Internal combustion engine cooling

Air-cooling describes the use of the air to cool an engine, and use of air to cool a liquid which itself is used as an engine coolant.

As engines generate mechanical power they also generate waste heat energy because they are not perfectly efficient. The engine must therefore be cooled to prevent it from cooking in its own heat.

Although some waste heat goes out with exhaust gases in most conventional internal combustion engines, further cooling is needed otherwise some components will get so hot that materials or lubricants will fail.

Basic principles

Most internal combustion engines are "air-cooled" or "liquid-cooled". Each principle has advantages and disadvantages, and particular applications may favor one over the other. For example, most cars and trucks use water-cooled engines, while most small airplane engines are air-cooled.

Most liquid-cooled engines use a mixture of water and other chemicals such as antifreeze and rust inhibitors. Some use no water at all, instead using a liquid with different properties, such as ethylene glycol. Although the term "liquid-cooled" is used here, most air-cooled engines also use some liquid oil cooling, and most liquid-cooled engines subsequently cool the hot liquid with air.

Conductive heat transfer is proportional to the temperature difference between materials. If an engine metal is at 300°C and the air is at 0°C, then there is a 300°C temperature difference for cooling. An air-cooled engine uses all of this difference.

In contrast, a liquid-cooled engine might dump heat from the engine to a liquid, heating the liquid to 150°C which is then cooled with 0°C air. Thus, in each step, the liquid-cooled engine has half the temperature difference and so may need as much as twice the cooling area.

Advantages and disadvantages

Cooling, however, is also limited by energy (heat) density. A small, very hot component is difficult to air cool because air has low heat density. If the air speed is low, then there is only a small mass of air to carry away heat. Since there is little mass, the air which is cooling the part gets nearly as hot as the part, and then the temperature difference is small so cooling is poor. Blowing more air over the part improves cooling, but blowing air fast creates noise and uses power. (Doubling the air speed may take eight times the power). In contrast, liquids have much higher heat density and so a comparative trickle of liquid can keep the part cool.

A major reason that heat density is important is that the most significant cause of engine failure in modern engines is hot spots. The engine as a whole may be cool enough, but if one part of the engine overheats, the engine eats itself. Slight overheating makes the engine wear out faster and gross overheating causes the engine parts to fail quickly. Common hot spots include parts of the cylinder head, exhaust valves, pistons, and cylinders.

Unfortunately, many hot spots are small and located where it is difficult to blow sufficient air over them. Furthermore, engine materials are not perfectly conductive, so it is often not possible to "cool at a distance" by building a metal bridge to a place where it is easy to blow a lot of air.

Liquid-cooling is thus a good solution to a difficult problem, but sometimes it is even difficult to move enough liquid coolant to keep a part cool. When liquid coolant gets to an engine hot spot, it may boil, expand to a gas, and momentarily stop the flow of coolant over the hot spot -- which then becomes even hotter. When more coolant reaches the hot spot, it simply boils.

The gas bubbles may disappear again as they mix with newly circulated coolant, and the hot spots may be damaged by localised boiling even though the radiator contents are not unusually hot. Indeed, the engine temperature sensor may indicate the engine is running cool overall, even though one part is dangerously overheated.

In an air-cooled engine, the coolant is already a gas and thus cannot boil. Thus, while air-cooling makes it harder to avoid hot spots, air-cooling also tends to limit sudden hot spot problems caused by boiling coolant.

Using air-cooling eliminates an entire engine subsystem and at the same time eliminates problems with coolant freezing. It simplifies engine design and can lead to markedly better engine reliability, though the benefits of reduced complexity must be traded against reliability problems caused by worse thermal control.

Using air-cooling also eliminates the coolant radiator and the weight of multi-wall engine parts needed to capture the coolant without leaks. Air-cooled engines are thus often lighter per unit power than liquid-cooled engines.

However, cooling fins on an air-cooled engine can be expensive to make compared to liquid-cooled engines, even though liquid-cooled engines require tricky hollow construction.

The liquid in a liquid-cooled engine also serves as sound insulation. An air-cooled engine must ensure good passage of air over the engine and thus it is difficult to substitute some other kind of sound deadening. In addition, using air to cool small hot parts means air must flow faster than for the radiator of a liquid-cooled engine, even though the air-cooled engine has much greater temperature differences to aid cooling. For both of these reasons, liquid-cooled engines are typically quieter for a given power output.

Liquid-cooling makes it easier to maintain each part of the engine at a given temperature in normal operation. Air-cooled engines can have the hot-spot problems described above, and getting enough air to the hot spots may cause other parts of the engine to run too cold. Liquids have less temperature rise as they absorb waste heat.

Thus, the coolant temperature varies less with engine load. In turn, more even temperature of liquid-cooling means better component tolerances can be maintained, which can improve both durability and emissions. It may be possible to achieve good temperature control with an air-cooled engine, but at the expense of more complicated thermal management and increased weight.

Similarly, peak power from a given displacement of air-cooled engine is typically limited compared to water-cooled counterparts, so liquid-cooled engines may be heavier but also fit in a smaller space.

Difficult to generalise

For all these reasons, it is difficult to make generalisations about air-cooled or liquid-cooled engines. air-cooled Volkswagen kombis are known for sometimes "eating engines", with both rapid wear in normal use and sometimes sudden failure when driven in hot weather. On the other hand, air-cooled Deutz diesel engines are known for reliability even in extreme heat, and are often used in situations where the engine runs unattended for months at a time.

It is usually more difficult to get either low emissions or low noise from an air-cooled engine, two reasons why most road vehicles use liquid-cooled engines. It is also often difficult to build large air-cooled engines, so nearly all air-cooled engines are under 500 kW, whereas large liquid-cooled engines exceed 80 mW (Wartsila-Sulzer RTA96C 14-cylinder diesel).

A reliable generalisation is that for a given power level it is more difficult to build a light liquid-cooled engine. Weight is one reason air-cooling is common in aircraft engines, though reliability through simplicity and ready access to cool air are two other reasons.