Diesel Engine Fuel and Fuel Emulsion Influence on Diesel Engine Performance and Emission

Abstract

The diesel engine’s superior energy conversion efficiency and cost-effective power source have made it a popular choice for a wide range of applications, including but not limited to agricultural machinery, transportation, and mining equipment. Diesel engines produce harmful emissions, including exhaust fumes that contain pollutants such as particulate matter and nitrogen oxides. These emissions are detrimental to the environment and public health, and as a result, strict standards are imposed to reduce them using modern technologies in diesel engine manufacturing, exhaust treatment systems improvement and fuel modifying technologies. This chapter reviewed the effect of wide verity of fuel in diesel engine performance and emission.

1. Introduction

Diesel engine has high energy conversion and economic power source compared to other engines which make it used in many applications, such as agricultural machines, transportation, and mining equipment [1]. On the other hand, diesel engines emit pollutant that is harmful for the environment, since regulated emissions like HC, CO, NOx and particulate matter (PM) were put in place [2, 3]. Increased concerns over environmental and emissions regulations have heightened the motivation to control diesel engine emissions. Consequently, there are three main approaches to control diesel engine emission which are: engine design techniques, exhaust gas after treatment and fuel. Injection timing has great influence on diesel engine emission. Advanced injection timing contributes to high NOx emission and low fuel consumption while delaying injection timing reduces NOx emission and increases fuel consumption [3]. The selective catalytic reduction as an aftertreatment system has proved significant reduction in both NOx and PM compared to engine without this system [4, 5, 6]. Similarly, The exhaust gas recirculation (EGR) contribute to reduce NOx and PM emission [7, 8, 9]. Correspondingly, The diesel particulate filter has proven dramatic decrease on PM emission [5]. The diesel oxidation catalysts are equipped to diesel engines to decrease regulated emission such as CO and HC [10].

Based on the above discussion, the objective of this chapter is to investigate the effect of fuel on diesel engine emission. Diesel engines are increasingly being developed and adopted to work with wide verity of fuel and fuel additives. The fuel, fuel blends, fuel additives and fuel emulsion effect on diesel engine regulated emission will be reviewed in this chapter.

2. Diesel engine fuel

Diesel engines could work with a wide range verity of fuel in which fuel ignition take a place without a spark as a result of air compression and fuel injection. Correspondingly, any fuel shows significant compression ignition features and other properties such as, viscosity, flash point, heat value and cetane number (CN). Fuel viscosity is simply a measure of flow resistance. According to ASTM D6751 the accepted fuel viscosity for the diesel engine varies between 1.9 to 6.0 cSt at 40°C [11]. Fuel with higher viscosity than diesel decreases the injection rate and less atomization and vaporization hence incomplete combustion tends to increase smoke opacity, HC and CO [12]. CN is a measure for fuel ignition quality. The CN of fuel affects the ignition delay time and the premixed combustion phase, which decrease by raising the CN in the fuel. Therefore, fuels with a high CN cause a reduction of NOx emission, whereas it improves high CO, HC and smoke emissions [13]. Additionally, A higher CN of biodiesel contributes to quieter engine operation and easier engine start-up [14, 15]. The energy content of fuel is a key element in fuel economy, as well as in an engine’s ability to produce power. Typically, fuel with higher heating value will provide better economic performance [16]. Overall, many fuels have been experimentally tested for running diesel engine such as biodiesel, diesel, kerosene, n-butanol and n-heptane [17, 18, 19, 20].

3. Fuel blends

Diesel is usually blended and used in different concentration with other fuels to improve diesel engine performance and emissions. Nageswara Rao et al. [18], tested Common Rail Direct Injection 4-stroke 4-Cylinder engine fueled with ethanol, biodiesel and diesel blends. The blend percentages were 50 vol% biodiesel with different percentages of ethanol and diesel. They concluded that increasing ethanol percentages in the fuel blend results in reducing brake thermal efficiency (BTE) and increasing brake specific fuel consumption (BSFC). The CO and HC emissions decreased while the NO_x emission increased. They reasoned that the higher latent heat of vaporization, lower CN (longer ignition delay), and lower heat content of ethanol are the main reason for those results. Many other studies on fuel blends have investigated their effect on diesel engine emission and performance as listed in Table 1.

