ROCK DOC - This alternative fuel not necessarily just hot air

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Kids delight in blowing up a balloon and letting it go. The air inside is under mild pressure, and when a youngster lets go of the neck of the balloon, air rushes outward. The escaping air propels the balloon forward like an erratic jet.

Remarkably enough, a car powered by the same energy source -- compressed air -- may be coming to a road near you.

At least one innovative auto company is investing in a small "air car," as these vehicles are known. Air cars have some wonderful advantages compared to our traditional internal combustion engine -- like the complete absence of air pollution coming from a tailpipe.

The idea of an air car is not so farfetched as it may sound. Your commuter car, my 1987 pickup, and a farmer's diesel tractor actually all run on a broadly similar idea.

Work with me for a moment, and I'll explain.

The internal combustion engines common around us look like they are powered by heat from burning fuel. But all the heat actually does is increase the pressure of gases in the engine's cylinder. It's the high pressure that pushes on the piston. Then the piston's motion powers the vehicle.

The heat isn't crucial, it's the pressure inside the cylinder that's the key.

Now imagine you could simply add highly compressed air into a car's cylinder to drive the piston. You wouldn't need heat, so there would be no need for gasoline or diesel fuel. And you could drive all day with no stinky fumes coming out your tailpipe.

For several decades, engineers have tinkered with the possibility of using air under high pressure to power the pistons of automobiles. The system can be made to work, especially if the air is under extreme pressures. (Those who know trucks will note that compressed air powers big-rig brakes and starters. So large trucks have a bit of airpower in their designs already.)

The pressures that are useful in a piston are generally much higher than those in a car tire. In scientific labs we often use very high-pressure tanks, as do welders and others in particular industries. If you work near an enormous tank of this high-pressure variety, and if it ruptures, your troubles are over. But I've never known that to happen.

If you've ever moved a high-pressure tank, you know they are heavy enough to give you a hernia. The steel "fuel tank" of compressed air in old test vehicles was so heavy it created real trouble for the engineering goal of powering a car on air alone.

But much lighter-weight materials based on carbon fibers that can hold air at high pressure are now on the market. So visionaries are taking another look at the "air car," and carmakers overseas are exploring options of bringing such cars to market.

But, of course, there is the question of where the compressed air will come from. There's no such thing as a free lunch, and the cost of running the air car is partly the cost of energy to compress the air.

As Popular Mechanics points out, it's generally electrical energy that's used to compress air. So air vehicles are essentially electric cars using the compressed air as a way of storing energy.

On the positive side, pollution that's created generating the electricity used to compress air could be distant from our cities. That's a real plus. (Although we geologists are fond of the smell of spilled gasoline and the choking fumes of exhaust on a hot day, normal human beings prefer to avoid all that filth.)

A lot of innovation is on the table these days in the car world, with major manufacturers investigating better electric cars, hybrid vehicles and natural-gas vehicles in the vein of what T. Boone Pickens advocates.

These are tough economic times, but interesting, too, and some folks are going to take advantage of entirely new ways of doing things to help move us forward.

I'm for that.

E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University. Peters can be reached at epeters@wsu.edu.

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