An Investigation of Low-Temperature Combustion Front Propagation Mechanisms

Blessinger, M. K. An Investigation of Low-Temperature Combustion Front Propagation Mechanisms. University of Wisconsin-Madison, 2015.

The combustion propagation mechanism of homogeneous charge compression ignition (HCCI) combustion was investigated using planar laser Rayleigh scattering thermometry. The experiments were performed in a small-bore, high-speed engine with Bowditch-style optical access uniformly fueled by ethylene and dimethyl ether. Image processing was seriously hindered by striations induced by beam steering, which made image processing difficult and whole sets of high-pressure HCCI data unusable. Analysis of the raw image results showed that the background image correction heavily influenced the temperature calculation; a wide range of temperatures (realistic or not) could be achieved. The combustion front thickness measurements were shown, however, to be immune to background image correction and could be performed on either density or temperature calculated from the scattering measurements.

The HCCI images were analyzed for sharp intensity gradients that would provide the highest chance of seeing evidence of flame propagation. The HCCI measurements yielded temperatures lower than a comparable spark ignition (SI) condition, and the resulting temperature gradients had values up to 8 K/?m. The HCCI combustion front thicknesses ranged as low as 80 ?m, which were comparable with the SI flame front thickness, to several millimeters (although these regions were not quantitatively investigated). The combustion front propagation speed was estimated for SI and HCCI conditions by acquiring two images spaced closely in time. The measurements relied on the presence of small particles in the field to correct for the chamber fluid motion; the combustion-induced in-cylinder flow field was quite significant, and the combustion front speed measurements were, therefore, unreliable.

A transition between successive autoignition and flame propagation was not observed when investigating the dependency between the combustion speed and the temperature gradient as had been suggested in the literature, but the uncertainties in the measured temperature gradient and, especially, the combustion speeds did not permit a conclusive determination about the role of flame propagation in HCCI. Based on the fact that HCCI combustion front thicknesses comparable to those for SI combustion were observed, however, it can be concluded that the presence of flame propagation in HCCI combustion is a very real possibility.