Bore, Stroke and Displacement - EngineKnowHow

Bore and Stroke

In an engine, the:

                Bore – The diameter of the cylinder

                Stroke – The distance the piston travels vertically in the cylinder

 

When the piston it at the top of its stroke, this position is called Top Dead Centre, or TDC.

When the piston it at the bottom of its stroke, this position is called Bottom Dead Centre, or BDC.

 

Bore and Stroke

 

In engines, the bore to stroke ratio, or B/S is typically between 0.75 – 1.5.  An engine can be termed:

        Square: Bore (B) = Stroke (S)

 

        Oversquare: Bore > Stroke –     Typically favours performance

  • The lower stroke allows for lower average piston speeds and therefore higher engines speeds, resulting in higher power.
  • The decreased stroke results in reduced friction in the cylinder.
  • The large bore diameter allows for increased valve diameters increasing the engine’s ability to induct and exhaust the fuel / air, measured as volumetric efficiency.

 

        Undersquare: Bore < Stroke – typically favours fuel efficiency

  • A low combustion chamber surface to volume ratio results in lower heat losses.
  • The reduced distance for the flame to travel increases combustion efficiency whilst minimising the engines tendency to knock allowing higher compression ratios.

 

Displacement

The engine displacement, or the swept volume is the volume swept by the piston or pistons within their cylinders.  This can be calculated by:

 

 

 

Or,


 

 

It is important to note that the engine displacement doesn’t include the volume of the combustion chamber at TDC.

Engine displacement is commonly noted as Litres (L), cubic centimetres (cc, cm3) and cubic inches (cu in, in3) where: