As in this example above lets look at the flow of air the restrictions and pressure points along the way
The air from the vacuum source creates a depression on the D side of the orifice plate air begins to be drawn through the orifice and begins to create another depression on the C side of the orifice plate as the air move further it creates a depression in the B chamber of the test & settling chamber.
Now if the test point is completely closed (Blocked off) the depression measured at A-B would be whatever the vacuum source is capable of pulling at the current settings and the pressure drop across C-D would be 0 thus the incline would read 0 flow (This is because both C & D are reading the same pressure on both ports at both sides of the orifice plate).
Now this is where it gets a little confusing as when we open the valve on the test point 1 we begin to leak air into the settling chamber and air begins to move through the system (picture this opening of your test point valve as leak.
The air leak at the test point causes the pressure to drop in chambers B & C, as air flows through the orifice plate. Lets say we are trying to test are part at 28” of depression the now open test point has cause the A-B scale to drop (the leak) so we must add more pull from the vacuum source thus creating a bigger pressure drop along the way “the air begins to flow through the system”.
Since now the pressure at point D is lower (More vacuum) than B-C (test side where air is leaking in through the valve) the incline (Delta-P) manometer begins to move showing this “Pressure Differential”. The math tells us that for a given orifice size a pressure difference across it the orifice would be flowing a specific amount of air.
Confusing but this is the basics of the orifice bench. Now as you open your test point more and more you have to apply more vacuum to maintain your test pressure and the greater the vacuum needed to pull the air through the orifice thus creating a grater pressure differential “Delta-P”. This causing the incline manometer to rise farther up the scale and again the math will show you are flowing more air.
The first key is the orifice plate is designed to work at a given Delta-P for the PTS DM bench 16” thus a 200cfm orifice plate will have a pressure differential across it of 16 inches when it is flowing 100% or 200cfm. If we were to do some simple math then if my test pressure is 28” and my Delta-P is 16” then at point D to atmosphere we should be able to measure very close to 28 + 16 or 44” of vacuum. This explains how the orifice Delta-P effect the usable power of the bench, if the orifice and instrumentation were set to read say a 6” Delta-P your vacuum needs would only be 34” so from a design perspective you might be able to use less motors.
Now you may ask why is the PTS DM set to use a 16” Delta-P this is for two specific reasons. At the time of design the smallest sensor available was a 16” sensor. And two combined with the electronics inside this sensor provides the highest level of accuracy across the digital range. Our sensor manufacturer has since come out with smaller sensors and we are looking at building prototypes of the PTS DM that would use a Delta-P of say 6” for smaller benches but again this may not be as sensitive as the 16” sensor application.
Ratio metric! What does this mean and how does it apply to the PTS Bench? This means that the basic operation of this particular style of flow bench uses a confined pressure differential of same air to determine flow. In other words the air Density is the same on both sides of the orifice eliminating the need for adjustments due to temperature, barometric pressure and humidity.
Though the PTS DM does use these figures in the configuration settings they would only be changed if you were making drastic changes in any specific area for test purposes. Example you wanted to see if your bench flowed the same in Denver as it does at home in Ocean City Maryland. The discussion of the effects of temperature and barometric pressure came about because of the setup and configuration of the SuperFlow SF 1XX series of benches. These benches do not use a “Confined Orifice” there orifice has one side facing the atmosphere so flow would follow a different path “Test piece > Vacuum Motor > chamber > orifice > atmosphere” thus the air flowing through the motor is heated versus atmosphere. In these benches the ambient temp & humidity along with the bench internal temp all play a role in the formula to determine Flow.
(Thanks Rick (1960FL) for taking the time to put this together!)
Flowbench 101
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Brucepts
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Flowbench 101
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gofaster
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Re: Flowbench 101
That is a great explanation, it probably should stand alone on the forum for quick and easy access for any newcomers. I think it would save a lot of time for them!
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blaktopr
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Re: Flowbench 101
Wikipedia version. The tractorsport forum is part of a link at the bottom for info. THE only one. 
Here is the direct link to see all the pics. There is info on port flow and pressures too.
http://en.wikipedia.org/wiki/Air_flow_bench
Air flow bench
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2007)
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 flow through the metering element is known and the flow through the test piece is the same, it is also known.
