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'[EE]: Exhaust Gas Temp Sensors and O2 sensors'
2001\02\13@062138 by Peter Betts

picon face
Alan,

Also being in the UK :-) I fully understand. Unfortunately the wideband
sensors are very expensive.

I have connected the standard O2 sensor into a PIC and measure the output.
You normally find that because the narrow band sensor is only accurate
around ideal air/fuel ratios (stochiometric) that the engine management
system toggles the fueling back and forth between rich and lean to keep the
average around the ideal. If you were to connect a voltmeter you'd see it
cycling up and down around 1V. This is the closed loop operation of the ECU.

Open loop is as soon as you hit the throttle pedal. This switches the
controlling of air/fuel mixtures away from using the O2 sensor and instead
uses an internal MAP. The O2 sensor can still be used to give an indication
if the mixture is tending towards rich or lean giving you a glimpse of
what's going on but it's by no means accurate. The O2 sensors are really
only there to make sure the mixture is correct for the efficient operation
of the catalytic converters.

I think an 8% excess of unused oxygen is about the limit for a combustion
engine. Less than that and you start to lean out and could cause serious
damage to the engine.

For an MOT (UK car emmisions test) then the standard O2 sensor could be used
I think. Take it to your local MOT place. Get them to take a reading at
about 2000->2500rpm and measure the O2 voltage at that point. Now you can
use this as a guide to keeping the car in check throughout the year.

Sometimes the car can go lean or rich (normally rich) due to bad O2 sensors
or dirty ones and you could pick this up probably.

Just some thoughts.

Pete

> {Original Message removed}

2001\02\13@072626 by Martin Hill

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> Alan,
>
> Also being in the UK :-) I fully understand. Unfortunately the wideband
> sensors are very expensive.
Indeed, about #500 for a UEGO sensor and from #300 for the
controller.

>
> I have connected the standard O2 sensor into a PIC and measure the output.
> You normally find that because the narrow band sensor is only accurate
> around ideal air/fuel ratios (stochiometric) that the engine management
> system toggles the fueling back and forth between rich and lean to keep the
> average around the ideal. If you were to connect a voltmeter you'd see it
> cycling up and down around 1V. This is the closed loop operation of the ECU.

This is what happens, but the rich-lean cycling is required for the
catalyst to work, as HCs and CO are oxidised when lean and NOx
is reduced when rich.

{Quote hidden}

If optimised for it you can run pretty lean, down to about 4% fuel by
mass, but you don't want to be doing that at high load.

Martin

{Quote hidden}

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