we use oil because there is no lubrication facility in case
of two stroke engines as they are double acting and the
fuel has to be mixed in the lubricating oil but in case of
4 stroke engines,they are separated by the moving piston.
2 stroke is much simpler machanism. It has inlet and
exhaust ports. There is no valve mechanism as in the 4
stroke engine. So lubrication in 2 stroke is much simpler
and just add oil into the petrol for lubricating the moving
parts like piston assy. Whereas 4 stroke requires more
machanism and seperate lubrication system.
In a 4 stroke engine, we have a dedicated system for lubrication. We have lubricating ports which supply the lubricating oil from the sump so that all the parts are thoroughly cooled. But, most small petrol two-stroke engines cannot be lubricated by oil contained in their crankcase and sump, since the crankcase is already being used to pump fuel-air mixture into the cylinder. So while fueling, we mix gasoline with an oil to be distributed throughout the engine for lubrication. The two-stroke oil is ultimately burned along with the fuel. But i=this can contribute to more smoke and distinctive odor.
2 stroke engine lack a dedicated cooling system as in 4 strake engine. the crankcase of 2 stroke engine doesnot contain lubricating oil.
As the mixture of oil and petrol reaches crankcase in 2 stroke ,the petrol gets vaporised and transferred to cylinder for combustion leaving the oil lubricating the parts of crankcase.
The connecting rod uses a roller bearing which requires less lubrication .
We are using oil with petrol in two stroke engine because
their are no force feed system to lubrication the
component.Two stroke engine have no inlet and exhaust valve
so no need of lubrication for cylinder head. the air/fuel
mixture follow via crankcase we need to lubricate only
crankcase ( cylinder wall, connecting rod, piston pin etc.)
It is just becuase in two stroke petrol engines the
crankcase is used to pressurized the Air Fuel mixture into
the combusting chamber and therefore if using the wet sump
system the combustion chamber will be filled with oil and
the spark plug will be coverd by burned oil that is why the
dry sump systerm is used.
in two stroke more heat is generated. in order to reduce
heat generation lubrication is needed.there is two power
cycles in when compared to 4 stroke ,more lubrication is
needed.this is the main drawback of 2 stroke .