1. Introduction
Diesel engines, owing to their superior reliability, cost effectiveness, adaptability, and thermal efficiency, are extensively employed in the transportation and thermal power generation industries. Nonetheless, they encounter a multitude of challenges, encompassing greenhouse gas emissions, environmental contamination, and concerns pertaining to the exhaustion and storage of fossil fuels [1,2,3]. In light of these factors, there has been a significant escalation in research efforts aimed at identifying alternative fuels sourced from renewable biomass feedstocks in recent years [4,5,6,7,8,9]. Among the alternative’s biofuels for an internal combustion engine, bio-alcohols and biodiesels have undergone extensive testing with the objective of mitigating our reliance on fossil fuels and curtailing the emission of exhaust gasses into the atmosphere [10,11]. First-generation oxygenated biofuels, including bioethanol and methanol, have been extensively utilized as substitute or supplementary fuels in engines for a considerable duration, attributed to their renewable nature and elevated octane number [12]. Bioethanol, despite its potential, poses several challenges for its utilization in spark-ignition engines due to its elevated O/C ratio, diminished energy density, subpar mixing stability, low lubricity, and high latent heat of vaporization [13,14,15,16]. The surge in the investigation of alternative fuels, propelled by significant advancements in biofuel production methodologies, has recently drawn researchers’ attention to furan-derived fuels such as 2-methylfuran (2-MF) and 2,5-dimethylfuran (DMF) [17,18]. A Cu nanoparticle-supported ZIF-8 catalyst for producing 2-methylfuran (2-MF) from furfural has been developed, achieving high conversion and selectivity, with potential for bio-oil applications [19]. In a separate study, the pyrolysis of a 1:1 blend of 2-methylfuran (MF) and a three-component jet fuel surrogate was investigated, revealing that MF alters decomposition pathways and reduces aromatic species and light hydrocarbons, with key reactions involving allyl and propargyl radicals contributing to benzene formation [20]. Specifically, fructose undergoes dehydration to transform into 5-hydroxymethylfurfural, which subsequently converts to 2-MF during the hydrogenolysis process. When compared to ethanol, another biofuel derived from fructose, 2-MF, exhibits several advantages that make it more suitable for engine applications. These benefits include a lower latent heat of vaporization and a higher energy density, both of which contribute to the formation of a combustible mixture [21,22]. Furthermore, 2-MF demonstrates physicochemical properties that are closely analogous to those of gasoline fuel. Additionally, due to its water insolubility, 2-MF exhibits stable storage. 2-MF also possesses an energy density that is approximately 34% higher than ethanol, which contributes to a reduction in SFC [23]. In summary, these advantages render 2-MF fuel a highly competitive alternative fuel that is used in ICEs.
Recent research has conducted several tests to analyze the burning and emission properties of the 2-MF biofuel in ICEs. An analysis of the combustion characteristics and emissions produced by various blends of 2-MF and diesel fuel in a compression ignition (CI) engine was carried out by Xiao et al. [12]. The outcomes revealed that 2-MF-diesel mixtures resulted in an ignition delay (ID) for fuel and a shorter burning duration relative to standard diesel fuel. Simultaneously, mixtures demonstrated higher thermal efficiency, carbon monoxide, and nitrogen oxide emissions than pure diesel. Additionally, the soot emission from the mixtures was notably lower than that from the base diesel fuel. In the study by Wei et al. [24], the combustion characteristics and emission profiles of a 10% volumetric blend of 2-MF and gasoline were examined in a single-cylinder, four-stroke, spark-ignition (SI) engine. The findings demonstrated that the incorporation of a minor proportion of 2-MF into gasoline resulted in a fuel mixture that exhibited a shorter combustion duration and enhanced brake power. This outcome positions the 2-MF–gasoline blend as a more efficacious substitute to ethanol–gasoline mixtures with a 10% ethanol concentration by volume. Liu H, Olalere R et al. [25] investigates the impact of engine parameters like spark timing, air-fuel ratio, injection timing, and pressure on MF combustion in a DISI engine, compared to gasoline and ethanol. Results show that MF has a wider spark timing range, is more resistant to lean burn, and requires longer injection durations than gasoline. Higher injection pressure helps mitigate this difference. Wang et al. [26] conducted a comparative analysis of the burning and emissions of 2-MF, 2,5-DMF, ethanol, and gasoline in SI engines. Their study revealed that 2-MF demonstrated superior knock suppression and improved combustion characteristics compared to both 2,5-DMF and gasoline fuel. However, the concern regarding increased NOx emissions associated with 2-MF use remains significant. Hoppe et al. [27] examined the feasibility of employing 2-MF biofuel in gasoline engines and found that 2-MF displayed preferable combustion stability and higher knock suppression ability, especially under low load and cold conditions. Pan et al. [28] found that 2-MF fuel under 15% exhaust gas recirculation (EGR) in an SI engine exhibited higher thermal efficiency (TE) and lower NOx emissions compared to gasoline.
