喷油对二冲程汽油机混合燃烧模式影响的模拟

摘要

The effect of fuel injection strategies on the mixture formation and stratified flame ignition (SFI) hybrid combustion process was studied in a poppet-valve two-stroke direct injection (DI) gasoline engine at partial loads through three-dimensional numerical simulation.The results show that the fuel and temperature stratification in the cylinder are the main factors affecting auto-ignition processes.With the increase of the second injection ratio at a fixed cycle fuel injection quantity,fuel-air equivalence ratio increases around the spark plug and decreases from the cylinder center to its wall.The ignition timing advances slightly and then delays.Combustion duration shows an opposite trend.In the meantime,indicated mean effective pressure,the maximum pressure rise rate and the peak pressure in the cylinder first increase and then drop.Compared with single fuel injection,double fuel injection facilitates SFI combustion more easily.Among all the cases,the flame propagation is the highest and the auto-ignition duration is the shortest at the second injection ratio of 30％.As the second injection ratio is greater than 70％,SFI combustion is difficult to be achieved because of extra lean mixture and relative low temperature.In this case,the mixture in the cylinder is mainly burned in the form of flame propagation.%利用三维数值模拟,研究了小负荷时两次喷油策略对气门式二冲程直喷汽油机缸内混合气形成和火花点火形成的火焰诱发自燃的混合燃烧(SFI)特性的影响.结果表明:缸内燃油和温度的分层形式是影响混合气自燃特性的重要因素.在固定循环喷油量下,随着第二次喷油比例的增大,火花点火时火花塞周围燃空当量比增大,从气缸中心区到外围区的燃油浓度减小,着火时刻先略有提前然后逐步推迟,同时燃烧持续期先减小后增加,而平均指示压力、最大压力升高率和最大气缸压力均呈先增加后降低的趋势.与单次喷油相比,两次喷油更容易实现SFI燃烧.其中,第二次喷油比例为30％时,火焰传播速度最快、自燃持续期最短.第二次喷油比例大于70％时,气缸周围的偏稀混合气在火焰传播过程中难以自燃,缸内以火焰传播为主.