Effects of alternative fuels on the combustion characteristics and emission products from diesel engines: A review

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Effects of alternative fuels on the combustion characteristics and emission products from diesel engines: A review ⁎

Peng Geng , Erming Cao, Qinming Tan, Lijiang Wei Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China

A R T I C L E I N F O

A BS T RAC T

Keywords: Alternative fuel Diesel engine Alcohol Biodiesel Natural gas Di Methyl Ether (DME) Performance Combustion & emission

Diesel engines are the main source of rapidly-growing energy consumption worldwide. Diesel consumption is responsible for serious air pollution, which includes nitrogen oxides (NOx), hydrocarbon (HC), carbon monoxide (CO) emissions and some particulate matter (PM) discharged from the combustion chamber. In the past few decades, alternative fuels, such as alcohol, biodiesel, natural gas, and Di Methyl Ether (DME), have been used in diesel engines to reduce energy costs and environmental pollution. As a result of alternative fuels directives, an increasing number of diesel engines have adapted dual fuel blends, and an enormous amount of research is focused on new and inadequately studied combustion and emission profiles. Compared to conventional diesel fuel, the application of dual fuels would add new parameters to combustion and emission profiles for diesel vehicles worldwide. This review aims to reveal (1) Known and anticipated combustion characteristics and emissions products from dual fuels. (2) Toxic properties and the expected influence on engine performance. (3) Identifying promising alternative fuels for emissions control in compression combustion engines. The results presented herein will show a significant reduction of regular gas and PM emissions by the use of alcohol/diesel dual fuel, while unregulated emissions such as methanol, ethanol, acetaldehyde, formaldehyde, ketone, have increased compared to those from diesel fuel. PM emissions decreased significantly with the increase of alternative fuels, such as alcohols, natural gas, biodiesel and DME, while regular gaseous emissions varied depending on the type alternative fuel and the engine conditions. As one new and cleaner substitute for diesel engines, DME operation has a longer injection delay, lower maximum cylinder pressure, a lower ratio of pressure rise, and shorter ignition delay in comparison with diesel operation–the opposite of alcohol/diesel and dual fuels. This review evaluates the effects of some alternative fuels (alcohol, biodiesel, natural gas and Di Methyl Ether (DME)) on combustion characteristics and emission products from diesel engines to meet future emission regulations using alternative fuel.

1. Introduction Diesel engines have been widely used in the world transportation sector for commercial and public transportation due to their improved durability and efficiency. However, nearly 30% of the world's greenhouse gas emissions are produced from the transportation sector, leading to global warming [1,2]. Meanwhile, diesel engines are the major sources of inhaled air-suspended particulate matters [3,4]. There are various types of hazardous substances in these inhaled suspended particles, such as organic carbon, trace elements, elemental carbon and inorganic ions [5–7]. These chemical components threaten human health and the ecosystem [8–11]. Numerous studies have provided evidence that these particulate matter (PM) emissions result in degenerative disorders [12,13] and diseases [14–17]. Urban environments receive more than half of the NOx emissions from the diesel

⁎

vehicles, creating a ground level ozone layer, acid rain and smog [18,19]. Thus, Environmental protection focuses on reducing the PM and NOx emissions from diesel vehicles [20]. To achieve the standard emission reduction requirements, some vehicles manufacturing communities have already devoted resources to decrease emissions from their diesel-engine vehicles. In this regard, the use of sustainable and alternative fuels such as alcohols, natural gas, biodiesel and dimethyl ether (DME) are considered as effective methods to reduce the NOx, greenhouse gas and PM emissions [21–23]. In recent studies, researchers suggested that alcohols, natural gas, biodiesel and DME could be the key alternative fuel technologies to reduce the harmful pollutant and greenhouse gases released from diesel-powered engines [24–27]. On the other hand, fossil fuels such as gasoline and diesel are dwindling [28]; studies indicate that the known reservoirs will suffi-

Corresponding author. E-mail address: [email protected] (P. Geng).

http://dx.doi.org/10.1016/j.rser.2016.12.080 Received 12 October 2015; Received in revised form 22 November 2016; Accepted 12 December 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.

