Shop Talk: The Cooling System (Part III): Engine coolant characteristics

By Paul Dilger

Coolant — A coolant is not just water; it’s a chemical formulation. Coolants should have a satisfactory freezing point and boiling point, efficient heat transfer, corrosion protection, rust protection, water pump lubrication, and chemical stability.

As chemical engineers discover and adapt better ways to improve the efficiencies of gas and diesel engine cooling systems, customers and technicians must also expand their service knowledge to take advantage of new technology. When studying cooling system components and the problems that develop within them, service technicians will need to understand the complexity of modern coolants. An engine coolant is a mixture of chemically engineered, water-diluted antifreeze solution designed for the particular cooling system application. Commercial antifreeze contains ethylene glycol or propylene glycol and chemical additives like corrosion inhibitors, rust inhibitors, foam suppressers, dyes and proprietary chemicals. The best all-around coolant for circumstances where freezing temperatures are possible is a mixture of 50-percent antifreeze and distilled water.

Figure 4 – Thermal conductivity of coolant solutions. (Chart courtesy of Prestone)Figure 1 is a graph that plots the relative freezing points of various concentrations of antifreeze and water. The vertical axis (left margin) lists the freezing temperature (degrees F) of an antifreeze/water coolant mixture. The horizontal axis (bottom margin) lists the ratio of antifreeze to water expressed as a percentage (%). Follow the curve from the vertical axis where 0-percent antifreeze (100-percent water) freezes at 32 degrees F to where the solid line changes to a dashed line. Note the curve from 60- to 80-percent antifreeze. This zone gives the lowest possible freezing temperatures, but it is a gray area and there are no guarantees. There are other factors, such as the amount of dissolved salts in non-distilled water, which can have an effect on the freezing point. Industry normally uses a 50-percent solution because it covers most applications down to about -32 degrees F. Colder areas may require an antifreeze percentage up to 60 to provide a little more protection. At concentrations above 75 percent, the freezing point increases, going up to -23 degrees F for 100-percent antifreeze, thus defeating the purpose of any higher concentrations.

Question — Is it all right to mix different brands or bases of antifreeze?

Answer — Normally, this is not a good idea, especially if you don’t know for sure whether an engine has any aluminum in it. When mixing antifreezes, you might find their additives are not compatible and gelling with silicon-silicate dropout may occur. The silicon-silicate gel can restrict the flow of coolant through the radiator tubes, engine block and cylinder head, and cause a cooling system failure.

Figure 2 is a graph plotting the boiling points of various concentrations of antifreeze in coolant. The vertical axis (left margin) lists the boiling points (degrees F). The horizontal axis (bottom margin) gives the ratio of antifreeze to water expressed as a percentage (%). As the graph illustrates, the boiling point of the coolant dramatically increases after the addition of more than 50-percent antifreeze to the solution.

Question — When would 100-percent antifreeze be best?

Answer — Never! Never! Never! The beneficial effect of raising the boiling point ends with about 70-percent antifreeze. During the summer, engines will run warmer to hotter; therefore, as the percentage of antifreeze increases, heat transfer decreases because antifreeze has a lower specific heat than water. Let’s see why!

Figure 3 explains the specific heat transfer of antifreeze coolant solutions. On the vertical axis (left side) of the graph is listed the specific heat of a coolant. First, understand that heat is energy. The more heat there is in a closed container, the higher the temperature. Heat is measured in British Thermal Units or BTUs. A BTU is the amount of heat required to raise one pound of water, one degree F. Begin at the left point of the line labeled “0% Antifreeze;” it’s directly above the 32-degree F solution temperature point on the horizontal axis (bottom margin) and aligns with 1.00-BTU specific heat on the left margin. Move along this line to the right toward the vertical “Atmospheric Boiling Point” curve. This curve sweeps down, showing the boiling point of coolant decreases as the percentage of antifreeze increases. (Compare the lines for 0-, 50- and 100-percent antifreeze.) For this reason, the higher the percentage of antifreeze, the lower the pressure cap rating needs to be. Lower pressure means less stress on the hoses and heat exchangers to prevent rupture. However, this results in less coolant heat transfer, so you have a higher boiling point and a higher operating temperature.

Now, look at the “50% Antifreeze” curve where it intersects both the “Freezing Point” and “Atmospheric Boiling Point” curves. Follow the “50% Antifreeze” curve to the left where the coolant’s specific heat is 0.70 BTUs (70 percent of the specific heat of pure water with 0-percent antifreeze). Thus, there is a 30-percent loss in heat transfer capacity with 50-percent antifreeze. Simply put, antifreeze is thicker or more viscous than water. Where water increases its capacity to transmit heat as the temperature rises, antifreeze decreases in its ability to transmit heat as the concentration of antifreeze in the coolant increases.

Some light-duty gas and diesel engines do not have a water pump; the coolant moves by thermal convection. These systems depend on hot coolant rising and cooler coolant settling. Heat is transferred more slowly in this kind of system. Under heavy loads and extremely hot operating conditions, a 100-percent antifreeze solution could quickly lead to engine failure.

Based on the data provided in Figures 1-3, we can conclude that a coolant solution of 60- to 80-percent antifreeze (40- to 20-percent water) by volume does not have a consistent freezing point at temperatures below -60 degrees F. Therefore, no further freezing protection can be expected when antifreeze is increased beyond 60 percent. The general recommendation for most geographical areas is a 50-percent antifreeze solution, which will give good protection down to -34 degrees F. Even though most areas may never get below 0 degrees F, a 50-percent mixture will give some inhibitor protection in a coolant additive package to protect the cooling system.

Figure 4 illustrates the speed or velocity at which heat is able to move through the solution. What this really means is how fast the solution can transmit heat from the metal cylinder walls to the coolant, and from the coolant to the radiator, and then from the radiator to air passing through the radiator. Take a few moments to study Figure 4 and compare the 0- and 100-percent antifreeze curves. Several factors should be noted. First, the thermal conductivity of water (0-percent antifreeze) increases with increasing temperature, and the thermal conductivity of 100-percent antifreeze decreases with increasing temperature. At 180 degrees F, the difference in thermal conductivity between 100-percent water and 100-percent antifreeze results in about 64-percent less heat transfer. Comparing 50-percent antifreeze to pure water, we see about 38-percent less heat transfer. Thus, 100-percent water will cool more efficiently than any coolant with a percentage of antifreeze. Remember that using 100-percent water without proper additives will cause iron to rust and aluminum to corrode. Rusted iron and corroded aluminum reduce heat transfer because the rust and corrosion transfer heat more slowly than pure iron or pure aluminum. This is where additive packages come into play to help solve an overheating problem.

EDITOR’S NOTE: For more comprehensive studies on OPE engine, electrical and hydraulic certification, visit Paul Dilger’s Web site at www.imslo.com. These study programs develop professional mechanics.