A key point is being overlooked?
The diagam in #15 is correct, of course, but notice:
Pressure at the inlet is just about ambient (include any motion-speed ram effects.) The velocity and pressure changes occur within a defined length of plumbing flow.Theoretically, the same volume of "air" should pass any point along the length in the same time. An obstruction that slows the velocity, at the neck, will also slow the rate of air entering at the inlet - considering just theoretical smooth flow through a venturii.
If the diameter at the inlet is twice the diameter at the 'neck,' inlet area is four times 'neck' area. Ideally, velocity through the neck must be four times inlet velocity to keep the same volume of flow at all points.
Bernoulli's statement is for smooth, steady flow conditions: pressure varies inversely with the square of velocity change between two reference locations in a defined condition.
P1/P2 = V2^2/V1^2
This is what I didn't see in the discussion so far - that the velocity change, from "ambient" to neck minimum area pressure, depends on the change in area between those locations in the intake flow path.. ...Presuming the same volume per unit of time. Say, for example, we have a .40 cu in engine turning 12,000 RPM... that's 200 rotations per second, right?
200 X .40 = 80 cu in per second - if all were perfect (it isn't) - that could be the flow rate to fill the cylinder completely each revolution. Of course, that never happens - too much else involved.
But, let's just say, ok? If inlet diameter is .5", its area is just under 0.2 sq in. Velocity to pass 80 cu in per second would be around 400 "/sec or 33.33... '/sec.
If the choke min diam is .25, its area is about .05 sq in (gee whiz! that's 1/4th of inlet area!) So, I guess the needed velocity would be something like 1600 "/sec, or 133.33... '/sec.
P1 is atmospheric, so long as we're simplifying wildly - say 14.7 psi at the entry with velocity of 33.33'/sec.
The equation becomes: P2/P1 = V1^2/V2^2 ---Solve for P2 --- P2 = V1^2 /(P1 *V2^2)
Works out to something like P2 = 1.1 psi. IF the venturii flow were perfect, smooth, and all presumptions of ideal conditions were present.
Also, consider that the shaft intake port is only open about half a revolution, so the flow in that brief time would need to average twice as fast.
Air has mass: inertia and momentum affect how well the charge gets through the shaft port.
MACH drag rise type of resistance to flow can get in play, too.
And these don't consider the rebound wave in the intake tract when the shaft valve slams shut.
Makes it all sound impossible? Nah, despite the aerodynamicist's opinion, THIS bumblebee does fly...
We make do with a lot of losses along the way, but we still have plenty left after them...
Hi Lou
I did not overlook these items you mentioned because there is little to nothing that can be done with them, however ,All the things you wrote above that affect the flow thru a venturi, also effect the flow thru a restrictor, or a tube , or any of the venturi variants like fuel post,spinklers ..etc.. If you make or buy a manometer vacuum Air suction pressure meter , and use it to test all of these things, including a tubed, you will find that the true venturi does it best.
There is more, as you say, to all of this, and the number of differant induction system you can come up with are staggering, I make true venturis, or the classic shape venturi when I can on all my engines, They do perform the best in all of my test.
Also Scott Bair did a very in depth look , and study into this, he found this also to be true.
In engines That I could not make true venturis for, because they could not accept them, because of whatever reason, I made true ventuis and added fuel post into them.
The OS Max engines are such motors, as they do not have the material to add a passage way next to the venturi, What we did was to make the venturi and add a OS Max 10 spraybar/fuel post combo to one side threaded into the venturi.
You could then use the venturi and could also have some control over size by rotating the threaded NVA in deeper or Shallower, thus making the effective hole size larger or smaller.
I used these on many of the OPS and OS engine I made for myself and sold to many others.
If I were to have to use a sprinkler tube as Andrew spoke about, it would have multi fuel post tube sticking out into the slipstream.
This was the only way I could get such a device to work as I would need it to. I discounted them early on because of the end of flight burping problem that I did not care for, the ditch that is machined all the way around the outside of the venturi for all the 360 degree sprinkler holes, would be a large area, it would fill with fuel and air during the last part of the flight and the engines would burp off and on for several laps sometimes before cutting off. I did use some of these after blocking off the holes around the venturi and just using either 1 or 2 of them that were opened slightly. And also quite frankly because they did not work as well as the classic venturis I was making with 1 fuel feedhole.
Randy