Source: Cycle World
Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)If you are using a flow bench to measure exhaust flow, you have the test cylinder head mounted on a dummy cylinder that is in turn attached to the bench. In the test, you blow air past the partly open exhaust valve and outward through the port. Normally you have built a device to hold the exhaust valve(s) at the desired lift (just as not all real-world flow occurs at full lift, so flow testing must test at various lifts).The strange thing about exhaust flow testing is that if you make a measurement with the bare port exhausting to atmosphere (forming what is called a “free jet”) then test a second time with a crude cone made of rolled-up paper stuck in the port, the flow is about 30 percent greater in the second case.If you sit and think about this a while, you can understand why this is. In the first case, the high-speed jet of air coming out of the bare port is pinched from all sides by inward-pressing atmospheric pressure, reducing the area of the jet and the amount of airflow. This is because the pressure of moving air is less than that of still air—an effect to which the name “Bernoulli’s Principle” has been given.In still air, the molecules of nitrogen (78 percent) and oxygen (21 percent) are rushing in all directions, colliding with each other. Air pressure measures the intensity and number of these collisions.If we now let that air escape into a region of lower pressure, that part of molecular energy that happens to be directed parallel to the flow is subtracted from the randomly directed energy of molecules in still air. Therefore the pressure in the jet is less than the pressure of still air. One way to see this is to blow through a reed valve. Intuitively, it seems that the pressure of blowing into the reed should push the petals wide open. But that’s not what happens: The flexible petals hardly open at all, and the reason is that atmospheric pressure is pushing them closed against the pressure of your breath trying to push them open.By sticking that paper cone into the exhaust port whose flow we are testing, we are protecting the free jet, whose pressure is lower, from the pinch—the inward-directed greater pressure of the surrounding air. And as the high-speed “exhaust” flow travels along the gradually enlarging cone, it gradually slows down and recovers pressure. The cone is therefore called a diffuser: a flow element that converts the kinetic energy of rapid movement back into directionless energy of pressure.It may seem crazy, but if you now cut that cone in half lengthwise and test again with just half of the cone in place, some beneficial effect on flow remains but is reduced from the original 30 percent boost to about 15. In this case, the half-a-cone protects the jet from half the pinch of surrounding air. This is “half-a-diffuser.”If you’ve spent time with airflow people, you’ve probably heard someone say, “The forgotten element of good intake airflow is what happens to the flowing air after it has passed through the valve.”The intake flow emerging into the cylinder from an open valve is a circular free jet, and as such it is subject to being pinched by the surrounding higher-pressure still air. But if that valve is located in a bowl-shaped combustion chamber, the curving surface of the chamber itself acts as half-a-diffuser, just as did the half-a-cone stuck into the bare exhaust port of a head being tested on a flow bench. When the flow emerging from under the valve attaches itself to the bowllike combustion chamber surface, that side of the flow is thereby protected from half of the pinch effect of the surrounding still air. Intake airflow can therefore be increased by as much as 15 percent by this protection.If the valve seat has a sharp edge, this can “trip” the flow, preventing it from attaching to the bowl, resulting in a loss of some flow.I noticed when I first tried flow testing on a four-valve head, that its intake flow coefficient—the flow in CFM per square inch of valve head area—was substantially less than that of single intake valves in bowl-shaped combustion chambers. The reason for this difference, I eventually understood, was that it is much harder or even impossible to get intake flow to attach to the flatter, less-bowllike shape of a four-valve pent roof chamber.Then how do modern four-valve engines out-flow the two-valve designs of the previous era? Notice that specific flow is cubic feet per minute per square inch of valve head. The four-valve achieves its high flow by having a lot of valve head area, not by having high port flow coefficients.The second thing a two-valve chamber can do well is burn its charge quickly. We know that combustion speed depends upon generating charge turbulence that quickly shreds and carries parts of the spark plug’s flame kernel throughout the chamber. That turbulence is achieved by storing the kinetic energy of the fast-moving intake flow in the form of axial swirl. Axial swirl is charge motion around the cylinder axis, and in a two-valve chamber this is easily created by aiming the flow not straight across a diameter, but more on a tangent. We have all had the experience of filling a bucket from a water hose, and have played with this effect to make the water in the bucket rotate.With the fresh charge rotating in this fashion, as the piston approaches top dead center (TDC) the ignition spark occurs, its arc duration creating a streak of flame. As the piston arrives at TDC, charge swirl breaks up into random turbulence that quickly converts the chemical energy of the fuel into heat and pressure.Because fast combustion shortens the time during which heat can be lost to the cooler metal surfaces containing it, a bit more combustion pressure is available to drive the piston.Not all two-valve designs achieve these ideals, but that’s another story.
