Wankel engine

The Wankel engine (, ) is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion. It was invented by German engineer Felix Wankel, and 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 rotates inside an oval-like epitrochoidal housing, around a central output shaft. The rotor spins in a hula-hoop fashion around the central output shaft, spinning the shaft via toothed gearing.

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 less parts over the aforementioned reciprocating piston internal combustion engines make the Wankel engine suited for applications such as chainsaws, auxiliary power units, loitering munitions, aircraft, jet skis and snowmobiles. In the past, the Wankel engine has also been used in road vehicles such as automobiles, motorcycles, and racing cars.

Concept

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 (see Figure 2.) and the ''Kreiskolbenmotor'' (KKM, "circuitous piston engine"), designed by Hanns-Dieter Paschke (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 is has an outer circular shape, and an inner figure 8 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.

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. 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 Prof. from Stuttgart University of Applied Sciences, the concept was defined mathematically. The supercharger he designed was used in the 500 cm³ two-cylinder engine of the NSU Delphin motorcycles. The engine produced a power output of at 8,500rpm. In the NSU Delphin III (see Figure 4.) the engine allowed a top speed of more than .

In 1954, NSU agreed upon developing a rotary internal combustion engine with Felix Wankel, based upon Wankel's design of the supercharger for their Delphin motorcycles. 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 (see figure 2.), first ran on 1 February 1957, at the NSU research and development department ''Versuchsabteilung TX''. It produced . Soon thereafter, a second prototype, the DKM 125 was built. It had a working chamber volume of 125 cm³ and produced at 17,000rpm. It could even reach speeds of up to 25,000rpm. However, these engine speeds caused distrortion in the outer rotor's shape, thus proving impractical.

Due to its complicated design with a stationary center shaft, the DKM engine was not practical. NSU engineer Walter Froede used Hanns-Dieter Paschke's design and converted the DKM into what would later be known as the KKM (see figure 5.). 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 hula-hoop motion of the rotor. NSU could not afford financing the development of both the DKM and the KKM, and eventually decided to drop the DKM in favour of the KKM, because the latter seemed to be the more practical design.

Wankel obtained the 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, the oil flow inside the engine, and the engine cooling. The first fully functioning KKM engine, the KKM 125, weighing in at only displaced 125 cm³ 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 (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 quite powerful engine with a decent potential, smooth operation and low noise emissions at high engine speeds. It was a single-rotor PP engine with a displacement of , a rated power of at 6,000rpm and a BMEP of .

Operation and design

The Wankel engine has a spinning eccentric power take-off shaft, with a rotory piston riding on eccentrics on the shaft in a hula-hoop fashion. The Wankel is a 2:3 type of rotary engine, i.e., it 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 against the external engine housing. The force of expanded gas pressure on the rotor exerts pressure to 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. 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. Therefore, Wankel engines typically have a high-voltage spark ignition system.

In a Wankel engine, one side of the triangular rotor completes the four-stage Otto cycle of intake, compression, ignition, and exhaust each revolution of the rotor (see figure 8.). 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.For a detailed calculation of the curvature of a circular arc approximating the optimal Wankel rotor shape, see As the rotor has three sides, this gives three power pulses per revolution of the rotor. All three faces of the Wankel's rotor operate simultaneously in one revolution. As the output shaft uses toothed gearing to turn three times faster than the rotor, one power pulse is produced at each revolution of the shaft. For comparison, the four-stroke piston engine completes the Otto cycle in two revolutions of its output shaft (crankshaft). The Wankel thus produces twice as many power pulses per output shaft revolution.

Wankel engines have a much lower degree of irregularity when compared against a reciprocating piston engine, making the Wankel engine run much smoother. This is because of the lower moment of inertia and fewer excess torque area the Wankel engine has due to its more uniform torque delivery. For instance, a two-rotor Wankel engine runs more than twice as smoothly as a four-cyclinder piston engine. 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.Kenichi Yamamoto: Rotary Engine, 1981, 3. 3. 2, Fig. 3.17 page -25- Hardened steel gears are used for extended operation above 7,000 or 8,000rpm. 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 10,000rpm, but so are four-stroke reciprocating piston engines of relatively small displacement per cylinder. In aircraft, they are used conservatively, up to 6,500 or 7,500rpm.

Chamber volume and displacement

Chamber volume

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

:$A_k=2 \cdot B \cdot (R+a) \cdot sin (60^\circ) = \sqrt 3 \cdot B \cdot (R+a)$

The rotor path $s$ may be integrated via the eccentricity $e$ as follows:

:$\sum \, ds= \int_^ e \cdot sin \frac 3 \alpha \, d \alpha = 3e$

Therefore,

:$V_k= A_k \cdot s = \sqrt 3 \cdot B \cdot (R+a) \cdot 3e$

For convenience, $a$ may be omitted because it is difficult to determine and small:

:$V_k= \sqrt 3 \cdot B \cdot R \cdot 3e$

A different approach to this is introducing $a'$ as the farthest, and $a$ as the shortest parallel transfer of the rotor and the inner housing and assuming that $R_1=R+a$ and $R_2=R+a'$. Then,

:$V_k= \sqrt 3 \cdot B \cdot (2 \cdot R_1+R_2) \cdot e$

Including the parallel transfers of the rotor and the inner housing provides sufficient accuracy for determining chamber volume.

Displacement

In a Wankel rotary engine, the eccentric shaft needs to make three full rotations (1080°) per combustion chamber in order to complete all four cycles of a four cycle engine. Since a Wankel rotary engine has three combustion chambers, all four cycles of a four cycle engine are completed within one full rotation of the eccentric shaft (360°).
    This is different from a four cycle piston engine, which needs to make two full rotations per combustion chamber in order to complete all four cycles of a four cycle engine. Therefore, in a Wankel rotary engine, the chamber volume has to be doubled in order to obtain the displacement $V_h$:

:$V_h=2 \cdot V_k \cdot i$,

with $i$ being the number of rotors per engine. The Wankel rotary engine's displacement $V_h$ is equivalent to a piston engine's displacement $V_h$.

Examples

;KKM 612 ( NSU Ro80)

*e=14 mm
*R=100 mm
*a=2 mm
*B=67 mm
*i=2

:$V_k = \sqrt 3 \cdot 67 \, mm \cdot (100 + 2 \, mm) \cdot 3 \cdot \, 14 \, mm \approx 498,000 \, mm^3 = 498 \, cm^3$

:$V_h = 2 \cdot 498 \, cm^3 \cdot 2 = 1,992 \ cm^3$

;Mazda 13B-REW (Mazda RX-7)

*e=15 mm
*R=103 mm
*a=2 mm
*B=80 mm
*i=2

:$V_k = \sqrt 3 \cdot 80 \, mm \cdot (103+2 \, mm) \cdot 3 \cdot \, 15 \, mm \approx 654,000 \, mm^3 = 654 \, cm^3$

:$V_h = 2 \cdot 654 \, cm^3 \cdot 2 = 2,616 \ cm^3$

Licenses issued

NSU licensed the design to companies around the world, with many companies implementing continual improvements. In 1960, NSU and the US firm Curtiss-Wright, signed a joint agreement. NSU was to concentrate on low and medium-powered Wankel engine development, with Curtiss-Wright developing high-powered engines, including aircraft engines of which Curtiss-Wright had decades of experience designing and producing. Curtiss-Wright recruited Max Bentele to head their design team.

Many manufacturers signed license agreements for development, attracted by the smoothness, quiet running, and reliability emanating from the uncomplicated design. Among them were Alfa Romeo, American Motors, Citroën, Ford, General Motors, Mazda, Mercedes-Benz, Nissan, Porsche, Rolls-Royce, Suzuki, and Toyota. In the United States in 1959, under license from NSU, Curtiss-Wright pioneered improvements in the basic engine design. In Britain, in the 1960s, Rolls-Royce's Motor Car Division pioneered a two-stage diesel version of the Wankel engine.

Deere & Company designed a version that was capable of using a variety of fuels. The design was proposed as the power source for United States Marine Corps combat vehicles and other equipment in the late 1980s.

In 1961, the Soviet research organization of NATI, NAMI, and VNIImotoprom commenced development creating experimental engines with different technologies. Soviet automobile manufacturer AvtoVAZ also experimented in Wankel engine design without a license, introducing a limited number of engines in some cars.Hege, p. 75.

By mid-September 1967, even Wankel model engines became available through the German Graupner aeromodelling products firm, made for them by O.S. Engines of Japan.

Despite much research and development throughout the world, only Mazda has produced Wankel engines in large quantities.

Engineering

Felix Wankel managed to overcome most of the problems that made previous attempts to perfect the rotary engines fail, by developing a configuration with vane seals having a tip radius equal to the amount of "oversize" of the rotor housing form, as compared to the theoretical epitrochoid, to minimize radial apex seal motion plus introducing a cylindrical gas-loaded apex pin which abutted all sealing elements to seal around the three planes at each rotor apex.

