Last week in
Monitoring Factory Oxy
Sensors, Part 1 we looked at factory narrow band sensors – the most common
sensors found in cars. Now it’s time to examine how you can monitor the
wide-band sensors that are becoming much more common, especially in hybrids and
super economical cars.
How Wide Band Sensors Work
As their name suggests, wide band sensors operate
over a much broader range of air/fuel ratios than narrow band sensors. This is
especially important in cars that run ultra-lean cruise capability, where the
air/fuel ratios must still be monitored by the ECU when they are beyond the
measuring capability of narrow band sensors.
Wideband sensors are used by aftermarket air/fuel
ratio measuring instruments, and there is plenty of material available on the
web about these sensors and how they work. The trouble is, most of the
explanations are largely incomprehensible to normal people!
However, to understand how to make use of an
existing, in-car wideband oxygen sensor, you don’t need to know anything much
about the sensor chemistry or control electronics.
A wideband oxygen sensor consists of two sections.
These comprise a pump cell and a reference cell. The pump cell has a chemical
effect on the behaviour of the reference cell, as indicated here by the green
arrow.
The reference cell’s output voltage is monitored
by the ECU. The ECU aims to achieve an output by this cell of 450mV – just under
half a volt. The other cell, the pump cell, is controlled by the ECU varying the
current flow through it.
If the reference cell’s output is not 450mV, the
ECU adjusts the current through the pump cell until the reference cell is again
back at 450mV. If the reference cell is at 450mV, no current is passed through
the pump cell. In addition, the direction of current flow through the
pump cell can be changed.
If the output of the reference cell is 450mV
without any current flow being needed through the pump cell, the air/fuel ratio
must be at stoichiometric (14.7:1 for normal petrol). If current flow is needed
in one direction before the reference cell gets to 450mV, then the mixtures must
be lean. If current flow is needed in the other direction, then the mixtures
must be rich. The magnitude of this positive or negative current flow reflects
how lean or rich the mixtures are.
So how does the ECU control the current flow? Have
a look at this diagram. We’ve added one more wire, a constant voltage supply.
If the ECU places a voltage of 2.7 volts on the
pump cell current supply wire (blue), then no current will flow through the pump
cell. (That’s because 2.7 volts is pushing against 2.7V.)
However, if the ECU raises the voltage on the blue
wire to (say) 3.2V, then current will flow through the cell in the direction of
the arrow.
Conversely, if the ECU lowers the voltage on the
blue wire to 2.3V, current will flow the other way.
Monitoring the Output of Wideband
Sensors
As described above, the amount (and direction) of
current passed through the pump cell by the ECU is indicative of the measured
air/fuel ratio. However, measurement of this current is most easily achieved by
measuring voltages rather than currents.
As we showed above, in order that current flows
through the pump cell in an anti-clockwise direction, the ref voltage has to be
higher than the pump cell voltage. Therefore, if we measure the voltage between
the pump cell and ref voltage wires, we’ll find a reading of (in this example)
+0.5V.
But if the current flows the other way, the
voltage will need read a minus value – in this case, -0.4V.
You can therefore see that if the output of the
oxygen sensor is to be measured, you would normally need a meter that reads both
positive and negative voltages. (And another point: with the meter organised in
this way, higher voltages indicate leaner mixtures – the opposite to a narrow
band sensor.)
Reading both positive and negative voltages can be
achieved by a normal multimeter, but (as with conventional oxy sensors), if the
reading is changing fast, a multimeter is hard to read accurately. It’s also
awkward to fit a multimeter on your dashboard!
So why not use the LED Mixture Meter that we
described in Part 1? There’s a big problem with trying to do so - this display
cannot show negative voltages, so all the mixtures richer than 14.7:1 will not
be shown. (In addition, the voltage range of the Mixture Meter is not likely to
be correct.)
However, there is a very simple trick that can be
employed to allow us to use a meter that measures only positive voltages.
We already know that if the pump cell voltage is
the same as the reference voltage (2.7V is the examples we’ve been using here),
no current will flow in the pump cell. If the pump cell voltage rises
above the reference voltage, current will flow in one direction. If the
pump cell voltage falls below the reference voltage, current will flow
the other way.
Therefore, by measuring just the pump cell
voltage, we still know the variation in mixture strength!
If the pump cell voltage is above 2.7V (or
whatever the reference voltage is), the mixtures must be lean. If the pump cell
voltage is below 2.7V, we know the mixtures must be rich. And, best of all,
these voltages all stay positive! But how can we easily display these values?
Using a LED Bar Graph Display
As covered in
Universal Bargraph Voltage Display, a very versatile
automotive LED bargraph kit already exists. It’s called the Automotive Voltage Monitor.
In kit form, the bargraph display costs just AUD$19.95; the kit is also
available pre-built for AUD$76.
What makes the bargraph kit particularly suitable
for this application is that the voltage at which the top and bottom LEDs light
up can be independently set, the display then automatically scaling the
‘in-between’ LEDs into ten equal steps.
