Study of direct water injection on knock suppressing and engine performance of a gasoline engine

Turbocharger technology has become an essential method for the internal combustion engine to improve fuel economy. However, too high a boost pressure leads to too high a temperature in the combustion chamber at the end of the compression stroke, which can cause knocking or worsen abnormal combustion. Water injection technology plays a role in cooling the intake air and components in the cylinder, which can control the combustion process and suppress knock. This work focused on the influence of direct water injection (DWI) on knock suppression and efficiency improvement using a small turbocharged gasoline engine, investigating the fuel-saving potential of DWI and the optimal strategy of DWI for different engine operating conditions. Taking knock intensity (KI) as the evaluation index, KI decreases from 0.052 to 0.04 MPa, and knock limit spark angle (KLSA) increases with increasing water injection. This work shows that the DWI strategy plays a critical role in earlier spark timing, optimized combustion phase, and improved efficiency. Fuel consumption decreases by 11.55% with the optimal DWI strategy at an operating condition of 2000 rpm and a brake mean effective pressure (BMEP) of 1.3 MPa. Moreover, the optimal water injection timing is at -180 degrees CA under various engine loads. Due to the stronger knock tendency under higher load conditions, the knock suppression is less effective for the same amount of water, so more water injection mass is necessary for higher efficiency.