Please cite this article as: Geng, P., Renewable and Sustainable Energy Reviews (2016), http://dx.doi.org/10.1016/j.rser.2016.12.080

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BTE BSFC LHV BMEP BD HCCI PCCI E PAH

Nomenclature NOx HC CO PM DME HLB LPG EGR

Nitrogen Oxides Hydrocarbon Carbon monoxide Particulate Matter Di Methyl Ether Hydrophile Lipophile Balance Liquefied Petroleum Gas Exhaust Gas Recirculation

Biodiesel is produced from renewable energy materials such as vegetable seeds and wheat, corn and sugar beet [56–58]. The main difference between fossil fuels and biodiesel is the oxygen content [59,60]. Biodiesel fuels can provide renewable and clean energy; thus, they are promising renewable and eco-friendly fuels [61,62]. DME is a liquefied gas with similar characteristics to liquefied petroleum gas (LPG) [63]. DME appears to be an efficient and excellent alternative fuel for use in diesel engines, producing fewer smoke emissions, due to its high oxygen content and non C-C bonds in its molecular structure [64]. Moreover, the NOx emissions can also be reduced with the proper DME injection design and combustion system [65]. The main purpose of this study is to provide a comprehensiveness on the effect of the above four most promising alternative fuels, regarding their performance, combustion and emission characteristics in diesel engines.

ciently meet the worldwide demand for another 39 years for oil, 61 years for natural gas and 216 years for coal [29–31]. The increasing global motorization and industrialization is leading to the energy dearth. Petroleum oil usage is a considerable challenge and the demand for alternative fuels is increasing as societies are struggling to ensure global energy security. Thus, the alternative fuels have been widely used in the internal combustion engine to solve the shortage of fossil fuels, such as the rise of natural gas via shale and fracking. Among these substitute fuels, alcohols, natural gas, biodiesel and DME are considered to be the four most promising and attractive alternatives, because they may be easily acquired, handled and stored [32–35]. Alcohols can be used in internal combustion engines with both the blended and the dual-fumigation fuel operational mode [36,37]. In the blended mode, alcohols are mixed with diesel before being injected into the in-cylinder [38,39]. Fuel additives are required to stabilize the blend. The co-solvents and emulsifier is the fatty alcohol with long carbon chains and its normal HLB (HydrophileLipophile Balance) is from 1.6 to 6. In the dual-fumigation mode, the diesel engine requires only minor modifications by adding a separate fuel tank, lines, a low-pressure fuel injector and the necessary controls [40,41]. However, fuel additives are not required in this case. Consequently, a large amount of alcohol can be used under every engine condition [42,43]. Natural gas is another potential alternative to fossil fuels for use in the diesel-fueled vehicles [44–48]. It is the mixture of various hydrocarbon molecules such as methane, ethane, butane, propane and inert diluents like carbon dioxide and molecular nitrogen. However, its availability varies geographically and throughout the year. In addition, specific treatment is required during transportation and production [49–53]. It can be mixed thoroughly with air to form the homogenous fuel/air mixture for combustion in the cylinder and significantly reduces the emissions from the engine exhaust [54,55].

2. The physicochemical properties of alternative fuels 2.1. Alcohols Alcohols such as methanol and ethanol have been widely studied as alternative fuels for blending with conventional diesel fuels because of their renewable resource base and oxygenated properties, which can significantly reduce the PM emissions from diesel engines [66]. According to Table 1, compared to diesel, the alcohols’ vaporization latent heat and self-ignition temperature are much higher and decrease with increasing number of carbons whereas their octane number is much lower and increases with the number of carbons molecules. As an important parameter for combustion, the higher carbons alcohols with lower vaporization latent heat were not considered. Compared with other alcohols, methanol and ethanol are much more desirable for partly replacing diesel because of their lower price and emissions [67]. In recent studies, when the diesel engine operated at the high/full load, the soot emission can be significantly reduced by switching to methanol/ethanol consumption due to their low carbon number [68]. Moreover, by the use of methanol/ethanol, the NOx emissions also decrease significantly because alcohols tolerate high exhaust-gas recirculation ratios (EGRs) [69].

Table 1 Comparison of properties of diesel fuel, methanol, ethanol, propanol, butanol and pentanol [70–72]. Properties

Diesel

Methanol

Ethanol

Propanol

Pentanol

Chemical formula

CnH2n or CnH2n+2 (n=13~17) 837 0 197.21

CH3OH

C2H5OH

C3H7OH

C5H11OH

791.3 49.93 32.04

789.4 34.73 46.07

803.7 26.62 60.10

814.8 18.15 88.15

42.65

20.08

26.83

29.82

32.16

45–50 210–235 254

2 65 385

11 79 363

12 97 350

20 138 300

375

1162.64

918.42

727.88

308.05

Density (kg/m3) Oxygen (%) (wt.) Molecular weight (g/mol) Lower heating value (MJ/kg) Octane number Boiling point (°C) Self-ignition temperature (°C) Vaporization latent heat (kJ/kg)

Brake Thermal Efficiency Brake Specific Fuel Consumption Low Heat Value Brake Mean Effective Pressure Biodiesel-Diesel Homogeneous Charge Compression Ignition Premixed Charge Compression Ignition Engine's speed surface Polycyclic Aromatic Hydrocarbon

Table 2 Composition and properties of natural gas [128].

