Air Flow 101 - applicable to engines
03-21-2002, 08:50 PM
#1
Air Flow 101 - applicable to engines
Following the rect vs. oval port thread got me thinking that some members might like a little background on airflow in pipes, ducts and of course ports. There's a lot more going on inside an engine besides simple steady state air flow (pulsing, resonance, swirl, mixture quality etc.) but we have to start somewhere.
Air flows between two points when there is a pressure difference between the two points. This pressure difference is called Static Pressure (SP). You must expend energy to create this SP, and nothing happens without it. This pressure difference between the two points results in a force being applied to the air and the acceleration of the air mass.
Imagine the piston drawing down (energy expended) and creating a vacuum (negative static pressure) under the closed throttle plates. Now open the throttle plates to allow the difference in pressure between the atmosphere and the cylinder to act on the air in the port. Air in the port is set in motion.
The air mass or volume flowrate in CFM (Q) and the velocity of the flow in fpm (V) are related according to the equation Q = V x A, where A is the cross-sectional area of the pipe in square feet. Therefore, if Q is constant, velocity goes up as area goes down.
Now that the air is travelling at a specific velocity it exerts a Velocity Pressure (VP) in the direction of air flow. This means that some of the Static Pressure (SP) has been converted into Velocity Pressure (VP). The amount of velocity pressure is equal to the energy required to accelerate the air.
So everything is moving along nicely in our straight pipe, and if we reduce the diameter of the pipe, static pressure goes down, velocity pressure goes up, and if we increase diameter back to its original value, both static pressure and velocity pressure go back to their original value, right? WRONG! Velocity and velocity pressure go back to their original value, but we lose some of our static pressure. Why?
Because air flow in pipes encounters resistance to flow due to friction and turbulence. Friction losses are caused by the roughness of the surface and turbulence losses occur whenever the air flow changes direction or velocity. This means that some of our original pressure difference gets used up fighting the resistance to flow created by these losses. In fact these losses are expressed in the same units as static pressure (psi or "H2O).
This is important to us because a pump, fan, or other air-moving device (engine) operating at a given RPM usually provides us with a fixed, limited amount of pressure difference (or force) to apply to the air we want to move. The more static pressure we waste on these losses, the less we have to convert into air motion. Less air = less power.
Now here's where it gets interesting. It turns out that these losses are proportional to velocity pressure, as shown in the following equation:
Pressure loss = K x VP
where K is the loss coefficient for the element causing the loss.
Velocity pressure is in turn proportional to velocity squared. So if we reduce the size of the port, velocity goes up, velocity pressure goes up by the change in velocity squared and therefore resistance to air flow goes up by the change in velocity squared. Sounds like a good reason for large ports, doesn't it?
Velocity is bad for air flow, because you use up too much of your static pressure fighting the increased resistance. In steady state flow, increasing port area reduces velocity, and according to the above equation, reduces the pressure loss, leaving more of the original pressure difference for setting air in motion. More air = more power.
So why is everyone always talking about velocity in the port as a good thing? Well it's mainly because air flow in engines is not steady state, and we are continually stopping and starting the flow. Small ports keep the column of air smaller so there is less mass to accelerate, and once accelerated to a higher velocity, the air is more likely to continue at that speed. This is a good thing for filling the cylinder in an engine. Which brings us to the subject of inertia.
When the column of air in the pipe is at rest, it wants to stay at rest. Once the column of air is moving, it wants to stay moving. Starting the air mass moving from zero velocity cost us some energy. If we then slam the valve shut, this energy is wasted, isn't it? Well, not exactly.
By carefully timing the closing of the intake valve we can allow the column of air to continue to fill the cylinder even after the piston is past BDC, and no longer providing the pressure difference that started the column moving. This is sometimes called inertial supercharging (thanks JimV) and can result in greater than 100% cylinder filling or volumetric efficiency. This is why the cam timing event that has the most effect on the location of the torque peak is intake valve closing. Since this post is definitely too long already, that's enough about cam timing.
So back to pressure losses and how to reduce them. If you are injecting fuel directly into the cylinder and don't care about velocity as a way of keeping fuel droplets from the carb in suspension, then bigger IS better. If you are supercharging and don't care about "tuning" the port for resonance and improved cylinder charging at a given RPM then bigger IS better. But if you do care about these two factors then you have to reduce pressure loss another way, without increasing area and reducing velocity.
Looking at the pressure loss equation the answer is obvious; you must reduce the loss coefficient of the elements causing the loss. If we ignore friction losses, there are three elements that cause losses:
1) Changes in direction (elbows)
2) Changes in velocity (expansions and contractions)
3) Obstructions to flow (valves)
This is what porting is all about. Reducing the loss coefficient to reduce pressure loss, without resorting to larger area and slower velocity. Increasing the radius of an elbow can reduce the loss coefficient by up to 50%. This is what raised ports accomplish. Smoothing the transition when area is reduced (from carb plenum to intake runner) and when area is increased (from valve seat to combustion chamber) can also reduce the loss coefficients in those areas. Raising a valve farther off the seat reduces the loss coefficient for this obstruction.
