Optical and Numerical Investigation of Pre-Injection Reactions and Their Effect on the Starting of a Diesel Engine

Ultraviolet chemiluminescence has been observed in a diesel engine cyclinder during compression, but prior to fuel injection under engine starting conditions. During a portion of the warm-up sequence, the intensity of this emission exhibits a strong correlation to the phasing of the subsequent combustion. Engine exhaust measurements taken from a continuously misfiring, motored engine confirm the generation of formaldehyde (HCHO) in such processes. Fractions of this compound are expected to be recycled as residual to participate in the following combustion cycle. Spectral measurements taken during the compression period prior to fuel injection match the features of Emeleus' cool flame HCHO bands that have been observed during low temperature heat release reactions occurring in lean HCCI combustion. That the signal from the OH* bands is weak implies a buildup of HCHO during compression. To investigate the combined effects of wall temperature, equivalence ratio and HCHO concentration on combustion phasing, a two-zone CFD-kinetics model was employed utilizing the second version of the Lawrence Livermore National Laboratory detailed n-heptane mechanism [1,2]. The effect of these three parameters was simulated over a parameter space spanning equivalence ratios from 0.1 to 4, wall temperatures from 60 °C to 140 °C, and HCHO concentrations between 0 and 300 ppm. Results suggest HCHO has a retarding effect on autoignition under lean conditions, yet an enhancing effect at equivalence ratios greater than 0.5. The advancing effect is most pronounced at rich, lower temperature conditions. It is estimated that under certain conditions, the presence of HCHO in concentrations of 300 ppm, as is possible following a misfire, has the equivalent effect on combustion phasing as an increase in the surface wall temperature by 12 °C or by a one °C increase in the charge temperature prior to compression.