Comprehensive overview on diesel additives to reduce emissions, enhance fuel properties and improve engine performance

Renewable and Sustainable Energy Reviews 74 (2017) 891–901

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

Comprehensive overview on diesel additives to reduce emissions, enhance fuel properties and improve engine performance Ahmad Fayyazbakhsh, Vahid Pirouzfar

MARK

⁎

Young Researchers and Elite Club, Central Tehran Branch, Islamic Azad University, Tehran, Iran

A R T I C L E I N F O

A BS T RAC T

Keywords: Diesel fuel Engine performance Fuel properties Oxygenated additives Reduced emissions

The present review investigates modification of diesel fuel formulation and development of a new model to enhance engine performance, improve fuel properties and reduce exhaust emissions. Emissions arising from the fuel can be controlled by blending an oxygenated fuel (renewable fuel) with the diesel fuel. The blending oxygenated fuels namely Methanol, Ethanol, and n-Butanol are examined in addition to their effects. This review paper studies the implication of different torques and various engine speeds. In some conditions, it can even cause an increase in the content of carbon monoxides (CO), carbon dioxide (CO2) and nitrogen oxides. This review showed that the engine speed has a negative effect on all of the air pollutants, so that increasing of the engine speed leads to reduction of the air pollutants. However, the engine load gives rise to most exhaust emissions. Adding the oxygenate fuels increases brake specific fuel consumption (BSFC), while brake thermal efficiency (BTE) decreases. In some researches, a nano-metal additive has been used in the fuel for improving the engine performance. In case of using the nano-metal additives to the diesel fuel (a nano-metal with small thermal conductivity coefficient), the engine performance is seen increased.

1. Introduction There is a growing demand for energy due to the increasing population which can lead to greater air pollution. On the other hand, it is clear that there are limited sources of fossil-based fuels as a sustainable energy. As a result of world industrialization, the demand for oil-based fuels (fossil fuels) has increased dramatically [1–3]. Apart from the economic matters, the widespread use of fossil fuels is responsible for a long-term environmental problem in the form of climate changes and the global warming. The main source of energy in different forms originates from the combustion. Recently, depletion of the fossil fuels due to their continuous use has become the first priority concern for all people in the world whose lives depend on this source of energy for all their activities. Diesel engines have a considerable share of transportation. At the same time, along with the large-scale use of the diesel fuels, lung diseases and cancer invasion have been increased dramatically [4–6], such that a comprehensive study on this issue seems necessary. Today, the fossil fuels constitute about 80% of the total needed energy with almost 50% of it being related to the local transportation [7]. The diesel fuel mainly contains aliphatic hydrocarbons of C8–28 with boiling temperatures varying from 130 to 370 °C [8]. The exhaust emissions from the diesel engines contain various types of air pollutants such as total hydrocarbon content (THC), oxides

⁎

of nitrogen (NOx), particulate matter (PM), carbon monoxide (CO). The oxides of nitrogen and sulfur which are emitted by internal combustion engines can result in acid rains [6,7]. The main source of greenhouse gas emissions are the fossil fuels [9–11]. The worldwide concern of the environmental pollutants has triggered intensified researches for new alternative sources of energy. Widespread applications of the fossil fuels and the environmental issues associated with their use have directed us to replace them with reasonable price, high efficiency and renewable sources [10–13]. To find a proper convincing answer to this question how to select, distribute and use the data, one needs to focus on both engine technology and fuel efficiency [14–19]. However, the blended renewable bio fuels or the oxygenated fuels with the capability of reduction of the exhaust emissions are among the most important topics in the literature [20–25]. Alcoholic fuels are of potential capability for being used as oxygenated fuel additives with fossil-based fuels (such as gasoline to improve the octane number and to reduce the air pollutant emissions, as well as diesel fuels to control the soot emissions) for the diesel engines. They can be an alternative source of environment friendly fuels to reduce the exhaust gas emissions using the renewable energy in different countries of the world. Previous researches and studies were just focused on addition of vegetable oil, cooking oil, bio diesel, methanol and ethanol to the oilbased fuels. The vegetable oil, cooking oil and biodiesel were all useable

Corresponding author. E-mail address: [email protected] (V. Pirouzfar).

http://dx.doi.org/10.1016/j.rser.2017.03.046 Received 16 October 2016; Received in revised form 2 January 2017; Accepted 8 March 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

Renewable and Sustainable Energy Reviews 74 (2017) 891–901

A. Fayyazbakhsh, V. Pirouzfar

Nomenclature HC NOx PM CO CO2

BSFC THC BTE CI PAHs GHG MXEE

Hydrocarbon Nitrogen oxides Particle matter carbon monoxides carbon dioxide

Brake Specific Fuel Consumption Total hydrocarbon content Brake thermal efficiency Cetane Index Polycyclic aromatic hydrocarbons Greenhouse gas 2-methoxy ethyl ether

be said that the incoming air improves the efficiency of the internal combustion engines [65]. Taking into account the significant latent heat of vaporization, increasing of the oxygen content of the fuel containing methanol increases the amount of NOX (in some temperature conditions, especially at temperatures than needed for combination of nitrogen and oxygen) [19–21].

