2008 Chevrolet Tahoe Two-Mode Hybrid First Drive

Two modes are better than one

Published: 05/07/2007  - by Michelle Krebs, Edmunds.com Senior Analyst

The 2008 Chevrolet Tahoe Hybrid looks nothing like the NASA space shuttle, yet it's a similar leap into the future, and a similar collaboration among a bunch of science guys made it happen.

GM's 2008 Chevrolet Tahoe Hybrid and 2008 GMC Yukon Hybrid are the first vehicles outfitted with an advanced two-mode hybrid powertrain developed through unprecedented cooperation by BMW, DaimlerChrysler and General Motors.

Hybrid variants of the Tahoe and Yukon deliver significantly improved fuel economy over their conventional counterparts — about 25 percent better, GM tells us. At the same time, these full-size SUVs can still haul seven or eight passengers plus cargo, and they can tow up to 6,000 pounds. Yet the only different thing you'll notice about driving them is the number of fuel stations you can pass by without feeling the need to top off the gas tank.

A Tiny Science Geek Does the Work
Two-mode hybrids aren't exactly new. GM has had the two-mode hybrid system working in transit buses since 2003. Today, about 700 of those buses are operating in 60 cities in North America and have just been introduced in Europe.

But the Tahoe and Yukon represent the technology's first application to a personal-size vehicle. It comes from a unique collaboration among BMW, DaimlerChrysler and General Motors, which have established a special center in Troy, Michigan, where some of the brightest minds from these companies have been brought together.

The two-mode hybrid is a complex integration of electric motors, high-performance electronics, wiring, energy management and hybrid-system control units. As a result, GM had a devil of a time trying to help us understand why this two-mode hybrid technology is better than existing hybrids.

So here's an elementary way to think about it: Imagine there's a science geek under the hood of the Chevy Tahoe. The geek's job is to keep the vehicle running at optimum fuel-efficiency. In fact, the geek thinks about this task 100 times every second.

Compared to the geeks that live under the hoods of other hybrids, the GM geek has a much bigger toolbox. And the science lies in manipulating all these tools without becoming hopelessly confused.

Inside the Geek's Toolbox
The basic building block of the Tahoe Hybrid is a version of GM's 6.0-liter Vortec V8. Then there's a 300-volt battery that sits beneath the second-row seats. (Both the second- and third-row seats still fold and tumble like those in the conventional Tahoe.) Like a conventional hybrid, there are two electric motors, but they are very compact and are packaged within the transmission.

In fact, the real science of the two-mode hybrid lies in the transmission. It's kind of like a continuously variable transmission (CVT), only there are very sophisticated controls to deliver speed while carefully balancing the amount of power that comes from both the gasoline V8 and the electric motors.

This is where the geek comes in. The geek figures out the situation and decides if it wants to use the electric motors or bypass them. GM engineers reckon that during city driving, the electric motors operate about 75 percent of the time, and the transmission bypasses them about 25 percent of the time. During highway driving, these percentages reverse, with the electric motors being engaged only about 25 percent of the time.

Two Modes Are Better Than One
For the Chevy Tahoe, each of its two modes has a specific duty in order to deliver the best fuel economy for the situation. During low-speed, light-duty driving, the system works like a conventional single-mode hybrid, stopping the V8 whenever it can and relying on one or both electric motors.

During highway cruising, the system uses one or both electric motors to provide a power boost when necessary. Meanwhile, the V8 engine incorporates a range of fuel-saving technologies, including variable valve timing and cylinder deactivation.

The magic really lies in the transmission, which shuts off the V8 engine when it can, and then tries to keep the engine running at a constant, fuel-saving rpm the rest of the time. And the transition between the two modes is completely seamless. That science geek under the hood really knows his stuff.

All That Typical Hybrid Stuff
Like other hybrids, the Tahoe and Yukon are equipped with regenerative brakes that capture energy normally wasted during braking and use it to recharge the battery. A new air-conditioning unit requires less power and thus uses less fuel. And Tahoe Hybrid rides on lightweight aluminum wheels with tires that have low rolling resistance.

A hybrid powertrain requires more airflow for cooling, so the front grille and front airdam have been suitably modified. Meanwhile the front fascia, running boards and rear spoiler have been reconfigured for a more slippery aerodynamic profile, while the chassis rake has been altered as well. As a result, the Tahoe Hybrid now has a 0.34 Cd, compared to the conventional version's 0.39 Cd.

Since the new hybrid powertrain adds about 400 pounds to the Tahoe's curb weight, there are a number of useful measures to reduce the vehicle's weight. To begin with, the hood and rear liftgate are now aluminum. The front bucket seats are thinner, though they turn out to be just as comfortable and improve rear-seat legroom by a fraction.

Driving in the Science Mode
Our test-drive of the prototype Tahoe at GM's proving grounds in Milford, Michigan, proved to be uneventful. And we mean that in a good way. It's always best if a new technology is transparent and requires no adaptation, because drivers are always reluctant to change their behaviors.

The GM proving ground had been set up with orange cones and stop signs to simulate city stop-and-go traffic. Our Tahoe behaved much as other hybrids do. It predictably ran on electric power as we accelerated and then fell silent when we came to a stop. Occasionally, we edged past 20 mph and the gasoline engine joined in. The transitions between electric and gas power were quiet and smooth, almost imperceptible.

Later we drove the Tahoe Hybrid on the long, high-speed loop across the Milford asphalt, which simulates a two-lane country road with some curves, undulations and rough pavement. Except during acceleration and deceleration, the Tahoe Hybrid ran on the gas engine, mostly with V8 power. Toward the 50-mph mark on steady stretches, cylinder deactivation imperceptibly changed the V8 into a V4.

The Tahoe Hybrid also proved to have exceptional braking performance. The pedal felt firm, and the GM engineers claim the Hybrid comes to a halt in a shorter distance than the conventional Tahoe.

Coming to a Dealer Near You
The 2008 Chevrolet Tahoe Hybrid goes on sale late this year, and it'll be joined by a GMC Yukon Hybrid. Both will be available with both two- and four-wheel drive. GM hasn't announced pricing, but we expect the price difference between a conventional Tahoe and the Tahoe Hybrid to be substantial, more like the price premium you pay for a Lexus RX 400h over its conventional counterpart.

Chrysler is next up for the two-mode hybrid and the Chrysler Aspen and Dodge Durango should incorporate the technology in early 2008. BMW hasn't yet made any announcement about its use of the technology. The hybrid is also planned for a future Mercedes-Benz model, and we expect the Chevrolet Silverado and GMC Sierra full-size pickups, plus the Cadillac Escalade to receive the two-mode hybrid technology.

Yet the real significance here is the system itself. The two-mode hybrid promises to deliver worthwhile fuel economy benefits across a wide range of driving situations. Just as important, it seems to be small enough and light enough to be easily packaged into vehicles of different sizes.

Edmunds attended a manufacturer-sponsored event, to which selected members of the press were invited, to facilitate this report.