The literature study on PPC in optical engine reveals investigations on OH chemiluminescence and combustion stratification. So far, mostly PRF fuel is studied and it is worthwhile to examine the effect of fuel properties on PPC. Therefore, in this work, fuel having different octane rating and physical properties are selected and PPC is studied in an optical engine. The fuels considered in this study are diesel, heavy naphtha, light naphtha and their corresponding surrogates such as heptane, PRF50 and PRF65 respectively. Without EGR (Intake O2 = 21%), these fuels are tested at an engine speed of 1200 rpm, fuel injection pressure of 800 bar and pressure at TDC = 35 bar. SOI is changed from late to early fuel injection timings to study PPC and the shift in combustion regime from CI to PPC is explored for all fuels. An increased understanding on the effect of fuel octane number, physical properties and chemical composition on combustion and emission formation is obtained. High-speed images of the combustion process are analyzed for each and every fuel and in-cylinder phenomenon is associated with rate of heat release and in-cylinder pressure. Based on the intensity of the images, stratification analysis is performed.
The results of the analysis show that CA50 decreases for all fuels from late to early SOI wherein PPC is realized. According to the reactivity of fuels, intake air temperature is increased to comply with the combustion phasing of baseline diesel. When studying the effect of physical properties of fuels, premixed effect and lean combustion are observed for PRF0 compared to diesel. The engine emissions of THC and CO are higher for PRF0 than diesel, while soot concentration is reduced. Diesel showed more stratified combustion than PRF0 despite having same RON due to the effect of physical properties. The effect of fuel octane number on PPC is suppressed due to temperature effect; intake air temperature is increased to 140°C and 90°C for PRF65 and PRF50. PRF0 lacked LTR phase and combustion was noted to be more premixed than PRF50 and PRF65 at SOI = -10 CAD (aTDC). The intensity of the combustion images is brighter for high RON fuels than PRF0 due to physical effects, while octane number effect is not realized due to higher intake air temperature. While THC and CO emissions decreased with the increase in RON, NOX emission increased due to increased intake air temperature. When comparing real fuels, soot concentration is lower for light naphtha when compared to diesel and heavy naphtha.
