Electrically assisted turbochargers are a promising technology for improving boost response of turbocharged engines. These systems include a turbocharger shaft mounted electric motor/generator. In the assist mode, electrical energy is applied to the turbocharger shaft via the motor function, while in the regenerative mode energy can be extracted from the shaft via the generator function, hence these systems are also referred to as regenerative electrically assisted turbochargers (REAT). REAT allows simultaneous improvement of boost response and fuel economy of boosted engines. This is achieved by optimally scheduling the electrical assist and regeneration actions. REAT also allows the exhaust turbine to operate within a narrow range of optimal vane positions relative to the unassisted variable geometry turbocharger (VGT). The ability to operate within a narrow range of VGT vane positions allows an opportunity for a more optimal turbine design for a REAT system. This is because the design compromises necessary for turbochargers that must operate with wider range suboptimal VGT vane positions can be eliminated. This raises a critical design question. What additional benefits can be exploited by using a REAT system with a redesigned, more efficient, exhaust turbine.
In this paper the impacts of the improved turbine efficiencies, of a REAT system, on the performance of a 6.7L Diesel engine are investigated via high fidelity GT-SUITE model. Results are compared against the performance of a REAT system with the base turbocharger design and a conventional (unassisted) turbocharger with the improved design. Results from a first principles fundamental analysis show that higher turbine efficiency reduces the pre-turbine pressure and therefore reduces engine pumping loss. This benefit, however, decreases with increasing electrical assist levels. This is because electrical assist has an effect similar to turbine efficiency improvement. In addition, a REAT system with a high-efficiency turbine also improves the electrical energy balance or state of charge (SOC), since the electrical energy demand reduces from the improved ability of the turbine to transfer work. FTP-75 drive cycle simulation results show that with 5% increase in VGT efficiency, only ~0.1% BSFC improvement and 7.7% reduction in electrical energy deficit are achieved, when the total electrical assist energy over the total engine work output is ~2%. On the other hand, REAT with a high-efficiency turbine is more prone to deficits in high-pressure exhaust gas recirculation relative to the nominal system.