© The Author(s), published by EDP Sciences, 2025

1 Introduction

The energy requirements in many areas such as transportation, power generation, and agricultural applications, etc. is still mostly met by diesel engines. Reliability and efficiency of diesel engines are key factors that generate significant interest in them [1]. However, this types of engines are notorious for the release of harmful gases into the environment. Recently, engine manufacturers have been compelled by strict regulations to maintain emission levels below certain limits. Oxides of nitrogen (NO_x), carbon monoxide (CO), carbon dioxide (CO₂), and Unburned Hydrocarbons (UHC) are pollutants released by diesel engines operated with conventional fossil fuels which have detrimental effects on both human health and the environment [2]. On the other hand, as a well-known phenomenon, fossil fuel reserves are limited and are predicted to be extinct in the near future. Energy demand arising from industrialization and human population growth rapidly increase depletion rate of fossil fuels [3, 4]. All the above-mentioned issues have made the interest and search for more environmentally friendly, renewable, and sustainable fuels inevitable. Biodiesel has great potential to replace fossil fuels because of its similar properties, making it suitable for use in diesel engines with minor or no engine modification. It is not surprising that many scientists have investigated the utility of biodiesel in diesel engines instead of conventional fuels because biodiesel has numerous notable advantages. Biodiesel is a strong alternative to petroleum products, which face the threat of scarcity. Being biodegradable, sustainable, and renewable, along with their environmental and economic friendliness, make them attractive fuels [5]. Poorer engine performance due to the lower heating value and reduced atomization caused by higher viscosity of biodiesel are some of the major drawbacks of it [6]. Although the additional oxygen content in the molecular structure of biodiesel contributes to reduced CO and UHC emissions by providing extra oxidation, the improved combustion facilitated by these additional molecules unfortunately has adverse effects on the generation of NO_x. Enhancement of combustion leads to higher in-cylinder temperatures. Elevated temperature levels in the combustion area is one of the most significant factors contributing to NO_x formation [7, 8].

NO_x emissions are highly hazardous, posing a threat not only to the environment but also to human health. They can trigger the formation of fog, lead to acid rain, contribute to lung diseases, and reduced resistance to respiratory infections [9]. Therefore, it is essential to employ various techniques to decrease NO_x emissions and mitigate these potentially serious damages. Recently, Exhaust Gas Recirculation (EGR) is a pre-treatment method and Selective Catalytic Reduction (SCR) as an after-treatment technique have been utilized to reduce NO_x emissions. There have also been various attempts to lessen these emission levels, including split injection and Lean NO_x Trap (LNT) methods [10, 11].

While the most of the NO_x reduction methods realize their duty effectively, they can negatively affect engine performance and other emissions. Water-Emulsified Fuel (WEF) is another effective solution for NO_x reduction. Although it may seem strange to introduce water into an engine, it is possible to reduce NO_x without significantly compromising engine performance. This technique also simultaneously reduces NO_x and Particulate Matter (PM) [12, 13].