Let’s take a specific example. On the hybrid Honda
Insight, over a range of different driving situations, the monitored pump cell
voltage varies between 2.0V and 3.8V. Therefore, the bar graph display needs to
be set so that the bottom LED lights at 2.0V and the top LED at 3.8V.
It’s as simple as that!
(In other cars, where the reference voltage may be
different, the top and bottom LED adjustments can be altered to suit.)
After the range has been set correctly, the
Automotive Voltage Monitor is then connected to power and ground, with its input
signal wire connected to the pump cell control lead.
OK - but let’s take a few steps back.
Locating the Right Wire
The easiest way of finding the wire on which the
varying voltage signal is present is to directly measure it with a multimeter,
with one lead grounded to the chassis and the other lead used to do the probing.
The oxy sensor should be plugged in and working and the car running.
On one oxygen sensor wire you will find a voltage
of about 0.45 volts. It changes only a little with driving, and is nearly
constant at idle. That’s likely to be the reference cell voltage.
On another wire you’ll find a fixed voltage – in
the Honda’s case, 2.7V. This voltage does not vary with driving - it’s the
constant voltage.
Finally, you’ll find a wire on which there is a
small voltage variation (eg 2 volts total) that, with different driving styles,
occurs above and below the reference voltage. This is the pump cell
control - the one to monitor for watching air/fuel ratio variations.
In addition, there is likely to be three or four
other wires. These are for a calibration resistor and the oxygen
sensor heater. Both the heater and the trim resistor are likely to have one
side grounded to the chassis.
On a Car
So how does it all work on a real car? Let’s have
a look at the Honda Insight.
This diagram shows the Honda’s wide band sensor
connections to the ECU. (Note that the diagram has been revised slightly to fit
into a smaller screen space – the factory wiring colour codes shown are at the ECU not the
sensor.)
The heater wiring is indicated with a green arrow,
and the calibration resistor with a red arrow.
The centre lead (marked with the blue arrow) must
be the reference wire (2.7V) and so therefore the pump cell control lead must be
one of the wires marked with purple arrows. In fact, it is the right-hand wire (green on
the actual car) that is the correct wire to monitor.
Clearly, it makes things a lot easier if you have
a wiring diagram available, but even in its absence, some good detective work
with a multimeter should still locate the correct wire.
Numbers?
As
with the narrow band display we covered in Part 1 of this series, the bargraph
display described here does not read out the numerical air/fuel ratio. That is,
it doesn’t show 13:1 – or even 16.7:1.
Many
people will see this as a major deficiency, but I do not. Unless you are tuning
the action of an engine management interceptor or revised engine management
software, you have no need to know the exact numerical values. Knowing whether
the car is running at stoichiometric, rich, very rich, lean or ultra-lean is
what’s really wanted to be known.
However,
if you know the leanest that the car ever runs, and the richest the car ever
runs, some numbers can be worked out.
In
my Honda, use of an external MoTeC air/fuel ratio meter shows that, when the
injectors are actually operating, the richest air/fuel ratio is about 12:1 and
the leanest, about 25:1.
As
I have the display set up, that corresponds to one end LED being 12:1 and a LED
seven along the bargraph being 25:1. Therefore, the intervening LEDs can be
approximated as each being indicative of 2 air/fuel ratios up or down from these extremes. (Note: however, wide band
sensors don’t have identical response either side of stoichiometric.)
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Display
The Automotive Voltage Monitor kit uses
rectangular LEDs mounted along one side of the board. The kit can therefore be
mounted in a small box, with the LEDs protruding through a slot cut in one side
wall. The completed assembly is small enough to mount on top of the steering
column, or in some other convenient place on the dash.
Alternatively, you may choose to remote-mount all
the LEDs, connecting them via ribbon cable to the module. If you do this,
remember that the LEDs have to be wired with the correct polarity, and mounted
in the correct sequence. Remote mounting the LEDs means that they can be
squeezed-in almost anywhere. You can even use 3mm round LEDs (making the
finished display even smaller) and arrange the colours of the LEDs however you
wish.
Another approach is to do what we did, and that is
to remote-mount only a couple of the LEDs (arrowed). In the Honda Insight in
which the system was installed, it was decided that only two conditions needed
to be seen. The first was when the car was in lean cruise – that is, running
with an air/fuel ratio of about 25:1. The second condition was when the engine
management dialled-up a very rich air/fuel ratio – about 12.5:1.
So how is this achieved? With the upper and lower
pots on the Automotive Voltage Monitor kit set so that one end-LED of the full
bargraph display illuminated at the lowest voltage ever measured (2.0V - rich)
and the other end LED illuminated at the highest voltage ever seen (3.8V –
lean), the LED that turned on in lean cruise was found to be the fourth from the
‘3.8V’ end.
