Flame Speeds of Low-Cetane Fuels in a Diesel Engine

A theoretical and experimental investigation of the flame speeds of low-cetane fuels during the initial stage of heat release in a Diesel engine is described. This information is important for developing a fundamental background in the understanding of fuel injection rate-controlled heat release in a Diesel engine. In this study, a theoretical model based on droplet size, turbulent intensity, and equivalence ratio was developed for the flame propagation through a fuel droplet/air matrix. The results of the theoretical model were compared to experimental high-speed photographs of flame growth in a Diesel engine. For successful injection rate controlled heat release to occur using pilot injdection, the model determined that the combustion zone due to a pilot fuel spray must flow to a distance of at least 30 orifice diameters from the nozzle tip before the main injection event can take place. Results of the model were verified by experiment for the two limiting cases of X/D less than 30 and X/D greater than 30. As expected, rate-controlled heat release was to achieved for the case of X/D less than 30. However, for the case of X/D greater than 30, the main fuel injectin ignited upon injection into the cylinder, and heat release was controlled by rate of injection.