It is most desirable if each cylinder is producing the same power. It
is even more desirable if the cylinder does not suffer from bad events such
as abnormal pressures, detonation or pre-ignition. Aircraft engines
are interesting because each cylinder is its own engine. Assuming that
each cylinder volume is the same, rings seal equally, cams produce
equal valve timing and movement, all plugs produce equal spark value
and timing, induction air is delivered equally, fuel is distributed equally and
it is atomized equally so that the flame burns the same at an equal air/fuel
ratio(A/F) producing the desired power then the engine is a happy
engine.
The burn in each cylinder should produce useful pressure against the piston
only during a certain crank angle range. In order to accomplish that the
flame must be started at the right time and burn at a proper speed.
Thus the importance of spark timing and A/F. The fixed timing (say 25 DBTDC)
issued by a magneto is a comprise, but it works well enough over a broad range
of A/F - just not as efficient as it could be.
High power and best power occur sufficiently ROP (EGT >100F of
peak) so that some destructive events are less likely as is also true at
lower power settings (say below 75%). That is, one can further alter
the F/A at such lower power.
However, the mixture control affects all cylinders and if each is running
at different power because of different A/F then there is a problem. I.E.
reducing the mixture causes most cylinders to be 50F LOP and one is 30F ROP (bad
because of cylinders pressures) will also lead to roughness because of the
different power being produced.
Fuel flow is an indication of F/A.
Thus, the GAMI Lean Test. Starting at power below 75% (i.e. NA
engine, at or above 6500 MSL) and quite ROP, reduce fuel flow (FF) by say .3 gph
and note the EGTs of each cylinder. Continue until each cylinder has gone
say 30F LOP or until roughness but not until the engine stops.
Chart the FF vs EGTs and note the FF at each cylinder peak. If the
overall spread is greater than .3 gph, the cylinders are not producing
equal power and they are a candidate for some adjustment before good
operation LOP can be accomplished. The GAMI approach is to alter the
injectors so the FF matches the air available for each cylinder,
thus matched A/Fs.
Of course, if the spread is .3 gph or less, the engine is in good
shape.
Other methods to achieve an acceptable spread is to alter the induction air
(difficult because the flow may be different at different throttle settings) or
alter the atomization (F/A) by controlling the air going to the injector
(like supercharged or turbocharged engines through the use of shrouded
injectors).
While ROP/LOP affect the burn speed, so does Compression Ratio
(CR). Thus, even greater efficiency can be obtained by optimizing the
spark timing to better match the F/A and CR. Lower power, as measured by
resultant MAP/RPM, can benefit from advancing the spark ahead of the fixed value
to account for slower burning. High power/CR can sometimes benefit
from retarding the spark.
See the attached chart as an example of the benefit of operating LOP.
One must remember that low drag airplanes (Lancairs) are not penalized very much
by running LOP to extend range.
Scott Krueger
IO320
In a message dated 11/2/2010 7:50:51 A.M. Central Daylight Time,
douglasbrunner@earthlink.net writes:
Terrence,
LOP
starts when the mixture is LOP of course!
Sorry,
for the smart ass response. There are a number of good article on engine
management and LOP operation written by John Deakin on “Pelican’s Perch” on
avweb.
There
is also an excellent course on engine management given by the GAMI folks
called “Advanced Pilot Seminars” - http://www.advancedpilot.com/ I
took it and learned a lot.
The
short answer to your question is that you want to be approximately 50 deg LOP
in most situations and it is best if all your cylinders are LOP to the same
degree.
Doug
Brunner
N241DB
Jeff,
May I ask for a little more detail on when the LOP
condition starts? Do all EGTs have to be LOPP, or is it when just the
leanest one goes? And, how lean constitutes enough to be considered
LOP?