Lets get one thing straight. Fuel injection does nothing for the BSFC
or the
HP an engine will generate. Particularly with the same intake manifold
the single carb version uses. Only if the manifold is designed for fuel
injection with long equal length separate runners tuned for best BMEP
(torque) at a certain RPM will you see a HP increase and a slight
improvement
in BSFC. There will be no improvement over carbs with separate venturi
for each runner. No way can you improve the thermodynamic efficiency of
an engine by matching the fuel flow to each cylinder. It does not change the
average
mixture for a given HP.
I agree and disagree. First, I agree that a cylinder doesn't care how
the fuel-air charge that it breathes gets mixed. Whether a carburetor,
injector, or Aunt Edna did the mixing is irrelevant. Absent really
atrocious mixing (i.e., fuel not atomized), nothing in the method of
creating the fuel-air charge will change the BSFC.
My disagreement lies in that we don't just fly one cylinder. We fly
between four and six cylinders in most air-cooled flat engines. Due to
variations in runner lengths, turbulence, flow turning, etc., each of these
cylinders will see different airflows and mixtures. Carbureted engines,
where one venturi feeds all intake runners simultaneously, are especially
bad in this regard (BTW, I've never heard of a carbureted GA engine where
there is a "separate venturi for each runner"). With port fuel injected
engines, you have the opportunity to fine tune the mixtures that each
cylinder sees by swapping out nozzle restrictors that are slightly leaner or
richer on individual cylinders. This lets you tune the engine so that all
EGT's peak at the same time, which allows LOP operation while maintaining
engine smoothness. Note that you can only get it perfect at a certain
throttle angle, MAP and RPM (preferably, your normal cruise values).
Variations, especially of throttle angle, will upset the balance.
The reason gami can make wild ass claims and get away
with it is because things like BSFC are too hard to measure with out the
controlled conditions of a dyno.
The only thing that GAMI does is sell nozzle restrictor kits that have
been through a tuning process as described above. There is no magic to the
GAMIjector. You can achieve the same result if you have access to a supply
of restrictors of various internal diameters, good multi-cylinder EGT
instrumentation, and some time. Tuning allows you to operate lean of peak.
The dramatic change in going from .45 BSFC (typical of a rich running
engine) to .375 BSFC (attainable by a stock TCM IO-550 running lean of peak
according to TCM's engine curves) is due to improved operating technique . .
. it is not a "wild ass claim."
If you run your Lycoming lean of peak the warrantee is void and you risk
burning the very expensive $250+ each sodium cooled exhaust valves
Actually, when you run 50 degrees lean of peak, your exhaust valves run
cooler that when you run 50 degrees rich of peak. This is because, while
EGTs are the same, the CHTs for the lean of peak condition are quite a bit
lower (45°F lower according to the TCM IO-550 curves!). It is the delta
between exhaust valve temperature and CHT that conducts heat out of the
exhaust valves and cooler CHTs are better.
Another difference between lean of peak operation and rich of peak
operation is that lean of peak provides a slightly oxygen rich exhaust,
while rich of peak provides a fuel rich exhaust. There is a theory that an
oxygen rich exhaust stream can affect exhaust valve life due to corrosion,
but I haven't seen any definitive studies on it. Anyone have access to data
on this?
Naturally, you don't lean like this at high power settings (i.e., takeoff
or high-power, low-altitude ops), although the reason for keeping mixtures
very rich in these conditions has as much to do with avoiding detonation by
slowing combustion as it does with keeping things cool.
If I had read this before I responded to your last message it would save me
a lot of
work :-) Get the book called the The Sky Ranch Engineering Manual.
Here are some graphics from that book.
The warrantee on Lycoming engines is still void if you run it lean of peak.
That gami stuff is still BS. You still cannot manually control the engine that
close and expect not to screw up once in a while. It only takes once.
All this is totally irrelevant to a rotary engine as there are no exhaust
valves and
you can lean the mixture until the engine quits without hurting the engine.
Its like the price of auto gas being half what it is for 100LL. The exact BSFC
becomes irrelevant. The wankel rotary becomes the aircraft engine
of choice.
Paul Lamar
Paul,
I love the engineering of this lean of peak stuff, and understand all the
theory, and that it has been testing on the dyno and in the air.
Unfortunately, somewhere something went wrong in real life. The
Continental in the Malibu has experienced a significant engine damage
rate. Incompetent pilots? Could be. Poor QA? Possibly. Maybe only
a tiny percent have problems, but the service history is poor enough that
Piper went to a Lyc and then a turbine in the Malibu. I suspect the turbine
is wonderful, but the fuel burn should be pretty horrific. Small turbines
are notoriously thirsty, very poor fuel specifics.
The possibility to have an electronic fuel injection system run the
engine in the lean of peak range safely is pretty tantilizing. The fact
that it is possible to get below 0.40 BSFC verifies my long-held
position that the basic engine designs are nearly optimum - asside from
the stupid maintenance and QA issues. These engines have excellent
breathing and are very light and relatively robust. You'll never see a
crankshaft or connecting rod in a car that is anything close to the beef
of an aircraft engine.
With FADEC coming online soon, perhaps we'll see the potential actually
realized without engine damage. It would be a good thing.
Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor
Lots of people have problems with aircraft exhaust valves.
Tracy Saylor and John Lynch both went through Lycoming IO-360
exhaust valves in about 350 hours from new engines. John does not
race and he keeps an immaculate airplane with a scanner. He is
also a very aware pilot and engineer.
If you are on the lean side of peak and you descend with out changing
the knob (forgot) the engine gets leaner right away. Up to peak EGT very quickly.
If you are low enough to open the throttle more and you do..... bye bye exhaust valve.
Perhaps you needed to climb back up and got distracted.
Once it starts to leak the heat transfer from valve head to seat (the main mode
of heat transfer) is reduced. The head gets hotter and hotter and the valve leaks
more and more... divergently unstable cooling system!!! Pops like a pop corn!
Paul Lamar
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