Propulsive effect of microwave-induced plasma jet on spark ignition of CO2-diluted CH4-air mixture

To explore the application potential of microwave-assisted spark ignition (MAI) under exhaust gas recirculation conditions, the ignition performance of MAI under CO2 dilution is tested with CH4-air premixed spherical flames. Experiments are carried out in a constant volume combustion chamber with CO2 dilution ratio of 0–20% and equivalence ratio varied from 0.5–1.0. The microwave pulse repetition frequency (PRF) and trigger delay are altered to evaluate the interaction time window between microwave and flame kernel according to the occurrence of bright spot. Results show that MAI extended the lean limit from 0.7 to 0.6 when the mixture is diluted with 8% CO2. At equivalence ratio 1.0, the CO2 dilution limit is extended from 8% in SI mode to 20% in MAI mode. Delaying the microwave trigger beyond 1 ms, the bright spot no longer occurs implying that the time window is about 1 ms. The time window is extended to 2 ms with 1 kHz microwave triggered synchronously with spark, and increasing the microwave PRF to 10 kHz prolongs the time window to 3 ms. However, as CO2 added, the time window is obviously shortened and no more than 1 ms, indicating that CO2 dilution accelerates the decay of electron density in the flame kernel, so that the enhancement is mainly reflected in the first microwave pulse with 1 kHz PRF. The single microwave pulse of 1 kHz induces stronger plasma jet propelling the flame front, and its direction directly decides the ignition success rate near ignitable limit. On the whole, under high ratio of CO2 dilution, increasing the intensity and controlling the direction of microwave-induced plasma jet is beneficial to MAI performance.