- One of the techniques that's often used for individual cylinder tuning is the addition of
individual cylinder exhaust gas temperature sensors and as their name implies, these senors
measure the temperature of the exhaust gases.
They're typically mounted quite close to the cylinder head exhaust header flange and they're
mounted in the primary runners of your exhaust manifold.
The idea behind them, by measuring the exhaust gas temperature of the combustion
charge exiting the exhaust valves, this is going to give us some insight into what's going on
inside of the cylinder.
Now particularly years ago when individual cylinder lambda sensors or wideband air/fuel
ratio controllers in general were a little bit rarer and much more expensive, this was a
technique that was widely leant on for individual cylinder fuel tuning and in
particular we used this technique on a number of our world record holding drag cars.
The issue here, particularly with a small capacity engine when we're pushing it to the
limit with very high boost pressure and also if we're running on methanol fuel which is
not very tolerant of a lean mixture, our tuning window becomes very narrow so we want
to make sure that every single cylinder is running a safe rich air/fuel ratio for
reliability.
Typically with our fuel tuning we are still relying, even with EGT sensors on a single
exhaust lambda sensor that's fitted post turbo or post collector.
The problem with this is that it's reporting the average air/fuel ratio across all of the
cylinders so for example on a 4 cylinder engine it's quite possible that we'll have 2
cylinders that are right on our air/fuel ratio target, one too rich and one too lean
but the average being reported by the collector or post turbo air/fuel ratio
sensor is still going to be right on our target.
Now in a road car or a lightly tuned engine that's not making huge power, that's probably
not a huge concern but as our boost pressure climbs, our tuning window does become
much narrower.
The principle of operation with the exhaust gas temperature sensors or how we utilise them
is that as our air/fuel ratio richens up the exhaust gas temperature cools down because
our combustion charge temperature is reduced.
Conversely as we lean our air/fuel ratio out towards the stoichiometric air/fuel
ratio, the combustion charge temperature increases and so does our exhaust gas
temperature.
That's all well and good but there are another few intricacies to consider here.
For example the amount of fuel and air being burnt in different cylinders will also affect
the exhaust gas temperature.
What this means is that even with a consistent air/fuel ratio across all of the cylinders on the
engine, if we have one cylinder which is being favoured with more air than the other
and we're adding more fuel to go along with this, the exhaust gas temperature
because we have more fuel and air being burned in that cylinder, is likely to be higher.
Also the exhaust gas temperature is not just affected by our air/fuel ratio, it's also affected
by ignition timing, it can also be affected by the coolant distribution around the
cylinder head and the engine.
So it's not a really rock solid guide on what our air/fuel ratio is.
Still, it is a technique that is used and it's certainly better than having absolutely
no idea so let's have a look at a couple of exhaust gas temperature sensors, we'll see
what they are and we'll talk about how they work.
Here we've got a couple of quarter inch diameter exhaust gas temperature sensors.
These also are available in 3/16th which is a smaller diameter.
I personally haven't found them to be that reliable in the smaller diameter so I tend to
stick to the larger quarter inch.
This particular sensor here is what's referred to as an enclosed tip sensor.
No big surprise there, the actual sensing tip is encased.
On the other hand, the sensor we've got here is an exposed tip so we can actually see the
junction between the two dissimilar metals which is what we'll talk about in a second,
that is exposed to the exhaust gas flow.
Now there's pros and cons with both types of sensor there.
In my own personal experience with the high boost drag engines that we've used
these on, we've found that the exposed tip sensor, while it does respond a little bit
faster to changes in exhaust gas temperature, tends to be a little bit less reliable and we've
actually had the sensors fail and even the exposed tip break off and go through the
turbocharger.
Normally doesn't end too well so I tend to favour the enclosed tip.
However, for a naturally aspirated engine, perhaps where the exhaust gas temperature
isn't quite going to be so great, you may be able to get away with the exposed tip.
The advantage of the exposed tip is that because the sensing junction is right out
there in the exhaust gas flow, it can respond faster to changes.
The other thing I've seen between the 2 different types of sensor is that the exposed tip tends
to actually read a little bit hotter.
And this is one of the questions we quite often get asked, what exhaust gas temperature
is considered safe?
And unfortunately that's a difficult question to answer.
Obviously I've just talked about there the fact that the exposed tip sensor tends to read a little
bit higher but the actual exhaust gas temperature reading that we're looking at is also going to
vary depending on where abouts in the exhaust manifold we place the sensor or how far from
the exahust valve it's placed, the further away, the cooler the temperature tends to be.
Also how far we've got the sensor protruding out into the exhaust gas flow.
If it's sitting just off the primary wall, we'll tend to get a cooler reading than if
it's exposed right out into the centre of the exhaust gas flow.
