Wankel Rotary Engines, Rotary Engines History, Rotary Engine Engineering

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The rotary engine was an early type of internal combustion aircraft engine, used mostly in the years shortly before and during World War I. It is also used in a few motorcycles and cars.

The Wankel rotary engine is a type of internal combustion engine, invented by German engineer Felix Wankel, which uses a rotor instead of reciprocating pistons.

This design delivers smooth high-rpm power from a compact, lightweight engine.

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 as with the Radial engine, the crankshaft remains stationary and the entire cylinder block rotates around it.
Wankel Engine in Deutsches Museum Munich, Germany
Felix Wankel Rotary Engine in Deutsches Museum Munich, Germany.
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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 a very 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 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.

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Rotary Engine History

Rotary Engine History was started by Lawrence Hargrave who first developed a rotary engine in 1889 using compressed air, intending for it to be used in powered flight.

In 1951, Wankel began development of the Rotary Engine at NSU (NSU Motorenwerke AG), where he first conceived his Rotary Engine in 1954 (DKM 54, Drehkolbenmotor) and later the KKM 57 (the Wankel rotary engine, Kreiskolbenmotor) in 1957. The first working prototype DKM 54 was running on February 1, 1957 at the NSU research and development department Versuchsabteilung TX.

Considerable effort went into designing rotary engines in the 1950s and 1960s. They were of particular interest because they were smooth and quiet running, and because of the reliability resulting from their simplicity.

In the United States, in 1959 under license from NSU, Curtiss-Wright pioneered minor improvements in the basic engine design. In Britain, in the 1960s, Rolls Royce Motor Car Division at Crewe, Cheshire, pioneered a two-stage diesel version of the Wankel engine.

Also in Britain Norton Motorcycles developed a Wankel rotary engine for motorcycles, which was included in their Commander and F1; Suzuki also made a production motorcycle with a Wankel engine, the RE-5. In 1971 and 1972 Arctic Cat produced snowmobiles powered by 303 cc Wankel rotary engines manufactured by Sachs in Germany. John Deere Inc, in the U.S., designed a version that was capable of using a variety of fuels. The design was proposed as the power source for several U.S. Marine combat vehicles in the late 1980s.

After occasional use in automobiles, for instance by NSU with their Ro 80 model, Citroën with the M35, and GS Birotor using engines produced by Comotor, as well as abortive attempts by General Motors and Mercedes-Benz to design Wankel-engine automobiles, the most extensive automotive use of the Wankel engine has been by the Japanese company Mazda.

After years of development, Mazda's first Wankel engined car was the 1967 Mazda Cosmo. The company followed with a number of Wankel ("rotary" in the company's terminology) vehicles, including a bus and a pickup truck. Customers often cited the cars' smoothness of operation.

However, Mazda chose a method to comply with hydrocarbon emission standards that, while less expensive to produce, increased fuel consumption, just before a sharp rise in fuel prices. Mazda later abandoned the Wankel in most of their automotive designs, but continued using it in their RX-7 sports car until August 2002 (RX-7 importation for North America ceased with the 1995 model year).
First Wankel Engine DKM54 (Drehkolbenmotor)
First Wankel Rotary Engine DKM54 (Drehkolbenmotor), at the Deutsches Museum in Bonn, Germany.
Wankel Engine NSU KKM 57P (Kreiskolbenmotor)
Wankel Rotary Engine NSU KKM 57P (Kreiskolbenmotor), at Autovision und Forum, Germany.
NSU Wankel Spider, the first line of cars sold with Wankels
NSU Wankel Spider, the first line of cars sold with Wankels.
Rolls Royce R6 two stage Wankel Diesel engine
Rolls Royce R6 two stage Wankel Diesel engine.

The company normally used two-rotor designs, but received considerable attention with their 1991 Eunos Cosmo, which used a twin-turbo three-rotor engine.

Since its introduction in the NSU Motorenwerke AG (NSU) and Mazda cars of the 1960s, the engine has been commonly referred to as the rotary engine, a name which has also been applied to several completely different engine designs.

