How the Wankel Works Rev. 2 copyright 2007 by Paul Lamar

The Wankel rotary consists of a three sided rotor moving in a modified oval
chamber or rotor housing. The shape of the chamber is a mathematical
construction called an epitrochoid. These curves were studied by Durer
(1525), Desargues (1640), Huygens (1679), Leibniz, Newton (1686), L'Hospital
(1690), Jakob Bernoulli (1690), la Hire (1694), Johann Bernoulli (1695),
Daniel Bernoulli (1725), Euler (1745, 1781). An epitrochoid appears in
Durer's work Instruction in Measurement with Compasses and Straight Edge
(1525). Nothing new here.

It is also called a Roulette. If you have ever seen a Roulette wheel in
motion at a casino you have seen a Wankel engine in motion sort of. To
describe the Mazda rotary epictrochoid shaped housings the diameter of the
ball is about 1.181 and the diameter of the track (Roulette wheel race) is
9.375 inches. The circuitous path a point on the surface of the ball makes
as it rotates defines the epitrochoid shape. This 9.375 inch dimension is
also the height of the inside of the Mazda rotor housing. The necked down
dimension of the Mazda rotor housing is 9.375 minus twice the ball diameter
or about seven inches. The radius of the ball is called the "e" dimension of
the Wankel engine. It is also the stroke of the e-shaft so to speak. Think
of the rotary as a large bore piston engine with a very short stroke except
it does not reciprocate.  Of course an apex seal's path MUST match the inner
surface epitrochoid curve of the rotor housing as the rotor MUST rotate at
one third of the e-centric shaft rotation.  Also the radius of the rotor
MUST be seven times the "e" dimension. A variation in any of these
relationships would cause the engine to jam.

Written by Dave Mix.
The gears inside the rotary are a means of  "timing"  the position and
orientation of the rotor.  This is very much akin to the way  "timing"
gears ( or chains or cog belts )  "time"  the position of the CAMshaft in a
piston engine.  *** POWER is not transmitted in either case, only the exact
POSITION is determined.***  This  "timing"  is necessary to put the piston
engine valves in the proper place at the proper time.  In the rotary this
"timing" is necessary to put the rotor in the proper place and orientation
at the proper time.

There are at least two holes in the walls of one side of the rotor housing.
In the other side of the rotor housing there is at least one spark plug. One
hole is used as an intake port and the other as an exhaust port. Just like a
one cylinder two cycle piston engine. The rotor rotates around the center of
the eccentric on the output shaft while at the same time rotating around its
own axis. This is very similar to the way the earth rotates around the sun
while rotating around its own axis. The engine is sometimes called a
planetary engine for this reason.  The rotor rotates one third as fast about
its own axis as it does around the center of the output shaft. Actually what
is happening is the center or CG of the rotor is moving in a circle around
the center of the output shaft while the output shaft rotates. Very very
clever on the part of Felix Wankel to invent and engine that uses this
ingenious motion.

Many engineers and mechanics that should know better still think the rotor
translates, reciprocates or wobbles in the rotor housing. Nothing could be
further from the truth. The key is to think only of the center of the rotor.
That is the center of gravity of the rotor and what that does is all that
matters. It rotates around the center of the output shaft in a circle pure
and simple. The Wankel is a subtle engine. No doubt about it.

This motion is very important when it comes time to balance the engine. Since
there are no reciprocation parts what so ever it is possible to completely
balance the Wankel engine just like a turbine or electric motor and unlike
any piston engine despite some claims to the contrary. The rotor is balanced
around it own axis. The front rotor balances the rear rotor as they are 180
degrees apart on the output shaft. That leaves a small couple which is
finally completely balanced by the counter weights.

As this motion is taking place the tips of the triangular shaped  rotor are
passing the ports in the housing. As one tip, call it tip 1, passes the
intake port in the rotor housing the face behind tip 1 starts to draw in the
fuel air mixture. Call it face or chamber A.  When the next tip call it tip
2 passes the intake port the mixture is captured and starts to compress in
chamber A. As chamber A continues around the housing it encounters the spark
plug and the mixture inside it is fully compressed.  The spark plug fires
and the burning gasses in chamber A start to expand imparting the pressure
energy in the burning mixture to the output shaft. As the rotor continues on
around tip 1 passes the exhaust port and the burnt mixture starts to leave
chamber A.

This completes the four cycles for chamber A. In the mean time chamber B and
chamber C are completing various cycles of this same sequence
simultaneously. Since the rotor is rotating one third as fast as the output
shaft only one firing event and one chamber's volume worth of air is mixed
with fuel and processed per revolution of the output shaft. This is the same
as a one cylinder two cycle piston engine of the same displacement or a two
cylinder four cycle piston engine of twice the displacement.

This three chamber simultaneous action  is the reason the Wankel  engine is
so light and compact for the power it is able to generate. Furthermore the
forces in each chamber due to the pressure in each chamber are completely
contained in the one piece cast iron rotor. In a four cycle piston engine
this is not the case. The compression force must pass down a connecting rod,
into the crankshaft and up another connecting rod into another chamber that
is expanding and extracting power from the burning mixture. This makes
piston engine force paths and consequent stresses far far more complicated
and problematic than the Wankel engine. In fact the Mazda Wankel engine is
so robust, structurally speaking, it is possible to get as much as 400 HP
per rotor. This HP per rotor number is still increasing while auto racers
around the world are still developing the engine.

Although the Wankel has similarities with a two cycle engine,  particularly
in regard with the way the porting mechanism works, it is in no way a two
cycle engine. All four cycles are continuously taking place simultaneously
and those are intake, compression expansion and exhaust.
 
Paul Lamar