high altitude

[Mattias Nyberg]

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