ROTARY ENGINE AND ITS APPLICATION IN AUTOMOBILE SEMINAR SUBMITTED IN PARTIAL FULFILLMENT OF BACHELOR OF ENGINEERING DEGREE OF THE JODHPUR NATIONAL UNIVERSITY JODHPUR
GUIDED BY: -
SUBMITTED BY:-
PROF. S.N.GARG
AMRUTIYA DHAVAL
(H.O.D OF MECHANICAL)
(4TH YEAR, 8TH SEM)
DEPARTMENT OF MECHANICAL ENGINEERING FACULTY OF ENGINEERING & TECHNOLOGY JODHPUR NATIONAL UNIVERSITY JODHPUR (RAJ.) 1
(SESSION 2011-2012)
ACKNOWLEDGEMENT
It is indeed a pleasure for me to express my sincere gratitude to those who have always helped me throughout my seminar report work. I would like to thank my report guide PROF. S.N.GARG (H.O.D of mechanical engg.)who helped me selecting the report topic, understanding the subject, stimulating suggestions, encouragement and also for writing of this thesis. I am sincerely thankful for this valuable guidance and help to enhance my presentation skill.
AMRUTIYA DHAVAL
2
CERTIFICATE
I here certify that the seminar entitled ROTARY ENGINE AND ITS APPLICATION IN AUTOMOBILE being submitted by AMRUTIYA DHAVAL KUMAR M. in partial fulfillment for the award of the degree of Bachelor of Technology in Mechanical Engineering to the Department of Mechanical Enfineering, Faculty of Engineering & Technology, Jodhpur National University, Jodhpur is a record of the candidates own work carried out by him under my supervision supervision and guidance the matter embodied in the seminar seminar has not been submitted submitted by him for the award of any degree.
Date: Place: -JODHPUR
Prof. S.N.Garg H.O.D Mech. Engg. Faculty of E&T JNU,Jodhpur
3
INDEX Introduction of Rotary Engine
6
6
History
Principles The
of Rotary Engine
8
Parts of Rotary Engine
9
Rotary
Engine Assembly
Material
used
Sealing
11 13 13
Working
13
17
Ignition
Cooling
system
Different Fuel
and challenges challenges
consumption and hydrogen emission
Rotary
engine geometry
19 21 22 22
Different
types of rotary engine
25
Research
paper
31
Advantages
41
Disadvantages
41
Application
42
Utilities
42
Conclusion
42
References
43
4
FIGURE LIST Rotary Engine Figure 1 Rotary Engine Figure 2 Rotar Figure 3 Housing Figure 4 [Output Shaft; Not e the eccentric lobes.] Figure 5 One of the two end pieces of a two-rotor Wankel engine Figure 6 The part of the rotor housing that hold s the rotors Figure 7 The center piece conta ins another intake port for each rotor. Figure 8 Figure 9 schematic f ig. ig. of rotary engine Figure 10 intak e stage Figure 11 compression stage Figure 12 combustion stage Figure 13 Exhaust stage Figure 14 [The Wankel cycle: Intake (blue), Compression (green), Ignition (red), Exhaust (yellow)] Figure C-1 cooling system and lubrication system Figure 15 Rotary Engine Nomenclature Figure 16 Nomenclature For Epitrochoid parametric equation Figure 17 successive phases in the execution of the otto cycle in the rotary engine Fiugre 18 minimum working fluid with flat-flank ed rotary engine Figure 19 maximum work ing-fluid volume for a flat-flank ed rotary engine Figure 20 Geometry of circular arc of rotor Figure 21 Exploded view of twin rotary engine Figure 22 Influence of flank rounding onclea rance and compression ratio for an eccentricity ratio of 0.16 Figure 23 After top center pilot, dual injector conf igur iguration Figure 24 Stratif ie ied-Charge Rotary Engine performance Figure 25. Comparison of energy densities between internal combustion engines and primary batteries. Figure 26 Rotary Engine Operation Figure 27 Mini-Rotary Engine Test Bench Figure 28 Dynamometer calibration chart for 50 W Maxon b rushless electric motor. Figure 29 MN30 Mini-Rotary Engine Figure 30 Modif ie ied MN30 Rotor with Apex Seals Figure 31 CWRU Micro-Rota ry Engine Left: Three.waf er er Si mold Right: Molded SiC rotor Figure 32 CWRU Micro-Rota ry Engine Housing
5
ROTARY ENGINE
Introduction of rotary engine:
The Rotary engine is a type of internal combustion engine which uses a rotor to convert pressure into a rotating motion instead of using reciprocat ing pistons. Its four-stroke cycle is generally generated in a space between the inside of an oval-like epitrochoid-shaped housing and a roughly triangular rotor (hypotrochoid). This design delivers smooth high-rpm power f rom a compact, lightweight engine. The engine was invented by German engineer Felix Wankel. He began its development in the early 1950s at NSU Motorenwerke AG (NSU) before completing a work ing, running prototype in 1957. NSU then subsequently licensed the concept to oth er companies across the globe, who added more efforts and improvements in the 1950s and 1960s. Because of its compact, lightweight design, Wank el rotary engines have been installed in a variety of vehicles and devices such as automobiles and racing cars, aircraft, go-karts, personal water crafts and aux ilia ry power units. In a piston engine, the same volume of space (the cylinder) alternately does four diff ere erent jobs - intak e, compression, 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 l ike having a dedicated cylinder for each of the four jobs, with the piston moving continually f rom one to the next. The rotary engine (originally conceived and developed by Dr. FelixWankel) is sometimes called a Wankel engine.[1]
Fig.1 Rotary Engine
History: In 1951, Wankel began development of the engine at NSU (NSU Moto renwerke AG), where he f irs irst conceived his rotary engine in 1954 (DKM 54, Dr ehkolbenmoto ehkolbenmotor ) and later the KKM 57 (the Wankel rotary engine, Kr eiskolbenmoto eiskolbenmotor ) in 1957. The f irs irst working prototype DKM 54 as 6
running on February 1, 1957 at the NSU research and development department V er suchsabteilung suchsabteilung 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 f rom their simplicity. In the United States, in 1959 under license f rom NSU, Curtiss-Wright pioneered minor improvements in the basic engine design. In Britain, in the 1960s, Rolls Royce Motor Car Division at C rewe, Cheshire, pioneered a twostage diesel version of the Wankel engine. Also in Britain Norton Moto rcycles developed a Wank el rotary engine for 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 197 2 Arctic Cat produced snowmobiles powered by 303 cc Wankel rotary engines manufactured by Sachs in Germany. John Deere Inc, in the U.S., designed a version that was capabl e of using a variety of f ue uels. 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 80 model, Citroën with the M35, and GS B irotor using engines produced by Co motor, as well as 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 f irs irst Wankel engined car was the 1967 Mazda Cosmo. The company normally used two-rotor designs, but received considerable attention with their 1991 Eunos Cosmo, which used a twinturbo 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 f rom the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and f ur urther power gains. The RENESIS is capable of delivering 238 horsepower (177 kW) f rom its 1.3 L displacement with better f ue uel economy, reliability, and environmental f rie riendliness than any other Mazda rotary engine in history.[5]
D r. FelixWankel 7
Principles of a Rotary Engine: Like a piston engine, the rotary engine uses the pressure created when a combination of air and f ue uel is burned. In a piston engine, that p ressure is conta ined in the cylinders and forces pistons to move back and fo rth. The connecting rods and crankshaft convert the reci procating motion of the pistons into rotat ional motion that can be used to power a car. In a rotary engine, the pressure of combustion is contained in a chamber formed by part of the housing and sealed in by one face of the triangular rotor, which is what the engine uses instead of pistons.
