Ignition,soot formation,and end-of-combustion transients in diesel combustion under high-EGR conditions

Abstract

The ignition, soot formation, and end of combustion transients of n-heptane and #2 diesel jets were investigated in an optically accessible constant-volume combustion vessel under high exhaust-gas recirculation (EGR) environments. A wide range of EGR levels were simulated by systematically decreasing the ambient oxygen concentration from 21 to 8 per cent, while holding other experimental conditions constant. Characteristics of the effect of EGR on the ignition transient include: development of a cool flame early after injection for all EGR levels, an increase in the premixed-burn (high-temperature combustion) ignition delay inversely proportional to ambient oxygen concentration, ([O₂]⁻¹), and lower apparent heat-release rates during the premixed-burn with increasing EGR. The timing of soot formation is strongly dependent upon EGR, and the time between ignition and the first soot formation increases with decreasing ambient oxygen concentration. Soot-forming fuel jets are shown to become soot-free at high-EGR conditions by reducing the injection duration to be less than the soot formation time, but longer than the ignition delay time (negative ignition dwell). While past studies show success in reducing soot formation when the injection duration is less than the ignition delay (positive ignition dwell), this result shows that high EGR can suppress soot formation even with negative ignition dwell, thereby permitting higher-load operation by using longer injection durations. At the end of injection, increasing EGR presents difficulties in completing combustion because of the lower ambient oxygen concentration. Despite eventually reaching the same pressure rise (i.e., combustion efficiency) more time is required for the higher EGR conditions to mix fuel with sufficient oxygen to complete combustion.