A reciprocating air flow 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 in fact the air flow is compressed by a second, smaller piston to a pressure of up to 175 PSI.
Like a little internal combustion engine, a conventional piston compressor has a Piston Air Compressor crankshaft, a connecting rod and piston, a cylinder and a valve head. … As the piston moves down, a vacuum is established above it. This allows outside air at atmospheric pressure to drive open the inlet valve and fill up the area above the piston.