2

Component

v/v (%)

Component

v/v (%)

CH4 C2H6 C3H8 C4H10 Density(kg/m3)

91.72 ± 1.7 5.5 ± 1.6 1.98 ± 0.8 0.44 ± 0.5 0.788

C5H12 N2 CO2

0.03 ± 0.03 0.322 ± 0.3 0.03 ± 0.03

Lower heating value (MJ/kg) Stoichiometric air/fuel ratio

49.5 ± 0.2 17.20

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biodiesel, LPG, palm methyl ester and other fuels [83]. As shown in Table 4, the DME's cetane number is higher than that of other specific hydrocarbon fuels. Consequently, DME is a good alternative fuel for auto-ignition. Compared with diesel, DME can achieve ultra-low emissions and has better energy combustion efficiency than diesel [84]. Due to Sulphur constituents and properties of diesel fuels, there is a presence of trade-off between NOx and PM emissions, while there is no such concern in the DME combustion due to its no PM emissions [85]. Moreover, DME evaporation and atomization are improves which leads to reduced emissions and lower costs.

2.2. Natural gas Among the alternative fuels, natural gas is considered to be an essential energy source for internal combustion engines [73]. Compared with other alternative fuels, there are some distinct and desirable advantages of natural gas including relatively reduced capital costs and low green-house gas emissions. The properties of natural gas are shown in Table 2. Natural gas can be used in diesel engines with a high compression ratio, due to its high octane number. In recent years, energy shortage and environmental pollution have drawn increasing attention from governments worldwide towards natural gas as the alternative fuel for heavy-duty diesel engines, as well as stationary engines [74]. It is because that the natural gas is induced into the intake manifold or the cylinder directly and mixed with fresh air to form a homogeneous mixtures, which is then ignited by pilot diesel fuel or the spark plug [75]. Therefore, efficient combustion is achieved and emissions from the exhaust gas and substantially reduced. Moreover, natural gas can be applied on the in-use vehicles without significant modifications, offering important economic and environmental benefits [76].

3. Application mode There are two main alternative fuel application modes in compression-ignition engines: dual fuel combustion mode and blended fuel combustion mode. 3.1. Dual fuels

As a prominent alternative fuel candidate, biodiesel is refined from various vegetable oils: palm, sunflower, rapeseed, cottonseed, peanut and soybean. It has been widely used in heavy-duty diesel vehicles and marine engines [77]. Its properties are presented in Table 3. Biodiesel can be obtained by reacting vegetable oils with alcohol, and the alkali catalysts like KOH and NaOH are added through the transesterification process. The purpose of this process is to reduce the oxygen content and viscosity of the vegetable oil. Biodiesel contains organic matter with high molecular weight, such as ether, aldehyde, ketone, phenol and alcohol [78]. The density of biodiesel is lower than that of water, and it can be stored for a long duration due to its stability [79]. Its Sulphur content is far below that of diesel, and there are no aromatic hydrocarbons included in biodiesel; thus, the emissions from biodiesel combustion are far less harmful to people's health. Moreover, the cetane number of biodiesel is higher than that of diesel, improving its combustion performance [80].

Dual fuel combustion offers several advantages and has been extensively studied [90,91]. The Schematic diagram of dual fuel mode system is shown in Fig. 1. The pressurized alternative fuels are induced into the manifold and then mixed with fresh air to form homogeneous mixtures [92]. These mixtures are ignited by pilot diesel or spark plugs, when the piston is placed around the top dead center. The amount of injected alternative fuel varies according to the engine power output, while the amount of diesel fuel remains constant under the different operational conditions [93]. Alternative fuels used with the dual fuel combustion mode include alcohols, natural gas and DME. In previous studies [94–96], the dual fuel mode performed reasonably well, with alternative fuels such as natural gas as the primary fuel and diesel as the pilot fuel. Compared with the conventional diesel engine, the dual fuel engine produces significantly reduced NOx and PM emissions. However, there are some difficulties associated with the dual-fuel mode combination. When the diesel engine operates at low and medium loads, the fuel combustion efficiency is low. Consequently, the unburned HC and CO emissions are significantly increased with the dual fuel combustion mode [97,98]. Therefore, further research into the dual-fuel combustion mode is required.

