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To my understanding, cylinder walls are lubricated by retaining a tiny amount of oil in the honed or otherwise manufacturing prepared cavities in the cylinder walls, so that when the piston rings travel above the cavities, it forms a closed space of pressurized oil that the rings float on.

How is this tiny amount of oil not burnt off during the power stroke to the point of not providing enough lubrication for the following exhaust stoke?

As a side note, I suppose this oil combustion the way I described it is one of the reasons for engine oil consumption.

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  • The rings hold oil and wipe the cylinder with it on every up and down movement, and no not all of it gets burned away.
    – Moab
    May 17, 2020 at 21:46

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Oil from the crankshaft rotation is thrown against the cylinder walls during engine operation. This is oil escaping from the rod bearing. When the piston comes down in the cylinder, the bottom rings (usually a pair, called oil control rings) scrapes the cylinder pretty much clean of oil. There is a very small amount left behind on the cylinder walls which lubricates the rings. This oil is burnt off during the combustion process, but really there is so little of it, it doesn't cause issues. Remember, the oil is coming from the bottom and is scraped downward during engine operation by not only the oil control rings, but by the compression rings as well. This doesn't leave a lot of oil on the cylinder walls. Since the rings and pistons are lubricated from the bottom, they will draw oil with them upward as the piston travels upward during the exhaust stroke. This happens the same way during the compression stroke.

As you've probably figured out, the top ring sees the least amount of lubrication. It is most commonly made from cast iron, which self-lubricates to an extent. Cast iron also is very durable against wear of this type.

Here is what a cross section view of the piston, cylinder wall, and piston rings look like:

Image taken from University of Windsor page

Image from University of Windsor.

Please note, the above image is representational, though it is a pretty good representation. Every manufacturer can and will do their setups differently.

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  • That makes a lot of sense with oil seeping past the compression rings and drawing oil with them as the pistons moves up. What I can’t grasp is how the oil splash leaving the rod bearing provides oil around the circumference of the cylinder, considering the geometry of the big end of the rod. Intuitively it seems the front and back sides get oil and not the rest, but the non-uniform splash might be distributed by the rings as well? Also not needing it to be completely distributed for adequate lubrications either
    – Erik
    May 17, 2020 at 18:09
  • And the cylinder walls are maybe 400 F, certainly not the very high temperature of the combustion gases. And the oil had been processed above 800 F , the oil vapor pressure is very low at 400/500 F
    – blacksmith37
    May 17, 2020 at 21:00
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    @Erik - Believe me when I say, the oil gets splashed around ... a lot. A lot more than you'd think. When a piston is traveling up-down-up in a cylinder 50 times per second at 3000 RPM, things are a little wild. Oil not only comes off the rod/crank interface, it'll get hit by it as well, then when it lands, it splashes too, which creates secondary landing points. The oil control rings will spread it out as they come down, as will the piston skirts. There's a lot of stuff going on. The entire cylinder wall will get covered in oil.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    May 17, 2020 at 21:37

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