Wankel engine
From Wikipedia, the free encyclopedia
The Wankel rotary engine is a type of internal combustion engine, invented by German engineer Felix Wankel, which uses a rotor instead of reciprocating pistons. This design delivers smooth high-rpm power from a compact, lightweight engine.
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[edit] Naming
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Since its introduction in the NSU Motorenwerke AG (NSU) and Mazda cars of the 1960s, the engine has been commonly referred to as the rotary engine, a name which has also been applied to several completely different engine designs.
[edit] How it works
In the Wankel engine, the four strokes of a typical Otto cycle occur in the space between a rotor, which is roughly triangular, and the inside of a housing. In the basic single-rotor Wankel engine, the oval-like epitrochoid-shaped housing surrounds a three-sided rotor (similar to a Reuleaux triangle, a three-pointed curve of constant width, but with the middle of each side a bit more flattened). The central drive shaft, also called an eccentric shaft or E-shaft, passes through the center of the rotor and is supported by bearings. The rotor both rotates around an offset lobe (crank) on the E-shaft and makes orbital revolutions around the central shaft. Seals at the corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers. Fixed gears mounted on each side of the housing engage with ring gears attached to the rotor to ensure the proper orientation as the rotor moves.
As the rotor rotates and orbitally revolves, each side of the rotor gets closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the strokes of a piston in a reciprocating engine. The power vector of the combustion stage goes through the center of the offset lobe.
While a four-stroke piston engine makes 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 each driveshaft rotation, i.e. one power stroke per rotor orbital revolution and three power strokes per rotor rotation. Thus, 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 also generally have a much higher redline than a reciprocating engine of similar size since the strokes are completed with a rotary motion as opposed to a reciprocating engine which must use connecting rods and a crankshaft to convert reciprocating motion into rotary motion.
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 engine of 1.5 to 2 times the displacement; some racing regulatory agencies view it as offering such a pronounced advantage that they ban it altogether.[citation needed] After Mazda won the 24 hours of Le Mans in 1991 with their 787B car powered by a 4-rotor Wankel engine, the FIA (Federation Internationale de l'Automobile) decided to ban rotary-engine cars from racing in that league.[citation needed]
[edit] Advantages
Wankel engines have several major advantages over reciprocating piston designs, in addition to having higher output for similar displacement and physical size. Wankel engines are considerably simpler and contain far fewer moving parts. For instance, because valving is accomplished by simple ports cut into the walls of the rotor housing, they have no valves or complex valve trains; in addition, since the rotor is geared directly to the output shaft, there is no need for connecting rods, a conventional crankshaft, crankshaft balance weights, etc. The elimination of these parts not only makes a Wankel engine much lighter (typically half that of a conventional engine with equivalent power), but it also completely eliminates the reciprocating mass of a piston engine with its internal strain and inherent vibration due to repeated acceleration and deceleration, producing not only a smoother flow of power but also the ability to produce more power by running at higher rpm.
In addition to the enhanced reliability due to the elimination of this reciprocating strain on internal parts, the engine is constructed with an iron rotor within a housing made of aluminum, which has greater thermal expansion. This ensures that even a severely overheated Wankel engine cannot seize, as would likely occur in an overheated piston engine. This is a substantial safety benefit in aircraft use.
A further advantage of the Wankel engine for use in aircraft is the fact that a Wankel engine can have a smaller frontal area than a piston engine of equivalent power.
The simplicity of design and smaller size of the Wankel engine also allows for savings in construction costs, compared to piston engines of comparable power output.
50% longer stroke duration as a four stroke engine, therfore more time to complete the combustion.
No empty stroke.
As another advantage, the shape of the Wankel combustion chamber and the turbulence induced by the moving rotor prevent localized hot spots from forming, thereby allowing the use of fuel of very low octane number or very low ignition power requirement without preignition or detonation, a particular advantage for hydrogen cars. Mazda has recently placed a hydrogen-burning rotary engine in one version of its RX-8 sports car and Mazda 5.
