The Piston - Feel the Force

Taking a generic 600cc inline four engine with a bore of 67mm, stroke of 42,5mm, and max power at 13,500rpm as an example – let's look a bit closer. At maximum power every minute the little piston goes up and down 13,500 times, which equates to 225 times a second, giving it an average speed of 19.125meter a second.

Okay, so 19.125m/s doesn't sound that fast, but this is an average speed. The piston is going up, coming to a halt, returning to the bottom of its stroke, coming to a halt again then returning again to the top. And it's doing this 225 times a second, which is why the average speed is quite slow, it has to do a lot of stopping and starting reducing its overall average, so how fast does it actually go? The peak speed at 13,500rpm is 36.4m/s. But remember, it is only traveling a distance of 0,0425meter.

The piston's peak acceleration is 51.49km/sec, which is very high acceleration. To put it in context a bullet accelerates out of a small pistol at around 0.884km/sec, a road car's piston's peak acceleration is around 30km/sec and a current MotoGP motorcycle can go as high as 100km/sec.
Although a piston weighs around 150grams, when it changes direction it is subjected to massive change in force, around 5895G-force, or 5895 times the force of gravity. The highest G a roller coaster creates is about 5.5G, a fighter pilot will pass out at around 8G, a extreme sneeze is around 2.9G and the maximum a human body can take is around 40G.

Pistons are hard, and it is in an effort to reduce these huge forces that race motorcycles run lightweight pistons.
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