InTRODUCTION The Rotary engine is a type of internal internal combustion combustion engine which uses a rotor to convert pressure into a rotating motion instead of using reciprocating pistons. pistons . Its four-stroke four-stroke cycle is generally generated in a space between the inside of an oval-like epitrochoid-shaped epitrochoid -shaped housing and a roug roughl hly y trian triangu gular lar rotor rotor ( hypotrochoid). hypotrochoid ). This This desig design n deliv deliver ers s smoo smooth th high-rpm high- rpm power from a compact, lightweight engine. The engine was invented by German engineer Felix Wankel. Wankel . He began its development in the early 1950s at NSU Motorenwerke AG (NSU) befo before re comp comple leti ting ng a wo work rkin ing, g, runn runnin ing g prot protot otyp ype e in 1957 1957.. NSU NSU then then subsequently licensed the concept to other companies across the globe, who added more efforts and improvements in the 1950s and 1960s. Because of its compact, lightweight design, Wankel rotary engines have been installed in a variety of vehicles and devices such as automobiles and racing cars, cars , aircraft, aircraft , go-karts, go-karts , personal water crafts and auxiliary power units. units . In a pist piston on engi engine ne,, the the same same volu volume me of spac space e (the (the cylind cylinder) er) altern alternat ately ely does does four four diffe differen rentt jobs jobs -- intak intake, e, compre compress ssio ion, n, combustion and exhaust. A rotary engine does these same four jobs, but each one happens in its own part of the housing. It's kind of like having a dedica dedicated ted cylin cylinde derr for for each each of the the four four jobs jobs,, with with the the pisto piston n movin moving g continually from one to the next. The rotary engine (originally conceived and developed by Dr. Felix Wankel) is sometimes called a Wankel engine.
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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 1957. The first working prototype DKM 54 was running on Febr Februa uary ry 1 , 1957 at the the NSU rese resear arch ch and and devel evelop opme ment nt depa depart rtme ment nt Versuchsabteilung TX . Considerable effort went into designing rotary engines in the 1950s and 1960s. They were of particular interest because they were smooth and quiet running, and because of the reliability resulting from their simplicity. In the United States, in 1959 under license from NSU, Curtiss-Wright pioneer pioneered ed minor minor improve improvemen ments ts in the basic engine engine design. In Britain, Britain, in the 1960s, Rolls Royce Motor Car Division at Crewe, Cheshire, pioneered a twostage diesel version of the Wankel engine Also in Britain Norton Motorcycles developed a Wankel rotary engine for motorcycles, motorcycles , which was included in their Commander and F1; Suzuki also made a production motorcycle with a Wankel engine, the RE-5. In 1971 and 1972 Arctic Arctic Cat produce produced d snowmob snowmobiles iles powered powered by 303 cc Wankel Wankel rotary rotary engines engines manufactured by Sachs in Germany. John Deere Inc, in the U.S., designed a version that was capable of using a variety of fuels. The design was proposed as the power source for several U.S. Marine combat vehicles in the late 1980s. After occasional use in automobiles, for instance by NSU with their Ro their Ro 80 model, Citroën with the M35, M35, and GS Birotor using engines produced by Co motor , as well as abortive attempts by General Motors and Mercedes-Benz to design Wankel-engine automobiles, automobiles , the most extensive automotive use of the Wankel engine has been by the Japanese company Mazda. Mazda. After years of development, development, Mazda's Mazda' s first Wankel engined car was the 1967 Mazda Mazda Cosmo Cosmo.. The The comp compan any y norm normal ally ly used used twotwo-ro roto torr desi design gns, s, but but received considerable attention with their 1991 Eunos Cosmo, Cosmo , which used a twinturbo three-rotor engine. In 2003, Mazda introduced the RENESIS engine with the new RX-8. 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 ports,, bette betterr airfl airflow ow,, and and furthe furtherr powe powerr gains gains.. The The RENE RENESI SIS S is capab capable le of delivering 238 horsepower (177 kW) from its 1.3 L displacement with better fuel economy, economy, reliability, reliability, and environmental environmental friendliness friendliness than any other Mazda rotary engine in history.
Naming: 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. designs .
The Parts of a Rotary Engine: A rotary engine has an ignition system and a fuel-delivery system that are similar to the ones on piston engines. If you've never seen the inside of a rotary engine, be prepared for a surprise, because you won't recognize much. 1.