In the early days, special, dedicated production machines had to be built for different housing dimensional arrangements. 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 the problem.

Wankel engines have a problem not found in reciprocating piston four-stroke engines in that the block housing has intake, compression, combustion, and exhaust occurring at fixed locations around the housing. In contrast, reciprocating engines perform these four strokes in one chamber, so that extremes of "freezing" intake and "flaming" exhaust are averaged and shielded by a boundary layer from overheating working parts. The use of heat pipes in an air-cooled Wankel was proposed by the University of Florida to overcome this uneven heating of the block 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.'Rotary Engine', Kenichi Yamamoto; Toyo Kogyo, 1969, pag 65-66

The boundary layer shields and the oil film act as thermal insulation, leading to a low temperature of the lubricating film (approximate maximum on a water-cooled Wankel engine. This gives a more constant surface temperature. The temperature around the spark plug is about the same as the temperature 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. For a while, engineers were faced with what they called "chatter marks" and "devil's scratch" in the inner epitrochoid surface. 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. Scratches disappeared after the introduction of more compatible materials for seals and housing coatings. 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 metal insert/ copper sleeve in the housing, instead of a plug being screwed directly into the block housing.'The Wankel Engine', Jan P. Norbye, NSU develops the Wankel, page 139, and Citroën, page 305; Chilton, 1971. Toyota found that substituting a glow-plug for the leading site spark plug improved low rpm, part load, specific fuel consumption by 7%, and also emissions and idle.SAE paper 790435 A later alternative solution to spark plug boss cooling was provided with a variable coolant velocity scheme for water-cooled rotaries, which has had widespread use, being patented by Curtiss-Wright,, 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 with the last-listed for better air-cooled engine spark plug boss cooling. These approaches did not require a high-conductivity copper insert, but did not preclude its use. Ford tested a Wankel engine with the plugs placed in the side plates, instead of the usual placement in the housing working surface (, 1978).

Recent developments

Increasing the displacement and power of a Wankel engine by adding more rotors to a basic design is simple, with a limitation in the number of rotors, because power output is channeled through the last rotor shaft, with all the stresses of the whole engine present at that point. For engines with more than two rotors, coupling two bi-rotor sets by a serrate coupling (such as a Hirth joint) between the two rotor sets has been tested successfully.

Research in the United Kingdom under the SPARCS (Self-Pressurising-Air Rotor Cooling System) project, found that idle stability and economy were obtained by supplying an ignitable mix to only one rotor in a multi-rotor engine in a forced-air cooled rotor, similar to the Norton air-cooled designs.

The Wankel engine's drawbacks of inadequate lubrication and cooling in ambient temperatures, short engine lifespan, high emissions, and low fuel efficiencies were tackled by Norton Wankel rotary engine specialist David Garside, who developed three patented systems in 2016.
* SPARCS
* Compact-SPARCS
* CREEV (Compound Rotary Engine for Electric Vehicles)

SPARCS and Compact-SPARCS provide superior heat rejection and efficient thermal balancing to optimize lubrication. A problem with Wankel engines is that the engine housing has permanently cool and hot surfaces when running. It also generates excessive heat inside the engine which broke down older lubricating oil quickly. The SPARCS system reduces this wide differential in heat temperatures in the metal of the engine housing, and also cools the rotor from inside the body of the engine. This results in reduced engine wear prolonging engine life. As described in Unmanned Systems Technology Magazine, "SPARCS uses a sealed rotor cooling circuit consisting of a circulating centrifugal fan and a heat exchanger to reject the heat. This is self-pressurized by capturing the blow-by past the rotor side gas seals from the working chambers."

CREEV is an ‘exhaust reactor’, containing a shaft & rotor inside, of a different shape to a Wankel rotor. The reactor, located in the exhaust stream outside of the engine's combustion chamber, consumes unburnt exhaust products without using a second ignition system before directing burnt gasses into the exhaust pipe. Horsepower is given to the reactors shaft. Lower emissions and improved fuel efficiency are achieved. All three patents are currently licensed to UK-based engineers, AIE (UK) Ltd.

Torque delivery

A peripheral intake port gives the highest mean effective pressure; however, side intake porting produces a more steady idle,Yamamoto, Kenichi. ''Rotary engine'', fig 4.26 & 4.27, Mazda, 1981, p. 46. because it helps to prevent blow-back of burned gases into the intake ducts which cause "misfirings", caused by alternating cycles where the mixture ignites and fails to ignite. Peripheral porting (PP) gives the best mean effective pressure throughout the rpm range, but PP was linked also to worse idle stability and part-load performance. Early work by Toyota led to the addition of a fresh air supply to the exhaust port, and proved also that a Reed-valve in the intake port or ductsSAE paper 720466, Ford 1979 patent improved the low rpm and partial load performance of Wankel engines, by preventing blow-back of exhaust gas into the intake port and ducts, and reducing the misfire-inducing high EGR, at the cost of a small loss of power at top rpm. David W. Garside, the developer of the Norton rotary engine, proposed that earlier opening of the intake port before the top dead center (TDC), and longer intake ducts, improved the low rpm torque and elasticity of Wankel engines. That is also described in Kenichi Yamamoto's books. Elasticity is also improved with a greater rotor eccentricity, analogous to a longer stroke in a reciprocating engine. Wankel engines operate better with a low-pressure exhaust system. Higher exhaust back pressure reduces mean effective pressure, more severely in peripheral intake port engines. The Mazda RX-8 Renesis engine improved performance by doubling the exhaust port area compared with earlier designs, and there have been studies of the effect of intake and exhaust piping configuration on the performance of Wankel engines.Ming-June Hsieh et al. SAE papers Side intake ports (as used in Mazda's Renesis engine) were first proposed by Hanns-Dieter Paschke in the late 1950s. Paschke predicted that precisely calculated intake ports and intake manifolds could make a side port engine as powerful as a PP engine.

Materials

Unlike a piston engine, in which the cylinder is heated by the combustion process and then cooled by the incoming charge, Wankel rotor housings are constantly heated on one side and cooled on the other, leading to high local temperatures and unequal thermal expansion. While this places great demands on the materials used, the simplicity of the Wankel makes it easier to use alternative materials, such as exotic alloys and ceramics. With water cooling in a radial or axial flow direction, and the hot water from the hot bow heating the cold bow, the thermal expansion remains tolerable. Top engine temperature has been reduced to , with a maximum temperature difference between engine parts of by the use of heat pipes around the housing and in side plates as a cooling means.

Among the alloys cited for Wankel housing use are A-132, Inconel 625, and 356 treated to T6 hardness. Several materials have been used for plating the housing working surface, Nikasil being one. Citroën, Mercedes-Benz, Ford, A P Grazen and others applied for patents in this field. For the apex seals, the choice of materials has evolved along with the experience gained, from carbon alloys, to steel, ferritic stainless, and other materials. The combination between housing plating and apex and side seals materials was determined experimentally, to obtain the best duration of both seals and housing cover. For the shaft, steel alloys with little deformation on load are preferred, the use of Maraging steel has been proposed for this.

Leaded gasoline fuel was the predominant type available in the first years of the Wankel engine's development. Lead is a solid lubricant, and leaded gasoline is designed to reduce the wearing of seals and housings. The first engines had the oil supply calculated with consideration of gasoline's lubricating qualities. As leaded gasoline was being phased out, Wankel engines needed an increased mix of oil in the gasoline to provide lubrication to critical engine parts. A SAE paper by David Garside extensively described Norton's choices of materials and cooling fins.

Sealing

Early engine designs had a high incidence of sealing loss, both between the rotor and the housing and also between the various pieces making up the housing. Also, in earlier model Wankel engines, carbon particles could become trapped between the seal and the casing, jamming the engine and requiring a partial rebuild. It was common for very early Mazda engines to require rebuilding after . Further sealing problems arose from the uneven thermal distribution within the housings causing distortion and loss of sealing and compression. This thermal distortion also caused uneven wear between the apex seal and the rotor housing, evident on higher mileage engines. The problem was exacerbated when the engine was stressed before reaching operating temperature. However, Mazda Wankel engines solved these initial problems. Current engines have nearly 100 seal-related parts.

The problem of clearance for hot rotor apexes passing between the axially closer side housings in the cooler intake lobe areas was dealt with by using an axial rotor pilot radially inboard of the oils seals, plus improved inertia oil cooling of the rotor interior (C-W , C. Jones, 5/8/63, , M. Bentele, C. Jones. A.H. Raye. 7/2/62), and slightly "crowned" apex seals (different height in the center and in the extremes of seal).

