2008 Chevrolet Tahoe Hybrid - Road Test

The three-ton economy machine: You don’t need lofty mpg to save a lot of gasoline.

BY CSABA CSERE, PHOTOGRAPHY BY J.G. RUSSELL
March 2008

Mention hybrid, and most people think of fuel misers, such as the 40-to-50-mpg Toyota Prius. But a hybrid powertrain can save fuel in vehicles of any size, as demonstrated by this Chevrolet Tahoe, which represents the first example (along with the GMC Yukon and 2009 Cadillac Escalade)
of hybrid technology applied to a large SUV. As such, it gets a
big-time fuel-economy bump, but as a 5840-pound vehicle, it isn’t—and
can’t be—a fuel miser by anyone’s definition.

This Tahoe is the first “full hybrid” from General Motors, meaning
that it can run purely on electric power at low speeds. It uses a new
two-mode hybrid system that is a joint development of GM, BMW,
Chrysler, and Daimler and is fundamentally different from the hybrid
powertrains used by Toyota and Honda.

The transmission resembles a conventional four-speed automatic to
which two 80-hp electric motors are integrated by marrying them to the
transmission’s three planetary gearsets. Despite the two-mode name, the
transmission, which fits into the same space as the automatic in
conventional Tahoes, seems to operate in three distinct modes.

This novel transmission is coupled to a high-efficiency version of the
6.0-liter small-block Chevy V-8 to maximize fuel saving. Designated
LFA, the V-8 operates on the Atkinson cycle, which uses a late
intake-valve closing to improve efficiency by increasing the expansion
ratio of the power stroke without creating an unduly high compression
ratio. Variable cam timing allows the powertrain computer to maximize
this effect at different engine speeds and loads.

The one downside of this Atkinson-cycle strategy is that it compromises low-rpm torque. That’s why the hybrid gets a 6.0-
liter
V-8 rather than the standard 5.3, which might have seemed the more
obvious high-efficiency choice. Like the 5.3, however, the 6.0 also
gets Active Fuel Management, which means the engine can shut off four
cylinders and operate in a V-4 mode during light load conditions.

Powering the electric motors is a 288-volt, 6.5-amp-hour battery
through a controller under the hood. This piece of heavy-duty circuitry
also runs two auxiliary components: the A/C compressor and the power
steering. When the V-8 shuts down, electricity keeps these devices
operating.

The electric motors in the transmission operate as generators under
braking to recharge the battery, up to 0.30 g of deceleration. Brake
harder than that, and the conventional hydraulic brakes engage to slow
down as quickly as tire traction permits.

The hybrid hardware adds a weight penalty of about 350 pounds, but
that’s offset by lighter seats and wheels, a smaller 12-volt battery,
no starter motor, and an aluminum hood, liftgate, front bumper beam,
shaft, and engine. Sure enough, our Tahoe hybrid weighed 5840 pounds,
exactly the same as the Tahoe LTZ that we tested two years ago, making
this Tahoe the first hybrid we’ve seen that doesn’t weigh more than its
conventional counterpart.

Like most other hybrids on the market, the Tahoe gets several
changes designed to improve its overall efficiency. A deep front air
dam, a rear spoiler, front and rear wheel spats, and reworked front and
rear fascias, running boards, and D-pillar moldings help reduce the
drag coefficient from 0.36 to 0.34.

How much efficiency does this hybrid magic provide? By EPA measures, the hybrid Tahoe four-by-four is rated at 20 mpg city and
highway. The standard Tahoe four-by-four’s ratings are 14 city and 19
highway. Our as-tested mileage for the hybrid was 19 mpg, way better
than the 12 mpg we got with the regular Tahoe, although both machines
were driven on random, not necessarily comparable cycles.

We rounded up a GMC Yukon SLT four-by-four equipped with the
5.3-liter V-8, just like a base Tahoe, and drove a mixed
urban-and-highway loop in convoy with the Tahoe hybrid to get a more
direct mileage comparison. The loop averaged 43 mph, and the hybrid got
23 mpg compared with 18 for the Yukon. The Tahoe hybrid’s fuel economy
improved by just five percent on the highway cycle, but by the other
measures, it delivers 27-to-58-percent-better fuel efficiency.

That’s a useful improvement, given that the hybrid is a few 10ths
quicker to 60 mph and through the quarter than the standard Tahoe. It
also has a 6000-pound towing capacity and seating for eight, although
the third-row seat is hardly attractive to anyone more than five feet
tall.

The Tahoe hybrid driving experience will be transparent to most
drivers. Although the engine shuts down almost every time you stop, the
Tahoe moves off instantly under electric power as soon as you touch the
accelerator, with the engine firing quietly soon after. The
transmission shifts through its modes smoothly, and if you sometimes
sense engine-rpm changes that are a tad unfamiliar, the experience is
still much closer to a conventional ­automatic’s than to that of the
mechanical CVT.

Drivers with elevated sensitivity will notice that the electric
power steering is light on effort and even lighter on feel, but the
steering is accurate, and positioning the hybrid requires no great
attention. The hybrid’s brakes feel somewhat nonlinear, but it’s no
problem to scrub speed smoothly.

Driving for optimal economy is achieved through light touches on the
accelerator and brake, behavior encouraged with an inscrutable
“economy” gauge. Some labeling would make this unit more
comprehensible.

If you select “instant fuel economy” on the trip computer, you can see
when the V-8 switches to four-cylinder mode. The hybrid’s electrical
boost is used to increase the time spent in this mode, but it doesn’t
take much of a grade to cause the engine to go to V-8 mode, even at
moderate speeds. Still, with a roughly 10-mpg jump when the engine
switches from V-8 to V-4, even occasional four-cylinder operation is
helpful.

The Tahoe 4wd hybrid comes only in a well-optioned version for
$53,295. That’s about five grand more than a comparably equipped
regular Tahoe. If you drive mostly on the highway, you’ll never make up
the extra cost in fuel savings. But if you drive 15,000 miles per year,
mostly in the city, you’ll save more than 300 gallons of gasoline per
year. At $3 per gallon, you’ll make up the extra hybrid cost in about
five years. And that’s a quicker payback than you’ll see in a Prius or
a Civic hybrid.

source: www.caranddriver.com