was actually open. Sounds like a patentable device to me except
it is not as you read it on here first :)
Paul Lamar
Even after looking at the picture, I still don't understand the problem
with averaging.. Maybe you could describe the issue in a different way.
Thanks,
Matt-
No problem Matt.
Think of these curves as the side view of some mountains
and valleys. Averaging would be like drawing a straight line half
way up the intake mountains... valleys included. What you might get is
just atmospheric or static pressure (top yellow line) as the valleys
below the top yellow line are below atmospheric pressure and those above
are above atmospheric pressure. What you want is atmospheric pressure
PLUS the area of the side view of the mountain between to the two
green lines and above the top yellow line. Namely the additional air
going into the engine
when the port is open due to the dynamic ram effect. The mass of air
going into the engine is directly proportional to the side view area of
the mountain above the top yellow line PLUS the top yellow line.
Tracy refers to this as total pressure. This represents the 20% of
additional air you get with a highly tuned intake. This is also why the
volumetric efficiency of a p-port tuned intake rotary engine is roughly
120%. This is also probably why we are 20% lean on the top end.
BTW Chrysler referred to the effect years ago as RAM charging and the
roots of the name are still used by Daimler/Chrysler.
I hope this clears it up. Perhaps someone else can give it a try
as well. Never hurts to have someone else explain things in different
words.
Paul Lamar
snip
Hi Paul,
Okay... I think I finally get it.. Your point: The problem is that as
the volumetric efficiency of the engine relatively improves as RPM
reaches
the P-port tuning peak value, causing the average value of the manifold
pressure to fall [?? rise] - the engine is effectively taking deeper
breaths, so
increased average vacuum [not vacuum], even WOT.. The lowered
averaged manifold
pressure (lower MAP reading) causes the computer to erroneously lean the
mixture. Is that what you are saying?
I still think this effect can be tuned out effectively (and more
simply) -
the fuel delivery curve should end up reflecting the increase in
volumetric efficiency associated with changes in RPM.
And, I still think that the problem you encountered was more likely
caused
by the FI computer taking a quick snapshot of the MAP at the wrong time -
at a time when the MAP value was really low. I think you already said
this.. We just disagree on how to fix it.
Regards,
Matt-
VE N34RD, C150 N714BK
------------------------------------------------------------
Not exactly. Think of things in absolute pressure terms. We are walking
around in 15 psi pressure at sea level. That is 30 inches of mercury
using
a manifold pressure gage. Our body would be crushed if it were not for
the same pressure being in our lungs.
Now what happens in the intake manifold due to an organ pipe tuning
effect is the PEAK pressure pulse may go up to 200% of
15 psi or 30 psi (60 inches of Hg). 100% more than wide open throttle.
When the port is closed at the RPM tuning point the pressure also goes
below 15 psi (30 inches of Hg) for awhile. Perhaps all the way to zero
pressure. The average is still 15 psi (30 inches of Hg). Remember
the throttle is still wide open. The key word is "PEAK". What we are
interested in is the amount or mass of air that flows into the engine.
That is proportional to the area under the peak plus the
the air flowing into the engine due to atmospheric pressure alone.
If the shape of the pulse were a right triangle the area would
be 1/2 base times height or 7.5 psi so to speak. That would
be 50 percent more air or the equivalent to 22.5 psi (45 inches of Hg).
In the real world we don't really see that much air. What we see is
about 20% more or the equivalent to 18 psi (36 inches of of Hg).
Using the term vacuum confuses the issue. A pure vacuum is just the
absence of pressure and what one would get in outer space.
BTW pressure at sea level expressed in psi is merely the weight of
a long, one by one inch, column of air up to the start of space
expressed in pounds. In other words the column weighs 15 pounds.
I don't mean to be condescending here I just don't know the level of
Physics education of every one on here. I am trying to explain things
from the most basic level possible.
I DO NOT disagree with you on merely changing the map table entry to
correct for this effect. It would be better however if we did not have
to do that 128 times. One for each .12 psi increment in the intake
manifold pressure. It would be better if the computer would
set the mixture on the basis of the actual mass of air entering
the engine at any RPM. That is exactly what a mass air flow sensor
does in an automotive EFI system or a carburetor does for that matter.
I think Tracy could do it by taking five or ten equally spaced pressure
samples during the time the port is actually open and integrating them.
Then add or subtract that number from the atmospheric pressure in the
intake manifold. The computer is fast enough to do this no problem. It
can probably get a sample in a few millionths of a second since it runs
at 33 million instructions per second more or less. I am sure it is now
twiddly its thumbs most of the time.
To Tracy credit there is an auto tuning mode where the
computer looks at the O2 mixture monitor installed in the exhaust
pipe and adjust the MAP table entries accordingly. I have not tried
that yet. That might solve the problem. I can see a scenario when
descending where that might not work. If it were me I would
also look at the EGT to get a better feel for what the
engine was doing.
Paul Lamar
Hi Paul,
I'll have to cogitate a bit more on the physics of the system.. The
dynamic effects are interesting to consider..
As far as the auto tuning mode.. Tracy designed in a setup for tuning
high MP while high RPM, and another setup for tuning low MP while high
RPM (descending). As you suggested, the fuel demands are much different
between these states, and it appears Tracy has a method for taking care
of it. I was recently reading about this being an issue for John
Slade. Read about it at the very bottom of the following page:
http://www.canardaviation.com/cozy/chap29f.htm
Regards,
Matt-
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