Performance and In-cylinder Combustion Analysis of Gasoline Ethanol Fueled Spark-Ignition Engine

Saikrishnan et al. [7] examined the impact of ethanol-gasoline blends (E0, E5, and E15) on the performance and emissions of a four-stroke, three-cylinder SI engine, using E0 as a benchmark. The study assessed various parameters, including mechanical efficiency, BTE, and specific fuel consumption, as well as emissions of carbon monoxide (CO), hydrocarbons (HC), carbon dioxide (CO₂), and nitrogen oxides (NOx), under different torque conditions while keeping the engine speed constant. The results indicated that the use of ethanol-gasoline blends led to increases in BTE, brake power, and fuel consumption. Meanwhile, emissions of CO and HC were reduced, while NOx emissions showed an increase with the ethanol-gasoline blends.

Nwufo et al. [8] conducted a comprehensive study on a single-cylinder spark ignition engine operating on ethanol-gasoline blends. The research investigated various ratios of ethanol-gasoline blends, assessing combustion characteristics, exhaust emissions (HC, CO, O₂, and CO₂), and engine performance metrics including brake power, engine torque, brake-specific fuel consumption, BTE, and brake mean effective pressure. Tests performed at full load across different engine speeds revealed that incorporating ethanol into gasoline improves combustion efficiency and substantially lowers emissions, resulting in enhanced engine performance.

In their experimental investigation, Srinivas Rao et al. [9] investigated the effects of fuel additives on the performance and emission characteristics of a spark ignition engine operating on petrol. Toluene, benzene, and ethanol were used as fuel additives at a concentration of 20% by volume. Performance assessments carried out on a single-cylinder, four-stroke stationary gasoline engine showed that ethanol-blended gasoline outperformed pure gasoline, especially under higher load conditions. While toluene exhibited marginally lower performance at lower loads, it demonstrated enhanced performance relative to pure gasoline at higher loads. Conversely, benzene consistently demonstrated poor engine performance across all load values. The short-term experimental results suggested the successful substitution of petrol with 20% toluene or 20% benzene additives without necessitating alterations in engine design.

An overview of the effects of mixing gasoline and ethanol for usage in spark ignition engines was given by Stein et al. [10]. The study found that the high latent heat of vaporization (HoV), research octane number (RON), and sensitivity of ethanol contribute to a substantial enhancement in knock resistance when its proportion in an ethanol-gasoline blend is increased. Nevertheless, in-use SI engines are unable to fully utilize the fuel's knock resistance due to their limitation caused by peak cylinder pressure (Pmax) at higher loads. As a long-term fuel, a mid-level blend of ethanol (more than E20 and less than E40) seems appealing.

Pal [11] focused on the emissions and performance of a four-cylinder SI engine equipped with a multi-point fuel injection (MPFI) system while using various ethanol-gasoline blends (Gasohol). It assessed multiple performance indicators, including thermal efficiency and brake power, as well as exhaust emissions such as CO, HC, NOx, and CO₂. The findings suggest that low-ethanol blended gasoline can act as a cleaner fuel, significantly reducing CO, HC, and NOx emissions. Performance metrics indicated improvements in both power and thermal efficiency, along with a slight increase in specific fuel consumption. However, there was also an increase in exhaust gas temperature and peak cylinder pressure, which underscores the benefits of the natural oxygen content present in ethanol molecules.

Delvi et al. [12] conducted an experimental study on the performance, emissions, and combustion characteristics of different ethanol-gasoline (Gasohol) blends, specifically E25, E30, and E35, using a single-cylinder spark ignition engine. Tests with different loads, engine speeds, and compression ratios were run, and the outcomes were noted. The outcome demonstrated enhanced engine performance metrics. It was found that ethanol-gasoline blends exhibit greater thermal brake efficiency compared to gasoline. The E35 blend achieved a maximum BTE of 27.87%.

In an investigative study conducted by Al-Hasan [1], in which a single-cylinder, naturally aspirated, four-stroke spark ignition engine was evaluated using gasoline blended with ethanol at various concentrations (2.5%, 5%, 10%, 15%, and 20%). The engine's performance was assessed and compared to its operation on pure gasoline. Under stoichiometric conditions, increasing the ethanol content led to a rise in indicated engine power, while indicated specific energy consumption (ISEC) showed a slight reduction.

