An experimental study of flame kernel evolution in lean and diluted methane-air mixtures at engine-like conditions

An experimental study was performed to investigate the flame kernel propagation and combustion characteristics of methane (CH4)-air mixture at spark-ignition (SI) engine-like conditions. Tests were performed in a constant-volume combustion vessel (CV) with a product engine spark plug mounted at the top to simulate the engine combustion chamber. In this work, the equivalence ratio of 0.7 for lean methane-air mixture was used as test fuel, and different levels of inert gas were introduced to simulate exhaust gas recirculation (EGR) as in actual engine operations. The visualization of flame kernel formation and flame propagation was performed with a schlieren optical system and flame propagation speed was measured and analyzed accordingly. Global combustion characteristics are acquired and analyzed by utilizing the pressure history of combustion events recorded by high speed and fast response pressure measurements. The flame kernel expansion rates have been reported with quiescent and turbulent conditions. Global combustion characteristics, such as heat release rate, cumulative heat release, and mass fraction burned (MFB) as functions of time, are also reported. The suppression effect of pressure on flame propagation showed the flame slowdown while turbulence played a significant role in changing the combustion mode of the flame kernel and shortened combustion duration. These sets of experimental work and measurements are expected to be useful for development and validation of computational fluid dynamic (CFD) codes that simulate combustion processes for combustion ignition processes inside SI engines