Fuel economy and emissions

The shape of the Wankel combustion chamber is more resistant to preignition operating on lower- octane rating gasoline than a comparable piston engine.Hege, p. 10. The combustion chamber shape may also lead to incomplete combustion of the air-fuel charge using gasoline fuel. This would result in a larger amount of unburned hydrocarbons released into the exhaust. The exhaust is, however, relatively low in NOx emissions, because combustion temperatures are lower than in other engines, and also because of exhaust gas recirculation (EGR) in early engines. Sir Harry Ricardo showed in the 1920s that for every 1% increase in the proportion of exhaust gas in the admission fuel mixture, there is a reduction in flame temperature. This allowed Mazda to meet the United States Clean Air Act of 1970 in 1973, with a simple and inexpensive "thermal reactor", which was an enlarged chamber in the exhaust manifold. By decreasing the air-fuel ratio, unburned hydrocarbons (HC) in the exhaust would support combustion in the thermal reactor. Piston-engine cars required expensive catalytic converters to deal with both unburned hydrocarbons and NOx emissions. This inexpensive solution increased fuel consumption.

Sales of Wankel engine cars suffered because of the oil crisis of 1973 raising the price of gasoline leading to lower sales. Toyota discovered that the injection of air into the exhaust port zone improved fuel economy reducing emissions. The best results were obtained with holes in the side plates, however, in the exhaust duct, there was no noticeable influence. The use of a three-stage catalyst, with air supplied in the middle, as for two-stroke piston engines, also proved beneficial in meeting emissions regulations.SAE Automotive Engineering International, 'Controlling 2-Stroke Emissions', Feb 2000, pag 27-32

Mazda improved the fuel efficiency of the thermal reactor system by 40% with the RX-7 introduction in 1978. However, Mazda eventually shifted to the catalytic converter system. According to Curtiss-Wright research, the factor that controls the amount of unburned hydrocarbon in the exhaust is the rotor surface temperature, with higher temperatures producing less hydrocarbon. Curtiss-Wright widened the rotor, keeping the rest of engine's architecture unchanged, thus reducing friction losses and increasing displacement and power output. The limiting factor for this widening was mechanical, especially shaft deflection at high rotative speeds.SAE paper 710582 Quenching is the dominant source of hydrocarbon at high speeds, and leakage at low speeds.

Curtiss-Wright produced the RC2-60 engine, which was comparable to a V8 engine in performance and fuel consumption.https://airandspace.si.edu/collection-objects/wright-aeronautical-wankel-rc2-60-rotary-engine/nasm_A19870228000 Unlike NSU, Curtiss-Wright had solved the rotor sealing issue in 1966 with seals lasting .

Automobile Wankel engines are capable of high-speed operation. It was discovered that an early opening of the intake port, longer intake ducts, and a greater rotor eccentricity can increase torque at lower rpm. The shape and positioning of the recess in the rotor, which forms most of the combustion chamber, influences emissions and fuel economy. The results in terms of fuel economy and exhaust emissions varies depending on the shape of the combustion recess which is determined by the placement of spark plugs per chamber of an individual engine.

Mazda's RX-8 car with the Renesis engine met California State fuel economy requirements, including California's low emissions vehicle (LEV) standards. This was achieved by a number of innovations. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This solved the problem of the earlier ash buildup in the engine, and thermal distortion problems of the side intake and exhaust ports. A scraper seal was added to the rotor sides, and some ceramic parts were used in the engine. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing the exhaust port area. The side port trapped the unburned fuel in the chamber, decreased the oil consumption, and improved the combustion stability in the low-speed and light load range. The HC emissions from the side exhaust port rotary engine are 35–50% less than those from the peripheral exhaust port Wankel engine, because of near zero intake and exhaust port opening overlap. Peripheral ported rotary engines have a better mean effective pressure, especially at high rpm and with a rectangular-shaped intake port.SAE paper 288A However, the RX-8 was not improved to meet Euro 5 emission regulations and was discontinued in 2012.

Mazda is still continuing the development of the next-generation of Wankel engines. The company is researching engine laser ignition, which eliminates conventional spark plugs, direct fuel injection, sparkless HCCI ignition and SPCCI ignition. These lead to greater rotor eccentricity (equating to a longer stroke in a reciprocating engine), with improved elasticity and low revolutions-per-minute torque. Research by T. Kohno proved that installing a glow-plug in the combustion chamber improved part load and low revolutions per minute fuel economy by 7%. These innovations promise to improve fuel consumption and emissions.

To improve fuel efficiency further, Mazda looked at using the Wankel as a range-extender in series-hybrid cars, announcing a prototype, the Mazda2 EV, for press evaluation in November 2013. This configuration improves fuel efficiency and emissions. As a further advantage, running a Wankel engine at a constant speed gives greater engine life. Keeping to a near constant, or narrow band, of revolutions eliminates, or vastly reduces, many of the disadvantages of the Wankel engine. Mazda stated they will be introducing in early 2023 an electric Wankel hybrid, the MX-30 R-EV, with the engine turning only a generator.https://www.capomazda.com/blog/introducing-the-2023-mazda-mx-30/

In 2015 a new system to reduce emissions and increase fuel efficiency with Wankel Engines was developed by UK-based engineers AIE (UK) Ltd, following a licensing agreement to utilize patents from Norton rotary engine creator, David Garside. The CREEV system (Compound Rotary Engine for Electric Vehicles) uses a secondary rotor to extract energy from the exhaust, consuming unburnt exhaust products while expansion occurs in the secondary rotor stage, thus reducing overall emissions and fuel costs by recouping exhaust energy that would otherwise be lost. By expanding the exhaust gas to near atmospheric pressure, Garside also ensured the engine exhaust would remain cooler and quieter. AIE (UK) Ltd is now utilizing this patent to develop hybrid power units for automobiles and unmanned aerial vehicles.

Laser ignition

Traditional spark plugs need to be indented into the walls of the combustion chamber to enable the apex of the rotor to sweep past. As the rotor's apex seals pass over the spark plug hole, a small amount of compressed charge can be lost from the charge chamber to the exhaust chamber, entailing fuel in the exhaust, reducing efficiency, and resulting in higher emissions. These points have been overcome by using laser ignition, eliminating traditional spark plugs, and removing the narrow slit in the motor housing so the rotor apex seals can fully sweep with no loss of compression from adjacent chambers. This concept has a precedent in the glow plug used by Toyota (SAE paper 790435), and the SAE paper 930680, by D. Hixon et al., on 'Catalytic Glow Plugs in the JDTI Stratified Charge Rotary Engine'. The laser plug can fire through the narrow slit. Laser plugs can also fire deep into the combustion chamber using multiple lasers. So, a higher compression ratio is permitted. Direct fuel injection, to which the Wankel engine is suited, combined with laser ignition in single or multiple laser plugs, has been shown to enhance the motor even further reducing the disadvantages.

Homogeneous charge compression ignition (HCCI)

Homogeneous charge compression ignition (HCCI) involves the use of a pre-mixed lean air-fuel mixture being compressed to the point of auto-ignition, so electronic spark ignition is eliminated. Gasoline engines combine homogeneous charge (HC) with spark ignition (SI), abbreviated as HCSI. Diesel engines combine stratified charge (SC) with compression ignition (CI), abbreviated as SCCI. HCCI engines achieve gasoline engine-like emissions with compression ignition engine-like efficiency, and low levels of nitrogen oxide emissions (NO) without a catalytic converter. However, unburned hydrocarbon and carbon monoxide emissions still require treatment to conform with automotive emission regulations.

Mazda has undertaken research on HCCI ignition for its SkyActiv-R rotary engine project, using research from its SkyActiv Generation 2 program. A constraint of rotary engines is the need to locate the spark plug outside the combustion chamber to enable the rotor to sweep past. Mazda confirmed that the problem had been solved in the SkyActiv-R project. Rotaries generally have high compression ratios, making them particularly suitable for the use of HCCI.

Spark Controlled Compression Ignition (SPCCI)

SPCCI incorporates spark and compression ignition. A spark is always used, to control exactly when combustion occurs. Depending on the load, it may be only spark ignition, other times SPCCI.

The spark ignites a small pulse of richer mixture injected into the combustion chamber. A fireball is created, acting like an air piston, and increasing the pressure and temperature. Compression-ignition of the very lean mixture occurs, with a rapid and even and complete burn leading to a more powerful cycle. The compression-ignition aspect makes the lean burn possible, improving engine efficiency by up to 20–30%. It gives a rotary the ability to switch from the ideal, stoichiometric, 14.7:1 air-to-fuel mixture of a conventional gasoline-burning engine to the lean-burn mixture of over 29.4:1. The engine is in the lean-burn mode about 80% of running time. The combustion timing is controlled by the flame from the spark plug.

According to Mazda, SPCCI combines the advantages of both petrol and diesel engines and gives high efficiency across a wide range of rpms and engine loads. Combined with a supercharger the compression ignition delivers an increase in torque of 20–30%.

