Rotary engine

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 effect of rotating such a large mass was an inherent large gyroscopic flywheel effect, smoothing out the power and reducing vibration. Vibration had been such a serious problem on other conventional piston engine designs that a heavy flywheels had to be added. Because the cylinders themselves functioned as a flywheel, rotary piston engines typically had a power-to-weight ratio advantage over more conventional engines.

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.

Contents 1 History in aircraft 2 Use in cars and motorcycles 3 See also 4 External links

History in aircraft

The first effective rotaries were built by Stephen Balzer, who was interested in the design for two main reasons:

In order to generate 100 hp (75 kW) at the low RPM at which the engines of the day ran, the pulsation resulting from each combustion stroke was quite large. In order to damp out these pulses, engines needed to mount a large flywheel, which added weight. In the rotary design the engine itself doubled as its flywheel, thus rotaries were lighter than similarly sized engines of regular design.

The cylinders had good airflow over them even when sitting still, which was an important concern given the alloys they had to work with at the time. Balzer's early engines did not even use cooling-fins, a feature of every other air-cooled design, and one that is complex and expensive to manufacture.

Another advantage, not realized at first, is that the pistons do not actually reciprocate; rather, they travel in a circle around the common center of the connecting rods' "big ends", and only appear to reciprocate from the rotating frame of reference of the cylinders, which travel in a circle whose center is offset from that of the pistons (the centre of the crankshaft is offset from the engines centre of rotation). This lack of reciprocating mass 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 Louis and Laurent Seguin's Gnome series from 1908. Believed to have been inspired by the American Adams-Farwell automobile's rotary engine concept, they started their development with the seven cylinder Gnme Omega No.1, which still exists and is in the collection of the Smithsonian's National Air and Space Museum. A production version of the Omega then soon reached the aviation market, still as 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 them 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; since 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 gave the characteristic sputtering sound as though the engine was nearly stalling, though it did not stall as quickly as conventional engines due to its great rotational inertia.

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, partially because the engine was always "full throttle", and also because the valve timing was often less than ideal. 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 from the larger and heavier engine were overwhelming and they proved to be largely unworkable (the planes would be slow to turn, or roll, in one direction. A skilled pilot, however, could use the gyroscopic effect to his advantage by making very quick turns in the opposite direction!) Inline engines were able to increase power through increased RPM, another trick the rotary couldn't match due to increased wind resistance on the cylinder heads as they rotated, an increase of 36% from a then-normal 1200 to 1400 rpm, energy that was not being put into the propeller. As construction methods improved engines running at 2000 rpm became common, and the rotary became a dead-end.

One clever attempt to rescue the design was made by Siemens AG, who spun the propeller, crankcase and cylinders counterclockwise at 900 rpm and the engine's internal parts (the crankshaft, connecting rods and pistons) in a clockwise direction at the same speed through bevel gearing at the rear of the crankcase, resulting in an engine that ran at 1800 rpm and had little net torque. It apparently was also the only rotary engine to use a regular style of throttleable carburetor, just as in an in-line engine. Used on the superb Siemens-Schuckert D.IV, the Siemens-Halske Sh.III created what is considered by many to be the best aircraft of the war. By the end of the year only a single new rotary powered aircraft was designed, Fokker's own D-VIII, designed solely to provide some use for their Oberursel factory's backlog of now-useless Ur II 110hp engines, themselves clones of the Le Rhne 9J rotary. 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.

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 6.5 kg. 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.