The effect of close post injection on combustion characteristics and cooling loss reduction

In previous study, post injection has shown a significant benefit in the reduction of soot emissions. Among them, the close post injection which only has short dwell between the main injection is supposed to be able to increase the thermal efficiency. The reduction of the cooling loss due to the main flame impingement is considered as the reason to the increase of thermal efficiency. However, there is a lack of study related to the influence of post injection timing and quantity on the combustion characteristics. Therefore, in this study, the engine performance and emission characteristics of injection strategy with close post injection was investigated. The post injection timing was modified from near zero interval to around 4.5°CA from the end of main injection. In addition, the post injection quantity was modified from 3, 9, to 15 mm3/cycle and the intake pressure was modified from 170, 200, to 220 kPa while maintaining the total injection quantity at 39 mm3/cycle. In order to clarify the mechanism of cooling loss by applying post injection and the effect of injection dwell to the cooling loss and combustion characteristics, an optical engine was used to observe the combustion process. As a result, a certain dwell is needed to fully exploit the effect of cooling loss reduction by post injection.