Study of the engine configuration effect on the maximum achievable load in CAI using water injection

The approach of this research is to enlarge the knowledge about the methodologies to increase the maximum achievable load degree in the context of gasoline CAI engines. This work is the continuation of a previous work related to the study of the water injection effect on combustion, where this strategy was approached. The operating strategies to introduce the water and the interconnected settings were deeply analyzed in order to optimize combustion and to evaluate its potential to increase the maximum load degree when operating in CAI. During these initial tests, the engine was configured to enhance the mixture autoignition. The compression ratio was high compared to a standard gasoline engine, and suitable fuel injection strategies were selected based on previous studies from the authors to maximize the reactivity of the mixture, and get a stable CAI operation. Once water injection proved to provide encouraging results, the next step dealt in this work, was to go deeper and explore its effects when the engine configuration is more similar to a conventional gasoline engine, trying to get CAI combustion closer to production engines. This means, mainly, lower compression ratios and different fuel injection strategies, which hinders CAI operation. Finally, since all the previous works were performed at constant engine speed, the engine speed was also modified in order to see the applicability of the defined strategies to operate under CAI conditions at other operating conditions. The results obtained show that all these modifications are compatible with CAI operation: the required compression ratio can be reduced, in some cases the injection strategies can be simplified, and the increase of the engine speed leads to better conditions for CAI combustion. Thanks to the analysis of all this data, the different key parameters to manage this combustion mode are identified and shown in the paper.