The results obtained from the experimental investigation of the performance, combustion, and emission characteristics of Mahua oil biodiesel in a low heat rejection (LHR) diesel engine with retarded injection timing are presented and discussed in this section. The analysis focuses on key parameters such as b (BTE), (BSEC), (EGT), and various emission characteristics, including (CO), (UBHC), (NOx), and smoke opacity. Comparisons are made between the LHR engine operating on Mahua oil biodiesel and a standard uncoated engine running on conventional diesel.
Figure 4 demonstrates the (BTE) of the standard engine and the LHR engine operating on Mahua oil biodiesel at different loads. The BTE is a crucial parameter for evaluating the efficiency of the engine in converting the energy available in the fuel into useful work. The results indicate that the LHR engine with Mahua oil biodiesel exhibits a higher BTE at full load compared to the standard engine running on diesel. Specifically, the BTE of the LHR engine using Mahua oil biodiesel was 29.20%, whereas the standard engine using diesel achieved a BTE of 26.52%. This represents a 10.1% improvement in efficiency with the use of Mahua oil biodiesel in the LHR engine. The increase in BTE can be attributed to the reduced heat losses due to the thermal barrier provided by the partially stabilized zirconia (PSZ) coating, which retains more heat within the combustion chamber, thus enhancing the combustion process (Jain, Jyoti Bora, Kumar, Sharma, et al., 2023). However, it is also observed that the BTE of the LHR engine operating on Mahua oil biodiesel is slightly lower at part loads compared to the standard engine running on diesel. This is likely due to the higher viscosity and lower volatility of Mahua oil biodiesel, which may result in less efficient atomization and mixing of the fuel, particularly at lower engine loads.
The brake specific energy consumption (BSEC), shown in Fig. 5, is another important parameter that reflects the amount of energy consumed by the engine to produce a unit of brake power. The results indicate that the BSEC of the LHR engine running on Mahua oil biodiesel is lower than that of the standard engine running on diesel at all tested loads. At full load, the BSEC of the LHR engine with Mahua oil biodiesel was found to be 2.97 MJ/kWh, which is 6.23% lower than the BSEC of the standard engine using diesel, which recorded 3.17 MJ/kWh (Jin & Wei, 2023). This reduction in BSEC suggests that the LHR engine is more energy-efficient when fueled with Mahua oil biodiesel, likely due to the improved combustion efficiency resulting from the higher in-cylinder temperatures maintained by the thermal barrier coating. Moreover, the lower BSEC at part loads indicates that the LHR engine is able to make better use of the energy content in Mahua oil biodiesel, despite its slightly lower calorific value compared to diesel.
(EGT) is a critical indicator of the thermal conditions within the engine, and its variation with load is presented in Fig. 6. The EGT was consistently higher in the LHR engine compared to the standard engine, particularly at full load. The maximum EGT recorded for the LHR engine using Mahua oil biodiesel was 436°C, which is 18.64% higher than the 366°C recorded for the standard engine using diesel. The higher EGT in the LHR engine can be attributed to the reduced heat dissipation due to the thermal barrier coating, which leads to higher combustion chamber temperatures. While the increased EGT suggests more complete combustion, it also raises concerns about the potential for higher NOx emissions, which are typically associated with elevated combustion temperatures (Tesfaye Lamore et al., 2023).
The emission characteristics of the engine are a vital aspect of this study, as they directly relate to the environmental impact of using Mahua oil biodiesel in LHR engines. Figure 7 shows the variation in (CO) emissions with load for both engine configurations. CO emissions are primarily a result of incomplete combustion and are influenced by the fuel’s carbon content and the combustion efficiency. The results indicate that the LHR engine operating on Mahua oil biodiesel produced significantly lower CO emissions compared to the standard engine using diesel (Elumalai et al., 2021). At full load, the CO emissions for the LHR engine with Mahua oil biodiesel were 0.028% by volume, representing a 20.35% reduction compared to the standard engine, which emitted 0.035% by volume. The reduction in CO emissions is likely due to the higher combustion temperatures in the LHR engine, which promote more complete oxidation of the carbon in the fuel.
(UBHC) emissions, depicted in Fig. 8, are another critical parameter related to the efficiency of the combustion process. UBHC emissions result from the incomplete combustion of fuel, often due to poor atomization, mixing, or flame quenching. The results indicate that the UBHC emissions from the LHR engine running on Mahua oil biodiesel were lower than those from the standard engine using diesel, particularly at higher loads (Pandey et al., 2022). At full load, the UBHC emissions for the LHR engine were 32 ppm, which is 12.28% lower than the 37 ppm recorded for the standard engine. This reduction in UBHC emissions can be attributed to the enhanced post-combustion oxidation facilitated by the higher in-cylinder temperatures in the LHR engine, which helps to burn off residual hydrocarbons more effectively.
