A reciprocating surroundings compressor is a positive displacement compressor that uses a crankshaft-driven piston and cylinder to compress the atmosphere. … A two-stage compressor Piston Air Compressor contains an additional step where the surroundings is compressed by a second, 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 moves down, a vacuum is created above it. This enables outside air flow at atmospheric pressure to push open up the inlet valve and fill the area above the piston.
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