Downsizing yet again

Savour the last of the great hybrid SUVs

Economist.com - Jun 6th 2008

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DETROIT is finally getting the message. General Motors announced this week that it will close four truck plants and seek a buyer for its Hummer division. American carmakers are at last moving from gas-guzzling sport-utility vehicles (SUVs) and pick-up trucks toward fuel-sipping family cars.

Ironically, this abrupt reversal—forced on GM by a looming recession and petrol at $4 per gallon—threatens some of the industry's best fuel-saving technology. GM's “two-mode” hybrid drive, introduced in January in the huge new Chevrolet Tahoe and GMC Yukon SUV twins, is one of the most significant advances in petrol-electric technology since the first Toyota Prius hit the street a decade ago.

The problem is that unlike the ingenious Synergy drive used in Toyota's petrol-electric Prius, Camry, Highlander and Lexus models, or the less ambitious Integrated Motor Assist in Honda's Civic, GM’s hybrid transmission was engineered primarily for trucks and buses. Brilliant as it is, it does not lend itself well (at least in its current guise) to small family cars.

In leviathans like the truck-based Chevrolet Tahoe and GMC Yukon, the two-mode drive is paired with a husky six-litre engine to deliver 20mpg (compared with 14mpg for non-hybrid versions of the same vehicles). That doesn't sound much when you think of the Prius's 45mpg. But it’s not bad for a 5,500lb eight-seater that can tow a 6,000lb trailer. Getting 20mpg in the city is even more impressive when you consider that BMW's diminutive Mini gets only 26mpg.

But saving fuel in stop-and-go urban traffic is not the only thing the nifty GM transmission does well. Unlike other hybrid drives, the two-mode transmission delivers considerably better mileage on the highway as well as in the city. The two-mode drive gets its name from the way it reaps the hybrid's benefits at both the low and the high ends of its speed range.

With their electric motors contributing little or nothing to the wheels at high speed, the Toyota and Honda hybrids lose much of their advantage on the highway. Some hybrids fare even worse than conventional vehicles on steady runs, because of the extra dead weight they have to lug around.

Like the Toyota system, GM’s two-mode drive is a “strong” hybrid. That means it can run on its petrol engine alone, on its electric motor and batteries alone, or on a combination of both. By contrast, Honda uses a “mild” form of hybrid. True believers claim such vehicles are not real hybrids, as they cannot be driven on electric power alone.

A mild hybrid is simply a conventional car with an oversized starter motor and a 42-volt battery. Fuel savings come from switching off the engine while the car is coasting, braking or stopped. Thanks to the beefier starter motor, which functions also as a flywheel and alternator, the engine can be restarted seamlessly the instant the gas pedal is pressed.

By running the starter motor backwards as an alternator, most of the braking energy can be captured to top up the battery. Mild hybrids achieve a 10-15% better fuel economy compared with a strong hybrid’s 40% or more.

The only thing GM's two-mode transmission has in common with other hybrid drives is that it combines the best of an internal combustion engine with the best of an electric motor. Petrol engines come into their own at higher speeds when their throttles are wide open; electric motors deliver oodles of urge at much lower speeds.

The two-mode transmission is unique in having three planetary gear sets, four multi-plate clutches and two hydraulic oil pumps, instead of one of each in a conventional automatic box. More importantly, it also contains a pair of 60 kilowatt (80 horsepower) electric motors.

From a standstill, with the engine switched off, an electric motor at the back of the transmission winds the vehicle up to 30mph—provided the driver goes gently on the accelerator pedal. The same electric motor is used to make the vehicle go backwards when in reverse.

When needed, a second electric motor at the front of the transmission fires up the engine. The two electric motors then combine forces to deliver extra torque for hard acceleration and pulling heavy loads.

While cruising gently in top gear, the first electric motor alone helps relieve the petrol engine of some of its chores by chipping in an extra 30 horsepower or so. That, plus the fact that four of the engine's eight cylinders are automatically shut off when the vehicle is cruising steadily and needing nothing like the engine's full 330 horses, is the key to the two-mode’s good highway mileage.

During braking, one or both of the electric motors work as an alternator, converting unwanted momentum into electricity for recharging the 300-volt battery pack.

Such clever technology does not come cheap. At $53,000, a hybrid Tahoe costs $4,000 more than a conventional Tahoe. Amid collapsing demand for SUVs generally, GM has sold only a quarter as many hybrid SUVs as it anticipated back in January.

The complexity and cost of such drive-trains has made many in the industry think twice about strong hybrids. Mild hybrids like Honda's may offer only modest fuel savings, but they are considerably cheaper to make. Selling for less than a Toyota Prius, the new mild-hybrid version of the Chevrolet Malibu has been a runaway success, despite having only 2mpg better fuel economy than a conventional Malibu.

The mild hybrid's fortunes could improve dramatically if lithium-ion battery packs—jumbo versions of the power supplies used in laptops and mobile phones—could be prevented from overheating and exploding. Engineers reckon a mild hybrid with a lithium battery could match a strong hybrid like the Prius with its existing nickel-metal hydride battery.

Twice Toyota has delayed launching its third-generation Prius because of safety problems with its proposed lithium battery. The buzz now is that a sleeker, better and possibly cheaper Prius will arrive in late 2009, but with an improved nickel-metal hydride battery instead of a lithium one.

Meanwhile, German carmakers have been pressing ahead on battery development. BMW has shown off a mild version of its X5 SUV equipped with a lithium battery. Not to be left out, Mercedes has unveiled a lithium-powered mild hybrid based on its S400 car.

By all accounts, Mercedes has licked lithium's thermal runaway problems by integrating the battery pack into the car's cooling system. Continental, a German tyre and component manufacturer, is preparing to mass produce the patented battery for Mercedes. Presumably, other battery makers are doing similar things.

The question, then, is whether the motor industry decides mild hybrids with lithium batteries are good enough—and abandons advanced solutions like GM's two-mode transmission—until fuel-cell vehicles finally come of age. If so, the 80mpg family car is further off than many of us had hoped.

Copyright © The Economist Newspaper Limited 2008

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