lml@lancaironline.net Mailing List Archive

Fred,

I too, share your mystification. I think you've stated your observation very well. I have heard that on a Lycoming, the oil can get into the gearcase and the gears "pump" the oil up to the top. The top shaft - the camshaft - has a slinger that supposedly separates the oil from the blowby. The extra oil from being full gets by the slinger and out it goes. I'm not sure I believe that. The Continental has a different breather and oil separator design and the story there is that the oil gets into the crankcase. Yes, I suppose the oil could turn into a foam and therefore rise in level when running. But that has to be balanced by the effect of oil drain-back in that there is probably a quart or so of oil "in transit" draining back to the sump. Therefore, while running the oil level will be significantly lower than with it shut off. The increase in volume due to foam would have to be substantial. The bottom line is that I'm not sure what to believe. One would think that with all of us running these engines we would all know exactly how much oil gets "blown overboard" with a full sump. I'll have to say that after thousands of hours observing mostly Lycoming engines I'm not sure I can tell the difference. Here's a thought, though: Engines are rarely, if ever, filled exactly to the top since it would require the addition of a fractional quart of oil. Maybe we look a the dipstick, see approximately 11 quarts and then put in a quart. It will likely end up at something less than 12 quarts (after all, if it read more than 11 we wouldn't put in the quart). Then not too many hours later we see it is back down to 11 quarts. It didn't burn a quart - it was never at 12 to begin with. Or?

There is a Lycoming legend that says during takeoff and landings the engine burns more oil because the nose-up climb attitude lets oil get back into the gearcase. I propose that the reason for the high oil consumption is the power-off condition during landing. In this case the high manifold vacuum draws oil past the intake valve guides and rings and during the intake stroke. A lot of engines will burn much more oil at light load than at full load. So the things that affect oil consumption can make up a long list.

Gary Casey

From Fred:

"Everybody"knows that when you put "too much" oil in your aircraft engine (that is, fill it up to the top level), it will blow oil overboard.

But I am mystified after seeing how our engines are constructed.

On my Continental IO-550, the oil in the pan can not "see" the crankshaft spinning overhead because the crank is nearly entirely enclosed by the crankcase housing, with only narrow semicircular slits in the casting allowing oil to drain from around the crank and bearings down to the oil pan. So there is very little windage effect possible to disturb the surface of the oil in the sump.

There is a cracked Lycoming 0-360 crankcase in my hangar, and it has narrow slots oriented parallel to the crank centerline through which oil drains to the crankcase so in this design the prospect of windage reaching the oil surface in the crankcase seems remote indeed.

Yet, adding two quarts of oil which may raise the oil level half an inch on a big engine makes the difference between oil going overboard or not going overboard.

Why? What seems obvious is now not so obvious when I think about it and try to visualize what is going on inside.

The only idea that came up in discussion is that there is a layer of oil foam on top of the oil in the sump, and it builds up deep enough to block the drain slots or allow oil foam to churn around the crankshaft. That seems like a stretch, except that a friend who used to work at Garrett told me he saw a Garrett turbocharger on the test stand, and what came out the bottom of the turbo was not oil, but more like brown mayonnaise that was sucked up by the scavange pump and returned to the engine. But turbos with bearings at slight negative pressure (and suckng air through seals) are not crankshafts at positive pressure.

Reading about oil foaming ascribes it to water in the oil, an abnormal condition. Engine oil have anti foaming agents.

So to Walter or anyone: please explain how adding one to two quarts of oil causes oil to be blown out?

Fred Moreno, mystified (as usual)