DIY_WB O2 Theory of operation

[Ray Drouillard]

I'll give you a quickie explanation.

The UEGO sensor consists of two elements: a standard O2 sensor and an ion
pump.  The ion pump is used to pump oxygen from the air into a small
chamber, where it is mixed with exhaust gas.  There is a standard HEGO
sensor in that chamber that measures the amount of oxygen.

A standard HEGO sensor measures a very small range of mixtures.  It gives a
wide voltage swing over a very narrow range of mixtures.  This sensitivity
actually works to our advantage in a UEGO sensor.

If the exhaust gas contains oxygen, the ion pump is used to pump oxygen out
of the chamber.  It is adjusted to pump out just enough to bring the
standard HEGO element into the middle of its operating range (around 2-3
volts).  If the exhaust gas contains hot unburned hydrogen/hydrocarbons,
oxygen is pumped into the chamber, where it combines with the hydrocarbons.
Just enough is pumped in to the chamber to bring the standard HEGO element
into the middle of its operating range.

The mixture is measured by measuring the current that is required to keep
the HEGO element in the middle of its range.  A simple op-amp circuit is
used as a feedback loop to keep the ion pump properly adjusted (though the
actual "practical" circuit is somewhat more complicated).  In the diagram,
the upper left op-amp is used to provide the feedback.  The output is fed
through an amplifier and a resister, then to the ion pump.  If you look at
the resister just before the ion pump output, you'll notice that it is
connected (on both sides) to an op-amp circuit.  This is a differential
amplifier, and the output voltage is proportional to the current through
that resister (V=IR).  The output of the differential amp is fed into
another amplifier.  This amplifier uses the calibration resistor on the
sensor to allow for differences in the sensors, and it buffers the signal.

There is other circuitry designed to supply the heater and keep the
temperature constant, protect the sensor from overloads, and other similar
functions.  The circuit uses a double-ended power supply with a "phony
ground".  "Ground" for the circuit is actually something like four volts
above the vehicle ground (I used the same trick in my senior project).


Ray Drouillard



----- Original Message -----
From: <david.roland at gm.com>
To: <diy_efi at diy-efi.org>
Sent: Thursday, October 18, 2001 7:58 AM
Subject: Re: DIY_WB O2 Theory of operation


> Does anybody have a good source of information on how a WB O2 sensor
works?
> I've got the PCB, an NTK sensor and I am  ordering the components. I would
like
> to know more about the sensor itself before I start.
>
> Thanks in advance
>
> David Roland


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