Chemiluminescence imaging of pre-injection reactions during engine starting

The thermal and chemical state of residual gas is known to influence the likelihood of autoignition, ignition delay and combustion phasing of the subsequent diesel engine cycle. To elucidate the role of residual gases in these processes, ultraviolet chemiluminescent reactions and their spectra are observed during the pre-injection, compression period in a dynamometer-driven, optically-accessible, diesel engine operated with a single fuel injection event. During a cold start sequence, while the engine is motored and fuel is injected without firing, the pre-injection chemiluminescence (PIC) intensity increases from cycle to cycle. This leads to a second mode of intermittent firing cycles which arc observed to follow a higher intensity of PIC. In the third mode, decreased PIC intensity is measured in firing cycles that are preceded by partial misfires. In the fourth mode, firing is continuous, but with a high IMEP coefficient of variation (COV). Here, PIC intensity is found to strongly correlate with advanced combustion phasing. As firing continues, it is observed that COV, PIC intensity and the phasing correlation decrease. Upon fuel shutoff, PIC intensity decays with time. Spectral measurements confirm that reactions of low temperature combustion intermediates, including chemiluminescent formaldehyde (HCHO*) and CHO* comprise the observed PIC.