Determination and Representation of Turbocharger Thermodynamic Efficiencies

The boost pressure demands call for high efficiency turbochargers. Perfect matching to an engine and controlling in operation is a prerequisite, especially if highly diluted mixture is used. The main impact on four-stroke engine efficiency is performed via gas exchange work, Correct turbocharger representation, usually performed by maps, should be delivered by turbocharger manufacturers and applied in simulation optimizations. The robust calibration methods are required for fast-running real time simulations used for model-based control. The paper clarifies the relations between apparent and real turbocharger isentropic efficiencies at steady-flow testbed and their impact on engine cycle optimization by simulation. Simple procedures excluding the impact of heat transfer inside a turbocharger are described. The described methods are based on the use of overall turbocharger efficiency. If details are needed, cold turbine flow tests and direct evaluation of compressor power using Eulerian theorem should be applied. Reynolds number influence should be taken into account. The regression model of turbine shaft heat transfer is used for correction of friction losses. The possibilities of turbine map measurement are analyzed for achieving wide range of blade speed ratio values using different turbine inlet temperature. The reasons for turbine physical model instead of map-based approach are explained on several specific examples. Apparent turbine efficiency, measured during engine tests, and possibilities to correct it by simulation are illustrated using examples of different exhaust systems with high level of pressure pulses.