Fuel blend	Emission	Performance	Reason	Ref.
Diesel 85% + heated grape seed oil 30% + propanol 5%	4.22% NOx reduction.; 5.61% smoke opacity reduction.; 6.17% HC increase; 4.23% CO increase; Compared to diesel.	3.26% BSFC increase.; 5.50% BTE reduction; Compared to Diesel.	Low thermal value, high viscosity and poor evaporation properties compared to diesel were thought to be influential on diesel engine emission and performance.	[21]
Polanga biodiesel + diesel (B0, B5, B10, B15, B20, B25, B100)	Increasing biodiesel amount on blend increases NOx emission.; PM emission decreases with biodiesel amount increase on the blend.	There is a slight decrease in engine BTE. The BSFC was not tested in this experimental study.	Increase the amount of biodiesel increases the fuel viscosity leading to poor air-fuel mixture. NOx emission in biodiesel rises due to higher combustion temperatures, longer combustion durations, and higher oxygen.	[22]
Canola biodiesel + diesel blends	Diesel amounts decrease in the blend increases NOx emission. On the other hand, both HC and co emission decrease with biodiesel amount increase in the blend.	N/A	Oxygen content in biodiesel improves combustion temperature leads to more NOx emission.	[23]

Table 1.

Fuel blend effect on diesel engine performance and emission.

4. Fuel additives

Using additives is an effective method to modify the fuel properties. Some additives are used as CN improvers such as acetone and diethyl ether [16, 23]. Other additives are used to decrease fuel viscosity, density and cold flow properties. Table 2 summarized many studies that have been conducted to control diesel engine performance and emissions.

Ref.	Additives	Results
[24]	Methanol, ethanol, diethyl ether (DEE) were added to B40 blend	Methanol and ethanol reduced NOx emission and considerably increased CO and HC emission emissions, DEE was found to improve NOx and smoke compared to its base fuel.
[25]	Acetone	The best concentration of acetone in diesel fuel was found to be 3% due to enhancing the engine performance and reducing the emissions without affecting the engine stability.; The engine emissions were reduced at all loads with low concentration of acetone.
[26]	Silicon dioxide (SiO2) nanoparticle additives to methanol	The addition of SiO2 nanoparticle in methanol could enhance the combustion and performance characteristics, and suppress the emissions of diesel engines.
[27]	Nano additive in water (5%; WiDE); 25 ppm cylindrical carbon nanotube (CNT) and 25 ppm spherical Al2O3 nano-additives	Therefore, 5%WiDE based Al2O3-CNT hybrid nano fuel has a high potential to be an alternate conventional fuel for diesel engines without any design modification.
[28]	Four-carbon alcohol (n-butanol) and ester (ethyl ethanoate) were separately blended with pure diesel (D100) at three blending ratios.	At low loads, up to 20.9% rise in brake specific fuel consumption (BSFC) was recorded but EE15 (15% ethyl ethanoate, 85% diesel) achieved 15.35% lower BSFC at 60% load.; NOx emissions reduced for all blends, with n-butanol blends achieving 30.6% less NOx emissions compared to D100. However, CO emissions increased by up to 37.6% for n-butanol and 32.9% for ethyl ethanoate.
[29]	Higher alcohols and graphene oxide nano-additives (GO) to Jatropha biodiesel	The NOx, CO, UHC, and smoke concentrations are dropped curiously by 20, 30, 30, and 40%, respectively, with introducing of higher alcohols with a conventional oil-jatropha blend.; The CO, UHC, and smoke intensity are lowered noticeably by 15%, 60%, 70, and 80%, respectively, whereas NOx is expanded by 13% with implanting of GO.
[30]	ZnO nanoparticles	Reduction of smoke by 11.86%, CO by 5.7%, UHC by 28%, and NOx by 14.93%, along with the; enhancement of BTE by 2.47%, were noticed at maximum load with 100 ppm particles.
[31]	The Tri ethylene glycol mono-methyl-ether (TGME)	CO, CO2, and NOx emissions decreased by 76.77, 40.9, 1.31%, respectively.; Brake power, and brake thermal efficiency increased by 10.54 and 12.77%, the generated power cost amount decreased by 20.16%.
[32]	Diethyl Ether	BTE increases as the power is increased. BSFC is being seen to decrease with the increment in power.; NOx emission increases whereas HC emission decreases with increment in power.; Overall study concludes with B60BD30DEE10 showing the best feasible results out of all the blends and pure diesel.; CO emission increases for blends as compared with pure diesel for lower values of power but as the power is increased, after some specific value of brake power CO emissions for blended fuels decreases and goes below than that for pure diesel fuel.
[33]	Alcohol and Nano additives	Higher alcohol-biodiesel-diesel blends were recommended for engines as an alternative substitute for conventional diesel.; Metal base additives have improved combustion efficiency and reduced emissions by enhancing combustion efficiency.; Further, metal base additives increased the NOx emission, but the remaining emissions are getting decreased under all operating conditions.
[34]	1-Hexanol, D70H30/H2	It was observed that the higher the proportion of 1-Hexanol, the lower the engine performance.; for the blend D70H30/H2, around 8.24% rise in brake specific fuel consumption, slight rise in hydrocarbon, 2.80% reduction in brake thermal efficiency, and 16.70% reduction in nitrogen oxides (NOx) emission.
[35]	Various nanoparticles blended with PB20FTPO10 (20 palm biodiesel+10% filtered tire pyrolysis oil); MgO nano-fuel, graphene; and Al2O3 nano-fuel models.	The tribological behavior of the nano fuels made of MgO, graphene and Al2O3 had been improved. In contrast to MgO and graphene nano fuel, the optimized Al2O3 nano fuel demonstrated the best tribological performance with the lowest concentration, price, load and speed.

