A novel turbo-assisted mild-hybrid configuration for a city car: Compressor electric drive characterization

The paper reports the preliminary results of a study to develop and prototype a novel compact hybrid powertrain for a small/medium urban vehicle. The aim is to improve a standard turbocharged gasoline engine's performance by utilizing various energy recovery systems. Most notable is a novel turbocharger with mechanically decoupled compressor and turbine. During operation, the turbine and compressor, which are connected to two different electric machines, contribute a net energy input to the battery pack. The mechanical decoupling also enables both machines to operate close to their maximum efficiency over the expected real operating range. A properly designed engine control unit (ECU) monitors and controls the angular velocities of the two machines separately, eliminating the need for a wastegate valve enabling a net surplus of electrical energy generated by the turbine. The study focuses on the compressor and its electric motor, which are two essential components of the suggested design. The two components must be developed together and their performance curves closely matched due to the extremely high speed required by the design criteria that dictate high power density. The compressor used is a modified Garrett GT12, and the electric motor is a high-speed prototype brushless unit. The design parameters and their impact on the operating ranges of both the compressor and the electric motor are described. After a description of the conceptual and technological issues encountered during the design, the proposed solutions are discussed in detail, with preliminary results reported.