The rotary engine is a type of internal combustion engine with a substantially different design than that of the much more common piston engine. In a conventional piston engine, a compressed fuel-air mixture is ignited in a closed chamber by a spark plug, causing the rapid expansion of superheated gas. Repeating this cycle causes the characteristic up-down motion of pistons in an engine, which is converted into rotational motion used to drive the wheels of an automobile or the propeller of a plane or boat.
First conceived by German engineer Felix Wankel in the early 20s, the rotary engine consists of a rotating triangular component embedded within an ovoid housing. The apexes of the triangle form a seal against the housing, creating three isolated chambers, which collectively perform the four basic functions of any internal combustion engine - intake, compression, ignition, and exhaust. As the triangle rotates, the three isolated chambers rotate as well, allowing a given volume to be exposed to an intake valve, spark plug, and exhaust port as they rotate through the interior of the rotary engine.
In the first stage, a fuel-air mixture is injected into a pocket created between one edge of the triangle and its oval housing. The triangle rotates inside the rotary engine until the seal isolates the volume of fuel from the intake valve, and exposes the fuel to a spark plug. The spark plug fires, igniting the fuel-air mixture, which expands rapidly and causes the rotating component to spin, driving a crankshaft. In the final phase of the rotation, the expanding gases escape through an exhaust port, and the volume is now clear again, allowing it to receive more fuel from the intake port and repeat the entire process. Each of the three volumes alternatively is injected full of fuel, which is then combusted, and then exposed to an exhaust port.
Although the Wankel rotary engine is theoretically more elegant than a piston engine, and requires less moving parts, the wear and tear caused on the apexes of the triangle moving rapidly within the housing of the rotary engine has proven unacceptable to most auto companies considering the creation of production models. Today, Mazda is the only company which continues to sell cars that use rotary engines. Fundamentally, the idea of an engine which replaces the jerky up-down motion of a reciprocating piston engine with smooth rotational motion is quite appealing, but practical considerations have thus far prevented them from displacing the conventional engine. Perhaps with the introduction of new materials or insights, the rotary engine will one day become the most prominent form of internal combustion engine.