2-stroke engine

[elpea]

How does the engine on something like a strimmer or outboard motor work? Why do you have to mix oil and petrol together?

[Catso]

The internals of an engine have to be lubricated, or the moving parts would soon overheat and sieze up. An engine just couldn't work without some lubrication.

In a 4-stroke engine (EG car) a reservoir of oil is kept internal to the engine, in the sump. An oil pump takes oil from the sump, and pumps it through a complicated series of channels so it gets to every bit that needs lubricating. It then drops back into the sump, mainly through gravity. The oil goes round and round this circuit repeatedly.

In a 2-stroke, usually because of the design needs of simplicity and lightness, there is no oil circulating system. The oil is provided in the fuel, and every time the engine draws in fuel, to burn it, it also takes in some oil, which provides one-off lubrication of the internals of the engine. Most of the oil then gets burnt, with the petrol. Hence the smoky, smelly exhaust of 2-strokes.

[elpea]

many thanks, catso! that's brilliant

[elpea]

can you describe what the relevance of 2 or 4 stroke is please?

[Catso]

2- vs 4-stroke doesn't have to have any link to the petrol/oil mixing thing, but often does. In everyday circumstances, 2-strokes need the oil/fuel mix, 4-strokes don't. But it ain't necessarily so - there may be wierd engines that don't conform to this.

2-strokes mean that the engine (well, each cylinder of) produces one power stroke (IE one up & down movement) for every two strokes of the piston. IE 50% efficient, in a manner of speaking. 4-strokes means each cylinder does four strokes for each power stroke.

Which, by the by, is another reason why 2-strokes have a higher power to weight ratio than an equivalent 4-stroke, they have twice as many power strokes.

[Adam80]

http://science.howstuffworks.com/two-stroke.ht m/printable