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.

1 hour ago, Patcon said:

But a good first post!

Thanks.

48 minutes ago, TomoHawk said:

That theory presumes you have an engine that is working 100% perfectly and you have a 100% perfect combustion.

Funnily enough, it doesn't - it mainly depends on the chemistry of the fuel and air mixture which hasn't changed for a very long time.

I'm sure there are conditions in which the accuracy is compromised significantly, but it's not within the normal degradation of an engine say in the first few 100k miles of it's life.  That's why many of the spark ignition cars of the last 20 odd years are still on the road passing their emissions tests using this technology to regulate their fueling.

They also have more than just the one oxygen sensor, in a few places, and modern electronic or digital engine controllers that make up for sensor degradation and other compromises.  I see no reason why you can rely on only one oxygen sensor. to get an overall and precise air-fuel mixture determination.

Edited November 24, 20186 yr by TomoHawk

I have a 20 year old mr2 with 3 sensors and 20 year old modern engine management electronics. The first sensor measures the output of cylinders 1 & 4, the second measures the output of cylinders 2 & 3 and the last measures the aggregate after the catalytic converter to judge whether the cat is working or not.
I believe this is representative of multisensor setups. Each sensor is trusted to measure its bit independently.
Sensors of course do fail.
The management system strategy for this is to constantly cycle mixture up and down a bit to check if the sensor reports it. If it doesn't it reports a fault.

There's no reason to not believe a single sensor and controller and gauge will report reasonably accurately the average AFR of the combustion chambers it's monitoring.

You are right, about the use of the multi-sensor setup, and how it's all used to get an "average"  or overall  fuel mix that works reasonably well, and it agrees with my opinion that just the one sensor can't do the whole job.  It makes you wonder why so people put their whole confidence in just the one sensor, how there's a whole aftermarket industry that makes a LOT of profit on it.

Edited November 24, 20186 yr by TomoHawk

36 minutes ago, TomoHawk said:

You are right, about the use of the multi-sensor setup, and how it's all used to get an "average"  or overall  fuel mix that works reasonably well, and it agrees with my opinion that just the one sensor can't do the whole job.  It makes you wonder why so people put their whole confidence in just the one sensor, how there's a whole aftermarket industry that makes a LOT of profit on it.

Actually the ECU relies on each O2 sensor independently. They are used as much to determine faults and OBD codes as real time fueling. They are also used to cross check each other. Not so much for accuracy but for function. I have found that when it an O2 sensor fails it's dead, not out of spec. So if my wideband O2 fails which it has in the past I get no AFR readings. 

I don’t get this thread. Why are we arguing the science behind a device that is so widely accepted in the industry. It works - use your time for something else IMHO

2 hours ago, madkaw said:

I don’t get this thread. Why are we arguing the science behind a device that is so widely accepted in the industry. It works - use your time for something else IMHO

When do we get to discuss if the coolant temp sensors are correct or not? AFM? Thermostat?

Oh right, we should do something else now....

Edited November 25, 20186 yr by zKars

Because using an oxygen sensor as the sole sensor for the Air-fuel ratio is like measuring tyre pressure using the temperature of the rubber tyre.

It always bothered me that there  isn’t an indicator on the dash to verify I am in 3rd gear and not 5th. Just because I push the shifter further over - how do I really know that I am in 5th and not 3rd? Just because I follow the pattern on that stupid little knob doesn’t mean I have actually changed anything

10 hours ago, madkaw said:

Just because I push the shifter further over - how do I really know that I am in 5th and not 3rd?

The engineers included a built in audible warning signal that you are in 3rd when you should be in 5th,  it sounds like a high pitched turbine combined with breaking metal, very distinctive sound.

Haha!!      From the Zcon thread:

On ‎11‎/‎18‎/‎2018 at 2:14 PM, Mark Maras said:

As the smile starts to spread across my face I shift into 3rd and then 4th gear and put the gas pedal to the floor just to keep up with Bill and Jai. Soon I think, OK, time for 5th gear. Slip it into neutral over to the right and up. Goes in easily, I let out the clutch and immediately realize that it was in 3rd and also realize this is a 4 speed.

 

Edited November 25, 20186 yr by Captain Obvious

Measuring the coolant temperature is easily done with a temperature sensor like a thermocouple or the LM35 temperature sensor.  The  Lm35 gives a signal in millivolts proportional to the temperature in degrees C, and you can make an offset circuit for it as well, because 0C related to 0.5Volts, or display it directly using an Arduino program.

Is there a practical electronic sensor for intake manifold vacuum.  That's also important for having the engine work efficiently.