Monday, May 14, 2012
Sunday, May 13, 2012
Ural Motorcycles
Ural Motorcycles
The IMZ - URal story
The Ural story begins in 1939, during the USSR's pre-World War II planning. Despite the Molotov/von Ribbentrop Pact, the Soviet Union knew it would soon be going to war against Adolf Hitler, the ruthless dictator of the German Third Reich. Joseph Stalin ordered the military to prepare all areas of operation, including the ground forces that would defend the Russian motherland against invading German Panzers, ground troops, and Special Forces. Having seen the effects of the Blitzkrieg against the Polish Army, mobilization was of paramount importance to the USSR.
A meeting was held at the USSR Defense Ministry to discuss what motorcycle model was most suitable for the Red Army. The Army had wanted to modernize its equipment after termination of the military conflict with Finland, as the motorcycles it had been using had not worked satisfactorily. Their technology was out-dated and the manufacturing quality left much to be desired.
The official version reads that, after long discussion and debate, the BMW R71 motorcycle was decided to most closely match the Red Army's requirements. Five units were covertly purchased through intermediaries in neutral Sweden and smuggled to Russia. Soviet engineers in Moscow busily dismantled the 5 BMWs. They copied every detail of the BMW design and made moulds and dies to produce their own engines and gearboxes in Moscow. Everything about the bike was reverse engineered. Early in 1941, the first trial samples of M-72 motorcycles were shown to Stalin, who immediately approved production of the motorcycles.
(Incidentally, one of these original BMWs survives and is on display at the factory museum. Harley-Davidson also copied the BMW design, and delivered about 1,000 Harley-Davidson XA (Experimental Army) flat-twin shaft drive motorcycles to the US Army during World War II. Meanwhile, in Japan, Riyushko was busy copying Harley-Davidson V twins!)
A more likely story is that the BMW factory supplied the construction drawings and casting moulds. As a result of the Molotov / Ribbentrop Pact, transfers of technology had taken place in support of their Soviet "friends" in different areas. Soviet engineers toured German aircraft factories and brought back complete cannons as samples. The OPEL Kadett was given to the Soviets just prior to the war; however, it commenced series production only toward the end of the war as the Moskvitch 400. In 1941, BMW began series production of R75, and did not resume production of R71. Supplying the Soviets with the superceded R71 model may have seemed a good idea at the time.
A factory in Moscow was soon producing hundreds of Russian M-72 sidecar motorcycles. The Nazi Blitzkrieg was so fast and effective that Soviet strategists worried that the Moscow factory was within easy range of German bombers. It was decided to move the motorcycle plant further east, out of bombing range, into the middle of the resource rich Ural Mountain region. The chosen site was the small trading town of Irbit, located on the fringe of the vast Siberian steppes in the Ural Mountains. Irbit had, before the Revolution of 1917, been an important Trade and Fair center in Russia.
The only substantial building in town was a brewery. It was soon converted into research and development headquarters, where long hours were spent preparing for the construction of a massive new production complex for the M-72. On October 25, 1942, the first M-72s were sent into battle. Over the course of World War II, 9,799 M-72 motorcycles were delivered to the front for reconnaissance detachments and mobile troops.
The history of Ural began with the glory of helping to defeat the terror of Hitler's armies on the Russian and European battlefields. After World War II the Factory was renovated, and in 1950, the 30,000th motorcycle was produced. In the late 1950s, a plant in the Ukraine took over the manufacture of Urals for military use, and the Irbit Motorcycle Works (IMZ) began to build Urals for domestic, civilian consumption. The popularity of the outfits grew steadily among Russians, and in the 1960s, the plant was turned over to full non-military production.
The export history of URALs started in 1953
The first Urals were exported in 1953, at first mainly to developing countries. In the late 1960s, deliveries to developed countries began, and since then more and more Urals have appeared on the road on every continent. Urals are a unique combination of price, ageless styling, and sidecar functionality.
In November 1992, the State-owned factory transformed into Uralmoto Joint Stock Company. Uralmoto was a privatized entity, 40% of which was divided among management and employees through a grant, 38% of which was sold by auction with privatization vouchers (which went mostly to management and employees), and 22% of which was retained by the government.
In early 1998, Ural was bought by private Russian interests; it is no longer a State-owned company. (Shortly after the purchase, in 2000, the government shares were redistributed to investors.) New ownership has brought new management, fresh ideas and production techniques, modernized design and updated technology, and above all, a commitment to quality control at all points of production. Ural motorcycles have been given a new lease on life. While the outward appearance of the engine retains the look of a classic Ural, quality control techniques and use of better alloying and casting, better engineering tolerances, better paint and chrome, make for a stronger, better bike. Everything good and unique about the old Urals has been maintained, including the inherently balanced design of a horizontally opposed flat twin engine with roller bearings in a solid frame.