In 2003, Mazda introduced the RENESIS engine with the new RX-8. The RENESIS engine relocated the ports for exhaust and intake from the periphery of the rotary housing to the sides, allowing for larger overall ports, better airflow, and further power gains. The RENESIS is capable of delivering 238 hp from its 1.3 L displacement with better fuel economy, reliability, and environmental friendliness than any other Mazda rotary engine in history.

Although VAZ, the Soviet automobile manufacturer, is known to have produced Wankel-engine automobiles, and Aviadvigatel, the Soviet aircraft engine design bureau, is known to have produced Wankel engines for aircraft and helicopters, little specific information has surfaced; what has been seen indicates a general similarity to Wankel designs by NSU, Comotor, and Mazda.

The People's Republic of China is also known to have experimented with Wankel engines, but even less is known in the West about the work done there, other than one paper, #880628, delivered to the SAE in 1988 by Chen Teluan of the South China Institute of Technology at Guangzhou.

Although many manufacturers licensed the design, and Mercedes-Benz used it for their C111 concept car, only Mazda has produced Wankel engines in large numbers. American Motors (AMC) was so convinced "...that the rotary engine will play an important role as a powerplant for cars and trucks of the future..." according to its Chairman Roy D. Chapin Jr., that the smallest U.S. automaker signed an agreement in 1973 to build Wankels for both passenger cars and Jeep vehicles, as well as the right to sell any rotary engines it produces to other companies.

It even designed the unique Pacer around the engine, even though by that time AMC had decided to buy the Wankel engines from GM instead of building them itself. However, the engines never reached production by the time the Pacer was to hit the showrooms. Part of the demise of this feature was the rising fuel crisis and concerns about emission legislation in the United States.

General Motor's Wankel engine did not comply with emission levels, so in 1974 the company canceled its development. This meant that the Pacer's drivetrain design had to be reconfigured to house the venerable AMC Straight-6 engines with rear-wheel drive.

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Theory of Operation

In the Wankel rotary engine, the four strokes of a typical Otto cycle occur in the space between a rotor, which is roughly triangular, and the inside of a housing.

In the basic single-rotor Wankel engine, the oval-like epitrochoid-shaped housing surrounds a three-sided rotor (similar to a Reuleaux triangle, a three-pointed curve of constant width, but with the middle of each side a bit more flattened).

The central drive shaft, also called an eccentric shaft or E-shaft, passes through the center of the rotor and is supported by bearings.

The rotor both rotates around an offset lobe (crank) on the E-shaft and makes orbital revolutions around the central shaft.

Seals at the corners of the rotor seal against the periphery of the housing, dividing it into three moving combustion chambers.

Fixed gears mounted on each side of the housing engage with ring gears attached to the rotor to ensure the proper orientation as the rotor moves.
The Wankel cycle
The Wankel cycle. The "A" marks one of the three apexes of the rotor. The "B" marks the eccentric shaft and the white portion is the lobe of the eccentric shaft. The shaft turns three times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft.

As the rotor rotates and orbitally revolves, each side of the rotor gets closer and farther from the wall of the housing, compressing and expanding the combustion chamber similarly to the strokes of a piston in a reciprocating engine. The power vector of the combustion stage goes through the center of the offset lobe.

While a four-stroke piston engine makes one combustion stroke per cylinder for every two rotations of the crankshaft (that is, one half power stroke per crankshaft rotation per cylinder), each combustion chamber in the Wankel generates one combustion stroke per each driveshaft rotation, i.e. one power stroke per rotor orbital revolution and three power strokes per rotor rotation. Thus, power output of a Wankel engine is generally higher than that of a four-stroke piston engine of similar engine displacement in a similar state of tune and higher than that of a four-stroke piston engine of similar physical dimensions and weight.

Wankel engines also generally have a much higher redline than a reciprocating engine of similar size since the strokes are completed with a rotary motion as opposed to a reciprocating engine which must use connecting rods and a crankshaft to convert reciprocating motion into rotary motion.

