IT would take a magician of considerable skill to turn a large, thirsty SUV into a gas-sipping hybrid, but General Motors has tried to pull just such a big green bunny out of its hat. For 2008, the company's engineers have installed an innovative hybrid system in a couple of the company's larger sport utilities, the Chevrolet Tahoe and GMC Yukon, and plan to add the system to other models, including the Cadillac Escalade and full-size pickups, in the coming months. The technology itself, which began with a clean sheet of paper, is impressive. It is called a two-mode hybrid system because it offers two types of operation, one intended for city driving and one for the highway. Based on the hybrid-drive technology in big GM buses, the two-mode was jointly developed by GM, BMW and the formerly co-joined DaimlerChrysler. All four companies (Daimler and Chrysler have separated) will be using two-mode technology in future vehicles. GM was first to market with the Tahoe and Yukon hybrids, which do manage to be greener without much degradation of their heavy-duty characteristics. They can haul a big boat or trailer and accommodate, in theory, up to nine passengers. And while the hybrid SUV's get better overall mileage than their conventional gasoline-only siblings — in the high teens in my experience, compared with perhaps 14 — the difference may be less than potential buyers expect. Ultimately, real-world fuel economy may be less than what was promised either by the company or by the federal ratings on the window sticker. A prospective purchaser will need to consider whether the mileage gains are enough to offset the hybrids' substantially higher prices. More fundamentally, shoppers may want to reflect on whether they really need large trucklike vehicles like these instead of car-based crossovers that are inherently more efficient — like GM's own GMC Acadia. GM explains its approach — to start offering the system in some of its largest passenger vehicles — by noting that mileage gains in such thirsty models can save the most gallons of fuel (and the most dollars at the pump). In contrast, the gains from a hybrid powertrain are relatively smaller in a relatively small, efficient car like a Honda Civic. After driving the new GM hybrids, one must wonder whether it might have made more sense to direct this considerable engineering expertise to a mainstream sedan or crossover that could actually achieve 40 mpg or so. The two-mode hybrid system should not be confused with previous GM hybrids like the Saturn Green Line models or the new Chevy Malibu Hybrid. These are not capable of running on electricity alone; instead, they have an electric motor that gives an assist to the rather small gasoline engine when more power is needed. They also have, like most hybrids, an automated system that shuts off the engine when the car is stopped. The two-mode also differs from the most popular hybrid, the Toyota Prius, which can be driven on the battery alone for brief periods. The Prius's electric motor is relatively small, providing only a slight boost. The Prius was designed to be a high-mileage but relatively low-performance vehicle, and it performs that role fairly well. The two-mode system, on the other hand, was designed from scratch with a clear goal: to preserve the muscular qualities of the big trucks in which it is installed. Simply put, the system consists of a 300-volt nickel-metal-hydride battery; two variable transmissions, or EVT's, each with a 60-kilowatt electric motor; and a 6-liter 332-horsepower V-8 engine. That gas engine can operate as a V-4, shutting half of the cylinders down when they are not needed to save fuel. The engine is essentially the 6-liter V-8 found in large GM pickups, though it has been heavily modified for better economy. All this is controlled by a computer that constantly calculates the most efficient way to deliver power from the eight (or four) cylinders and the two variable transmissions. One of these transmissions is infinitely variable, providing a nearly unlimited combination of gear ratios for low speeds and light loads, and one has four fixed gear ratios. At low speeds and with light loads, the vehicle can be driven by the battery alone or by an infinite combination of gear ratios from one of the EVT's. For heavier loads and trailer towing, the second EVT selects the most economical fixed ratio — and half of the engine's cylinders can shut down. In any driving situation, the electric motors can kick in for a burst of power. Reverse gear uses the battery alone. So that the vehicle can operate in the most efficient way possible, many other engineering changes were made from the conventional Tahoe and Yukon. A 300-volt battery operates the air-conditioning compressor so that climate-control is available even when the engine is off. A 42-volt battery runs the power steering and the engine oil pump; a 12-volt battery runs the other systems. Under braking, energy is recaptured and sent to the 300-volt battery. The changes to these two-mode SUV's do not stop with the complicated hybrid electrical and transmission system. Since weight and aerodynamic drag are the most important factors in achieving high mileage, changes to the styling (a front air dam, running boards and a special rear-end design) improved the aerodynamics. Aluminum sheet metal on the hood and rear panels, along with thinner seats, helped to offset some of the 400 or so pounds added by the hybrid system and its batteries. Another important change