The most successful rotary engine is the Wankel engine. Developed by the German engineer Felix Wankel in 1956, it has a disk that looks like a triangle with bulging sides rotating inside a cylinder shaped like a figure eight with a thick waist. Intake and exhaust are through ports in the flat sides of the cylinder. The spaces between the sides of the disk and the walls of the cylinder form combustion pockets. During a single rotation of the disk each pocket alternately grows smaller, then larger, because of the contoured outline of the cylinder. This provides for compression and expansion. The engine runs on a four-stroke cycle.
The Wankel engine has 48% fewer parts and about a third the bulk and weight of a reciprocating engine. Its main advantage is that advanced pollution control devices are easier to design for it than for the conventional piston engine. Another advantage is that higher engine speeds are made possible by rotating instead of reciprocating motion, but this advantage is partially offset by the lack of torque at low speeds, leading to greater fuel consumption.
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