An essential element in the phenomenon of combustion, the squish is a particular structure in the inner part of the cylinder head, shaped like an inclined circular wheel. This geometry optimises engine performance significantly and virtually all engines for racing karts have it
The “Squish” is the area of the cylinder head, generally in the shape of a circular wheel, which surrounds the central part which is more or less hemispherical. This wheel has a minimum inclination with respect to the piston crown, and it is in close proximity to it, which in engines for racing karts varies between a little less than 1 mm and about 1.5 mm, depending on the characteristics of the engine.
The squish wheel (also called “squish band” or “squish ring”) can vary in width, but its surface area, it should be remembered, practically never exceeds 50% of the area of the cylinder section. The angle of inclination also varies and, obviously, depends on the corner of the piston crown, which in
turn is directly related to the curvature of its head. It is clear that, if you change the piston or the cylinder head, you have to ensure that these angles are the same, in order to maintain the performance characteristics and reliability of the engine unchanged, which in fact relate to the squish.
All competition engines 2 now have the squish, including karts, and its effect, which allows better and more uniform combustion of the air-fuel mixture, is crucial in optimising performance. Therefore, it is without doubt interesting to know the mode of action, the advantages it gives and its critical elements, also taking into account the possibility of an adjustment parameter which, however, is not easy and there is a need for caution.
The head of the internal cylinder
A first indication of how the “squish” works comes from its name, an onomatopoeic English term that reproduces the sound of the mixture which, compressed by two surfaces, flies away. In fact, on a practical level, when the piston has almost reached the top dead centre (ATDC) in the final part of the compression phase, it is positioned few tenths of a millimetre from the wheel of the squish. This leads to a strong compression of the air-fuel mixture that, thanks to the angle between the squish and piston crown surfaces, flies towards the centre of the combustion chamber. This is the first positive effect of the squish: pushing the mixture farthest from the spark-plug towards the area where the flame front can reach it earlier, shortening the combustion time and burning the maximum amount possible, giving an improvement in engine performance.
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