Wankel engine

This article is about a particular pistonless rotary engine. For other pistonless rotary engines, see pistonless rotary engine. For piston designs arranged in a rotary configuration, see rotary engine.

A partially disassembled Mazda Wankel engine shown at the Deutsches Museum in Munich, Germany

The Mazda RX-8, a sports car 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.

All parts rotate consistently in one direction, as opposed to the common reciprocating piston engine, which has pistons violently changing direction. In contrast to the more common reciprocating piston designs, the Wankel engine delivers advantages of simplicity, smoothness, compactness, high revolutions per minute, and a high power-to-weight ratio. This is primarily because three power pulses per rotor revolution are produced compared to one per revolution in a two-stroke piston engine and one per two revolutions in a four-stroke piston engine. Although at the actual output shaft, there is only one power pulse per revolution, since the output shaft spins three times as fast as the actual rotor, as can be seen in the animation below, it makes it roughly equivalent to a 2-stroke piston engine of the same displacement. This is also why the displacement only measures one face of the rotor, since only one face is working for each output shaft revolution.

The engine is commonly referred to as a rotary engine, although this name also applies to other completely different designs, primarily aircraft engines with their cylinders arranged in a circular fashion around the crankshaft. The four-stage cycle of intake, compression, ignition, and exhaust occur each revolution at each of the three rotor tips moving inside the oval-like epitrochoid-shaped housing, enabling the three power pulses per rotor revolution. The rotor is similar in shape to a Reuleaux triangle with the sides somewhat flatter.

Concept and design

The design was conceived by German engineer Felix Wankel. Wankel received his first patent for the engine in 1929. He began development in the early 1950s at NSU, completing a working prototype in 1957. NSU subsequently licensed the design to companies around the world, who have continually added improvements. The engines produced are of spark ignition, with compression ignition engines having only been built in research projects.

The Wankel engine has the advantages of compact design and low weight over the most commonly used internal combustion engine employing reciprocating pistons. These advantages have given 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. The power-to-weight ratio has reached under one pound per horsepower in certain engines.

History

Early developments

The first DKM Wankel engine designed by Felix Wankel, the DKM 54 (Drehkolbenmotor), at the Deutsches Museum in Bonn, Germany: the rotor and its housing spin.

The first KKM Wankel Engine designed by Hanns Dieter Paschke, the NSU KKM 57P (Kreiskolbenmotor), at Autovision und Forum, Germany: the rotor housing is static.

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.

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 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. 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".

Licenses issued

In 1960, NSU, the firm that employed the two inventors, and the US firm Curtiss-Wright, signed a joint agreement. NSU were to concentrate on low- and medium-powered Wankel engine development and Curtiss-Wright developing high-powered engines, including aircraft engines of which Curtiss-Wright had decades of experience designing and producing. Curtiss-Wright recruited Max Bentele to head their design team.

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

Citroën did much research, producing the M35, GS Birotor and RE-2 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.

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

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

Developments for motorcycles

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 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. In 1967, NSU began production of a Wankel-engined luxury car, the Ro 80. However, NSU had not produced reliable apex seals on the rotor, 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. Audi, after the takeover of NSU, built in 1979 a new KKM 871 engine with side intake ports and 750 cc per 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.

Mazda's first Wankel engine, at the Mazda Museum in Hiroshima, Japan

Mazda, however, claimed to have solved the apex seal problem, and operated test engines at high speed for 300 hours without failure. 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, by 1966 Curtiss-Wright had solved the rotor sealing issue with seals lasting 100,000 miles (160,000 km).

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 had also 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. The Renesis is capable of 238 hp (177 kW) with improved fuel economy, reliability, and lower emissions than previous Mazda rotary engines, all from a nominal 1.3 L displacement. However, 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 improved Euro 5 emission standards, leaving no automotive company selling a Wankel-powered vehicle. The company is continuing development of the next generation of Wankel engines, the SkyActiv-R with a new rear wheel drive sports car model announced in October 2015 although with no launch date given. Mazda states that the SkyActiv-R solves the three key issues with previous rotary engines: fuel economy, emissions and reliability. Mazda announced the introduction of the series-hybrid Mazda2 EV car using a Wankel engine as a range extender, however no date of introduction has been announced.

