'Two Mode' Hybrid: 5 Facts

By Bill Siuru

In 2004, General Motors, DaimlerChrysler, and BMW formed a joint program to develop a hybrid electric system that could be adapted for use in each company’s products. The result is the two-mode hybrid system that's based on the highly successful Allison parallel hybrid transmission used in large transit buses now operating in many U.S. cities. What they came up with is impressive by any measure. Here’s what you need to know.

Why Two Modes?
The two-mode hybrid system has two operating modes, one optimized for city driving and the other for highway travel. The two-mode provides propulsion with the electric motors and battery pack alone, with the engine only, or a combination of the two systems. It can start up and travel at speeds of up to 25 mph on silent electric power, at least for short distances. At higher speeds and heavier loads, it switches to the second mode, running on the internal combustion engine with the electric motors available if needed for added power to handle acceleration, climbing steep hills, or towing a trailer.

Already in Showrooms
General Motors offers its two mode system in the Chevrolet Tahoe, GMC Yukon, and Cadillac Escalade SUVs as well as the Chevrolet Silverado and GMC Sierra pickups. It will be available in the 2009 Saturn Vue SUV. Chrysler’s two-mode hybrid models include the Chrysler Aspen and Dodge SUVs. BMW has shown its Concept X6 ActiveHybrid based on the new X6 Sport Activity Coupe with the system. It could be in production in 2009 or 2010. Mercedes-Benz plans to use the two-mode system in the 2009 ML 450 Hybrid SUV. Others will be coming in the future including ones mated to diesel engines.

How it Works
The GM two mode hybrid system features a pair of 80 horsepower electric motors integrated into an electrically continuously variable transmission (ECVT). Electrical energy is stored in a nickel-metal-hydride (NiMH) battery pack. In low speed mode, the ECVT functions with continuously variable gear ratios as power is supplied by the engine and electric motors. In the high speed mode, the transmission’s four clutches transition from variable ratio mode to one of four fixed gear ratios, like a regular automatic transmission. The other automakers’ versions work similarly, but with differences in the components used.

Engines are Important
Not all the fuel savings come from the two mode hybrid transmission. For GM and Chrysler, it also comes from shutting down half the engine’s cylinders when less power is needed. GM two mode hybrids use the 6.0L V-8 Gen IV engine with Active Fuel Management (AFM) that runs on only four cylinders at times, such as when cruising at highway speeds. Chrysler hybrids use a 5.7-liter HEMI V-8 with a Multi-Displacement System (MDS) that similarly shuts down four of the engine’s eight cylinders under light load conditions.

Impressive Fuel Savings
General Motors claims that the hybrid Tahoe, Yukon, and Escalade get 25 percent better fuel economy on the combined EPA city/highway cycles compared to the smaller 5.3-liter V-8 that’s standard in the Tahoe and Yukon. All these two mode hybrids have a 6,000 lb towing capacity, eight passenger seating, and are available in both rear- and all-wheel drive versions. Chrysler is also claiming a 25 percent improvement in fuel economy over the non-hybrid 5.7-liter HEMI V-8.