Various properties of engine coolants

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.

 

Substance	Specific Heat
Ethylene Glycol	.57
Propylene Glycol	.59
Water	1.00

 

 Latent Heat is a property that relates to the extra energy removed or added to accomplish a state transition.

 

Cylinder heads heat very unevenly and extreme hot spots in the vicinity of the exhaust valves. Coolant in contact with these areas will instantly boil, drawing a great deal of heat from the component. This local boiling is called "nucleate boiling". 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. A high latent heat is a desirable property in a coolant. Here are the latent heats of the three principal coolants:

 

Vapor Pressure

As heat transfers to a liquid, its molecules become more energetic. This energy tends to force the molecules apart and convert the liquid into a gas. The liquid will boil if these internal forces exceed the atmospheric pressure acting on the fluid. The boiling point of a liquid may be increased by raising ambient pressure. Pressure caps adding 4, 7, 15, or more pounds of pressure to the system raises the boiling point of the coolant is raised.

 

Boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.

 

Fluid will become gaseous when the kinetic energy of its molecules exceeds the external forces acting on it. The more pressure, the higher the boiling point. When vapor pressure exceeds ambient atmospheric pressure, fluids boil. At sea level, ambient air pressure is 14.7 PSI. When the temperature of water reaches 212F, its vapor pressure will be 14.7 PSI, and boiling begins. At higher altitude the ambient air pressure is less and water boils at a lower temperature.

 

Surface tension. Water is composed of hydrogen and oxygen molecules, which carry a positive and negative charge respectively. The polar nature of water creates a strong bond between water molecules. In the absence of air resistance and gravity, water will “clump” together in a spherical shape. Of all geometric shapes spheres have the smallest amount surface area per unit volume. Altering the shape a the clumped molecule requires an external force to covercome the internal forces keeping the “clump” together. This force must be overcome.

When water interacting with one hits one of the extreme hot spot , it quickly heats to the boiling point and water vapor is formed. This is called nucleate boiling. Once the bubble is formed, it should disperse so that additional liquid coolant can reach the hot spot. Unfortunately, surface tension of the surrounding fluid resists until the bubble grow large enough and energetic enough to break free of the surface. Reducing surface tension is a very important goal in coolant selection.