1972 GM Rotary engine cutaway shows twin-rotors

American Motors (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 Wankels for both passenger cars and Jeeps, as well as the right to sell any rotary engines it produced to other companies. American Motors' president, William Luneburg, did not expect dramatic development through to 1980. However 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. Plans called for the engine to be used in the AMC Pacer, but development was pushed back. American Motors designed the unique Pacer around the engine. By 1974, AMC had decided to purchase the General Motors Wankel instead of building an engine in-house. Both General Motors and AMC confirmed the relationship would be beneficial in marketing the new engine, with AMC claiming that the General Motors' Wankel achieved good fuel economy. However, General Motors' engines had not reached production when the Pacer was launched onto the market. The 1973 oil crisis played a part in frustrating the uptake of the Wankel engine. Rising fuel prices and talk about proposed US emission standards legislation also added to concerns.

By 1974, General Motors 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 General Motors' Wankel project required AMC to reconfigure the Pacer to house its venerable AMC straight-6 engine driving the rear-wheels.

In 1974, the Soviet Union created a special engine design bureau, which, in 1978, designed an engine designated as "VAZ-311". In 1980, the company commenced delivery of the VAZ-411 twin-rotor Wankel engine in VAZ-2106s and Lada cars, with about 200 being manufactured. Most of the production went to the security services. The next models were the VAZ-4132 and VAZ-415. Aviadvigatel, the Soviet aircraft engine design bureau, is known to have produced Wankel engines with electronic injection for 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 won't replace the piston in "my lifetime".

Design

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 3 times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft.

In the Wankel engine, the four strokes of an Otto cycle piston engine occur in the space between a three-sided symmetric rotor and the inside of a housing. In each rotor of the Wankel engine, the oval-like epitrochoid-shaped housing surrounds a rotor which is triangular with bow-shaped flanks (often confused with a Reuleaux triangle, a three-pointed curve of constant width, but with the bulge in the middle of each side a bit more flattened). The theoretical shape of the rotor between the fixed corners is the result of a minimization of the volume of the geometric combustion chamber and a maximization of the compression ratio, respectively. The symmetric curve connecting two arbitrary apexes 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 and is supported by fixed bearings. The rotors ride on eccentrics (analogous to crankpins) 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 corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers. The rotation of each rotor on its own axis is caused and controlled by a pair of synchronizing gears 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 1/3 turn for each turn of the eccentric shaft. The power output of the engine is not transmitted through the synchronizing gears. 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 centre of the eccentric part of the output shaft.

The easiest way to visualize the action of the engine in the animation at left 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, there are three cavities per housing, all repeating 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 1.5 times faster than that of the oscillating parts. Th eccentric shafts do not have the stress related contours of crankshafts. The maximum revolutions of a rotary engine is limited by tooth load on the synchronizing gears. 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. However, as gas pressure participates in seal efficiency, racing a Wankel engine at high rpm 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 1.5 to 2 times the displacement of one chamber per rotor, even though there are three lobes per rotor (because the rotor is only completing 1/3rd rotation per 1 rotation of the output shaft, so there is only one power stroke working per output revolution, the other two lobes are simultaneously ejecting a spent charge and intaking 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 due to the perceived advantages of the design in racing applications.

Engineering

Apex seals, left NSU Ro 80 Serie and Research and right Mazda 12A and 13B

Left: Mazda L10A camber axial cooling
Middle: Audi NSU EA871 axial water cooling only the hot bow
Right: Diamond Engines Wankel radial cooling only the hot bow

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

In the early days, special, dedicated production machines had to be built for different housing dimensional arrangements. However, patented 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. Pre-heating of certain housing sections with exhaust gas improved performance and fuel economy, also reducing wear and emissions.

