An Investigation of Thermal Stratification in HCCI Engines Using Chemiluminescence Imaging 2006-01-1518

Chemiluminescence imaging has been applied to investigate the naturally occurring charge stratification in an HCCI engine. This stratification slows the pressure-rise rate (PRR) during combustion, making it critical to the high-load operating limit of these engines. Experiments were conducted in a single-cylinder HCCI engine modified with windows in the combustion chamber for optical access. Using this engine, chemiluminescence images were obtained from three different view angles. These included both single-shot images with intensified CCD cameras and high-speed (20kHz) sequences with an intensified CMOS video camera. The engine was fueled with iso-octane, which has been shown to be a reasonable surrogate for gasoline and exhibits only single-stage ignition at these naturally aspirated conditions.

The chemiluminescence images show that the HCCI combustion is not homogeneous but has a strong turbulent structure even when the fuel and air are fully premixed prior to intake. Images were acquired with three fueling strategies to change the possible sources of charge stratification. The results indicate that the inhomogeneities are caused primarily by thermal stratification due to heat transfer during compression, combined with turbulent transport. High-speed movie sequences show that this thermal stratification causes the combustion to occur as a sequential autoignition of progressively cooler regions, slowing the PRR.

To investigate the spatial distribution of the thermal stratification, and the relative importance of boundary-layer and bulk-gas stratification, images were acquired through windows at the top of the cylinder wall from two view angles. Analysis of these images shows that sequential autoignition occurs throughout the bulk gases as well as the boundary layer. However, from the onset of hot ignition up past the time of the maximum PRR, the vast majority of the combustion occurs in the central part of the charge. Therefore, thermal stratification must extend into the bulk gases to be effective in controlling the maximum PRR. Thermal stratification between the bulk gases and the boundary layer was found to have a lesser role in reducing the maximum PRR.