air fuel ration petrol and lpg

Submitted: Monday, Feb 15, 2010 at 21:44
ThreadID: 76054 Views:5893 Replies:4 FollowUps:8
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Hi all i have a device that connects to my oxy sensor and tells me the air fuel ratio live .

The question is running on petrol when it shows 14.7 and i switch it over to lpg should it still show 14.7 or a different reading?

If it should be different please explain why?

Thanks
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Reply By: Battery Value Pty Ltd - Monday, Feb 15, 2010 at 22:20

Monday, Feb 15, 2010 at 22:20
Interesting question!
I've an old Paj, running 100% of the time on LPG, open loop.

Your O2 sensor causes the ECU to make the AFR stoich, which is around 14.7 for petrol and 15.8 for LPG.

So your gadget reading out the AFR is actually looking at the output voltage of the O2 sensor and interprets an average of 0.45V as stoich. The voltage will actually swing rapidly between low and high (0 to 1.1V max), and if the average voltage is >0.45 the ECU will interpret this as rich and vice versa.

Since it's a closed loop system, the ECU will always try and maintain a 0.45V reading from the O2 sensor, regardless which fuel type is involved. But maybe your ECU is tuned for a slightly rich burn for LPG (>0.45 from the O2 sensor) which would show up as a lower AFR on your gadget, when on LPG.

Best regards, Peter
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Follow Up By: zacc - Monday, Feb 15, 2010 at 22:27

Monday, Feb 15, 2010 at 22:27
It does fluctuate as you accelerate and decelarate and so forth i just used the 14.7 as an example .

I have set the gas up to read the same as petrol but i can adjust via laptop .
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Reply By: Battery Value Pty Ltd - Monday, Feb 15, 2010 at 22:42

Monday, Feb 15, 2010 at 22:42
There we go, AFR 14.7 for both should make the mix stoich in both situations (although the real AFR for LPG will still be 15.8, it's just reported as 14.7 because you can't tell your gadget to display AFR for LPG and petrol differently I guess?)

I don't know if there would be anything to be gained by tweaking the AFR up or down for LPG?
From memory, I think it's better to err at the rich side because LPG lean burn is quite hot with a very quick flame front?
Maybe someone with a bit more LPG tuning experience will reply to this...

Best regards, Peter
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Follow Up By: zacc - Monday, Feb 15, 2010 at 22:48

Monday, Feb 15, 2010 at 22:48
Thanks for your reply it answered my question.
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Reply By: That Troopy Bloke (SA) - Monday, Feb 15, 2010 at 23:19

Monday, Feb 15, 2010 at 23:19
NFI :-)

My head hurts.


Cheers
Glenn
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Follow Up By: Battery Value Pty Ltd - Tuesday, Feb 16, 2010 at 08:35

Tuesday, Feb 16, 2010 at 08:35
Hang on, you're not saying that you have absolutely NFI what happens under the bonnet while your' having fun behind the wheel? ;)

@ zacc,

no worries, glad to have answered it for you.

Best regards, Peter
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Follow Up By: That Troopy Bloke (SA) - Tuesday, Feb 16, 2010 at 19:59

Tuesday, Feb 16, 2010 at 19:59
I drive a troopy, I don't need to know what happens under the bonnet....it just works.

What I had NFI about was all the ABC stuff....let alone the numbers :-)

Cheers
Glenn
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Reply By: Robin Miller - Tuesday, Feb 16, 2010 at 19:24

Tuesday, Feb 16, 2010 at 19:24
Troppy

You were supposed to say that you can shift the reading sideways by applying a laplace transform in the time domain - effectively by adding a 100ms offset delay after the 02 sensor swings down - but just before it drops below 150 millivolts in practise.
Robin Miller

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Follow Up By: That Troopy Bloke (SA) - Tuesday, Feb 16, 2010 at 19:54

Tuesday, Feb 16, 2010 at 19:54
Robby

You took the words right out of my mouth :-)

Cheers
Glenn
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Follow Up By: Gazal Champion - Tuesday, Feb 16, 2010 at 20:36

Tuesday, Feb 16, 2010 at 20:36
Robin, can I borrow your sonic screwdriver please??.
Your obviously a fan of the Doctors.
Regards, Bruce.
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restless and lost on a track that I know. HL.

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Follow Up By: Robin Miller - Tuesday, Feb 16, 2010 at 21:11

Tuesday, Feb 16, 2010 at 21:11
I have a secret - what I said was a description of a device I invented which is currently attached to my Patrols ECU.
Robin Miller

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Follow Up By: Pete Jackman (SA) - Wednesday, Feb 17, 2010 at 13:35

Wednesday, Feb 17, 2010 at 13:35
Did you have to wire it through the flux capacitor?
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