The boundary layer shields and the oil film act as thermal insulation, leading to a low temperature of the lubricating film (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.

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. 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. 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, 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).

Four-stroke reciprocating engines are not very suitable for use with hydrogen fuel. The hydrogen can misfire on hot parts like the exhaust valve and spark plugs. Another problem concerns the hydrogenate attack on the lubricating film in reciprocating engines. In a Wankel engine, this problem is circumvented by using a ceramic apex seal against a ceramic surface, so that there is no oil film to suffer hydrogenate attack. The piston shell must be lubricated and cooled with oil. This substantially increases the lubricating oil consumption in a four-stroke hydrogen engine.

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 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 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.

• SPARCS.
• Compact-SPARCS.
• CREEV (Compound Rotary Engine for Electric Vehicles)

SPARCS and Compact-SPARCS provides superior heat rejection and efficient thermal balancing to optimise lubrication. 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." CREEV is a ‘exhaust reactor’ that consumes unburnt exhaust products delivering lower emissions and improved fuel efficiency. All three patents are currently licensed to UK-based engineers, AIE (UK) Ltd.

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 (232 °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.

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 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 reduced 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.

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. 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.

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 shape of the Wankel combustion chamber is more resistant to preignition operating on lower-octane rating gasoline than a comparable piston engine. The combustion chamber shape may also lead to relatively incomplete combustion of the air-fuel charge. 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 some inherent 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, which was already a weak point for the Wankel engine, and the oil crisis of 1973 raised the price of gasoline. Toyota discovered that injection of air into the exhaust port zone improved fuel economy and reduced emissions. The best results were obtained with holes in the side plates; doing it in the exhaust duct had no noticeable influence. The use of a three-stage catalysts, with air supplied in the middle, as for two-stroke piston engines, also proved beneficial.

Mazda had improved the fuel efficiency of the thermal reactor system by 40% when the RX-7 was introduced in 1978. However, Mazda eventually shifted to the catalytic converter system. 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. 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. Quenching is the dominant source of hydrocarbon at high speeds, and leakage at low speeds.

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 low rpm. The shape and positioning of the recess in the rotor, which forms most of the combustion chamber, influences emissions and fuel economy. The results in terms of fuel economy and exhaust emissions varies depending on the shape of the combustion recess which is determined by the placement of spark plugs per chamber of an individual engine.

Mazda's RX-8 car with the Renesis engine met California State fuel economy requirements, including California's low emissions vehicle (LEV) standards. This was achieved by a number of innovations. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This solved the problem of the earlier ash buildup in the engine, and thermal distortion problems of 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. However, the RX-8 was not improved to meet Euro 5 emission regulations and was discontinued in 2012.

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, and sparkless HCCI ignition. These lead to greater rotor eccentricity (equating to a longer stroke in a reciprocating engine), with improved elasticity and low revolutions-per-minute torque. Research by T. Kohno proved that installing a glow-plug in the combustion chamber improved part load and low revolutions per minute fuel economy by 7%. These innovations promise to improve fuel consumption and emissions. To improve fuel efficiency further, Mazda 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.

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. 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 and unmanned aerial vehicles.

Laser ignition

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

Homogeneous Charge Compression Ignition (HCCI)

Homogeneous charge compression ignition (HCCI) involves the use of a pre-mixed lean air-fuel mixture being compressed to the point of auto-ignition, so electronic spark ignition is eliminated. Gasoline engines combine homogeneous charge (HC) with spark ignition (SI), abbreviated as HCSI. Diesel engines combine stratified charge (SC) with compression ignition (CI), abbreviated as SCCI. HCCI engines achieve gasoline engine-like emissions with compression ignition engine-like efficiency, and low levels of nitrogen oxide emissions (NO 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 SkyActiv Generation 2 program. A constraint of rotary engines is the need to locate the spark plug outside the combustion chamber to enable the rotor to sweep past. Mazda confirmed that the problem had been solved in the SkyActiv-R project. Rotaries generally have high compression ratios, making them particularly suitable for the use of HCCI.

