the usage of alcohol fuel instead of gasoline caused to
272
decrease the NOx, and to increase CO2 emission because of the improved and completed combustion
273
(Balki et al., 2014; Mustafa and Balki, 2014; Wu et al., 2016).
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The increase of methanol increases the formaldehyde emissions and methanol emission increases
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with the increase of engine load under different speeds (Liu et al., 2007; Wang et al., 2015). Injection
276
and ignition parameters also have significant influence on the combustion and emission of engines
277
fueled with alcohol/gasoline blends. Advancing methanol injection timing decreased the HC and CO,
278
while increased the NOx emission (Qu et al., 2015). Retarding ignition timing decreased the HC and
279
NOx emissions and the effect of ignition timing changes on CO emission is small (Qu et al., 2015). The
280
HC, CO, and NOx emissions of rich mixture are higher than those of lean mixture. Increasing intake air
281
temperature decreased the HC and CO emissions. Retarding methanol injection timing, advancing
282
ignition timing, using lean mixture and reducing intake air temperature can decrease the formaldehyde
283
emission (Li et al., 2010b).
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Alcohols can also be applied on diesel engines through two methods: dual fuel mode or
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emulsification/micro-emulsification. For dual fuel mode, methanol is mainly used. Compared with
286
conventional diesel, the methanol/diesel dual fuel combustion mode significantly reduces NOx and PM
287
emissions (Pan et al., 2015; Park et al., 2017; Wang et al., 2008; Wei et al., 2015). Park et al. (2017).
288
When the amount of methanol is increased, cylinder pressure and temperature decreased. The
289
resulting decrease in the combustion efficiency lowered the NOx emission and BTE of the engine.
290
Wang et al. (2008a) found that the increase in methanol mass fraction lowers the polytropic index of
291
compression and the temperatures at BDC and TDC, as well as the oxygen concentration in the
292
mixture. This prolongs the ignition delay under the same engine load and speed condition by
293
comparison with diesel operation. The heat release rate changes from dual-peak mode to single-peak
294
mode. The high methanol mass fraction will realize a simultaneous reduction in both smoke and NOx
295
under all the operating conditions. Meanwhile, the NOx-smoke trade-off curve disappears in
296
combustion of the dual fuel, but CO and HC increase. Wei et al. (2015) investigated the combustion
297
and emission characteristics of a dual fuel diesel engine with high premixed ratio of methanol (PRM).
298
High PRM prolonged the ignition delay but shortened the combustion duration and decreased the
299
in-cylinder gas temperature. Both NOx and dry soot emissions were significantly reduced, while HC,
300
CO, formaldehyde emissions and NO2 in NOx increased significantly with the increase of PRM (Wei et
301
al., 2015). Pan et al. (2015) and Geng et al. (2014) found that there was a strong coupling between the
302
intake air temperature and the methanol fraction to performance and emissions of the engine. At dual
303
fuel operation mode, decreasing intake air temperature reduced the indicated thermal efficiency (ITE)
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and exhaust gas temperature and the trend was more evidently as methanol energy fraction increased.
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Decreasing of intake air temperature also prolonged the ignition delay, which caused a later
306
combustion phasing and smaller peak cylinder pressure. By the induction of methanol, NOx, NO and
307
smoke emissions decreased markedly, while NO2, CO, THC, formaldehyde and methanol emissions
308
increased. However, increasing the intake air temperature would inhibit the NO2, THC, CO,
309
formaldehyde and methanol emissions and increase NO, NOx and soot emissions. Overall,
310
methanol/diesel dual-fuel combustion performs better in terms of engine performance and emissions
311
reduction under rich mixture conditions (Amin et al., 2015). Chen et al. (2017b) and Li et al. (2016)
312
investigate the effects of diesel injection parameters on the rapid combustion and emissions of the
313
diesel-methanol dual-fuel engine. The experimental results show that the diesel injection parameters
314
affect rapid combustion fraction (α) greatly, which increases as the diesel injection pressure rises while
315
decreases as the diesel injection timing advanced or diesel injection quantity increases. Liu et al. (2015)
316
indicated that at low injection pressure, the IMEP of dual fuel mode is lower than that of pure diesel
317
combustion mode. COVIEMP of dual fuel mode firstly decreases and then increases with the increasing
318
of injection pressure. Their researches shows both of NOx and smoke emissions are reduced while CO
319
and HC emissions increased obviously in dual fuel mode (Liu et al., 2015). Smoke emission can be
320
further reduced by coupling the high diesel injection pressure and the advanced diesel injection timing.
