Single stage reciprocating compressor with PV diagram

The above figure shows the P-V diagram for single stage reciprocating air compressor without clearance.

During the suction stroke the air is drawn into the cylinder along line 4-1 at constant pressure P1 which is slightly below the atmosphere.

At point 1, the piston completes the suction stroke and starts its compression stroke. At this time, all the valves are closed; the air inside the cylinder is compressed along the curve 1-2.

At point 2, the pressure P2 is reached which is slightly higher than the receiver pressure. At this point discharge valve opens delivery of compressed air takes place along line 2-3 at constant pressure P2.

The piston has now reached at top of cylinder and again starts its suction stroke & the pressure in the cylinder will be lowered again P1 & the cycle of operations will be repeated. The net work done required is represented by area 1-2-3-4.