The main bike models built in the plant today are the heavy-duty Ural sidecar motorcycles, designed with rough Russian roads in mind, and the custom Wolf. There are many places in Russia where only horses and Ural motorcycles can be used to transport gear where you need it. Ural motorcycles are equipped with four-stroke air-cooled flat-twin engines, a four-speed gearbox with reverse gear, shaft drive, two disc dry clutch, spring shock absorbers, and drum brakes. New solo and sidecar models have been developed recently to better suit the tastes of Western markets.
Ural is the only Russian manufacturer of heavy capacity motorcycles, and one of few manufacturers of sidecar motorcycles in the world. Besides sales of Ural motorcycles on the Russian market, they have also been exported to Australia, Britain, the United States, France, the Netherlands, Belgium, Spain, Greece, Norway, Finland, Sweden, Germany, Egypt, Iran, South Africa, Brazil, Uruguay, Paraguay, and numerous other countries. Over 3.2 million motorcycles have been delivered since the first M-72 rolled off the production floor.
The future looks bright for Ural, constantly improving its role as versatile and economical form of transport that is fun to ride and easy to maintain. The story is far from over.
Friday, February 17, 2012
Rotary engine
Rotary engine
-Cat-and-mouse engines
-Eccentric-rotor engines
-Multirotor engines
-Revolving-block engines
-Engines of other types
- Bibliography
Internal combustion engine that duplicates in some fashion the intermittent cycle of the piston engine, consisting of the intake-compression-power-exhaust cycle, wherein the form of the power output is directly rotational.
Four general categories of rotary engines can be considered: (1) cat-and-mouse (or scissor) engines, which are analogs of the reciprocating piston engine, except that the pistons travel in a circular path; (2) eccentric-rotor engines, wherein motion is imparted to a shaft by a principal rotating part, or rotor, that is eccentric to the shaft; (3) multiple-rotor engines, which are based on simple rotary motion of two or more rotors; and (4) revolving-block engines, which combine reciprocating piston and rotary motion. Some of the more interesting engines of each type are discussed in this article.
Cat-and-mouse engines
Typical of this class is the engine developed by T. Tschudi, the initial design of which goes back to 1927. The pistons, which are sections of a torus, travel around a toroidal cylinder. The operation of the engine can be visualized with the aid of Fig. 1, where piston A operates with piston C, and B with D. In chamber 1 a fresh fuel-air mixture is initially injected while pistons C and D are closest together. During the intake stroke the rotor attached to pistons B and D rotates, thereby increasing the volume of chamber 1. During this time the A-C rotor is stationary. When piston D reaches its topmost position, the B-D rotor becomes stationary, and A and C rotate so that the volume of chamber 1 decreases and the fuel-air mixture is compressed.
Intake-compression-power-exhaust cycle of the Tschudi engine.
Fig. 1 Intake-compression-power-exhaust cycle of the Tschudi engine.
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When the volume of chamber 1 is again minimal, both rotors move to locate chamber 1 under the spark plug, which is fired. The power stroke finds piston D moving away from piston C, with the A-C rotor again locked during most of the power stroke. Finally, when piston D has reached bottom, the B-D rotor locks, the exhaust port has been exposed, and the movement of piston C forces out the combustion product gas. Note that four chambers exist at any time, so that at each instant all the processes making up the four-strike cycle (intake-compression-power-exhaust) are occurring.
The motion of the rotors, and hence the pistons, is controlled by two cams which bear against rollers attached to the rotors. The cams and rollers associated with one of the rotors disengage when it is desired to stop the motion of that rotor. The shock loads associated with starting and stopping the rotors at high speeds may be a problem with this engine, and lubrication and sealing problems are characteristic of virtually all the engines discussed herein. However, the problem of fabricating toroidal pistons does not appear to be as formidable as was once believed.
An engine similar to the Tschudi in operation is that developed by E. Kauertz. In this case, however, the pistons are vanes which are sections of a right circular cylinder. Another difference is that while one set of pistons is attached to one rotor so that these two pistons rotate with a constant angular velocity, the motion of the second set of pistons is controlled by a complex gear-and-crank arrangement so that the angular velocity of this second set varies. In this manner, the chambers between the pistons can be made to vary in volume in a prescribed manner. Hence, the standard piston-engine cycle can be duplicated. Kauertz tested a prototype which was found to run smoothly and to deliver 213 hp (160 kW) at 4000 rpm. Here again, however, the varying angular velocity of the second set of pistons must produce inertia effects that will be absorbed by the gear-and-crank system. At high speeds over extended periods, problems with this system are likely to be encountered.
An advanced version of the cat-and-mouse concept called the SODRIC engine has been developed by K. Chahrouri. Unlike the Tschudi engine, in which the four processes of intake-compression-power-exhaust are distributed over 360° of arc, the SODRIC engine performs these same four processes in 60° of arc. Hence, six power strokes per revolution are achieved, resulting in very substantial improvements in engine performance parameters. For example, Chahrouri has estimated that 225 hp (168 kW) can be achieved at only 1000 rpm using an engine having a toroid radius of 8 in. (20 cm) and 1-in.-radius (25-mm) pistons. Chahrouri has also improved upon the method by which alternate acceleration-deceleration of the pistons is achieved, and power is transmitted to the output shaft by using noncircular gears.
