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A Wankel engine with its rotor and geared output shaft.
The Mazda RX-8 sports car is the last production car to be powered by a Wankel engine.
Norton Classic air-cooled twin-rotor motorcycle

The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion.

Compared to the reciprocating piston engine, the Wankel engine has more uniform torque and less vibration and, for a given power, is more compact and weighs less.

The rotor, which creates the turning motion, is similar in shape to a Reuleaux triangle, except the sides have less curvature. Wankel engines deliver three power pulses per revolution of the rotor using the Otto cycle. However, the output shaft uses toothed gearing to turn three times faster giving one power pulse per revolution. This can be seen in the animation below. In one revolution, the rotor experiences power pulses and exhausts gas simultaneously, while the four stages of the Otto cycle occur at separate times. For comparison, in a two-stroke piston engine there is one power pulse for each crankshaft revolution (as with a Wankel engine output shaft) and, in a four-stroke piston engine, one power pulse for every two revolutions.

The four-stage Otto cycle of intake, compression, ignition, and exhaust occurs each revolution of the rotor at each of the three rotor faces moving inside the oval-like epitrochoidal housing, enabling the three power pulses per rotor revolution.

The definition of displacement applies to only one face of the rotor as only one face is working for each output shaft revolution.

The engine is commonly referred to as a rotary engine, although this name is also applied to other completely different designs, including both ones with pistons and pistonless rotary engines.

Concept

The Wankel KKM motorcycle: The "A" marks one of the three apices of the rotor. The "B" marks the eccentric shaft, and the white portion is the lobe of the eccentric shaft. The shaft turns three times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft.
Schematic of the Wankel:
  1. Intake
  2. Exhaust
  3. Stator housing
  4. Chambers
  5. Pinion
  6. Rotor
  7. Crown gear
  8. Eccentric shaft
  9. Spark plug.

The Wankel engine has the advantages of compact design and low weight over the more common internal combustion engine, which employs reciprocating pistons. These advantages give rotary engine applications in a variety of vehicles and devices, including automobiles, motorcycles, racing cars, aircraft, go-karts, jet skis, snowmobiles, chainsaws, and auxiliary power units. Certain Wankel engines have a power-to-weight ratio over one horsepower per pound.[2] Most engines of the design are of spark ignition, with compression ignition engines having been built only in research projects.

In the Wankel engine, the four strokes of an Otto cycle occur in the space between each face of a three-sided symmetric rotor and the inside of a housing. The oval-like epitrochoid-shaped housing surrounds a triangular rotor with bow-shaped faces similar in appearance to a Reuleaux triangle.[3] The theoretical shape of the rotor between the fixed apexes is the result of a minimization of the volume of the geometric combustion chamber and a maximization of the compression ratio, respectively.[4][5] The symmetric curve connecting two arbitrary apices of the rotor is maximized in the direction of the inner housing shape with the constraint that it not touch the housing at any angle of rotation (an pistons. These advantages give rotary engine applications in a variety of vehicles and devices, including automobiles, motorcycles, racing cars, aircraft, go-karts, jet skis, snowmobiles, chainsaws, and auxiliary power units. Certain Wankel engines have a power-to-weight ratio over one horsepower per pound.[2] Most engines of the design are of spark ignition, with compression ignition engines having been built only in research projects.

In the Wankel engine, the four strokes of an Otto cycle occur in the space between each face of a three-sided symmetric rotor and the inside of a housing. The oval-like epitrochoid-shaped housing surrounds a triangular rotor with bow-shaped faces similar in appearance to a Reuleaux triangle.[3] The theoretical shape of the rotor between the fixed apexes is the result of a minimization of the volume of the geometric combustion chamber and a maximization of the compression ratio, respectively.[4][5] The symmetric curve connecting two arbitrary apices of the rotor is maximized in the direction of the inner housing shape with the constraint that it not touch the housing at any angle of rotation (an arc is not a solution of this optimization problem).

The central drive shaft, called the "eccentric shaft" or "E-shaft", passes through the center of the rotor being supported by fixed bearings.[6] The rotors ride on eccentrics (analogous to crankpins in piston engines) integral to the eccentric shaft (analogous to a crankshaft). The rotors both rotate around the eccentrics and make orbital revolutions around the eccentric shaft. Seals at the apices of the rotor seal against the periphery of the housing, dividing it into three moving In the Wankel engine, the four strokes of an Otto cycle occur in the space between each face of a three-sided symmetric rotor and the inside of a housing. The oval-like epitrochoid-shaped housing surrounds a triangular rotor with bow-shaped faces similar in appearance to a Reuleaux triangle.[3] The theoretical shape of the rotor between the fixed apexes is the result of a minimization of the volume of the geometric combustion chamber and a maximization of the compression ratio, respectively.[4][5] The symmetric curve connecting two arbitrary apices of the rotor is maximized in the direction of the inner housing shape with the constraint that it not touch the housing at any angle of rotation (an arc is not a solution of this optimization problem).