Therefore, it is clear that researchers have thoroughly examined the effects of ethanol-gasoline blends on engine performance and combustion parameters. Nevertheless, the authors of the current study propose that a complete understanding of gasoline-ethanol blends in engines can be attained by conducting an in-depth analysis of both engine performance and combustion characteristics. This method seeks to offer a more comprehensive view of the topic.

2.1 Latest technological advancements for ethanol-blended gasoline

In this paper, Purayil et al. [13] performed a thorough analysis of how ethanol-gasoline and methanol-gasoline blends affect the performance of lean-burn spark ignition engines, particularly focusing on the knock limit related to ethanol-enriched fuels. The results indicate that the addition of ethanol and methanol to gasoline raises the hydrogen knock limit, especially in cases of retarded spark timing. However, this method also results in a decline in BTE and peak in-cylinder pressure. Furthermore, while these blends reduced CO₂ and NOx emissions, they led to an increase in CO emissions, highlighting the connection between engine performance, fuel type, and environmental consequences.

In the context of increasing environmental regulations and the necessity to enhance air quality, Meng et al. [14] addressed the significant issue of vehicle exhaust emissions. The study aimed to investigate the role of engine oil as a contributor to environmental pollutants during the operation of motor vehicles and emphasized the importance of evaluating its impact on emissions when used with gasoline, ethanol, and E85 fuels. The research examined the effects of new engine oil (NEO) and waste engine oil (WEO) in various proportions on emissions. The results indicated that the inclusion of engine oil notably elevated the emissions of carbon monoxide (CO) and particulate matter. For example, adding 1% NEO to E85 fuel resulted in a 25% increase in CO emissions, along with increases of 4.91% in particulate matter (PM) and 2.23% in particle number (PN). Additionally, the study utilized thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) to investigate the pyrolysis of engine oil and the ash morphology resulting from combustion, respectively. These findings underscored the necessity for further research on the effects of engine oil on pollutant emissions when utilizing ethanol-blended fuels.

Zhao et al. [15] examined the optimization of a multifuel combined supply system that can utilize ethanol, gasoline, and oxyhydrogen to enhance engine performance and lower emissions. The results indicated that the stratified combustion achieved with ethanol direct injection (EDI) is superior to that of gasoline direct injection (GDI). The use of oxyhydrogen negative pressure inhalation (ONPI) increased both the maximum pressure and the indicated mean effective pressure (IMEP), while reducing emissions of CO, HC, and particulate matter; however, it resulted in higher NOx emissions. On average, EDI reduced NO emissions by 28.39% compared to GDI. Therefore, the optimal control strategy focused on improving performance and reducing emissions could be advantageous for the use of ethanol-blended fuels.

Isobe et al. [16] explored the effect of low-temperature heat release (LTHR) on the laminar burning velocity (LBV) of ethanol-blended gasoline surrogate fuels in a spark-ignited flex-fuel engine. The study's results indicated that higher compression ratios (CR) and exhaust gas recirculation (EGR) can enhance BTE. In a single-cylinder engine, it was established that LTHR occurred before the main combustion event, with rising temperatures positively affecting the LBV, while partial oxidants negatively impacted it. Furthermore, the findings suggested that increasing the ethanol blending ratios could lead to a higher LBV during the later stages of combustion, which may improve engine performance and contribute to decarbonization efforts.

2.2 Drawbacks and challenges with ethanol-blends in spark ignited engines

This section provides a literature review that investigates the disadvantages and challenges associated with the use of ethanol blends in SI engines, including a modest rise in NOx emissions [17]. Additionally, engine performance tends to be subpar, and specific fuel consumption may deteriorate when using ethanol blends, as noted in references [1, 2, 11, 18]. Furthermore, there is a slight increase in unregulated pollutants such as aldehydes [3], along with material compatibility concerns for higher blends exceeding E20 [4], and issues related to cold Startability, as mentioned in reference [16].

