The steady and pulsating performance results of a turbocharger mixed-flow turbine are presented. The results are taken at an equivalent speed of 70% (42,000rpm) for a pulse frequency range of 20 to 80 Hz. All instantaneous parameters required for unsteady performance evaluation are measured and discussed. Significant improvements to the measurement of instantaneous actual power have been carried out. Large variations in the operating point of the turbine occur in each pulse cycle, a velocity ratio range of 0.43 to 1.28 is seen for a 20 Hz pulse, this range reduces as the pulse frequency increases and unsteady effects become more prominent. During periods of turbine freewheeling, negative efficiencies can arise due to momentum transfer from the turbine to the working gas, although detrimental to the efficiency the energy content in these regimes are low. The use of a modified Strouhal number (MSt.) and a pressure modified Strouhal number (PMSt.) has proved useful in assessing when the onset of unsteadiness of the flow will become significant, a value of 0.1 has been used as an appropriate limit to steadiness. The results suggest that for a typical engine speed range the rotor may be considered quasi-steady whilst the turbine stage is predominately operating in an unsteady regime. Inference from the experimental data would suggest it is adequate to capture the performance of a turbine under pulsating flow using a ‘quasi-steady’ model when the MSt. < 0.1, and a ‘filling and emptying’ code when a PMSt. < 0.1 and above this value a ‘wave action’ model is more appropriate.

The flow in turbocharger turbines is highly unsteady in nature as it responds to the exhaust manifold of an internal combustion engine. This paper investigates the significance of unsteadiness by examining first its relevance to real engine situations and then its effect on turbocharger turbine operation. The engine simulations carried out show the relevance of the Strouhal number effect for real turbocharger applications, which has been demonstrated experimentally on a turbine stage test stand. Therefore, for realistic multiple-cylinder-engine configurations with different exhaust gas pipe lengths and firing frequencies the importance of the actual unsteady behavior needs careful assessment. The effect upon the turbine itself is examined by modeling the laboratory arrangement to replicate the test stand configuration and operation using a one-dimensional wave action code. The 1D model is validated against experimental results obtained using a new permanent magnet eddy-current dynamometer for a mixed flow turbine suitable for a medium-sized automotive application covering an equivalent speed range of 50–100%, U2/Cis of 0.3–1.1 and a pulse frequency of 20–80 Hz. The turbine model has been refined using unsteady experimental data and so enables the capture of unsteady effects in engine design codes. The beneficial effect of the ability of this model to predict turbine mass flow is discussed.

This paper describes the development of a high-power high-speed eddy current dynamometer for turbocharger turbine research. Turbochargers are present in virtually all automotive diesel engines and it is essential to determine the characteristics of the turbine and compressor for inclusion into the engine model. Accurate measurement of the turbocharger turbine torque forms a vital part of this process. Earlier developments of dynamometers for this purpose are summarized and their limitations commented upon. The paper then describes the principle behind a new permanent magnet disc-type eddy current dynamometer including the relevant electromagnetic theory and analysis. The overall performance is given together with the results of an uncertainly analysis; a wide load range of 1.7-62.2 kW has been demonstrated at 60 000 r/min, and it allows characterization of turbine performance over the entire operating range, including the ability to measure unsteady torque.

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