The term Stoichiometric ratio describes the chemically correct air-fuel ratio necessary to achieve complete combustion of the fuel. It is represented by the Greek letter, lambda.
The exhaust gas oxygen sensor is also called the lambda sensor, since it can be used to maintain the air-fuel ratio at lambda equal to 1, within very close limits. It can be installed in the exhaust manifold, where it measures the percentage of oxygen in the exhaust gases.
1.00 is Stoich
.99 is rich
1.01 is lean
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First a couple of misconceptions. Narrow band O2 sensors do not adjust to 14.7 AFR and Catalytic convertors do not work best at 14.7 AFR. The narrow band O2 sensor adjusts to Stoichiometric of fuel and Cats work best at Stoich.
The 14.7 AFR was correct, but it is calculated at Stoich.
Stoich = Lambda
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Stoich for gas is 14.7 AFR. Stoich for Ethanol is ~9.00AFR. That means that Stoich for E10 would be (14.7 x 0.90) + (9.00 x 0.10) = 14.13AFR.
Now the confusion on tuning by AFR instead of Lambda. WOT for 12.78 AFR or 0.87 lambda (0.87 = x / 14.7 AFR so x= 12.78AFR) at WOT. That's for gasoline, not E10. E10 requires lower AFR as seen in the example above. For E10, it would be 12.2AFR (14.1 x 0.87 = 12.2).
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A Wide-Band is not an AFR gauge. It is a Lambda gauge.
Most Wide Band controllers calculate AFR based of Lambda to give user a number he wants to see...
E10 Stoich is 14.1 -- Lambda for E10 = 1.0
E0 Stoich is 14.7 -- Lambda for E0 = 1.0
Notice Lambda is always 1.0 when adjusted by narrow band O2 sensor?
I typical Wide-Band Gauge is programmed to display 14.7 at 1.0 Lambda. So AFR is not accurate with E10 fuel, at cruise or at WOT PE!
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Found this post from Greg Banish and found it interesting.
To work effectively with Ethanol blends, you really need to switch to units of Lambda to avoid confusion. (lambda = AFR/stoichAFR)
Almost any hydrocarbon fuel makes peak torque somewhere near lambda=0.9. For pure gasoline this is an actual AFR of (14.68)*(0.9)=13.2 AFR. With e10, it would be calculated based on lambda with a different stoich point: (14.13)*(0.9)=12.7 AFR. Both mixtures, although numerically different, look the same to the combustion process.
e10 isn't a huge difference, but e85 get much more significant: (9.85 stoich)*(0.9)=8.9 AFR, which still looks the same as 13.2 with gasoline as far as the combustion process is concerned.
Wideband sensors also don't know what %ethanol blend you're burning. The UEGO (wideband) sensor just measures oxygen percent and delivers a lambda number. The cheaper widebands just multiply this measured lambda by 14.6 to give inexperienced users a number they're used to seeing in magazines. Don't tune to AFRs (instead of lambda) using ethanol blends, it will only mislead you.
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Greg Banish
Calibrated Success, Inc.
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