BANDERA, Tex. -- Cattle, grazing in a pasture studded with limestone and live oaks in the hills of Texas, give way to a big-rig sport utility vehicle as we rock and roll across the rugged landscape on a twin-rut track etched into sandy loam.
To keep the tires tracking through this sand and stone demands a sturdy vehicle that's not only hiked high in the suspension but outfitted with a traction system that applied the big engine's strength to all four wheels.
For this off-road excursion we're steering a special version of GMC's full-size Yukon SUV rigged with four-wheel-drive (4WD) traction.
Well, it's a large SUV with a huge V8 engine aboard and 4WD equipment but it's also a hybrid electric vehicle (HEV).
The HEV version of Yukon uses a special two-mode hybrid propulsion system developed by General Motors to improve fuel economy scores.
GM constructs the HEV-SUV off the GMC Yukon, a truck-based SUV of the full-size and half-ton class, and makes an equivalent model for Chevrolet as the Tahoe Hybrid.
The HEV version of Yukon (or Tahoe) looks virtually identical to a conventional Yukon (or Tahoe), save for strips of door decals denoting the hybrid nature and special fender and tailgate badges in chrome.
Riding on a chassis constructed with boxed frame rails stretching from tip to tail for platform rigidity, Yukon Hybrid stretches more than 16 feet long from the point of the prow to the trailing edge of a tailgate and it's well over six feet wide and six-plus-feet high at the rooftop, with a wheelbase length almost ten feet long.
Such dimensions create a big structure with plenty of room in the cabin for riders and cargo.
Its two-box body features a massive prow with a fat-lip fascia and faux skid plates, a chrome-striped grille and oversized headlamp clusters on front corners.
There's a keen rake to the windshield and a faint power dome shape to the hood, squarish wheelwells on flanks with chiseled fender flares and smooth sides on doors, save for a line of protective molding.
Overall, the Yukon Hybrid looks fairly conventional for a big SUV, although the powertrain tucked inside Yukon's engine compartment is anything but conventional.
First, there's a powerful 6.0-liter V8 aboard that runs on gasoline.
It knocks out 332 hp at 5100 rpm and high torque up to 367 lb-ft at 4100 rpm.
Further, the Yukon Hybrid SUV carries some additional power sources.
Included are two 60-kW electric traction motors.
These electric motors are packaged with three planetary gearsets and four hydraulic wet clutches as components of GM's new electrically variable transmission (EVT).
A sophisticated control unit -- dubbed the Hybrid Optimizing System (HOS) -- manages all energy produced by the on-board gas/electric engines and applies it directly to Yukon's wheels in infinitely variable measures, as well as providing four fixed-gear ratios so the operation mimics a conventional automatic transmission.
Power to run the two electric motors flows from a 300-volt nickel-metal hydride battery pack, the Energy Storage System (ESS) which is housed in Yukon's cabin below the second-row bench seat. The ESS also stores energy produced during regenerative braking and may be charged by the V8 engine with one of the electric motors used as a generator.
GM describes the high-tech apparatus for Yukon Hybrid as a two-mode hybrid propulsion system.
At low speed the wagon can move forward or backward using an electric motor or the gas-fired V8 or a combination of the V8 and electric motor.
At highway speed Yukon's second mode works with all eight cylinders of the gasoline engine pumping when commanded, or with only four cylinders firing through GM's Active Fuel Management (AFM) technology to pare the number of cylinders in the combustion process and conserve on fuel.
And when the HEV-SUV stops, the V8 engine shuts down, leaving only electric motors running.
The point of using all of this equipment on a husky SUV like Yukon is to trim the powertrain's fuel consumption.
GM tests indicate that Yukon Hybrid earns fuel economy figures up to 22 miles per gallon for highway runs in the two-wheel-drive (2WD) version, or up to 20 miles per gallon with the optional 4WD system.
And for in-town driving, the consumption numbers run to 21 mpg for the 2WD HEV-SUV and 20 mpg for the 4WD version.
For a big SUV that stretches almost 17 feet long and tips the scales at close to three tons, these fuel economy scores are huge -- representing as much as a 50 percent improvement for city driving over a conventional Yukon with a 340-hp 5.3-liter V8 aboard.
So there's obvious savings to be earned from Yukon Hybrid's elevated fuel economy figures, but the front-end cost to add all of the hybrid equipment on Yukon amounts to almost $5,000.
GM sets the MSRP for a 2009 Yukon Hybrid at $50,900 (2WD) or $53,700 (4WD).
Variances with Yukon Hybrid include the electric power steering (EPS) system with an electrically driven 42-volt variable-assist power steering rack. It eliminates the conventional hydraulic apparatus along with the power losses of an engine-driven pneumatic pump, and also pares pounds.
The brake system consists of a big disc at every wheel and a four-wheel anti-lock brake system (ABS) for straight-line brake tracking, plus a traction control system (TCS) and StabiliTrak, GM's anti-skid device.
And the HEV has that regenerative braking system to capture electricity generated through vehicle braking and decelerating to recharge the ESS so it never needs to be plugged in for recharging like an electric vehicle.
Yukon Hybrid also has modifications to pare the body weight and improve aerodynamics.
* Its prow dips by 10 mm for aerodynamic efficiency, and the front hood and rear liftgate are made of aluminum to trim the weight.
* Running boards are tapered fore and aft to decrease wind resistance, and close inspection of spokes on the 18-inch Ultra Bright aluminum wheels reveals routed edges for aero-efficiency.