Compression-Ignition Rotary

Rolls Royce R6 two stage rotary compression ignition engine

There has been research into compression ignition engines and the burning of diesel heavy fuel in rotaries 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 and Yanmar compression-ignition approach was to use a two-stage unit, with one rotor acting as compressor, while combustion takes place in the other. Conversion of an standard 294 cc per chamber spark-ignition unit to use heavy fuel was described in SAE paper 930682, by L. Louthan. SAE paper 930683, by D. Eiermann, resulted in the Wankel SuperTec line of compression-ignition rotary engines.

Compression-ignition engine research is being undertaken by Pratt & Whitney Rocketdyne, which was commissioned by DARPA to develop a compression-ignition Wankel engine for use in a prototype VTOL flying car called the "Transformer". The engine, based on an earlier concept involving an unmanned aerial vehicle called "Endurocore", powered by a Wankel diesel. plans to utilize Wankel rotors of varying sizes on a shared eccentric shaft to increase efficiency. The engine is claimed to be a 'full-compression, full-expansion, compression-ignition-cycle engine'. An October 28, 2010 patent by Pratt & Whitney Rocketdyne, describes a Wankel engine superficially similar to Rolls-Royce's earlier prototype, that required an external air compressor to achieve high enough compression for compression-ignition-cycle combustion. 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 specialises 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 utilised, 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 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.

Advantages

NSU Wankel Spider, the first line of cars sold with a rotor Wankel engine

Mazda Cosmo, the first series two rotor Wankel engine sports car

Prime advantages of the Wankel engine are:

• A far higher power to weight ratio than a piston engine
• Approximately one third of the size of a piston engine of equivalent power output
• 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
• It can use fuels of wider octane ratings
• Does not suffer from "scale effect" to limit its size. On some Wankel engines the sump oil remains uncontaminated by the combustion process, so no oil changes are required. The oil in the mainshaft is totally sealed from the combustion process. The oil for Apex seals and crankcase lubrication is separate. In piston engines the crankcase oil is contaminated by combustion blow-by through the piston rings

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

The surface-to-volume-ratio in the moving combustion chamber is so complex that a direct comparison cannot be made between a reciprocating piston engine and a Wankel engine. The flow velocity and the heat losses are quite different. Surface temperature characteristics are completely different; the film of oil in the Wankel engine acts as insulation. Engines with a higher compression ratio have a worse surface-to-volume ratio. 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 on 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 increased. The Wankel, therefore, has higher volumetric efficiency and lower pumping losses through the absence of choking valves. Because of the quasi-overlap of the power strokes, that cause the smoothness of the engine and the avoidance of the four-stroke cycle in a reciprocating 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 rpm's. This difference is more pronounced when compared to four-cylinder reciprocating engines and less pronounced when compared to higher cylinder counts.

In addition to the removal of internal reciprocating stresses by the complete removal of reciprocating internal parts typically found in a piston engine, the Wankel engine is constructed with an iron rotor within a housing made of aluminium, which has a greater coefficient of thermal expansion. This ensures that even a severely overheated Wankel engine cannot seize, 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, that 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 aerodynamic nose to be designed around the engine. A cascading advantage is that the smaller size and lower weight of the Wankel engine allows for savings in airframe 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, or 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. From the point of view of oil refiners this may be an advantage in fuel production costs.

Due to a 50% longer stroke duration than a reciprocating four-cycle engine, there is more time to complete the combustion. This leads to greater suitability for direct fuel injection and stratified charge operation. A Wankel rotary engine has stronger flows of air-fuel mixture and a longer operating cycle than a reciprocating engine, achieving a concomitantly thorough mixing of hydrogen and air. The result is a homogeneous mixture and no hot spots in the engine, which is crucial for hydrogen combustion.

