Most engine failures (some say up to 80 per cent of them) can be attributed to overheating or related problems, yet most people pay little or no attention to the cooling system of their cars.
Ask anyone why a pressure cap is being used on the radiator and one gets all sorts of answers. Even mechanics often have no clue. People who maintain expensive machines or vehicles have to be begged, threatened or bribed to use a coolant inhibitor or anti-freeze. Occasionally anti-freeze would be used in the winter period only, a clear indication that they do not know why an inhibitor should be added to the coolant. Leaks don’t get repaired, hoses and “V” belts are replaced until they fail completely – or when a “blown-up” engine is being replaced. You often see people topping up radiators every morning, or worse, when the engine is hot.
Often people act this way out of ignorance. They think that they are doing the right things, but they are actually creating problems. The cooling system is a science in its own right and to cover all aspects, one could write volumes. It is essential however, that we should understand the basics.
The Cooling System
The cooling system of every internal combustion engine performs a vital function – the dissipation of heat in order to maintain normal engine operation. Internal combustion engines develop power through the conversion of fuel into heat and then into mechanical energy. Only one-third of this heat is converted into power, while another third goes out of the exhaust system. This leaves one third to be eliminated by the cooling system.
The cooling system handles its one-third share in the following manner. Heat is conducted from the combustion chamber through the surrounding metal to the coolant passages where the coolant picks up the heat and carries it to radiator. The radiator, hoses, fan and other accessible components of the cooling system are easy to inspect but heat transfer areas inside the engine are not. There is usually no signs that problems are developing until it is too late and the engine is damaged. By then the only course of action is to repair and replace engine parts.
The average diesel engine generates sufficient heat at maximum load to melt the metal parts in minutes if the cooling process fails. Naturally this does not happen due to engine seizure. Melting point of iron is 1 530°C and aluminium is 685°C.
Overheating is normally caused by:
1. Insufficient coolant
1.1 No coolant or very low coolant levels will obviously prevent heat transfer.
1.2 Coolant losses are normally caused by burst hoses, leaking seals, gaskets etc.
2. Restriction of coolant flow
2.1 A faulty thermostat can cause restriction of coolant flow; radiator tubes blocked internally, faulty water pump etc.
2.2 Also due to air bubbles admitted to the system by low coolant levels or pin-sized holes at the suction side of the water pump. These bubbles displace coolant at heat transfer surfaces and can also cause air locks at the water pump, thus reducing coolant flow.
3. Restriction of air flow
3.1 An unkempt radiator exterior reduces airflow.
3.2 Also slipping fan belts, wrong type of fan or blades at an incorrect angle etc.
3.3 Position of radiator in relation to the fan or cowling is also important factors that can affect the airflow.
These causes for overheating are mostly very obvious and the temperature gauge would normally tell the story – but by then it’s often too late! Damage can be prevented or minimised by an effective Engine Protection System, however.
A serious, but often overlooked reason for damage by overheating of engine components, is caused by improper heat transfer due to deposits of scale and other matter on heat transfer surfaces such as combustion chambers, cylinder liners etc. Scale deposits of 0,3 mm on the cylinder liners will cut heat transfer by 40 per cent!
Heat transfer surfaces offer no easy inspection, any sign that problems are developing until the damage is done.
These deposits are formed in the following ways:
1. Scale from water-borne minerals. (Continuously topping up!)
2. Products of erosion i.e. rust.
3. Products of chemical incompatibility.
To prevent scale forming the following steps should be taken:
1. Use water of a known softness.
2. Use the correct inhibitor or antifreeze and maintain the strength. A solution of less than 30 per cent concentrations does not provide sufficient corrosion protection and a concentration of 50 per cent is recommended. Never exceed a 67 per cent concentration as it will adversely affect freeze protection and heat transfer rates.
3. Ensure that no coolant loss is taking place to eliminate the need to top up continuously. (Hoses in a good condition and properly clamped; correct pressure cap; correct expansion gap maintained in radiator.)
a) Pressure cap.
The cooling system is pressurised to increase the boiling point of water. (Coolant)
Antifreeze is used in the cooling system to:
1. Lower the freezing point
2. Increase the boiling point
3. Prevents oxidation (Rust)