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Jack,
There are about 400 million hours of highly successful engine operating
experience, with aircraft spark ignition piston engines operating at cruise
BMEPs in the range of 175 psi. Those engines normally went well past 3000
hours between overhauls and or jug changes. Oh! And by the way, they were
heavily boosted engines, so "longevity" is not an issue in a turbo-charged
engine that is properly engineered.
In piston engine combustion science, it is a fundamental (and common)
epistemological error to assume that all BMEPs of 175 PSI cause the same
kinds of stress (distress) on the engine. In my experience, one compounds
that error by simply throwing out some arbitrary BMEP number as a basis
for comparing one engine condition to another, hoping to "conclusively"
prove to somebody some issue with respect to piston engine design,
operation, or durability. The "Brake Mean Effective Pressure" term is an artifact. It is purely a
calculated number, from other numbers. Further, it is, as its name
indicates, an "average" of a real world engineering parameter that is much
more important when considered in its detailed characterization than it is
useful when reduced to is "mean", where it becomes only somewhat useful, but
capable of being highly misleading. There is simply a lot more to this subject than can be reduced to a single
or series of BMEP numbers. Example (these numbers are approximations, from memory, from recent
experience on the test stand):
BMEP Peak Combustion Pressure
Theta(p-p) Peak Torsional Crankshaft Stress Reversals
Condition A) 175 1100 psi 5-8
degrees ~5 x mean torsional stress reversals
Condition B) 175 850 psi
17-19 degrees ~3.5 x mean torsional stress reversals
Where "Theta(p-p)" is the rotational angle between TDC and the peak of the
combustion pressure event.
From the point of view of an experienced person evaluating the structural
integrity of the engine, Condition A) would suggest a very short operational
life, whereas, Condition B) would suggest a much longer operational life.
Of course, the engine was making exactly the same horsepower in each
instance.
In point of fact, I have personally operated a general aviation piston
engine under conditions close to those described in B), above, for
extended periods of time, and then torn down the engine and dimensionally
inspected all of the components. The cylinder barrels were still within new
limits (not just service limits, but NEW limits) including the choke area.
On the other hand, I have seen lots of engines operated under conditions
that would be the equivalent of what would be found about 1/3rd of the way
from A to B, and most of those engines had to be topped before 500 hours.
Regards, George Braly
LML website: http://members.olsusa.com/mkaye/maillist.html
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Please send your photos and drawings to marvkaye@olsusa.com.
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