Characterizing the Effect of Combustion Chamber Deposits on a Gasoline HCCI Engine

Homogenous Charge Compression Ignition (HCCI) engines offer a good potential for achieving high fuel efficiency while virtually eliminating NOx and soot emissions from the exhaust. However, realizing the full fuel economy potential at the vehicle level depends on the size of the HCCI operating range. The usable HCCI range is determined by the knock limit on the upper end and the misfire limit at the lower end. Previously proven high sensitivity of the HCCI process to thermal conditions leads to a hypothesis that combustion chamber deposits (CCD) could directly affect HCCI combustion, and that insight about this effect can be helpful in expanding the low-load limit. A combustion chamber conditioning process was carried out in a single-cylinder gasoline-fueled engine with exhaust rebreathing to study CCD formation rates and their effect on combustion. Burn rates accelerated significantly over the forty hours of running under typical HCCI operating conditions. Variations of burn rates diminished after approximately 36 hours, thus indicating equilibrium conditions. Observed trends suggest that deposits change dynamic thermal boundary conditions at the wall and this in turn strongly affects chemical kinetics and bulk burning. In addition, this work presents a methodology for investigating the thermal diffusivity of deposits without their removal. The experimental technique relies on a combination of instantaneous surface temperature and CCD thickness measurements. Results demonstrate a strong correlation between deposit thickness and the diffusivity of the CCD layer.