A reciprocating air compressor is a positive Piston Air Compressor displacement compressor that uses a crankshaft-driven piston and cylinder to compress the surroundings. … 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 little internal combustion engine, a conventional piston compressor includes a crankshaft, a connecting rod and piston, a cylinder and a valve head. … As the piston moves down, a vacuum is created above it. This enables outside surroundings at atmospheric pressure to push 
open up the inlet valve and fill up the region above the piston.