Purpose of barometer

MikeT · Post by **MikeT** » 23 May 2010, 19:23

Just a passing thought that didn't pass quick enough.... Can anyone tell me...

Why is the atmospheric pressure important to my ECU when it has no control of air intake or fuel?

Brigsygtt · Post by **Brigsygtt** » 23 May 2010, 19:42

What car is it on, derv or petrol & is it forced induction or n/a? I assume you mean maf or map?

The sensor will talk to the ecu to tell it airflow & i imagine it will adjust the fuelling/ign/timing etc according to load etc.

MikeT · Post by **MikeT** » 23 May 2010, 20:16

1.9TD.
It's not a map or maf, just an atmospheric barometric sensor in the ECU box.

The thing is, the ECU has no direct control on fuelling, other than fast idle, EGR and timing AFAIK. Perhaps it's timing related.

Post by **CitroJim** » 23 May 2010, 21:59

Yes, timing related Mike as that's all the ECU controls.

Fact is, at high altitudes where the air is thin the ignition delay (i.e the time between the fuel being injected and starting to burn) is longer because of the less oxygen available.

Therefore if the ECU knows barometric pressure it can advance the timing to compensate. It might do little for power but will help emissions.

Post by **Paul-R** » 24 May 2010, 11:13

But surely, as it's a TD then the turbo will be able to compensate for lack of oxygen.

I read years ago that turbos compenated for the loss of air pressure, and therefore power, as you climbed higher and higher. I'm thinking partucularly of the Audi Quattros climbing Pike's Peak in the 80s.

KP · Post by KP » 24 May 2010, 11:27

True to a certain extent but still the higher you go the less air there is for the turbo to pull in....

MikeT · Post by **MikeT** » 24 May 2010, 13:05

Hi Jim, that was all I could think of but I didn't realise timing is a smoke factor except at higher revs.

Bit of trivia; The mechanical equivalent is the Bosch pump ADA - which is simialr in appearance and function to the LDA (ufo) but works the other way - to reduce fuelling relevant to air pressure.

Taking it to an extreme, can advancing/retarding the timing go some way to curing off-boost smoke?

MikeT · Post by **MikeT** » 25 May 2010, 12:41

Thanks to Toby for showing me the way, I managed to find some Citroen literature on the subject. They call it an "Altimeter Corrector". It is also used to calculate EGR control which is interesting as this obviously affects AFR as well (presuming the EGR is working).

It would be interesting to experiment with different corrector inputs to see how much and when the timing changes in relation though being at sea level, I guess my ECU's already operating in the more advanced area of the map.

Interesting stuff and thanks to all that responded.