Engine Cooling System

As previously mentioned gasoline when burned in combustion with oxygen releases a great deal of heat. As also previously mentioned internal combustion engines are not very efficient at fully utilizing that heat energy. If all that excess heat energy were released through the exhaust this would not be that big mobile car detailing an issue even the heat generated by friction in the mechanical process would be easily managed. Unfortunately, much of that heat does not exit with the exhaust but is absorbed into the cylinder.

Even with the appropriate grade of gasoline, knock can occur not from spontaneous combustion through compression but rather through pre-ignition from the heat of the cylinders, the pistons, the heads and even the spark plug tip. Something is needed to absorb a good deal of that heat away from these areas (I say a good deal rather than as much as possible because a certain level of heat increases the efficiency of the engine by keeping the gasoline in an easier to burn vaporized state rather than allowing it to condense).

Water will be a recurring solution in these papers. For cooling purposes water in a liquid state is an excellent coolant. It has a very high specific heat, which means to increase water by one degree it takes more energy than most other liquids that adapt themselves well to an engine’s environment. This high specific heat means that for a given volume it will absorb (and will absorb quickly) a lot of the excess heat remaining in the engine components from the combustion process and friction. This is why most engines use a water mix in their cooling systems.

Three of water’s characteristics are also its limits. It freezes at a relatively high temperature when compared to the environments many vehicles operate in during at least some part of the year. It boils into a gas at a relatively low temperature when compared to both other liquids (even though they may have lower specific heat) and the desirable operating temperature of an internal combustion engine. And lastly it readily either dissolves or suspends many undesirable other materials.

The problem with water freezing at such a high temperature is that water expands when it freezes and cooling systems by necessity are closed, this expansion will damage the system. Secondly, the cooling system relies on a pump which will not move solids through the system. The solution is anti-freeze. At varying levels of mixture with anti-freeze the coolant mix’s freezing temperature can be lowered to levels below all but the most extreme climates that it would be called on to operate under. Electric block heaters are required when the engine is not running in those more severe circumstances.

The problem with water boiling into a gaseous state at a low temperature is that once its specific heat has absorbed a given level of heat it vaporizes into steam. Once in a gaseous state most compounds have lost their ability to absorb high levels of heat quickly. Gases have low specific heat and as a result tend to increase in temperature even further while absorbing very little heat. This is not a desirable condition in an engine. Vapor on the cylinder wall is especially a problem for turbo-charged engines, since turbo-charged engines produce so much heat vapor along the cylinder wall allows hot spots on the cylinders that will ignite the fuel mixture and is the primary cause of coolant failure related knock. Water also expands when it boils into steam but can be contained by most engine components at water’s boiling point. The first solution to water’s low boiling point is that this ability for the components to handle the pressure of gases is that under pressure water’s boiling point is raised.

Under pressure and without room to expand water will remain a liquid and continue to absorb heat at a high rate. For a given system volume, closed systems systems that can handle pressure will be more efficient with heat management than a system that cannot be pressurized. This reference to volume provides the next solution to better heat management. A larger cooling system can circulate more coolant and absorb (and ultimately transfer to the air through the radiator) more heat than a smaller one. There is a limit to just how large a cooling system can be though both in space and weight limitations. However, if you are experiencing problems with overheating there is likely a larger radiator available for your car that will help. Lastly most anti-freeze mixes will have the secondary benefit of also increasing the boiling point of water.

The last issue is the ease with with water dissolves or suspends other materials. In an engine many of these materials, especially minerals, have harmful side effects. They corrode engine internals, leave deposits and provide electrical conductivity within the coolant. Again anti-freeze provides a solution containing anti-corrosive and deposit cleaning additives (but does not help with electrical conductivity) to provide a level of protection. However, the additives are limited in their ability to deal with these problems and the real solution here is frequent changes in the coolant mixture.

In summary, the best balanced solution for dealing with heat management in a turbo-charged engine is:

  • Proper coolant mixture
  • Proper maintaining of the system’s ability to hold pressure
  • Appropriate sized system – more power will generate more heat and more heat will at some point require a larger system
  • Change the mixture regularly

For lack of a better place to discuss the topic, motor oil also plays a role in heat management. However, its role largely relates to preventing heat through lubrication. As a lubricant the function of motor oil is to reduce friction between moving parts and the heat that would otherwise result. (Energy within an engine seems to always be trying to transfer back into heat.) Once combustion has occurred the objective is to use all the energy that was transferred into mechanical power in that form.

To the extent that oil reduces energy transfer into heat the engine will have a higher mechanical power output and run cooler. Despite how well motor oil performs its function, energy is still transferred by the friction. To the extent that the motor oil absorbs that heat it needs to go through the same cycle as engine coolant to manage that load of heat. Generally, motor oils do not perform their lubrication function once they reach a temperature of 250 degrees Fahrenheit. With the exception of pressure, oil heat management is the same as for water – proper mixture (viscosity), appropriate sized system (sump and when necessary cooler size) and change the motor oil regularly (it does wear out).

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