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This article was first published in 2004.
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Is it possible to tune the air/fuel ratios in a
car using nothing but a normal heated oxygen sensor and a multimeter? Well yes,
sure it's possible - but how accurate are the results going to be?
Recently we had a chance to put it to the test, comparing a 'multimeter tune'
with the air/fuel ratios measured by a professional Autronic air/fuel ratio
meter. And not content with that, we also used the opportunity to change the
spring tension in a vane airflow meter and test a prototype of a new DIY kit
interceptor.
Whew!
The occasion was the tuning of the air/fuel ratios
right through the load range on a 1985 BMW 735i. Being (just) pre-unleaded, the
BMW doesn't use an oxygen sensor feedback loop, so whatever mixtures are set are
the ones that stay - you can't wait for the self-learning to help you out at all
the lower loads. The tuning tool was one of the prototype DIY electronic kit
interceptors that I am developing with Silicon Chip magazine. It's a
device that allows real-time interception of the airflow meter signal, allowing
very fine and accurate tuning of the air/fuel ratios. (We expect that the very
low cost kit will be out in the second quarter of 2004.)
The need for the tune came about because after
being purchased, it was found that the BMW was running stinkingly rich - as in,
an average fuel economy of near 30 litres/100 km!
The Vane Airflow Meter
Like many older fuel injected cars, the BMW runs a
vane airflow meter. As the name suggests, this design places a door (the vane)
in the passage of the intake air. As more air flows through to the engine, the
door is pushed open wider and wider. The primary vane is connected to a
secondary vane which works in a closed chamber, helping damp any flutters of the
first (sensing) vane. The vane assembly is connected to a potentiometer which
allows the airflow meter to output a voltage signal that relates to airflow.
Vane Airflow Meters - Not What They Seem!
There
are a few other things to note about vane airflow meters:
(1)
Unlike a hot wire design, the vane airflow meter measures volume airflow, not
mass. It's mass which is important, and so a temp sensor is included in the
airflow meter. With airflow volume and temperature available, mass can be
calculated. (Atmospheric pressure? - for really accurate calculations, it's
needed as well!)
(2)
The insides of the meter - both the mechanicals and electricals - are carefully
designed so that the meter is very sensitive around low loads, where good
driveability requires a high resolution. This means that, strictly speaking, the
output signal is not directly proportional to combustion air intake volume.
(3)
It is a complete and utter fallacy that vane airflow meters are restrictive. In
fact, measurement shows that a large vane airflow meter poses less full-load
restriction than a similar sized hotwire airflow meter. (We'll cover this
intriguing point in more detail another time.)
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Some vane airflow meters work across a 0-12 volt
output signal range but ones from later cars (as in this BMW Motronic system)
work across the 0-5V range that is pretty well now standard for
voltage-outputting airflow meters. (Note: in this context a "0-5V" range doesn't
mean that the meter's output signal will literally be from nought to five volts.
More usually, it will be from about 1 to about 4 volts, though some airflow
meters will output a signal as high as 5.5V.)
With a vane airflow meter, adjusting the output
signal in a coarse way is dead-easy. All that is required is to remove the black
plastic cover (it's glued into place and so a sharp knife is needed
to cut the bead) and then adjust the spiral spring tension. The spring is used
to pretension the vane, so loosening the spring allows the door to open further
for a given airflow volume (richening the mixtures) and tightening the spring
means that the door will open less for a given airflow (leaning the mixtures).
The spring tension is easily able to be adjusted
by using a screwdriver to temporarily pry out of the teeth of a cog (shown as
'1' on this diagram) a small piece of spring wire that normally prevents it
rotating; with this out, the cogged wheel can be adjusted freely to provide more
or less tension.
Make sure that if you are doing this you don't
lose your 'place'. Mark where the wheel is to start with and adjust it only a
few teeth either way of that starting point.
OK, so the first step in changing the mixtures was
to adjust the airflow meter spring tension. But how much to adjust it?
Use that Finger!
One of the beauties of a vane airflow meter is
that the vane can be physically moved when the engine is running. In the case of
the BMW, the rubber elbow in front of the airflow meter could be removed and a
small piece of hooked wire used to pull back the vane while the engine was
idling. (Remember, the BMW was running rich and so we wanted the door to open
less for a given flow volume.)
