Aircraft engines need some form of cooling to avoid damage to the engine. The heat generated by the combustion process is only put to work partially and the rest will warm the engine and must be directed away through some form of cooling system. This will keep the engine within temperature limits as to ensure reliability and long service life.
Most commonly used piston aircraft engines have some sort of air cooling but there are some types that are using a liquid cooling system only or a combination of both, all with their own (dis)advantages. Gas turbine engines use secondary or external air to cool internal engine parts.
The lubrication system is also part of the cooling system, circulation oil will keep the internal parts like bearings and such on a certain temperature within specifications. This page here will concentrate on external engine cooling.
The cooling system is designed to reduce and control the temperature of the engine and in particular the cylinder barrels and heads, which contain the combustion chamber and valves. Other parts like bearings and pistons are usually cooled by recirculating oil with its own (sometimes thermostatically controlled, recommended!) cooler.
If there was no cooling at all the temperatures would rise to such high levels that detonation would be guaranteed with even more internal damage as a result. If left uncontrolled, the metals inside the engine would be red hot and even oil will start to boil and loose its effectiveness. Engine failure is then unavoidable.
Most of the excess heat is lost through the exhaust (this explains why the turbo gets red hot at high power settings) and oil cooler, but some 33 % must be dissipated by some form of cooling, be that air or some form of liquid solution.
Radial engines are a perfect example of air cooling, all cylinders are equally exposed to the airflow and there is an even temperature distribution. But the form of the engine constitutes high drag due to the large frontal area. Inline four, six or eight cylinder aero engines are almost exclusively air cooled, except for the Rotax, Viking and Subaru engines and some aero diesels. It is a good compromise to get a low drag form and there is no weight penalty compared to liquid cooling designs.
Air cooled cylinders have a large number of cooling fins cast around the heads and barrels. This increases the total cooling area, but it can be too effective in winter time or at altitudes above the freezing levels, a winterization kit is then used to control the incoming airflow. To guide the air from the intakes to the engine ducts, baffles and plates are used to maintain a positive air pressure above the engine underneath the top cowling. These items are very important to maintain the correct engine temperatures and an even cooling of all cylinders.
Cool air is taken in at the front of the engine and after cooling the cylinders, the warm (and expanded) air needs to be exhausted. This is done through openings in the lower cowling, sometimes controlled by cowl flaps. These pilot operated flaps are open during high power/ low speed operations (letting more air through during climb and taxi), they will also increase the parasite drag of the aircraft when in the open position. During normal cruise and descent the cowl flaps should be closed.
This type of cooling has a weight penalty but this is offset by the advantage that all cylinders are more even in temperature, they cannot be shock cooled during high speed/ low power descends and the coolant can be thermostatically controlled. This means that the engine is quicker to warm up and remains on a constant operating temperature at all times. Which extends into more reliability, lower fuel consumption and longer engine life, to name but a few advantages.
To keep the engine oil (or coolant) at the optimum operating temperature using a thermostat is almost a requirement. Some have used tape to block airflow through the radiators, but this is essentially a poor mans solution. We strongly recommend to install a thermostat of some sort to keep engine temperatures within operating limits. Have a look at the next link for some ideas: Silent Hektik from Germany sells Rotax temperature solutions.
Modern engines (Rotax, Viking) are all liquid cooled. Indeed, there is more weight and you need to check extra fluid lines and a radiator or two, but the advantages are there and the system can be very reliable if maintained properly. Personally I never had any problems with the cooling system of, for example, a Rotax four stroke engine.
Experimental Aircraft Info
