EGT for mixture setting

[wmcgonegal at rr.etc.ncr.doe.ca]

My understanding is that (in general) the exhaust gas temperature is lower with
a rich mixture and raises to a point as you get leaner, then starts to drop as
you continue going lean.  This may or may not be useful in trying to determine
your AF ratio, but it is useful in determining when your pistons are about to
seize in a two stroke.  Polaris put a system on one of their production
snowmobiles this year that monitors the exhaust gas temperature while you are
at full throttle (like when racing across a lake).  If the temperature gets
dangerously high the monitor shuts down the ignition for an instant (like a rev
limiter).

Using a AFR sensor in a standard two stroke is not very practical as a mixture
of burnt and unburned gets blown out the exhaust.  I had considered using EGT
feed back in a fuel injection system for two strokes to richen up the mixture
if the temperature got to hot.  I have read about a similar technique used by
people who race two stroke motocross bikes in the desert.  When they started to
sense a lean condition, they would operate a finger controlled choke until the
engine cooled a bit in order to prevent seizures.

>The last time I checked, 8 thermocouples ran about $240.   The readout box,
>which would give all 8 temps at once would cost about 2000-2500 to pay someone
>to construct.

You could by a 486 computer and data acquisition card for less then that and end
up with something a lot more flexible for a DIYer.

Starting Line Products sells an adjustable EGT monitor called a Stutter Box for
sleds ($136US for single cylinder, $181 for twin, $227 for triple).  They also
sell reasonably priced EGT gauges starting at $51US (2") $68US (3"),and $34US
for a probe.  SLP is at
http://www.srv.net/shop/slp/slp.html
email: slp at srv.net
Royal Distributing (800-265-2970) also sells these gauges (3") $150CAN with a
probe.  Omega Engineering (800-TC-OMEGA) sells MANY different types of
thermocouple probes and gauges as well.

It is not overly difficult to construct a thermocouple conditioning circuit.
One only needs an amplifier and a cold junction compensation circuit.  You can
even do away with the compensation if your circuit is always at the same
temperature.  There are many simple, documented circuits for reading
thermocouples if you feel up to constructing one.

Will McGonegal
Mobile Sources Emissions Division
Environment Canada

wmcgonegal at rr.etc.ncr.doe.ca