Wankel engine: Difference between revisions

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The Wankel engine is a type of

to convert pressure into rotating motion.

The Wankel engine has the advantages of compact design, smoothness, lower weight and less parts over the more common reciprocating

When hydrogen is used to fuel the Wankel the efficiency rises again by 23%.

The simple rotational design of the engine delivers a smoother more uniform torque delivery leading to less vibration. The 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.

National agencies that tax automobiles according to displacement and regulatory bodies in

consider the Wankel engine to be equivalent to a four-stroke piston engine of up to two times the displacement of one chamber per rotor. The displacement given in engine specifications is typically a single working chamber which is one face of one rotor, or one face multiplied by the number of rotors.

Concept

In 1951, the NSU research department in Germany led by Dr. Walter Freode and the Lindau Technical Development Agency led by Felix Wankel, began working on compressors. Wankel conceived the design of a triangular rotor in the compressor. With the assistance of Prof. Baier from Stuttgart University of Applied Sciences, the concept was defined mathematically. The supercharger he designed was used in the record breaking two-stroke engine of the NSU Baumm II car and also in the NSU Delphin III motorcycle. Wankel who had received his first patent for a rotary type of engine in 1929, realized that the triangular rotor of the supercharger could have intake and exhaust ports added producing an internal combustion engine.^[2]

Wankel convinced NSU the supercharger could be a rotary internal combustion engine, with NSU giving approval for development in 1954. The initial development was undertaken at the Lindau Technical Development Agency, leading to a working prototype at NSU, the DKM design, being completed in 1957.^[3][4][5]

Early developments

Bonn, Germany

: the rotor and its housing spin

In 1954, NSU in Germany began the development of the engine, derived from a supercharger designed by Felix Wankel, with two prototypes being built. The first, the DKM motor, was developed by Felix Wankel. The second, the KKM motor, was developed by Hanns Dieter Paschke. The Paschke engine was adopted as the basis of the modern Wankel engine, yet named after Felix Wankel.^[6]

The basis of the DKM type of motor was that both the rotor, which was attached to the output shaft, and the rotor's housing both spun in a pure circular motion on independent axes. There was an exterior static engine housing encapsulating the two revolving parts. Being more naturally balanced the DKM motor reached up to 17,000 rpm. However, the engine needed to be stripped to change the three spark plugs as they were screwed into each side of the triangular spinning rotor. The first working prototype, DKM 54, produced 21 hp (16 kW) first running on February 1, 1957, at the NSU research and development department Versuchsabteilung TX.^[4][7]

The second prototype, the KKM 57 (the Wankel rotary engine, Kreiskolbenmotor) was designed and constructed by NSU engineer Hanns Dieter Paschke in 1957 to simplify the design. The KKM engine was simpler reducing the housings from two to one. The revolving rotor housing was eliminated, with the triangular rotor spinning inside a fixed static engine housing. From spinning in a pure circular motion, the rotor revolved around the central output shaft in an elliptical orbit in a hula-hoop fashion, pushing the shaft around via fixed tooth gearing. The KKM was developed without the knowledge of Felix Wankel, who remarked, "you have turned my race horse into a plow mare".^[8] The NSU chairman chairman Dr. von Heydekampf replied: “If only we actually had a plough horse!” Wankel complained that more stresses would be placed on the apex seals due to the eccentric hula-hoop motion of the rotor. The DKM motor's rotor was attached to the output shaft revolving in a pure circular motion, as Wankel wanted it, rather than spinning like a hula-hoop around the output shaft, as the KKM did. The hula-hoop spinning motion reduced the revolutions per minute.

Dr. Walter Freode head of the NSU research department dropped the DKM 54 in 1958 with all research funds concentrated on Paschke's KKM 57 motor. The KKM Wankel rotary engine started up for the first time on 7th July 1958. KKM 57 was a 125cc single rotor engine producing 25 HP at 11,000 rpm.^[9][10]

Operation and design

The engine is of

having been built only in research projects.

