David B. Wittry, with a diagram of his "scissors-action" internal combustion engine: "This engine design overcomes the excessive fuel-consumption and pollution problems that were an inseparable part of an earlier rotary engine."

Photo- ERIC MANKIN

The U.S. Patent Office has issued patent number 5,433,179 for a pistonless engine designed by David B. Wittry, a professor in the School of Engineering.

Wittry's design is based on a form of rotary engine known as the "scissors-action" internal combustion engine. Instead of using a piston, it uses two rotors, each equipped with a pair of vanes set 180 degrees apart. These thick vanes (two per rotor) fit together to form the shape of an "X" when viewed on end.

As the engine goes through its cycle, the rotors alternately rotate, in effect chasing each other around the cylinder. This action alternately broadens and narrows the "X" shape, like a pair of scissors the blades of which are alternately opened wide and then almost completely closed.

The vanes rotate past two openings in the cylinder wall: the fuel-air mixture goes in through one opening, and the exhaust passes out through the other. In a traditional piston engine, valves are needed to open or close these openings; in Wittry's rotary engine, the vanes themselves fulfill the function.

Scissors-action engines have been proposed before , but Wittry's design provides a better linkage to keep the rotors moving in sync, cools the rotors more effectively and improves efficiency by letting the burning fuel that powers the engine push the rotor an extra distance each time the spark plug fires.

Traditional internal combustion engines have pistons or rotors that move exactly as far on the compression stroke, which prepares the fuel-air mixture for firing, as they do on the expansion stoke, when the hot gas forces the piston down or the rotors to move.

In Wittry's engine, the burning gases expand into a volume greater than that of the fuel-air mixture that has previously been compressed, getting more work out of the burned fuel. The greater expansion also means that exhaust gases are cooler - a big plus for any internal combustion engine.

Wittry said his improved engine is as efficient as a diesel, but far quieter and nearly vibration free. Moreover, easy adjustments to the mechanism can enable the same engine to run on different fuels (such as gasoline, compressed natural gas, methanol, ethanol, propane, butane and even hydrogen).

Like earlier rotary engines, Wittry's model will be far lighter and cheaper to build than piston engines. But unlike the earlier versions, it won't guzzle fuel - thanks to its high-efficiency, extra-expansion power stroke.

"For applications such as lawnmowers, chain saws or outboard motors, it seems ideal," he said. "It should be far quieter than the two-stroke cycle engines now used for these applications, and it should produce fewer emissions due to its four-stroke cycle." The new engine also has potential for snowmobiles, jet skis, motorcycles and perhaps even automobiles, he added.

Wittry is assembling a prototype working engine that will weigh 10 pounds and develop approximately 3 horsepower.

"This engine design overcomes the excessive fuel-consumption and pollution problems that were an inseparable part of an earlier rotary engine design, the Wankel engine used in Mazdas," he said.

At the School of Engineering, Wittry holds joint appointments in the departments of Materials Science and Electrical Engineering/Electrophysics. He has 14 earlier patents - mostly in materials science instrumentation - to his credit.