Prior research has underscored the influence of lower CN of 2-MF, a furan-derived oxygenated fuel, on the formation of NOx during the premixed combustion phase in diesel engines (DEs). This is primarily attributed to the elevated in-cylinder temperatures associated with fuels possessing lower CN. Specifically, fuels with lower CN exhibit extended ID, allowing additional time for the fuel–air mixture to blend prior to the onset of combustion. This prolonged blending period results in increased in-cylinder temperatures during the premixed combustion phase. Given that NOx formation is a high-heat process involving the reaction of nitrogen and oxygen, these elevated in-cylinder temperatures may contribute to increased NOx formation. The relationship between CN, in-cylinder temperature, and NOx formation is crucial for understanding the efficiency and environmental impact of fuels [29]. Consequently, numerous studies have been undertaken to augment the CN of biofuels to curtail the ignition delay duration, thereby inhibiting NOx formation. Achieving the desired CN is essential to comply with fuel quality standards and ensure optimal engine performance. Additives can be employed to enhance the autoignition properties of biofuels, with 2-EHN being a commonly used additive for this purpose [30]. In recent research focusing on DEs, several studies have explored the possibility of employing 2-EHN as a cetane improver. Salmani MH, Hussain I et al. [31] investigate the performance and emissions of diesel blends with ethanol and 2-EHN, finding that E20B exhibits improved BTE and indicated power and also increased emissions of CO, HC, and CO2 at high loads and compression ratios. Qian et al. [32] delved into the impact of incorporating varying proportions of dimethyl carbonate fuel and 2-EHN into traditional diesel fuel using a four-cylinder and turbocharged DE. The inclusion of 0.5% 2-EHN resulted in an increased in-cylinder pressure, reduced ID period, and lowered CO and NOx emissions. H. Kumar et al. [33] carried out research examining the impacts of the 2-EHN cetane improver in a diesel/bioethanol blend at concentrations of 1000 and 2000 ppm (E20). The results indicated that E20EHN1000 yielded higher HC emissions but comparatively lower emissions when contrasted with other fuel samples. Imdadul et al. [34] assessed the efficiency and emission levels of fuel mixtures containing n-butanol and diesel, enhanced with 2-EHN as a cetane rating improver. The incorporation of 2-EHN into the mixture resulted in a decrease in NOx emissions and an enhancement in BTE. This can be ascribed to the presence of free radicals in the combustion chamber (CC), which are provided by 2-EHN. These free radicals expedite the oxidation process, leading to a reduction in ignition delay and ultimately enhancing combustion efficiency. Pan et al. [28] elevated the CN of the dimethyl carbonate/diesel mixture (DMC) by introducing 2-EHN at rates of 0.5%, 1%, and 2%. Subsequently, they investigated their impacts in a four-cylinder turbocharged DE. The findings demonstrated that the presence of 2-EHN led to a decrease in the duration of the ignition delay of DMC and a reduction in the maximum pressure rise rate (MPRR). The utilization of 2-EHN resulted in an improvement in both brake thermal efficiency and BSFC. BTE increased by 0.2% and 1%, respectively, in comparison to DMC20 under the same load conditions and with a 0.5% and 2% EHN addition. Ciniviz et al. [35] inferred from their findings that 2-EHN can reduce the ignition delay. They further suggested that the utilization of 2-EHN led to decreased emissions and BSFC while enhancing BTE. In their investigation, they assessed the impact of adding 2-EHN to a diesel engine running on a mixture of ethanol and diesel with a 10% volumetric ethanol component. In the EDB, the fractions of 2-EHN employed were 2, 4, and 6% by volume. The test outcomes revealed a rise in the cetane number (CN) of the EDB as the concentration of 2-EHN increased. Studies generally indicated that adding 2-EHN to the EDB led to a decrease in the quantity of CO and NOX in exhaust gasses, albeit with a negative impact on SFC and brake power. Zhang et al. [36] conducted experiments that demonstrated that the incorporation of 2% 2-EHN into mixtures of 2,5-dimethylfuran and diesel resulted in an 80 percent decrease in soot emissions. Additionally, the maximum pressure rise exhibited a linear increase with the quantity of 2-EHN. The introduction of 2-EHN resulted in a minor increase in NOx emissions while significantly reducing THC emissions. According to their findings, adding 2% 2-EHN to the D40 blend (40 vol.% DMF) effectively reduced the compromise between soot emissions and combustion noise.
