Tech Talk: Kart Cooling Systems Part 1

PCR aria copia
A typical air-cooled engine with radial fins on the cylinder head and horizontal fins on the cylinder

In the year 2000 the CIK authorised the introduction in selected classes of watercooling systems on 100cc kart racing engines that had formerly been air-cooled. Such a change determined both advantages and disadvantages. Better reliability on the one hand but with increased complexity. This month as an introduction to the subject of cooling systems we shall first take a look at air-cooling. Why a cooling system? When an internal combustion engine is working it burns fuel and produces, through this process, great quantities of energy in the form of heat and high pressure. Pressure in the combustion chamber works on the piston, forcing its downward movement and consequently all movement in the engine from the connecting rod to the crankshaft. On the other hand, heat produced increases the temperature of the engine’s parts such as the cylinder and the cylinder head. These parts are made of special steel alloys that resist extremely high temperatures. However, such high temperatures causes the expansion of the materials and leads to changes in the very precise geometry of the engine. Moreover, an increase in the internal temperature in the combustion chamber can lead to the phenomena of detonation (uncontrolled explosions of the mixture with a consequent seizure of the engine). To avoid excessively high temperatures heat must be extracted from the engine somehow and this is done by the cooling system. There are different kinds of cooling systems, either with air (forced or free) or with liquid (water or special liquid refrigerants). If we consider the gas flow inside the combustion chamber we notice that the burnt gases exiting the exhaust port give a higher temperature in this area. In fact in the rest of the combustion chamber we have the lateral transfer ports and the TT that have a cooling effect thanks to the fresh air entering the volume.

Cooling systems, both air and liquid, therefore tend to concentrate on cooling the exhaust port side of the engine. Air-cooling system This system extracts heat from the engine through the direct contact between the air and the external surface of the engine, particularly the cylinder and the cylinder head area, the hottest parts of the engine. Since in karts the engine is uncovered and out in the external air flow, the solution is a free air cooling system meaning that there is no fan blowing air against the engine but rather the movement of the kart that creates the air flow. The heat exchange between air and engine depends on the difference in temperature between the two, on the area of contact between the air and the metallic surface of the engine and the coefficient of heat exchange between air and metal. The first thing to consider is the heat exchange coefficient between air and metal that indicates the capacity of heat to pass from air to metal and vice versa. This coefficient is very small for air (but much greater for liquids such as water). The only way to increase the quantity of heat moving from the engine to the external air is to increase the contact surface area of the cylinder, since the coefficient of heat exchange and the temperatures of the air and the metal are not controllable. This is the reason why in aircooled engines, such as kart engines, the surfaces of the cylinder and the cylinder head are covered with large fins.

The capacity of these fins to liberate heat is proportional to their surface area and to their thickness. Go over a certain area and above a certain thickness though and the heat released by the fins remains constant. The distance between the fins is also fundamental. Fins too close together tend to stop the airflow passing by, with the result that the temperature of the air between the fins increases and the difference between the air and the metal temperature decreases, resulting in a reduced heat flow. Dimensions and geometry of fins As indicated, the cooling system has to concentrate more on the side of the engine with the exhaust port and that is why the fins on on an air-cooled engine will extend more on the side where the exhaust duct is connected. In addition to this the exhaust side on most kart engines is on the opposite side to the cooling air flow and so the air has already passed over the engine and increased in temperature before it reaches the exhaust side. A solution to this has been found by Sonik with their horizontal 100cc engine that allows the airflow to hit the exhaust side of the engine directly. Such a solution changes other engine parameters however and, perhaps because the disadvantages outweigh the advantages, the main kart engine manufacturers have not followed this layout. Fins on the majority of engines are orientated radially on the cylinder head and horizontally on the cylinder with different shapes of fin profiles such as rectangles, triangles, trapezoids and biconcave forms. This last solution has proved the most valid in transmitting heat but producing such a profile is expensive and so is often substituted by the triangular or trapezoidal shape. After having determined the amount of heat the engine has to shed, the geometry and the profile of the cooling fins is defined as is the quantity of fins as well as their distance apart and their length.

Sonik’s ‘lay-down’ engine allows cooling air to hit the exhaust side directly

Advantages of air-cooling The great advantage of air-cooling, especially the free air system, is its simplicity. The absence of added elements means that you avoid just about any malfunction. Only fins have to be added to the engine, increasing weight that would be increased anyway by a water-cooling system. Other advantages are that the engine reaches the right working temperature very quickly from cold. On the other hand, air-cooling is limited in its cooling capabilities, for instance in very hot weather and rough working conditions. This can lead to greater working temperatures stressing the engine more with the likelihood of deformation, meaning less reliability and also reduced performance. Also the fins of the air-cooling system amplify the vibrations of the engine as extra noise. A water-cooled engine is less noisy. Accepting the disadvantage of more complexity, the karting world in general has decided to increase the number of water-cooled engines in the last few years.