Interesting Read About Air Fuel Ratio, (the most interesting and informative thus far) Options

[jsayre914]

Maybe some of you guys already know this, while i was browsing around on the net i found this interesting write up and some of you guys might want to read this up...

I'm taking this from another forum, if nothing else it makes for a good read:

Leaning out AFR due to increased octane.
Note that the octane is in RON.

What do you guys think?

His idea is fundamentally flawed- The (simplified) interpretation of octane rating is that the scale of 0 to 100 was originally devised by assigning a value of 0 to n-heptane (a fuel prone to knock), and a value of 100 to iso-octane (a fuel resistant to knock) Other fuels are then compared to that standard (and its test engine) to give their octane result
Although octane rating may affect Stoich it cant be simplified as a percentage of octane (iso-octane) as he assumes.

For example Methanol has a RON of 113 octane but has a stoich of @6.5:1 and Ethanol has a RON of 116 octane and has a stoich of @9:1. Both these fuels have alot more oxygen in them hence the lower stoic values.

Also "octane" is incorrect described as a chemical compound in this instance, because we are talking about RON "octane rating" of gasoline fuel

He has confused iso-octane with gasoline and if we used iso-octane as a fuel and it "happened" to have a stoich of 14.7 then I guess he is on the right track. But iso-octane (100% iso-octane) has a stoich @15.13 and gasoline has a number of other chemicals (Toluene @11.5:1) and oxygenates which drive the stoic value DOWN

ACTUAL STOICH VALUES FOR TYPICAL FUELS (AA Burluka et al, SAE 2004-01-2998)

95RON Shell gasoline- 14.49
98RON Shell gasoline- 14.35
100% Iso-octane- 15.13


Typical compositions both 90.5RON (not including all the additives)

type 1: typical of what the tuner is using for his calc
iso-octane 90.5%
n-hepatne 9.5%

type 2: what gas stations typically use
iso-octane 54%
n-heptane 13%
toluene 25%
cyclohexane 8%





AND if you have fuel higher than 100 RON that's still gasoline- it doesn't mean that there is different oxygen content (based off percentages) as we don't exactly know what has been used to increase the octane rating. Also there are a lot of high octane unleaded fuels which use quite a bit of oxygen enhancing chemicals.


I think that although he has tried to be exacting in his calculations, his basis is too in-exact to make me run 16.37:1 on the 98oct fuel I use.
We do however, through experimentation, know we can run mid 15s for cruise/light load without issues. We still have to allow for real-life issues, like changes in temperature etc, so we don't do permanent damage over long periods of time.



Below is taken from Tuners Bebo site


What is stoichiometric?
For the best fuel economy your air fuel ratios should be at the most efficient which is where all the fuel is being burnt and there is little or no excess fuel which will produce carbon monoxide (CO) emissions.

For a long time the accepted stoichiometric air fuel ratio has been 14.7:1. I'm guessing that was calculated using a different fuel to what we now put in our cars so it's time to re-calculate the CORRECT stoichiometric AFR.

First to gather the info needed.

IMPORTANT POINTS

There is 20.95% Oxygen (02) in the air we breath. (This is the same at sea level or at the top of Mt Everest).
98 Octane fuel is (supposedly) 98% Octane (95 = 95% etc)
Octane is a hydrocarbon written as C8H18


ATOMIC MASSES

Carbon = 12.011
Oxygen = 15.994
Hydrogen = 1.00794
(So C8H18 = 114.23092 and O2 = 31.988)

The stoichiometric chemical equation for the combustion of hydrocarbons is,
CxHy + (x+(y/4))O2 =} xCO2 + (y/2)H2O

Therefore the equation for octane is,
C8H18 + 12.5O2 =} 8CO2 +9H2O

If there was too much fuel or not enough O2 the Carbon monoxide would be produced. For example if the O2 is reduced from 12.5 to 11 the equation would look like this,
C8H16 + 11O2 =} 5CO2 + 9H2O + 3CO

And if there is even less O2 again there would be carbon emissions (black smoke)
C8H16 + 8O2 =} 9H2O + 7CO + C (10.7:1 AFR)

This means that for every 114.23092kg of C8H18 there needs to be 399.85kg of O2 or 1908.59kg of air.

1908.59/114.23092 = 16.71 parts air to 1 part octane.

So 16.7:1 is the stoichiometric AFR for 100 octane but since we don't get 100 octane here we should work it out for 98, 95 and 91 octane.

98 = 1908.59/(114.23092/0.98) = 16.37 AFR
95 = 1908.59/(114.23092/0.95) = 15.87 AFR
91 = 1908.59/(114.23092/0.91) = 15.20 AFR

Working backwards we can also work out for what octane rating the 14.7 stoichiometric value would be correct and comes out at 88% octane.

How about C16 (or C16H34 in it's full name).
C16H34 + 24.5O2 =} 16CO2 + 17H2O

Therefore for every 226.44596kg of C16 there needs 783.706kg of O2 or 3740.84kg of air.

3740.84 / 226.44596 = 16.52 AFR for 100% C16.

So I guess that's why cars seem to use more fuel than they need to. Cars are tuned to run at 14.7 AFR which means on average they're using 7.5% too much fuel.



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[Dave_Darling]

That's called DFCO, Decel Fuel Cut-Off. The 914 had it early on, but Porsche found that it actually increased emissions because of "uneven cooling". So they eliminated that feature in late 71 or so.

--DD