Fig. 2 Rotary Engine The rotor follows a path that look s like something you'd create with a Sp irograph. 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 expansion and contraction that d raws air and f ue uel into the engine, compresses it and makes usef ul power as the gases expand, and then expels the exhaust. We'll be taking a look inside a rotary engine to check out the parts, but f irs irst let's take a look at a new model car with an all-new rotary engine.[5]
8
Mazda
RX-8:
Mazda has been a pioneer in developing production cars that use rotary engines. The RX-7, which went on sale in 1978, was probably the most successf ul rotary-engine-powered car. But it was preceded by a series of rotary-engine ca rs, trucks and even buses, starting with the 1967 Cosmo Sport. The last year the RX-7 was sold in the United States was 1995, but the rotary engine is set to make a comeback in the near f uture. The Mazda RX-8 , a new ca r f rom Mazda, has a new, award winning rotary engine called the RENESIS. Named International Engine of the Year 2003, this naturally aspirated two-rotor engine will produce about 250 horsepower.[8]
The Parts of a Rotary Engine: A rotary engine has an ignition system and a f ue uel-delivery system that a re 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.[10] 1. Rotor : The r oto otor has three convex faces, each of wh ich acts like a piston. Each face of the rotor has apocket in it, which increases the displacement of the engine, allowing more space for air/f ue uel mixture. At the apex of each face is a metal blade that forms a seal to the outside of the combustion chamber. There are also metal rings on each side of the rotor that seal to the sides of the combustion chamber. The rotor has a set of internal gear teeth cut into the center of one side. These teeth mate with a gear that is f ixed to the housing. This gear mating determines the path and d irection the rotor takes through the housing.[10]
9
Fig.3 Rotar 2.
Housing :
The housing is roughly oval in shape (it's actually an epitroc hoid).The shape of the combustion chamber is designed so that the three tips of the rotor will always stay in conta ct with the wall of the chamber, forming three sealed volumes of gas. Each part of the housing is dedicated to one part of th e combustion process. [Rotor] The four sections are: · Intake · Compression · Combustion · Exhaust The intake and exhaust ports are located in the ho using. There are no valves in these ports. The exhaust port connects directly to the exhaust, and the intake port connects directly to the throttle.[10]
Fig.4 Housing
10
3.
Shaft:
The output shaft has round lobes mounted eccentrically, meaning that they are off se set f rom the centerline of the shaft . Each rotor f its over one of these lobes. The lobe acts sort of like the crankshaft in a piston engine.As the rotor follows its path around the housing, it pushes on the lobes.Since the lobes are mounted eccentric to th e output shaft , the force that the rotor applies to the lobes creates torque in the shaft , causing it to spin.[10]
Fig.5[Output Shaft;
Note the eccentric lobes.]
Rotary Engine Assembly:
A rotary engine is assembled in layers. The two-rotor engine we took apart has f ive ive main layers that a re held together by a ring of long bolts. Coolant flows through passageways surrounding all of the pieces. The two end layers conta in the seals and bearings for the output shaft . They also seal in the two sections of housing that conta in the rotors. The inside surfaces of these pieces are very smooth , which helps the seals on the rotor do their job. An intake port is located on each of these end pieces.[12]
Fig.6 One of the two end pieces of a two-rotor Wankel engine 11
The next layer in f rom the outside is the oval-shaped rotor housing, which contains the exhaust ports. This is the part of the housing that conta ins the rotor
Fig.7 The part of the rotor housing that holds the rotors (Note the exhaust port location.) The center piece conta ins two intak e ports, one for each rotor. It also separates the two rotors, so its outside surfaces are very smooth.
Fig.8 The center piece conta ins another intak e port for each rotor.
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In the center of each rotor is a large internal gear that rides around a smaller gear that is f ixed to the housing of the engine. This is what determines the orbit of the rotor. The rotor also rides on the large circular lobe on the output shaft .[12]
Materials used: Wankel engine is constructed with an iron rotor within a ho using made of al uminum, which has greater thermal 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. While this places high demands on the materials used, the simplicity of the Wankel makes it easier to use alternative materials like exotic alloys and ceramics. With water cooling in a radial or axial flow direction, with the hot water f rom the hot bow heating the cold bow, the thermal expansion remains tolerable.[1]
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 ca rbon pa rticles could become trapped between the seal and th e casing, jamming the engine and requiring a partial rebuild. Modern Wankel engines have not had th ese problems for many years. Further sealing problems arise f rom the uneven thermal distribution within the housings causing distortion and loss of sealing and compression. This thermal distortion also causes uneven wear between the apex seal and the rotor housing, quite evident on higher mileage engines. Attempts have been made to normalize the temperature of the housings, minimizing the distortion, with diff ere erent coolant circulation patt erns and ho using wall thicknesses.[1]
Working:
Fig.9 schematic f ig. ig. of rotary engine
13
The "A" marks one of the three apexes of the rotor. The "B" marks the eccentric shaft and th e white portion is the lobe of the eccentric shaft . The shaft turns three times for each rotat ion of the rotor around the lobe and once for each orbital revolution around the eccentric shaft . [10] 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-shaped housing surrounds a three-sided rotor (similar to a Reuleaux triangle, a three-pointed curve of constant w idth, 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 rotat es around an off se set 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 orientat ion as the rotor moves.[10] The best way to visualize the action of the engine in the animation atleft is to look not at th e rotor itself , but the cavity created between it and thehousing. The Wankel engine is actually a variable-volume progressing-cavitysystem. Thus there are 3 cavities per housing, all repeating the same cycle.[10]
As the rotor rotates and orbitally revolves, each side of the rotor gets closer and fa rther f rom 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 off se set lobe.[10] 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 rotat ion 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 Wank el engine is generally higher than that of a fo ur-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 a utomobiles according to displacement and regulatory bodies in automobile racing variously consider the Wankel engine to be equivalent to a fourstroke engine of 1.5 to 2 times the displacement; some racing regulatory agencies ban it altogether.[10]
Rotary Engine Power: Rotary engines use the four-stroke combustion cycle, which is the same cycle that four-stroke piston engines use. But in a rotary engine, this is accomplished in a completely diff ere erent way. 14
The heart of a rotary engine is the rotor. This is roughly the equivalent of the pistons in a piston engine. The rotor is mounted on a large circular lobe on the output shaft . This lobe is off se set f rom the centerline of the shaft and a cts like the crank handl e 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.[11] As the rotor moves through the housing, the three chambers created bythe 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.[11]
