Mean piston speed
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 had the highest piston speed for any production car (25.2 m/s) until the B7 Audi RS4 (2006-2008) which was powered by a 4.2 liter, naturally aspirated V8 with a redline of 8250 RPM, at which point the mean piston speed is (25.7 m/s).
The BMW S38B38 3.8 litre six cylinder engine from the 1993-1995 e34 M5 sports saloon has a redline of 7200rpm and stroke of 90mm giving a mean piston speed of 21.6m/s.
- 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
- Some extreme examples are NASCAR Sprint Cup Series and Formula one engines with ~25 m/s and Top Fuel engines ~30 m/s