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.
It is used primarily for testing the intake and exhaust ports of cylinder heads of internal combustion engines.
A flow bench 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.
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.
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.
Flow benches ordinarily use one of three types: orifice plate, venturi meter and pitot/static tube, all of which deliver similar accuracy.
Most commercial machines use orifice plates due to their simple construction and the ease of providing multiple flow ranges.
Air flow conditions must be measured at two locations, across the test piece and across the metering element.
Additional manometers can be installed for use with hand held probes, which are used to explore local flow conditions in the port.
The air flow bench can give a wealth of data about the characteristics of a cylinder head or whatever part is tested.
It is here that air flow bench norms differ from fluid dynamics or aerodynamics at large.
Each of these methods are valid for some purpose but none of them represents the true minimum area for the valve/port in question and each results in a different flow coefficient.
With the information gathered on the flow bench, engine power curve and system dynamics can be roughly estimated by applying various formulae.
For incompressible flow (below 230 Ft/s or 70 M/s this equation gives a less than 1% error corresponding to a test pressure of 12" of water or 306mm of water) it is calculated as follows: For one set of English units Where: For SI units Where: This represents the highest speed of the air in the flow path of a normally shaped port, at or near the section of minimum area (through the valve seat at low values of L/D for instance).
That would not apply to other shapes such as a venturi tube where the local speed in the throat can be much higher than indicated by the pressure drop across the whole system.
(Note, on the graph, that, in this case, when the intake valve opens, the cylinder pressure is above atmospheric (nearly 50% above or 1.5 bar or 150 kPa).
Evaporating fuel passing through the port-runner has the effect of adding gas to and lowering the temperature of the air stream along the runner and giving the outlet flow rate slightly higher than the flow rate entering the port-runner.
The very high temperature causes the viscosity of the gas to increase, all of which alters the Reynolds number drastically.
Exhaust port size and flow information might be considered as vague, but there are certain guidelines which are used when creating a base-line to optimum performance.