Rotor:
The rotor has three convex faces, each of which acts like a piston. Each ach face of the rot rotor has a pocket in it, which increases the dis displac place ement ent of the the engin gine, allowing allowing more space space for air/fue air/fuell mixture. At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamb chamber er.. There There are are also also metal metal ring rings s on each each side side of the the roto rotor r that seal to the sides of the combustion chamber.
[Rotor]
The rotor has a set of internal gear teeth cut into the center of one side. These teeth mate with a gear that is fixed to the housing. This gear mating determines the path and direction the rotor takes through the housing. 2.
Housing: The housing is roughly oval in shape (it's actually an epitrochoid).The epitrochoid ).The shape of the combustion chamber is designed so that the three tips of the rotor will always stay in contact with the wall of the chamber, forming three sealed volumes of gas. Each part of the housing is dedicated to one part of the combustion process.
The four sections are: • • • •
Intake Compression Combustion Exhaust
The The inta intake ke and and exha exhaus ustt port ports s are are located in the housing. There are no valves in these ports. The exhaust port connects directly to the exhaust, and the intake port connects directly to the throttle. 3.
[Housing]
Shaft: Shaft: The output shaft has round lobes mounted eccentrically, meaning that they are offset from the centerline of the shaft. Each rotor fits over one of these lobes. The lobe acts sort of like the crankshaft crankshaft in a piston engine. engine. As the rotor follows its path around the housing, it pushes on the lobes. Since the lobes are mounted eccentric to the output shaft, the force that the rotor applies to the lobes creates torque in the shaft, causing it to spin.
[Output Shaft; Note the eccentric lobes.] Now let's take a look at how these parts are assembled and how it produces power.
Principles of a Rotary Engine: Like a piston engine, the rotary engine uses the pressure created when a combi combinat nation ion of air and fuel fuel is burne burned. d. In a pisto piston n engin engine, e, that that press pressure ure is contai contained ned in the cylin cylinde ders rs and and forces forces pisto pistons ns to move move back back and forth. forth. The The connecting rods and crankshaft convert the reciprocating motion of the pistons into rotational motion that can be used to power a car. The rotor follows a path that looks like something you'd create with a Spiro graph. graph . This path keeps each of the three peaks of the rotor in contact with the housing, creating three separate volumes of gas. As the rotor moves around the chamber, each of the three volumes of gas alternately expands and contracts. It is this this expa expansi nsion on and and contr contrac actio tion n that that draws draws air and and fuel fuel into into the the engin engine, e,
comp compre ress sses es it and make makes s then expels the exhaust.
usef useful ul
power as
the
gases
expand,
and
How How it i t wor works ks: The "A" marks one of the three apexes of the rotor. The "B" marks the eccentric shaft and the white portion is the lobe of the eccentric shaft. The shaft turns three times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft. 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 housing. In the basic single-rotor Wankel engine, the oval-like epitrochoid-sh epitrochoid -shaped aped housing housing surroun surrounds ds a three-si three-sided ded rotor (similar to a Reuleaux triangle, triangle , a three-pointed curve of constant width, 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 shaf t. shaf t. Seals at the corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers. chambers . Fixed Fixed gear gears s mount mounted ed on each each side side of the hous housing ing engage engage with with ring ring gears gears attached to the rotor to ensure the proper orientation as the rotor moves. The best 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. housing. The Wankel Wankel engine engine is actually actually a variable variable-vol -volume ume progress progressinging-cav cavity ity system. Thus there are 3 cavities per housing, all repeating the same cycle. As the rotor rotates and orbitally revolves 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. 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 strok stroke e per per rotor rotor orbit orbital al revol revolut utio ion n and and thre three e powe powerr strok strokes es per per roto rotor r rotation. rotation . Thus, power output power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement displacement in a similar state of tune tune and and highe higherr than than that that of a four-s four-str troke oke piston piston engin engine e of simila similarr physi physica call dimensio dimensions ns and weight. weight. Wankel Wankel engines also generall generally y have have a much much higher higher redline than than a reci recipr proc ocat atin ing g engi engine ne of simi simila larr size size sinc since e the the stro stroke kes s are are completed completed with a rotary motion as opposed to a reciprocating 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 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 ban it altogether.
