Countertalk: Knowledge Building: Two-Mode Hybrid Technology

While all the current attention has been paid to Japanese automakers’ hybrid vehicles, it is the cooperative activity of GM, DaimlerChrysler, and BMW Group that is poised to have a significant impact on who will be building the most hybrids by volume, and what nameplates the aftermarket will be seeing on hybrids in the future.

This is because these three automakers have teamed up to develop the world’s first two-mode hybrid system for passenger vehicles to more rapidly advance the state of hybrid technology in the industry.

The two-mode hybrid system has been established as the starting point for the collaboration. Variants planned include rear, front, and all-wheel drive versions for cars, trucks, and other vehicles.

The two-mode hybrid will be launched in 2007 in two of GM’s most popular full-size SUVs, the 2008 Chevrolet Tahoe and the 2008 GMC Yukon. When combined with Active Fuel Management, the two-mode hybrid is expected to enable these SUVs to deliver a composite (city and highway mpg) fuel economy improvement of 25%. It is also scheduled to come out in 2008 on the Cadillac Escalade, as well as the Chevrolet Silverado and GMC Sierra crewcab pickup trucks. Using this system on full-size SUVs and pickups is part of GM’s strategy to apply advanced technologies on high-volume, higher fuel-consuming vehicle segments first.

The system uses two electrically continuously variable transmission (ECVT) modes of operation that optimize power and torque for various conditions. The addition of the second mode to the drive system improves efficiency and reduces the need for large electric motors, which are used in single-mode systems. The smaller motors are lighter and more easily packaged in the space of a typical automatic transmission.

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, which operates from vehicle launch through the second fixed gear ratio.

* Compound-split ECVT mode, or continuously variable Mode 2, which 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.

Key Advantages

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% less efficient than a mechanical path. This usually requires either a substantial compromise in vehicle capability or reliance on larger electrical motors, which can create cost, weight, and packaging issues.

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 both 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.

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.