Intake: The intake phase of the cycle starts when the tip of the rotor passes the intak e port. At the moment when the intak e port is exposed to the chamber, the volume of that chamber is close to its minimum. As the rotor moves past the intake port, the volume of the chamber expands, drawing air/f ue uel mixture into the chamber. When the peak of the rotor passes the intake port, that chamber is sealed off and compression begins.[11]
Fig.10 intake stage
Compression: As the rotor continues motion around the housing, the volume of the chamber gets smaller and the air/f ue uel mixture gets compressed. By the time the face of the rotor has made it around to the spark plugs, the volume of the chamber is again close to its minimum. This is when combustion starts.[11]
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Fig.11 compression stage
Combustion: Most rotary engines have two spark plugs. The combustion chamber is long, so the flame would spread too slowly if there were only one plug. When the spark plugs ignite the air/f ue uel mixture, pressure quickly builds, forcing the rotor to move. The pressure of combustion forces the rotor to move in the direction that makes the chamber grow in volume. The combustion gases continue to expand, moving the rotor and creating power, until the peak of the rotor passes the exhaust port.[11]
Fig.12 combustion stage
Exhaust: Once the peak of the rotor passes the exhaust port, the high-pressure combustion gases are f ree ree to flow out 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 stroke for each revolution of the output shaft .[11]
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Fig.13 Exhaust stage
Fig.14 [The Wankel cycle: Intake (blue), Compression (green), Ignition (red), Exhaust (yellow)]
Ignition:
Since the working chambers of each rotor f ire ire in the same geographic location in the engine, only one set of spark plugs are needed per rotor housing. Due to the complexities of combusting a long chamber, two spark plugs are used in each housing. The lower one is called the "leading" spark plug, while the top one is called the "trailing" spark plug. Additionally, the f ront rotor housing is denoted as "1" and th e rear rotor housing is denoted as "2". So, in short hand, the Trailing (top) plug on the f ront rotor housing is ref erre erred to a s "T1" , the Leading (lower) plug in the f ront rotor housing is ref erre erred to as "L1", and so forth.[6]
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As the working chamber approaches Top Dead Center (TDC), the leading plug f ires ires f irs irst, starting the ignition of the a ir-f ue uel mixture and contributing most to the generation of pow er. The trailing plug typically f ires ires 10 to 15 degrees later and eff ec ectively completes the combustion of the remaining air-f ue uel mixture above the minor axis of the trochoid housing. Additionally, the leading plug f ires ires a second time late in the power stroke, which is called a "wa stespark". The wastespark is done to simplif y the ignition system by allow ing both leading plugs to be f ire ired "on the same channel" (one coil, one signal) . Basically, both leading plugs always f ire ire at the same time, so there is an extra (wasted) spark during the power stroke. The trailing plugs cannot be f ire ired in wastespark mode due to the location of th e plug above the minor axis -- when one trailing plug is f iri iring, the other trailing plug is already in the next working chamber, where it would preignite the incoming air-f ue uel mixture.[6] As you may have f igure igured out, there are three discrete ignition "channels" -- one for the leading plugs and two separate ones for the trailing plugs. Early, distributor-controlled, rotary engines could get away with only two coils (one each for leading and t railing ignitions) due to the coil engery being redirected through the distributor to the appropriate trailing spark plug. Later, crank angle sensor-controlled, rotary engines (starting with the 2nd generation RX-7) require three separate coils due to the direct-f ire ire setup.[6] TIMING
COMMENTS
Timing of the engine, be it ignition or intak e/exhaust porting, is based on degrees of eccentric shaft rotat ion, rather than rotor rotation. The diff ere erence is that the rotor rotat es at one third the forward rate of the eccentric shaft , so three degrees of eccentric shaft rotat ion translates to one degree of rotor rotation relative to the housing. It is also helpf ul to remember that each working chamber of a rotary engine has two Top Dead Centers (minimum chamber volume) and two Botto m Dead Centers (maximum chamber volume). Timing is usually expressed relative to either TDC or BDC, in degrees of eccentric shaft rotation Before or After those critical points. 5° BTDC, for example, means the event in question occurs 5° of eccentric shaft rotat ion Before the applicable Top Dead Center of a given working chamber. Further, one may ref er er to Advancing the timing, which simply means to increase degrees BTDC or BBDC, or decrease degrees ATDC or ABDC. Retarding the timing is the converse. Relative measurements such as these are diff icu icult to grasp at f irs irst, but they make the most sense in application.[6]
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COOLING SYSTEM FOR ROTOR:
Table 1 suggests a classif ic ication of the Wankel type engines f rom the point of view of the cooling system employed for rotor and rotor bearing The best known concept to dat e is the oil cooled rotor (OCR) which is usually associated with a l iquid cooling system of engine housings, i.e., rotor housing, f ront ho using and rear housing. This was the original solution developed by NSU/Wankel, and was successf ully applied in production type engines by Mazda, John Deere, and others. For these reasons we considered this solution representing the current f ull potential of the Wankel engine and we credited it with 100% rank (see Table 1). For light duty applications, the charge cooled rotor (CCR) off ers ers signif ic icant manufacturing cost reduction and added simplicity by eliminating the oil cooling system. Combined with a l iquid cooling system for the engine housing, the CCR system off ers ers only 80% of the maximum obtainable power when compared to the OCR system applied to the same basic engine design, e.g. volumetric displacement, engine rated speed, port arrangement, etc.[9] Poor performance of the CCR type engine is due to the higher temperature of the rotor, rotor bearing and eccentric shaft and to th e diminished volumetric eff icie iciency as a result of heat transf er er f rom the above mentioned engine parts to the f res resh charge mixture. The engine performance is even lower when the CCR solution is coupled with an a ir cooling system for the engine housings. Some CCR engines are using the f res resh charge mixture to cool the eccentric shaft and rotor bearing. This method is usually employed when gasoline (mixed with oil) or natural gas are used as f ue uel. In the f irs irst case, the f ue uel evaporation helps the engine's internal cooling[9]. Table 1 is far f rom being exhaustive. For example,the introduction of an intercooler for the charge mixture in association with the CCR solution opens
19
f ig. ig. C-1 cooling system and lubrication system another branch of the classif ic ication tree. Also, there are a f ew Wankel engines which are using the bleed air to cool the rotor and the rotor bearing. This combination again extends the classif ic ication related to the rotor and rotor bearing cooling system.[9] 20
Table 2 shows the influence of the rotor cooling design concept on estimated engine performance, total eff icie iciency potential and p roduction cost. The evaluation was limited to the basic design concept discounting the influence of engine accessories such as intercoolers, etc.
