- Alternative meaning: Wankel engine or Quasiturbine
The rotary engine was a common type of internal combustion aircraft engine in the early years of the 20th century. It was also used in a few motorcycles and cars.
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, 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 Quasiturbine is base on a different principle than the Wankel engine, but can be used in similar applications.
The effect of rotating such a large mass was the creation of a large gyroscopic flywheel, smoothing out the power and reducing vibration. Vibration was such a serious problem on early engines that a flywheel would otherwise have to be added as a separate block of metal, so the rotaries had a somewhat better power-to-weight ratio than other designs.
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 and even one cylinder rotaries.
History in Aircraft
The first effective rotaries were built by Stephen Balzer, who was interested in the design for two main reasons:
- at the low RPM engines of the day ran at, the force of any one explosion needed to generate 100 hp (75 kW) was quite large. In order to damp out these pulses, engines needed to mount a large flywheel, which was basically dead weight. In the rotary design the engine itself doubled as its flywheel, so rotaries were automatically lighter than similarly sized engines.
- the cylinders had good airflow over them even when sitting still, which was an important concern given the alloys they had to work with. Balzer's early engines did not even use cooling-fins, a feature of every other air-cooled design and one that is complex to manufacture.
Another advantage, not realized at first, is that the pistons do not actually reciprocate, but orbit a common center. This leads to smoother running. Balzer's first designs were ready for use in 1899, at which time they were the most advanced in the world. Other aircraft engines would not catch up in performance for a decade. 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 Lauren Seguin's "Gnome" from 1908. Originally a 5-cylinder 50 hp (37 kW) engine, the production versions were scaled up to a 7-cylinder 50 hp (37 kW), which soon reached 80 hp (60 kW), and then 110 hp (80 kW). The engine was at this later standard when WWI started, 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, include the German Oberursel firm, later purchased by Fokker. It was not at all uncommon for French Gnomes to meet German versions in combat.
The Gnome (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 than only a small force was needed to open them, and releasing the force closed them 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 the burning oil. Finally, the Gnome had no throttle or carburetor, with the fuel 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.
Throughout the early period of the war, the power-to-weight ratio of the rotaries remained ahead of that 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 poor fuel consumption because the engine was always "full throttle". In combat the huge "flywheel" the rotary had originally been designed to create turned out to result in tricky handling due to gyroscope effects as well. But they maintained their edge through a series of small upgrades, and many newer designs continued to use them.
1918 saw the introduction of the inline powered Fokker D.VII. Through superb design the D-VII was able to dogfight with the rotaries, and outclimb and outrun them with ease due to its 185 hp (140 kW) engine. Aircraft had evolved so that speed had become the most important aspect of ability, leading to the need for more power. Larger rotaries were attempted, but the gyroscopic effects were overwhelming and they proved to be largely unworkable. Inline engines were able to increase power through increased RPM, another trick the rotary couldn't match due to the massive amount of metal being spun. By the end of the year only a single new rotary was designed, Fokker's own D-VIII, designed solely to provide some use for their Oberursel factory. When the war ended, the rotary disappeared almost instantly, with WWI engines being used for training for a short time until their poor fuel economy drove the users to newer engines.
The two original benefits were now no longer valid at the end of the war. Air cooling proved to be entirely "doable" as production techniques improved to the point where finning was no longer a concern. In addition as the size of the engines grew, the propellers themselves became large enough to act as the flywheel. Higher RPM also dramatically reduced the need for a flywheel at all. This left the rotary with high fuel and oil consumption compared to other designs, and not much else.
Perhaps more seriously, having such a large weight spinning in an aircraft produces significant gyroscopic effects. Maneuvering an aircraft with this kind of engine required a lot of skill since it did not always respond to controls as expected. It is said that the rotary powered Sopwith Camel could execute a turn to the right at double the rate to the left, by manuvering the plane around the massive gyroscopic momentum of the engine. On the downside the Camel was also known for killing new pilots for the same reason. With even larger rotaries, the planes proved almost unflyable.
Use in Cars and Motorcycles
Although the 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 motorcycle with a radial rotary engine inside the front wheel. Another motorcycle with a radial rotary engine was the Redrup Radial, which had a rotating 3 cylinder engine in its frame.
In 1904, the Barry engine was built in Wales, a rotating 2 cylinder boxer engine inside a motorcycle frame, weighting 6.5kg. 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.
The most recent automotive rotary engine was developed by Texas machinist, Frank Turner. It was noticed by Malcolm Bricklin and licensed for use in the Bricklin SV-1 vehicle. Although the engine was never used, it is connected with that legendary car.
In November 2004, the Quasiturbine rotary engine has been demonstrated on a go-kart.
See Also
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