Piston displacement of an Air compressor..

As far as I can establish, they're not quite the same. FAD (free air delivery) seems to relate to the volume of air taken in at ambient pressure. This would only be the same as the piston displacement if the pressure in the cylinder (at bottom dead center) was equal to the ambient pressure (or if some cunning airflow engineering was used to keep air entering the cylinder during the first part of the compression stroke), and if the volume above the cylinder was zero at TDC, and if the valve operation was perfect.

I may be talking a load of rubbish, here; I know bugger-all about compressors .

I don't think you're far out. FAD is always rather less than piston displacement. From memory, not having data to hand, difference is about 10%.

Cheers......Codey

Piston displacement is what volume your cylinder will displace with one stroke of the piston. Your FAD (l/s) is volume output at a predetermined rpm so if your piston is moving up and down at 1 stroke per second and your cylinder volume is one liter then yes. If you really need to know the volume of the piston, determine the rpm divide by 60 sec then multiply this number by the FAD this should give you the volume.

This is only true in the ideal case of the air in the cylinder at the end of the inlet stroke being at ambient/atmospheric pressure (and temperature), the valves operating perfectly and the total displaced volume being ejected at the end of the exhaust stroke.

In general, these conditions will not be true, so that the FAD/second will not equate to the piston displacement/second.

Yup, your right, but we are talking air here and not a metering set up. To really get down to a 'true' measurement of displacement you would need fluid. I don't know if its really necessary, the best military in the world thinks 'close is good enough', on the big stuff that is. Cheers.

Dear Friend,

Free Air Delivery and Piston Displacement is DIFFERENT.

DHAYANANDHAN.S