Table 2.

Fuel additives effect on diesel engine performance and emission.

5. Emulsion fuel

Emulsion fuel is a mixture of water and fuel that is blended with emulsifiers. The emulsifiers is usually consist of two different surfactants that responsible for minimizing surface tension between immiscible liquids [16, 36, 37]. Polar liquid like water has different charges at the end of each molecule, those molecules could dissolve only in a polar solvent. Contrary, non-polar liquids like fuel have an equal charge at each end of their molecules and they could dissolve only in a non-polar agent [38]. Generally, surfactants are liquids that have an imbalanced concentration of polar (hydrophilic) and nonpolar (lipophilic) molecules. Each surfactant has a value called hydrophilic- lipophilic balance (HLB) varying between 0 to 20. HLB measures surfactant degree of hydrophilicity or lipophilicity, that determined by calculating molecular percentages weight of the hydrophilic and lipophilic portion of the surfactant molecule [39]. The emulsified diesel and biodiesel have significant effect on reducing NO_x and PM emissions while it improves CO emission [40, 41, 42, 43]. The emulsified fuel generally reduces the combustion temperature, and improves the fuel atomization inside the combustion chamber hence lower NO_x and PM emissions but higher CO emission compared to the base fuel [16, 38].

5.1 Emulsion types

Generally, there are two approaches of emulsifying fuel, Figure 1, known as: 2-phase and 3-phase fuel emulsion. 2-phese emulsion could be water-in-oil (W/O), or Oil-in-water (O/W) [45]. The 3-phase emulsion and also known as multi-phase emulsion consists of water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O) [46]. The name of emulsion describes the liquid added to the other. For example, O/W emulsions the oil is the dispersed phase that is distributed into the continuous phase, water. Similarly, in a 3-phase emulsion the liquid is added to emulsion whether the liquid added oil or water. Compared 2-phase emulsion to 3-phase emulsion Lin et al. [47] found that 2-phase emulsion provides higher lower heating value and lower droplet size. In the other hand, 3-phase emulsion is more viscous, and it attributes lower CO and NO_x emission compared to 2-phase emulsion [42].

Figure 1.

