Implementing full electric turbocharging systems on highly boosted gasoline engines

To secure the highly challenging 2°C climate change limit, the automotive sector is expected to further improve the efficiency of the internal combustion engines. Over the past decade, internal combustion engine downsizing through turbocharging has become one of the major solutions that the industry has offered to fulfil its carbon commitment. Although the various new turbocharging technologies has changed the sluggish image of conventional turbocharged engines, the turbocharger system is far from perfect. From the perspective of engine energy flow, the copious amount of waste energy is habitually harvested by the turbine with low efficiency, subsequently the turbine power transmitted to the compressor is used solely to charge the engine. When this power for charging is excessive for the set engine operating condition, it either is consumed by throttling or is directly discharged through the wastegate, both as a pure enthalpy loss. To more efficiently harness the waste energy without deteriorating other engine performance parameters, a full electric turbocharging technology is proposed by Aeristech Ltd. The system is composed of an electric turbo generator and an electric compressor connected only through electrical system. Without the constraint of a mechanical turboshaft, the compressor and the turbine can be operated at different speeds. The electrically driven compressor can be free floating when boost is not required and the motor can provide the boost promptly only when higher load is requested. Meanwhile, the electric turbine can be controlled by the generator to operate at any set speed, allowing maximum efficiency for energy harvesting. This paper presents a simulation study of the capability of the decoupled eTurbocharging system to charge a highly boosted 2 litre gasoline engine. The single stage eTurbocharger configuration and the eTurbocharger plus a mechanical turbocharger configuration were evaluated and compared. The simulation results have revealed that the two stage eTurbocharging system has the potential to reduce CO2 emission in the proximity of 1 percent in different drive cycles compared to conventional wastegate turbocharger and the benefit would be much higher for future real world driving cycle. The single stage configuration was shown to be impractical in that the power level of the turbine generator will not only limit the engine power output, but also have negative impact on the system design, cooling and cost implied. Meanwhile, the two stage configuration where the eCompressor acts as a supplementary boost device at low end and transient device came out as a better solution with overall advantage in manageable power level, system efficiency, transient response and implied cost.