Typical Flow Bench schematic This article may need to be wikified to meet Wikipedia's quality standards. Please help by adding relevant internal links, or by improving the article's layout. (September 2009)
Contents [hide]
1 Air pump
2 Metering element
3 Instrumentation
4 Flow bench data
5 Limitations
5.1 Steady state flow vs dynamic flow
5.2 Pressure differential
5.3 Air only vs mixed gas/fuel mist flow
5.4 Bulk flow vs flow velocity
5.5 Even room temperature vs uneven high temperature
5.6 Physical and mechanical differences
5.7 Exhaust port conditions
6 Summary
7 See also
8 References
9 External links
[edit] Air pump
The 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 centrifugal dynamic type, which is familiar to most as a vacuum cleaner. Multistaged axial-flow fan types and positive displacement types (piston and rotary) could also be used with suitable provisions for dampening the pulsations. The pressure ratio of a single fan blade is too low and cannot be used.
[edit] Metering element
There are several possible types of metering element in use. Flow benches ordinarily use 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. Although the venturi offers substantial improvements in efficiency, its cost is higher
[edit] Instrumentation
Air flow conditions must be measured at two locations, across the test piece and across the metering element. The pressure difference across the test piece allows the standardization of tests from one to another. The pressure across the metering element allows calculation of the actual flow through the whole system.
The pressure across the test piece is typically measured with a U tube manometer while, for increased sensitivity and accuracy, the pressure difference across the metering element is measured with an inclined manometer. One end of each manometer is connected to its respective plenum chamber while the other is open to the atmosphere.
Ordinarily all flow bench manometers measure in inches of water although the inclined manometer's scale is usually replaced with a logarithmic scale reading in percentage of total flow of the selected metering element which makes flow calculation simpler.
Temperature must also be accounted for because the air pump will heat the air passing through it making the air down stream of it less dense and more viscous. This difference must be corrected for. Temperature is measured at the test piece plenum and at the metering element plenum. Correction factors are then applied during flow calculations. Some flow bench designs place the air pump after the metering element so that heating by the air pump is not as large a concern.
Additional manometers can be installed for use with hand held probes, which are used to explore local flow conditions in the port.
[edit] Flow bench data
The air flow bench can give a wealth of data about the characteristics of a cylinder head or whatever part is tested.
The result of main interest is bulk flow. It is the volume of air that flows through the port in a given time. Expressed in cubic feet per minute or cubic meters per second/minute.
Valve lift can be expressed as an actual dimension in decimal inches or mm. It can also be specified as a ratio between a characteristic diameter and the lift L/D. Most often used is the valve head diameter. Normally engines have an L/D ratio from 0 up to a maximum of .35. For example a 1 inch diameter valve would be lifted a maximum of 0.350 inch. During flow testing the valve would be set at L/D .05 .1 .15 .2 .25 .3 and readings taken successively. This allows the comparison of efficiencies of ports with other valve sizes, as the valve lift is proportional rather than absolute. For comparison with tests by others the characteristic diameter used to determine lift must be the same.
Flow coefficients are determined by comparing the actual flow of a test piece to the theoretical flow of a perfect orifice of equal area. Thus the flow coefficient should be a close measure of efficiency. It cannot be exact because the L/D does not indicate the actual minimum size of the duct.
A real orifice plate showing how the fluid would actually flow
A theoretical orifice plate showing perfect flow which is used as a standard for comparing the efficiencies of real flows
An orifice with a flow coefficient of .59 would flow the same amount of fluid as a perfect orifice with 59% of its area or 59% of the flow of a perfect orifice with the same area (orifice plates of the type shown would have a coefficient of between .58 and .62 depending on the precise details of construction and the surrounding installation).[1]
Valve/port coefficient is non dimensional and is derived by multiplying a characteristic physical area of the port and by the bulk flow figures and comparing the result to an ideal orifice of the same area. It is here that air flow bench norms differ from fluid dynamics or aerodynamics at large. The coefficient may be based on the inner valve seat diameter, the outer valve head diameter, the port throat area or the valve open curtain area. 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. The great difficulty of measuring the actual minimum area at all the various valve lifts precludes using this as a characterisc measurement. This is due to the minimum area changing shape and location throughout the lift cycle. Because of this non standardization, port flow coefficients are not "true" flow coefficients, which would be based on the actual minimum area in the flow path. Which method to choose depends on what use is intended for the data. Engine simulation applications each require their own specification. If the result is to be compared to the work of others then the same method would have to be selected.
