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’.