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How to Improve SI Engine Performances by Means of Supercritical Water Injection
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on October 7, 2019 by SAE International in United States
The efficiency of ICEs is strongly affected by the heat losses of exhaust gases and engine cooling system, which account for about 60% of the heat released by combustion. Several technologies were developed to recover waste heat in ICEs, from turbochargers to ORCs, Stirling cycles and piezoelectric generation. A promising approach is to transfer the waste heat to a fluid, like water, and inject it into the combustion chamber. In such a way, the recovered energy is partially converted into mechanical work, by improving both engine efficiency and performance. In this work, the engine benefits obtained by using supercritical water as the vector to recover heat losses are analysed. Water has been chosen since it has a relatively high heat capacity and can be extracted directly from exhaust gases. A quasi-dimensional model has been implemented to simulate the ICE work cycle. Specifically, in this paper a spark ignition ICE, four-stroke with port fuel injection (PFI) has been considered. The model accounts for gas species properties (Janaf tables and CoolProp libraries) and includes valves opening/closing laws, a wall heat transfer model (based on Annand-type correlation), a water injection model and a combustion model (based on the Weibe function). 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 SOI closer to TDC and shorter WID provide higher engine performance. As an example, with W/F ratio equal to 7 and water injected at 220 bar and 700 K, an increase of engine work up to 15% and 24% is obtained with a compression ratio equal to 10 and 14, respectively.