The Synergy Effect of Ignition Energy and Spark Plug Gap on Methane Lean Combustion with Addressing Initial Flame Formation and Cyclic Variation.

Controlling carbon emissions could be a win-win for both the environment and humans, and the use of low-carbon fuels is the key to being carbon-neutral in traffic transportation. Natural gas can achieve low carbon emissions and obtain high efficiency, but the poor lean combustion performance may result in large cycle-by-cycle variations. In this study, the synergy effect of high ignition energy and spark plug gap on methane lean combustion was optically studied under low-load and low-EGR conditions. High-speed direct photography combined with simultaneous pressure acquisition was used to analyze early flame characteristics and engine performance. The results show that high ignition energy can improve the methane engine's combustion stability, especially under high excess air coefficient conditions, and the main reason is that the initial flame formation is improved. However, the promoting effect may become marginal when the ignition energy increases above a critical value. As for the effect of spark plug gap, it varies with the ignition energy, and there exists an optimal spark plug gap for a given ignition energy. In another word, high ignition energy must combine with a large spark plug gap; thus, the promoting effect on combustion stability can be maximized and the lean limit can be extended. The statistical analysis of the flame area shows that the speed of initial flame formation is more important in determining combustion stability. As a result, a large spark plug gap (1.20 mm) can extend the lean limit to 1.4 under high ignition energy conditions. The current study shall give some insights into the spark strategies for natural gas engines.