National agencies that tax automobiles according to displacement and regulatory bodies in automobile racing variously consider the Wankel engine to be equivalent to a four-stroke engine of 1.5 to 2 times the displacement; some racing regulatory agencies ban it altogether.

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Advantages and Disadvantages of Wankel Rotary Engines

Advantages of Wankel Rotary Engines

Wankel rotary engines have several major advantages over reciprocating piston designs, in addition to having higher output for similar displacement and physical size.

Wankel rotary engines are considerably simpler and contain far fewer moving parts. For instance, because valving is accomplished by simple ports cut into the walls of the rotor housing, they have no valves or complex valve trains; in addition, since the rotor is geared directly to the output shaft, there is no need for connecting rods, a conventional crankshaft, crankshaft balance weights, etc.

The elimination of these parts not only makes a Wankel engine much lighter (typically half that of a conventional engine of equivalent power), but it also completely eliminates the reciprocating mass of a piston engine with its internal strain and inherent vibration due to repeated acceleration and deceleration, producing not only a smoother flow of power but also the ability to produce more power by running at higher rpm.

In addition to the enhanced reliability by virtue of the complete removal of this reciprocating stress on internal parts, the engine is constructed with an iron rotor within a housing made of aluminium, which has greater thermal expansion. This ensures that even a severely overheated Wankel engine cannot seize, as would likely occur in an overheated piston engine. This is a substantial safety benefit in aircraft use since no valves can burn out.

A further advantage of the Wankel engine for use in aircraft is the fact that a Wankel engine can have a smaller frontal area than a piston engine of equivalent power.

The simplicity of design and smaller size of the Wankel engine also allows for savings in construction costs, compared to piston engines of comparable power output.

Due to a 50% longer stroke duration compared to a four stroke engine, there is more time to complete the combustion. This leads to greater suitability for direct injection.

Disadvantages of Wankel Rotary Engines

Compared to four stroke piston engines, the time available for fuel to be injected into a Wankel engine is significantly shorter, due to the way the three chambers rotate. The fuel-air mixture cannot be pre-stored as there is no intake valve.

This means that to get good performance out of a Wankel engine, more complicated fuel injection technologies are required than for regular four-stroke engines. This difference in intake times also causes Wankel engines to be more susceptible to pressure loss at low RPM compared to regular piston engines.

In terms of fuel economy, Wankel engines are less efficient than four stroke piston engines. Problems also occur with exhaust gases at a peripheral port exhaust, where the prevalence of hydrocarbon can be higher than from the exhausts of four stroke piston engines.

The reason Wankel-cycle engines have higher fuel consumption than four stroke piston engines is that the combustion chambers in a Wankel are quite large. This lowers the thermal efficiency and thus the fuel economy. Additionally, some fuel may get too far from the flame front during combustion to be fully burned. This is why there can be more carbon monoxide and unburnt hydrocarbons in a Wankel's exhaust stream.

All Mazda made Wankel rotaries, including the new Renesis found in the RX8 burn a small quantity of oil by design; it is metered into the combustion chamber in order to preserve the apex seals. Owners must periodically add small amounts of oil, slightly increasing running costs; though it is still reasonable when compared to many reciprocating piston engines.

Unlike a piston engine, where the cylinder is cooled by the incoming charge after being heated by combustion, Wankel rotor housings are constantly heated on one side and cooled on the other, leading to high local temperatures and unequal thermal expansion.

While this places high demands on the materials used, the simplicity of the Wankel makes it easier to experiment with alternative materials like exotic alloys and ceramics. With water cooling in a radial or axial flow direction, with the hot water from the hot bow heating the cold bow, the thermal expansion remains tolerable and increased the thermal efficiency.

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Rotary Engine Engineering

Rotary Engine Engineering was persued by Felix Wankel who managed to overcome most of the problems that made previous rotary Otto cycle engines fail by developing a configuration with vane seals that could be made of more durable materials than piston ring metal that led to the failure of previous rotary designs.

