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 air. … A two-stage compressor includes an additional step where in fact the surroundings is compressed by another, smaller piston to a pressure of up to 175 PSI.
Like a small internal combustion engine, a conventional piston compressor has a crankshaft, a connecting rod and piston, a cylinder and a valve head. … As the piston movements down, a vacuum is established above it. This 
enables outside atmosphere at atmospheric pressure to push open up the inlet valve and fill the area above the piston.