in the diesel engines [26–33], while the methanol and ethanol could be used in most of the oil-based engines and have the potential to be used as an additive in most of the fossil fuels [34–39]. Butanol can also be added to the diesel fuel with no need to incorporate other additives, due to high cetane number and great molecular weight of the nbutanol. Moreover, in case of blending with diesel fuel at any temperature, a two-phase composition does not form. Pirouzfar et al. [8] studied the influence of various tertiary additives (nitro methane, nitro ethane and 2-methoxy ethyl ether) blended with the ethanoldiesel on the exhaust emissions (CO, CO2, HC and NOX). The results were obtained from the blended fuels through the free acceleration test. They showed that the nitro ethane has a negative effect on all of the air pollutants as compared to the other additives. However, they reported that increasing of the ethanol to the diesel-ethanol blend causes poorer chemical-physical properties of the fuel. This paper reviews the latest tests on the additives which are present in the diesel fuel. The effect of additives is compared in two aspects: reduction of the emissions and improvement of the fuel efficiency. Moreover, the effect of adding different alcohols and various nitrogenated fuels will be investigated on the fuel chemical-physical properties which include direct and indirect effects on the particles dispersed from the exhaust emissions. The current study concentrates on the fuel properties (cetane number and viscosity), exhaust emissions (CO, CO2, HC, NOX and soot) and engine performance (brake specific fuel consumption and brake thermal efficiency) in various speeds and engine loads. The oxygenated fuel in advantageous in terms of the BSFC, but has a negative impact on the brake thermal efficiency. However, the oxygenated fuel are also effective on all the fuel properties (density, viscosity, flash point and cetane number). The engine load and engine speed have no effect on the fuel properties. For improving the engine performance, one can add the nano-metal additives to the diesel fuel. Due to the lower thermal conductivity coefficient of the nano-metal additives as compared to that of the fuel blend, a nano-metal with a low thermal conductivity coefficient such as silica, alumina, manganese and cerium must be used. This review study, tried to demonstrate the effect of different additives on the engine performance, fuel chemical-physical properties and the exhaust emissions. The other purpose of this research is to review the effect of adding various additives on the diesel fuel at the same time.

2.2. Ethanol Ethanol is a fuel that is produced from biological materials and is known as a renewable fuel [40]. Ethanol can be produced in various ways such as distillation fermentation of natural materials [41] or sugar beet. Due to its high octane number, ethanol can be added to gasoline in order to increase its octane number [42,43]. Therefore, some studies have been carried out for the application of ethanol as a fuel in the spark ignition engines or even as an additive in the diesel fuels [5,44]. Different methods have been developed so far for adding the ethanol to the diesel fuel, for example: ethanol suspension (making an emulsifying dilution) [45–47], spraying of the ethanol to the diesel fuels [48–52], and bilateral injections [53]. The advantage of ethanol in comparison with the case where just the pure diesel fuel is used, ethanol can add to the heat of the combustion chamber and increase the output heat from it. The combination of various alcohols including ethanol and diesel fuel, leads to make a clean fuel [54]. In the past, there was not a stable and single-phase diesel-ethanol fuel by direct injection of the ethanol [55]. However, this problem was solved to some extent by adding ethyl ester [56], octyl nitrate [57], methyl esters [58], nitro methane, nitro ethane and 2-methoxy ethyl ether [8] as other additives or stabilizers. 2.3. Butanol Butanol is a four-carbon alcohol. This alcohol can also be blended as an additive with the diesel fuel. The main advantage of butanol over the other alcohols is that butanol has higher cetane number. The corrosion caused by the additives in the butanol is lower than the one in the other alcohols (i.e. ethanol and methanol), because the ratios of oxygen to carbon and also oxygen to hydrogen are rather small in butane with a greater heat capacity as compared to that of the ethanol and methanol [59,60]. The properties of butanol are close to those of the base fossil fuel to some extent: its ignition temperature is lower

2. Fundaments of using alcohol Alcohol contains hydroxyl groups which are attached to a carbon atom. Chemical and physical properties of a neat diesel fuel and alcohol are listed in Table 1.

Table 1 Chemical and physical properties of neat diesel fuel and alcohols. Properties

Pure diesel

Ethanol

n-Butanol

Methanol

Cinematic viscosity at 40 °C (cm2 /s.10-6) Density at 20 °C (kg/m3) Cetane number oxygen molecules C/H Flash point (°C) Boiling point (°C)

3.35 [11]

1.2 [80]

3 [11]

0.75 [3]

837 [68] 50 [68] 0 [68] 0.45 [3] 45 [52] 180–360 [67] 250 [67]

788 [68] 5–8 [68] 34.8 [68] 0.33 [3] 13–14 [5] 78 [67]

810 [48] 25 [48] 21.6 [48] 0.4 [3] 35–37 [5] 118 [47]

790 [3] 3–5 [3] 50 [3] 0.25 [3] 11 [3] 64.7 [3]

840 [67]

585 [47]

1100 [3]

2.1. Methanol Methanol is more oxygenated in comparison with butanol and ethanol, due to its higher ratio of oxygen to carbon and also oxygen to hydrogen. Methanol is produced from fossil fuels (by syn-gas process) and biomass. Gas fuel including hydrogen, CO and a little CO2, are used to produce ammonia or methanol. A great deal of research work is done recently to produce methanol by reacting CO2 and hydrogen retrial, which is not affordable [62–64]. Because of the significant heat of vaporization, temperature of the incoming air may decrease, so it can

Latent energy (kj/kg)

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4. Methods for combination of alcohol and diesel fuel

than that of ethanol and, methanol (about 385) [61]. Additionally, butanol can burn much simply when blended with the diesel fuel. Butanol has a lower latent heat of vaporization as compared to those of ethanol and methanol, while the percentage of wasted energy in butanol is lower than that of the other alcohols. Moreover, butanol can be added to the diesel fuel through fermentation of the biomass particularly disposable residues of trees which are rich of cellulose [61].

