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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.

Increasing global efficiency of direct injection spark ignition (DISI) engine is nowadays one of the main concerns in automotive research. A conventional way to reduce DISI engine fuel consumption is through downsizing. This approach is well suited to the current homologation cycle as NEDC, but has the drawback to induce over-consumptions in customer real driving usage. Moreover, the driving cycles dedicated to EURO 6d and future regulations will evolve towards higher load operating conditions with higher particulate emissions. Therefore, efficiency of current DISI has to be strongly increased, for homologation cycle and real driving conditions. This implies to deeply understand and improve injection, mixing and flame propagation processes.