Abstract
Gasoline compression ignition (GCI) engines achieve higher engine efficiency and lower NOx/soot emissions than diesel compression ignition engines through partially premixed charge combustion. The thesis aims to evaluate double-injection strategies for GCI combustion, their effects on efficiency/emissions, and ignition process. Two engines were used: a single-cylinder metal engine for performance/emissions testing and an optical engine sharing the same geometry. From the performance/emissions tests, it was shown that the double-injection strategy implementing early near-BDC and late near-TDC injections shows higher efficiency and lower emissions than the single-injection strategy. The GCI combustion showed high sensitivity to advanced second injection timing with advanced combustion phasing leading to increased engine efficiency, reduced smoke/uHC/CO emissions but higher noise and NOx. For fixed combustion phasing, the increased first injection proportion causes lower peak heat release rate due to increased homogeneity of the first-injection charge and thereby achieving lower smoke/NOx/noise emissions but higher uHC/CO. Regarding ignition quality, higher octane fuel at fixed mixture homogeneity showed higher power output due to increased charge premixedness of the second-injection. Given the strong influence to mixture preparation and subsequent ignition processes, visualisation of combustion luminosity and OH* chemiluminescence was performed. The results show that single-injection leads to multiple auto-ignition kernel development from which isolated flame growth occurs while for double-injection, isolated flame growth is not clearly defined due to increased mixture homogeneity. Detailed measurements using PLIF imaging of fuel, HCHO, and OH showed that single-injection leads to low-temperature reaction from bowl-wall region due to extended ignition delay. The high-temperature reaction also starts from the bowl-wall region; however, this transition involves multiple ignition kernels that progressively merge to form large high-temperature reaction zones. In comparison, double-injection shows dispersed fuel distribution, higher HCHO consumption rate and faster OH development across all reaction zones, indicating faster low- to high-temperature reaction transition. These fundamental findings explain how double-injection-based GCI combustion generates less NOx/soot emissions than single-injection while achieving better combustion stability.