Global Hybrid Cooperation (formerly called Advanced Hybrid System 2 or AHS2) is a set of hybrid vehicle technologies jointly developed by General Motors and DaimlerChrysler, with BMW joining in 2005. It uses two sets of gears in an automatic transmission: One for the internal combustion engine and another to multiply the power of a pair of electric motors. General Motors has stopped using the "AHS2" name as of 2006, preferring to call it simply a "two-mode hybrid system".
Toyota's Hybrid Synergy Drive is similar in that it also combines the power from a single engine and a pair of electric motors, although it uses only one planetary gearset. Honda's Integrated Motor Assist uses a more traditional internal combustion engine and transmission where the flywheel is replaced with an electric motor.
The goal of the cooperation is to build a flexible powertrain system that can be scaled to fit various vehicle models and brands. Each company can tailor the hybrid system to their own vehicle requirements, but will have the benefit of shared components and suppliers to keep costs low.
When GM and DaimlerChrysler engineers realized how similar their hybrid work was, they decided to join forces and share technology. The GM/DaimlerChrysler partnership was announced on December 13, 2004 with Dieter Zetsche of DaimlerChrysler joining Rick Wagoner of GM on stage with a prototype. The agreement was not signed until the following August, however.
GM is reportedly responsible for development of rear- and four-wheel drive truck and front wheel drive car systems while DaimlerChrysler is focused on a rear wheel drive luxury car application.
It was announced on September 7, 2005 that BMW would also join the alliance, likely using archrival DaimlerChrysler's rear wheel drive system.
The three companies have formed an organization called Global Hybrid Cooperation with engineering and management centered at the GM, DaimlerChrysler and BMW Hybrid Development Center in Troy, Michigan.
- Input-split mode — At low speeds, the vehicle can move with either the electric motors, the internal combustion engine, or both, making it a so-called full hybrid. All accessories will still remain functioning on electric power, and the engine can restart instantly if needed. This mode is operational using the first and second gear ratios of the transmission.
- Compound-split mode — At higher speeds or heavier loads, the internal combustion engine always runs, and the system uses advanced technologies like Active Fuel Management and late intake valve closing to optimize fuel efficiency.
The sophisticated fuel-saving system also incorporates four fixed gear ratios for high efficiency and power-handling capabilities in a broad variety of vehicle applications. During the two ECVT modes and four fixed gear operations, the hybrid system can use the electric motors for boosting and regenerative braking.
The four fixed gears overlay two ECVT modes for a total of six operating functions:
- Input-split ECVT mode, or continuously variable Mode 1, operates from vehicle launch through the second fixed gear ratio.
- Compound-split ECVT mode, or continuously variable Mode 2, operates after the second fixed gear ratio.
- First fixed-gear ratio with both electric motors available to boost the internal combustion engine or capture and store energy from regenerative braking, deceleration and coasting.
- Second fixed-gear ratio with one electric motor available for boost/braking,
- Third fixed-gear ratio with two electric motors available for boost/braking.
- Fourth fixed-gear ratio with one electric motor available for boost/braking.
Although the transmission mechanically has only four conventional gear ratios, the electric motors allow it to function as a continuously variable transmission. This variable ratio functions in addition to the torque multiplication of the planetary gears.
Despite the "two-mode" marketing pitch, however, it is the packaging of the first application of the system which is unique. A special automatic transmission incorporates two 60 kW (80 hp) DC electric motors, two planetary gearsets, and two selectively-engaging friction clutches. This system amplifies the output of the electric motors similarly to the way in which a conventional transmission amplifies the torque of an internal combustion engine. It also transfers more of the engine's torque to the wheels, making the transmission more efficient even without the electric motors in use. Finally, the whole system fits into the space of, and indeed appears as, a conventional GM 4L60-E automatic transmission.
A 300 volt battery pack is housed elsewhere in the vehicle to store energy. Most applications will also include 120 volt AC power outlets, as on the 2004 Chevrolet Silverado Hybrid.
GM will unveil a new version of the Saab BioPower Hybrid Concept at the 2006 British International Motor Show. The earlier version of the E100-capable flex-fuel hybrid concept featured a 300-volt Lithium-ion battery pack, a 38kW rear-mounted electric motor (rear drive unit - RDU), a 15kW integrated starter/generator (ISG) and all-wheel-drive with electric power transmission to the rear wheels. The new version of the concept builds on this by incorporating the Global Hybrid Cooperation two-mode transmission while retaining the RDU. The concept thus combines a flex-fuel engine capable of producing 260 horsepower(191 kW) with three electric motors that can generate a total of 148kW.
The Global Hybrid Cooperation two-mode transmission seems to resemble at least some, if not most, aspects of the SEL Transmission, researched and documented by TU Chemnitz under a public research grant, in July 2000.
