A new meaning to 'wind-cheating'

In a quest to create its own super family sedan, General Motors Corp. says it has designed the slipperiest midsize car ever.

At 0.163 Cd, the car even beats GM's own EV1 electric vehicle, hailed at its launch as the world's most aerodynamic production car.

The aerodynamic efficiency exercise is a component of GM's Partnership for a New Generation of Vehicles (PNGV) effort. PNGV is a coalition of U.S. government and auto industry interests whose goals are to develop the technologies to produce a high-mileage, 5-passenger family sedan. The PNGV car is projected to produce 80 mpg (2.9L/100 km) fuel efficiency; cost no more to own and maintain than today's average midsize sedan (originally targeted were the Ford Taurus, Chevrolet Lumina and Chrysler Concorde); meet or exceed 2004 safety and emissions standards; and achieve 80% re-cyclability.

To do this, engineers at GM's Advanced Technology Vehicles - the folks that gave us the EV1 - determined that they must develop a powertrain that operates 50% more efficiently than a conventional one, with about a 40% thermal efficiency. More so, they must double the vehicle's efficiency by reducing mass by 40%, rolling resistance by one-third and aerodynamic drag by 30%.

"We couldn't set out to do what we're doing without the research we've already done on the EV1," says Mike Kutcher, GM PNGV team leader.

In fact, the GM ATV team's benchmark was the EV1. To achieve the 30% reduction in aerodynamic drag, the team established an initial requirement of 0.18 Cd, just below the EV1's 0.19 Cd.

To meet its target, the team stuffed the hybrid electric drivetrain, including its 1.2L direct-injection diesel engine, in the rear. Its rear-engine, rear-drive design allows for the car's frontal area to be dictated by the passengers and not drivetrain size; that way, the car can more closely resemble a teardrop, the most aerodynamic shape.

But moving the engine to the rear means transfering all engine-cooling functions as well. In GM's PNGV model, air enters through rear fender vents in a very controlled manner and then is exhausted through the rear of the car.

This innovative rear thermal system produces 70% less aero drag than a conventional front-breathing car, says GM.

The car also features a totally smooth underbody. The rear-engine design eliminates the need for a long exhaust mounting, making room for a full belly pan to be installed. Wind also is carefully deflected around the tires.

Ron York, GM PNGV director, adds that the model is aerodynamically efficient from every angle, too, including how it faces crosswinds, an obscure value (obscure except to aero engineers, obviously) known as Cd-bar. "We've paid close attention to (overall) efficiency instead of producing a Cd stunt," he says. The Cd-bar number is 0.180 - and bully for you if you can do anything with it, although GM's engineers say that value is about half that of a typical car.

The vehicle's overall 0.163 Cd is 55% better than the average midsize family sedan drag coefficient, equating to a 12 mpg (0.5L/100 km) improvement. It also is 16% better than the marvelously slippery EV1 and 11% below ATV's aggressive original target Cd number.

Much of the over-achievement can be accredited to the GM stylists included as a part of the PNGV team. When the original design had gone through over 100 hours of wind tunnel testing, it was sent to the stylists to see if they could make the car look somewhat less like a potato.

They insisted on adding body lines and raising the front part of the roofline. The engineers argued that adding any frontal area would increase the car's Cd.

So both designs were tested in the wind tunnel. The stylists won. Most of the additions they made to the design did little to disturb the car's aero number, and in some cases even improved it.

GM will display its PNGV design at the 2000 North American International Auto Show in Detroit this coming January.

Mr. York and the ATV group are anxiously awaiting public reaction to the definitely different - but oddly appealing - "geo-mechanical" look of their low-drag masterpiece. Whether or not the public "likes" it may be meaningless if they eventually want 80-mpg cars, but it begs the question of whether some of ATV's mileage-enhancing design and styling efforts might find their way onto mainstream GM production cars.

"Where we use proprietary teams and compete is in design and integration," says Mr. York rather evasively.

"We may do a little horsetrading (with production-car design teams). We'll have to see."