The rotary engine is a type of reciprocating internal combustion engine, usually designed with an odd number of cylinders per row in a radial configuration. In most designs, the engine's crankshaft remained stationary in operation, while the entire crankcase and its attached cylinders rotated around it as a unit. Its main application was in aviation, although it also saw use in some motorcycles and automobiles.
Rotary aero engines were 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By the early 1920s, rotary aero engines had largely been superseded by other engine configurations.
Description Distinction between "rotary" and "radial" engines A rotary engine is essentially an Otto cycle engine, usually with cylinders arranged radially around a central crankshaft like a conventional radial engine, but instead of having a fixed crankcase and cylinders with a rotating crankshaft, the crankcase and cylinders rotate as a unit relative to the crankshaft. In the most common form, the crankshaft was fixed solidly to the airframe, and the propeller was bolted to the front of the crankcase.
This difference also has a significant impact on design (lubrication, ignition, fuel admission, cooling, etc.) and operation (see below).
The Musée de l'Air et de l'Espace in Paris has on display a special, "sectioned" working model of an engine with seven radially disposed cylinders. It alternates between rotary and radial modes to demonstrate the difference between the internal motions of the two types of engine.
Arrangement Like "fixed" radial engines, rotaries were generally built with an odd number of cylinders (usually 5, 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.
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.
Advantages and drawbacks Three key factors contributed to the rotary engine's success at the time:Air Board Technical Notes, RAF Air Board, 1917. Reprinted by Camden Miniature Steam Services, 1997 Smooth running: Rotaries delivered power very smoothly because (relative to the engine mounting point) there were no reciprocating parts, and the relatively large rotating mass of the crankcase/cylinders (as a unit) acted as a flywheel. Improved cooling: when the engine was running, the rotating crankcase/cylinder assembly created its own fast-moving cooling airflow, even with the aircraft at rest. Weight advantage: rotaries shared with other radial configuration engines the advantage of a small, flat crankcase. The superior air-cooling imparted by the moving engine also meant that cylinders could be made with thinner walls and shallower cooling fins. Their power-to-weight ratio was further enhanced by the flywheel effect of the rotating engine assembly.
As stationary engines became more reliable and achieved better specific weights and fuel consumption, rotary aero engines were gradually superseded. Rotary aero engines generally used a total-loss oiling system. In order to reach the whole engine, the lubricant was supplied to the crankcase through the hollow crankshaft, but the centrifugal force of the revolving crankcase made collection and recirculation difficult. In commonly used systems, the lubricant entered the crankcase with the fuel-air mixture, as in many two-stroke engines. Increases in power generally came with increases in mass and size,for instance, compare Gnome Monosoupape to Bentley BR2 increasing the gyroscopic effects of the rotating mass of the engine. Aircraft with these engines could therefore have unusual handling characteristics, especially for inexperienced pilots. Increasing amounts of power were consumed in overcoming the air resistance of the spinning engine. Some rotary engines had limited or difficult engine controls (see below), which could result in fuel waste.
The late World War I Bentley BR2 was the largest and most powerful rotary engine; it reached a point beyond which this type of engine could not be further developed, and it was the last of its kind to be adopted into RAF service.
Rotary engine control
Monosoupape rotaries It is often asserted that rotary engines had no throttle and hence power could only be reduced by intermittently cutting the ignition using a "blip" switch. This was particularly characteristic of the "Monosoupape" (single-valve) type, which took most of its air into the cylinder through the exhaust valve, which remained open for a portion of the downstroke of the piston. Thus, the mixture of fuel and air in the cylinder could not be controlled by throttling the crankcase intake. The fuel control of a Monosoupape provided only a limited degree of speed regulation, as opening it made the mixture too rich, while closing it made it too lean; either extreme could quickly stall the engine or cause damage. Early models featured a form of variable valve timing in an attempt to provide greater control, but this caused the valves to burn and was therefore abandoned.
The only way of running some Monosoupape engines smoothly at reduced revolutions was with a switch that changed the normal firing sequence, so that each cylinder fired only once per two or three engine revolutions while the engine remained more or less in balance. However, the fuel charge and oil continued to enter the cylinders and were exhausted unburned. Prolonged use of the switch could result in large quantities of unburned fuel and oil collecting in the exhaust and lower cowling, creating a fire hazard.
"Normal" rotaries Most rotaries had conventional inlet valves, so that the fuel and lubricating oil were taken into the cylinders already mixed with air, as in a conventional four-stroke engine. Although the spinning crankcase precluded the use of a conventional carburettor capable of maintaining a constant fuel-air ratio over a range of throttle openings, 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 position and then adjust the fuel-air mixture using a separate "fine adjustment" lever that controlled the air supply valve, in the manner of a manual choke control. 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 engine types and required considerable practice.
Throttling a running engine back to reduce revolutions was possible by closing the fuel valve to the required position while readjusting the fuel-air mixture to suit. This process also required careful adjustment, so reducing power, especially when landing, was often accomplished instead by intermittently cutting the ignition using the blip switch.
Cutting cylinders using ignition switches had the drawback of allowing fuel to continue passing through the engine, oiling the spark plugs and making smooth restarting problematic. The unburned oil-fuel mixture could also collect in the cowling, creating a fire hazard when ignition was restored. Part or all of the bottom of the generally circular cowling on many rotary-engined aircraft was cut away or fitted with drainage slots.
Pilots of surviving or reproduction aircraft fitted with rotary engines still find that the blip switch is useful while landing, as it provides a more reliable, quicker way to initiate power if needed, rather than risk a sudden engine stall, or the failure of a windmilling engine to restart at the worst possible moment.
History Rotary steam engines were discussed in an 1847 Scientific American article as being impractical for general use.
Millet
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 and Company London in 1900.
Hargrave Lawrence Hargrave first developed a rotary engine in 1889 using compressed air, intending to use it in powered flight. Materials weight and lack of quality machining prevented it becoming an effective power unit.
Balzer Stephen M. Balzer of New York, a former watchmaker, constructed rotary engines in the 1890s. He was interested in the rotary layout for two main reasons: To generate 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 was at rest—which was important, as the low airspeed of aircraft of the time provided limited cooling airflow, and alloys of the day were less advanced. Balzer's early designs even dispensed with cooling fins, though 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 famous De Dion-Bouton company produced an experimental 4-cylinder rotary engine in 1899. Though intended for aviation use, it was not fitted to any aircraft.
Adams-Farwell
The Adams-Farwell firm's automobiles, with the firm's first rolling prototypes using 3-cylinder rotary engines designed by Fay Oliver Farwell in 1898, led to production Adams-Farwell cars with first the 3-cylinder, then very shortly thereafter 5-cylinder rotary engines later in 1906, as another early American automaker utilizing rotary engines expressly manufactured for automotive use. Emil Berliner sponsored its development of the 5-cylinder Adams-Farwell rotary engine design concept 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.
Gnome
The Gnome engine was the work of the three Seguin brothers, Louis, Laurent and Augustin. They were talented 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, in turn, built licensed Gnome engines for German aircraft during World War I.