One side of the triangular rotor completes the four-stage Otto cycle of intake, compression, ignition, and exhaust each revolution of the rotor. As there are 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 spins three times faster than the rotor, only 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 produces twice as many power pulses.

Each stage of the Otto cycle occurs in the same chamber at different intervals in the four-stroke piston engine, while in the Wankel engine the stages of the Otto cycle occur simultaneously at separate fixed points around the engine's housing. The output shaft uses toothed gearing to turn three times faster than the rotor, giving one power pulse per output shaft revolution. Using faster burning and cleaner fuels such as hydrogen enhances the efficiency of the engine. The Wankel engine has reached a power-to-weight ratio of over one horsepower per pound.^[11]

In the Wankel engine, the four stages of an

of the rotor is determined by the inner housing shape ensuring the rotor does not touch the housing at any angle of rotation.

The central output shaft, called the "eccentric shaft" or "E-shaft", passes through the center of the housing being supported by fixed bearings.[15] The rotor rides on eccentrics which is analogous to crankpins in piston engine, an integral part of the eccentric shaft, which is analogous to a crankshaft. The rotor rotates around the eccentric output shaft in a hula-hoop movement. Seals at the apices of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers.^[13] The rotation of the rotor on its own axis is caused and controlled by a pair of synchronizing gears^[15] A fixed gear mounted on one side of the rotor housing engages a ring gear attached to the rotor and ensures the rotor moves exactly one-third turn for each turn of the eccentric output shaft. The power output of the engine is not transmitted through the synchronizing gears.^[15] The rotor moves in its rotating motion guided by the 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.

The easiest way to visualize the action of the engine in the animation is to look not at the rotor itself, but at the cavity created between it and the housing. The Wankel engine is actually a variable-volume progressing-cavity system. Thus, the three cavities per housing all repeat the same cycle. Points A and B on the rotor and E-shaft turn at different speeds—point B circles three times as often as point A does, so that one full orbit of the rotor equates to three turns of the E-shaft.

As the rotor rotates orbitally in a hula-hoop fashion, each side of the rotor is brought closer to and then away from the wall of the housing, compressing and expanding the combustion chamber. Analogous to a piston in a reciprocating piston engine. The power vector of the combustion stage goes through the center of the offset lobe.

While a

than reciprocating engines of similar power output. This is due partly to the smoothness inherent in its circular motion, the lack of a mechanical valvetrain employing reciprocating poppet valves, and the rotor spinning at one-third of the speed of the output shaft.

The eccentric output shafts do not have the stress-related contours of crankshafts. The maximum revolutions of a Wankel engine are limited by tooth load on the synchronizing gears.[16] Hardened steel gears are used for extended operation above 7000 or 8000 rpm. In practice, Wankel engines in production automobiles are not operated at much higher output shaft speeds than reciprocating piston engines of similar output power, and cycle speeds (one-third of Wankel output shaft speed and one-half of four-stroke crankshaft speed) are similar to conventional engines; for example, the "12A" Wankel in the 1970 RX-2 produced peak power at 7,000 RPM (39 engine cycles per second),^[17] while the reciprocating piston engine in the same year of the same model family (Capella) produced peak power at 6,000 RPM (50 engine cycles per second).^[18] Mazda Wankel engines in auto racing are operated above 10,000 rpm, but so are four-stroke reciprocating piston engines of relatively small displacement per cylinder. In aircraft, they are used conservatively, up to 6500 or 7500 rpm, but as gas pressure participates in seal efficiency, racing a Wankel engine at high revolutions under no-load conditions can destroy the engine.

