Heat transfer has a profound influence on homogeneous charge compression ignition combustion. When a thermal barrier coating is applied to the combustion chamber, the insulating effect magnifies the wall temperature swing, decreasing heat transfer during combustion. This enables improvements in both thermal and combustion efficiency without the detrimental impacts of intake charge heating. Increasing the temperature swing requires coatings with lower thermal conductivity and heat capacity. A promising avenue for simultaneously decreasing both thermal conductivity and capacity is to increase the porosity fraction. A proprietary solution precursor plasma spray process enables discrete organization of the porosity structure, called inter-pass boundaries, which in turn produces a step-reduction in thermal conductivity for a given porosity level. In this investigation, yttria-stabilized zirconia is used to create four different thermal barrier coatings to study the potential of structured porosity as means of improving the “temperature swing” behavior in a homogeneous charge compression ignition engine. The baseline coating is “dense YSZ,” applied using a standard air-plasma spray process. Next, significant reductions of the thermal conductivity are achieved by utilizing the solution precursor plasma spray process to create inter-pass boundaries with a moderate overall porosity. Performance, efficiency, and emissions are compared against both a baseline configuration with a metal piston and an air-plasma spray “dense YSZ” coating. Experiments are carried out in a single-cylinder gasoline homogeneous charge compression ignition engine with exhaust re-induction. Experiments indicate that incorporating structured porosity into thermal barrier coatings produces tangible gains in combustion and thermal efficiencies. However, there is an upper limit to porosity levels acceptable for homogeneous charge compression ignition engine application because an elevated porosity fraction leads to excessive surface roughness and undesirable fuel interactions. Comparison of the coatings showed the best results with coating thickness of up to 150  µm. Thicker coatings led to slower surface temperature response and attenuated swing temperature magnitude.

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In-cylinder surface temperature has significant impacts on the thermo-kinetics governing the Homogeneous Charge Compression Ignition (HCCI) process. Thermal Barrier Coatings (TBCs) enable selective manipulation of combustion chamber surface temperature profiles throughout a fired cycle. In this way, TBCs enable a dynamic surface temperature swing, which prevents charge heating during intake while minimizing heat rejection during combustion. This preserves volumetric efficiency while fostering more complete combustion and reducing emissions. This study investigates the effect of a Yttria-Stabilized Zirconia (YSZ) coating on Low Temperature Combustion (LTC) engine combustion, efficiency, and emissions. This is an initial step in a systematic effort to engineer coatings best suited for LTC concepts. A YSZ coating was applied to the top of the aluminum piston using a powder Air Plasma Spray process, Final thickness of the coatings was approximately 150 microns. The coated piston was subsequently evaluated in the single-cylinder HCCI engine with exhaust re-induction. Engine tests indicated significant advancement of the autoignition point and reduced combustion durations with the YSZ coating. Hydrocarbon and carbon monoxide emissions were reduced, thereby increasing combustion efficiency. The combination of higher combustion efficiency and decreased heat loss during combustion produced tangible improvements in thermal efficiency. When the effects of combustion advance were removed, the overall improvements in emissions and efficiency were lower, but still significant. Overall, the results encourage continued efforts to devise novel coatings for LTC.

A modified form of the sequential function specification method (SFSM) is developed with specific consideration given to multiple time scales in an effort to avoid overregularization of the solution estimates. The authors extend their approach to solve the inverse heat conduction problem (IHCP) associated with the application of thermal barrier coatings (TBC) to in-cylinder surfaces of an internal combustion engine. Subsurface temperature measurements are used to calculate surface heat flux profiles. The modified inverse solver is validated ex situ using a custom fabricated radiation chamber. The solution methodology is extended in situ to evaluate temperature data collected from a single-cylinder research engine operating in homogeneous charge compression ignition (HCCI) mode. Crank angle resolved, thermal barrier coating surface temperature and heat flux profiles are produced—enabling correlation of thermal conditions at the gas-wall boundary with engine performance, emission, and efficiency metrics.

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