TWO-STROKE EN­GINES

Motorcycle News (UK) - - This Week -

In a two- stroke en­gine the vac­uum cre­ated by the pis­ton ris­ing sucks a mix­ture of air/ fuel/ two- stroke oil into the crank­case un­der the pis­ton via a valve (gen­er­ally ro­tary or reed). While this is hap­pen­ing, the pis­ton is com­press­ing the pre­vi­ous in­take mix­ture of air/ fuel/two-stroke oil, which is then ig­nited by the spark­plug, forc­ing the pis­ton back down again.

As it trav­els down­wards, the pis­ton’s mo­tion in­creases the pres­sure on the mix­ture in the crank­case, caus­ing it to travel via an in­take port to the com­bus­tion cham­ber above the pis­ton. This new fuel helps push out the burnt gases via the ex­haust port be­fore the pis­ton rises again, clos­ing the ex­haust port as it com­presses the fresh air/fuel/two-stroke oil mix­ture and sucks in an­other load be­neath it, thus start­ing the whole se­quence again.

It’s called a two- stroke be­cause of the pis­ton’s two move­ments (strokes) up and down the cylin­der be­tween each com­bus­tion se­quence. A four-stroke sep­a­rates the same func­tions into four dis­tinct phases (suck, squeeze, bang, blow) – but that’s an­other story.

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