Disadvantages

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

• Rotor sealing. 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 leads 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 being exposed to fuel, allowing for 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 engine. The "heat pipes" effectively direct hot exhaust gas to the cooler parts of the engine, with resulting 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 engine from 159 °C to 18 °C.
• 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 troichoid housing in light-load operation when imbalances in centrifugal force and gas pressure occur. At low engine-rpm ranges, or under low-load conditions, 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 troichoid housing, which meant that the seals remain flush to the housing. Using the Wankel engine at sustained higher revolutions helps eliminate apex seal lift off, and makes it very viable in applications such as electricity generation. In motor vehicles, the engine will be suited to series-hybrid applications.
• Slow combustion. Fuel combustion is slow, because the combustion chamber is long, thin, and moving. Flame travel occurs almost exclusively in the direction of rotor movement, adding to the quenching that is 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. 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 oxidized 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. This is due to seal leakages, and the shape of the combustion chamber, which results in poor combustion behavior and mean effective pressure at part load, low rpm. Meeting the emissions regulations requirements sometimes mandates a fuel-air ratio that is not conducive to good fuel economy. Acceleration and deceleration in average driving conditions also affect fuel economy. However, operating the engine at a constant speed and load eliminates excess fuel consumption.
• High emissions. As unburnt 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 ultra low California emissions standards. The Mazda Renesis engine, with both intake and exhaust side ports, suppressed the loss of unburned mix to exhaust formerly induced by port overlap.

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 factor reducing its efficiency.

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 last 1080° for a Wankel engine (three revolutions of the output shaft) versus 720° for a four-stroke reciprocating engine, but the four strokes are still the same proportion of the total.

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, 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 stroke) and 2.6 liters after two revolutions (four total faces, two faces per rotor going through a full power stroke). 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 stroke after one revolution of the mainshaft, and 2.6 liters through its power strokes after two revolutions of the mainshaft. A Wankel rotary engine is still a four-cycle engine, and pumping losses from non-power strokes still apply, but the absence of throttling valves and a 50% longer stroke 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 stroke 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 which pushes back the flame front. With the conventional one or two-spark-plug system and homogenous mixture, this squeeze stream prevents the flame from propagating to the combustion chamber's trailing side in the mid and high engine speed ranges. Kawasaki dealt with that problem in its US patent US 3848574 , and Toyota obtained a 7% economy improvement by placing a glow-plug in the leading site, and using Reed-Valves in intake ducts. This poor combustion in the trailing side of chamber is one of the reasons why there is more carbon monoxide and unburnt hydrocarbons in a Wankel's exhaust stream. A side-port exhaust, as is used in the Mazda Renesis, avoids one of the causes of this because the unburned mixture cannot escape. The Mazda 26B avoided this problem through the use of a three spark-plug ignition system. (At the 24 Hours of Le Mans endurance race in 1991, the 26B had significantly lower fuel consumption than the competing reciprocating piston engines. All competitors had the same amount of fuel available due to the Le Mans limited fuel quantity rule.)

A peripheral intake port gives the highest mean effective pressure, however, side intake porting produces a more steady idle, because it helps to prevent blow-back of burned gases into the intake ducts which cause "misfirings", caused by alternating cycles where the mixture ignites and fails to ignite. Peripheral porting (PP) gives the best mean effective pressure throughout the rpm range, but PP was linked also to worse idle stability and part-load performance. Early work by Toyota led to the addition of a fresh air supply to the exhaust port, and proved also that a Reed-valve in the intake port or ducts 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, who proposed that earlier opening of the intake port before top dead center (TDC), and longer intake ducts, improved 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 has been specific study of the effect of intake and exhaust piping configuration on the performance of Wankel engines.

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.

Applications

Automobile racing

Mazda 787B

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

The Sigma MC74 powered by a Mazda 12A engine was the first engine and only team from outside Western Europe or the United States to finish the entire 24 hours of the 24 Hours of Le Mans race, in 1974. Yojiro Terada was the driver of the MC74. Mazda is the only team from outside Western Europe or the United States to have won Le Mans outright and the only non-piston engine ever to win Le Mans, which the company accomplished in 1991 with their four-rotor 787B (2.622 L or 160 cu in—actual displacement, rated by FIA formula at 4.708 L or 287 cu in).