Compression-ignition Wankel

Research has been undertaken into rotary compression ignition engines. The basic design parameters of the Wankel engine preclude obtaining a compression ratio sufficient for Diesel operation in a practical engine. The Rolls-Royce''Autocar'' magazine, week ending Dec 17, 1970 and Yanmar compression-ignitionSAE paper 870449 approach was to use a two-stage unit (see figure 14.), with one rotor acting as compressor, while combustion takes place in the other.Wolf-Dieter Bensinger: Rotationskolben-Verbrennungsmotoren, Springer, Berlin/Heidelberg/New York 1973, . p. 141 Both engines were not functional.

Hydrogen fuel

As a hydrogen/air fuel mixture is quicker to ignite with a faster burning rate than gasoline, an important issue of hydrogen internal combustion engines is to prevent pre-ignition and backfire. In a rotary engine, each cycle of the Otto cycle occurs in different chambers. Importantly, the intake chamber is separated from the combustion chamber, keeping the air/fuel mixture away from localized hot spots. Wankel engines also do not have hot exhaust valves, which eases adapting them to hydrogen operation. Another problem concerns the hydrogenate attack on the lubricating film in reciprocating engines. In a Wankel engine, the problem of a hydrogenate attack is circumvented by using ceramic apex seals.1980 BMF report hydrogen Audi EA871 comparison to a hydrogen reciprocating piston engine page 11. Page 8 higher lubricating oil consumption caused by hydrogen

In a prototype Wankel engine fitted to a Mazda RX-8 to research hydrogen operation, Wakayama et al. found that hydrogen operation improved thermal efficiency by 23% over petrol fuel operation. However, the exhaust gas behaviour significantly worsened due to high NOx emissions caused by lean combustion, which meant that the vehicle failed to comply with Japan's SULEV emissions standard. In order to comply with emissions regulations, a supplementary stoichiometric mode had to be used, which lowered the thermal efficiency of the engine.Wakayama N, Morimoto K, Kashiwagi A, et al.
Development of Hydrogen Rotary Engine Vehicle
'. Whec 2006; 16: 13–16https://core.ac.uk/download/pdf/47252483.pdf

Advantages

Prime advantages of the Wankel engine are:
* A far higher power-to-weight ratio than a piston engine
* Easier to package in small engine spaces than an equivalent piston engine
* No reciprocating parts
* Able to reach higher engine speeds than a comparable piston engine
* Operating with almost no vibration
* Not prone to engine-knock
* Cheaper to mass-produce, because the engine contains fewer parts
* Superior breathing, filling the combustion charge in 270 degrees of mainshaft rotation rather than 180 degrees in a piston engine
* Supplying torque for about two-thirds of the combustion cycle rather than one-quarter for a piston engine
* Wider speed range giving greater adaptability
* Does not suffer from the "scale effect" to limit its size.
* Easily adapted and highly suitable to use hydrogen fuel.
* On some Wankel engines the sump oil remains uncontaminated by the combustion process, so no oil changes are required. The oil in the mainshaft is totally sealed from the combustion process. The oil for Apex seals and crankcase lubrication is separate. In piston engines, the crankcase oil is contaminated by combustion blow-by through the piston rings.

Wankel engines are considerably lighter and simpler, containing far fewer moving parts than piston engines of equivalent power output. Valves or complex valve trains are eliminated by using simple ports cut into the walls of the rotor housing. Since the rotor rides directly on a large bearing on the output shaft, there are no connecting rods and no crankshaft. The elimination of reciprocating mass, and the elimination of the most highly stressed and failure-prone parts of piston engines, gives the Wankel engine high reliability, a smoother flow of power, and a high power-to-weight ratio.

The surface-to-volume ratio in the moving combustion chamber is so complex that a direct comparison cannot be made between a reciprocating piston engine and a Wankel engine. The flow velocity and the heat losses are quite different. Surface temperature characteristics are completely different; the film of oil in the Wankel engine acts as insulation. Engines with a higher compression ratio have a worse surface-to-volume ratio. When comparing the power-to-weight ratio, physical size, or physical weight to a similar power output piston engine, the Wankel is superior.

A four-stroke cylinder produces a power stroke only every other rotation of the crankshaft, with three strokes being pumping losses. This doubles the real surface-to-volume ratio for the four-stroke reciprocating piston engine and the displacement increases. The Wankel, therefore, has higher volumetric efficiency and lower pumping losses through the absence of choking valves.Ansdale, pp. 121–133. Because of the quasi-overlap of the power strokes, which produces a smoother engine, the Wankel engine is very quick to react to power increases, giving a quick delivery of power when the demand arises, especially at higher RPMs. This difference is more pronounced when compared to four-cylinder reciprocating engines and less pronounced when compared to higher cylinder counts.

In addition to the removal of internal reciprocating stresses by the complete removal of reciprocating internal parts typically found in a piston engine, the Wankel engine is constructed with an iron rotor within a housing made of aluminium, which has a greater coefficient of thermal expansion. This ensures that even a severely overheated Wankel engine cannot seize.

From the combustion chamber shape and features, the fuel octane requirements of Wankel engines are lower than in reciprocating piston engines. The maximum road octane number requirements were 82 for a peripheral-intake port Wankel engine, and less than 70 for a side-inlet port engine.SAE paper 720357 From the point of view of oil refiners this may be an advantage in fuel production costs.

Due to a 50% longer stroke duration than a reciprocating four-cycle engine, there is more time to complete the combustion. This leads to greater suitability for direct fuel injection and stratified charge operation.

Disadvantages

Thermodynamic disadvantages

Wankel rotary engines mainly suffer from poor thermodynamics caused by the Wankel engine's design with its huge surface area and poor combustion chamber shape. As an effect of this, the Wankel engine has a slow, and incomplete combustion, which results in high fuel consumption and bad exhaust gas behaviour.

In a Wankel rotary engine, fuel combustion is slow, because the combustion chamber is long, thin, and moving. Flame travel occurs almost exclusively in the direction of rotor movement, adding to the poor quenching of a fuel/air mixture, being the main source of unburnt hydrocarbons at high engine speeds. The trailing side of the combustion chamber naturally produces a "squeeze stream" that prevents the flame from reaching the chamber trailing edge combined with the poor quenching of a fuel/air mixture. Direct fuel injection, in which fuel is injected towards the leading edge of the combustion chamber, can minimize the amount of unburnt fuel in the exhaust.

Mechanical disadvantages

Although many of the disadvantages are the subject of ongoing research, the current disadvantages of the Wankel engine in production are the following:
; Rotor sealing: The engine housing has vastly different temperatures in each separate chamber section. The different expansion coefficients of the materials lead to imperfect sealing. Additionally, both sides of the seals are exposed to fuel, and the design does not allow for controlling the lubrication of the rotors accurately and precisely. Rotary engines tend to be overlubricated at all engine speeds and loads, and have relatively high oil consumption and other problems resulting from excess oil in the combustion areas of the engine, such as carbon formation and excessive emissions from burning oil. By comparison, a piston engine has all functions of a cycle in the same chamber giving a more stable temperature for piston rings to act against. Additionally, only one side of the piston in a (four-stroke) piston engine is exposed to fuel, allowing oil to lubricate the cylinders from the other side. Piston engine components can also be designed to increase ring sealing and oil control as cylinder pressures and power levels increase. To overcome the problems in a Wankel engine of differences in temperatures between different regions of housing and side and intermediary plates, and the associated thermal dilatation inequities, a heat pipe has been used to transport heat from the hot to the cold parts of the engine. The "heat pipes" effectively direct hot exhaust gas to the cooler parts of the engine, resulting in decreases in efficiency and performance. In small-displacement, charge-cooled rotor, air-cooled housing Wankel engines, that has been shown to reduce the maximum engine temperature from , and the maximum difference between hotter and colder regions of the engine from .
; Apex seal lifting: Centrifugal force pushes the apex seal onto the housing surface forming a firm seal. Gaps can develop between the apex seal and trochoid housing in light-load operation when imbalances in centrifugal force and gas pressure occur. At low engine-rpm ranges, or under low-load conditions, the gas pressure in the combustion chamber can cause the seal to lift off the surface, resulting in combustion gas leaking into the next chamber. Mazda developed a solution, changing the shape of the trochoid housing, which meant that the seals remain flush with the housing. Using the Wankel engine at sustained higher revolutions helps eliminate apex seal lift off, lending it viable in applications such as electricity generation. In motor vehicles, the engine is suited to series-hybrid applications. NSU circumvented this problem by adding slots on one side of the apex seals, thus directing the gas pressure into the base of the apex. This effectively prevented the apex seals from lifting off.

Although in two dimensions the seal system of a Wankel looks to be even simpler than that of a corresponding multi-cylinder piston engine, in three dimensions the opposite is true. As well as the rotor apex seals evident in the conceptual diagram, the rotor must also seal against the chamber ends.

