EGT

[Gregory R. Travis]

On Tue, 1 Apr 1997, Tom Cloud wrote:

> (BTW, saw your TLC pictures.  Mebbe you otter consider
> going domestic **** to those on the Bronco list, see:
> http://gtravis.ucs.indiana.edu/Personal/Images/lndcru.gif

Fat chance.  I just acquired another project car - a 1967
Volvo 1800 (little "sports" car)  The engine comes out next
week for a 0.187" overbore.  Vroom!

> Anyways ..... I wanna know what you can tell me (us)
> about EGT sensors.  What are they made of (type TC, what
> are they encased in, etc), where does one get them, is there
> any readily available, reasonably priced (stupid question)
> instruments to read them?  What can be reasonably deduced
> from them?

EGT probes are common as dirt in aircraft circles.  There are at
least two different companies that make them.

Typically, they consist of the actual thermocouple which is encased
in a stainless-steel tube about 1/8" in diameter.  The tube is closed
at one end and has a flange at the other.  Exiting the tube, at the
flange end, are two wires, each with an ordinary spade terminal.

On aircraft, a small hole (just slightly larger than the diameter of
the EGT tube) is drilled in each exhaust pipe (there is one for each
cylinder on aircraft) about 2-3" below the mounting flange on the
cylinder itself.  The probe is inserted into the pipe and held in place
with a stainless hose clamp.  The hose clamp also has a small hole through
which the probe passes.  The hose clamp sandwiches the probe's flange between
it and the exhaust pipe.

Some aircraft use just one EGT probe - located at the collector where the
separate exhaust pipes merge into the "muffler."  This is typical of a
factory installation.  However, as I mentioned earlier, the "state of the
art" is to have a probe PER cylinder.

The probes just output a small voltage that's proportional to the exhaust
temperature.  On aircraft, things get expensive on the INSTRUMENT side where
we have everthing from $200.00 analog needles, which just report probe voltage
on an EGT "scale", to $2000.00 multi-segment LED bar graphs and computer
temperature monitoring.

The probes themselves are relatively inexpensive, by aircraft standards.
About $30-40 each.  They're available at a number of aviation supply houses
but I get mine from Chief Aircraft in Grant's Pass, Oregon.  Don't have the
number handy - sorry.  They do have an 800 number.

Probes seem to last anywhere from 600-2000 hours in aircraft use.  Failure
is usually mechanical, not electric.  In other words, the little stainless
tube and/or the hose clamp are abraded or corroded away.

The folklore, on aircraft, is that absolute temperature measurements are
not important.  What's important is the point at which the temperature,
reported by the EGT as voltage, PEAKS.  That's the point at which the
mixture is at stochiometric.  We use multiple EGT probes in aircraft because
it's common for the cylinders to each peak at different mixture settings
(due to mixture maldistribution in the intake manifolds).  What we're
interested in there is the EGT DISTRIBUTION between cylinders.

EGT probe placement has a significant effect on the absolute temperature
measured.  A probe 1" from the exhaust valve will read a lot higher than
one 3" from the valve.  For that reason, it's extremely important that
all probes, in a multi-probe installation, be located equally distant from
their respective valves.  The actual distance isn't so important, but having
them all the SAME distance is.

Hope that helps,

greg

greg		greg at indiana.edu	http://gtravis.ucs.indiana.edu/