Most air compressors work by what’s called positive displacement. This is different from compressors that use rotating impellers to generate air pressure. Instead, air pressure is increased by reducing the size of the space that contains the air. Usually, this is done with a reciprocating piston. Like a small internal combustion engine, a conventional piston compressor has a crankshaft, a connecting rod and piston, a cylinder and a valve head. The crankshaft is driven by either an electric motor or a gas engine. While there are small models that are comprised of just the pump and motor, most portable air compressors have an air tank to hold a quantity of air within a preset pressure range. The compressed air in the tank drives the air tools, and the motor turns on and off to automatically maintain pressure in the tank. At the top of the cylinder, there is a valve head that holds the inlet and discharge valves. Both are simply thin metal flaps, one mounted underneath and one mounted on top of the valve plate. As the piston moves down, a vacuum is created above it. This allows outside air at atmospheric pressure to push open the inlet valve and fill the area above the piston. As the piston moves up, the air above it compresses, holds the inlet valve shut and pushes the discharge valve open. The air moves from the discharge port to the tank. With each stroke, more air enters the tank and the pressure rises.

Portable air compressors use a pressure switch to stop the motor when tank pressure reaches a preset limit, about 125 psi for many single-stage models. Most of the time, though, that much pressure isn’t needed. Therefore, the air line will include a regulator that is set to match the pressure requirements of the tool being used. A gauge before the regulator monitors tank pressure and a gauge after the regulator monitors air-line pressure. In addition, the tank has a safety valve that opens if the pressure switch malfunctions. The pressure switch may also incorporate an unloader valve that reduces tank pressure when the compressor is turned off. Many articulated-piston compressors are oil lubricated. The pistons have rings that help keep the compressed air on top of the piston and keep the lubricating oil away from the air. Rings, though, are not completely effective, so some oil will enter the compressed air in aerosol form. Having oil in the air isn’t necessarily a problem. Many air tools require oiling, and inline oilers are often added to increase a uniform supply to the tool. On the negative side, these models require regular oil checks, periodic oil changes and they must be operated on a level surface. While solutions to the airborne oil problem include using an oil separator or filter in the air line, a better idea is to use an oil free portable air compressor that uses permanently lubricated bearings in place of the oil bath.