Posted to Technical Theory Forum on 4/23/2012
There are three issues to be clarified:
1. This is not an oxygen sensor. Any time anyone says it is,
or implies that that is how it works, or that this is even
its purpose is causing misinformation to be disseminated.
And, what is worse, these exact sensors are used as real
oxygen sensors in other applications - so it gets tricky as
there is already information out there that can be used in
conflict to their application as lambda sensors. In the
presence of oxygen, they produce no voltage. In the absence
of oxygen, they produce no voltage. It is only in the
presence of combustibles and not enough oxygen to complete
oxidation of them that they produce voltage.
2. In order to meet the conditions above, in a complex gas
stream containing both combustibles and oxygen (and carbon
monoxide and hydrogen count as combustibles - it is not just
the fuel vapor) - the combustibles have to be a low enough
concentration that the sensor's natural catalytic action can
complete combustion enough to use up all the free oxygen in
the gas mix. That is, the combustion has to run to
completion in order for the sensor to create voltage in a
rich mixture. (It only creates voltage in a rich mixture -
oxygen has nothing to do with it.)
3. If the second condition above is not met, the sensor
cannot accurately read the lambda state of the gas mix - and
will respond in error. In the case of a bad misfire, the
catalytic action of the sensor is not sufficient to complete
the reactions, and no voltage is created. The ECU dumps more
fuel in there until the catalytic action can use up the free
oxygen is depleted enough for the combustibles to be sensed.
The idea that nothing will cause the voltage to go low
except for oxygen is an odd way to look at a sensor. Sensors
are only sensors when they produce a response. (An open
wire, for example, also produces no voltage.) Voltage is
created by the sensor only when oxygen ions flow through the
sensor, and that only occurs when there is something pulling
My point is that stressing the absence of response is simply
not the way to look at the function of a sensor - and will
naturally lead to problems - as there are a multitude of
conditions that will cause no response - but only one (for a
good sensor) that will cause a response.
This is intended to be aa lambda sensor , and for a lambda
sensor, there are only two important gas conditions:
1. Rich - excess combustibles. In this case, (as long as the
chemical reactions can be run to completion) the sensor
produces a voltage.
2. Lean - excess oxygen. In this case, (again, as long as
the chemical reactions can be run to completion) - the
sensor does not respond.
(There are those who insist that there is a third - that
where the mixture is neither rich nor lean - the balance
point of perfect lambda. While this is the theoretical
ideal, in actual practice, it both never happens nor is it
measureable. If the gas mix is perfectly in balance, and the
chemical reactions can be run to completion, the sensor
theoretically would produce no output.)
My points are two:
1. This is not an oxygen sensor, nor is it intended to be.
(Even though this is a simple way to teach - it does not
explain what is really going on - so you are bound to run
into conditions where it does not behave as anticipated.) It
is intended to be used to tell when the air/fuel mixture is
2. The catalytic action is as important as the intrinsic
mechanism of the sensor itself - as it only responds to a
deficiency (a need for) oxygen in an equilibrium (chemical
reactions completed) condition.
I know that there is a lot of discussion about this sensor -
to the point of being absurd. However, it actually has a
simple purpose and function, as is pretty easily understood
- but not until the misconceptions are cleared away.
That is why I jump on posts that maintain the
Remember that Humberto's original question was a query about
the Brettschneider formula - and the fact that he got high
CO in (what he calculated) was a lean mixture.
This is an interesting case where both the lambda sensor
(due to incomplete catalytic reaction) and the
Brettschneider formula (due to the high level of HC
concentration) fell apart - and both erred substantially in
the lean direction - as the true mixture was quite rich
(about 15% rich), but had really terrible combustion
efficiency (about 55%, as I recall, as opposed to 95%
So - the point is that even under pretty off-the-wall
conditions, the theory behind both the Brettschneider
formula and the lambda sensor hold up.
Enough said on my side, I think.
In essence, we all pretty much know how these things work -
and are simply approaching the situation from our own
Being in the gas analysis environment, though, I look at
these topics in a global (any condition goes) sense, rather
than just looking at one specific application - so my focus
is pretty broad.
Robert from California
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