Chris Cosman wrote:
At what RPM would you be getting supersonic airflows?
Good question. Lets say the displacement is
80 cubic inches per revolution.
Volumetric efficiency E = 100% Mazda rotaries can do better
than that by a bunch.
Snips of my original erroneous approach to the problem.
Taylor says on page 203 Vol. 1 inlets average .5 Mach for high VE
engines.
Another way of calculating it is to use the max speed of the rotor face
moving away from the port adjusted by the port area to rotor face
area ratio. It is really hard for me to figure the max rotor face
velocity
right at the moment :) There are some charts for this in the book
"Rotary Engine" as I recall.
For a piston engine peak inlet air speed in fps is peak piston speed
in fps times piston diameter squared divided by port diameter squared.
Gas speed = PS X (D^2/d^2)
According to the book "Scientific Design of Intake & Exhaust Systems".
Page 78.
They don't say whether D and d is in feet or inches.
I suspect inches. I guess the ratio would be the same
so it may not matter.
You can figure the peak piston speed by knowing the stroke and RPM.
Peak piston speed inches per minute = stroke X pi X RPM
Note also this is not the speed of the pressure wave which does indeed
move somewhere around Mach one depending on a couple of factors.
Paul Lamar
I think your approach is correct.
So... to properly tune intakes you'd need to account for the speed of
the pressure wave
minus the speed of the medium it's travelling through, is that correct?
Chris C.
microcosman.com/aircraft
I disagree. I think my original approach was and still is incorrect.
I think the approach they take in the book Scientific Design of Intake &
Exhaust Systems IS correct.
Yes on the one hand pressure is increased in one direction and decreased in the
other. It becomes very complicated. I would have to summarize the book Scientific
Design of Intake & Exhaust Systems or scan in the whole book.
I suggest everybody just get a copy that is interested in this subject.
Paul Lamar
Chris brings up an interesting point and I wrote a little BASIC program
to explore the situation. The assumptions here are the intake port
just opened and the air flow velocity instantly became 400 FPS.
Not accurate by any stretch. A possible refinements would
be to assume that the air flow velocity was sinusoidal in nature.
Starts at zero rises to 400 FPS and ends at zero when the port closes.
Intake situation at 6000 RPM. 100 RPS. Total time for one rev is .01 sec.
Given Air velocity 400 FPS or 4800 IPS.
Given Wave velocity 1000 FPS or 12,000 IPS
------------------------------------------
Time Angle Air trav'ed Wave trav'ed Wave trav - Wave trav +
Sec E-shaft Inches Inches Inches Inches
0.0005 18 2 6 4 8
0.0010 36 5 12 7 17
0.0015 54 7 18 11 25
0.0020 72 10 24 14 34
0.0025 90 12 30 18 42
0.0030 108 14 36 22 50
0.0035 126 17 42 25 59
0.0040 144 19 48 29 67
0.0045 162 22 54 32 76
0.0050 180 24 60 36 84
0.0055 198 26 66 40 92
0.0060 216 29 72 43 101
0.0065 234 31 78 47 109
0.0070 252 34 84 50 118
0.0075 270 36 90 54 126
0.0080 288 38 96 58 134
0.0085 306 41 102 61 143
0.0090 324 43 108 65 151
0.0095 342 46 114 68 160
0.0100 360 48 120 72 168
Note that the wave traveled minus the air flow velocity
more or less 36 inches at the e-shaft angle of 180 degrees which
IMHO confirms the dynamic chamber theory more or less.
IOW the wave arrives at the other rotor intake port just as
it is opening.
This is a computer simulation which at this point in time is
not really an accurate view of what is going on in the manifold.
A lot of assumptions are made such as air flow velocity and
the speed of sound in the the intake manifold which can vary.
This is pretty much a work still in progress.
What we really need is some instrumentation with a high speed
pressure sensors and air flow velocity sensors inside a real
manifold on the dyno.
Paul Lamar
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