Cooling Faq, Part
2: Coolants- Better Living Through Chemistry
It may come as a surprise that the theoretical maximum thermal efficiency of a gasoline motor is less than 60%. In practice, the efficiency of a real motor is much lower. It sounds grim, but your engine is really among the most efficient means of producing power. Compare this to photoelectric cells, which are about 7% efficient.
If only a fraction of the energy in your fuel is actually producing useful work, where does the rest go? Some goes out the tailpipe in the form of heat, noise and vibration, some is handled by the oil and lubrication system, and some goes to pumping loss internal to the engine. But most waste energy is handled by the cooling system.
The working fluid in your radiator is probably a combination of water and glycol antifreeze. In this FAQ, we will explore some of the chemistry involved in coolants. We will describe the physical properties of liquids that make them good heat transfer media, and the relative advantages and disadvantages of various mixtures.
Physical Properties of Liquids
Liquids have a few properties that need to be understood before we can proceed. Chemistry isn't everyone's favorite subject, so well keep it as simple as possible:
- Specific heat. Specific heat is the amount of heat that it takes to raise the temperature of a standard amount of fluid a standard degree of temperature. For example, one BTU is the amount of heat needed to raise one pound of water one degree F. To raise the temperature of a pound of water ten degrees, we need ten BTUs. This is true any place between the freezing point and boiling point of water. A fluid with a higher specific heat has greater capacity to absorb heat, which is a desirable property for a coolant. Here are the specific heats of the three principal coolants:
Substance Specific Heat Ethylene Glycol .57 Propylene Glycol .59 Water 1.00
- Latent heat. When a fluid is about to freeze or boil, extra energy must be removed or added to accomplish the state transition. If you were trying to boil one pound of water, every BTU you add will get you one degree closer to the boiling point. But just at the boiling point, you need to add much more heat to push it over the top. Latent heat is a very valuable property, because it gives you an easy way to add or remove lots of heat: just get a fluid to boil or freeze, and you've done it.
The latent heat effect is used in two ways in your cooling system. First, cylinder heads heat very unevenly. There are some very hot spots in the vicinity of the exhaust valves, for example. Coolant coming in contact with these areas will instantly boil, drawing a great deal of heat from the component. This local boiling is called "nucleate boiling". The other way that latent heat is used is more subtle. After cooling the head and block, the coolant passes through the intake manifold water jacket. This causes droplets of gasoline to vaporize, drawing heat from the coolant. (Vaporizing gasoline in this way actually improves the drivability of the motor, but that's another FAQ). Generally, a high latent heat is a desirable property in a coolant. Here are the latent heats of the three principal coolants:
Substance Latent Heat of Vaporization Ethylene Glycol 195 cal/gm Propylene Glycol 170 cal/gm Water 540 cal/gm
- Vapor Pressure. As heat is added to a liquid, its molecules become more energetic. This energy will tend to force the molecules apart, and turn the liquid into a gas. The liquid will boil if these internal forces exceed the atmospheric pressure acting on the fluid. Thus, the boiling point of a liquid can be increased by increasing ambient pressure. This is why the cap your radiator is a pressure cap. By adding 4, 7, 15, or more pounds of pressure to the system, the boiling point of the coolant is raised.
Note that the vapor pressure of ethylene glycol is well below atmospheric pressure at normal working temperatures: if you were to fill your radiator with pure ethylene glycol (not really recommended), you would not need a pressure cap.
- Boiling point. As we have discussed, fluid will become gaseous when the kinetic energy of its molecules exceeds the external forces acting on it. In other words, the more pressure, the higher the boiling point. At the point that vapor pressure exceeds ambient atmospheric pressure, the fluid will boil. A sea level, ambient air pressure is 14.7 PSI. If we increase the temperature of a sample of water to 212F, its vapor pressure will be 14.7 PSI, and we will begin to get boiling. But what happens if we are in Denver, where ambient air pressure is lower due to the higher altitude? If you add heat to your sample of water, vapor pressure can only build to prevailing atmospheric pressure before boiling occurs. So the water boils at a LOWER temperature in Denver, 202F. This has a profound effect on your cooling system, as anyone who travels through mountainous areas knows.
Substance Boiling Point @ Sea Level Ethylene Glycol 387 Propylene Glycol 370 Water 212
- Surface tension. Water is composed of hydrogen and oxygen, which carry a positive and negative charge respectively. The polar nature of water creates a fairly strong bond between water molecules. In the absence of air resistance and gravity, a quantity of water will tend to clump together, and will form a spherical shape, since a sphere has the smallest surface area per unit volume. You can see this when a small amount of water forms a drop. Altering the shape of this drop requires some external force, because the internal forces which keep the water in a clump must be overcome. To see this in action, try floating a double edge razor blade on a cup of water. If the blade is carefully lowered to the surface, it will not sink, because its mass isn't sufficient to overcome the internal forces which are trying to keep the water together.
