PB AFR meter

[Guest Ash]

 

 

In one of Pete's replies he mentioned about how, when in closed-loop mode, the LED readout somehow moved (to quote his phrase) Knightrider style.

 

Okay, so I've never seen one of these units, but could someone please explain what is meant by this?

 

At first I thought it was some kind of gimmick built into the unit just for effect. But today I had this thought come into mind that maybe the leds are moving up and down because of the output signal from the stock oxy sensor. If that is the case, could someone tell me please the upper and lower limits.

 

Yours,

J

[Simon]

Ash,

Pete described what was happening in one of the posts in your recent discussion.

 

I quote

'The transfer function of the stock O2 sensors gives you a voltage vs AFR curve which is very very steep around 14.7. In fact very small changes in voltage can cause large changes in the output of the sensor. This is what you see when you observe the closed loop operation and watch the LED's (or voltage) pulsing up and down between rich and lean.'

 

As regards what the upper and lower ranges are, then Pete is probably the best person to respond.

 

Simon

[Guest Ash]

 

 

Thanks, Simon, I read that paragraph before and didn't understand it then and still cannot get my head around it.

 

If Pete is about could you please explain the sentence... "In fact very small changes in voltage can cause large changes in the output of the sensor."  

 

What I don't understand is what voltage and which sensor.

 

Yours,

J

[Simon]

Ash,

I quote

The transfer function of the stock O2 sensors gives you a voltage vs AFR curve which is very very steep around 14.7. In fact very small changes in voltage can cause large changes in the output of the sensor. This is what you see when you observe the closed loop operation and watch the LED's (or voltage) pulsing up and down between rich and lean.

Unquote

 

I guess he means the O2 sensor produces a voltage and that varies with the reading it is measuring, the curve produced by the maths of it all is probably hyperbolic (could be parabolic - can't remember my maths now)

 

[Guest Ash]

 

 

I think Pete is ignoring the post.

 

Basically, I wanted to investigate how his AFR meter could indicate a wildly varying A/F ratio under light cruise when my calibrated UEGO sensor setup indicated not more than 2/10ths of a ratio under the same conditions.

 

If you are still listening, Pete, perhaps you could shed some light on this.

 

Yours,

J

 

 

[Rich J]

All A/F meters attached to stock O2 sensors do this on cruising and idle is something to do with the ECU using the 02 sensor to adjust the fueling for economy. Happened on both cars I have had a A/F meter attached to the stock sensor (two different A/F meters also), the difference in O2 levels is onlt slight but causes a big change in voltage (and therefor a big shift in the lights that are on)

 

 

[Guest Ash]

 

 

Forgive me, I have had little experience of the AFR meters in question as I always thought attaching one to the stock sensor bore no merit. Again, Pete, if you are listening could I respectfully ask for some input here.  

 

From what I can gather, the AFR meter is nothing more than a linear voltmeter that is optimised for a range of zero to +1 volt. However, the output from a narrow-band oxy sensor, of the like fitted as stock to the MKIV, is anything but linear.

 

As such, the moment the A/F ratio drops a couple of tenths of a ratio down from 14.7 A/F then the oxy sensor voltage will drop rapidly down to its low level output voltage which (from memory) is typically 0.1V.

 

But at 0.1 volt, wouldn't the LED's on the AFR meter be reading a full-on rich condition... even though the true A/F ratio may be MUCH leaner than this at 14.3 A/F (say)?

 

Yours,

J  

[THOR Racing]

All,

 

>Forgive me, I have had little experience of the AFR meters in question as I always thought attaching

>one to the stock sensor bore no merit. Again, Pete, if you are listening could I respectfully ask for

>some input here.

 

Sorry for appearing not to respond but I cannot view this BBS all the time.

I get sporadic bursts at looking and answering questions.

 

 

>From what I can gather, the AFR meter is nothing more than a linear voltmeter that is optimised for a

>range of zero to +1 volt. However, the output from a narrow-band oxy sensor, of the like fitted as

>stock to the MKIV, is anything but linear.

 

Correct. You understand the problem.

The AFR is only linear for a small region around 14.7

 

 

>As such, the moment the A/F ratio drops a couple of tenths of a ratio down from 14.7 A/F then the

>oxy sensor voltage will drop rapidly down to its low level output voltage which (from memory) is

>typically 0.1V.