Rotary engines have a thermodynamic problem not found in reciprocating four-stroke engines in that their "cylinder block" operates at steady state, with intake, compression, combustion, and exhaust occurring at fixed housing locations for all "cylinders". In contrast, reciprocating engines perform these four strokes in one chamber so that extremes of freezing intake and flaming exhaust are averaged and shielded by a boundary layer from overheating working parts.

Freezing temperatures from evaporating fuel prevail at the intake, while ignition reaches temperatures of about 2300 kelvins, a range that is wider than lubricants and most engine materials can withstand. Cooling, the boundary layer and the quench zone prevent the oil film in a Wankel rotary engine from overheating. The intake and exhaust stroke lowers the efficiency of a reciprocating four stroke engine, therefore the most effective reciprocating engine is a two-stroke diesel.

Four-stroke reciprocating engines are less suitable for hydrogen. The hydrogen can misfire on hot parts like the exhaust valve and spark plugs. Another problem concerns the hydrogenate attack on the lubricating film in reciprocating engines.

In a Wankel engine this problem is circumvented by using a ceramic apex seal against a ceramic surface: no oil film means no hydrogenate attack. Since a piston ring of ceramic material is not possible, the problem remains with the reciprocating engine. The piston shell must be lubricated and cooled with oil. This increases the lubricating oil consumption in a four-stroke engine substantially.





Early engine designs had a high incidence of sealing loss, both between the rotor and the housing and also between the various pieces making up the housing. Also, in earlier model Wankel engines carbon particles could become trapped between the seal and the casing, jamming the engine and requiring a partial rebuild. (This can be prevented in older Mazda engines by always allowing the engine to reach operating temperature).

It was common for very early Mazda engines to require rebuilding after 50,000 miles. Modern Wankel engines have not had these problems for many years. Further sealing problems arise from the uneven thermal distribution within the housings causing distortion and loss of sealing and compression. This thermal distortion also causes uneven wear between the apex seal and the rotor housing, quite evident on higher mileage engines. Attempts have been made to normalize the temperature of the housings, minimizing the distortion, with different coolant circulation patterns and housing wall thicknesses.

Fuel Consumption and Hydrocarbon Emissions

Just as the shape of the Wankel combustion chamber prevents preignition, it also leads to incomplete combustion of the air-fuel charge, with the remaining unburned hydrocarbons released into the exhaust. While manufacturers of piston-engine cars were turning to expensive catalytic converters to completely oxidize the unburned hydrocarbons, Mazda was able to avoid this cost by enriching the air/fuel mixture and increasing the amount of unburned hydrocarbons in the exhaust to actually support complete combustion in a 'thermal reactor' (an enlarged open chamber in the exhaust manifold) without the need for a catalytic converter, thereby producing a clean exhaust at the cost of some extra fuel consumption.

Unfortunately for Mazda, their choice increased fuel consumption just as world petrol prices rose sharply.

In Mazda's RX-8 with the Renesis engine, fuel consumption is now within normal limits while passing California State emissions requirements. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing exhaust port area. The Renesis engine even meets California's Low Emissions Vehicle or LEV standards.

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Rotary Engines Automobile Racing

In the racing world, Mazda has had substantial success with two-rotor, three-rotor, and four-rotor cars.

Private racers have also had considerable success with stock and modified Mazda Wankel-engine cars.

The Sigma MC74 powered by a Mazda 12A engine was the first engine and team from outside Western Europe or the United States to finish the entire 24 hours of the 24 Hours of Le Mans race, in 1974.
3-Rotor Eunos Cosmo engine
3-Rotor Eunos Cosmo engine.

Mazda is the only team from outside Western Europe or the United States to have won Le Mans outright and the only non-piston engine ever to win Le Mans, which the company accomplished in 1991 with their four-rotor 787B (2622 cc actual displacement, rated by FIA formula at 4708 cc).

The following year, rules were changed at Le Mans which made the Mazda 787 ineligible to race. Mazda is also the most reliable finisher at Le Mans (with the exception of Honda, who has entered only three cars in only one year), with 67% of entries finishing.

