Legitimacy of AFR gauges

[TomoHawk]

I have an L28E with L-Jet EFI.

I was thinking of building an AFR (air-fuel ratio) gauge using some common electronics stuff.   It lights up LEDs from a signal generated by a narrow-band oxygen sensor in the exhaust pipe.

But, I question the legitimacy and accuracy of the gauge.     How does oxygen in the exhaust tell you how much air and fuel is going into the cylinders?  There is no sensor that will do that, and it would go into the intake manifold, not the exhaust pipe.  The best you can do is to use a modern engine, with a programmable controller to measure the air (via the AFM) and calculate the amount of fuel inject from the fuel pressure and the injection timing, then calculate the AFR. Measuring the oxygen in the exhaust does not tell you what is going into the cylinders.  The best you can do with that method is to guess at what the AFR is, with presumptions of engine perfection (you would need an engine that is perfectly built, in perfect operating condition, producing a 100% perfect combustion of what did go into the cylinder.)

So using one of the so-called AFR meters is  a bunch of hooey, and the value it gives you cannot be trusted.  Using a narrow-band oxygen sensor, the signal will jump around, from a relatively lean (say 15:1) to a relatively rich (about 12:1) mixture- IF you believe the theory of these sensors. It's logical to say that the amount of oxygen detected by the sensor, is proportional to an amount f air, but it cannot tell you the AFR.  It only tells you there is oxygen/air in the exhaust.   

I have read a lot about wide-band sensors, but I'd say it is the same as any other oxygen sensor. 

The AFR display I will be building, if you are electronically-skilled, is based on the LM3814 LED driver chip, and lights LEDs, linearly, in proportion to the signal.  And the best you can do is to adjust the mixture so the display moves around enough to get an "average." then do some on-road or dynomometer testing to get your desired mixture.  Or, you could use an exhaut gas analyzer to adjust the mixture based on theCO content, whic is similar to the oxygen content adjustments.

[Stanley]

Nice project. Do you need to calibrate depending on sensor distance from exhaust ports ?

[TomoHawk]

Tjereis no "calibration" needed.  You install the oxygen sensor in the exhaust pipe and hook it up to the gauge.

[Zed Head]

I thought about building my own system and just using a look-up table, a real paper one, to mark up a voltmeter.  

Not sure where "legitimacy" comes in to the conversation.  It's just a number from a sensor, some basic scientific principles, and some math.

[TomoHawk]

The problem is that an oxygen sensor in the exhaust stream can't tell you what the fuel mixture in the intake manifold is, precisely.  You make a LOT of presumptions, and people accept what the gauge says as an exact, scientific answer.

 You can buy pre-built displays cheaply from electronic kit companies, but it's just as easy to build one yourself, if you have the skill. Because the  signal can change quickly, you would be better off color-coding the voltmeter, for rich, about-right, and lean.

[Zed Head]

6 minutes ago, TomoHawk said:

 people accept what the gauge says as an exact, scientific answer.

 you would be better off color-coding the voltmeter, for rich, about-right, and lean.

I don't.

That's what I was going to do.

Then realized it didn't matter for my plans.  I only have to pass emissions every two years and my engine is unmodified.

[TomoHawk]

You may be interested in this inexpensive thing, to try for fun.

DISPLAY FUEL MIXTURE MODULE EGO 10LED

Edited November 21, 2018 by TomoHawk

[Captain Obvious]

23 hours ago, TomoHawk said:

Using a narrow-band oxygen sensor, the signal will jump around, from a relatively lean (say 15:1) to a relatively rich (about 12:1) mixture- IF you believe the theory of these sensors. It's logical to say that the amount of oxygen detected by the sensor, is proportional to an amount f air, but it cannot tell you the AFR.  It only tells you there is oxygen/air in the exhaust.   

I'm no expert on O2 sensors, but I don't believe the output will be jumping around unless the mixture is jumping around. When used in the control loop in a car, the only reason the O2 sensor output swings back and forth from rich to lean is because the controller is adjusting the mixture and making the output swing from rich to lean.

If you hang that 3914 based meter on your car and you're running lean, it'll always show lean. It won't be flipping around.

 

[Stanley]

My A/F meter flips around when it's first started and the engine is cold. Don't know if it's because the sensor hasn't warmed up or if it's due to the mix being irregular because the intake runners haven't warmed up. Or maybe both. Once the engine has run about 15 seconds the meter is steady and the numbers on the gauge respond instantly with adjustment of the choke lever. Kind of handy (with the SU's) to avoid running too rich or lean at startup. Need to adjust the lever several times to keep AFR where I want it until it warms up.

Noticed it also responds to stuff I don't think of as fuel. The AT modulator was bad, leaking ATF into the balance tube occasionally until I replaced it last month. I was taking a drive and saw the gauge suddenly drop to 10. Looked in the rearview and saw a cloud of blue smoke. Guess the engine had just sucked in a gob.

 

[madkaw]

10 hours ago, Stanley said:

My A/F meter flips around when it's first started and the engine is cold. Don't know if it's because the sensor hasn't warmed up or if it's due to the mix being irregular because the intake runners haven't warmed up. Or maybe both. Once the engine has run about 15 seconds the meter is steady and the numbers on the gauge respond instantly with adjustment of the choke lever. Kind of handy (with the SU's) to avoid running too rich or lean at startup. Need to adjust the lever several times to keep AFR where I want it until it warms up.

Noticed it also responds to stuff I don't think of as fuel. The AT modulator was bad, leaking ATF into the balance tube occasionally until I replaced it last month. I was taking a drive and saw the gauge suddenly drop to 10. Looked in the rearview and saw a cloud of blue smoke. Guess the engine had just sucked in a gob.

 

Yes the heater needs to heat up the sensor so the initial readings might be off. Containment’s will ruin a sensor . Coolant from a HG failure will do it for sure, but oil will ruin the sensor too. 

[Captain Obvious]

10 hours ago, Stanley said:

My A/F meter flips around when it's first started and the engine is cold. Don't know if it's because the sensor hasn't warmed up or if it's due to the mix being irregular because the intake runners haven't warmed up. Or maybe both.

Yeah, I'm not surprised the first couple seconds are unreliable. After that though, it sounds like it tracks the mixture very nicely. That's cool. I wish I had something like that installed.

But back to the point I was trying to make originally... People talk about the mixture oscillating quickly between rich and lean. I propose that the only reason that's occurring is the control system is MAKING that occur. And that's because that's the only way they can measure the mixture. They are using a narrow band sensor and shooting for "an average" mixture at stoichiometric.

If you stick an O2 sensor in the Z's exhaust stream and hold the engine condition steady, then you will measure a steady voltage out of the sensor. It won't be flipping around. It will probably be pegged rich, or pegged lean, but it'll be steady.    

[TomoHawk]

That's the advantage of a simple display like this:  to see if there is a pattern in the exhaust for the different throttle settings.  It should be an overall steady value until you press the throttle pedal enough to operate the TPS, then you ought to see the display change to a slightly lean indication for mid-throttle, then richer for an open throttle, then back to rich at idle.  If your AFM and ECU are operating correctly, the display should be fairly steady otherwise.