Rotary piston engine

For articles on non-piston rotary combustion engines, see:

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 piston engine designs that a separate piece of metal had to be added as a flywheel, 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.

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 point of view of the cylinders, which travel in a circle whose center is offset from that of the pistons. 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 Lauren Seguin's Gnome series 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 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, giving the characteristic sputtering sound as though the engine was nearly stalling.

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 from the larger and heavier engine 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 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 one way at 900 rpm and the engine the other at the same speed through gearing, resulting in an engine that ran at 1800 rpm and had little net torque. 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 was designed, Fokker's own D-VIII, designed solely to provide some use for their Oberursel factory's backlog of now-useless 110hp engines. 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.