不同气缸首先着火起动后发动机排气特性模拟

Simulation of the Characteristics of Exhaust Gas Flow in the Exhaust System of a Compressed Natural Gas Engine Ignited Each Cylinder at the First Cycle During Start

分别利用GT-Power和FIRE软件建立了天然气发动机一维仿真模型和排气系统三维模型,并实现了二者的耦合计算.模拟结果得到了试验验证.对4缸天然气发动机起动过程中不同气缸首先着火后,排气系统内气体流动和温度分布特性进行了模拟研究.结果表明:起动阶段第1个循环未着过火的气缸在排气过程中存在废气回流到气缸的现象,并推迟了废气向排气系统排出;不同气缸首次着火后,各缸排气歧管结构的差异引起排气系统中废气的流动和温度分布的不同.优化选择首个着火气缸进行起动,可以使催化器入口排气温度场分布更均匀,其中,第2缸首先着火起动后催化器入口流速均匀性指数最高,温度分布均匀,高温气体所占截面面积的比例最大.但不同气缸首次着火起动对排气流动和温度分布的影响主要表现在前3个发动机工作循环,此后这种差异消失.

Based on GT-Power, the one-dimensional（1D） code, and FIRE, the three-dimensional（3D） code, respectively, a combined model to simulate the working processes of a four-cylinder compressed natural gas （CNG） engine and the engine exhaust system was developed. Simulation results were validated by experiments. The exhaust flow characteristics and temperature distribution in the exhaust system during engine start were numerically studied when each cylinder was ignited at the first cycle. Results show that the back flow from the exhaust pipe into the unignited cylinders occurs, which delays the exhaust discharge into the exhaust system. After the first selected cylinder is ignited, the difference in the configuration of each exhaust manifold affects the exhaust gas flow charac- teristics and temperature distribution in the exhaust pipe. The optimal selection of the first ignition cylinder during start can improve the homogeneity of exhaust temperature distribution at the inlet section of a catalytic converter. Among the four cases of the first ignition cylinder selection, the highest temperature at the inlet of the catalytic con- verter is achieved when the second cylinder of the engine is ignited first during start, conducive to the fastest light-off of the catalytic converter. However, the effect of first ignition cylinder on the distribution of exhaust flow and tem- perature only happens in the first three operating cycles and disappears soon.