Analysis of a Variable Geometry Diffuser Turbocharger to Increase Engine Operating Flexibility
The operating range of a centrifugal compressor is limited by the surge and choke flow conditions that defines its operating range at low and high flow rates respectively. Fitting a conventional full length vaned diffuser creates a throat and the increased blockage reduces the flow range. In addition, incidence effects at low flow rates can have an adverse effect on the surge condition. In general, conventional vaned diffusers lead to an improvement in efficiency and pressure ratio over that possible with a vaneless diffuser; the operating range, however, is reduced. The deficiency in the operating range can be eliminated by the adoption of variable geometry diffusers. In this study, performance of a turbocharger centrifugal compressor for different diffuser vane setting angles was investigated numerically and experimentally, with a focus on experimental results. The numerical simulation results showed that the diffuser vane setting angle influences compressor operating range and its efficiency. Higher pressure ratio and a wider high efficiency island were achieved when the vanes were moved 10° closer to the impeller in comparison to its baseline vane settings. To verify the computational results, a redesign on an existing ET-18 turbocharger was done. The redesign allows the diffuser vanes to rotate in real time while operating the turbocharger throughout its speed range. The test data of the VGT turbocharger show a significant improvement in the operating range at design speed or higher while rotating the vanes closer to the impeller. The draw back is a small loss in pressure and isentropic efficiency. While turning the vanes away from the compressor blade, the pressure ratio increased but the operation range was narrowed. This paper reports the quantified results of the VGT tests, and clearly shows how the turbocharger can be used to expand the engine operating range.
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