Various characteristic measurements used to determine flow coefficients
Using extra instrumentation (manometers and probes) the detailed flow through the port can be mapped by measuring multiple points within the port with probes. Using these tools, the velocity profile throughout the port can be mapped which gives insight into what the port is doing and what might be done to improve it.
Of less interest is mass flow per minute or second since the test is not of a running engine which would be affected by it. It is the weight of air that flows through the port in a given time. Expressed in pounds per minute/hour or kilograms per second/minute. Mass flow is derived from the volume flow result to which a density correction is applied.
With the information gathered on the flow bench, engine power curve and system dynamics can be roughly estimated by applying various formulae. With the advent of accurate engine simulation software, however, it is much more useful to use flow data to create an engine model for a simulator.
Determining air velocity is a useful part of flow testing. It is calculated as follows:
For one set of English units * UNITS for SI or English are wrong, different results on conversion!-(The English formula is Verified correct. See ref.)
Where:
V = Velocity in feet per minute
H = Pressure drop across test piece in inches of water measured by the test pressure manometer
d = density of air in pounds per cubic foot (0.075 pound per cubic foot at standard conditions)[2]
For SI units
Where:
V = Velocity in meters per second
H = Pressure drop across test piece in pascals measured by the test pressure manometer
d = density of air in kilograms per cubic meter (1.20 kilograms per cubic meter at standard conditions)
This represents the highest speed of the air in the flow path, at or near the section of minimum area (through the valve seat at low values of L/D for instance).
Once velocity has been calculated, the volume can be calculated by multiplying the velocity by the orifice area times its flow coefficient.
[edit] Limitations
A flow bench is capable of giving flow data which is closely but not perfectly related to actual engine performance. There are a number of limiting factors which contribute to the discrepancy.
[edit] Steady state flow vs dynamic flow
A flow bench tests ports under a steady pressure difference while in the actual engine the pressure difference varies widely during the whole cycle. The exact flow conditions existing in the flow bench test exist only fleetingly if at all in an actual running engine. Running engines cause the air to flow in strong waves rather than the steady stream of the flow bench. This acceleration/deceleration of the fuel/air column causes effects not accounted for in flow bench tests.
Comparison of flow bench test pressure to actual engine pressures predicted by an engine simulation program
This graph, generated with an engine simulation program, shows how widely the pressures vary in a running engine vs. the steady test pressure of the flow bench.
(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). This will cause reverse flow into the intake port until pressure in the cylinder falls below the ports pressure).
[edit] Pressure differential
The coefficient of the port may change somewhat at different pressure differentials due to changes in Reynolds number regime leading to a possible loss of dynamic similitude. Flow bench test pressure are typically conducted at 10 to 28 inches of water (2.5 to 7 kPa) while a real engine may see 190 inches of water (47 kPa) pressure difference.
[edit] Air only vs mixed gas/fuel mist flow
The flow bench tests using only air while a real engine usually uses air mixed with fuel droplets and fuel vapor, which is significantly different. 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. A port which flows dry air well might cause fuel droplets to fall out of suspension causing a loss of power not indicated by flow figures alone.
[edit] Bulk flow vs flow velocity
Large ports and valves can show high flow rates on a flow bench but the velocity can be lowered to the point that the gas dynamics of a real engine are ruined. Overly large ports also contribute to fuel fall out.
[edit] Even room temperature vs uneven high temperature
A running engine is much hotter than room temperature and the temperature in various parts of the system vary widely. This affects the actual flow, fuel effects as well as the dynamic wave effects in the engine which do not exist on the flow bench.
[edit] Physical and mechanical differences
The proximity, shape and movement of the piston as well as the movement of the valve itself significantly alters the flow conditions in a real engine that do not exist in flow bench tests.
