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Tech Talk: Cooling Systems Part 2

cylinder and head water#E28

The coolant enters the cylinder head and then exits at the bottom of the cylinder

The average temperature of a kart engine is a balance between the heat produced by combustion (which heats up the engine’s parts) and the sum of heat expelled by the cooling system and the quantity ‘absorbed’ by the cool fresh mixture entering the combustion chamber (heat that exits the engine). In particular, some elements and volumes of the engine need good cooling to avoid engine seizure. Water-cooling Water-cooling reduces the engine’s temperature using water, or, more usually, a refrigerating liquid or coolant with good cooling and anti-corrosion properties. Some coolants also have special characteristics that avoid cavitation, a phenomenon that produces air inside the liquid present in the water pump, reducing its efficiency. Liquids have a much higher heat transmitting coefficient (greater capacity to transmit heat) than air. This means that a water or liquid cooling system has a much greater potential in reducing engine temperature. Moreover, it is less dependent on weather conditions as air-cooling is more dependant on the ambient air temperature. The possibility of keeping engine temperatures lower gives the opportunity of having smaller tolerances between the mechanical elements of the engine because the deformation and expansion of the metal will be smaller. Elements of a water-cooling system A water-cooling system on a kart is made up of a rotating pump, two elastic rings mounted on a circular guide positioned on the rear axle, special rubber pipes fixed to the engine and to the radiator by metal jubilee clips and a radiator mounted on the chassis. Of course the engine will also have to be adapted in its shape to create passages for the liquid around the parts that need to be cooled, especially the cylinder and cylinder head. The rotating pump is generally made of plastic, but sometimes metal, and has a diameter of around 10cm. Its movement is guaranteed by the rotation of the rear axle via the two elastic rings.

thermocouple

Thermocouples are used to monitor coolant temperature

So the faster the kart travels the faster the pump rotates and the faster the liquid in the cooling circuit is pumped around. The radiator is where the water is cooled using external air. In fact a radiator is made of an aluminium structure of many very thin layers that give an extremely large area of contact between air and water. This is needed because the heat transmitting coefficient of air is extremely low, this is also the reason why air-cooled engines have such large, long and numerous fins. The large area of contact between air and water in the radiator gives a good and rapid transmission of heat between the hot water exiting the engine and the cool external air so that the temperature of the water after passing through the radiator will be decreased significantly. Cooling circuit The water or coolant is poured into the radiator. The pump rotates and the liquid (which exits the radiator with a low temperature) moves from the radiator, through the pump and then to the cylinder head and around the cylinder The passage through which liquid enters the cylinder is positioned by the cylinder head on the exhaust side. This is because that is the area where temperatures are higher so the coolest liquid is required. The liquid then exits the engine at the bottom of the cylinder having absorbed the heat of the engine, reducing cylinder temperature and increasing the liquid’s temperature. Finally the liquid, that by now will have become hot, returns to the radiator to be cooled. Thermocouple Thermocouples are used to help check the condition of the cooling system and the temperature of the engine since they are sensors that quickly measure the temperature of the coolant.

water pump

The rotation of the rear axle drives the pump

The thermocouple is positioned on the pipe through which the coolant exits the engine (hottest point), so the temperature measured is very close to the temperature of the coolant inside the cylinder, since it does not cool down much by flowing along the rubber pipes. We will see in the next issue how tolerances change in a kart engine depending on its temperature and what a too high or too low engine temperature does to engine performance. Checking the system To ensure good cooling of an engine using a water-cooling system we should always check the following: 1. Check that there is coolant inside the radiator. 2. Check that the elastic rings are in good condition since they do wear out and can break. If both rings break the pump stops turning and working and the coolant does not move through the cooling circuit. 3. Check that there are no leaks in the circuit otherwise coolant could be lost during driving. High temperatures can increase the pressure inside the circuit and leakage would also increase. 4. Check that the pump is working by opening the circuit, for example where the rubber pipe enters the cylinder head. See if by rotating the tyres and the rear axle that coolant comes out of the pipe. If it does, then the pump is working. 5. Always fill the radiator with coolant, then turn the rear axle to fill the circuit. Then pour more coolant into the radiator if required. 6. If, when running the kart, the engine loses power and starts to run roughly, stop and check the cooling circuit, whether coolant is running in the circuit and if in fact there is any coolant at all. Such rough running could be due to too high temperatures in the engine’s combustion chamber.