非同步排气门正时对GDI汽油机缸内流场和工作性能的影响

Effects of Asynchronous Exhaust Valve Timing on In-cylinder Flow Field and Operation Performance of GDI Engine

利用三维CFD软件建立了某1.5T缸内直喷汽油机的模型,分析比较了在2 000 r/min全负荷时非同步排气门正时策略下缸内流场、燃油雾化和燃烧过程变化的规律。结果表明:采用非同步排气门正时,缸内流场与原机有明显不同。湍流动能呈现"三峰"变化,缸内的平均湍流动能均大于原机(排气门1提前20°开启时变化最大);相比原机,缸内气体的平均流速有所增加,提高了燃油蒸发速率,上止点时缸内混合气更加均匀,但也会使得缸内的残余废气质量增大。排气门1提前20°开启时,缸内的燃烧速度比原机略快,缸内最高燃烧压力增加了0.3 MPa,但温度比原机略低,因此氮氧化物的排放低于原机,CO与原机持平。排气门1延迟20°开启效果不明显。综合考虑,排气门1提前20°开启能提升原机的动力性和排放性能。

The model of a 1.5T gasoline direct injection engine was built with 3D CFD software and the laws of in-cylinder flow field,fuel atomization and combustion process were compared and analyzed at full load of 2000 r/min based on the asynchronous exhaust valve timing.The results showed that the flow field in the cylinder was different from that of the original engine when the non-synchronized exhaust valve timing strategy was adopted.The turbulent kinetic energy exhibited a characteristic of three peaks and its average value was greater than that of the original engine which had the greatest change when the exhaust valve 1 opened at the advance angle of 20°CA.Compared with the original engine,the average flow velocity of in-cylinder mixture increased,the fuel evaporation rate improved,the in-cylinder mixture on the top dead center became more uniform,but the residual gas increased.In addition,the in-cylinder combustion accelerated and the maximum pressure increased by 0.3 MPa.The in-cylinder temperature decreased and hence NO x emission reduced,but CO emission kept the same.However,delaying the exhaust valve 1 by 20℃A would make no difference.Accordingly,advancing the exhaust valve 1 by 20℃A could improve the power and emission characteristics of the original engine.