4 – stroke Engine
In 4st engines same oil is used for crankcase and cylinder lubrication.
So they are exposed to high temperature.
In order to maintain their property the lube oil used in 4st engines are
- high viscosity to remain viscous at high temperature,
- good load carrying capacity,
- high thermal conductivity to transfer the heat, and
- high flash point to prevent any explosion.
If blow passes occur with heavy fuel oil in a trunk-type engine, incomplete combustion products reach directly into the crankcase and can cause contamination of lube oil with acid. Thus, high TBN oil must be used in this type of engine to neutralize the acid contamination.
The “TBN” value is higher for 4-stroke oil. Since there is no separation between the space of the cylinder and the space of the crank in the trunk style engines, the blow past process will easily contaminate and acidize the oil.
TBN for lube oil for 4 -Stroke engine is 30mg/koh
While In 2 stroke engines, the two different oils are used for crankcase and cylinder lubrication.
It is because in 2-stroke, cylinder and crankcase are separated by diaphragm.
Since the crankcase oil of 2-stroke engines are not exposed to high temperature they have relatively low flash point than used in 4-stroke but high enough to prevent crankcase explosion, less alkaline, low viscosity as compared to 4st engine.
TBN value of lube oil if crankcase for 2 – stroke engine is 30 mg/koh.