An engine system may include a turbocharger to increase charge density during combustion, to increase power output and/or engine-operating efficiency. The turbocharger may include a compressor driven by a turbine, where the rotational speed of the compressor is controlled at least partially by the rate of gasses flowing through the turbine. The rotational speed of the compressor may affect the charge density during combustion, the efficiency of the compressor, and the peak cylinder pressure. Therefore, it may be desirable to independently control the rotational speed of the compressor to match the engine operating requirements over the full performance range of the engine.
One solution for controlling the rotational speed of the compressor is to include a bypass path around the turbine that is controlled by a wastegate valve. Ideally, the system would be designed such that the bypass would be closed at all steady-state operating conditions. However, to meet the operating and emission requirements of the engine, during some operating conditions, the engine system is operated with the bypass at least partially open. For example, the wastegate valve opening may be increased to increase the flow of gasses through the bypass path, decrease the flow of gasses through the turbine, and decrease the speed of the compressor. However, routing gasses through the bypass path may result in thermodynamic energy being lost, e.g., engine-operating efficiency being decreased, when the gasses are not used to drive the turbine.