Thermodynamics of indirect water injection in internal combustion engines: Analysis of the fresh mixture cooling effect

Water injection is a well-known efficient way to improve the performance of internal combustion engines. Amazingly, most of previous studies have yet only assess this process in an experimental manner, depriving us of an understanding of its specific influence on different operating phases of the engine – density of the aspirated fresh mixture, work required by the compression stroke, and so on – but also of the possibility to predict its effects if set up on an existing engine. Thanks to a theoretical framework specifically developed, and similar to the one commonly used for the analysis of air conditioning systems, we start in this article to untangle in a theoretical manner the different consequences of water injection on internal combustion engines. This first study is specifically focused on the fresh mixture density increase, due to the vaporisation of liquid water in the intake manifold. Results show that in the best scenarios, we cannot expect to increase the amount of fuel finally aspirated into the cylinders by more than 10%. The methodology presented here can be of a precious help for the optimisation of such process if applied to existing or future engines.