|An 80 horsepower rated Le Rhône 9C, a typical rotary engine of WWI. The copper pipes carry the fuel-air mixture from the crankcase to the cylinder heads, collectively as an intake manifold.|
The rotary engine was an early type of internal-combustion engine, usually designed with an odd number of cylinders per row in a radial configuration, in which the crankshaft remained stationary and the entire cylinder block rotated around it. Its main application was in aviation, although it also saw use in a few early motorcycles and automobiles.
This type of engine was widely used as an alternative to conventional inline engines (straight or V) during World War I and the years immediately preceding that conflict. They have been described as "a very efficient solution to the problems of power output, weight, and reliability".
By the early 1920s, however, the inherent limitations of this type of engine had rendered it obsolete, with the power output increasingly going into overcoming the air-resistance of the spinning engine itself. The rotating mass of the engine also had a significant gyroscopic precession: depending on the type of aircraft, this produced stability and control problems, especially for inexperienced pilots. Another factor in the demise of the rotary was the fundamentally inefficient use of fuel and lubricating oil caused in part by the need for the fuel/air mixture to be aspirated through the hollow crankshaft and crankcase, as in a two-stroke engine.
A rotary engine is essentially a standard Otto cycle engine, but instead of having a fixed cylinder block with rotating crankshaft as with a conventional 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 the airframe, and the propeller was simply bolted onto the front of the crankcase.
Three key factors contributed to the rotary engines success at the time:
- Smooth running: Rotaries delivered power very smoothly because (relative to the engine mounting point) there are no reciprocating parts, and the relatively large rotating mass of the cylinders acted as a flywheel.
- Weight advantage: many conventional engines had to have heavy flywheels added to smooth out power impulses and reduce vibration. Rotary engines gained a substantial power-to-weight ratio advantage by having no need for an added flywheel.
- Improved cooling: when the engine was running the rotating cylinder block created its own fast-moving cooling airflow, even with the aircraft at rest.
Most rotary engines were arranged with the cylinders pointing outwards from a single crankshaft, in the same general form as a radial, but there were also rotary boxer engines 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, to provide smooth running. Rotary engines with an even number of cylinders were mostly of the "two row" type.
Distinction between "Rotary" and "Radial" engines 
Rotary and radial engines look strikingly similar when they are not running and can easily be confused, since both have cylinders arranged radially around a central crankshaft. Unlike the rotary engine, however, radial engines use a conventional rotating crankshaft in a fixed engine block.
Rotary engine control 
Monosoupape rotaries 
It is often asserted that rotary engines had no carburetor and hence power could only be reduced by intermittently cutting the ignition using a "blip" switch. This was literally true only of the "Monosoupape" (single valve) type in which the air supply was taken in through the exhaust valve, and so could not be controlled via the crankcase intake. The "throttle" (fuel valve) of a monosoupape therefore provided only a very limited degree of speed regulation, as opening it made the mixture too rich, while closing it made it too lean (in either case quickly stalling the engine, or damaging the cylinders). Early models featured a pioneering form of variable valve timing in an attempt to give greater control, but this caused the valves to burn and therefore it was abandoned.
The only way of running a Monosoupape engine smoothly at reduced revs was with a switch that changed the normal firing sequence so that each cylinder fired only once per two or three engine revolutions, but the engine remained in perfect balance. As with excessive use of the "blip" switch: running the engine on such a setting for too long resulted in large quantities of unburned fuel and oil in the exhaust, and gathering in the lower cowling, where it was a notorious fire hazard.
"Normal" rotaries 
Most rotaries however, had normal inlet valves, so that the fuel (and lubricating oil) were taken into the cylinders already mixed with air - as in a normal four-stroke engine. Although a conventional carburetor, with the ability to keep the fuel/air ratio constant over a range of throttle openings was precluded by the spinning cylinder block, it was possible to adjust the air supply through a separate flap valve or "bloctube". The pilot needed to set the throttle to the desired setting (usually full open) and then adjust the fuel/air mixture to suit using a separate "fine adjustment" lever that controlled the air supply valve. Due to the rotary engine's large rotational inertia, it was possible to adjust the appropriate fuel/air mixture by trial and error without stalling it, although this varied between different types of engine, and in any case it required a good deal of practice to acquire the necessary "knack". After starting the engine with a known setting that allowed it to idle, the air valve was opened until maximum engine speed was obtained.
