Basic question - sorry to ask !

[DieselMonkey]

Guys,

This must sound like a very silly question, but how are cylinder bores lubricated ?

I am aware that 'splash' lubrication is an old chainsaw / lawnmower engine type of lubrication because of the size aspect, but i've had a good study of the common modern diesel and cant come to any conclusion.

Obviously the crank doesnt run in the sump oil so where does the lubrication come from.

Answers much appreciated. DM

[VWRacer]

Yep, good old splash is all they get...both from the crankshaft splashing about in the oil, as well as from the oil squirters spraying oil on the underside of the pistons. I'll post a pic shortly.



Edit: I don't have a TD engine apart at the moment, but here's a photo of a squirter from a turbocharged Mitsubishi engine. The VW ones are functionally identical.

[racer_x]

Don't the rods also have an oil galley up the middle of them, which would put some oil up in the wrist pin bore, and that would go out the end of the wrist pin bore and lubricate the cylinder bore.

[VWRacer]

Quite so! My point was that there is no 'organised' effort to get oil up onto the cylinder walls. It just sort'a happens as a byproduct of all that monkey motion inside the engine...  :wink:

[fspGTD]

Actually, I think this is an interesting question.

The cylinder hone pattern, composing a cross-hatch pattern of grooves throughout the cylinder walls, is where the oil sits and is what lubricates the two compression rings above the oil scraper ring.  The correct hone pattern is therefore critical for proper long-term lubrication and sealing of the piston rings, and it's expected on a good running engine that's had its cylinder head removed to see honing grooves visible in the cylinder walls.

The cross-hatch pattern also likely helps any oil that does find it's way to the cylinder walls to distribute evenly around the cylinder - as rings scrape up and down over them, the oil migrates around due to the diamond-like honing pattern.

A good bit of engine oil likely gets slung onto the cylinder walls out of the pressure-lubricated big-end rod bearings as the crank swings up approaching TDC on the compression stroke and the exhaust stroke.

No IDI VW Diesel that I am aware of has rifle-drilled rods.

The oil jets, found on turbo-diesels only, although certainly don't hurt piston wall lubrication, are not necessary for it.  NA diesel engines without the oil jets go many miles and do not seem to have any cylinder wall lubrication problems.  Their main purpose is for piston cooling.

It also helps in a diesel engine that any uncombusted liquid diesel fuel above the piston has lubricant properties.  On gas engines, excess uncombusted fuel dissolves oil and does not lubricate the bores like it does on diesels.  This is one reason why diesel bottom ends last longer than gas engines.