Piston Speed - EngineKnowHow

The stroke of the engine is determined by the offset between attachment point of the conrod to the crankshaft relative to the centre of the crankshaft.  This distance is known as the crank offset with the stroke calculated as:

In addition, the angle between the attachment point of the conrod to the crankshaft and the centre-line of the piston is known as the crank angle.  The crank angle for a four stroke engine is typically between 0 – 720° as this correlates with two complete rotations of the crankshaft.

The mean piston speed is an indicator for engine performance, with a high mean piston speed resulting in a higher potential speed of air into the cylinder.  This increased speed increases the mass of air in the cylinder resulting in higher torque and therefore power.   It is also an indication of how well the engine has been designed to handle friction and the inertia of rotating components.

The mean piston speed is the average speed of the piston over one crankshaft revolution as is calculated via:

 

 

As the mean piston speed increases, the following parameters will also increase:

  • Engine wear
  • Mechanical friction
  • The friction of the air flowing into the cylinder and combustants out of the cylinder
  • Engine noise
  • The need to increase valve diameters due to increased airflow potential
  • Loading on components due to the increasing acceleration and deceleration

 

Below are several examples of available engines and their associated mean piston speeds.

 

Engine Engine Type Max. Engine Speed Mean Piston Speed
Cosworth – 3L, V10, Petrol Formula 1 19,000rpm 33m/s
Honda – 2L, I4, Petrol Car Petrol 9,200rpm 25m/s
Audi – 3L, V6, Diesel Car Diesel 5,200rpm 16m/s
Volvo – 12.8L, I6, Diesel Truck Diesel 2,100rpm 11m/s