This ‘lean cruise’ LED was removed and extension
wires soldered to the PCB pads to allow a LED showing this condition to be
mounted on the steering column. This LED lights only in lean cruise –
even leaner mixtures (eg indicating injector shut off) cause the next LEDs along
in the bargraph to light, so the ‘lean cruise’ LED stays off (or flashes only
momentarily as the mixtures slide past this point).
At the rich end of the scale, the on-board pot was
set so that the end LED illuminated. Again, by means of extension wires, this
LED was then moved to the steering column. (In fact, round 5mm LEDs were used
rather than the original kit’s rectangular LEDs.)
To ensure that these ‘monitor’ LEDs were in fact
reflecting the designated air/fuel ratios, checking was carried out using a
tail-pipe mounted MoTeC professional air/fuel ratio meter. This showed that yes,
the ‘lean cruise’ and ‘rich’ LEDs worked perfectly – in fact, they were faster
to respond than the MoTeC meter.
So what if you don’t have an expensive MoTeC
meter? The LEDs can still be calibrated correctly if:
-
You set the on-board posts so that the lowest and
highest voltages that ever occur are displayed on the respective end LEDs of the
bargraph display.
-
You drive the car in a variety of conditions,
while carefully observing on the full bargraph display what mixtures are being
run in what conditions. Doing this makes it easy to see when lean cruise occurs
(if in fact it does in your car), and when richer mixtures are
used.
-
You remote mount those LEDs that show the
conditions you want to monitor.
Note: from a cold start, the display can take a
minute or so to come alive, the time the oxy sensor takes to come up to
operating temp.
Adding
Another Monitor LED
In
the main text we’ve said that on the Honda Insight, two LEDs from the bargraph
display were remote-mounted on the steering column – one showing lean cruise,
and the other showing rich mixtures. So why in the pics are there three
dashboard LEDs?
In
this car, the other very useful indication of what is going on is to show the
action of the Exhaust Gas Recirculation valve. This is displayed by the third
LED.
As
we covered in
Tweaking the EGR, Part 1 and
Tweaking the EGR, Part 2 the Honda (and probably
many other cars) can be improved in fuel economy by running, at small throttle openings, greater-than-standard
amounts of Exhaust Gas Recirculation (EGR).
The
Honda uses a 12V pulse-width modulated EGR valve and by wiring the LED (and
suitable dropping resistor) in parallel with the EGR valve coil, the LED
illuminates when the EGR valve is open. Furthermore, it increases in intensity
as the valve opens to a greater degree.
The
only tricky aspect was setting the value of dropping resistor so that, at
maximum brightness, this LED looked the same as the LEDs being driven by the
Automotive Voltage Monitor. To achieve this, a 1 kilo-ohm pot was wired as a
variable resistor and this was adjusted until the correct brightness was
achieved. Note: you cannot use a LED without the dropping resistor – it will
immediately die if connected straight across 12V.
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Using the Display
As with the simple Mixture Meter that monitors a
narrow band sensor (covered in Part 1 of this series), the monitor for wideband
sensors can be used to aid fuel economy and also show that the engine management
system is working correctly.
The first step is to simply watch the display over
a wide variety of driving conditions. From this, you’ll soon learn when the car
is in closed loop, when it is in lean cruise (if ever), when the mixtures are
enriched, and when the injectors are switched off. (For more explanation on
these terms, see Part 1).
In most cars, the greatest gain in fuel economy
will be made if the fuel enrichment that occurs at high loads is rarely seen!
For example, when climbing a hill, you might find that lifting your foot just a
fraction causes the ECU to switch back to closed loop from enriched mode.
Lifting your foot may make almost no difference to your speed up the hill, but
will reduce fuel consumption.
Moving to higher gears earlier may result in the
car staying in closed loop; in other cars it will be better to rev slightly
higher at a reduced throttle angle before changing up.
In some cars, the injector shut-off does not
function unless revs are above a certain level. When travelling down a long hill
with no throttle being applied, in these cars it can be better to drop down a
gear, so lifting no-load revs and triggering the injector shut-off.
In other cars, the clear indication of when the
car is in lean cruise allows the driver to better keep the vehicle in that mode
– to put this another way, it’s much easier to see in what conditions the car
enacts lean cruise and in what conditions it leaves this mode.
In performance applications, the simple bargraph
monitor of the wideband sensor will give an accurate indication of the relative
mixture strength.
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It
appears that when in closed loop, the output of a wideband sensor does not
fluctuate in level like a narrow band sensor. This is almost certainly because
the ECU doesn’t need to actually pass through 14.7:1 before the sensor output
changes.
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Conclusion
With the techniques covered in this series, both
narrow band and wideband oxygen sensors can now be easily and cheaply monitored.
The monitoring displays are not only suitable for spotting engine management
faults as they develop, but by showing the driver the most economical techniques
to use, can also improve real-world fuel economy. Finally, in cars that are
modified, the wideband sensor display can be used for the tuning of relative
mixtures, and to show if a dangerous high load lean condition is occurring.
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