Engine Cooling System | How It Works

How it Works: Engine Cooling System

Of the five basic functions of an internal combustion engine, that is, compression, combustion, lubrication, ignition and cooling, cooling over-rides and controls the successful function of the other four. It is the stabilizing influence of the cooling system that allows the others to operate successfully. The purpose of the cooling system is to control metal temperatures within safe linjits by removing excess heat from the engine. Heavy driving with today's high output engines can generate enough heat to keep a five-roomed house at comfortable temperatures in a sub-zero climate.

With the increase in demand for more horsepower per pound of engine weight, it is necessary to work all materials a little nearer to their endurance limit. Today's motors put out substantially more power per weight than did those of even ten years ago. The design engineers have a difficult job in relating the co-efficiency of expansion and contraction of the various metals used. They can only do this on normal rates and usage but as corrosion by-products and scale build up in the cooling system, so the co-efficiency of expansion and contraction gradually alter, leading to the inevitable failure, or shorter life, of valves, rings, gaskets, etc.

To control engine operating temperatures, water in the cooling system must circulate constantly through all passages in the block and head, and the walls enclosing the water must be good heat conductors. The most efficient operating temperature of a motor is near 200degreesF and to prevent boiling, the modern cooling system is pressurised. Each one pound of pressure within the system raises the boiling point of the coolant by 3degrees and so with an engine fitted with a 15 pound pressure cap, it is possible to work the coolant to 257degrees without it actually boiling. The other advantage of a pressurised cooling system is increased water pump efficiency. The main components of the cooling system and their function and operation are as follows:

The Pressure Cap

We have already explained how important pressure is within the system and the pressure cap is the key to the successful operation of the cooling system. Pressure builds naturally in the system from the expansion of the coolant as it heats up. Expansion by heat from 60degrees to 180 degrees is approximately three percent It is from this expansion that we obtain pressure and it is pressure within the system that allows us the safety margin from boiling under varying conditions of operation. The pressure cap is designed to lift when a predetermined pressure is reached, and when it lifts, a small amount of coolant is lost down the overflow. Built into the centre of the pressure cap is a vacuum valve that allows air to be drawn into the system when the coolant contracts when cooling off and this prevents collapse of radiator and heater hoses. Pressure caps should be tested at least twice a year and should be discarded if they will not hold the pressure stamped on them.

The Thermostat

The thermostat has four basic functions to perform. It permits rapid warm-up of the motor to operating temperature.

It permits rapid warm-up of the motor to operating temperature
It controls the temperature of the coolant within working limits.
It prevents fluctuation of pressure in the radiator by controlling the flow of water to the radiator and this prevents constant lifting of the pressure cap with the resultant coolant loss
It prevents localised steam pockets within the head and block by maintaining an even flow of coolant to the radiator. Under normal operating conditions, the thermostat is hardly ever open or shut but constantly moves (or flutters) and the temperature variations in the block can be as much as 20 degrees F in spite of this control. It is recommended that thermostats be checked every twelve months and a thermometer be used to ensure it opens with five degrees F of the stated opening temperature. A thermostat should never be removed permanently from an engine as coolant loss and severe steam pocketing will result.

The Water Pump

Water pumps should be inspected for corrosion of the pump vanes and deterioration of the seal as both of these conditions can lead to pressure loss and aeration within the system.

Fan belts and radiator hoses.

These should be inspected at regular intervals. A slipping or slack fan belt leads to poor coolant circulation and overheating. Hoses should be checked for clogging and general deterioration which also leads to poor coolant circulation. All hose connections should be checked periodically as loose hose connections can also cause aeration in the system. From the moment water is added to a new motor, the following actions start.

Rust and Corrosion

The action of iron and water and oxygen results in rust. The degree of rust and the amount of corrosion depends on the PH factor of the water used. To stop the corrosive action of water, it is necessary to keep the PH factor at or above PH 7, and it should be assumed that most water is below PH 7 or slightly acid. Therefore, the tendency is for the water solution to corrode cooling system metals unless protected by an inhibitor. The formation of rust within the system is the forerunner of many problems. It forms a preventive crust which stops normal heat transfer from the metal to the water. If not treated and allowed to build up within the system, severe overheating and boiling will occur.