Three lobes exist per rotor (because the rotor is completing only one-third rotation per one rotation of the output shaft, so only one power stroke occurs per working per output revolution, the other two lobes are simultaneously ejecting a spent charge and taking in a new one, rather than contributing to the power output of that revolution). Some racing series have banned the Wankel altogether, along with all other alternatives to the traditional reciprocating-piston, four-stroke design.^[19]

Licenses issued

Hiroshima, Japan

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.^[20] 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

In 1961, the Soviet research organization of NATI, NAMI, and VNIImotoprom commenced development creating experimental engines with different technologies.^[23] Soviet automobile manufacturer AvtoVAZ also experimented in Wankel engine design without a license, introducing a limited number of engines in some cars.^[24]

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.^[25]

In the early days, special, dedicated production machines had to be built for different housing dimensional arrangements. However, patented designs such as U.S. patent 3,824,746, G. J. Watt, 1974, for a "Wankel Engine Cylinder Generating Machine", U.S. patent 3,916,738, "Apparatus for machining and/or treatment of trochoidal surfaces" and U.S. patent 3,964,367, "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.^[26] Pre-heating of certain housing sections with exhaust gas improved performance and fuel economy, also reducing wear and emissions.^[27]

The boundary layer shields and the oil film act as thermal insulation, leading to a low temperature of the lubricating film (approximate maximum 200 °C or 392 °F 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.^{[28][29][30][31]}

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.^[32] 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.^[33] 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,^[34] 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 (CA 1036073 , 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.^[35][36]

• 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."^[37][38]

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.^{[39][40][41][42][43]}

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 129 °C (264 °F), with a maximum temperature difference between engine parts of 18 °C (32 °F) by the use of heat pipes around the housing and in side plates as a cooling means.^[26]

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. Experienced users advise, even in engines with electronic fuel injection, adding at least 1% of oil directly to gasoline as a safety measure in case the pump supplying oil to combustion chamber-related parts failed or sucked in air.^{[citation needed]} Lead tetraethyl (TEL) burns in the engine to form carbon dioxide, lead oxide, and water. Since the lead oxide would deposit in the combustion chamber, TEL is used together with ethyl bromide or ethylene chloride, which convert the lead oxide into lead bromide or lead chloride, which evaporate more easily. ^[44] A SAE paper by David Garside extensively described Norton's choices of materials and cooling fins.

Several approaches involving solid lubricants were tested, and even the addition of LiquiMoly (containing MoS₂), at the rate of 1 cc (1 mL) per liter of fuel, is advised. Many engineers^[who?] agree that the addition of oil to gasoline as in old two-stroke engines is a safer approach for engine reliability than an oil pump injecting into the intake system or directly to the parts requiring lubrication. A combined oil-in-fuel plus oil metering pump is always possible.^{[45][failed verification]}

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 50,000 miles (80,000 km). 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.^{[citation needed]} 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.^[4]

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 US 3261542 , C. Jones, 5/8/63, US 3176915 , 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 Wankel engine had problems with fuel efficiency and emissions when using gasoline as the fuel. Gasoline mixtures are slow to ignite, have a slow flame propagation speed and a higher quenching distance on the compression cycle of 2 mm compared to hydrogen's 0.6 mm. Combined, these factors waste fuel that would have created power, reducing efficiency. The gap between the rotor and the engine housing is too narrow for gasoline on the compression cycle, but sufficiently wide for hydrogen. The narrow gap is needed to create compression. When the engine uses gasoline, unburnt gasoline is ejected into the atmosphere through the exhaust. This is not a problem when using hydrogen fuel, as all the fuel mixture in the combustion chamber is burnt which gives near zero emissions raising fuel efficiency by 23%.^[46][47]

The shape of the Wankel combustion chamber is more resistant to

to deal with both unburned hydrocarbons and NOx emissions. This inexpensive solution increased fuel consumption.