2-Ethylhexyl Nitrate (2-EHN) is an oxygenated compound commonly employed as a cetane enhancer in fuels. It improves the ignition characteristics of low cetane number fuels, such as 2-MF. The inclusion of 2-EHN in diesel–2-MF blends is particularly significant for enhancing combustion efficiency and minimizing the formation of undesirable emissions. By raising the cetane number of these blends, 2-EHN promotes quicker ignition and smoother combustion, addressing the challenges associated with the low CN of 2-MF.
Diesel–2-MF blends present several environmental advantages, including reduced soot emissions and the use of renewable biofuels. However, they also introduce certain complexities, notably the potential increase in Nox emissions. Previous studies have shown that blending 2-MF with diesel can elevate NOx emissions, which is primarily attributed to the higher combustion temperatures and altered chemical kinetics resulting from 2-MF’s combustion properties [37]. These factors can lead to enhanced NOx formation under specific operating conditions.
As mentioned earlier, numerous experiments have been conducted to lower NOx emissions from various fuel types. According to the collective findings of these investigations, adding 2-EHN to a fuel with a lower CN greatly raises the CN. It lowers the production of NOx. However, the existing literature lacks information on the impact of adding 2-EHN to diesel/2-methylfuran mixtures on the combustion, engine performance, and emissions characteristics of a diesel engine.
The primary objective of this study is to assess the impact of 2-EHN on the combustion behavior, engine performance, and emission characteristics of diesel–2-Methylfuran (2-MF) blends. Specifically, the research will examine the effects of varying concentrations of 2-EHN on combustion efficiency, with a particular focus on BTE, BSFC, Nox emissions, soot formation, and overall engine performance. Additionally, the study will investigate how different 2-EHN concentrations influence ignition characteristics and emission profiles under various engine loads conditions. By understanding these relationships, the study aims to optimize the trade-offs between fuel efficiency and emissions, providing insights into the potential of diesel–2-MF–2-EHN blends as sustainable fuel alternatives.
2. Experimental
2.1. Engine Configuration and Instrumentation
The experiments were conducted using a modified four-cylinder, four-stroke, water-cooled, direct-injection compression ignition (DICI) engine. As depicted in Figure 1, the engine was equipped with a common rail fuel injection system. Detailed specifications of the engine are provided in Table 1. Torque output was controlled using an eddy current (EC) dynamometer, ensuring a consistent engine speed of 1800 rpm (±5 rpm). An Electronic Control Unit (ECU) regulated key engine parameters, including fuel injection timing and fuel mass. The ECU was programmed to maintain a fixed injection timing at a 7.5° crank angle (CA) before top dead center (TDC).
Cylinder pressure was monitored via a Kistler 6025C pressure transducer mounted on the cylinder head. The pressure signals were amplified and transmitted to a CB-466 combustion analyzer for processing. Pressure data were collected over 100 consecutive cycles, with a resolution of 0.25 crank angle degrees (CAD) per data point. The intake air temperature and pressure were controlled by an air conditioning system and compressor, maintaining an intake temperature of approximately 25 °C (±1 °C). Coolant and oil temperatures were stabilized at 85 °C (±1 °C), while the lubricating oil temperature was regulated at 87 °C (±2 °C) during the test runs, using temperature controllers. Temperature measurements were recorded using K-type thermocouples. Exhaust gas emissions were measured using an AVL gas analyzer, with a measurement accuracy of 1 ppm for HC and CO, and a precision of 0.1% for (NOx). Smoke emissions were monitored using an NH-T6 smoke opacimeter, which provided measurements with an accuracy of 0.01 m−1. Exhaust gas samples were drawn from the exhaust port and delivered to the gas analyzer via tubing for quantitative analysis.
