Two-mode fixed-gear transmission
Two-mode fixed-gear transmission. Click image to enlarge

By Jim Kerr

When we think of hybrids, we usually think of small, fuel-efficient vehicles best suited for commuting. That concept may change soon. Three auto manufacturing giants – BMW, DaimlerChrysler and GM have been working together on new hybrid technology to suit bigger vehicles. Known as the Global Hybrid Cooperation, engineers from all three companies are sharing knowledge and expertise at the Troy Michigan Hybrid Development Centre. Their mandate is to build a flexible powertrain system that can be scaled to size to fit various vehicle models and brands. Each company will be able to tailor the hybrid system to their own vehicle requirements, but will have the benefit of shared components and suppliers to keep costs low.

The first hybrid design from this group will appear on 2007 models. Dodge has announced they will have it on the market in 2008 on the 2007 Durango and we will likely see large numbers of vehicles on the road from all three manufacturers. The reason? These hybrid vehicles will offer up to 25 percent fuel economy savings and still have the ability to tow or haul heavy loads.

The heart of this new hybrid design is a two-mode system that fully integrates a combination of electric motors with a fixed-gear transmission. The system is known as a two-mode system because the two electric motors can be used to produce low speed and high speed CVT (continuously variable transmission) operation. This CVT operation is combined with four fixed mechanical gear ratios to produce six operating functions. The first function is during normal acceleration, where the electric motors operate in CVT input-split mode combined with mechanical first and second gear operation. At higher speeds, the electric motors operate in CVT compound-split mode in combination with second, third and fourth mechanical gears.

The other four functions are best described as either boost or regenerative braking supplied by both electric motors in each of the four mechanical gear ratios. It sounds kind of complex, and the engineering behind it is, but the concept is quite beautiful in its simplicity. It combines the best of an electric CVT and a mechanical transmission into one unit that is about the size of a conventional transmission.

The concept transmission pictures show a planetary gear set in the front with the first electric motor behind it, followed by another planetary gear set and the second electric motor. A final planetary gear set is located at the rear of the hybrid transmission. Multi-plate clutches are used to control the middle and rear planetary gear sets, similar to what is found in a conventional automatic transmission.

A sophisticated electronic control module designed by the Hybrid Cooperation group constantly optimizes the entire hybrid powertrain system to select the most efficient operation point for the power level demanded by the driver.

The fixed mechanical gear ratios offer a more efficient path for power transfer than powering just by electric motors. They also allow the electric motors to be smaller and less dependent on engine torque or size. Therefore, this hybrid unit adapts well to a wide range of engines and drivetrains.

This hybrid technology is first being designed for rear-wheel drive or four-wheel drive applications although the technology is easily transferable to front-wheel drive applications. The design can be adapted to existing internal combustion engines and offers the advantages of improved performance and improved fuel economy. It is
particularly beneficial in demanding applications that require larger engines, such as towing, hill climbing or carrying heavy loads. Maybe your next truck will be a hybrid.

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