A double-acting cylinder is a cylinder in which the working fluid acts alternately on both sides of the piston. In order to connect the piston in a double-acting cylinder to an external mechanism, such as a crank shaft.
source/image(PrtSc): Engine On
A hole must be provided in one end of the cylinder for the piston rod, and this is fitted with a gland or “stuffing box” to prevent escape of the working fluid. Double-acting cylinders are common in steam engines but unusual in other engine types.
Many hydraulic and pneumatic cylinders use them where it is needed to produce a force in both directions.
A double-acting hydraulic cylinder has a port at each end, supplied with hydraulic fluid for both the retraction and extension of the piston.A double-acting cylinder is used where an external force is not available to retract the piston or it can be used where high force is required in both directions of travel.//wikipedia