2.2. Test Conditions and Fuels
In the conducted study, a fuel blend was created by combining 2-methylfuran and diesel. The mixture, denoted as M30, consisted of 30% 2-methylfuran and 70% diesel. Following this, the additive 2-EHN was introduced into the M30 blend at varying concentrations of 1.5% and 2.5%. These modified blends were labeled as M30E1.5 and M30E2.5, respectively. The diesel used in the experiment was sourced from China Petroleum and Chemical Corporation (Beijing, China). At the same time, 2-MF (99% purity) was supplied by TZHL Biological Technology Co., Ltd. The 2-EHN additive was provided by Tianjin Kermel Chemical Reagent Co., Ltd., (Tianjin, China). Table 2 outlines the physicochemical properties of these four fuel compositions, ensuring transparency and adherence to originality standards. Throughout the experimental procedures, the engine underwent varying test loads set at 0.13, 0.38, 0.63, 0.88, and 1.13 MPa brake mean effective pressure (BMEP) while consistently operating at a constant speed of 1800 rpm. The timing of the injection did not change; it was at 7.5 crank angle (°CA) degrees before the TDC, controlled by the management software integrated into the Electronic Control Unit (ECU). Temperature controllers were utilized to accurately regulate the lubricating oil and coolant temperatures, maintaining them at 85 °C (±1 °C) and 86 °C (±1 °C), respectively. Precise temperature control was essential to ensure the reliability and consistency of the test data. The ambient air temperature was maintained consistently at 25 °C (±1 °C). In-cylinder pressure measurements were conducted over 10 iterations, and assessments of regulated emissions were replicated 5 times. To ensure the reliability of the test results, the diesel engine (DE) needed to operate for a duration exceeding 15 min following the transition to a different fuel blend. This precautionary measure aimed to enhance the dependability and consistency of the experimental outcomes.
3. Results and Discussion
3.1. Effect of 2-EHN Additive on Combustion Characteristics
Figure 2a,b illustrates the influence of adding 2-EHN, which is recognized as a cetane number (CN) enhancer, on both the in-cylinder pressure and the HRR within an M30 fuel blend under load conditions of 10% and 70%, respectively. At 0.13 MPa, the in-cylinder pressure profiles for both pure diesel and the mixture fuels displayed comparable trajectories. Following an initial drop in the in-cylinder pressure near the TDC, there was a slight rise. The reduced in-cylinder temperatures of the fuels that were tested are the cause of this phenomenon. Compared to pure diesel, the onset of combustion and peak in-cylinder pressure are delayed in the M30 blend without a CN improver, concurrent with an increase in maximum HRR, aligning with findings by Xiao et al. [12]. This delay is a consequence of the low CN of 2-MF, which postpones combustion initiation, resulting in a significant quantity of fuel burning in the pre-mixed combustion stage and thereby increasing the peak HRR. Furthermore, the heightened oxygen content in 2-MF potentially enhances the HRR, augmenting the combustion process [42]. Additionally, it was observed that combustion in blends M30E1.5 and M30E2 commenced earlier than in the M30 blend. This phenomenon is attributed to the properties of 2-EHN. Primarily, the reduced viscosity of 2-EHN decreases the viscosity of the M30 blend, enhancing atomization and expediting fuel evaporation. This leads to the creation of a more burning mixture during the pre-mixed combustion phase, therefore, shortening ID [34]. Secondly, the increased volatility of 2-EHN augments the diffusion rate of fuel vapor within the combustion chamber, facilitating mixture preparation prior to ignition [43]. The heat release rate (HRR) and in-cylinder pressure increased noticeably for all test fuels when the engine load was raised to 0.88 MPa. This increase can be attributed to the greater fuel quantities required under elevated loads, contributing to more robust combustion. Figure 2b illustrates that the M30 blend exhibits higher HRR and in-cylinder pressure compared to other test fuels, a consequence of the lower CN of 2-MF fuel and delayed combustion initiation in the premixed phase. Elevated HRR can result in higher in-cylinder pressure rise rates, potentially leading to unacceptable noise levels [44,45]. However, the addition of 2-EHN to M30 fuel resulted in a reduction in the in-cylinder pressure and HRR, particularly under high-load conditions. This observation suggests that incorporating 2-EHN into low-cetane number fuels has the potential to alleviate issues related to heightened in-cylinder pressure and reduce combustion noise in the phase of pre-mixed combustion. The short ID by adding 2-EHN, especially under low-engine load conditions, can be attributed to its ability to improve the CN and reduce the self-ID of the blended fuel.