321
However, NOx emission may be increased in this case (Park et al., 2017). The methanol
322
co-combustion ratio (CCR) is defined as the ratio of methanol energy to the total energy in the dual-fuel
323
mode. θin is the injection timing. Comparison between emissions of diesel/methanol dual fuel (DMDF)
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engine and normal diesel is presented in Fig. 5(a) and Fig. 5(b).
325
(a)
326
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Fig. 5 Comparison of emissions between diesel and DMDF (Li et al., 2016). (a) CO and HC emissions. (b) NOx and soot
328
emissions.
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Alcohols can hardly be mixed with diesel, because the alcohols are polar solvents and diesel is
330
non-polar fuel. Cosolvents and surfactants must be used to help the mixing of diesel and alcohols,
331
forming emulsified fuel or micro-emulsified fuel. Emulsified fuel is in the condition of milkiness and
332
opaque, and the mixing state is unstable. Micro-emulsified fuel is transparent and stable. Some studies
333
confirmed that diesel/biodiesel blend fuels can mixed with a low volume ratio alcohols, forming
334
micro-emulsified fuel. Further studies (Aydin et al., 2015, 2017; Shi et al., 2015) have found that
335
biodiesel/ethanol/diesel fuel blends can be directly used on a diesel engine for lower PM and THC
336
emissions. However, a major drawback is that ethanol is immiscible in diesel over a wide range of
337
temperatures and water content because of their difference in chemical structure and characteristic,
338
these can result in fuel instability (Prommes et al., 2007).
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Both methanol and ethanol can be applied on SI or CI engines. The ultimate aim is to improve the
340
oxygen content and thus the combustion efficiency. As a result, HC, CO and PM emissions are
341
reduced. Methanol/diesel dual fuel engine is an exceptional case, which is similar with NG/diesel dual
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fuel. The BTE of methanol/diesel dual fuel engine declines compared to diesel, producing lower NOx
343
and soot emissions and higher HC and CO emissions.
344
4
345
Most researchers reported that use biodiesel in a conventional diesel engine brings about a
346
considerable reduction in CO, CO2 and PM (Hasan and Rahman, 2017; He et al., 2007; Lou and Tan,
347
2016; Miri et al., 2016; Xu et al., 2012). Feedstocks of biodiesel are very abundant and they can be
348
classified into three generations. The first generation refers to the edible crops such as soybean,
349
rapeseed and palm oil, the second includes oil plants such as jatropha curcas, pistacia chinensis and
350
sapium sebiferum, and the third is microalgae such as Scenedesmus obliquus. Production principle
351
can be summarized into a common formula, shown in Fig. 6. Abundant feedstocks result in significant
352
difference in chemical composition of biodiesel and thus its properties. Table 1 lists the composition
353
and properties of some typical biodiesel (Jain and Sharma, 2011; Serrano et al., 2013; Wang et al.,
354
2012). Biodiesel mainly contained the same five components: methyl palmitate (C17H34O2), methyl
355
stearate (C19H38O2, C18:0), methyl oleate (C19H36O2, C18:1), methyl linoleate (C19H34O2, C18:2) and
356
methyl linolenate (C19H32O2, C18:3) (Shi et al., 2018). The molecular structure of the above
357
components is exhibited in Fig. 7. Cetane number of methyl palmitate, methyl stearate, methyl oleate,
358
methyl linoleate and methyl linolenate are respectively 86, 101, 59, 38 and 23. It can be concluded that
359
the higher the mass fraction of saturated fatty acid methyl esters (S-FAMEs) in biodiesel is, the higher
360
the cetane number is and the better the oxidative stability is, while the worse the low temperature
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Emissions of automobiles fueled with biodiesel based on engine technology
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fluidity is. To ensure the basic use on diesel engines, the low temperature fluidity of biodiesel must be
362
guaranteed and as a result the mass fraction of unsaturated FAMEs (U-FAMEs) is generally higher
363
than 70% by weight. Consequently, cetane number of biodiesel in common use is a little higher than
364
that of diesel, which contributes to a better ignitability. Fig. 8 presents the heat release and in-cylinder
365
temperature of a common rail diesel engine fueled with biodiesel and diesel. Biodiesel has higher
366
viscosity and distillation temperature, leading to a poor homogeneity of mixture gas and atomization
367
quality. Consequently, the HRR of biodiesel in the pre-mixed combustion phase is lower than that of
368
diesel, and so does the PHRR. With the proceeding of combustion, biodiesel produces more active
369
radicals and accelerates the speed of chemical reaction, resulting in higher HRR compared to diesel in
370
the diffusion combustion phase. As a result, the in-cylinder temperature of biodiesel is higher and thus
371
the NOx emission.