[edit] Exhaust port conditions
The flow simulated on a flow bench bears almost no similarity to the flow in a real exhaust port. Here even the coefficients measured on flow benches are inaccurate. This is due to the very high and wide ranging pressures and temperatures. From the graph above it can be seen that the pressure in the port reaches 2.5 bar (250 kPa) and the cylinder pressure at opening is 6 bar (600 kPa) and more. This is many times more than the capabilities of a typical flow bench of 0.06 bar (6 kPa).
The flow in a real exhaust port can easily be sonic with choked flow occurring and even supersonic flow in areas. The very high temperature causes the viscosity of the gas to increase, all of which alters the Reynolds number drastically.
Added to the above is the profound effect that downstream elements have on the flow of the exhaust port. Far more than upstream elements found on the intake side.
It is for these reasons that most published information on exhaust port flow is vague. Particularly when concerning how much flow would be required in any given situation. Often quoted is that the exhaust should flow 60% of intake flow but this is only crude guesswork.
[edit] Summary
In spite of its limitations the flow bench, in skilled hands, still provides an excellent source of information to guide the engine designer/builder in deciding which modifications to apply to the ports and other components. They are also essential to provide accurate port flow coefficient data that is required for the use of engine simulation software. Together, the air flow bench and engine simulation software provide a powerful microscope to examine the detailed inner workings of running engines. Details which are impossible to view any other way.
Air flow benches are relatively inexpensive to buy and it is fairly simple to build a scientifically valid instrument for home use.
[edit] See also
Air flow meter
[edit] References
^ Fundamentals of Fluid Mechanics 4th Ed Munson Young -Wiley P514-515
^ Dwyer Air Velocity Instruments manual
[edit] External links
Free demo engine simulator used to generate graph above
Plans for a home built flow bench
Forums for those interested in the design and construction of flow benches
Retrieved from "http://en.wikipedia.org/wiki/Air_flow_bench"
Categories: Engine tuning instruments | Aerodynamics
Hidden categories: Articles needing additional references from September 2007 | All articles needing additional references | Articles that need to be wikified from September 2009 | All articles that need to be wikified
Here is the direct link to see all the pics. There is info on port flow and pressures too.
http://en.wikipedia.org/wiki/Air_flow_bench
Air flow bench
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2007)
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 flow through the metering element is known and the flow through the test piece is the same, it is also known.
Typical Flow Bench schematic This article may need to be wikified to meet Wikipedia's quality standards. Please help by adding relevant internal links, or by improving the article's layout. (September 2009)
Contents [hide]
1 Air pump
2 Metering element
3 Instrumentation
4 Flow bench data
5 Limitations
5.1 Steady state flow vs dynamic flow
5.2 Pressure differential
5.3 Air only vs mixed gas/fuel mist flow
5.4 Bulk flow vs flow velocity
5.5 Even room temperature vs uneven high temperature
5.6 Physical and mechanical differences
5.7 Exhaust port conditions
6 Summary
7 See also
8 References
9 External links
[edit] Air pump
The 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 centrifugal dynamic type, which is familiar to most as a vacuum cleaner. Multistaged axial-flow fan types and positive displacement types (piston and rotary) could also be used with suitable provisions for dampening the pulsations. The pressure ratio of a single fan blade is too low and cannot be used.
[edit] Metering element
There are several possible types of metering element in use. Flow benches ordinarily use 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. Although the venturi offers substantial improvements in efficiency, its cost is higher
[edit] Instrumentation
Air flow conditions must be measured at two locations, across the test piece and across the metering element. The pressure difference across the test piece allows the standardization of tests from one to another. The pressure across the metering element allows calculation of the actual flow through the whole system.
The pressure across the test piece is typically measured with a U tube manometer while, for increased sensitivity and accuracy, the pressure difference across the metering element is measured with an inclined manometer. One end of each manometer is connected to its respective plenum chamber while the other is open to the atmosphere.
Ordinarily all flow bench manometers measure in inches of water although the inclined manometer's scale is usually replaced with a logarithmic scale reading in percentage of total flow of the selected metering element which makes flow calculation simpler.
Temperature must also be accounted for because the air pump will heat the air passing through it making the air down stream of it less dense and more viscous. This difference must be corrected for. Temperature is measured at the test piece plenum and at the metering element plenum. Correction factors are then applied during flow calculations. Some flow bench designs place the air pump after the metering element so that heating by the air pump is not as large a concern.