As discussed earlier in this section, there are different methods to add ethanol to the diesel fuel. One of these methods is direct injection in which the ethanol is added to the diesel fuel directly. Combination of the diesel with the alcohol (ethanol) incorporates no physical problem, but at temperatures lower than 10 C°, integration is not reached properly which creates a two – phase dilution. However, as noted earlier, the ethanol could be added to the diesel fuel without producing any two – phase composition [61,65]. In another method, one should blend the ethanol droplets as a kind of emulsion with a novel emulsifier [45,66], so the fuel efficiency and combination stability increase. Chen and colleagues [67] worked on an emulsion fuel having 15% glucose and found this combination more stable and discovered that distribution of the fine particles is decreased significantly, while according to the following stoichiometry formula, it was observed that the CO2 content is increased considerably.

3. Tertiary and nano-metal additives When blending the oxygenate fuels with the diesel fuels, the chemical-physical properties of the fuel including flash point, density, viscosity and actually cetane number might be altered. The oxygenate fuels has a low cetane number, flash point, viscosity and density [1–4]. For solving this problem and especially increasing the cetane number, one needs to use tertiary additives which are known as the cetane number improvers. The main and most important tertiary additives available are nitro methane, nitro ethane, 2-methoxy ethyl ether, methyl ester and octyl nitrate. Tertiary additives have high performance improving the cetane number and preventing formation of a two-phase blend. However, the tertiary additives can enhance the flash point, viscosity and density as well. Goldsborough et al. [161] worked on effect of an alkyl nitrate that is 2-ethyl-hexyl nitrate as a cetane number improver and finally showed that these nitrogenate additives promote the cetane number significantly. Blending of the oxygenated fuels and the tertiary additives causes a poor brake power, but may increase the brake specific fuel consumption on the other hand. For increasing of the brake power, one should use a nano-metal additive. Adding the nano-metal additive to the fuel, thermal conductivity coefficient of the fuel is decreased, so that blending with a metal with lower thermal conductivity would be a promising alternative for improving the brake power. The main potential nano-metal additives are: manganese, cerium, alumina and silica. Chandrasekaran et al. [62] studied the effect of using nano additives on the diesel fuel. They used Mahua oil methyl ester as a nano additive and concluded that 20MEOM fuel blends can improve the brake power of a diesel engine. Soukht Saraee et al. [63] worked on the impact of blending cerium oxide and cerium dioxide on the engine performance. Their research showed that the brake specific fuel consumption, HC emission and NOX emissions were decreased by adding the nano additives, but the engine power is enhanced by adding both of the nano additives. This enhancement was affected from increasing of the hot spot value in the engine. Basha [64] presented a research that used di-ethyl ether as nano additives. He said that the nano additives could increase the engine power. In recent years, the use of nano metal as additives in diesel fuel improves the chemico-physical properties, such as high thermal conductivity, mass diffusivity, and ratio of surface area/volume, when blended in any base fluid medium. Based on the previous research, it is found that nano metal additives with biodiesel, diesel and blends improve the kinematic viscosity, flash point, and other properties, owing to upon the volume of the nano metal additives [163]. Selvan et al. [162] research the experimental investigation in two phases, to study emission characteristics and the performance of a CI engine, when using cerium oxide nano particles as nano additives in diesel–biodiesel–ethanol blends and pure diesel. In the first phase of their experiments research, the stability of diesel– biodiesel–ethanol fuel blends and pure diesel with the blending of cerium oxide nano particles was analyzed, and in the other phase, the performance characteristics were studied. The improvement in brake thermal power and reduction in ignition delay are reported. Goldsborough et al. [161] worked on effect of an Alkyl nitrate that is 2-ethyl-hexyl nitrate as a Cetane number improver. They showed that this nitrogenate additives has a high effect on improve of Cetane number.

C6 H45 O6 → 2C2 H5 OH + 2CO2 Moses et al. [68] worked on a micro – emulsion aqueous ethanol containing 5% of water and diesel fuel. They finally reported that this compound is very stable and it burns a while after the diesel fuel base. The spraying method could be considered for combining the alcohol with the diesel fuel. In this method the alcohol is sprayed into the diesel engine by injection. In a typical engine with a diesel blend – alcohol fuel, the combustion takes place in spite of the intake air [68]. The advantages of this method over the other methods could be cited with the following reasons: 1. The composition of diesel – alcohol fuel obtained from this method is more stable in comparison with the same combination produced from the previous conventional techniques [68,69]. 2. The amount of the injected alcohol into fuel is controlled according to the engine need [70–74] 3. Aqueous alcohols could be blended with the diesel fuel [75–78] Yao et al. [79] worked on a composition of diesel – methanol fuel. They used the spraying method to blend the methanol and the diesel fuel and finally concluded that the method reduces the amount of particulate matter (PM) and NOX, but the HC and CO contents increase. Zhang et al. [80] focused on a diesel engine with natural direct injection. They used the spraying method for combination of the methanol with the diesel fuel. In two tests by this method they concluded that the contents of PM and NOX decrease, while the amount of HC and CO increase. However, reduction of NOX takes place as compared to the combination of methanol-diesel which are combined using a method other than spraying [74,80,81]. As discussed previously, to combine some alcohols (especially methanol and ethanol) with the diesel fuel, one should add another additive to the composition, so by using this method it is possible to produce a stable composition and improve auto ignition properties of the fuel. This additive could act as an interface between the diesel fuels to prevent creation of a two-phase composition. Pirouzfar et al. [8] worked on the ethanol diesel fuel with the help of three different types of an additive like 2-methoxy- ethyl ether, nitro methane and nitro ethane, and their obtained results revealed fuel stability, as well as improvement of the cetane number and BSFC. If the n-butanol (normal butanol) is used as an additive, it will be impossible to use another additive, because the n-butanol has greater cetane number and molecular weight as compared to the other alcohols. These properties make cetane number acceptable without transformation to a two-phase compound.

