MechDesigner:
The Peritrochoid Curves are produced by a Point on a gear when:
The shape of the Peritrochoid Curve depends on the ratio of the teeth. With gears, the ratio of the teeth is always a rational number. Therefore the pattern of the curve repeats eventually. You can easily draw the shape of a Wankel engine's casing. 

The Revolving Gear is OUTSIDE The Fixed gear is INSIDE The ratio of teeth is 3:2 There are four separate curves in the image. You can use this ratio and shape to define the combustion chamber of a Wankel Engine.


The Revolving Gear is OUTSIDE The Fixed gear is INSIDE The ratio of teeth is 4:3 There are four curves. 

The Revolving Gear is OUTSIDE The Fixed gear is INSIDE The ratio of teeth is 11:12 There are four curves. 