Squish (piston engine)

Squish is an effect in internal combustion engines which creates sudden turbulence of the air-fuel mixture as the piston approaches top dead centre (TDC).^[1][2]

In an engine designed to use the squish effect, at top dead centre the piston crown comes very close (typically less than 1 mm^[2]) to the cylinder head. The gases are suddenly "squished" out within the combustion chamber, creating turbulence which promotes thorough air-fuel mixing, a factor beneficial to efficient combustion. Squish effect may be found in side-valve, OHV and OHC engines, including engines with a Heron cylinder head. Squish effect may be found in any fuel type internal combustion piston engine. Squish piston engines are also found in both two stroke and four stroke engines.

Turbulence in the combustion chamber due to this squish helps with air-fuel mixing, cylinder wall heat transfer, thermal efficiency, and overall engine performance. Heat transfer is aided when the combustion gasses swirl around and heat the cylinder wall, allowing the cooling system to work more efficiently.^[3] This efficiency and swirling can also reduce the amount of soot production.^[4]