Additional manometers can be installed for use with hand held probes, which are used to explore local flow conditions in the port.
[edit] Flow bench data
The air flow bench can give a wealth of data about the characteristics of a cylinder head or whatever part is tested.
The result of main interest is bulk flow. It is the volume of air that flows through the port in a given time. Expressed in cubic feet per minute or cubic meters per second/minute.
Valve lift can be expressed as an actual dimension in decimal inches or mm. It can also be specified as a ratio between a characteristic diameter and the lift L/D. Most often used is the valve head diameter. Normally engines have an L/D ratio from 0 up to a maximum of .35. For example a 1 inch diameter valve would be lifted a maximum of 0.350 inch. During flow testing the valve would be set at L/D .05 .1 .15 .2 .25 .3 and readings taken successively. This allows the comparison of efficiencies of ports with other valve sizes, as the valve lift is proportional rather than absolute. For comparison with tests by others the characteristic diameter used to determine lift must be the same.
Flow coefficients are determined by comparing the actual flow of a test piece to the theoretical flow of a perfect orifice of equal area. Thus the flow coefficient should be a close measure of efficiency. It cannot be exact because the L/D does not indicate the actual minimum size of the duct.
A real orifice plate showing how the fluid would actually flow
A theoretical orifice plate showing perfect flow which is used as a standard for comparing the efficiencies of real flows
An orifice with a flow coefficient of .59 would flow the same amount of fluid as a perfect orifice with 59% of its area or 59% of the flow of a perfect orifice with the same area (orifice plates of the type shown would have a coefficient of between .58 and .62 depending on the precise details of construction and the surrounding installation).[1]
Valve/port coefficient is non dimensional and is derived by multiplying a characteristic physical area of the port and by the bulk flow figures and comparing the result to an ideal orifice of the same area. It is here that air flow bench norms differ from fluid dynamics or aerodynamics at large. The coefficient may be based on the inner valve seat diameter, the outer valve head diameter, the port throat area or the valve open curtain area. 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. The great difficulty of measuring the actual minimum area at all the various valve lifts precludes using this as a characterisc measurement. This is due to the minimum area changing shape and location throughout the lift cycle. Because of this non standardization, port flow coefficients are not "true" flow coefficients, which would be based on the actual minimum area in the flow path. Which method to choose depends on what use is intended for the data. Engine simulation applications each require their own specification. If the result is to be compared to the work of others then the same method would have to be selected.
Various characteristic measurements used to determine flow coefficients
Using extra instrumentation (manometers and probes) the detailed flow through the port can be mapped by measuring multiple points within the port with probes. Using these tools, the velocity profile throughout the port can be mapped which gives insight into what the port is doing and what might be done to improve it.
Of less interest is mass flow per minute or second since the test is not of a running engine which would be affected by it. It is the weight of air that flows through the port in a given time. Expressed in pounds per minute/hour or kilograms per second/minute. Mass flow is derived from the volume flow result to which a density correction is applied.
With the information gathered on the flow bench, engine power curve and system dynamics can be roughly estimated by applying various formulae. With the advent of accurate engine simulation software, however, it is much more useful to use flow data to create an engine model for a simulator.
Determining air velocity is a useful part of flow testing. It is calculated as follows:
For one set of English units * UNITS for SI or English are wrong, different results on conversion!-(The English formula is Verified correct. See ref.)
Where:
V = Velocity in feet per minute
H = Pressure drop across test piece in inches of water measured by the test pressure manometer
d = density of air in pounds per cubic foot (0.075 pound per cubic foot at standard conditions)[2]
For SI units
Where:
V = Velocity in meters per second
H = Pressure drop across test piece in pascals measured by the test pressure manometer
d = density of air in kilograms per cubic meter (1.20 kilograms per cubic meter at standard conditions)
This represents the highest speed of the air in the flow path, at or near the section of minimum area (through the valve seat at low values of L/D for instance).
Once velocity has been calculated, the volume can be calculated by multiplying the velocity by the orifice area times its flow coefficient.
[edit] Limitations
A flow bench is capable of giving flow data which is closely but not perfectly related to actual engine performance. There are a number of limiting factors which contribute to the discrepancy.
