Skalywag wrote:Question is: "Are piston aircraft engines designed to have fuel lubricate any part of the cylinder? If so is there somewhere I can find this engineering data in black and white?"

As far as I understand, engines are lubricated by oil...Cylinders rely on oil for lubrication just like the rest of the engine? Some IA's I know say anyone that believes that fuel lubricates anything are not well versed on how an engine works, while some other IA's vehemently insist that you must dowse the cylinder with extra gas to lubricate the top end...???

Will someone out there please help lay this one to bed? Is there any validity to this claim that fuel lubricates anything?

-Thanks

Skalywag wrote:Question is: "Are piston aircraft engines designed to have fuel lubricate any part of the cylinder?
As far as I understand, engines are lubricated by oil...Cylinders rely on oil for lubrication just like the rest of the engine? Some IA's I know say anyone that believes that fuel lubricates anything are not well versed on how an engine works, while some other IA's vehemently insist that you must dowse the cylinder with extra gas to lubricate the top end...???

Is there any validity to this claim that fuel lubricates anything?

-Thanks

The cylinder and pistons and rings need very, very little oil compared to the rest of the engine, and in most applications they get far too much. Oil shot into the bottom of the piston by holes in con rod caps, for instance, is for cooling, not lubrication.

Oil rings can only control so much oil. If too much is flung into the cylinder, the rings begin to hydroplane on it and oil consumption goes way up, along with the spark plug fouling and valve stem coking and sticking that comes with burning oil. The risk of destructive detonation also goes way up when oil is part of the combustion process.

I spent 12 years in the compressor rebuilding business. Oil control is critical in air compressors, since it will contaminate the whole air brake system with sticky varnish and will cause serious coking and clogging of the discharge passages in the cylinder head. Since there is no combustion happening in there, any oil getting past the rings causes serious trouble. I found that by sticking closely to the manufacturer's bearing clearance specifications I could keep the oil pumping troubles to almost nothing, and warranty claims for oil pumping dropped right down to nothing.

The trick is to keep those clearances very close so that much less oil escapes the bearings to be flung into the cylinder. The crosshatch in the cylinder wall, as microscopic as it is, holds enough oil for piston and ring lube and will not release the oil into the combustion process (in the engine) or into pumped air (in the compressor). The cooler cylinder wall prevents most of the very thin bit of oil from being boiled off.

If the compressor suffered an oil-supply failure, the bearings always failed first, long before the pistons or rings ever seized.

So we need to reduce the amount of oil getting past the rings, not increase it. We don't need much. Aircraft engines will use more oil not because they're dinosaurs but because they're air cooled, and so their temperature range is much larger than a liquid-cooled engine, so the clearances must be larger. Ring gaps and piston-to-cylinder clearances in the Lyc or Continental will be bigger than in your typical Toyota, for sure, and it will use more oil than the Toyota. Yet, when I replaced the Lycs in the fleet, i always followed to the letter the break-in procedures provided by Lycoming and we seldom had to add oil between 50-hour inspections. Many guys will baby a new or overhauled engine, exactly the wrong thing to do, and it will use far too much oil. That engine need high pressures and temperatures to get the rings and cylinders to seat properly, a nice way of saying that they must grind away at each other until they mate closely. Babying it results in the rings riding on high spots that get glazed and shiny and hard, leaving gaps where the cylinder has low spots, so that oil sneaks past the rings and gets into the combustion chamber. The wear stops once the glazing happens and the engine forever after uses oil. It's not, as I say, an old-technology problem; it's an engine-operation problem.

Dan Thomas