As mileage increases, some of this rust encrustation becomes loose from " i the block and head and when the motor overheats or boils finds its way to the top of the core effectively blocking the tubes and stopping proper coolant circulation. Scale. This is another and often more deadly enemy of heat transfer. Scale is often confused with rust but is actually an opposite. Scale forms an even film over the hottest parts of the motor and effectively interferes with heat transfer from the metal in the head and block. The higher the mineral content of water, especially ‘if alkaline, and the higher the operating temperature of the engine, then scale formation is rapidly accelerated. Metallurgists say that one sixteenth of an inch of scale is equivalent to four inches of cast iron in heat transferability. Scale is more dangerous in the modern engine than rust, as the overheating is not evident to the driver until the excessive heat, which is prevented from leaving the metal, distorts the head, or the gasket fails, or a valve seat cracks - at this stage it is virtually impossible to remove the scale.

Corrosion

Another problem in the cooling system of today's high output engines is the corrosion of the soft metal parts of the system. This is caused by electrolytic or galvanic action and comes from a flow of current between dissimilar metals. This is similar to the action which takes place-in a battery and the result is metal loss. This metal loss can be observed on water pump impellers and bodies, on wet sleeves in diesel motors, and it can also cause the loss of solder from the radiator. With the increasing use of aluminium and alloy parts in motors, corrosion protection is essential. Cases have already occurred of alloy water pump housings failing in fifteen thousand miles. When aluminium corrodes it forms a sludge instead of the powdery results of iron corrosion and this sludge is far more difficult to remove than rust.

As pressure is increased in the radiator, so it is also increased in the block and head. Recent tests have shown that - in the average engine, whilst the radiator pressure may be fourteen pounds, pressures in the back of the block and head under normal operating conditions may be as high as twenty pounds, but when engine revolutions are increased to the maximum under high speed sustained driving, in many engines this pressure in the back of the block and head will go as high as thirty or fourty pounds. This means that the coolant is always endeavouring to find a way out of its area and inevitably it goes into the cast metal itself, the gasket faces and finally the oil.

Visualise the motor constantly expanding and contracting, subject to jarring pressure fluctuations and it becomes easy to see why core plugs fail, why gaskets leak and why coolant finds its way into the oil. There are many authorities including oil and chemical companies who state that gasket leakage is far more prevalent than realised and also that water contamination of the oil is the cause of many problems accepted in the past as "just one of those things". The leakage of coolant into the oil results in oil film failure and, should the coolant contain anti-freeze, then a gummy sludge forms which can block oil passages.

Coolant level in the radiator should be checked only when the engine is cold, and should be approximately one inch below the neck of the radiator. This is normal. Any water added to a radiator which shows this level will only be expelled through the overflow as the coolant expands with heat. Constant topping up of the radiator brings the additional risk of adding other impurities and also dilutes the inhibitor. Air, steam and hot water are the worst enemies of rubber and this is borne out by the fact that top hoses have a far shorter life than bottom hoses. Frequent top hose failure can often be traced to the cooling system being defective and not holding pressure, allowing excessive air to pass along the top of the hose. This hot air is very destructive to rubber.

The major symptom of all cooling systems troubles is overheating and, eventually, boiling and, if the following is understood, a lot of problems can be checked in their minor stages.

Water loss is a danger signal.
Rusty coolant is a red flag.

Pressure caps, fan belts, hoses and thermostats all need to be checked regularly on a definite time or mileage basis. Any time a radiator has to be removed for any reason at all, the block must be flushed immediately to remove all soft rust and scale deposits. The effort required, and the cost of regular cooling system maintenance as recommended by all leading manufacturers is insignificant when compared with the end result and the cost of neglect. It is essential that all modern engines be treated with a recommended inhibitor and sealer. This should be renewed every five or six thousand miles when the block, head and radiator should be reverse flushed. Cooling system neglect is the root of most failures in both older and modern engines.