The “cat-and-mouse” and “scissors” characterizations of these engines should be clear once the picture of pistons alternately running away from, and catching up to, each other is firmly in mind. Other engines of this type, including the Maier, Rayment, and Virmel designs, differ principally in the system used to achieve the cat-and-mouse effect.
It should be noted that since the length of the power stroke is readily controlled in these engines, good combustion efficiencies (close to complete combustion) should be attainable.
Eccentric-rotor engines
The rotary engine which has received by far the greatest development to date is the Wankel engine, an eccentric-rotor type. The basic engine components are pictured in Fig. 2. Only two primary moving parts are present: the rotor and the eccentric shaft. The rotor moves in one direction around the trochoidal chamber, which contains peripheral intake and exhaust ports.
Operation and basic components of the Wankel engine.
Fig. 2 Operation and basic components of the Wankel engine.
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The operation of this engine can be visualized with the aid of Fig. 2. The rotor divides the inner volume into three chambers, with each chamber the analog of the cylinder in the standard piston engine. Initially, chamber AB is terminating the intake phase and commencing its compression phase, while chamber BC is terminating its compression phase and chamber CA is commencing its exhaust phase. As the rotor moves clockwise, the volume of chamber AB approaches a minimum. When the volume of chamber AB is minimal, the spark plug fires, initiating the combustion phase in that chamber. As combustion continues, the point is reached where the exhaust port is exposed, and the products of combustion are expelled from chamber AB.
To increase the chamber volumes, each segment of the rotor rim is recessed (Fig. 2). During the combustion-expansion phase, unburned gas tends to flow at high velocity away from the combustion zone toward the opposite corner. As a result, this engine has a tendency to leave a portion of the charge unburned, similar to the problem encountered in ordinary piston engines. In addition to reducing the engine performance, this unburned gas is a source of air pollution. Efforts have been directed toward increasing the turbulence in each chamber, thereby improving the mixing between the burned and unburned gases, leading to better combustion efficiency.
However, Wankel engines have demonstrated impressive advantages when contrasted with standard engines. Some of these are listed below.
The Wankel has superior power-to-weight ratio; that is, it generally produces more or at least comparable horsepower per pound of engine weight when compared with conventional piston engines.
To increase power output, additional rotor-trochoidal chamber assemblies can readily be added, which occupy relatively little space and add little weight. In piston engines, cylinder volumes must be increased, leading to substantial increases in weight and installation space.
The rotor and eccentric shaft assembly can be completely balanced; since they usually rotate at constant velocity in one direction, vibration is almost completely eliminated and noise levels are markedly reduced.
As with the cat-and-mouse engines, the intake and exhaust ports always remain open, that is, gas flow into and from the engine is never stopped, so that surging phenomena and problems associated with valves which open and close are eliminated.
Tests indicate that Wankel engines can run on a wide variety of fuels, including ordinary gasoline and cheaper fuels as well.
After considerable development, reasonably effective sealing between the chambers has been achieved, and springs maintain a light pressure against the trochoidal surface.
The Wankel has so few parts, relative to a piston engine, that in the long run it will probably be cheaper to manufacture.
The initial application of the Wankel engine as an automotive power plant occurred in the NSU Spider. In the early 1970s, however, the Japanese automobile manufacturer Mazda began to use Wankel engines exclusively. However, relatively high pollutant emissions, coupled with low gasoline mileage for automobiles of this size and weight, resulted in poor sales in the United States. Mazda ceased marketing Wankel-powered automobiles in the United States in the mid-1970s. Several American automobile manufacturers have experimented with Wankel-powered prototypes, but no production vehicles have emerged.
The Wankel engine is being used as a marine engine and in engine–electric generator installations, where its overall weight and fuel consumption have proved to be superior to those of a diesel engine or gas turbine generating equivalent power. Other projected applications include lawnmower and chainsaw engines. This wide range of applications is made possible by the fact that almost any size of Wankel engine is feasible.
An engine conceptually equivalent to the Wankel was developed jointly by Renault, Inc., and the American Motors Corp. It is sometimes called the Renault-Rambler engine. In this case, however, the rotor consists of a four-lobe arrangement, operating in a five-lobe chamber (Fig. 3). When a lobe moves into a cavity, which is analogous to the upward motion of the piston in the cylinder, the gas volume decreases, resulting in a compression process. The operation of this engine is detailed in Fig. 3. The fact that each cavity has two valves (intake and exhaust) represents a significant drawback. However, sealing between chambers may be simpler than in the Wankel; since each cavity acts as a combustion chamber, heat is evenly distributed around the housing, resulting in little thermal distortion.
Renault-Rambler engine.
Fig. 3 Renault-Rambler engine.
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Simple multirotor engine.
Fig. 4 Simple multirotor engine.
