A positive displacement machine is a fluid energy machine in which the fluid flows through the machine in individual volumes of the same size. It differs in this respect from the turbomachine , which is continuously flowed through by a fluid.
The two main subsets of the displacement machine are
- the piston engines and
- the screw machines ( screw pumps , screw motors ), these are occasionally also listed as a subspecies of the rotary piston machines ( rotary piston pump , rotary piston engine ).
-  Fluid technology department, Technical University of Dortmund: Research focus a. a. Screw machines; How the screw machine works