Effects of the In-Cylinder Environment on Diffusion Flame Lift-Off in a DI Diesel Engine

Abstract

The diffusion flame lift-off length of isolated, free diesel jets in quiescent atmospheres is known to have a strong influence on soot formation by affecting fuel/air mixing prior to combustion. In realistic engine environments, the proximity and temperature of in-cylinder surfaces, in-cylinder gas flows (swirl), and interactions between adjacent jets may affect the behavior of the flame lift-off, and thereby affect soot formation. To better understand the influence of these factors on the lift-off length and on soot formation, optical imaging diagnostics were employed to measure the flame lift-off length in an optically-accessible heavy-duty Direct Injection (DI) diesel engine. A two-camera OH chemiluminescence diagnostic was developed and employed to measure the flame lift-off length for a range of injector nozzle geometries and engine operating conditions. A two-camera OH Planar Laser-Induced Fluorescence (OH-PLIF) diagnostic for side-on illumination of the jet,was also developed and utilized to aid in the interpretation of the OH chemiluminescence data. The diesel flame lift-off lengths in the optical engine were shorter than those observed previously for single, isolated jets in a quiescent atmosphere. Also, high cycle-to-cycle variation was observed in the lift-off length data from the engine, yielding considerable uncertainty in most of the reported trends. However, on average, the lift-off length was 7% longer on the windward side of the jet relative to the weak in-cylinder swirl flow. The proximity of the firedeck to the jets (14° down-angle) did not cause preferential vertical asymmetry in the flame shape, on average. A reduction in the interjet spacing angle from 90° (i.e., 4-holes) to 45° (8-holes) was correlated with a 35% reduction in the lift-off length, along with decreased sensitivity to ambient gas temperature and density. The mechanism by which the proximity of adjacent jets affects the lift-off length was not firmly identified, but fluid mechanical/thermal coupling between jets and/or internal injector fuel flow characteristics may be important. In realistic Dl diesel engine environments, high cycle-to-cycle variability, decreased temperature dependence, and shortening of the lift-off length may hinder soot reduction methods that rely on manipulation of the mean flame lift-off length.