893

Renewable and Sustainable Energy Reviews 74 (2017) 891–901

894

Tertiary additive Alcohol percentage

– – ppm % ppm

ppm

ppm

g/kwh ppm % g/kwh g/kwh g/kwh

218g/kwh 340g/kwh 165g/kwh 470g/kwh 236 – 300g/kwh 345g/kwh 0.36 4% 305g/kwh 294g/kwh – – 272g/kwh – 187g/kwh – 238g/kwh g/kwh ppm g/kwh ppm

BSU FSN

g/kwh g/kwh HSU %

g/kwh FSN

ppm

%

0.7 275 0.18 580 – 380 ~+0.03 22.7 450 −13 0.225 0.235 3.5 – 85.5 – 1250 0.18 420 g/kwh

0.21 – −18 – 20.03 – – 1.05 0.43 – 0.71 0.72 20 −23 – 0.82 0.025 – – g/kwh ppm g/kwh ppm ppm ppm ppm g/kwh ppm % g/kwh g/kwh g/kwh % ppm ppm ppm ppm ppm 5.85 580 1.55 175 676 180 540 13.5 840 −12 2.8 3.15 14 8 503 830 804 100 800 [99] [82] [90] [61] [8] [122] [93] [142] [53] [101] [84] [84] [95] [96] [83] [102] [66] [146] [137] Shi et al. Rakopoulos et al. Sayin et al. Choi et al. Pirouzfar et al. Choi et al. Jincheng et al. Andres et al. Jilin et al. Karabektas et al. Cenk sayin Cenk sayin Gnanomorthi et al. Sahin and Akso Rakopoulos et al. Bang-Quan et al. Liu et al. Seyfi Ileri and Kocar

Engine application Ref.

5.1.2. Brake thermal efficiency Brake thermal efficiency (BTE) is one of the most important and effective factors in selecting an additive. Adding the oxygenated additives causes degradation of the brake thermal efficiency, because the latent heat of vaporization of the oxygenated fuels is greater than that of the diesel fuel. Therefore, by

Research group

Table 2 Summary of emissions, fuel properties and performance test results from additive–diesel blends.

Engine load

Engine speed

5.1.1. BSFC BSFC (brake specific fuel consumption) is affected by three parameters: 1) oxygen content of the additive. 2) Useful output power. 3) Engine speed. When the oxygenated fuels are added to the diesel fuel, the BSFC increases because blending of such fuels with the diesel fuel leads to greater oxygen content that contributes to a more complete ignition. Rakopoulos [82] in his article about the impact of combination of normal butanol with fuel efficiency concluded that adding the alcohol to the diesel fuel improves the fuel efficiency in the engine. They have also shown that as a result of increasing the power of the fuel used, thermal efficiency of the fuel is decreased in the engine. In another study, Rakopoulos et al. [82] worked on adding ethanol to the diesel fuel. Their results indicated that increasing of the engine speed has a direct effect on the BSFC, and the BSFC can be improved by increasing the engine speed. Most studies using the biodiesel blends report improved BSFC for all operating conditions. Once the engine speed increases, the fuel will get more chance to have a more complete ignition. This means improved efficiency of the fuel in the engine that is also equal to greater BSFC [83–88]. Balamurugan and Nalini [89] investigated enhancement of the engine performance by using both normal butanol and normal propanol alcohols. Their observations indicated that the engine fuel efficiency increases by adding each additive. Improvement of the engine output power causes a substantial decrease in the fuel efficiency. In addition, they showed that almost at any percentage of the engine load percentage, when the level of fuel efficiency in the pure diesel is halved, then the fuel efficiency in the pure diesel will be greater than that of various combinations of the diesel-alcohol. In their experiments, the effect of the normal butanol and propanol was more than that of the normal butanol for improvement of the fuel efficiency. If the fuel could be burnt completely, it would then be possible to increase the fuel efficiency in the engine. This definition indicates that when the alcohol is oxygenated more and gets shorter branches, it's properties improve the fuel efficiency in the engine at similar situations. Table 3 summarizes results showing that the alcohols can have different effects on the BSFC. However, it is clear that when blending the alcohols with the gasoline, the amount of BSFC could be significantly increased. Since the alcohols give rise to the oxygen content of the fuel, they contribute the fuel to have complete combustion.

250 Nm 1900 rpm at brake mean effective pressure 2.57 bar 2000% Nm – 50% 200,000% rpm 370 Nm 2200 rpm 25.4% Nm 1500 rpm – 2000 rpm 43 Nm – 96 Nm 1600 rpm – 2000 rpm – 1400 rpm – 1600 rpm carbonat at brake power 3 kW 135 Nm 3000 rpm at bmep:3.56 bar 1500 rpm at bmep:0.5 MPa 1700 rpm 80 % 1400 rpm inlet temp 360 K 1200 rpm – 3000 rpm

NOx emission

The effects of various additives on the engine performance are discussed below.

Methyl soyate – – – Nitro methane – at peclete number 4 – CLZ emulsifier – – – Ethyl acetate+Diethyl – – – – Diethyl ether Methyl ester 20%

5.1. Engine performance

Ethanol 20% Butanol 16% Methanol 10% Butanol 10% Ethanol 10% Butanol 10% Ethanol 20% Butanol 10% Ethanol 10% isoButanol 10% Methanol 5% Ethanol 5% Ethanol 20% Butanol 4% Ethanol 5% Ethanol 10% Methanol Ethanol 50% BHA 500 ppm

PM emission

CO emission

BSFC

BTE

Adding alcohol to the diesel fuel needs a balance between the emission reduction and engine power in order to reach the optimal point. Emissions from the diesel fuel are the most critical problem using this kind of fuel. In this study it was tried to address some researches that blended the alcohols with the gasoline to control the air pollution and improve the engine performance (Table 3). It was also focused on the emission reduction, improvement of the fuel properties and enhancement of the engine power by adding various alcohols. The most important incentives using alcohol for the purpose of emission reduction are: production of oxygenated alcohols, reaching small molecular weight and the most importantly, achieving shorter molecular chain in comparison with that of the diesel fuel.

