Legitimacy of AFR gauges

[TomoHawk]

I think what I'd like to see is scientific PROOF that you can quantify the FR in the intake manifold be measuring the oxygen in the exhaust.  Just because lots of people use it (either the narrow-or wide-ban sesor and AFR gauge) doesn't mean it works and is accurate and recise. 

[Patcon]

6 hours ago, TomoHawk said:

I think what I'd like to see is scientific PROOF that you can quantify the FR in the intake manifold be measuring the oxygen in the exhaust.  Just because lots of people use it (either the narrow-or wide-ban sesor and AFR gauge) doesn't mean it works and is accurate and recise. 

All modern closed loop systems are built around O2 sensors. The fueling is modulated by the ECU based on the O2 sensors. If it didn't work and was unreliable than it wouldn't be the default engine model. Everything from Ford to Ferrari to Bugatti do it this way...unless I am mistaken.

[gundee]

11 hours ago, TomoHawk said:

 Just because lots of people use it (either the narrow-or wide-ban sesor and AFR gauge) doesn't mean it works and is accurate and recise. 

Lots of people? How about every modern car manufacturer. Its what passes emission standards and gives you modern gas mileage values. Because it works.

[TomoHawk]

The point is that people claim the sensor/gauge combination give an accurate and precise  measurement, which it can't.  It's based in faulty presumptions, and the engine is controlled by high-speed electronics to get an "average" that just happens to be acceptable.

You might as well measure the AFR from the temperature of the tyres. Or, you might as well make the claim of a "clean-burning fuel,"  when the sole purpose of the internal-combustion engine is to burn fuel to create carbon dioxide and water, and nitrogen compounds.

Until a sensor that directly measures the air-to-fuel relationship in the intake manifold, you cannot claim to measure the air-to-fuel ratio by the waste gases of combustion.

Edited November 22, 2018 by TomoHawk

[rcb280z]

18 minutes ago, TomoHawk said:

you cannot claim to measure the air-to-fuel ratio by the waste gases of combustion

Try explaining that to the state of California.

[TomoHawk]

That's too ambiguous to answer.

[zKars]

Every sensor anyone has built, or will ever build, to read any physical property, does not read 'perfectly' with no errors in accuracy or precision. They almost all, regardless of what they measure or how they do it, as based on a scientific response models that connect, for example, how resistance changes occur with varying temperature. It ain't perfect. They are designed to deliver a specified accuracy and repeatability. We as consumers do not see this data when we buy a gauge.

 They are however, calibrated to internationally accepted standards (which also have errors, but much smaller ones) that everyone who builds this type of sensor is required to calibrate their sensors to, so that they can be responsible and accountable engineering businesses. Some comply, some don't. Companies that buy these sensors tend to buy from trusted sources. You can't as GM or Nissan, afford to buy whatever sensors you like from the cheapest vendor when they lay at the heart of how the engine runs.

Nothing you build at home will even remotely approach the precision, accuracy and repeatability or response time of what commecial O2 sensors and their related electronics now provide. It is a wonderful experience to try to do better, you will learn a great deal while you do it. 

Edited November 22, 2018 by zKars

[TomoHawk]

I agree with that, for the most part.  It's just that people buy a narrow-band, or wide band gauge and the corresponding OXYGEN sensor, and set it up, thinking they will get Air-fuel mixture readings from the exhaust gases.

[LeonV]

Seems somebody failed Chemistry class...

[TomoHawk]

Well, since you are infinitely knowledgeable in gas analysis, explain, exactly, how you convert an oxygen signal from a narrow-band sensor to an air-fuel ratio.

[jonbill]

In theory, what you do exactly is measure the voltage generated by the sensor, look it up in a table to find the experimentally determined and theoreticaly backed o2 concentration that corresponds to and then look that up in another table to find what the AFR would have been in the chamber to result in that residual o2 concentration in the exhaust.
That o2 concentration to AFR table is also experimentally determined and theoretically backed.
The experiments show that, consistently enough, for a given AFR in the chamber, o2 concentration in the exhaust will be constant regardless of other variables.

For a narrow band sensor, its responses are only trustworthy in a narrow range either side of lambda=1 or stoichiometric AFR. Outside that it can only tell you lean or rich.
A wideband sensor is accurate in a wider band of AF ratios, around 10:1 to 20:1.

An odd first post I know.

[Patcon]

3 hours ago, jonbill said:

In theory, what you do exactly is measure the voltage generated by the sensor, look it up in a table to find the experimentally determined and theoreticaly backed o2 concentration that corresponds to and then look that up in another table to find what the AFR would have been in the chamber to result in that residual o2 concentration in the exhaust.
That o2 concentration to AFR table is also experimentally determined and theoretically backed.
The experiments show that, consistently enough, for a given AFR in the chamber, o2 concentration in the exhaust will be constant regardless of other variables.

For a narrow band sensor, its responses are only trustworthy in a narrow range either side of lambda=1 or stoichiometric AFR. Outside that it can only tell you lean or rich.
A wideband sensor is accurate in a wider band of AF ratios, around 10:1 to 20:1.

An odd first post I know.

But a good first post!