The Wankel engine (, ) is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion. The concept was proven by German engineer Felix Wankel, followed by a commercially feasible engine designed by German engineer Hanns-Dieter Paschke. The Wankel engine's rotor 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 gear. The midpoint of the rotor moves in a circle around the output shaft, rotating the shaft via a cam.
In its basic gasoline-fuelled form, the Wankel engine has lower thermal efficiency and higher exhaust emissions relative to the four-stroke reciprocating engine. This thermal inefficiency has restricted the Wankel engine to limited use since its introduction in the 1960s. However, many disadvantages have mainly been overcome over the succeeding decades following the development and production of road-going vehicles. The advantages of compact design, smoothness, lower weight, and fewer parts over reciprocating internal combustion engines make Wankel engines suited for applications such as chainsaws, auxiliary power units (APUs), loitering munitions, aircraft, personal watercraft, snowmobiles, motorcycles, racing cars, and automotive range extenders.
Concept
The Wankel engine is a type of rotary piston engine and exists in two primary 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 triangular rotor, and the outer rotor, which has a circular outer shape, and a figure-eight inner shape. The center shaft is stationary, and torque is taken from the outer rotor, which is geared to the inner rotor. In a KKM engine, the outer rotor is part of the stationary housing, and is thus not a moving part. The inner shaft is a moving part with an eccentric lobe for the inner rotor to spin around. The rotor spins around the center of the lobe and around the axis of the eccentric shaft in a hula hoop-like fashion, resulting in the rotor making one complete revolution for every three revolutions of the eccentric shaft. Torque is taken from the eccentric shaft, making it a much simpler design to adapt to conventional powertrains.
Development Felix Wankel designed a rotary compressor in the 1920s in the German City Lahr and received his first patent for a rotary type of engine in 1934. He realized that the triangular rotor of the rotary compressor could have intake and exhaust ports added, producing an internal combustion engine. Eventually, in 1951, Wankel began working at German firm NSU Motorenwerke to design a rotary compressor as a supercharger for NSU's motorcycle engines. Wankel conceived the design of a triangular rotor in the compressor. With the assistance of Professor from Stuttgart University of Applied Sciences, the concept was defined mathematically. The supercharger he designed was used for one of NSU's two-stroke single-cylinder engines. The engine produced a power output of at 12,000rpm.
In 1954, NSU agreed to develop a rotary internal combustion engine with Wankel based upon his supercharger design. Since Wankel was known as a "difficult colleague", the development work for the DKM was carried out at Wankel's private Lindau design bureau. According to John B. Hege, Wankel received help from his friend Ernst Höppner, who was a "brilliant engineer". The first working prototype, DKM 54, first ran on 1 February 1957 at NSU's Versuchsabteilung TX research and development facility. It produced . Soon after that, a second prototype of the DKM was built. It had a working chamber volume Vk of and also produced at 17,000rpm. It could even reach speeds of up to 25,000rpm. However, these engine speeds distorted the outer rotor's shape, thus proving impractical. According to Mazda engineers and historians, four units of the DKM engine were built; the design is described to have a displacement Vh of 250 cm3 (equivalent to a working chamber volume Vk of 125 cc). The fourth unit built is said to have received several design changes, and eventually produced at 17,000 rpm; it could reach speeds up to 22,000 rpm. One of the four engines built has been on static display at the Deutsches Museum Bonn.
Due to its complicated design with a stationary center shaft, the DKM engine was deemed impractical. Wolf-Dieter Bensinger explicitly mentions that proper engine cooling cannot be achieved in a DKM engine, and argues that this is the reason why the DKM design had to be abandoned. NSU development chief engineer Walter Froede solved this problem by using Hanns-Dieter Paschke's design and converting the DKM into what would later be known as the KKM. The KKM proved to be a much more practical engine, as it has easily accessible spark plugs, a simpler cooling design, and a conventional power take-off shaft. Wankel disliked Froede's KKM engine because of its inner rotor's eccentric motion, which was not a pure circular motion as Wankel had intended. He remarked that his "race horse" was turned into a "plough horse". Wankel also complained that more stresses would be placed on the KKM's apex seals due to the eccentric motion of the rotor. NSU could not afford to finance developing both the DKM and the KKM, and eventually decided to drop the DKM in favour of the KKM since the latter seemed to be the more practical design.
Wankel obtained US patent 2,988,065 on the KKM engine on 13 June 1961. Throughout the design phase of the KKM, Froede's engineering team had to solve problems such as repeated bearing seizures, oil flow issues, and cooling issues. The first fully functioning KKM engine, the KKM 125, weighed in at only , displaced , and produced at 11,000rpm. Its first run was on 1 July 1958.
