Assessment of In-Cylinder Thermal Barrier Coatings over a Full Vehicle Drive Cycle

In-cylinder thermal barrier coatings (TBCs) have the capability to reduce fuel consumption by reducing wall heat transfer and to increase exhaust enthalpy. Low thermal conductivity, low volumetric heat capacity thermal barrier coatings tend to reduce the gas-wall temperature difference, the driving potential for heat transfer from the gas to the combustion chamber surfaces. This paper presents a coupling between an analytical methodology for multi-layer coated wall surface temperature prediction with a fully calibrated production model in a commercial system-level simulation software package (GT-Power). The wall surface temperature at each time step was calculated efficiently by convolving the engine wall response function with the time-varying surface boundary condition, i. e., in-cylinder heat flux and coolant temperature. This tool allows the wall to be treated either as spatially uniform with one set of properties, or with independent head/piston/liner components. Steady state results are presented for five modern, engine-specific coating architectures selected from the literature. The coating performance, on a system-level basis, was found to be engine-condition dependent. The optimum material for one performance parameter may not provide any benefit to another. To comprehensively compare TBCs relative to the uncoated baseline, a full transient drive cycle simulation was performed for two coatings using the 20-minute certification Non-Road Transient Cycle (NRTC). Experimental boundary conditions along with ECU data were provided from production engine test cell data. A reduction of in-cylinder heat transfer and fuel consumption was found for both coatings, while exhaust enthalpy was increased by 0.5% in spite of the fuel mass saving. For a piston-coated scenario, a maximum of 1.5% reduction in fuel consumption and consequently a similar level of brake specific CO₂ reduction was realized over the drive cycle, depending on the coating architecture.