The Wankel engine (/ˈvaŋkəl̩/, VUN-kell) 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.^[1] 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, rotating the shaft via a cam.

In its basic gasoline fuelled form, the Wankel engine has lower thermal efficiency and higher exhaust emissions when compared to the four-stroke reciprocating piston engine. The thermal inefficiency has restricted the engine to limited use since its introduction in the 1960s. However, many disadvantages have mainly been overcome over the succeeding decades as the production of road-going vehicles progressed. The 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. The Wankel engine was also used to power motorcycles and racing cars.

Concept

Rotary engine types

Figure 2.
The first DKM Wankel engine designed by Felix Wankel, the DKM 54 (Drehkolbenmotor), at the Deutsches Museum in Bonn

Figure 3.
A Wankel engine with its rotor and geared output shaft

Figure 5.
The first KKM Wankel Engine based on a design by Hanns-Dieter Paschke, the NSU KKM 57P (Kreiskolbenmotor), at Autovision und Forum

Figure 6.
First production Wankel engine; installed in an NSU Spider

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 (see Figure 2.) and the Kreiskolbenmotor (KKM, "circuitous piston engine"), designed by Hanns-Dieter Paschke^[2] (see Figure 3.), 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.^[3]
• In a KKM engine, the outer rotor is part of the stationary housing (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 its 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,^[4] making it a much simpler design to be adopted to conventional powertrains.^[5]

Wankel engine development

Felix Wankel designed a rotary compressor in the 1920s, and received his first patent for a rotary type of engine in 1934.^[6] 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.^[7] With the assistance of Prof. Othmar Baier [de] from Stuttgart University of Applied Sciences, the concept was defined mathematically.^[8] The supercharger he designed was used for one of NSU's 50 cm³ one-cylinder two-stroke engines. The engine produced a power output of 13.5 PS (10 kW) at 12,000 rpm.^[9]

In 1954, NSU agreed to develop a rotary internal combustion engine with Felix Wankel, based upon Wankel's supercharger design for their motorcycle engines. 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".^[10] The first working prototype, DKM 54 (see figure 2.), first ran on 1 February 1957, at the NSU research and development department Versuchsabteilung TX. It produced 21 PS (15 kW).^[11][12] Soon after that, a second prototype of the DKM was built. It had a working chamber volume V_k of 125 cm³ and also produced 21 kW (29 PS) at 17,000 rpm.^[13] It could even reach speeds of up to 25,000 rpm. However, these engine speeds distorted the outer rotor's shape, thus proving impractical.^[14] According to Mazda Motors engineers and historians, four units of the DKM engine were built; the design is described to have a displacement V_h of 250 cm³ (equivalent to a working chamber volume V_k of 125 cm³). The fourth unit built is said to have received several design changes, and eventually produced 29 PS (21 kW) 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 (see figure. 2).^[15]

Due to its complicated design with a stationary center shaft, the DKM engine was impractical.^[4] 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.^[16] 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 (see figure 5.).^[4] 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.^[5] 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 hula-hoop motion of the rotor. NSU could not afford to finance the development of both the DKM and the KKM, and eventually decided to drop the DKM in favor of the KKM, because the latter seemed to be the more practical design.^[17]

Wankel obtained the US patent 2,988,065 on the KKM engine on 13 June 1961.^[18] Throughout the design phase of the KKM, Froede's engineering team had to solve problems such as repeated bearing seizures, the oil flow inside the engine, and the engine cooling.^[19] The first fully functioning KKM engine, the KKM 125, weighing in at only 17 kg (37.5 lb) displaced 125 cm³ and produced 26 PS (19 kW) at 11,000 rpm.^[20] Its first run was on 1 July 1958.^[21]

In 1963, NSU produced the first series-production Wankel engine for a car, the KKM 502 (see Figure 6.). 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 PP engine with a displacement of 996 cm³ (61 in³), a rated power of 40 kW (54 hp) at 6,000 rpm and a BMEP of 1 MPa (145 lbf/in²).^[22]

Operation and design

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., its housing's inner side resembles a two lobes oval-like epitrochoid (equivalent to a peritrochoid),.^[25] In contrast, its rotary piston has a three vertices trochoid shape (similar to a Reuleaux triangle). Thus, the Wankel engine's rotor constantly forms three moving working chambers.^[26] The Wankel engine's basic geometry is depicted in figure 7. Seals at the rotor's apices seal against the housing's periphery.^[27] 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.

All practical Wankel engines are four-cycle (i.e., four-stroke) engines. In theory, two-cycle engines are possible, but they are impractical because the intake gas and the exhaust gas cannot be properly separated.^[16] The operating principle is similar to the Otto operating principle; the Diesel operating principle with its compression ignition cannot be used in a practical Wankel engine.^[28] Therefore, Wankel engines typically have a high-voltage spark ignition system.^[29]

In a Wankel engine, one side of the triangular rotor completes the four-stage Otto cycle of intake, compression, expansion, and exhaust each revolution of the rotor (equivalent to three shaft revolutions, see Figure 8.).^[30] The shape of the rotor between the fixed apexes is to minimize the volume of the geometric combustion chamber and maximize the compression ratio, respectively.^[27][31] As the rotor has three sides, this gives three power pulses per revolution of the rotor.

Wankel engines have a much lower degree of irregularity when compared to a reciprocating piston engine, making the Wankel engine run much smoother. This is because the Wankel engine has a lower moment of inertia and less excess torque area due to its more uniform torque delivery. For instance, a two-rotor Wankel engine runs more than twice as smoothly as a four-cylinder piston engine.^[32] The eccentric output shaft of a Wankel engine also does not have the stress-related contours of a reciprocating piston engine's crankshaft. The maximum revolutions of a Wankel engine are thus mainly limited by tooth load on the synchronizing gears.^[33] Hardened steel gears are used for extended operation above 7,000 or 8,000 rpm. In practice, automotive Wankel engines are not operated at much higher output shaft speeds than reciprocating piston engines of similar output power. Wankel engines in auto racing are operated at speeds up to 10,000 rpm, but so are four-stroke reciprocating piston engines with relatively small displacement per cylinder. In aircraft, they are used conservatively, up to 6500 or 7500 rpm.

Chamber volume

In a Wankel rotary engine, the chamber volume ${\displaystyle V_{k}}$ is equivalent to the product of the rotor surface ${\displaystyle A_{k}}$ and the rotor path ${\displaystyle s}$. The rotor surface ${\displaystyle A_{k}}$ is given by the rotor tips' path across the rotor housing and determined by the generating radius ${\displaystyle R}$, the rotor width ${\displaystyle B}$, and the parallel transfers of the rotor and the inner housing ${\displaystyle a}$. Since the rotor has a trochoid ("triangular") shape, sinus 60 degrees describes the interval at which the rotors get closest to the rotor housing. Therefore,

${\displaystyle A_{k}=2\cdot B\cdot (R+a)\cdot sin(60^{\circ })={\sqrt {3}}\cdot B\cdot (R+a)}$^[34]

The rotor path ${\displaystyle s}$ may be integrated via the eccentricity ${\displaystyle e}$ as follows:

${\displaystyle \sum <!--\; \sum \; -->ds={\int <!--\; \int \; -->}_{\mathrm{\alpha <!--\; \alpha \; -->}=0^{\circ <!--\; \circ \; -->}}^{\mathrm{\alpha <!--\; \alpha \; -->}={270}^{\circ <!--\; \circ \; -->}}e\cdot <!--\; \cdot \; -->sin\frac{}{}}$Zdroj:https://en.wikipedia.org?pojem=Wankel_engine
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