Air cooling vs. liquid cooling:
In liquid-cooled engines, cooling channels are provided in the engine block and cylinder head through which coolant flows. The coolant does not actually cool; instead, it absorbs heat from the components and carries this heat to the radiator. There, the coolant is cooled by the ram air while driving. The cooled coolant then flows past the hot engine components again to absorb more heat.
Advantages of liquid cooling compared to air (ram air) cooling:
- More uniform cooling. The flow of the coolant can be controlled precisely. Heat dissipation can be regulated better, resulting in smaller temperature differences. Coolant absorbs heat very quickly, can store a lot of heat and is quite thin, allowing it to be pumped quickly through the cooling system.
- Less engine noise. The liquid around the cylinders acts as sound insulation.
- When the engine is switched off after a long drive, it stays at temperature longer (beneficial during short breaks, as the engine will be less cold after a restart). The coolant ensures that the engine cools down more slowly. During slow cooling, less material stress occurs than when some engine parts cool down faster than others.
Types of coolant:
We find coolant in several types and colours:
- Green or blue (G11). Contains silicates (salts) and can be used in older engines. Most commonly used, e.g. BMW and most MINIs.
- Red / Yellow (G12). This coolant is suitable for aluminium engines and the silicates have been replaced by Organic Acid Technology (OAT). Yellow coolant is found, among others, in Renaults;
- Purple / colourless (G12+). This universal coolant contains improved additives compared to red or yellow coolant;
- Red / Pink (G12++, G13). Contains silicate additives and provides extra aluminium protection in modern engines, is suitable for longlife applications and is mainly used by Volkswagen / Audi, etc.
Before topping up coolant, you must first check which type (colour) belongs in the system. Topping up and/or mixing an incorrect type of coolant can be harmful to engine components.
Coolant components:
Coolant consists of a mixture of demineralised water, antifreeze (glycol) and additives. These additives provide protection to engine components and parts of the cooling system and are referred to as “dopes”.
- Water: lime and chlorine are removed from the demineralised water so that no blockages occur and engine components are not affected.
- Antifreeze: antifreeze (glycol) is added to coolant to lower the freezing point. Without antifreeze, the coolant would freeze at 0 °C and engine components could be damaged (cracked) by the expansion of frozen water. Adding antifreeze lowers the freezing point to between -25 °C and -40 °C. As antifreeze, Mono Ethylene Glycol (MEG) or Mono Propylene Glycol (MPG) can be used, of which MEG has a greater heat-absorbing capacity and is therefore used most often, and MPG is the least harmful to the environment. The mixing ratio between water and antifreeze determines the freezing point.
– a mix of 30:70 (30% antifreeze and 70% water) is often sufficient in temperate climates;
– in climates where the temperature often drops far below freezing, the mixtures are adjusted to 60:40 or 70:30. - Additives: the additives in coolant are called additives. They provide even better properties for the coolant:
– anti-foam dope: when the coolant foams, less heat is absorbed and circulation is impeded. Foaming can also cause cavitation. Cavitation occurs when air bubbles suddenly implode due to a pressure change. The pressure waves that result can even break off small pieces of metal from components;
– anti-sludge dope: the clumping together of components can lead to blockages;
– anti-corrosion dopes: because the parts of the engine and the cooling system consist of different metals, electrolytic corrosion can occur.
The anti-corrosion dope prevents this corrosion by reducing conductivity;
– lubrication: adding a lubricant prevents wear of the coolant pump seal. Without lubricant, more friction occurs between the shaft and the seal, squeaking noises can occur and the seal can wear out more quickly and start leaking.
Determining antifreeze concentration:
During a (major) service of a car, the antifreeze concentration of the coolant is checked. Over time, the amount of antifreeze may have decreased, for example after topping up with water. The antifreeze concentration can be checked in two ways:
- A refractometer can be used to measure the refraction of light through liquids. That is why it is also called an optical meter. The higher the antifreeze content, the slower light passes through it and the stronger the bending effect on light. By placing a few drops of coolant on the glass, the break point can be viewed against the light via the viewing glass. You then look through the eyepiece into the light. The boundary line between white and blue (see the image below) indicates down to which temperature the coolant offers protection. In the example, this is -28 °C. When tap water without additives is measured, the boundary will drop to 0 °C;
- The antifreeze hydrometer measures the specific gravity of the coolant. Antifreeze has a higher specific gravity than water. The mixing ratio affects the weight. The higher the weight, the more antifreeze is present.
If it is found that the antifreeze concentration is too low, the coolant must be replaced. Adding pure antifreeze to the existing coolant is not recommended. The next paragraph shows why it is better to replace the coolant.
Coolant replacement interval:
Coolant is often not replaced periodically, as is the case with engine oil. Manufacturers usually do not specify a replacement interval. Nevertheless, it is advisable to replace the coolant every few years. Over time, the dopes wear out and no longer offer protection against foaming, sludge, corrosion and the coolant no longer has a lubricating effect. When corrosion starts to occur in the engine block, metals dissolve slightly in water. The differences in the tendency to dissolve create a kind of battery effect. An electrical voltage arises between the different metals and a small current circuit can be created in which one of the two metals corrodes. If the coolant also contains salts or acids, for example when tap water has been used, conductivity is increased.
In addition to the increasing risk of corrosion, contamination also increases. Through sweating gaskets, wear of the water pump and tiny metal particles due to erosion from the liquid flow, the coolant becomes contaminated and this causes more wear.
It is advisable to replace the coolant every two years, or at most every four years and a maximum of 50,000 km. After repairs have been carried out to, for example, the radiator, coolant pump or thermostat, the drained coolant is best not reused. Filling with new coolant is preferred.
The images below show coolant in which the dopes have worn out and in which dilution has presumably taken place with tap water. The coolant turns brown, as do all the components and channels in the cooling system. This “rust water” can be flushed out, after which the system can be refilled with new coolant. If this is not done in time, defects may occur, such as leakage of the water pump, leaking seals and gaskets, clogged (heater) radiator, defective thermostat and deposits on the walls of the cooling system.
Replacing coolant:
There is often a drain plug at the bottom of the radiator. If this is not present, the lower radiator hose can be disconnected to drain most of the cooling system. Some engines also have drain plugs on the engine block itself. Consult the instruction manual in advance to find the locations of the drain plugs.
After draining, the system can be filled. While bleeding is very easy on one car, on another one or more bleed screws must be partially opened to let the remaining air escape from the system via the highest point. There are engines where bleeding is so difficult that the car has to be tilted backwards with the engine running in order to let the air bubbles escape via the highest point (the radiator cap or expansion tank).
The best way to fill the cooling system is to first evacuate it. Using a cap with valves and compressed air, the entire cooling system can be put under vacuum. By then opening the valve of the coolant container, the new coolant is drawn into the system. In this case, bleeding is not necessary.
Once the system has been filled and bled, there is a fairly simple way to check whether the circulation is in order. In the expansion reservoir a clear return flow must be visible. The heater must also become sufficiently warm. Is the engine at operating temperature but the heater does not or hardly gets warm? Or does the temperature briefly rise when you accelerate, but the air then becomes colder again? Then you can be sure there are still air bubbles in the system.
Newest generation coolant:
Conventional anti-corrosion additives are not able to sufficiently protect the combinations of different materials. Erosion of the water pump occurs in the short term and the cooling channels become clogged because the coolant turns into a kind of gel. In addition, plastics can harden, causing them to break. To prevent this, the latest generation of coolants is equipped with additives made from organic carboxylic acids, which do not form a protective layer over metals and therefore do not attack the metal.