Study of water injection on suppressing knock in a high compression ratio and supercharged hybrid gasoline engine

Abstract

This study investigated the effects of a direct water injection from both macroscopic characteristics and chemical kinetics aspects in high compression ratio engines, specifically regarding knocking concerns. On the one hand, −80 °CA is thought to be the ideal water injection timing (WIT) in terms of lowering mixture temperature, knock suppression, and water droplet dispersion. The mixture temperature is lowered to below 760 K by −80 °CA of WIT. When ignition timing (IT) is maintained at 2 oCA or −3 oCA, respectively, the water-fuel ratio (WFR) of 0.4 and 0.6 is the threshold for the impact of water droplet mass on combustion. The knock intensity decreases with increasing water-fuel ratio, and the pressure rise rate threshold of 0.6 bar/°CA is a knock-occurrence criterion. On the other hand, spontaneous combustion of end mixtures first occurs beneath the exhaust valve, transporting burned gases and raising temperatures. The IC4H7 concentration is higher in the mid-to-high temperature areas near the cylinder liner. Elevated concentrations of CH2O indicate high-temperature reactions, while increased levels of CH3 and HCO signify auto-ignition onset. Whether the IT is 2 oCA or −3 oCA, both IC4H7 concentration and OH concentration observe a declining trend with the increase of WFR.