Numerical analysis of the effects of port water injection in a downsized SI engine at partial and full load operation

The water injection (WI) technique represents a viable way to improve the performance of spark ignition (SI) engines. The main benefit of using this technique is to take advantage of the high heat of vaporization of water. If injected into the intake port, its vaporization cools the air–fuel mixture entering the engine. This could lead to greater spark advances, with optimal combustion phasing, and to a reduction of the combustion temperature with consequent lower heat losses. Moreover, due to the vaporization cooling effect, the heat capacity ratio of the in-cylinder gases increases. These three main factors allow improving the thermal efficiency of the engine. In this paper, the numerical simulation has been utilized in order to assess the influence of water injection on the performance of a “downsized” turbocharged spark-ignition engine. Calculations have been carried out using a 1-D model validated with respect to the experimental data of the analyzed engine. A knock model has been implemented in the 1-D model in order to identify the knock-limited parameters in engine optimization. Threshold values for knock index and exhaust turbine inlet temperature have been imposed as optimization constraints. The model has been subsequently modified by adding water injection into the intake ports. A simple thermodynamic approach has been utilized in order to obtain a first estimation of effects of water evaporation on the temperature of the fresh charge entering the cylinder and on that of the unburned mixture at the end of the compression stroke. The authors analyzed the effects of water injection at various operating points. An optimization procedure of the engine parameters in case of WI operation is presented and its effects on combustion characteristics and engine performance are described in detail. The engine control parameters have been optimized in order to obtain minimum specific fuel consumption. The most remarkable results show that, at high load operation, it is possible to increase both spark advance and air-tofuel ratio with a consequent improvement in terms of both brake mean effective pressure (BMEP) and brake specific fuel consumption (BSFC). In particular, at full load, the BSFC registered a 12% mean improvement in the engine speed range, while BMEP mean increase was about 9%. The obtained results show water injection technique could be a useful tool in performance improving of turbocharged spark-ignition engines.