A review of water injection application on spark-ignition engines

The increasingly stringent emission and fuel economy regulations force the advancement of internal combustion engines towards higher power density and lower emissions. Knock and thermal limits are bottlenecks that prevent improvement of the spark ignition (SI) engines' thermal efficiency. Due to its significant knock mitigation and cooling abilities, water injection technology has great potentials in enhancing thermal efficiency of SI engines and thus, regains research attention in recent years. This paper aims to present a comprehensive review of water injection applications on SI engines. Various methods for the implementation of water injection in SI engines are introduced and compared. Methods to maximize the working efficiency of water injection and its detailed physical and chemical effects on combustion and emissions are discussed. It was found that, with different purposes, water injection can improve the combustion and performance of SI engines. Typically, better combustion phasing and thus higher thermal efficiency can be achieved especially under high load conditions. Emissions such as nitrogen oxides and soot can be largely suppressed. However, crucial issues such as water supply and wall wetting still restrict wide application of water injection technology. The detailed kinetic mechanisms of chemical effects and coupling of physical and chemical effects are needed to be investigated.