Effects of late injection on lean combustion characteristics of methane in a high compression ratio optical engine

Energy crisis and environment pollution have made engines fueled with natural gas getting more and more attention, but practical engines fueled with natural gas often suffer from low engine power output and thermal efficiency. In this study, with addressing the role of late high-pressure injection, engine performance and flame propagation were studied in a high compression-ratio optical engine fueled with methane under lean combustion condition. First, the experiments with different excess air coefficients under two representative injection timing conditions were performed. The results show that with the increase of excess air coefficient, the indicated mean effective pressure (IMEP) is decreased. When the excess air coefficient is high, the combustion even becomes unstable under early injection conditions, which is due to the low flame speed. Second, different injection timings were employed under lean condition, and the results show that the combustion process is significantly affected by injection timing. Advanced combustion phase and higher heat release rate are observed under late injection condition. Finally, further analysis of flame images and pressure shows that late injection can increase the air intake quantity but also in-cylinder turbulence intensity. The highest flame speed is obtained under the latest injection condition with an increment above 50%, and flame propagation direction is also changed under different injection timings. Besides the compression pressure before spark is higher for late injection, meaning high volumetric efficiency. Therefore, late injection with high pressure is an effective way to improve the power output and thermal efficiency of natural gas engine.