|
Paul,
All dyno testing of automotive engines I have done suggests
that friction is linear relative to rpm as you suggest. That is of course unless
you deprive a sliding surface of lubrication. All friction studies I have
done were on "motoring" engines therefore any added ring drag due to compression
loads were not comprehended. On the relatively constant speed of our aircraft
engines, I can't imagine it would have any proportional effect. I suspect
prop effientcy and other similar factors are bigger guns on reducing bsfc
numbers that the internal engine parts when it comes to minor engine speed
changes. Certainly timing would be a factor on naturally aspirated engines
at altitude.
Craig Berland
Friction or Coulomb* loss is independent of sliding rate.
However, the friction loss in pounds, when multiplied by rate, yields hp.
As rpm goes up, so does the friction hp. Also, as rpm goes up, engine power, at
a given MEP, also goes up proportionatly. It would seem that the friction hp
would maintain a certain percentage of the overall power input, and I'm not sure
how this would cause a change in SFC. Also, I'm not sure of the effect of
increased MEP on the coefficient of friction. Does it go up with the
increased pressure of the rings against the cylinder walls at higher MEP?
Keeping MEP max certainly cuts down on pumping loss! I seek
enlightenment!
*Charles Augustin de Coulomb
|