Throttling a running engine back to reduce revs was possible by closing off the fuel valve to the required position while re-adjusting the fuel/air mixture to suit. This process was also tricky, so that "throttling back", especially when landing, was often accomplished by temporarily cutting the ignition using the blip switch.
Cutting of cylinders using ignition switches had the drawback of allowing fuel to continue to pass through the engine, causing the spark plugs to oil up and prevent the engine from restarting. A raw fuel/oil mix would also collect in the cowling. As this could cause a serious fire when the switch was released it became common practice for part or all of the bottom of the basically circular cowling fitted to most rotary engines to be cut away, or fitted with drainage slots.
By 1918 a Clerget handbook advised that all necessary control was to be effected using the fuel and air controls, and the engine was to be stopped and started by turning the fuel on and off. The landing procedure recommended involved shutting off the fuel using the fuel lever, while leaving the blip switch on. The windmilling propeller allowed the engine to continue to spin without delivering any power as the aircraft descended. It was important to leave the ignition on to allow the spark plugs to continue to spark and keep them from oiling up, so that the engine could (if all went well) be restarted simply by re-opening the fuel valve. Pilots were advised to avoid the use of ignition cut out switches as it would eventually damage the engine.
Pilots of surviving or reproduction aircraft fitted with rotary engines still find, however, that the blip switch is useful while landing rotary-engined aircraft, as it allows pilots a more reliable, quicker source of power in case it should be needed, rather than risking a sudden engine stall, or the failure of a windmilling engine to restart as expected, at the worst possible moment.
Félix Millet showed a 5-cylinder rotary engine built into a bicycle wheel at the Exposition Universelle in Paris in 1889. Millet had patented the engine in 1888, so must be considered the pioneer of the internal combustion rotary engine. A machine powered by his engine took part in the Paris-Bordeaux-Paris race of 1895 and the system was put into production by Darracq in 1900.
Lawrence Hargrave first developed a rotary engine in 1889 using compressed air, intending it to be used in powered flight. Weight of materials and lack of quality machining prevented it becoming an effective power unit.
- 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 low airspeed attainable by aircraft of the time provided limited cooling airflow, and alloys of the day were less advanced than they are now. Balzer's early designs even dispensed with cooling fins, although subsequent rotaries did have this common feature of air-cooled engines.
Balzer produced a 3-cylinder, rotary engined car in 1894, then later became involved in Langley's Aerodrome attempts, which bankrupted him while he tried to make much larger versions of his engines. Balzer's rotary engine was later converted to static radial operation by Langley's assistant, Charles M. Manly, creating the notable Manly-Balzer engine.
De Dion-Bouton 
The Adams-Farwell was another early US rotary engine which was being manufactured for use in automobiles by 1901. Emil Berliner sponsored its development as a lightweight power unit for his unsuccessful helicopter experiments. Adams-Farwell engines later powered fixed-wing aircraft in the US after 1910. It has also been asserted that the Gnôme design was derived from the Adams-Farwell, since an Adams-Farwell car is reported to have been demonstrated to the French Army in 1904. In contrast to the later Gnôme engines, and much like the later Clerget 9B and Bentley BR1 aviation rotaries, the Adams-Farwell rotaries had conventional exhaust and inlet valves mounted in the cylinder heads.
The Gnome engine was the work of the three Seguin brothers, Louis, Laurent and Augustin. They were gifted engineers and the grandsons of famous French engineer Marc Seguin. In 1906 the eldest brother, Louis, had formed the Société des Moteurs Gnome to build stationary engines for industrial use, having licensed production of the Gnom single-cylinder stationary engine from Motorenfabrik Oberursel, who would in turn build licensed Gnome engines for German aircraft during World War I.
