Emission regulations worldwide demand better low-end torque from internal combustion engines. This pushes the operating condition of turbocharger turbine to lower U/C values, where U is the blade tip speed and C is the turbine isentropic spouting velocity which increases with turbine expansion ratio. Traditional radial and some mixed flow turbines, dictated by their rotor design, have their efficiency peaks at U/C value around 0.70, a value considerably larger than desired. In this paper, we deliberate the measures to shift the peak efficiency of turbine rotor toward a lower U/C value than 0.7. The underlying physics of these measures are first explained, CFD and test results where available are then given. Implications to mechanical design, manufacturing and others are also discussed. Finally, an example of a turbine rotor design is given implementing these measures. Test results showed better efficiency was obtained from the design at lower U/C values than from the baseline.

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