Interesting P&H Driver Link and a MAP Question

[steve ravet]

This was hard for me to understand at one point.  Barometric pressure
affects both intake and exhaust.  It's measured and accounted for by the
MAP sensor on the intake side, but not on the exhaust side.  Gar had a
good post for explaining it, attached below:

--------------
Ahhh, I think I see the conundrum...yes, baro affects both MAP on
intake AND total exhaust backpressure, but it's effect on MAP *is*
measured and taken into account, while it's effect on the ass end is
NOT (without baro).

So I think to answer your question, "If you put an engine in
a sealed room and raised/lowered the pressure would the VE change?",
the answer is of course YES, assuming in BOTH cases you tweaked the
throttle to HOLD the same MAP reading; then if the ambient BARO isn't
measured, the fueling will be off some. All the ECM knows is absolute
intake plenum pressure (assuming of course all other temp and rpm
variables are held the same), which you've made sure is the same in
your "sealed room" experiment, right?

The conundrum comes because you're actually imagining the MAP
changing, which it *would* do if you kept the throttle at the same
pos, and lowered the ambient pressure in your sealed room. Thus, you
kept the same *differential* pressure between intake and exhaust, and
in THAT case, no I don't see how the VE would change, except you're
now at a different MAP which would influence the VE of course. But
that's NOT the case we're dealing with here; we're dealing with the
case where the MAP *is* assumed to be the same, but the ambient is
different; and then asking the question, what's the effect of that
case.

Here's another way to look at the above. Forgetting that the ecm knows
anything about the throttle position for the moment, just imagine two
cases:
        [case 1]: you have 10psia in the intake plenum that the MAP is
reading, and the ambient is 15psia (you're at sea level),

        [case 2]; you have 10psia again in plenum, but now the ambient
is
12.5psia (you're now at say 5000ft elevation).

If all the other variables that the ECM sees are kept the same, will
the fueling be changed? Nope, it knows nothing about ambient pressure
changing; it just knows the MAP it's seeing in both cases is the same.
But, will the VE be different? Yep. Cuz the pumping losses in case 2
are less.

Of course, I could be having an Altzheimer Moment, and be completely
a-tilt, so please y'all correct where due.
--------------------------


Ken Bauman wrote:
> 
> OK, I once thought I had it understood, but know I guess I don't. I'm concerned
> that a system be able to easily handle altitude changes such as the example of
> going up the mountain.

-- 
Steve Ravet
steve.ravet at arm.com
ARM,Inc.
www.arm.com
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