Hybrid Electric System: Overview
The hybrid electric system consists of 3 key components: the internal combustion engine, the electronically controlled continuously variable transaxle (CVT), and the high voltage traction battery. For a detailed description of each component, refer to HYBRID ELECTRIC CONTROL HARDWARE . In this powertrain configuration, there are 2 power sources that are connected to the driveline: a combination of the engine and the generator which uses a planetary gear set to connect to each other, and the electric traction motor which is connected to the drive wheels.
The high voltage traction battery is an electric energy storage device. The electric energy is used by the generator motor and the traction motor.
The planetary gear set functions as an electronically controlled CVT between the carrier gear (engine) and the ring gear (traction motor) which is connected to the drive wheels. This is achieved by controlling the sun gear (generator) speed and direction. The reason this is an electronically controlled CVT is due to the property of the planetary gear set in which the torque relationships between the sun gear, the carrier gear, and ring gear are fixed for this mechanical design. Therefore, the planetary gear set can also be viewed as a device that splits the engine output power to the driveline and to the generator motor.
There are 2 paths for the engine to deliver its output power: from the engine to the carrier gear, to the ring gear, to the intermediate shaft (mechanical path), and from the engine to the carrier, to the sun gear, to the ring gear and to the intermediate shaft (electrical path). The combination of the mechanical and the electrical paths makes this powertrain similar to a conventional vehicle powertrain with a CVT.
The electric traction motor uses power supplied by the high voltage traction battery and provides propulsion to the vehicle independently from the engine. Both power sources, a combination of the engine and the generator and the electric traction motor, can propel the vehicle simultaneously and independently.
This powertrain configuration is able to achieve better than conventional powertrain fuel economy and lower emissions levels because:
- the engine operates in its most efficient operating regions whenever possible.
- the engine size can be reduced with the same vehicle performance because of the dual power sources.
- the engine operation can be better optimized since it can be stopped if operational conditions are not favorable to the fuel economy or emissions.
- the kinetic energy during braking can be captured and stored in the high voltage traction battery through regenerative braking.
The torque determination and energy management strategy controls and operates the powertrain system to satisfy driver demands, increase the fuel economy, and decrease emissions levels.
In order to achieve better fuel economy and lower emission levels, the powertrain control module (PCM) torque determination and energy management strategy operates the powertrain system with specified operational conditions. First, the torque determination and energy management strategy determines in real time how much torque the driver is requesting and how much torque each power source can deliver to the drivetrain. Then it chooses the most efficient power source for that operational condition. Some of the inputs to the energy management strategy include driver demand, traction battery state of charge, performance limitations of components, battery life (charging and discharging rate and cycling), driveability, ambient temperature, and barometric pressure.
The hybrid electric system is a torque based system. When the gear selector is placed in DRIVE, the driver is going to request a positive torque by pressing the accelerator pedal, or a negative torque by pressing the brake pedal. The positive torque is perceived as vehicle acceleration, and the negative torque as vehicle deceleration (braking).
The brake and the electric power assist steering systems remain fully functional when the engine is stopped by the PCM. This allows the driver to operate the vehicle in electric mode when the engine is off.