a and a 39 thereof because this area represented a portion of the lower planar surface of the discharge valve the area between a and b 39 of the prior discharge valve provides a substantially linearly increasing flow area due to the increasing diameter of the concentric conical surfaces defining the flowpath however with the discharge valve of the present invention the area between a 39 and b 39 is of an arcuate contour representing the surface of revolution of an arc and thus provides a significantly greater flow area over the entire range from a 39 to b 39 and which gradually decreases as the flow progress outwardly from b 39 because both the discharge valve of the present invention and that of the prior design have identical conical contours between b 39 and b the flow area provided between these points is identical thus as is dramatically and clearly illustrated in the graphical representation the provision of an arcuate or spherical surface portion along the lower edge of the discharge valve operates to significantly increase the resulting flow area through which discharge gas may exit from the cylinder although the re expansion volume of the cylinder is also increased slightly due to the space provided by the arcuate surface portion viewed another way assuming a given.