Inventors who are mixing water with fuel to power engines say they're onto something big. German-born inventor Rudolf Gunnerman [believes] that one of the world's most common compounds - good old tap water - can be blended with fuel to power your car, truck or lawn mower. Gunnerman claims to have devised a means to blend water with naphtha in order to power engines in a cleaner, cheaper, more efficient way. New ideas and better ideas are not necessarily found by universities or by large companies. New ideas and better ideas are found by people who look for them," says Gunnerman, 68. "Caterpillar" is the single word that brings a degree of credibility to Gunnerman's claims. The Peoria, Ill.-based heavy- equipment manufacturer entered a joint venture with Gunnerman in July 1994. Together, under the name Advanced Fuels, they've conducted experimental uses of the A-21 fuel - made up of 70 percent naphtha, a crude-oil byproduct, and 30 percent water. And now, Paccar Inc. is throwing its trucking weight in Gunnerman's corner. The Bellevue-based manufacturer of Kenworth and Peterbilt trucks recently sent a truck to Peoria for testing with the A-21 fuel. Paccar changed out the engine to add a Caterpillar engine and modified the cylinders and fuel injectors to handle more fluid volume. They also did a series of baseline tests of noise, cooling, drivability and fuel economy, said Jim Reichman, Paccar's technology-development manager. Back at Paccar's Mount Vernon technical center, Reichman is enthused. "We're pretty pleased with it," he said.