Of course, this is all pretty simple steady state stuff. Unfortunately, there's a valve opening and closing in this flow path, and other cylinders pulling off the same manifold, and cam timing issues and so on and so on. But reducing loss coefficients is how you make a small port give you more air flow without giving up the velocity that improves inertial supercharging and combustion efficiency.
Air flows between two points when there is a pressure difference between the two points. This pressure difference is called Static Pressure (SP). You must expend energy to create this SP, and nothing happens without it. This pressure difference between the two points results in a force being applied to the air and the acceleration of the air mass.
Imagine the piston drawing down (energy expended) and creating a vacuum (negative static pressure) under the closed throttle plates. Now open the throttle plates to allow the difference in pressure between the atmosphere and the cylinder to act on the air in the port. Air in the port is set in motion.
The air mass or volume flowrate in CFM (Q) and the velocity of the flow in fpm (V) are related according to the equation Q = V x A, where A is the cross-sectional area of the pipe in square feet. Therefore, if Q is constant, velocity goes up as area goes down.
Now that the air is travelling at a specific velocity it exerts a Velocity Pressure (VP) in the direction of air flow. This means that some of the Static Pressure (SP) has been converted into Velocity Pressure (VP). The amount of velocity pressure is equal to the energy required to accelerate the air.
So everything is moving along nicely in our straight pipe, and if we reduce the diameter of the pipe, static pressure goes down, velocity pressure goes up, and if we increase diameter back to its original value, both static pressure and velocity pressure go back to their original value, right? WRONG! Velocity and velocity pressure go back to their original value, but we lose some of our static pressure. Why?
Because air flow in pipes encounters resistance to flow due to friction and turbulence. Friction losses are caused by the roughness of the surface and turbulence losses occur whenever the air flow changes direction or velocity. This means that some of our original pressure difference gets used up fighting the resistance to flow created by these losses. In fact these losses are expressed in the same units as static pressure (psi or "H2O).
This is important to us because a pump, fan, or other air-moving device (engine) operating at a given RPM usually provides us with a fixed, limited amount of pressure difference (or force) to apply to the air we want to move. The more static pressure we waste on these losses, the less we have to convert into air motion. Less air = less power.
Now here's where it gets interesting. It turns out that these losses are proportional to velocity pressure, as shown in the following equation:
Pressure loss = K x VP
where K is the loss coefficient for the element causing the loss.
Velocity pressure is in turn proportional to velocity squared. So if we reduce the size of the port, velocity goes up, velocity pressure goes up by the change in velocity squared and therefore resistance to air flow goes up by the change in velocity squared. Sounds like a good reason for large ports, doesn't it?
Velocity is bad for air flow, because you use up too much of your static pressure fighting the increased resistance. In steady state flow, increasing port area reduces velocity, and according to the above equation, reduces the pressure loss, leaving more of the original pressure difference for setting air in motion. More air = more power.
So why is everyone always talking about velocity in the port as a good thing? Well it's mainly because air flow in engines is not steady state, and we are continually stopping and starting the flow. Small ports keep the column of air smaller so there is less mass to accelerate, and once accelerated to a higher velocity, the air is more likely to continue at that speed. This is a good thing for filling the cylinder in an engine. Which brings us to the subject of inertia.
When the column of air in the pipe is at rest, it wants to stay at rest. Once the column of air is moving, it wants to stay moving. Starting the air mass moving from zero velocity cost us some energy. If we then slam the valve shut, this energy is wasted, isn't it? Well, not exactly.
By carefully timing the closing of the intake valve we can allow the column of air to continue to fill the cylinder even after the piston is past BDC, and no longer providing the pressure difference that started the column moving. This is sometimes called inertial supercharging (thanks JimV) and can result in greater than 100% cylinder filling or volumetric efficiency. This is why the cam timing event that has the most effect on the location of the torque peak is intake valve closing. Since this post is definitely too long already, that's enough about cam timing.
So back to pressure losses and how to reduce them. If you are injecting fuel directly into the cylinder and don't care about velocity as a way of keeping fuel droplets from the carb in suspension, then bigger IS better. If you are supercharging and don't care about "tuning" the port for resonance and improved cylinder charging at a given RPM then bigger IS better. But if you do care about these two factors then you have to reduce pressure loss another way, without increasing area and reducing velocity.
Looking at the pressure loss equation the answer is obvious; you must reduce the loss coefficient of the elements causing the loss. If we ignore friction losses, there are three elements that cause losses:
1) Changes in direction (elbows)
2) Changes in velocity (expansions and contractions)
3) Obstructions to flow (valves)
This is what porting is all about. Reducing the loss coefficient to reduce pressure loss, without resorting to larger area and slower velocity. Increasing the radius of an elbow can reduce the loss coefficient by up to 50%. This is what raised ports accomplish. Smoothing the transition when area is reduced (from carb plenum to intake runner) and when area is increased (from valve seat to combustion chamber) can also reduce the loss coefficients in those areas. Raising a valve farther off the seat reduces the loss coefficient for this obstruction.
