Effects of in-cylinder water injection on knocking and combustion stability generated by expanding the lean burn limit

The integration of direct water injection (DWI) with the lean burn (LB) process can significantly enhance engine performance. This study reveals that an increase in the water-fuel (W/F) ratio suppressed knocking by extending the ignition delay time. Specifically, the knocking of stoichiometric combustion was suppressed, while that of deeper LB (excess air ratio (λ)˃1.5) was sensitive to a larger W/F ratio. The intensity of knocks during stoichiometric combustion decreased significantly (more than twofold) when the W/F ratio increased from 0.1 to 0.2. Under identical W/F conditions, a shorter ignition delay (CA0–10) and combustion duration (CA10–90) of the main combustion flame were identified as important factors for knocking when λ increased from 1 to 1.5. However, for λ values exceeding 1.5, CA10–90 emerged as the primary knocking determinant. Faster flame propagation and higher oxygen concentration caused by LB and an early spark promoted autoignition of the end-gas and increased the autoignition heat release rate, thus strengthening knock intensity (KI). Furthermore, a minimal amount of water (W/F ≤ 0.3) exerted negligible influence on the stoichiometric combustion stability. As the LB approached its limit, the average KI remained relatively consistent, with combustion stability emerging as the key factor in elevating the LB performance. A 0.3 W/F ratio and precise spark timing ensured that a balance exists between combustion stability and knocking, while simultaneously elevating λ to 2.0.