______________________________________________________________________________________
Page 80/83
UltraGauge
Blue
™
User Manual www.ultra-gauge.com
A
A
i
i
r
r
t
t
o
o
F
F
u
u
e
e
l
l
R
R
a
a
t
t
i
i
o
o
(
(
A
A
F
F
R
R
)
)
Occasionally we are asked if UltraGauge supports real-time AFR. We have in the past not supported it. Note that it is supported by
the UltraGauge MX and UltraGauge Blue as a user programmable parameter, but is not recommended. Please read on to understand
why.
Real-time AFR can be determined on those vehicles which support wideband O2 sensors. Wideband O2 sensors are less common,
but very much superior to the older narrow band O2 sensors. Narrow Band O2 sensors have a very narrow range of useable
operation around the Stoichiometric ratio. The Stoichiometric ratio is the point at which there is just the exact amount of Oxygen to
burn the available fuel. The narrow band sensors essentially can only tell the ECU that the mixture is lean or that it’s rich, but not
the degree. In fact if you monitor the O2 sensor output, it constantly switches from lean to rich, and rich to lean, as the ECU attempts
to keep the Air to Fuel mixture at the Stoichiometric point.
Wideband O2 sensors have a much broader linear range of operation and if monitored generally provide a relatively constant output
corresponding to the amount of oxygen in the exhaust. As a result, wideband O2 sensors can be used to provide real time AFR. The
ECU monitors the wideband O2 sensor and outputs the ratio Lambda.
Lambda = Actual AFR / Stoichiometric AFR.
When the AFR is ideal, Lambda is 1. When the mixture is Rich, actual AFR is reduced and Lambda is less than 1.
If the Stoichiometric AFR is known for the fuel in use, then the Actual AFR can be determined
Actual AFR = Lambda * Stoichiometric AFR = (Actual AFR / Stoichiometric AFR) * Stoichiometric AFR
But here in lies the problem. The Stoichiometric AFR is never known because the makeup of the fuel that comes from the pump is
not known. For example, this table provides the Stoichiometric AFR for various ideal fuels
Fuel
Stoichiometric AFR
Pure Gasoline
14.7:1
10% Ethanol Gas
14.04:1
15% Ethanol Gas
13.79:1
E85
9.75:1
Pure Ethanol
9:1
Diesel
14.6:1*
The problem is that pure gas is never pure, and a 10% blend is rarely 10%. That's why the pumps reads: "
May contain 10%
...". But
in reality, it could be 1% or 15%, or any percentage in between.
Without knowing the Stoichiometric AFR for the fuel in your tank, there is no way to use wideband O2 sensor and lambda to
determine exact value of AFR. Most AFR meters simply assume pure gasoline and use a value of 14.7:1. However, the O2 sensor
cares little that you are using pure gas or pure Ethanol. For both it will report a Lamda of 1.
So let's say you have E85 in the tank. What will your AFR meter read? It will read 14.7:1, because Lambda is 1. But we know the
AFR should be around 9.75:1. This is why reporting AFR can be so misleading and absolutely wrong.
The far better parameter to monitor is Lambda, as Lambda is independent of the fuel used. As long as Lambda
is very near or equal to 1, you know your mixture is correct (
Stoichiometric)
.
If for performance reasons, you still wish to
monitor AFR, because you wish to run rich, Lambda is still the better parameter to monitor as AFR will be distorted by the
Stoichiometric AFR assumed. Using the MX or UltraGauge Blue any Stoichiometric AFR you wish can be programmed, but it is
still best to simply use Lambda.
*
Diesel engines
do not run at the Stoichiometric point and the actual AFR varies from 18 to as much as 70 (lambda >>1).