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It can be concluded that engines of the eccentric-rotor type are an integral part of the internal combustion engine scene. Their inherent simplicity, coupled with their advanced state of development, make them attractive alternatives to the piston engine in a number of applications.
Multirotor engines
These engines operate on some form of simple rotary motion. A typical design, shown in Fig. 4, operates as follows. A fuel-air mixture enters the combustion chamber through some type of valve. No compression takes place; rather, a spark plug ignites the mixture which burns in the combustion chamber, with a consequent increase in temperature and pressure. The hot gas expands by pushing against the two trochoidal rotors. The eccentric force on the left-hand rotor forces the rotors to rotate in the direction shown. Eventually, the combustion gases find their way out the exhaust.
The problems associated with all engines of this type are principally twofold: The absence of a compression phase leads to low engine efficiency, and sealing between the rotors is an enormously difficult problem. One theoretical estimate of the amount of work produced per unit of heat energy put into the engine (by the combustion process), called the thermal efficiency, is only 4%.
The Unsin engine (Fig. 5) replaces the trochoidal rotors with two circular rotors, one of which has a single gear tooth upon which the gas pressure acts. The second rotor has a slot which accepts the gear tooth. The two rotors are in constant frictional contact, and in a small prototype engine sealing apparently was adequate. The recommendation of its inventor was that some compression of the intake charge be provided externally for larger engines.
The Walley and Scheffel engines employ the principle of the engine in Fig. 4, except that in the former, four approximately elliptical rotors are used, while in the latter, nine are used. In both cases the rotors turn in the same clockwise sense, which leads to excessively high rubbing velocities. (The rotors are in contact to prevent leakage.) The Walter engine uses two different-sized elliptical rotors.
Unsin engine, with two circular rotors.
Fig. 5 Unsin engine, with two circular rotors.
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Revolving-block engines
These engines combine reciprocating piston motion with rotational motion of the entire engine block. One engine of this type is the Mercer (Fig. 6). In this case two opposing pistons operate in a single cylinder. Attached to each piston are two rollers which run on a track that consists of two circular arcs. When the pistons are closest together, the intake ports to the chambers behind the pistons are uncovered, admitting a fresh charge (Fig. 6a). At this moment a charge contained between the pistons has achieved maximum compression, and the spark plug fires. The pistons separate as combustion takes place between them, which results in a compression of the gases behind the pistons (Fig. 6b). However, the pistons moving apart force the rollers to move outward as well. This latter motion can only occur if the rollers run on their circular track, which consequently forces the entire engine block to rotate. When the pistons are farthest apart, the exhaust ports are uncovered and the combustion gases purged (Fig. 6c). At this same time the compressed fresh charge behind the pistons is transferrred to the region between the pistons to prepare for its recompression and combustion, which must occur because of the continuing rotation of the block.
Cycle of the Mercer engine. (a) Intake of fresh charge. (b) Compression of charge, following firing...
Fig. 6 Cycle of the Mercer engine. (a) Intake of fresh charge. (b) Compression of charge, following firing of spark plug. (c) Exhaust of combustion gases and transfer to compressed fresh charge.
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No doubt some of the fresh charge is lost to the exhaust during the transfer process. Stresses on the roller assembly and cylinder walls are likely to be quite high, which poses some design problems. Cooling is a further problem, since cooling of the pistons is difficult to achieve in this arrangement.
One the other hand, the reciprocating piston motion is converted directly to rotary motion, in contrast with the connecting rod–crank arrangement in the conventional piston engine. Also, no flywheel should be necessary since the entire rotating block acts to sustain the rotary inertia. Vibration will also be minimal.
Cycle of the Rajakaruna engine, showing distortion of the four-sided chamber as the surrounding housing...
Fig. 7 Cycle of the Rajakaruna engine, showing distortion of the four-sided chamber as the surrounding housing rotates.
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Walker engine, with elliptical rotor and two C-shaped rocking heads.
Fig. 8 Walker engine, with elliptical rotor and two C-shaped rocking heads.
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Cycle of one pocket of combustion gases in the Heydrich vane-type rotary engine.
Fig. 9 Cycle of one pocket of combustion gases in the Heydrich vane-type rotary engine.
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Leclerc-Edmon-Benstead engine.
Fig. 10 Leclerc-Edmon-Benstead engine.
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The Selwood engine is similar in operation, except that two curved pistons 180° opposed run in toroidal tracks. This design recalls the Tschudi cat-and-mouse engine, except that the pistons only travel through 30° of toroidal track. This motion forces the entire block to rotate. The Leath engine has a square rotor with four pistons, each 90° apart, with a roller connected to each. As in the Mercer engine, the reciprocating motion of the pistons forces the rollers to run around a trochoidal track which causes the entire block to rotate. The Porshe engine uses a four-cylinder cruciform block. Again, rollers are attached to each of the four pistons. In this arrangement power is achieved on the inward strokes of the piston. Finally, the Rajakaruna engine (Fig. 7) uses a combustion chamber whose sides are pin-jointed together at their ends. Volume changes result from distortion of the four-sided chamber as the surrounding housing, which contains a trochoidal track, rotates. The huge pins are forced against the track. As usual, cooling and lubrication problems will be encountered with this engine, as will excessive wear of the hinge pins and track.