Piston rings in reciprocating engines are not perfect seals; each has a gap to allow for expansion. The sealing at the apexes of the Wankel rotor is less critical because leakage is between adjacent chambers on adjacent strokes of the cycle, rather than to the mainshaft case. Although sealing has improved over the years, the less-than-effective sealing of the Wankel, which is mostly due to lack of lubrication, remains a factor reducing its efficiency.

The trailing side of the rotary engine's combustion chamber develops a squeeze stream that pushes back the flame front. With the conventional one or two-spark-plug system and homogenous mixture, this squeeze stream prevents the flame from propagating to the combustion chamber's trailing side in the mid and high-engine speed ranges. Kawasaki dealt with that problem in its US patent ; Toyota obtained a 7% economy improvement by placing a glow-plug in the leading side, and using Reed-Valves in intake ducts. In two-stroke engines, metal reeds last about while carbon fiber, around . This poor combustion in the trailing side of the chamber is one of the reasons why there is more carbon monoxide and unburned hydrocarbons in a Wankel's exhaust stream. A side-port exhaust, as is used in the Mazda Renesis, avoids port overlap, one of the causes of this, because the unburned mixture cannot escape. The Mazda 26B avoided this problem through the use of a three spark-plug ignition system.

Regulations and taxation

National agencies that tax automobiles according to displacement and regulatory bodies in automobile racing use a variety of equivalency factors to compare Wankel engines to four-stroke piston engines. Greece, for instance, taxed cars based on the working chamber volume (the face of one rotor), multiplied by the number of rotors, lowering the cost of ownership. Japan did the same, but applied an equivalency factor of 1.5, making Mazda's 13B engine fit just under the 2-liter tax limit. FIA used an equivalency factor of 1.8 but later increased it to 2.0, using the displacement formula described by Bensinger.

Car applications

The first rotary-engined car for sale was the 1964 NSU Rotary Spider. Rotary engines were continuously fitted in cars until 2012 when Mazda discontinued the RX-8. Mazda has announced the introduction of a rotary-engined hybrid electric car, the MX-30 R-EV for a 2023 introduction.

NSU and Mazda

Mazda and NSU signed a study contract to develop the Wankel engine in 1961 and competed to bring the first Wankel-powered automobile to the market. Although Mazda produced an experimental rotary that year, NSU was the first with a rotary automobile for sale, the sporty NSU Spider in 1964; Mazda countered with a display of two- and four-rotor rotary engines at that year's Tokyo Motor Show. In 1967, NSU began production of a rotary-engined luxury car, the Ro 80. NSU had not produced reliable apex seals on the rotor, though, unlike Mazda and Curtiss-Wright. NSU had problems with apex seals' wear, poor shaft lubrication, and poor fuel economy, leading to frequent engine failures, not solved until 1972, which led to large warranty costs curtailing further NSU rotary engine development. This premature release of the new rotary engine gave a poor reputation for all makes, and even when these issues were solved in the last engines produced by NSU in the second half of the '70s, sales did not recover. Audi, after the takeover of NSU, built, in 1979, a new KKM 871 engine with side intake ports, a chamber, at 6,500rpm, and 220Nm (162ft-lb) at 3,500rpm. The engine was installed in an Audi 100 hull named "Audi 200", but was not mass-produced.

Mazda

Mazda claimed to have solved the apex seal problem, operating test engines at high speed for 300 hours without failure. After years of development, Mazda's first rotary engine car was the 1967 Cosmo 110S. The company followed with a number of Wankel ("rotary" in the company's terminology) vehicles, including a bus and a pickup truck. Customers often cited the cars' smoothness of operation. However, Mazda chose a method to comply with hydrocarbon emission standards that, while less expensive to produce, increased fuel consumption. Unfortunately for Mazda, this was introduced immediately prior to a sharp rise in fuel prices.

Mazda later abandoned the rotary in most of their automotive designs, continuing to use the engine in their sports car range only. The company normally used two-rotor designs. A more advanced twin-turbo three-rotor engine was fitted in the 1990 Eunos Cosmo sports car. In 2003, Mazda introduced the Renesis engine fitted in the RX-8. The Renesis engine relocated the ports for exhaust from the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and further power gains. Some early Wankel engines also had side exhaust ports, the concept being abandoned because of carbon buildup in ports and the sides of the rotor. The Renesis engine solved the problem by using a keystone scraper side seal and approached the thermal distortion difficulties by adding some parts made of ceramics.Masaki Ohkubo et al., SAE paper 2004-01-1790 The Renesis is capable of with improved fuel economy, reliability, and lower emissions than previous Mazda rotary engines, all from a nominal 1.3-L displacement, but this was not enough to meet more stringent emissions standards. Mazda ended production of their rotary engine in 2012 after the engine failed to meet the more stringent Euro 5 emission standards, leaving no automotive company selling a rotary-powered road vehicle. The company is continuing development of the next generation of Wankel engines, the SkyActiv-R. Mazda states that the SkyActiv-R solves the three key issues with previous rotary engines: fuel economy, emissions, and reliability. Mazda and Toyota announced they combined to produce a range-extending rotary engine for vehicles. Mazda will enter back into the rotary engine car market in March 2023, using the rotary as a generator in a MX-30 R-EV hybrid electric car.

Citroën

Citroën did much research, producing the M35 and GS Birotor cars, and the helicopter, using engines produced by Comotor, a joint venture by Citroën and NSU.

Mercedes Benz

Daimler-Benz fitted a Wankel engine in their C111 concept car. The C 111-II's engine was naturally aspirated, fitted with petrol direct injection, and had four rotors. The total displacement was , and the compression ration was 9.3:1 It provided a maximum torque of at 5,000rpm and a produced a power output of at 6,000rpm.

American Motors

American Motors Corporation (AMC), the smallest U.S. automaker, was so convinced "... that the rotary engine will play an important role as a powerplant for cars and trucks of the future ...", that the chairman, Roy D. Chapin Jr., signed an agreement in February 1973 after a year's negotiations, to build rotary engines for both passenger cars and military vehicles, as well as the right to sell any rotary engines it produced to other companies. AMC's president, William Luneburg, did not expect dramatic development through to 1980, but Gerald C. Meyers, AMC's vice president of the engineering product group, suggested that AMC should buy the engines from Curtiss-Wright before developing its own rotary engines, and predicted a total transition to rotary power by 1984.

Plans called for the engine to be used in the AMC Pacer, but development was pushed back. American Motors designed the unique Pacer around the engine. By 1974, AMC had decided to purchase the General Motors (GM) rotary instead of building an engine in-house. Both GM and AMC confirmed the relationship would be beneficial in marketing the new engine, with AMC claiming that the GM rotary achieved good fuel economy. GM's engines had not reached production, though, when the Pacer was launched onto the market. The 1973 oil crisis played a part in frustrating the use of the rotary engine. Rising fuel prices and speculation about proposed US emission standards legislation also added to concerns.

General Motors

By 1974, GM R&D had not succeeded in producing a Wankel engine meeting both the emission requirements and good fuel economy, leading to a decision by the company to cancel the project. Because of that decision, the R&D team only partly released the results of its most recent research, which claimed to have solved the fuel-economy problem, as well as building reliable engines with a lifespan above . Those findings were not taken into account when the cancellation order was issued. The ending of GM's rotary project required AMC, who was to purchase the engine, to reconfigure the Pacer to house its AMC straight-6 engine driving the rear wheels.

AvtoVAZ

In 1974, the Soviet Union created a special engine-design bureau, which in 1978, designed an engine designated as VAZ-311 fitted into a VAZ-2101 car. In 1980, the company commenced delivery of the VAZ-411 twin-rotor Wankel engine in VAZ-2106 cars, with about 200 being manufactured. Most of the production went to the security services."LADA – part II" Autosoviet, undated
retrieved on September 27, 2008."ЛИНИЯ ЖИЗНИ – ЭПИТРОХОИДА" 01.07.2001
, retrieved on September 27, 2008.

Ford

Ford conducted research in rotary engines, resulting in patents granted: , 1974, a method for fabricating housings; 1974, side plates coating; , 1975, housing coating; , 1978: Housings alignment; , 1979, reed-valve assembly. In 1972, Henry Ford II stated that the rotary probably would not replace the piston in "my lifetime".

Car racing

In auto racing, Mazda has had success with two-rotor, three-rotor, and four-rotor cars. Private racers have also had success with stock and modified Mazda Wankel-engine cars.