[edit] Steady state flow vs dynamic flow
A flow bench tests ports under a steady pressure difference while in the actual engine the pressure difference varies widely during the whole cycle. The exact flow conditions existing in the flow bench test exist only fleetingly if at all in an actual running engine. Running engines cause the air to flow in strong waves rather than the steady stream of the flow bench. This acceleration/deceleration of the fuel/air column causes effects not accounted for in flow bench tests.
Comparison of flow bench test pressure to actual engine pressures predicted by an engine simulation program
This graph, generated with an engine simulation program, shows how widely the pressures vary in a running engine vs. the steady test pressure of the flow bench.
(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). This will cause reverse flow into the intake port until pressure in the cylinder falls below the ports pressure).
[edit] Pressure differential
The coefficient of the port may change somewhat at different pressure differentials due to changes in Reynolds number regime leading to a possible loss of dynamic similitude. Flow bench test pressure are typically conducted at 10 to 28 inches of water (2.5 to 7 kPa) while a real engine may see 190 inches of water (47 kPa) pressure difference.
[edit] Air only vs mixed gas/fuel mist flow
The flow bench tests using only air while a real engine usually uses air mixed with fuel droplets and fuel vapor, which is significantly different. 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. A port which flows dry air well might cause fuel droplets to fall out of suspension causing a loss of power not indicated by flow figures alone.
[edit] Bulk flow vs flow velocity
Large ports and valves can show high flow rates on a flow bench but the velocity can be lowered to the point that the gas dynamics of a real engine are ruined. Overly large ports also contribute to fuel fall out.
[edit] Even room temperature vs uneven high temperature
A running engine is much hotter than room temperature and the temperature in various parts of the system vary widely. This affects the actual flow, fuel effects as well as the dynamic wave effects in the engine which do not exist on the flow bench.
[edit] Physical and mechanical differences
The proximity, shape and movement of the piston as well as the movement of the valve itself significantly alters the flow conditions in a real engine that do not exist in flow bench tests.
[edit] Exhaust port conditions
The flow simulated on a flow bench bears almost no similarity to the flow in a real exhaust port. Here even the coefficients measured on flow benches are inaccurate. This is due to the very high and wide ranging pressures and temperatures. From the graph above it can be seen that the pressure in the port reaches 2.5 bar (250 kPa) and the cylinder pressure at opening is 6 bar (600 kPa) and more. This is many times more than the capabilities of a typical flow bench of 0.06 bar (6 kPa).
The flow in a real exhaust port can easily be sonic with choked flow occurring and even supersonic flow in areas. The very high temperature causes the viscosity of the gas to increase, all of which alters the Reynolds number drastically.
Added to the above is the profound effect that downstream elements have on the flow of the exhaust port. Far more than upstream elements found on the intake side.
It is for these reasons that most published information on exhaust port flow is vague. Particularly when concerning how much flow would be required in any given situation. Often quoted is that the exhaust should flow 60% of intake flow but this is only crude guesswork.
[edit] Summary
In spite of its limitations the flow bench, in skilled hands, still provides an excellent source of information to guide the engine designer/builder in deciding which modifications to apply to the ports and other components. They are also essential to provide accurate port flow coefficient data that is required for the use of engine simulation software. Together, the air flow bench and engine simulation software provide a powerful microscope to examine the detailed inner workings of running engines. Details which are impossible to view any other way.
Air flow benches are relatively inexpensive to buy and it is fairly simple to build a scientifically valid instrument for home use.
[edit] See also
Air flow meter
[edit] References
^ Fundamentals of Fluid Mechanics 4th Ed Munson Young -Wiley P514-515
^ Dwyer Air Velocity Instruments manual
[edit] External links
Free demo engine simulator used to generate graph above
Plans for a home built flow bench
Forums for those interested in the design and construction of flow benches
Retrieved from "http://en.wikipedia.org/wiki/Air_flow_bench"
Categories: Engine tuning instruments | Aerodynamics
Hidden categories: Articles needing additional references from September 2007 | All articles needing additional references | Articles that need to be wikified from September 2009 | All articles that need to be wikified
Chris Sikorski
Chris@wetflowtech.com
Totallywirednow.com
Chris@wetflowtech.com
Totallywirednow.com
-
1960FL
- Posts: 1339
- Joined: Fri Jan 08, 2010 10:36 pm
- Location: Maryland
Re: Flowbench 101
Chris,
My description was not meant as a defacto on the flow bench but a Primer for those that come here and want to understand the basics of design and operation. If studied closely one will see the reason for my Primer was that the WIKI schematic was NOT in the studies of the majority of the PTS members. What I mean to say is the link to Wikipedia is a description of a NON Ratiometric design and therefore more in line with how the big blue baby benches function SF110 etc. (as you will see the air power source is in the middle of the flow path) That said there is lots of good reading and information on the flow bench and orifice plates, pitot tubes etc on Wikipedia.