This study aimed to comprehensively evaluate ethanol blends in SI engines so that the relative significance of the positive and the negative aspects of ethanol blending in SI engines is absolutely clear. This study supports the view that the environmental benefits of ethanol-blended gasoline fuels far outweigh the seemingly adverse impact of their lower fuel economy. More ethanol-blended fuel is required to achieve a gasoline-like engine performance due to lower calorific value, leading to lower fuel economy. This impact of a lower fuel economy for ethanol-blended fuels can be mitigated due to the lower unit price of ethanol over neat gasoline.

2.3 The objectives of the study

•To examine the effects of ethanol-gasoline blends (0%, 10%, 20%) on combustion characteristics such as pressure, HRR, combustion temperature, and mass burn fraction (MBF) in a SI engine.

•To evaluate performance metrics including torque, power output, BSFC, fuel flow rate, and exhaust temperature while utilizing ethanol-blended fuel.

•To study the dependence of combustion and performance parameters, providing insights into the optimal blend ratios for enhanced engine performance utilizing ANOVA.

This study explored the effects of ethanol blending with pure gasoline on the performance of SI engines under varying speeds and loads. The results of the analysis of variance (ANOVA) are detailed in Table 6, which indicates significant influences of both the blend percentage and engine RPM on several in-cylinder combustion parameters, such as peak combustion pressure (P_max), peak combustion temperature (T_max), IMEP, crank angle for maximum combustion pressure (AP_max), crank angle for 10% mass fraction burned (MFB10), and crank angle for 90% mass fraction burned (MFB90). The F-values for these parameters are notably high, suggesting that the variations in the responses can primarily be linked to the independent variables, which include the fuel blends and engine speeds.

Table 6. ANOVA results (F-value and p–value) for in-cylinder combustion parameters

The F-value, which represents the ratio of the variance explained by the model to the variance attributed to error, is a crucial indicator of the model's significance. A higher F-value signifies a substantial impact of the independent variables on the dependent variables. Moreover, all associated p-values are below 0.05, confirming statistical significance and indicating that the observed variations are unlikely to be due to random chance [21].

Figures 3-8 illustrate the variations and average changes in the engine performance parameters for all fuel blends and the three different engine speeds obtained from the experimental tests. The engine was operated at three different levels of speeds: low, medium, and high, which are represented by 1500 RPM, 4000 RPM, and 6000 RPM respectively. The combustion pressure data and HRR data collected from these tests are shown in Figures 3-8. An additional table, having comparison between each measured parameter is also provided (Table 6).

Figures 3-8 show the variations of cylinder pressure with respect to the crank angle for three different fuel blends (E0, E10, and E20) with varying engine speeds.

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Figure 3. Combustion pressure data at FTP conditions for low engine speed

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Figure 4. Combustion pressure data at FTP conditions for medium engine speed

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Figure 5. Combustion pressure data at FTP conditions for high engine speed

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Figure 6. HRR and combustion temperature data for low engine speed

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Figure 7. HRR and combustion temperature data for medium engine speed

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Figure 8. HRR and combustion temperature data for high engine speed

Table 7. Percentage change in various parameters w.r.t E0 for low, medium and high speed

The highlighted values (bold) in Table 7 represent the highest percentage changes observed in combustion pressures and temperatures. The impact of adding ethanol to gasoline on different engine performance parameters is discussed in detail below.

4.1 Effect on in-cylinder pressure

This subsection presents an analysis of in-cylinder pressures as per the three levels of speed zones explored in this study.

Low Speeds

At lower speeds, the in-cylinder pressure (Pmax) for E10 fuel is higher than that for E0 fuel while Pmax for E10 and E20 fuel is similar. The Pmax increases when ethanol is blended. This can be mainly attributed to the higher HRR. The IMEP is also greater for E10 fuel due to the increased cylinder pressures. The combustion temperatures for E10 and E20 are lower than those for E0, which is due to the higher heat of vaporization associated with ethanol. Furthermore, the oxygen content in ethanol results in a shorter combustion duration and a quicker combustion process compared to E0 fuel. It is noteworthy that the combustion performance is better for E10 than for E20, as the base engine is specifically optimized for E10 fuel. If the engine were adjusted for E20 fuel, it could potentially achieve better performance with E20 compared to E10.

