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April 16, 2003
News
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Exhaust Gas
Temperature De-Mystified |
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Reprinted with the permission of
National Kart News http://www.nkn.com/ and John Copeland of Fox
Valley Kart Shop - http://www.foxvalleykart.com/
Editor’s Note: I read this article some time ago
and it is right on the money for EGT tuning assistance for
100cc Controlled and Yamaha engines. Having your
Yamaha set lean enough without sticking
the motor can mean a big difference in lap times; I’ve seen as
much as two to three seconds. Since learning to apply the
techniques below and watching my EGT closely, I have saved
several motors from sticking and quickly manage to get
perfectly tuned. Thanks so much to Fox Valley's John Copeland,
Kurt Huber and Curt Paluzzi with National Kart News for
allowing EKN to share this with our readers.
- Rhonda
Mims-Brown
- by Kurt Huber and John Copeland
More and more serious racers are taking an alternative
approach to judging the condition and performance of their
engines. By using exhaust gas temperature they have
added a powerful diagnostic and tuning tool to their arsenal
of racetrack weapons. Exhaust Gas Temperature (EGT) measurement has been a
fact of life in other forms of motorsports for years, yet its
use in karting has been relatively limited until recently.
Here's the skinny.
If combustion was a perfect
process, the exhaust gas from an engine would contain only
nitrogen, water vapor, and carbon dioxide. But in the real
world it also contains carbon monoxide, hydrogen, unburned fuel, other hydrocarbons,
plus traces of aldehydes, alcohols, ketones, phenols, acids,
nitrogen oxides, carbon, and lots of other stuff. And that's
assuming that we're starting with conventional, legal fuel. There have been lots
of things written about illegal additives and how dangerous
they can be. Please think before you or anybody you know adds
anything extra to your fuel. Think about the price
you or your friends or family might pay, health-wise, just
because someone is looking for an edge. But this article is
not about what fuel is composed of, but
rather about the temperature of the combustion products and
how their measurement can be an even bigger edge.
In
the temperature measurement industry there are two basic types
of measurement devices. The first is called resistance
temperature detection or RTD. This type of device is basically
a very fine wire encased in a container, or bulb. As the
temperature of the bulb changes, the electrical resistance of
the wire changes. By passing a small current through this wire
and measuring the resistance, the temperature can be
determined. This is the method used by your Digatron cylinder head temperature gauge. And as
any of you knows who've used a CHT GAUGE for any length of
time, these CHT sensors, while accurate,
are relatively delicate. Rough service (like on a kart) is not
generally recommended. They also have a temperature limit that
makes them unsuitable for use measuring EGT.
The other major
means of temperature measurement is the thermocouple. The
thermocouple is a unique device. There are several different
types of thermocouples, using different materials for
different temperature ranges, but they all operate by the same
basic means. A thermocouple consists of two wires, of
different materials, welded or fused together. For the
temperature range we are most interested in, the type K
thermocouple is most suitable with a maximum temperature of
1900 degrees Fahrenheit. In a type K device one wire is an
alloy called CHROMEL®*, and the other an alloy called ALUMEL®*. A small portion of each wire is exposed and the two
are welded or fused together. That assembly is encased in an
electrically insulated sheath and the other ends of the wires
are connected to a very sensitive voltmeter. Now here's where
the thermocouple differs from the RTD. When the fused end of
the thermocouple wire is heated, it generates its own current.
It's only a matter of millivolts (that's one one-thousandth of
a volt), but the voltage generated is an accurate indicator of
the temperature of the end of the thermocouple. A real bonus
for motorsports is that these thermocouples are remarkably
sturdy and reliable. With no delicate parts to break, unless
you exceed their maximum temperature, they're pretty hard to
damage. In fact, every gas- fired furnace and water
heater uses one to tell the gas valve that the pilot
flame is lit.
The thermocouple probe is carefully
fitted into the exhaust system, relatively
close to the engine. For maximum accuracy
you want the tip of the thermocouple to be centered in the exhaust gas stream as it comes out
of the engine. But there is
considerable debate about how close to the engine it needs to be.
Digatron's information advises mounting the probe between 3
and 4 inches from the piston face. But many
snowmobile racers routinely set their EGT pickups as much as 8
inches from the exhaust port. In fact, it
really doesn't matter exactly where the probe is mounted,
although the closer to the exhaust port, the less the
ambient air temperature will cool the header and affect the
readings. One word of caution, however. Comparing EGT readings between engines
or karts whose EGT probes are not mounted
exactly the same distance from the piston will get you in
trouble. If you use EGT, mount the pickups in
all your headers at the same length. Otherwise you might just
misinterpret the readings.
On the subject of mounting
the EGT probe, there is some
concern among 4 cycle racers about disrupting the gas flow in the relatively
small diameter headers that are most common on 4 stroke
engines. Introducing the probe, with a diameter of about .125
inches, about 1/2 inch into a 1 inch diameter header will
consume about .0625 sq. inches of header cross section. That's
about 8% of the total area. To test just what effect this
might have on the absolute flow, we checked it on the flow
bench. Our testing revealed that fitting a Digatron EGT probe into a .990
diameter header reduced the flow by __%. This is approximately
the same flow as using a .960 header. You may want to factor
that into your pipe selection if you're
going to use EGT on your 4 cycle. You'll
also want to factor in the value of knowing what your air/fuel ratio is doing versus
whatever minor loss is exhaust flow there might be.
