In a message dated 12/29/2004 8:39:42 P.M. Central Standard Time,
Walter@advancedpilot.com writes:
**MAP
(displayed in inches of Hg) is not actually “absolute” since it
is ambient
pressure (AP) plus gauge sensed pressure (GP). That is
indicated MAP = AP + GP. At wide open throttle (WOT), GP can be
positive from pressure created via a ram air induction system and
limited by KIAS. GP can be negative (vacuum) at reduced
throttle
(reducing air in the induction system). Ram air may
overcome the
slight closing of the throttle to where the MAP =
AP.**
I can see how the GP could be higher than ambient in a
non-running
induction system undergoing ram air effect. Are you
saying that in a
RUNNING engine's intake that the GP can be higher than
ambient? If
so, can you explain the physics of this to
me. I must be missing
something. Are you measuring that the
RAM air effect can be so high as
to overcome the air sucked in by the
engine?
Walter,
I am only talking about running engines in flight, not on a laboratory
stand. At WOT and 200 KIAS utilizing a ram air induction system, the GP
component of indicated MAP is positive, perhaps even by as much as 2" Hg.
Thus, the MAP = AP + GP. Lets say at an altitude where the AP is 20" and
the GP is +2", the MAP is 22". At the same altitude, suppose the throttle
is reduced, yielding a GP of -2" (vacuum). thus the MAP would be 18"
(momentarily, as the ram would be reduced when the air speed bled off), a 4" Hg
swing. If the throttle was closed only slightly, enough to reduce the
total stream pressure 1" (to 21"), some ram air would still keep the MAP higher
than AP alone.
**
This gives rise to the question: Should I connect the AP port to the
same
static air used by the altimeter?**
It sure looks that way,
doesn't it?
Yes, and I wonder if pressurized aircraft do just that.
**Side Note: If an electronic ignition is
used and one of the
controlling parameters is MAP, the ambient pressure is
obtained from
the position of the sensor, generally located in the “brain”
box that
is often placed under the cowling and is sensing the cooling air
output
pressure. This is generally slightly above “static” ambient
pressure.
I misplaced the measurement I once made of this but I
remember it was
maybe about .25” Hg greater. At cruise, this is a
true difference of
about .5” in what the MAP indicator is showing and what
the electronic
ignition is using for timing adjustments.**
In
answer to your side note, there are some pitfalls in using MAP as a
determining factor in setting timing. The only factor which trumps
all
others is thetaPP. The optimal thetaPP is about 16. The
major factor
after that is peak pressure being maintained below the
detonation
margin. The use of multiple factors in attempting to
calculate those
answers is, at best, iffy. It's much better to
measure thetaPP than to
try to calculate it.. It's the only way I've
seen that has the
capability to maximize output and be certain that the
detonation margin
is maintained.
Regardless of the ideal, my commentary was only to point out that what you
see ain't necessarily what you got. The fact is that extant electronic
ignitions have an option that allows combinations of RPM and MAP (or a facsimile
thereof) to alter timing - usually by advancing the timing. An option with
the Lightspeed system is a display that shows the MAP used for making such
adjustments - it may not match the MAP display on on engine monitoring
system.
**Side note: Consider that ram air can raise the induction
air
pressure higher than that utilized by the fuel injectors (obtained,
for
example, from the upper cooling plenum) to help atomize the
fuel. This
may suppress the best atomization and shrouded injectors
utilizing the
higher ram air, available before the throttle body, may
improve
atomization. Such a system is similar to the way super/turbo
charged
engines operate.**
Yes, it appears that the optimal upper
deck pressure on injectors may
be in the area of a 3 psi
differential.
That's interesting, 3 psi is 6.1" Hg. - not possible for me no matter
how fast I go. The best I can hope for is 0 psi at WOT or by
flying with a greatly reduced throttle setting. But, at least it is
not negative psi. Does this mean that turbo/super charged systems
maintain such a differential? If only I could use the 12-port
electronic injectors supplied with my motorcycle and, they don't need no
stink'nr to help with atomization.
Scott Krueger
AKA Grayhawk
Lancair N92EX IO320 Aurora, IL
(KARR)