Rick
My description was not meant as a defacto on the flow bench but a Primer for those that come here and want to understand the basics of design and operation. If studied closely one will see the reason for my Primer was that the WIKI schematic was NOT in the studies of the majority of the PTS members. What I mean to say is the link to Wikipedia is a description of a NON Ratiometric design and therefore more in line with how the big blue baby benches function SF110 etc. (as you will see the air power source is in the middle of the flow path) That said there is lots of good reading and information on the flow bench and orifice plates, pitot tubes etc on Wikipedia.
Rick
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blaktopr
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Re: Flowbench 101
I understand Rick. The reason for that was to get more information here instead of guys searching around the net. Maybe Bruce can put it somewhere else more appropriate.
Chris Sikorski
Chris@wetflowtech.com
Totallywirednow.com
Chris@wetflowtech.com
Totallywirednow.com
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1960FL
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Re: Flowbench 101
Chris,
I was not trying to negate your post just make the point about the differance in the flow schematic.
If you do not mind i may clean up your post so it look more like the Wiki.
Rick
I was not trying to negate your post just make the point about the differance in the flow schematic.
If you do not mind i may clean up your post so it look more like the Wiki.
Rick
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blaktopr
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Re: Flowbench 101
Rick, thats no prob at all. Make it work best for the forum.
Chris Sikorski
Chris@wetflowtech.com
Totallywirednow.com
Chris@wetflowtech.com
Totallywirednow.com
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Brucepts
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Re: Flowbench 101
I've moved this topic to "Global" as EVERYONE needs to get an understanding of the basic principals of how an orifice style flowbench functions.
I seem to be having alot of discussion with people who are not grasping how orifice plates work inside and on-top of their flowbench.
I sell a set of plates that are dual marked in 16" and 28" and they are marked for a range of flows, I do this so people do not have to buy more plates than they need. Instead of buying 4 internal and 4 external plates they can use this set of 4 plates for inside or outside use.
The 28" number is for external use for calibration.
The 16" number is for internal use and this is the max CFM the plate will flow at 16" DP. You can not flow more CFM at 16" DP than the plate is rated for! (reference the above picture and text in Flowbench 101)
It does not matter if you test at 10", 28" or 100" the plates are rated for 16" DP for the CFM range, so if the plate says 340@16" it's only going to be able to measure 340@16" no matter what depression you test at. Once you hit 340CFM you have reached the end of the scale and the "water" is blown out of the gauge or in the case of the digital manometer it's maxed out the sensor.
You can not try and measure more CFM than the plate is marked for! And should stay 25cfm below the plates marked range for safety.
The 28" reference is for calibration at 28" of static pressure it has nothing to do with the 16" CFM number and you do not need to use the 16" number for testing on top of your flowbench for calibration. If you only buy plates for calibration they are only marked in 28" to avoid confusion for those who are not using them on a PTS Flowbench build.
Maybe in hind-sight I should have just had internal and external plates and not had a "set" which had dual flow as this would have kept the confusion down and I would sell more plates. The objective was to keep costs down but I seem to be having alot of issues with people not understanding the "orifice flowbench concept" from this post.
My plans/parts are sold with the assumption to some extent that you have done some research on this forum on the basic concepts of Flowbench 101 and have a good understanding of the basic concepts. A little "sweat-equitity" is done by the end-user to keep the costs down on my parts as they have less instructions such as plug a into b and install in c . . .
I have no problem with helping people along and the forum members have no problem offering help. What does worry me is the amount of people who think that the digital manometer has an endless CFM gauge and will continue to test something past the orifice plate/sensor range which ends up in possible sensor damage. I do not like to have to repair DM's damaged and I feel bad having to possibly charge someone due to me not explaining it fully but, I also can't keep eating this cost by sending out new DM's and getting the damaged ones back and trying to figureout what to do with them?