Sales of Wankel engine cars suffered because of the

Mazda improved the fuel efficiency of the thermal reactor system by 40% with the

Curtiss-Wright produced the RC2-60 engine, which was comparable to a V8 engine in performance and fuel consumption.[53] Unlike NSU, Curtiss-Wright had solved the rotor sealing issue in 1966 with seals lasting 100,000 miles (160,000 km).^[54]

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.^[55]

Mazda's

Mazda is still continuing the development of the next-generation of Wankel engines. The company is researching engine

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.^[62] 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.^[63]

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.^[37] 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^[39] and unmanned aerial vehicles.^[64]

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

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_x) 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

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%.[70]^[71]

Compression-ignition Wankel

Research has been undertaken into rotary compression ignition engines and the burning of diesel heavy fuel using spark ignition. The basic design parameters of the Wankel engine preclude obtaining a compression ratio higher than 15:1 or 17:1 in a practical engine, but attempts are continuously being made to produce a compression-ignition Wankel. The Rolls-Royce^[72] and Yanmar compression-ignition^[73] approach was to use a two-stage unit, with one rotor acting as compressor, while combustion takes place in the other.^[74] Conversion of a standard 294-cc-chamber spark-ignition unit to use heavy fuel was described in SAE paper 930682, by L. Louthan. SAE paper 930683 (BSFC 330 g/KWhr), by D. Eiermann, resulted in the Wankel SuperTec line of spark ignition rotary engines (BSFC 270-310 g/KWhr),^[75] not much less than motor 250/400 by Rudolf Diesel of the year 1897. The Curtiss-Wright RC2-47 with stratified charge injection achieves consumption values around 226g/kWh which is on par with the common rail diesel engine MTU MB 873-Ka 501.^[76][77]

Compression-ignition engine research is being undertaken by

The design differs from Rolls-Royce's compression-ignition rotary, mainly by proposing an injector both in the exhaust passage between the combustor rotor and expansion rotor stages, and an injector in the expansion rotor's expansion chamber, for 'afterburning'.

The British company Rotron, which specializes in unmanned aerial vehicle (UAV) applications of Wankel engines, has designed and built a unit to operate on heavy fuel for NATO purposes. The engines uses spark ignition. The prime innovation is flame propagation, ensuring the flame burns smoothly across the whole combustion chamber. The fuel is pre-heated to 98 degrees Celsius before it is injected into the combustion chamber. Four spark plugs are utilized, aligned in two pairs. Two spark plugs ignite the fuel charge at the front of the rotor as it moves into the combustion section of the housing. As the rotor moves the fuel charge, the second two fire a fraction of a second behind the first pair of plugs, igniting near the rear of the rotor at the back of the fuel charge. The drive shaft is water-cooled which also has a cooling effect on the internals of the rotor. Cooling water also flows around the external of the engine through a gap in the housing, thus reducing the heat of the engine from outside and inside eliminating hot spots.^[86]

Hydrogen fuel

Using hydrogen fuel in Wankel engines improved efficiency by 23% over gasoline fuel with near-zero emissions.

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 pulse of the Otto cycle occurs in different chambers. The rotary has no exhaust valves that may remain hot and produce the backfire that occurs in reciprocating piston engines. Importantly, the intake chamber is separated from the combustion chamber, keeping the air/fuel mixture away from localized hot spots. These structural features of the rotary engine enable the use of hydrogen without pre-ignition and backfire.

A Wankel engine has stronger flows of air-fuel mixture and a longer operating cycle than a reciprocating piston engine, achieving a thorough mixing of hydrogen and air. The result is a homogeneous mixture with no hot spots in the engine, which is crucial for hydrogen combustion.[88] Hydrogen/air fuel mixtures are quicker to ignite than gasoline mixtures with a high burning rate, resulting in all the fuel being burnt with no unburnt fuel being ejected into the exhaust stream as is the case using gasoline fuel in rotary engines. Emissions are near zero, even with oil lubrication of apex seals.

