Question on Fuel/Air ratio

[IFX]

What is the Air to Fuel Ratio on a car?
Does this number vary? I just read that it is 14:1?

[TxZHP04]

Amazing what you can find with a simple search...

http://en.wikipedia.org/wiki/Air_fuel_ratio

[IFX]

All you had to say was yes, and I did research this before I posted this.
I was asking if what I read was correct.
Jesus.

[TxZHP04]

In reality it's a complex subject and "yes" isn't a comprehensive answer (nor is it a completely correct answer to your question).

Since this is the forced induction forum, take a look at some of the dyno plots you can find here - many of them plot AFR. You'll notice they are well below 14:1. You might also notice that even stock, the AFR varies with rpm. Still think "yes" was the best answer?

[RACEMX-M3]

What is the Air to Fuel Ratio on a car?
Does this number vary? I just read that it is 14:1?

Air to Fuel ratio is a measurement of how much air vs fuel is passing through your engine at a given point in time. Your car should have sophisticated fuel injection software that dictates how much fuel to inject based on a variety of factors, i.e. air density, altitude, humidity, ambient air temps, load, air/fuel readings, etc... too much fuel and your car will run "rich" and will be down on power, too little fuel and your car will run "lean" and well, you definitely don't want to experience that. :tsk:

[SPDu4ea]

What is the Air to Fuel Ratio on a car?
Does this number vary? I just read that it is 14:1?

Scott answered the first part of the question, but I'll elaborate on the 2nd -- "does this number vary?"


Yes, it does. 14.7:1 is stoichiometric and is generally what you will see at idle/light cruising. 15-16:1 is on the lean side for gasoline, but it isn't necessarily bad. Under light loads, running in this range will greatly improve fuel economy. The downside, is that a leaner mixture burns much hotter -- which increases the propensity of oxygen to bond with nitrogen (NOx emissions). So from the factory, most cars really run richer than they *need* to at low load. Running richer than 14.7:1 will result in some fuel not being burned and increase Hydrocarbon and carbon monoxide emissions (as well as lowering fuel economy).

Under load, you want to run richer than stoich. Maximum power is usually achieved in the 12.5-13.5:1 range. However, most cars run richer than this just to be on the safe side. With forced induction, you'll typically see even richer AFRs: 11.0-12.0 range at WOT.

But there is a lot more into it than just that though. For example, if you have a laggy turbo, you might run a little over 13:1 AFR from vacuum to low boost, and then drop down under 12:1 when you start running higher boost. This is because hotter combustion will help spool the turbo faster (spool is achieved both by flow & by thermal energy -- one of many reasons to ceramic coat exhaust manifolds and turbine housings -- but I digress).

Anyway, the point is AFRs varies a great deal and should be tuned with all anticipated circumstances/goals in mind.


edit: Here is a generic chart showing the AFR ranges. Keep in mind, individual engines will be slightly different, but the general idea is the same

http://steveww.org/Images/Misc/96.jpg


edit: here's another graph -- with a lot more going on -- AFR's effect on power/emissions:

http://www.g33.co.uk/images/fic-af-emissions.gif



edit: another graph showing the catalytic converter's ideal efficiency range (which is vary narrow right around 14.7:1):

http://www.che.iitb.ac.in/faculty/pa/dinesh/link%20t6.gif
A: HC
B: CO
C: NOx.


You can see that the Catalytic Converter's efficiency at reducing NOx emission drops dramatically above 15:1 which is why car's generally don't run above 15:1 anymore (to the detriment of fuel economy -- we had compact cars in the 70s and 80s getting 40-50mpg but that pretty much stopped in the 90s due to NOx regulations [and heavier cars]).



edit: One last one -- hard to read -- but this is the effect of AFR on combustion temperatures. For some stupid reason, the scale is reverse of the others (left is lean, right is right)
http://www.n66ap.alexap.com/Mixture_article_files/image002.jpg

[IFX]

Race and SPD thank you for the responses.

So when a car starts up and it burns let's say white smoke, this means it's running a bit lean?
And black smoke being rich?

[TxZHP04]

http://autorepair.about.com/od/troubleshooting/a/TS_exhaust.htm