How does octane affect A/F's? If the octane numbers go up do the A/F numbers go down? I always increase my octane when I go to the dyno just as a small margin of safety. By doing that am I altering my monitored A/F readings?

 

Nobody???

 

As octane goes up the burn goes deeper into the cyl so that would mean that your A/F ratio should show leaner since you are getting a more complete combustion.Stan

 

Thanks Stan. Just the opposite of what I was thinking. I was worried that the higher octane would cool the burn and show an artificially rich A/F.

 

These thoughts are so hopelessly off track they don't even deserve a reply.

Read a book or something... the information is out there.

 

Good thing you're on your own track. Don't think you could run into anybody over there.

 

 

Please feel free to back up your disagreement with facts, it does make a person more credible

 

I would like to know what BOOK to buy?
The testing that I have done backs up my statement as far as the burning end of it.Real easy to duplicate.a tube a rubber ball that barely fits in the tube but can roll up and down it.get an eye dropper and put a few drops[of gas, race gas and so on] in the tube then take the ball and let it roll down the tube and back[tube has to have a cap on one end].Now,put a match to the end of the tube with the open end [to simulate a cyl] and watch how far each one burns.That is the distance that it burns IN your cyl also look at how it burns.Then you MIGHT understand 99 white lite.If you do do this WEAR safty glasses and DON'T get a cheap clear plastic tube.[REMEMBER gas almost explodes!!!]Stan

And I am NOT responsible for ignorance!!In doing what is stated above

 

Perhaps WhiteLite should remove the stick in his a$$ and replace it with Ruslow's clear tube. At least it will give the doctor an avenue to find his head.

 

I didn't mean to be mean, but that statement is very far off from what happens. The experiment isn't even good because it doesn't replicate the condiitions in cylinder (in cylinder has a much higher starting temperature and pressure).

Heywood's IC engines book is probably the most widely known book for fundamental combustion. A lot of it goes pretty deep into chemistry and such, but you can normally pick up the fine parts.

A fuel has a stoichiometric A/F ratio based upon the number of Hydrogen, Carbon, Nitrogen, and Oxygen atoms in the fuel. Most fuels are only H and C, but some area's of the nation that use MTBE or Ethanol as oxygenates have some O's as well. The stoich A/F ratio is what balances out the:
(A) CxHy + (B) O2 --> (C) CO2 + (D) H2O
This is only dependent on those variables.

Octane is just some crappy correlation of how the knock properties of the fuel compare with a certain mixture of n-Heptane (0 octane #) and 2,2,4-trimethylpentane (100 octane #) in a crappy old variable compression carburated engine. Its bascially a function of how the chains are put together in the molecule.

Between the grades of gasoline, the stoichiometry may be a tiny fraction different, and the octane # is obviously different. The only effect of octane on A/F ratio is if a different grade of fuel happens to have a slightly different stoich A/F ratio; it is not because of octane number in and of itself. Even if it does have a different stoich A/F, it will me immeasurable with the equipment at a dyno shop.

This is just plain old not accurate. The A/F ratio of an engine is just the mass of air over the mass of fuel that makes it into the engine. It doesn't matter how much burns, even if zero of it burns, the A/F is still the same. And everything in the cylinder will burn (except for crevices and such, but that isn't dependent on the fuel and doesn't even make a difference in power, only emissions). I've seen combustion efficiency data (HC emissions) to back that up. Even if you somehow quenched the flame halfway through the combustion proces, the ratio of air and fuel is still the same.

Also in your experiment, for the volume of air you have, you only need a very small amount of fuel. For example, 1 liter of air pressurized to 7 atmospheres only needs about 600 microliters of fuel. In your rubber tube you are probably putting more than that amount of fuel into a smaller volume that isn't even pressurized. The mixture is being extinguished when enough of the fuel vaporized do to the heat from the flame that the mixture is overly rich and won't burn anymore. That experiment is not what happens in an engine and probably is nowhere close to repeatable because of the tiny increments in fuel needed to keep the A/F ratio the same between test fuels.

So back to the original question, octane number doesn't do jack didly squat to the A/F ratio. There are some other minor things that change the way the engine might run between fuels, but everything is so small it doesn't even matter.

I investigated which fuel would make the most power in a motorcycle engine and found that there are a whole bunch of trade-offs that all equal each other out. A faster burning fuel needs less spark advance so it should also make more power. That same fuel also reduced the inlet temps through vaporization so the inlet density to the engine should be higher and will make more power. Except for the fact that I needed more of the fuel because it had a lower stoich A/F ratio, which displaces more oxygen, and made it have less power than some off-the-shelf reference fuel. And the exotic fuel was several dollars a gallon!! Its not magic, just chemistry and physics.

 

 

99 white light:

You made one obvious mistake in your logic.

You talked about A/F as intake valumes. This is almost correct. It is very easy to get from the computer. It will also result in an engine going boom.

What most tuners (and users) are interested in is the combusted A/F aka "How much gas did I burn?" or "What's the O2 sensor read?"

Ruslow is correct that this number will show leaner with a higher octane for the reason he stated.

This is also a much more important number than intake air volume and intake gas volume since it actually measures what your engine is doing.

Coldie

 

my head hurts after reading all this, and I didn't comprehend anything

Wish I did though

 

Ok, before people start blowing gerbils out their butt tubes on this one, let me chime in...

Octane has a barely measurable affect on A/F. It's so small that if you saw .1 A/F change, that would be a lot. Going from lower octane to higher octane or vica versa is not going to change your A/F ratio.

However.... If you change your timing based on the new octane, that WILL affect your A/F ratio. But that's A/F changing because of timing, not octane.

Also keep in mind that A/F is a measure of oxygen in the exhaust, and NOT hydrocarbons (fuel). Many people make the mistake of thinking A/F measures fuel. For example, if you had a misfiring cylinder, you wouldn't have a richer A/F from unburned fuel, you'd have a leaner A/F, from unburned O2.

There is no reason to over analyize this subject with formulas and text book data. All that's going to do is confuse the heck out of people.