The Ma-Ho engine, invented by G. Hofmann, uses four cylinders welded concentrically around a central shft. As the pistons oscillate in the cylinders as a result of the intake-compression-power-exhaust processes undergone by the fuel-air mixture, the pistons rotate a barrel cam to which they are connected by cam followers. Hence, the entire block, including the central shaft, is forced to rotate.
Engines of other types
Although the vast majority of rotary engines fall into one of the categories discussed above, several ingenious designs which do not are worthy of mention.
The Walker engine (Fig. 8) involves an elliptical rotor which rotates inside a casing containing two C-shaped rocking heads. The fuel-air mixture is drawn into combustion chambers on each side of the rotor; mixture cut-off occurs when the rotor is in the vertical position. As the rotor turns, the mixture undergoes compression, and combustion is initiated by spark plugs. Rotor momentum coupled with the expansion of the combustion gases forces the rotor to continue turning. Compensation for seal wear is made by adjusting the rocking heads closer to the rotor.
The Heydrich engine (Fig. 9) is a vane-type rotary engine which utilizes a small hole to “store” a quantity of high-temperature combustion gases from a previous firing. This gas is then used to ignite the subsequent fuel-air charge. The first charge is ignited by a glow plug. Floating seals make contact with the chamber wall.
The Leclerc-Edmon-Benstead engine (Fig. 10) has three combustion chambers defined by stationary vanes, a cylindrical stator, and end flanges. An output shaft passing through the chamber is geared to a circular, eccentrically mounted piston. As the chambers are fired sequentially, the movement of the piston forces the shaft to rotate. Slots in the flange control porting of the intake and exhaust gases. See also: Combustion chamber; Diesel cycle; Diesel engine; Gas turbine; Internal combustion engine; Otto cycle
Wallace Chinitz
Bibliography
R. F. Ansdale, Rotary combustion engines, Auto. Eng., vol. 53, no. 13, 1963, and vol. 54, nos. 1 and 2, 1965
W. Chinitz, Rotary engines, Sci. Amer., vol. 220, no. 2, 1969
H. E. Dark, The Wankel Rotary Engine, 1974
W. Froede, The Rotary Engine and the NSU Spider, Soc. Automot. Eng. Pap. 650722, October, 1965
K. Matsumoto et al., The Effects of Combustion Chamber Design and Compression Ratio on Emissions, Fuel Economy and Octane Number Requirement, SAE Pap. 770193, 1977
R. H. Thring, Gasoline engines and their future, Mech. Eng., vol. 15, no. 10, 1983
R. Wakefield, Revolutionary engines, Road Track, vol. 18, no. 3, 1966
F. Wankel, in R. F. Ansdale (ed.), Rotary Piston Machines: Classification of Design Principles for Engines, Pumps and Compressors, 1965
-Cat-and-mouse engines
-Eccentric-rotor engines
-Multirotor engines
-Revolving-block engines
-Engines of other types
- Bibliography
Internal combustion engine that duplicates in some fashion the intermittent cycle of the piston engine, consisting of the intake-compression-power-exhaust cycle, wherein the form of the power output is directly rotational.
Four general categories of rotary engines can be considered: (1) cat-and-mouse (or scissor) engines, which are analogs of the reciprocating piston engine, except that the pistons travel in a circular path; (2) eccentric-rotor engines, wherein motion is imparted to a shaft by a principal rotating part, or rotor, that is eccentric to the shaft; (3) multiple-rotor engines, which are based on simple rotary motion of two or more rotors; and (4) revolving-block engines, which combine reciprocating piston and rotary motion. Some of the more interesting engines of each type are discussed in this article.
Cat-and-mouse engines
Typical of this class is the engine developed by T. Tschudi, the initial design of which goes back to 1927. The pistons, which are sections of a torus, travel around a toroidal cylinder. The operation of the engine can be visualized with the aid of Fig. 1, where piston A operates with piston C, and B with D. In chamber 1 a fresh fuel-air mixture is initially injected while pistons C and D are closest together. During the intake stroke the rotor attached to pistons B and D rotates, thereby increasing the volume of chamber 1. During this time the A-C rotor is stationary. When piston D reaches its topmost position, the B-D rotor becomes stationary, and A and C rotate so that the volume of chamber 1 decreases and the fuel-air mixture is compressed.
Intake-compression-power-exhaust cycle of the Tschudi engine.
Fig. 1 Intake-compression-power-exhaust cycle of the Tschudi engine.
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When the volume of chamber 1 is again minimal, both rotors move to locate chamber 1 under the spark plug, which is fired. The power stroke finds piston D moving away from piston C, with the A-C rotor again locked during most of the power stroke. Finally, when piston D has reached bottom, the B-D rotor locks, the exhaust port has been exposed, and the movement of piston C forces out the combustion product gas. Note that four chambers exist at any time, so that at each instant all the processes making up the four-strike cycle (intake-compression-power-exhaust) are occurring.
