A single-stage piston compressor is built as shown in the diagram. The engine turns the flywheel, which rotates the crankshaft (1). The crank on the shaft moves the big end of the connecting rod (3) in a circular motion. This motion is passed through the rod to its small end, driving the crosshead (4), the piston rod (5), and the piston (7) back and forth in a straight line.
Operating Cycle
The crank and connecting rod move the piston repeatedly between two ends. The piston’s furthest point toward the crankshaft is called the inner dead center. Its furthest point toward the cylinder cover is called the outer dead center. The distance between these two points is the stroke S.
When the piston moves from the outer dead center toward the inner dead center, the space between the cylinder head and the piston grows. The gas inside expands and its pressure drops. Once the pressure falls below the pressure in the inlet pipe, the suction valve opens. Gas enters the cylinder. The piston reaches the inner dead center and the suction valve closes. The suction stage ends.
Next, the piston moves toward the outer dead center. The space in the cylinder becomes smaller. The gas is compressed and its pressure rises. When the pressure exceeds the pressure in the outlet pipe, the discharge valve opens. Gas is pushed out. The piston arrives at the outer dead center and the discharge valve closes.
In this way, each full turn of the crankshaft moves the piston back and forth once. The compressor completes one full cycle. This cycle includes expansion, suction, compression, and discharge.