4-cylinder single cylinder single-cylinder 4-cylinder 4-cylinder 4-cylinder single-cylinder 4-cylinder 4-cylinder single cylinder single cylinder Single cylinder single cylinder 4-cylinder 6-cylinder 4-cylinder 6-cylinder single cylinder 4-cylinder

5. Discussion

– 0.253 −7% 75 Ps 79 kW – – 0.257 0.33 −1.70% 0.248g/kwh 0.272g/kwh 3% – 0.32g/kwh –

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A. Fayyazbakhsh, V. Pirouzfar

Table 3 Summary of emissions and performance test results from additive–gasoline blends. Researcher

Ref no.

Oxygenate additive

Additive (%)

speed

Pressure (kPa)

Power (kW)

BSFC

CO

HC

NOX

Najafi et al.

[157]

Ethanol Ethanol Ethanol Ethanol

5 15 5 15

2000 2000 3500 3500

– – – –

20.5 23.5 35.7 38

260g/kw 255g/kw 257g/kw 250g/kw

4.8 v.d% 2.35 v.d% 4.68 v.d% 2.55 v.d%

235 ppm 148 ppm 175 ppm 137 ppm

– – – –

Li et al.

[158]

Isopropanol +But+Eth Isopropanol +But+Eth Isopropanol +But+Eth Isopropanol +But+Eth

0

–

300

22.5

370g/kw

–

310 ppm

1400 ppm

30

–

300

23.5

395g/kw

–

282 ppm

1230 ppm

0

–

500

26.5

330g/kw

–

343 ppm

1720 ppm

30

–

500

27.8

355g/kw

–

295 ppm

1490 ppm

0 3 10 0 3 10

2700 2700 2700 3400 3400 3400

– – – – – –

1.33 1.265 1.3 1.66 1.52 1.57

– – – – – –

8.7% 6% 5.5% 3.9% 3.5% 2.25%

365 ppm 301 ppm 275 ppm 252 ppm 238 ppm 220 ppm

– – – – – –

Alfasakhany

[159]

N-Butanol N-Butanol N-Butanol N-Butanol N-Butanol N-Butanol

to carbon is smaller in the n-Butanol [102–105]. Since in comparison between methanol and ethanol, the oxygen content of the methanol oxygen is greater than the others, the methanol can thus contribute to the diffusion process [92–94,106]. Actually increasing of the rotation speed of the engine adds to the engine temperature which gives rise to the CO2 production [97–101]. Because combustion is the basic step for production of the CO [65]. Byunchul et al. [61] studied the effect of butanol (5, 10 and 20 vol%) blended to the diesel fuel on the exhaust emissions and engine performance. They reported that increasing of the butanol (i.e. oxygen content) has led to greater emissions of the carbon monoxide.

increasing the percentage of the oxygenated additives, the minimum size of droplet needed for the micro-explosion of the oxygenated fuel, require a higher boiling temperature and shorter time for dispensation before the evaporation occurs. Sayin et al. [90] worked on the impact of methanol on both the engine emissions and performance. Their results revealed reduction of the brake thermal efficiency in comparison with that of the pure diesel fuel. They also stated that by increasing the useful output power or torque, the brake thermal efficiency increases as well. They noticed little improvement of the thermal efficiency in the longer injection times. Rakopoulos et al. [83] worked on the impact of adding butanol to the diesel fuel in a diesel engine. Adding butanol to the diesel fuel does not incorporate much effect on the thermal efficiency. They also declared that increasing of the pressure leads to significant improvement of the engine thermal efficiency. From Table 1 it can be understood that the difference between the latent heats of vaporization for the butanol and the diesel fuel are smaller than those of the other alcohols. Hence it's expected that the brake thermal efficiency grows a little by adding the butanol. An opposite trend is seen for the methanol (and ethanol to some extent). According to Table 1 it can be observed that there is a big difference between the latent heat of evaporation for the pure diesel fuel and the methanol. The figures listed in Table 3 show that all the alcohols have positive effect on the power generated in a gasoline engine.

5.2.2. GHG A greenhouse gas (GHG) is a gas in the atmosphere that emits and absorb radiation through the thermal infrared range. This mechanism is the basal cause of the greenhouse effect. Increasing of greenhouse gases (GHG) is a result of indirect land-use change affected by cropbased biofuels as one of the most important aspects of the biofuels in terms of energy security and environment policy. The main greenhouse gases in the atmosphere of the earth are nitrogen oxides, carbon oxides, ozone, methane, and water vapor. Without the greenhouse gases, the earth temperature would be about −18 °C instead of the present average temperature of 15 °C. Ebner et al. [160] studied the effect of a novel mechanism for converting the food waste to the ethanol. They showed that when the landfill emissions are avoided, the process shows an improvement over the conventional gasoline or ethanol in terms of the GHG emissions. Emissions from the engine exhausts were believed causing significant reduction of the greenhouse gases (CO2). The European Environment Agency has set as target for its state members the use of bio-fuel in the transportation system fuel minimum for 2% by the end of 2005 and 5.74% by the 2011. The use of biofuel especially alcohols, could partly be replaced with the fossil fuel which can reduce the noxious emission and mitigate the CO2 emissions. CO2 is one of the most important pollutants and it's the most abundant air pollutants in the temperature because of the so-called greenhouse effect phenomenon. The amount of this gas increases by using more oxygen-gases, especially the alcohols [105,107]. However, it is reduced with adding the oxygenated material to the diesel fuel in some situations [78,103] because the carbon is lower than the oxygen. Due to the alcohol combination it could be said that the ratio of carbon to hydrogen is smaller [103].

