In-Cylinder Thermal Barrier Coatings

Hannu Jääskeläinen, W. Addy Majewski

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• Thermal Barrier Coatings and Engine Performance
• Emission Control Systems

Thermal Barrier Coatings and Engine Performance

Thermal barrier coatings (TBC) were originally developed and commercialized for gas turbine and jet engine applications. Many investigations have been conducted into various aspects of applying such coatings to the walls of combustion chamber in internal combustion engines. The prime objective which has been sought is to achieve higher thermal efficiencies by reduction of heat rejection from the combustion chamber. Experiments with diesel and gasoline engines suggest that thin coatings produce higher engine efficiency than thick coatings, in spite of being less effective as heat insulators [700]. This behavior of TBCs has not been satisfactorily explained. It is believed that some detailed heat transfer characteristics must have a more profound effect on thermodynamic efficiency than the overall heat rejection rate from the engine.

One study noted that testing of 13 coated pistons with thermal barrier coatings of various materials, thicknesses, and surface preparations under gasoline compression ignition (CGI) conditions, showed an efficiency gain at 6 bar IMEPn (low load) and an efficiency penalty at 15 bar IMEPn (high load). This trend was explained by convective vive, originally presented by Woschni [6376][6377]. Under this phenomenon, there is a reduction in the quench distance of a reacting flame when the quenching surface temperature is elevated that results in exothermic reactions occurring in the boundary layer that leads to an increased heat transfer coefficient and heat transfer rate despite the lower temperature difference between the gas and the wall. While the work was unable to provide direct experimental evidence for convective vive, other possible explanations were ruled out including differences in heat release rate, radiation heat transfer, combustion chamber deposits, and surface catalytic effects. The experiments also showed that at low load, injection pressure did not impact the efficiency gain but at high load, the efficiency penalty increased with rail pressure. The authors suggest that in mixing controlled combustion systems, the thermal barrier coating should not be present on the combustion chamber surfaces that experience high fuel-wall interaction during combustion [6375].

Besides improved thermal efficiency, additional potential advantages of TBCs include improved engine durability, reduction in erosion and corrosion, less internal friction, lowered noise and reductions in exhaust emissions. A lot of work has been done on evaluating the effects of in-cylinder coatings on diesel engine performance and emissions. The results have been inconclusive and often contradictory. While most of published studies [146][144] report potential emission benefits, some [143][145] claim that the coatings have detrimental effects on fuel mixing and combustion, thus, deteriorating the performance and emissions. There is a significant variability in the coating effect between different engine types. The emission benefit of coatings appears to be related to their enhancing effect on the thermal efficiency of the engine. Therefore, higher emission effectiveness of coatings may have been possible in older technology engines which were characterized by relatively low thermal efficiency.

Effects of TBCs on particular diesel emissions compiled from published experimental data are listed in Table 1.

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