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Fig. 6 Chemical reaction formula for biodiesel production
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Fig. 7 Chemical structure of five major components in biodiesel
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Due to the high cetane number, SOC of biodiesel advances and the ignition delay shortens.
377
In-cylinder temperature of biodiesel is slightly higher than that of diesel (shown in phase Ⅰ from A to B,
378
Fig. 8). Lower LHV, higher viscosity and lower volatility commonly contribute to the slower combustion
379
and hence the lower PHRR and combustion temperature of biodiesel, compared to diesel (shown in
380
phase Ⅱ from B to C, Fig. 8). It can be concluded that the intensity of diffusion combustion for
381
biodiesel is obviously higher than that of diesel, leading to the higher PCT and combustion temperature
382
(shown in phase Ⅲ from C to D, Fig. 8). It is mainly because that the intramolecular oxygen increases
383
the OH intensity of biodiesel along the injector axial line and the active radicals surely accelerate the
384
combustion speed. Fast combustion promotes the complete combustion of biodiesel and the
385
combustion duration shortens. As a result, the in-cylinder temperature of biodiesel is lower than that of
386
diesel, as shown in the phase Ⅳ (Fig. 8).
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387 388 389 390
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(a)
(b)
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Fig. 8 Comparison of heat release and in-cylinder temperature between diesel and biodiesel. (a) Heat release. (b) Temperature.
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Table 1 Composition of biodiesel derived from three generation feedstocks. Vernicia
Idesia
Sapium
Scenedesmus
Xanthoceras
Armeniaca
(%)
fordii
polycarpa
sebiferum
obliquus
sorbifolia
sibirica
C14:0
/
/
/
/
/
0.03
C16:0
3.56
15.50
7.21
18.42
5.27
C16:1
/
6.65
/
2.31
/
C16:2
/
/
/
3.26
/
C18:0
2.62
1.39
2.28
3.43
1.92
C18:1
10.57
9.53
14.61
49.64
C18:2
14.64
64.81
31.72
11.30
C18:3
59.20
2.08
41.02
8.26
C20:0
/
/
/
/
C20:1
0.90
0.04
/
/
C20:3
/
/
/
Others
8.51
/
/
S-
6.18
16.89
9.49
93.61
83.11
90.51
Cetane number
37.0
45.0
CFPP
-11
-5
Oxidative
0.4
0.7
(Wang et al.,
(Wang et al.,
2012)
2012)
Ref.
Elaeis
chinensis
0.30
/
/
0.10
1.00
3.79
10.90
19.75
23.14
5.10
44.80
0.67
/
/
0.99
/
0.30
/
/
/
/
/
/
1.01
3.20
4.63
1.18
2.10
3.80
SC
curcas
M AN U
Rapeseed
guineensis
46.83
44.35
57.90
39.90
44.47
28.92
54.50
28.50
28.51
24.70
9.28
6.46
0.14
6.80
0.01
0.84
7.90
0.22
0.20
0.09
0.10
0.17
0.10
0.20
0.35
/
0.12
/
/
/
1
/
/
7.27
/
/
/
/
/
/
3.38
3.24
/
0.20
/
0.89
/
0.03
21.85
8.00
4.92
14.50
24.66
24.42
7.50
50.27
74.77
92.00
95.08
85.30
75.34
74.82
91.50
49.70
43.0
46.0
47.6
48.8
/
51.0
51.30
/
61.0
-11
-6
-8
-14
/
0
-3
/
12
0.8
1.2
1.7
2.7
2.9
3.3
4.2
4.6
7.7
(Wang et al.,
(Serrano et
(Jain and
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24.00
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Pistacia
65.23
EP
U
Jatropha
31.17
FAMEs
FAMEs
Soybean
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Composition w
(Wang et al., (Serrano et al., (Wang et al., 2012)
2013)
2012)
2012)
(Wang et (Serrano et (Wang et al.,
al., 2013) Sharma, 2011) al., 2012)
al., 2013)
2012)
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There are many researches find that rapeseed biodiesel and its blends have higher cetane number,
397
increased oxygen content, higher density and viscosity, but inferior lower heating value, and
398
compressibility when compared to diesel fuel. Miri et al. (2016), Aldhaidhawi et al. (2017) and Ismet et
399
al. (2012) investigated the performance, combustion and emission characteristics of a diesel engine
400
fueled with rapeseed biodiesel and its blends and compared with pure petroleum diesel fuel.
