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Date:         Mon, 1 Nov 1999 13:45:18 -0800
Reply-To:     Alistair Bell <albell@UVIC.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Alistair Bell <albell@UVIC.CA>
Subject:      Re: Digifant, barometers, and the AFM
Comments: To: daveb@CP.NET
In-Reply-To:  <381DE41E.F352FE2E@cp.net>
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. Dave said: snip > 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

From the Bosch publication on the KE-Jetronic system (CIS-motronic, caveat - no AFM as in Digifant, but does ahave a sensor plate which measures the amount of air drawn into the engine)

"Adaptation of the air-fuel mixture at high altitudes.

"The low density of the air at high altitudes necessitates a leaner air-fuel mixture.

At high altitudes, due to the lower air density, the volumetric flow measured by the air-flow sensor corresponds to a lower air-mass flow. Depending upon the facilities incorporated in the particlualr KE-Jetronic, this error can be compensated for by correcting the injection time. Over-enrichment is avoided and, therefore, excessive fuel consumption at high altitudes.

The altitude compensation is provided by a sensor which measures air pressure. In accordance with the prevailing air pressure, the sensor inputs a signal to the ECU which changes the pressure actuator current accordingly. This alters the lower chamber pressure, and therefore the pressure difference at the metering slits, and this results in a change in the injected fuel quantity. It is also possibel to incorporate continuous adaptation of the injected fuel quantity according to the changing air pressure"

Alistair


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