2.4. Dimethyl ether

3.2. Blended fuels

Dimethyl ether (DME) is a promising diesel alternative, which is mainly developed from natural gas and coal [81]. It can be applied to the compression ignition engine with minor modifications to the engine's configuration. Table 4 shows the key physical properties of DME and its hydrocarbon components. Limitations in pure DME in diesel engines include its low density, certain after-effects and its poor viscosity. Consequently, it is widely used blended with diesel. The high cylinder pressure in conjunction with the DME-diesel blends’ properties can sustain the fuel in the liquid state within the engine's supply system [82]. Moreover, DME can blend with not only diesel, but also

The blend-fuel combustion mode is suitable for alternative liquid fuels such as biodiesel and some alcohols, which can be dissolved in diesel [101,102]. Meanwhile, some alternative fuels such as methanol and ethanol can also be blended with diesel using co-solvents and emulsifiers [103,104]. The Schematic diagram of blending fuel mode system is shown in Fig. 2. For the blending fuel combustion mode, the problems encountered in the use of alternative fuels are phase separation. This, however, can be solved by adding the solvent into the blended fuels. Moreover, some ignition improver such as methanol and diethyl ether can also be added into the fuel mixture to compensate for the fuels’ cetane number. This application can be realized without major modifications of the diesel engine design and the fuel supply system, if the concentrations of the alternative fuels in the blended fuels are low. The use of alternative fuel-diesel blends in compression combustion engines and their effects on the engine performance, combustion characteristics and exhaust emissions have been investigated in previous studies. [105–108] In these studies, different percentages of blended fuels were selected. By adding alternative fuel into the diesel, soot and NOx emissions are significant reduced. However, due to the different properties of the alternative fuels, the CO and HC emissions exhibit no regular variations. Moreover, the engine performance and the exhaust emissions are affected by the engine speed and load in the blended-fuel combustion mode.

2.3. Biodiesel

Table 3 Properties of biodiesel and diesel fuel [128]. Property

Diesel

Biodiesel

Oxygen content (wt%) Carbon content (wt%) Hydrogen content (wt%) Specific gravity at 15 °C Gross heating value (MJ/kg) Flash point (°C) Viscosity at 40 °C (cSt) Stoichiometric air/fuel ratio Cetane number

0 86 14 852 45.76 67 1.57 14.60 49

10.32 77.0 12.18 896 40 105 2.99 12.33 53

3

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ratios. Some researchers also found that there is no significant impact on BSFC if alcohol fuels are used, while the diesel engine's behavior improves with EGR, when blends with alcohol content up to 15% are used [114–116]. For natural gas, the total BSFC variation with BMEP is calculated directly from the ratios between the diesel engine power output and the sum of the fuel mass flow rates. When the engine operates on the low and medium engine conditions, the total BSFC in the natural gas-diesel mode is higher than that of the conventional combustion mode. This is because the fuel/air ratio in the combustion chamber and the combustion temperature are lower, which leads to a reduction in the combustion rate. However, on the high-load engine condition, the total BSFC for the natural gas-diesel mode becomes lower than that of the diesel mode. The richer mixture and the high combustion temperature are the two major factors increasing the conversion of natural gas to work and the total BSFC decreases [117]. For biodiesel, the total BSFC is greater than that of diesel because of its lower calorific value and higher density. In previous studies, the average of the total BSFC for BD10 (10% biodiesel to 90% diesel) and BD20 (20% biodiesel to 80% diesel) were about 8% and 13% higher than those of diesel fuel. Because of its lower calorific value, more biodiesel was needed to produce the same amount of engine power compared with diesel fuel [118]. For DME, the total BSFC increased with the DME increase, when the injection time is fixed. The diesel diffusion-combustion decrease and the DME HCCI combustion increase improved the fuel combustion efficiency and enhanced the fuel conversion efficiency. Moreover, the total BSFC decreased with the increase of injection time, within certain limits due to the better mixture preparation before ignition combustion. However, a much longer injection time leads to a BSFC increase because of the highpressure rate and in-cylinder pressure [119].

