Aircraft technical Basics: Aircraft Engines - RAF Flying Training Manual - Chapter VII.- Engines: Cooling

COOLING

The need for cooling

8. If an engine were perfectly efficient all the heat produced in it would be turned into useful work and the problem of cooling would not arise. This is not possible, however, and a large part of the heat produced, by the burning of the charge which is continually going on inside the cylinder, finds its way into the metal and will, if no special steps are taken to get rid of it, make it so hot that its mechanical properties will be affected, the oil would then burn and the engine would seize up. An aero-engine develops a great deal of power for its size and this means that a correspondingly large amount of heat must be disposed of somehow. Moreover, not only must the average temperature of the various parts be kept down, but local hot spots must be avoided and the temperature be kept as even as possible. It is comparatively easy to deal with cylinders, which are in direct contact with the cooling medium, but some parts like pistons, valves and sparking plugs can only be indirectly cooled and special attention has to be paid to them in order to avoid hot spots. Pistons get rid of their heat by conduction through the metal of which they are made, this always has a high conductivity, to the cylinder walls from which it escapes to the coolant. Valves and sparking plugs will be discussed later. There are two principal methods of cooling the cylinders—air cooling and liquid cooling—but whichever is adopted it must be remembered that the heat eventually finds its way to the air through which the engine moves, the difference being that in one case it is taken away by direct contact with the air and in the other it is taken from the engine by a liquid coolant which passes it on to the air through a radiator.

Air cooling

9. On air cooled engines the cylinders are made with fins on the outside, baffles or deflectors are sometimes fitted around the cylinders in order to increase the surface of metal through which the heat can escape, and in order to make the air flow all over the surface. Air cooled engines have the advantages of simplicity and light weight, they are less vulnerable than liquid cooled ones but unfortunately it is difficult to control the flow of air so exactly as to ensure adequate cooling of the hotter parts of the engine without wasting air flow on the parts that do not need it and so causing unnecessary drag. By using suitable cowlings and baffles, however, the resistance can be reduced to something comparable to that of a liquid cooled engine.

Liquid cooling

10. In liquid cooled engines the cylinders are surrounded by jackets containing the liquid coolant which is circulated through them and the radiator by a pump. By careful design of the coolant passages the flow can be so controlled as to ensure more even cooling, the heat so absorbed is dissipated to the air through a separate radiator. The coolant used may be either water, a mixture of water and ethylene glycol (used to prevent freezing in cold weather) or ethylene glycol by itself. The last named has the advantage of boiling at a higher temperature than water, this not only enables the engine to run at a somewhat higher and more efficient temperature but requires a smaller radiator with correspondingly less drag. Liquid cooling has the advantage that the engine can be completely enclosed and only the radiator need project into the air stream, so that the great advantage of a liquid cooled engine is the low drag of the installation. On the other hand, there is the complication and extra weight of pipes, pumps and radiators, and risk of leaks, both in and outside the engine. The choice between liquid and air cooling is largely influenced by the comparative importance, for the purpose for which the engine is to be used, of low drag as against low weight, simplicity, and easy maintenance. (See fig. 97.)

Composite cooling

11. In some water cooling systems, the water is allowed to boil under certain conditions, the steam is condensed and the condensate returned to the water system. This is known as composite cooling, and a full description will be found in the handbooks of the aeroplanes in which it is used.