Influence of Engine Speed on Gasoline Compression Ignition (GCI) Combustion in a Single-Cylinder Light-Duty Diesel Engine

The present study aims to evaluate the effects of engine speed on gasoline compression ignition (GCI) combustion implementing double injection strategies. The double injection comprises of near-BDC first injection for the formation of a premixed charge and near-TDC second injection for the combustion phasing control. The engine performance and emissions testing of GCI combustion has been conducted in a single-cylinder light-duty diesel engine equipped with a common-rail injection system and fuelled with a conventional gasoline with 91 RON. The double injection strategy was investigated for various engine speeds ranging 1200~2000 rpm and the second injection timings between 12°CA bTDC and 3°CA aTDC. From the tests, GCI combustion shows high sensitivity to the second injection timing and combustion phasing variations such that the advanced second injection causes advanced combustion phasing and extended pre-combustion mixing time, and thereby increasing engine efficiency and decreasing ISFC. This leads to the reduced smoke/uHC/CO emissions but increased combustion noise and NO_x emissions, similar to the trends in conventional diesel combustion. It is found that the increased engine speed requires a higher fuel mass per injection to maintain similar IMEP values, leading to lower efficiency, higher ISFC, and increased combustion noise. The heat release rate increases with increasing engine speed but the combustion phasing is largely unchanged. A typical smoke-NO_x trade-off is found with increased smoke and decreased NO_x emissions at higher engine speed, primarily due to reduced charge premixing time, which suggests the partially premixed charge-based GCI combustion behaves similar to conventional diesel combustion with overall lower smoke and NO_x emissions.