5.2. Emissions The effect of different nitrogenate fuels and various oxygenated fuels on exhaust emissions is studied in this section. 5.2.1. CO Generally speaking, addition of the alcohols to the diesel fuel provides an excess oxygen in the fuel blend that contributes the inner cylinder to produce CO and also to oxide CO to CO2. Increasing of the oxygen content in the fuel leads to completion of the fuel combustion. However, because of the high oxygen content in the fuel, qualification for production of CO has become rather difficult. Carbon monoxide appeared due to the incomplete combustion of fossil fuels (especially diesel fuel). Most studies showed increasing of the oxygen content by combining the oxygenated fuels causes further distribution of the CO [91–101]. In fact, the effect of adding various additives is different from each other. For example, the effect of butanol is lower as compared to that of ethanol and methanol, because the ratio of oxygen 895

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than that of the ethanol fuel [88,90] and this reduced rate is greater than the time n-butanol is used in the fuel [83–96]. Since the alcohols have lower C/H, the soot emissions are reduced. The soot emissions increase for the greater engine loads, but the soot emission decrease with more ethanol and higher load.

5.2.3. HC HC is one of the main pollutants produced from incomplete combustion which enter into the air with the other combustion products [10]. Increasing of the HC degrades the efficiency of the diesel engines as well [108]. Adding the oxygenated materials to the diesel fuel adds to the HC amount [109–113] because the oxygen content in the diesel fuel to complete the combustion is rather high [114,115]. This finding could be confirmed by investigation on how the radicals react with each other. When the engine temperature comes down, the amount of all air pollutant especially HC also decreases significantly. The temperature is decreased by using a emulsifyer such as water, so the amount of HC production is decreased as well [67,116]. Choi and Reitzr [21] worked on the emissions reduction by using a combination of normal–butanol diesel. They reported that when the normal-butanol is added to the fuel (at various percentages), the distribution of unburned hydrocarbons in low percentages of normal butanol (up to 5%) reduces, but at higher percentages they attributed this effect to the minimum amount of droplet needed for microexplosions of the natural butanol. As a result, these blends would require higher temperatures to boil in addition to longer times for being sprayed prior to evaporation. However, in another work which was conducted by Choi et al. [61] in the same year, it was not mentioned and they noted that in any percentage of adding the oxygenated combinations to the diesel fuel, the amount of HC is decreased and some of them are transformed to straight chain hydrocarbons. Therefore, the generation rate of HC increases as discussed earlier. This in turn reduces the thermal efficiency of the engine and contributes to the formation of HC and CO in hot spots emerged in the engine. Of course, when we use the net diesel fuel, these two pollutants are made similarly in using of the pure diesel, when the required heat was prepared for their formation. Kannan et al. [164] studied the effect of blending ferric chloride as a nano additives to diesel fuel on emissions. They reported that blending this nano additives cause to decrease of smoke, UHC and carbon monoxide emissions, but increase of NO and carbon dioxides emissions.

5.2.5. NOX Nitrogen oxides are mainly composed of nitric oxide (NO) and nitrogen dioxide (NO2). The nitrogen oxides are considered as an important factor in creation of acid rain, chemical fog and smoke in urban air. While the dust is rich of nitrogen oxides in the urban air. The creation of NOX is dependent on the engine temperature and ignition delay [125], so the NOx forms in hot spots of the diesel engine. The three chief reactions producing thermal NOx was shown in Zeldovich Mechanism as below: N2 + O→NO + N N + O2 →NO + O N + OH →NO + H The temperature should be about 1600 °C for the oxygen and the nitrogen to react with each other, so if the temperature doesn't exist, the NOX won't form at all. So it's clear that in any case, an enhancement in the engine speed (engine temperature decreases and its emissions as well), reduces the amount of NOX [53,98–101]. By adding further oxygenated materials to the diesel fuel, the amount of NOX distribution is also increased [91,98–118]. However, in the additives where the ratio of oxygen to hydrogen and also that of oxygen to carbon is higher (e.g. methanol), generation of NOX is also greater. The NOX production has direct relation with the high pressure. So, greater amount of NOX is produced at the high pressure [87]. The high evaporation enthalpy of alcohols reduces the temperature during combustion, so this reduction of NOX occurs, though just to a limited extent [119,120]. By adding more alcohol to dilution, the combustion process occurs in shorter time and can't reduce the combustion heat [44]. Increasing of NOx by further addition of oxygenated additives reduces the cetane number which has direct and significant effect on ignition [51]. The successful method of NOX reduction is via lowering the peak cylinder pressure among retarded injection timing or using exhaust gas recirculation. In practical applications, the exhaust gas recirculation reduces the nitrogen oxide emissions for sure as reported in the literature [121–125]. On the other hand, considering the engine performance, soot and other exhaust emissions, lower local temperature and oxygen centralization in the cylinder faces the fuel to incomplete combustion and further results in a lower brake thermal efficiency and higher emissions of PM, PAHs, HC and CO [126–128]. In support to the previous researches, Table 2 summarizes the results from all Ethanol, normal Butanol and Methanol blended to diesel fuel studies on PM, nitrogen oxides, carbon oxides emissions and engine performance (brake thermal efficiency and brake specific fuel consumption). Moreover this table showed the effect of tertiary additives on all responses. This table showed that the oxygenated additives have a positive effect on the BSFC and the emissions of nitrogen oxides but have a negative effect on the emissions of BTE and PM. The effect of butanol on CO emissions is lower as compared to that of ethanol and methanol, because the amount of oxygen to the carbon in the n-Butanol is smaller. The effect of various additives on the amount of PM reduction is variable, so when the ratio of oxygen to carbon or oxygen to hydrogen is high, the amount of PM reduction will thus be the greatest. In the work of Mirzajanzadeh et al. [165] mixture (homogenous) amide-functionalized MWCNTs were applied as support for CeO2 as the catalytic and the hybrid catalyst (CeO2-MWCNTs) was blended to the diesel–biodiesel blends (B5and B20) at three different concentrations. CeO2 nano particles owing to