Randive et al. [14] investigated the performance and emission characteristics of a diesel engine operated with lemon peel oil and its emulsions. Emulsions were prepared with 5% and 10% water ratios using two different emulsifiers called Span 80 and Methyl-dihydroxypropylimidazolium chloride. Emulsion with 5% water content was found to be stable and performance and emission values were examined. As a result of micro-explosion of water, thermal efficiency has increased. Although NO_x emissions decreased, UHC and CO emissions increased because of the low calorific value obtained with the water addition. Zhang et al. [15] investigated the effects of adding low-level water on the spraying, combustion, and emission characteristics of a diesel engine. In this study, water-biodiesel emulsions and pure biodiesel were used. As a result of research, it was determined that adding water has considerable effects on fuel injection and combustion processes. Air and fuel mixtures are better provided due to micro explosions. Low water additive levels reduced NO_x, CO and CO₂ emissions. It is stated that the optimum amount of water to be added is 4% by weight. Vellaiyan [16] evaluated the biodiesel-diesel fuel emulsion effect and the effects of nanoparticles on the combustion, performance, and emission characteristics of a diesel engine. In this study, soybean oil biodiesel, water emulsions, and carbon nanotube nanoparticles were used. Emulsion fuels have improved specific fuel/energy consumption and exhaust gas temperature compared to pure biodiesel. Water/biodiesel fuels reduced NO_x and smoke emissions. In addition, 10% water-containing emulsion has reduced HC and CO emissions compared to 20% water-containing emulsion and pure biodiesel. Rao and Anand [17] examined the effects of water and AlO(OH) nanoparticle additives on the performance and emission characteristics of a compression ignition (CI) engine operated with biodiesel. The results showed that the thermal efficiency decreased and the amount of NO increased when using biodiesel compared to diesel fuel. However, water and nanoparticle additives improved the performance and emission values. Anbarasu and Karthikeyan [18] investigated the effects of canola biodiesel emulsion fuel on the performance and emissions of a diesel engine under different operating conditions. Emulsion fuels were formed using canola biodiesel, distilled water, and surfactant (a mixture of Span 80 and Tween 20) with a mechanical stirrer. The ratios used while preparing the emulsion were 83% canola biodiesel, 15% distilled water and 2% surfactant, and the mechanical mixing process was carried out at 1500 rpm for 30 min at room temperature. The experimental results showed that water emulsions enhance diesel engine performance and reduce harmful emissions. Khidr et al. [19] investigated the effects of water-emulsified biodiesel/diesel mixtures with alumina nanoparticles on a diesel engine. According to the results, the optimum blend was achieved with 10% biodiesel mixture containing 1% water and 150 mg/L of alumina. This blend resulted in a 9.1% improvement on Brake Thermal Efficiency (BTE) over diesel alone. Additionally, reductions of 40%, 26%, and 22% were observed for CO, smoke, and NO_x emissions, respectively. Vellaiyan et al. [20] evaluated the use of waste-derived biodiesel with CNT/MgO nanocomposite and water emulsion to improve the performance and emission characteristics of diesel engine. Biodiesel was produced from Citrus maxima peels. Water and nanocomposite were added to the biodiesel to enhance its efficiency and achieve environmental goals. Fuel combinations were determined using Box–Behnken design matrix. The optimal rates were 27.6%, 12.2%, and 64.9 ppm. Under optimum conditions, Brake Specific Fuel Consumption (BSFC) and BTE values of 300.93 g/kWh and 32.83% were achieved. Additionally, projections for emissions included 20.18 ppm for UHC, 0.046 vol.% for CO, 929 ppm for NO_x, and 16.05 vol.% for smoke. Reddy and Pal [21] performed a study to see the effects of hydrogen addition to a diesel engine operated by waste plastic oil-water emulsion. First, the engine was tested using 80% pure diesel + 20% waste plastic oil biodiesel (WPO20). Then, tests were conducted in dual fuel mode containing hydrogen + WPO (WPO20 + H₂). Finally, engine tests were conducted with water-emulsified WPO + hydrogen (WPO20 + W10 + H₂). The experimental results indicated that incorporating hydrogen enhanced BTE and significantly decreased the BSFC. There was also enhancement on combustion characteristics and emissions, with the exception of NO_x emissions. The addition of hydrogen led to a 24% increase in NO_x emissions compared with the WPO20 blend. However, water emulsification helped to reduce NO_x emissions by 19%. Gowrishankar and Krishnasamy [22] assessed the performance and emission of a light-duty diesel engine fueled with biodiesel–water emulsified fuel. They intended to control NO_x emissions released by biodiesel combustion with utilization of biodiesel–water emulsion (WBDE) and biodiesel–ethanol emulsion (EBDE). The water/biodiesel and ethanol/biodiesel ratios were 18% by mass. Polyglycerol Polyricinoleate (PGPR) was used as a novel surfactant in emulsion fuels which kept emulsions stable up to 4 months. Ignition delay and peak pressure levels were increased by emulsions compared with neat biodiesel. BTE improved by up to 7% with the WBDE at a brake mean effective pressure of 6.3 bar, while the EBDE achieved a 3% increase. WBDE reduced BSFC by 20%, whereas EBDE remained the same as biodiesel. Both WBDE and EBDE contributed to the reduction of NO_x and smoke emissions simultaneously. There was also a reduction in CO emissions by using WBDE. Tamam et al. [23] presented a study for determining the performance and emission of diesel engine with an emulsion fuel supply system. The system is emulsifier free and injected water mixes with diesel fuel via a novel mixing system. A 28.6% increase in BTE was obtained compared to reference diesel fuel. There was a reduction in BSFC with increasing the water content in the blend. Both NO_x and smoke opacity decreased with water emulsion. However, this led to higher CO emissions at low to middle loads, which then decreased at higher loads.

In this study, diesel–water and biodiesel–water emulsions containing distilled water (5% and 10% by vol.) and surfactant (2% by vol.) were prepared with the aid of a magnetic stirrer, and their fuel properties were determined. The effects of the prepared fuels on the performance and emission characteristics of the CI engine were then determined. The novelty of this study lies in the comparative analysis of the results obtained from WEFs prepared separately using both diesel and biodiesel fuels, which are not often encountered in the literature.