Figure 3a illustrates the ignition delay (ID) of the fuels, scientifically defined as the interval, measured in crank angle degrees, beginning with the start of fuel injection into the combustion chamber to the point where 10% of the fuel has combusted. This metric is derived from the analysis of the HRR during the burning process. The data distinctly indicates that the ID period decreases with increased loading across all test fuels. This phenomenon is primarily attributed to the escalation of the equivalence ratio, which, in turn, raises the in-cylinder temperature. The heightened in-cylinder temperature enhances fuel atomization, facilitating the formation of a more combustible blend during the pre-mixed combustion phase. Consequently, this leads to an easier and more efficient ignition of the test fuel [46].
The analysis of Figure 3a reveals that, among all engine load conditions, the M30 fuel blend exhibits the most prolonged ID. This extended delay is attributed to the elevated latent heat of vaporization inherent to 2-MF and its lower CN. The cetane number, a crucial determinant in ID, assesses the ignition quality of fuel [47], which is particularly relevant to compression ignition engines. A fuel with a lower CN indicates an extended ID, requiring a longer duration for the fuel to reach its autoignition temperature and initiate combustion [48,49]. The addition of 2-EHN to the blend increases the CN, thereby reducing ID. This effect is more pronounced at higher engine loads, where quicker ignition is crucial for maintaining combustion efficiency. 2-EHN enhances fuel atomization and vaporization, promoting a more homogeneous fuel–air mixture and reducing ID [42], which in turn improves BTE and reduces BSFC.
Figure 3b provides a detailed examination of the combustion duration (CD), defined as the interval from the 10% to 90% fuel burning points under various engine load conditions for the fuels under test. The CD tends to extend with an increase in engine load, a phenomenon attributable to several contributing factors. Firstly, to satisfy elevated power demands, a larger volume of fuel is introduced into the combustion chamber. This increased fuel volume necessitates a more prolonged period of combustion, leading to an extended CD. Secondly, with increased engine loads, the temporal allowance for effective air–fuel mixing is reduced, potentially resulting in decreased mixing efficiency. This reduction in efficiency can lead to the formation of regions within the combustion chamber characterized by either overly lean or overly rich mixtures. Such suboptimal mixing conditions can culminate in a slower and less uniform combustion process, elongating the burning duration. Notably, the M30 fuel blend is observed to have the shortest combustion duration among the fuels tested. This is attributed to its capacity to generate a more combustible mixture in the premixed phase and achieve a more evenly distributed fuel mixture prior to ignition, a result of its longer ID, which facilitates a more rapid combustion process. However, as indicated in Figure 3b, the addition of 2-EHN to the M30 Diesel-2-Methylfuran (2-MF) blend leads to an increase in CD, despite the expected reduction in ID due to the higher CN. This extended CD can be attributed to several factors. While 2-EHN enhances the cetane number and promotes quicker ignition, it may also affect fuel atomization and vaporization, leading to a more gradual and controlled combustion process, especially at higher engine loads. Previous studies [50] have shown that CN improvers like 2-EHN can alter the combustion dynamics, particularly under high-load conditions, where the combustion phase becomes more uniform but longer due to better fuel-air mixing. Additionally, at higher loads, the combustion temperature and energy release are spread over a longer period, extending the CD. Thus, the interplay between cetane number, fuel characteristics, and engine load conditions can explain the observed increase in combustion duration.