5.2.4. Soot emission Oxygenated fuels are known to reduce particulate matter (PM) emissions. Composition of the particle components changes with engine technology, fuel quality and combustion conditions. The particles are of different size. The smaller the particles, the more dangerous they are. The main cause of cancer (particularly lung and blood cancer) is PM [6]. Therefore, it must be tried to reduce the spread of the exhaust gases coming from the diesel engines, because this is the most dangerous part of their output soot. Meanwhile, it could be said that the key factor in dangerous features of the engine exhaust is PAH which the main reasons for PM formation itself. Using the alcohol-diesel organic fuel is of little importance to develop OH radicals and inert H2O2. There is a possibility of forming poly aromatic hydrocarbons (PAH) throughout this test. The carbon ratio per hydrogen of the diesel fuel is higher than that of ethanol, and it mitigates formation of the soot less than what happens in the fuel rich conditions. The particulate matter is composed of two components: 1soot; which has carbon and is solid; 2- organic solvents have this property that adsorb the hydrocarbons and release them into the atmosphere [117–121]. In general, oxygenation of the fuel by using an oxygenated material is applied to reduce the PM distribution. Many researchers have tried to use methanol, ethanol, butanol and other alcohols and even add vegetable or animal fats into the diesel fuel [122–124]. The effect of various additives on the amount of PM reduction is variable, so when the ratio of oxygen to carbon or oxygen to hydrogen is high, the amount of PM reduction is even greater. For example, the PM reduction in the methanol fuel [12,78,94] is more

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becomes equal to the pressure surrounding the liquid. The boiling point of a liquid difference is due to the surrounding environmental pressure. A liquid with high pressure has a higher boiling temperature than a liquid at atmospheric pressure and vice versa. The latent energy is the internal energy a system requires to undergo a phase transition. Its volume is specific to the substance under study. The volume can also vary with pressure and temperature [4]. A higher latent energy of the alcohols causes an increase in the energy needed to start. The density, or more precisely, the volumetric mass density of a substance is its mass per unit volume. The difference of density between the alcohols and the diesel fuel especially methanol is one of the main character that could lead the fuel to 2-phase compositions [1– 4].

their decreasing impact on peak temperature in the combustion chamber resulted in decreased production of nitrogen oxides (NOx).

5.3. Fuel properties A number of physical-chemical fuel properties are essential for adequate combustion of a diesel fuel. Blending of the alcohols with the diesel fuel changes certain key properties such as viscosity, density, flash point and cetane number. This study focuses on of the effects the alcohols blended with diesel have on cetane number and viscosity. The cetane number is one of the most important properties of a diesel fuel, very similar to the octane number that is associated with the gasoline fuel. A high cetane number brings about a good starting ability in a cold weather, in addition to low combustion noise and long life of the engine. Viscosity of the fuel is one of the important characteristics of a liquid fuel. It also plays a key role in lubrication of the fuel injection systems. Blending of the alcohols to the diesel fuel causes a slight reduction in the fuel viscosity. De cardo et al. [46] studied the impact of blending the ethanol with the diesel fuel on the physic-chemical properties of a fuel. They reported that ethanol, causes the cetane number stays above 45. The relation between the reduction of the cetane number and the ethanol blending was linear. Moreover, they found that with the ethanol contents of 10–20%, the viscosity does not meet the minimum requirements for the diesel fuels. Numerous techniques have been proposed to increase the cetane number of diesel fuels owing to smaller soot emission. Flash point is the lowest temperature that a fuel will ignite when exposed to an ignition source. Blending the alcohols with the diesel lowers the fuel flash point, but increasing of the tertiary additives can increase of the fuel viscosity. Pirouzfar et al. [8] concluded that the effect was positive on the tertiary additives (especially, nitrogenate fuel) and this fuel can give rise the cetane number and other mentioned properties. They showed that the nitrogenated fuels can enhance the cetane number to 57. They used three different cetane number improvers for this purpose. The cetane number improvers were 2-methoxy ethyl ether (MXEE), nitro methane and nitro ethane. However they showed that the nitro methane was more effective than the other two tertiary additives. The boiling temperature of a substance is the temperature at which the liquid changes to vapor [4] and the vapor pressure of the liquid

6. Combustion 6.1. Combustion noise Sound noise in the air-pressure fluctuations is really small. The understanding from the sound intensity is based on the ratio of difference in the pressure. A logarithmic scale is used in this respect as formulated below [129].

⎛ P ⎞ ⎛ P ⎞2 Sound pressure = 10 log ⎜ ⎟ = 20log ⎜ ⎟ ⎝ Pref ⎠ ⎝ Pref ⎠ Where the reference pressure is the lowest audible sound from the order of 20 µPa. The three primary sources of noise generation in a diesel engine are listed in Fig. 1.

