Abstract
This study shows the development of low-temperature and high-temperature reactions in a gasoline-fuelled compression ignition (GCI) engine realizing partially premixed combustion for high efficiency and low emissions. The focus is how the ignition occurs during the low- to high-temperature reaction transition and how it varies due to single- and double-injection strategies. In an optically accessible, single-cylinder small-bore diesel engine equipped with a common-rail fuel injection system, planar laser-induced fluorescence (PLIF) imaging of formaldehyde (HCHO-PLIF), hydroxyl (OH-PLIF), and fuel (fuel-PLIF) has been performed. This was complemented with high-speed imaging of combustion luminosity and chemiluminescence imaging of cool flame and OW. The diagnostics were performed for two different fuels including conventional diesel as a reference case and then a kerosene-based jet fuel which is a low-ignition quality fuel with cetane number of 30, firstly with single near top dead center (TDC) injection and then a double-injection strategy implementing very early injection and late injection in the same engine. For diesel combustion, it is shown that the cool-flame and HCHO signals appear from the jet axis before spreading downstream towards the bowl wall. The OH radicals present in the high-temperature reaction zones also show a similar development pattern with distinctive reaction zones forming from the jet axis and then near the bowl wall for each nozzle hole. When the reactions occur near the bowl wall, the HCHO and OH radicals coexist. Later, the high-reaction zones merge with each other due to jet-wall and jet-jet interactions. In comparison, the single-injection GO combustion shows HCHO signals appearing from the bowl-wall region due to extended ignition delay. The OH radicals develop out of this HCHO region and show a more sequential development pattern than diesel combustion. The single-injection GCI also involves multiple ignition kernels that progressively merge to form larger reaction zones. The double-injection GCI combustion has higher charge premixing than the other cases, and due to very early first injection, the mixture homogeneity is also much higher. This is evidenced by a higher consumption rate of HCHO and faster development of OH across the entire reaction zones, indicating faster low- to high-temperature reaction transition. These fundamental findings explain why GCI combustion generates less soot and NO than diesel combustion as well as how double-injection GCI combustion achieves better low-load stability than the single-injection.