Medium Speed

At medium speed, the pattern observed is consistent with that seen at lower speeds. The peak combustion pressure (Pmax) is greater for E10 and comparable to that of E20 fuel when compared to E0 fuel. The combustion duration is shorter, while the IMEP and HRR are higher for both E10 and E20 blends in comparison to E0 fuel.

High Speed

At high speed, the trend changes and is not similar to that seen in lower and medium speeds. The Pmax is higher for E0 fuel as compared to E10 and E20 fuel. This is because firstly, ethanol takes time to evaporate and secondly, its volatility is poor as compared to gasoline. Thus, the combustion of E0 is faster as compared to E10 and E20. The combustion time at higher speeds is lower as compared to combustion time at lower and medium speeds. Hence, ethanol blended fuels are outperformed by E0 fuel at higher speeds. However, if the engine is specifically optimized for high-speed operations, the same trend as seen for lower and medium speeds can be obtained. Therefore, it is evident that, increasing the proportion of ethanol blend in the gasoline leads to higher peak cylinder pressure and improved IMEP along with faster combustion and higher HRR with lowered combustion temperatures. Figures 9 and 10 show the in-cylinder pressure (Pmax) and crank angle at max in-cylinder pressures (APmax) for different ethanol blends.

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Figure 9. Speed v/s in-cylinder pressure

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Figure 10. Speed v/s APMAX

At low and medium speeds, the increased values of peak combustion pressure (Pmax) for E10 and E20 in comparison to E0 can be attributed to the higher laminar flame speed of alcohols, which leads to quicker flame propagation during combustion [22]. At low and medium speeds, the in-cylinder combustion pressure for E10 and E20 configurations is slightly greater and faster than that of E0. High flame speed in ethanol leads to higher maximum pressure. This action accelerates combustion and increases maximum pressure levels [23].

Higher cylinder pressure results from increased oxygen availability in the chemical composition, leading to improved combustion efficiency [12]. The ease of ethanol evaporation leads to a more homogeneous charge. At higher speeds, however, owing to the increased fuel supply, the combustion of E10 and E20 slows down as engine speed increases [24]. The incorporation of ethanol into gasoline significantly influences the properties of the fuel blend. One key effect is the enhancement of the octane number and the latent heat of vaporization, which helps to postpone the chain reactions in the end gas. Another effect is a decrease in the heating value. It should be noted that these effects play opposing roles in engine performance. The initial effect is likely to dominate until the ethanol concentration reaches 10%, at which point the second effect begins to take over [25]. This decrease in heating value of fuel results in lower indicated power. As the ethanol content increases from E0 to E20, the trend for APmax (the crank angle at which maximum pressure is reached) shows a decrease, suggesting that higher ethanol levels lead to faster burn rates. This observation is supported by the fact that increased ethanol content contributes to quicker burn rates due to its higher laminar flame speed. There is a slight difference in the two Pmax peaks, which suggest that the spark was initiated much later, owing to achieve the same engine power.

4.2 Effect on HRR

This subsection discussed the effect of ethanol addition on HRR. The trends shown in Figure 11 indicate that HRR increases from low to medium speed and then remains stable from medium to high speed. The HRR is measured as heat released per crank angle and measured in kJ/kg-deg unit. The HRR clearly indicates that incorporating ethanol into gasoline results in a greater amount of heat being released, particularly noticeable when the piston reaches top dead center. Overall, the ethanol blends generate significant heat due to their high enthalpy. The oxygen-rich nature of ethanol improves combustion, which predominantly takes place near the top dead center. Figure 11 also demonstrates that using blends in engines results in greater and more effective HRR [26]. Ethanol has a latent heat of vaporization that is 2.9 times greater than that of gasoline, which renders it less effective for the evaporative atomization of the blended fuel. As a result, the combustion chamber absorbs more heat because the heat released from the mixture is diminished. This lower temperature of the fuel mixture at the time of ignition causes a slower development of the flame core. Additionally, since the ignition time stays constant, the combustion process is postponed, leading to a larger crank angle at which peak pressure occurs. This trend becomes more noticeable with an increase in mixing ratio [27].

