I've been waiting for the "one best" approach to air-separation to show up, as I would like to reduce the oil streak on my (plane's) belly, but so far I don't see an evidence of such a thing.  Some discussions have been targeted at "crankcase pressure" being the problem, but I don't think that has much to do with anything except leakage out of various seams.  Generally, but not always, front crank seals won't be affected by a slight positive pressure as they should seal effectively with a relatively high pressure differential.  The crankcase pressure shouldn't be much higher than the pressure at the vent discharge, assuming the tube diameter is relatively large (3/4 inch or larger?).  High crankcase pressure is an indicator of too much blowby(my company builds crankcase pressure sensors used in locomotive engines to indicate excess engine wear).  Incidentally, the quantity of blowby is essentially unaffected by the pressure at the vent:  Remember that blowby occurs when combustion products at perhaps 800 psi leak past the rings - a change in crankcase pressure of a fraction of a psi won't change that flow by very much.  I suspect some of the problems encountered have to do with the way the separator is drained.  If the separator is very little higher than the engine drain location, the crankcase pressure doesn't have to be much higher than the separator pressure to prevent oil from returning to the engine.  Also, if the drain oil is led to a canister that is vented to the engine compartment, its pressure will rise during high speed cruise, again preventing oil from draining.  The separator, if it has enough capacity, could just hold the oil until shutdown, after which it would drain.  Another comment - some people are afraid to drain the recovered oil back into the engine and prefer to install a catch tank, saying the oil looks "bad."  I maintain that the oil in the catch tank IS the same as the oil in the engine, so if you don't like that oil you shouldn't like the rest of the oil either.  Actually, the reason it looks icky is that at the lower temperature in the catch tank it tends to absorb water and will turn color.  Pour it back in the engine and the heat will drive off the water and make it good as new.

Note that there is little, if any data or positive statements in the above discussion.  I would welcome a post with hard facts and numbers that could lead us out of the maze.  do oil separators work?  Not work?  Some work and some don't?  What's the difference?  Does the world need a "real" separator(I've got some ideas)?  I've been listening to discussions on this topic for over 20 years, though, and have yet to hear a convincing argument one way or the other.

Gary Casey

Fred said, in part:

I wrote earlier that my IO-550 was dumping oil out the air oil separator, and with some fiddling with the plumbing, it appeared I solved the problem.   High power climbs yielded no oil loss.  Ground tests showed no blow by.  Problem solved. 

Wrong.