Abstract: A pressurized gas engine with an axially rotatable rotor having aerodynamically smooth exterior and interior fluid flow surfaces and a central opening in one of a pair of end walls that define with a peripherally circular border wall a hollow interior. The rotor includes energy conversion means that convert fluid pressure energy into mechanical energy of rotation with the conversion means including a plurality of nozzles in and spaced around the border wall and exhausting rearwardly of the direction of rotation. The rotor is enclosed in a casing that is spaced from the rotor at the areas traversed by the nozzles a distance sufficient that the exhaust fluid within the casing does not serve as a drag or brake on the rotating rotor so that the engine attains high speeds in an efficient manner. The disclosure also includes the fact that the total area of the entrance opening means to the rotor is greater than the total area of the exit opening means with the preferred ratio being about 2 to 1.