Coming to the Hybrid Party

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 fuel 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.

GM Chairman and CEO Rick Wagoner and Chrysler CEO Dieter Zetsche examine a prop shaft from the two-mode hybrid transmission. The two electric motors can be seen on either end of the casing.

By November of 2003, things had already changed. 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 full-size, GM’s first 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 fuel just with the buses.”

The AHS2 is a scaled down version of GM’s Allison EV Drive hybrid bus transmission and fits in the same space as a standard six-speed automatic truck transmission.

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 fullhybrid 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 vehicles 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.”

The two-mode system schematic shows the input split used for launching the vehicle and low-speed operation and the compound split, that adds a second gear set for improved performance at cruising speeds or under heavy loads, like towing. Adding the second mode creates a larger mechanical path and a smaller electrical path (shown center) allowing the use of smaller electric motors.

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 launch and slower speeds: electric only, engine only, or any combination of the two.

The Graphyte concept, built in GM’s Advanced Studio in Coventry, England, represents what a GM hybrid SUV could look like. Graphyte is a front engine all-wheel-drive fivepassenger SUV. It sits on a 114-in. wheelbase and measures 187.7-in. in overall length and 78.9 in. in width. Power comes from a 300 hp 5.3L Vortec 5300 V-8 with Displacement on Demand and other modifications to utilize the AHS2 electronically controlled electric variable hybrid transmission. Two 60k electric motors are integrated into the transmission hooked to a 300v NiMH battery located under the rear seat.

Operating in the electric-only mode for extended periods of time at low speeds is the key to reducing fuel 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 fuel 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 fuel 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. In a typical single-mode input power-split transmission, like the ones found in the Toyota Prius and Ford Escape, power from the engine drives a planetary gear set that splits the power between the final drive axle and the electric motor. The first electric motor operates as a generator, turning that power into current.

The current can either recharge the battery or drive the second electric motor and turn it back into torque that is sent to the final drive axle. A single-mode system requires very large electric motors to create the necessary power to run the vehicle from launch to top speed and make up for the lack of transmission gearing.

The two-mode system essentially adds an electronic second gear or second planetary gear set to the electronically variable transmission. The first mode, or input-split is used only at launch and low speeds, so the initial gearing in the transmission can be smaller.

Unlike the input-split system in the Prius, where the electric motor becomes a generator at higher speeds and turns the power chores over to the gas engine, the second gear set in the twomode system allows the electric motor to continue to assist the engine, even at high speeds. The initial gear set runs up to a maximum speed of 60 to 70 mph. At higher speeds, or under heavy loads, the transmission shifts to the compound- split mode using the second planetary gear set. This reduces the overall current needed to run the vehicle allowing for the electric motors and inverter to be smaller.

Inside the AHS2 are two electric motors sandwiching two planetary gear sets. The gear sets and electric motors are separated by two electronic clutches.

In the input split mode, power is applied to the ring gear, clutch one is opened and clutch two is closed (grounded) which holds the ring gear on planetary two. The sun gear of the number one planetary gear is connected to the first electric motor and the carrier gear is tied to the output. The electric motors are connected via a serial bus from the battery pack and the second motor (generator) is also connected to the output.

In the compound-split mode, clutch two is opened and clutch one is closed, linking up the sun gear from planetary one to the ring gear of planetary two, creating a compound planetary system. This keeps the electric motor/generators operating at their best efficiency.

GM says that this system only needs about two-thirds the torque and about one-half the motor power for full hybrid capability. Reducing the size of the motors and inverter not only allows for packaging in a wider rang of vehicle applications, but overall mass is reduced as well as cost.

With the additional gearing operating like a transmission, the rest of system can be scaled up or down while still offering full hybridization and reap the benefits of regenerative braking, something not available with a standardtransmission. The system that will power the Chevrolet Tahoe/GMC Yukon in 2007 uses two 60k motors, and a 300 V NiMH battery pack. A 6.0L V-8 will run 13:0 to 1 compression and have late valve opening and variable valve timing making it an Atkinson cycle engine, for the best possible fuel economy. With an idle stop function, regenerative braking and the ability to run on electric power at slow speeds, GM expects a 25 percent improvement in fuel economy for its full-size SUVs. As an added advantage, Stephens says the hybrid truck actually performs better than a standard truck.

The hybrid SUV will be one of 12 different hybrid models (front-wheel-drive and rearwheel drive and all-wheel-drivet), that GM will launch before the end of the decade.

Following the mild hybrid pickups will be belt alternator starter options on the Saturn VUE and Chevrolet Malibu, two of GM’s highest volume platforms. That will be followed by the Tahoe and Yukon. GM also showed a concept Opel Astra at the North American International Auto Show powered by a 1.7L CDTI diesel engine bolted to a scaled-down version of the AHS2 electronically controlled electric variable hybrid transmission.