From: John Paulding (kb9wle@yahoo.com)
Date: Fri Sep 27 2002 - 23:04:51 PDT
Hi everyone,
I'm new to the list, and have been reading the threads
on pyrometers and mixture settings with interest. I
am an aircraft mechanic and although I have little
experience with diesels I do have lots of experience
with spark ignition engines.
In aviation we call a pyrometer an EGT (Exhaust Gas
Temperature) gauge. It is mainly used to help lean
out the mixture during cruise, but when probes are
installed on all the engine cylinders can be used to
spot poor combustion in one cylinder. During cruise,
the mixture is normally leaned to peak the gauge at
maximum temperature (1200 - 1250 degrees Farenheit).
This is best economy mixture (best MPG in a land
vehicle). 50 - 100 degrees Farenheit richer (cooler)
is usually the best power mixture. Naturally, if the
thermocouple is placed far downstream in the exhaust
it will read a lower temperature. Set your car
mixture like this, it is cheaper than buying a $500
tailpipe sniffer.
Diesels are different of course and I have not spent
enough time studying the engines to know how to set
the mixture on one. ( I think most people set mixture
on a diesel by smoke content at the midrange RPM.)
One thing I ran across recently was a table of
fuel/air mixtures versus smoke content for a
precombustion chamber diesel. (At 100% fuel, all the
fuel is burned by the engine, at 130% fuel, there is a
30% excess in fuel.) Normally, the EGT on any engine
should not exceed 1200 degrees.
100% Fuel - No smoke 110% Fuel - Light gray smoke
115% Fuel - Dark gray smoke 130% Fuel - Black smoke
As for buying an EGT gauge, try Wag Aero in
Wisconsin. Phone: 1-800-558-6868 They have EGT gauges
with thermocouple as low as $65.
I know I'm rambling, but I thought it would be useful.
John Paulding,
1976 Cessna 150M N-9417U, M35A2 coming soon
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