MAF vs MAP

[atsakiri at ford.com]

> |>Seems like the MAP sensor could be used to measure the transients in the 
> |>manifold.
[snip]

> |                           Decisions need to be made regarding
> | under what condition to use which sensor, how to switch modes, 
> | etc.

> I'm not talking about switching modes, I'm talking about using both sensors 
> as inputs to a control equation.  Throttle openings and closings can cause 
> transients in the manifold/plenum that the MAF sensor won't see, right?  
> There was a discussion a long time ago about equations to model these 
> transients in order to correct the MAF measurement.  It seems like using a 
> MAP sensor to measure transients would be easier than constructing a 
> mathematical model of the intake.  Isn't the throttle based enrichment 
> algorithm a simple way of approximating this transient condition?

I agree with you that a MAP sensor could possibly provide a 
benefit during transients.  I just haven't figured out how 
to do it yet.  :)  If you've got the control equation, by all
means DON'T send it to me!  Send it to the list or some Ford
lawyer.  [ :) again ]

If the MAF sensor is mounted upstream of the intake manifold,
throttle position changes will produce air flow changes 
at the MAF sensor before pressure changes in the manifold.
(Assumption:  throttle flow is not choked.)  The flow through
the throttlebody and the MAF sensor will be a function of
pressure ratio across the throttle and the flow area.  Even
if the pressure ratio doesn't change, the area surely will.

The manifold pressure will lag the inflow.  The flow rate at 
the  throttle and MAF sensor will change dramatically.  This
air flow however (the flow INTO the manifold) is not that 
which is inducted into the cylinders.  Some of that incoming
air serves to raise the manifold pressure.  After the transient
event, inflow and outflow converge again.


Anthony Tsakiris


MAF = mass air flow
MAP = manifold absolute pressure
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The opinions expressed are my own and not necessarily those of my employer.