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Two-Mode Hybrid

In 2005, General Motors, DaimlerChrysler and BMW formed the Global Hybrid Cooperation to jointly develop a hybrid powertrain system. The objective was a flexible hybrid system for use in each of the three manufacturers' vehicles. Each company is tailoring the basic system to their own needs, while benefiting from shared components to reduce costs.

Initial application of this 'Two-Mode Hybrid' system include rear-wheel drive or four-wheel drive vehicles. It is especially suited for the more demanding applications, such as hauling and towing heavy loads with steep hill climbing involved. It allows smaller, more fuel sipping engines. Up to a 25-percent fuel savings is claimed and is it equally efficient in city and highway driving. Expect to see the system to first appear in 2008 models, most likely available on the Chevrolet Tahoe, GMC Yukon and Dodge Durango SUVs. The system can also be used on front-wheel drive vehicles and light trucks.

The 'two-mode' title refers to the twin electric motors used to for low speed and high-speed operation. The low speed mode is used during low-speed, stop-and-go urban driving. The high-speed mode operates at highways speeds when full power is needed for hill climbing and trailer pulling. The electric motors are used for additional power and regenerative braking in both modes.

In the first mode, the electric motors operate in CVT (continuously variable transmission) input-split mode combined with mechanical first and second gear operation. At higher speeds, the electric motors operate in CVT compound-split mode in combination with second, third and fourth mechanical gears. The hybrid system switches modes automatically so the driver doesn't have to do anything. A key part of the development was the sophisticated electronic control module that constantly optimizes the entire hybrid system to select the most efficient operation point for the power demanded by the driving conditions.

Most 'single-mode' hybrid systems have only one torque-splitting arrangement and no fixed mechanical ratios. Thus, they transmit a significant amount of power through an electrical path that is 20 percent less efficient than a mechanical path. In contrast, fixed mechanical gear ratios offer a more efficient path for power transfer as well as allowing smaller electric motors that are less dependent on engine torque or size. The single mode hybrid usually results in either a significantly compromise in performance or require larger, heavier and more costly electrical motors.

The full hybrid system has an overall mechanical content and size similar to a conventional automatic transmission, yet can operate in infinitely variable gear ratios or one of the four fixed-gear ratios. It can used be cars and trucks already being produced. Existing engines can be used with relatively minimal modification. It enables the three automakers to package internal combustion engines with the full hybrid transmissions more cost-effectively and offer the fuel-saving technology across a wider range of vehicles.

HOW IT WORKS

The input split mode is used for launching the vehicle from a stop, driving at low speeds, and for towing, when more power is needed. The compound split mode is used for cruising at highway speeds when less power is needed.