Natural Gas Stable Combustion under Ultra-Lean Operating Conditions in Internal Combustion Engines

Ultra-Lean operating conditions have a great potential toward the increase of engine thermal efficiency and control of pollutant emissions in Internal Combustion Engines; however, cyclic variability and stability are still critical issues to be solved. The use of natural gas may partially solve these issues due to a more efficient mixing process, widening the flammability limits. An analysis of natural gas ultra-lean combustion process is presented in this work using local charge stratification to stabilize the process. In particular, the predictive capabilities of the numerical model are here validated up to ultra-lean (λ=2.0) mixtures. Moreover, an analysis of the ignition timing has been performed, to find the optimal trade-off between flame quenching and turbulent kinetic energy decay as produced by the jet in the vicinity of the spark location. The optimal ignition timing has been calculated in the range of 5ms.