fuel pressure vs altitude

[Garfield Willis]

On Thu, 11 May 2000 09:57:37 -0400, "Flanagan, Stephen CECOM RDEC STCD"
<Flanagas at mail1.monmouth.army.mil> wrote:

>Some of you aviation enthusiasts (Gar for one) might be able to give
>some incite.
>...
>The question I have for the group is how much should the fuel pressure
>be dropped for each increment in altitude change, i.e. say car is
>running perfect at sea level (perfect A/F ratio) and for argument sake 
>the density altitude is also at sea level, when I change to a higher 
>altitude area with different weather conditions, what is the ball park % 
>of fuel that should be removed for each 1000ft change in density altitude.
>
>Is there any literature out there that I could get my hands on that will 
>educate me in this area?

If you're interested in learning about all this, there is plenty of
meterological info on lapse rates of pressure and temperature with
altitude in any beginning pilot book (and ignoring humidity effects,
thus you have your density altitude); there's even a very
complex/accurate atmospheric model that NASA and Av experimenter guys
use when figuring density altitudes to the last nat's eyebrow (I can
send you a copy if you like, it's written in basic..ugh..somewhat
readable),

BUT...and here's the kicker, there are no models that work well ON LAND
at altitude, because there, local conditions (especially air temps)
don't follow the atmospheric lapse-rates, etc.

So at takeoff, one computes an effective density altitude based on local
baro and temp conditions, which is obtained from local airport
measurements, NOT simply from known altitude. When making large altitude
changes *inflight* however, (and not needing to asymptote to the nat's
eyebrow), pilots use rulesOthumb like:
	-3.5degF / 1000'		temp. lapse-rate
and
	-1" Hg / 1000'		baro lapse-rate

Now, having said that, you seem to indicate all you want is how much the
air density changes, ASSUMING you DO know the density altitude, via your
own ECU's measurements, such as BARO, MAP, IAT, etc. Then it's just the
simple gas law formula, PV = nRT, with "number density" D = n/V (moles
per liter), so P = kDT. (It's been awhile since I carved up anything
with these tools, so I could be a tad rusty applying them, but you get
the general idea; I know someone will correct me if I err :).

If you want the mass density "d", you multiply the molar mass aka
"molecular weight" of air (28.97 g/mol) times n/V, which gives you mass
density in grams/liter (0.08073 lbs/cu.ft. @ STP = 1.29 g/l; this is for
dry air; gawd I hope some of this is correct :). So you have:

	P = k * d * T		(the k's are just arbitrary constants here)
or
	d = k * P / T

which just says that the density d varies directly with the Pressure and
inversly with the Temp. Just that simple; PV = nRT is called the "ideal
gas law", but it's also close enough in practice to not need any
corrections for our normal fuel injection type conditions.

BTW, I'm not real clear on why you'd want to adjust the *fuel pressure*
for this; doesn't your engine controller have a baro or map sensor? If
so, it's going to make those corrections automagically; so if you're
dialed in real good at MSL, I imagine you should also be fine atop
Pike's Peak.

HTH,
Gar


----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org