Science: Power Without Pistons

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Conventional gasoline engines have a basic fault; their reciprocating parts (pistons, connecting rods, etc.) must be stopped and started thousands of times per minute. This wastes power, and it also calls for a heavy engine to stand up against the battering it gets. Last week NSU Werke motor company of Neckar-sulm, West Germany described a gas engine that has neither pistons nor valves. Invented by a mechanical genius named Felix Wankel, it was developed with financial help from Curtiss-Wright Corp., which provided a fervid earlier announcement of it (TIME, Dec. 7) but no mechanical details.

Instead of cylinders and pistons, the Wankel engine has a single combustion chamber shaped like a fat-waisted figure eight (see diagram). Inside, it is a three-cornered rotor with curved sides. A shaft passes through the rotor and makes it move on an eccentric orbit by means of two gears. All three corners of the triangle stay in contact with the wall of the chamber at all times. To make the contacts gaslight, each corner is tipped with an inset metal strip that, as the drive shaft revolves, is pushed tight against the cavity's inner walls by centrifugal force.

The whole engine is an ingenious exercise in mechanical geometry. As the rotor turns, its corners form three moving cavities that first increase in volume and then decrease. When a cavity, still small but growing, passes the intake port leading to the carburetor, it draws in fuel and air. Then the cavity decreases in volume, compressing the mixture. The engine's single spark plug fires; the exploding gas pushes the rotor and shaft. At the end of the power stroke, a corner of the rotor uncovers the exhaust port, and the burned gases are swept out of the engine. Meanwhile, two other cavities have been formed and are passing through the same cycle. Maximum shaft speed is 17,000 r.p.m., but the rotor turns over only one-third as fast.

The Wankel engine is not as efficient as a diesel, but its makers say that its fuel consumption is about the same as the economical Volkswagen. The most extraordinary thing about it is its small size. A 28.6-metric-h.p. model is 8 in. in diameter, 6 in. long, weighs only 22 lbs. The Volkswagen engine has about the same horsepower, but is many times bigger and weighs 198 lbs.

Critical in the engine's design were the metal "piston rings" at the tips of the triangle that keep the chambers gastight. But NSU says the metal strips show no wear after 300 hours of fullspeed operation. The engine uses a conventional carburetor and can be made to burn many kinds of fuel, including diesel oil. It is not for sale yet, but NSU expects to have it debugged and in large production in about two years.