The invention relates to a piston type axial expander (4) comprising: an intake cylinder head (10) for vapor under pressure comprising a vapor intake opening (100), an expansion zone comprising a plurality of cylinders (110), wherein a piston (111) sliding in each respective cylinder is connected to a shaft (40) by an inclined plate (20), each piston being parallel to said shaft. a plurality of poppet valves (12) arranged orthogonally to the shaft (40) in the intake cylinder head (10) allowing alternating intake of vapor into said cylinders (110), each valve (12) being controlled by a cam (21) arranged on the shaft (40), a lift mechanism (13) for each valve cooperating with the cam, a return mechanism for each valve.

The intake cylinder head (10) comprises: an enclosed and lubricated central zone (10A) comprising the cam and the lift and return mechanisms of the valves, a peripheral zone (10B) in which the intake opening (100) leads, extending around the central zone (10A).