By eliminating the oil cooling system with an oil pump, a heat exchanger, an oil sump and especially the oil sealing system, a cost advanta ge of up to 30% can be achieved for a LCCR engine when compared with the OCR solution. The CCR solution off ers ers an even better cost advantage but its concept is limited to the relatively small engines. To date, only CCR rotary engines up to 650cc have been developed and produced successf ully.The total eff icie iciency of the CCR engines can equal that of th e OCR engines due to the former's lower f ric riction losses as long as the overheating phenomena can be controlled. Overheating is especially worrissom at part load conditions. in this respect, the LCCR engine demonstrates a decisive advantage by exactly controlling its internal temperatures.[9]
Differences and C hallenges: There are several def ining characteristics that diff ere erentiate a rotary engine f rom a typical piston engine. Fewer
moving parts:
The rotary engine has far f ewer moving parts than a 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 , valves, valve springs, rockers, timing belt, timing gears and crankshaft . This minimization of moving parts can translate into better reliability f rom a rotary engine. This is why some aircraft manufacturers (including the maker of Sky car) pref er er rotary engines to piston engines.[11]
21
Smoother: All the parts in a rotary engine spin continuously in one direction, rather than violently changing directions like the pistons in a conventional engine do. Rotary engines are internally balanced with spinning counterweights that a re 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 rotat ion. This means that a single-rotor engine delivers power for three-quarters of 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 p iston engine).[11]
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.[11] 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-f ue uel 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 unb urned hydrocarbons, Mazda was able to avoid this cost by enriching the air/f ue uel mixture and increasing the amount of unburned hydrocarbons in the exhaust to a ctually 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 f ue uel consumption. 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.[11]
Rotary Engine Geometry
Fig.15 Rotary Engine Nomenclature
22
Fig.16 Nomenclature For
Fig.17 successive phases
Epitrochoid parametric equation
in the execution of the otto cycle in the rotary engine
23
The major elements of the rotary engine. the ho using and the rotor. are shown in crossrochoid choid or section in Figure 15. The housing inner surface has a mathematical form known as a t r o rochoid choid . A single-rotor engine housing may be thought of as two parallel planes separated epit r o by a cylinder of epitrochoidal cross-section. Following the notat ion of Figure 7.5, the parametric form of the epitrochoid is given by _ _ _ (1a) x = e cos 3 + R cos [ft | m] _ _ (1b) y = e sin 3+ R sin [ft | m] where e is the eccent r ricity i city and R is the rotor center-to-tip distance. For given values of e and R, Equations (1) give the x and y c y coordinates def ining the housing shape when is varied f rom 0 to 360 degrees. The rotor shape may be thought of as an equilateral triangle, as shown in Figures 15 and 17 (flank rounding and other ref inements are discussed later in the chapter). Because the rotor moves inside the housing in such a way that its three apexes are in constant conta ct with the housing periphery, the positions of the tips are also given by equations of the form of Equations (1): _ _ (2a) x = e cos 3 + R cos(+ 2n/3) [ ft | m] [ft | m] _ _ ( 2b) y = e sin 3 + R sin(+ 2n) where n = 0, 1, or 2, the three values identif yi ying the positions of the three rotor tips, each separated by 120°. Because R represents the rotor center-to-tip distance, the motion of the center of the rotor can be obta ined f rom Equations (2) by setting R = 0. The equations and Figure 17 indicate that the path of th e rotor center is a circle of radius e. Note that Equations (1) and (2) can be nondimensionalized by dividing through by R. This yields a single geometric parameter governing the equations, e/R, e/R, known as the ricity eccent r i city r atio. atio. It will be seen that th is parameter is critical to successf ul performance of the rotary engine. The power f rom the engine is delivered to an external load b y a cylindrical shaft . The shaft ax is coincides with the axis of the housing, as seen in Figure 15. A second circular cylinder, the eccent r ric, i c, is rigidly attached to the shaft and is off se set f rom the shaft ax is by a distance, e, the ecce ntricity. The rotor slides on the eccentric. Note that the axes of the rotor and the eccentric coincide. Gas forces exerted on the rotor are transmitted to the eccentric to provide the driving torque to the engine shaft and to th e external load. The motion of the rotor may now be understood in terms of the notation of Figure 16. The line labeled e rotates with the shaft and eccentric through an an gle 3 _, while the line labeled R is f ixed to the rotor and turns with it through an an gle _ about the moving eccentric center. Thus the entire engine motion is related to the motion of these two lines. Clearly, the rotor (and thus line R) rotates at one-third of the speed of the shaft , and there are three shaft rotat ions for each rotor revolution.[5]
24
Different types of RO TARY ENGINES:
(1) A Simple Model for a Rotary Engine:
Fig.18 minimum working fluid wit h
flat-flanked rotary engine
Additional important f eatures of the rotary engine can be easily studied by considering an engine with an equi lateral triangular rotor. Figure 18 shows the rotor in the position where a rotor flank def ines the minimum volume. We will call this position top center, TC, by analogy to the reciprocating engine. The rotor housing clear ance ance par amete amete r , d , is the diff ere erence between the housing minor radius, R -e, and the distance f rom the housing axis to mid-flank,
e + R cos 60 = e + R/2: d = (R -e) -e) - (e + R/2) = R/2 -2e
[ft | m]
_ _ _ (3)
Setting the clearance to zero establishes an upper limiting value for the eccentricity ratio: (e/R)cri e/R)crit = 1/4. Study of Equations (1), at the other extreme, shows that , for e/R = 0, the epitrochoid degenerates to a circle. In this case the rotor would spin with no eccentricity and thus produce no compression and no to rque. Thus, for the flat-flanked rotor, it is clear that usable values of e/R lie between 0 and 0 .25.
25
Fig.19 maximum working-fluid volume for a flat-flanked rotary engine
Now lets examine some other f undamental pa ra meters of the flat-flanked engine model. Consider the maximum mixture volume shown in Figure 19. For a given rotor width w , the maximum volume can be determined by calculating the area between the housing and the flank of the rotor. Using Equations (1), the diff ere erential area 2y dx c dx can be written as: dA max= 2y dx 2 2 = 2(e sin3 + R sin) d (e cos3 + R cos) [ft | m ] _ _ _ (4) 2
