缸内流动对对置活塞二冲程发动机换气效果的影响

Effect of in-cylinder flow on gas exchange efficiency of opposed-piston 2-stroke engines

换气过程中的缸内流动及废气分布对对置活塞二冲程(OP2S)发动机换气效果的影响规律目前缺乏充分的研究。采用CONVERGE建立了OP2S发动机的CFD模型并通过仿真计算研究了扫气口倾角、扫气口构型及转速通过改变扫气气流组织影响换气效果的规律。结果表明扫气口倾角对OP2S发动机缸内流动有显著且非单调的影响。扫气口倾角α≤5°时,扫气气流聚集于气缸轴线,缸壁附近存在周边废气滞留区;当扫气口倾角α>5°时,换气过程初期扫气气流在扫气口附近汇聚为环状进气涡流,在该进气涡流中心产生低压区,并导致气缸中心流动方向与外围的扫气气流方向相反,最终导致气缸轴线出现柱状的中心废气滞留区。随着扫气口倾角增大,周边滞留区逐渐减小消失,而中心滞留区出现并增强,两者的变化规律共同造成了残余废气系数的非单调变化规律。大扫气口倾角下复合式气口的扫气气流可兼顾气缸中心和外围,换气效果显著优于单层气口。相较于倒置组合气口,组合气口的缸内流动组织更为合理,扫气效率更高。

The effect of in-cylinder flow and residual distribution on gas exchange of opposed-piston 2-stroke(OP2S)engines is yet to be thoroughly studied.The effects of scavenge port angle,scavenge port structure and engine speed on scavenge efficiency by directing scavenge flow in OP2S engine were investigated by modeling and CFD simulation with CONVERGE software.Result showed that scavenge port angle had significant and non-monotonic effect on in-cylinder flow.For scavenge port angle a≤5°,scavenge flow was collected around cylinder axis and a border residual area appeared next to liner;for scavenge port angle a>5°,scavenging flow formed a circular downward flow around cylinder axis at the early stage of scavenging.A low-pressure area was induced by this circular flow at its center and drew nearby gas upwards,forming a column of central residual area along cylinder axis.As scavenge port angle increased,the border residual area declined and disappeared while central residual area came into being and got strengthened.The combined effects of both residual areas resulted in the non-monotonic trend in residual ratio.Composite scavenge port produced drastically lower residual ratio than simple port at relatively larger scavenge port angle,since uniform scavenge flow suppressed the low-pressure area,weakening central residual area.This effect was less prominent for reversed composite scavenge port,rendering it less effective than composite scavenge port.