of the rotor 19 experimentation has proven that values of 2 degrees for angle u03b2 and increment degrees for angle u03b3 provide a remarkably workable small gap between the rotors these optimum values resulted due to the face that when the value of angle u03b2 is fixed the gap is relatively too large when the value of angle u03b3 is 38 degrees while the gap is relatively too small when the value of angle u03b3 is 46 degrees similarly when the value of angle u03b3 is fixed the gap is relatively too large when the value of angle u03b2 is 0 degrees while the gap is relatively too small when the value of angle u03b2 is 4 degrees then after the outer profile of each rotor lobe 18 and its adjacent recess 17 has been determined in the above described manner the contour of the lobe at the angle jki is rounded by a specific radius of curvature r o the radius of curvature r o is determined by drawing a centerline through the center o 1 and the point k the center of the radius of curvature r o is located along the centerline such that the line l extending from the profile curve of the lobe 18.