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Figure 11. Speed v/s HRR

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Figure 12. Speed v/s maximum temperature

4.3 Effect on maximum temperature

This subsection discussed the effect of ethanol addition on maximum combustion temperature. The trends shown in Figure 12 indicate that maximum temperature increases across different fuel blends (EO, E10 and E20) from low to medium speed and then remains stable from medium to high speed. Increased ethanol content leads to increased pressure and temperature in the cylinder. Ethanol's high flammability and adiabatic flame speed are responsible for this effect. The combustion is completed in a short period of time, and thus, these effects provide a small increase in torque and power, as mentioned by some researchers [22].

4.4 Effect on MBF

The effect of ethanol addition on MBF is discussed as follows.

In this study, MBF traces were used to identify the different phases of combustion. The ignition delay is defined as the angle between the beginning of fuel injection and the point at which 10% of the MBF is reached.

Meanwhile, the combustion duration (CD) is defined as the time or crank angle interval between the points where 10% and 90% of the MBF occur. The ignition delay period consists of both chemical and physical delays that occur simultaneously.

Figures 13 and 14 show a line graph with trends of MBF percentages at 10% and 90% across different speeds (Low, Medium, High) for different conditions (E0, E10, E20). MBF 10% represent the start of combustion and the crank angle for that instant. As seen in the Figures 13 and 14, there is an ignition delay while using gasoline fuel for lower speed, but the delay remains constant at medium as well as high speeds. This trend is lower in value at low speeds but increases as it moves towards medium speed, and it remains stable through high speed. This could be due to the engine’s performance characteristics at low speeds, where it may be operating less efficiently. As the speed increases, the engine may operate more efficiently [28].

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Figure 13. Crank angle (MBF10%) v/s speed

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Figure 14. Crank angle (MBF 90%) v/s speed

At low and medium engine speeds, excessive combustion duration causes the burning process to continue even when the pistons reach BDC. Thermal energy increases the in-cylinder temperature resulting in a drop in the HRR. MBF 90% represents the end of combustion. This is useful in finding out the combustion duration. As seen in the Figures 13 and 14, the combustion duration is lower at low speeds i.e. at the start of combustion but it increases as we increase the engine speed. As we move towards high speed, the combustion duration increases due to increase in fuel quantity needed for combustion. Along with this, two more characteristics of ethanol blends influence MBF. Firstly, the heat of evaporation causes an increase in ignition delay by lowering combustion temperatures. The second factor is the presence of oxygen, which facilitates combustion. Both qualities work against each other during combustion. One researcher found that using oxygenated fuel in the spray reduced pyrolysis and enhanced oxidation, resulting in shorter combustion durations [29].

These trends also can be due to the varying effects of engine speed and ethanol content in the fuel on engine performance. The engine’s best characteristics can vary with speed and fuel type, reflecting the complex interplay of various factors such as fuel combustion efficiency, engine design, engine configuration, and operating conditions.

4.5 Effect on power

Power is seen to have increased as the engine speed was increased. But at medium and high speed, this difference is relatively low. As seen in the Figure 15, the power values for E0 are lower than E10 and E20 while at low speed. As the engine speed rises, the power values for the blended fuel exhibit only a slight increase. This can be explained by the lower specific heat of ethanol, which contributes to the increase in power output [11]. The high heat of vaporization of ethanol helps cool the fuel-air mixture and increases its density, resulting in a higher power output [30].

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Figure 15. Speed v/s power

4.6 Effect on torque

Ethanol possesses a greater heat of vaporization compared to gasoline, which cools the fuel-air mixture and enhances its density, ultimately resulting in a higher power output. Increasing ethanol content increases engine torque [31]. Figure 16 shows that as speed increases the torque values for E10 are always higher than pure gasoline. In contrast, E20 exhibits a minor reduction in torque values when compared to gasoline. A similar observation was made in a study conducted by Yusuf and Inambao [32] who investigated the engine performance and exhaust emissions of ethanol-gasoline blends. The authors observed that, adding ethanol to fuel causes problems in starting the engine and reduces engine performance at speeds above 4000 RPM (medium speed).

4.7 Effect on fuel flow rate

The effect of ethanol blends on fuel flow rate is shown in Figure 17. The fuel flow values increase for E20 fuel as compared to E10, whereas there is a slight decrease of approximately 5-8% for E10 fuel as compared to gasoline (E0).