Dividing by R and diff ere erentiating on the right-hand side, we obta in an equation for the dimensionless area in terms of the eccentricity rat io and th e angle _: 2 Amax/R = - 2 [(e/R)si e/R)sin3 + sin][3(e/R)si e/R)sin3 + sin]d ]d [dl] _ _ _(5) In order for the diff ere erential area to sweep over the maximum trapped volume in Figure 19, the limits on the angle _ must vary f rom 0° to 60°. Thus integration of Equation (5) with these limits and using standa rd integrals yields 2 2 1/2 _ _ _ (6) Amax/R = [(e/R) e/R) + 1/3]- 3 /4[1 -6(e/R)] e/R)] [dl] Similarly, using Figure 6 and the diff ere erential volume shown there, the nondimensionalized minimum area can be written as: 2 1/2 _ _ _ (7) Amin/R = [(e/R)2 e/R)2 + 1/3] - 3 /4 [1 + 6(e/R)] e/R)] [dl]
26
These maximum and minimum volumes (area-rotor width products) are analogous to the volumes trapped between the piston and cylinder at BC and TC in the four-stroke reciprocating engine. In that engine the diff ere erence between the volumes at BC and TC is the displacement volume, and their ratio is the compression ratio. A little thought should convince the reader that the analogy holds quantitatively for the displacement and compression ratio of the rotary engine. Therefore, subtracting Equation (7) f rom Equation (6) gives the displacement for a rotor width w for one flank of the flat-flanked engine as 2 3 3 1/2 disp = 3 ×3 w R (e/R) _ _ _ (8 ) e/R) [ft | m ] Thus the displacement increases with increases in rotor width, the square of the rotor radius, and with the eccentricity ratio, whereas the compression ratio is independent of size but increases with increase in eccentricity ratio.[5]
(2) The Circular-Arc-Flank Model:
Fig.20
F ig 21
Geometry of circular arc of rotor
Exploded view of twin rotary engine 27
Fig.22
Influence of flank rounding onclearance and compression ratio for an eccentricity ratio of 0.16 While the triangular rotor model represents a possible engine and is usef ul as a learning tool, such an engine would perform poorly compared with one having a rotor with rounded flanks. A more realistic model is one in which the triangular rotor is augmented with circular-arc flanks, as shown in Figure 20. The radius of curvature, r , of a flank could vary f rom inf inity, corresponding to a flat flank , to a value for which the arc touches the minor axis of the epitrochoid. Note that the center of curvature of an a rc terminated by two flank apexes depends on the value of r . It can also be seen f rom Figure 20 that r is related to the angle, _, subtended by the flank arc by 1/2 [ft | m] r sin(/2) = R sin(/3) = 3 R/2 or 1/2 [dl] _ _ _ (10) r /R = 3 /[2sin(/2)] Thus either the included angle, , or the radius of curvature, r , may be used to def ine the degree of flank rounding for a given rotor radius R. [ 5 ]
Clearance with Flank Rounding The additional a rea obta ined by capping a side of a t riangle with a circular arc is called a segment. The segment height, h, shown in Figure 20, is the diff ere erence between r and the projection of r on the axis of symmetry: [dl] _ _ _ (11) h/R = (r /R) /R)[1 - cos( /2)] Substitution of Equation (10) in Equation (11) yields 1/2 [dl] _ _ (12) h/R = 3 [1 -cos( /2)] / [2sin( /2)] It is evident f rom the f igure igure that the clearance for the rotor with circular arc flanks is the diff ere erence between the clearance of the flat-flanked rotor and h. Thus, using Equation (3), the clearance is given by 28
1/2
[dl] _ _ _ (13) d/R = 1/2 - 2(e/R) e/R) - 3 [1 - cos(/2)] / [2sin( /2)] In a real engine, of course, the clearance must be non-negative.[5]
Added Volume per Flank Due to Rounding The segment area is the diff ere erence between the pie-shaped area of the sector subtended by its included angle, , and the enclosed triangular a rea. The sector area, or volume per unit rotor width, is the f ra ction of the area of a circle of radius, r , subtended by the angle ; 2 2 i.e., r (/2) = r /2. Thus using Equation (10), the dimensionless segment volume is 2 2 2 2 [dl] As /R = (Asec - Atri ) /R = (r /R) /R) (. sin ) /2= (3/8)(- sin ) /sin /2 _ _ _ (14) Displacement
and Compression Ratio
It was pointed out ea rlier that the displacement of the flat-flanked engine is the diff ere erence between the maximum and minimum capture volumes, and is given by Equation(8). This is true also for the engine with rounded flanks. The additional volume added to the rotor by flank rounding subtracts f rom both of the flat-flanked maximum and minimum capture volumes, leaving the diff ere erence unchanged . Thus the displacement of one flank of a rounded-flank engine is 2 3 3 1/2 disp = 3 ×3 w R (e/R) _ _ _ (15) e/R) [ft | m ] Likewise, the ratio of the maximum and minimum capture volumes given by Equations (6) and (7), corrected for the segment volume f rom Equation (14) provides a relation for the rounded-flank engine compression ratio: Rotary engines usually have the maximum rounding possible consistent with adequate engineering clearances.[5]
Effect of t he Recess Volume The additional capture volume associated with the recess is seen in Figure 21. Its influence on the displacement and compression ratio may be reasoned in thesame way as with the segment volume. The recess increases both minimum and maximum mixture volumes by the same amount. It therefore has no eff ec ect on d isplacement and it decreases the compression ratio. Figure 22 shows the influence of flank rounding and recession on clearance and compression ratio. While flank recession reduces the compression ratio for given values of and e/R, e/R, it improves the shape of the long, narrow combustion pocket forming the minimum capture volume. Rotary engines usually have more than on e spark plug, to help overcome the combustion problems associated with this elongated shape.[5]
29
(3) Stratified-Charge Rotary Engine
F ig.23
After top center center pilot, dual injector configuration configuration
Fig.24
Stratified-Charge Rotary Engine performance The design and p erformance of stratif ie ied-charge rotary engines developed for commercial aviation propulsion and APU (a ux iliary power unit) application as well as for marine, industrial, and military requirements. Figure 23 shows a direct f ue uel injection conf igur iguration that ha s performed well under a wide range of speed, load , and environmental conditions and w ith a variety of liquid f ue uels. The ref ere erence reports a lack of octane and cetane sensitivities, so that d iesel, gasoline, and jet f ue uel can all be used with th is conf igur iguration. As air in the rotor recess passes below, the spark plug ignites a locally rich pilot stream that in turn ignites the f ue uel f rom the main injector. The net f ue uel-air ratio is lean, resulting in improved f ue uel economy over normal carburetion. Figure 24 presents data for f ullload brake horsepower and specif ic ic f ue uel consumption obta ined with Jet-A f ue uel for the twinrotor 2034R engine. The maximum takeoff power at 5800 rpm was 430 horsepower, with a brake specif ic ic f ue uel consumption (BSFC) of 0.44 lbm/BHP-hr.
