Hi Ron,
The FART is smelling Oxygen.. whether its CO1 or CO2 is irrelevant to me; i just
want the FART to tell me how much Oxygen it is smelling; if the FART says 14.5
then i know its a happy FART. As long as my FART is happy i am happy also...
Whether the mixture is observed to be slightly rich during taxi, couldn't care
less; its 5-10 minutes of mild operational demand time.
You cannot leak boost pressure back through the float needles on a 914 snd
expect
to keep moving; firstly that would intimate that your boost pressure is higher
than your fuel pressure; secondly you would run out of fuel on one or both
carburettors.
The float chambers need to remain equalised to plenum pressure; if they are
leaking
air pressure the engine will run lean; hence all the Rotax instructions;
and yes you tune to the leanest cylinder always; using an infrared pistol tester
is good enough to indicate this.
Remember you have a lot less room to get it wrong with turbo charging; things go
crunch a lot quicker with less warning. If you want to run really lean above
50% power as you mention then firstly you are already going to be at 30% power;
so you will give it more throttle; then you will experience more heat, and
its all down hill from this point.
Its like performing an engine out glide approach; the airframe has its optimum
airspeed for endurance and distance... 14.5 Stoich is that optimum Oxygen
content
for "all" petrol engines under "all" operating conditions, for maximum thermal
efficiency, end of story...
There are numerous ways to diagnose and monitor engine tune; for me its pretty
much field experience knowing the different harmonic tones generated by
different
engine cylinder formations under full and partial load; they indicate
air/mixture,
spark timing, or mechanical imbalances. The FART is purely a final quality
assurance confirmation for me befor signing off on maintenance service.
There are endless test points that can be used to monitor an engine; Oil
Pressure
and Head Temp are the most vital, the rest are nice to haves but not essential
and are just early warning meters for the less experienced. If you want perfect
analogue economy as good as any fuel injection device then you want to know
your manifold pressure and exhaust oxygen content, and maybe add as a cross
reference exhaust temperature probe, however to be as accurate as the O2 sensor
the EGT probe really should be placed right at the valve throat; so for a
912/914 that's 4 probes.
For a 914 the best point to probe EGT is upstream at the impeller, this area
between
the head and impeller is by default a fairly constant temp environment and
will rise and fall according to boost load and is a cross reference point for
oil and head temperature meters, the other point i would probe is inlet air
temp post intercooler, but personally i would still really on my O2 readings
as a final checkpoint.
All this above will give the operator the insight that the engine, mixture wise,
can or cannot run any better than is being indicated.
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