Next generation vehicles are under environmental and economic pressure to reduce emissions and increase fuel economy, while maintaining the same ride and performance characteristics of present day combustion engine automobiles. This has prompted researchers to investigate hybrid vehicles as one possible solution to this challenge. At Southwest Research Institute (SwRI), a unique parallel hybrid drivetrain was designed and prototyped. This hybrid drivetrain alleviates the disadvantages of series hybrid drivetrains by directly coupling the driving wheels to two power sources, namely an engine and an electric motor. At the same time, the design allows the engine speed to be decoupled from the vehicle speed, allowing the engine to operate at its most efficient state.
This paper describes the drivetrain, its components, and the test stand that was assembled to test the parallel hybrid drivetrain. The different modes of operation, the overall control strategy, and hardware data is presented for several driving profiles, including the city, highway, and EPA urban driving profile. Validation results between the hardware and the computer model of the drivetrain are also discussed. Results show a very satisfactory match between computer model and hardware. We have achieved a 60 mpg (28.05 km/liter) fuel economy using the most inefficient operating mode (assist mode) for a mid-sized vehicle. Concepts to further improve this fuel economy are also presented.