The motion of the rotors, and hence the pistons, is controlled by two cams which bear against rollers attached to the rotors. The cams and rollers associated with one of the rotors disengage when it is desired to stop the motion of that rotor. The shock loads associated with starting and stopping the rotors at high speeds may be a problem with this engine, and lubrication and sealing problems are characteristic of virtually all the engines discussed herein. However, the problem of fabricating toroidal pistons does not appear to be as formidable as was once believed.
An engine similar to the Tschudi in operation is that developed by E. Kauertz. In this case, however, the pistons are vanes which are sections of a right circular cylinder. Another difference is that while one set of pistons is attached to one rotor so that these two pistons rotate with a constant angular velocity, the motion of the second set of pistons is controlled by a complex gear-and-crank arrangement so that the angular velocity of this second set varies. In this manner, the chambers between the pistons can be made to vary in volume in a prescribed manner. Hence, the standard piston-engine cycle can be duplicated. Kauertz tested a prototype which was found to run smoothly and to deliver 213 hp (160 kW) at 4000 rpm. Here again, however, the varying angular velocity of the second set of pistons must produce inertia effects that will be absorbed by the gear-and-crank system. At high speeds over extended periods, problems with this system are likely to be encountered.
An advanced version of the cat-and-mouse concept called the SODRIC engine has been developed by K. Chahrouri. Unlike the Tschudi engine, in which the four processes of intake-compression-power-exhaust are distributed over 360° of arc, the SODRIC engine performs these same four processes in 60° of arc. Hence, six power strokes per revolution are achieved, resulting in very substantial improvements in engine performance parameters. For example, Chahrouri has estimated that 225 hp (168 kW) can be achieved at only 1000 rpm using an engine having a toroid radius of 8 in. (20 cm) and 1-in.-radius (25-mm) pistons. Chahrouri has also improved upon the method by which alternate acceleration-deceleration of the pistons is achieved, and power is transmitted to the output shaft by using noncircular gears.
The “cat-and-mouse” and “scissors” characterizations of these engines should be clear once the picture of pistons alternately running away from, and catching up to, each other is firmly in mind. Other engines of this type, including the Maier, Rayment, and Virmel designs, differ principally in the system used to achieve the cat-and-mouse effect.
It should be noted that since the length of the power stroke is readily controlled in these engines, good combustion efficiencies (close to complete combustion) should be attainable.
Eccentric-rotor engines
The rotary engine which has received by far the greatest development to date is the Wankel engine, an eccentric-rotor type. The basic engine components are pictured in Fig. 2. Only two primary moving parts are present: the rotor and the eccentric shaft. The rotor moves in one direction around the trochoidal chamber, which contains peripheral intake and exhaust ports.
Operation and basic components of the Wankel engine.
Fig. 2 Operation and basic components of the Wankel engine.
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The operation of this engine can be visualized with the aid of Fig. 2. The rotor divides the inner volume into three chambers, with each chamber the analog of the cylinder in the standard piston engine. Initially, chamber AB is terminating the intake phase and commencing its compression phase, while chamber BC is terminating its compression phase and chamber CA is commencing its exhaust phase. As the rotor moves clockwise, the volume of chamber AB approaches a minimum. When the volume of chamber AB is minimal, the spark plug fires, initiating the combustion phase in that chamber. As combustion continues, the point is reached where the exhaust port is exposed, and the products of combustion are expelled from chamber AB.
To increase the chamber volumes, each segment of the rotor rim is recessed (Fig. 2). During the combustion-expansion phase, unburned gas tends to flow at high velocity away from the combustion zone toward the opposite corner. As a result, this engine has a tendency to leave a portion of the charge unburned, similar to the problem encountered in ordinary piston engines. In addition to reducing the engine performance, this unburned gas is a source of air pollution. Efforts have been directed toward increasing the turbulence in each chamber, thereby improving the mixing between the burned and unburned gases, leading to better combustion efficiency.
However, Wankel engines have demonstrated impressive advantages when contrasted with standard engines. Some of these are listed below.
The Wankel has superior power-to-weight ratio; that is, it generally produces more or at least comparable horsepower per pound of engine weight when compared with conventional piston engines.
To increase power output, additional rotor-trochoidal chamber assemblies can readily be added, which occupy relatively little space and add little weight. In piston engines, cylinder volumes must be increased, leading to substantial increases in weight and installation space.
The rotor and eccentric shaft assembly can be completely balanced; since they usually rotate at constant velocity in one direction, vibration is almost completely eliminated and noise levels are markedly reduced.
As with the cat-and-mouse engines, the intake and exhaust ports always remain open, that is, gas flow into and from the engine is never stopped, so that surging phenomena and problems associated with valves which open and close are eliminated.
Tests indicate that Wankel engines can run on a wide variety of fuels, including ordinary gasoline and cheaper fuels as well.
