Fluid Motion into and in the Cylinder - EngineKnowHow

The shape and orientation of the intake port is an important aspect as it defines how the air or fuel / air is delivered to the cylinder and therefore the motion of the fuel / air in the cylinder.  Fluid motion into and within the cylinder is known as charge motion and is important due to:


  • Influencing turbulence in the cylinder which generally results in faster combustion, which:
    • increases combustion efficiency
    • reduces fuel consumption
    • reduces knock and preignition in spark ignition engines
  • The velocity at which the charge enters the cylinder
  • Directing fuel / air towards the spark plug or fuel injector for ignition
  • The transfer of heat which influences evaporation of the fuel, improving mixing of the fuel / air mixture
  • Can increase volumetric efficiency, although at the expense of turbulence


Turbulence is a key parameter in intake port design due to the benefits it offers in terms of fuel consumption however an intake port designed for turbulence will have a lower volumetric efficiency as the components which increase the turbulence of the intake flow limit the ability of the air / fuel to flow into the cylinder.  An optimised intake port would have a high level of turbulence whilst maintaining a high flow coefficient.

Engines developed for high performance have intake ports optimised to deliver the greatest mass of air to the cylinders under high load conditions, often compromising on turbulence and therefore part load fuel consumption to target higher volumetric efficiency.


Flow Phenomena

There are two predominant flow phenomena which can occur into the and within the cylinder, Tumble and Swirl.  Tumble style ports typically offer lower turbulence but higher flow then swirl ports.

Tumble within the Cylinder Engine: GD, Source Toyota

Intake Ports to deliver Tumble

Swirl within the Cylinder Engine: KD, Source ToyotaIntake Ports to deliver Swirl


Intake Ports are typically designed to offer a mixture of tumble and swirl to increase turbulence:


  • In spark engines the fuel is either injected during the intake stroke or added in a carburetor. Therefore spark ignition engines have a longer time for fuel / air mixing and favour tumble intake ports to add turbulence whilst not sacrificing to much flow for peak power.
  • In compression ignition engines fuel in injected during the compression stroke once the air has been compressed. Due to the short time for fuel / air mixing diesels typically prefer swirl ports to increase fuel / air mixing.  The swirl flow phenomenon also persists in the cylinder during compression and expansion and doesn’t collapse as much as tumble ports.


Some engines will also have flaps installed in the intake ports to increase the level of tumble or swirl.   Engine speed, throttle position, load and valve timing etc. will also heavily influence the fluid flow into the cylinder.  Therefore in some engines these flaps can be moved to alter the level of turbulence / flow across different parts of the engine map.


As the piston nears the roof of the cylinder, tumble and swirl will each continue to occur however a final common flow phenomena which occurs in the cylinder is squish, particularly in higher compression engines and can be further influenced by piston crown design.

Squish within the Cylinder Crown of a diesel piston to influence squish