Effects of Fuel Parameters and Diffusion Flame Lift-Off on Soot Formation in a Heavy-Duty DI Diesel Engine

To better understand the factors affecting soot formation in diesel engines, in-cylinder soot and diffusion flame lift-off were measured in a heavy-duty, direct-injection diesel engine. Measurements were obtained at two operating conditions using two commercial diesel fuels and a range of oxygenated paraffinic fuel blends. A line-of-sight laser extinction diagnostic was improved and employed to measure the relative soot concentration within the jet (“jet-soot”) and the rates of soot-wall deposition on the piston bowl-rim. An OH chemiluminescence imaging technique was developed to determine the location of the diffusion flame and to measure the lift-off lengths of the diffusion flame to estimate the amount of oxygen entrainment in the diesel jets.

Both the jet-soot and the rate of soot-wall deposition were found to decrease with increasing fuel oxygen-to-carbon ratio (O/C) over a wide range of O/C. Good agreement between soot-wall deposition and jet-soot measurements indicates that using the soot-wall deposition diagnostic to indirectly measure jet-soot is valid. This is especially useful when the jet-soot is too optically thick to be measured directly. OH chemiluminescence imaging indicated little differences in the flame lift-off and corresponding oxygen entrainment between fuels. Commercial diesel fuels displayed higher sooting tendencies than the zero-oxygen-content paraffinic fuel blend. Since the estimated oxygen entrainment rates were similar, differences in soot formation are most likely due to the effects of fuel chemical structure. A linear extrapolation of the soot-wall deposition data for the oxygenated, paraffinic fuels predicts that zero soot formation may be achieved when the fuel atomic O/C exceeds 0.4.