Coolant and cooling systems.
When the engine on a vehicle runs it burns fuel, and at the same time loses a great deal of energy due to the un used heat that must be dissipated if an engine failure is to be avoided. Leaving aside the few vehicles on our roads that are air cooled the vast majority of vehicles use water to assist in the process.
There is a natural tendency for anything heated to expand and therefore become less dense as can be seen in hot air balloons etc, at the most basic level the engine uses this effect to aid circulation of the water. The water needs to circulate because if it did not it could simply boil. All cooling systems work by collecting the spare heat in the water (or coolant) and then getting rid of this heat by passing it through a radiator where the cooler atmosphere passes over it and allows an exchange of heat.
So hot water comes out of the engine at the top and flows into the top of the radiator and as it cools and becomes denser sinks to the bottom where it starts the journey all over again. Early vehicles just used this thermal circulation but it is very inefficient. The reason is that a large amount of water is needed and hot spots are extremely likely, it is for this reason that all cars have a pump added to assist the process. The pump is normally engine driven and so takes a little power from the engine normally driven by a belt off the front of the engine.
But water boils at 100 degrees C at sea level and lower when at altitude, most engines like to run at about 85 degrees C but when sitting in traffic may well get hotter, because of this a pressure cap is added to raise the boiling point, and to stop the water evaporating from the cooling system. Modern engines routinely run with up to I bar of pressure when hot.
NB The reason that it is extremely dangerous to remove the cap from a hot pressured system is two fold, firstly there is pressure in the system and the water is likely to squirt out, but secondly if the engine is sitting at over 100 degrees the water will boil once it returns to atmospheric pressure. This is accentuated by what is known as heat sink. Heat sink is simply the transfer of the heat in the engine to non circulating water causing a rise in temperature. This effect can often be witnessed in car parks on very hot days as parked cars vent their superheated steam.
Moving on to coolant; at itís most simple water is normally the base but it is usually adulterated by adding "Antifreeze". But preventing the water freezing is only part of what it does, and along with most of the other components of the engine antifreeze has developed.
Glycol based antifreeze is the norm but in the past alcohol has proved effective, however the development of Aluminium engines meant that corrosion has become much more of an issue especially in hard water areas where the dissolved chalk lead to a Alkaline solution that is particularly damaging to an Aluminium engine.
Modern antifreeze raises the boiling point of the water and is normally mixed at a strength of 33% it has anti corrosion and anti foaming agents and is designed to last a very long time, strangely however the addition of antifreeze makes the solution marginally less efficient at cooling the vehicle, and in some situations other products are added to lower the surface tension which can make a big difference.
Increasingly all in one coolants are being sold pre-mixed using de-mineralised water and containing all of the additives in the optimum ratios. These cost a bit more but seem pretty cheap when compared to the cost of an engine.
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