Air-cooled engines rely on the circulation of air directly over hot parts of the engine to cool them. Most modern internal combustion engines are cooled by a closed circuit carrying liquid coolant through channels in the engine block, where the coolant absorbs heat, to a heat exchanger or radiator where the coolant releases heat into the air, and so on, ad infinitum. Thus, while they are ultimately cooled by air, because of the liquid-coolant circuit they are known as water-cooled. In contrast, heat generated by an air-cooled car engine is released directly into the air. Typically this is facilitated with metal fins covering the outside of the cylinders which increase the surface area that air can act on. It is worth noting that in all combustion engines, a great percentage of the heat generated (around 44%) escapes through the exhaust, not through either a liquid cooling system nor through the metal fins of an air-cooled engine (12%). About 8% of the heat energy finds its way into the oil, which although primarily meant for lubrication, also plays a role in heat dissipation via a cooler.

Road vehicles

Many motorcycles use air-cooling for the sake of reducing weight and complexity. Few current production automobiles have air-cooled engines, but notable past models include the Volkswagen Beetle and related models, Citroën 2CV, the Chevrolet Corvair, the Porsche 911 and others.

Engine Coolants

Antifreeze is used in internal combustion engines, and for many other heat transfer applications, such as electronics cooling and chillers for HVAC. Compounds are added to water to reduce the freezing point of the mixture to below the lowest temperature that the system is likely to be exposed to, and to inhibit corrosion in cooling systems which often contain a range of electrochemically incompatible metals (aluminum, cast iron, copper, lead solder, etc.). The term 'colligative agent' is to be preferred as, in warm climates, the benefit of these compounds is to increase the boiling point of the coolant, which should then be more properly referred to as 'anti-boil', and as anti-freeze decreases and increases both properties, respectively, 'colligative agent' more accurately describes the liquid. The term 'engine coolant' is widely used in industry.

Methanol

Until the late 1930s, methanol was the most widely used antifreeze. While effective in preventing the coolant from freezing, its low boiling point and low specific heat capacity led to considerably less cooling than water alone. Also, the concentration of methanol would tend to be reduced over time due to its greater tendency to evaporate than the water with which it was mixed.

Ethylene glycol

Ethylene glycol solutions became available in 1937 and were marketed as "permanent antifreeze", since the higher boiling points provided advantages for summertime use as well as during cold weather. They are still used today. Ethylene glycol antifreezes are poisonous and should be kept away from any person or animal (children and especially dogs) that might be tempted by its sweet taste. They form calcium oxalate crystals in the kidneys and can cause acute renal failure and death. All spills should be cleaned, or else an area in which it may be present should be kept inaccessible to those who might ingest it.
Should ingestion of antifreeze occur, ethanol (alcoholic beverages) can be administered until proper treatment can be started in order to slow the conversion of methanol to formaldehyde and formic acid which are the substances responsible for methanol's toxicity. In practice, ethanol can be administered intravenously by doctors to counter ethylene glycol and methanol poisoning, but now that another antidote is available (fomepizole), it is used less and less for the purpose.
In order to prevent ingestion, bittering agent (denatonium benzoate) is usually added to engine coolant to make it taste unpleasant. In the United States, there is legislation before Congress (H.R.2567/S.1110) that would make the use of a bittering agent mandatory.

Propylene glycol

Propylene glycol, on the other hand, is considerably less toxic and may be labelled as "non-toxic antifreeze". It is used as antifreeze where ethylene glycol would be inappropriate, such as in food-processing systems or in pipes in homes, as well as numerous other settings. It is also used in food, medicines, and cosmetics, often as a binding agent. Propylene glycol is "generally recognized as safe" by the Food and Drug Administration (FDA) for use in food. However, propylene glycol-based antifreeze should not be considered safe for consumption. In the event of accidental ingestion, emergency medical services should be contacted.

Other Developments

In the 1980s inventor Jack Evans discovered the advantages of using a waterless coolant. His final formulation is a mixture of ethylene glycol and propylene glycol. This coolant has a high boiling point of 370 °F and is not corrosive, solving many of water's problems including freezing.
Most commercial antifreeze formulations include corrosion inhibiting compounds, and a colored dye (commonly a green, red or blue fluorescent) to aid in identification. A 1:1 dilution with water is usually used, resulting in a freezing point of approximately −40 °F (−40 °C). In warmer areas weaker dilutions are used.
Glycol antifreeze solutions should generally be replaced with fresh mixture every two years. Many modern cars now come filled with organic acid technology (OAT) antifreeze (such as Dex-Cool), which has an extended service life of five years. Although these still contain glycol, OAT solutions may not be compatible with conventional inorganic-based coolants containing glycol (e.g., with silicates, borates, phosphates) and, if changing from one type to the other, the cooling system must be thoroughly flushed with clean water. Typically OAT antifreeze contains a red or pink dye to differentiate it from the conventional inorganic coolants (blue or green). Some of the newer technology OAT coolants claim to be compatible with all types of OAT and inorganic-based coolants; these are typically green or yellow in colour.