[edit] Disadvantages
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Short intake time for portinjection, only 270° against 720° for a reciprocating four stroke engine and 360° for a reciprocating two stroke engine. For direct injection has the wankel engine the advantage of 270° stroke duration, a reciprocating four stroke engine has only a stroke duration of 180°. 50% longer stroke duration as four stroke engine, therfore pressure loss at low revolutions may be higher. Oil consumption, but significantly lower as a reciprocating two stroke engine. High HC-values with periport outlet, can avoided with longstroke, Fuel Stratified Injection or side exhaust ports. Also, in terms of fuel economy, they have been nowhere near as efficient as a traditional engine of similar output; while some may argue that the NSU Wankel Spider disproves this, most experts would agree that the 27mpg rating of that car was due largely to its incredibly light weight. The NSU Marine Ro135 has a specific fuel consumption of 205gPsh in the year 1968. 1991 won Mazda with the 787B with 26B wankel the 24h of Le Mans. The compentioner reciprocating engines from Ford, Mercedes Benz, Jaguar, Porsche... were thirstier.
[edit] Engineering
Felix Wankel managed to overcome most of the problems that made previous rotary Otto cycle engines fail by developing a configuration with vane seals that could be made of more durable materials than piston ring metal that led to the failure of previous rotary designs.
Rotary engines have a thermodynamic problem not found in reciprocating four stroke engines in that their "cylinder block" operates at steady state, with intake, compression, combustion, and exhaust occurring at fixed housing locations for all "cylinders". In contrast, reciprocating engines performs 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. Freezing temperatures from evaporating fuel prevail at the intake while ignition reaches temperatures of about 2300 kelvins [1], a range that is wider than lubricants and most engine materials can withstand. Cooling, the boundary layer and the quench Zone prevent the oil film in a Wankel rotary engine for overheating. [2], [3] The empty stroke lowers the efficiency of a reciprocating four stroke engine, therefore the most effective reciprocating engine is a two stroke Diesel. Four stroke reciprocating engines are less suitable for hydrogen. The hydrogen can misfire on hot parts like exaust valve and spark plugs. Another big problem represents the hydrogenate attack on the lubricating film in reciprocating engines. This problem solved with ceramic apex seal against a ceramic surface at a wankel engine. No oil film, therfore no hydrogenate attack. Since a piston ring out ceramic is not possible, this solution method remains locked for the reciprocating engine. The piston shell must be lubricated and cooled with oil. This increases the lubricating oil consumption in a four stroke engine substantially.
[edit] Materials
Unlike a piston engine, where the cylinder is cooled by the incoming charge after being heated by combustion, Wankel rotor housings are constantly heated on one side and cooled at the other, leading to very high local temperatures and unequal thermal expansion. While this places high demands on the materials used, the simplicity of the Wankel makes it easier to experiment with alternative materials, e.g. exotic alloys and ceramics. We used water in a radial or axial cooling system, with the hot water from the hot bow we heated the cold bow. Therefore the thermal expansion remains tolerable.
[edit] 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. (This can be prevented in older Mazda engines by always allowing the engine to reach operating temperature). It was common for very early Mazda engines to require rebuilding after 50,000 miles. Modern Wankel engines have not had these problems for many years.
[edit] Fuel consumption and hydrocarbon emissions
Just as the shape of the Wankel combustion chamber prevents preignition, it also leads to incomplete combustion of the air-fuel charge, with the remaining unburned hydrocarbons released into the exhaust. While manufacturers of piston-engine cars were turning to expensive catalytic converters to completely oxidize the unburned hydrocarbons, Mazda was able to avoid this cost by enriching the air/fuel mixture and increasing the amount of unburned hydrocarbons in the exhaust to actually support complete combustion in a 'thermal reactor' (an enlarged open chamber in the exhaust manifold) without the need for a catalytic converter, thereby producing a clean exhaust at the cost of some extra fuel consumption.
