Internal Combustion Piston Lubrication

[Grayfox]

Hi,
Can someone explain the lubrication mechanism for a piston in a typical automotive internal combustion engine? If the walls in the piston chamber are coated with oil, the lubricant should also burn along with the petroleum during the combustion phase (leaving problematic residues), so, is the oil somehow "squeezed" out of the chamber as the piston shoots back down after detonation? And if so how is this accomplished? Perhaps the skirt of a piston relies on a solid lubricant coating? Or does the motor oil simply dissolve and carry away the soot and burnt residues from the chamber?

 

[xxChrisxx]

Hi,
Can someone explain the lubrication mechanism for a piston in a typical automotive internal combustion engine? If the walls in the piston chamber are coated with oil, the lubricant should also burn along with the petroleum during the combustion phase (leaving problematic residues), so, is the oil somehow "squeezed" out of the chamber as the piston shoots back down after detonation? And if so how is this accomplished? Perhaps the skirt of a piston relies on a solid lubricant coating? Or does the motor oil simply dissolve and carry away the soot and burnt residues from the chamber?

The mechanism for the oil is 'hydrodynamic lubrication'.

Piston rings control the film of oil coating the chamber (you may hear someone describe one an a oil scraper ring). Some nominal amount of oil is burnt with each combustion cycle, they don't leave residues as the mass fraction is so low the oil gets fully combusted.

 

[Grayfox]

@ Charlatanza
According to engineeringtoolbox, the flashpoint of motor oil is only about 485 degrees F, I'm sure the combustion of gasoline would be hot enough to burn the thin layers of oil in the chamber.

 

[Grayfox]

Thanks guys for your quick replies so far.
@Chris: So you say there is an oil scraping mechanism and whatever is left over and burnt is simply expelled as exhaust? Could you possibly elaborate more on these rings and where they scrape the oil to during what phase of the combustion cycle?

 

[xxChrisxx]

http://en.wikipedia.org/wiki/Piston_ring

The oil scraper ring controls the oil film thickness along the bore of the cylinder. It controls film thickness at all times during the the engines operation, when moving down it will be scraping the oil away from the combustion chamber.

 

[Grayfox]

Oh now I see. That makes sense.

 

[Ranger Mike]

As usual xxChrisxx is exactly correct and to the point. if i might elaborate on a portion of your question- Can someone explain the lubrication mechanism for a piston in a typical automotive internal combustion engine...motor oil pressurized by the engines oil pump is routed through oil passages to the crank shaft and piston connecting rod bearings. These babbit type bearings have specific clearances regarding the diameter and side width relative to the crank shaft journal diameter and width. These clearances or " gaps" must be kept as this is how amount of oil that squirts out of the oil passage is controlled. Oil squirts out of the side gaps and sprays the bottom of the piston, to include the piston wrist pin, coats the cylinder, inside of the engine block ,,sprays everywhere..its a true hot mess. Too much oil will squirt out and coat the cylinder bore and the piston ring oil scraper will not be able to clean the bore properly. Too little oil will melt the piston ( oil must hit the bottom of the piston to carry off combustion heat), starve the piston wrist pin,,will greatly increase piston / bore wear, ring life and increase engine heat dramatically. Don't forget that engine oil is the second means of cooling the engine and this is just as important as radiator coolant. So the oil control process is critical to engine life. Finally, pistons are not round. They are oval and expand to become close to round to seal the round cylinder bore. Surface texture of the piston skirt must be controlled tightly as well to yield maximum life.

 

[Grayfox]

Thanks for your elaboration Ranger Mike, this is precisely what I was interested in.