This finding can be explained by the fact that the blended fuel has a lower heating value per unit mass compared to gasoline. Consequently, a larger volume of fuel enters the combustion chamber for the blended fuels.

4.8 Effect on BSFC

In Figure 18, BSFC is seen to improve for E10 as compared to pure gasoline during low speed, although E10 fuel shows a decrease in BSFC during medium and high speed. E20 shows a constant increase as compared to pure gasoline and E10. As seen in Figure 18, as the ethanol proportion of the blends increases, so does the BSFC. This is because more fuel is required to generate the same amount of power as ethanol has a lower calorific value than gasoline.

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Figure 16. Speed v/s torque

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Figure 17. Speed v/s fuel flow rate

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Figure 18. Speed v/s BSFC

4.9 Effect on exhaust temperature

In Figure 19, the exhaust temperatures are seen to decrease for ethanol blends. Ethanol blends show the lowest temperature during low speed, whereas, as the speed increases, the difference between gasoline and blends is close to 5-10%. This can be attributed to ethanol combustion, which produces more exhaust gas per heating value input, leading to increased heat capacity and exhaust temperatures [10].

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Figure 19. Speed v/s exhaust temperature

4.10 Comparisons with existing literature

To enhance understanding and facilitate comparisons, the outcome of this study is compared with contemporary studies and presented as follows.

In the first comparison, Mohammed et al. [33] investigated the effects of ethanol-gasoline blends (from 10% to 40%) on brake power, thermal efficiency, volumetric efficiency, and BSFC in a single-cylinder, naturally aspirated engine at various speeds. The results showed a 14.67% increase in brake power at 1500 RPM, while BSFC dropped by 17.21% at 2500 RPM for the E40 blend compared to pure gasoline. Additionally, BTE improved by 31.12%. Although this study used a different engine type, testing cycle, and ethanol blending ratio than that of Mohammed et al. [33], both studies found an increase in power output and a reduction in BSFC. However, while Mohammed et al. [33] reported an increase in torque, the present study observed a decline in torque for E10 and E20 blends.

In the second comparison, Ramadhas et al. [34] utilized a four-cylinder SI engine to evaluate emissions from ethanol-gasoline blends, specifically E5, E10, and E20. The findings indicated a 2.5% increase in torque for the E20 blend at 3000 RPM. Furthermore, brake-specific energy consumption decreased by 25%, 10%, and 15% for the E5, E10, and E20 blends, respectively, under low-load conditions. While the engine type and ethanol blending percentages in this study differ from those in study [34], the latter reported an increase in torque, whereas the present study observed a decrease in torque with higher ethanol content.

In the third comparison, Ye et al. [35] studied the influence of 10%, 20%, and 30% ethanol blends on a fuel-injected gasoline engine. The brake power was decreased by 2.34%, 5.63 and 13.615% for the respective blends (E10-E30) compared to pure gasoline. The BSFC was increased by 10%, 18%, and 43% for E10, E20, and E30, respectively, at higher load. The exhaust gas temperature was decreased by 1.20%, 3.60%, and 8.40% for 10-30 blends percentages, respectively, at 3000 RPM. An increase in the brake power, BSFC and a decrease in the exhaust gas temperature were reported in the investigation performed by Ye et al. [35] in this current study as well.

In the fourth comparison, Najafi et al. [36] performed a study on a SI engine using various ethanol blend fractions, including E5, E7.5, E10, E12.5, and E15. The findings revealed a 3.92% increase in torque for E15 at 3500 RPM, while BSFC rose by 2.8% for E5 at the same speed. Additionally, brake power increased by 16.21% for E15 compared to E5 at 4000 RPM. The study noted an increase in torque at medium speeds, whereas the current study observed an increase in torque at low speeds. The trend in BSFC was consistent across both studies.

In the fifth comparison, Balki et al. [37] examined the impact of ethanol at full throttle in a spark ignition engine. They observed a 3.7% increase in torque when using ethanol compared to gasoline, while brake specific fuel consumption (BSFC) rose by 58%. The trend of increasing BSFC was noted to correlate with the percentage of ethanol blend in both studies. The authors indicated that torque improved with higher blend percentages at low speeds, whereas the current study found an increase in torque at medium speeds.