Thermal Characterization of Combustion Chamber Deposits on the HCCI Engine Piston and Cylinder Head Using Instantaneous Temperature Measurements

Extending the operating range of the gasoline HCCI engine is essential for achieving desired fuel economy improvements at the vehicle level, and it requires deep understanding of the thermal conditions in the cylinder. Combustion chamber deposits (CCD) have been previously shown to have direct impact on near-wall phenomena and burn rates in the HCCI engine. Hence, the objectives of this work are to characterize thermal properties of deposits in a gasoline HCCI engine and provide foundation for understanding the nature of their impact on autoignition and combustion. The investigation was performed using a single-cylinder engine with re-induction of exhaust instrumented with fast-response thermocouples on the piston top and the cylinder head surface. The measured instantaneous temperature profiles changed as the deposits grew on top of the hot-junctions. The change in peak temperature phasing was found to be closely correlated with the deposit thickness, thus paving the way for future in-situ tracking of deposit growth. Analysis of data provided insight about the impact of deposit thickness on local heat fluxes at the CCD/metal interface, and enabled calculation of the CCD thermal diffusivity. The results produced distinctly different correlations for the cylinder head surface versus the piston top.