The engine in the National Guard truck was humming along, spewing dirty exhaust. But it was also producing something unusual -- pure drinking water.

Bend Research Inc., of Bend, Ore., has developed a membrane that separates the water vapor out of engine exhaust. The membrane's skin, which does the filtering, is so thin that it would take 1,000 of them to equal the thickness of a piece of typing paper. After the water trickles into a container, a carbon filter removes remaining impurities.

Bend's technique is good news for the Army, which funded the project. "The military is worrying about having to fight in an environment that is biologically, chemically, and even nuclearly contaminated," says Rod Ray, Bend's project director. Under these combat conditions, says Mohsin Singapore, a mechanical engineer with the U.S. Army Tank Automotive Command, "if you see a water supply on the battlefield, you are not sure if the water is good to drink."

Military aircraft could use the technique, too. Spy planes on long trips might use it to provide water for operations on board. Navy dirigibles, which rise as they burn fuel and become lighter, may recapture water to slow their ascent.

The military isn't the only market. Motors that pump oil or produce electricity in arid areas from the tundra to the desert may someday be able to recover one gallon of filtered water from every two gallons of fuel they burn. Bend, which had sales of $2 million in 1983, expects that the technique will be commercially available within the next few years. We still can't squeeze water from a stone, but now it seems like we have the next best thing.