The Sigma MC74 powered by a Mazda 12A engine was the first engine and only team from outside Western Europe or the United States to finish the entire 24hours of the 24 Hours of Le Mans race, in 1974. Yojiro Terada was the driver of the MC74. Mazda was the first team from outside Western Europe or the United States to win Le Mans outright. It was also the only non-piston engined car to win Le Mans, which the company accomplished in 1991 with their four-rotor 787B (—actual displacement, rated by FIA formula at ). In the C2 class, all participants had only the same amount of fuel at their disposal, besides the unregulated C1 Category 1. This category only allowed naturally aspirated engines. The Mazdas were classified as naturally aspirated to start with 830 kg weight, 170 kg less than the supercharged competitors. The cars under the Group C1 Category 1 regulations for 1991 were allowed to be another 80 kg lighter than the 787B. In addition, Group C1 Category 1 had only permitted 3.5-liter naturally aspirated engines and had no fuel quantity limits.

Formula Mazda Racing features open-wheel race cars with Mazda Wankel engines, adaptable to both oval tracks and road courses, on several levels of competition. Since 1991, the professionally organized Star Mazda Series has been popular format for sponsors, spectators, and drivers. The engines are all built by one engine builder, certified to produce the prescribed power, and sealed to discourage tampering. They are in a relatively mild state of racing tune so they are reliable and can go years between motor rebuilds.

The Malibu Grand Prix chain, similar in concept to commercial recreational kart racing tracks, operates several venues in the United States where a customer can purchase several laps around a track in a vehicle very similar to open wheel racing vehicles, but powered by a small Curtiss-Wright rotary engine.

In engines having more than two rotors, or two-rotor race engines intended for high-rpm use, a multi-piece eccentric shaft may be used, allowing additional bearings between rotors. While this approach does increase the complexity of the eccentric shaft design, it has been used successfully in Mazda's production three-rotor 20B-REW engine, as well as many low-volume production race engines. The C-111-2 4 Rotor Mercedes-Benz eccentric shaft for the KE Serie 70, Type DB M950 KE409 is made in one piece. Mercedes-Benz used split bearings.

As a vehicle range extender

Due to the compact size and the high power-to-weight ratio of a Wankel engine, it has been proposed for electric vehicles as range extenders to provide supplementary power when electric battery levels are low. There have been a number of concept cars incorporating a series hybrid powertrain arrangement. A Wankel engine used only as a generator has packaging, noise, vibration and weight distribution advantages when used in a vehicle, maximizing interior passenger and luggage space. The engine/generator may be at one end of the vehicle with the electric driving motors at the other, connected only by thin cables. Mitsueo Hitomi the global powertrain head of Mazda stated, "a rotary engine is ideal as a range extender because it is compact and powerful, while generating low-vibration".

In 2010, Audi unveiled a prototype series-hybrid electric car, the A1 e-tron, that incorporated a small 250-cc Wankel engine, running at 5,000 rpm, which recharged the car's batteries as needed, and provided electricity directly to the electric driving motor. In 2010, FEV said that in their prototype electric version of the Fiat 500, a Wankel engine would be used as a range extender. In 2013, Valmet Automotive of Finland revealed a prototype car named the EVA, incorporating a Wankel powered series-hybrid powertrain car, utilizing an engine manufactured by the German company Wankel SuperTec. The UK company, Aixro Radial Engines, offers a range extender based on the 294-cc chamber go-kart engine.

Mazda of Japan ceased production of direct-drive Wankel engines within their model range in 2012, leaving the motor industry worldwide with no production cars using the engine. The company is continuing development of the next generation of their Wankel engines, the SkyActiv-R. Mazda states that the SkyActiv-R solves the three key issues with previous rotary engines: fuel economy, emissions and reliability. Takashi Yamanouchi, the global CEO of Mazda said: "The rotary engine has very good dynamic performance, but it's not so good on economy when you accelerate and decelerate. However, with a range extender you can use a rotary engine at a constant 2,000rpm, at its most efficient. It's compact, too." No Wankel engine in this arrangement has yet been used in production vehicles or planes. However, in November 2013 Mazda announced to the motoring press a series-hybrid prototype car, the Mazda2 EV, using a Wankel engine as a range extender. The generator engine, located under the rear luggage floor, is a tiny, almost inaudible, single-rotor 330-cc unit, generating at 4,500rpm, and maintaining a continuous electric output of 20 kW. In October 2017, Mazda announced that the rotary engine would be utilised in a hybrid car with 2019/20 the targeted introduction dates.

Mazda has undertaken research on Spark Controlled Compression Ignition ( SPCCI) ignition on rotary engines stating that any new rotary engines will incorporate SPCCI. SPCCi incorporates spark and compression ignition combining the advantages of gasoline and diesel engines to achieve environmental, power, and fuel consumption goals.

Mazda confirmed that a rotary-equipped range extended car would be launched. It will give full EV running with battery charging from the grid, with the engine performing the dual functions of a range-extender and battery charger when the battery charge is too low. When running on the engine, the electric motor is used to assist in acceleration and take off from stationary. Mazda announced a launch date of March 2023 for the MX-30 R-EV Range Extender car.

The Japanese motoring press have reported that Toyota and Mazda will announce hybrid sports cars for 2026 launches.

Motorcycle applications

The small size and attractive power-to-weight ratio of the Wankel engine appealed to motorcycle manufacturers. The first Wankel-engined motorcycle was the 1960 'IFA/MZ KKM 175W' built by German motorcycle manufacturer MZ, licensed by NSU.

Norton

In Britain, Norton Motorcycles developed a Wankel rotary engine for motorcycles, based on the Sachs air-cooled rotor Wankel that powered the DKW/Hercules W-2000 motorcycle. This two-rotor engine was included in the Commander and F1. Norton improved on the Sachs's air cooling, introducing a plenum chamber. Suzuki also made a production motorcycle powered by a Wankel engine, the RE-5, using ferro TiC alloy apex seals and an NSU rotor in a successful attempt to prolong the engine's life.

In the early 1980s, using earlier work at BSA, Norton produced the air-cooled twin-rotor Classic, followed by the liquid-cooled Commander and the Interpol2 (a police version).
translation
. Subsequent Norton Wankel bikes included the Norton F1, F1 Sports, RC588, Norton RCW588, and NRS588. Norton proposed a new 588-cc twin-rotor model called the "NRV588" and a 700-cc version called the "NRV700". A former mechanic at Norton, Brian Crighton, started developing his own rotary engined motorcycles line named "Roton", which won several Australian races.

Despite successes in racing, no motorcycles powered by Wankel engines have been produced for sale to the general public for road use since 1992.

Yamaha

In 1972, Yamaha introduced the RZ201 at the Tokyo Motor Show, a prototype with a Wankel engine, weighing 220 kg and producing from a twin-rotor 660-cc engine (US patent N3964448). In 1972, Kawasaki presented its two-rotor Kawasaki X99 rotary engine prototype (US patents N 3848574 &3991722). Both Yamaha and Kawasaki claimed to have solved the problems of poor fuel economy, high exhaust emissions, and poor engine longevity, in early Wankels, but neither prototype reached production.

Hercules

In 1974, Hercules produced W-2000 Wankel motorcycles, but low production numbers meant the project was unprofitable, and production ceased in 1977.

Suzuki

From 1975 to 1976, Suzuki produced its RE5 single-rotor Wankel motorcycle. It was a complex design, with both liquid cooling and oil cooling, and multiple lubrication and carburetor systems. It worked well and was smooth, but being rather heavy, and having a modest power output of , it did not sell well.

The two different design approaches, taken by Suzuki and BSA may usefully be compared. Even before Suzuki produced the RE5, in Birmingham BSA's research engineer David Garside, was developing a twin-rotor Wankel motorcycle. BSA's collapse put a halt to development, but Garside's machine eventually reached production as the Norton Classic.

Wankel engines run very hot on the ignition and exhaust side of the engine's trochoid chamber, whereas the intake and compression parts are cooler. Suzuki opted for a complicated oil-cooling and water-cooling system, with Garside reasoning that provided the power did not exceed , air-cooling would suffice. Garside cooled the interior of the rotors with filtered ram-air. This very hot air was cooled in a plenum contained within the semi-monocoque frame and afterwards, once mixed with fuel, fed into the engine. This air was quite oily after running through the interior of the rotors, and thus was used to lubricate the rotor tips. The exhaust pipes become very hot, with Suzuki opting for a finned exhaust manifold, twin-skinned exhausted pipes with cooling grilles, heatproof pipe wrappings, and silencers with heat shields. Garside simply tucked the pipes out of harm's way under the engine, where heat would dissipate in the breeze of the vehicle's forward motion. Suzuki opted for complicated multi-stage carburation, whilst Garside choose simple carburetors. Suzuki had three lube systems, whilst Garside had a single total-loss oil injection system which was fed to both the main bearings and the intake manifolds. Suzuki chose a single rotor that was fairly smooth, but with rough patches at 4,000 rpm; Garside opted for a turbine-smooth twin-rotor motor. Suzuki mounted the massive rotor high in the frame, but Garside put his rotors as low as possible to lower the center of gravity of the motorcycle."Cycle World" Magazine March 1971

Although it was said to handle well, the result was that the Suzuki was heavy, overcomplicated, expensive to manufacture, and at 62bhp short on power. Garside's design was simpler, smoother, lighter and, at , significantly more powerful.''Bike'' magazine, autumn 1974

Van Veen

Dutch motorcycle importer and manufacturer Van Veen produced small quantities of a dual-rotor Wankel-engined OCR-1000 motorcycle between 1978 and 1980, using surplus Comotor engines. The engine of the OCR 1000, used a re-purposed engine originally intended for the Citroën GS Birotor car.