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

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

In engines having more than two rotors, or two rotor race engines intended for high-rpm use, a multi-piece eccentric shaft may be used, allowing additional bearings between rotors. While this approach does increase the complexity of the eccentric shaft design, it has been used successfully in the 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, Typ DB M950 KE409 is made in one piece. Mercedes-Benz used split bearings.

Motorcycle engines

Norton Interpol2 prototype

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.

In 1972, Yamaha introduced the RZ201 at the Tokyo Motor Show, 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.

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

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.

Dutch motorcycle importer and manufacturer Van Veen produced small quantities of their dual-rotor Wankel-engined OCR-1000 between 1978 and 1980, using surplus Comotor engines.

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

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

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

Wankel engines run very hot on the ignition and exhaust side of the engine's trochoid chamber, whereas the intake and compression parts are cooler. Suzuki opted for a complicated oil-cooling and water cooling system, with Garside reasoning that provided the power did not exceed 80 hp (60 kW), air-cooling would suffice. Garside cooled the interior of the rotors with filtered ram-air. This very hot air was cooled in a plenum contained within the semi-monocoque frame and afterwards, once mixed with fuel, fed into the engine. This air was quite oily after running through the interior of the rotors, and thus was used to lubricate the rotor tips. The exhaust pipes become very hot, with Suzuki opting for a finned exhaust manifold, twin-skinned exhausted pipes with cooling grilles, heatproof pipe wrappings and silencers with heat shields. Garside simply tucked the pipes out of harm's way under the engine, where heat would dissipate in the breeze of the vehicle's forward motion. Suzuki opted for complicated multi-stage carburation, whilst Garside choose simple carburetors. Suzuki had three lube systems, whilst Garside had a single total-loss oil injection system which was fed to both the main bearings and the intake manifolds. Suzuki chose a single rotor that was fairly smooth, however with rough patches at 4,000 rpm; Garside opted for a turbine-smooth twin-rotor motor. Suzuki mounted the massive rotor high in the frame, but Garside put his rotors as low as possible to lower the center of gravity of the motorcycle.

Although it was said to handle well, The result was that the Suzuki was heavy, overcomplicated, expensive to manufacture, and (at 62 bhp) a little short on power. Garside's design was simpler, smoother, lighter and, at 80 hp (60 kW), significantly more powerful.

Aircraft engines

Diamond DA20 with Diamond Engines Wankel

Sikorsky Cypher Unmanned aerial vehicle (UAV) powered with a UEL AR801 Wankel engine

ARV Super2 with the British MidWest AE110 twin-rotor Wankel engine

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

1. Rotors cannot seize, since rotor casings expand more than rotors;
2. The engine is less prone to the serious condition known as "engine-knock", which can destroy the plane's 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 some cars and motorcycles, a Wankel aero-engine will be sufficiently warm before full power is asked of it because of the time taken for pre-flight checks. A Wankel aero-engine spends most of its operational time at high power outputs, with little idling. This makes ideal the use of peripheral ports. An advantage is that modular engines with more than two rotors are feasible. If icing of any intake tracts is an issue, there is plenty of waste engine heat available to prevent icing.

The first Wankel rotary-engine aircraft was the experimental Lockheed Q-Star civilian version of the United States Army's reconnaissance QT-2, basically a powered Schweizer sailplane, in the late 1960s. The plane was powered by a 185 hp (138 kW) Curtiss-Wright RC2-60 Wankel rotary engine. The same engine model was also used in a Cessna Cardinal and a helicopter, as well as other airplanes. In Germany in the mid-1970s, a pusher ducted fan airplane powered by a modified NSU multi-rotor Wankel 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 Wankel engines, model aircraft-sized versions were pioneered by a combine of the well-known Japanese O.S. Engines firm and the then-extant German Graupner aeromodeling 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 differing versions from 1970 to the present day, solely by the O.S. firm after Graupner's demise in 2012.