There are lots of myths and questions surrounding EGT and its use. Some folks
figure that, if you have a Cylinder Head Temperature gauge (CHT), you already have all
the information you need, and that EGT is redundant. While you
can certainly get by on just head temp, CHT and EGT each tell you slightly
different things, and using them together tells you some
things that neither one could tell you alone. EGT has some advantages
because of its basic construction and its mounting location. A
thermocouple responds very quickly. Because the CHT sensor has to respond to
the temperature on the outside of the head, it cannot respond
to changes in combustion temperature as fast as the EGT probe that is directly
in the exhaust gas stream. Secondly, the EGT probe is not exposed to
the outside air; it is not affected by changes in outside
temperature. By comparison, since the CHT is measuring the
temperature of the cylinder head casting
itself, and since the cylinder head is one of the
engine's primary means of
shedding heat to the air, the cooler the air, the cooler the
CHT reading and vice versa.
For quick, consistent temp readings, EGT is definitely worth a
look.
But what exactly are we trying to determine with
these temp sensors, anyway? EGT and CHT are simply ways of
trying to judge the relative fuel/air ratio. We all know
how critical it is to have the carb mixture correct, whether by
changing the jet in a 4 cycle, or by adjusting the carb needles on a 2 cycle. And
it's generally agreed that the leaner the mixture, the hotter the engine will run. But what is
really happening inside there? Does hotter always mean better,
or just sometimes?
Well, the truth is, it's mainly a
matter of air. Many of you have had the experience of hitting
the set-up just right in practice and then waiting
excitedly for the race, certain you're going to blow 'em all
away this time. But when the time comes for your race to
start, suddenly you've lost that wonderful top-end RPM you had in practice, or the clutch just won't pull like
it did in practice, or some other
problem pops up to spoil your day. You haven’t changed a
thing, but the air may have changed things for you! As the air
temperature goes up, or the humidity goes down, or a storm
front blows in, the density of the air changes, and that
changes the fuel/air ratio that your carb delivers. If you don't
recognize what's happening and adjust accordingly, you’re
going to suffer.
So how can you stay on top of the
effect that changing air conditions is having without bringing
your own weatherman with you to the track? With an EGT gauge you can take a lot of
the guesswork out of carb tuning. 'Remember we
said that it was generally agreed that a leaner fuel/air ratio was always
hotter. And when we asked if hotter was always better? Well,
you guessed it, neither one is true. If you get the fuel/air ratio too lean, the combustion
temperature will actually go down! Let's look at another
example of this, one that you can actually see with the naked
eye. An Oxy-Acetylene torch will burn with a wide variety of
fuel/air ratios. Generally
when you light the torch the mixture will have too much
fuel (acetylene) for the
amount of oxygen that's flowing. The flame will be yellow and
produce a lot of smoke, and not be very hot, relatively
speaking. But as you turn up the oxygen valve, the yellow
flame and smoke disappear, the flame turns bright blue, and
the flame temperature goes up dramatically. So leaner here is
definitely hotter. But as you continue to turn up the oxygen,
the flame begins to shrink, and the flame temperature actually
goes down, even though it's leaner! Eventually, if you keep
turning up the oxygen, the flame will just go out! Believe it
or not, the same thing happens inside your engine.
"Wait a minute,"
you say. "I know that when I lean the engine out it just keeps
getting hotter until it sticks!" If all you have to go by is
CHT you're absolutely right.
When your engine gets too lean, the skyrocketing
temperature you see on the CHT is probably not really
an indication of hotter combustion. Most likely it's a warning
sign of DETONATION. Detonation is the collision of two flame
fronts inside the combustion chamber, where there should be
just one, and it's the single biggest cause of heat related engine failures. Savvy
drivers can often sense that an engine is slowing down and
richen up the mixture to control the
detonation. But you don't need decades of experience to spot
detonation before it puts you on the trailer for the day. Just
like with the Oxy-Acetylene torch, when the mixture gets too lean, the flame temperature
goes down! Detonation floods the combustion chamber with heat,
so the CHT goes up, but with CHT and EGT readings, if you see CHT rising and EGT going down, it's a sure
sign of detonation.
A quick adjustment will restore
the power and save that expensive rebuild. Even with just EGT, it's a lot easier to
get the most out of your engine without burning it
down. EGT should climb as the RPMs
come up on the straight, and then drop when you lift for the
corner. If it drops when you're pulling off a hard corner, or
under acceleration, you're on the detonation expressway back
to the shop for a rebuild. Detonation is a fascinating
subject, one that is too complicated to be handled adequately
here. We'll save that for another article. But trust that it
is something to avoid, and the best way to avoid it is to
watch the exhaust gas temperature.
So
to summarize, we know we want to run the fuel/air ratio as close to
ideal as possible. And we know that the ideal fuel/air ration should
produce the hottest combustion flame. While the cylinder head temperature
gives us some indication of the combustion temperature, it can
be misleading because of air temperature or other weather
conditions. Because of the mass of the cylinder head, CHT can take a few seconds
to register a change in internal temperature. And CHT alone is not the best
indicator of detonation. Exhaust gas temperature does all
these things better that CHT; better, faster, and
more reliably. So what's holding you back? If someone came up
with a clutch that was better,
faster, and more reliable, you'd be after it in a second. Why
is this any different? Remember, the more you know, the faster
you go.
*CHROMEL® and ALUMEL® are registered trademarks of
Hoskins Manufacturing Company. |
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Kurt
Huber and John Copeland
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Copyright © 2002 Ekartingnews.com. All
Rights Reserved.
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