Cooling system is designed to keep the engine at its optimal operating temperature. It cools down the engine when the optimal temperature is exceeded. Automobile engine is a heat engine and apparently it is a waste of energy to remove heat from an engine but it is due to the reliability requirements and metallurgical properties of the metals used in the engine construction that excessive heat has to be removed.
Engine Performance
Engine performance is directly linked to the cooling system. Engine uses a rich mixture when the engine (or coolant) is relatively cool and then moves to lean the mixture as engine heats up. If an engine is overheating, the ECU or PCM would reduce the ignition advance and/or fuel injection pulse duration, both leading to a loss of power. Engine temperature is measured indirectly through the coolant temperature.
Choice of Coolant
A coolant is also known as anti-freeze in everyday parlance but it gives an incomplete picture of its functions. A coolant should not freeze in winter when temperatures are below zero degree Celsius but it should also have a higher boiling point (at least higher than water, which is 100°C at sea level). Additionally, it should have a low vapor pressure, high specific heat capacity, high enthalpy of vaporization and very low electrical conductivity. Pure water has a high specific heat capacity but has a high vapor pressure (unsuitable), a lower boiling point (unsuitable), high freezing point (unsuitable) and unsuitable electrical conductivity. Therefore, water is mixed with other chemical coolants like glycols to produce an acceptable mixture. Water can be used temporarily though, in case the engine develops a coolant leak on the road and you need to top up the coolant. In Pakistan (1990s), it was common to see truck drivers (very rugged Bedford trucks) pouring fresh water into hot radiators on the road side. Not only water is not a good candidate as a coolant (except in some races) but cold water should not be poured into a hot engine.
Expansion Tank Design
The engine cooling system will have an expansion tank and/or overflow tank and possibly a recovery tank. Expansion and overflow tanks are of more importance. Engines with cross-flow radiators usually have an expansion tank. With vertical flow raidators, the expansion tank is built in the top tank of the radiator. Recovery tank is meant for recovering coolant that has passed through the pressure cap. These tanks have two hoses attached to them, the entry and exit ones. The exit hose is usually at the bottom of the tank but the entry hose is situated at top or mid-height of the tank. It must be verified that the entry coolant hose is submerged in the coolant so as not to draw air into the cooling system. If it is not the case, then a correctly sized and shaped brass or copper pipe or a high temperature rubber tube should be placed in the right position inside the expansion tank. With air in the cooling system, the engine would keep overheating with the risk of reduced head gasket life. If the expansion tank has a pressure cap, then it is a pressurized tank and therefore it can develop cracks leading to less cooling capacity and coolant loss. With engine at its operating temperature, check the expansion tank (plastic ones) for signs of cracks like bubbles coming out etc. In the image below, you can see a crack on the upper surface of the tank,
In the image below, you can see an expansion tank and a 6mm diameter brass tube. Expansion tank has one inlet, one outlet (bigger one) and an opening for the pressure cap. This tank did not have an internal tube that would keep the inlet hose submerged in the coolant, so we can add ourselves an extra tube to the inlet port.
Bend one end of the brass tube because it would remain submerged in the coolant. Insert the brass tube into the inlet hose port of the tank and cut the extra length. Of course use an o-ring or rubber tube for sealing the brass tube.
Here is the result, we can see the brass tube inside the tank.
Pressure Cap
The pressure cap has two functions, i.e., to open/vent the tank when the pressure inside the expansion tank or radiator exceeds a threshold (engine hot) or to let outside air get into the tank when a vacuum develops there (engine cold). The pressure cap should be according to the manufacturer's specification and should be tested from time to time. This cap should start to open (or vent) at specified pressure (e.g., at 1.2 bar or 1.4 bar (gauge pressure and not absolute). If this pressure cap is faulty (e.g., stuck) and does not allow pressure to escape, it would damage the tank over which it is mounted and/or the coolant pump or even the cooling system hoses. If the special cooling system tester is not available, then an old expansion tank can be used to test the pressure cap. Block the exit hose of the expansion tank, put the pressure cap on it and attach a hand pump or foot pump (equipped with a manometer) to the entry hose of the tank. Start pumping and at the threshold pressure, the cap should open, vent the excessive pressure and stabilise at the required level. This procedure for testing the pressure cap is shown below.
The pressure cap should also be tested for vacuum opening with a vacuum pump and an old expansion tank (not shown).
