Optical Study on Spark Plug Gap in Extending Methane Lean Combustion Limits under High Ignition Energy Conditions

Lean combustion has the potential to achieve high thermal efficiency for internal combustion engines. However, natural gas (NG) engines often suffer from slow burning rates and large cyclic variations when adopting lean combustion. In this study, the effects of spark plug gaps (SPGs) on methane lean combustion are optically investigated under high ignition energy conditions. Synchronization measurements of in-cylinder pressure and high-speed photography are performed for combustion analysis. The results show that large SPGs with high ignition energy exhibit great improvement in engine combustion stability and power capability. Under ultra-lean conditions, a large SPG with a high ignition energy of 150–200 mJ can extend the lean limit to 1.55. Combustion images indicate that this is contributed by the enlarged initial flame kernel, which promotes early flame propagation. Besides, an empirical criterion is adopted to quantify the underlying mechanism, and the results confirm that a more stable early flame development with a faster burning rate can be obtained by a larger SPG and higher ignition energy under lean conditions. Therefore, a large SPG is an effective way to improve combustion stability and thermal efficiency for NG engines.