An engine coolant performs three basic functions:

It transfers heat from the engine block, through the radiator to the air.  Only about one third of the energy developed by burning fuel, be it petrol or diesel, goes towards moving the vehicle.  Another third goes out the exhaust.  The final third has to be removed from the engine block, or the engine will overheat and seize.  This is where the engine coolant performs its first function, as a heat transfer medium.

Antifreeze has a higher boiling temperature, only by a few degrees C, but that increases the efficiency of the cooling system.  But more importantly, antifreeze will have a lower freezing point, and this will prevent the coolant freezing and cracking the engine block if the engine is stopped at temperatures below zero degrees C.

But the third, and most often overlooked function of an engine coolant, is to prevent the corrosion of the various metals within the engine and cooling system.  To understand just how quickly an unprotected cooling system will corrode, put a steel bolt in a bottle of plain water and watch it start to rust within just a few days.

There are a number of different types of engine coolants and antifreezes.  Antifreeze contains glycol for freezing protection and comes as a concentrate or as a pre-mix.  The concentrate must be mixed 50/50 with de-ionised water before use, the pre-mix is already mixed with de-ionised water and ready for use.  Antifreeze also contains a corrosion inhibitor that protects against engine/cooling system corrosion and cavitation.  Then there is corrosion inhibitor, this comes as a concentrate or pre-mix as well. Corrosion inhibitor is just the corrosion protection package found in the glycol based antifreeze, it is just mixed with de-ionised water.  The absence of glycol in a corrosion inhibitor makes it unsuitable for use where the engine is likely to be stopped in freezing conditions, water based corrosion inhibitors are used in applications such as stand-by generators and marine engines.

Over the last 20 years there have been dramatic changes in coolant/antifreeze corrosion inhibitor technology.  Older coolant/antifreezes used a chemical corrosion inhibitor package based mainly on metallic salts, there were nitrites, nitrates, silicates, molybdates, borates, phosphates and a whole heap of other chemicals.  These chemicals deposited on the surfaces of the engine cooling system and formed a physical corrosion barrier between the metal of the engine and cooling system and the coolant/antifreeze.

But, and isn’t there always a “but!”  Once deposited on the engine cooling surfaces these chemicals were used up, the coolant/antifreeze had to be drained and filled with fresh coolant/antifreeze every year or two.  Some engine manufacturers wanted specific chemicals in their coolant/antifreeze and installed Supplementary Coolant Additive (SCA) filters in the cooling system to keep replenishing the level of these chemicals in the coolant/antifreeze as they were used up.  Also, the layer of chemicals deposited on the cooling surfaces tended to crack and break off, causing hard particles which contributed to water pump seal failure.

Because older engines were constructed mainly from iron, the older style of coolant/antifreeze were highly alkaline, a coolant/antifreeze with an alkalinity of about 10.5 gave excellent iron corrosion protection – that’s pretty alkaline, just less alkaline than the ammonia solution used as a cleaner around the house.  But manufacturers started to use more and more aluminium in their engines and cooling systems and highly alkaline coolant actually corrodes aluminium.  Modern coolant/antifreeze will have a pH of only 8.0 to 8.5, that’s about the alkalinity of an egg white.

Around the early to mid 1990s, GM introduced their DEX-COOL specification written around a coolant/antifreeze product developed by Texaco using an Organic Acid Technology (OAT) coolant inhibitor which also contained no silicates or phosphates .  Ford followed with their similar Mercon OAT coolant/antifreeze specification.  OAT coolant/antifreezes are also referred to as Extended Life Coolants (ELC), they are suitable for 5 to 6 years service before they need to be changed.

But, and isn’t there always a “but!”  One of the organic acids used in GM’s original DEX-COOL product, 2-ethylhexanoic acid (2-EHA), is a plasticiser which makes rubbers and plastics soften and swell, its inclusion in DEX-COOL has been associated in GM coolant system recalls involving millions upon millions of dollars.  Because of the early problems with DEX-COOL, OAT coolant technology has substantially evolved.  Different organic acids have been developed as coolant inhibitors and some years ago Hybrid Organic Acid Technology (HOAT) coolants, which combined the best features of OAT coolants/antifreezes with a boost of silicate and/or phosphate inhibitors, came on to the market.

OAT and HOAT coolant/antifreezes prevent corrosion and give long service life because the organic acid salts in their corrosion inhibitor package stay in solution.  The organic acid salts could be described as small molecular strands of rubber with an electric charge at one end.  Corrosion is largely a galvanic process, in simple terms, when a potential corrosion site occurs on the cooling surface of the engine these organic acid salts are electrostatically attracted to the site and build up a barrier against further corrosion.  However, this anti-corrosion barrier is only temporary.  When the normal movement of electrons within the metal of the cooling system material naturally neutralises the corrosion site, the organic acid salts return into the coolant/antifreeze solution, they are not used up.

So, what coolant should you use for your diesel engines?  Unfortunately there’s no one product which meets all coolant system requirements.  If freezing protection is not required then an OAT water based corrosion inhibitor might be suitable and save money.  If antifreeze is required, then does the engine manufacturer specify that the coolant/antifreeze contains nitrite or not, or that the coolant/antifreeze doesn’t contain phosphate.  Discuss your coolant needs with your supplier.  But you might have to make compromises.  You might decide to use different coolant/antifreezes for different engines, or choose to use one coolant/antifreeze which satisfies the greater number of engines right across your fleet.

 And colour?  Old style antifreezes used to be mainly coloured green.  You just had to ring up and ask for “the green antifreeze”.  But now coolant/antifreezes are made in so many different colours.  GM DEX-COOL and Ford Mercon are red or orange, depending how you look at them.  But different coolant/antifreeze manufacturers now use different colours for what might well be very similar products.  There are red, orange, yellow, green, blue and purple coolant/antifreezes out there, every colour of the rainbow.  Colour means nothing these days, so discuss your corrosion protection, coolant/antifreeze needs with your supplier and use a quality coolant inhibitor or antifreeze that meets your diesel engines’ needs.

Written by: Steve Streater, Technical Advisor – Fluids and Lubricants TDX Ltd