Thermo-Siphoning and Thermostat
An engine when shut-down after a drive will still be very hot for some time (at least an hour) and it is always beneficial to have some cooling going on. Turbo-charged engines generate too much heat and therefore use electric cooling pumps that operate for some time after the engine shut-down. For naturally-aspirated engines, usually there is no such pump and the only means can be the thermo-siphoning. Thermo-siphoning is the circulation of coolant due to temperature difference with hot coolant going up (to the radiator upper hose) and cold coolant going down.
A thermostat which starts to open/close around 85-90°C would not allow thermo-siphoning (thermostat is closed). A solution to this is to drill two small (around 2 mm diameter) holes in the thermostat. These holes would allow thermo-siphoning to occur. Only one such hole would suffice if the thermostat is designed to open around 70-75°C. Drilling hole(s) in a thermostat also helps to bleed the cooling system of the trapped air easily. A closed thermostat does not allow air trapped in the engine cooling circuit to get out.
Radiator
The fins of radiator often get deformed/damaged and this affects the cooling capability of radiator, therefore, the fins should be straightened using either a special fin straightening tool or a flat head screwdriver.
Radiators also have a maximum pressure rating, which should be equal to or more than the pressure rating of the radiator cap or expansion tank pressure cap. Also the standard single-core radiator need to be upgraded to a dual-core radiator if engine that has been modified to produce more horsepower is overheating. A dual-core radiator would also improve cooling by having increased volume of coolant itself. Radiators are also classified as single-pass or dual-pass (with respect to coolant) and cross-flow or vertical flow.
Heater Radiator
In addition to the front-mounted coolant radiator, there is a second radiator which is part of the heating/defogging system for the inside of the vehicle. In modern vehicles, engine coolant circulates permanently in this radiator and the rate of cooling through this radiator is controlled by a shutter and heater fans which have different speeds. To help fight the overheating problem, you can set this radiator to full hot position even if you are not using the heater fans themselves. Full hot position would open the shutter wide thus allowing more fresh air to enter in contact with the radiator. In case of overheat, you can turn on the heater fans too for a short time.
Engine Oil
Engine oil also plays a role in cooling the engine apart from its primary function of providing lubrication. The back of the pistons are cooled by oil jets. Therefore it is important to check the oil level too in connection with the cooling system. Oil is cooled by the flow of air on the oil pan below the engine, therefore, don't put any obstacles in front of the oil pan which would disturb the air flow. Also dont forget to clean the external surface of the oil pan from time to time because it can accumulate a thick layer of oily dirt/dust which would certainly reduce the rate of heat transfer. In the pictures below, both dirty and cleaned oil pans are shown.
Some vehicles (especially diesels) have an oil-coolant heat exchanger to harmonize their temperatures as quickly as possible. Additionally, if an engine has been modified to produce 40-50 additional horsepower, then it would be desirable to fit a separate oil radiator.
Coolant Pump and Belt Tensioning
Coolant Pump is the most important the part of the cooling system. It is the pump that circulates coolant through the cooling circuit. To ensure that coolant is circulating, check that there is flow through the hose that goes to the expansion tank at engine idle. No flow would indicate a defective pump.
Coolant or water pumps have a weep hole and if coolant starts to leak from that hole, then it needs to be replaced. A new pump will come with its new bearing and it would help reduce noise and vibrations (if any). Coolant pump is driven by the engine crankshaft via a belt drive. Correct tensioning of this belt is required for reliable engine operation and long belt life. Most people install belts without any special tools using either experience (belt sound when started) or guesswork. The reason it still works is that the required belt tension is not a single value, it is a range of acceptable values with minimum and maximum. Also a used belt should be tightened to almost 80 percent tension of a new belt.
Cooling Fans
Cooling fans are mounted in several locations and are powered either by electric motors or through a belt drive. In most designs, these fans are located behind the coolant radiators but sometimes they are also mounted in front of the coolant radiators. When mounted in front of the coolant radiators, they would surely block some air flow, reducing the cooling capacity of radiator. Electrically-driven cooling fans are switched on by a thermo-contact when the engine temperature exceeds a fixed threshold. These electric fans can work either in parallel or in series mode, depending upon the rotational speed (of fans) required. My own research project during Masters of Engineering degree was designing low-noise cooling fans using blade sweep. The blades of these fans are given a forward, backward or radial sweep for silent operation.
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