Experimental Investigation of Single and Two-Stage Ignition in a Diesel Engine

This paper presents an experimental investigation conducted to determine the parameters that control the behavior of autoignition in a small-bore, single-cylinder, optically-accessible diesel engine. Depending on operating conditions, three types of autoignition are observed: a single ignition, a two-stage process where a low temperature heat release (LTHR) or cool flame precedes the main premixed combustion, and a two-stage process where the LTHR or cool flame is separated from the main heat release by an apparent negative temperature coefficient (NTC) region. Experiments were conducted using commercial grade low-sulfur diesel fuel with a common-rail injection system. An intensified CCD camera was used for ultraviolet imaging and spectroscopy of chemiluminescent autoignition reactions under various operating conditions including fuel injection pressures, engine temperatures and equivalence ratios. The chemiluminescent spectra were measured to confirm the presence of excited-state formaldehyde, (HCHO*), CH* and OH*, the spatial distribution of which were subsequently observed during the ignition period by filtered imaging. Experiment results based on the apparent rate of heat release (ARHR), indicate that ignition at lower engine temperatures and injection pressure of 400 bar is a single-stage process, however by increasing the injection pressure to 500 bar ignition becomes a two-stage process separated by a NTC region. Under warmed engine conditions, ignition follows the two-stage process at all fuel injection pressures examined. The ARHR correlated with the measured chemiluminescent intensity of HCHO*, both decreasing in the NTC region. The effect of injection pressure on the transition from a single to a two-stage ignition process is believed to result from different fuel spray evaporation rates determining local equivalence ratios during the temperature and pressure window of the LTHR mechanisms.