Mean piston speed

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The mean piston speed is the average speed of the piston in a reciprocating engine. It is a function of stroke and RPM. There is a factor of 2 in the equation to account for one stroke to occur in 1/2 of a crank revolution (or alternatively: two strokes per one crank revolution) and a '60' to convert seconds from minutes in the RPM term.

MPS = 2 * Stroke * RPM / 60

For example, a piston in an automobile engine which has a stroke of 90 mm will have a mean speed at 3000 rpm of 2 * (90 / 1000) * 3000 / 60 = 9 m/s.

It is a good indicator of the class and performance of an engine relative to its competitors. Honda S2000 has the highest piston speed for any production car (25.2 m/s).

The BMW S38B38 3.8 litre six cylinder engine from the 1993-1995 e34 M5 spoerts saloon has a redline of 7200rpm and stroke of 90mm giving a mean piston speed of 21.6m/s.

Classes

low speed diesels 

~8.5 m/s for marine and electric power generation applications

medium speed diesels 

~11 m/s for trains or trucks

high speed diesel 

~14 m/s for automobile engines

medium speed petrol 

~16 m/s for automobile engines

high speed petrol 

~20–25 m/s for sport automobile engines or motorcycles

competition 

Some extreme examples are NASCAR Sprint Cup Series and Formula one engines with ~25 m/s and Top Fuel engines ~30 m/s

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