Water Injection to Improve Direct Injection Spark Ignition Engine Efficiency 2019-01-1139

The increasing use of downsized turbocharged gasoline engines for passengers cars and the 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 retarding spark advance thereby penalizing the combustion efficiency. At high engine speeds, knock still occurs but is less problematic. However, in order to comply with thermo-mechanical properties of the turbine, excess fuel is injected to limit the exhaust gas temperature while maximizing engine power, even with cooled exhaust manifolds. This also implies a decrease of the combustion efficiency and an increase in pollutant emissions.

Water injection is one way to overcome both limitations. With its high specific heat, water may be a 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 occurrence. 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, in order to identify configurations already implemented on different gasoline engines, and to use these results to build our test program and improve our configuration. The second part 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 greater fuel consumption reduction. Both port and direct water injection allowed to eliminate the need for overfueling, while respecting the maximum exhaust gas temperature. This allowed about 13% of fuel consumption reduction. Finally, EGR and water were compared and an in depth analysis carried out using the split of losses method. The results clearly show that the levers for fuel consumption reduction are different between these two diluents.