2 Materials and methods

Production of biodiesel, preparation of WEFs, fuel property analysis, and engine experiments were carried out at Automotive Engineering Department Laboratories of Çukurova University.

2.1 Biodiesel production

Sunflower oil is used for biodiesel production was purchased from a local market. The transesterification method was preferred for production. In this method, a chemical reaction occurs between triglyceride and alcohol in the presence of a suitable catalyst [24]. Reactants and products are illustrated in the following Figure 1.

Figure 1 The transesterification reaction [24].

In the transesterification reaction, CH₃OH (methyl alcohol) and NaOH (sodium hydroxide) were used as the alcohol and catalyst, respectively. The oil/alcohol molar ratio used was 6/1, and the amount of catalyst used was 0.5% of the weight of the oil. The biodiesel production details are presented in Figure 2.

Figure 2 Steps of biodiesel production.

2.2 Emulsions

Emulsions are formed when one of two immiscible liquids disperses as spherical droplets within the other. Surfactant materials lower the interfacial tension between liquids, enabling the dispersed phase, called the discontinuous phase, to remain suspended as particles within a continuous phase liquid, thus facilitating the formation of stable mixtures [25]. As shown in Figure 3, two-phase emulsions can be essentially categorized as water-in-oil (W/O) and oil-in-water (O/W) emulsions. In W/O emulsions, oil is the continuous phase, and water is the dispersed phase, whereas in O/W emulsions, water is the continuous phase, and oil is the dispersed phase.

Figure 3 W/O and O/W emulsions.

Surfactants contain hydrophilic and lipophilic groups. The hydrophilic group is the polar part that is soluble in water and has an affinity to water, whereas the lipophilic part is the non-polar portion that is soluble in oil and has an affinity to oil (Fig. 4).

Figure 4 Surfactant structures in W/O and O/W emulsions.

Hydrophilic and lipophilic groups are present in various surfactant substances in different proportions, and this ratio is expressed as the Hydrophilic-Lipophilic Balance (HLB) value. The HLB value determines the application area of the surfactant substance [14, 25]. Table 1 lists the appropriate HLB values of surfactant substances for various applications.

2.3 Diesel–water and biodiesel–water emulsions

In this study, both diesel and sunflower biodiesel fuels were mixed with distilled water in the presence of a surfactant to prepare WEFs in the W/O phase. Water droplets do not directly contact with engine parts since they are surrounded by oil in W/O formation. On the other hand, the O/W form may lead to deteriorated engine operation because it allows more water to get into direct contact with the engine system [25–27].

Randive et al. [14] reported that most researchers used non-ionic Span 80 (Sorbitan Monooleate) with an HLB value of 4.3 when preparing W/O emulsions. However, they mentioned that fuel stability could be improved by using both a high HLB value and a low HLB value surfactant together, as compared to using a single surfactant. Therefore, Span 80 with an HLB value of 4.3 and Tween 80 (Polyoxyethylene Sorbitan Monooleat) with an HLB value of 15 were utilized in order to enhance the stability of emulsions for a longer period (Fig. 5). Anbarasu and Karthikeyan also stated that the combination of Span 80 and Tween 80 surfactants is effective in forming more stable emulsions [18]. Table 2 shows the properties of Span 80 and Tween 80. The prepared W/O emulsions, their notations, and content ratios are summarized in Table 3.

Figure 5 Tween 80 and Span 80.

WEFs were prepared by mixing diesel/biodiesel, distilled water, and surfactants using a mechanical stirrer at a speed of 1500 rpm for 1 h at room temperature. Figure 6 shows the fuel specimens.

Figure 6 Diesel–water and biodiesel–water emulsion fuels.

2.4 Fuel properties determination

The physicochemical properties of the WEFs and diesel fuel were determined according to TS EN 14214 and EN 590 standards, respectively, using the equipment in the Fuel Analysis Laboratories of the Automotive Engineering Department at Çukurova University.

The properties evaluated were density, viscosity, calorific value, and flash point of each fuel. The devices used for analyses were as Kyoto Electronics DA-130 type density meter (i), Tanaka AKV-202 type automatic kinematics viscosity meter (ii), IKA-Werke C2000 bomb calorimeter (iii), and Tanaka automated Pensky–Martens closed cup Flash Point tester (iv). The demonstration of devices is given in Figure 7.

