DaimlerChrysler, BMW, and GM today announced a new full hybrid system that “represents a major automotive industry milestone due to the unprecedented fully integrated combination of electric motors with a fixed-gear transmission.” The technology is said to be ready for production beginning next year. 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 system 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.

Key Advantages

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.

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