 

Yep, something like that.

It's about a 600mV change around the centre point of 14.7 , which is sitting somewhere near 500mV for 14.7 (Stochiometric) (so +/- 300mV)

 

So anything say between 800mV and 1V is non-linear and roughly indicates rich

(don't quote me if I've got this the wrong way around, I can't remember without the data in front of me)

and anything below 200mV indicates LEAN (again sorry if I've just mixed the RICH/LEAN bit up)

 

The general idea as I mentioned before is measuring the voltage from the stock O2 sensor gives you an indication of the state of the fueling.

How good/accurate an indication this is should be investigated further.

 

 

>But at 0.1 volt, wouldn't the LED's on the AFR meter be reading a full-on rich condition... even though

>the true A/F ratio may be MUCH leaner than this at 14.3 A/F (say)?

 

Could be, but the region isn't that narrow. Probably more like 0.1V == 13:1

But it's all a bit wooly. I've got a graph at hoe somewhere.

 

Oh and the idea that it's a PB AFR meter is a little OTT.

This is just a LED voltmeter really measuring the stock O2 sensor.

They are all like this.

This thread should be really named "Using the Stock O2 sensor for AFR measurements",

using PB somehow indicates my device is question when it's really the validaty of using the stock sensor.

 

The advantage I have is I can program the PIC chip inside to remap the O2 sensor output to expand

the non-linear regions and compound the linear region.

i.e. ignore most of the 200mV-800mV output and concentrate on the extremes.

This may yield a better result, but would have to be checked against a reference then.

 

I have got a circuit to control a 5 wire heated wideband O2 sensor (using a Honda Civic sensor) and I'll be making this up at some point.

The AFRi device is not in question. It's a programmable voltmeter and duty cycle device (for the injectors)

 

 

Pete

[Rich J]

I have got a circuit to control a 5 wire heated wideband O2 sensor (using a Honda Civic sensor) and I'll be making this up at some point.

 

Pete

 

Do the Honda wideband sensors work on the 0?V to 1V range, I have an A/F ratio meter and I was going to put it on the Integra but wasn't sure if the wideband O2 sensor fitted would be giving ou the right signal.

[Guest Ash]

 

 

I dug up some more interesting info. The link is given below:

 

http://fjoinc.com/automotive/WBO2details.htm#ox1

 

In particular, I quote:

 

"..............

 

Common AFR Meters:

 

Common dash mounted AFR meters make use of one, three or four wire factory installed oxygen sensors which are incapable of providing a wideband output. These sensors, by design, will read a high voltage with an air/fuel mixture below stoichiometric (14.7:1) and a low voltage above this point. Outside this narrow operating range, the meter has no way of accurately and reliably differentiating between 12:1 and 13:1, or conversely, 15:1 and 18:1. These readings are also greatly affected by exhaust gas temperature and sensor location. A 2:1 ratio variation is not uncommon. It is important to remember that automobile manufacturers installed these sensors to maintain a stoichiometric (14.7:1) mixture under cruising conditions, as this is the point where optimum catalytic converter efficiency is achieved.

 

........."

 

I thought the section beginning, "Outside this narrow operating range........" was quite interesting as it basically backs my own point.

 

The other interesting thing was the graph of narrow-band sensor output. Taking the RICH condition, from 10 A/F to 14.7 A/F, there is only about 0.2 of a volt difference in the signal. And I assume it was this region you were speaking of, Pete, in your previous post where you state.....

 

"The advantage I have is I can program the PIC chip inside to remap the O2 sensor output to expand the non-linear regions and compound the linear region. i.e. ignore most of the 200mV-800mV output and concentrate on the extremes. This may yield a better result, but would have to be checked against a reference then."

 

The big problem I immediately see, in comparing your statements against the output of the graph, is that of temperature.

 

The extreme region, as you call it, appears to have somewhat differing characteristics depending on whether the sensor was at 400, 750 or 900 degrees Celcius. Plus, there would be different interim characteristics as the sensor would naturally change temperature depending upon how the car was being driven.

 

It was interesting to note further that over such a wide temperature range of between 400 degs to 900 degs the output at A/F 14.7 was virtually unchanged. Even at 900 degrees, at the point where 14.7 A/F was detected, the output voltage was straight for around 0.25V. Which is more than ample for detection purposes.  

 

Yours,

J

 

 

Yours,

J