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GM's full-size hybrid is the real deal. But is it better than a diesel?
BY DAVE VANDERWERP, October 2007
This is GM's self-proclaimed "no excuses hybrid." You can have it all: a gargantuan SUV with seating for eight, big 6.0-liter V-8, towing, hauling, four-wheel-drive, and fuel economy. That's right, GM's first "full" hybrid boosts the 4WD Chevrolet Tahoe/ GMC Yukon's fuel economy by a whopping 43 percent on the EPA city cycle and five percent on the highway figure, to a V-6 Toyota Camry–shaming 20 mpg city rating, as well as a 20 mpg highway rating (21/22 city/highway on 2WD models).
As part of this "no excuses" business, GM is proud that, unlike some single-purpose hybrids (think two-seater Honda Insight), the Tahoe maintains all existing functionality and features. Mark this one down: the Tahoe is the first hybrid to come with keyless starting; heck, it's even standard. That's right Mr. and Mrs. Overindulgent American Greenie, the press of a button fires up six liters of gas-guzzling V-8 goodness in the driveway. You may be worried about your carbon footprint, but a cold leather seat is no way to start the morning.
What, you thought Toyota would think of this first?
The Impressive Mechanicals
Making this fuel-economy leap possible are a host of changes, but the most interesting is GM's patented "two-mode" hybrid transmission that packages in the same space as one of the company's venerable four-speed automatics. What sets this system apart from the popular Toyota hybrid arrangement is that the Tahoe can run as a continuously variable transmission (CVT) and an automatic through one of four fixed gear ratios. With their theoretically infinite ratios, CVTs are much better than conventional automatics at keeping engines spinning at the most efficient speed, but at highway speeds, that advantage can be nullified by the internal friction inherent to CVT operation. In hybrids, the energy loss can be even worse because the drivetrain must also spin an electric motor. The Tahoe, however, uses a computer to switch over to a more energy efficient mechanical path (one of the fixed ratios in the automatic) to minimize wasted energy whenever possible. Whereas the Prius is always processing its power through the CVT, even at inefficient highway speeds, the Tahoe's system is better and can simply bypass the electric motors and lock onto a gear.
The power comes from GM's familiar pushrod 6.0-liter V-8—chosen instead of the 5.3-liter, GM says, because the larger engine is able to run in four-cylinder mode more of the time—as well as two 80-hp, 12,000-rpm AC electric motors fed by a 300-volt Panasonic DC battery pack (1.8 kW/h) housed under the second row. However, to further boost efficiency, the hybrid's small-block V-8 uses a late-closing intake valve tuning which sacrifices some low-end torque as well as top-end power. That's why the Tahoe's output is 332 hp and 367 lb-ft, down from its typical 366 hp and 380 lb-ft. Of course, the electric motors are there to fill in at the low end, even though they don't add anything to the peak power output of the overall system.
A Host of Tweaks
All the hybrid hardware adds weight—about 350 pounds in the Tahoe's case, GM says—which is not the direction to go in order to maximize efficiency of an already porky SUV. To counteract this, and admirably maintain a 5850-pound weight (5650 for 2WD models), the Tahoe changes a number of pieces to aluminum: the hood, front bumper beam, driveshaft, and rear liftgate. It also gets thinner, lighter seats, and forged-aluminum wheels. Even the recommended tire pressures are bumped up by two psi to eke out every last bit of efficiency.
To maximize aerodynamics—trimming the coefficient of drag from 0.36 to 0.34—the hybrid's lower front fascia is devoid of holes and extends further to the ground to decrease the amount of turbulent air passing beneath the truck, and a roof-mounted spoiler at the rear smoothes airflow over the top. 4WD models are lowered 0.4 inch in the front.
