Simulation Study on the Effect of In-Cylinder Water Injection Mass on Engine Combustion and Emissions Characteristics

The rapid development of the automobile industry has brought energy and environmental issues that scholars are increasingly concerning about. Improving efficiency and reducing emissions are currently two hot topics in the internal combustion engine industry. Direct water injection technology (DWI) can effectively reduce the cylinder temperature, which is due to the absorption of the heat by the injecting liquid water. In addition, lower temperature in the cylinder will reduce the formation of NO. In this paper, a CFD simulation of DWI application in a lean-burning single-cylinder engine with pre-chamber jet ignition was carried out. And the engine was experimentally tested for the simulation model validation. And then the effect of DWI strategy with different injecting water mass on the combustion and emissions characteristics are analyzed. Physically, injected water not only absorbs heat but also provides heat insulation. The results are shown under the rotating speed of 2800 r/min and IMEP of 11.5 bar, without changing the ignition advance angle. A small amount of water, with almost constant cylinder temperature, dissociates into more OH and H, which may promote combustion and improve the thermal efficiency combined with heat insulation effect. An appropriate amount of water plays a dominant role in heat absorption, resulting in decrease of temperature in-cylinder. NO emission will significantly reduce under the water mass is 5mg and the injection timing is -150°CA ATDC. The ITE decreases by 3.3% while the NO emission decreases by about 80%. Excessive water significantly reduces cylinder temperature, leading to unstable combustion and even flameout. It may cause insufficient combustion and reduce the ITE of the engine.