The Mazda RX-7 has won more IMSA races in its class than any other model of automobile, with its one hundredth victory on September 2, 1990. Following that, the RX-7 won its class in the IMSA 24 Hours of Daytona race ten years in a row, starting in 1982. The RX7 won the IMSA Grand Touring Under Two Liter (GTU) championship each year from 1980 through 1987, inclusive.

Formula Mazda Racing features open-wheel race cars with Mazda Wankel engines, adaptable to both oval tracks and road courses, on several levels of competition. Since 1991, the professionally organized Star Mazda Series has been the most popular format for sponsors, spectators, and upward bound drivers. The engines are all built by one engine builder, certified to produce the prescribed power, and sealed to discourage tampering. They are in a relatively mild state of racing tune, so that they are extremely reliable and can go years between motor rebuilds.

The Malibu Grand Prix chain, similar in concept to commercial recreational kart racing tracks, operates several venues in the United States where a customer can purchase several laps around a track in a vehicle very similar to open wheel racing vehicles, but powered by a small Curtiss-Wright rotary engine.

In engines having more than two rotors, or two rotor race engines intended for high-rpm use, a multi-piece eccentric shaft may be used, allowing additional bearings between rotors. While this approach does increase the complexity of the eccentric shaft design, it has been used successfully in the Mazda's production three-rotor 20B-REW engine, as well as many low volume production race engines. (The C-111-2 4 Rotor Mercedes-Benz eccentric shaft for the KE Serie 70, Typ DB M950 KE409 is made in one piece! Mercedes-Benz used split bearings.)

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Aircraft Rotary Engines

The first Wankel rotary engine aircraft was the experimental Lockheed Q-Star civilian version of the United States Army's reconnaissance QT-2, basically a powered Schweizer sailplane, in 1968 or 1969.

It was powered by a 185 hp (138 kW) Curtiss-Wright RC2-60 Wankel rotary engine.

Aircraft Wankels have made something of a comeback in recent years. None of their advantages have been lost in comparison to other engines.
Diamond DA20 with Diamond Wankel Engines
Diamond DA20 with Diamond Wankel Engines.


Many companies and hobbyists adapt Mazda rotary engines (taken from automobiles) to aircraft use; others, including Wankel GmbH itself, manufacture Wankel rotary engines dedicated for the purpose.

Wankel engines are also becoming increasingly popular in homebuilt experimental aircraft, due to a number of factors. Most are Mazda 12A and 13B automobile engines, converted to aviation use. This is a very cost-effective alternative to certified aircraft engines, providing engines ranging from 100 to 300 horsepower at a fraction of the cost of traditional engines.

These conversions first took place in the early 1970s. With a number of these engines mounted on aircraft, as of 10 December 2006 the National Transportation Safety Board has only seven reports of incidents involving aircraft with Mazda engines, and none of these is of a failure due to design or manufacturing flaws.

During the same period they have issued several thousand reports of broken crankshafts and connecting rods, failed pistons and incidents caused by other components which are not found in the Wankel engines. Rotary engine enthusiasts derisively refer to piston aircraft engines as "reciprosaurs," and point out that their designs are essentially unchanged since the 1930s, with only minor differences in manufacturing processes and variation in engine displacement.

Peter Garrison, Contributing Editor for FLYING Magazine, has said that "the most promising engine for aviation use is the Mazda rotary." Mazdas have indeed worked well when converted for use in homebuilt aircraft. However, the real challenge in aviation is producing FAA-certified alternatives to the standard reciprocating engines that power most small general aviation aircraft. Mistral Engines, based in Switzerland, is busy certifying its purpose-built rotaries for factory and retro-fit installations on certified production aircraft.

With the G-190 and G-230-TS rotary engines already flying in the experimental market, Mistral Engines hopes for FAA and JAA certification in 2007 or early 2008. Mistral claims to have overcome the challenges of fuel consumption inherent in the rotary, at least to the extent that the engines are demonstrating specific fuel consumption within a few points of reciprocating engines of similar displacement. While fuel burn is still marginally higher than traditional engines, it is outweighed by other beneficial factors.