Drives Like, Well, a Quirky Tahoe
The Tahoe can propel itself on electric-only power (up to 32 mph) and, during our time, did so quite often. Pulling into a subdivision, for example, the Tahoe would shut down the V-8 and stay in electric mode for a half-mile or more as we wound our way through the low-speed streets. The gas engine also shuts down at stoplights, but pulling away at a rate that won't annoy those behind you will make it fire back up almost immediately.
GM says the hybrid can run its V-8 in four-cylinder mode at speeds up to 75 mph, although we can't verify that claim. We almost never saw the four-cylinder indicator in the trip computer at anything near highway speeds and, even at much lower speeds, any slight grade would quickly re-fire the four dormant cylinders.
Not driving any differently than normal, we managed a respectable 18 mpg over a 250-mile weekend, far better than the 12 mpg we got in a standard 5.3-liter V-8 Tahoe. And the hybrid's acceleration is comparable to that of the 5.3-liter V-8 as well, charging to 60 mph in just over eight seconds. But the power delivery, now that's different. At the low end, the hybrid feels a bit weak, but comes on with a surge of power in the mid-range. And there's still that familiar drone that accompanies many vehicles using CVTs to keep their engines revving at high, constant rpm for an extended period under hard acceleration.
Using the same 17-inch tires from other Tahoes, the hybrid actually has better stopping distances, GM says, because the anti-lock braking system was able to be optimized for a single tire. The feel of this electromechanical brake system that captures energy to charge the battery under deceleration, however, is lacking, and smoothly braking at your desired rate involves much guesswork. Trying to brake at the threshold of ABS activity proved nearly impossible because, at a predetermined point in the pedal's travel, the system goes to full-on panic-stop mode. Although, how many Tahoe buyers even know what threshold braking is?
Ride-and-handing felt comparable to regular Tahoes—fairly agile for a body-on-frame, solid-rear-axle SUV—but the electric power steering has even less feel than the regular Tahoe's. GM has yet to officially announce pricing but it will not be the most expensive Tahoe, the company claims. Since a fully-loaded Tahoe LTZ comes in comfortably above $50K, we expect the hybrid to start around $48,000 or so when they hit showrooms shortly.
Does It Make Sense?
The Tahoe/Yukon is only the start of GM's two-mode hybrid system. GM has already forged partnerships with Chrysler and BMW, so expect to see Dodge Durango/Chrysler Aspen hybrids as well as the X6 hybrid in the near future. In the GM fold, the Cadillac Escalade hybrid is next, followed by this system in its full-size Chevrolet Silverado/GMC Sierra pickup trucks.
And the system certainly works, producing substantial fuel-economy gains. But, during our time with the Tahoe, we couldn't ignore this burning question: Why not just have a diesel Tahoe instead?
Consider this: the similarly sized, seven-passenger Mercedes-Benz GL320 is powered by a 215-hp, 398-lb-ft, 3.0-liter V-6 turbo-diesel. That 5Best Trucks–winning SUV gets an EPA rating of 18/24 city/highway and handily out-thrifted the Tahoe by 20 percent in our hands, netting 22 mpg over a similar driving cycle. Despite having a far better seating package, the GL320 suffers from none of the Tahoe's quirks (numb steering, lack of brake feel, and abnormal power delivery). Yes, starting at $53,775, the GL320 costs more than the Tahoe hybrid. However, the GL is cheaper than an Escalade, and will likely be both less expensive and more fuel efficient than the '09 Escalade hybrid.
For sure, both clean-diesel and hybrid technology are extremely expensive to develop. But we imagine that once a diesel engine is developed, it is likely far easier to install it in multiple vehicles (as Mercedes has done, putting this diesel in everything from the GL-, R-, and M-class SUVs to the E-class sedan) without painstakingly retuning the regenerative brakes, power delivery, and packaging; and without all the added complexity and relatively unknown service life of the electric motors and battery pack.
In light of all this, it seems like hybrids are definitely still fighting an uphill battle to prove themselves as the near-term fuel-efficiency technology-of-choice.