30
Throughout a range of loads and altitude conditions the engine operates with a f ue uel-air ratio between 0.035 and 0.037, well below the stoichiometric value. The ref ere erence reports a best thermal eff icie iciency of 35.8% (BSFC = 0.387 lbm/BHP-hr) at 3500 rpm and 225-horsepower output.[5]
Closure: Continued engineering research on the rotary engine has resulted in performance improvements through improved seals, lean-burn combustion, f ue uel injection, integral electronic control, improved intak e design, weight reduction, and turbocharging.Despite vehicle weight increases, the Mazda RX-7 w ith a two- rotor 80-in.3 displacement engine improved 9.4% in f ue uel consumption and 8% in power output between 1984 and 1987 . During this time period, the addition to the engine of a turbocharger with intercooling increased its power output by 35%. The Mazda RX-Evolv, a year-2000 concept car, has a naturally-aspirated rotary engine called .RENESIS.. The two-rotor, side intak e and exhaust engine is reported to ha ve reduced emissions and improved f ue uel economy and to ha ve attained 280 horsepower at 9000 rpm and 226 N-m torque at 8000 rpm.[12]
Research Paper:
DESIGN
AND EXPERIMENTAL RESULTS OF SMALL-SCALE ROTARY ENGINES
ABSTRACT A research project is currently underway to develop small-scale internal combustion engines f ue ueled by liquid hydrocarbons. The ultimate goal of the MEMS Rotary Internal Combustion Engine Project is to develop a l iquid hydrocarbon f ue ueled MEMS-size rotary internal combustion micro-engine capable of delivering power on the order of milli-watts. This research is part of a larger effort to develop a po rtable, autonomous power generation system with an order of magnitude improvement in energy density over alkaline or lithium-ion batteries. The rotary (Wank el-type) engine is well suited for the fabrication techniques developed in the integrated chip (IC) community and ref ined by the MicroElectroMechanical Systems (MEMS) f ie ield. Features of the rotary engine that lend itself to MEMS fabrication are its planar construction, high specif ic ic power, and self-valving operation. The project aims at developing a "micro-rotary" engine with an epitrochoidal-shaped housing under 1 mm3 in size and with a rotor swept volume of 0.08 mm 3. To investigate engine behavior and design issues, larger-scale "mini-rotary" engines have been fabricated f rom steel. Mini-rotary engine chambers a re approximately 1000 mm 3 to 1700 mm 3 in size and their displacements range f rom 78 mm 3 to 348 mm 3.[7] A test bench for the mini-rotary engine has been developed and experiments have been conducted with gaseous-f ue ueled mini-rotary engines to examine the eff ec ects of sealing, ignition, design, and thermal management on eff icie iciency. Preliminary testing has shown net power
31
output of up to 2.7 W at 9300 RPM. Testing has been performed using hydrogen-air mixtures and a range of spark and glow plug designs as the ignition source. Iterative design and t esting of the miniengine has lead to improved sealing designs. These particular designs are such that they can be incorporated into the fabrication of the micro-engine.[7] Design and fab rication of a f irs irst generation meso-scale rotary engine has been completed using a SiC molding process developed at Case Western Reserve University. The fabrication of the micro-rotary engine is being conducted in U.C. Berkeley's Microfabrication Laboratory.[7] Testing of the mini-engine has lead to th e conclusion that th ere are no f undamental phenomena that wo uld prevent the operation of the micro-engine. However, heat loss and sealing issues are key for eff icie icient operation of th e micro-engine, and they must be taken into account in the design and fabrication of the micro-rotary engine. The mini-rotary engine design, testing, results and appl icat ions will be discussed in this paper.[7] NOMENCLATURE Biot Number Bi e Engine Eccentricity Convection Coeff icie h icient k Ratio of Specif ic ic Heats ksolid Conduction Coeff icie icient l Length Output Shaft Speed nE Compression Ratio rv CutoffRatio rc Rotor Generating Radius R Residence Time Tres Tehem Chemical Time Temperature (Melting) Tm U Velocity Vs Swept Volume (Displacement) W Engine Width Theoretical Compression Ratio Eth
32
INTRODUCTION
Figure 25. Comparison of
energy densities between internal combustion engines and primary
batteries. MicroElectroMechanical Systems (MEMS) devices are mechanical elements constructed using the manufacturing techniques and materials used in the integrated circuit / microchip industry [1,2]. Typically, MEMS fabrication uses lithography to mask the silicon substrate, followed by etching and deposition to create the high aspect ratio f eatures in the substrates. The combination of small scale and pot entially inexpensive mass production is the Intake primary attraction of the technology. [7] Early products in the MEMS f ie ield (accelerometers and gyroscopes) have given way to other, more complex, engineering systems (head-mounted displays, optical communication systems, and micro-fluidic systems). As new manufacturing techniques and materials have been developed, MEMS-scale thermal devices are now practical [3]. These microscale engines have numerous applications, because the power output can be electrical or mechanical (torque). Applications range f rom mobile electrical power supplies (potential batt ery replacements) to mechanical power.[7] The ultimate goal of the MEMS Rotary Internal Combustion Engine Project is to fab ricate a micro-rotary 'Wank el' engine, which produces -10-100 mW of mechanical power. The engine chamber will f it in a cubic millimeter, with a swept volume of 0.08 mm 3. A power generation system using the micro-rotary engine would have 5 times to 14 times the energy density of a primary batt ery [4] (see Fig. 25). The MEMS scale "micro-rotary" engine will be constructed primarily of Silicon (Si), Silicon Carbide (SIC), and S ilicon Dioxide (SiO2). All parts subjected to high temperatures and stresses will be built either using molded SiC or a Si substrate with a thin SiC coating [7]
33
The research program is planned in progressive steps leading to the fabrication of a MEMSscale engine. A series of larger scale "mini-rotary" engines, with swept volumes of 78 mm 3 to 348 mm 3, have been constructed to investigate design and combustion issues as the engine scale is reduced. In add ition, fabrication of the microrotary engine has begun. This paper describes the challenges of building a small-scale rotary engine f rom testing, design, and MEMS fabrication viewpoints and suggests solutions to these challenges.[7]
ROTARY ENGINE LAYOUT AND DESIGN
F ig. 26 Rotary Engine Operation
A rotary engine was selected for development as the basis of a MEMS-scale power generation system due to several factors: - the planar design of the rotary engine lends itself to MEMS fabrication - the rotor controls the timing of the intak e and exhaust, eliminating the need for the complex valve actuating and valve timing systems found in 4-stroke reciprocating engines - the power output is in the form of rotary motion of the shaft , which is necessary for powering either a micro-vehicle or an electric generator[7] The rotary engine consists of a t riangular rotor rotat ing within an epitrochoidal-shaped housing (see Fig. 26). The rotor centerline is off se set f rom the housing centerline by an eccentric, e. As the rotor rotat es about the center of the epitrochoid, all three apexes of the rotor remain in conta ct with the walls, forming three sealed chambers. In large-scale systems, an elabo rate sealing system is used to prevent leakage: 1) over the face of the rotor (face seals) and 2) around the rotor ends (apex seals). The seals incorporated into the present work will be addressed later in this paper.[7] The rotary engine operates on a 4-stroke cycle, as illustrated in Figure 26. As the rotor apex passes over the intak e port, the increasing volume in the chamber draws in a f res resh f ue uel / air charge (intak e). The rotor continues to rotat e, and the next apex closes the intake port and compresses the f ue uel / air mixture (compression). The f ue uel / air mixture is ignited using a spark plug, glow plug, or compression ignition. The resulting pressure rise acts off the rotor axis due to the eccentricity, e, leading to a torque f rom which the shaft work is extracted (power). After the power stroke, the rotor apex uncovers the exhaust port. As the rotor continues to rotate, the exhaust gases are e jected f rom the engine by the decreasing volume (exhaust). The 34