Louis was joined by his brother Laurent who designed a rotary engine specifically for aircraft use, using Gnom engine cylinders. The brothers' first experimental engine was a 5-cylinder model which developed 34 hp (25 kW), and which was a radial rather than a rotary. They then turned to rotary engines in the interests of better cooling, and the world's first production rotary engine, the 7-cylinder, 50 hp (37 kW) "Omega" was shown at the 1908 Paris automobile show. The first Gnome Omega built still exists, and is now in the collection of the Smithsonian's National Air and Space Museum. The Seguins used the highest strength material available - recently developed nickel steel alloy - and kept the weight down by machining components from solid metal, using the best American and German machine tools to create the engine's components; the cylinder wall of a 50 hp Gnome was only 1.5 mm thick, while the connecting rods were milled with deep central channels to reduce weight. While somewhat low powered in terms of horsepower per litre, its power-to-weight ratio was an outstanding 1 hp (0.75 kW) per kg.
The following year, 1909, the inventor Roger Ravaud fitted one to his Aéroscaphe, a combination hydrofoil/aircraft, which he entered in the motor boat and aviation contests at Monaco. However, it was Henry Farman's use of the Gnome at the famous Rheims aircraft meet that year which brought it to prominence, when he won the Grand Prix for the greatest non-stop distance flown - 180 kilometres (110 mi), also a world record for endurance flight. The very first successful seaplane flight, of Henri Fabre's Le Canard, was powered by a Gnome Omega on March 28, 1910 near Marseille.
Production of Gnome rotaries increased rapidly, with some 4,000 being produced before World War I, and Gnome also produced a two-row version (the 100 h.p. Double Omega), the larger 70 hp Gnome Lambda and the 160 hp two-row Double Lambda. By the standards of other engines of the period, the Gnome was considered not particularly temperamental, and was credited as the first engine able to run for ten hours between overhauls.
In 1913 the Seguin brothers introduced the new Monosoupape ("single valve") series, which replaced inlet valves in the pistons by using a single valve in each cylinder head which doubled as inlet and exhaust valve. The engine speed was controlled by varying the opening time and extent of the exhaust valves using levers acting on the valve tappet rollers, a system which was later abandoned due to causing burning of the valves. The weight of the Monosoupape was slightly less than the earlier two-valve engines and it used less lubricating oil. The 100 hp Monosoupape was built with 9 cylinders, and developed its rated power at 1,200 rpm. The later 160 hp nine-cylinder Gnome 9N rotary engine used the Monosoupape valve design, and was the last known rotary engine design to use such a cylinder head valving format.
Rotary engines produced by the Clerget and Le Rhône companies used conventional pushrod-operated valves in the cylinder head, but used the same principle of drawing the fuel mixture through the crankshaft, with the Le Rhônes having prominent copper intake tubes running from the crankcase to the top of each cylinder to admit the intake charge.
The 80 hp (60 kW) seven-cylinder Gnome was the standard at the outbreak of World War I, as the Gnome Lambda, and it 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 Motorenfabrik Oberursel firm who designed the original Gnom engine. Oberursel was later purchased by Fokker, whose 80 hp Gnome Lambda copy was known as the Oberursel U.0. It was not at all uncommon for French Gnomes, as used in the earliest examples of the Bristol Scout biplane, to meet German versions, powering Fokker E.I Eindeckers, in combat, from the latter half of 1915 on.
The only attempts to produce twin-row rotary engines in any volume were undertaken by Gnome, with their Double Lambda fourteen-cylinder 160 hp design, and with the German Oberursel firm's early World War I clone of the Double Lambda design, the U.III of the same power rating. While an example of the Double Lambda went on to power one of the Deperdussin Monocoque racing aircraft to a world-record speed of nearly 204 km/h (126 mph) in September 1913, the Oberursel U.III is only known to have been fitted into a few German production military aircraft, the Fokker E.IV fighter monoplane and Fokker D.III fighter biplane, both of whose failures to become successful combat types were partially due to the poor quality of the German powerplant, which was prone to wearing out after only a few hours of combat flight.
World War I 
The favourable power-to-weight ratio of the rotaries was their greatest advantage. While larger, heavier aircraft relied almost exclusively on conventional in-line engines, many fighter aircraft designers preferred rotaries right up to the end of the war.
Rotaries 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. Oil consumption was also very high. Due to the primitive nature of the carburetion and the absence of a true sump the lubricating oil had to be added to the fuel/air mixture. This meant that the engine fumes were heavy with smoke caused by partially burnt oil. Castor oil was the lubricant of choice, as its lubrication properties were not affected by the presence of the fuel, its gum-forming tendency being irrelevant in a total-loss lubrication system. An unfortunate side-effect was that World War I pilots inhaled and swallowed a considerable amount of the oil during flight, leading to persistent diarrhoea. Flying clothing worn by rotary engine pilots was routinely soaked with oil.
