About volumetric efficiency
The volumetric efficiency of an air compressor is less than it may appear because you have to take in to account ‘clearance volume’. This is why the volume of air compressed inside the cylinder for is lower than the theoretically-calculated volume.
The gas is compressed and then delivered via the discharge valve, however it doesn’t discharge all the gas and some high-pressurised gas remains inside the ‘clearance volume’.
This happens because as the piston begins to move downwards the suction valve doesn’t open immediately because of this high-pressure gas. As the piston moves downwards the high-pressure gas in the clearance volume reduces due to expansion, and only when the pressure reaches a certain level does the suction-valve open.
Therefore, this gas used in the piston’s suction stroke goes unused and reduces the volumetric efficiency of the compressor.
How is it defined?
A compressor’s volumetric efficiency (V) is defined by the ratio of actual volume of gas that it could suck if there was no such thing as clearance volume. Therefore, a compressor with less clearance volume should be more volumetrically efficient. The volumetric efficiency directly affects the capacity because the capacity depends directly on the cylinder’s piston displacement.
A compressor’s volumetric efficiency can also depend on the compressor’s ratio. When the disparity between suction and discharge pressure increases, refrigerant remains trapped inside the clearance volume for longer and prevents the suction valve opening. Maintaining your compressor appropriately can prevent this.