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10 September 2022 • 15 min. read

It is the main component of the kart’s liquid cooling circuit and cools the water to stabilize the temperature of the engine. To use it at its best it is essential to know its structure and the principles on which its operation is based.


Two-stroke or four-stroke internal combustion engines heat up due to the combustion that takes place inside the combustion chamber. Combustion generates thrust on the piston crown which is partly transformed into rotary motion by the connecting rod-crank system and partly into heat to be disposed of. The heat escapes from the exhaust pipe but also accumulates on the engine walls and must therefore be quickly disposed of, to avoid overheating and consequent seizure of the internal components. There are two ways to remove residual heat from the engine: directly through air that hits the engine (air cooling), or through a closed circuit that surrounds the engine, inside which flows a coolant that “collects” the heat and transports it to the radiator, which in turn transfers it to the air (liquid cooling; for more information read “Technique – Cylinder cooling”).

Air cooling is the simplest. It does not require maintenance or components external to the engine, but it is not very effective if the power involved is high. In fact, the more power an engine has, the greater the heat generated, which needs to be disposed of. This is why, for the most performing karts, the choice of the type of cooling falls on “liquid” cooling. In this article we will talk about the radiator, the main liquid cooling component.


1. An example of an engine, in this case an air-cooled 60 cc.
2. An example of a water-cooled engine, of the same displacement as the one above (60 cc).

With liquid cooling, heat is removed from the cylinder by passing the coolant in contact with its outer surface. Subsequently, the liquid is cooled by the ambient air, by means of a water-air heat exchanger, known in jargon as a “radiator”. A radiator consists of a radiant mass composed of finned tubes and two tanks to which the liquid delivery and return hoses are connected. A centrifugal pump is used to circulate the liquid inside the radiator, set in motion by the kart’s axle. The coolant therefore serves as a vehicle for transporting heat from the small surface of the cylinder and the engine head to the large surface, exposed to the air, of the radiator.

The structure of an engine has cavities or channels, which ensure the most thermally stressed areas by the liquid are reached. In karting the only coolant allowed is water, because according to the regulations any other mixture, such as ethylene glycol commonly used in cars, is prohibited for safety reasons, since in the event of a spill it can make the track extremely slippery. During the engine’s operation, the heating of the liquid causes an expansion and therefore a pressurization of the same, up to about 0.9 bar. This helps to increase the boiling temperature by a few degrees, avoiding the phenomenon of cavitation, that is, the formation of steam bubbles that can cause a decrease in the efficiency of the system and erosion phenomena of the internal parts.


The coolant serves as a vehicle for transporting heat from the engine to the radiator.

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