I'm often asked if you could replace the water in your cooling system with oil, with the supposed advantage being that it would never boil over. The answer is a definite "no"; your engine would overheat in a matter of minutes. You must use some kind of water-based fluid, and there is none better than a 50-50 mix of regular anti-freeze and water.

Here's why. The purpose of the coolant is to get as much heat as possible out of the cylinders and cylinder heads. If you think back to your high school physics class, recall the following:

  • the amount of energy required to raise the temperature of liquid water from 0 to 100 degrees Celsius (32 to 212 degrees Fahrenheit) is 100 calories per gram
  • the energy required to vaporize water already at 100 degrees is 539.4 calories per gram (called the latent heat of vaporization).

So we can see that over 5 times more energy is required just to turn water into steam at the same temperature, than is required to heat water from the freezing point all the way up to the boiling point.

Your car's cooling system is designed to take advantage of this principle (and so is your own body- this is why sweating cools you off). The water touching the cylinders actually gets boiled off as it absorbs the heat from the metal, thus transferring the maximum amount of heat possible. This gas quickly turns back into a liquid as it passes its heat into the surrounding water, and the cycle continues.

Since the boiling point of oil is much higher than water, it is unable to take advantage of this phase change and therefore must rely on temperature change only. There simply isn't enough surface area in your engine to transfer all the necessary heat in this manner, and so your engine will overheat.