From: hotrod@dixie.com (The Hotrod List)
Subject: Oxygen sensor
Date: Tue, 05 Jan 93 21:34:08 GMT
Organization: Dixie Communications Public Access.  The Mouth of the South.
To: hotrod@dixie.com
Reply-To: hotrod@dixie.com
Posted-Date: Tuesday, Jan 05 16:34:04
X-Sequence: 3188
X-Gifs-To: met@sunset.cse.nau.edu
X-Gifs-From: ftp.nau.edu

Before Christmas I had posted a note about the oxygen sensor system
that I built for tuning and diagnostic purposes.  William Street
asked for more information.

First, the theory: the oxygen sensor (more properly known as a
lambda sensor) measures the potential difference which results
from a chemical reaction between the exhaust gasses on its one
side and fresh air on the other side.  If a high-impedance
voltmeter is connected to the sensor then it will produce a
voltage which depends on the richness/leanness of the air-fuel
mixture.  For the sensors in popular use today a stoich AFR
produces 0.5 volts; richer mixtures produce higher voltages.
Another way to use the sensor is to apply a voltage to the
sensor and measure the current that flows.  If the mixture
is rich then current will flow from the sensor to the voltage
source; if it is lean then current flows in the other direction.

Note that there are some prerequisites: the sensor can only tell
you about the gasses that it is exposed to - if you have a
catalytic converter or an air pump and you measure after one of
these then you are not measuring what is coming out of the engine.
If you have an older simple vehicle without a catalytic converter
then you can make the measurement at the end of the tailpipe
(although you may need to disconnect the air pump to get meaningful
measurements).  If you have a catalytic converter then you must
mount the sensor before the converter to get meaningful results.
This usually means drilling a hole into the head pipe and brazing
a nut onto the pipe.

You must be careful to avoid leaded fuel and silicone.  Lead or
silicone will destroy the sensor and render it useless.  Make sure
that any RTV used in the engine or around the sensor is rated as
sensor-safe.

To be able to use the sensor you need two things: a means of
mounting it in the exhaust stream, and a set of electronics to
allow measuring the voltage or current flow produced by the sensor.

I made a tailpipe mount by scrounging a piece of exhaust pipe that
slips over the end of my tailpipe.  This pipe is about a foot long.
I drilled a hole in the center of it where I brazed on an 18mm no-foul
adapter with its extension sawed off.  The adapter is not necessary;
it was just a convenient way for me to obtain an 18mm nut.  I mounted
a Borg-Warner OS-109 heated oxygen sensor in the 18mm adapter so that
the tip of the sensor is in the exhaust stream.  The sensor is
connected to about 25 feet of 4 conductor cable which runs to a
small box containing the electronics.  This is a long enough cable
so that the box can be conveniently placed in the engine compartment
or on the dashboard.

I built the electronics so that I can run the sensor either in the
voltage-generation mode or in the current-producing mode.  The
circuitry for the voltage-sensing mode is simpler (and is also
required in the current-producing mode) but this mode gives less
useful AFR range.  The electronics uses conventional op amp
techniques to produce a DC current which is displayed on an
analog panel meter.  I used a surplus meter with a 0-1 ma movement
because it was available and reasonably priced.  New analog meters
are incredibly expensive these days.

The meter that I used is a 0-1 ma movement but the display
requirement in the measure-current mode is for a zero-center
meter.  I obtain this by summing in a signal which makes the
meter rest at half scale when there is no current flowing in the
sensor.  The sensor output deflects the meter above/below
center for rich/lean mixtures.  The system is not calibrated
in therms of AFR but it can be calibrated in terms of voltage
and/or current.  If you have calibration data from the sensor you
could interpret the voltage/current as AFR.  I find that merely
knowing which side of stoich and how the AFR changes under changing
engine conditions is sufficient for my purposes.

The box is powered from a cigarette lighter cord and plug.  I used
a long cord so that the box could be located in the engine compartment.
It needs a total of about 1.5 amps with a cold sensor; the current
drops to about 0.75 amps when the sensor has heated up.  Warmup
takes about 1 minute.  The electronics only takes about 10 ma so
its current consumption is irrelevant.  The response time of the
sensor is very rapid when it is warmed up - you can see individual
exhaust pulses when the engine is idling.  The time constant of the
meter will smooth things out when the engine is at cruising RPM.

If you have any questions then you can post them to this list or
e-mail me directly.

Bob Hale                                      ...!ucsd!btree!hale
...!btree!hale@ucsd.edu                       ...!ucsd!btree!hale@uunet.uu.net


----------
Posted by: emory!UCSD.EDU!btree!hale (Bob Hale)
Newsgroups: alt.hotrod,wiz.hotrod