不同喷射时刻下缸内直喷天然气发动机的燃烧特性

开展了天然气高压缸内直喷发动机不同喷射时刻时的燃烧特性研究。研究结果表明：燃料喷射时刻对发动机性能及排放有较大影响，喷射太迟会导致天然气和空气混合时间短，混合效果差，燃烧持续期长，放热速率慢。喷射过早会导致充量系数下降，燃料容易进入燃烧室狭缝间隙处，造成较高的HC排放。对于给定转速，发动机存在一个最佳燃料喷射提前角，此时缸内最高压力值最大，最大压力升高率和最大放热率最大，放热速率快，燃烧过程等容度好，火焰发展期、快速燃烧期和燃烧持续期短，发动机热效率高，HC、CO排放也维持较低水平。

Combustion Characteristics of a Direct Injection Natural Gas Engine under Various Fuel Injection Timings

The combustion characteristics of a direct injection natural gas engine under various fuel injection timings were investigated. The results showed that fuel injection timings had a large influence on engine performance, combustion and emissions. Over-late injection would make insufficient time for the fuel-air mixing, bringing poor quality of mixture formation and subsequently resulting in slow combustion rate and long combustion duration. In contrast to this, over-early injection would reduce the volumetric efficiency, meanwhile it would provide enough time for fuel to possibly enter into the crevices of combustion chamber and bring high HC emissions. For a given engine speed, there exists an optimum fuel injection timing where the maximum cylinder pressure, the maximum rate of pressure rise and the maximum rate of heat release get their highest values, and the shortest combustion and heat release duration can be achieved while maintaining the low level of HC and CO emissions.