Potential of variable diffuser vanes for extending the operating range of compressors and for improving the torque performance of turbocharged engines

Abstract

Turbocharging plays a significant role in internal-combustion engines. For engines in the future or for engines operating at a high altitude, compressors which are able to deliver a high pressure ratio are preferable. However, the poor low-end torque characteristics of turbocharged engines, which are often restricted by the narrow operating range of compressors at a high pressure ratio, result in a severe problem for turbocharging. The use of variable diffuser vanes is an effective method to increase the operating range, but the potential of an extended operating range at a high pressure ratio and improvement in the torque performance of engines is unclear. Nowadays, the pressure ratio of a turbocharger compressor may be only 1–4. Because of the increase in the pressure ratio, estimating the potential is ultimately worthwhile. In this paper the performances of a centrifugal compressor with different diffuser vane angles are investigated, the range extension and the improvement in the torque performance which benefited from variable diffuser vanes are estimated and the mechanisms for range extension are revealed. The approach includes steady three-dimensional Reynolds-averaged Navier–Stokes simulations and theoretical analysis. Adjusting the vane angle from −10° to 10° improves the operating range of a compressor from 23.5% (with fixed vanes) to 54.9% at a pressure ratio of 4.8. The range extension is obtained by utilizing the shifts in the choke line and the surge line. A method of assessing the choking component based on the simulation results is proposed. The diffuser, the flow stability of which was enhanced comparatively by closing it (pivoting the vanes by −10° and −5°), contributes mainly to reducing the surge flow. With this range extension, the improvement in the maximum torque is estimated to be 78%.