a pressure gas engine such as a compressed air turbine in which gas flow to a pressure responsive rotor is controlled by a movable valve that is operated by a bellows with means connecting each side of the bellows to the gas in the turbine for supplying substantially equal gas pressure to both the inside and the outside of the bellows , a gas control member that closes a gas inlet independently of the bellows operated valve , a bleed line leading from one side of the bellows so as to regulate the pressure on that side and a speed responsive throttle valve in the bleed line to meter the amount of flow through the bleed line thereby maintaining the desired speed , all regulated and dependent upon the setting of an adjustable throttle valve .