A reciprocating air flow Piston Air Compressor compressor is a positive 
displacement compressor that runs on the crankshaft-driven piston and cylinder to compress the atmosphere. … A two-stage compressor contains an additional step where the air flow is compressed by a second, smaller piston to a pressure of up to 175 PSI.
Like a small internal combustion engine, a typical piston compressor has a crankshaft, a connecting rod and piston, a cylinder and a valve head. … As the piston techniques down, a vacuum is established above it. This enables outside air at atmospheric pressure to drive open the inlet valve and fill the region above the piston.