Pull the door back too far and the car ran with
the staggers - too lean. Push the door a little further open and the car again
ran with the staggers - too rich. In the middle of this range, the car idled
sweetly.
Now, even with no air/fuel ratio measurement at
all, the 'middle sweet spot' for idle is going to be reasonably close for
air/fuel ratios of about stoichiometric - ie 14.7:1. It might be 15:1 or 14:1,
but if you feel what the engine is doing as the vane is manually moved back and
forth, it won't be anything like 12:1 or 17:1.
Or so went my theory, anyway.
I then measured the output voltage of the airflow
meter, so that I could see the voltage output of the airflow meter at this
'sweet spot' for idle. (This is easy to do - just connect a multimeter set to
'volts' between ground and whatever wire on the meter gives a variable voltage
as you waggle the vane.)
I then adjusted the spring tension of the vane so
that at idle, this same voltage was outputted by the meter - but this time
without my having to manually move the vane.
After this adjustment, the car smelt far better
(previously, it was so rich your eyes would water from the exhaust fumes!) and
idled far better as well.
However, while the changing of the spring tension
had leaned idle mixtures, it had also leaned mixtures across the whole
load range. And that's where the use of the interceptor came in.
The Interceptor
The prototype interceptor - called the Digital
Fuel Adjuster - allows major changes to be made to the airflow meter output
signal...if that's required. So in the case of the BMW, the spring tension
readjustment could have been carried out electronically. The reason that it
wasn't is that it's always best to have the meter reading within its normal
range (analogous to the swapping-in of a larger airflow meter on an engine
that's had a major power boost) and
then make the adjustments.
The task of the Digital Fuel Adjuster (DFA) was to
fine-tune the mixtures over the whole load range - something it can do with
ease.
It took only a short time to wire it into place -
only four connections need to be made - and then tuning could start. Or could
it? In this case, some dynamic form of air/fuel ratio monitoring was needed.
The Air/Fuel Indicator
As we have covered in detail in other articles
(The Technology of Oxygen Sensors
and
Cheaply Monitoring Air/Fuel Ratios),
normal zirconia oxygen sensors output a voltage that relates to the air/fuel
ratio. However, as this type of sensor is designed primarily to tell the ECU
when mixtures are richer or leaner than stoichiometric (ie on most fuels, an
air/fuel ratio of about 14.7:1), the voltage abruptly jumps from high to low as
mixtures go from being rich to lean. In addition, the voltage outputted by the
sensor is very sensitive to exhaust gas temperature.
So for the following reasons, measuring air/fuel
ratios with a simple zirconia oxygen sensor is questionable in its accuracy:
-
The output voltage is very non-linear with respect
to air/fuel ratio
-
Temperature variations can make a huge change to
the measured output
-
For a given temperature, the voltage output that
corresponds to a given air/fuel ratio varies from sensor to sensor
This diagram shows the output of a typical oxy
sensor. You can see that either side of the 'jump', the sensor still outputs a
slightly varying voltage as the air/fuel ratio changes. For a fixed
temperature, this variation in voltage can be used to quantify mixtures.
OK, so let's turn all that back into English. For
most sensors, anything around 400 - 600mV can be classed as stoichiometric
mixtures, which is fine for idle and all light load cruising. Under load,
anything less than 600mV will be bad news - too lean. Instead, you want to see
voltages which are at the top end of the voltage scale - whatever that is for
the particular oxy sensor you are using.
Sound imprecise? Yes, it can be - but remember, in
a minute we're going to compare the tuning results of this technique with the
measurements of an expensive air/fuel ratio meter.
The Oxygen Sensor and Reading its
Output
As briefly covered above, the temperature of the
oxy sensor can have a major affect on its output voltage... even for the same
air/fuel ratio. The easiest way of limiting this variation is to place a heated
oxygen sensor in the tailpipe during the tuning. Locating it away from the
engine prevents the huge temp spikes (up to 900 degrees C) which an oxy sensor
close to the engine can be subjected to. Using a heated oxygen sensor, however,
makes sure that it still gets hot enough to output sensible voltages.
