Effects of EGR rate and excess air ratio on the combustion characteristics of compressed natural gas engine

The influence of EGR (exhaust gas recirculation) rate and excess air ratio (λ) on the combustion and emissions of a twofold dilution compressed natural gas (CNG) engine CA6SE3-21E4N were investigated experimentally to solve the limited decreasing-ability on NO_x emissions with air dilution. The results show that as EGR rate and λ increase, the peak of cylinder pressure and heat release rate decrease and delay, the heat release centroids and the start of combustion move backward, the main combustion duration becomes longer and then the flame propagation velocity slows down. The cyclic variations intensify versus the EGR rate and λ increase. With smaller λ than that of air dilution under equivalent NO_x emission levels, the twofold dilution makes the engine operating more stably. Considering the trade-off between NO_x emissions and fuel economy, values as optimal control point can be obtained under 1750r/min, 75% load conditions, namely, λ is 1.35, EGR rate is 7%.