401
Researches results reveal that rapeseed biodiesel, either pure or blended with diesel, has lower heat
402
release rate, reduced ignition delay and lower thermal efficiency. Meanwhile, effective power and
403
torque decrease at all engine loads. Regarding gaseous emissions, rapeseed biodiesel increase NOx
404
emissions, other emissions such as those of CO and particulate matter PM are usually found to
405
significantly decrease with rapeseed biodiesel content.
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Qi el al. (2009; 2010) and Özener et al. (2014) investigated the effect of biodiesel produced from
407
soybean crude oil on the combustion characteristics, performance and exhaust emissions of a diesel
408
engine. The results showed that biodiesel exhibited the similar combustion stages to that of diesel,
409
however, biodiesel showed an earlier start of combustion. At lower engine loads, the peak cylinder
410
pressure, the peak rate of pressure rise and the peak of heat release rate during premixed combustion
411
phase were higher for biodiesel than for diesel. At higher engine loads, the peak cylinder pressure of
412
biodiesel was almost similar to that of diesel, but the peak rate of pressure rise and the peak of heat
413
release rate were lower for biodiesel. The power output of biodiesel was almost identical with that of
414
diesel. It is also observed that there is a significant reduction in CO and smoke emissions at high
415
engine loads, while NOx emissions increased. Moreover, biodiesel provided significant reduction in CO,
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HC, NOx and smoke under speed characteristic at full engine load (Qi et al., 2009). Özer et al. (2016)
417
indicated that the maximum heat release rate and maximum in-cylinder pressure were mostly
418
increased with the combined effects of biodiesel fuel addition and EGR application. Improvements on
419
the THC emissions were obtained by the use of 20 vol.% soybean biodiesel fuel blend and increase of
420
EGR rate at the low and partial engine loads, while deterioration occurred at the high engine load when
421
the EGR was increased to over 5% rate.
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Wei et al. (2017) investigated the influence of waste cooking oil (WCO) biodiesel on the combustion,
423
emissions characteristics of a diesel engine. The increase of in-cylinder pressure is mainly due to the
424
advanced start of combustion and more complete combustion when using biodiesel. Lower maximum
425
heat release rate is due to the less intense combustion in the premixed combustion phase. Earlier start
426
of combustion is mainly attributed to the higher bulk modulus and higher viscosity of biodiesel.
427
Biodiesel reduces the weighted particle mass concentration and the weighted geometric mean
428
diameter of the particles. Enweremadu and Rutto (2010) found that the engine performance of the
429
WCO biodiesel and its blends were only marginally poorer compared to diesel. From the standpoint of
430
emissions, NOx emissions were slightly higher while HC emissions were lower for WCO biodiesel when
431
compares to diesel fuel. Compared with other vegetable oils and petroleum diesel fuels, palm oil is
432
associated with better engine performance and shorter ignition delay. Use of palm oil also reduces
433
exhaust emission of HC, CO and smoke and exhaust gas temperatures, while significantly improve
434
levels of NOx (Leevijit et al., 2017; Mosarof et al., 2015; Ndayishimiye and Tazerout, 2011).
435
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However, some researches showed that the addition of biodiesel reduces NOx emissions and
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increases in soot emissions (Nair et al., 2016; Yasin et al., 2015; Yusaf et al., 2011). Sun et al. (2010)
437
report that inconsistencies of NOx emission appear among studies due to different with engine type,
438
engine technology, and fuel feedstock.
439
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Emissions of automobiles fueled with ethers based on engine technology
DME, CH3-O-CH3, has a higher cetane rating and oxygen content than diesel and has good
441
evaporation characteristics in the combustion chamber. Meanwhile, it has no direct C-C bonds which
442
produces considerably less pollutants like HC, smoke and particulate matter (PM) than conventional
443
fuels. This property makes it very attractive as a clean fuel for transportation and domestic utilization.
444
Therefore, it is an excellent, efficient alternative fuel for diesel engines. At present, most of
445
investigations focused on the pure DME combustion or effects of DME quality on CI engine fuel
446
economy and emissions (Hou et al, 2014; Su et al., 2014; Su and Chang, 2014; Park and Lee, 2013;
447
Youn et al., 2011). DME has good solubility with diesel (Wang et al., 2008b) (Fig. 9(a)). It is also found
448
that compared with diesel, BSECs of blend fuels firstly decrease and then increase, as shown in Fig.
449
9(b). Consequently, the sequence of BTE is diesel < DME10 < DME15 > DME20. This condition is
450
similar with PODEn. Adequate addition of DME can surely promote the complete combustion and
451
increase the BTE. Both NOx and soot emissions of DME/diesel blend fuels are lower than those of
452
diesel (Fig. 10).