Figure 7 Test devices for fuel properties determination.

2.5 Test engine and emission device

Engine tests were conducted on a 4-stroke, 4-cylinder, turbocharged, and water-cooled diesel engine. The test engine and the schematic representation of test bench are shown in Figure 8. In addition, Table 4 presents the technical properties of the test engine.

Figure 8 (i) Test engine, (ii) test engine schematic view.

An eddy current dynamometer (i) and S-type load cell (ii) were used to determine the engine torque and power (Fig. 9). A rotary encoder, as shown in Figure 10, was used as the speed sensor in the tests.

Figure 9 (i) Eddy current dynamometer, (ii) S type load cell.

Figure 10 Rotary encoder.

Exhaust emissions were measured with the MRU Air Delta 1600 V exhaust gas analysis device shown in Figure 11. The measurement accuracies of the device are ±20 ppm for CO, ±0.5% for CO₂, ±5 ppm for NO, and ±5 ppm for NO_x.

Figure 11 Emission device.

3 Results and discussion

3.1 Properties of fuels

The fuel properties of diesel–water emulsions, biodiesel–water emulsions, pure diesel, and pure biodiesel are given in Table 5.

Density, kinematic viscosity, calorific value, and flash point determination tests were repeated three times for each test fuel, and their average values were used. The obtained results were compared with EN 590 and EN 14214 standards, and it was determined that the fuels fell within the standard value ranges.

The density of B is 4.56% higher than that of the fuel D. It was observed that the densities of BW5 and BW10, obtained by adding water to B, increased compared with B. Similarly, the densities of DW5 and DW10, formed by adding water to D, also increased in comparison with D. B has 1.64 times higher viscosity than D. It has been observed that adding water to fuels, similar to the density, results in an increase in the viscosity. When the calorific values of the fuels are examined, it is observed that the calorific value of B is 12.07% lower than that of D. It was observed that DW5 and DW10 had lower calorific values compared to D, and BW5 and BW10 had lower calorific values than B. The flash point, which is the temperature at which a fuel produces enough vapor to ignite a flammable mixture, and is particularly important for fuel storage, has been measured within the standard value ranges for all fuels.

3.2 Performance

The engine tests were conducted within the speed range of 1200–2200 rpm under full load conditions. To stabilize the test engine, it was run for 15 min before each data acquisition, and then data collection was initialized. This procedure also ensured the complete consumption of any residual fuel in the fuel line from the previous realized test.

Figures 12 and 13 depict the variation of engine torque and brake power with engine speed for each test fuel. Compared to D, the use of B has resulted in a 10.33% decrease in engine torque and 10.5% decrease in brake power. The possible reason for this effect could be the lower calorific value of B [28].

Figure 12 Torque variation with engine speed.

Figure 13 Brake power variation with engine speed.

WEFs prepared using diesel and biodiesel cause engine torque and brake power reductions. DW5 and DW10 fuels resulted in average engine torque decreases of 6.27% and 9.82%, and average brake power decreases of 6.49% and 10.31%, respectively, compared to D. In addition, BW5 and BW10 fuels led to an average engine torque decrease of 4.63% and 8.12%, and average brake power decrease of 4.73% and 8.38%, respectively, compared to the B.

As the water content increases in the WEF, the calorific values tend to decrease. It is believed that the reduction in average engine torque and average engine brake power for DW5 and DW10 compared with D, and for BW5 and BW10 compared with B may be attributed to the decrease in the calorific value of the fuel caused by the water content within the emulsion [29].

3.3 Emissions

Figures 14 and 15 illustrate the variation in engine speed with CO and NO_x emissions for each test fuel. Compared with D, the use of B resulted in a 4.23% reduction in CO emissions and a 16.52% increase in NO_x emissions. This average decrease in CO emission can be attributed to the higher oxygen content in B fuel, leading to an increased oxidation of CO emissions into CO₂ [30]. In addition, the extra oxygen content in the chemical structure of B has improved the combustion quality, resulting in higher cylinder temperatures and an increase in NO_x emissions [31].

Figure 14 CO variation with engine speed.

Figure 15 NOx variation with engine speed.

DW5 and DW10 fuels have resulted in 1.61% and 2.85% reductions in average CO emissions compared to D, while causing 8% and 12.61% reductions in NO_x emissions, respectively. Additionally, BW5 and BW10 fuels have led to 1.56% and 3.67% decreases in average CO emissions compared with B, and 3.87% and 10.47% reductions in NO_x emissions, respectively.

