A study on supercritical water injection as waste heat recovery system in internal combustion engines

The rush towards solutions able to increase the sustainability of energy conversion systems in terms of efficiency and emissions is becoming even more significant in view of the transition to the zero-emission carbon target by 2050. In this context, waste heat recovery (WHR) is a possible solution to increase the systems global efficiency. In particular, the recovery of the high energy content from the exhaust has gained attention, especially for internal combustion engine applications. In the framework of WHR systems, a novel application of the concept is heating the water, as a working fluid, up to the supercritical condition to be injected directly into the combustion chamber to increase the cycle work. The approach includes the recovery of the needed water from the exhaust gases for closed-loop operations. To the authors' knowledge, no experimental studies have been published in the literature that investigate the feasibility of using supercritical water injection (SWI) in internal combustion engines. Specific WHR layout and SWI systems have been developed and integrated into a single-cylinder spark ignition engine. A proper experimental test campaign has been designed to assess the SWI potential on engine performance and emissions, and, as a supporting diagnostic tool, a quasi-dimensional model has been developed. The results evidence that although supercritical conditions have been reached in the heating system, the injection occurs in subcritical conditions because of the heat losses in the injector body. Direct water injection affects the combustion process in terms of heat release and charge cooling with effects on cycle work and NOx, depending on various factors such as water/fuel ratio (W/F) and injection timings. The technological limits of the integrated systems are discussed and supported by suited numerical studies, demonstrating that the theoretical advantages are not obvious and disclosing the possible solutions or strategies to improve the system performance.