Hybrid Drive Systems: Description and Operation

HYBRID ELECTRIC SYSTEM

Overview
The hybrid electric system consists of three 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 two 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. Hybrid Electric Control Hardware

The high voltage traction battery is an electric energy storage device. The electric energy is used by the generator motor and the traction motor.

Hybrid Electric System:

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 two 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.

Internal Combustion Engine
The 2.5 liter variable compression engine uses the Atkinson cycle for its operation. Sometimes referred to as the five-stroke cycle, it uses a normal intake stroke but at the start of the compression stroke, the intake valve stays open. This allows a backflow stroke of air from the cylinder into the intake manifold which decreases manifold vacuum that requires energy to overcome. Although the intake valve stays open in the compression stroke, it closes early enough to produce sufficient cylinder pressure for combustion. The Atkinson cycle engine is designed to operate in a smaller RPM band than a conventional engine, and it runs mainly at the most efficient RPM range for the maximum fuel economy. The Atkinson cycle is up to 10% more efficient than a conventional 4-stroke gasoline engine, but it has reduced low engine RPM torque. The engine is mechanically linked to the electronically controlled CVT by the crankshaft and transaxle input shaft. The engine primarily provides torque to the output shafts, charges the high voltage traction battery using the generator motor, and runs the A/C compressor when requested. It also produces heat for the passenger compartment climate control.

Electronically Controlled Continuously Variable Transaxle (CVT)
The primary objective of the electronically controlled CVT is to deliver torque to the drive axles of the vehicle. The CVT transmits input torque from the internal combustion engine, or uses electrical power from the high voltage traction battery. The electrical power is converted into mechanical power by the traction motor and the generator motor. The electronically controlled CVT operates in several different modes, and under certain conditions can transmit power from the traction motor and engine simultaneously or independently. For additional information, refer to Hybrid Electric Control Software, operating modes. Hybrid Electric Control Software

The key electronically controlled CVT components are:
- planetary gear set
- generator motor
- traction motor
- transaxle control module (TCM)

NOTE: Do not attempt to make repairs to any components within the transaxle.

Planetary Gear Set
The planetary gear set, the generator motor, and the traction motor are internal to the electronically controlled CVT. The planetary gear set mechanically links the internal combustion engine, the electric traction motor, and the electric generator together. It distributes power between the three elements that it connects. The engine is connected to the carrier, the generator motor is connected to the sun gear, and the traction motor is connected to the ring gear of the planetary gear set.

Planetary Gear Set:

Generaltor Motor
The generator motor is a three-phase permanent magnet AC motor connected to the sun gear of the planetary gear set. The generator power inverter (internal to the transaxle) receives a DC current from the high voltage traction battery. The DC current is inverted to an AC current, which is controlled by the TCM and the generator control unit (GCU). Depending on the mode of operation, the generator motor can rotate in the same (clockwise) or reverse (counter clockwise) direction as the internal combustion engine. The TCM also monitors generator position, speed, phase current, power inverter temperature, coil temperature, and voltage. It processes a variety of PCM and traction battery control module (TBCM) inputs to determine generator function. The inputs are divided into two categories: the hardwired signals and the network messages. The inputs used to determine generator function include the total torque desired, the engine speed desired" the generator mode, the generator shutdown, and the vehicle mode. The generator operates in one of the following generator modes: torque, speed, engine cold start, engine normal start, engine high speed start, and engine enhanced speed control stop. The generator is used as a starter for the internal combustion engine, charges high voltage traction battery, and controls engine speed. The TCM reports the generator error status to the PCM. The PCM initiates an appropriate limited operating strategy (LOS) mode and sets the diagnostic trouble code (DTC) P1A0D (Hybrid powertrain control module - generator disabled) based on the error status it received. The TCM also sets a DTC which indicates the cause of 1he generator motor concern.

Generator Motor:

Traction Motor
The traction motor is a three-phase permanent magnet AC motor connected to the ring gear of the planetary gear set. The traction motor is connected to the drive wheels through a series of gears and rotates whenever the drive wheels rotate. The traction motor power inverter (internal to the transaxle) receives a DC current from the high voltage traction battery. The DC current is inverted to an AC current, which is controlled by the TCM and the motor control unit (MCU). The traction motor can deliver positive torque by propelling the vehicle in the forward or reverse direction. It can also provide negative torque by functioning as a generator during the regenerative braking. The TCM receives input from the position sensor as well as the motor coil temperature sensor. The TCM monitors the motor coil temperature and sets a DTC if the temperature exceeds a maximum threshold value. The TCM also monitors motor position, speed, phase current, power inverter temperature, and voltage. The TCM processes a variety of the inputs from the PCM and the traction battery control module (TBCM) to determine motor function. The inputs are divided into two categories: the hardwired signals and the network messages. The inputs used to determine motor function include the total torque desired, the motor inverter shutdown, and the vehicle mode. The traction motor is used to provide torque to the axle shafts and recharge the high voltage traction battery during the regenerative braking. The TCM reports the motor error status to the PCM. The PCM initiates an appropriate LOS mode and sets the DTC P1A0E (Hybrid powertrain control module - traction motor disabled) based on the error status received. The TCM also sets a DTC which indicates the cause of the traction motor concern.

Traction Motor:

High Voltage Traction Battery

WARNING: To prevent the risk of high-voltage shock, always follow precisely all warnings and service instructions, including instructions to depower the system. The high-voltage hybrid system utilizes approximately 300 volts DC, provided through high-voltage cables to its components and modules. The high-voltage cables and wiring are identified by orange harness tape or orange wire covering. All high-voltage components are marked with high-voltage warning labels with a high-voltage symbol. Failure to follow these instructions may result in serious personal injury or death.

The high voltage traction battery stores energy for later use by the traction motor and the generator motor. It is connected to both the traction motor and the generator motor by the high voltage cables. The traction motor uses the traction battery power when it propels the vehicle. The generator motor uses the traction battery power when it starts the internal combustion engine. The traction battery also provides energy to the DC/DC converter, which steps down the high voltage to maintain the low voltage system charge. For more information on the charging system, refer to the Hybrid Drive Systems High Voltage Converter/Inverter. Refer to the Hybrid Drive Systems High Voltage Traction Battery for more information on the high voltage traction battery and diagnostics.