基于负阀重叠EGR策略的HCNG发动机仿真

为降低HCNG发动机NOx排放,采用负阀重叠EGR策略,利用AVL-Fire软件对HCNG发动机不同进气门开启角(θIVO)下的进气过程和燃烧过程进行了三维仿真计算,对比分析了采用负阀重叠前后发动机缸内EGR分布和燃烧过程。仿真结果表明:负阀重叠EGR策略下,排气门关闭角(θEVC)固定为340°曲轴转角不变,当θIVO为380°曲轴转角时,既可避免发生回火又能保证一定的进气量及充气效率;采用负阀重叠后,在压缩冲程后期,缸内EGR率呈梯度分布(靠近火花塞位置EGR率较低),更有利于着火及火焰传播;采用负阀重叠可降低缸内最高燃烧压力及最高温度,但会减少进入气缸的新鲜工质,降低发动机功率;通过负阀重叠实现内部EGR可降低NOx排放,但会导致着火困难,燃烧速度变慢;提高点火能量可缩短着火落后期和燃烧持续期,加快燃烧速度。

Simulation of HCNG Engine Based on Negative Valve Overlap EGR Strategy

In order to reduce NOx emission of hydrogen‐enriched compressed natural gas (HCNG) engine ,3D simulation of intake and combustion process under different intake valve open (IVO) timings was conducted with AVL‐Fire software based on EGR strategy of negative valve overlap (NVO) .The simulation results show that the EGR strategy can not only avoid the backfire but also guarantee a certain intake mass flow and charging efficiency when the IVO timing is 380° and the exhaust valve closure (EVC) timing is 340° .EGR concentration shows a gradient distribution of low value near the spark plug at the late of compression stroke by using NVO ,which is beneficial to ignition and flame propagation .Moreover ,the NVO can decrease the maximum in‐cylinder pressure and temperature ,but reduces the fresh working gas with the loss of engine power .In addition , NVO can realize internal EGR with NOx reduction ,but leads to difficult ignition by slowing the combustion .Ignition delay and combustion duration shortens by improving the ignition energy ,and then the combustion accelerates .