A rotary engine is an internal combustion engine, like the engine in your car, but it works in a completely different way than the conventional piston engine.
In a piston engine, the same volume of space (the cylinder) alternately does four different jobs -- intake, compression, combustion and exhaust. A rotary engine does these same four jobs, but each one happens in its own part of the housing. It's kind of like having a dedicated cylinder for each of the four jobs, with the piston moving continually from one to the next.
The rotary engine (originally conceived and developed by Dr. Felix Wankel) is sometimes called a Wankel engine, or Wankel rotary engine.
Wankel first conceived his rotary engine in 1924 and finally received a patent for it in 1929. He worked through the 1940s to improve the design. Considerable effort went into designing rotary engines in the 1950s and 1960s. They were particularly interesting because of their smooth, very quiet running, and their reliability resulting from their simplicity.
In Britain, Norton Motorcycles developed a Wankel rotary engine for motorcycles, which was included in the Norton Commander (Motorcycle); Suzuki also produced a production motorcycle with a Wankel engine, the RE-5. John Deere Inc, in the US, had a major research effort in rotary engines and designed a version which was capable of using a variety of fuels without changing the engine. The design was proposed as the power source for several US Marine combat vehicles in the late 1980s.
After occasional use in automobiles, for instance by NSU with their Ro 80 model and CitroÃƒÂ«n with the GS Birotor, and abortive attempts by General Motors and Mercedes Benz to design Wankel engined 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 generally loved them, notably the smoothness. However they had the very bad luck of being released during the middle of efforts to decrease emissions and increase fuel economy. Mazda later abandoned the Wankel from most of their automotive designs, but continued using it in their RX-7 sports car into the 1990s. The company normally used two-rotor designs, but received considerable attention with their 1991 Eunos Cosmo, which used a twin-turbo three-rotor engine. In 2003, Mazda relaunched the rotary with the new RX-8. In the racing world, Mazda has had substantial success with both two-rotor and four-rotor cars, and private racers have also had considerable success with stock and modified Mazda Wankel-engined cars.
In the Wankel engine, the four strokes of a typical Otto cycle engine are arranged sequentially around an oval, unlike the reciprocating motion of a piston engine. In the basic single rotor Wankel engine, a single oval (technically a trochoid) housing surrounds a three-sided rotor (a Reuleaux triangle) which turns and moves within the housing. The sides of the rotor seal against the sides of the housing, and the corners of the rotor seal against the inner periphery of the housing, dividing it into three combustion chambers.
The Wankel cycle: Intake (blue), Compression (green), Ignition (red), Exhaust (yellow)
As the rotor turns, its motion and shape and the shape of the housing cause each side of the rotor to get closer and farther from the wall of the housing, compressing and
expanding the combustion chamber similarly to the "strokes" in a reciprocating engine. However, whereas a normal four stroke cycle engine produces one combustion stroke per cylinder for every two revolutions, i.e. one half power stroke per revolution per cylinder, each combustion chamber of each rotor in the Wankel generates one combustion 'stroke' per revolution, i.e. three power strokes per rotor revolution. Since the Wankel output shaft is geared to spin at three times the rotor speed, this becomes one combustion 'stroke' per output shaft revolution per rotor, twice as many as the four-stroke piston engine, and similar to the output of a two stroke cycle engine. Thus, power output of a Wankel engine is generally higher than that of a four-stroke engine of similar engine displacement in a similar state of tune, and higher than an engine of similar physical dimensions. In international sports car racing the FIA considers a Wankel engine to be equivalent to a four-stroke engine of twice the displacement; other racing series seem to have settled on 1.8 times the displacement. The Wankel engine is quite different from a more recent concept called the Quasiturbine.