Unfortunately for Mazda, their choice to use extra fuel to combust excess hydrocarbons increased fuel consumption just as world gasoline prices rose sharply. This resulted in drop in sales and another blow to the reputation of Mazda and the rotary engine.
In Mazda's RX-8 with the Renesis engine, fuel consumption is now within normal limits while passing California State emissions requirements. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing exhaust port area. The Renesis engine even meets California's Low Emissions Vehicle or LEV standards.
[edit] History
In 1951, Wankel began development of the engine at NSU (NSU Motorenwerke AG), where he first conceived his rotary engine in 1954 (DKM 54, Drehkolbenmotor) and later the KKM 57 (the Wankel rotary engine, Kreiskolbenmotor) in the year 1957. The first working prototype DKM 54 was running on February 1, 1957 at the NSU research and development department Versuchsabteilung TX.[1]
Considerable effort went into designing rotary engines in the 1950s and 1960s. They were of particular interest because they were smooth and very quiet running, and because of the reliability resulting from their simplicity.
In the United States, in 1959 under license from NSU, Curtiss Wright Corp. pioneered minor improvements in the basic engine design. Curtis-Wright takes 50% of the license income made in the USA and limited its development activity to a minimum.
In Britain, in the 1960s, Rolls Royce Motor Car Division at Crewe, Cheshire, pioneered a two-stage Diesel version of the Wankel engine.
Also in Britain Norton Motorcycles developed a Wankel rotary engine for motorcycles, which was included in their Commander and F1; Suzuki also produced a production motorcycle with a Wankel engine, the RE-5. In 1971 and 1972 Arctic Cat produced snowmobiles powered by 303cc Wankel rotary engines manufactured by Sachs in Germany. John Deere Inc, in the US, had a major research effort in rotary engines and designed a version which was capable of using a variety of fuels without changing the engine. The design was proposed as the power source for several US Marine combat vehicles in the late 1980s.
After occasional use in automobiles, for instance by NSU with their Ro 80 model, Citroën with the M35 and GS Birotor using engines produced by Comotor, and abortive attempts by General Motors and Mercedes-Benz to design Wankel-engine automobiles, the most extensive automotive use of the Wankel engine has been by the Japanese company Mazda.
After years of development, Mazda's first Wankel engined car was the 1967 Mazda Cosmo. 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 just before a sharp rise in fuel prices. Mazda later abandoned the Wankel in most of their automotive designs, but continued using it in their RX-7 sports car until August of 2002 (RX-7 importation for North America ceased with the 1995 model year). The company normally used two-rotor designs, but received considerable attention with their 1991 Eunos Cosmo, which used a twin-turbo three-rotor engine. In 2003, Mazda introduced the RENESIS engine with the new RX-8. The RENESIS engine relocated the ports for exhaust and intake from the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and further power gains. The RENESIS is capable of delivering 250 hp from its minute 1.3 L displacement at better fuel economy, reliability, and environmental friendliness than any other Mazda rotary engine in history.
Although VAZ, the Soviet automobile manufacturer, is known to have produced Wankel-engine automobiles, and Aviadvigatel, the Soviet aircraft engine design bureau, is known to have produced Wankel engines for aircraft and helicopters, little specific information has surfaced in the outside world; what has been seen indicates a general similarity to Wankel designs by NSU, Comotor, and Mazda, therefore it is likely that many Western patents were infringed[citation needed] upon by these designs, the probable reason for their being hidden.
The People's Republic of China is also known to have experimented with Wankel engines, but even less is known in the West about the work done there, other than one paper, #880628, delivered to the SAE in 1988 by Chen Teluan of the South China Institute of Technology at Guangzhou.
Although many manufacturers licensed the design, and Mercedes-Benz used it for their C111 concept car, only Mazda has produced Wankel engines in large numbers.
