Altitude Compensation

[tom cloud]

>>From:             Mark Pitts <saxon at zymurgy.org>
>
>> The point is even tho' the density drops, if you compress that air to sea
level density, there is less oxygen in it, thus you need to lean out at
altitude.

        [ SNIP ]
>
>No, I don't think so. If you took air from 10K and compressed it to 
>sea level, the ratios of the components would be identical or nearly 
>identical. The reason aircraft have to lean at altitude is (if not 
>alititude compensated) is lower air density and therefore lower O2. 
>
>Mechanical injection systems vary fuel delivery pretty much by 
>throttle position, so you have to manually reduce the fuel delivered 
>as the pressure (and mass per volume) of air goes down.
>
>(I may be on thin ice here.) Carbs draw fuel based on the velocity of 
>air flowing past the venturi, and not the actual mass, so they also 
>richen with altitude.
>
>---
>David Parrish

what do i know ??  As I understand:  the ratio of O2 and N2 are
the same at any altitude us puny homo saps (I know, speak for my-
self) can survive at (oops, poor english).

The term "lean out" in the apps above really means to supply less fuel.
This is necessary since the air is less dense and therefore less O2.
Seems to me you're trying to hold the A/F ratio essentially the same --
or at least know what it is to determine richer / leaner.  So, makes
sense to me that you always want to know how much O2 there is so you'll
know how much petrol to mix with it.  The velocity of the air flow
(eg. carb) doesn't tell you this.  So, when you go from sea level to the
mountains, you have to change the jets. Barometric pressure (BAP / MAP)
doesn't tell you either.  I would think the EGO would tell you (albeit a
few milliseconds after the fact) whether the mixture was near stoich or
not), though it doesn't give direct O2 availability data either.

Now, it's been a long time since my physics, and I have to go on
what little reasoning powers I have left, but it seems to me that the
quantity of O2 available can be adequately determined from the volume
of air flow (MAF) and density (BAP or MAP).  If one doesn't have MAF,
he could interpolate from TPS and the differential of BAP and MAP (i.e.
the absolute pressure on the throttle opening).  Then, assuming atmos
pressure doesn't change very quickly, one's system could 'remember' the
EGO's feedback reference point of stoich and extrapolate lean / rich
from there.

Now, if the ratio of O2 in the atmosphere really did change at different
altitudes (hard to imagine) we would really be screwed.

But, what do I know?? (It's been said before.)

Tom