A Lesson on Antifreeze

"Topping up" the antifreeze solution in a car's cooling system is a routine maintenance item for most modern cars.

An antifreeze is an additive which lowers the freezing point of a water-based liquid and increases its boiling point. An antifreeze mixture is used to achieve freezing-point depression in cold environments and also achieves boiling-point elevation ("anti-boil") to allow higher coolant temperature. Freezing and boiling points are colligative properties of a solution, which depend on the concentration of the dissolved substance.

Because water has good properties as a coolant, water plus antifreeze is used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters. The purpose of antifreeze is to prevent a rigid enclosure from bursting due to expansion when water freezes. Commercially, both the additive (pure concentrate) and the mixture (diluted solution) are called antifreeze, depending on the context. Careful selection of an antifreeze can enable a wide temperature range in which the mixture remains in the liquid phase, which is critical to efficient heat transfer and the proper functioning of heat exchangers.

Salts are frequently used for de-icing, but salt solutions are not used for cooling systems because they can cause severe corrosion to metals. Instead, non-corrosive antifreeze is commonly used for critical de-icing, such as for aircraft wings.

Automotive and internal combustion engine use

Fluorescent green-dyed antifreeze is visible in the radiator header tank when car radiator cap is removed

Most automotive engines are "water"-cooled to remove waste heat, although the "water" is actually antifreeze/water mixture and not plain water. The term engine coolant is widely used in the automotive industry, which covers its primary function of convective heat transfer for internal combustion engines. When used in an automotive context, corrosion inhibitors are added to help protect vehicles' radiators, which often contain a range of electrochemically incompatible metals (aluminum, cast iron, copper, brass, solder, et cetera). Water pump seal lubricant is also added.

Antifreeze was developed to overcome the shortcomings of water as a heat transfer fluid. In some engines freeze plugs (engine block expansion plugs) are placed in areas of the engine block where coolant flows in order to protect the engine from freeze damage[citation needed] if the ambient temperature drops below the freezing point of the antifreeze/water mixture. These should not be confused with core plugs, whose purpose is to allow removal of sand used in the casting process of engine blocks (core plugs will be pushed out if the coolant freezes, though, assuming that they adjoin the coolant passages, which is not always the case).

On the other hand, if the engine coolant gets too hot, it might boil while inside the engine, causing voids (pockets of steam), leading to localized hot spots and the catastrophic failure of the engine. If plain water were to be used as an engine coolant, it would promote galvanic corrosion. Proper engine coolant and a pressurized coolant system can help obviate the problems which make plain water incompatible with automotive engines. With proper antifreeze, a wide temperature range can be tolerated by the engine coolant, such as −34 °F (−37 °C) to +265 °F (129 °C) for 50% (by volume) propylene glycol diluted with water and a 15 psi pressurized coolant system.

Early engine coolant antifreeze was methanol (methyl alcohol). Methanol was widely used in windshield fluids, however, in Europe, due to new REACH legislation, the use of methanol in windshield fluids is limited to 5% and in the near future will be further reduced to 3%. As radiator caps were vented, not sealed, the methanol was lost to evaporation, requiring frequent replenishment to avoid freezing of the coolant. Methanol also accelerates corrosion of the metals, especially aluminum, used in the engine and cooling systems. Ethylene glycol was developed, and soon[when?] replaced methanol as an engine cooling system antifreeze. It has a very low volatility compared to methanol and to water. Before the 1950s, coolant systems were unpressurized, and the engine was often cooler than modern automotive engines. By pressurizing the coolant system with a radiator cap, the boiling point of the fluid is increased, permitting higher engine temperatures and better fuel efficiency. Pressurized systems do not appreciably change the freezing point. Later propylene glycol was introduced due to its environmental credentials.


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