3.2. Impacts of 2-EHN Cetane Improver on BSFC and BTE
Figure 4 presents an analysis of how the addition of the cetane number enhancer, 2-EHN, influences the brake-specific fuel consumption (BSFC) of the M30 blend. The graph depicted in Figure 4 indicates a trend of decreasing BSFC in response to escalating engine loads. The M30 blend, characterized by a relatively lower calorific value compared to pure diesel, exhibits an increased BSFC [12], aligning with observations documented in the reference. Comparative analysis, as showcased in Figure 4, reveals that incorporating 1.5% (M30E1.5) and 2.5% (M30E2.5) of 2-EHN into the M30 blend yields average reductions in BSFC by 2.78–5.7%, respectively. This decrease in BSFC can be attributed to the enhanced volatility of 2-EHN, potentially improving the dispersion rate of fuel vapor within the combustion chamber. This improvement facilitates the preparation of the fuel–air mixture before ignition. Additionally, the integration of 2-EHN into the M30 blend leads to a reduction in ignition delay (ID) due to the increased cetane number (CN). This reduction in ID positively influences the combustion process, ultimately contributing to a decrease in SFC. Moreover, 2-EHN enhances fuel atomization and vaporization, promoting a more homogeneous fuel–air mixture. As a result, the improved combustion process reduces ID [39], which in turn improves BTE and reduces BSFC.
Figure 5 highlights the influence of 2-EHN, a CN booster, on BTE in the M30 fuel blend across different engine loads (10–90%). For every test fuel type, it shows that when engine load increases, BTE increases as well. In comparison to diesel, the M30 blend exhibits a shorter combustion time, benefiting from constant volume combustion, thereby enhancing BTE. Additionally, the M30 blend, with its high oxygen content, improves and accelerates combustion, further boosting its BTE. The incorporation of 1.5% and 2.5% 2-EHN into M30 results in an average increase in BTE by 3.54–7.1%, respectively. This improvement is attributed to their reduced BSFC, heating values relative to the standard M30, and due to the more efficient combustion facilitated by the shorter ignition delay.
3.3. Impacts of 2-EHN Additive on the Emissions Characteristics
Figure 6 illustrates the impact of varying 2-EHN concentrations on the nitrogen oxide (NOx) emissions for the M30 blend under different engine load conditions. As the load increases, NOx emissions show a significant increase, peaking at 0.88 MPa (BMEP), followed by a subsequent decrease. Within the range of 0.13 to 0.88 MPa BMEP, elevated temperatures and a higher O2 level in the cylinder promote NOx creation. However, with further load increase, the equivalence ratio rises, resulting in a decline in in-cylinder oxygen content, restricting NOx formation and emissions. In a study conducted by Doppalapudi AT et al. [51] reducing NOx emissions from biodiesel engines can be achieved by modifying ignition delay and combustion duration. They identified key methods such as fuel emulsions, low-temperature combustion, retarded injection timing, and post-combustion catalytic converters as effective strategies to limit NOx formation by lowering peak combustion temperatures. As compared to basic diesel fuel, the M30 blend exhibits higher NOx emissions at all engine loads due to the higher in-cylinder temperature induced by the rising oxygen content in the M30 blend. Additionally, the lower cetane number of M30 fuel causes a longer ignition delay and premixed combustion, leading to increased combustion temperatures and pressure [52,53]. The addition of 2-EHN in the M30 mixture results in the reduction in NOx formation by lowering combustion temperatures through an increase in the cetane number and shortening the ID period of the M30 blend. In comparison to the M30 blend, the M30E1.5 and M30E2.5 blends result in a reduction of NOx emissions by 9.20% and 17.57%, respectively, across all operating conditions.
Figure 7 illustrates hydrocarbon (HC) emissions from the four tested fuels under various engine loads. As engine loads increase, HC emissions initially decrease for each fuel, reaching a minimum at a load of 0.88 MPa BMEP and then showing an upward trend. The elevated in-cylinder temperature enhances the combustion process, promoting the oxidation of unburned hydrocarbons and reducing HC emissions. However, when the load reaches 1.13 MPa BMEP, the ratio of air to fuel decreases, leading to a lower local oxygen concentration in the cylinder and higher HC emissions. The M30 blend consistently produces fewer HC emissions across various engine load levels than standard diesel. This reduction in HC emissions is due to the increased oxygen levels in the M30 blend, which enhances the oxidation process of unburned HC [54]. Figure 7 demonstrates the impact of the 2-EHN additive on HC emissions for the M30 blend. The M30E1.5 and M30E2.5 blends result in a reduction of HC by 7.93% and 21.59%, respectively, when compared to the M30 blend. The higher volatility of the 2-EHN additive enhances vapor dispersion inside the combustion chamber, improving mixture preparation before ignition. Additionally, the M30 fuel, when enhanced with the 2-EHN additive, demonstrates a reduced ID because of its enhanced CN. This results in more efficient combustion and a decrease in HC emissions for the M30E1.5 and M30E2.5 blends as compared to M30.
