June 19, 2006 Details of the hybrid drive system under co-development by General Motors, DaimlerChrysler and BMW have been released and the new system is quite a technological feat due to the full integration 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. The system also incorporates four fixed gear ratios for high efficiency and power-handling capabilities. During the two ECVT modes and four fixed gear operations, the hybrid system can use the electric motors for boosting and regenerative braking. Traditional “one-mode” hybrid systems typically have only one torque-splitting arrangement and no fixed mechanical ratios. 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. Working collaboratively, the group has 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.
The new system means that 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 automakers to package internal combustion engines with the full hybrid transmissions more cost-effectively and scale 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. 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 is being readied for production beginning next year.
In summary, the four fixed gears overlay two ECVT modes for a total of six operating functions:
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.
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.
The result is a hybrid technology that provides superior fuel economy, performance and load carrying capability.
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