Investigation of water spray evolution process of port water injection and its effect on engine performance

In this study, a 1.5L turbocharged gasoline direct injection (GDI) engine was modified by installing a port water injection (PWI) system adjacent to the intake valve to simulate the "quasi-direct" water injection. Experiments was performed at 1500 rpm wide throttle open (WOT) condition to investigate the effect of PWI on knock suppression, and 4850 rpm WOT condition to test the removal of fuel enrichment through PWI. Then, numerical simulation was conducted to investigate the water spray evolution process and subsequent influence on mixture formation. The experimental results showed that PWI could effectively suppress knock and decrease combustion temperature. Therefore, at 4850 rpm WOT condition, the engine was able to operate at a stoichiometric air/fuel ratio with moderate advancement of spark timing. The combined effect finally resulted in nearly 6% thermal efficiency improvement. At 1500 rpm WOT, 3.8% efficiency gain was achieved solely due to knock mitigation. Nitrogen oxides (NOx), soot and hydrocarbon (HC) emissions also showed a decreasing trend with the increase of water injection amount. The simulation results indicated that about 80% of total injected water collided on the inner surface of the intake port which became the major source of water vapor. The portion of water vaporized in the air is small. Sufficient time was important for intake port water film evaporation. PWI also resulted in in-cylinder wall wetting. The in-cylinder water wall wetting in 4850 rpm was sober than that at 1500 rpm due to stronger intake air motion and higher cylinder temperature. Although port water injection imposes limited impacts on the whole in-cylinder equivalent ratio, it can induce part fuel-rich zones inside the combustion chamber.