Two-Mode Hybrids

What Is A Two-Mode Hybrid?

How the Two-Mode Hybrid Works

A two-mode is a hybrid vehicle that operates in two different ways.

The first mode works like a conventional full mode hybrid. A full mode hybrid is a vehicle containing an electric motor and an internal combustion engine. In a full mode, these two systems can work separately or together.

They key difference in the two-mode hybrid is in its second mode. The second mode can change the various amounts of engine power and motor functions to respond to vehicle task and traffic conditions.

The Partnership Made It Possible

GM, Chrysler, BMW and to some degree, Mercedes-Benz partnered up to produce the two-mode system. It was a partnership that involved some very sophisticated engineering and development. So what do we really have here?

The two-mode hybrid system replaces a conventional automatic transmission system with a shell that contains a pair of electric motors and several sets of gears. This means all the gears, bands and clutches associated with your regular automatic transmission are all gone. This two-mode is really a low speed or low load system combined with a heavy load high-speed system. The two modes of operation can be described as a low load/low-speed mode, and a higher speed/heavy load mode.

Breaking Down The Modes

The first mode (lower speeds/light loads): This part of the hybrid's system can operate employing either the electric motors or the internal combustion engine, separately. Both systems can also operate together or in combination. In this first mode, the combustion engine can be shut off and all the power needed to operate the car can be drawn exclusively form electric power. If more power is needed for a heavier load, the hybrid system will simply restart the internal combustion engine. One of the hybrid motors behaves as a generator that keeps the battery charged. The other system functions as a motor for propulsion of the vehicle.

The second mode (at higher speeds/heavy loads): When more power is needed for greater speeds or heavier loads, the hybrid's internal combustion engine always runs. The hybrid system uses cylinder deactivation technology. General Motors refers to this cylinder deactivation as active fuel management. Chrysler has another name for it - multi-displacement system. This cylinder deactivation technology and variable-valve timing work together to increase engine efficiency. In the second mode, things get more complex. As the motor, generator and gear sets go on, offline the torque has to be maintained, and the horsepower has to be kept at a maximum. So let's break this down.

Just before the second mode kicks in, both motor and generator act as motors and give the vehicle's engine a boost. The vehicle contains four fixed gears. Once the vehicle's speed starts to increase, combinations of those four fixed gears are both employed and disengaged to multiply and maintain engine torque. As this is happening both the motor and generator switches back to generator mode. This trading mechanism between the vehicle's motor, generator and four gears continue as the vehicle's speed and load changes depending on load, road and traffic conditions.

An Efficient And Powerful Combonation

This well balanced combination of motor, generator and gears create a smooth flowing system. It allows the two-mode system to operate with a transmission that provides constant velocity. The transmission provides a solid, heavy-duty torque through the hybrid's gear sets. The system is also packaged much more economically within the conventional automatic transmission body. This reduces overcrowding in the hybrid's engine bay. This contrasts greatly with a large externally mounted motor and generator.

The end result is a very fuel-efficient vehicle that can provide hauling power and speed at any moment.

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