Keeping Your Cool

This is the time of year when people suddenly wake up to the fact that it is getting mighty cold, and had therefore better do something about the car's cooling system before it freezes up. In some cases it is too late and the resultant damage that a frozen cooling system can cause has already happened. The most popular question is how much anti-freeze to use? It
would seem that cooling systems and coolant are yet another much misunderstood part of a car. With this in mind, I thought it would be interesting to point out a few facts with brief
explanations on the duties and problems that the cooling system has to perform.
Now contrary to popular opinion, water is a far better coolant on its own without any antifreeze (glycol in some form or other). Surprised? It is astounding how many folk believe that a
greater mix of glycol in water will improve the cooling ability of the system - when all they have to do is read the instructions! Old adage - if all else fails, read the instructions. Very wrong I'm afraid. It is true that the addition of glycol will raise the boiling point of water slightly as well as reducing its freezing point - but this can possibly make problems rather than solving them.
By and large, most production cars are built with cooling systems with a larger capacity than is actually required to allow for a fairly strong glycol/water mix. This not only helps reduce
the possibility of freezing, but also to reduce the corrosion of aluminium based components used in the cooling system and cylinder heads (where aluminium heads are used) and the
formation of rust on any iron parts. The reduction of this corrosion also helps prevent blocking of the radiator cooler/slower running sections. This capacity for cooling is more accurately described as its heat rejection capability. The capability becomes suspect with bigger or more powerful (tuned) engines, so increases in capacity or effectiveness will be needed. However, for now, I really only want to explain a bit about the cooling system in general and how it works. I will cover more specific applications and problem solving and modifications in another issue. So on with the show, as they say ...
OK, so what is the requirement of the cooling system? Essentially the internal combustion automotive engine is not particularly efficient. It creates energy by burning a fuel/air mixture -
but much of this energy must be rejected from the combustion chamber (and to a certain degree the cylinder walls) and components via the coolant, through the radiator into the
atmosphere. The rejection of this heat is essential to prevent component failure through thermal fatigue. These components include pistons, bore walls, valves, and cylinder head. The combustion chamber must be cooled sufficiently to prevent pre-ignition and detonation problems that are exaggerated these days by the low octane/unleaded fuel and the ever increasing demand by authorities for lean burning engines. Higher combustion chamber temperatures require ignition retardation to prevent the onset of the aforementioned and
therefore reducing engine output - particularly torque. further torque reductions are caused by higher inlet temperatures when an engine is running hot by creating a less dense fuel/air
mixture. Thus far we can see that an inefficient cooling system rescues the heat flow from the combustion chamber area to the coolant and radiator, therefore reducing the efficiency of the engine.
So how is this heat flow/transfer effected? This is relatively straight forward in principle, but certain areas create problems in practice. Basically, the coolant passes over the metal
surfaces where heat is transferred as the coolant is at a lower temperature than the combustion area. The coolant then passes through the radiator where the heat is rejected into the
atmosphere. This is all essentially carried out by thermal conductivity. Water has excellent heat transfer properties in. its liquid state, but has very high surface tension. This surface tension limits its ability to 'wet' the metal surfaces of thewater jacket. This water tension can create other problems where localised hot-spots occur around the combustion chamber. These hotspots form vapour bubbles by boiling the water, despite the fact that the bulk of the water is below boiling point. The vapour bubbles formed on the metal surfaces then cause an insulated area which greatly impedes heat transfer, reducing the cooling systems efficiency and thereby increasing the combustion chamber temperature. This, in turn, will cause
eventual failure of components - the piston usually being the first to suffer. The speed at which these failures occur are dependant on the severity of the hot-spot problem and dynamic
loads on the engine.
Now for some interesting comparisons. Water possesses amazing heat transfer properties in comparison to virtually any other liquid cooling medium. It is far superior to glycol - based coolants (as in anti-freeze) and is vastly cheaper!! Tests carried out by specialists have concluded that water has almost two and a half times greater thermal conductivity when compared to glycol type coolants. The improvement of glycol's thermal conductivity is almost directly proportional to the amount of water added. Most heat in the cooling system in transferred by convection - from hot metal to a cooler liquid (as in an engine block) then from a hot liquid to cooler metal surfaces (as in the radiator), thus to cooler air. Therefore the
thermal conductivity of the coolant is of ultra importance. Since it has been found that a 50/50 solution of water and glycol has about 70% of the thermal conductivity of water only, it is easy to see that the combustion chamber temperatures will be naturally higher where a mix is used.
There are other factors that affect the cooling system's performance as a whole, such as the viscosity of the coolant, and the convection coefficient of the coolant in a tube - a complex
relationship between the thermal conductivity, viscosity, tube diameter and turbulent flow of the system. A 50/50 glycol/water mix has roughly four times the viscosity of water, and as
previously mentioned, about 70% of the thermal conductivity. Using these factors, it has been established that this 50/50 mix has approximately 50% of the convection coefficient of water only. Water only as a coolant is capable of twice as much heat transfer as a 50/50 mix. Amazing!
Despite water being such an excellent liquid coolant, where freezing temperatures are going to be experienced some anti-freeze is essential. As to how much is dependant on the type of
anti-freeze and the sub zero temperatures expected. Read the manufacturers recommendations carefully, and use the least amount required. Water expands by approximately 9% when frozen which can obviously cause quite severe damage! As mentioned earlier, the use of anti-freeze can reduce internal corrosion and help lubricate water pumps - but avoid the milky soluble oil types. These actually impede heat transfer considerably by coating everything with a thin film of oil, and can also create softening (and premature deterioration) of the rubber hoses. The coolant should be changed at the very least once a year as antifreeze becomes acidic from the exudation of the glycol to acids. This will then create internal corrosion, particularly to aluminium based components (such as thermostat housings and water pumps), even radiators where special alloy cores are used.
Just for the record, a 50/50 mix would raise the boiling point of water to approximately 130Q C when using a 15 psi radiator cap. Another interesting statistic is that for every 1 % of glycol added to water, the cylinder head temperature could be raised by around 1 Q, so a 45/50Q F (7-lOQC).
As I said earlier, I will go into greater detail of what to do where extra or better cooling is required and how this can be achieved in a forthcoming issue.
Keith Calver