The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion. The concept was invented and proven by German engineer Felix Wankel, and the commercially feasible engine designed by German engineer Hanns-Dieter Paschke. The Wankel engine’s rotor, which creates the turning motion, is similar in shape to a Reuleaux triangle, with the sides having less curvature. The rotor spins inside a figure-eight-like epitrochoidal housing, around a fixed toothed gearing. The midpoint of the rotor moves in a circle around the output shaft, spinning the shaft via a cam.
Due to its inherent poor thermodynamics, the Wankel engine has a significantly worse thermal efficiency and worse exhaust gas behaviour when compared against the Otto engine or the Diesel engine, which is why the Wankel engine has seen limited use since its introduction in the 1960s. However, its advantages of compact design, smoothness, lower weight, and fewer parts over the reciprocating piston internal combustion engines make the Wankel engine suited for applications such as chainsaws, auxiliary power units, loitering munitions, aircraft, jet skis, snowmobiles, and range extenders in cars. In the past, the Wankel engine has also been used in road vehicles such as motorcycles, and racing cars.
The Wankel engine is a type of rotary piston engine and exists in two basic forms, the Drehkolbenmotor (DKM, “rotary piston engine”), designed by Felix Wankel and the Kreiskolbenmotor (KKM, “circuitous piston engine”), designed by Hanns-Dieter Paschke, of which only the latter has left the prototype stage. Thus, all production Wankel engines are of the KKM type.
In a DKM engine, there are two rotors: the inner, trochoid-shaped rotor, and the outer rotor, which has an outer circular shape, and an inner figure eight shape. The center shaft is stationary, and torque is taken off the outer rotor, which is geared to the inner rotor.
In a KKM engine, the outer rotor is part of the stationary housing (and thus not a moving part). The inner shaft is a moving part and has an eccentric lobe for the inner rotor to spin around. The rotor spins around its own center, and around the axis of the eccentric shaft in a hula hoop fashion, resulting in the rotor making one complete revolution for every three revolutions of the eccentric shaft. In the KKM engine, torque is taken off the eccentric shaft, making it a much simpler design to be adopted to conventional powertrains.
The Wankel engine has a spinning eccentric power take-off shaft, with a rotary piston riding on eccentrics on the shaft in a hula-hoop fashion. The Wankel is a 2:3 type of rotary engine, i.e., two-thirds of its ideal total geometrical volume can be attributed to displacement. Thus, its housing’s inner side resembles an oval-like epitrochoid, whereas its rotary piston has a trochoid (triangular) shape (similar to a Reuleaux triangle), and the Wankel engine’s rotor always forms three moving working chambers.
The Wankel engine’s basic geometry is depicted in figure 7. Seals at the apices of the rotor seal against the periphery of the housing. The rotor moves in its rotating motion guided by gears and the eccentric output shaft, not being guided by the external chamber. The rotor does not make contact with the external engine housing. The force of expanded gas pressure on the rotor exerts pressure on the center of the eccentric part of the output shaft.
Felix Heinrich Wankel (13 August 1902 – 9 October 1988) was a German mechanical engineer and inventor after whom the Wankel engine was named.
Wankel was born in 1902 in Lahr in what was then the Grand Duchy of Baden in the Upper Rhine Plain of present-day southwestern Germany.
During World War II, Wankel developed seals and rotary valves for German air force aircraft and navy torpedoes, as well as for companies such as BMW and Daimler-Benz. After the war, the region was occupied by France. Wankel was imprisoned by French authorities for several months in 1945 and his laboratory was closed by French occupation troops. Wankel’s work was confiscated and he was prohibited from doing any more work. However, by 1951, he got funding from the Goetze AG company to furnish the new Technical Development Center in his privately owned house in Lindau on Lake Constance. He began development of the engine at NSU Motorenwerke AG, leading to the first running prototype on 1 February 1957. Unlike modern Wankel engines, this 21 horsepower version had both the rotor and housing rotating. His engine design was first licensed by Curtiss-Wright in New Jersey, United States.
On 19 January 1960 the rotary engine was presented for the first time to specialists and the press in a meeting of the German Engineers’ Union at the Deutsches Museum in Munich. In the same year, with the KKM 250, the first practically applied rotary engine was presented in a converted NSU Prinz automobile. At around this time the term “Wankel engine” became synonymous with the rotary type of engine, whereas previously it was referred to as the “Motor nach System NSU/Wankel”. At the 1963 IAA motor show in Frankfurt, the NSU company presented the NSU Wankel-Spider, the first consumer vehicle with a rotary engine, which went into production in 1964. Great attention was received by the NSU in August 1967 for the very modern NSU Ro 80 sedan, which had a 115-horsepower engine with two rotors. It was the first German car named “Car of the Year” in 1968.