Aircraft Wankel engines are increasingly being found in roles where the compact size, high power-to-weight ratio and quiet operation are important, notably in drones and unmanned aerial vehicles. Many companies and hobbyists adapt Mazda rotary engines, taken from cars, to aircraft use. Others, including Wankel GmbH itself, manufacture Wankel rotary engines dedicated for that purpose. One such use is the "Rotapower" engines in the Moller Skycar M400. Another example of purpose-built aircraft rotaries are Austro Engine's 55 hp (41 kW) AE50R (certified) and 75 hp (56 kW) AE75R (under development) both appr. 2 hp/kg.

Wankel engines are also becoming increasingly popular in homebuilt experimental aircraft, such as the ARV Super2 which can be re-engined with the British MidWest AE series aero-engine. Most are Mazda 12A and 13B automobile engines, converted to 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 engines. These conversions first took place in the early 1970s. With a number of these engines mounted on aircraft, as of 10 December 2006 the National Transportation Safety Board has only seven reports of incidents involving aircraft with Mazda engines, and none of these were a failure due to design or manufacturing flaws.

Peter Garrison, contributing editor for Flying magazine, has said that "in my opinion ... the most promising engine for aviation use is the Mazda rotary." 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, G-300 rotary engine development ceased, with the company citing cash flow problems.

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

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

The French company Citroën had developed Wankel powered RE-2 helicopter in the 1970s.

Since Wankel engines operate at a relatively high rotational speed, at 6,000 rpm of output shaft, the Rotor makes only 2,000 turns. With relatively low torque, propeller driven aircraft must use a propeller speed reduction unit to maintain propellers within the designed speed range. Experimental aircraft with Wankel engines use propeller speed reduction units, for instance the MidWest twin-rotor engine has a 2.95:1 reduction gearbox. The rotational shaft speed of a Wankel 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.

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

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

In 2013 e-Go aeroplanes, based in Cambridge, United Kingdom, announced that its new single-seater canard aircraft, the winner of a design competition to meet the new UK single-seat deregulated category, will be powered by a Wankel engine from Rotron Power, a specialist manufacturer of advanced rotary engines for unmanned aeronautical vehicle (UAV) applications. The first sale was 2016. The aircraft is expected to deliver 100 knots (190 km/h; 120 mph) cruise speed from a 30 hp (22 kW) Wankel engine, with a fuel economy of 75 mpg_‑imp (3.8 L/100 km; 62 mpg_‑US) using standard motor gasoline (MOGAS), developing 22 kW (30 hp).

The DA36 E-Star, an aircraft designed by Siemens, Diamond Aircraft and EADS, employs a series hybrid powertrain with the propeller being turned by a Siemens 70 kW (94 hp) electric motor. The aim is to reduce fuel consumption and emissions by up to 25 percent. An onboard 40 hp (30 kW) Austro Engines Wankel rotary engine and generator provides the electricity. A propeller speed reduction unit is eliminated. The electric motor uses electricity stored in batteries, with the generator engine off, to take off and climb reducing sound emissions. The series-hybrid powertrain using the Wankel engine reduces the weight of the plane by 100 kg compared with its predecessor. The DA36 E-Star first flew in June 2013, making this the first ever flight of a series-hybrid powertrain. Diamond Aircraft state that the technology using Wankel engines is scalable to a 100-seat aircraft.

Range extender

Structure of a series-hybrid vehicle. The grey square represents a differential gear. An alternative arrangement (not shown) is to have electric motors at two or four wheels.

