Air flow bench

An air flow bench is a device used for testing the internal aerodynamic qualities of an engine component and is related to the more familiar wind tunnel.

Used primarily for testing the intake and exhaust ports of cylinder heads of internal combustion engines. It is also used to test the flow capabilities of any component such as air filters, carburetors, manifolds or any other part that is required to flow gas. It is one of the primary tools of high performance engine builders and porting cylinder heads would be strictly hit or miss without it.

A flow bench consists of an air pump of some sort, a metering element, pressure and temperature measuring instruments such as manometers, and various controls. The test piece is attached in series with the pump and measuring element and air is pumped through the whole system. Therefore, all the air passing through the metering element also passes through the test piece. Because the volume flow rate through the metering element is known and the flow through the test piece is the same, it is also known. The mass flow rate can be calculated using the known pressure and temperature data to calculate air densities, and multiplying by the volume flow rate.

The air pump used must be able to deliver the volume required at the pressure required. Most flow testing is done at 10 and 28 inches of water pressure (2.5 to 7 kilopascals). Although other test pressures will work, the results would have to be converted for comparison to the work of others. The pressure developed must account for the test pressure plus the loss across the metering element plus all other system losses. The greater the accuracy of the metering element the greater is the loss. Flow volume of between 100 and 600 cubic feet per minute (0.05 to 0.28 m³/s) would serve almost all applications depending on the size of the engine under test.

Any type of pump that can deliver the required pressure difference and flow volume can be used. Most often used is the dynamic-compression centrifugal type compressor, which is familiar to most as being used in vacuum cleaners and turbochargers, but multistaged axial-flow compressor types, similar to those used in most jet engines, could work as well, although there would be little need for the added cost and complexities involved, as they typically don't require such a high flow rate as a jet engine, nor are they limited by the aerodynamic drag considerations which makes a narrow-diameter axial compressor more effective in jet engines than a centrifugal compressor of equal air flow. Positive displacement types such as piston compressors, or rotary types such as a Roots blower could also be used with suitable provisions for damping the pulsations in the air flow (however, other rotary types such as twin screw compressors are capable of providing a steady supply of compressed fluid). The pressure ratio of a single fan blade is too low and cannot be used.

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