Lubrication of a two-stroke petrol engine is quite different from that of a four-stroke engine, as it is not possible to have separate lubricating oil sump. Hence a two-stroke petrol engine is always lubricated by a petroil mixture. The lubricating oil is mixed with petrol in suitable proportion as per the manufacturer's recommendation. In the latest generation Two-stroke engines, the lubricating oil is directly injected by an accurate metering device from a separate tank into the petrol in quantities dependent on the speed and load of the engine.

When the petrol mixture enters the crankcase, because of the high temperatures prevailing, the petrol fraction vaporizes leaving a thin oil film of lubricant on the crankcase, cylinder walls, crankshaft and bearings.

The primary requirement of two-stroke engine oil is its ability to readily mix with petrol and burn without leaving too much ash. The oil should reduce engine wear, spark plug fouling, combustion chamber deposits, piston ring sticking, exhaust port blocking and silencer deposits. In addition the oil should have the ability to keep the engine clean at high engine operating temperatures and prevent bearing corrosion at lean oil-fuel ratios.

Four stroke two wheelers have a different lubrication requirement than a two stroke two wheeler since it does not use the Petrol mixed with oil. These engines are like the four stroke engines used in Cars. A separate Oil sump in the engine lubricates the various moving parts in the engine.

The main differences between the four stroke Engine in a Two Wheeler and in a Car are:

• Two wheeler Engines are Single Cylinder Engines where as Car engines are multi cylinder (3 or 4 or 6 cylinder engines).
• The Engines in two wheeler are air-cooled engines where as Car engines are Water / Coolant cooled Engines. The air drag across the engines as the vehicle moves cools the engines in two wheeler.
• In two wheeler, the Engine, the clutch and the Gear trains are housed in a single unit and lubricated by the same oil where as in a car, Engine, Clutch and gearboxes are separate unit and lubricated by separate oils.

These difference in Engine configuration demands a separate quality of Oil for use in four stroke Engines in two wheeler. The air cooled Engines suffers the most in terms cooling efficiency wherein the vehicles running at slow speed, such as city driving condition, leading to higher Engine operating temperature compared to a car Engines where the Engine is cooled by the coolant leading to more or less constant operating Temperature irrespective of the Speed of the Vehicle. This higher operating temperature encountered demands oils with very high thermal and Oxidation stability and also higher viscosity grades.

To achieve higher & higher fuel economy in a Car Engine, the oils are formulated with additives which reduces the friction drastically between moving parts in Engines. However use of this oil in a Two Wheeler, which has a wet clutch incorporated in the Engine, will cause starter clutch slippage (Engine start failure) and also Poor running due to higher engine stall speeds
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To overcome this the Oil for four stroke two wheeler Engine demands controlled friction characteristics as specified by the JASO (Japanese Automotive Standards Organization)

For detailed working of a four stroke engine, Please see the car lubricants section.