Two-Mode Tahoe Hybrid: Smooth and Quiet

By Tom Berg, Truck Editor | September 28, 2010
Chevrolet Tahoe Hybrid

Chevrolet Tahoe Hybrid looks pretty much stock except for special badges. Lightweight body parts, including an aluminum hood, contribute a bit to 21-mpg city fuel economy, about 50 percent better than a Tahoe with a 5.3-liter straight-gasoline V-8. 
GMC Sierra Chevrolet Silverado pickups

The 2-Mode Hybrid system will be available in GMC Sierra and Chevy Silverado pickups in early 2009. 
Chrysler Aspen Dodge Durango

Two-motor, multi-ratio transmission, developed by a GM-Chrysler-BMW joint venture, is also offered in Chrysler Aspen and Dodge Durango SUVs. 
GMC Sierra Chevrolet Silverado pickups

A hefty 6-liter 332-horsepower Vortec V-8 acts as aV-4 whenever possible and shuts down frequentlyin stop-and-go driving. The hybrid's electric system,partly housed in the far corner of the engine compart-ment, operates at 300 volts.
GMC Sierra Chevrolet Silverado pickups

Tach needle rests on Auto Stop as the engine naps at a red light; it restarts instantly when needed. Readout shows unimpressive economy during heavy-footed demonstrations, but the hybrid's supposed to get 20 to 21 mpg in town.

It's quiet! That's the first thing I thought as I began driving this Chevrolet Tahoe with its 2-Mode Hybrid system. It's so quiet that safety experts worry it'll sneak up on pedestrians, and in California they're suggesting minimum noise standards for hybrids, if you can believe it. And it's smooth, so smooth that I hardly knew what the engine and transmission were doing except by watching the tachometer.

More importantly, it was saving fuel, or was supposed to be. This Tahoe was being hammered by enthusiastic automotive press reporters at a GM event last May in Las Vegas, and a readout under the tach showed only 14 mpg. However, a two-wheel-drive Tahoe or GMC Yukon Hybrid should get 21 miles per gallon in the city and 22 on the highway (or 20/21 as a 4x4) versus 15/17 or so for one with a straight gasoline-and-automatic transmission power train, according to General Motors Corp., citing the federal EPA's test numbers.

This gasoline-electric hybrid could well revive the idea of driving a full-size sport-utility vehicle as a personal conveyance, something that folks are turning away from since the run-up in fuel prices. It could also cut operating expenses for anyone who uses a pickup truck for work, because this is the same system that'll be available on Silverado and Sierra 1500-series pickups early next year. A GM Hybrid SUV is rated to pull 6,000 pounds, and that'll probably be higher in the half-ton pickups. Chrysler LLC also offers the 2-Mode system in two SUVs and is readying it for certain pickups.

The 2-Mode Hybrid is so named because its 4-speed automatic transmission has two electric motor-generators and two sets of infinitely variable ratios, plus fixed ratios in 1st and 2nd gears under high-load conditions. The motors act as generators during coasting and braking, sending electricity to a bank of nickel-metal-hydride batteries, from where the juice is sent back to the motors to help acceleration. They whine faintly while operating, making for an eerie experience at first, but one quickly becomes accustomed to it.

A joint venture involving GM, Chrysler and BMW shared the work and expense of developing the complex device, which GM calls an Electronically Variable Transmission, and the three companies have begun selling vehicles that use it. In North America it's available in the Chrysler Aspen and Dodge Durango SUVs, and in the 2010 model year it will be an option in Dodge Ram 1500 pickups. Chrysler employs its own 5.7-liter Hemi V-8 engine, along with its own electrical parts and electronic controls. To a driver, Chrysler's 2-Mode Hybrid operates like GM's.

GM mates the 2-Mode's transmission to a 6-liter Vortec V-8, which has plenty of horsepower (332) and torque (364 pounds-feet), yet it's well muffled, as is the cabin in this limo-like truck.

Full torque from both the motors and the engine is available when needed, so it's a proverbial stump puller. Under light loads the V-8 engine becomes a V-4 whenever it can, closing exhaust valves on those unused cylinders and employing variable valve timing in many situations. And it shuts down frequently in stop-and-go driving.

From a dead stop, the vehicle moves out under electric propulsion — that's where most of the gasoline is saved — and the engine fires up when speed begins to build and/or the driver puts his foot into it. If the driver uses a light foot, the motors can accelerate the Tahoe to 30 mph, GM says. With this one I couldn't get above 13 mph before the engine restarted.

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