Help me understand Air Fuel ratio

[AndreiD]

Hi guys,

I was keeping an eye on the Air Fuel ratio on a longer trip that I did, and I saw a lot of variation depending on the throttle input. Ford says the compression ratio is 12:1 right?

I think I never seen that number. I've seen only 14:1 or even higher. Under no throttle (lift off) it reads 20:1. Why? And why does it fluctuate so much? Is it because of the O2 sensors that constantly read different O2 values and adjust accordingly?

Then again, what happens if you go catless and delete the O2 sensors? Will it fluctuate the same and if yes, by what?

Cheers!

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[firestarter2]

Compression ratio inst A/F ratio.

All things being equal under cruising you want 14.7:1. or higher

Under acceleration you want between 11.5:1 and say 14:1 depending if you are forced inducted or natural aspirated. The additional fuel helps to reduce the chance of detonation under load.

Under lift off depending on the car a lot of time there is actually no fuel at all being injected (Think decelerating, or coasting down a hill). It may say 20:1 but actually no fuel.

Depending on the load, rpm, temp, timing, cam position the amount of fuel will vary so usually the A/F is all over the place unless you are cruising.

This is overly simplified.

 

[Tank]

I am not an expert but my layman's understanding is:
Air fuel ratio is the mixture going into your cylinder. This is indicted on your display.
Compression ratio is how much your piston compresses the air fuel mixture prior to detonation. The compression ratio is not indicated in your display.
Though the amount of fuel might be much reduced, there is always some fuel present in the mixture for detonation. The amount of fuel in the mixture increases proportionally with the throttle.

 

[krt22]

The others have explained the difference be A/F ratio and compression ratio.

As far as the rear 02, that is simply to measure cat efficiency for emissions purposes, the A/F ratio comes from the pre-cat 02, you don't want to remove that one otherwise your car will run like shit (if at all lol)

 

[Grimace427]

I am not an expert but my layman's understanding is:
Air fuel ratio is the mixture going into your cylinder. This is indicted on your display.
Compression ratio is how much your piston compresses the air fuel mixture prior to detonation. The compression ratio is not indicated in your display.
Though the amount of fuel might be much reduced, there is always some fuel present in the mixture for detonation. The amount of fuel in the mixture increases proportionally with the throttle.

You are pretty much correct. Just want to point out a terminology error though, the air/fuel mixture is ignited and not detonated. Detonation is a term used when an uncontrolled ignition event occurs inside the chamber, most often when the mixture begins to burn while the piston is still coming up on the compression cycle.

 

[nastang87xx]

Compression ratio and AFR are in no way tied to each other.

Compression ratio is how much the cylinder compresses the AF mixture from bottom dead center to top dead center.

AFR: 12/1 - 12 parts air to 1 part fuel
Compression: 12:1 - 12 times compressed at top dead center from bottom dead center

Regardless of what your AFR is, compression ratio will always compress to 12 x whatever came into the bore whether it's 12 parts air 1 part fuel, or 100000 parts air and 1 part fuel. Compression is compression.

 

[AndreiD]

Thank all of you guys for clearing up definitions and terminlogies. Sorry if I came up with stupid quesitons...

Cheers!

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