After considerable development, reasonably effective sealing between the chambers has been achieved, and springs maintain a light pressure against the trochoidal surface.
The Wankel has so few parts, relative to a piston engine, that in the long run it will probably be cheaper to manufacture.
The initial application of the Wankel engine as an automotive power plant occurred in the NSU Spider. In the early 1970s, however, the Japanese automobile manufacturer Mazda began to use Wankel engines exclusively. However, relatively high pollutant emissions, coupled with low gasoline mileage for automobiles of this size and weight, resulted in poor sales in the United States. Mazda ceased marketing Wankel-powered automobiles in the United States in the mid-1970s. Several American automobile manufacturers have experimented with Wankel-powered prototypes, but no production vehicles have emerged.
The Wankel engine is being used as a marine engine and in engine–electric generator installations, where its overall weight and fuel consumption have proved to be superior to those of a diesel engine or gas turbine generating equivalent power. Other projected applications include lawnmower and chainsaw engines. This wide range of applications is made possible by the fact that almost any size of Wankel engine is feasible.
An engine conceptually equivalent to the Wankel was developed jointly by Renault, Inc., and the American Motors Corp. It is sometimes called the Renault-Rambler engine. In this case, however, the rotor consists of a four-lobe arrangement, operating in a five-lobe chamber (Fig. 3). When a lobe moves into a cavity, which is analogous to the upward motion of the piston in the cylinder, the gas volume decreases, resulting in a compression process. The operation of this engine is detailed in Fig. 3. The fact that each cavity has two valves (intake and exhaust) represents a significant drawback. However, sealing between chambers may be simpler than in the Wankel; since each cavity acts as a combustion chamber, heat is evenly distributed around the housing, resulting in little thermal distortion.
Renault-Rambler engine.
Fig. 3 Renault-Rambler engine.
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Simple multirotor engine.
Fig. 4 Simple multirotor engine.
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It can be concluded that engines of the eccentric-rotor type are an integral part of the internal combustion engine scene. Their inherent simplicity, coupled with their advanced state of development, make them attractive alternatives to the piston engine in a number of applications.
Multirotor engines
These engines operate on some form of simple rotary motion. A typical design, shown in Fig. 4, operates as follows. A fuel-air mixture enters the combustion chamber through some type of valve. No compression takes place; rather, a spark plug ignites the mixture which burns in the combustion chamber, with a consequent increase in temperature and pressure. The hot gas expands by pushing against the two trochoidal rotors. The eccentric force on the left-hand rotor forces the rotors to rotate in the direction shown. Eventually, the combustion gases find their way out the exhaust.
The problems associated with all engines of this type are principally twofold: The absence of a compression phase leads to low engine efficiency, and sealing between the rotors is an enormously difficult problem. One theoretical estimate of the amount of work produced per unit of heat energy put into the engine (by the combustion process), called the thermal efficiency, is only 4%.
The Unsin engine (Fig. 5) replaces the trochoidal rotors with two circular rotors, one of which has a single gear tooth upon which the gas pressure acts. The second rotor has a slot which accepts the gear tooth. The two rotors are in constant frictional contact, and in a small prototype engine sealing apparently was adequate. The recommendation of its inventor was that some compression of the intake charge be provided externally for larger engines.
The Walley and Scheffel engines employ the principle of the engine in Fig. 4, except that in the former, four approximately elliptical rotors are used, while in the latter, nine are used. In both cases the rotors turn in the same clockwise sense, which leads to excessively high rubbing velocities. (The rotors are in contact to prevent leakage.) The Walter engine uses two different-sized elliptical rotors.
Unsin engine, with two circular rotors.
Fig. 5 Unsin engine, with two circular rotors.
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Revolving-block engines
These engines combine reciprocating piston motion with rotational motion of the entire engine block. One engine of this type is the Mercer (Fig. 6). In this case two opposing pistons operate in a single cylinder. Attached to each piston are two rollers which run on a track that consists of two circular arcs. When the pistons are closest together, the intake ports to the chambers behind the pistons are uncovered, admitting a fresh charge (Fig. 6a). At this moment a charge contained between the pistons has achieved maximum compression, and the spark plug fires. The pistons separate as combustion takes place between them, which results in a compression of the gases behind the pistons (Fig. 6b). However, the pistons moving apart force the rollers to move outward as well. This latter motion can only occur if the rollers run on their circular track, which consequently forces the entire engine block to rotate. When the pistons are farthest apart, the exhaust ports are uncovered and the combustion gases purged (Fig. 6c). At this same time the compressed fresh charge behind the pistons is transferrred to the region between the pistons to prepare for its recompression and combustion, which must occur because of the continuing rotation of the block.
Cycle of the Mercer engine. (a) Intake of fresh charge. (b) Compression of charge, following firing...
Fig. 6 Cycle of the Mercer engine. (a) Intake of fresh charge. (b) Compression of charge, following firing of spark plug. (c) Exhaust of combustion gases and transfer to compressed fresh charge.
