FOUR-STROKE EN­GINES

EX­PLAINED

Motorcycle News (UK) - - This Week -

Last week we looked at the sim­ple, but clever, two-stroke en­gine, and this week we’re dou­bling the com­plex­ity.

In four-stroke en­gines there are four cy­cles be­tween each com­bus­tion se­quence, split­ting the two-stroke’s mul­ti­task­ing pair of strokes into their four com­po­nent func­tions, and re­sult­ing in the in­nu­endo-laden process of suck, squeeze, bang, blow.

On the first stroke the pis­ton draws air/fuel mix­ture into the en­gine via the in­take valves on its down­ward stroke (suck) be­fore the valve closes and the pis­ton rises to com­press the mix­ture (squeeze). This com­pressed gas is then ig­nited by the spark­plug fir­ing, caus­ing a mas­sive ex­plo­sion (bang), which then forces the pis­ton back down.

As it rises again, the ex­haust valve opens and the ris­ing pis­ton ex­pels the burnt gases via the ex­haust (blow), and the cy­cle starts again. So for each fir­ing cy­cle the pis­ton rises twice and falls twice – hence ‘four-stroke’.

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