Of course, this is all pretty simple steady state stuff. Unfortunately, there's a valve opening and closing in this flow path, and other cylinders pulling off the same manifold, and cam timing issues and so on and so on. But reducing loss coefficients is how you make a small port give you more air flow without giving up the velocity that improves inertial supercharging and combustion efficiency.
Last edited by tomcat; 03-22-2002 at 03:22 PM.
03-22-2002, 12:42 AM
#5
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Damn, Tomcat, you're putting some technical stuff out. 
Those long cold winters must give you to much time to think.
Keep'em coming makes for good reading.
mike
Those long cold winters must give you to much time to think.
Keep'em coming makes for good reading.
mike
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03-22-2002, 01:57 PM
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I always thought that large intake ports would work really well for supercharging too (not to mention the low factory compression ratio)--I think thats one reason why the big blocks make more power so easily. I've also wondered how the engine speed relates to the inertial mass effects of the larger air column--seems it would have less of an effect the faster you turn (the air has less of a chance to react).
03-22-2002, 02:03 PM
#10
I have to apologize for the length of that post. I get carried away in my desire for "completeness" and making things "easy to understand". In fact I just added more to it. But I've read so many magazine articles about heads and cams that just don't take the time to explain things properly. Rather than take short cuts, and over-simplify things, and jump to conclusions, I'd rather start at square one.
It's not that everybody needs to know exactly how and why things work. We can just learn from experience what works, and pass on our hard earned knowledge in perfectly acceptable non-technical language like "the air in large ports is lazy". After a while, we get a gut feeling for what works and what doesn't. But when you read the Rect vs. Oval port thread and see how hard it is to kill the idea that bigger is automatically better, you see that the basic principles are not well known. So why not educate those that don't have the opportunity to learn from experience?
The enemy is pressure loss. The easiest way to kill him is bigger ports, valves and cams. But you sacrifice velocity and all the good things that go with it. So reducing loss coefficients is the way to go. This applies to more than just head porting too.
Finally, the basics are just that, nothing more. They can't predict all the possible combinations and outcomes. Experience (meaning actual data) is always the final reality. Anyway, I accept the good-natured ribbing and appreciate the opportunity this forum gives me to play high school physics teacher.
Hey Corey - You're right on large ports for Roots or screw compressor supercharging. First of all, you are expending a little more crank power to increase the pressure difference with the supercharger, which overcomes the pressure losses in the "pipe". You can get away with the larger ports to reduce the pressure loss, because the mixing of fuel and air by the compressor rotors compensates for the lower velocity (and less mixing) in the ports. Blowthrough superchargers like Procharger or Vortech do not provide this "egg beater" effect, so I think you might not be able to use as big a port.
At higher RPM there is less time for acceleration of the air mass and inertial cylinder filling. That's why cam duration is increased for high RPM engines. The down side is that at low speed the intake valve stays open too long and the piston starts pushing air back out of the cylinder into the intake manifold. Result, rough idle and poor low RPM power. The obvious answer is variable valve timing.
It's not that everybody needs to know exactly how and why things work. We can just learn from experience what works, and pass on our hard earned knowledge in perfectly acceptable non-technical language like "the air in large ports is lazy". After a while, we get a gut feeling for what works and what doesn't. But when you read the Rect vs. Oval port thread and see how hard it is to kill the idea that bigger is automatically better, you see that the basic principles are not well known. So why not educate those that don't have the opportunity to learn from experience?
The enemy is pressure loss. The easiest way to kill him is bigger ports, valves and cams. But you sacrifice velocity and all the good things that go with it. So reducing loss coefficients is the way to go. This applies to more than just head porting too.
Finally, the basics are just that, nothing more. They can't predict all the possible combinations and outcomes. Experience (meaning actual data) is always the final reality. Anyway, I accept the good-natured ribbing and appreciate the opportunity this forum gives me to play high school physics teacher.
Hey Corey - You're right on large ports for Roots or screw compressor supercharging. First of all, you are expending a little more crank power to increase the pressure difference with the supercharger, which overcomes the pressure losses in the "pipe". You can get away with the larger ports to reduce the pressure loss, because the mixing of fuel and air by the compressor rotors compensates for the lower velocity (and less mixing) in the ports. Blowthrough superchargers like Procharger or Vortech do not provide this "egg beater" effect, so I think you might not be able to use as big a port.
At higher RPM there is less time for acceleration of the air mass and inertial cylinder filling. That's why cam duration is increased for high RPM engines. The down side is that at low speed the intake valve stays open too long and the piston starts pushing air back out of the cylinder into the intake manifold. Result, rough idle and poor low RPM power. The obvious answer is variable valve timing.
Last edited by tomcat; 03-22-2002 at 02:24 PM.