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.^[89][90]

All these points lend the Wankel engine as ideal for hydrogen fuel burning. Mazda built and sold a vehicle that took advantage of the rotary's suitability to hydrogen fuel, a dual-fuel Mazda RX-8 Hydrogen RE that could switch on the fly from gasoline to hydrogen and back.^[91][47]

Advantages

Prime advantages of the Wankel engine are:^[54]

• 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 revolutions per minute than a 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
• Can use fuels of wider octane ratings
• 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.^[92]

Wankel engines are considerably lighter and simpler, containing far fewer moving parts than piston engines of equivalent power output.

.

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. The surface-to-volume ratio of a reciprocating piston diesel engine is much poorer than a reciprocating piston gasoline engine, but diesel engines have a higher efficiency factor. Hence, comparing power outputs is a realistic metric. A reciprocating piston engine with equal power to a Wankel will be approximately twice the displacement. 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.[93]^[94] The Wankel, therefore, has higher volumetric efficiency and lower pumping losses through the absence of choking valves.^[95] 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

. This ensures that even a severely overheated Wankel engine cannot seize, as is likely to occur in an overheated piston engine. This is a substantial safety benefit when used in aircraft. In addition, the absence of valves and valve trains increases safety. GM tested an iron rotor and iron housing in their prototype Wankel engines, which worked at higher temperatures with lower specific fuel consumption.

A further advantage of the Wankel engine for use in aircraft is that it generally has a smaller frontal area than a piston engine of equivalent power, allowing a more

nose to be designed around the engine. A cascading advantage is that the smaller size and lower weight of the Wankel engine allow for savings in air-frame construction costs, compared to piston engines of comparable power.

Wankel engines operating within their original design parameters are almost immune to catastrophic failure. A Wankel engine that loses compression, cooling, or oil pressure, will lose a large amount of power and fail over a short period of time. It will, however, usually continue to produce some power during that time, allowing for a safer landing when used in aircraft. Piston engines under the same circumstances are prone to seizing or breaking parts, which will almost certainly result in catastrophic failure of the engine, and the instant loss of all power. For this reason, Wankel engines are very well-suited to snowmobiles, which often take users into remote places where a failure could result in frostbite or death, and in aircraft, where abrupt failure is likely to lead to a crash or forced landing in a remote place.

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.^[96] From the point of view of oil refiners this may be an advantage in fuel production costs.^[97][98]

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

operation.

Disadvantages

Although many of the disadvantages are the subject of ongoing research, the current disadvantages of the Wankel engine in production are the following:[99]

Rotor sealing

This is still a minor problem as 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 231 °C to 129 °C, and the maximum difference between hotter and colder regions of the engine from 159 °C to 18 °C.^[100]

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.^[101] 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.

Slow gasoline combustion

Fuel combustion is slow using gasoline fuel, 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 gasoline/air mixture of 2mm, being the main source of unburned hydrocarbons at high rpm. 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 gasoline/air mixture. This problem does not occur using hydrogen fuel as the quenching is 0.6mm. 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. Where piston engines have an expanding combustion chamber for the burning fuel as its oxidizes and decreasing pressure as the piston travels toward the bottom of the cylinder during the power stroke is offset by additional leverage of the piston on the crankshaft during the first half of that travel, there is no additional "leverage" of a rotor on the mainshaft during combustion and the mainshaft has no increased leverage to power the rotor through the intake, compression and exhaust phases of its cycle.

Bad fuel economy using gasoline fuel

This is due to the shape of the moving combustion chamber, which results in poor combustion behavior and mean effective pressure at part load and low rpm. This results in unburnt fuel entering the exhaust stream; fuel that is wasted not being used to create power. Meeting the emissions regulations requirements sometimes mandates a fuel-air ratio using gasoline fuel that is not conducive to good fuel economy. Acceleration and deceleration in average driving conditions also affects fuel economy. However, operating the engine at a constant speed and load improves fuel consumption.^[62][102] The small air cooled housing, charge cooled rotor Wankel engines are well adapted to alcohol in gasoline mixes, as E5 and E10 sold in Europe.^[103]

