We all know that an internal combustion engine burns fuel to produce power which spins the wheels in an automobile. But the second law of thermodynamics makes sure that only a part of the energy of the fuel can be converted to useful power. The rest converts to heat, some of which leaves the engine through the exhaust, while some of it tends to remain and heat the parts in proximity. If this heat is not taken care of, it can cause damage. And so, we have coolant running around the engine to prevent it from overheating. This coolant then flows into the radiator, where it loses its heat to the surrounding air.
But how does this heat transfer happen? The name radiator sure is misleading in this context- because it is not at all designed to lose heat through radiation. Rather, it applies the other two modes of heat transfer known to us- conduction and convection. The heat from the coolant first passes from the inner surface of the radiator tubes to the outer surface and subsequently to the fins through conduction. Then, convection causes the fins to lose heat to the flowing air.
Let’s dig a little deeper into this. In an IC Engine, the energy of the fuel is divided into three parts: horsepower, exhaust heat and cooling system load, which is the term for heat handled by the radiator. The heat to be lost per unit time (measured in BTUs) by the radiator at a particular rpm is almost constant. What we need is a radiator which keeps the inlet temperature (of coolant coming from engine) low enough to prevent the engine from overheating. In simple words, this means that both a good and a bad radiator transfer the same BTUs per minute to the surroundings. However, the good radiator does so while keeping the engine temperature well within safe limits, and the bad radiator fails to keep this temperature under check.
The heat transferred by convection is directly proportional to the difference of average radiator core temperature (the mean of the radiator inlet and outlet temperatures) and the ambient air temperature. It also depends on the surface area available for heat transfer and the speed of flowing air. The job of increasing the surface area within the space available is achieved with the help of fins and the job of increasing airflow speed is done by installing fans which suck air into the radiator. These factors lead to a smaller temperature difference for the same heat transfer, which in turn causes the average core temperature and correspondingly the engine temperature to reduce. One thing to be noted here is that even though increasing fin density would result in better radiator performance, it has to be kept within a limit, otherwise it poses restrictions to the airflow, thus negating the advantage of increased surface area.
The heat loss rate of the coolant can be calculated by multiplying the temperature drop of coolant in the radiator with the mass flow rate of coolant and its specific heat. Since this heat loss rate is equal to the cooling system load, we can conclude that increasing the coolant flow rate (by using a stronger coolant pump) will lead to a decreased temperature drop, thus increasing the radiator outlet temperature and reducing the inlet temperature, giving better cooling performance.
TAnother thing to put light upon is the dimensioning of the radiator. The frontal surface area of the radiator is usually kept large, to expose more coolant to undisturbed airflow, but is limited by the space availability. On the other hand, the thickness of the radiator is a quantity to be well optimized. As the air traverses the thickness of the radiator, it gets warmed up, and on the backside, its cooling efficiency diminishes. The thickness should be kept only to the point where cooling is still effective.
The performance of the radiator also depends largely upon the material used for building it. The material should be such that it allows high conduction as well as convection rates. The most commonly used materials which have such properties are copper, brass and aluminium. All these factors sum up to keep your engine cool and running.