Mistral points out that the Wankel rotary is an engine that has very few moving parts, making it more dependable. In addition it has a much better power-to-weight ratio and is smaller, thus enabling more efficient engine cowl design. Finally, the engine runs with a smoothness more akin to turbine engines than gas powered "recips", thus reducing airframe vibration and occupant fatigue.

Since Wankel engines operate at a relatively high rotational speed with relatively low torque, propeller aircraft must use a Propeller Speed Reduction Unit (PSRU) to keep conventional propellers within the proper speed range. There are many experimental aircraft flying with this arrangement.

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History in Aircraft

Lawrence Hargrave first developed a rotary engine in 1889 using compressed air, intending for it to be used in powered flight. Weight of materials and lack of quality machining prevented it becoming an effective power unit.

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 own flywheel, thus rotaries could be lighter than similarly sized engines of regular design.

  • The cylinders had good airflow over them even when the aircraft in which they were mounted were 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.

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 Gnôme series from 1908.

This design was developed from a German single-cylinder stationary engine intended for light industrial use, the Gnom, which the brothers were producing under license from Motorenfabrik Oberursel.
Le Rhône 9C, a typical rotary engine of WWI
Le Rhône 9C, a typical rotary engine of WWI. The copper pipes carry the fuel-air mixture from the crankcase to the cylinder heads.

They essentially took several Gnom cylinders and combined them on a common shaft to produce a seven-cylinder rotary, the Gnôme Omega No.1 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 eventually 110 hp (80 kW). The engine was at this later 80 hp (60 kW) standard when World War I started, as the Gnôme Lambda, 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, including the German Oberursel firm who designed the original Gnom engine. Oberursel was later purchased by Fokker, whose Gnôme Lambda copy was known as the Oberursel U.I. It was not at all uncommon for French Gnômes to meet German versions in combat.

The Gnôme (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 that only a small force was needed to open them, and releasing the force closed the valve 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 burning oil. Castor oil was the lubricant of choice, its gum-forming tendency being irrelevant in a total-loss lubrication system. An unfortunate side-effect was that World War I pilots inhaled and swallowed a considerable amount of the oil during flight, leading to persistent diarrhoea.

Finally, the Gnôme had no throttle or carburetor. Since the fuel was 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.

The rotating mass of the engine also made it, in effect, a large gyroscope, which resulted in tricky handling. The Sopwith Camel, for example, was known to turn very nimbly to the right, but rather sluggishly to the left. Nevertheless, rotaries maintained their edge through a series of small upgrades, and many newer designs continued to use them.

A few of the nine cylinder rotaries managed to accomplish a partial "throttle" functionality by switching off either three or six cylinders (or other numbers of them), instead of all nine of them, when the "coupe switch" was depressed to cut the spark. It is believed that both German and Allied WW I rotaries had this ability, as some surviving documentation regarding the Fokker Eindecker shows a rotary selector switch to cut out a selected number of cylinders on its rotary engine.

The Gnôme Monosoupape series of engines is known to have this sort of switching available to it, and has been demonstrated long after WW I by a 160 hp Monosoupape powered reproduction Sopwith Camel at Old Rhinebeck Aerodrome while in flight in the 1990s.

As the war progressed, aircraft designers demanded ever-increasing amounts of power. Inline engines were able to meet this demand by improving their RPM, as more "bangs per minute" meant more power delivered. Improvements in valve timing, ignition systems and lighter materials made these higher RPM possible, and by the end of the war the average engine had increased from 1,200 RPM to 2,000. However the rotary was not able to use the same "trick," due to the drag of the cylinders through the air as they spun.

For instance, if an early-war model of 1,200 RPM increased to only 1,400, the drag on the cylinders increased 36%, as air drag increases with the square of velocity. At lower speeds the drag could simply be ignored, but as speeds increased the rotary was putting more and more power into spinning the engine, and less into spinning the propeller.

One clever attempt to rescue the design was made by Siemens AG. The crankcase and cylinders spun counterclockwise at 900 RPM while the crankshaft spun clockwise at the same speed. This was achieved by the use of bevel gearing at the rear of the crankcase, resulting in the Siemens-Halske Sh.III, running at 1800 RPM with little net torque.