Wankel engines have several major advantages over reciprocating piston designs besides the higher output for similar displacement and physical size. They 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 valve trains; in addition, since the rotor is geared directly to the output shaft, there is no need for connecting rods, a conventional crankshaft, or balance weights, etc. The elimination of these parts not only makes a Wankel engine much lighter, typically half that of a conventional engine with equivalent power, it eliminates the reciprocating mass of a piston engine with its higher strain due to repetitious acceleration and deceleration as well as its inherent vibration, producing not only a smoother flow of power, but the ability to run at higher rpm, thereby producing more power.
In addition, this simplicity and small size allows for a savings in construction costs.
As another advantage, the shape of the Wankel combustion chamber and the turbulence induced by the moving rotor prevent localized hot spots from forming, thereby allowing the use of fuel of very low octane number without preignition or knock. This is a particular advantage for Hydrogen cars.
The design of the Wankel engine with its numerous sliding seals and its housing, typically built as a sandwich of cast iron and aluminum pieces which expand and contract by different degrees when exposed to heating and cooling cycles in use, led to a very high incidence of loss of sealing, both between the rotor and the housing and also between the various pieces making up the housing. Further engineering work by Mazda brought these problems under control, but unfortunately, they were then confronted with a sudden global concern over both hydrocarbon emission and a rise in the cost of gasoline, the two most serious drawbacks of the Wankel engine.
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. At first, while manufacturers of piston engined cars were turning to expensive catalytic converters to completely oxidize the unburned hydrocarbons, Mazda was able to avoid this cost by paradoxically enriching the air/fuel mixture enough to produce an exhaust stream which was rich enough in hydrocarbons to actually support complete combustion in a 'thermal reactor' (just 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 switch to this solution was immediately followed by a sharp rise in the cost of gasoline worldwide, so that not only the added fuel cost of their 'thermal reactor' design, but even the basically lower fuel economy of the Wankel engine caused their sales to drop alarmingly.
Another disadvantage of the Wankel engine is the difficulty of expanding the engine to more than two rotors. The complex shapes of the rotor, housing, and output shaft and the way they fit together requires that engines with more than two rotors use an output shaft made of several sections assembled during the assembly of the rest of the engine. While this technique has been used successfully in Wankel powered racing cars, it negates a great deal of the relative simplicity and lower cost of the Wankel engine construction.
The Wankel's superb power-to-weight ratio makes it particularly well suited to aircraft engine use. There was intense interest in them in this role in the 1950s when the design was first becoming well known, but it was at this same time that almost the entire industry was moving to the jet engine, which many believed would be the only engine in use within a decade. The Wankel suffered from a lack of interest, and when it later became clear that the jet engine was far too expensive for all roles, the general aviation world had already shrunk so much that there was little money for new engine designs. Nevertheless, interest in them for small aircraft has continued.
Wankels have made something of a comeback in recent years. None of their advantages have been lost in comparison to other engines, and the introduction of better materials has helped the tip-seal problem. They are being found increasingly in roles where their compact size and quiet running is important, notably in drones, or UAVs. More recently, these UAV-designed engines are being found increasingly in other roles, such as personal water craft and auxiliary power units for aircraft.
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 the basic advantages of the Wankel over the four-stroke internal combustion engine are not relevant. 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 pretensioner system of the Volkswagen VW_New Beetle. In this car, when deceleration sensors sense a potential crash, small blank 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.
ROTARY COMBUSTION ENGINES VERSUS ROTARY ENGINES
The Wankel is a specific type of rotary combustion engine, of which many other designs have been devised
 all having the same basic concept; to avoid the reciprocating motion of the piston with its inherent vibration and rotational-speed-related mechanical stress. Sometimes these engines like the Quasiturbine or the Wankel engine specifically are referred to in somewhat erroneous shorthand as rotary engines, although the term rotary engine was first used to describe a type of aircooled reciprocating aircraft engine with a static crankshaft attached to the airplane, and a bank of cylinders which rotate around the crankshaft and are attached to the propeller, the opposite design to the normal automotive piston engine. They might more properly be called "rotating" engines, but the terminology for the time being is entrenched.
1. Processes and Materials of Manufacture by R.A. LINDBERG