Indirect Assessment of Isentropic Efficiency in Turbocharger Turbines via Mechanical Efficiency Evaluation under Quasi-Adiabatic Test

The isentropic efficiency estimation of small radial turbines is an important aspect of turbocharger performance evaluation. Because of inaccuracies in measuring the outlet temperature due to the non-homogeneous flow field distribution, it is common practice to refer to the thermomechanical efficiency, defined as the product of mechanical and turbine isentropic efficiencies. This paper proposes a method for the indirect evaluation of turbine isentropic efficiency through specific experimental tests. In particular, the evaluation of friction losses in the bearings can be assessed thanks to experimental investigations in quasi-adiabatic condition. By maintaining the turbine inlet temperature and the average temperature of lubricating oil and water-cooling circuit equal to the compressor outlet temperature, a negligible heat transfer between turbine and compressor can be achieved. Therefore, the heat transferred to the lubricating oil can only be attributed to the friction in the bearings. Once the mechanical and thermomechanical efficiency has been experimentally assessed, the isentropic efficiency of the turbine can be evaluated with good accuracy. The knowledge of this quantity is essential to optimize the matching between different components of propulsion systems, and to correctly evaluate turbine outlet temperature. Furthermore, this last information allows the correct estimation of the inlet temperature of the aftertreatment systems.