Delivery control for rotary piston compressor - has closable ducts in housing end wall portion traversed by piston between BDC and TDC

The delivery control is intended for a rotary piston compressor having a piston and a housing defining working chamber between them. At least one housing end wall is provided with closable ducts in the area which is swept by the side wall of the piston between bottom and top dead centre. A bore may be provided across the ducts, with a slide value element movable in the bore. The value may be pneumatically actuated from outside, e.g. by boiler pressure. The ducts may connect with a common chamber communicating with the compressor inlet. There may be a number of rows of ducts in the end wall of a working chamber.