[edit] Automobile racing
In the racing world, Mazda has had substantial success with two-rotor, three-rotor, and four-rotor cars, and 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 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. 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 (2622 cc actual displacement, rated by FIA formula at 4708 cc). The following year, rules were changed at Le Mans which made the Mazda 787 ineligible to race. Mazda is also the most reliable finisher at Le Mans (with the exception of Honda, who has entered only three cars in only one year), with 67% of entries finishing.
The Mazda RX-7 has won more IMSA races in its class than any other model of automobile, with its one hundredth victory on September 2, 1990. Following that, the RX-7 won its class in the IMSA 24 Hours of Daytona race ten years in a row, starting in 1982. The RX7 won the IMSA Grand Touring Under Two Liter (GTU) championship each year from 1980 through 1987, inclusive.
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.[4]
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.)
[edit] Aircraft engines
The first 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 1968 or 1969. It was powered by a 185 hp (138 kW) Curtiss-Wright RC2-60 Wankel rotary engine.
Aircraft Wankels have made something of a comeback in recent years. None of their advantages have been lost in comparison to other engines. They are increasingly being found in roles where their compact size and quiet operation is important, notably in drones, or UAVs. Many companies and hobbyists adapt Mazda rotary engines (taken from automobiles) to aircraft use; others, including Wankel GmbH itself, manufacture Wankel rotary engines dedicated for the purpose.
Wankel engines are also becoming increasingly popular in homebuilt experimental aircraft, due to a number of factors. 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 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 is of a failure due to design or manufacturing flaws. During the same period they have issued several thousand reports of broken crankshafts and connecting rods, failed pistons and incidents caused by other components which are not found in the Wankel engines. Rotary engine enthusiasts derisively refer to piston aircraft engines as "reciprosaurs," and point out that their designs are essentially unchanged since the 1930s, with only minor differences in manufacturing processes and variation in engine displacement.
Peter Garrison, Contributing Editor for FLYING Magazine, has said that "the most promising engine for aviation use is the Mazda rotary." Mazdas have indeed worked well when converted for use in homebuilt aircraft. However, the real challenge in aviation is producing FAA-certified alternatives to the standard reciprocating engines that power most small general aviation aircraft. Mistral Engines, based in Switzerland, is busy certifying its purpose-built rotaries for factory and retro-fit installations on certified production aircraft. With the G-190 and G-230-TS rotary engines already flying in the experimental market, Mistral Engines hopes for FAA and JAA certification in 2007 or early 2008. 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.
Mistral points out that the Wankel rotary is an engine that has very few moving parts, making it more dependable. In addition it has a much better power-to-weight ratio and is smaller, thus enabling more efficient engine cowl design. Finally, the engine runs with a smoothness more akin to turbine engines than gas powered "recips", thus reducing airframe vibration and occupant fatigue.
Since Wankel engines operate at a relatively high rotational speed with relatively low torque, propeller aircraft must use a Propeller Speed Reduction Unit (PSRU) to keep conventional propellers within the proper speed range. There are many experimental aircraft flying with this arrangement.
[edit] Other uses
Small Wankel engines are being found increasingly in other roles, such as go-karts, personal water craft and auxiliary power units for aircraft. The Graupner/O.S. 49-PI is a 1.27 hp (947 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 (13.4 ounces).
The simplicity of the Wankel makes it ideal for mini, micro, and micro-mini engine designs. The MicroElectroMechanical Systems (MEMS) Rotary Engine Lab at the University of California, Berkeley has been developing 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 is to eventually develop an internal combustion engine that will deliver 100 milliwatts of electrical power; the engine itself will serve as the rotor of the generator, with magnets built into the engine rotor itself.
The largest Wankel engine was built by Ingersoll-Rand; available in 550 hp (410 kW) one rotor and 1100 hp (820 kW) two rotor versions, displacing 41 liters per rotor with a rotor approximately one meter in diameter, it was available between 1975 and 1985. It 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, and thereby 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 use of natural gas as fuel; this was particularly well chosen, as one of the major uses of the engine was to drive compressors on natural gas pipelines.