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Youn et al. (2011) investigated the combustions and emissions characteristics of DME compared to
454
conventional diesel fuels. In combustion characteristics, the peak combustion pressure and the ignition
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delay of DME fuel is higher and faster than those of ultra low sulfur diesel (ULSD), respectively. The
456
NOx emission of DME fuel shows slightly higher than that of diesel at the same engine load condition,
457
while HC and CO emissions were lower. Also, the soot emission of DME fuel is nearly zero level (Su
458
and Chang, 2014; Park and Lee, 2013). Hou et al. (2014) investigated combustion and emissions
459
characteristics of a turbocharged compression ignition engine fueled with DME and biodiesel blends.
460
The result shows that with the increase of DME proportion, ignition delay, the peak in-cylinder pressure,
461
peak heat-release rate, peak in-cylinder temperature decrease, and their phases retard. Compared to
462
biodiesel, NOx emissions of blends significantly decrease; HC emissions and CO emissions increase
463
slightly (Su et al., 2014). Zhao et al. (2014) investigated the effects of DME (dimethyl ether) premixing
464
ratio and cooled external EGR (exhaust gas recirculation) rate on combustion, performance and
465
emission characteristics of a DME-diesel dual fuel premixed charge compression ignition (PCCI)
466
engine. The result showed that HCCI combustion of the premixed gas promoted the in-cylinder
467
pressure and temperature, resulting in an earlier SOC and a shortened diesel ignition delay. The
468
decrease in the diesel fuel during the diffusion combustion improved the mixing uniformity between the
469
fuel and air. Thus, the combustion became more complete and the brake thermal efficiency improved.
470
A higher DME premixing ratio caused lower smoke and NOx emissions but higher HC and CO
471
emissions. PCCI engine with EGR exhibited an obvious postponed SOC and prolonged combustion
472
duration. Thus, the maximum values of in-cylinder pressure, mean charge temperature, heat release
473
rate and pressure rise rate all decreased. As the EGR rate increased, NOx emission decreased, but
474
smoke, CO and HC emission increased. Wang et al. (2013, 2014) found that both port DME quantity
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and injection timing remarkably influenced the combustion process and exhaust emission of engine.
476
They had little impact on the peak position of HRR during low temperature reaction (LTR) phase.
477
However, the peak value of HRR increased and the crank-angle corresponding to the HRR peak
478
advanced with an incremental DME quantity or an early injection during high-temperature reaction
479
(HTR) phase. The peak value of HRR dropped with an incremental DME quantity or a late injection
480
during the diffusion combustion phase. Peak values of in-cylinder pressure and temperature increased
481
with an incremental DME quantity or an early injection. For the fixed injection timing, NOx emissions
482
presented a decreasing trend with a rise of DME quantity but this decreasing trend ceased at a higher
483
DME quantity. Smoke emission reduced, but CO and HC emissions increased with a rise of DME
484
quantity. Su et al. (2016) also found that the ethanol fraction have a more obvious effect on the
485
indicated mean effective pressure (IMEP) for advanced in-cylinder injection timings than around the
486
top dead center (TDC) conditions. The application of the DME-ethanol dual-fuel combustion strategy
487
caused a significant reduction of indicated specific NOx without deterioration of indicated specific soot.
488
In addition, a high ethanol fraction led to a low NOx for the same premixed combustion duration, while
489
HC and CO emissions increased slightly.
490
(a)
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(b)
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Fig. 9 Comparison of mutual solubility and BSEC between DME and diesel (Wang et al., 2008b). (a) Mutual solubility. (b) BSEC.
28
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(a)
(b)
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Fig. 10 Comparison of NOx and soot emissions between DME and diesel (Wang et al., 2008b). (a) NOx emission. (b) Soot
496
emission.
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Although DME is usually thought to be an alternative fuel for CI engines, the SI DME engine could be
498
started successfully and realize the stabile running. Shi et al. (2018) investigated the combustion and
499
emissions characteristics of a SI engine fueled with gasoline-DME blends. Test results showed that the
500
addition of dimethyl ether resulted in the raised indicated mean effective pressure for the gasoline
501
engine. Over increased and decreased spark timing tended to cause the dropped indicated mean
502
effective pressure. The coefficient of variation in indicated mean effective pressure was diminished
503
with the spark timing advances and dimethyl ether addition. NOx and HC emissions were dropped with
504
the spark timing decrease. NOx emissions from the dimethyl ether-mixed gasoline engine are
505
decreased with the decrease of spark angle. Ji et al. (2011) indicated that thermal efficiency, NOx and
506
HC emissions are improved with the increase of DME addition level. The combustion performance was
507
improved when DME addition fraction was less than 10%. CO emission first decreased and then
508
increased with the increase of DME enrichment level at stoichiometric condition.
