General Motors, DaimlerChrysler and BMW have announced that the full hybrid system they are co-developing will be a major automotive industry milestone due to the unprecedented fully integrated combination of electric motors with a fixed-gear transmission.

In a statement, the companies said that as a result of its low- and high-speed electric continuously variable transmission (ECVT) modes, the system is commonly referred to as the two-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 companies say their new hybrid system has an overall mechanical content and size similar to a conventional automatic transmission. A sophisticated electronic control module constantly optimises the entire hybrid powertrain system to select the most efficient transmission operation point for the power level demanded by the driver, be it a fixed gear ratio or a continuously variable point.

They say that their two-mode hybrid has several advantages over traditional hybrid systems. Traditional one-mode hybrids transmit a significant amount of power through an electrical path that is 20% less efficient than a mechanical path, as is the case in the two-model hybrid.

One-mode hybrids therefore require larger electrical motors, which can create cost, weight and packaging issues.

Furthermore, 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.

IN addition, 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 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.