How to Improve SI Engine Performances by Means of Supercritical Water Injection

The efficiency of ICEs is strongly affected by exhaust gases and engine cooling system heat losses, which account for about 50% of the heat released by combustion. A promising approach is to transfer this exhaust heat to a fluid, like water, and inject it into the combustion chamber under supercritical conditions. In such a way, the recovered energy is partially converted into mechanical work, improving both engine efficiency and performance. A quasi-dimensional model has been implemented to simulate an SI engine with supercritical water injection. Specifically, a spark ignition ICE, four-stroke with Port Fuel Injection (PFI) has been considered. The model accounts for gas species properties, includes valves opening/closing, wall heat transfer, a water injection model and a combustion model. The influence of some injection parameters, i.e. Water/ Fuel ratio (W/F), Start Of water Injection (SOI) and Water Injection Duration (WID), on engine performances and efficiency is discussed in details. The results show that an increase of W/F ratio has the strongest impact on the performances with respect to SOI and WID, i.e. higher W/F ratio, SOI closer to TDC and shorter WID provide a higher engine performance. For instance, with W/F ratio equal to 7 and water injected at 230 bar and 700 K, an increase of engine efficiency up to 11.5% is obtained. The parametric analysis is performed by injecting water when the chemical heat release is complete, in order to avoid the interaction between injected water and combustion. Finally, the effects of spark advance on water injection efficiency gain have been investigated. By advancing the spark timing the combustion heat release is advanced, hence the supercritical water can be injected earlier. This leads to an increase of efficiency gain with earlier SOI.