Concept of two-phase water-in-oil and oil-in- water and three-phase oil-in-water-in-oil and water-in-oil-in-water emulsions [44].

5.2 Emulsion production methods

Emulsion could be done in different approaches. The most common approach is the external force. In this method, the emulsion is made by using mechanical stirrer machine, and it is used widely for emulsifying fuel industries [16]. This device converts the electric energy to mechanical energy (sound wave vibration), Figure 2 illustrates schematic diagram of this device. The other method of making emulsion is ultrasonic vibrator. To achieve satisfied fuel emulsion, the ultrasonic vibrator is usually done by setting its frequency at range of 20 kHz – 40 kHz and the power at range from 150 W to 700 W [47, 49].

Figure 2.

Schematic diagram for preparation [48].

For making fuel emulsion, first mix two surfactants to gain suitable HLB value. The required HLB value is mainly obtained experimentally and different oil types require different HLB value that achieves significant emulsion [50]. Generally, the required HLB value for W/O emulsion is raging between 3 and 8 while to achieve suitable emulsion of O/W emulsion the HLB value required is ranging between 9 and 12 [51]. There is wide verity of surfactants used for emulsion But in fuel emulsion more common using sorbitan monoleate (span) and polyoxyethylene sorbitan monoleate (tween) surfactants [38]. Blending the two surfactants could be conducted to obtain suitable HLB value by using the following formula [16]:

$$\mathit{\text{Tween}}80\%=\frac{100\left(x-{\mathit{HLB}}_{\mathit{\text{span}}80}\right)}{{\mathit{HLB}}_{\mathit{\text{Tween}}80}-{\mathit{HLB}}_{\mathit{\text{span}}80}}$$E1

$$\mathit{\text{span}}80\%=100-\mathit{\text{Tween}}80\%$$E2

Where,

X = required HLB value,

HLB_{Tween 80} is 15 for Tween 80.

There are many other indications that need to be considered to achieve better results for emulsions. Higher water quantity in emulsion affects the emulsion results and it leads to less emulsion stability [16]. Accordingly, the flow rate of surfactant blend with suitable HLB value to the continues phase affects both emulsion stability and droplet size [38]. Finally, the amount of surfactants varies from 0.5 to 2% of the total volume [38, 48, 51].

5.3 Emulsion fuel stability

The emulsion is stabilized whenever no separation could be detected [48]. The water and surfactant amount on emulsion are the main two indicators that affect emulsion fuel stability. Elsanusi [16] has done different emulsion fuel types using external force method and measured their stability and the results are shown in Figure 3.

Figure 3.

Emulsion fuel stability [16].

Figure 3 illustrates that fuel EB0W5% (Emulsion Biodiesel 0%, Diesel 100%, water 5%) shows better emulsion stability compared to EB0W15% (Emulsion Biodiesel 0%, Diesel 100%, water 15%). Additionally, the biodiesel amount in the emulsion blend was found to affect emulsion stability at the same study. The droplet size of fuel emulsion affects the emulsion stability. A review study conducted on recent progress on mixing technology for water-emulsion fuel, showed that the mixing time effect the emulsion droplet size and smaller droplet size of emulsion leads to increase emulsion stability [52]. A similar result was obtained from another study, they measured the emulsion droplet size using Malvern Mastersizer 2000 and emulsion diesel with 5% water showed smaller droplet size and higher emulsion stability see [38].

5.4 Emulsified fuel benefits

The utilization of emulsified fuel in diesel engines offers several benefits that could positively impact both the engine’s regulated emissions and performance. Alahmer et al. [53], studied the engine performance fueled using emulsified diesel and they concluded that engine brake specific fuel consumption (BSFC) increases compared to diesel but when subtracting the amount of water from emulsion was found slightly less amount of diesel consumed for the same comparison. The lower heating value of fuel emulsion is the main reason for higher BSFC. The emulsified fuel was found to slightly increase brake thermal efficiency (BTE) in many studies [54, 55, 56]. The micro-explosion of emulsion that is enhanced by water content evaporation in the diesel engine combustion chamber making secondary atomization, promotes combustion efficiency, see Figure 4.

