Improving the lean burn performance of a high compression ratio methanol engine by multiple-injection

Abstract

Methanol is a promising fuel for achieving carbon neutrality in sectors requiring internal combustion engines. The multiple-injection approach is effective in reducing cycle-to-cycle variations and improving the performance of methanol engines under lean burn conditions. This study proposes a concept for transforming diesel engines into methanol engines with flexible injection and lean burning capabilities. This study fills the research gap in the study of lean methanol combustion in swirl combustion chambers. The effects of 22 different multiple-injection strategies on combustion characteristics, emission performance, and combustion stability are experimentally assessed in a direct-injection spark ignition engine with an excess air ratio (λ) of 1.35. Each multiple-injection strategy is comprehensively evaluated using a merit function and compared with the baseline case that uses single injection. The results show that the double- and triple-injection strategies substantially shorten the durations of combustion and flame development. Therefore, a higher effective expansion ratio is obtained. Meanwhile, the cycle-to-cycle variation in the IMEP decreases from 4.2% of the baseline to 1.49% at T8. CO and HC emissions deteriorate; however, NOx and CO2 emissions under triple injection are lower than those under the baseline. The T7 case with triple injection achieves the highest thermal efficiency, which is 6.8% higher than that for the baseline case. Overall, triple injection achieves higher merit function values, which indicates a better capability to improve lean-burn performance than double injection.