The relationship of surface tension to your cooling system is subtle, but critical. There are points in your engine which are hotter than others, such as the area around your exhaust valves. When water hits one of these areas, its quickly heated to the boiling point. A small bubble of water vapor is formed. This is called nucleate boiling. Once the bubble is formed, we'd like it to disperse, so that additional liquid coolant can reach the hot spot. But the surface tension of the surrounding fluid resists until the bubble is large enough and energetic enough to break away from the surface. Reducing surface tension is a very important goal in coolant selection.
- Specific Gravity. Some substances are heavier than others. This is intuitively obvious in the case of solids: a volume of steel is heavier than an identical volume of aluminum. The same is true of liquids. Specific gravity measures the density of a substance relative to water.
In our discussion of specific heat, we related the capacity of a substance to absorb heat to it's mass. Since the volume of our cooling system is constant, filling it with a denser substance will allow it to carry more heat. The higher the specific gravity of a substance, the better:
The standard old saw among mechanics is "water cools best". This even has some scientific basis, since water has the highest specific heat and latent heat among the available coolants. But on the other hand, water has the highest surface tension, lowest specific gravity, and lowest boiling point, which would suggest that there may be problems with pure water as a coolant. I decided to conduct some controlled experiments to determine the facts.
Experimental design. The experiment was structured as follows. For each coolant mixture tested, a fixed volume of coolant was used (one pint). The stovetop was thermostatically controlled to 900F, and was allowed to reach its full operating temperature prior to each trial. The test coolant was placed in an aluminum pie tin, at 70F room temperature. The time required to heat the fluid to boil was observed.
BTU output of the stove was calculated based on measuring the time required to boil a one pound sample of water, mixed with surfactant. This calculation is almost certainly incorrect, but since we are mainly interested in relative results, it's OK..
Since significant nucleate boiling occurs as soon as the pan comes in contact with the stovetop, a true boiling condition was determined when the temperature of the coolant reached its limiting temperature, i.e., when the entire volume of coolant was at the boiling point. A fast acting thermocouple and calibrated meter was used to determine this.
The coolants tested included pure water, pure ethylene glycol, 50/50 EGW, water modified with a surfactant, and 50/50 modified by a surfactant. These are the most common mixes.
Rationale: The pie tin is a fair model of a cooling system. The stovetop represents the heat generated by the engine, while the exposed surface of the coolant provides heat transfer to the atmosphere, emulating the radiator in a car. Using a standard volume of coolant reflects the way an actual cooling system is filled, and therefore includes the full systemic effect of specific gravity. The pie tin has minimal mass, so the heat transfer properties of the pan need not be considered. The longer the coolant can function without "boiling over", the more effective it will be in a real engine.
Shortcomings: In the experimental model, fluid remains static. The effect of circulation through a water pump may be significant. The systems is unsealed and unpressurized, which undoubtedly affects the results. In particular, results with pure water and aqueous glycol solutions are distorted, since significant evaporation occurs during the test.
Test 1. The above graph summarizes the first experiment, a trial of pure water vs. pure ethylene glycol vs. 50/50. It's clear that a 50/50 mix of water and ethylene glycol not only has a higher boiling point than water, but extends the heat carrying capacity of the system, which can be determined by calculating the area under the curve up to the boiling point. (Pure ethylene glycol was not tested to its boiling point, since it is very flammable.) The systemic heat carrying capacity is displayed in this graph:Summary of Results. Water does not cool best. But water with a surfactant is an excellent coolant. Pure ethylene glycol gives the best boilover protection, but should never be used in non aqueous solutions because it's inflammable. Given that most drivers are concerned with freeze protection as well as cooling, a 50/50 mix, plus a surfactant is the best choice.
Test 2. There are products called surfactants which improve the cooling ability of water by lowering it's surface tension. The next test attempts to measure the effectiveness of Redline Water Wetter, a commonly available surfactant. For comparison, a test was run with dishwashing liquid, another commonly available surfactant. The quantity of surfactant added to one pint of our test mix, per manufacturer's spec, was 1 fluid oz. Note that surfactants do not change the boiling point of the working fluid, but do affect it's ability to transfer heat.
Test 3. Various combinations of surfactant, ethylene glycol, and water were tested. The use of surfactant only applies to aqueous solutions.
Test4. Non-aqueous glycol. At least one manufacutrer makes a pure glycol coolant that is designed to be used without water. Since glycol has a relatively low specific heat, a concern with using such products is that operating temperatures would increase. To test this hypothesis, an measurement was made of the number of BTU's required to raise various coolant mixtures to a standard temperature (212F). The negative effect of surface tension is evident in the results for water vs. water+Water Wetter. The results for ethylene glycol are especially interesting, since they suggest a specific heat of .78, vs. an expected result of .57. This may mean that commercial coolants have a different composition than standard, or that there's some factor (like convection cooling) that isn't being measured in the experiment. Based on these results, pure glycol should not raise normal operating temperatures.
Copyright©2002 Michael Frank
About the author - Michael Frank is Vice President and Webmaster of the Jaguar Touring Club. His red 2+2 E-Type is a familiar sight at JCNA events in the Northeast. Currently, he is building a reproduction of the 1963 Cunningham Lightweight E-Type .
Updated : 6/24/2002
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