Efficiency of automotive electric supercharging compressors

The automotive electric supercharger provides high torque at low engine speeds. During accelerations, it fills the power gap due to the turbocharger's inertia and avoids turbo lag. Its operation is of short duration because once this deficit has been juggled, it must be stopped in order not to generate unnecessary electricity consumption and to avoid overheating of the electric motor which would compromise its lifetime. Usually the power of automotive turbocharger is calculated assuming an adiabatic air compression and the adiabatic efficiency is assessed. At low speed or for low compression ratio, which is the case of electric supercharger, this hypothesis is no more valid and special experiments have to be done. On our test bench the compressor is driven by a turbine supplied with cold compressed air. Between the compressor and the turbine, a torque meter is inserted to measure the power given to the compressor shaft. The tests were conducted with a non-insulated compressor and an insulated compressor for eight iso-speeds. Additional tests were carried out to assess the actual power supplied to the fluid including bearing losses measurements and to assess the influence of different preload springs used to counter the axial thrust of the compressor. This article presents and analyses the results of this work.