Internal combustion engines naturally generate heat in the cylinders as part of the combustion process. Most of that heat escapes the cylinders in the form of hot exhaust gas, but some also escapes through conduction across its walls. Most metal conducts this heat to radiate out into the atmosphere, but there is a limit to how much a metal can radiate heat based on its surface area. If the heat radiated outside the cylinder is much less than what is left inside, the engine can overheat. Overheating can result in uneven thermal expansion of parts, corrosion, and thermal stress, all of which can lead to mechanical failure.
Therefore, engine designs implement cooling systems. The main work of an aircraft engine cooling system is to reduce the excess heat generated in the cylinder. It should not reduce the waste heat too much, as this can negatively affect the engine’s performance. Generally, engine cooling systems should reduce excess waste heat by about 30%.
Small engines with relatively low power output tend to have air-cooled engines. These are seen in motorcycles, small tractors, scooters, and propeller aircraft. These engines are cheap, easy to construct, and lightweight. Air cooling systems work off of air velocity and surface area on heat transfer between two mediums or bodies. When two mediums are contact, heat is transferred from the one with high temperature to the one with low temperature. Therefore, in an engine, heat is first transferred from a cylinder to its walls, and then is taken away from the walls by air by means of natural convection. The walls of the cylinder heat the surrounding air, and the hot air rises to make space for cool air to take its place and continue the cooling process. To speed this process, some engines will mount a fan in the engine shaft to speed up the cooling air’s movement. Some engines also mount larger cooling surfaces like fins to provide more surface area and increase the rate of the cooling process.
A large number of automobile and industrial engines are water-cooled. These engines include a radiator (a type of heat exchanger), an expansion tank, cooling fan, water pump, thermostat, bypass valve, cylinder jacket, and pressure cap. In a water-cooled engine, water flows from bottom to top in a cylinder jacket, with water flowing in series to the cylinder heads. This means that some of the hot water created by cooling the cylinder cycles back to the cylinders and exhaust valve by design. This is done to prevent thermal shock and stress to the cylinder head. Water is used to cool solid parts like the head and cylinder, while moving parts like the piston rely on lubrication oil for cooling.
The biggest problem with water-cooling systems is that water expands exponentially when it freezes into ice, a serious concern in cold-weather regions. When water freezes and expands, this can cause fractures and damage to the pipes and valves of the aircraft engine cooling system. To counter this, antifreeze chemicals are mixed in with the water to lower the water’s freezing temperature, with ethylene glycol and polypropylene glycol being the most common choices.
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