An Innovative Measurement Technique for the Direct Evaluation of the Isentropic Efficiency of Turbocharger Turbines

Turbocharging plays a key role not only in improving automotive engine performance, but also in reducing fuel consumption and exhaust emissions for Spark Ignition and diesel engines. In depth experimental investigations on turbochargers are therefore necessary to better understand their performance. The availability of experimental information on realistic turbine steady flow performance is an essential requirement for optimizing engine-turbocharger matching calculations developed in simulation models. This is most evident with regards to the turbine efficiency, as its swallowing capacity can be accurately assessed by measuring the mass flow, inlet temperature and pressure ratio across the machine. In fact, in the case of a turbocharger radial flow turbine, the isentropic efficiency evaluated directly starting from the measurement of the thermodynamic parameters at the inlet and outlet sections can give significant errors. This inaccuracy is mainly related to the difficulty of a correct evaluation of the turbine outlet temperature due to the flow field and the temperature distribution at the machine outlet. The purpose of this work is to obtain a reliable measurement of the turbine outlet temperature thanks to a specific device installed before the standard measurement section to dissipate the flow structures dominated by vorticity, thus obtaining a uniform distribution of the flow fields and of temperature to the measurement section. This measurement allows to optimize turbocharger experimental performance implemented in simulation models, obtaining a better control of the after-treatment device generally adopted downstream of the turbine. To this aim, a non-intrusive 3-hole probe was adopted to perform measurement of the flow field, pressure, and temperature downstream the turbine. The main results obtained through the non-standard measurements are compared with those achieved through a direct measurement of turbine outlet temperature by three probes inserted in pipe with a different protrusion. The experimental campaign concerns investigations also developed in almost adiabatic conditions and to be adopted to carry out a realistic measurement of the turbine outlet temperature in a simpler and less time-consuming way.