Table 4 Properties of specific hydrocarbon fuels and DME [86–89]. Property

DME

Propane

Methanol

Diesel oil

Cetane number Boiling point (°C) Liquid density @ 20 °C, gm/cm3 Vapor pressure @20 °C, bar Wobbe index, kJ/m3 Sp/ Gravity of gas (vs. Air) Calorific value, lower heating value, Kcal/kg Calorific value, lower heating value, Kcal/nm3 Flammability limits in Air, vol%

55–60 −25.1 0.67 5.1 46,198 1.69 6900

5 −42 0.509 8.4 69,560 1.52 11,100

5 64.6 0.79 – – – 4800

40–55 180–360 0.84 – – – 10,200

14,200

21,800

–

–

3.4–17

2.1–0.4

5.5–36

0.6–7.5

4. Engine performance 4.1. Engine brake torque To estimate the performance of the diesel engine, it is important to investigate the main performance parameters, i.e. the engine brake torque, the brake thermal efficiency (BTE) and the brake specific fuel consumption (BSFC). The engine brake torque is the turning force produced by the cylinder pressure from the piston's crankshaft. The engine's charge, the average effective cylinder pressure and the stroke length have an effect on the engine torque. When the engine operates constantly, the engine torque varies according to the alternative fuel, due to the generated effective pressure and the fuel's properties. The BSFC and the BTE are calculated by the following equations:

BSFC =

Fuel consumption Output power

3600*Brake power BTE = Fuel consumption*LHV

(1)

4.2. Brake thermal efficiency The effects of the alternative fuels on the diesel engine's performance are determined by the relation between the energy input and output [124–126]. To evaluate this, the BTE to the engine's speed surface (E) on the maximum-load engine condition is integrated [127,128]. The diesel engine is assumed to operate under common working conditions.

(2)

In the present studies [110,111], the torque curves are similar for the same alternative fuels. The effects of the different types of alternative fuels on the BSFC were shown in Fig. 3. For alcohols, the engine power curves slightly towards the left and thus there is a slight increase in the maximum brake torque with the increase of alcohol content in the testing fuels [112]. The engine's losses significantly diminish with increasing number of carbons in the alcohols. The same engine torque increases by approximately 1.5%, with 4% ethanol-diesel fuel blends, for various engine compression ratios [113]. Moreover, the engine's behavior deteriorates at the highest and lowest alcohols/fuel

nf

E=

∫ni

BTE (n ) dn

(3)

where Ei is the value of BTE to engine speed for all the tests and E0 is that of the diesel fuel. The effects of different types of alternative fuels on BTE were shown in Fig. 4. For alcohols, the BTE significantly

Fig. 1. Schematic diagram of dual fuel mode system [99,100].

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Fig. 2. Schematic diagram of blending fuel mode system [109] 1. Test engine, 2. Test platform, 3. DC dynamometer, 4. Laminar air flow element, 5. Balance, 6. Diesel fuel line, 7. Inlet air heat, 8. In-cylinder pressure sensor, 9. Combustion analyzer, 10. Needle-lift sensor, 11. Needle-lift sensor amplifier, 12. Diesel fuel line sensor, 13. Diesel fuel line pressure sensor amplifier, 14. Encoder, 15. Data acquisition system, 16. Smoke meter, 17. Heated emission sampling line, 18. Emissions sampling system, 19. Emission analyzers rack cabin, 20. Function gases (O2, H/He, NO).

Fig. 3. Effects of different types of alternative fuels on BSFC [120–123].

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Fig. 4. Effects of different types of alternative fuels on BTE [129–132].

BSFC reduction. The chemical energy of the biodiesel-diesel mixture is converted to brake mechanical energy with the same efficiency. However, differences in the brake thermal efficiency between the biodiesel blends and the diesel fuel at low and medium engine loads are less than those at high engine loads, because the biodiesel content increases significantly with the engine load. For DME, the DME HCCI combustion increase and the diffusion combustion decrease improved the fuel conversion efficiency and the combustion efficiency. In previous studies, kinetically-controlled HCCI combustion improved the BTE which reduced the fuel consumption compared with the conventional diffusion combustion.

increases with the increase of alcohol content in the test fuels. The combustion improves due to the presence of oxygen, which leads to a decrease of heat losses and an increase in combustion efficiency because of the lower boiling point of alcohols compared to that of diesel. Moreover, the ignition delay increase leads to an increase in the release energy rate, and thus the heat losses from the combustion are reduced, as less time is required for this heat to be transferred from the cylinder to the coolant. Furthermore, the premixed combustion part becomes higher by the use of alcohol fuels due to their lower carbon number, consequently, the percentage of constant volume combustion increases and the leaner combustion occurs with low heat losses. For natural gas, the BTE decreases significantly by using dual fuel, compared with the conventional combustion mode, when the diesel engine operates on the low and medium load conditions. This is because the total test-fuel flow rate increases significantly during the combustion process with the natural gas-diesel mode, reducing the flame propagation speed and the combustion temperature. However, on the high load engine condition, the BTE increases by the use of the dual fuel due to the elevated temperature. For biodiesel, the BTE decreases with the biodiesel content. For each test fuel, the BTE increases significantly with the increase of the engine load due to the