Full Text:
Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)
If you are using a flow bench to measure exhaust flow, you have the test cylinder head mounted on a dummy cylinder that is in turn attached to the bench. In the test, you blow air past the partly open exhaust valve and outward through the port. Normally you have built a device to hold the exhaust valve(s) at the desired lift (just as not all real-world flow occurs at full lift, so flow testing must test at various lifts).
The strange thing about exhaust flow testing is that if you make a measurement with the bare port exhausting to atmosphere (forming what is called a “free jet”) then test a second time with a crude cone made of rolled-up paper stuck in the port, the flow is about 30 percent greater in the second case.
If you sit and think about this a while, you can understand why this is. In the first case, the high-speed jet of air coming out of the bare port is pinched from all sides by inward-pressing atmospheric pressure, reducing the area of the jet and the amount of airflow. This is because the pressure of moving air is less than that of still air—an effect to which the name “Bernoulli’s Principle” has been given.
In still air, the molecules of nitrogen (78 percent) and oxygen (21 percent) are rushing in all directions, colliding with each other. Air pressure measures the intensity and number of these collisions.
If we now let that air escape into a region of lower pressure, that part of molecular energy that happens to be directed parallel to the flow is subtracted from the randomly directed energy of molecules in still air. Therefore the pressure in the jet is less than the pressure of still air. One way to see this is to blow through a reed valve. Intuitively, it seems that the pressure of blowing into the reed should push the petals wide open. But that’s not what happens: The flexible petals hardly open at all, and the reason is that atmospheric pressure is pushing them closed against the pressure of your breath trying to push them open.
By sticking that paper cone into the exhaust port whose flow we are testing, we are protecting the free jet, whose pressure is lower, from the pinch—the inward-directed greater pressure of the surrounding air. And as the high-speed “exhaust” flow travels along the gradually enlarging cone, it gradually slows down and recovers pressure. The cone is therefore called a diffuser: a flow element that converts the kinetic energy of rapid movement back into directionless energy of pressure.
It may seem crazy, but if you now cut that cone in half lengthwise and test again with just half of the cone in place, some beneficial effect on flow remains but is reduced from the original 30 percent boost to about 15. In this case, the half-a-cone protects the jet from half the pinch of surrounding air. This is “half-a-diffuser.”
If you’ve spent time with airflow people, you’ve probably heard someone say, “The forgotten element of good intake airflow is what happens to the flowing air after it has passed through the valve.”
The intake flow emerging into the cylinder from an open valve is a circular free jet, and as such it is subject to being pinched by the surrounding higher-pressure still air. But if that valve is located in a bowl-shaped combustion chamber, the curving surface of the chamber itself acts as half-a-diffuser, just as did the half-a-cone stuck into the bare exhaust port of a head being tested on a flow bench. When the flow emerging from under the valve attaches itself to the bowllike combustion chamber surface, that side of the flow is thereby protected from half of the pinch effect of the surrounding still air. Intake airflow can therefore be increased by as much as 15 percent by this protection.
If the valve seat has a sharp edge, this can “trip” the flow, preventing it from attaching to the bowl, resulting in a loss of some flow.
I noticed when I first tried flow testing on a four-valve head, that its intake flow coefficient—the flow in CFM per square inch of valve head area—was substantially less than that of single intake valves in bowl-shaped combustion chambers. The reason for this difference, I eventually understood, was that it is much harder or even impossible to get intake flow to attach to the flatter, less-bowllike shape of a four-valve pent roof chamber.
Then how do modern four-valve engines out-flow the two-valve designs of the previous era? Notice that specific flow is cubic feet per minute per square inch of valve head. The four-valve achieves its high flow by having a lot of valve head area, not by having high port flow coefficients.
The second thing a two-valve chamber can do well is burn its charge quickly. We know that combustion speed depends upon generating charge turbulence that quickly shreds and carries parts of the spark plug’s flame kernel throughout the chamber. That turbulence is achieved by storing the kinetic energy of the fast-moving intake flow in the form of axial swirl. Axial swirl is charge motion around the cylinder axis, and in a two-valve chamber this is easily created by aiming the flow not straight across a diameter, but more on a tangent. We have all had the experience of filling a bucket from a water hose, and have played with this effect to make the water in the bucket rotate.
With the fresh charge rotating in this fashion, as the piston approaches top dead center (TDC) the ignition spark occurs, its arc duration creating a streak of flame. As the piston arrives at TDC, charge swirl breaks up into random turbulence that quickly converts the chemical energy of the fuel into heat and pressure.
Because fast combustion shortens the time during which heat can be lost to the cooler metal surfaces containing it, a bit more combustion pressure is available to drive the piston.
Not all two-valve designs achieve these ideals, but that’s another story.
