Rotary engine
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The rotary engine was an early type of internal combustion aircraft engine, used mostly in the years shortly before and during World War I. It has also been used in a few motorcycles and cars. By the early nineteen twenties the rotary aircraft engine was becoming obsolete, mainly because it could not operate efficiently at speeds greater than approximately 1600 RPM.
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[edit] Description
In concept, a rotary engine is simple. It is a standard Otto cycle engine, but instead of having an orthodox fixed cylinder block with rotating crankshaft as with the radial engine, the crankshaft remains stationary and the entire cylinder block rotates around it. In the most common form, the crankshaft was fixed solidly to an aircraft frame, and the propeller simply bolted onto the front of the cylinder block.
The effect of rotating the bulk of the engine's mass was an inherent large gyroscopic flywheel effect, smoothing out the power and reducing vibration. Vibration had been such a serious problem on other conventional piston engine designs that heavy flywheels had to be added. Because the cylinders themselves functioned as a flywheel, rotary piston engines typically had a power-to-weight ratio advantage over more conventional engines. Most rotary engines were arranged with the cylinders pointed outwards from a single crankshaft, in the same general form as a radial, but there were also rotary boxer engines[1] and even one-cylinder rotaries.
Like radial engines, rotaries were generally built with an odd number of cylinders (usually either 7 or 9), so that a consistent every-other-piston firing order could be maintained, providing smooth running.
[edit] Distinction between "Rotary" and "Radial" engines
Rotary and radial engines look strikingly similar when they are not running and can easily be confused. Both have cylinders arranged radially around a central crankshaft. In a radial engine the power output is provided through a conventional rotating crankshaft. In a rotary engine the engine itself, including its cylinders, rotates around a stationary crankshaft.
[edit] History in aircraft
Lawrence Hargrave first developed a rotary engine in 1889 using compressed air, intending for it to be used in powered flight. Weight of materials and lack of quality machining prevented it becoming an effective power unit.[2]
The first effective rotaries were built by Stephen Balzer, who was interested in the design for two main reasons:
- In order to generate 100 hp (75 kW) at the low rpm at which the engines of the day ran, the pulse resulting from each combustion stroke was quite large. To damp out these pulses, engines needed a large flywheel, which added weight. In the rotary design the engine acted as its own flywheel, thus rotaries could be lighter than similarly sized conventional engines.
- The cylinders had good cooling airflow over them, even when the aircraft in which they were mounted were at rest, which was important, as the alloys of the day were less advanced than is currently the case. Balzer's early designs relied on this feature to avoid the use of cooling fins, a normal feature of air-cooled engines.
Balzer's first designs were ready for use in 1899. He then became involved in Langley's Aerodrome attempts, which bankrupted him while he tried to make much larger versions.
The next major advance in the design was Louis and Laurent Seguin's Gnôme series from 1908. This design was developed from a German single-cylinder stationary engine intended for light industrial use, the Gnom, which the brothers were producing under license from Motorenfabrik Oberursel. They essentially took several Gnom cylinders and combined them on a common shaft to produce a seven-cylinder rotary. The Gnôme Omega No.1 still exists and is in the collection of the Smithsonian's National Air and Space Museum. A production version of the Omega then soon reached the aviation market, still as a 7-cylinder 50 hp (37 kW), which soon reached 80 hp (60 kW), and eventually 110 hp (80 kW). The engine was at this later 80 hp (60 kW) standard when World War I started, as the Gnôme Lambda, and the Gnome quickly found itself being used in a large number of aircraft designs. It was so good that it was licensed by a number of companies, including the German Oberursel firm who designed the original Gnom engine. Oberursel was later purchased by Fokker, whose 80 hp Gnôme Lambda copy was known as the Oberursel U.0. It was not at all uncommon for French Gnômes, as used in the earliest examples of the Bristol Scout biplane, to meet German versions, powering Fokker E.I Eindeckers, in combat, starting during the latter half of 1915.
