Abstract: An apparatus (1) and a method of converting a portion of the specific energy of a fluid in gas phase into mechanical work are described, the apparatus (1) comprising: at least one housing (3, 3?) which is provided with at least one gas-supply portion (7, T) and at least one exhaust portion (9, 9?)/each of the at least one housing (3, 3?) comprising: a blade wheel (5) which is rotatably arranged in the housing (3, 3?) and which includes: a shaft (51) enclosed by a drum (53); at least two blades (55) which are movably arranged to the drum (53) so that a portion (57) of the blades (55) is arranged to be moved towards the internal casing surface (31) of the housing (3, 3?) in such a way that the drum (53), the internal casing surface (31) of the housing (3) and the blades (55) define chambers (59) arranged to contain gas, an effective area of a blade (55) which is immediately upstream of the exhaust portion (9, 9?) being larger than an effective area of a blade (55) which is immediately upstream of the gas-supply