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 includes an additional step where the surroundings is compressed by a second, smaller piston to a pressure as high as 175 PSI.
Like a small 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 enables outside air flow at atmospheric pressure to press open up the inlet valve and fill the region above the piston.