Bernoulli and 350 ft/s, 146 cfm/in²
Here is the source of some of my confusion on this subject. In book after book and in video after video, the theoretical limits of 350 ft/s and 146 cfm/in² are used. Several months ago, in pitot tube probing the combustion chamber side of the intake valve (see picture below), I was getting local velocities of well over 350 ft/s. I thought, well, that's OK as long as the average velocity is below the limit of 350ft/s. Look at the sixteen pitot tube readings in pink. The average is 370 ft/s.
The answer is twofold. One reason is because of the venturi effect. Air is speeding up to get through the venturi created by the valve and valve seat. The other answer is because of the 28 in H2O pressure drop across the intact port. The 350 ft/s velocity limit is the limit at the density of air at the inlet of the port (14.696 psi). Because the air in the chamber is at a lower pressure (13.675 psi) than the air at the inlet to the port, the density in the chamber is lower. So, as air travels down the intake port, it's pressure is reducing and therefore its density is also reducing. Since the mass flow rate of air through the port cannot change, the velocity has to increase as the air gets closer to the chamber. So, even if the area of the intake port is constant, the velocity will be increasing as the air travels down the port.
One other confusing part of this pitot testing in the chamber was the average cfm/in² readings. These can be seen in blue. Again, the average number of 154 cfm/in² seems to violate the maximum theoretical value of 146. But again, the reason it's doing so is because of the venturi effect and the reduction in density associated with high air speeds. The 154 number would yield a discharge coefficient of 154/146=1.05 if the 146 is used as the theoretical maximum. It should not be possible to get discharge coefficients higher than 1, but it is common at low lifts. I believe the explanation has to do with scfm vs acfm. The 154 number is cfm at a lower density than the 146 number. If both numbers were converted to standard conditions (scfm), I believe that a discharge coefficient of greater than 1 could never be achieved.
Discharge coefficients are about as intimidating as shaping a short side radius. I think the reason is because of the complexities of scfm vs acfm and the difficulty of measuring the valve curtain area. More on that later.
Here is the source of some of my confusion on this subject. In book after book and in video after video, the theoretical limits of 350 ft/s and 146 cfm/in² are used. Several months ago, in pitot tube probing the combustion chamber side of the intake valve (see picture below), I was getting local velocities of well over 350 ft/s. I thought, well, that's OK as long as the average velocity is below the limit of 350ft/s. Look at the sixteen pitot tube readings in pink. The average is 370 ft/s.
The answer is twofold. One reason is because of the venturi effect. Air is speeding up to get through the venturi created by the valve and valve seat. The other answer is because of the 28 in H2O pressure drop across the intact port. The 350 ft/s velocity limit is the limit at the density of air at the inlet of the port (14.696 psi). Because the air in the chamber is at a lower pressure (13.675 psi) than the air at the inlet to the port, the density in the chamber is lower. So, as air travels down the intake port, it's pressure is reducing and therefore its density is also reducing. Since the mass flow rate of air through the port cannot change, the velocity has to increase as the air gets closer to the chamber. So, even if the area of the intake port is constant, the velocity will be increasing as the air travels down the port.
One other confusing part of this pitot testing in the chamber was the average cfm/in² readings. These can be seen in blue. Again, the average number of 154 cfm/in² seems to violate the maximum theoretical value of 146. But again, the reason it's doing so is because of the venturi effect and the reduction in density associated with high air speeds. The 154 number would yield a discharge coefficient of 154/146=1.05 if the 146 is used as the theoretical maximum. It should not be possible to get discharge coefficients higher than 1, but it is common at low lifts. I believe the explanation has to do with scfm vs acfm. The 154 number is cfm at a lower density than the 146 number. If both numbers were converted to standard conditions (scfm), I believe that a discharge coefficient of greater than 1 could never be achieved.
Discharge coefficients are about as intimidating as shaping a short side radius. I think the reason is because of the complexities of scfm vs acfm and the difficulty of measuring the valve curtain area. More on that later.