Speed density/altitude adjustments

[atsakiri at ford.com]

>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

One reason is that volumetric efficiency is affected by exhaust backpressure,
which is in turn largely determined by ambient pressure (for a fixed set of
exhaust hardware).  So, the "nvol" term in your equation, typically mapped at
sea level, should be adjusted for altitude.  Without the adjustment, even steady
state estimation of air flow will be in error.

For mass air flow systems, flow is actually measured (at least the flow into the
manifold), so the steady state air flow estimate needs no adjustment for
altitude.

Also, for a four stroke engine, there's a "2" missing in the equation.
I realize you may not have intended to get that detailed, but just in case
someone else tries to run some numbers, I think it should be more like:


 ma =  da * rpm/60 * Vd/2 * nvol

where units are

 kg    kg      rev   1 min     m^3     1 cycle
 -- = ----- *  --- * ------ * ------ * ------- * nvol
 s     m^3     min    60 s     cycle    2 rev

             


---                                                                          
Anthony Tsakiris                                                             
                                                                             
The opinions expressed are my own and not necessarily those of my employer.