High emissions

As unburned fuel when using gasoline fuel is in the exhaust stream, emissions requirements are difficult to meet. This problem may be overcome by implementing direct fuel injection into the combustion chamber. The Freedom Motors Rotapower Wankel engine, which is not yet in production, met the stringent California emissions standards.^[104] The Mazda Renesis engine, with both intake and exhaust side ports, suppressed the loss of unburned mix to exhaust formerly induced by port overlap.^[105]

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.^[106]

In a Wankel engine, the fuel-air mixture cannot be pre-stored because there are consecutive intake cycles. The engine has a 50% longer stroke duration than a reciprocating piston engine. The four Otto cycles take 1080°, which is three revolutions of the output shaft, for a Wankel engine, however three Otto cycles occur in that 1080°, giving one power pules per 360° of the output shaft. While it is 720° for the four stages of the Otto cycle in a four-stroke reciprocating engine, giving one power pulse per 720° of the output shaft.

There are various methods of calculating the engine displacement of a Wankel. The Japanese regulations for calculating displacements for engine ratings use the volume displacement of one rotor face only, and the auto industry commonly accepts this method as the standard for calculating the displacement of a rotary. When compared by specific output, however, the convention resulted in large imbalances in favor of the Wankel motor. An early revised approach was to rate the displacement of each rotor as two times the chamber.

Wankel rotary engine and piston engine displacement, and corresponding power, the output can more accurately be compared by displacement per revolution of the eccentric shaft. A calculation of this form dictates that a two-rotor Wankel displacing 654 cc per face will have a displacement of 1.3 liters per every rotation of the eccentric shaft (only two total faces, one face per rotor going through a full power revolution) and 2.6 liters after two revolutions (four total faces, two faces per rotor going through a full power revolution). The results are directly comparable to a 2.6-liter piston engine with an even number of cylinders in a conventional firing order, which will likewise displace 1.3 liters through its power revolution after one revolution of the mainshaft, and 2.6 liters through its power revolution after two revolutions of the mainshaft. A Wankel rotary engine is still a four-stage engine, and pumping losses from non-power revolutions still apply, but the absence of throttling valves and a 50% longer stage duration result in a significantly lower pumping loss compared to a four-stroke reciprocating piston engine. Measuring a Wankel rotary engine in this way more accurately explains its specific output, because the volume of its air-fuel mixture put through a complete power stage per revolution is directly responsible for torque, and thus the power produced.

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.

A peripheral intake port gives the highest mean effective pressure; however, side intake porting produces a more steady idle,^[110] 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^[60] 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 ducts^[111] 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.^[112]

All Mazda-made Wankel rotaries, including the Renesis, found in the RX-8, burn a small quantity of oil by design, metered into the combustion chamber to preserve the apex seals. Owners must periodically add small amounts of oil, thereby increasing running costs. Some sources, such as rotaryeng.net, claim that better results come with the use of an oil-in-fuel mixture rather than an oil metering pump. Liquid-cooled engines require a mineral multigrade oil for cold starts, and Wankel engines need a warm-up time before full load operation as reciprocating engines do. All engines exhibit oil loss, but the rotary engine is engineered with a sealed motor, unlike a piston engine that has a film of oil that splashes on the walls of the cylinder to lubricate them, hence an oil "control" ring. No-oil-loss engines have been developed, eliminating much of the oil lubrication problem.^{[citation needed]}

Car applications

It has been suggested that this article should be split into a new article titled Wankel engine use in cars. (discuss) (December 2022)

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.^[113]

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

, after the takeover of NSU, built, in 1979, a new KKM 871 engine with side intake ports, a 750-cc chamber, 170 hp (130 kW) at 6,500 rpm, and 220 Nm (162 ft-lb) at 3,500 rpm. 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.

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

Citroen

Citroën did much research, producing the

, a joint venture by Citroën and NSU.

Mercedes Benz

Mercedes-Benz fitted a Wankel engine in their C111 concept car.