Small hybrid vehicles like the Prius, Escape Hybrid and Honda Insight can't tow heavy loads, and sustained uphill driving will eventually discharge the batteries enough to turn off the electric motor assist, which reduces performance and increases fuel consumption. Traditional hybrid systems typically have only one torque-splitting arrangement and no fixed mechanical ratios, often called “one-mode” hybrids. Due to their less capable mechanical content, one-mode hybrids need to transmit a significant amount of power through an electrical path that is 20 percent less efficient than a mechanical path. This usually requires substantial compromise in vehicle capability or reliance on larger electrical motors, which can create cost, weight and packaging issues.
The Global Hybrid Cooperation's two-mode hybrid system reduces power transmission through the less efficient electrical path. Consequently, the electric motors are more compact and less dependent on engine size.
The combination of two ECVT modes and four fixed gear ratios eliminates the drawbacks of one-mode hybrid systems to allow for efficient operation throughout a vehicle’s operating range, at low and high speeds. It also allows for application across a broader variety of vehicles. It is particularly beneficial in demanding applications that require larger engines, such as towing, hill climbing or carrying heavy loads.
Existing internal combustion engines can be used with relatively minimal alteration because the full hybrid system imposes no significant limitation on the size or type of engine. It enables the three global automakers to package internal combustion engines with the full hybrid transmissions more cost-effectively and offer the fuel-saving technology across a wider range of vehicles. A key factor in ensuring optimum development is the focus on a flexible system design that can be scaled to the size, mass and performance needs of the various vehicle concepts and brands. The extensive sharing of components and the collaborative relationship with suppliers will enable the alliance partners to achieve economies of scale and associated cost advantages that will also benefit customers.
The two-mode hybrid system could boost fuel economy of big SUVs and large luxury cars by at least 25 percent. This would allow GM's SUVs, such as the 6,000-pound Chevrolet Tahoe Hybrid, to get about 26 mpg in combined city/highway driving, and to keep ahead of any government-imposed increases in corporate average fuel economy.
Strategic Advantages of the Project
- The two-mode hybrid system will boost fuel economy of big SUVs and large luxury cars, perhaps by at least 25 percent.
- The three partner automakers save development time and money.
- Offering a fuel efficient full-sized SUV could give GM and Dodge a major competitive advantage over Ford and the imports, and help GM sell its most profitable vehicles.
- The two-mode hybrid system could boost the engineering reputations of all three companies.
- Hybrids offer the BMW and Mercedes an alternative in case diesel engines don't become popular in the United States as expected.
Initial applications are suitable for front-engine, rear- and four-wheel-drive vehicle architectures, but the full hybrid system has the flexibility to be used in front-engine, front-wheel-drive architectures in the future as well.
The system was first used in the General Motors transit buses deployed in 2004.
- City bus system by Allison Transmission
- Albuquerque, New Mexico (twelve busses announced December 21, 2004)
- Indianapolis, Indiana IndyGo (two busses announced January 24, 2005)
- Yosemite National Park National Park Service (18 Gillig busses announced April 25, 2005)
- Shreveport, Louisiana SporTran (one bus announced June 9, 2005)
- Charlotte, North Carolina Charlotte Area Transit System (2 busses announced June 9, 2005)
- Springfield, Massachusetts Pioneer Valley Transit Authority (one bus announced October 14, 2005)
- Aspen, Colorado Roaring Fork Transportation Authority (seven busses announced December 9, 2005)
- Consortium of 11 transit agencies in California, Nevada and New Mexico (157 busses announced March 20, 2006)
The longitudinal system for light trucks from General Motors will be manufactured at Baltimore Transmission by GM's Allison Transmission division. The nickel-metal hydride batteries will be manufactured by Panasonic EV of Japan.
The system was to be introduced for the 2007 model year in the full-sized GM SUVs, but these were delayed for one year for unspecified reasons.
- Rear wheel drive truck system
- Front wheel drive car system
- Opel Astra Diesel-Electric Hybrid hybrid concept (shown at the 2005 NAIAS)
- Rear wheel drive car system
Mercedes-Benz and BMW have not stated which vehicles will receive the transmission or when it will debut in their vehicles, but it could be used with their largest luxury cars to compete with the Lexus GS 450h sedan.
- All wheel drive car system
- Saab BioPower Hybrid Concept (to be shown at the British International Motor Show)
- "Gas-Guzzling Hybrids"
- "Chevrolet Tahoe with Two-Mode Full Hybrid Exploits Fuel Savings, SUV Driving Pleasure and Performance"
- "Global Alliance for Hybrid Drive Development: Cooperation between BMW, DaimlerChrysler and GM"
- GM Adds Two-Mode Hybrid Transmission to Saab BioPower Concept
- New hybrid technology coming for bigger vehicles
- The New Two-Mode Hybrid System from the Global Hybrid Cooperation, General Motors press release / April 28, 2006
- Hybrid Cars and Vehicles — MixedPower.com
- Comparative Assessment of Hybrid Vehicle Power Split Transmissions
- Electrically variable transmission having two planetary gear sets with one fixed interconnection, US Patent Application 2006/0111213 A1