The central drive shaft, called the "eccentric shaft" or "E-shaft", passes through the center of the rotor being supported by fixed bearings.[6] The rotors ride on eccentrics (analogous to crankpins in piston engines) integral to the eccentric shaft (analogous to a crankshaft). The rotors both rotate around the eccentrics and make orbital revolutions around the eccentric shaft. Seals at the apices of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers.[4] The rotation of each rotor on its own axis is caused and controlled by a pair of synchronizing gears[6] 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 shaft. The power output of the engine is not transmitted through the synchronizing gears.[6] The rotor moves in its rotating motion guided by the gears and the eccentric shaft, not being guided by the external chamber; the rotor must not rub 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 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 revolving, each side of the rotor is brought closer to and then away from the wall of the housing, compressing and expanding the combustion chamber like the strokes of a piston in a reciprocating piston engine. The power vector of the combustion stage goes through the center of the offset lobe.

While a four-stroke piston engine completes one combustion stroke per cylinder for every two rotations of the crankshaft (that is, one-half power stroke per crankshaft rotation per cylinder), each combustion chamber in the Wankel generates one combustion stroke per driveshaft rotation, i.e. one power stroke per rotor orbital revolution and three power strokes per rotor rotation. Thus, the power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement in a similar state of tune; and higher than that of a four-stroke piston engine of similar physical dimensions and weight.

Wankel engines generally are able to reach much higher engine revolutions than reciprocating engines of similar power output. This is due partly to the smoothness inherent in circular motion, and the fact that the "engine" rpm is of the output shaft, which is three times faster than that of the oscillating parts. The eccentric shafts do not have the stress-related contours of crankshafts. The maximum revolutions of a rotary engine are limited by tooth load on the synchronizing gears.[7] Hardened steel gears are used for extended operation above 7000 or 8000 rpm. Mazda Wankel engines in auto racing are operated above 10,000 rpm. 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.

National agencies that tax automobiles according to displacement and regulatory bodies in automobile racing variously 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, though 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.[8]

In 1951, NSU Motorenwerke AG in Germany began development of the engine, with two models being built. The first, the DKM motor, was developed by Felix Wankel. The second, the KKM motor, developed by Hanns Dieter Paschke, was adopted as the basis of the modern Wankel engine.[9]

The basis of the DKM type of motor was that both the rotor and the housing spun around on separate axes. The DKM motor reached higher revolutions per minute (up to 17,000 rpm) and was more naturally balanced. However, the engine needed to be stripped to change the spark plugs and contained more parts. The KKM engine was simpler, having a fixed housing.

The first working prototype, DKM 54, produced 21 hp (16 kW) and ran on February 1, 1957, at the NSU research and development department Versuchsabteilung TX.[1][10]

The KKM 57 (the Wankel rotary engine, Kreiskolbenmotor) was constructed by NSU engineer Hanns Dieter Paschke in 1957 without the knowledge of Felix Wankel, who later remarked, "you have turned my race horse into a plow mare".[11]

Licenses issued

In 1960, NSU, the firm that employed the two inventors, and 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.[12] 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. Amongst them were Alfa Romeo, American Motors, Citroën, Ford, General Motors, Mazda, The basis of the DKM type of motor was that both the rotor and the housing spun around on separate axes. The DKM motor reached higher revolutions per minute (up to 17,000 rpm) and was more naturally balanced. However, the engine needed to be stripped to change the spark plugs and contained more parts. The KKM engine was simpler, having a fixed housing.