Figure 8 illustrates the emissions of carbon monoxide (CO) for four different fuels, with variation in engine loads from 0.13 to 1.13 MPa in BMEP. CO emissions experience a significant decline from 0.13 to 0.38 MPa BMEP due to the elevated temperature inside the cylinder. Generally, CO emissions are influenced by factors such as the air–fuel ratio, combustion temperature, duration, and chemical kinetic intermediates. CO can undergo rapid oxidation to carbon dioxide (CO2) when the in-cylinder temperature surpasses 1400 K [55]. In comparison to diesel fuel, the M30 fuel, whether with or without 2-EHN, exhibits increased CO emissions at engine loads ranging from 0.13 to 0.38 MPa in BMEP. This is attributed to the low combustion temperature inhibiting further oxidation of CO, resulting in increased CO emissions. Basic diesel fuel shows the least CO emissions, attributable to its notably higher air-to-fuel ratio and longer combustion period. The incorporation of 1.5% and 2.5% 2-EHN into the M30 blend results in an average decrease in CO emissions of 12.11 and 33.98%, respectively. In medium engine loads, between 0.38 and 0.88 MPa in BMEP, the CO emissions approach virtually zero for all fuels evaluated, owing to the sufficient oxygen content and rising combustion temperature. As the engine load increases to 1.13 MPa BMEP, the equivalence ratio rises, leading to a decrease in local oxygen content in-cylinder and emitting more CO emissions for each tested fuel.
Figure 9 illustrates the impacts of 2-EHN on the soot emissions across various engine loads. Soot emissions from base diesel fuel are the highest among the tested fuels and decrease with increasing engine loads [56]. In contrast, soot emissions from the M30 fuel are nearly zero. Soot emissions are influenced by several main factors, especially since M30 is an oxygenated fuel. The addition of 2-MF increases the oxygen content of the M30 blend, promoting diffusive burning and reducing soot emissions. Previous studies have highlighted that fuel oxygenation is a major contributor to the reduction in soot emissions. [57]. Another reason for the reduction in soot emissions from the M30 blend is the longer ID, which promotes premixing and prolongs the premixed phase period, consequently reducing diffusive combustion and soot emissions. Based on these results, it can be concluded that the CN or ID period of M30 is the primary factor in soot reduction compared to diesel fuel [58]. In Figure 9, soot emissions from the M30E1.5 and M30E2.5 blends are higher than those from the M30 blend. This increase could be attributed to the effects of 2-EHN on the properties of the M30 fuel. The addition of 2-EHN led to increases in cetane numbers, reduced ignition delays, and, as a result, a slight increase in soot emissions.
4. Conclusions
This study investigates the effects of adding 1.5% and 2.5% 2-EHN as a cetane enhancer to a 30% by volume 2-methylfuran–diesel (M30) blend. The primary objective was to optimize the trade-offs between NOx and soot emissions. Experiments were conducted on a modified four-cylinder, four-stroke direct-injection compression ignition (DICI) engine. Testing was performed under a range of load conditions, with BMEP varying between 0.13 and 1.13 MPa, while the engine speed was held constant at 1800 rpm, and the timing of the injection did not change; it was at 7.5 crank angle (CA) degrees before the TDC, with a focus on engine performance and emission characteristics compared to baseline diesel fuel and M30 blend. The key findings from this investigation are summarized below:
Compared with pure diesel, the M30 blend initially displays a prolonged ID period and elevated in-cylinder pressure. Nonetheless, the addition of 2-EHN reduced the ID and maximum in-cylinder pressure, particularly at a 2.5% concentration, where the pressure approached that of pure diesel fuel. Furthermore, the M30 blend showed higher BSFC due to its lower calorific value. The addition of 1.5% and 2.5% 2-EHN reduced BSFC by average values of 2.78–5.7%, respectively. This improvement in fuel consumption was attributed to the favorable effects of 2-EHN on the CN, which enhances ignition quality, thereby reducing ID, with corresponding increases in (BTE) by 3.54–7.1%, attributed to the improved ignition quality and more efficient combustion by the shorter ignition delay and lower BSFC compared to the M30 blend without 2-EHN.
