The forthcoming Chevrolet Tahoe and GMC Yukon two-mode hybrids will offer a fuel economy improvement of approximately 40% in city driving compared to conventional models of the SUVs, according to GM.
The V-8 powered two-mode hybrids are combined with GM’s Active Fuel Management cylinder deactivation technology. The Tahoe Hybrid will go on sale in the fourth quarter of 2007 as a 2008 model, along with the GMC Yukon two-mode Hybrid. Pricing and production volumes have not been announced.
Because of its low- and high-speed electric continuously variable transmission (ECVT) modes, the system is commonly referred to as the two-mode hybrid. However, the system also incorporates four fixed-gear ratios for high efficiency and power-handling capabilities in a broad variety of vehicle applications. (Earlier post.)
During the two ECVT modes and four fixed-gear operations, the hybrid system can use the electric motors for boosting and regenerative braking.
In the first mode, at low speed and light loads, the vehicle can operate in three ways: electric power only, engine power only or in any combination of engine and electric power. When operating with electric power only, it provides all the fuel savings benefits of a full hybrid system.
The second mode is used primarily at highway speeds. In addition to electric assist, the second mode provides full eight-cylinder engine power when conditions demand it, such as when passing, pulling a trailer or climbing a steep grade. The second mode also integrates sophisticated electronic controls, such as Active Fuel Management, cam phasing, and late-intake valve closure, allowing even more efficient engine operation.
A sophisticated controller determines when the vehicle should operate in either mode of the 2-mode drive system. Input from the controller determines the necessary torque for the driving conditions and sends a corresponding command to the engine and electric motors. The engine and electric motors transfer torque to a series of gears in the transmission, which multiply torque similar to a conventional automatic transmission to propel the vehicle.