Which mechanism compresses air in a small, positive displacement compressor?

• A. Rotor blades
• B. Diaphragm
• C. Piston movement
• D. Spiral screws

Answer: (C)

Positive-displacement compression works by trapping a fixed volume of air and shrinking that volume to raise its pressure. In a small unit like this, the air is compressed mainly by a piston moving inside a cylinder. As the piston travels inward, the space available for the trapped air gets smaller, so the pressure rises and the air is pushed out through the discharge. The intake fills on the opposite stroke, then the next compression cycle begins. This straightforward piston movement is the common mechanism in small portable compressors because it creates the necessary volume change to deliver compressed air. Other mechanisms describe different types of compressors: rotor blades imply dynamic compression, a diaphragm pump uses a flexible membrane, and spiral screws correspond to a rotary screw design that traps and compresses air between intermeshing screws—more typical of larger, continuous-flow systems.