Impact of Compressor and Turbine Operating Range Extension on the Performance of an Electric Turbocharger for Fuel Cell Applications

In this study, an electric turbocharger for fuel cell applications is investigated with regards to the extension of both compressor and turbine operating range by means of geometric changes to the turbomachinery components, namely variable nozzle and diffuser vane angles. Therefore, the interaction of the electric turbocharger subsystem with the fuel cell stack is investigated over the full operating range to judge the overall efficiency, system dynamics and stability. Initially, selected options for extending the performance maps of both compressor and turbine are presented and discussed. The numerical methods used for predicting the performance maps are then described. Subsequently, the entire machine is simulated under both steady-state and transient operating conditions using the in-house tool ASTOR (AircraftEngine for Transient Operation Research). Based on the wider operating ranges of compressor and turbine, promising setups are identified and investigated in further detail to select the best choice for the operation of the electric turbocharger. The impact of isolated component modifications is shown initially and substantial improvements of the operating range are shown. The modification of the compressor diffuser leading edge angles in a fixed-geometry diffuser increases the range of covered operating points from 4 to 7 while at the same time improving the compressor surge margin during steady state operation above the required safety margin of 20%. Additionally, the system efficiency can be increased by 0.2%. The application of a positive angle variable nozzle turbine significantly shifts the operating line towards higher mass flows, thus increasing the surge margin especially in the high speed range where the transient effects during deceleration of the machine are the greatest. By applying combined modified compressor with pivoting vanes and and variable nozzle turbine geometry, the operating range can be extended even further. The surge margin can be kept above 20% during the initial critical part of a transient deceleration manoeuvre. Nevertheless, a decrease of 0.5% in overall system efficiency has to be accepted due to the measures.