I'm working on a video showing the use of my plates and will post it up when it's finished which should make this a little clearer.
I seem to be having alot of discussion with people who are not grasping how orifice plates work inside and on-top of their flowbench.
I sell a set of plates that are dual marked in 16" and 28" and they are marked for a range of flows, I do this so people do not have to buy more plates than they need. Instead of buying 4 internal and 4 external plates they can use this set of 4 plates for inside or outside use.
The 28" number is for external use for calibration.
The 16" number is for internal use and this is the max CFM the plate will flow at 16" DP. You can not flow more CFM at 16" DP than the plate is rated for! (reference the above picture and text in Flowbench 101)
It does not matter if you test at 10", 28" or 100" the plates are rated for 16" DP for the CFM range, so if the plate says 340@16" it's only going to be able to measure 340@16" no matter what depression you test at. Once you hit 340CFM you have reached the end of the scale and the "water" is blown out of the gauge or in the case of the digital manometer it's maxed out the sensor.
You can not try and measure more CFM than the plate is marked for! And should stay 25cfm below the plates marked range for safety.
The 28" reference is for calibration at 28" of static pressure it has nothing to do with the 16" CFM number and you do not need to use the 16" number for testing on top of your flowbench for calibration. If you only buy plates for calibration they are only marked in 28" to avoid confusion for those who are not using them on a PTS Flowbench build.
Maybe in hind-sight I should have just had internal and external plates and not had a "set" which had dual flow as this would have kept the confusion down and I would sell more plates. The objective was to keep costs down but I seem to be having alot of issues with people not understanding the "orifice flowbench concept" from this post.
My plans/parts are sold with the assumption to some extent that you have done some research on this forum on the basic concepts of Flowbench 101 and have a good understanding of the basic concepts. A little "sweat-equitity" is done by the end-user to keep the costs down on my parts as they have less instructions such as plug a into b and install in c . . .
I have no problem with helping people along and the forum members have no problem offering help. What does worry me is the amount of people who think that the digital manometer has an endless CFM gauge and will continue to test something past the orifice plate/sensor range which ends up in possible sensor damage. I do not like to have to repair DM's damaged and I feel bad having to possibly charge someone due to me not explaining it fully but, I also can't keep eating this cost by sending out new DM's and getting the damaged ones back and trying to figureout what to do with them?
I'm working on a video showing the use of my plates and will post it up when it's finished which should make this a little clearer.
Bruce
Who . . . me? I stayed at a Holiday in Express . . .
Who . . . me? I stayed at a Holiday in Express . . .
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buddy b
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Re: Flowbench 101
Bruce , just to make sure I am using my bench correctly , sence I am a new guy .If I have the plate that is rated for 450 cfm @ 28" inside and I'm testing a head .I pull the depression to 28" and the cfm's is 260 or so . I'm ok , correct?I thought that is what you told me . I have been useing my bench alot with no problems but after reading this I'm second guessing my self .The reason is you talk about the 16" rating . I can test at a 28" depression , right?
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1960FL
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Re: Flowbench 101
Buddy you do not understand the functioning of the bench, 28" is your test pressure and has nothing to do with the Flow of the bench. The plates that are marked @ 16” are the ones that dictate the MAXIMUM flow range of the bench when used inside the bench.
So in your example your 450@28 plate is only good to 340CFM when used inside of the bench! As the DM flow sensor CANNOT read more than 16” across the plate.
You can test parts at whatever depression you want but the CFM markings @ 16” on the plate dictate the Absolute MAXIMUM CFM FLOW through the bench when used inside.
Rick.
PS Please play with this spreadsheet and learn to understand the affects of pressure on flow through the orifice.
So in your example your 450@28 plate is only good to 340CFM when used inside of the bench! As the DM flow sensor CANNOT read more than 16” across the plate.
You can test parts at whatever depression you want but the CFM markings @ 16” on the plate dictate the Absolute MAXIMUM CFM FLOW through the bench when used inside.
Rick.
PS Please play with this spreadsheet and learn to understand the affects of pressure on flow through the orifice.
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