General Motor's hybrid program has had more starts and stops than a belt alternator starter. In February of 2003, answering critic's allegations that GM had abandoned hybrid technology in favor of furl cells, GM announced a three-hybrid system strategy that would begin with the launch of a two-motor advanced hybrid system set to debut in a Saturn VUE In late 2005. A mild hybrid system developed for the full-size Chevrolet Silverado and GMC Sierra pickups would be available to commercial customers in late 2003 followed by retail sales in 2004 and Chevy's Equinox SUV would get a belt starter alternator system in 2006.

By November of 2003, things had already changeck GM announced that they were canceling both the strong hybrid Saturn VUE and soft hybrid Equinox, concentrating its efforts on its full-size truck and SUV platforms. A small number of mild-hybrid pickups were finally made available for commercial application In the summer of 2004, nearly a year behind schedule.

Speaking of flail-size, GM's fist commercial hybrid vehicle, shown that November, was a 50-foot city bus. Working in conjunction with transmission-builder Allison, GM developed a two-mode hybrid/electric system. The Allison EP40/EP50 electric-drive system uses two 100 kW electric motors bolted to an 8.9L diesel engine (replacing an 11L diesel) with a 588-volt nickel metal-hydride battery that promises a 60 percent savings in fuel economy.

GM currently has 335 hybrid buses operating in 18 cities around the U.S. In October, GM delivered a bus to Shanghai for evaluation by the Chinese government and Stephens says that the company expects to make more announcements soon with plans to deliver hybrid bus technology around the world by the end of the decade.

"We would really like to replace about 13,000 buses in the nine largest cities," Stephens says, "because we'd save 40 million gallons of furl just with the buses."

It's the two-mode bus hybrid drivetrain that makes up the basis for GM's latest hybrid strategy.

"We're trying to target the highest fuel consuming vehicles first," says Tom Stevens, group vice president, GM Powertrain. "The best way to save the most fuel is to go after the highest fuel users first."

GM and DaimlerChrysler announced in December that they were working in conjunction on a scaled-down version of the Allison EP40/EP50 hybrid system for use in full-size SUVs. AHS2 is designed for rear-drive applications and fits within the envelope of a standard 6-speed automatic.

GM will launch a two-mode hybrid Tahoe/Yukon late in 2007 followed with full-hybrid Chevy Silverdao and GMC Sierra pickups in 2008. DCX will launch a hybrid Durango in early 2008.

"The coming Durango and Yukon hybrids are an example of what the joint development intends to do," says Stephens. "The vehicle share technologies in the electrically variable transmission though each adoption has been tailored to the vehicle's engine and platform, giving the vehicles distinct character and dynamics."

Stephens says that the collaboration should significantly reduce cost and development times. The high cost of doing hybrid business has kept the number of offerings low.

"Working together, we have the ability to expand the number of hybrids brought to market," Stephens says, adding that higher volumes will help to further reduce costs.

Dr. Thomas Weber, DaimlerChrysler Board of Management with responsibility for Research, Technology and Development, Mercedes Car Group, says that within the DaimlerChrysler Corporation, Mercedes Benz will have the design lead and will be the first to put it in their cars.

"In Europe we see big advantage from the diesel side," says Weber, "DCX will have hybrids all over the world and will possibly be available in all Mercedes cars."

Weber says that Mercedes 4-cylinder, 6-cylinder and V-8 diesel and gas engines will get the hybrid transmissions.

GM and DCX, who began initial talks in October of 2003, have 60 patents pending on the scaleable technology, that will eventually be used for both front-drive and rear-drive applications.

Two-mode hybrid system

GM's two-mode system operates one of three ways at hunch and slower speeds: electric only, engine only, or any combination of the two. Operating in the electric-only mode for extended periods of time at low speeds is the key to reducing had consumption in heavy stop-and-go traffic.

The compound-split mode is used when the vehicle is traveling at higher speeds like highway driving. In compound-split mode the system relies on the assist from the electric motor and Displacement on Demand cylinder deactivation to improve had consumption.

The power control system seamlessly switches from input to compound split mode. The electric motors not only regulate the power that's flowing through the transmission, but are also capable of extending cylinder deactivation well beyond what's possible without the hybrid assist. An idle-stop function saves had by shutting the engine off during deceleration and when stopped, and regenerative braking supplies electricity to recharge the battery or to run the vehicle electric-only. What makes the two-mode system ideal for large SUVs is there is still full V-8 power available when higher loads demand it.