Geometric shape of the rotor housing
 

Anyone know the exact geometric shape of the rotor housing? I forget the exact name, and I wanted to take an old rotor housing into class to show the geometry students.

thanks

he there little rotary buddy its called an epitrochoid

Isn't it a "petrichoid"

peritrochoid is what your after :)

+1 http://auto.howstuffworks.com/rotary-engine.htm