Experimental and Numerical Study of a Methane-Fueled Pre-chamber System in Rapid Compression Machine

Pre-chamber jet ignition is a promising combustion technology to accelerate burning velocity and improve combustion stability in natural gas engines. In this study, firstly, the ignition characteristics and the dilution limits in passive pre-chamber system with different orifice diameters were experimentally investigated based on a rapid compression machine (RCM). Methane (CH4) was used as the test fuel and CO2/N-2 was used as the dilution components to simulate exhaust gas recirculation conditions in engines. Real-time combustion pressures were recorded and combustion images were captured using high-speed photography to visualize the entire jet ignition and combustion process. The experimental results showed that two different ignition behaviors were observed regarding to different orifice diameters. And no capacity of extending CO2/N-2 dilution limits in the passive pre-chamber system compared to conventional spark ignition system. Then three-dimensional numerical simulations were conducted based on the experimental results. Simulation results demonstrated two ignition modes, pre-chamber jet flame ignition and pre-chamber jet autoignition, were presented due to the different jet temperatures, concentrations of immediate species, and turbulence intensities. Lastly, based on the numerical model, the effects of pre-chamber structure parameters on the ignition modes and potentials of different dilution state of mixture in pre-chamber for extending dilution limits were numerically studied. It was found that the ratio of total effective cross-sectional area of orifices to pre-chamber volume (A(t)/V-PC) was the key structure parameter of pre-chamber systems to determined different ignition modes. And there was a narrow critical range of A(t)/V-PC to achieve pre-chamber jet autoignition (PJAI) mode, beyond which PJFI mode or ignition failure occurred. The actual dilution rate of mixture in the pre-chamber affected the ignition probability and stability in the main chamber. Improving the chemical reactivity of mixture in the pre-chamber was beneficial to effectively extend the dilution limits due to the stronger thermal, chemical, and turbulent effects of pre-chamber jet.