Rotary Engine Power: Rotary engines use the four-stroke four-stroke combustion cycle, which is the same cycl cycle e that that four four-s -str trok oke e pist piston on engi engine nes s use. use. But But in a rota rotary ry engi engine ne,, this this is accomplished in a completely different way. The heart of a rotary engine is the rotor. This is roughly the equivalent equivalent of the pistons in a piston engine. The rotor is mounted on a large circular lobe on the output shaft. This lobe is offset from the centerline of the shaft and acts like the crank handle on a winch, giving the rotor the leverage it needs to turn the output shaft. As the rotor orbits inside the housing, it pushes the lobe around in tight circles, turning three times for every one revolution of the rotor. As the rotor moves through the housing, the three chambers created by the rotor change size. This size change produces a pumping action. Let's go through each of the four strokes of the engine looking at one face of the rotor.
Intake: The intake phase of the cycle starts when the tip of the rotor passes the intake port. At the moment when the intake port is exposed to the chamber, the volume of that chamber chamber is close to its minimum. As the rotor moves past the intake port, the volume of the chamber expands, drawing air/fuel mixture into the chamber. When the peak of the rotor passes the intake port, that chamber is sealed off and compression begins.
Compression: As the rotor continues motion around the housing, the volume of the chamber gets smaller and the air/fuel mixture gets compressed. By the time the face of the rotor has made it around to the spark plugs, plugs , the volume of the chamber is again close to its minimum. This is when combustion starts.
Combustion: Most rotary engines have two spark plugs. The combustion chamber is long, so the flame would spread too slowly if there were were only only one plug. plug. When When the the spark spark plugs plugs ignite ignite the air/f air/fuel uel mixture, pressure quickly builds, forcing the rotor to move. The pressure of combustion forces the rotor to move in the the dire direct ctio ion n that that make makes s the the cham chambe berr grow grow in volu volume me.. The The
combustion gases continue to expand, moving the rotor and creating power, until the peak of the rotor passes the exhaust port.
Exhaust: Once the peak of the rotor passes the exhaust port, the high-p high-pres ressu sure re comb combust ustion ion gases gases are are free free to flow flow out out the the exhaust. As the rotor continues to move, the chamber starts to contract, forcing the remaining exhaust out of the port. By the time the volume of the chamber is nearing its minimum, the peak of the rotor passes the intake port and the whole cycle starts again. The neat thing about the rotary engine is that each of the three faces of the rotor is always working on one part of the cycle -- in one complete revolution of the rotor; there will be three combustion strokes. But remember, the output shaft spins three times for every complete revolution of the rotor, which means that there is one combustion combustion stroke for each revolution of the output shaft.
[The Wankel cycle: Intake (blue), Compression (green), Ignition (red), Exhaust (yellow)]
Differences and Challenges: There are several defining characteristics that differentiate a rotary engine from a typical piston engine.
Fewer_moving_parts : The rotary engine has far fewer moving parts than a comparable comparable four stroke piston engine. A two-rotor rotary engine has three main moving parts: the two rotors and the output shaft. Even the simplest four-cylinder piston engine has at least 40 moving parts, including pistons, connecting rods, camshaft, camshaft , valves, valve springs, rockers, timing belt, timing gears and crankshaft. This minimization of moving parts can translate into better reliability from a rotary engine. This is why some aircraft manufacturers (including the maker of Sky of Sky car ) prefer rotary engines to piston engines.
Smoother: All the parts in a rotary engine spin continuously in one direction, rather than violently changing directions like the pistons in a conventional engi engine ne do. do. Rota Rotary ry engi engine nes s are are inte intern rnal ally ly bala balanc nced ed with with spin spinni ning ng counterweights that are phased to cancel out any vibrations. The power delivery in a rotary engine is also smoother. Because each combustion event lasts through 90 degrees of the rotor's rotation, and the output shaft spins three revolutions for each revolution of the rotor, each combustion event lasts through 270 degrees of the output shaft's rotation. This means that that a single single-ro -rotor tor engin engine e delive delivers rs powe powerr for three three-q -qua uarte rters rs of each each revolution of the output shaft. Compare this to a single-cylinder piston engine, in which combustion occurs during 180 degrees out of every two revolutions, or only a quarter of each revolution of the crankshaft (the output shaft of a piston engine).
Slower: Since the rotors spin at one-third the speed of the output shaft, the main moving parts of the engine move slower than the parts in a piston engine. This also helps with reliability.
