As always, I offer a $500.00 reward to anyone who can show proof
that intake
air velocity is not important to intake system tuning.
Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com
Here is a very good illustration from Taylor. This is volumetric
efficiency as
a function of Mach number (Z) for a number of actual engines with a
number of differnt
intake manifolds and carbs. As you can see at lower RPMs the vol eff
is increasing
hence the power is increasing as the flow increases and as the intake
pipe length
is becoming closer to its tuned length what ever that is. As the Mach
number goes
above about 0.45 Mach things start going down hill. 0.45 is a magic
Mach number in
intakes and this number shows up in a lot of Taylor's work. If you
want lots
of power keep the Mach number less than 0.45 and that means larger
pipes
for more power at higher RPMs.
OK Tracy put the check in the mail NOW!!!
Paul Lamar
Paul, didn't you just make Tracey's point that velocity does matter?
Hans
Its a question of semantics. That is not what he meant, What he meant was
air
velocity can be too low for a given RPM. I maintain that for any RPM
the vol eff will get better as the pipe gets bigger and the air velocity
goes down. There is no question that up to a point (0.45 Mach) higher
velocity means more air in the engine as the RPM increases. That is
consistent of course with how a pump works. More RPM more air. An ICE
is just a glorified air pump of course. I think the myth got started
in the old days when you needed a lot of velocity to mix fuel with
the air in a venturi. That does not apply with fuel injection.
Several of us were sitting on Tracy's porch during the rotary round up
and Ed Anderson brought this up. I shouted: "Ed! Don't spread myths
like that!" Most people got up and walked away. Poor Ed had to sit
there awhile and listen to me rant and rave about it. :-)>
Robin took a picture.
Paul Lamar
I agree, I DID mean that air velocity can be too low (for any given rpm).
And I agree that I thought you were saying
You said, " I maintain that for any RPM, the vol eff will get better as the
pipe gets bigger and the air velocity goes down." Note that you did not put
a lower limit on this assertion, i.e., lower is better. What I am saying
(still) is that the velocity is a necessary component to high VEs.
There is no lower limit. Think how well a piston engine would breath
if there were no cylinder head or valves to get in the way.
At bottom dead center, due to the inertia of the column of air above the
piston,
the mass of air would be greater than the mass of air down there
when the piston was stationary for awhile at bottom dead center.
The VE would be greater than 100%.
Basically a peripheral port rotary comes close to that ideal.
Can't write the check yet because the data provided so far clearly shows
that velocity IS important and if it is too low, volumetric efficiency will
suffer.
I don't think it is a matter of semantics, I think it is a question of
relativity. How high is high? The chart you provided shows VE
improvements up to almost Mach .5 Note that this is on fairly dated engines
and that the later engines with the highest VEs benefited from the highest
velocities. The rotary is up in the 1.25 VE range so I'm sure the optimum
velocity would be well above any of those in the Taylor chart. Again, my
real point is that it is the energy in the moving column of air (velocity)
that makes high volumetric effeciencies (in normally aspirated engines)
possible.
Tracy Crook
tcrook@rotaryaviation.com
www.rotaryaviation.com
I thought you would try to wiggle out of it.
That's OK. I will come down there and live with you for a week and you will
have to feed me :-)>
Paul Lamar
Paul,
I think Tracy's point is that inertial effects due to intake pipe tuning
causes the greater
than 1.0 Volumetric efficiency, so it should indicate that more inertial
"supercharging" can
be expected with higher velocity. Of course, flow resistance losses will
reduce this
as the velocity goes up and when we hit M=1.0 it is all over, no more flow
increase.
I think that with NO inlet tract, 1.0 is the upper limit of volumetric
efficiency, but with
a tuned inlet port you can take advantage of resonance and get VE > 1. Higher
velocity = higher inertia (good) higher velocity = high pressure drop
(bad), vel = M=1.0,
a choked inlet system. The trick is sorting out the slopes of the two
opposing
effects (inertial tuning vs pressure drop) and see where the 'sweet spot' is.
Bill Freeman
Not Mach 1 according to Taylor it is Mach 0.45
Skin friction losses in the pipe well below Mach 0.45 are in addition to
the Mach number effect. Large pipe.. lower velocity.. skin friction is
proportional to velocity squared... lower drag.
I think a good analogy is a woofer speaker.
The larger the cone the more efficient the speaker. It couples
better to the air around it. It is related to impedance matching.
It is easier to start a large mass of air moving with a large
hole and large piston. When the piston or rotor attempts
to get as much mass of air as possible moving it is better to do it
with a large hole and a large pipe.
Another analogy is the large rotor diameter on a helicopter. Max
thrust at zero vertical velocity.
The Mazda Lemans engine is simple and absolute proof of this concept.
When somebody builds a rotary engine that generates more power
than the Mazda Lemans engine at the same RPM with a smaller pipe
and smaller port I will believe other wise. 240 HP for a two rotor
at 6000 RPM is a worlds record so far for NA rotary engines.
Nobody else comes close. The VE is well above 100% over a wide RPM
range. Peak torque or max Ve occurs at 6500 RPM with a very flat
but slightly decreasing VE and torque above that up to 9000 RPM.
Almost a straight line between 620 Nm at 6500 to 580 Nm at 9000 RPM.
The slight decrease in VE and torque I believe is caused by the increased
pipe skin friction due to increased air velocity (~V^2) out weighing the
decreasing length of the tuned pipe.
If it were not for keeping the apex seals in the rotor slots I believe
the rotary would generate even more HP per rotor at the same RPM
as the Lemans engine with even larger pipes and wider intake ports.
SAE Mazda paper 920309.
Paul Lamar
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