rotor apex then uncovers the intak e port again, tak ing in a f res resh charge. Since there are three chambers formed by the rotor and epitrochoid, this cycle occurs three times for every revolution of the rotor. The rotor spins at 1/3 the speed of the output shaft , resulting in one power pulse for every revolution of the shaft . Ref ere erences to engine speed in this paper ref er er to the output shaft rotat ional speed.[7] The three primary engine parameters are: the rotor generating radius (R), the rotor width (w), and the eccentricity (e). Once these are established, all other engine parameters (displacement, theoretical compression ratio, maximum tip velocity, and tip angle) a re also established [7,8]. The compression ratio in a working rotary engine is also never as high as the theoretical compression ratio. All rotary engines have a rotor cutout to allow th e combustion reaction to propagate at the onset of the power stroke. The theoretical compression ratio does not tak e the rotor cutout into account, because the cutouts are diff ere erent for each engine.[7] A series of mini-rotary engines of intermediate size (78 mm 3 to 348 mm 3) have been built and used to study combustion, fluid, and design issues as the engine is reduced in size. The design of the mini rotary mimics the proposed design of the micro-rotary. Each engine has diff ere erent power outputs and builds upon the lessons learned f rom the previous engine.[7]
COMBUSTION, THERMODYNAMICS, AND SCALING The rotary engine cycle can be described by either the Otto o r Diesel cycle, depending on the ignition source (spark or compression ignition, respectively). The eff icie iciency of both the Otto cycle engine and Diesel cycle engine is dependent on the compression ratio. As can the compression ratio is dependent only upon the rotor generating radius (R) and the ecce ntricity (e) rather than the length scale of the engine. Therefore, decreasing the size of an engine does not aff ec ect the theoretical compression ratio or the theoretical thermal eff icie iciency the theoretical thermal eff icie iciency of a spark ignited rotary engine (Otto Cycle) is 69%. For the same compression ratio, and assuming a cutoff ratio of 4:1 (typical for a diesel engine), the theoretical thermal eff icie iciency of a compression ignited rotary engine (Diesel Cycle) is 56%. However, there are other factors that aff ec ect the design and eff icie iciency of the engine due to the change f rom the macro scale to micro scale. Issues include residence time vs. reaction time, increased heat loss, and quenching.[7] Residence time (swept time of rotor through combustion chamber) in a rotary engine is independent of size. Rather, the residence time is related to the operating speed of the engine. The power stroke of the rotary engine occurs over 90 ° of the rotor turn. At 40,000 RPM , the f ue uel / air mixture has 375 µsec to react before the exhaust port is uncovered. At normal combustion temperatures, hydrocarbons have a characteristic reaction time of 10 µsec Assuming tccs/tchem =10 for complete reaction, the maximum reaction t ime limited engine speed for a rotary engine is 150,000 RPM. Since the micro-rotary engine is designed to operate at an engine speed of 40,000 RPM, the reaction time is not a factor, if the temperature is kept 35
elevated. As the length scale of the engine decrea ses, the surface area to volume ratio increases . [7] The reduced temperature gradient between the hot (combustion / exhaust) and cold (intake) sides of the engine will reduce the eff ec ect of thermal expansion of the housing. An isothermal housing implies that measures must be taken to control the amount of heat added to the incoming f ue uel / air mixture. While pre-heating the intak e f ue uel / air mixture will aid in obtaining combustion in the microscale, care must be taken to avoid autoignition in the inlet port. A f ur urther concern is that the heat added to the intake mixture will reduce the thermal eff icie iciency due to the reduced TH/T c ratio of the cycle.[7]
Flame quenching occurs due to cooling of the reaction, as well as radical destruction at the walls. The minimum distance in the micro-rotary engine occurs at the point of maximum compression, when the f ue uel/air mixture is compressed between the rotor cutout and th e side of the epitrochoid. The smallest linear distance of the micro-rotary engine is well below the quenching distance for any f ue uel. It has been determined that a fla me can be stabilized in tubes well below the quenching diameter with an external heat supply to heat the tube wall. In a supporting experiment, this strategy has been successf ully employed using only the exhaust gas to heat the tube wall. Based on these f indings, the obvious solution to the quenching problem is to increase the temperature of the engine wall.[7] The use of a SiC housing helps with this solution since there are no material constraints (SiC Tm= 31000(2). In fact, quenching can be totally eradicated if the wall temperature can be maintained at the adiabatic temperature of the f ue uel. If thermal management is not suff icie icie nt to support gas phase combustion, the combustion reactions can be enhan ced through catalytic surface reactions. It should be noted that fo r catalytic reactions, the increasing surface-area-tovolume ratio leads to overall higher conversion eff icie iciency. For this purpose, the housing and rotor walls can be coated with a catalyst (e.g., Platinum). From the point of view of combustion eff icie iciency, the critical factor in MEMS combustion systems is the increased heat loss at small length scales. Heat loss decreases combustion temperatures, which leads to reduced reaction rate and quenching. Several strategies are being reviewed to reduce heat loss:[7] - Exhaust Gas Re circulation (EGR): A strategy that re circulates the combustion gases around the outside of the combustion chamber to decrease heat loss f rom the combustion chamber and p re-heat the incoming f ue uel-air mixture. The eff ec ectiveness of this strategy has been demonstrated - Stacking the engines to reduce the heat losses f rom the center engine(s). The center engine(s) will operate at nearly adiabat ic conditions due to insulation and the higher melting temperatures of SiC - An insulating channel f illed with low conduction material, such as SIO2, surrounding the engine A signif ic icant problem facing large-scale rotary engines is leakage, either past the rotor tips or over the rotor face. Leakage reduces the engine eff icie iciency (by reducing the compression 36
ratio) and increases the eff ec ect of incomplete combustion. Sealing mechanisms have been created to reduce this problem in large-scale engines. These sealing systems consist of springloaded tabs at the rotor apexes and across the rotor face. The tabs maintain contact between the rotor and epitrochoid walls and sealing for the chambers. Apex seals have been fabricated in the mini-engine, shown to be eff ec ective, and designed for the micro-rotary engine. A face seal system has not yet been developed that is compatible w ith MEMS fabrication. However, it should be noted that the smallest commercially available rotary engine (the 5000 mm 30. S. Graupner) does not use a face seal.[7] MINI-ROTARY
ENGINE TEST BENCH
At the scale of the mini-rotary engine, there a re no commercially available diagnostic engine test stands. Therefore, a test bench (Fig. 27). diagnostic engine test stands. Therefore, a test bench (Fig. 3) was designed and fab ricated to test the mini-engine operation. The test bench consists of an electric motor / dynamometer, optical tachometer, ignition system (for spark plug use), and flywheel. A magnet on the flywheel is used to t rigger the spark. Ignition and spark timing is achieved with a Hall Eff ec ect sensor mounted on a rotary dial and a spark ignition system manufactured by CH Electronics, Inc. Engine speed is measured using a Mona rch Instruments ACT-3 tachometer with a ROS-5W remote optical sensor. The mini-engine is rigidly coupled to the dynamometer via a steel shaft.
Fig. 27 Mini-Rotary Engine Test Bench To test combustion in the mini-rotary engine, a pre-mixedhydrogen-air mixture is used rather than a l iquid hydrocarbon, given hydrogen's ease of ignition. A gaseous f ue uel mixture is advantageous for this study because it does not require the complexity of a f ue uel carburetion system. The gaseous f ue uel and a ir are mixed upstream of the engine in a T-junction. The f ue uel is metered using valves and rotameters upstream of the T-junction.[7] To measure engine power, a dynamometer has been developed using a Maxon TM b rushless electronically commutated motor and a rectif ier ier circuit. Power generated by the mini-rotary engine spins the dynamometer, which acts as a generator and p roduces electrical power. The rectif ier ier circuit converts the dynamometer's three-phase output to a DC voltage potential.
37
Rheostats are used to apply a load to th e dynamometer. The rheostats can be adjusted to produce the appropriate load, based on the mini-rotary engine being tested.[7] To calibrate the voltage across the dynamometer load to mechanical power output, the dynamometer is driven by an electric motor while connected to a to rque arm and load cell. Figure 28 shows the calibration curve for the Maxon 50 W brushless DC motor.
