General Motors, DaimlerChrysler and BMW-–the Global Hybrid Cooperation--say their co-developed hybrid system should be ready for production next year.
The trio pronounced their system “unprecedented” based on its fully integrated combination of electric motors with a fixed-gear transmission. It’s a two-mode hybrid, with low- and high-speed electric continuously variable transmission (ECVT) modes, but it also incorporates four fixed gear ratios for high efficiency and power-handling capabilities in a variety of vehicle applications. During the two ECVT modes and four fixed gear operations, the system can use 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 (CV mode 1) operates from vehicle launch through the second fixed gear ratio. Compound-split ECVT mode (CV mode 2) operates after the second fixed gear ratio. In the first fixed-gear ratio, both electric motors are available to boost the internal combustion engine or capture and store energy from regenerative braking, deceleration and coasting. The second fixed-gear ratio has one electric motor available for boost/braking, The third fixed-gear ratio makes two electric motors available for boost/braking, and the fourth fixed-gear ratio has one electric motor available for boost/braking.
The full hybrid system has an overall mechanical content and size similar to a conventional automatic transmission, but it can operate in infinitely variable gear ratios or one of the four fixed-gear ratios. An 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 automakers said their system provides advantages in combined (city and highway) fuel economy, dynamics and towing capability, since traditional--1-mode--hybrid systems typically have only one torque-splitting arrangement and no fixed mechanical ratios, and must transmit a significant amount of power through an electrical path that is 20% less efficient than a mechanical path. As a result, vehicle capability can be compromised or larger electrical motors required.
The hybrid system can be used with existing internal combustion engines with relatively minimal alteration, according to the automakers, because the full hybrid system imposes no significant limitation on the size or type of engine. The development partners expect to package internal combustion engines with full hybrid transmissions more cost-effectively and offer the fuel-saving technology across a wider range of vehicles.