The first working prototype, DKM 54, produced 21 hp (16 kW) and ran on February 1, 1957, at the NSU research and development department Versuchsabteilung TX.[1][10]

The KKM 57 (the Wankel rotary engine, Kreiskolbenmotor) was constructed by NSU engineer Hanns Dieter Paschke in 1957 without the knowledge of Felix Wankel, who later remarked, "you have turned my race horse into a plow mare".[11]

In 1960, NSU, the firm that employed the two inventors, and 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.[12] 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. Amongst them were Alfa Romeo, Ame

Many manufacturers signed license agreements for development, attracted by the smoothness, quiet running, and reliability emanating from the uncomplicated design. Amongst them were Alfa Romeo, American Motors, Citroën, Ford, General Motors, Mazda, Mercedes-Benz, Nissan, Porsche, Rolls-Royce, Suzuki, and Toyota.[1] 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.[13]

Citroën did much research, producing the M35, GS Birotor, and RE-2 [fr] helicopter, using engines produced by Comotor, a joint venture of Citroën and NSU. General Motors seemed to have concluded the Wankel engine was slightly more expensive to build than an equivalent reciprocating engine. General Motors claimed to have solved the fuel-economy issue, but failed in obtaining in a concomitant way to acceptable exhaust emissions. Mercedes-Benz fitted a Wankel engine in their C111 concept car.

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.[14]

In 1961, the Soviet research organization of NATI, NAMI, and VNIImotoprom commenced development creating experimental engines with different technologies.[15] Soviet automobile manufacturer AvtoVAZ also experimented in Wankel engine design without a license, introducing a limited number of engines in some cars.[16]

By mid-September 1967, even Wankel model engines became available through the German Graupner aeromodeling 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.

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 ferroTiC alloy apex seals and an NSU rotor in a successful attempt to prolong the engine's life.

Developments for cars

Mazda and NSU sign

Mazda and NSU signed a study contract to develop the Wankel engine in 1961 and competed to bring the first Wankel-powered automobile to market. Although Mazda produced an experimental Wankel that year, NSU was first with a Wankel automobile for sale, the sporty NSU Spider in 1964; Mazda countered with a display of two- and four-rotor Wankel engines at that year's Tokyo Motor Show.[1] In 1967, NSU began production of a Wankel-engined luxury car, the Ro 80.[17] 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 Wankel engine development. This premature release of the new Wankel 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.[1] Audi, 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 at 3,500 rpm. The engine was installed in an Audi 100 hull named "Audi 200", but was not mass produced.

[1] After years of development, Mazda's first Wankel-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. Curtiss-Wright produced the RC2-60 engine, which was comparable to a V8 engine in performance and fuel consumption. Unlike NSU, Curtiss-Wright had solved the rotor sealing issue with seals lasting 100,000 miles (160,000 km) by 1966.[18]

Mazda later abandoned the Wankel in most of their automotive designs, continuing to use the engine in their sports car range only, producing the RX-7 until August 2002. The company normally used two-rotor designs. A more advanced twin-turbo three-rotor engine was fitted in the 1991 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.[19] The Renesis is capable of 238 hp (177 kW) with improved fuel economy, reliability, and lower emissions than previous Mazda rotary engines,[20] all from a nominal 1.3-L displacement, but this was not enough to meet more stringent emissions standards. Mazda ended production of their Wankel engine in 2012 after the engine failed to meet the more stringent Euro 5 emission standards, leaving no automotive company selling a Wankel-powered vehicle.[21] The company is continuing development of the next generation of Wankel engines, the

Mazda later abandoned the Wankel in most of their automotive designs, continuing to use the engine in their sports car range only, producing the RX-7 until August 2002. The company normally used two-rotor designs. A more advanced twin-turbo three-rotor engine was fitted in the 1991 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.[19] The Renesis is capable of 238 hp (177 kW) with improved fuel economy, reliability, and lower emissions than previous Mazda rotary engines,[20] all from a nominal 1.3-L displacement, but this was not enough to meet more stringent emissions standards. Mazda ended production of their Wankel engine in 2012 after the engine failed to meet the more stringent Euro 5 emission standards, leaving no automotive company selling a Wankel-powered vehicle.[21] 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.[22] Mazda and Toyota announced they combined to produce a range extending rotary engine for vehicles.[23][24][25]

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 Wankel engines for both passenger cars and Jeeps, as well as the right to sell any rotary engines it produced to other companies.[26][27] 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 Wankel engines, and predicted a total transition to rotary power by 1984.[28] Plans called for the engine to be used in the AMC Pacer, but development was pushed back.[29][30] American Motors designed the unique Pacer around the engine. By 1974, AMC had decided to purchase the General Motors (GM) Wankel instead of building an engine in-house.[31] Both GM and AMC confirmed the relationship would be beneficial in marketing the new engine, with AMC claiming that the GM Wankel achieved good fuel economy.[32] 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 Wankel engine. Rising fuel prices and talk about proposed US emission standards legislation also added to concerns.