Fig. 28 Dynamometer calibration chart for 50 W Maxon brushless electric motor. Voltage drops across the rectif ier ier circuit and across the engine coils were taken into account. During engine operation, the output voltage of the motor is measured and related to the power output through the calibration curve.[7] Temperature measurements were also made upstream of the engine and at th e housing near the combustion chamber using thermocouple or inf rared imaging. Pressure measurements upstream of the engine were also made to accurately measure f ue uel and a ir flow rates. MN30 MINI-ROTARY
ENGINE DESIGN AND TESTING
The MN30 generation mini-rotary engine is shown in Figure 29. The basic engine is simple in design, consisting of 7 pa rts: f ront plate, epitrochoid housing, back plate, rotor, internal gear, spur gear, and shaft . Two bearings, mounted in the f ront and ba ck plate, position the shaft . For speed of fabrication, the mini-rotary engine is made f rom steel. The most accurate means of manufacture at th is scale is through electrodischarge machining (EDM). Note that the initial design of the MN30 engine does not include apex or face seals.[7]
38
Fig.29 MN30 Mini-Rotary Engine
Fig.30 Modified MN30 Rotor with Apex Seals The MN30 mini-rotary engine has a displacement of 78 mm 3. Preliminary sealing tests determined that th e engine suff ere ered f rom 20% leakage during operation due to poor tolerances during manufacturing. From the leakage tests, it was apparent that an ap ex sealing system was necessary. In order to improve sealing, slots were cut in the rotor apexes and apex seals inserted. The springs consist of brass tabs with leaf springs formed f rom spring steel (see Fig. 30). Even these simple designs signif ic icantly improved leakage. [7] Combustion tests were performed on the MN30 mini-engine with apex seals. In these tests, an electric motor was used to rotate theengine, while a stoichio metric H2-air or H2-C3H8-air mixture was supplied. The ignition system used was a spark plug. Ignition timing was changed f rom 15 degrees BTDC to 15 degrees ATDC. While the engine generated no net power output, there was a 50% reduction in the power required by the electric motor to turn the mini-engine with combustion. Exhaust temperatures also changed with spark timing, with peak exhaust temperatures at 7.5 degrees BTDC.[7]
39
MEMS-ROTARY
ENGINE FABRICATION
Design and fab rication of the MEMS-scale rotary engine (microrotary engine) has begun. A larger scale "meso-rotary" engine was fabricated at the Case Western Reserve University (CWRU) MEMS Research Center using Si and SiC. The CWRU micro-engine has a displacement of 1.2 mm 3 and a rotor diameter of 3 mm, compared to the ultimate goal of 0.08 mm 3 displacement and 1 mm rotor diameter. The larger engine was fabricated to t est the limits of the CWRU SiC fabrication process. The rotor is made of molded SiC f rom a three-waf er er Si mold (see Fig. 31). The Si housing is fabricated f rom three separate 500 mm waf ers ers deep reactive ion etched (DRIE) to form the f eatures and then f usi usion bonded together (see Fig. 32).[7]
Fig.31 CWRU Micro-Rotary Engine Left: Three.wafer Si mold Right: Molded SiC rotor
Fig.32 CWRU Micro-Rotary Engine Housing The CWRU micro-engine highlights some of the diff icu iculties in fabricat ing an engine using MEMS techniques. Even tho ugh the tolerance obtained in MEMS is typically on the order of microns, the percentage tolerance when compared to the length scale is high. For a high precision 40
system such as the rotary engine, the tolerance required is on the order of 0.1% (
Advantages :
1.Wankel engines have several major advanta ges over reciprocating piston designs, in addition to having higher output for similar displacement and physical size. It is simple and ha s f ewer moving parts. 2. The rotor is geared directly to the output shaft , there is no need for connecting rods, a conventional crankshaft , crankshaft balance weights, etc 3. Smoother flow of power but also the ability to produce more power by running at higher rpm. 4. Fuel of very low octane number can be used without pre ignition or knock. 5. Its substant ial saf ety benef it makes it usef ul in aircraft 6. There is no valve operation. 7. 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. 8. It has smaller f rontal area than a p iston 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.[12]
Disadvantages : 1 .The f ue uel-air mixture cannot be pre-stored as there is no intake valve. 2. Time available for f ue uel to be injected into a Wank el engine is signif ic icantly shorter. 3. More complicated f ue uel injection technologies are required. 4. In terms of f ue uel economy, Wankel engines are generally less eff icie icient than four stroke piston engines 5. Sealing loss is high. 6. The compression ratio is lower. This lowers the thermal eff icie iciency and th us the f ue uel economy. 7. It is diff icu icult to expand the engine to more than two rotors. 8. There can be more carbon monoxide and unburned hydrocarbons in a Wankel's exhaust stream. 9. Wankel engines are very sensitive to misf ires ires since the engine will lose momentum f rom the lost stroke and get slammed back into movement f rom the next chamber f iri iring. Care of the ignition system is of utmost importance to avoid the problem.[12]
41
Applications:
1. Used in aircraft. 2. Racing car. 3. For mini, micro, and micro-mini engine designs. 4. The most exotic use of the Wankel design is in the seat belt pre-tensioner system of some Mercedes-Benz. 5. Go-karts, personal water craft and a uxiliary power units for aircraft.[12]
UTILITY
The Wankel's superb power-to-weight ratio makes it particularly well suited to aircraft engine use. There was intense interest in them in this role in the 1950s when the design was f irs irst becoming well known, but it was at this same time that almost the entire industry was moving to the jet engine, which many believed would be the only engine in use within a decade. The Wankel suff ere ered f rom a lack of interest, and when it later became clear that the jet engine was far too expensive for all roles, the general aviation world had already shrunk so much that th ere was little money for new engine designs. Nevertheless, interest in them for small aircraft has continued. Wankels have made something of a comeback in recent years. None of their advanta ges have been lost in comparison to other engines, and the introduction of better materials has helped the tip-seal problem. They are being found increasingly in roles where their compact size and quiet running is important, notably in drones, or UAVs. More recently, these UAV-designed engines are being found increasingly in other roles, such as personal water craft and a uxiliary power units for aircraft. Aside f rom 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 the basic advanta ges of the Wankel over the four-stroke internal combustion engine are not relevant. In a design using a Wankel supercharger on a Wank el 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 the Volkswagen VW_New Beetle. In this car, when deceleration sensors sense a potential crash, small blank cartridges are triggered electrically, and the resulting pressurized gas f ee eeds into tiny Wankel engines which rotat e to take up the slack in the seat belt systems, anchoring the driver and pa ssengers f irm irmly in the seat before any collision. [13}
conclusion
Application of adiabat ic (low heat re jection) engine technology to the rotary (Wank el type) engine is highly dependent upon the materials used for the basic engine components.
Fundamental work on increasing the permissible operating temperature of the apex seal/rotor housing tribological system is required before the adiabat ic technology can be successf ully applied to a complete engine.
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Successf ul low heat re jection major engine components have been designed, analyzed, fabricated and t ested in a Mazda gasoline rotary engine and low heat re jection components fabricated for the NASA 1007R stratif ie ied charge engine.
REFERENCES:
(1) www.wikipedia.com (2) www.praxair.com (3) www.lindegroup.com (4) www.vorras.com (5) www.rotaryengines.com (6) www.google.com (7) Steven R King Durability of Natural Gas Fueled Rotary Engine SAE 870048 (8) www.mazdarx7.com (9) www.wankleccpapers.com (10) Internal combustion engine by M.L. Mathur (11) www.scribd.com (12) www.howstuffworks.com
(13) www.seminarprojects.com
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