Date:         Mon, 9 Jun 1997 08:27:35 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         mholser@Adobe.COM (Malcolm Holser)
Subject:      Re: O2 sensor question

> > The best power is at lambda 0.90.  The sensor has its entire voltage
> change between
> 0.98 and 1.01 -- almost no range at all
> 
> I followed you up until here.
> 
Hmmm.  "Lambda" is the expression used generally as the "ideal" air/fuel
mixture.  At lambda of 1.00 means there is precisely enough oxygen to
completely oxidize the gasoline.  Generally, the whole charge delivered
to the cylinders does not burn, and for best power you want some extra gas.
This allows for more "push" as the gas is not all burned up before the power
stroke is finished.  For best emmissions, you want a leaner mixture, since
you don't want to be putting out unburned and partially burned gasoline in
the form of CO and HC (which is what gets measured at smog inspections).

Lower lambda values mean richer mixtures, higher mean leaner mixtures.
Lambda is a Greek letter, like an upside-down "y".  Volvo used to have
this on the badge at the rear of their cars, with "Lambda Sond" written
as well, advertising the fact they added the O2 sensor.  A lambda of 1.00
is perfect mixture, and is also commonly called "stoichiometric ratio".
For most gasoline, this is 14.7:1 by mass (or *weight*).  14.7 kilos of
air per kilo of gasoline.  That's a whole lot of air!

The sensor, as somebody else said, is sort of like a "fuel cell" battery.
It generates a voltage when hot, and when one side is exposed to oxygen and
the other is not.  the arrangement is like a thimble sticking down into
your exhaust.  One side is in the exhaust stream, while the other is in the
outside air effectively (which is why you can't simply shove one up the 
tailpipe).  When there is oxygen on *both* sides, no voltage is generated.
When there is oxygen only on one side, it generates about .9 volts.  

If you are running slightly rich, all of the oxygen is used up and it turns
on.  If you are running slightly lean, there is some oxygen left around
and it turns off.  An ideal thing would be to be able to read how close you
are to the desired mixture, but this sensor really only tells when you are
*not*.  It gives no voltage at lambda 1.02 and full voltage at 0.98. With
such a narrow range, you cannot really hope to hit the target with any fuel
metering system, either a carb or FI.  So the goal is to wander around in
the neighborhood -- seeing the sensor go from zero to .9 volts "flickering".
If it stays low, you are lean, if it stays high, you are rich, but you can't
get it to stay halfway.

For tuning for emissions this is ideal.  If you run lean, you rapidly start
generating NOx.  If you run rich, you generate excessive HC and CO.  But
best efficiency is *not* achieved here.  It is more effecient to completely
burn the fuel charge, and minimum fuel consumption is achieved at about a
lambda of 1.05.  This would give a zero reading on an O2 sensor.  Best power
is achieved by wasting a bit of fuel (and best cooling -- lean engines run
*hot*) and is achieved at a rich mixture with lambda 0.90.  A sensor is
full-on at this point.

So the standard O2 sensor used in cars for closed-loop FI feedback control is
pretty useless for most other things.  It can be used, in its "flicker"
mode to get a car set up for best emissions levels, and this is a pretty good
compromise between mileage and power as well, but for air-cooled engines
you want both the power and the cooling richer mixtures give.  And to set
the mixture slightly rich (say at .94), you need better sensors -- like a real
"exhaust gas analyzer" or even a CO meter.  Goods shops generally use such
things now, and all mechanics who service emission contraols have to have them
to stand any hope of doing a good job.


*ALL* of this info is from the Bentley Book called "Bosch Fuel Injection and
Engine Management" book by Charles O. Probst.  I recommend that book for
every VW owner with a desire to learn more about their FI system.


> Enjoy the crewcab! It is the best vehicle I have ever owned.

I think my wife already has intentions of keeping it for herself!  Now I need
to add in air conditioning -- it was really hot driving through Bakersfield.

malcolm