Descriptive statistical analysis of cyclic combustion variability and performance metrics in a hydrogen-enriched CNG spark-ignition engine at low speed

Carbon emissions from internal combustion engines are a global concern, with cyclic variation posing a major challenge in gas engines, particularly under diluted conditions and low engine speed. High cycle-to-cycle variations lead to irregular torque delivery, compromising efficiency and smoothness. Hydrogen enrichment and advanced spark timing offer promising solutions for reducing cyclic variation and lowering the carbon-to-hydrogen (C/H) ratio. Experiments were performed on a heavy-duty spark ignition engine operating at 900 RPM under varying conditions: hydrogen content in CNG fuel (0–50 %), EGR rates (2.5–11.2 %), ignition timings (4–40° CA bTDC), and load levels (30–70 %). Key performance metrics such as brake thermal efficiency, torque, emissions (NOx, CO2, CO, THC), and combustion pressure were analyzed. Statistical evaluations of 250 combustion cycles were conducted using ANOVA for parameters including indicated mean effective pressure (IMEP), peak pressure, coefficient of variation (COV), and combustion duration (MFB 0–10 % and 0–90 %). Metrics considered include the mean, standard deviation, minimum, maximum, and 25th and 75th percentiles. Results showed that at 30 % engine load without dilution, each 10 % increment in hydrogen increased brake thermal efficiency by 1.88 % while, it decreased CO2 emissions and COV by 6.45 % and 6.40 %, respectively. Moreover, the dispersion range of IMEP dropped by 9.55 % at 30 % load and 8.48 % at 70 % load with each 10 % hydrogen addition. These findings highlight the significance of hydrogen enrichment and ignition optimization in enhancing performance and reducing cyclic variations in heavy-duty spark ignition engines at low speed.