is the choice of a 3.08 final-drive ratio, compared with the 3.73 ratio in conventional Tahoes and Yukons. This alone reduces engine speed on the highway and, in my judgment, is responsible for a significant part of the improved fuel economy. Indeed, I would wager that a regular gas-engine Tahoe or Yukon with a 3.08 final drive ratio and a six-speed automatic transmission (instead of the usual four-speed) might well match the highway fuel economy of the hybrid versions. And the gains could be accomplished with minimal added cost or complexity, though such a vehicle would still get dismal mileage in town. Does all this engineering complication (and innovation) work in normal use? The quick answer is yes. The Yukon two-mode accelerates with verve. The “transmission” shifts imperceptibly and right when it should. The electric steering is competitive with that of other large SUV's. The noise level is low, the slight whine in all-electric mode is not obtrusive, and the regenerative braking feels right. A hybrid driver will notice little difference from the regular Yukon except for the eerie quiet when the vehicle is stopped and the gas V-8 is not running. As is common with hybrids, a graphic display shows the power mode for the vehicle. More usefully, there is a small fuel-efficiency gauge that notes the “sweet spot” between too heavy a foot on the accelerator and too much braking, so that by keeping the gauge's needle at 12 o'clock the vehicle is running at maximum economy. I found that this device made it much easier to drive in the most economical manner. It would be useful in all cars and trucks. Bells and whistles aside, how much better is fuel economy from this hybrid? For comparison, I drove a two-mode Yukon and a conventional gas-engine Tahoe LT3. Except for their powertrains they were equipped the same. The Yukon/Tahoe hybrid comes in only one, fully loaded, version. The two-wheel-drive Tahoe model lists for $50,490. With four-wheel drive, the Tahoe lists for $53,295. (Comparable Yukons are a few hundred dollars more.) My tested four-wheel-drive Yukon had the only two options available: a $995 sunroof and a $1,295 rear-seat DVD system. The total price topped $56,000. I also tested a nonhybrid Tahoe with four-wheel drive. It had a 5.3-liter V-8 (which is also capable of running as a V-4) and the usual four-speed automatic transmission. Its many options (mostly standard on the hybrid) totaled $11,145. The sticker price was $49,995. With four-wheel drive the hybrid is rated at 20 mpg in both city and highway driving. (Two-wheel-drive versions are 20/21.) The gasoline-engine Tahoe with four-wheel drive is rated 14/19 (14/20 with two-wheel drive). These numbers are the basis for GM's claim that the hybrid achieves 40 percent better city mileage, and that overall mileage is 25 percent better. My own experience did not match those claims. I drove both SUV's in a similar manner, on highways and city streets, with an emphasis on mixed driving. As a driver attuned to economy — in the 1960s, I competed in the Mobil Economy Run — I try to coax the best mileage from a car most of the time. Try as I might, I could not get the Yukon Hybrid's mileage, as shown on the trip computer, up to an average of 20 mpg It ranged from a low of 18.6 mpg to a high of 19.4 mpg My overall average was 19.3 mpg on the computer and 19.5 mpg based on my calculations of the actual fuel consumed. There were a few times when I was able to drive on the battery alone (in traffic jams or on a level city street), and that obviously helped the overall mileage. What was remarkable to me was the consistency of the mileage in all types of driving. The nonhybrid Tahoe was not so consistent between suburban and highway driving. I have driven several GM sport utilities equipped like the Tahoe and have found that over many miles in a variety of conditions, they end up averaging around 14 mpg (Though sustained city driving lowers that number appreciably.) On the 2008 Tahoe, I averaged 14.9 mpg My take on these numbers is that an owner of one of the new two-mode hybrids is likely to average about 18 or 19 mpg while a conventional Tahoe or Yukon will end up around 14. So the new hybrid SUV's represent a lot of work (on GM's part) and added expense (for the customer) for a gain of 4 to 5 mpg For an owner who drives 15,000 miles a year, that amounts to roughly $850 to $1,000 in savings, based on a gas price of $3.60 a gallon. And the hybrid version costs roughly $10,000 more than a comparably equipped gasoline model. Nor is the hybrid any cleaner, in terms of smog emissions, than many gas-powered cars and crossovers that carry PZEV ratings (for partial zero emission vehicle). The standard-engine versions can run on E-85 ethanol, but at a significant penalty in fuel economy. While I marveled at the engineering and sophistication of this two-mode hybrid, I remain somewhat baffled by the too-heavy, too-costly vehicles in which the technology made its debut. So, perhaps, is the public. Only 655 Tahoe and Yukon hybrids were sold in the first quarter of 2008, though GM has insisted that it still plans to sell up to 12,000 a year. This fall, Chrysler is bringing out two-mode versions of big SUV's, the Dodge Durango and Chrysler Aspen. Further in the future, the picture is brighter for innovative technology like this. GM has announced plans to install a two-mode system in the smaller, lighter Saturn Vue crossover. Perhaps a passenger sedan will get it next. – The New York Times __
