Thermal efficiency improvement of a spark ignition engine by stratified water distribution

•The different water spatial distribution states was distinguished experimentally.•The effects of stratified and homogeneous water states were investigated.•Experimental results shows the correlations among knock combustion, ignition delay and water concentration gradient. The stratified water distribution was introduced on a single cylinder direct injection spark ignition engine by changing the direct Water Injection Timing (WIT) and Water Fuel Ratio (WFR) to control the water distribution states before ignition. Experiments were conducted to fundamentally understand the stratified water effects on combustion process control. The results show that different water injection timing could result in wall wetting state, homogeneous state, and stratified state. Stratified water distribution has more obvious suppression effects on knock combustion than other states. Furthermore, optimized combustion process of stratified water distribution case by advancing spark timing is with higher thermal efficiency than homogeneous case. Finally, with the experiment of increasing water injection amounts at stratified state, thermal efficiency increasing was realized by 2.1 %. The high correlation between ignition delay and knocking combustion was demonstrated, which substantiate that water stratified injection has much talent on knock combustion suppressing and thermal efficiency increasing. In further research, the stratified water effects by split injection strategy is hopeful to reduce the water amounts and avoiding oil dilution.