A reciprocating atmosphere compressor is a positive displacement compressor that runs on the crankshaft-driven piston and cylinder to compress the air flow. … A two-stage compressor 
contains an additional step where in fact the air flow is compressed by another, smaller sized piston to a Piston Air Compressor 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 allows outside air flow at atmospheric pressure to press open the inlet valve and fill the area above the piston.