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.
In the 1940s Cyril Pullin developed the Powerwheel, a wheel with a rotating one-cylinder engine, clutch and drum brake inside the hub, but it never entered production.
Cars with rotary engines were built by American companies Adams-Farwell, Bailey, Balzer and Intrepid, amongst others.
Stephen Balzer of New York, a former watchmaker, constructed rotary engines in the 1890s. He was interested in the rotary layout for two main reasons:
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.
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.
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.
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.
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:
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.
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.
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.
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 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.