Coolant Composition

Appropriate Base Fluids for Automotive Cooling

The most appropriate fluid selection for the purpose of removing heat from the automotive engine is one which possesses a number of characteristics.  Ideally, and most importantly, the fluid should have a high heat capacity and low viscosity – meaning that it can absorb the heat in the engine and flow fast, away to the radiator whereupon it is cooled by airflow. The simplest such fluid is water. Water possesses a very high heat capacity, is readily available and cheap, and is therefore one of the most effective cooling substances known to man.

Water does present problems though. Water freezes at 0°C (32°F) and boils at 100°C (212°F), which is a narrow range for the operating engine, particularly in colder climates where freezing temperatures are common. Another significant problem with the use of water alone in a cooling system, is that it is corrosive by nature, and this corrosive influence of water needs to be addressed.

The addition of other base fluids to water can significantly alter its boiling and freezing temperatures. Monoethylene Glycol (MEG, or otherwise simply known as “Glycol”) has been known to have such an effect on water for many years. The addition of varying quantities of MEG has a profound impact on the freezing point of water. Therefore, it is possible to blend a base fluid comprised of parts of water (which enhance cooling effect, and reduce cost), with MEG (to enhance freezing point depression and boiling point elevation performance), to produce a fluid that is effectively in liquid state throughout all temperatures expected in the normal operation of an automotive engine, even in cold climates.

The elevation of the boiling point is even further enhanced by applying the system to elevated pressure. This has a profound impact on elevating the boiling point of the fluid, which is important at operating temperatures. A good quality, well maintained, integrous cooling system with a good quality radiator cap will apply sufficient pressure to elevate the boiling point of a cooling solution to over 139°C.

There are two varieties of glycol: monoethylene glycol (MEG) and propylene glycol (PG). Both can be used in automotive or heavy-duty engine coolants or heat transfer fluids, although it is generally accepted that ethylene glycol based fluids, when used as directed, offer the greatest performance advantage.

Corrosion Mechanisms

Compared to glycol and water content, the corrosion inhibitor packages form a small percentage in the overall composition of coolant. The function of the inhibitor package is absolutely critical to the performance of the fluid. The combination of water, glycol and heat forms a potent mixture that is able to attack any unprotected metal and component surfaces.

Such corrosion can take a variety of forms. The most common form of corrosion is Oxidation (otherwise simply known as rust) which can be widespread over entire metal surfaces.

Other forms of corrosion are also possible in common engine cooling systems including:
a) Cavitation corrosion; caused by the implosion of air bubbles in the cooling fluid.
b) Pitting corrosion; caused by aeration of the coolant fluid.
c) Galvanic/ electrolytic corrosion; caused by a temporary voltage difference between two disparate metals in the coolant hardware.

Properly formulated coolants will contain inhibitors which suppress and limit the development of each of these corrosion mechanisms.

Any form of corrosion can liberate particulates in the fluid, which can be very abrasive (leading to further wear), and if accumulated over time, can clog vital ducts within the radiator, reducing the radiator’s efficiency. This can subsequently reduce engine performance or in severe cases cause engine breakdown.

Other Minor Coolant Ingredients

Coolant formulators add a number of other minor ingredients which all perform significant and important roles in the performance of the fluid, although may be unrelated to heat transfer or corrosion protection.

Some of these minor performance additives are:
- Scale inhibitors which prevent and remove the formation of scale (which can be common where poor quality water is used).
- Antifoam ingredients which prevent the formation of bubbles and aeration and which is key to minimising pitting corrosion.
- Dyes to differentiate coolant types.  There are a number of conventions of coolant dyes based on the technology types.
- Bitterant (or denaturing fluids) which limit the likelihood of accidental ingestion of the coolant fluid.