It was also apparently the only rotary engine to use a normal carburetor that could be controlled by a conventional throttle, just as in an in-line engine. Used on the Siemens-Schuckert D.IV fighter, the new engine created what is considered by many to be the best fighter aircraft of the war.

One new rotary powered aircraft, Fokker's own D.VIII, was designed at least in part to provide some use for their Oberursel factory's backlog of now-useless Ur.II 110 hp engines, themselves clones of the Le Rhône 9J rotary. By the time the war ended, the rotary engine had become obsolete, and on the whole it disappeared from use quite quickly.

The British Royal Air Force probably used rotary engines for longer than most other operators - the post-war standard fighter, the Sopwith Snipe used the Bentley BR2 rotary, and the standard trainer, the Avro 504K, had a universal mounting to allow several types of low powered rotary, of which there was a large surplus supply. The cheapness of war-surplus engines had to be balanced against their poor fuel economy, and the expense of their total loss lubrication system.

By the mid twenties rotaries had been more or less completely displaced even in British service, largely by the new generation of air-cooled radial engines.

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Rotary Engines Other Uses

Small Wankel engines are being found increasingly in other roles, such as go-karts, personal water craft and auxiliary power units for aircraft. The simplicity of the Wankel makes it ideal for mini, micro, and micro-mini engine designs.

The Graupner/O.S. 49-PI is a 1.27 hp (947 W) 5 cc Wankel engine for model airplane use which has been in production essentially unchanged since 1970; even with a large muffler, the entire package weighs only 380 grams (13.4 ounces).
Norton Interpol 2 Wankel prototype
Norton Interpol 2 Wankel prototype.

The MicroElectroMechanical Systems (MEMS) Rotary Engine Lab at the University of California, Berkeley has been developing Wankel engines of down to 1 mm in diameter with displacements less than 0.1 cc. Materials include silicon and motive power includes compressed air. The goal is to eventually develop an internal combustion engine that will deliver 100 milliwatts of electrical power; the engine itself will serve as the rotor of the generator, with magnets built into the engine rotor itself.

The largest Wankel engine was built by Ingersoll-Rand; available in 550 hp (410 kW) one rotor and 1100 hp (820 kW) two rotor versions, displacing 41 liters per rotor with a rotor approximately one meter in diameter, it was available between 1975 and 1985.

It was derived from a previous, unsuccessful, Curtiss-Wright design, which failed because of a well-known problem with all internal combustion engines; the fixed speed at which the flame front travels limits the distance combustion can travel from the point of ignition in a given time, and thereby the maximum size of the cylinder or rotor chamber which can be used. This problem was solved by limiting the engine speed to only 1200 rpm and use of natural gas as fuel; this was particularly well chosen, as one of the major uses of the engine was to drive compressors on natural gas pipelines.

From 1974 to 1977 Hercules produced a limited number of motorcycles powered by Wankel engines. The tooling was later used by Norton to produce the Norton Commander model in the early 1980s. The best-known example of a Wankel-powered motorcycle, however, was the Suzuki RE5, produced in 1975 and 1976. This 500cc (actual) displacement motorcycle could have been a great touring bike except for the poor fuel mileage of 32-36 mpg. Examples are still frequently found on online auction sites.

Aside from being used for internal combustion engines, the basic Wankel design has also been utilized for air compressors, and superchargers for internal combustion engines, but in these cases, although the design still offers advantages in reliability, the basic advantages of the Wankel in size and weight over the four-stroke internal combustion engine are irrelevant. In a design using a Wankel supercharger on a Wankel engine, the supercharger is twice the size of the engine.

Perhaps the most exotic use of the Wankel design is in the seat belt pre-tensioner system of some Mercedes-Benz cars and the Volkswagen New Beetle. In these cars, when deceleration sensors sense a potential crash, small explosive cartridges are triggered electrically and the resulting pressurized gas feeds into tiny Wankel engines which rotate to take up the slack in the seat belt systems, anchoring the driver and passengers firmly in the seat before any collision.

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