Experimental investigation of the performance and emissions of a dual-injection SI engine with natural gas direct injection plus gasoline port injection under lean-burn conditions

Abstract Conventional gasoline SI engines have low efficiency and poor lean burn characteristics at lower loads. The objective of this study is to improve the combustion and emission performance of gasoline SI engines under lean burn conditions through dual-injection mode with gasoline port injection and natural gas direct injection(GPI + NDI). In this work, total fuel energy is kept constant for all investigated test points. The sole gasoline mode with port injection is used as the baseline and compared with the GPI + NDI mode. The results show that the proposed GPI + NDI mode provides stable and promising performance. In particular, the GPI + NDI mode achieves stable combustion with COVIMEP lower than 1.5 under the condition of λ = 1.4 where the sole gasoline mixture cannot stably combustion, which broadens the lean burn limit of the gasoline engine at low load. Compared with pure gasoline mode, GPI + NDI mode with appropriate RCNG and direct injection timing can increase efficiency by 0.7 and 0.94 percentage points at λ = 1 and 1.2, and BSFC drops by 3.1% and 3.6%. HC continues to decrease with the increase of RCNG. CO emissions at λ = 1 decrease first and then increases with the increase of RCNG, and when λ is higher than 1.2, CO emissions reach extremely low levels. Furthermore, there is a significant effect on the limitation of the total particle number(TPN)with the increase of RCNG. When λ = 1 and 1.2, the TPN has a downward trend with the RCNG increases. Since there is enough oxygen to ensure the complete oxidation of the particles, TPN is not sensitive to changes in RCNG at λ = 1.4.