The state-of-the-art full hybrid system, whose components are being co-developed by General Motors Corp., DaimlerChrysler and the BMW Group for production beginning next year, represents a major automotive industry milestone due to the unprecedented fully integrated combination of electric motors with a fixed-gear transmission.
As a result of its low- and high-speed electric continuously variable transmission (ECVT) modes, the system is commonly referred to as the 2-mode hybrid. However, 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.
In summary, 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.
The result is trend-setting hybrid technology that provides superior fuel economy, performance and load carrying capability.
The full hybrid system being co-developed by General Motors, DaimlerChrysler and the BMW Group has an overall mechanical content and size similar to a conventional automatic transmission, yet this full hybrid transmission can operate in infinitely variable gear ratios or one of the four fixed-gear ratios.
A sophisticated electronic control module constantly optimizes the entire hybrid powertrain system to select the most efficient operation point for the power level demanded by the driver.
When compared to conventional hybrid systems, this avant-garde hybrid technology, relying on both the ECVT modes and the four fixed gear ratios, provides advantages in combined (city and highway) fuel economy, dynamics and towing capability.
Traditional hybrid systems typically have only one torque-splitting arrangement and no fixed mechanical ratios. These systems are 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 requires usually substantial compromise in vehicle capability or reliance on larger electrical motors, which can create cost, weight and packaging issues.
General Motors, DaimlerChrysler and the BMW Group have conceived a full hybrid system featuring four fixed mechanical ratios, within the two ECVT modes, to reduce power transmission through the less efficient electrical path. Consequently, the electric motors are more compact and less dependent on engine size.
This 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.
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.
Global Hybrid Cooperation
General Motors, DaimlerChrysler and the BMW Group have formed a cooperative effort called the Global Hybrid Cooperation, which is actively developing this next generation hybrid powertrain system. In an alliance of equals, all three partners are pooling expertise and resources to jointly and efficiently develop hybrid technology. Each company will individually integrate the full hybrid system into the design and manufacturing of vehicles in accordance with their brand specific requirements.
In Troy, Michigan, the “GM, DaimlerChrysler and BMW Hybrid Development Center” houses together engineers and specialists from all three companies to develop the complete hybrid system and the individual components — electric motors, high-performance electronics, wiring, energy management, and hybrid system control units. In addition, the “GM, DaimlerChrysler and BMW Hybrid Development Center” will be responsible for system integration and project management.
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. Currently full hybrid systems are under development for front- and rear-wheel-drive passenger cars, and light-duty truck and SUV applications.