Double-acting engines use both the top and bottom faces of a piston for power strokes. Steam engines commonly use a slide valve to feed high pressure steam onto one face, driving the expanded steam on the opposing side out. The slide valve will then be moved to reverse the process. Some marine double-acting diesels have/do exist, however it is not common. Some larger marine diesels will use the downward stroke to compress charge air for combustion in other cylinders, as well as to cushion the piston as it approaches the bottom of the stroke, however no power is returned to the piston face.
Double-acting engines use crosshead pistons, to maintain a vertical alignment of the piston, as well as to allow the piston rod to pass through one of the expansion chambers (sealed, similar to a stuffing box).