超高滚流对高热效率汽油机燃烧过程影响研究

以某自然吸气高压缩比直喷汽油机为研究对象,利用一维软件和三维软件对汽油机的工作过程进行了数值模拟,探究了超高滚流进气系统在高热效率燃烧系统中对燃烧性能的趋势性影响。研究中设计了三款不同滚流比气道和四种计算方案,对不同计算方案的瞬态滚流比、瞬态湍动能、缸内速度场、当量比分布、放热率和缸内平均压力等进行了对比分析。研究结果表明:滚流比的大小影响缸内混合气的分布和湍动能的强度,较强的滚流比可以将更多能量保留到上止点附近,提高点火时刻缸内平均湍动能,从而影响缸内燃烧,进而影响发动机的性能;带凹坑的活塞形状设计可以使湍动能在不同滚流比下都保持在相对中心位置,使火花塞处在湍动能较高区域,有利于点火之后火焰的迅速传播;在此燃烧系统中,滚流比增大到一定程度后,缸内的平均压力峰值难再提高,对性能的增益已接近极限,过高的滚流比对发动机性能的提升已无明显作用。

A Study on the Effect of Ultrahigh Tumble on the Combustion System of a High Thermal Efficiency Gasoline Engine

Numerical simulation of the gasoline engine working process is carried out with one dimensional and threedimensional software based on a natural aspiration direct injection engine with high compression ratio to analyze the trend and effect of the ultrahigh tumble intake system on combustion performance in combustion system of high thermal efficiency. Three kinds of in take port with different tumble ratio and four kinds of calculation cases are designed in the study. Comparisons of transient tumble ratio, transient turbulence energy, velocity field in cylinder, equivalence ratio, heat release rate and mean pressure in cylinder among different cases are made. Results show that the tumble ratio affects the distribution of the gas mixture in cylinder and the strength of the turbulence energy. Higher tumble ratio can help keep more energy stay near the top dead center, which can enhance the average turbulence energy in cylinder at spark time, affecting the combustion process including flame development and propagation and the engine performance. The piston with the pit can keep the turbulence energy at the center at different tumble ratios, which means that the turbulence energy around the spark plug is higher, accelerating the propagation of the flame. In the combustion system, when the tumble ratio increases to some extent, it is hard to enhance the average peak pressure in cylinder and better the engine performance. Thus, the excessive tumble ratio has little effect on engine performance optimization.