Ignition Delay of Bio-Ester Fuel Droplets

Most studies have shown that biodiesel results in an increase in NOx emissions with respect to petroleum diesel. The complete mechanism behind the NOx increase from biodiesel is not completely understood but evidence suggests that it is caused by differences in both the physical properties and the chemical oxidation mechanisms between biodiesel and petroleum diesel. To date, the contribution to the biodiesel NOx increases related to differences in chemical kinetics has received very little attention. Similarly, the chemical kinetic mechanism responsible for the dramatic decreases in PM emissions from biodiesel is also poorly understood. As a first step in understanding the chemical kinetic mechanisms behind biodiesel NOx and PM processes, appropriate surrogate fuels must be identified that have similar chemical structure (and/or autoignition characteristics) to the long chain methyl esters found in biodiesel. In the present study, droplet ignition delay experiments were conducted using a variety of methyl esters and the results were compared to commercial soy methyl ester biodiesel. Specifically, fuel droplets were injected into a tube furnace containing atmospheric air at temperatures up to 1000 C. The ignition event was characterized by measurement of UV emission from hydroxyl radical (OH*) chemiluminescence. In addition to the biodiesel surrogate fuels, experiments were conducted using methanol, for which a validated, detailed kinetic mechanism exists. The experimental results for methanol were compared against a detailed, time-dependent numerical model. Droplet ignition experiments were chosen as a means to evaluate liquid biodiesel surrogate fuels because spherically symmetric droplet combustion represents the simplest two-phase, time-dependent chemically reacting flow system that can be solved numerically with detailed chemical kinetics and transport.