In the AutoblogGreen Garage: 2008 Chevy Tahoe Two-Mode Hybrid

Click the Tahoe hybrid for a high-res gallery

General Motors has had a checkered relationship with hybrids in the last few years. Through the 1990s they did a lot of development work in the area, particularly through the federally funded Partnership for a New Generation of Vehicles (PNGV). Ultimately, Toyota and Honda were the first to actually bring hybrid electric drivetrains to market. GM eventually brought their first light duty hybrid system to market with the mild parallel hybrid that was offered on the Silverado and Sierra pickups for a couple of years. That was followed by the belt-alternator-starter system that debuted in late 2006 on the Saturn Vue. Neither of these systems could in any way be considered a commercial success although GM claims to have learned some real world lessons from them.

Late in 2007, GM finally launched production of the Chevy Tahoe and GMC Yukon hybrids, the company's first "strong" or full hybrid systems offered on light duty vehicles. The hybrid full-size SUVs have been controversial among hybrid fans who are of course dubious about why GM is bothering to apply this technology to these big, heavy vehicles. Even with the hybrid system, the rear wheel drive Tahoe still only manages an EPA combined rating of 21 mpg, less than half of the 46 mpg that a Prius gets. There is, however, a method to the apparent madness which we'll come back to. Previously we've had a couple of opportunities for short drives of the Tahoe hybrid at GM's Milford Proving Ground. Now that we've had a chance to live with it for a week you can read all about it after the jump.

The Tahoe (and it's GMC Yukon twin) are the first light duty applications of GM's Two-Mode hybrid system that was originally developed for transit buses by Larry Nitz and his engineering team at GM's Allison Transmission division in the late '90s(BTW Allison was sold off last year, but GM kept the intellectual property rights to the Two-Mode system). The bus system debuted in 2003. As we all know by now, non-plug-in parallel hybrids provide their biggest benefit during stop and go driving when they can capitalize on the deceleration to recapture kinetic energy and charge the battery. During these conditions, the battery maintains a higher state of charge allowing it to provide pure electric drive and electric boost.

During higher speed highway driving the batteries typically provide relatively little benefit, most of which comes from having a more efficient base engine. This is one area where the Two-Mode system holds an advantage over other hybrids. The original bus version of the system derived its name from having two different gear ratio ranges for electronically variable transmission (EVT). All the current strong hybrids use a planetary gear EVT as does this one. The GM system has a second planetary gear set providing a wider operating range for the hybrid functionality. When GM adapted the system to light duty vehicles they also added in an extra clutch that allows the power flow to bypass the electric motors at higher speeds, running directly through the more efficient mechanical gears. In actual implementation, the full powertrain of the Tahoe actually has a lot more than two modes which we'll get back to later.

The Tahoe is, without a doubt, a big vehicle, a much bigger vehicle than the vast majority of people actually need. When GM announced the hybrid SUVs, green car fans groaned at thought of hybrid technology being "wasted" on a nearly three ton truck. But there is a method to GM's "madness." Most full hybrids get anywhere from 25-35 percent better efficiency than conventional equivalents. The problem is that as the starting mileage increases, the same percentage increase provides progressively lower fuel savings. For a Tahoe going from 16 mpg to 21 mpg, driving 15,000 miles a year will save 223 gallons. Going from 30 mpg to 39 mpg only saves 121 gallons annually. Admittedly, the Tahoe still uses a lot more fuel overall, but the savings are impressive nonetheless. Even at $3.50 a gallon there are still many people who need or want a full size SUV. If someone insists on driving such a large vehicle, at least they will make a significant dent in their fuel consumption with a hybrid version.

The Two-Mode hybrid system is complex and very expensive for GM to produce. General Motors doesn't say exactly how much it costs, but for these first applications it's a safe bet that the company is losing quite a bit of money on the system. To help minimize the loss, the system is installed on well-equipped vehicles that people are willing to spend more for. As such, the Tahoe has amenities like heated leather seats, automatic climate control and a navigation system as standard equipment. The ergonomics of the GMT900 trucks are generally pretty good, but the quality of some of the materials seems lower than you might expect in a vehicle costing over $50,000. The look and texture as well as the fit of the plastic panels is fine, certainly better than those found in the Jeep Grand Cherokee. However the hard plastic on the door panels is somewhat off-putting.

The first two rows of seats offer plenty of space and comfort. The front seats don't really offer any lateral support, but frankly not many drivers are going to be tossing a Tahoe down a twisty country road. GM implemented a variety of weight saving measures in the Tahoe and Yukon to compensate for some of the 400lbs of extra mass added by the hybrid hardware. The front seats are visibly thinner than the units on the standard Tahoe while components like the hood and tailgate have been redone in aluminum rather than steel. The alloy wheels are also lighter than the regular units thanks to hollow spokes.

