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Gasoline engines are not very efficient at turning chemical energy into mechanical power. Most of the energy in the gasoline (approximately 70%) is converted into heat. It is the job of the cooling system to take care of that heat. In fact, the cooling system on a car driving down the freeway dissipates enough heat to heat two average-sized houses! The primary job of the cooling system is to keep the engine from overheating by transferring this heat to the air, but the cooling system also has several other important jobs.
Never open the radiator of a car that has just been running. The cooling system of a car is under high pressure with fluid that is usually hotter than boiling water. Look for the cooling system reserve tank, somewhere near the radiator. It is usually translucent white so you can see the fluid level without opening it. (Do not confuse it with the windshield washer tank). The reserve tank will have two marks on the side of it. "FULL HOT" and "FULL COLD." If the level frequently goes below "full cold" after adding fluid, you probably have a leak which should be checked as soon as possible. Today's engines are much more susceptible to damage from overheating, so do not neglect this important system.
Cars operate in a wide variety of temperatures, from well below freezing to well over 100°F. So whatever fluid is used to cool the engine has to have a very low freezing point, a high boiling point, and it has to have the capacity to hold a lot of heat.
Water is one of the most effective fluids for holding heat, but water freezes at too high a temperature to be used in car engines. The fluid that most cars use is a mixture of water and ethylene glycol (C2H6O2), also known as antifreeze. By adding ethylene glycol to water, the boiling and freezing points are improved significantly.
| Significant Tempratures for mixtures of Water and C2H6O2 | Pure Water | 50/50 C2H6O2/Water |
70/30 C2H6O2/Water |
|---|---|---|---|
| Freezing Point | 32°F | -35°F | -67°F |
| Boiling Point | 212°F | 223°F | 235°F |
The temperature of the coolant can sometimes reach 250° to 275°F (121° to 135°C). Even with ethylene glycol added, these temperatures would boil the coolant, so something additional must be done to raise its boiling point.
The cooling system uses pressure to further raise the boiling point of the coolant. Just as the boiling temperature of water is higher in a pressure cooker, the boiling temperature of coolant is higher if you pressurize the system. Most cars have a pressure limit of 14 to 15 pounds per square inch (psi), which raises the boiling point another 45°F (25°C) so the coolant can withstand the high temperatures.
All coolants must be diluted with water at the proper ratios and should not be used full-strength. Full-strength antifreeze actually has a lower freeze point than when mixed with water. Generally, standard ethylene glycol type antifreeze should be changed every two years or 24,000 miles. Even though the coolant freeze protection may test OK with a hydrometer (freeze protection only drops with extreme dilution, not with age), the additives break down over time. When changing coolant, it also presents an opportune time to replace bad cooling system hoses. Leaking, brittle, spongy, cracked, or rotted hoses should be replaced before new antifreeze is installed. Hose clamp connections should also be checked to ensure that they're secure and free from leaks.