A pistonless rotary engine is an internal combustion engine that does not use pistons in the way a reciprocating engine does, but instead uses one or more rotors, sometimes called rotary pistons. An example of a pistonless rotary engine is the Wankel engine.

The term rotary combustion engine has been suggested as an alternative name for these engines to distinguish them from early (generally up to the early 1920s) aircraft engines and motorcycle engines also known as rotary engines. However, both continue to be called rotary engines and only the context determines which type is meant. In particular, the only commercial producer of (pistonless) automobile rotary engines As of 2005, Mazda, consistently refers to its Wankel engines as rotary engines. O.S. Engines, which produces a Wankel model airplane engine, refer to it as a wankel rotary engine.

Pistonless rotary engines

The basic concept of a (pistonless) rotary engine avoids the reciprocating motion of the piston with its inherent vibration and rotational-speed-related mechanical stress. As of 2006 the Wankel engine is the only successful pistonless rotary engine, but many similar concepts have been proposed and are under various stages of development. Examples of rotary engines include:

Production stage
Development stage
Conceptual stage


All such engines have the potential to improve on the piston engine in the areas of:

  • Higher power-to-weight ratios.
  • Mechanical simplicity.
  • Less vibration.
  • Sealing system has no revolutions-limit; piston rings fail after the engine's revolutions-limit.

While typically larger than the piston of an engine of corresponding capacity, a rotor may perform many strokes per revolution. The Wankel produces twelve strokes per revolution of the rotor (four strokes per chamber times three chambers) (although the spindle rotates three times faster than the rotor or three times over the twelve strokes), as opposed to two strokes for each crankshaft rotation of a single-cylinder single acting piston engine, or four strokes for a double-acting cylinder such as found in some steam engines. The quasiturbine and MYT engine deliver sixteen strokes for every rotor (and spindle) revolution.


Although in two dimensions the seal system of a Wankel looks to be even simpler than that of a corresponding multi-cylinder piston engine, in three dimensions the reverse is the case. Not only the rotor apex seals evident in the conceptual diagram, but also the rotor must seal against the chamber ends.

Piston rings are not perfect seals. Each has a gap in fact to allow for expansion. Moreover the sealing at the Wankel apices is less critical, as leakage is between adjacent chambers on adjacent strokes of the cycle, rather than to the crankcase. However, the less effective sealing of the Wankel is one factor reducing its efficiency, and confining its success mainly to applications such as racing engines and sports vehicles where neither efficiency nor long engine life are major considerations. In earlier models, Wankel engines should never be started and run unless the engine has reached operating temperature -- starting a car and moving it a few yards, e.g. from a garage to a driveway, can lead to a jammed engine. In these situations it is better to push the car and not start the engine. This is due to the engine flooding with fuel, which can lead to hydrolock of the motor. This "flooding" is caused by the excess amount of fuel injected into the engine in its "cold" running circuit. The flooding issue has been largely fixed through changes in the ECU programming and a faster starter motor.

Fifty percent longer stroke duration than a piston engine (Wankel engine).

The Quasiturbine has similar disadvantages with its concave combustion chamber, and in the AC design the sharp angles of the carriers hamper the propagation of the flame front, leading to incomplete combustion. The stroke duration is too short for a complete combustion.



The simplest design, either proposed or in use, is the Wankel. Its only moving parts are a three-sided rotor turning on an eccentric shaft; there are neither camshaft nor valves. The rotor is not fixed to the eccentric shaft, but rather turns it by means of an internal gear on the inside of the rotor engaging a smaller conventional gear on the side plate. The rotor is positively located by the eccentric shaft and by the geometry of the rotor and engine chamber. A Wankel engine fires three times for every revolution of the rotor, and every revolution of the rotor equates to three rotations of the eccentric shaft. The Wankel engine can be balanced perfectly, with counterweights.