In 1963, NSU produced the first series-production Wankel engine for a car, the KKM 502. It was used in the NSU Spider sports car, of which about 2,000 were made. Despite its "teething troubles", the KKM 502 was a powerful engine with decent potential, smooth operation, and low noise emissions at high engine speeds. It was a single-rotor peripheral port engine with a displacement of , a rated power of at 6,000rpm and a brake mean effective pressure (BMEP) of .
Evolution
Felix Wankel managed to overcome most of the problems that interfered with prior attempts to perfect the Wankel engine, by designing the apex seals with a tip radius equal to the amount of "oversize" of the rotor housing shape relative to the theoretical epitrochoid, to minimize radial apex seal motion, and cylindrical gas-loaded apex pins which strengthened the seals.
In the early days, unique, dedicated production machines had to be built for different housing dimensions. However, patented designs such as , G. J. Watt, 1974, for a "Wankel Engine Cylinder Generating Machine", , "Apparatus for machining and/or treatment of trochoidal surfaces" and , "Device for machining trochoidal inner walls", and others, solved such production issues.
Wankel engines have a problem not present in reciprocating piston four-stroke engines in that intake, compression, combustion, and exhaust occur at fixed locations, causing a very uneven thermal load on the rotor housing. In contrast, four-stroke reciprocating engines perform these four strokes in one chamber, so that the extremes of the cold intake and hot exhaust are averaged out and shielded from working parts by a boundary layer. The University of Florida proposed the use of heat pipes in an air-cooled Wankel to overcome this uneven heating of the housing.SAE paper 2014-01-2160 Pre-heating of certain housing sections with exhaust gas improved performance and fuel economy, also reducing wear and emissions. Boundary layer shields and lubricant film act as thermal insulation, leading to a lower temperature of the film (approximately a maximum of on a water-cooled Wankel engine) and a more constant surface temperature. The temperature around the spark plug is about the same as in the combustion chamber of a reciprocating engine. With circumferential or axial flow cooling, the temperature difference remains tolerable.
Problems arose during research in the 1950s and 1960s as engineers were faced with what they called "chatter marks" and "devil's scratch" in the inner epitrochoid surface, resulting in chipping of the chrome coating on the trochoidal surfaces. They discovered that the cause was the apex seals reaching a resonating vibration, and the problem was solved by reducing the thickness and weight of the apex seals as well as using more suitable materials. Scratches disappeared after introducing more compatible materials for seals and housing coatings. Kenichi Yamamoto experimentally lightened apex seals with holes, identifying weight as the main cause and leading Mazda to use aluminum-impregnated carbon apex seals in their early production engines. NSU used carbon antimony-impregnated apex seals against a chrome housing surface; upon developing an "Elnisil" coating to production maturity, it returned to a metal sealing strip for the Ro 80. Mazda continued to use a chrome surface, but applied to a steel jacket in the aluminum housing. This allowed Mazda to return to the 3mm and later even 2mm thick metal apex seals.Yamamoto, Kenichi (1971). Rotary Engine. Toyo Kogyo. Page 60-61 Another early problem was the build-up of cracks in the stator surface near the plug hole, which was eliminated by installing the spark plugs in a separate conductive copper insert instead of screwing them directly into the block housing.
Toyota found that substituting glow plugs for leading-area spark plugs improved low-RPM partial-load specific fuel consumption by 7%, as well as emissions and idle performance.SAE paper 790435 A later alternative solution to spark plug boss cooling was a variable coolant velocity scheme for water-cooled rotaries, which was patented by Curtiss-Wright and saw widespread use., M. Bentele, C. Jones, F. P. Sollinger, 11/7/61 and , C. Jones, R. E. Mount, 4/29/63 and , C. Jones, 7/27/65 These approaches did not require a copper insert, but did not preclude its use. Ford tested a Wankel engine with the plugs placed in the side plates instead of the housing working surface (, 1978).
Operation and design
The Wankel engine has a spinning eccentric power take-off shaft with an eccentric lobe around which the rotor revolves. The rotor's crown gear has one and a half times the number of teeth as the gear that is fixed to the housing (a 2:3 gear ratio). The rotor and housing constantly form three moving working chambers. The rotor does not make contact with its housing, so seals at the rotor's apices press against the housing's periphery to prevent pressure loss. The increase in pressure from combustion pushes against the rotor face, in turn transferring force to the eccentric part of the output shaft.
All practical Wankel engines are Otto cycle (i.e., four-stroke) engines, with each of the three rotor faces undergoing its own intake, compression, expansion, and exhaust cycles. The shape of the rotor between the fixed apices is to minimize the volume of the geometric combustion chamber and maximize the compression ratio, respectively.For a detailed calculation of the curvature of a circular arc approximating the optimal Wankel rotor shape, see In theory, two-cycle engines are possible, but they are impractical because the intake gas and the exhaust gas cannot be properly separated. As the Diesel cycle with its compression ignition cannot be used in a practical Wankel engine, Wankel engines typically have a high-voltage spark ignition system.