A single-mode hybrid system is sufficient for small high-mileage cars but, when you're dealing with husky, fuel-slurping trucks, you need more. What we now know as GM's two-mode hybrid system was originally developed for urban buses that began service in 2003. As the technology trickled down to Chevrolet and GMC SUVs, pickups, and (future) crossovers, GM resisted the urge to coin a new name even though the actual number of driving modes increased as the hardware evolved.
Matching last year's Chevy Tahoe and GMC Yukon applications, two-mode applied to the GMC Sierra pickup consists of a 6.0-liter V-8 engine teamed with two 80-horsepower AC electric motors, three planetary gear sets, and four multi-plate clutches. Everything but the engine and a 300-pound, 300-volt nickel metal hydride battery pack is stashed inside an aluminum case that looks like an enlarged automatic transmission. The aluminum block and head Vortec V-8 operates with late intake valve closing (called an Atkinson cycle) and Active Fuel Management (GM's name for cylinder shut-down) to maximize mileage. The engine's peak output is 332 hp@ 5100 rpm and 367 lb-ft of torque at 4100, a slight reduction from the standard Silverado's 6.0-liter output. Since both electric motors pitch in when needed, the total hybrid-powertrain output and efficiency are both higher than what's delivered by a conventional powertrain.
After all systems are warmed up, the Sierra's engine shuts down at stop lights and during low-level acceleration to 30 mph. During light-load cruising, four of the eight cylinders take a break. In normal driving, the gas and electric mix is seamless with the tach needle rising in synch with the speedometer as the drive system shifts between two continuously-variable ranges. Except for an occasional hint of electric-motor whine, there's little audible evidence of this hybrid system in action. It's notably quieter overall than any conventional powertrain.
With a trailer hitched and the throttle floored, the system climbs through four fixed ratios with a slight hesitation noticeable at 38 and 73 mph. There are instances when the tach needle hovers in a narrow rpm range while the speedometer continues climbing. Uninhibited by a trailer or law enforcement, this truck accelerates to sixty mph in 9.7 seconds and through the quarter-mile in 17.3 seconds at 85 mph on its way to a governed top speed of 99 mph.
Mileage benefits are significant. The 4wd editions scored 20 mpg in EPA city and highway tests or about 5 mpg better than a conventional full-sized V-8-powered pickup. The 2wd edition carries an even better 21 city, 22 highway mileage sticker.