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.^[122][123] 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.^[124]

Plans called for the engine to be used in the AMC Pacer, but development was pushed back.^[125][126] 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.^[127] 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.^[128] 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 530,000 miles (850,000 km). 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.^[129]

USSR

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.^[130] In 1980, the company commenced delivery of the VAZ-411 twin-rotor Wankel engine in VAZ-2106 and Lada cars, with about 200 being manufactured. Most of the production went to the security services.^[131][132] The next models were the VAZ-4132 and VAZ-415. A rotary version of the Samara was sold to the Russian public from 1997. Aviadvigatel, the Soviet aircraft-engine design bureau, is known to have produced rotary engines with electronic injection for fixed-wing aircraft and helicopters, though little specific information has surfaced.

Ford

Ford conducted research in rotary engines, resulting in patents granted: GB 1460229 , 1974, a method for fabricating housings; US 3833321  1974, side plates coating; US 3890069 , 1975, housing coating; CA 1030743 , 1978: Housings alignment; CA 1045553 , 1979, reed-valve assembly. In 1972, Henry Ford II stated that the rotary probably would not replace the piston in "my lifetime".^[133]

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.^[134]

The Sigma MC74 powered by a Mazda 12A engine was the first engine and only team from outside

The

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

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.^[140][141] In 2010, FEV said that in their prototype electric version of the Fiat 500, a Wankel engine would be used as a range extender.^[142] 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.^[143] The UK company, Aixro Radial Engines, offers a range extender based on the 294cc-chamber go-kart engine.^[144]

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.^{[118][145][146]} 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."^[62] 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 30 hp (22 kW) at 4,500 rpm, and maintaining a continuous electric output of 20 kW.^{[147][148][149]} In October 2017, Mazda announced that the rotary engine would be utilised in a hybrid car with 2019/20 the targeted introduction dates.^{[150][139][151]}

Mazda has undertaken research on Spark Controlled Compression Ignition (

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.[152]^[120] Mazda announced a launch date of March 2023 for the MX-30 R-EV Range Extender car.^[153][154]

The Japanese motoring press have reported that Toyota and Mazda will announce hybrid sports cars for 2026 launches.^[155]

Motorcycle applications

It has been suggested that this article should be split into a new article titled Wankel engine use in motorcycles. (discuss) (December 2022)

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.^[156]

Norton

In Britain,

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

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,[159] no motorcycles powered by Wankel engines have been produced for sale to the general public for road use since 1992.

Yamaha

In 1972,

, a prototype with a Wankel engine, weighing 220 kg and producing 60 hp (45 kW) 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.^[160]

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 62 hp (46 kW), it did not sell well.^[161]

The two different design approaches, taken by Suzuki and BSA may usefully be compared. Even before Suzuki produced the RE5, in

.

Wankel engines run very hot on the ignition and exhaust side of the engine's

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 80 hp (60 kW), significantly more powerful.[163]

Van Veen

Dutch motorcycle importer and manufacturer

Non-road vehicle applications

Aircraft

Diamond DA20

with a Diamond Engines Wankel

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:

1. Rotors cannot seize, since rotor casings expand greater than rotors;
2. The engine is less prone to the serious condition known as "engine-knock", which can destroy a plane's piston engines in mid-flight.
3. The engine is not susceptible to "shock-cooling" during descent;
4. The engine does not require an enriched mixture for cooling at high power;
5. Having no reciprocating parts, there is less vulnerability to damage when the engine revolves at a higher rate than the designed maximum. The limit to the revolutions is the strength of the main bearings.

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.^[165] 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

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,^[171] solely by the O.S. firm after Graupner's demise in 2012.^[172]

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

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 300 horsepower (220 kW) 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.^{[citation needed]}

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."^[176] 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. As of June 2010^[update], G-300 rotary engine development ceased, with the company citing cash flow problems.^[177]

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.^[178][179]

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^[180] 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.^[181]

Since rotary engines operate at a relatively high

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 7,500 rpm, the rotors are turning at a much slower 2,500 rpm.

