The effect of oxygenates on soot formation in rich heptane mixtures: A shock tube study

There is a strong environmental interest in blending diesel fuels with oxygenates to reduce soot emissions. To investigate the influence of oxygenates on diesel soot emissions, soot formation in fuel-rich n-heptane/oxygen mixtures with added dimethyl ether, acetone, butanal, or 3-pentanone was investigated behind reflected shock waves at pressures from 20 to 30 atm and temperatures from 1600 to 1900 K. Soot formation histories were observed by simultaneously measuring the soot-induced laser light extinction at 633 nm and the light emission by soot particles at 670 nm. Uniform reflected shock conditions over the long test times needed to form soot were achieved by using a new driver insert method to modify shock tube performance. In measurements made under these uniform conditions, the soot formation window near 1700 K was found to be narrower than previously measured. A significant reduction in the overall soot yield was found with the addition of small quantities of oxygenates as well. On a per oxygen-mass basis, butanal was found to be the most effective additive in reducing soot among the oxygenates studied, consistent with the smoke point tests of Pepiot-Desjardins et al. [Pepiot-Desjardins P, Pitsch H, Malhotra R, Kirby SR, Boehman AL. Structural group analysis for soot reduction tendency of oxygenated fuels. Combust Flame 2008;154:191–205]. It was also found that normal alkyl-group chain length in oxygenates has little impact on soot reduction.