An experimental study has been carried out to provide qualitative and quantitative insight into gas to wall heat transfer in a gasoline fueled Homogeneous Charge Compression Ignition (HCCI) engine. Fast response thermocouples are embedded in the piston top and cylinder head surface to measure instantaneous wall temperature and heat flux. Heat flux measurements obtained at multiple locations show small spatial variations, thus confirming relative uniformity of incylinder conditions in a HCCI engine operating with premixed charge. Consequently, the spatially-averaged heat flux represents well the global heat transfer from the gas to the combustion chamber walls in the premixed HCCI engine, as confirmed through the gross heat release analysis. Heat flux measurements were used for assessing several existing heat transfer correlations. One of the most popular models, the Woschni expression, was shown to be inadequate for the HCCI engine. The problem is traced back to the flame propagation term which is not appropriate for the HCCI combustion. Subsequently, a modified model is proposed which significantly improves the prediction of heat transfer in a gasoline HCCI engine and shows very good agreement over a range of conditions.

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The accumulation and burn-off of combustion chamber deposits create uncontrolled shifting of the homogeneous charge compression ignition operability range. This combustion chamber deposit–created operational variability places increased control burden on a multi-mode engine. However, the operational variability can be mitigated by manipulating combustion chamber deposit accumulation. A magnesium zirconate thermal barrier coating was applied to the piston of a homogeneous charge compression ignition engine in an effort to reduce combustion chamber deposit accumulation through elevated piston surface temperatures. While reduced combustion chamber deposit thicknesses were observed on the magnesium zirconate piston periphery, combustion chamber deposit accumulation in the bowl region increased relative to aluminum piston operation. Additionally, combustion chamber deposit thicknesses on the aluminum cylinder head were reduced during operation with the magnesium zirconate coated piston. Chamber-wide alterations to combustion chamber deposit accumulation taken together with the increased burn duration and hydrocarbon emissions measured during operation with the magnesium zirconate piston indicate significant interaction between the directly injected fuel spray and thermal barrier coating porosity. The porosity and surface roughness of the magnesium zirconate thermal barrier coating are speculated to create fuel pooling/absorption within the piston bowl, increasing combustion chamber deposit accumulation in the bowl and leaning the remaining fuel–air charge. The charge leaning lengthens the magnesium zirconate burn duration and reduces cylinder head combustion chamber deposit accumulation. Furthermore, hydrocarbon emissions were increased during magnesium zirconate operation due to late desorption and subsequent incomplete burning of fuel from piston bowl and the presence of incombustibly lean areas in the remaining cylinder charge.

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