So as you can see, there's a few aspects there and using it for an absolute value for safety
can be a little bit problematic on this basis, however when we're familiar with a particular
engine and a particular type of sensor and location, we do tend, over time to build up
a bit of an understanding of what a safe exhaust gas temperature for that combination
is going to be.
While we've talked about the sensors briefly there I haven't really discussed how they work
or the operating principle behind them.
And this is important to understand.
So what we're looking at here is a junction between 2 dissimilar metals, on one side we
have a metal called alumel and on the other side we've got a metal called chomel.
And when that dissimilar junction with the metals is exposed to temperature,
it creates a very small voltage that is relative to the temperature that the junction is
exposed to.
Now a subtle aspect here that's important to understand.
We actually have 2 junctions, we've got our hot junction which is at the tip of the sensor
but we've also got a cold junction which is at the other end of the sensor, the connector,
let's have a look at that.
So this particular sensor here, we can see is not terminated so this could go into something
like a DTM connector.
On the other hand, the other sensor we've got here, this has our typical thermocouple
style connector with 2 pins of different sizes so we can't plug them in around the wrong way.
So regardless, this junction here becomes the cold junction.
What this means is that we're not actually just measuring the temperature at the exposed tip,
what we're really doing is measuring the difference between the temperature at the
exposed tip and the temperature that the cold junction is exposed to.
Now that might sound like I'm splitting hairs but quite often that cold junction, particularly
with the second sensor we just looked at with the plug in connector, that cold junction is
likely to be in the engine bay.
Particularly if the cold junction is relatively close to the exhaust manifold or the turbocharger
then it's quite likely that it still may be exposed to temperatures in the range of 50 too 80°C or
even higher.
So that understandably has a big effect on our actual measured exhaust gas temperature.
Now there are ways around this as well.
A number of the thermocouple amplifiers that are used with these sensors, and we'll talk a bit
more about thermocouple amplifiers in a second, will be provided with either a built
in cold junction temperature compensation sensor or alternatively you may be able
to add one yourself.
So in this instance it actually compensates for the temperature at the cold junction correcting
the reading that we're seeing.
I know it sounds a little complicated there but it's just important to understand the
implications of that cold junction.
On that note, if we are using a sensor like this which can be terminated in either a
DTM style connector or maybe even an Autosport connector, there are still implications
around the cold junction.
If we simply terminate it at a DTM connector and then use conventional wire to run the
sensor down into our ECU or dash logger, then the cold junction will become the connector.
In some instances we may not want to do that.
It is possible to extend the thermocouple wiring down to the ECU or datalogger using
the proper thermocouple wiring but we also need to match this with correct terminals for our
connector, otherwise we'll still end up with that cold junction inside of the engine bay.
Really comes down to what you're trying to achieve and understanding the implications
of your decisions around how you go about that.
Less of an issue if you are plugging straight into a thermocouple amplifier that provides cold
junction compensation though.
Now we've talked about thermocouple amplifiers but we haven't gone into too
much detail.
On their own, the thermocouple is typically not that much use.
The voltage that it generates as it's exposed to a difference in temperature is very small
and for the ECU or datalogger to be able to actually make sense of that signal, it needs
to be amplified into a 0-5 volt signal.
This is where a thermocouple amplifier comes in.
We've got a couple of options here to look at.
The first one we've got here is a Haltech TCA8 and as its name implies, this can take up to
8 thermocouples into the bottom.
The thermocouples plug in using the common yellow 2 pin connector that we've
already looked at.
On the top of the TCA8 we can see we've got a simple connector.
And this particular unit actually works via CAN.
This means that that connector only needs to be provided with 12 volts and
ground plus CAN high and CAN low and we can then transmit the thermocouple
information to any CAN enabled device.
This means that we don't necessarily need to be using a Haltech product for our dash logger
or ECU provided we know the CAN message template for the TCA8, we can read this
into any device that has a user configurable CAN template.
So it's a nice neat package and it's really easy to work with.
The alternative though is to use an analog voltage thermocouple amplifier.
Let's have a look at one of those.
This style of sensor is available from a variety of manufacturers.
This one is from Motorsport Electronics so we can see that we've got a remote 2 pin connector,
again this mates to the standard thermocouple connector.
It's terminated in a 3 pin DTM connector and this simply needs to be provided with a regulated
sensor 5 volt, a sensor ground and then an analog voltage output which varies between
0 and 5 volts.
This actually has a built in cold junction compensation in it as well.
So we're going to get accurate data at our ECU.
One consideration with using an individual thermocouple amplifier like this is that
we're going to obviously need a spare analog voltage input for each EGT sensor that we
want to add.