The Gnôme (and its copies) had a number of features that made it unique, even among the rotaries. Notably, the fuel was mixed and sprayed into the center of the engine through a hollow crankshaft, and then into the cylinders through the piston itself, a single valve on the top of the piston let the mixture in when opened. The valves were counter balanced so that only a small force was needed to open them, and releasing the force closed the valve without any springs. The center of the engine is normally where the oil would be, and the fuel would wash it away. To fix this, the oil was mixed in liberal quantities with the fuel, and the engine spewed smoke due to burning oil. Castor oil was the lubricant of choice, its gum-forming tendency being irrelevant in a total-loss lubrication system. The result is that the engines threw a mist of unburnt fuel and castor oil; the pilot's scarf being used to wipe this from their goggles. Finally, the Gnôme had no throttle or carburetor. Since the fuel was being sprayed into the spinning engine, the motion alone was enough to mix the fuel fairly well.
Of course with no throttle, the engine was either on or off, so something as simple as reducing power for landing required the pilot to cut the ignition. "Blipping" the engine on and off with the "blip switch" gave the characteristic sputtering sound as though the engine was nearly stalling, though it did not stall as quickly as conventional engines due to its great rotational inertia. For that reason a few of the nine-cylinder rotaries accomplished a partial "throttle" functionality by switching off a number of the cylinders instead of all nine of them (typical configurations ran on 1, 3, or 6 cylinders when not using all 9)[3], when the "coupe switch" was depressed to cut the spark. This feature is known to exist in the Gnôme Monosoupape series, and long after WWI was demonstrated by a 160hp Monosoupape-powered reproduction Camel at Old Rhinebeck Aerodrome in flight in the 1990s, some surviving documentation regarding the Fokker Eindecker shows a rotary selector switch to cut out a selected number of cylinders suggesting the German rotaries also had this feature. The more standard arrangement found in other rotary engines is a traditional two piece air/fuel throttles, however these engines could also be controlled with a blip-switch. Indeed the blip-switch is commonly used during landing these days as despite the original factory manuals often discouraging cutting the ignition dead and emphasizing the use of the throttle the blip switch allows pilots a more reliable, quick source of power that lends itself to modern airfields.[3]
Throughout the early period of the war, the power-to-weight ratio of the rotaries remained ahead of their competition. They were used almost universally in fighter aircraft, while traditional water cooled designs were used on larger aircraft. The engines had a number of disadvantages, notably very high fuel consumption, partially because the engine was typically run at full throttle, and also because the valve timing was often less than ideal.
The rotating mass of the engine also made it, in effect, a large gyroscope. During level flight the effect wasn't especially apparent, however under turning it was far more pronounced. Due to the direction of the force right-turns required some degree of effort and happened slowly, while left-turns were almost instantaneous.[4] IN some aircraft this could be advantageous in situations such as dogfights while the Sopwith Camel suffered to such an extent it had a dangerous reputation[citation needed].
Even before the First World War attempts were made to overcome the inertia problem of rotary engines. As early as 1906 Charles Benjamin Redrup had demonstrated to the Royal Flying Corps at Hendon a 'Reactionless' engine in which the crankshaft rotated in one direction and the cylinder block in the opposite direction, each one driving a propeller. A later development of this was the 1914 reactionless 'Hart' engine designed by Redrup in which there was only one propeller connected to the crankshaft, but it rotated in the opposite direction to the cylinder block, thereby largely cancelling out rotational inertia. This proved too complicated for the Air Ministry and Redrup changed the design to a static radial engine which later flew in Vickers FB12b and FB16 aircraft. [5]
As the war progressed, aircraft designers demanded ever increasing amounts of power. Inline engines were able to meet this demand by improving their upper rev limits, which meant more power. Improvements in valve timing, ignition systems, and lightweight materials made these higher revs possible, and by the end of the war the average engine had increased from 1,200 rpm to 2,000. The rotary was not able to do the same due to the drag of the cylinders through the air. For instance, if an early-war model of 1,200 rpm increased to only 1,400, the drag on the cylinders increased 36%, as air drag increases with the square of velocity. At lower speeds, drag could simply be ignored, but as speeds increased, the rotary was putting more and more power into spinning the engine, and less into driving the propeller.
