Potential of water direct injection in a CAI/HCCI gasoline engine to extend the operating range towards higher loads

CAI (Controlled AutoIgnition) systems, also named HCCI (Homogeneous Charge Compression Ignition), are a promising way to improve gasoline engines. This combustion mode is more efficient than the standard SI (Spark Ignition) combustion and, additionally, it has very low emissions, especially NOx emissions, which represent a source of problems nowadays. The main problem of this combustion mode is the constrained operating range, caused, on the one hand, by the difficulty to ignite the fuel since it has to be auto-ignited by the control of the mixture reactivity, and, on the other hand, by its high heat release rates, causing high pressure gradients and, in some circumstances, knocking combustion. In this paper, the possibility to use directly injected water into the combustion chamber as a reactivity suppressor in order to extend the constrained load range of CAI operation is evaluated. For this study, a four-stroke single-cylinder gasoline engine has been modified to allow CAI combustion by means of adapted valve trains enabling to keep hot residual gases inside the cylinder, which will provoke the fuel autoignition. Additionally, a water direct injection system has been installed in the engine to carry out this study. The results show that water injection is an efficient strategy to increase the maximum affordable load in CAI conditions, since the reactivity of the mixture can be suitably controlled, thus reducing the pressure gradients and the knocking tendency of the combustion process, also keeping good levels of combustion stability. Nevertheless, the engine has to be significantly boosted and the necessary intake pressure compared to a conventional SI operation mode in stoichiometric conditions is much higher.