The concept behind the air compressor is a simple one and easily understandable to anyone who is into old cars and such. The reason the pump on the top of your compressor looks like a simplified motorcycle engine is because it basically is. The vast majority of compressors, from the tiny 12v tire inflator you stash in your trunk to the big 80 gallon shop unit, basically compress air with an air cooled piston engine. In fact, there are kits to turn air cooled VW bug motors into gas powered air compressors that could be mounted in the bed of a truck; two cylinders power it and two compress the air.
When the electric motor kicks in, the piston is drawn down the stroke of the cylinder. A simple spring-loaded one way valve opens allowing clean atmospheric air in, usually sucking thru some kind of air filter to keep out dirt and dust just like your car. The piston then moves up, compressing the air, and a 2nd valve opens to let the now pressurize air into the tank, or out into your tire in the case of a tire inflator or the one outside at the gas station. When a gauge signals that the tank has the correct pressure, the motor stops.
Because air tools, spray guns and the like only allow the air out thru a restriction, the tank stays pressurized for a while. If an air hose were to break, the air would all come out pretty quick and return to atmospheric pressure in seconds. The CFM rating on a compressor indicates how many cubic feet of air per minute can leak out via the tool you are using before the pump would be unable to keep up with the demand at a given pressure.
Multi-Stage and Screw Compressors
There are variations on how air compressors work. There are multi stage reciprocating compressors, and nearly silent rotary screw compressors.
Two stage compressors typically have a v-twin looking head on top of the tank. The first cylinder works just like a single stage, but instead of feeding the tank, it feeds the intake on a second smaller cylinder, the second cylinder, then compresses it even more. There are also three piston versions that work the same way, as well as single stage multi-cylinder units for more air flow at lower pressures.
Rotary screw compressors work more like a Roots-type supercharger. Two rotors intermesh, forcing air into a smaller space along the length of the rotors, and finally out into the tank. Because of how they work, they produce more of a constant “whir” then the “chuga-chuga” of a reciprocating pump. This makes them popular for applications where noise is an issue. This type of compressor is also generally used in larger industrial application because of its increased efficiency and ability to deliver larger volumes of air at high pressure.
Care and Maintenance
No matter how you compress the air, you have to deal with the physics of thermal dynamics and moisture. Squishing the air makes it hot, it’s an unavoidable by-product of compression, which is why some units have cooling fins on the supply line to the tank, or even a separate intercooler. All air has moisture in it too and compressing it results in condensation, which is why you need to drain the tank occasionally, and use an air dryer in applications like spraying paint. Just like an engine, compressors need to be lubricated, and if you are spraying paint, you need to strip the oil out of the airstream as well as the water. Luckily, most air dryers function this way already.