甲醇直喷及高压缩比对点燃式发动机性能的影响

Effects of Methanol Direct Injection and High Compression Ratio on the Performances of a Turbocharged Spark-Ignition Engine

将一台增压直喷米勒循环汽油机改制成高压缩比(13.8)甲醇直喷(MDI)点燃式发动机,在转速为2 750 r/min下研究了甲醇直喷和高压缩比对发动机燃烧、排放及有效热效率(BTE)的影响.结果表明:相同边界条件下,甲醇直喷在中大负荷(平均有效压力(BMEP)≥0.8 MPa)能够提高燃烧速度、缩短燃烧持续期,甲醇直喷由于低压缩温度和高燃料辛烷值,高压缩比下各负荷燃烧均不受爆震限制;甲醇直喷在各负荷下均有利于降低NO_(x)、CO及HC,相同边界条件下甲醇直喷在不同负荷的未燃损失均降低1%以上;甲醇直喷结合高压缩比的最高有效热效率工况点BMEP从原机的1.1 MPa提升到1.5 MPa,当量比燃烧有效热效率从37.4%提升到42.6%,有效热效率提升主要是因为排气损失、传热损失及未燃损失均降低,而进一步提升有效热效率主要受发动机爆发压力极限限制.

A turbocharged Miller cycle gasoline direction injection(GDI)engine was modified to a methanol direction injection(MDI)engine with high compression ratio(13.8),and experimental study was conducted to investigate the effects of MDI on combustion,emission and brake thermal efficiency(BTE)at speed of 2750r/min from low to high loads.Results show that MDI gives a shorter combustion duration at middle to high loads(BMEP≥0.8MPa).Low in-cylinder temperature caused by MDI and high octane number of methanol allows the engine operating at the optimum combustion phase without knock occurrence.Furthermore,MDI reduces NO_(x),CO and HC emissions at different loads,and the unburned fuel loss is reduced by greater than 1%at all loads under the same control parameters.The operating point with the maximum BTE shifts from BMEP=1.1MPa of the original engine with a lower compression ratio of 11.5 to BMEP=1.5MPa of the MDI engine with higher compression ratio.The maximum BTE under stoichiometric combustion also increases from 37.4%to 42.6% due to lower losses of exhaust heat,heat transfer and unburned fuel loss.The main obstacle to higher BTE is the maximum allowable in-cylinder pressure.