# Piston speed

The piston speed is the speed (v) with which the piston of a reciprocating engine covers the way from top dead center (TDC) to bottom dead center (BDC) or vice versa.

The piston speed in piston machines is not uniform:

• The linear movement of the piston is coupled to the rotating movement of the crankshaft . Therefore, depending on the connecting rod ratio λ, there is an approximately sinusoidal curve of the piston speed for each stroke. The movement deviates from a pure sine curve, since it is superimposed with further movements with double the frequency. The maximum piston speed is z. B. for λ = 0.25 at a crank angle of about 76 ° and is about 1.6 times the mean piston speed. The mean piston speed assumes a constant speed over the stroke.

The average piston speed is normally used for the calculation.

This is calculated in the MKS system of units as follows:

${\ displaystyle v_ {m} = 2 \ cdot n _ {\ mathrm {s}} \ cdot s _ {\ mathrm {m}}}$ ${\ displaystyle v_ {m}}$ = Mean piston speed in m / s
${\ displaystyle n _ {\ mathrm {s}}}$ = Engine speed in 1 / s
${\ displaystyle s _ {\ mathrm {m}}}$ = Piston stroke in m

or in common units:

${\ displaystyle v_ {m} = n \ cdot s: 30000}$ ${\ displaystyle v_ {m}}$ = Mean piston speed in m / s
${\ displaystyle n}$ = Engine speed in 1 / min
${\ displaystyle s}$ = Piston stroke in mm

The maximum piston speed is essentially limited by the lubricating oil , the material combination of piston and cylinder and the thermal load on the piston (power), which is why the average piston speed only exceeds 20 m / s in racing engines.

In practice, the piston speed in Langhuber engines is similar to that in Kurzhuber engines . There are also hardly any differences between two-stroke and four-stroke engines. Even the small gasoline, methanol, and nitro engines in model airplanes have piston speeds similar to large diesel engines. Standard values ​​for mean piston speeds in mass-produced engines are between 10 and 15 m / s.

## literature

• Wilfried Staudt: Handbook Vehicle Technology Volume 2. 1st edition, Bildungsverlag EINS, Troisdorf, 2005, ISBN 3-427-04522-6 .
• Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3rd edition, Westermann Schulbuchverlag GmbH, Braunschweig, 2000, ISBN 3-14-221500-X .
• Jan Drummans: The car and its technology. 1st edition, Motorbuchverlag, Stuttgart, 1992, ISBN 3-613-01288-X .