Comparison of 2-ethylhexyl nitrate and fuel composition induced changes in the diesel spray ignition process

Abstract

The autoignition processes of three fuels under quiescent, direct injection, heavy-duty diesel engine conditions were investigated and compared. Two of the fuels were blended from three single component fuels to have cetane numbers (CN) of 45 and 55. The third fuel was made by adding 2-ethylhexyl nitrate to the CN-45 fuel blend to give a second CN-55 fuel. The results indicate that cool-flame, first-stage ignition chemistry induces a significant temperature rise in the igniting region of the spray. For longer ignition delay conditions (i.e. cold-start and light-load), this thermal boost (up to 300 K), coupled with hot air entrainment, is critical for driving the ignition process into the high-temperature chemistry, second stage of ignition. Increases in CN cause an earlier start of the first stage and thus an earlier thermal boost and faster ignition. For shorter ignition delay conditions (i.e. high-load), the ignition chemistry develops very fast and the ignition period becomes dominated by physical processes. Consequently, differences in ignition due to CN become less significant at higher load conditions. The results also show that the two fuels with a CN of 55 have different ignition processes, indicating that fuels with the same CN do not necessarily follow the same path to ignition. The first stage of ignition for the CN-55 fuel containing 2-ethylhexyl nitrate mirrored that of the parent CN-45 fuel, except that it was shifted earlier in time. The 2-ethylhexyl nitrate results support previous observations indicating that 2-ethylhexyl nitrate provides radicals early in the ignition process, but does not change the basic ignition chemistry of the parent fuel.