Rotary engine
The rotary engine is an early type of internal combustion engine, usually designed with an odd number of cylinders per row in a radial configuration. 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 a few early motorcycles and automobiles.
This type of engine was widely used as an alternative to conventional inline engines during World War I and the years immediately preceding that conflict. It has been described as "a very efficient solution to the problems of power output, weight, and reliability".
By the early 1920s, the inherent limitations of this type of engine had rendered it obsolete.
Description
Distinction between "rotary" and "radial" engines
A rotary engine is essentially a standard Otto cycle engine, with cylinders arranged radially around a central crankshaft just like a conventional radial engine, but instead of having a fixed cylinder block with rotating crankshaft, 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 to the front of the crankcase.This difference also has much impact on design and functioning.
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, 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:- Smooth running: Rotaries delivered power very smoothly because there are no reciprocating parts, and the relatively large rotating mass of the crankcase/cylinders 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 in comparison with engines that required an added flywheel for smooth running.
- Rotary engines had a fundamentally inefficient total-loss oiling system. In order to reach the whole engine, the lubricating medium needed to enter the crankcase through the hollow crankshaft; but the centrifugal force of the revolving crankcase was directly opposed to any re-circulation. The only practical solution was for the lubricant to be aspirated with the fuel/air mixture, as in most two-stroke engines.
- Power increase also came with mass and size increases, multiplying gyroscopic precession from the rotating mass of the engine. Thus aircraft with these engines had stability and control problems, especially for inexperienced pilots.
- Power output increasingly went into overcoming the air resistance of the spinning engine.
- Engine controls were tricky, and resulted in fuel waste.
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 only true of the "Monosoupape" type, which took most of the 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 via the crankcase intake. The "throttle" 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. 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 more or less in balance. However, the fuel charge and oil was still pulled into the cylinders and exhausted normally, it simply wasn't burned. Running the engine with the switch on 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 had normal inlet valves, so that the fuel was 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 crankcase; 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 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 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 letting fuel continue to pass through the engine, oiling up the spark plugs and making smooth restarting problematic. Also, the raw oil-fuel mix could 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 on most rotary engines to be cut away, or fitted with drainage slots.
By 1918 a Clerget handbook advised maintaining all necessary control by using the fuel and air controls, and starting and stopping the engine by turning the fuel on and off. The recommended landing procedure involved shutting off the fuel using the fuel lever, while leaving the blip switch on. The windmilling propeller made the engine 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 be restarted simply by re-opening the fuel valve. Pilots were advised to not use an ignition cut out switch, as it would eventually damage the engine.
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
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
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
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.