In the most popular Mazda family of engines, the 13B, this consists of two rotors displacing 654 cc (cubic centimeters) per each rotor face, a total of 1308 cc or 1.3 liters). A Wankel engine has no empty stroke like a reciprocating four stroke piston engine, therefore a Wankel engine needs only half the volume of a reciprocating four stroke engine.

There are various methods of calculating the engine displacement of a Wankel; the Japanese regulations calculating displacements for engine ratings calculate on the basis of the volume displacement of one rotor face only. This is widely accepted as the standard method of calculating the displacement of a rotary, however comparing a piston engine to a Wankel rotary using this displacement convention is flawed and results in large imbalances in specific output in favor of the Wankel motor. Many believe this is for marketing purposes on Mazda's part.

If looking for the maximum possible displacement, a twin rotor Wankel rotary displacing 654cc per combustion chamber (such as the Mazda 13B) has a maximum displacement of 3924cc (3.9 liters). This is because there are three possible combustion chambers per rotor, each displacing 654cc at full expansion. 654cc per face, three faces per rotor (1962cc/2 liters), two rotors per engine (3924cc/3.9 liters). Other notable rotaries such as the '2 liter' 3-rotor Mazda 20b in the Eunos Cosmo (total displacement: 5886cc/5.9 liters) and '2.6 liter' 4-rotor Mazda R26b from the Le Mans winning 787b (total displacement: 7848cc/7.8 liters) can have maximum displacement calculated in the same way. Taking combustion chamber volume and multiplying by the total number of possible combustion chambers per engine. However comparing rotaries to piston engines using this method is futile as the Wankel's subjects its full displacement to a power stroke after three full rotations of the eccentric shaft. This means comparing a hypothetical 3.9 liter piston engine to a '1.3 liter' Wankel rotary (with a maximum possible displacement of 3.9 liters) using this method will result in the piston engine's theoretical specific output being approximately 50% higher than the Wankel rotary because the piston engine will displace its 3.9 liters through a power stroke one revolution (50%) sooner than the Wankel rotary.

For comparison purposes between a Wankel Rotary engine and a piston engine, displacement (and thus power output) can more accurately be compared on a displacement per revolution (of the eccentric shaft) basis. This dictates that a two rotor Wankel displacing 654cc per face will have a displacement of 1.3 liters per every rotation of the eccentric shaft(only two total faces, one face per rotor going through a full power stroke) and 2.6 liters after two revolutions (four total faces, two faces per rotor going through a full power stroke). This is directly comparable to a 2.6 liter piston engine with an even number of cylinders in a conventional firing order which will also displace 1.3 liters through its power stroke after one revolution of the crankshaft, and 2.6 liters through its power strokes after two revolutions of the crankshaft. Measuring a Wankel rotary engine in this way more accurately explains its specific output numbers, as the volume of its air fuel mixture put through a power stroke per revolution is directly responsible for torque and thus horsepower produced.


The Sarich orbital engine has a larger number of moving parts than the Wankel. The six-chamber design used for the prototype has, conceptually, eight moving parts within the engine chamber as opposed to two for the Wankel. However it also requires six spark plugs, one per combustion chamber, as opposed to one per rotor for the Wankel (although two are commonly used in practice for performance reasons). The Sarich was developed to the point of being demonstrated running briefly as a bench-test with no load before the design was abandoned.


The Quasiturbine AC design is more complex still than the Sarich. Even with only two wheels per carriage, there are at least nineteen moving parts within the engine chamber including the shaft and differential, and possibly more depending on the design of the differential. In common with the Wankel, the Quasiturbine only requires a single spark plug. A prototype of the Quasiturbine AC design was constructed and turned by an external engine for 40 hours, but ignition was never achieved.

The Quasiturbine SC design is greatly simplified from the AC, but still has at least seven moving parts within the chamber, including the shaft and again possibly more depending on the design of the differential. The SC design has been demonstrated as a steam and pneumatic engine, but As of 2005 not as an internal combustion engine. Prototype steam engines have run for periods of up to a few hours. Quasiturbine has the disantvantage of a short stroke duration, this limited the maximal revolutions.