509
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PODEn, with the structure CH3-O-(CH2O)n-CH3 and with no C-C bond, are promising blend fuels for
29
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diesel due to low viscosities and pour points, high oxygen contents and high CNs. The “n” of CH2O
511
group is from 1 to 8, and the main composition is from 2 to 6. Properties of PODEn components are
512
listed in Table 2. PODEn can also be soluble with diesel at any proportion. Added into the diesel
513
blending with 10% to 20%, PODEn can significantly reduce the diesel cold filter point, can improve the
514
diesel combustion in the engine quality, and improve thermal efficiency (Shi et al., 2012). Feng et al.
515
(2013) found that the ignition delay of PODE3-8-diesel mixed fuel is shortened, the fuel consumption is
516
increased, but the effective thermal efficiency is improved, compared with diesel fuel.
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Table 2 Properties of PODEn components (Chen et al., 2017a). Density (g/cm3)
Component
0.96
PODE3: CH3O(CH2O)3CH3
1.02
PODE4: CH3O(CH2O)4CH3 PODE5: CH3O(CH2O)5CH3
Oxygen content (%)
63
45.3
156
78
47.1
1.06
202
90
48.2
1.10
242
100
49.0
1.13
280
104
49.6
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PODE6: CH3O(CH2O)6CH3
Cetane number
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PODE2: CH3O(CH2O)2CH3
Boiling point (℃)
Yang et al. (2015), investigated the performances of engine fueled with PODE2-4 blend fuel. The
519
result showed that with the increase of the mixing ratio of PODE2-4, the diesel engine power and torque
520
drop, but thermal efficiency increase. Xie et al. (2017) made a study about PODEn and its high
521
proportion of diesel blended fuel on the combustion and emission of the engine. It was found that the
522
engine was pulsed with pure PODEn, and its effective thermal efficiency was improved and discharged
523
relative to diesel oil. So PODEn can be used as an alternative fuel for diesel alone (Burger, 2010; Feng,
524
2013). The NOx-soot trade off relationship can be dramatically improved (Burger et al., 2010). The NOx
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and soot emissions can meet Euro Ⅴ standards at high load and Euro Ⅵ standards at medium load.
526
Oxygenated fuel is an important method to inhibit the formation of soot emissions and improving air
527
entrainment by blending high volatility fuel is another approach. PODEn has lower distillation
528
temperature than diesel and thereby higher volatility (Liu et al., 2016a, 2017b). Furthermore, PODEn
529
has lower viscosity than diesel. As a result, blending PODEn in diesel is helpful to improve the forming
530
quality of air-fuel mixture and the spray quality.
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Furthermore, Liu et al. (2017b) found the combustion efficiency can be dramatically improved which
532
leads to lower HC and CO emissions. In order to reduce emissions and improve thermal efficiency of
533
diesel engines, blends of GDP (gasoline/diesel/PODEn) were proposed and studied. GDP blends have
534
shorter ignition delay, lower max pressure rise rate and COVIMEP (coefficient of variation of indicated
535
mean effective pressure) than GD blends. GDP blends also have higher combustion efficiency and
536
thermal efficiency than GD blends, even slightly higher than diesel fuel. Pellegrini et al. (2013) studied
537
PODE3-5 on a diesel engine and the results showed that the use of 12.5% PODE3-5 mixture reduced
538
PM emissions; high mixing ratio PODE3-5/diesel could simultaneously optimize NOx, PM and noise, but
539
may cause problems with the engine hardware. Chen et al. (2017a) found that both soot and ultrafine
540
particles (UFP) emissions obviously decreased with the increase of PODEn ratio. At low engine loads,
541
the reduction effect for UFP is especially significant, as shown in Fig. 11.
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(a)
(b)
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Fig. 11 Comparison of UFPs between diesel and its blends with PODEn (Chen et al., 2017a).
549
(a) Number concentration. (b) Volume concentration.
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BTEs of PODEn/diesel blend fuels firstly increase compared to diesel with the blending ratio of
551
PODEn and then decrease, which is similar with DME. This condition is attributed to the lower LHV of
552
PODEn. The above reviews concluded that with the blending of PODEn, HC, CO and smoke emissions
553
decrease, while NOx emission increases. Compared to P20, BTE of P30 decreases and thus reduce
554
the NOx emission. High oxygen content still has obvious effect in reducing smoke emission.
