Experimental Study of Influence on Microwave Plasma Ignition Combustion Performance of Pulse Microwave Signals

It has been demonstrated by a large number of studies that microwave ignition can greatly extend the lean-burn limit in the internal combustion engines. This paper investigates the influence of the pulse period and duty cycle of pulsed microwave signals on the combustion performance of CH4-air mixtures during microwave ignition in a constant-volume combustion chamber for a lean-burn equivalence ratio $\varphi =0.6$ and the same energy input as for conventional spark ignition. Combustion performance was evaluated with respect to combustion pressure, lean-burn limit, OH concentration, initial flame kernel, and composition of combustion emissions. The results indicate that the duty cycle (i.e. average power) strongly influences the formation and development of the initial microwave discharge, while the pulsewidth influences the generation of non-equilibrium plasma which contributes to sustained combustion. The increasing the duty cycle can improve the ignition success rate, extend the lean-burn limit, and reduce the flame development time. Moderately increasing pulsewidth can further extend the lean-burn limit, particularly for elevated initial pressures.