Gas flows through the inlet check valve and fills the larger diameter first-stage cylinder. Pressurized hydraulic fluid, acting on the hydraulic piston, strokes the piston assembly to the left compressing the gas in the first-stage cylinder. Gas in the first-stage cylinder flows through the check valves into the smaller diameter second-stage cylinder.

At the end of Stage I compression, the four-way valve changes position and directs pressurized hydraulic fluid to the left side of the hydraulic piston. The piston assembly moves to the right compressing gas in the second-stage cylinder. Gas flows out of the second-stage cylinder into the discharge gas line. The piston assembly reverses direction at the end of the second-stage stroke and the cycle repeats.

Control system of these compressors are mechanical flip-flap. Since control system and actuation of piston is hydraulic, so shutdown of system is hydraulic, too. This type of shutdown is the most reliable system.