Thermal Profiling of Automotive Turbochargers in Durability Tests

A major portion of the development of an automotive powertrain system is devoted to robustness and durability testing to ascertain the viability of the design. The configuration of these tests is designed to test the expected life limiting mechanism for the system. Since these tests are used to verify the system before product release they are standardised. For turbochargers, thermo-mechanical fatigue is often considered as life limiting failure mechanism for the turbine section, therefore, these tests involved repeated and continuous cycling of the turbocharger for hundreds of hours.

Some instrumentation is required to monitor the test operation and help understand the test conditions. Thermocouples are applied to components to record the operating temperature throughout the test. While thermocouples can provide accurate temperature information, this is only at the location at which they are installed. The installation of the thermocouples can be practically challenging and expensive in inaccessible areas so typically a limited number are installed in accessible locations. Furthermore, the cyclic and extended operation of the tests can cause the thermocouples to fail. Therefore, the already limited coverage that the thermocouples provide can be further reduced by failure of the instrumentation. As a result, there may be very limited temperature data at the end of the test. If a failure occurred in the system during the testing, the lack of temperature data can inhibit the understanding of the cause. Further testing may be required and delay product release, which add significant expense to the product development.

The Thermal History Coatings and Paint (THC&P) developed by Sensor Coating Systems can offer a new and unique solution for this purpose. The THC&P are applied to the surface of a component and when heated the coating or paint permanently changes according to the maximum temperature of exposure. A laser-based instrumentation system is then used to measure the coating or paint, and through calibration, the maximum temperature profile of the surface can be recorded. Although this technique is relatively new it has been used in several turbomachinery, and other applications to capture the spatial temperature distribution of critical components. However, the turbocharger durability test presents new challenges or the technique. It has not been tested in this type of application and the extended and repeated cycling operation can test the durability of the coating and will influence the response of the coating, hence, the temperature measurements. The internal surfaces of the turbocharger will also be exposed to the exhaust gases of the combustion process.

In this paper, the capability of the THC for this application was investigated. For the first time, the effect of cyclic operation and exhaust gas atmospheres on the measurement from the THC is reported. The measurement capability was demonstrated on three turbine housings with increasingly complex test profiles based on a standard cyclic durability engine test. Two of the turbine housings were tested on a gas stand, one for a single cycle, another for 10 cycles. The final turbine housing was tested on an engine for 1200 cycles. The results show that the surface temperature profile of all three turbine housings can be accurately recorded and the results are validated against the installed thermocouples. The demonstration indicates that the THC can be used to acquire substantially more temperature information than thermocouples alone. This information can be used to improve the interpretation of the durability test and hence accelerate new product release.