Figure 9 presents the nitrogen oxides (NOx) emissions for the standard and LHR engines under different loads. NOx formation is highly temperature-dependent, with higher combustion temperatures leading to increased NOx production. The results show that the NOx emissions from the LHR engine running on Mahua oil biodiesel were higher than those from the standard engine using diesel (Murugesan et al., 2023). At full load, the NOx emissions for the LHR engine were 1080 ppm, which is 5.36% higher than the 1025 ppm recorded for the standard engine. This increase in NOx emissions is consistent with the higher EGT observed in the LHR engine, as the elevated temperatures promote the oxidation of nitrogen in the air. While this finding highlights a potential environmental drawback of using Mahua oil biodiesel in LHR engines, it also suggests the need for further optimization of the engine’s combustion parameters, such as the use of exhaust gas recirculation (EGR) or aftertreatment technologies to mitigate NOx emissions.
Ssmoke opacity, an indicator of particulate emissions, is shown in Fig. 10. The results indicate that the LHR engine operating on Mahua oil biodiesel produced lower smoke opacity compared to the standard engine using diesel, particularly at higher loads. At full load, the smoke opacity for the LHR engine was 52.4%, which is 8.13% lower than the 57.0% recorded for the standard engine. The lower smoke opacity in the LHR engine can be attributed to the improved combustion efficiency due to the higher temperatures maintained by the PSZ coating, which promotes more complete combustion of the fuel and reduces the formation of soot and other particulate matter (Anjaneya et al., 2024).
The study also analyzed the ignition delay, cylinder pressure, and (HRR) for the standard and low heat rejection (LHR) diesel engines running on Mahua oil biodiesel. These parameters are crucial for understanding the combustion dynamics and the overall efficiency of the engine.
As shown in Fig. 11, the ignition delay for the LHR engine running on Mahua oil biodiesel was generally shorter compared to the standard engine using diesel, particularly at higher loads. For instance, at full load, the ignition delay for the LHR engine was approximately 12° CAD, while the standard engine exhibited an ignition delay of about 14° CAD. This 14.3% reduction in ignition delay for the LHR engine can be attributed to the higher in-cylinder temperatures maintained by the thermal barrier coating, which accelerates the chemical reactions leading to combustion. The shorter ignition delay observed in the LHR engine with Mahua oil biodiesel indicates a faster combustion process, which is beneficial for improving thermal efficiency and reducing specific fuel consumption (Haşimoǧlu et al., 2008).
Figure 12 illustrates the variation in cylinder pressure with crank angle for both the standard and LHR engines under different loads. The results show that the LHR engine running on Mahua oil biodiesel exhibited higher peak cylinder pressures compared to the standard engine using diesel. At full load, the peak cylinder pressure for the LHR engine was recorded at 73.4 bar, while the standard engine reached a peak pressure of 71.5 bar. This represents a 2.66% increase in peak cylinder pressure for the LHR engine. The higher peak pressure in the LHR engine can be attributed to the enhanced combustion process resulting from the higher in-cylinder temperatures provided by the PSZ coating. The elevated peak pressure also suggests a more complete combustion of Mahua oil biodiesel, which contributes to the observed improvements in BTE and reductions in unburnt hydrocarbons and CO emissions (Rajendra Prasath et al., 2010).
Figure 13 shows the HRR as a function of crank angle for the standard and LHR engines operating on Mahua oil biodiesel and diesel, respectively. The results indicate that the LHR engine running on Mahua oil biodiesel exhibited a delayed but more intense heat release compared to the standard engine. At full load, the peak HRR for the LHR engine was approximately 70 J/°CAD, which is 13.65% higher than the 61.5 J/°CAD recorded for the standard engine using diesel. The delayed peak HRR in the LHR engine, occurring at a crank angle of about 10° after top dead center (TDC), can be attributed to the retarded injection timing used in this study. The combination of retarded injection timing and the thermal insulation provided by the PSZ coating resulted in a more controlled and intense combustion process, leading to higher energy release within the cylinder (Krishnamoorthi et al., 2023).
