Oxygen sensors are a very useful tuning aid.
(sometimes called Lambda or O2 sensors)
How do you know if you have the correct
air/fuel (A/F) ratio?
One way is with exhaust temp (EGT), but be careful here. If you tune
with EGT you must be careful, it's possible to get exhaust temps where
you think you want them, but actually you are so far lean that exhaust
temps go down, along with power and probably your engine. If you do
this, start REALLY rich and slowly lean the engine out to make sure
you don't fall off the cliff. Not the safest way to tune. You can tune
by reading your spark plugs (mmmm, yummy chocolate brown spark plug
tips). But this method is crude at best and is not real time, requires
you to shut the motor off under WOT conditions, and generally should
be used as a last resort (if your plugs are melted your are too lean
or are detonating).
Oxygen sensors and air to fuel (A/F) ratios.
Ideal (stochiometric) A/F ratio is 14.7:1. This more or
less means that for every particle of air in your engine, there is
enough fuel for it to burn correctly. If you have a lean mixture
( too much air ) the A/F ratio increases and your exhaust temps go
up and power goes down. This is a dangerous area, especially for
turbocharged engines and 2-strokes. When your A/F ratio is too high
and your exhaust temps go above 1400 degrees F or so you have an
engine full of melted pistons - it took us 12 pistons in our early
turbo snowmobile days to learn this for good, so trust us!!.
Tips on making maximum power.
You want to be a little rich, to make sure there is plenty of fuel
for the available air, 12.5:1 seems to be a good place to be. Anything
richer and exhaust temps go down and so does power. Normal Oxygen sensors
have a voltage output and come in various combinations from a single
wire to up to a 4 wire heated sensor. Their output voltage characteristics
are shown below. That little squiggle thing above A/F ratio is the
greek letter for Lambda.
As you can see, the sensor is almost like a switch.
It's output voltage lowers as your A/F ratio increases, but at 14.7:1,
it drops dramatically. If you use this sensor to tune, you can tell
if your lean or rich, but you can't tell by how much. It's not designed
to do more than this, so it's cheap and the manufactures can slap
it in their cars without a lot of cost. If you hook a voltmeter or
an A/F gauge (which is just a pretty volt meter) up to this, when
the engine is operating in closed loop mode (using the sensor to
adjust fuel mixture for emissions), you will see the voltage pinging
back and forth from lean to rich. It's trying to get the engine to
stay at 14.7:1, but doesn't know how much to adjust the A/F ratio
because the sensor just doesn't provide that information. It just
shows the engine is lean, so the computer richens up the mix. Then
checks the sensor again, oops, now we are rich, lean up the mix,
check the sensor, oops, too lean, ping pong ping pong....
The solution is a wide band O2 sensor, or UEGO. This is actually made
of 2 different types of sensors built into 1 package, and requires
specialized circuitry to interface to it, it's not simply a voltage
output any more, so don't bother with the volt meter. It's pretty
expensive, so most car manufacturers don't use them (some lean
burn Hondas and the new 1.8T VW's do). We bought powertrain electronics'
AFM1000 UEGO sensor interface hardware to power our UEGO sensor,
check out it's output voltage vs. A/F ratio. The output is linear
relative to air fuel, so you can get an accurate reading.
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