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Fig. 12 Schematic diagram of soot formation process (Dale and Kenth, 2007).
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Fig. 12 indicates the soot formation process of diesel engine (Dale and Kenth, 2007). Alkynes are
558
polymerized into aromatic hydrocarbons (PAHs) through fuel pyrolysis. The growth of PAHs leads to
559
the formation of soot (core formation). On the whole, blending oxygenated fuel in diesel may provide
560
oxygen in the fuel-rich area of the diesel jet, which can inhibit the soot formation. Common ethers used
561
as diesel additive were DME with the molecular formula of CH3OCH3 and PODEn of CH3O(CH2O)nCH3.
562
There are no C-C bonds in both DME and PODEn and these ethers have the best effects in soot 32
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reduction. Zhu et al. confirmed that the R-(C=O)O-R ′ group in biodiesel was less efficient in
564
suppressing the soot precursor’s formation than the R-OH group in n-pentanol (Zhu et al., 2016).
565
Further, it was confirmed that soot precursor generated in the biodiesel pyrolysis was proportional to
566
the concentration of unsaturated fatty acid methyl ester (the number of C=C double bonds) (Wang et
567
al., 2016b).
568
6.
569
Greenhouse gas (GHG) emissions derived from vehicles are the significant contributors to the global
570
warming and the climate change as shown in Fig.13. Life cycle assessment (LCA) methodology is
571
commonly used to evaluate the well-to-wheel greenhouse gas emissions of alternative fuels.
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Greenhouse gas emissions of alternative fuels based on life cycle assessment
Fig. 13 The transport sector as a major contributor to global energy-related CO2 emissions (Ashnani et al. 2015).
574
Ou and Zhang (2013) found that CHG-powered and LNG-powered vehicles emit 10%-20% and
575
5%-10% less GHGs than gasoline- and diesel-fueled vehicles respectively, which has the similar
576
results with Rose et al. (2013) that a 24% reduction of GHG emissions (CO2-equivalent) realized by
577
switching from diesel to CNG. Since NG has a lower carbon content than petroleum, gas to liquid
578
(GTL)-powered vehicles emit approximately 30% more GHGs than conventional fuel vehicles. The
33
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579
carbon emission intensity of the LNG energy chain is highly sensitive to the efficiency of NG
580
liquefaction and the form of energy used in that process (Ou and Zhang, 2013). It was confirmed that biodiesel appears attractive since its use results in significant reductions of
582
GHG emissions in comparison to gasoline and diesel (Nanaki and Koroneos, 2012). Nocker and Torfs
583
(1998) made a comparison of LCA and external-cost analysis for biodiesel and diesel, which found that
584
both approaches confirm that although biodiesel offers advantages in terms of greenhouse gas
585
emissions, and it has similar or higher impacts on public health and the environment. However, from a
586
LCA perspective, it is not an accurate way for only focusing on the use phase of the fuels. Carneiro et
587
al. (2017) found that some biodiesel production pathways perform satisfactorily in terms of GHG
588
emissions compared to other biofuels, but some others can be even worst than fossil diesel, but
589
energy and GWP performances still can be improved if production pathways are carefully chosen and
590
optimized. Further, Collet et al., (2014) found that a large fraction of environmental impacts and
591
especially GHG emissions stem from the production of the electricity required for producing, harvesting
592
and transforming algae, in that case the source of electricity as well as algae production technology
593
may also play an important role in GHG reduction.
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As for ethanol, Blottnitz and Curran (2007) reported that bio-ethanol results in reductions in resource
595
use and global warming. A life cycle environmental impacts of selected U.S. ethanol production and
596
use pathways in 2022 was conducted and indicates that one kilometer traveled on E85 from the
597
feedstock-to-ethanol pathways evaluated has 43%-57% lower GHG emissions than a car operated on
598
conventional U.S. gasoline (base year 2005) (Hsu, et al. 2010). Even though bio-ethanol production
34
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from sugarcane is considered to be a beneficial and cost-effective greenhouse gas (GHG) mitigation
600
strategy, it is still a matter of controversy due to insufficient information on the total GHG balance of this
601
system (Lisboa et al. 2011). In sum, the utilization of alternative fuels including CNG, biodiesel, and
602
ethanol is helpfuel to control the GHG emissions.
603
7.
