Chevrolet Silverado Mild Hybrid Pickup Offers Fuel Savings

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GM’s implementation strategy for hybrid vehicle technology isn’t geared to make bold headlines. Rather, it’s a long-term, step-by-step approach that tackles lower mile-per-gallon vehicles with the highest margin for improvement first. While the marketing department might be scratching their heads on how to make news against the onslaught of high mileage full-hybrid compacts and mid-size cars, GM’s steady game plan could potentially have the biggest impact in the long run.

The company’s first commercial/consumer entry – a full-size mild-hybrid V-8 pickup – may seem like a baby step, but considering potential volume, the Chevy Silverado and GMC Sierra Hybrids could have a huge impact in terms of overall fuel savings and emissions reductions.

GM estimates overall fuel savings of 12 percent in these pickups with their relatively simple mild-hybrid system. Our four-wheel-drive evaluation Silverado carried EPA numbers of 17 mpg city and 19 mpg highway (18 city and 21 highway on the two-wheel-drive model), and the test truck managed to meet or beat those numbers on a regular basis. GM’s comparable, conventionally-powered four-wheel-drive Vortec 5300 V-8 powered Chevrolet and GMC pickups rate EPA numbers of 15 mpg city and 19 mpg highway. Those two extra miles-per-gallon add up in a hurry if you use your vehicle much around town and they’re truly significant from a commercial fleet perspective. Perhaps more importantly, emissions reductions brought about by the hybrid truck’s engine shut-off feature represent a good start toward cleaner air around major metropolitan areas.

GM plans to follow the Silverado/Sierra Hybrids with amore sophisticated “strong” hybrid version of the full-size Tahoe and Yukon sport utility vehicles in the 2007 model year. Those vehicles will use an evolution of the Allison hybrid technology now powering transit buses, combining this with the automaker’s Displacement on Demand cylinder deactivation technology to yield a 30 percent increase in fuel economy. Strong hybrid iterations of GM’s full-size pickups will follow in 2008.

“GM’s strategy is to go after the highest fuel consuming vehicles first,” explains GM Powertrain group vice president Thomas G. Stephens, adding that “we believe this strong hybrid on a full size truck will save twice as much fuel per mile as a comparable small hybrid vehicle, with no compromise in performance or utility.”

Green Car Journal recently spent a week behind the wheel of a 2005 Chevrolet Silverado Hybrid, flogging it in real-world driving situations. To a point, the truck is quite ordinary, much like any other full-size pickups we’ve driven. In other words, GM’s mild-hybrid system is well integrated for daily use. Fuel efficiency increases and emissions reductions are largely due to the truck’s ability to stop and restart its V-8 engine under specific driving conditions. 

The truck doesn’t have a regular starter motor or alternator. In their place is a compact 14 kW starter-generator, or electric induction motor, mounted between the engine and transmission in the transmission bell housing. The starter-generator incorporates a rotor bolted directly to the engine crankshaft and a stator that’s attached to the back of the engine block. High efficiency compact coils, formed by laser welding copper bars together instead of traditional copper wire windings, allow tidy packaging.  Mounted in a concentric arrangement in series between the engine and transmission with a 258 mm diameter torque converter, the starter generator doesn’t add length to the overall powertrain dimensions.

The unit provides several functions in the mild hybrid scheme. First, the starter generator delivers fast, powerful, and quiet engine starting, enabling the stop/start function. Second, the unit generates electrical current during coast-down to act as a regenerative braking sys-tem for recharging the battery bank. Finally, the starter-generator system helps smooth out driveline surges.

Energy for the 42-volt system is stored in three valve-regulated, deep cycle lead-acid batteries mounted in a single box beneath the rear seat of the extended cab pickup. There is a small service panel on the passenger side of the box, allowing access to the shut-off switch. The rear seat flips up and with both rear doors open, the battery module is easy to service. Lead-acid batteries were chosen over nickel-metal-hydride because they’re less costly to replace at the end of their life cycle. These batteries have a 50-55 amp hour capacity and feature absorbent glass mat technology designed to meet the specific requirements of GM’s mild-hybrid application.

The system’s starter-generator control module, located underhood on the passenger fender, controls the flow of energy and torque into and out of the starter-generator unit.  Acting as the brains behind the entire system, the control module generates enough heat to warrant a dedicated cooling system. In addition to controlling the starter-generator functions, it also controls the accessory power module, allowing it to generate four types of power: 42-volt AC for the starter function, 42-volt DC, 14-volt DC, and 120-volt AC to power the pickup’s handy power outlets.

Yep, we said power outlets. The Silverado Hybrid has four 120 volt AC power out-lets, two in the cab located at the front of the battery box, and two weather sealed outlets mounted on the curb side rear bed wall. Controlled by a button on the dash, these allow the truck to act as a mobile generating station with 20 amps of continuous current total to all outlets. The bed-mounted outlets are ideal for contractors on a remote jobsite. In fact, a small fleet of GM Hybrid pickups were pressed into service as power generating stations during clean-up operations after the hurricanes that struck Florida this past hurricane season.

Hybrid engineers strive to make the driving experience as normal as possible, if not transparent. The Silverado Hybrid’s operation is so seamless that if the radio is on, passengers may not notice the engine go offline while braking at a traffic light. Though we understood the engine shut-off routine prior to driving the truck, it’s still a bit disconcerting when the oil pressure gauge goes to zero and the tach needle falls to the peg as you pull up to a stop. Take your foot off the brake and the engine restarts immediately, running before your foot reaches the accelerator pedal.

One very unique sound that may be noticed while driving a GM Hybrid pick-up is the electro-hydraulic power steering system, which is powered by the 42 volt battery pack.  This system provides power steering function and power assist for the hydraulic brake system to allow braking and steering while the engine is shut down. It also reduces the load on the gasoline engine since there isn’t a traditional power steering pump to drive.  With the engine off, turning the wheel delivers a muted electric motor sound much like the servos on a radio controlled model car. 

With many miles behind the wheel of GM pickups over the years, we were struck by how “normal” the Silverado Hybrid functions. Except for a little added weight, there simply isn’t a downside to the Hybrid option. Performance, ride, and handling qualities are difficult to differentiate from its standard gasoline counterpart.

Primary power comes from GM’s Vortec 5300 V-8, which  delivers 295 horse-power and 335 lbs-ft of torque. The 325 cubic-inch, iron block overhead valve V-8 is a workhorse that can handle a big load when needed. Acceleration is strong, as you might expect from 295 horses powering a 5008 pound 4x4 pickup. Ride quality is quite good as well, thanks to the four-wheel drive model’s torsion bar independent front suspension. Steering feel with the electro-hydraulic system is also classic GM truck, with solid tracking and good off-center feel.

The hybrid option adds an even $2,500 to the price of an Extended Cab GM pickup.  Overall, that seems reasonable considering the added fuel savings, emissions reduction, and power generating capability that’s exclusive to GM’s hybrid pickup in the full-size truck field. Once this pickup gains broader understanding of its benefits and place in the hybrid world, increased sales in the mainstream pickup market are sure to follow, which surely is part of GM’s plan.