Others may have gotten the jump on hybrid-electric vehicles, but hybrid-development partnersCorp., DaimlerChrysler AG and AG were sure they had a better idea where HEV technology was concerned.
“We’re convinced the concept we’re following is superior (to HEV leaderMotor Corp.’s),” DC’s Andreas Truckenbrodt, executive director-Hybrid Powertrain Programs, declared at a spring 2006 media backgrounder in New York.
The new hybrid powertrain system, developed through a cooperative launched less than two years earlier and ready to begin appearing in vehicles in 2007, integrated electric motors with an electronic continuously variable transmission (ECVT) and a step-ratio automatic transmission that featured four fixed gearsets.
The “synergistic combination” of ECVT and a conventional automatic offered six distinct operating modes and allowed for infinitely variable gear ratios, according to engineers, and it all fit into existing vehicle drivetrain layouts.
Compared with existing HEV technology, the GM/DC/design was more economical in highway driving and with larger-engine vehicles, the partners contended, and was well tuned to truck applications because it produced more torque to provide for a higher towing capacity.
The so-called Two-Mode Hybrid system developed by the three auto makers fit into a conventional transmission housing and could be scaled in size to accommodate the appropriate vehicle and engine type. The technology worked with 4- or all-wheel-drive configurations.
The key feature with the GM-DC-BMW Two-Mode system and its stepped gearing capability, according to engineers, was 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 typically on the market, 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 said, was its potential for wasting some of the power generated by the ICE. Electrical path efficiency typically was only about 70%, while the mechanical path could deliver more than 90% of the available power to the drive axle.
And in available ECVT-based systems, there were few driving situations in which no power was being transmitted through the electrical path. That meant under some conditions, such as at high speeds in highway driving, power ended up being wasted through the electrical path.
HEV systems already on the road also required large, expensive electrical motors, usually with a combined power rating that was 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 pointed 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 and pressuring profit margins.
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 told Ward's, but it still wouldn’t optimize the power flow along the electrical path in the same way the new Two-Mode setup would.
GM had been using a Two-Mode powertrain in hybrid buses in operation in urban fleets around the country for some time, and that served as a template for the new light-vehicle hybrid system.
“But we’ve made further improvements,” noted 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:
- In input-split ECVT mode (or continuously variable Mode 1), the unit propels the vehicle from launch through the second fixed gear ratio.
- Compound-split ECVT mode (continuously variable Mode 2) takes up operation after the second fixed gear ratio.
- First fixed-gear with both electric motors mode supplements power to the drive wheels or allows energy to be captured and stored from regenerative braking, deceleration and coasting.
- Second fixed-gear ratio and one electric motor mode can be used for a boost in power or regenerative braking.
- Third fixed-gear ratio with two electric motors mode provides a power boost or regenerative braking.
- Fourth fixed-gear ratio with one electric motor mode can provide more drive power or regenerative braking.
In addition to improving torque and towing power, developers said the system also benefits combined city/highway fuel economy – current HEVs offer little fuel-economy gain in highway driving, although engineers declined 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 added, because the hybrid system places no significant limits on the size or type of engine required. That meant it would be relatively easy for the three auto makers to match the hybrid powertrain to their individual existing engines, including large-displacement V-8s.
It also was said to work well with cylinder deactivation technology that is emerging on many of the industry’s larger engines and – as proven with the GM bus program – could accommodate diesels.
“It depends on the demand for that and how much customers are willing to spend,” Truckenbrodt said of the possibility of marrying the system with a diesel in light vehicles. “But it is certainly an option.”
Adding the planetary gearsets makes for more components in the package, Nitz pointed out, but it remains an inexpensive way to go.
“There are more mechanical parts (with the Two-Mode system),” he noted. “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 (cost) is in hybrids.”
Engineers wouldn’t detail how much the operation would save in cost and weight vs. the HEV systems already in use, or how much fuel economy would be improved. The system weighed more than a GM 6-speed automatic transmission, they said, but officials declined to specify how much more.
They also were unwilling to detail the battery technology used, although indications were it would be nickel-metal-hydride (NiMH).
“It’s the kind of technology available to us now,” Nitz said of the batteries, reminding the system had been in joint development for about two years, while lithium-ion technology – believed to be an advancement over NiMH – was just emerging.
Engineers do reveal the system uses a 300-volt battery pack and two 60-kW electric motors. They said the motors were roughly half the size used in competing HEVs on the market.
The system did have 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 was called, was formed in December 2004 between GM and DC. BMW joined in September 2005. A spokesman for DC said the JV remained open to the possibility of additional partners.
The cooperative came about, the auto makers noted, when engineers for the three companies were presenting concepts for hybrids at a technical conference and discovered they all were heading down the same path. Developers floated the idea of forming a JV to upper management and the cooperative was formed.
“We could have done this on our own,” said Truckenbrodt of the hybrid system. “But we thought it was much smarter to look for a partner for dollar reasons.”
Added BMW’s Wolfgang Epple, vice president-Hybrid Program: “There are new rules of the game (making it necessary) to cooperate in certain areas.”
He pointed to varying fuel economy and emissions requirements worldwide and changing customer expectations that make it tough 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 said.
The alliance’s hybrid development center in Troy, MI, was used to house engineers from 20 different nations under the guidance of three different management teams. It consisted of both company-specific and co-located operations.
All developmental, sourcing and production decisions were being made together by the three partners, and patents developed in the cooperative were jointly held.
“There’s no contract that says one company has a bigger share (of the JV) than the others,” said Epple. “Everything we do, we have to have a consensus.”
The Global Hybrid Cooperative was 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 application was set to come with the ’08 Chevrolet Tahoe. That would be followed shortly after by a hybrid Dodge Durango fromGroup. The system also was to be used in the Cadillac Escalade and GMC Yukon beginning in 2008. All the GM vehicles were to be built at the auto maker’s Arlington, TX, plant, and the Durango would come from Chrysler’s Newark, DE, facility.
The smaller unit would be used in GM, DC and BMW luxury cars. BMW earlier had shown a hybrid version of its X3 cross/utility, but pointed out that a hybrid X3 wasn’t necessarily in the plans. It did say it will offer HEVs in Europe, as well as the U.S.
In all, according to engineers, there were multiple programs planned, “and each has different plan values.” That meant the GM vehicles would exhibit driving characteristics associated with that company’s brands, while the systems would meet different criteria in, Mercedes and BMW vehicles – and luxury cars would boast performance characteristics different from those of trucks.
GM was to produce the larger, RWD hybrid systems at its Baltimore Transmission Operations in White Marsh, MD. The auto maker planned to 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 didn’t disclose production plans for the smaller unit aimed at passenger cars, nor was it willing to detail volume plans for either unit.
The three auto makers pointed out there was nothing that prevented expanding application into smaller vehicles. But engineers said, in the long run, a full hybrid system may not be the best choice for that segment of the market.
“There’s nothing here that couldn’t be applied to smaller vehicles,” said Truckenbrodt. “The question is what makes economical and market sense. A mild hybrid (where only the ICE actually drives the wheels and an electric motor is used to restart the engine) might make more sense.
“But 80% of hybrids out there today are full hybrids,” he noted, acknowledging competitive pressure may force DC and others to “go this way.”
“The question is where is the hybrid market going,” Nitz said. “We’re working together to answer this.”
The system also could be geared toward performance rather than fuel economy, similar to the approachwas taking with its Lexus GS and LS sedans.
“There’s plenty of electric motor here to give you a performance-type boost,” Nitz said.