Experimental Study On Lean Combustion And Knock In Spark Ignition Methanol Engine

In recent years,as a clean alternative fuel,methanol has been widely concerned in reducing harmful exhaust emissions of vehicle engines and improving atmospheric environment.In view of the advantages of methanol,such as good antiknock performance,fast flame propagation speed and wide lean burn limit,this paper explores the factors influencing the engine working stability under lean burn condition at part load and solves the problem of rough operation of engine under heavy load,so as to improve the combustion stability and thermal efficiency of methanol engine under lean burn condition and restrain the increasing knock trend under heavy load,It has important engineering application value.Based on a prototype gasoline engine,a single cylinder test machine with spark ignition compressed air assisted inlet injection of methanol is developed.The effects of cooling water temperature,injection time,ignition advance angle and ignition energy on lean combustion characteristics of Spark Ignition Methanol Engine under part load were studied by experiments.The results show that: higher cooling water temperature can reduce the heat loss,improve the evaporation and atomization effect of methanol fuel,and form a better mixture concentration;there is an optimal injection time window,so that the injected methanol air mixture can enter the cylinder when the intake valve is in a large lift,and form a more uniform mixture after the intake and compression stroke,and the combustion phase is stable There is an optimal ignition advance angle,which makes the combustion efficiency of the engine the highest and improves the stability and power performance of the engine.With the increase of ignition energy,the number of incomplete combustion and misfire cycles is reduced,and the lean combustion stability and thermal efficiency are improved.In the boundary expansion test of lean burn,a wider range of lean burn can be achieved by coupling the optimal cooling water temperature,jet start time,ignition advance angle and high energy ignition energy,and the excess air coefficient of lean burn limit can reach 1.63.At the same time,aiming at the knock combustion of methanol engine under the condition of heavy load,this paper used the method of extracting knock characteristic frequency band based on wavelet packet analysis and the evaluation index of knock intensity based on energy entropy,and solved the problem of increasing knock trend by using lean combustion under the condition of heavy load.The results show that the knock characteristic frequency band of the spark ignition methanol engine prototype is11.25 ～ 18.75 k Hz,and the energy in this frequency band increases rapidly with the occurrence of knock.Taking the maximum value of energy entropy of different highfrequency oscillation pressure signals as the evaluation index of knock intensity,it has a good linear correlation with the knock factor KF based on Siemens VDO algorithm(the correlation coefficient is above 0.9).The experimental results show that with the increase of excess air coefficient,the proportion of knock combustion cycle gradually decreases,the trend of knock is effectively suppressed,and the problem of rough operation of methanol engine is alleviated.