Materi Mat erials als used used:: Wankel Wankel engine engine is construc constructed ted with an iron rotor rotor within within a housi housing ng made made of aluminum, aluminum, which has greater thermal expansion. expansion . 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 on the other, leading to high local temperatures and unequal thermal expansion. expansion . While this places high demands on the mate materia rials ls used used,, the simpli simplicit city y of the the Wanke Wankell make makes s it easie easierr to use use alternative materials like exotic alloys and ceramics. ceramics . With water cooling in a radial or axial flow direction, with the hot water from the hot bow heating the cold bow, the thermal expansion remains tolerable.
Seal Se alin ing g: 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. Modern Wankel engines have not had these problems for many years. Further sealing problems arise from the uneven thermal distribution within the housin housings gs caus causing ing distor distorti tion on and and loss loss of sealin sealing g and and comp compres ressio sion. n. This This thermal distortion distortion also causes uneven wear between the apex seal and the rotor housing, quite evident on higher mileage engines. Attempts have been made to norma normaliz lize e the tempe tempera ratur ture e of the hous housing ings, s, minimi minimizi zing ng the disto distorti rtion on,, with with different coolant circulation patterns and housing wall thicknesses.
Fuel consumpt co nsumption ion and an d hydrocarbon hydro carbon emissions emission s: 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 unburn unburned ed hydro hydroca carb rbons ons relea release sed d into into the the exha exhaust ust.. While While manuf manufact acture urers rs of piston-engine cars were turning to expensive catalytic converters converters to completely oxidiz oxidize e the unbu unburne rned d hydroc hydrocarb arbons ons,, Mazda Mazda was was able able to avoid avoid this this cost cost by enri enrich chin ing g the the air/ air/fu fuel el mixtu ixture re and and incr incre easin asing g the the amou amount nt of unbu unburn rned ed hydroc hydrocar arbon bons s in the the exha exhaust ust to actu actuall ally y suppor supportt comp complet lete e combu combusti stion on in a 'thermal reactor' (an enlarged open chamber in the exhaust manifold) manifold ) without the need for a catalytic converter, thereby producing a clean exhaust at the cost of some extra fuel consumption. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. chamber. This approach allowed Mazda to eliminate eliminate overlap between intake and exhaust port openings, while simultaneously increasing exhaust port area.
Advan Ad vanta tages ges :
Wankel engines have several major advantages over reciprocating reciprocating piston designs, in addition to having higher output for similar displacement and physical size. It is simple and has fewer moving parts. The The rotor rotor is geare geared d direc directly tly to the the output output shaft, shaft, there there is no need need for connecting rods, rods , a conventional crankshaft, crankshaft , crankshaft balance weights, weights , etc Smoother flow of power but also the ability to produce more power by running at higher rpm higher rpm.. Fuel of very low octane number can be used without pre ignition or knock. It’s substantial safety benefit makes it useful in aircraft There is no valve operation. The engine is constructed with an iron rotor within a housing made of aluminum, aluminum, which has greater thermal greater thermal expansion. expansion . This ensures that even a severely overheated Wankel engine cannot seize.
It has 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.
Disad Di sadvan vantag tages es : The fuel-air mixture cannot be pre-stored as there is no intake valve. Time available for fuel to be injected into a Wankel engine is significantly shorter. More complicated fuel injection technologies are required. In terms of fuel economy, Wankel engines are generally less efficient than four stroke piston engines Sealing loss is high. The compression ratio is lower. This lowers the thermal efficiency and thus the fuel economy. It is difficult to expand the engine to more than two rotors. There can be more carbon monoxide and unburned hydrocarbons in a Wankel's exhaust stream. Wank Wankel el engi engine nes s are are very very sens sensit itiv ive e to misf misfir ires es sinc since e the the engi engine ne will will lose lose momentum from the lost stroke and get slammed back into movement from the next chamber firing. Care of the ignition system is of utmost importance importance to avoid the problem.
Applications:
Used in aircraft. Racing car. For mini, micro, and micro-mini engine designs. The most exotic use of the Wankel design is in the seat belt pre-tensioner system of some Mercedes-Benz. Mercedes-Benz . Go-karts, Go-karts , personal water craft and auxiliary power units for aircraft.
CONTENTS: INTRODUCTION HISTORY THE PARTS OF A ROTARY ENGINE WORKING PRINCIPLE DIFFERENCES AND CHALLENGES MATERIALS USED SEALING FUEL CONSUMPTION & EMISSION ADVANTAGES DISADVANTAGES APPLICATIONS REFERENCES
REFERENCES: www.wikipedia.com www.praxair.com www.linde
group.com www.vorras.com msn Encarta www.google.com
