Knocking Suppression by Stratified Stoichiometric Mixture With Two-Zone Homogeneity in a DISI Engine

Knocking is the main obstacle of increasing the compression ratio in order to improve the thermal efficiency of gasoline engines. This paper proposes a concept of a stratified stoichiometric mixture (SSM) with two-zone homogeneity (TZH) for suppressing knocking and validated the concept by means of a numerical simulation and an experimental study in a DISI engine. The results show that the SSM can effectively suppress knocking and the knocking intensity decreases when the zone around the spark plug is richer and the end-gas zone is leaner. The less rich zone (fuel/air equivalent ratio of ϕ ≤ 1.2) of the SSM can speed up the initial burning velocity in order to avoid a thermal efficiency decrease, while the over rich zone (ϕ > 1.2) would decrease the combustion velocity when knocking was suppressed. The SSM leads to higher CO and lower HC and NO_x emissions, which can be effectively after-treated using a three-way catalyst. The SSM can also reduce the decrease of power output compared to the method of retarding spark timing for suppressing knocking and has better fuel economy and fewer emissions than the method of enriching the mixture. The TZH can effectively alleviate combustion deterioration and soot formation due to the stratified mixture combustion. As a result, the SSM with TZH suppresses knocking, thus simultaneously lowering fuel consumption and emissions.