The sailplane manufacturer Schleicher uses an Austro Engines AE50R Wankel^[182][183] in its self-launching models ASK-21 Mi, ASH-26E,^[184] ASH-25 M/Mi, ASH-30 Mi, ASH-31 Mi, ASW-22 BLE, and ASG-32 Mi.

In 2013,

The DA36 E-Star, an aircraft designed by

Other uses

Small Wankel engines are being found increasingly in other applications, such as

The simplicity of the rotary engine makes it well-suited for mini, micro, and micro-mini engine designs. The

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.

Yanmar of Japan produced some small, charge-cooled rotary engines for chainsaws and outboard engines.^[201] 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.^[202]

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 35 or 45 hp (26 or 34 kW) 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,^[203] 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.^{[204][needs update]}

Non-internal combustion

In addition to use as an internal combustion engine, the basic Wankel design has also been used for

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

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

• Yamaguchi, Jack K (2003). The Mazda RX-8: World's First 4-door, 4-seat Sports Car Plus Complete Histories of Mazda Rotary Engine development and Rotary Racing Around the World. Mazda Motor.
  ISBN 4-947659-02-5
  .
• Yamaguchi, Jack K (1985). The New Mazda RX-7 and Mazda Rotary Engine Sports Cars. New York: St. Martin's Press.
  ISBN 0-312-69456-3
  .
• Norbye, Jan P. (1973). "Watch out for Mazda!". Automobile Quarterly. XI.1: 50–61.
• Biermann, Arnold E.; Ellerbrock Jr., Herman H. (1941). "Report No. 726 The design of fins for air-cooled cylinders" (PDF). NACA. Archived from the original (PDF) on 2020-12-04. Retrieved 2018-05-05.
• Yamamoto, Kenichi (1981). Rotary Engine. Toyo Kogyo.
• Grazen, Alfred E. P., Patents CA 602098 and CA 651826 (plating for working surface in Suzuki RE-5)
• Societé Anonyme Automobiles Citroën (1969), Spanish patents 0374366 and 0375053 on improvements in procedures for covering a friction surface (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
• Ansdale, R. F. (1968). The Wankel RC Engine, Design and Performance. Iliffe.
  ISBN 0-592-00625-5
  .
• 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.
  ISBN 0-8019-5591-2
• 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;
  ISBN 978-3-642-52173-7
• 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,
  ISBN 0 85298 395 6
  , 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.
  ISBN 1855321 81 5
• Karl Ludvigsen: Wankel Engines A to Z, New York 1973.
  ISBN 0-913646-01-6
• 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: US 3848574 , 1974 -Kawasaki; GB 1460229 , 1974 -Ford; US 3833321 , 1974; US 3981688 , 1976. -Ford; CA 1030743 , 1978; CA 1045553 , 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

Wikimedia Commons has media related to Wankel engine.

• U.S. patent 2,988,008
• How Wankel Engines Work, How Stuff Works, 29 March 2001, retrieved 2012-08-14
• Wankel Engine, Animated Engines, Keveney
• How and why an engine must rev smoothly, UK: Citroën Net, retrieved 2012-08-14^{[permanent dead link]}
• Aaron Cake's page on Wankel engine, CA
• J I Martin-Artajo SI Rotary engine mockup, ES, archived from the original on 2021-11-14
• J C Lefeuvre's Moto-Turbine-Radiale rotary engine installed in a motorcycle, 7 August 2010, FR
• Scott, David (March 1960). "Auto Engine Without Pistons". Popular Science: 82.
• Norbye, Jan P. (January 1967). "Rivals to the Wankel: A Roundup of Rotary Engines". Popular Science: 80. Kauertz, Tschudi, Virmel, Mercer, Selwood, Jernaes examples.{{cite journal}}: CS1 maint: postscript (link)
• The Norton Rotary; summary of the development of the Norton series of Wankel engines, by David Garside