By 1974, GM R&D had not succeeded in producing a Wankel engine meeting both the emission requirements and good fuel economy, leading 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 Wankel project required AMC to reconfigure the Pacer to house its venerable AMC straight-6 engine driving the rear wheels.[33]

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.[34] 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.[35][36] The next models were the VAZ-4132 and VAZ-415. A rotary version of the Samara was sold to Russian public from 1997. Aviadvigatel, the Soviet aircraft-engine design bureau, is known to have produced Wankel engines with electronic injection for fixed-wing aircraft and helicopters, though little specific information has surfaced.

Ford conducted research in Wankel engines, resulting in patents granted: GB 1460229 , 1974, 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".[37]

Felix Wankel managed to overcome most of the problems that made previous 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.[38]

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

Rotary 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.[39] Pre-heating of certain housing sections with exhaust gas improved performance and fuel economy, also reducing wear and emissions.[40]

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 r

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

Rotary 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.[39] Pre-heating of certain housing sections with exhaust gas improved performance and fuel economy, also reducing wear and emissions.[40]

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.[41][42][43][44]

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 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 plug being screwed directly into the block housing.[45] 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.[46] 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,[47] 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 rotary engine with the plugs placed in the side plates, instead of the usual placement in the housing working surface (CA 1036073 , 1978).

Increasing the displacement and power of a rotary engine by adding more rotors to a basic design is simple, but a limit may exist 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 was 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

Research in the United Kingdom under the SPARCS (Self-Pressurising-Air Rotor Cooling System) project, found that idle stability and economy was 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 rotary engine specialist David Garside, who developed three patented systems in 2016.[48][49]

SPARCS and Compact-SPARCS provides superior heat rejection and efficient thermal balancing to optimise lubrication. A problem with rotary engines is that the engine housing has permanently cool and hot surfaces when running. It also generates excessive heat inside the engine which breaks down lubricating oil quickly. The SPARCS system reduces this wide differential in heat temperatures in the metal of the engine housing, and also cooling 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-pressurised by capturing the blow-by past the rotor side gas seals from the working chambers."[50][51] 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. Horse power 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.[52][53][54][55][56]

Materialsthermal 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.[39]

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. Citroen, Mercedes-Benz, Ford, A P Grazen and others applied for pa

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. Citroen, 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, ferrotic, 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 seal 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. 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 MoS2, at the rate of 1 cc (1 mL) per liter of fuel, is advised (LiquiMoly). Many engineers 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.[57]

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 rotary engines solved these initial problems. Current engines have nearly 100 seal-related parts.[1]

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

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).

The Wankel engine has problems in fuel efficiency and emissions when burning gasoline. 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, leftover 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 nearly no emissions and raises fuel efficiency by 23%.[58][59]

The shape of the Wankel combustion chamber is more resistant to preignition operating on lower-octane rating gasoline than a comparable piston engine.[60]The shape of the Wankel combustion chamber is more resistant to preignition operating on lower-octane rating gasoline than a comparable piston engine.[60] 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 mix, there is a 7 °C 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 rotary engine cars suffered because of the oil crisis of 1973 raising the price of gasoline leading to lowering of sales. Toyota discovered that injection of air into the exhaust port zone improved fuel economy reducing emissions. The best results were obtained with holes in the side plates; doing it in the exhaust duct had no noticeable influence.[46] The use of a three-stage catalysts, with air supplied in the middle, as for two-stroke piston engines, also proved beneficial meeting emissions regulations.[61]

Mazda had 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.[6] According to the 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.[62] Curtiss-Wright showed also that the rotor can be widened, 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.[63] Quenching is the dominant source of hydrocarbon at high speeds, and leakage at low speeds.[64]

Automobile Wankel rotary engines are capable of high-speed operation. However, it was shown 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.[65]

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 side intake and exhaust ports. A scraper seal was added in 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 Wankel 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.[66][67][68] However, the RX-8 was not improved to meet Euro 5 emission regulations and was discontinued in 2012.[69]

Mazda is still continuing development of 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%.[70] These innovations promise to improve fuel consumption and emissions.[71]

To improve fuel efficiency further, Mazda is looking 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.[72]

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 utilise 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.[50] 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 utilising this patent to develop hybrid power units for automobiles[52] and unmanned aerial vehicles.[73]

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.[71][74][75]

Homogeneous charge compression ignition (HCCI)