Study of water direct injection on knock control and combustion process of a high compression ratio GDI engine

To improve the engine thermal efficiency and reduce emissions, a single-cylinder gasoline direct injection (GDI) engine was installed with a water direct injection (WDI) system in the current research. The effects of water injection ratio (WIR) and water injection timing (WIT) on engine combustion, knock intensity, and emissions were studied. The maximum braking torques (MBT), and the corresponding spark timings (ST) with different WDI strategies were investigated. The results showed that the WDI could effectively inhibit engine knock through increased WIR or delayed WIT, while the indicated thermal efficiency (ITE) was reduced. With increasing WIR, the decreased in-cylinder temperature led to reduced NOx emission but increased HC emission. CO emission gradually decreased but increased again when WIR was over 40%. With WIR of 50% at test basic ST and MBT, NOx emission reduced by 48% and 18.8%, HC emission increased by 7.3% and 36.1%​​, but the ITE decreased by 1.3% and increased by 3% respectively. With the delay of WIT at test basic ST conditions, ITE and the emissions of NOx, CO, and HC decreased, CA50 gradually retarded, the exhaust temperature steadily increased. IMEP increased by 9.5%, and ITE increased by 3.5% at WIT of −60°CA ATDC when ST was optimized to MBT. NOx emission of basic ST and MBT decreased by 34.4% and 11.5% compared with the basic conditions, respectively. The optimized trade-off between ITE and emission was achieved with the WIT of −100°CA ATDC and WIR of 50%. The ITE and IMEP under WDI conditions can be further improved at the sacrifice of emissions with the optimized earlier combustion phase, but the increased range depended on knock suppression. The correlation between WIR and knock was more significant than that of WIT, indicating more robust knock suppression.