直喷汽油机反转起动次循环起动参数优化

Optimization of Starting Parameters for the Second Cycle During GDI Engine Reversed Starting Process

为了使直喷汽油机在无起动机反转直接起动时达到更好的效果,在一台壁面引导式直喷汽油机上研究了对反转成功起动具有重要影响的次循环在不同起动参数下的着火和速度特性。试验结果表明：当水温80℃、残余轨压1.8 MPa、膨胀缸活塞初始位置于上止点后120°、首循环起动边界一定时,基准过渡点前3°次循环点火具有最高的缸压峰值和转速峰值,分别为3.08 MPa和545r/min;一定范围内混合气越浓,次循环着火特性和速度特性越好,缸压峰值在次循环过量空气系数为0.5时比1.0高出32.8%,转速峰值高出19.5%;基于油束和活塞顶凹坑的匹配,喷油正时在上止点前100°左右时表现最优。改变水温、轨压和膨胀缸活塞初始位置等被动参数,最佳点火时刻仍为基准过渡点前3°,最佳过量空气系数范围一般为0.6-0.7,膨胀缸活塞初始位置在上止点后100°之前时,喷油正时选择在上止点后100°为最佳,膨胀缸活塞初始位置在上止点后100°之后时,喷油时刻越早越好。

To achieve better starting performance for GDI （gasoline direct injection） engine reversed direct-starting without starter, an experiment for ignition and speed characteristics of the second cycle affecting successful reversed direct-starting is conducted by changing starting parameters in a wall-guided GDI engine. The results show that the highest cylinder pressure of 3.08 MPa and the highest speed of 545 r/min for the second cycle are achieved at 3° CA （crank angle） before the basic transition point with water temperature 80 ℃, residual rail pressure of 1.8 MPa, expansion stroke piston position of 120° CA ATDC （after top dead center） and the same starting boundary of the first cycle. And better ignition and speed characteristics of the second cycle can be obtained with richer mixture in a specific range. For instance, the cylinder pressure peak with excess air coefficient 0. 5 gets 32.8% higher than excess air coefficient 1.0, and the speed peak 19.5% higher. The best starting performance appears when injection timing is taken as 100° CA BTDC （before top dead center） based on the effective match between spray and surface of piston bowl. The best ignition timing remains 3° CA before the basic transition point within the best range of excess air coefficient as 0.6-0.7 under different temperatures, injection pressures and initial piston positions. Specially, the best injection timing for the second cycle is 100° CA BTDC when the expansion stroke piston position is before 100° CA ATDC, and the injection timing should be as early as possible when the initial piston position is after 100° CA ATDC.