The rotating mass of the engine also made it, in effect, a large gyroscope. During level flight the effect was not especially apparent, however when turning the gyroscopic precession became noticeable. Due to the direction of the engine's rotation the left-turns required some degree of effort and happened relatively slowly, combined with a tendency to nose-up, while right-turns were almost instantaneous, with a tendency for the nose to drop. In some aircraft this could be advantageous in situations such as dogfights, while the Sopwith Camel suffered to such an extent that it required left rudder for both left and right turns and could be extremely hazardous if full power was used over the top of a loop at low airspeeds. Trainee Camel pilots were warned to attempt their first hard right turns only at altitudes above 1,000 ft (300 m). Predictably, the Camel's most famous German foe, the Fokker Dr.I triplane, also used a rotary engine, usually the Oberursel Ur.II clone of the French-built Le Rhone 9J 110 hp powerplant.
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 negative effects. This proved too complicated for reliable operation and Redrup changed the design to a static radial engine which was later tried in the experimental Vickers F.B.12b and F.B.16 aircraft, unfortunately without success.
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 rotating cylinders through the air. For instance, if an early-war model of 1,200 rpm increased its revs to only 1,400, the drag on the cylinders increased 36%, as air drag increases with the square of velocity. At lower rpm, drag could simply be ignored, but as the rev count rose, the rotary was putting more and more power into spinning the engine, with less remaining to provide useful thrust through the propeller.
One clever attempt to rescue the design, in a similar manner to Redrup's British "reactionless" engine concept, 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, so the set was effectively running at 1800 rpm. This was achieved by the use of bevel gearing at the rear of the crankcase, resulting in the eleven-cylindered Siemens-Halske Sh.III, with less drag and less net torque. Used on several late war types, notably the Siemens-Schuckert D.IV fighter, the new engine’s low running speed, coupled with large, coarse pitched propellers, gave types powered by it outstanding rates of climb.
One new rotary powered aircraft, Fokker's own D.VIII, was designed at least in part to provide some use for the Oberursel factory's backlog of otherwise redundant 110 hp (82 kW) Ur.II engines, themselves clones of the Le Rhône 9J rotary.
Because of the Allied blockade of shipping, the Germans were increasingly unable to obtain from India the castor oil necessary to properly lubricate their rotary engines. Substitutes were never entirely satisfactory - causing increased running temperatures and reduced engine life.
By the time the war ended, the rotary engine had become obsolete, and it disappeared from use quite quickly. The British Royal Air Force probably used rotary engines for longer than most other operators - the 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 the use of several different types of low powered rotary, of which there was a large surplus supply. However, the cheapness of war-surplus engines had to be balanced against their poor fuel efficiency and the operating 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 "stationary" radials.
Use in cars and motorcycles 
Although rotary engines were mostly used in aircraft, a few cars and motorcycles were built with rotary engines. Perhaps the first was the Millet motorcycle of 1892. A famous motorcycle, winning many races, was 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 three-cylinder 303 cc rotary engine fitted to a number of motorcycles by Redrup.
Other rotary engines 
Besides the configuration described in this article with cylinders moving around a fixed crankshaft, several other very different engine designs are also called "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.
In the late 1970s a concept engine called the Bricklin-Turner Rotary Vee was being tested. The Rotary Vee is similar in configuration to the elbow steam engine. The Rotary Vee uses piston pairs connected as solid V shaped members with each end floating in a pair of rotating cylinders clusters. The rotating cylinder cluster pair are set with their axes at a wide V angle. The pistons in each cylinder cluster move parallel to each other instead of a radial direction, This engine design has not yet gone into production. The Rotary Vee was intended to power the Bricklin SV-1.
See also 
- Petrol engine
- Monosoupape engine
- Manly-Balzer engine
- Nutating disc engine
- Wankel rotary engine
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- Smithsonian NASM Gnôme Omega No.1 page
- Smithsonian NASM Le Rhône 9J page
- 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
- Video of 1909 Gnome Omega Engine - Run April 2009
- Bricklin-Turner Rotary Vee Engine
- Bi-rotary engine from Franky Devaere