high altitude

[Jens Knickmeyer]

<Mattias Nyberg> wrote:
> 
> 
> I've heard that the engine control system compensates the
> the amount of fuel at high altitudes because the air density
> is lower. If this function is not
> implemented, the result will be a rich mixture. I don't
> understand why this compensation is needed.
> 
> In a speed density system, the air mass flow is
> calculated:
> 
>    ma = da * rpm/60 * Vd * nvol 
> 
> where
>    ma = air mass flow rate
>    da = air density
>    Vd = displaced volume
>    nvol = volumetric efficiency
> 
> The air density (da) can be calculated from the gas law:
> 
>    da = p/(RT)
> 
> where pressure (p) and temperature (T) of the inlet air is
> measured so the density should be known.
> 
> So the question is, why do we need to compensate for high
> altitudes and how is it done?
> (The same question arises in a system which measures the
> air mass flow.)
> 
> Mattias Nyberg
> 
> matny at isy.liu.se
> 
As far as I understand, air _mass_ flow compensates high altitudes
(and therefore different air mass) in the way you describe. A
correction has to be made for the air flow sensors (like on the
Bosch K-jetronic). These sensors measure only the flow, whereas
the newer sensors (with hot wire or hot film) measure air mass.

Feel free to correct,

Jens.
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           Jens Knickmeyer
Technische Universitaet Braunschweig
         Mikroporzessorlabor
 38106 Braunschweig near Wolfsburg
       knick at ibr.cs.tu-bs.de
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