Date:         Mon, 1 Nov 1999 11:03:58 -0800
Reply-To:     daveb@CP.NET
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Dave Bayer <daveb@CP.NET>
Subject:      Digifant, barometers, and the AFM
Content-Type: text/plain; charset=us-ascii

Roger Bowman wrote:
>
> I saw the same thing when I played digifant recently.  It looks like a
> little "blow-out" valve or reed valve.
[snip]
> Which opens the question up to Barometric Sensing.  Lots of FI systems have
> a baro sensor, or something to tell the FI what the air density is (hot wire
> systems just look at the wire...)  What about digifant II?

        As someone recently said, the air density is taken into account
by the AFM. It is an air MASS detector - the momenteum (mass x velocity)
of the air is what pushes the vane back, less dense air will push less
against the vane as said air rushes past to fill the low pressure spot
in the intake manifold... The Probst Bosch FI book details it nicely
and is a very good read for anyone who wants to know how their FI
system works.

        There is no need for a barometer because the FI system is not
interested in air density, it's interested in the mass so the ecu can
compute the correct amount of fuel for the proper stociometeric ratio.
A barometer isn't going to help the ecu discern altitude (and thus the
amount of oxygen in the incoming air stream) as barometric pressure
varies as much due to the weather as due to altitude if I remember
correctly. Some sort of O2 sensor pre combustion might help with
this, but it would have to be another type of O2 sensor as the ones
in cars work by having a different amount of oxygen on the two sides...

dave