A numerical study on knock combustion suppression using targeted fuel injection in an SI engine

A novel concept to suppress knock combustion using the targeted fuel injection in a spark-ignition (SI) engine is proposed, which utilizes a customized injector to inject a small portion of the total fuel toward the end-gas region in the late compression stroke so that charge cooling effect by fuel vaporization is imposed on the local mixtures. CFD studies were carried out to evaluate the effects of various targeted fuel injection strategies on the processes of charge cooling, air-fuel mixing and combustion in a 1.2-L gasoline SI engine with a high compression ratio. A knock prediction model adopting a level set G-equation model coupled with a transported Livengood-Wu integral correlation was applied to predict the knock event by capturing the occurrence of end-gas auto-ignition. It was found that the local gas temperatures in the sprayed region decreased and knock combustion tendency was suppressed indeed, and the degree of the reduction in local gas temperature and knock tendency was affected by the selected targeted injection strategy. However, the results also indicated that excessive fuels injected could result in worsen air-fuel mixing that led to incomplete combustion. Hence, the injection strategy needs to be optimized by balancing the knock tendency and combustion efficiency. The study further shown that the spark timing could be advanced by 3.3° crank angle in a particular injection strategy case by comparing with the non-targeted-injection case, which demonstrated the effectiveness of the proposed concept.