Grey gear of Z1 teeth is stationary. The pink crank is fixed to the pink gear of Z2 teeth. Z1 = 2.Z2

When blue crank turns, the point on the pink crank traces an ellipse (in green).

Semi minor axis = A - R

Semi major axis = A + R

A: center distance of the grey and pink gears.

R: radius of the pink crank.

It is possible to improve the mechanism in order that A and D are adjustable.