Research On Performance And Strengthening Regime Of Microwave-assisted Spark Ignition

Microwave-Assisted spark Ignition(MAI)is one novel ignition technology which is of great potential to improve the ignition performance of engines under lean or exhaust gas recirculation conditions.Previous researches based on engine platform showed that MAI can obviously improve the serious cycle to cycle variations of engine under lean conditions,and was effective in extending the lean limit of engine.Researches based on constant volume combustion chamber(CVCC)showd that the addition of microwave accelerated the early flame development and thus enhanced the ignition.However,the enhancement of microwave was weakended as ambient pressure increased,and it is a critical issue that greatly restricted the practical application of MAI.In addition,an integrated spark plug was adopted in most previous studies on MAI,which is actually not conducive to the observation of early flame development,causing that the research in MAI regime,such as the regime for spark plasma absorbing microwave energy,is still not clear enough,and the strategy in improving MAI performance is still lacking.To solve the problems above,present study has designed and built a visualization platform for MAI at first,and then set the ignition electrodes as pin-pin configuration using the self-designed microwave antenna,so that the developing characteristic of spherical flame can be used to mirror the enhancement of microwave.Based on the testing platform,the MAI tests are widly conducted under different ambient conditions,and the effect of microwave peak powe and pulse repetition frequency(PRF)on MAI performance are also explored.The coupling regime between microwave spark plasma during MAI is then deeply discussed by combining the observations and measurements.Tests fuled with methane and acetylene show that MAI can accelerate the flame kernel development under lean or rich conditions.However,the enhancement of MAI weakends as the equivalence ratio of mixture approaches to stoichiometric ratio.A main reason for the phenomenon may be that flame speed increases so that the flame front leaves the region of strongest microwave field too fast,causing that there is no enough time for flame absorbing microwave energy.In addition,microwave enhancement is more obvious under low ambient pressures,and it decreases with the increases in ambient pressure.In this situation,increasing the microwave peak powe and PRF is beneficial to improving the efficiency for spark absorbing the microwave energy,and thus is helpful to strengthen the MAI performance.Generally,the microwave mainly influences the early flame development and once the flame reaches its self-sustained stage,the energy released from combustion will be the dominant.MAI tests under CO2 dilution show that the contact between of microwave induced bright spot or plasma jet and flame front can transmit active particles,heat and momentum to front and accelerate the local flame development.Once the plasma or bright spot is out of contact with the flame front,the microwave effect will be reduced.The test with high dilution ratio also shows that the direction of microwave-induced plasma jet can affect the ignition success rate of the mixture.MAI tests with Ar/N2/CO2 as carry gas show that there is a time window for the interaction between microwave and spark plasma during MAI.During the time window,the spark can absorb microwave energy and a bright spot or plasma jet occur.However,once the time window is ended,the microwave energy is no longer absorbed.Compared with the situation under carry gas of N2 or CO2,Ar is more conducive to enlongating the time window.The kinetics of microwave assisted spark ignition is also modelled basing on a 0-D two-temperature well mixed reactor solver.Results show that in Ar/N2 case,the addition of microwave can significantly conteract the decay of electron density,however,this effect of microwave is not obvious in CO2 case.Furthermore,the faster and more the microwave energy emitted in before the ignition timing,the more conducive to shorten the ignition delay.If same microwave energy is added in,increasing the PRF is helpful to slow the electron decay.