The transmission is a simple planetary gearset that can via software direct ICE rpm and torque to the generator (MG1) and the drive shaft. ICE rpm and torque can be split between MG1 and the drive shaft independently. The traction motor (MG2) is connected to the drive shaft and can be used as a motor or generator. When used as a motor, MG2 propels the car. When used as a generator, MG2 is simulating ICE braking (foot off the brake) or slowing the car down when pressing the brake pedal (regeneration). MG1 can also be used as a motor or generator. When used as a motor, MG1 can be used to reduce ICE rpm. This allows ICE to operate more efficiently (high torque, lower rpm).
The software that controls the transmission has several modes of operation which has been discussed in previous threads and meets driver's requests (accelerating / braking / gear shifting and so forth). The software has been updated several times. The presumption is that the software updates either enhance efficiency or reliabiltiy. I believe that increasing the electric mode operational speed up to 85 mph allows ICE to operate in positive split mode more frequently which should enhance higher speed efficiency since electric mode can now be used to deplete the HVB at higher speeds when efficient to do so and then to charge the HVB back up when efficient to do so. IMO, this change is beneficial to hypermilers and those using cruise control at higher speeds. Of course, lower speeds is still more efficient with respect to FE because aero drag kills FE at high speed. From the Manual:
Torque Determination and Energy Management
The TCM is responsible for torque determination and energy management functions. The TCM monitors gear selector position (PRNDL), brake pedal position (BPP) and accelerator pedal position (APP). The TCM then makes a torque command determination. Positive torque is perceived as vehicle acceleration and negative torque is perceived as braking. Based on the amount of torque requested by the driver, the TCM decides which power source has to deliver the torque to meet the driver demand while the powertrain system is running most efficiently.
There are five fundamental operating modes in the hybrid electric system:
- series mode
- electric mode
- positive split mode
- negative split mode
- engine cranking mode
The system operates in this mode when the engine is running and the vehicle is not moving. This is the preferred mode whenever the high voltage traction battery is charging, passenger compartment temperature control, high voltage traction battery temperature control or catalyst warm up is necessary.
The system operates in this mode when the vehicle is propelled by the electrical power stored in the high voltage traction battery. The torque is supplied to the output shafts by the traction motor. This is the preferred mode whenever the desired torque is low and can be produced more efficiently by the electrical system than the engine. The electric mode is also used in reverse because the engine can deliver torque only in a forward direction.
Positive Split Mode
The system operates in this mode when the engine is running and powering the generator motor which produces the electricity. The power from the engine is split between the path through the generator motor and the path to the output shafts of the vehicle. The electricity produced by the generator motor charges the high voltage traction battery or powers the traction motor. In this mode the traction motor can operate as a motor or as a generator to make up the difference between engine torque and desired torque at the wheels. This mode is preferred whenever the traction battery needs to be charged or at moderate loads at low speeds.
Negative Split Mode
The system operates in this mode when the engine is running but the generator motor is reducing the engine speed. This mode is never preferred but occurs if the engine is running, the vehicle speed is high, the high voltage traction battery is charged.
Engine Cranking Mode
The generator motor provides the engine cranking function to start or restart the internal combustion engine. When the PCM requests the engine cranking mode, the generator motor rapidly accelerates the engine speed up to about 950 RPM in about 0.3 seconds. When the engine speed reaches a calibrated speed the PCM commands the delivery of fuel and spark at the appropriate time.