基于当量燃烧的天然气发动机燃烧室优化研究

在一台采用废气再循环(exhaust gas recirculation,EGR)策略的当量燃烧天然气发动机上开展了不同挤气比、压缩比的活塞对燃烧、热效率和排放影响的对比试验研究。结果表明:在50%负荷与中低转速75%负荷下,增大EGR率拓展了爆震边界,使得主燃烧相位(CA50)提前,指示热效率提高;而在100%负荷及高转速75%负荷下,EGR率的增大对燃烧持续期的延长作用更为明显,且CA50后移,指示热效率降低。增大压缩比和适当增大挤气比有利于增强缸内湍流运动,加快天然气火焰传播速度,使CA50更靠近上止点,热功转换效率提高,最高指示热效率提高了0.24%,NO_x和CH_4排放分别升高了2.30g/(kW·h)、0.55g/(kW·h)。进一步增大挤气比会受到爆震的限制,最佳点火时刻推迟,燃烧定容度小,燃烧持续期延长,最高指示热效率下降了0.51%,NO_x和CH_4排放分别降低了2.20g/(kW·h)、0.44g/(kW·h),CO排放升高了0.36g/(kW·h),因此挤气比存在一个优化的范围。

Optimization Study on Combustion Chamber of a Stoichiometric Combustion Natural Gas Engine

Based on a stoichiometric combustion natural gas engine with EGR strategy, a comparative experimental study on the effect of different squish ratio (SR63, SR68, SR72) and compression ratio (CR11.5, CR12) on combustion, thermal efficiency and emissions was carried out. The result shows that at 50% load and 75% load of low and medium speed, the increase of the EGR rate expands the knock boundary, advances main combustion phase(CA50), and increases the indicate thermal efficiency. At 100% load and 75% load of high speed, the increase of the EGR rate has more obvious effect on prolonging combustion duration, moves CA50 backward, and decreases the indicate thermal efficiency. Increasing the compression ratio and appropriately increasing the squish ratio are beneficial to enhance the turbulent motion in the cylinder, accelerate the gas flame propagation speed, make CA50 closer to the top dead center, and the heat conversion efficiency is high. The highest indicate thermal efficiency increased by 0.24%. NOx and CH4 emission is increased by 2.3 g/(kW·h), 0.55 g/(kW·h) respectively. Further increasing the squish ratio will be limited by knock. Maximum brake torque timing (MBT) is postponed, combustion constant volume is small, combustion duration is extended, the highest indicate thermal efficiency decreased by 0.51%. NOx and CH4 emission is reduced by 2.2 g/(kW·h), 0.44 g/(kW·h) respectively, CO emission is increased by 0.36 g/(kW·h). So there is an optimized range for the extrusion ratio.