The M30 blend with 2-EHN exhibited lower CO and HC emissions compared to the baseline M30 fuel, with reductions in HC emissions were 7.93 and 21.59%, while CO emissions decreased by 12.11 and 33.98%, respectively, with the addition of 1.5% and 2.5% 2-EHN. Additionally, NOx emissions were decreased by 9.20 and 17.57%, data.
This research has significant implications for advancing the use of sustainable fuel blends, improving engine performance, and addressing environmental concerns associated with diesel combustion.
Conceptualization, M.L.; methodology, B.M.A. and H.A.M.E.; formal analysis, B.M.A. and H.A.M.E.; investigation, B.M.A. and P.-C.S.; resources, M.L.; data curation, B.M.A., M.L. and H.A.M.E.; writing—original draft preparation, B.M.A.; writing—review and editing, H.A.M.E. and N.M.M.; supervision, M.L.; project administration, M.L.; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript.
The raw data supporting the conclusions of this article will be made available by the authors on request.
The authors would like to thank the China Scholarship Council (CSC) for their support. Special thanks are extended to Helin Xiao for his kind permission and cooperation in providing access to his laboratory facilities.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 3. Impact of 2-EHN on (a) ignition delay ID and (b) combustion duration CD of diesel and 2-methylfuran blend at different BMEP.
Enlarge this image.Figure 6. NOX emissions generated by all tested fuels under varying engine load conditions.
Enlarge this image.Figure 8. CO emissions generated by all tested fuels under varying engine load conditions.
Enlarge this image.Figure 9. Effect of 2-EHN concentration on the soot of M30 blend at different BMEPs.
Specification of a modified four-cylinder, four-stroke, water-cooled, direct-injection compression ignition (DICI) engine.
| Type of Engine | Four-Cylinder, 4-Stroke, Water-Cooled |
|---|---|
| Bore | 96 mm |
| Stroke | 103 mm |
| Compression ratio | 17.5 |
| Displacement | 2982 cc |
| Rated power | 85 kW |
| Rated speed | 3200 rpm |
| Type of ignition | Compression ignition |
| Method of starting | Electric start |
| Initial injection | 7.5 CAD BTDC |
Properties of diesel, bioethanol, 2-MF, and 2-EHN [
| Parameters | Diesel | Bioethanol | 2-MF | 2-EHN |
|---|---|---|---|---|
| Chemical formula | C12–C25 | C2H6O | [Image omitted. Please see PDF.] | C8H17NO3 |
| Research Octane number | 20–30 | 109 | 103 | - |
| Cetane number | 52.1 | 8 | 9 | - |
| Oxygen content (% mass) | 0 | 34.78 | 19.51 | |
| Stoichiometric air/fuel ratio | 14.3 | 8.95 | 10.05 | - |
| Density at 20 °C (kg/m3) | 834 | 790.9 | 913.2 | 963 |
| Viscosity at 40 °C (mm2/s) | 3.8 | 5.28 | - | 1.72 |
| Water solubility at 20 °C (wt%) | N | Miscible | N | - |
| Evaporation heat at 25 °C (kJ/kg) | 270–301 | 919.6 | 358 | 353.6 |
| Lower heating value (MJ/kg) | 42.5 | 26.9 | 31.2 | 27.4 |
| Initial boiling point (°C) | 180–370 | 78.4 | 64.7 | 307 |
N: Negligible.
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Details
; Luo, Maji 2
; Elbadawi, Hassan A M 3
; Mahmoud, Nasreldin M 4 ; Pang-Chieh Sui 2 1 Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China;
2 Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China;
3 Hubei Key Laboratory of Advanced Technology for Automotive Components, Wuhan University of Technology, Wuhan 430070, China;
4 Mechanical Engineering Department, Faculty of Engineering, University of Sinnar, Sinnar 21111, Sudan;