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Rusty,
I went looking at my notes on this and didn't find my own calcs (I wonder if I really did my own calcs now???) but even better I found Ed's note to guess who? Here is that note:
Sent: Sunday, December 21, 2003 11:21 AM
Subject: [FlyRotary] Re: Dumb intake question
then I'll give it a shot.
So based on that - it looks like your length from port to butterfly inlet of TB ranges from 13.1" for 6500 RPM to 11.5 RPM for 7500 RPM. This provides time for the pulse to be generated by the closing of the port and to arrive after that port opens again after bouncing back from the throttle body once. So from that Length you would need to subtract your block to port distance of 2.5" and the length of your throttle body.
Excellent! This is just the sort of info I was hoping for, but there's one thing I still don't understand. When you say the pulse bounces back from the TB, what is it bouncing off of? I believe the throat of the TB is straight, with no venturi effect at all, and we'll assume that the intake runner matches the TB size perfectly. That just leaves the open butterfly, which has a pretty small profile. Is it really getting a strong bounce from the open butterfly? What if you had a slide type opening, like the Ellison TB, where there is nothing left in the path of the air? Would there just be no bounce?
Thanks a bunch,
Rusty (special needs student)
See, you should have come to class that day{:>), Rusty
What happens is when a Finite Amplitude Wave (FAW) hits a change in cross-sectional area (like an opening from your tube to the atmoshpere or a plenium - or a decrease to a closed tube) energy is reflected back down the tube. The amount depends on the extent of cross section change, but opening to the atmosphere reflects most of the energy. If the cross sectional area it sees is Larger then a FAW wave of the opposite type is reflected. If the cross sectional area is smaller, a FAW wave of the same type is reflected.
So in your case, a compression wave is generated when the intake port closes, the wave travels up the tube until it sees the atmosphere (at the throat of your throttle body), there a large amount of the energy (Most of the wave's energy) is reflected back as a expansion wave (meaning its negative pressure with regard to atmosphere). However, when it gets to the port the port has closed, so a wave of the same type (Expansion or negative pressure) is reflected back to the TB throat, again where it sees a larger cross section. A larger cross section reflects the opposite type wave, so this time the wave reflected back to the port is a compression (positive pressure) wave and this when time it arrives, it finds the intake port now open and helps to shovel more air into the combustion chamber. Now this is not as desirable as having it arrive just as the port is closing to overcome reversion - but of some aid.
Ideally, you would want to take the pulse created by an opening port (in the rotary because that pulse is stronger) and then have it bounce back just before that port closes. But, that is a duration of approx 288 degs which would take at 7500 rpm about .0064 Seconds which would require a length of approx 46 inches for a two bounce system. This (I think) is one of the reasons that Mazda interconnected the two intake ports - it reduces the intake length required because the two points of interest on the different rotors are approx 93 deg apart rather than 288 deg they are on the same rotor - so the time to travel and therefore the intake is much shorter.
So you say - lets do a four bounce, well the problem there is when the pulse comes back to the intake port - it finds it open (remember in this case, it is from Port opening to port closing so the port is open each time the pulse arrives - the opposite of the short runner case). Since the port is opened into the chamber in this chase, the FAW sees a larger cross sectional area and reverses its type which sends an expansion wave back to the TB. This reflects back a compression wave which helps shove a bit more air into the chamber but is reflected back as a expansion wave. This would continue several times (2,4) and each time some of the pulse's energy would be lost into the atmosphere and more loss into the chamber. So by the time the port is in its closing phase and pushing some of its intake charge back out (Reversion) the pulse is probably too weak to provide much assistance in overcoming the reversion.
FWIW
Ed Anderson
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Rusty,
Is the engine 'seeing' a LONG intake tube at WOT? In other words, when the TB is WOT the engine is seeing an intake tube length of 20" - 24" (from looking at the picture) due to the CEET tubing instead of 12" - 14" (from the rotor to the TB). If the intake is functioning as LONG then the engine will not rev. Rotaries are sensitive to intake length. Try a flight with only the TB and air horns.
Kenb Powell
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Being as how I'm firmly NA, I've received one message
"loud and clear": take extra care to make my intake path as
smooth and unrestricted as I possibly can.
Dale R.
Great. I'm glad to see that some of my babble will reduce the pain and suffering of others :-)
BTW, I've decided to make an airbox that will bolt to the TB, and incorporate the airhorns that are currently in place. It will have a 3" inlet duct going back to the right radiator duct if possible. If I can't fit the 3" duct under the radiator, then I'll have to make a new scoop. Just won't know until I get the parts in hand. I'm (stubbornly) going to continue trying to use CEET (lower temp, cheaper SCEET) tubing, even though it has some turbulent flow when bent. If it works well, I'll eventually fit a smooth tube of some type.
Cheers,
Rusty (running out of rev numbers)
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