Water injection to improve direct injection spark ignition engine efficiency 2019-01-1139
The increasing use of downsized turbocharged gasoline engine for passengers cars and new European homologation cycles (WLTC and RDE) both impose an optimization of the whole engine map. More weight is given to mid and high loads, thus enhancing knock and overfueling limitations. At low and moderate engine speeds, knock mitigation is one of the main issues, generally addressed by decreasing spark advance and thus lowering the combustion efficiency. At high engine speeds, knock still occurs but is less pregnant. However, in order to comply with thermo-mechanical properties of the turbine, excess fuel is injected to limit the exhaust gas temperature while maximizing the power. This also implies a decrease of the combustion efficiency and an increase in pollutant emissions.
Water injection can be one way to overcome both these limitations. With its high specific heat, water might be the solution to accurately control the temperature evolution in the cylinder. At low or medium speed, it can help to cool down the working gas and prevent knock occurence. At high speed, water can replace overfueling and play a similar role but in a cheaper and cleaner way.
This study proposes to evaluate the potential of water injection to improve gasoline engine efficiency. The first part of this study is dedicated to a literature review of water injection concepts. The purpose is to identify different configurations already implemented on different gasoline engines, and to use these results to build our test program and improve our configuration. The second part of the study is focused on single-cylinder engine tests. A downsized direct injection spark ignition single-cylinder engine was used with different water injection configurations, port or direct injection. Water and EGR (Exhaust Gas Recirculation) were also compared and combined.
This study highlights the positive impact of water injection on knock mitigation and overfueling limitation. Direct water injection is the preferred configuration for knock mitigation due to a better gain on fuel consumption. Both port and direct water injection allowed to eliminate the need for overfueling, while respecting the maximum exhaust gas temperature. This allowed a gain of about 11% in terms of fuel consumption. At last, EGR and water were compared and deeply analyzed by the split of losses method. The results clearly show that the levers for fuel consumption reduction are different between these two diluents.