Unlike the first two rows of seats, the third row suffers compared to some competitors. The Tahoe (and the entire GMT900 lineup) use a solid rear axle. As a result, the floor pan needs to be higher up to provide clearance for movement of the rear differential. GM's own Lambda platform CUVs are similar in size to the Tahoe but use an

independent rear suspension. This allow the floor to be lower. The rear seats of the Lambdas and other IRS vehicles sit higher up off the floor providing for a more natural seating position. The third row seat of the Tahoe is on the floor, resulting in a knees up sitting position that's OK for kids but grownups won't want to spend much time back there. The live axle also means the seats don't fold flat into the floor. They are however, easily removable and left behind in the garage if you need the extra cargo space. In the Tahoe, the hybrid battery doesn't eat into usable space thanks to its position under the middle row seat.

So just how well does the the Tahoe hybrid work? So far most of what I've said applies as much to a standard Tahoe as it does to the hybrid. In the past few months I've driven the Toyota Highlander, Lexus LS600h and Tahoe hybrids, all during a particularly cold Michigan winter. In the two previous cases it seemed that the vehicles were somewhat reluctant to run in electric only mode and shut off the engine infrequently when coming to a stop. While driving those vehicles, the temperatures stayed pretty frigid. Not so with the Tahoe. Most of the mornings that the Tahoe was in the ABG garage, the overnight temperatures dipped into the low teens or single digits. By mid-day, the temperatures rose into the mid thirties.

It became apparent that hybrids (and EVs) don't mix well with cold temperatures and the nickel metal hydride battery under the second row seat of the Tahoe. Anyone who has left a phone, camera or laptop in a car overnight during winter knows that cold batteries don't like to share any spare electrons they might have. When the Tahoe was cold, even without turning on seat heaters, defoggers or other power draining accessories, the Tahoe simply would not auto stop, or run in EV mode. Later in the day when the battery had warmed up, the behavior was very different. Coming to a stop or coasting at speeds up to 35mph, the engine would shut down and keeping a light foot on the gas pedal would allow battery-only driving for a mile or more.

The Tahoe has a new braking system developed by TRW (disclaimer: in my previous career before becoming a full-time writer, I was an engineer at TRW and worked on the brake system for this vehicle) that provides blending of friction and regenerative braking. The system also provides hydraulic brake boost so that no vacuum pump is needed. When the truck is in motion, the system provides a very firm pedal feel that is easily modulated, especially when compared to previous GM trucks which often felt mushy and unresponsive. When sitting parked, it takes little effort to hold the vehicle still, but pushing harder on the pedal actually yields a weird sort of spongy artificial feel. The electronically-controlled braking system also provides full traction and stability control functionality. GM has tuned in quite a bit of creep in the system. Releasing the brake, the truck starts to pull ahead in a manner stronger than most other hybrids that could be considered a little too aggressive. This might be to keep the heavy Tahoe from rolling backwards when sitting on a slope.

It's kind of spooky rolling along in such a big heavy vehicle in almost complete silence. Even when the engine is running the Tahoe hybrid is quieter than a standard version. In order to minimize the difference between electric and engine on operation, GM put a lot effort into minimizing NVH from all the vehicle systems and it clearly worked. We're not talking Lexus-like isolation, but the Tahoe was definitely smoother and quieter than the Highlander. The engine starts and stops are very smooth with none of the shuddering apparent on smaller front wheel drive hybrids. Maneuvering such a large vehicle requires a significant adjustment for someone used to smaller cars. The Tahoe has a rear backup camera and the large mirrors provide good visibility to the sides, but there are still plenty of blind spots. Changing lanes or pulling into tight parking spaces definitely requires extra care.

During the time I drove the Tahoe hybrid it averaged just shy of 20mpg which is a remarkable achievement for such a big heavy vehicle with the capability to tow 6,000 pounds. During the first day I had it when the weather was warmer, it actually got up to 22.5 mpg but for the remainder of the week, the frigid temperatures often limited the usefulness of the electric drive. The driver information center in the instrument cluster can display whether the engine is running on four or eight cylinders while the main screen in the center stack displays the power flow between engine, battery and wheels. Even in cold temperatures, an easy gas foot can leave the engine running in four cylinder mode much of the time.

The Two-mode branding is actually somewhat mis-leading since this system actually has many more operating modes. With the various combinations of four and eight cylinder running, electric drive, two EVT ranges, and highway cruising, the permutations seem endless. Reports of from other people who have driven the Two-mode SUVs have indicated that it's not hard to achieve 23 mpg or more in warmer temperatures. One other reviewer actually claimed to have achieved 26 mpg in a week of driving. I have to admit still being skeptical of that one, but in the right conditions it certainly seems possible. Even with the 20 mpg I got, the Tahoe handily beats anything else in its class including GM's own Lambda CUVs (Saturn Outlook, GMC Acadia and Buick Enclave).

If you don't actually need such a gargantuan vehicle, I would still suggest going for something smaller and lighter. After all, you don't need a Tahoe to schlep two or three kids to school and swing by Whole Foods. If you live in more northern climates with even colder, longer winters than Michigan had this year, you might want to hold off to see if GM offers the new 4.5L diesel V-8 in the SUVs next year. In extreme cold that might be better option than the hybrid. The Texans who build the Tahoe and Yukon at GM's Arlington assembly plant won't have to worry about single digit or sub-zero temperatures very often.