• • •

Gas-flow Combustion Mechanical process

Generally speaking, the diesel engine, being a very intricate system comprising various dynamic forces acting on a structure of varying hardness, modifying and response characteristics, remains by far inferior to its gasoline correlate. Particularly for the diesel-engine vehicles, the unbecoming combustion knock might also be a matter of irritation for passengers and pedestrians. Gas-flow noise, usually low

Fig. 1. Engine noise generation [129].

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more pilots the inter-variability will increase. In all instances the exhaust gas recirculation reduced inter-cycle variability [153–156].

frequency reined, is dependent on the intake and exhaust processes, including a turbocharger and a cooling fan. Mechanical noise, on the other hand, contains the contribution of both rotating and reciprocating engine ingredients. It emanates from gears, tappets, valve trains, timing drives, fuel injection instruments and bearings, and also from gas forces creating piston slap. The low cranking speed appeared to have the dominant impact on combustion noise development and its absolute values [130–137]. The bio-fuels blends have small effect on continuous performance of the engine and the overall combustion noise radiation. Combustion behavior and stability among the first transient cycles were mainly affected by the bio-diesel blend and reduced by the alcohols blend [138]. Giakoumis et al. [14] reported that during transient, the value of radiation noise at the early cycles of the transient event is higher than its value during the steady-operation at the same fueling and speed conditions. The impact of the small match between the cylinder wall temperature and that of the operating conditions is responsible for a considerable value of the noise development during the steady-state operation [139]. During a cold start in particular, due to the lower cylinder wall temperatures, the above phenomena are even more significant so that the engine makes sound more than its warm-up condition. Slow increase of the engine speed on start contributes to elevated noise levels. When the alcohols are blended into the fuel blend, a longer ignition delay is caused, due to their lower cetane number that is reliable for a further rise in the combustion noise over the pure diesel fuel operation. According to the limited tests on both genres of biofuels performed so far, more researches seem necessary, with wider variety of the biofuels and higher blending ratios, in order to attain more clear results [140–142]. Dhaenens et al. [141] reported that increasing the engine speed cause higher sound pressure level. Rakopoulos et al. [143] reported that the bio-fuel blends have limited impact on the transient performance of the diesel engine and the combustion noise radiation. However, the low cranking speed appeared to have a dominant effect on development of the combustion noise and its absolute amount.

7. Major issues and further research requirements Reduction of the air pollutions is the main subject for future the researches. New models that can lead to much lower soot emissions should be studied in the future researches. The following are also topics which are of great potential for the future studies in this respect: Optimization of energy and cost Use new nitrogenate and nano additives to diesel fuel for improving the chemical-physical properties and also engine performance. Effect of preheating the fuel on the combustion characteristics, pollutions and engine performance. Influence of the nitrogenate additives or additives known as cetane number improvers on the engine performance and air pollutions. 8. Conclusion The following are the main conclusions which can be made from the current review: 1) Adding the alcohols is necessary to reduce the soot content and to reduce the soot content even more, an alcohol of higher oxygen content must be used. 2) Adding the alcohols at certain temperatures creates a two-phase composition. To solve this problem one should use additives which act like an interface between the pure diesel fuel and the alcohol and prevent formation of a two-phase fuel at any temperature. 3) In case it is not possible to use another additive, one can use normal butanol, because the n-botanol has a higher cetane number. These properties cause the n-botanol combined with the diesel fuel not experience transformation to a two-phase composition. 4) If the produced fuel has high percentage of oxygen, the distribution of NOX grows, and of course if the percentages of oxygen is low, the amount of soot spread increases. In order to control the dispersion ratio, it should be tried to make a proportion between the oxygen and these two pollutants, but it must be said that the first priority is still the soot reduction. 5) In addition to application of the alcohols for the purpose of soot reduction, it is possible to use the low-percent alcohols to reduce the HC dispersion ratio, because the low-percent alcohols increase the required temperature for occurrence of the micro-explosions. 6) Combustion noise radiation during an acceleration transient event, like the one experienced continuously during a daily driving, has been found to be enhanced when a normal blend is used, although no experimental result is available for the methanol and ethanol.

6.2. Cylinder pressure and heat release rate The combustion process of a typical diesel engine is partially premixed combustion (the first heat release unit) and partially diffusive [144–150]. These phenomena are complex mechanisms to identify and they actually depend on the compression ratio, fuel, fuel injection timing engine load, temperature and intake boosting the pressure. Gnanamoorthi and Devaradjane [95] showed that a high ratio of ethanol-diesel blend occurs in the premix stage and just a few part of this blend occurs during the diffusive stage. Wei et al. [105] reported that, when methanol is injected at the intake port, methanol starts to vaporize and absorb the heat of intake gas leading to lower gas temperature and hence lower pressure of the inner cylinder at the compression stroke. The dissimilar physical-chemical properties of each fundament of the blends result in transformation of the fuel delivery, dynamic injection timing [148], fuel fumigation dispersion, wall impingement rate, ignition timing (delay), as well as fuel evaporation and blending rates [151]. As a result, the premixed and diffusion parts of combustion modify, while the presence of oxygen in the bio-fuel blends is accounted for various local fuel–air equivalence ratios, triggering or prevention of the combustion. Fang et al. [152] studied the effect of pilot injection and exhaust gas recirculation on the combustion and the emissions in a combustion engine with homogeneous charge compression ignition-direct injection. The diesel engine used in their study was a heavy duty four-cylinder engine with a common rail injection system. The impact of exhaust gas recirculation and pilot injection quantity on inter-cycle mutability is investigated in this section. The inter-cycle variability was discussed in terms of the coefficient of variation of the indicated mean effective pressure and also peak pressure. They showed that raising the pilot quantity reduces the inter-cycle variability to a certain threshold and then with

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