Figure 4.

Primary and secondary atomization in spray flame of emulsified fuel [57].

Fuel emulsion has dramatic influence on diesel engine regulated emissions. The water content in fuel emulsion reduces combustion temperature hence lower NO_x emission since higher combustion improves NO_x emission. The equivalent temperature effect on diesel engine NO_x is shown in Figure 5 [58]. Similarly, smoke opacity of diesel engine tends to be decreased. Osama et al. [38], explained that the micro-explosion, secondary atomization, and vaporization by the injectors are the main reasons for smoke opacity reduction.

Figure 5.

Regions of NOx and soot formation in local equivalence ratio versus local temperature space [58].

The CO and HC emissions are caused by incomplete combustion. Emulsified diesel reduces the in-cylinder temperature hence lower combustion efficiency “higher percentage of heat losses” hence high CO emission compared to base fuel [16, 38, 57, 59, 60, 61, 62]. Interestingly, the emulsion fuel does not have a major effect on HC emission compared to base fuel and that is obtained from micro-explosion resulting from better atomization of emulsified fuel [16, 38].

5.5 Fuel emulsion downside

Emulsified diesel increases CO emission due to reducing combustion temperature [38, 42]. Fuel emulsion were complained that lead to potential issues such as: corrosion of tank storage and fuel pumping system in diesel engine [63], low cold flow properties of emulsified fuel since the fuel temperature affects the emulsion stability [16].

6. Conclusion

This investigated the effect of fuel on diesel engine emission and performance. Diesel engines are increasingly being developed and adopted to work with wide verity of fuel and fuel additives. The fuel, fuel blends, fuel additives and fuel emulsion effect on diesel engine regulated emission and performance were reviewed. The main finding of this conducted study as following:

• Diesel engines could run with wide verity of fuel, and each fuel provides different results in terms of engine performance and emissions. Biodiesel emits slightly higher NO_x emission while lower HC and CO emission achieved compared to diesel. The BSFC of diesel engine for biodiesel were found to be higher compared to diesel whereas BTE were shown to be approximately like diesel results. Kerosene n-heptane were found to decrease HC, CO emissions and smoke opacity, while NO_x showed a slight increase. Additionally, kerosene as fuel decreases the BSFC and raises BTE.

• Fuel blends affect diesel engine emission and performance. Blending diesel with biodiesel improves engine BTE and BSFC, reduces engine HC, CO, and smoke opacity of diesel engine while it affects negatively the NO_x to diesel. On the other hand, blending diesel or biodiesel with methanol and ethanol was found to provide opposite results. Blending n-heptane with diesel or biodiesel has proven lower HC, CO and smoke opacity but increases NO_x emission compared to base fuel.

• Fuel additives play a major role in modifying fuel properties. Some additives are considered as CN improvers and increasing CN improves combustion quality. Other additives are used to reduce fuel viscosity which improves fuel atomization through injection process hence better combustion and lower PPM, HC, CO and BSFC and higher BTE.

• All types of fuel emulsion have shown dramatic decrease on NO_x and smoke emissions and improves the combustion quality by providing better atomization and decreasing the combustion temperature. Additionally, emulsion fuel has shown a slight increase in BSFC but less fuel consumption if water amount in emulsion subtracted. However, emulsion fuel improves HC, and CO emission and the BTE was found to be slightly lower compared to base fuel. Emulsion fuel droplet size and stability are the two factors affecting diesel engine performance and emission. Smaller droplet size better combustion and lower emissions. Emulsion fuel has some downsides like corrosion and low cold flow properties.

Acknowledgments

The authors are deeply grateful to Industrial Technology College in Misurata for their generous financial support towards our research. Their funding has been instrumental in the successful completion of our project and has significantly contributed to its outcomes. Industrial College’s commitment to supporting research and innovation is commendable, and we are honored to have been a recipient of their support. We extend our heartfelt appreciation to the Misurata University for their guidance and expertise. We would like to express our sincere appreciation and gratitude to Nina Miocevic for her invaluable assistance and exceptional communication throughout our publication process.