Combustion of the air-fuel mixture in the cylinders generates heat which produces high pressure, to force the piston down in the power stroke. Not all of this heat can be converted into useful work on the piston, and it must be removed to prevent seizure of moving parts. This is the role of the cooling system. Most engines are liquid-cooled.
A liquid-cooled system uses coolant - a fluid that contains special chemicals mixed with water. Coolant flows through passages in the engine, and through a radiator. The radiator accepts hot coolant from the engine, and lowers its temperature. Air flowing around, and through the radiator takes heat from the coolant. The lower-temperature coolant is returned to the engine through a pump.
Air cooling is common on smaller internal combustion engines. Some engines use cooling fins. Their design makes the exposed surface area as large as possible, which allows more heat energy to radiate away, and be carried off in convection currents in the air. Some engines also use a fan to direct air over the fins.
The internal combustion engine works by changing heat energy into kinetic energy. There are many ways to do this, some better than others. But no matter how efficiently it is done, and no matter the size of the engine, the heat energy generated never completely changes into kinetic energy. Some energy is always lost.
This is certainly true in internal combustion engines where only about a third of the heat generated is transformed into the mechanical energy that moves the piston and turns the crankshaft. Another third goes out the exhaust, wasted. The rest tries to spread round the engine.
Heat always moves from areas of higher temperature to areas of lower temperature, which can be a problem. To control this movement, it is necessary to understand how heat is transferred.
Heat travels in just 3 ways.
- The way it moves through solids is called conduction.
- Through liquids and gases, it is called convection. It follows paths called convection currents.
- Through space, it moves by radiation.
Coolant prevents an engine from overheating in use and from freezing when idle.
The amount of heat generated by an engine is the equivalent of that required to heat a large house in winter in very cold climates. As engines and vehicles become smaller and more powerful they generate even more heat in a confined space, and aerodynamically efficient body designs tend to direct air away from, rather than into, the engine bay.
When an engine stands idle in cold weather, water in the cooling system will expand as it freezes, and this can have sufficient force to crack the engine block or radiator.
An effective coolant must therefore contain good heat transfer properties, have a higher boiling point and lower freezing point than water, prevent corrosion and erosion, resist foaming, be compatible with cooling system component materials, be compatible with hard water, resist sedimentation, and be as chemically stable as possible.
A concentrate, usually made of Ethylene Glycol together with some protective additives, is mixed with water to produce coolant. Propylene Glycol, which is non-toxic, is sometimes used in the mixture, as well as, or even instead of, the more toxic Ethylene Glycol. Glycol does not absorb heat as effectively as water, but when added to water it has the ability to lower the fluid’s freezing point as well as raise its boiling point.
A common Glycol to water ratio used is 50:50. This will lower the freezing point of the fluid to minus 39°
C – minus 38°
F, or 70 degrees below freezing – and raise the boiling point to 108°
C – 226°
F. Manufacturers can recommend other specific mixture ratios, but below 33% Glycol the coolant will give inadequate freeze protection, and above 65% Glycol the mixture has inadequate heat absorption.
There are three types of additives used in coolants: conventional, or inorganic additives; organic additives, and a hybrid mix of the two.
A fully formulated coolant is comprised of a careful balance of ethylene or propylene glycol with rust inhibitors, corrosion inhibitors, scale inhibitors, pH buffers for the acid to alkali balance, anti foaming agents, and reserve alkalinity additives.
Coolant should be changed at recommended intervals, because some of the additives will age and deteriorate over time, reducing the effectiveness of the coolant. While some coolants are compatible with others, changing the chemical balance in the cooling system can affect coolant performance, so mixing different types of coolant is not recommended.