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No doubt some of the fresh charge is lost to the exhaust during the transfer process. Stresses on the roller assembly and cylinder walls are likely to be quite high, which poses some design problems. Cooling is a further problem, since cooling of the pistons is difficult to achieve in this arrangement.
One the other hand, the reciprocating piston motion is converted directly to rotary motion, in contrast with the connecting rod–crank arrangement in the conventional piston engine. Also, no flywheel should be necessary since the entire rotating block acts to sustain the rotary inertia. Vibration will also be minimal.
Cycle of the Rajakaruna engine, showing distortion of the four-sided chamber as the surrounding housing...
Fig. 7 Cycle of the Rajakaruna engine, showing distortion of the four-sided chamber as the surrounding housing rotates.
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Walker engine, with elliptical rotor and two C-shaped rocking heads.
Fig. 8 Walker engine, with elliptical rotor and two C-shaped rocking heads.
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Cycle of one pocket of combustion gases in the Heydrich vane-type rotary engine.
Fig. 9 Cycle of one pocket of combustion gases in the Heydrich vane-type rotary engine.
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Leclerc-Edmon-Benstead engine.
Fig. 10 Leclerc-Edmon-Benstead engine.
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The Selwood engine is similar in operation, except that two curved pistons 180° opposed run in toroidal tracks. This design recalls the Tschudi cat-and-mouse engine, except that the pistons only travel through 30° of toroidal track. This motion forces the entire block to rotate. The Leath engine has a square rotor with four pistons, each 90° apart, with a roller connected to each. As in the Mercer engine, the reciprocating motion of the pistons forces the rollers to run around a trochoidal track which causes the entire block to rotate. The Porshe engine uses a four-cylinder cruciform block. Again, rollers are attached to each of the four pistons. In this arrangement power is achieved on the inward strokes of the piston. Finally, the Rajakaruna engine (Fig. 7) uses a combustion chamber whose sides are pin-jointed together at their ends. Volume changes result from distortion of the four-sided chamber as the surrounding housing, which contains a trochoidal track, rotates. The huge pins are forced against the track. As usual, cooling and lubrication problems will be encountered with this engine, as will excessive wear of the hinge pins and track.
The Ma-Ho engine, invented by G. Hofmann, uses four cylinders welded concentrically around a central shft. As the pistons oscillate in the cylinders as a result of the intake-compression-power-exhaust processes undergone by the fuel-air mixture, the pistons rotate a barrel cam to which they are connected by cam followers. Hence, the entire block, including the central shaft, is forced to rotate.
Engines of other types
Although the vast majority of rotary engines fall into one of the categories discussed above, several ingenious designs which do not are worthy of mention.
The Walker engine (Fig. 8) involves an elliptical rotor which rotates inside a casing containing two C-shaped rocking heads. The fuel-air mixture is drawn into combustion chambers on each side of the rotor; mixture cut-off occurs when the rotor is in the vertical position. As the rotor turns, the mixture undergoes compression, and combustion is initiated by spark plugs. Rotor momentum coupled with the expansion of the combustion gases forces the rotor to continue turning. Compensation for seal wear is made by adjusting the rocking heads closer to the rotor.
The Heydrich engine (Fig. 9) is a vane-type rotary engine which utilizes a small hole to “store” a quantity of high-temperature combustion gases from a previous firing. This gas is then used to ignite the subsequent fuel-air charge. The first charge is ignited by a glow plug. Floating seals make contact with the chamber wall.
The Leclerc-Edmon-Benstead engine (Fig. 10) has three combustion chambers defined by stationary vanes, a cylindrical stator, and end flanges. An output shaft passing through the chamber is geared to a circular, eccentrically mounted piston. As the chambers are fired sequentially, the movement of the piston forces the shaft to rotate. Slots in the flange control porting of the intake and exhaust gases. See also: Combustion chamber; Diesel cycle; Diesel engine; Gas turbine; Internal combustion engine; Otto cycle
Wallace Chinitz
Bibliography
R. F. Ansdale, Rotary combustion engines, Auto. Eng., vol. 53, no. 13, 1963, and vol. 54, nos. 1 and 2, 1965
W. Chinitz, Rotary engines, Sci. Amer., vol. 220, no. 2, 1969
H. E. Dark, The Wankel Rotary Engine, 1974
W. Froede, The Rotary Engine and the NSU Spider, Soc. Automot. Eng. Pap. 650722, October, 1965
K. Matsumoto et al., The Effects of Combustion Chamber Design and Compression Ratio on Emissions, Fuel Economy and Octane Number Requirement, SAE Pap. 770193, 1977
R. H. Thring, Gasoline engines and their future, Mech. Eng., vol. 15, no. 10, 1983
R. Wakefield, Revolutionary engines, Road Track, vol. 18, no. 3, 1966
F. Wankel, in R. F. Ansdale (ed.), Rotary Piston Machines: Classification of Design Principles for Engines, Pumps and Compressors, 1965
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