In this video I will explain how to watt straight line generator work. As you see, it is 4 bar linkage type. What is special?
As this picture above, link AB and CD can hinge at A and D. Link BC can hinge at B that follow link AB, hinge at C that follow link CD. The midpoint I of connecting link BC traces a figure eight pattern over the full mechanism excursion.
You can see this video below for more detail.