Rand cam

The Rand cam engine uses sliding vanes to implement the four stroke cycle. It is primarily being developed by Reg Technology.

Rotary Atkinson

The Rotary Atkinson cycle engine has only three moving parts within the chamber and has one power stroke per revolution. However unlike the Wankel which uses the Otto cycle, this engine uses the more efficient Atkinson cycle. Multi fuels can be used including gasoline, diesel and hydrogen.


Trochilics the science of rotating mechanical devices describes the array of TrochilicEngines ranging from Stirling cycle, internal combustion, to high-pressure gas or steam and with adaptive alterations to gaseous or fluid pumping. The piston is composed of two mirror image gull wing segments intermeshed and rotating about a common central axis. Varying the relative segment velocities in rotation, forms four variable quadrants. The quadrants are functionally a four-cylinder engine requiring no mechanically driven valves. Each segment is integrally connected to a rotating gear cage that converts the undulating piston motion to a linearly rotating output shaft. The segmented piston has a preferred direction of rotation imposed by the mechanically leveraged action of the gear cage. Trochilic engines do not employ compression rings, as conventional engines. This design approach improves efficiency through the reduction of friction losses and reduced engine wear. The air-fuel mix is aspirated, compressed, ignited and burnt between each rotor's forward and back faces as each rotor advances or retreats relative to the other during operation, varying the volume of the chamber continuously. Currently being developed by the Trochilic engine team.

In the MYT engine, the rotary pistons are toroid-sections (curved cylinders sliding inside the toroidal stator) and connected to either of two inner discs. This principle of operation can be traced back to the 1968 Tschudi engine. The main problems of this type of engine is getting a constant rotation on the output shaft from the two oppositely accelerating and decelerating rotors (planetary gears are used on some versions of Trochilic Engines, while the MYT makes use of a more complex connection system using camshafts) and preventing the rotors from turning in the wrong direction. On the other hand, these designs do not suffer from the sealing problems of Wankel engine or Quasiturbine, and use very few moving parts (5 in the simpler model of Trochilic Engine).


The Engineair engine, invented by Angelo Di Petro in Australia in 1999, and developed by Engineair since then, is based on a cylindrical rotary piston. The piston rolls around the cylindrical stator wall cushioned by a thin film of air. Six expansion chambers are created by curved vanes in slots in the stator contacting the surface of the piston (or shaft driver). Air pressure on its outer wall forces the shaft driver to move eccentrically, thereby rotating the motor shaft by means of two rolling elements mounted on the shaft with bearings. Engine speed and torque are simply controlled by throttling air inlet and exhaust via a variable slotted timer mounted on the output shaft. Large torque is instantly available at zero RPM and can be precisely controlled to give soft start and acceleration control. About a dozen moving parts.

See Also

Piston engine configurations
This box: view  talk  edit
Type BourkeControlled combustionDelticOrbitalPistonPistonless (Wankel) •
RadialRotarySingleSplit cycleStelzerTschudi
Inline types H · U · Square four · VR · Opposed · X
Stroke cycles Two-stroke cycleFour-stroke cycleSix-stroke cycle
Straight Single · 2 · 3 · 4 · 5 · 6 · 8 · 10 · 12 · 14
Flat 2 · 4 · 6 · 8 · 10 · 12 · 16
V 4 · 5 · 6 · 8 · 10 · 12 · 16 · 20 · 24
W 8 · 12 · 16 · 18
Valves Cylinder head portingCorlissSlideManifoldMultiPistonPoppet
SleeveRotary valveVariable valve timingCamless
Mechanisms CamConnecting rodCrankCrank substituteCrankshaft
Scotch YokeSwashplateRhombic drive
Linkages EvansPeaucellier–LipkinSector straight-lineWatt's (parallel)
Other HemiRecuperatorTurbo-compounding