5. Combustion characteristics Different types of alternative fuels have an effect on the combustion characteristics, including the calculated heat release rate and the cylinder pressure. The ignition delay is defined as the crank angle variation between the injection start and ignition timing. Ignition timing is determined by locating the zeroing crank angle of cylinder pressure's second derivative. The accumulated heat release rate is defined as the crank angle of 90% heat release (CA90), and the 6

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Fig. 5. Effects of different types of alternative fuels on in-cylinder pressure and heat release rate [145–148].

but improves later on. The premixed fuel/air mixture burns rapidly after the ignition delay. Moreover, the engine load leads to the accumulation of much more gaseous fuel in the combustion chamber and the decrease of the ignition delay. For biodiesel, the addition of biodiesel accelerates combustion at the beginning for all engine loads due to the rapid cylinder pressure rise on the pressure traces at the end of the compression stroke [139–141]. Moreover, the biodiesel blends’ combustion starts earlier than that of diesel fuel. The biodiesel-blended fuels’ ignition values occur at slightly earlier crank angles because of their high viscosity, density and bulk modulus. By adding biodiesel, the heat release rate in the premixed combustion phase is generally reduced when the diesel engine operates under the low and high load conditions. This is because the amount of injected fuel was lower due to the short ignition delay and the low heating value of the biodiesel fuel blends. For DME, the PCCI combustion heat-release process is relative complex [142–144]. The earlier direct-injection accelerates combustion in the cylinder. When DME is induced into the intake manifold, HCCI combustion occurs because of the superior auto-ignition ability and high cetane number of DME. A negative temperature coefficient, low temperature reaction and high temperature reaction are observed with the HCCI combustion mode. Therefore, the features of a staged combustion event are demonstrated on the PCCI operation with DME used as the premixing fuel.

combustion duration is defined as the crank angle variation between the ignition timing and CA90. The alternative fuels’ effects on the in-cylinder pressure and the heat release rate are presented in Fig. 5. For alcohols, there are many differences in the in-cylinder pressure and the heat release rate between the diesel fuel mode and the alcohol-diesel mode. Compared with traditional diesel combustion, compression pressures decrease significantly with increasing alcohol content in the test fuels [133– 135]. This is because the alcohols’ vaporization latent heat is higher than that of diesel. Thus when alcohols such as methanol and ethanol are injected into the intake port, they vaporize and absorb the heat of the intake air, which leads to a decrease of the in-cylinder temperature and pressure at the compression stroke. When the diesel engine operates at low speed and load conditions, the maximum cylinder pressure slightly changes to low and middle premixed alcohol ratios, decreasing from high premixed alcohol ratios. The alcohols’ high vaporization latent heat decreases the in-cylinder temperature and slows the combustion phase excessively, at high premixed alcohol ratios, when the test engine operates at low speed and load. For natural gas, the dual fuel mode with natural gas-diesel generates the lower combustion pressure peaks, compared to the conventional diesel fuel mode [136–138]. At low engine loads, the natural gas-diesel mode reduces the cylinder combustion pressure, due to the greater ignition delay, the higher specific heat capacity and the leanness of the gas/air mixture. Heat is not release efficiently in the beginning of the combustion, due to the accumulated vapor during the ignition delay, 7

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engine, the CO emissions decrease significantly when the biodiesel ratio is reduced [159]. Some researchers also found that CO emissions are lower to those from the diesel fuel mode due to the lower viscosity and density in biodiesel which could improve the spray formation and the fuel atomization [160]. For DME, PCCI operation NOx emissions are slightly lower than those of the diesel fuel operation, and the decreasing trend follows the DME content increase but disappears when the DME quantity is higher [161]. There are two major factors leading to NOx reduction at the beginning of the combustion process. The first reason is that the ignition delay is shortened due to the higher in-cylinder pressure and temperature with the premixing DME HCCI operation. The second is that the oxygen content in the gaseous mixture is reduced by the gases from the HCCI combustion such as the internal EGR. Hereafter, the NOx emissions stabilize, because the DME causes a rapid combustion reaction with high temperature, which increases the charge temperature. The HC and CO emissions in this case rise with DME quantity [162]. However, the HC and CO emissions could be reduced with an earlier injection, which would promote the oxidation reaction of the HC and CO emissions [163].

