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Hefty hybrids

HYBRID TECHNOLOGY is best known for reaching astronomical gas mileage figures in small cars by supplementing gasoline-engine power with self-generated electrical power. Now the hybrid world is expanding, as General Motors takes it to the country with full-size hybrid pickup trucks.

Both the Chevrolet Silverado and the GMC Sierra have crew-cab models with optional two-mode hybrid systems. Naturally, their purpose is entirely different from the little city commuter cars, but the principle is the same.

In a hybrid vehicle, the gasoline engine supplies power to generate electricity that is stored in a battery pack, from where it drives two electric motors for supplemental power to ease the demands on the gas engine. In cars such as the Toyota Prius, Honda Civic and Insight, and Ford Escape and Fusion, the electric power compensates for the limited displacement of the smaller gas engines. In various larger vehicles, a larger gas engine might mean the fuel economy improvement is minimal because the goal is more power.

The two-mode hybrid system in the Silverado and Sierra aims for the latter goal. A small gas engine, with a more potent electrical system, might someday power half-ton pickups to reach 30 mpg, but in the Silverados and Sierras, the purpose was to make them work smoothly and have a large gas engine powerful enough to take on the heavy lifting required on a farm or a ranch.

Hybrid technology

The Silverado/Sierra hybrid system is not similar to the Toyota system, which uses the gas engine just to generate electrical power and depends on the electric power to move the car. It is similar to the Honda system, which has a disc-shaped generator between the transmission and the engine, and the two can run independently.

The Silverados and Sierras have a large, 6-liter pushrod V8, plus a pair of 60W electric motors that are coordinated via a transmission that has four fixed-ratio and two variable-ratio gear sets. General Motors worked with Chrysler and BMW to come up with the concept, and GM claims 40% improvement in city fuel economy and 25% in combined city-highway driving.

The system's 300v nickel-metal hydride battery pack generates its own power through the regenerative energy created by braking or coasting to slow down, as the two electric motors reverse to become generators. The stored energy can, in turn, power one of the two electric motors to move the truck up to 30 miles per hour, as long as the driver has a light toe on the gas pedal. That same electric motor supplies all the torque when moving in reverse, and it is activated to bolster the power during highway cruising, when four of the eight gas-engine cylinders shut down.

The other electric motor comes to life to restart the gas engine, which shuts down in Auto Stop mode whenever the truck comes to a stop. Both electric motors join the gas engine for maximum towing of about 6,000 lbs.

Road tests

In normal driving, it is not readily apparent you have a hybrid system operating, except for a few hesitations in the transmission, as power shifts back and forth, from the silent low-speed electric-only operation, to the various gas and assisted variations.

The base Silverado with the 5.3-liter V8 has EPA fuel economy estimates of 14 mpg city/20 mpg highway. I tested a Silverado Hybrid with a 6.0-liter V8 and the Hybrid's electric-motor boost (which improves it to 332 hp peaking at 5,100 rpm and 367 ft-lbs. of torque at 4,100 rpm). This vehicle has EPA estimates of 20 mpg for both the city and highway. During road tests, I attained 17 mpg in combined city-highway driving.

The Silverado didn't feel as quick accelerating as the same vehicle with the V8 alone, even the 5.3. And the transmission, with its fixed ratio, feels like a continuously variable transmission (CVT) more than a normal automatic, but except for some hesitations, the performance was smooth.

The base price of a Silverado 1500 4WD Crew Cab Hybrid is $41,170.

GM's two-mode hybrid system allows it to catch up to some degree, after spotting Toyota and Honda more than a decade's head start in setting and resetting the hybrid efficiency bar extremely high. But even Honda and Toyota have never put a hybrid system into a vehicle weighing 5,882 lbs. The hybrid system in the standard Silverado and Sierra might help hybrid technology make the transition into the realm of the full-size, heavy-duty pickup truck.

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