In this case, the oxygen sensor used to do the
tuning was sourced from the local tip. Seriously. There I had found someone
disposing of (what looked to be) a Holden V6 exhaust, complete with a 4-wire
oxygen sensor in each pipe. They were happy to give it to me instead of the
landfill. The heater wires (in a heated sensor!) can be easily found by
measuring all the resistances of the wires coming from the oxy sensor - across
two of them there will be a low but stable resistance. In the case of the AC
AFS75 sensor, the heater coil had a resistance of 4.8 ohms. These heater wires
can be connected straight to the battery - after a few minutes, the sensor will
be warm to touch.
And the other two wires? They connect to a digital
multimeter reading millivolts. There are a couple of things to note about the
meter. Firstly, it should be of the high input impedance sort - and that's
pretty well any decent quality digital multimeter. (This characteristic prevents
the meter loading down the signal.) Secondly, it should be able to read a
quickly changing signal. This characteristic does vary a lot from meter to meter
- those with bar graph displays (in addition to the normal digits) are normally
quick sampling.
In my case I used an (expensive) Fluke 123 digital
multimeter/scope which in addition to reading very quickly, can also draw a line
graph showing the input voltage over time.
Tuning
As expected, the oxy sensor voltage at idle was in
the 400-600mV range, so little change was made with the Digital Fuel Adjust
interceptor at idle and light loads. However, at medium loads the oxy sensor
indicated that the mixtures were too lean - voltages were on the low side of the
'jump', rather than the high side. These were easily richened up with the DFA
and the car noticeably improved, becoming more throttle responsive. I then
progressed to high loads, finding again that the mixtures were apparently a bit
lean - about 750mV on the meter.
(To find out how high the oxy sensor would go in
output voltage, I used the DFA to dump in a LOT of fuel and in that way, got a
peak voltage from the sensor of 830mV.)
To be safe at high loads, I wanted a voltage over
800mV, and so altered the mixtures until this was achieved.
Where possible I used the same driving procedure
for each measurement run, helping stabilise the temp of the sensor before making
the measurements.
After those mixtures changes, the car drove very
well and I was confident that it wasn't running lean anywhere. Of course, it may
have been a bit rich, but over-rich will only cause a slight power reduction;
too lean can be engine-destroying.
The Autronic Meter
Lachlan Riddell of ChipTorque was kind enough to
make available his Autronic air/fuel ratio meter for some on-road testing. And
it was with great excitement that I connected up the meter and inserted its
dedicated probe into the BMW's exhaust - how good or bad would my multimeter
tuning prove to be?
In fact, it was very close. At idle and light
loads I had achieved 14.5-14.7 air/fuel ratios, and at medium and full loads the
BMW was running a bit rich - 12.5 and 11:1, where 13.5 and 12.5:1 (respectively)
were preferable. But the multimeter tune was within 1.5 air/fuel ratios of
optimal everywhere, and was 'wrong' on the safe (ie rich) side rather than lean
side.
The graph below shows the two maps of tuning
adjustments made with the DFA - the red bars shows the adjustments made with the
multimeter tune, and the red bars the adjustments with the Autronic meter tune.
As can be seen, the greatest difference is at high loads, where the car was
running richer than necessary.
Conclusion
It's important to note a few points.
Firstly, the BMW (while still developing about
150kW from a naturally aspirated 3-litres) is not a highly stressed turbo
engine. This means it will be able to tolerate incorrect mixtures far better
than some engines. Secondly, the engine is standard (although the exhaust is
quite loud - perhaps some muffler baffles have gone out the tailpipe!) and so
the fuel requirements are going to be fairly close to the mixtures provided by
the airflow meter. In other words, the shape of the fuelling curve will be much
the same as the factory system was designed to cope with.
Both of these factors will not apply to some
modified engines.
So to assume from this story that anyone with a
multimeter and a heated oxy probe up the (car's!) bum can successfully tune any engine on the road
is simply not the case.
But it's also fair to say that even on heavily
modified engines, idle and light load mixtures can be set fairly well using this
technique.
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