Effect of Swirl Ratio and Wall Temperature on Pre-Injection Chemiluminescence During Starting of an Optical Diesel Engine

Fuel wall impingement commonly occurs in small-bore diesel engines. Particularly during engine starting, when wall temperatures are low, the evaporation rate of fuel film remaining from previous cycles plays a significant role in the autoignition process that is not fully understood. Pre-injection chemiluminescence (PIC), resulting from low-temperature oxidation of evaporating fuel film and residual gases, was measured over 3200 mu sec intervals at the end of the compression strokes, but prior to fuel injection during a series of starting sequences in an optical diesel engine. These experiments were conducted to determine the effect of this parameter on combustion phasing and were conducted at initial engine temperatures of 30, 40, 50 and 60 degrees C, at swirl ratios of 2.0 and 4.5 at 1000 RPM. PIC was determined to increase and be highly correlated with combustion phasing during initial cycles of the starting sequence. This suggests that under starting conditions, combustion phasing is determined by wall film evaporation rates and residual concentrations rather than the fuel injection event. As surface temperatures rise further, wall film evaporation accelerates, and increasing wall film fractions evaporate prior to the end of combustion. As evaporated fuel film is consumed by combustion, PIC decreases. The injection event begins to control the combustion process only when a significant portion of the delivered fuel is evaporated within the combustion period. The sensitivity of combustion phasing to PIC is observed to increase at reduced temperatures, indicating the greater influence of wall film evaporation rates on starting under reduced temperature conditions. Comparison to combustion phasing sensitivity predicted by kinetic models is consistent with the autoignition characteristics of lean mixtures. Combustion phasing was found to be more sensitive to PIC at higher swirl levels under all temperatures.