Brake Mean Effective Pressure   Volumetric Flow Rate  
  Indicated Mean Effective Pressure   Friction Mean Effective Pressure  
Compression/Expansion Ratio   Combustion   Pumping Losses   Mean Piston Speed   Total Bore Area   Volumetric Efficiency
  Gas Dynamics  

The mean piston speed (vmps) is the average speed of the piston in a reciprocating engine which is related to the stroke (S) and the crankshaft angular displacement for one stroke (θs) at a given angular velocity (ω).

vmps =
ω S 
animated piston engine

By design, most reciprocating engines have an angular displacement of 1/2 revolution (π rad) per stroke.

Mean piston speed is proportional to:

Mean piston speed is a much more important (and limiting) feature of an engine than its RPM.

Fuel consumption will be the limiting factor for most «working» engines, so their speeds are kept as low as possible. For racing engines, power is the goal, so we try to get the highest mean piston speed possible. The upper limit is being imposed by the area of the intake valve(s) with respect to the bore area. This is what restricts the flow and is usually fixed by the cylinder head design. The other limit is imposed by the strength of the mechanical components.

Typical Mean Piston Speed
engine typevmps
@ max power
small industrial engine7 m/s
marine & generator8.5 m/s
train & truck11 m/s
automobile (low speed)14 m/s
automobile (typical)16 m/s
automobile (hi-performance)18 m/s
racing (endurance)22 m/s
racing (short race)25 m/s
drag racing30 m/s

Wankel engine

The mean «piston» speed is a rather meaningless value in the case of a Wankel engine. As a trial, this site will consider the more meaningful mean rotor tip speed instead. The rotor tip motion against the housing is similar to the piston motion against the cylinder wall in a reciprocating engine.

By design, the rotor of a Wankel engine rotates 3 times slower than the output shaft. So the mean rotor tip speed ( vmrts ) can be found with the rotor angular velocity ( ωR ) and the rotor radius ( R ):

vmrts = ωR R  
vmrts =

Where ω is the angular velocity of the output shaft.

Connecting rod length (more)