Finn Ogle, of Westland High School, asks :-

What is the difference between Two Stroke Oil and Engine Oil?

Adrian Aldridge, the Lubricants Technical Manager for Shell New Zealand Limited, responded.

It is essential that the correct oil be used for a given type of engine.

Engine Oils are used in Four Stroke Engines in which the oil is held in a compartment below the engine and is recirculated to lubricate the engine for many hours of operation. Therefore engine oils have complex chemical additives, such as detergents and anti-wear additives.

Multigrade engine oils also contain polymers to control their viscosity (thickness) at different temperatures so that the engine is easier to start in cold weather but well lubricated when running hot.

In Two Stroke Engines the lubricating oil is mixed with the fuel and therefore has to be compatible with petrol. It is continuously being replenished by new lubricant entering with the fuel. Spent and surplus lubricant is burnt in the combustion chamber, hence the blue smoke often seen coming from small two stroke motorcycles.

Therefore any additives in two stroke oil must burn cleanly, otherwise the exhaust port of the engine would clog up with a resulting loss of power. Multigrade oils must never be used in Two Stroke engines as the burnt polymer would soon clog the engine.

Water cooled two strokes engines, such as used in outboard motors, are very sensitive to ash forming oils. Therefore Two Stroke Oils are often dyed to help tell the Outboard Oils from the Air Cooled Two Stroke Oils and also from other engine oils. The dye also serves to show when Two Stroke Oil has been added to petrol to make a petrol/oil mix.