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Wankel Rotary Engine

Why (and how) an engine must rev smoothly

Since time immemorial, research has been aimed at finding substitutes for classic sources of energy - the muscle power of Man or draft animals.

The earliest machines used wind or water power; then came the steam engine, followed by the internal combustion engine.  With the internal combustion engine, designers have had to ensure that the output from the machine produces a rotary motion, this being necessary for almost any practical application of the energy produced.

[Fig 1]

[Fig 2]

There are two basic types of internal combustion engines - the conventional reciprocating piston engine and the rotary piston engine; the former being characterised by a connecting crank mechanism (the crankshaft) which transforms the linear motion of the piston into circular motion. 

[Fig 1]

Rotary piston engines do not employ a crankshaft; the linear piston is replaced by a rotary piston coupled to a rotating shaft which performs an uniform or variable rotary movement without being affected by alternating inertial forces due to changes in piston velocity, particularly at top dead centre and bottom dead centre.  Since the motion produced is rotary, it can be utilised directly without having to be transformed. 

[Fig 2]

History of an invention

Ramelli, an Italian engineer invented a water pump which continues to be used in oil pumps and compressors.


Pappenheim, a German engineer invented the gear pump which is still used to lubricate engines.  This gear pump made it possible to dispense with the reciprocating slide valves used by Ramelli.  

[Fig 3]
Pappenheim drove his machine by an overshot water wheel set in motion by a stream and was used to feed water fountains.  The emperor Ferdinand II granted him a “privilege” - the equivalent of a patent in respect of this invention.
Even in the 17th century, engineers were trying to solve the problem of “leak-proofing” between moving parts and this problem continues to be the Achilles heel of the rotary piston engine although Mazda would seem to have reduced this problem to manageable proportions.


Otto von Guericke built a vacuum pump which employed leather washers to prevent leakage between cylinder and piston.

James Watt who invented the steam engine’s connecting rod crank mechanism which made it possible to convert the piston’s reciprocating motion into rotary motion designed an oscillating piston machine in which a wing-shaped rotary blade performed an almost complete revolution uncovering inlet ports in a chamber separated off by a curved radial wall. 

[Fig 4]


One of Watt’s co-workers, Murdock, adapted Pappenheim’s gear pump to create a rotary piston steam engine. 

[Fig 5]


Elijah Galloway built the first rotary piston engine with inner epicycloid and enveloping outer line. 

[Fig 6]


Jones, modified Pappenheim’s gear pump and produced a double rotor with only two teeth per gear.  Rootes compressors and pumps employ this principle. 

[Fig 7]


Alotham and Franchot designed a vane compressor comprising a bust rotating inside a cycloidal housing.  

[Fig 8]


The American, Cooley, lodged a patent for a rotary piston engine with an internal epicycloid and enveloping outer chamber.  

[Fig 9]


The Englishman, Umpleby, transformed Cooley’s steam engine into an internal combustion engine but experienced problems with gas tightness.  

[Fig 10]


A Swedish patent was granted to Wallinder and Skoog in respect of a true rotary piston thermal engine with toothed meshing, enveloping interior hypocycloid and internal five pointed rotor with a 5:6 rotation ratio which could be used as either a two or four stroke internal combustion engine.  

[Fig 11]


Sensaud and Lavaud, (the French engineers responsible for the Traction Avant automatic gearbox which was a failure) applied for a patent for a rotary piston engine with internally meshing gears  in a hypocycloid housing and a 5:6 reduction ratio.  Both Renault and CitroŽn, at the instigation of the French Air Ministry  provided backing for this project and a number of engines were built at Batignolles.  Unfortunately the engine failed to live up to expectations and the project was abandoned.  

[Fig 12]


The Swiss manufacturer Bernard Maillard built an air compressor based on a British patent for a rotary piston machine with a 2:3 ratio and internal hypocycloid surfaced chambers.  Leakage under pressure made it impossible to use this design as a thermal engine.  

[Fig 13]

© 1999 Julian Marsh