The effect of direct water injection on a diesel-gasoline reactivity controlled compression ignition engine

Abstract

Reactivity Controlled Compression Ignition (RCCI) combustion is a promising method to achieve ultra-low nitrogen oxide and soot emissions. However, the main problem of this strategy is the limited operating range, which is mainly caused by high pressure rise rate. In this study, the possibility of using direct water injection as an approach to decrease pressure rise rate is appraised. To that end, a Lagrangian-Eulerian approach is used to simulate a gasoline-diesel RCCI engine with the use of OpenFOAM. The effects of water injection timing and the mass ratio of injected water to diesel fuel on the engine performance are investigated. The study also includes the injection of water into a Homogenous Charge Compression Ignition (HCCI) engine to compare the effect of direct water injection on RCCI and HCCI methods. According to the results, the optimal case with the water injection timing of −10°ATDC and mass ratio of 3 achieves a 29% reduction in maximum pressure rise rate at the cost of a 1.4% decrease in engine power and a 1% increase in overall emissions. Sensitivity analysis shows that direct water injection has the most impact on maximum pressure rise rate and the lowest impact on gross indicated efficiency. A trade-off between maximum pressure rise rate and emissions can be achieved by adjusting spray angle or water injection timing in the RCCI concept. However, in the HCCI strategy, direct water injection invariably results in significantly increasing overall emissions regardless of the spray angle and water injection timing.