604
Diversification of fuels for automobiles is an inevitable strategy and trend of social and economic
605
sustainable development. Environmental pollution effects of automobiles fueled with alternative fuels
606
are extremely complicated, determined by fuel properties, engine technology and application modes.
607
In sum, improvement of BTE generally accompanies with the increase of NOx emission. High oxygen
608
content will surely prohibit the formation of polycyclic aromatic hydrocarbons and soot.
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Conclusions and suggestions
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(1) Natural gas is the most important and successful alternative fuel for automobiles. NG/gasoline
610
bi-fuel automobiles have higher BTE than gasoline ones, producing less HC, CO, and PM emissions,
611
while more NOx emission. Pure NG automobiles have similar regulations with bi-fuel mode compared
612
to gasoline automobiles. NG/diesel dual fuel automobiles are commonly compared with diesel ones,
613
emitting higher HC and CO emissions and lower NOx and PM emissions with BTE decreasing. HCNG
614
improves the BTE of NG engine and as a result the NOx emission increases significantly.
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(2) Methanol and ethanol are generally applied on SI automobiles, through mixing with gasoline in
616
certain volume proportion together with the additive. High intramolecular oxygen contents accelerate
617
the combustion speed, shorten the combustion duration and thus improve the BTE, promoting the
35
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complete combustion. Accordingly, HC and CO decrease obviously. Methanol and ethanol can also be
619
used on diesel engines, although their ignitability is poor due to the low cetane number. However, high
620
oxygen contents are helpful for inhibiting the formation of PM and their effects on the combustion
621
process are similar. The solubility of alcohols in diesel is very poor and many additives including
622
cosolvents and surfactants must be used, leading to the poor application.
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(3) Biodiesel is an ideal and renewable alternative fuel for diesel and its physical and chemical
624
properties are close to those of diesel. The most important advantage is that biodiesel can be mixed
625
with diesel in any volume ratio. Although the feedstocks for the production are extremely abundant,
626
most biodiesel has five major compositions and as a result its intramolecular oxygen content is 10% or
627
so by weight, accounting for more than 70% by weight. To balance the trade-off relationship between
628
properties including oxidation stability (induction period) and ignitability (cetane number) and low
629
temperature fluidity (freezing point), the mass fraction of U-FAMEs is controlled no less than 70%. High
630
viscosity and low volatility of biodiesel result in poor homogeneity of mixture gas and spray quality.
631
Biodiesel has lower BTE than diesel at low loads, and higher at medium and high loads. Biodiesel
632
produces higher NOx emission than diesel, while less HC, CO, and PM emissions.
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633
(4) DME and PODEn do not contain C-C bonds and have high oxygen contents. They are considered
634
as the most promising blend fuel in diesel. Both of them have higher BTE than diesel, due to the more
635
concentrated heat release and more completed combustion. DME blending in diesel can reduce both
636
NOx and soot emissions. PODEn blending can also decrease the soot or PM emission of diesel engine
637
significantly, while increase the NOx emission in general. In general, increasing the blending ratio of
36
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DME and PODEn reduce the soot or PM emissions, mainly due to the increasing of intramolecular
639
oxygen content. However, BTE does not increase throughout with the blending ratio and HC and CO
640
will increase when BTE decreases.
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(5) In most cases, high BTE means high NOx emission and low soot (or PM) emission for diesel
642
automobiles, exhibiting a trade-off relationship between NOx and soot (or PM). Automobiles equipped
643
with dual fuel engines, including both NG/diesel and methanol/diesel dual fuel, simultaneously reduce
644
the NOx and PM (or soot) emissions compared to diesel. The decline of BTE leads to the higher HC
645
and CO emissions.
646
Conflict of interest
647
The authors do not have any conflict of interest with other entities or researchers.
648
Acknowledgments
649
This study was supported by National Engineering Laboratory for Mobile Source Emission Control
650
Technology (NELMS2017B02), and the Special Fund for Basic Scientific Research of Central Colleges,
651
Chang’an University (310822172203).
652
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653
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Dr. Yisong Chen is lecturer in Chang’an University, School of Automotive. He received his PhD degree in
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vehicle engineering from Hunan University in China at 2014, done postdoctoral research at Tsinghua
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University form 2014 to 2015. His research interests include alternative fuels of automobiles, life cycle
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assessment of automotive and strategic research into the automotive industry in China.
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Dr. Hao Chen is an associate professor in Chang’an University, School of Automotive. He obtained his
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PhD degree and Master degree in vehicle engineering from Chang’an University. He is interested in
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the fields of alternative fuels of automobiles, diesel engine, engine and fault diagnosis, life cycle
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assessment of automotive, etc.
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