
This patented hybrid technology optimizes fuel economy for various driving conditions. A sophisticated control module helps determine the amount of power or torque that's required at a given moment and seamlessly selects the appropriate operating mode. MODE 1: Used at low speeds and with light loads. The vehicle operates in three ways: electric power only, engine power only, or using a combination of electric and engine power. MODE 2: Used primarily at highway speeds. Full V8 engine power is used when conditions demand it such as when passing, trailer towing, or climbing steep grades.

In Hybrid Mode 1, used mainly at low speeds and with light loads, Sierra Hybrid and Yukon Hybrid can operate using electric power alone, supplied by the on-board 300-volt battery. Mode 1 can also utilize engine power alone or a combination of electric and engine power. Fuel consumption is reduced in stop-and-go traffic by shutting off the engine and operating on electric power alone.
In Hybrid Mode 2, used primarily at highway speeds, Sierra Hybrid and Yukon Hybrid use the Vortec V8 engine with Active Fuel Management, sometimes with supplemental electric power as conditions warrant. Full V8 engine power is available whenever needed, such as when passing, towing a trailer, or climbing a steep grade. When the extra power isn't needed, Active Fuel Management deactivates four of the eight cylinders with seamless precision. When load demands increase, all cylinders are reactivated to provide 8-cylinder operation and maximum power.

The innovative GM Two-Mode Hybrid system receives supplemental power from two 60kW electric motors housed within the electronically variable transmission. Conventional features are also integrated into the system: For example, a tow/haul mode provides the best operating point for the powertrain to deliver fuel economy and performance while providing smooth shifting.
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