Non-road vehicle applications

Aircraft

In principle, rotary engines are ideal for light aircraft, being light, compact, almost vibrationless, and with a high power-to-weight ratio. Further aviation benefits of a rotary engine include:

# The engine is less prone to the serious condition known as "engine-knock", which can destroy a plane's piston engines in mid-flight.
# The engine is not susceptible to "shock-cooling" during descent;
# The engine does not require an enriched mixture for cooling at high power;
# Having no reciprocating parts, there is less vulnerability to damage when the engine revolves at a higher rate than the designed maximum.

Unlike cars and motorcycles, a rotary aero-engine will be sufficiently warm before full power is asked of it because of the time taken for pre-flight checks. Also, the journey to the runway has minimum cooling, which further permits the engine to reach operating temperature for full power on take-off.MidWest Engines Ltd AE1100R Rotary Engine Manual A Wankel aero-engine spends most of its operational time at high power outputs, with little idling. This makes ideal use of peripheral ports. An advantage is that modular engines with more than two rotors are feasible, without increasing the frontal area. Should icing of any intake tracts be an issue, there is plenty of waste engine heat available to prevent icing.

The first rotary engine aircraft was in the late 1960s being the experimental Lockheed Q-Star civilian version of the United States Army's reconnaissance QT-2, essentially a powered Schweizer sailplane. The plane was powered by a Curtiss-Wright RC2-60 Wankel rotary engine. The same engine model was also used in a Cessna Cardinal and a helicopter, as well as other airplanes. The French company Citroën developed a rotary powered helicopter in the 1970s. In Germany in the mid-1970s, a pusher ducted fan airplane powered by a modified NSU multi-rotor rotary engine was developed in both civilian and military versions, Fanliner and Fantrainer.

At roughly the same time as the first experiments with full-scale aircraft powered with rotary engines, model aircraft-sized versions were pioneered by a combination of the well-known Japanese O.S. Engines firm and the then-extant German Graupner aeromodelling products firm, under license from NSU/Auto-Union. By 1968, the first prototype air-cooled, single-rotor glow plug-ignition, methanol-fueled 4.9 cm³ displacement OS/Graupner model Wankel engine was running, and was produced in at least two different versions from 1970 to the present day, solely by the O.S. firm after Graupner's demise in 2012.

Aircraft rotary engines are increasingly being found in roles where the compact size, high power-to-weight ratio, and quiet operation are important, notably in drones and unmanned aerial vehicles. Many companies and hobbyists adapt Mazda rotary engines, taken from cars for aircraft use. Others, including Wankel GmbH itself, manufacture rotary engines dedicated to that purpose. One such use is the "Rotapower" engines in the Moller Skycar M400. Another example of purpose-built aircraft rotaries are Austro Engine's AE50R (certified) and AE75R (under development) both appr. 2 hp/kg.

Rotary engines have been fitted in homebuilt experimental aircraft, such as the ARV Super2, a couple of which were powered by the British MidWest aero-engine. Most are Mazda 12A and 13B automobile engines, converted for aviation use. This is a very cost-effective alternative to certified aircraft engines, providing engines ranging from 100 to at a fraction of the cost of traditional piston engines. These conversions were initially in the early 1970s. With a number of these engines mounted on aircraft, as of 10 December 2006 the National Transportation Safety Board has only seven reports of incidents involving aircraft with Mazda engines, and none of these were a failure due to design or manufacturing flaws.

Peter Garrison, a contributing editor for ''Flying'' magazine, has said that "in my opinion ... the most promising engine for aviation use is the Mazda rotary.""Revisiting Rotaries", Peter Garrison, ''Flying'', 130, #6 (June 2003), pp. 90 ff. Mazda rotaries have worked well when converted for use in homebuilt aircraft. However, the real challenge in aviation is to produce FAA-certified alternatives to the standard reciprocating engines that power most small general aviation aircraft. Mistral Engines, based in Switzerland, developed purpose-built rotaries for factory and retrofit installations on certified production aircraft. The G-190 and G-230-TS rotary engines were already flying in the experimental market, and Mistral Engines hoped for FAA and JAA certification by 2011. , G-300 rotary engine development ceased, with the company citing cash flow problems.

Mistral claims to have overcome the challenges of fuel consumption inherent in the rotary, at least to the extent that the engines are demonstrating specific fuel consumption within a few points of reciprocating engines of similar displacement. While fuel burn is still marginally higher than traditional engines, it is outweighed by other beneficial factors.

At the price of increased complication for a high-pressure diesel type injection system, fuel consumption in the same range as small pre-chamber automotive and industrial diesels has been demonstrated with Curtiss-Wright's stratified charge multi-fuel engines, while preserving Wankel rotary advantages Unlike a piston and overhead valve engine, there are no valves which can float at higher rpm causing loss of performance. The rotary is a more effective design at high revolutions with no reciprocating parts, far fewer moving parts, and no cylinder head.

Since rotary engines operate at a relatively high rotational speed, at 6,000rpm of output shaft the rotor spins only at 2,000rmp. With relatively low torque, propeller-driven aircraft must use a propeller speed reduction unit to maintain propellers within the designed speed range. Experimental aircraft with Wankel engines use propeller speed reduction units, for example, the MidWest twin-rotor engine has a 2.95:1 reduction gearbox. The rotational shaft speed of a rotary engine is high compared to reciprocating piston designs. Only the eccentric shaft spins fast, while the rotors turn at exactly one-third of the shaft speed. If the shaft is spinning at 7,500rpm, the rotors are turning at a much slower 2,500rpm.

Pratt & Whitney Rocketdyne has been commissioned by DARPA to develop a diesel rotary engine for use in a prototype VTOL flying car called the "Transformer". The engine, based on an earlier unmanned aerial vehicle Wankel diesel concept called "Endurocore".

The sailplane manufacturer Schleicher uses an Austro Engines AE50R Wankel in its self-launching models ASK-21 Mi, ASH-26E, ASH-25 M/Mi, ASH-30 Mi, ASH-31 Mi, ASW-22 BLE, and ASG-32 Mi.

In 2013, e-Go airplanes, based in Cambridge, United Kingdom, announced that its new single-seater canard aircraft, the winner of a design competition to meet the new UK single-seat deregulated category, will be powered by a rotary engine from Rotron Power, a specialist manufacturer of advanced rotary engines for unmanned aeronautical vehicle (UAV) applications. The first sale was in 2016. The aircraft is expected to deliver cruise speed from a rotary engine, with a fuel economy of using standard motor gasoline (MOGAS), developing .

The DA36 E-Star, an aircraft designed by Siemens, Diamond Aircraft and EADS, employs a series hybrid powertrain with the propeller being turned by a Siemens electric motor. The aim is to reduce fuel consumption and emissions by up to 25%. An onboard Austro Engines rotary engine and generator provides the electricity. A propeller speed reduction unit is eliminated. The electric motor uses electricity stored in batteries, with the generator engine off, to take off and climb reducing sound emissions. The series-hybrid powertrain using the Wankel engine reduces the weight of the plane by 100 kg compared with its predecessor. The DA36 E-Star first flew in June 2013, making this the first-ever flight of a series-hybrid powertrain. Diamond Aircraft state that the technology using rotary engines is scalable to a 100-seat aircraft.

Other uses

Small Wankel engines are being found increasingly in other applications, such as go-karts, personal water craft, and auxiliary power units for aircraft. Kawasaki patented mixture-cooled rotary engine (US patent 3991722). Japanese diesel engine manufacturer Yanmar and Dolmar-Sachs of Germany had a rotary-engined chain saw (SAE paper 760642) and outboard boat engines, and the French Outils Wolf, made a lawnmower (Rotondor) powered by a Wankel rotary engine. To save on production costs, the rotor was in a horizontal position and there were no seals in the downside. The Graupner/ O.S. 49-PI is a 5-cc Wankel engine for model airplane use, which has been in production essentially unchanged since 1970. Even with a large muffler, the entire package weighs only .

