A Better Hybrid?

Others may have gotten the jump on hybrid-electric vehicles, but hybrid-development partners General Motors Corp., DaimlerChrysler AG and BMW AG think they have a better idea when it comes to HEV technology.

“We're convinced the concept we're following is superior (to HEV leader Toyota Motor Corp.'s),” declares DC's Andreas Truckenbrodt, executive director-Hybrid Powertrain Programs.

The new hybrid powertrain system, developed through a cooperative launched less than two years ago and ready to begin appearing in vehicles in 2007, integrates electric motors with an electronic continuously variable transmission (ECVT) and a step-ratio automatic transmission that features four fixed gearsets.

The “synergistic combination” of ECVT and a conventional automatic offers six distinct operating modes and allows for infinitely variable gear ratios, engineers say, and it all fits into existing vehicle drivetrain layouts.

Compared with current HEV technology, the GM/DC/BMW design is more economical in highway driving and with larger-engine vehicles, the partners contend, and is well tuned to truck applications because it produces more torque to provide for a higher towing capacity.

The so-called Two-Mode Hybrid system developed by the three auto makers fits into a conventional transmission housing and can be scaled in size to accommodate the appropriate vehicle and engine type. The technology works with 4- or all-wheel-drive configurations.

The key feature with the GM-DC-BMW Two-Mode system and its stepped gearing capability, engineers say, is its ability to deliver all of the internal combustion engine's (ICE's) power directly to the driving wheels under several different operating conditions, eliminating power loss through the less-efficient electrical path and saving fuel.

In most hybrids, power from the ICE is transmitted directly through an ECVT between two paths, mechanical and electrical. The mechanical path transfers the engine power to the drive axle, while the electrical path flows power through an electric motor or motors, which can act as a generator to charge the hybrid system's battery or drive the wheels.

The problem with that system, engineers from the three companies say, is its potential for wasting some of the power generated by the ICE. Electrical path efficiency is only about 70%, while the mechanical path delivers more than 90% of the available power to the drive axle.

And in today's ECVT-based systems, there are few driving situations in which no power is being transmitted through the electrical path. That means under some conditions, such as at high speeds on the highway, power ends up being wasted through the electrical path.

HEV systems on the road today also require large, expensive electrical motors, usually with a combined power rating that is more than the horsepower rating of the ICE.

Also, production HEVs to now have been proven successful only in small to midsize vehicles, the partners point out. As vehicle size is increased — something GM, DC and BMW are keenly focused on in relation to their large luxury cars and big SUVs and pickups — the electric motors, particularly the one connected to the final drive, become larger, heavier and more expensive, complicating packaging.

Adding 2-speed gearing to a single-mode HEV helps increase torque and reduce the size of the electric motors, engineers for the GM/DC/BMW development alliance say, but it still doesn't optimize the power flow along the electrical path in the same way the new Two-Mode setup does.

GM has been using a Two-Mode powertrain in hybrid buses in operation in urban fleets around the country, and that served as a template for the new system.

“But we've made further improvements,” notes GM's Larry Nitz, executive director-Hybrid Powertrain Engineering.

The GM-DC-BMW system overlays two ECVT modes for low- and high-speed operation, along with the four fixed gears, offering a total of six operating modes:

The powertrain chooses its operating mode based on vehicle speed and load.

In addition to improving torque and towing, developers say the system boosts combined city/highway fuel economy. Current HEVs offer little gain in highway driving, although engineers decline to give estimates on the potential improvement. Earlier, GM said the system could improve fuel economy vs. a conventional powertrain by as much as 25% overall.

The package allows existing ICEs to be used with minimal changes, the partners add, because the hybrid system places no significant limits on the size or type of engine required. That means it is relatively easy for each auto maker to match the powertrain to individual existing engines, including large-displacement V-8s.

It also is said to work well with cylinder deactivation technology and can accommodate diesels. “It depends on the demand for that and how much customers are willing to spend,” Truckenbrodt says of integration with a diesel engine.

