喷射策略对直喷氢气发动机燃烧与排放的影响

Effect of injection strategy on combustion and emission of direct injection hydrogen engine

为了提高缸内直喷氢气发动机的热效率,利用三维软件CONVERGE建立氢气发动机单缸仿真模型,分析了不同喷射策略对缸内混合气形成、燃烧与排放的影响。结果表明:随着喷射压力的增大,喷射持续时间缩短,喷氢持续期内湍动能增大,有利于提高混合气均匀性,但喷氢结束后缸内湍动能迅速衰减,对混合气的混合影响作用变小。喷射压力为4 MPa时,缸内形成的分层混合气促进了燃烧进行,使CA50更接近压缩上止点,热效率增大,但升高的燃烧温度使得NO_(x)排放增加。随着喷射正时延迟,混合时间缩短,均匀性呈现下降趋势,利用活塞顶部凹坑可提高混合气均匀性,上止点前100°CA时热效率提高,延迟至上止点前80°CA,均匀性过低,缸壁聚集大量浓混合气使传热损失增加,热效率降低。

In order to improve the thermal efficiency of direct injection hydrogen engine,a single cylinder simulation model of hydrogen engine was established by using three-dimensional software CONVERGE.The effects of different injection strategies on the formation,combustion and emission of in-cylinder mixture were analyzed.The results show that with the increase of injection pressure,the injection duration shortens,and the turbulent kinetic energy increases during the hydrogen injection duration,which is conducive to improving the mixture uniformity.However,after hydrogen injection,the turbulent kinetic energy in the cylinder decreases rapidly,and the effect on mixture becomes smaller.When the injection pressure is 4 MPa,the stratified mixture formed in the cylinder promotes the combustion,making CA50 closer to the compression top dead center(TDC),and the thermal efficiency increases,but the increased combustion temperature increases NO_(x) emissions.With the delay of the injection timing,the mixing time is shortened,and the uniformity shows a downward trend.The uniformity of the mixture can be improved by using the pit on the top of the piston.When 100°CA before top dead center(BTDC),the thermal efficiency is improved,but it is delayed to 80°CA BTDC,the uniformity is too low,and a large amount of concentrated mixture is gathered on the cylinder wall,which increases the heat transfer loss and reduces the thermal efficiency.