As our example of the operating cycle, we will describe one stage of compression in a single-stage, single-acting compressor. The cycle of operation, or compression cycle, within an air compressor cylinder includes two strokes of the piston: an intake stroke and a compression stroke. (Refer to fig. 14-4.) The intake stroke (view A) begins when the piston moves away from top dead center (TDC). The air remaining in the clearance space above the piston expands rapidly (view B) until the pressure in the cylinder falls below the pressure on the opposite side of the inlet valve, which is at atmospheric pressure. At this point (view C), the difference in pressure causes the inlet valve to open, and air is admitted to the cylinder. Air continues

Figure 14-3.-Air compressor cylinder arrangements.

Figure 14-4.-The compression cycle.

to flow into the cylinder until the piston reaches bottom dead center (BDC). The compression stroke (view D) starts as the piston moves away from BDC and continues until the piston reaches TDC again. When the pressure in the cylinder equals the pressure on the opposite side of the air inlet valve, the inlet valve closes. The air trapped in the cylinder continues to be compressed as the piston moves toward TDC. When the pressure in the cylinder (view E) becomes great enough, it will force the discharge valve to open against the discharge line pressure and the force of the valve springs. (The discharge valve opens shortly before the piston reaches TDC.) During the remainder of the compression stroke, the air that has been compressed in the cylin-der is discharged at almost constant pres-sure through the open discharge valve.

The basic operating cycle just described is completed twice for each revolution of the crank-shaft in double-acting compressors, once on the down stroke and once on the up stroke.