Adding the planetary gearsets makes for more components in the package, Nitz says, but it remains an inexpensive way to go.

“There are more mechanical parts (with the Two-Mode system),” he says. “But planetary gearsets are not expensive to build. GM makes millions of planetary gears a year. If we can make the electric motors half as big (as those used in competitive systems)… that's where the (savings) is in hybrids.”

Engineers won't detail how much the operation will save in cost and weight vs. the HEV systems now in use, or how much fuel economy will be improved. The system weighs more than a GM 6-speed automatic transmission.

They also are unwilling to detail the battery technology used, although indications are it will be nickel-metal-hydride (NiMH).

“It's the kind of technology available to us now,” Nitz says of the batteries, reminding the system has been in joint development for two years, while lithium-ion technology — believed to be an advancement over NiMH — is just emerging.

Engineers do reveal the system uses a 300-volt battery pack and two 60-kW electric motors. They say the motors are roughly half the size used in competing HEVs. The system has stop/start functionality to shut down the ICE in traffic and restart it again via the input electric motor.

The Global Hybrid Cooperation, as the collaborative effort is called, was formed in December 2004 between GM and DC. BMW joined in September 2005. A spokesman for DC says the JV remains open to the possibility of additional partners.

The cooperative came about when engineers for the three companies were presenting hybrid concepts at a conference and discovered they all were heading down the same path. Developers floated the idea of forming a JV to management.

“We could have done this on our own,” Truckenbrodt says of the hybrid system. “But we thought it was much smarter to look for a partner for dollar reasons.”

Adds BMW's Wolfgang Epple, vice president-Hybrid Program: “There are new rules of the game (making it necessary) to cooperate in certain areas.”

He points to varying fuel economy and emissions requirements worldwide and changing customer expectations that make it hard for auto makers to finance the wide variety of new technology requirements on their own. “This gives us the volume that leads to economies of scale,” he says.

The alliance's hybrid development center in Troy, MI, houses engineers from 20 different nations under the guidance of three different management teams. It consists of both company-specific and co-located operations.

All developmental, sourcing and production decisions are made together by the three partners, and patents developed in the cooperative are jointly held.

“There's no contract that says one company has a bigger share (of the JV) than the others,” Epple says. “Everything we do, we have to have a consensus.”

The Global Hybrid Cooperation is collaborating on two hybrid systems based on the same concept, a larger unit for use in fullsize trucks and a smaller one designed to package into rear-wheel-drive luxury cars and other vehicles.

GM's first applications will come next year with the '08 Chevrolet Tahoe. That will be followed shortly after by a hybrid Dodge Durango from Chrysler Group. The system also will be used in the Cadillac Escalade and GMC Yukon beginning in 2008. All the GM vehicles will be built at the auto maker's Arlington, TX, plant, and the Durango will come from Chrysler's Newark, DE, facility.

The smaller unit will be used in GM, DC and BMW luxury cars. BMW showed a hybrid version of its X3 cross/utility, but says a hybrid X3 isn't necessarily in the plans. It does say it will offer HEVs in Europe, as well as the U.S.

Engineers say there are multiple programs planned, each unique. That means the GM vehicles will exhibit driving characteristics associated with that company's brands, while the systems will meet different criteria in Chrysler, Mercedes and BMW vehicles — and luxury cars will boast performance characteristics that are different from those of the trucks.

GM will produce the larger, RWD hybrid systems at its Baltimore Transmission Operations in White Marsh, MD. The auto maker will invest $118 million and add as many as 87 jobs to the 440 already at the transmission plant as part of the launch.

The partnership isn't disclosing production plans for the smaller unit aimed at passenger cars.

The three auto makers say there is nothing that prevents expanding application into smaller vehicles. But engineers say, in the long run, a full hybrid system may not be the best choice for that segment of the market.

The system also could be geared toward performance rather than fuel economy, similar to the approach Toyota is taking with its Lexus GS and LS sedans.