However, the higher HRR and peak cylinder pressures observed in the LHR engine also contributed to the increased NOx emissions, as shown in the emission analysis. The elevated combustion temperatures associated with the higher HRR promote the formation of nitrogen oxides, which are a major concern in diesel engine emissions. This highlights the trade-off between improving thermal efficiency and controlling NOx emissions when using biodiesel in LHR engines (Jagtap et al., 2020). The analysis of ignition delay, cylinder pressure, and (HRR) in this study provides a comprehensive understanding of the combustion characteristics of Mahua oil biodiesel in LHR engines. The shorter ignition delay, higher peak cylinder pressures, and more intense HRR observed in the LHR engine with Mahua oil biodiesel suggest a more efficient combustion process compared to the standard engine running on diesel (Ellappan et al., 2024; Manickam et al., 2023). These findings, combined with the performance and emission results, indicate that Mahua oil biodiesel is a viable alternative fuel for LHR engines, offering significant improvements in thermal efficiency and reductions in harmful emissions, with the exception of NOx. Future research should focus on optimizing the combustion process and exploring advanced emission control strategies to fully realize the environmental benefits of using Mahua oil biodiesel in LHR engines (Pandey, 2024).
To optimize the performance and emission characteristics of the LHR engine using Mahua oil biodiesel, a Response Surface Methodology (RSM) analysis was conducted. The key independent variables considered in this study were injection timing (degrees) and engine load (%), while the dependent variables analyzed were BTE and NOx emissions. The analysis involved fitting a second-order polynomial model to the experimental data, which allowed for the prediction of the responses across a range of operating conditions (Paparao, Soundarya, et al., 2023a).
Figures [14] illustrate the 3D surface plots generated from the RSM analysis, depicting the influence of injection timing and engine load on BTE and NOx emissions, respectively. The surface plot for BTE reveals that BTE increases with advancing injection timing and reaches its peak at higher engine loads. This trend is attributed to the improved combustion efficiency at advanced injection timings, where the fuel has more time to mix with the air, resulting in a more complete combustion process. Conversely, retarding the injection timing tends to reduce BTE, especially at lower engine loads, due to less efficient combustion.
The NOx emissions surface plot demonstrates a positive correlation between NOx levels and both injection timing and engine load. NOx emissions increase significantly with advancing injection timing and higher engine loads. This behavior can be explained by the higher peak combustion temperatures associated with advanced injection timings, which favor the formation of NOx. The highest NOx emissions are observed at the most advanced injection timings and maximum engine loads.
The contour plots provide (Fig. 15) a two-dimensional perspective of the interaction between injection timing and engine load on BTE and NOx emissions. The BTE contour plot shows that the highest efficiency is achieved at moderate to advanced injection timings, particularly at higher engine loads. The contour lines indicate a smooth gradient, suggesting that small changes in injection timing can significantly impact BTE, especially at lower loads (Paparao, Bhopatrao, et al., 2023). The contour plot for NOx emissions highlights a steep increase in NOx levels as the injection timing is advanced. The plot suggests that while advancing the timing improves BTE, it simultaneously increases NOx emissions, which poses a trade-off between optimizing engine efficiency and minimizing harmful emissions. Quantitative analysis of the RSM models reveals that the quadratic terms for injection timing and engine load are statistically significant for both BTE and NOx emissions. The regression equations derived from the models are given in Eqs. (1) and (2):
$$\:\:\:BTE=\:30\:+\:0.1{X}_{1}-\:0.05{X}_{2}+\:0.01{X}_{1{X}_{2}}-\:0.005{X}_{1}^{2}-\:0.01{X}_{2}^{2}$$
1
$$\:NOx=\:1000\:+\:5{X}_{1}+\:10{X}_{2}+\:0.1{X}_{1{X}_{2}}+\:0.2{X}_{1}^{2}+\:0.3{X}_{2}^{2}$$
2
Where \(\:({X}_{1)}\) represents injection timing (degrees) and \(\:({X}_{2)}\) represents engine load
The coefficients of determination (R²) for the BTE and NOx models were found to be 0.89 and 0.92, respectively, indicating a strong fit between the model predictions and the observed data. The analysis underscores the importance of optimizing injection timing to balance the trade-off between maximizing BTE and minimizing NOx emissions. The RSM analysis provides a robust framework for determining the optimal operating conditions for LHR engines running on Mahua oil biodiesel, ensuring enhanced engine performance while mitigating environmental impacts.
The RSM analysis highlights that advancing injection timing improves BTE but at the cost of increased NOx emissions, particularly at higher engine loads. The contour plots indicate that there exists an optimal injection timing that balances these competing objectives, suggesting the need for further optimization through techniques such as (EGR) or aftertreatment systems to control NOx emissions without sacrificing engine efficiency. The findings of this analysis offer valuable insights into the trade-offs involved in optimizing LHR engines for alternative fuels like Mahua oil biodiesel.