Effect of direct water injection during compression stroke on thermal efficiency optimization of common rail diesel engine

Abstract

The increasingly strict emissions regulations put forward higher request of NOx emissions control to diesel engines. Water addition is considered to be an effective way to reduce NOx emissions. However, most studies around the world concerned mainly about the effect of water addition on NOx reduction. The effect of water addition on diesel engine efficiency was not paid as much attention as on emissions. A series of experiments were carried out on test bench based on a two cylinder diesel engine with the modifications of adding common rail system and direct water injection system. The effects of direct water injection on diesel combustion and indicated thermal efficiency optimization were studied and the mechanism of thermal efficiency improvement due to compression stroke water injection was revealed. The influences of water injection duration and water injection timing are also studied. Results showed that, compression stroke direct water injection is beneficial to thermal efficiency. Under test conditions up to 4.08% indicated thermal efficiency enhancement is achieved. The thermal efficiency enhancement is mainly attributed to the reduction of negative compression stroke’s work due to the heat absorption of water evaporation and the increment of positive power stroke’s work due to the addition of working fluid. For small water injection duration, earlier injection timing in the compression stroke leads to higher thermal efficiency, while injection at late stage in the compression stroke is more suitable for larger water addition quantity. Thermal efficiency rises at first until reaches its peak value, and then drops with increasing water injection duration. Further increasing water injection duration deteriorates combustion and has negative impact on thermal efficiency. The highest thermal efficiency enhancement is achieved under 0.4ms water injection duration and 180°CA BTDC water injection timing condition.