Also if we are going to be adding individual EGT sensors for each cylinder on a 4, 6 or
perhaps an 8 cylinder engine then the packaging of the individual thermocouple
amplifier can get a little bit messy when dealing with so many thermocouples
that we're going to be adding.
In that case it may make more sense for you to consider one of the multi thermocouple
amplifiers such as the TCA8 that we've looked at.
As I mentioned at the start, EGT sensors are typically used for individual cylinder tuning
but we've also mentioned that EGT often isn't a really rock solid guide on air/fuel ratio.
To highlight how this works, we'll have a look now at some data from one of our in house
test vehicles, our Toyota 86 which has both individual cylinder EGT and individual
cylinder lambda, so let's have a look at that data.
The data we've got here in MoTeC's i2 analysis software is taken from a run on
our dyno so let's have a look at what we've got.
So we start obviously at the top here with our ramp run and we're running from around
about 2000 RPM out to 7000 RPM.
Our next plot here is the throttle position and we can see that right through that ramp
run, the throttle is basically wide open as you'd expect.
Our next graph down is our inlet manifold pressure, this is actually an engine that's
fitted with a turbocharger so we can see the turbo boost is relatively low,
we're running around about 50 kPa of positive boost pressure.
Next set of graphs we've got here are our individual cylinder lambda sensors and
these come from a Link CAN to lambda sensor being fed into the MoTeC CDL 3 dash.
So we can see in the start of the run here where we haven't got any boost, our air/fuel
ratio or lambda's sitting around about 0.95 as we move through the run we're targeting
richer which would be typical as our boost comes in, down to around about 0.78 lambda.
Our next set of graphs down the bottom is our exhaust gas temperature so we can see what
each cylinder is doing.
Now the important thing to note here is if we look at our lambda, particularly near the
end of the run up at 7000 RPM, we can see that our lambda is actually spread by about
0.02 so it's around about a 2% spread across all 4 cylinders so that's pretty accurate,
they're pretty closely tuned there.
If we look on the other hand though at our EGT we can see that our EGT is varying by almost
30° from hottest to coldest.
Now the principle we know here is that when our air/fuel ratio is leaner or leans out towards
stoichiometric, the exhaust gas temperature will climb so what we would expect here,
given that we are operating on the rich side of stoich, as we move leaner the
exhaust gas temperature should climb.
If we look at our data, this isn't actually holding true.
We can see in our data here that our hottest cylinder, the green cylinder which
is cylinder 2 is sitting at about 729°C, let's call it 730°C.
However if we move up to our lambda we can see that cylinder 2 in terms of the
air/fuel ratio it's running is 0.77 lambda which is equal with the other richest
cylinder.
So our richest cylinder in this case is actually also our hottest.
Now that's not to say that EGT is useless, it's definitely better than having no insight
at all as to what's going on inside of the engine, however we do need to understand,
its limitations when it comes to accurately inferring our air/fuel ratio from the EGT data.
The other aspect we do need to be mindful of when we are fitting the EGT sensors is
exactly where abouts we are putting them.
And this can have a big impact on the accuracy from cylinder to cylinder.
In order to get really accurate results, it's essential that the EGT sensor is
exactly the same distance from the exhaust valve on each cylinder and that the
protrusion into the exhaust manifold is also identical, I've already touched on the
fact that these 2 aspects can affect our EGT reading.
So where do we put these?
Generally we want the sensors mounted relatively close to the exhaust manifold
header flange for the fastest response.
Even so, the EGT sensor compared to lambda is relatively slow to respond.
The other aspect there is the protrusion and while there is no absolute rock solid
rule here, on a turbocharged engine I tend to fit the sensor so that it's only
protruding just into the exhaust manifold runner.
On the other hand, with a naturally aspirated engine I'm more inclined to fit the sensor so
that it protrudes right into the centre of the runner.
If we extend the sensors right into the centre of the runner on a turbocharged engine,
we can end up with reliability issues and they can also pose a restriction to exhaust gas
flow, particularly if we're using the larger quarter inch diameter sensor on a relatively
small exhaust primary.
Interestingly one of the uses that's possibly overlooked often with individual cylinder
EGT sensors is the ability to figure out what cylinder you've got a misfire on.
Typically if we've got a misfire we're going to end up seeing the exhaust gas
temperature on that cylinder drop quite rapidly so it's a really quick and easy way
to zero in on exactly which cylinder we've got a problem on rather than going
through the task of removing all of the plugs individually, looking at each one and maybe
swapping coils between cylinders to try and fix a problem.
So there's some insight into EGT sensors, how they work, how we can utilise them, how we can
integrate them into our ECU or logger package and also the limitations of the
data that we're seeing from them.
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