The Car and Truck Fleet and Leasing Management Magazine

Two-Mode Hybrids Point to a New Direction

April 2008, by Paul Dexler - Also by this author

General Motors created somewhat of a sensation when it announced the launch of the full-size hybrid Chevrolet Tahoe and GMC Yukon SUVs and the Silverado hybrid pickup at the Los Angeles Auto Show in November. Chrysler followed with news that the system will also be offered in the Chrysler Aspen and Dodge Durango full-sized SUVs and the Dodge Ram pickup.

Previous hybrids have been aimed at the market segment that demands the highest fuel economy numbers. GM and Chrysler turned that on its head by introducing the new system on vehicles that needed the greatest boost in fuel economy numbers.

At the Detroit Auto Show in January, the third member of the two-mode hy-brid consortium showed off its vehicle. BMW displayed the Concept X6 Active Hybrid SUV.

The Two-Mode System

The system is built into the case of the new electrically variable transmission (EVT), which is no larger than a normal automatic transmission. Inside, however, are two electric motors and two gear sets. The modes blend continuously variable operation (for low-load, low-speed driving situations) with fixed-gear operation (for high-load conditions such as towing or highway driving).

A computer, called the hybrid operating system (HOS) manages the distribution of gasoline and electric power to where it is needed for most efficient operation. The system includes three major components:

  • Electrically variable transmission (EVT).
  • Energy storage system (ESS), the drive battery.
  • An engine with cylinder deactivation technology.

The EVT functions are controlled by the HOS, which constantly searches for optimum transmission operation to meet current conditions. The HOS also bases its decisions on allowing the gasoline engine to take full advantage of its cylinder deactivation system to remain in reduced-cylinder mode as long as possible for maximum fuel economy.

While the gasoline-only versions of the two-mode hybrids also offer cylinder deactivation, the hybrid system allows the vehicle to run in reduced-cylinder mode more often and for longer periods — because of the boost from the electric motors. This is the key to the hybrids' 25- to 30-percent improvement over the gasoline-only models in combined city/highway fuel economy.

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