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Glossary of terms you will come across during your Phase 2 training.

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Padraic Kelly


Coolant is the liquid that absorbs heat from the engine and then dissipates it through the radiator. It can also dissipated through the cab heater in the drivning compartment of the vehicle.

The main component of coolant is water, for the following reasons:

Pure water as coolant does have the following disadvantages:

  • Water boils at 100oC, and turns to steam.
  • Water freezes at 0oC, crystallises and expands. This will cause it to damage whatever system it is enclosed in.
  • Water causes corrosion.

Water as coolant does have big advantages, that is why we use it, but we must add things to it, and do things to the environment in which we store and work it:

  • Water boils at 100oC and turns to steam. This temperature is below what many modern engines want to operate at. In order to raise the boiling point of the water, we pressurise the cooling system.
  • If we allow the water to freeze, it will crystallise and turn to solid ice. When it does this, it will expand and crack the system in which it is contained. In the case of the cooling system, this means the engine block and hoses can get damaged.
    To avoid this, we add anti-freeze to the water.
  • Water causes corrosion, so we must add some sort of inhibitors such as rust inhibitors.
  • Water causes scale - just look at the element in an old electric kettle. To avoid scale build up inside the water passages of the engine, we add inhibitors. Scale build up will restrict proper coolant flow and slow down heat transfer.
  • Whilst pure water is a poor conductor of electricity, the water found in cooling systems is not pure, and it will conduct electricity. A liquid that conducts electricity is known as an elecrolyte.
    In addition, inside a cooling system, there are many different types of metals.
    This makes a liquid cooled engine one big battery: dissimilar metals immersed in an electrolyte. Because of this, coolant must contain some sort of electrolytic inhibitor.

Checking Coolant

All modern cooling systems are sealed and as a result they shouldn’t ever really need topping up between car services provided everything in your vehicle’s engine is functioning correctly. It is a pressurised system and as such should only be checked or topped up when the vehicle's engine is cold.

Benefits of coolant:

  • No corrosion - By replacing corrosive water with engine coolant solution, you will prevent corrosion damage.
  • No scale - Complete elimination of scale build up inside the water passages of the engine. In addition, "hard" fresh-water causes scale form, and this will restrict proper coolant flow and heat transfer.
  • No electrolysis in the engine.
  • Anti-freeze protection.