Cylinder wall lubrication

[Skyline1]

Hi,

can someone explain how cylinder walls are lubricated? I mean how oil is sprayed on cylinder wall underneath piston head. Is oil sprayed by cranckshaft or are there special nozzles to lubricate cylinder? Pics or video of the process would be very helpful!

Thanks!

[bta 120]

Copy and paste the link seems to describe what you are looking for diesel and gas engines http://books.google.com/books?id=0-H...diesel&f=false

[GoFaster]

The crankshaft has internal drilled holes that lubricate the big-end rod bearings. Oil flings off the crank bearings *everywhere*, and this is more than sufficient to coat the cylinder walls in oil. (Additionally, the TDI engines have dedicated oil-squirter jets that spray oil at the bottom of the pistons, but the purpose of this isn't so much lubrication as it is to keep the temperature of the pistons under control.)

The lower set of piston rings are the oil control rings. Their purpose is to scrape *excess* oil off the cylinder wall and either push it further down the cylinder (where it lubricates the sides of the pistons) or push it through a series of small holes in the base of the groove for the oil control rings into the inside of the piston.

The oil that remains on the cylinder wall is the oil that's retained by the deliberate, very slight microscopic roughness ("cross-hatch") of the cylinder wall, and this is enough to lubricate the two compression rings.

[Skyline1]

How much oil is sufficient to lubricate the cylinder? I am wondering because in compression cycle oil mist lubricate cylinder wall and in power cycle excess oil is scraped off the cylinder wall, but how is cylinder lubricated in exhaust cycle? Is the oil that left on the cylinder wall during combustion burnt? Is there enough oil left on wall during coubustion to lubricate things during exhaust cycle?
If we stop lubricating the cylinder wall, how many cycles would piston do without damaging?

[s4phillips]

Oil is splashed onto the cylinder walls continuously as the the engine runs. The cylinder, piston and rings require very little oil. The majority of the oil is scraped off each time the piston has a downward movement, regardless of which stroke it might be. The very thin oil film that remains is all that is required to complete the cycle. Compared to the plain (Babbitt) bearings used on the crankshaft or the lower rod, there is not much load on the piston or cylinder, thus they require very little oil.

[GoFaster]

Originally Posted by Skyline1

How much oil is sufficient to lubricate the cylinder? I am wondering because in compression cycle oil mist lubricate cylinder wall and in power cycle excess oil is scraped off the cylinder wall, but how is cylinder lubricated in exhaust cycle? Is the oil that left on the cylinder wall during combustion burnt? Is there enough oil left on wall during coubustion to lubricate things during exhaust cycle?
If we stop lubricating the cylinder wall, how many cycles would piston do without damaging?

The lubrication system underneath the piston doesn't much care about what's happening on top of the piston. The piston is moving "up" or it's moving "down". That's all.

When the piston is near the top, the cylinder is exposed to all the splashing and spraying underneath, and gets coated in oil. When the piston is coming down, the excess oil gets scraped off, and whatever remains lubricates the rings on the down-stroke and the following up-stroke. Doesn't much matter what's happening above the top of the piston.

[DPM]

Is the OP thinking of a 2-stroke?

[Herm TDI]

The fuel itself (#2 diesel) is a light oil. This is a major factor that contributes to the long service life of these tuff little engines.
I've seen TDI AHU engines with over 300K> miles on them and still running strong with good solid compression #'s.

[Dimitri16V]

what about the piston wrist pin ? is the rod oil passage rifle drilled ?

[Drivbiwire]

Originally Posted by Herm TDI

The fuel itself (#2 diesel) is a light oil. This is a major factor that contributes to the long service life of these tuff little engines.
I've seen TDI AHU engines with over 300K> miles on them and still running strong with good solid compression #'s.

The fuel never contacts the cylinder. Combustion is over before the piston barely starts it's downward motion. All that is left after ignition delay is the expanding exhaust gas and particulate matter post ignition.

Also, fuel is injected during the period where there is no relative motion of the piston, so again even prior to injection there is no exposure of the cylinder to diesel fuel pre-ignition.

As mentioned the cross hatch retains enough oil to lubricate the upper rings. The Top ring being the hottest has a slight amount of oil trapped in the land to lubricate it hence the reliance on synthetic oil due to the extreme heat it has to endure with the miniscule amount of oil available to it. The upper ring also has a chrome coating to increase its durability as the closest ring to the combustion gasses and pressure.

Oil characteristics become an important part in engine durability. NOACK volatility, Flash point all play into the resilience of the oil on the cylinder walls which is exposed to the combustion gasses during the pistons downward motion.

[Skyline1]

OK, you helped me a lot. As you gave me some answers, I came up with a new question. I read that the oil film on cylinder wall is getting thicker as piston moves faster. It is also thicker if oil viscosity is higher. If this is true why engine wear occurs at very high RPM? Is there to much heat generated at high RPM so viscosity become to low?

[Herm TDI]

Pete....never mind...its pointless

[GoFaster]

Originally Posted by Dimitri16V

what about the piston wrist pin ? is the rod oil passage rifle drilled ?

Ordinarily the wrist pin is only splash lubricated. It's below the oil control rings on the piston, in the region where there is lots of oil splashing around. The wrist pin has a lot less relative motion than the crank pin does.

[GoFaster]

Originally Posted by Skyline1

OK, you helped me a lot. As you gave me some answers, I came up with a new question. I read that the oil film on cylinder wall is getting thicker as piston moves faster. It is also thicker if oil viscosity is higher. If this is true why engine wear occurs at very high RPM? Is there to much heat generated at high RPM so viscosity become to low?

I would like to know what your motivation is for asking these questions. "Tribology" is an entire field of study for mechanical engineers. Google the term "hydrodynamic lubrication". It's not necessarily true that the thickness of the oil film on the cylinder wall has any relationship to the speed of the engine. It's not necessarily true that engine wear will be higher at high engine speeds, although it CAN be true, and eventually if you increase the speed further and further, eventually one will reach the mechanical limits of SOMEthing and it will break (or the heat generated will exceed the capacity of the cooling system, or some other system will reach a limit of some sort). But as far as wear rates are concerned ... I can show you high-RPM motorcycle engines with very low wear on piston rings, cylinder walls, and crank journals.

I have a 20 year old motorcycle engine apart in my shop right now, and it was a track bike for the last several years. This is the first time the engine has ever been apart; I can tell. There is almost no piston-ring or cylinder-wall wear. The camshafts look perfect. The lifters look perfect. The gearbox looks perfect. The main bearings look perfect and are within specs for a new engine. The pistons look perfect, no scuffing at all. All but one of the con-rod big-end bearings look perfect ... but the one that isn't perfect, is completely trashed and the crankshaft is damaged because of that trashed journal. All this is because an oil-control metering jet wasn't properly installed 20 years ago at the factory, and unscrewed itself to the point that the jet came loose and starved that journal of oil.