6. Emissions 6.1. Regulated emissions Compared with other types of internal combustion engines, diesel engines are the main power sources because of their high combustion efficiency and power performance. Engineering equipment, heavy automobiles and motor vehicles are widely driven by diesel engines. However, emissions from diesel engines, such as NOx and PM, are harmful to peoples’ health and pollute the environment seriously. To reduce the emissions from diesel engines, techniques such as supercharge, exhaust after-treatment and application of alternative fuels are employed. The use of high oxygen-content alternative fuels, such as certain oxygenates, with diesel fuel has been investigated to reduce emissions. For alcohols, previous studies indicate that alcohol-diesel blends can reduce some exhaust emissions without affecting the engine's performance [149]. Alcohols offer some advantages, such as lower NOx emissions, lack of sulphur and soot formation, as well as lower ozone formation potential [150]. However, there are also some disadvantages in the ignition of the air-fuel mixture, mainly because of its greater ignition delay, low cetane number and high latent heat of vaporization [151]. NOx emissions depend on the oxygen concentration in the combustion zone and the combustion temperature. They can be reduced significantly by using alcohol fuel due to its high latent heat of evaporation which leads to the reduction of the combustion temperature. However, NOx emissions could also be increased due to the alcohol's high oxygen content. Moreover, the combustion process time is shortened because the cooling effect of the high latent heat of vaporization does not significantly reduce the combustion temperature. In general, there is a correlation between alcohol content in mixtures and the NOx formation [152,153]. The HC emissions increase significantly with alcohol content in the test fuel due to the high amounts of single hydrogen radicals in the diesel-alcohol fuel in-cylinder charge. The CO emissions are the products of incomplete combustion, which occurs under conditions of low combustion temperature and poor incylinder charge mixing. The CO emissions from diesel engine fueled by alcohol-diesel blends are greater than diesel fuel. For natural gas, the HC emissions from engine fueled with a natural gas-diesel blend are greater than diesel fuel [154]. This is because the air-fuel ratio and the low temperature suppress the turbulent flame propagation from the ignition zones of the test fuel. In addition, gas-air mixtures is introduced in the crevices during the compression stroke, where it remains unburned. Another factor increasing HC emissions in this case is the overlapping between the exhaust and intake valves to facilitate scavenging, which leaves part of the air-gas mixture unburned. Compared with conventional diesel fuel, the CO emissions increase significantly with the natural gas content under all engine load conditions [155,156]. The reason is that the natural gas burns incompletely because of lack of ignition sources as well as the lower air- fuel equivalence ratio. The NOx emissions are affected by the combustion temperature, combustion duration and oxygen concentration [157]. At partial engine loads, because of the lack of oxygen in the mixture and the lower combustion rate, the NOx emissions decrease significantly with increasing natural gas content. When the diesel engine operates on the high load condition, the NOx concentration significantly increases. This is due to the higher temperature and pressure peaks and the rapid combustion. For biodiesel, biodieseldiesel blends have the higher NOx emissions compared with conventional diesel fuel because of their higher oxygen concentration, combustion temperature and longer combustion duration [158]. Under full load, the HC emissions increase with biodiesel content because of its viscosity in the test fuel spray quality, which leads to the poor combustion in the cylinder. However, a low biodiesel-diesel ratio produces less HC emissions when the engine operates on a partial load. This is because the lower viscosity and the biodiesel oxygen content results in a clean and complete combustion. Compared with the diesel

