Engine oil is used in the lubrication process, at the same place petrol is also there.
What prevents engine oil mixing with petrol in motor bikes?
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The piston rings prevent the two from mixing freely.
Note that it isn't possible to have the two perfectly isolated from each other. There will be a little bit of oil that gets burned during consumption, and some exhaust gases will find their way into the crankcase.
As these rings wear, the amount of oil lost to the combustion chamber increases. If a four-stroke engine is burning a lot of oil, worn piston rings are a likely candidate
Oil Scavenging Rings
There are several rings on a piston. Depending on the application and use of the piston it may have up to five ring grooves near the crown. Here are some points of interest.
The top ring is always the compression ring. It does prevent some oil from getting involved in the combustion process but it is not it's primary duty. The compression rings primary objective is to prevent exhaust gasses from slipping by the rings and getting into the crankcase.
The second ring is usually also a compression ring.
This Ring Scavenges Oil
Between the two oil scavenging rings and behind the expander there are small holes drilled into the ring groove. Oil that builds up between the two oil scavenge rings will get back into the crankcase via these small holes.
To Answer Your Question
What prevents engine oil from mixing with petrol in motor bikes?
Oil Scavenging Rings
The uneven brass colored ring is the expander. It prevents the two oil scavenging rings from touching in the ring landing and allows oil to buildup and pass through the holes in the piston that give access to the crankcase.
Yes the piston rings. The small metallic bands on the piston head.... From the bottom...As the piston rises and drops (compresses) it pulls oil up to help with lubrication. If the rings were not in place it would flood the chamber. Also the rings do expand slightly with heat as does all metal. The top has a "headgasket" preventing oil and water from entering into the combustion chamber or even mixing for that matter.