One clever attempt to rescue the design was made by Siemens AG. The crankcase (with the propeller still fastened directly to the front of it) and cylinders spun counterclockwise at 900 rpm, as seen externally from a "nose on" viewpoint, while the crankshaft and other internal parts spun clockwise at the same speed. This was achieved by the use of bevel gearing at the rear of the crankcase, resulting in the Siemens-Halske Sh.III, running at 1800 rpm with little net torque. It was also apparently the only rotary engine to use a normal carburetor, able to be controlled by a conventional throttle, just as in an in-line engine. Used on the Siemens-Schuckert D.IV fighter, the new engine created what is considered by many to be the best fighter aircraft design of the war.
One new rotary powered aircraft, Fokker's own D.VIII, was designed at least in part to provide some use for their Oberursel factory's backlog of now-useless 110 hp Ur.II engines, themselves clones of the Le Rhône 9J rotary. By the time the war ended, the rotary engine had become obsolete, and on the whole it disappeared from use quite quickly. The British Royal Air Force probably used rotary engines for longer than most other operators - RAF's standard post-war fighter, the Sopwith Snipe, used the Bentley BR2 rotary, and the standard trainer, the Avro 504K, had a universal mounting to allow several types of low powered rotary, of which there was a large surplus supply. The cheapness of war-surplus engines had to be balanced against their poor fuel economy and the expense of their total loss lubrication system.
By the mid-1920s, rotaries had been more or less completely displaced even in British service, largely by the new generation of air-cooled radials.
[edit] Use in cars and motorcycles
Although rotary engines were mostly used in aircraft, there were also a few cars and motorcycles with rotary engines. The most famous motorcycle (probably because of winning many races) is the Megola, which had a rotary engine inside the front wheel. Another motorcycle with a rotary engine was Charles Redrup's 1912 Redrup Radial, which was a 303cc rotary engine fitted to a number of motor-cycles by Redrup which had rotating three-cylinder engines in their frames.
In 1904, the Barry engine, also designed by Charles Redrup was built in Wales, a rotating 2 cylinder boxer engine[1] inside a motorcycle frame, weighing 6.5 kg. In the 1940s Cyril Pullin developed the Powerwheel, a wheel with rotating one-cylinder engine, clutch and drum brake inside the hub but it never went into serial production.
Cars with rotary engines were built (among others) by American companies Adams-Farwell, Bailey, Balzer and Intrepid.
[edit] Other rotary engines
Besides the configuration described in this article with cylinders moving around a fixed crankshaft, several other very different engine designs can also be described as rotary engines. The most notable pistonless rotary engine, the Wankel rotary engine has also been used in cars (notably by NSU in the Ro80 and by Mazda in a variety of cars such as the RX-series which includes the popular RX-7 and RX-8), as well as in some experimental aviation applications.
[edit] Notes and references
- ^ a b Charles Benjamin Redrup. Retrieved on 2008-04-11.
- ^ Hargrave, Lawrence (1850 – 1915). Australian Dictionary of Biography Online.
- ^ a b Willie, Chad. Rotary Engines and why they spin (HTML). Old Rhinebeck Aerodrome. Retrieved on 2008-05-01.
- ^ McCutcheon, Kimble D.. Gnome Monosoupape Type N Rotary (PDF). Aircraft Engine Historical Society. Retrieved on 2008-05-01.
- ^ William Fairney (2007). The Knife and Fork Man - The Life and Works of Charles Benjamin Redrup. Diesel Publishing. ISBN 978-0-9554455-0-7.
[edit] External links
- Animation of Gnome Rotary in action
- Ray Williams' operable miniature rotary engine website
- A rotary engine that runs solely on compressed air
- Charles Redrup's range of engines
[edit] See also
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