6.2. Unregulated emissions Unregulated emissions, such as aldehydes, unburned alcohols and benzene emissions, are intermediate products of oxygenated compounds and hydrocarbons in alternative fuels such as alcohols, biodiesel and natural gas. Formaldehyde and acetaldehyde are the two major abundant aldehydes released by the engine exhaust. In previous studies, aldehyde emissions increased first and then decreased with increasing engine load. When the diesel engine operates on the low and medium engine conditions, the aldehyde emissions of biodiesel and alcohol fuel blends are higher to those produced by diesel fuel. The natural explanation is that biodiesel contains some short chain chemicals, which form the short chain aldehydes, namely acetaldehyde and formaldehyde in the cylinder. Aldehyde emissions increase with alcohols content. The aldehyde emissions from the diesel engine exhaust are mainly produced from the incomplete combustion of hydrocarbons such as methanol and ethanol. The increase of alcohols leads to the reduction of the combustion temperature, and thus the formation of the intermediate combustion products like aldehydes is enhanced [164,165]. The unburned alcohol emissions such as methanol and ethanol decrease significantly with increasing load because of the higher combustion temperature. Using alcohol fuels, the cylinder combustion is delayed because of their lower cetane number and higher latent heat of evaporation when compared with diesel. Thus, the unburned alcohols increase due to the combustion of the fuel blends in the late expansion stroke. Moreover, benzene emission becomes a major concern in this case because of its high mutagenicity, carcinogenicity and teratogenicity. The benzene emissions decrease with increasing engine load because of the higher combustion temperature at higher engine load. Benzene emission from biodiesel combustion is lower than that from diesel combustion because of the characteristics of non-light-aromatics. However, when the engine operates on the low load condition, benzene emission increases with alcohol contents in the blended fuels, because of the lower combustion temperature. On the other hand, the higher oxygen content of alcohols improves the engine combustion efficiency and thus leads to the decreasing benzene formation. 6.3. PM emissions Diesel engine is one of the main sources of particulate matter (PM) emissions because of its characteristics of diffusive combustion. PM emissions are harmful to human health and lead to many diseases such as cancer, lung diseases and elevated blood pressure with the corresponding complications [166–168]. The use of alternative fuels, such as alcohols, natural gas and biodiesel, is widely gaining more and more 8

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attention. For alcohols, PM emissions decreased significantly with the increase of the alcohols content in the test fuels. There are several reasons leading to the reduction of PM emissions from alcohol/diesel fuels. Firstly, PM emissions are hardly ever produced in the combustion process of alcohols. Thus, the PM emissions’ formation is inhibited because of the reduction of diesel fuel burning in the diffusive combustion. Secondly, there are less unsaturated micro-molecules such as C2H2 formed in the oxidation with high temperature and alcohol. In general, the unsaturated micro-molecule is the precursor of the polycyclic aromatic hydrocarbon (PAH) [169]. In addition, alcohols could decrease of the intake air's temperature because of their high latent heat of vaporization increasing the excess air coefficient [170,171]. Finally, the ignition delay is prolonged because of the formation of a homogenous air/alcohols mixture [172,173]. Consequently, more diesel burns in the premixed combustion. All of these factors reduce PM emissions in the combustion process. For natural gas, in the conventional diesel combustion, the PM emissions are at low levels, when the diesel engine operates on the low and medium conditions, while increases significantly at high load, due to the increase of the diesel fuel injected mass. However, in natural gasdiesel combustion, the PM emissions remain low at all engine loads because the carbon/hydrogen ratio in natural gas is low and the premixed; homogeneous mixture is well inside the cylinder before the ignition. For biodiesel, the PM emissions from biodiesel fuel exhaust are lower than those from diesel combustion. PM emissions are formed from the incomplete burning of hydrocarbons and carbon particles in the diesel fuel. Moreover, the biodiesel's high oxygen content could be contributing to the reduction of PM emissions. For DME, the PM emissions decrease significantly with increasing DME quantity. The peak diffusive phase value decreases significantly. The DME quantity increase creates a mixture with higher homogeneity; thus, the PM emissions become less. Notably, the DME fuel cannot produce the PM emissions and thus it is positive to reduce the PM emissions. 7. Conclusions In this review paper, the combustion characteristics and emission products of four potential alternative fuels are presented. Compared to conventional diesel fuel, the application of dual fuels can add new parameters to the combustion and emission profiles. The known and anticipated combustion characteristics and emissions products from the dual fuels, their toxic properties and the expected influence on the engine's performance are revealed. According to the summary and analysis of the previous references, the following conclusions can be drawn from this review. 1. There is a significant reduction of regular gas and PM emissions by the use of an alcohol/diesel dual fuel. However, unregulated emissions such as methanol, ethanol, acetaldehyde, formaldehyde and ketone are increased compared to diesel fuel. 2. The PM emissions decrease significantly with increasing alternative fuel quantity, whereas the regular gaseous emissions vary according to the type of the alternative fuel and the engine conditions. 3. A new clean substitute for the diesel engine is the DME, which offers a longer injection delay, lower maximum cylinder pressure, lower pressure rise ratio and shorter ignition delay. These properties are opposite to those provided by using an alcohol/diesel dual fuel. 4. In essence, alcohols, natural gas, biodiesel and DME as alternative fuels have great potential in the energy shortage world. References [1] Dalkmann H, Brannigan C. Transport and climate change. A sourcebook for policy-makers in developing cities: module 5e Gesellschaft fur Technische Zusammenarbeit-GTZ Eschborn; 2007.

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