An experimental and numerical study of ultra-lean combustion characteristics of methanol direct injection using various coil spark ignition strategies

Methanol is significantly emerging as a promising alternative fuel in the pursuit of carbon neutrality. This study aims to analyze the combustion characteristics of methanol in a spark-ignition (SI) engine operating under high compression ratios and ultra-lean conditions through both experimental and simulation approaches. The objective is to derive optimized combustion efficiency by employing various ignition strategies based on discharge energy. To this end, experiments were conducted using a Rapid Compression Expansion Machine (RCEM) to replicate realistic engine environments. The effects of discharge energy and spark duration across different spark coil configurations were investigated through both experimental methods and computational fluid dynamics (CFD) simulations. The experimental results showed that the use of multiple spark coils achieved an energy release rate of approximately 239 mJ/s, more than twice that of the single-coil configuration. Simulation results were in good agreement with the experimental findings. In the single-coil setup, combustion indicators such as in-cylinder pressure were significantly influenced by variations in spark duration. In contrast, the multi-coil configuration demonstrated higher peak values in several combustion characteristics, but once a certain threshold of discharge energy was exceeded, the improvements in performance became marginal. This suggests that simply increasing the spark duration is not always an effective strategy for optimizing the spark channel in methanol SI engines. Furthermore, the multi-coil ignition strategy significantly reduced the ignition delay, contributing to improved combustion stability under lean-burn conditions. As a result, the multi-coil strategy achieved an indicated thermal efficiency (ITE) approximately 10% higher than that of the single-coil approach, with the maximum efficiency observed at 30.4% in the 10-coil strategy.