Chevrolet Tahoe HybridClick to enlarge picture

The Chevrolet Tahoe and GMC Yukon will be the first GM vehicles to feature a dual-mode hybrid system.

The first new hybrid system to make an appearance is called a two-mode. You may not have heard of it, but the two-mode hybrid powertrain currently drives 720 public transportation buses in 59 U.S. cities and 18 shuttle buses in Yosemite National Park. The two-mode will make news by year's end when a smaller version is introduced in two of America's best-selling full-size SUVs, the Chevrolet Tahoe and GMC Yukon.

Originally developed by General Motors' Allison Transmission division for the bus application, GM formed an alliance with BMW and DaimlerChrysler called the Global Hybrid Corporation to jointly develop the two-mode for passenger cars and trucks. GM Chairman Rick Wagoner said the new hybrid system will "one-up the competition because of its advanced design."

This co-developed, full-hybrid system is referred to as a two-mode hybrid because of its low- and high-speed electric variable transmission (EVT) modes. It also incorporates four fixed-gear ratios for high efficiency and power-handling capabilities.

Some of the functions of the new two-mode system are the same as those found on Toyota and Ford one-mode hybrids: the vehicle shuts off when at a standstill; batteries are charged during braking; and the vehicle can be powered by either the electric motor, the engine (gasoline or diesel) or a combination of both.

Where the two-mode stands out is its delivery of fuel economy in highway driving, not just the stop-and-go of commuting. Connected to a V8 engine that uses cylinder cutoff technology, the system is expected to increase fuel economy by 25 percent. That means the 5,200 pound Chevy and GMC SUVs could get highway fuel economy of nearly 26 mpg.

Another plus for the two-mode hybrid is its adaptability. It can be used in with multiple engine/drivetrain configurations, and can be mated to engines powered by gasoline, E85 or diesel fuels regardless of cylinder configuration—four, six or eight cylinders.

Following the two GM SUVs, the Chrysler Group will introduce the partnership's two-mode technology in the Dodge Durango and Chrysler Aspen in early 2008. Later in the year, GM will expand the offering to the Chevrolet Silverado and GMC Sierra full-size pickups as well as the Cadillac Escalade and Saturn Vue SUVs.

BMW and Mercedes will also offer the two-mode hybrid vehicle, but neither carmaker has stated what the vehicles are or when they will be marketed.

Granted, the "two-mode hybrid system" moniker doesn't have the sparkle of "Hybrid Synergy Drive" that Toyota calls its single-mode hybrid design, but look for the two-mode system to make a substantial impact on the hybridization of the automobile.

Read:  Back to Future Hybrids