Effect of adding ceramic thermal barrier coating on the turbocharger efficiency, external and internal heat transfer

Abstract

Turbocharger is a device installed on an internal combustion engine to boost its thermal efficiency. A turbocharger consists of three main components, namely the turbine, central housing, and compressor. The common material for commercial turbine housing is cast iron, for its lower cost yet resilience at elevated temperature. Given the high exhaust temperature a turbocharger is exposed to, energy loss in the form of heat transfer is inevitable. It is known to H turbine efficiency by up to 30%. This research aims to determine the turbocharger efficiency in the presence of thermal barrier coating (TBC) on the inner surface of turbine volute. Particularly, this work will focus on the internal and external heat transfer of the turbine and its impact on efficiency. The subject turbocharger is a commercial single-scroll vaneless unit commonly used in gasoline passenger vehicle. Yttria-Stabilized Zirconia (YSZ) is chosen as the TBC material, due to high melting point (around 2700°C), good thermal insulation property and very low thermal expansion compared to other ceramic materials. The YSZ was applied to the inner surface of turbine volute via plasma coating technique. However, due to the large disparity in thermal expansion between YSZ and cast iron, the TBC is prone to cracking at elevated exhaust temperature. Thus, an Inconel 718 turbine housing, with closer thermal expansion to YSZ, was refabricated for the use of this study The turbocharger performance was experimentally measured on the LoCARtic turbocharger gas stand. The turbine inlet temperature (TIT) was varied at 150, 350, 550, 650 and 750°C, while the compressor operating condition was maintained throughout the testing for equivalent comparison. From the result, the turbocharger efficiency drops when TIT is increased, and the turbine pressure ratio becomes lower. Overall, the external heat transfer loss is found to reduce 7% to 40% and no significant difference noticed on the internal heat transfer.