- Key People:
- Felix Wankel
- Related Topics:
- rotary engine
Wankel engine
What is unique about the Wankel engine’s design?
Are Wankel engines still in use?
Who developed the Wankel engine and when?
Do any cars use a Wankel engine?
Wankel engine, type of internal-combustion rotary engine distinguished by an orbiting triangular rotor that functions as a piston. Developed in Germany, it is radically different in structure from conventional reciprocating piston engines (engines in which pistons move up and down in cylinders). This engine was conceived by Felix Wankel, a specialist in the design of sealing devices, and experimental units were built and tested by a German firm beginning in 1956.
Structure and function
Instead of pistons that move up and down in cylinders, the Wankel engine has an orbiting rotor in the shape of an equilateral triangle. The rotor turns in a closed chamber, and the three apexes of the rotor maintain a continuous sliding contact with the curved inner surface of the casing. The casing is not a perfect cylinder; one of its diameters is longer than the other. The curve-sided rotor forms three crescent-shaped chambers between its sides and the curved wall of the casing. The volumes of the chambers vary with rotor position. A chamber attains its maximum volume when the side of the rotor forming it is parallel with the shorter diameter of the casing; the volume is reduced to a minimum when the rotor side is parallel with the longer diameter. Shallow pockets recessed in the flank of the rotor control the shape of the combustion chambers and establish the compression ratio (degree to which the fuel mixture is compressed before ignition) of the engine.
In turning about its central axis, the rotor follows a circular orbit about the geometric center of the casing. The orbiting rotation is caused by a central hole in the rotor in which an internal gear is fitted to mesh with a stationary pinion (a fixed gear) set immovably into the center of the casing. The rotor is guided by fitting its central hole to an eccentric (a lobe that is off the center of the circle it is fitted in) on the output shaft that passes through the center of the stationary pinion. This eccentric also harnesses the rotor to the shaft so that torque is applied when gas pressure is exerted against the rotor flanks as the fuel and air charges burn. A 3-to-1 gear ratio causes the output shaft to turn three times as fast as the rotor turns about the eccentric. Each quarter turn of the rotor completes an expansion or a compression, permitting intake, compression, expansion, and exhaust to be accomplished during one turn of the rotor. The only moving parts are the rotor and the output shaft.
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The fuel mixture is supplied by a carburetor and enters the combustion chambers through an intake port in one of the end plates of the casing. An exhaust port is formed in one of the flattened sides of the casing wall, and a spark plug is located in a pocket communicating with the chambers through a small throat in the opposite side of the casing wall. The rotor and its gears and bearings are lubricated and cooled by oil circulating through the hollow rotor. The metal seals at the tips of the rotor are lubricated by a small amount of oil added to the fuel (as little as 1 part oil to 200 parts fuel). Water flows through cooling channels in the engine housing, entering next to the spark plug, where the engine gets the hottest.
Keeping the engine airtight is a considerable design challenge. To solve this, sliding metal strips sit in slots at the three corners of the rotor and are pushed constantly against the engine wall by small springs. The flat sides of the rotor are also sealed using curved metal rings tucked into grooves along the edges, which are pressed against the side walls by flat springs.
Applications
The major advantages of the Wankel engine are its small space requirements and low weight per horsepower, smooth and quiet operation, and low manufacturing costs resulting from mechanical simplicity. The absence of inertial forces from reciprocating parts found in a typical piston-and-cylinder arrangement and the elimination of spring-closed poppet valves permit operation at much higher speed than is practical for reciprocating piston engines—an advantage because shaft speed must be high for optimum performance. The induction of fresh fuel mixture and exhaust are more effective because the ports are opened and closed more rapidly than with poppet valves, and gas flow through them is almost continuous. Heat transfer and the resulting cooling requirement are low because the jacketed surface is small. Lower weight and a lower center of gravity make it much safer in an automobile in the event of a collision.
Fuel economy concerns and the cost of meeting emissions standards have kept Wankel engines from widespread use in production vehicles. Mazda ended production of its rotary-powered RX-8 in 2012 but resumed mass production of rotary-engine vehicles in 2023 with the MX-30 Rotary-EV, a plug-in hybrid in which the engine generates electricity rather than driving the wheels directly. Wankel-type engines also remain in specialized applications where their compact size and low weight are advantageous, particularly in unmanned aircraft and other aerospace uses.
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