ARLINGTON, Tex. -- Along I-30, a.k.a. the Tom Landry Freeway shooting across the Dallas suburb of Arlington, we're steering the world's first full-size SUV that's a hybrid electric vehicle (HEV). General Motors constructs this HEV-SUV off the Chevrolet Tahoe, a truck-based sport utility wagon of the full-size and half-ton class, and makes an equivalent model for GMC as the Yukon Hybrid.
The HEV version of Tahoe (or Yukon) looks virtually identical to a conventional Tahoe (or Yukon), save for strips of door decals denoting the hybrid nature and special fender and tailgate badges in chrome. Its two-box body features a predominant prow with a fat-lip fascia and faux skid plates, a twin-port horizontal grille and squarish headlamp clusters on the corners.
There's a sharp rake to the windshield and a faint power dome shape to the hood, squared wheelwells on flanks with sheered fender flares and smooth sides on doors, save for a line of protective molding. However, the powertrain tucked inside Tahoe's squarish engine bay is certainly not conventional.
First, there's a big and powerful 6.0-liter V8 aboard that runs on gasoline. It punches out 332 hp at 5100 rpm and tall torque to 367 lb-ft at 4100 rpm. Next, the Tahoe Hybrid 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 Tahoe'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 Tahoe'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 Tahoe Hybrid as a two-mode hybrid propulsion system. At low speed Tahoe Hybrid 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 Tahoe'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 engaged 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 big SUV like Tahoe is to trim the powertrain's fuel consumption.
GM tests indicate that Tahoe 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 four-wheel-drive (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 Tahoe with a 340-hp 5.3-liter V8 aboard.
The 5.3-liter V8 Tahoe brings fuel economy numbers of 14 mpg for in-town driving and 20 mpg on the highway (2WD) or 19 mpg highway (4WD). Our own test while driving a 2WD version of Tahoe Hybrid non-stop for 200 miles on Texas interstate highways resulted in a fuel burn score of 22.5 miles per gallon.
So there's obvious savings to be earned from Tahoe Hybrid's elevated fuel economy figures. However, the front-end cost to add all of the hybrid equipment on Tahoe amounts to almost $5,000.
GM sets the MSRP for a 2008 Tahoe Hybrid at $50,045 (2WD) or $52,855 (4WD). Variances from convention with Tahoe Hybrid include the unique electric power steering (EPS) system, which consists of 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. Ultimately the EPS contributes an estimated 0.5 mpg fuel economy boost.
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.
* Tahoe Hybrid has some 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 weight.
* The running boards are tapered fore and aft to decrease wind resistance, and close inspection of spokes on the 18-inch cast aluminum wheels reveals routed edges for aero-efficiency.
* In Tahoe's cabin, thin-profile front bucket seats shed pounds and also shave inches to improve the knee room for backseat riders.
* And the customary spare wheel and tire and jack and tools have been deleted from the HEV to reduce vehicle mass. Instead, you'll find an easy-to-use tire inflator kit stowed behind a trim panel in the cargo bay.