Finn Lassen wrote:

Thanks Ed.

Very impressive. Somehow the keys got chopped off. What are the three

curves?

Three different sensors? Brand names/models?

Even more interesting, how did you get the data?

Again, I'll show my ignorance. Is 15.7 the point where there is no

excess oxygen

and no excess fuel - ideal combustion? If so, why does a richer mixture

give

better performance?

Yes, I'm leaning towards going back to rich again. That hot exhaust is

butchering

my exhaust system and cowling, not to mention heating up my feet.

Finn

  No, the three curves are all from the same sensor.  Another factor that
affects the curve is exhaust temperature.  So the three curves you see are
all for the same sensor, but different temperatures.  The blue curve is for
exhaust temp of 750F, the green is 1200F, and the red for 1500F.  I selected
the red curve (1500F) as closest to my exhaust temp when I want best power,
so best power 12.6:1 A/F mixture on the red curve is where the line crosses
at approx 0.9 volts.  But actually, since my exhaust is hotter than 1500F at
full power, the curve is probably skewed even more to the left and when my
LED for 0.9 volt lights up I am probably pulling an AF of greater than
12.6:1 or running a little rich.  It is probably possible to work up a
calibration circuit to compensate for the changes in exhaust temp and I
suspect the more expensive AF mixture monitors probably do that.  However,
for our needs its probably not necessary.

   Actually, 14.7:1 is the mixture that supposedly gives you the best
combustion for EPA purposes that is the part of the curve that runs mostly
vertical.  The 15.7:1 point is what might be described as "Economy Cruise",
on the lean side for max range but less power.

My understanding is that the stoichiometric point is seldom achieved in real
world combustion and that the oxygen content of the exhaust varies from
exhaust pulse to exhaust pulse.  It appears that the objective is to get the
oxygen content to the point where the chemical composition of the exhaust
can be handled by the catalytic converter.  There apparently is always some
unused oxygen and fuel in an exhaust pulse even at the stoichiometric point.
By running richer, the odds of burning all (most?) of the oxygen increases,
however, the odds of having unoxygenated fuel also increases (of course bad
for emissions).  So you get the additional power by increasing the odds of
fuel and oxygen meeting by running rich.  That's the best answer I can come
up with.

  http://www.dakotadigital.com/tech/ODY-13-2.htm   Here is a link that
provides a better explanation that I am capable of.

Ed

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