From 1974 to 1977 Hercules produced a limited number of motorcycles powered by Wankel engines. The tooling was later used by Norton to produce the Norton Commander model in the early 1980s. The best-known example of a Wankel-powered motorcycle, however, was the Suzuki RE5, produced in 1975 and 1976. This 500cc (actual) displacement motorcycle could have been a great touring bike except for the poor fuel mileage of 32-36 mpg. Examples are still frequently found on online auction sites.
Aside from being used for internal combustion engines, the basic Wankel design has also been utilized for air 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.
Perhaps the most exotic use of the Wankel design is in the seat belt pretensioner system of some Mercedes-Benz cars[2] and the Volkswagen New Beetle. In these cars, when deceleration sensors sense 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 any collision.
[edit] Trivia
- Wankel Rotary Engine is a phrase included in a Monty Python audio sketch, titled "Are You Embarrassed Easily?" on Monty Python's Previous Record (1972) and on Monty Python's The Final Rip Off . It is here considered to sound rude due to the word "Wankel," which sounds obscene but is actually the name of a person.
- When it was designed in 1974, the American Motors Pacer was originally designed to incorporate a Wankel engine designed by GM, but this idea was scrapped before they hit the showrooms in 1975. Part of the demise of this feature was the rising fuel crisis and concerns about emission legislation in the United States. The Wankel engine did not comply with emission levels, so in 1974 GM canceled its development. When the idea was abandoned, it left AMC struggling to find an alternative. The design was reconfigured to house the AMC inline six-cylinder engine.
[edit] References
- ^ http://www.spiegel.de/auto/aktuell/0,1518,459789,00.html
- ^ Mercedes-Benz. Occupant Safety Systems 11-12. Retrieved on February 1, 2007.
- Yamaguchi, Jack K. (2003). The Mazda RX-8: World's First 4-door, 4-seat Sports Car Plus Complete Histories of Mazda Rotary Engine development and Rotary Racing Around the World. Mazda Motor Corporation. ISBN 4-947659-02-5.
- Yamaguchi, Jack K. (1985). The New Mazda RX-7 and Mazda Rotary Engine Sports Cars. St. Martin's Press, New York. ISBN 0-312-69456-3.
- Jan P. Norbye (1973). "Watch out for Mazda!". Automobile Quarterly XI.1: 50-61.
- Theory, Design, and Principles of Operation. Monito.com. Retrieved on February 24, 2005.
- Compendium of Production and Experimental Wankel Engine Data. Monito.com. Retrieved on February 24, 2005.
[edit] See also
[edit] External links
- U.S. Patent 2988008
- Rebuilding Rotary Engines Videos - Informational Videos on rebuilding a rotary engine
- The Mazda Wankel Rotary Engines For Aircraft Website
- Rotary Engine Illustrated - Animations of Engines, Evolution of Engine Parts, Pieces of Engine Pieces.
- New York Rotary Association - New Yorks Biggest Rotary Engine Auto Club (NYRA)
- Wankel Kart Engine in Action on Video
- How Wankel Engines Work
- Wankel Rotary Combustion Engines (WRCE)
- Animated Engines: Wankel Engine
- Wankel's biography and engine
- How and why an engine must rev smoothly
- UAV Engines Lichfield UK
- Wankel Supertec
- Rotary-engined Suzuki RE-5 motorcycle
- Rotary-engined Norton F1 motorcycle
- Freedom-Motors - Fuel Efficient, low emissions Wankel rotary engines
- EngineAir.com - A related wankel engine design powered by compressed air
- Wankel rotary engine
| Piston engine configurations | |
|---|---|
| Straight | Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14 |
| V | 2, 4, 5, 6, 8, 10, 12, 16, 20, 24 |
| Flat | 2, 4, 6, 8, 10, 12, 16 |
| W | 8, 9, 12, 16, 18 |
| Other inline | H, VR, Opposed, U (Square), X |
| Other | Radial, Rotary, Pistonless (Wankel) |
