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).
By design, most reciprocating engines have an angular displacement of 1/2 revolution (
Mean piston speed is proportional to:
- Friction losses (higher fuel consumption)
- Volumetric flow rate (higher power)
- piston acceleration & deceleration (higher stress on mechanical components)
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.
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|
Where ω is the angular velocity of the output shaft.