It has been observed that WEFs have a reducing effect on CO and NO_x emissions, and an increase in the water content within the emulsion enhances the reduction rate. A decrease in CO emissions with emulsion usage is believed to occur due to a better formation of the mixture capable of reacting due to micro-explosions, as well as more efficient combustion [32, 33].

NO_x emissions occur in the cylinder as a result of the reaction between oxygen and nitrogen at high temperatures. The water present in the emulsions transitions into the vapor phase, where it draws heat from the surroundings and causes a cooling effect. As the cylinder temperature decreases, a reduction in NO_x emissions is observed. In the study conducted by Hegde et al., similar results were obtained, supporting the findings [34].

4 Conclusion

In this study, WEFS were prepared by adding 5% and 10% water by volume (DW5, DW10, BW5, and BW10) to diesel (D) and biodiesel fuels produced from sunflower oil (B), accompanied by surfactant materials (Span 80 + Tween 80).

All fuels were first analyzed to determine their fuel physicochemical properties and were found to be within the standard value ranges. Subsequently, performance and emission tests were conducted under full load conditions in a 4-stroke, 4-cylinder, turbocharged, water-cooled diesel engine at engine speeds ranging from 1200 to 2200 rpm. The obtained results are listed below:

• The density of B is 4.56% higher than that of D, and its viscosity is 1.64 times higher.

• DW5 and DW10 have higher density and viscosity values than D, and BW5 and BW10 have higher density and viscosity values than B. The increase became greater with increasing water content.

• The calorific value of B is 12.07% lower than D.

• DW5 and DW10 had lower calorific values than D, and BW5 and BW10 had lower calorific values than B. The decrement level became greater with increasing water content.

• All fuel properties were measured within the standard ranges specified in EN 590 and EN 14214.

• Compared to the D, the use of B resulted in 10.33% decrease in engine torque and 10.5% decrease in brake power.

• DW5 and DW10 fuels caused a 6.27% and 9.82% decrease in the average engine torque compared to D, and a 6.49% and 10.31% decrease in average brake power, respectively. BW5 and BW10 fuels caused a 4.63% and 8.12% decrease in average engine torque compared to B, and a 4.73% and 8.38% decrease in average brake power, respectively.

• Compared with D, the use of B resulted in a 4.23% reduction in CO emissions and a 16.52% increase in NO_x emissions.

• DW5 and DW10 fuels have caused 1.61% and 2.85% reductions in average CO emissions compared to D, and 8% and 12.61% reductions in NO_x emissions, respectively. Additionally, BW5 and BW10 fuels have led to 1.56% and 3.67% decreases in average CO emissions compared to B, along with 3.87% and 10.47% reductions in NO_x emissions, respectively.

5 Future recommendations

For the future studies on WEFs, several recommendations can be sequenced as stated below:

• More effective surfactants and stabilizers should be explored to improve WEF stability.

• The impact of different water percentages on engine performance and emissions should be studied to find the optimal balance.

• Modifications to existing engines or the development of new designs compatible with WEFs should be considered.

• The long-term effects of WEFs on engine components should also be assessed.

• Integrating WEFs into hybrid or electric vehicles can further reduce fuel consumption and emissions.

• Numerical models can be developed to simulate how micro-explosions occur, focusing on droplet behavior and fuel atomization.

• The sound and vibration characteristics of WEF operated diesel engines can be investigated.

Acknowledgments

This manuscript was produced from an MSc thesis and was supported by Çukurova University Scientific Research Project Coordination (FYL-2020-13048) project.

References

All Tables

All Figures

	Figure 1 The transesterification reaction [24].
In the text

	Figure 2 Steps of biodiesel production.
In the text

	Figure 3 W/O and O/W emulsions.
In the text

	Figure 4 Surfactant structures in W/O and O/W emulsions.
In the text

	Figure 5 Tween 80 and Span 80.
In the text

	Figure 6 Diesel–water and biodiesel–water emulsion fuels.
In the text

	Figure 7 Test devices for fuel properties determination.
In the text

	Figure 8 (i) Test engine, (ii) test engine schematic view.
In the text

	Figure 9 (i) Eddy current dynamometer, (ii) S type load cell.
In the text

	Figure 10 Rotary encoder.
In the text

	Figure 11 Emission device.
In the text

	Figure 12 Torque variation with engine speed.
In the text

	Figure 13 Brake power variation with engine speed.
In the text

	Figure 14 CO variation with engine speed.
In the text

	Figure 15 NOx variation with engine speed.
In the text