Mazda2 EV prototype

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

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

Mazda of Japan ceased production of direct drive Wankel engines within their model range in 2012, leaving the motor industry worldwide with no production cars using the engine. The company is continuing development of the next generation of their Wankel engines, the SkyActiv-R, with a new rear-wheel-drive sports car model announced in October 2015. The company later withdrew this proposed model. Mazda states that the SkyActiv-R solves the three key issues with previous rotary engines: fuel economy, emissions and reliability. Takashi Yamanouchi, the global CEO of Mazda said: "The rotary engine has very good dynamic performance, but it's not so good on economy when you accelerate and decelerate. However, with a range extender you can use a rotary engine at a constant 2,000rpm, at its most efficient. It's compact, too." No Wankel engine in this arrangement has yet been used in production vehicles or planes. However, in November 2013 Mazda announced to the motoring press a series-hybrid prototype car, the Mazda2 EV, using a Wankel engine as a range extender. The engine, located under the rear luggage floor, is a tiny, almost inaudible, single-rotor 330cc unit, generating 30 hp (22 kW) at 4,500rpm, and maintaining a continuous electric output of 20 kW. In October 2017 Mazda announced that the rotary engine would be utilised in a hybrid car with 2019 the introduction date.

Other uses

UEL UAV-741 Wankel engine for a UAV

Small Wankel engines are being found increasingly in other applications, such as go-karts, personal water craft, and auxiliary power units for aircraft. Kawasaki patented mixture-cooled rotary engine (US patent 3991722). Japanese diesel engine manufacturer Yanmar and Dolmar-Sachs of Germany had a rotary-engined chain saw (SAE paper 760642) and outboard boat engines, and the French Outils Wolf, made lawnmower (Rotondor) powered by a Wankel rotary engine. To save on production costs, the rotor was in a horizontal position and there were no seals in the down side. The Graupner/O.S. 49-PI is a 1.27 hp (950 W) 5 cc Wankel engine for model airplane use, which has been in production essentially unchanged since 1970. Even with a large muffler, the entire package weighs only 380 grams (13 oz).

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

Ingersoll-Rand built the largest-ever Wankel engine, with two rotors, which was available between 1975 and 1985, producing 1,100 hp (820 kW). A one rotor version was available producing 550 hp (410 kW). The displacement per rotor was 41 liters, with each rotor being approximately one meter in diameter. The engine was derived from a previous, unsuccessful Curtiss-Wright design, which failed because of a well-known problem with all internal combustion engines: the fixed speed at which the flame front travels limits the distance combustion can travel from the point of ignition in a given time, thereby limiting the maximum size of the cylinder or rotor chamber which can be used. This problem was solved by limiting the engine speed to only 1200 rpm and the use of natural gas as fuel. That was particularly well chosen, since one of the major uses of the engine was to drive compressors on natural gas pipelines.

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

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

In the early 1970s Outboard Marine Corporation sold snowmobiles under the Johnson and other brands, which were powered by 35 or 45 hp (26 or 34 kW) OMC engines.

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

The American M1A3 Abrams tank will use an auxiliary rotary-engined power unit, 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 high octane military grade jet fuel.

Non-internal combustion

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

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

See also

• General Motors Rotary Combustion Engine
• Gunderson Do-All Machine
• Jonova engine
• Mazda RX-8 Hydrogen RE
• Mazda Wankel engine
• Mercedes-Benz M950F
• Mercedes-Benz C111
• Pistonless rotary engine
• Quasiturbine
• RKM engine

References

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External links

• U.S. patent 2,988,008
• How Wankel Engines Work, How Stuff Works, retrieved 2012-08-14
• Wankel Engine, Animated Engines, Keveney
• How and why an engine must rev smoothly, UK: Citroën Net, retrieved 2012-08-14
• Aaron Cake's page on Wankel engine, CA
• J I Martin-Artajo SI Rotary engine mockup, ES
• J C Lefeuvre's Moto-Turbine-Radiale rotary engine installed in a motorcycle, FR
• Scott, David (March 1960). "Auto Engine Without Pistons". Popular Science: 82. Retrieved August 2, 2017.
• Norbye, Jan P. (January 1967). "Rivals to the Wankel: A Roundup of Rotary Engines". Popular Science: 80. Retrieved August 2, 2017. 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
• [1] Article on Wankel Motorcycle history.