The simplicity of the rotary engine makes it well-suited for mini, micro, and micro-mini engine designs. The Microelectromechanical systems (MEMS) Rotary Engine Lab at the University of California, Berkeley, has previously undertaken research towards the development of rotary engines of down to 1 mm in diameter, with displacements less than 0.1 cc. Materials include silicon and motive power includes compressed air. The goal of such research was to eventually develop an internal combustion engine with the ability to deliver 100 milliwatts of electrical power; with the engine itself serving as the rotor of the generator, with magnets built into the engine rotor itself. Development of the miniature rotary engine stopped at UC Berkeley at the end of the DARPA contract. Miniature rotary engines struggled to maintain compression due to sealing problems, similar to problems observed in the large-scale versions. In addition, miniature engines suffer from an adverse surface-to-volume ratio causing excess heat losses; the relatively large surface area of the combustion chamber walls transfers away what little heat is generated in the small combustion volume resulting in quenching and low efficiency.

Ingersoll-Rand built the largest-ever rotary engine, with two rotors, which was available between 1975 and 1985, producing . A one-rotor version was available producing . The displacement per rotor was 41 liters, with each rotor being approximately one meter in diameter. The engine was derived from a previous, unsuccessful Curtiss-Wright design, which failed because of a well-known problem with all internal combustion engines: the fixed speed at which the flame front travels limits the distance combustion can travel from the point of ignition in a given time, thereby limiting the maximum size of the cylinder or rotor chamber which can be used. This problem was solved by limiting the engine speed to only 1200 rpm (400 kW per rotor) and the use of natural gas as fuel. That was particularly well chosen since one of the major uses of the engine was to drive compressors on natural gas pipelines. The engines used "homogeneous charge", and some ran for over 30,000 hours.

John Deere acquired the Curtiss-Wright rotary division in February 1984, also making large multi-fuel prototypes, some with an 11-liter rotor for large vehicles. The developers attempted to solve the flame front issue (which hampered Ingersoll-Rand's homogeneous combustion) by stratifying the fuel in the combustion chamber. The technology was transferred to RPI in 1991.

Yanmar of Japan produced some small, charge-cooled rotary engines for chainsaws and outboard engines. One of its products is the LDR (rotor recess in the leading edge of the combustion chamber) engine, which has better exhaust emissions profiles, and reed-valve controlled intake ports, which improve part-load and low rpm performance.

In 1971 and 1972, Arctic Cat produced snowmobiles powered by Sachs KM 914 303-cc and KC-24 294-cc Wankel engines made in Germany.

In the early 1970s, Outboard Marine Corporation sold snowmobiles under the Johnson and other brands, which were powered by OMC engines.

Aixro of Germany produces and sells a go-kart engine, with a 294-cc-chamber charge-cooled rotor and liquid-cooled housings. Other makers are: Wankel AG, Cubewano, Rotron, and Precision Technology USA.

The American M1A3 Abrams tank may use an rotary Diesel APU,17 kW Rotary JP-8 APU developed by the TARDEC US Army lab. It has a high-power-density 330-cc rotary engine, modified to operate with various fuels such as standard military JP-8 jet fuel.

Non-internal combustion

In addition to use as an internal combustion engine, the basic Wankel design has also been used for gas compressors, and superchargers for internal combustion engines, but in these cases, although the design still offers advantages in reliability, the basic advantages of the Wankel in size and weight over the four-stroke internal combustion engine are irrelevant. In a design using a Wankel supercharger on a Wankel engine, the supercharger is twice the size of the engine.

The Wankel design is used in the seat belt pre-tensioner system in some Mercedes-Benz and Volkswagen cars. When the deceleration sensors detect a potential crash, small explosive cartridges are triggered electrically, and the resulting pressurized gas feeds into tiny Wankel engines which rotate to take up the slack in the seat belt systems, anchoring the driver and passengers firmly in the seat before a collision.

See also

* General Motors Rotary Combustion Engine
* Gunderson Do-All Machine
* Mazda RX-8 Hydrogen RE
* Mazda Wankel engine
* Mercedes-Benz M950F
* Mercedes-Benz C111
* O.S. Engines, the only licensed maker of Wankel model engines
* Pistonless rotary engine
* Quasiturbine
* RKM engine
* Liquidpiston

Notes

References

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* (Nikasil).
* F Feller and M I Mech: "The 2-Stage Rotary Engine—A New Concept in Diesel Power" by Rolls-Royce, The Institution of Mechanical Engineers, Proceedings 1970–71, Vol. 185, pp. 139–158, D55-D66. London
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* Frank Jardine (Alcoa): "Thermal expansion in automotive engine design", ''SAE Journal'', Sept 1930, pp. 311–319, and also SAE paper 300010.
* P V Lamarque, "The Design of Cooling Fins for Motor-Cycle Engines", ''The Institution of Automobile Engineers Magazine'', London, March 1943 issue, and also in "The Institution of Automobile Engineers Proceedings", XXXVII, Session 1942–1943, pp. 99–134 and 309–312.
* W. M. Holaday and John Happel (Socony-Vacuum Oil Co): 'A Refiner's Viewpoint on Motor Fuel Quality', SAE paper 430113
* Walter G Froede: 'The NSU-Wankel Rotating Combustion Engine', SAE Technical paper 610017
* M. R. Hayes & D. P. Bottrill: 'N.S.U. Spider -Vehicle Analysis', Mira (Motor Industry Research Association, UK), 1965.
* C Jones (Curtiss-Wright), "Rotary Combustion Engine is as Neat and Trim as the Aircraft Turbine", SAE Journal, May 1968, Vol 76, nº 5: 67–69. Also in SAE paper 670194.
* Jan P Norbye: "Rivals to the Wankel", Popular Science, Jan 1967; 'The Wankel Engine. Design, development, applications'; Chilton, 1972.
* T W Rogers et al. (Mobil),"Lubricating Rotary Engines", Automotive Engineering (SAE) May 1972, Vol 80, nº 5: 23–35.
* K Yamamoto et al. (Mazda): "Combustion and Emission Properties of Rotary Engines", Automotive Engineering (SAE), July 1972: 26–29. Also in SAE paper 720357.
* L W Manley (Mobil): "Low-Octane Fuel is OK for Rotary Engines", Automotive Engineering (SAE), Aug 1972, Vol 80, nº 8: 28–29.
* W-D Bensinger (Daimler-Benz), "Rotationskolben-Verbrennungsmotoren", Springer-Verlag 1973;
* Reiner Nikulski: "The Norton rotor turns in my Hercules W-2000", "Sachs KC-27 engine with a catalyst converter", and other articles in: "Wankel News" (In German, from Hercules Wankel IG)
* "A WorldWide Rotary Update", Automotive Engineering (SAE), Feb 1978, Vol 86, nº 2: 31–42.
* B Lawton: 'The Turbocharged Diesel Wankel Engine', C68/78, of: 'Institution of Mechanical Engineers Conference Publications. 1978–2, Turbocharging and Turbochargers, , pp 151–160.
* T Kohno et al. (Toyota): "Rotary Engine's Light-Load Combustion Improved", Automotive Engineering (SAE), Aug 1979: 33–38. Also in SAE paper 790435.
* Kris Perkins: ''Norton Rotaries'', 1991 Osprey Automotive, London.
* Karl Ludvigsen: ''Wankel Engines A to Z'', New York 1973.
* Len Louthan (AAI corp.): 'Development of a Lightweight Heavy Fuel Rotary Engine', SAE paper 930682
* G Bickle et al. (ICT co), R Domesle et al. (Degussa AG), "Controlling Two-Stroke Engine Emissions", Automotive Engineering International (SAE), Feb 2000, pp 27–32.
* BOSCH, "Automotive Handbook", 2005, Fluid's Mechanics, Table: 'Discharge from High-Pressure Deposits'.
* Anish Gokhale et al.: "Optimization of Engine Cooling through conjugate heat transfer simulation and analysis of fins"; SAE paper 2012-32-0054.
* Patents: , 1974 -Kawasaki; , 1974 -Ford; , 1974; , 1976. -Ford; , 1978; , 1979, -Ford.
* Dun-Zen Jeng et al.: 'The Numerical Investigation on the Performance of Rotary Engine with Leakage, Different Fuels and Recess Sizes', SAE paper 2013-32-9160, and same author: 'The intake and Exhaust Pipe Effect on Rotary Engine Performance', SAE paper 2013-32-9161
* Wei Wu et al.: 'A Heat Pipe Assisted Air-Cooled Rotary Wankel Engine for Improved Durability, Power and Efficiency', SAE paper 2014-01-2160
* Mikael Bergman et al. (Husqvarna): 'Advanced Low Friction Engine Coating applied to a 70 cc High Performance Chainsaw', SAE paper 2014-32-0115
* Alberto Boretti: 'CAD/CFD/CAE Modelling of Wankel Engines for UAV', SAE Technical Paper 2015-01-2466

External links

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The Norton Rotary
summary of the development of the Norton series of Wankel engines, by David Garside

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Piston ported engines
Pistonless rotary engine
Motorcycle engines
German inventions
20th-century inventions