1. Purposes and Functions of the Cooling System

1.7. Pressurization of Engine Coolant

LO 11-02: Identify and explain the purposes and functions of the cooling system.

 

Pressurization of Engine Coolant

Cooling systems are pressurized for two important reasons. The first is to maintain pressure greater than atmospheric pressure at the water pump inlet. Without pressure to the water pump inlet, pumps operate less efficiently. If the pump inlet is not under pressure, the cooling system can become air-bound due to coolant that vaporizes under low pressure and can prevent coolant flow. Related to this is cavitation, which is a major problem in diesel cooling systems. Cavitation is erosion in cylinder block walls, heads, injector tubes, and liner sleeves caused by the vaporization of coolant produced by the combination of heat and low coolant pressure. Cavitation occurs more easily if cooling system pressures are abnormally low.

The second important reason for pressurizing the cooling system is that it increases the boiling point of water. Permitting water to turn to vapor or steam near the cylinders and in the cylinder heads dramatically lowers the ability of the cooling system to absorb heat. Even momentary loss of direct contact with coolant causes severe thermal stress on these components, resulting in the cracking of heads and engine blocks. Increasing cooling system pressure prevents engine damage and loss of coolant due to boiling by increasing the boiling point of coolant. Every 1 pound per square inch (psi; 7 kilopascal [kPa]) increase in cooling system pressure increases the boiling point by approximately 3° F (−16° C). This means 10 psi (69 kPa) of pressure on the cooling system raises its boiling point to 242° F (117° C) Figure 13. Pressurizing the cooling system also prevents water from boiling away from the cooling system. The cooling system is pressurized by both coolant expansion and trapped air, including steam trapped in the radiator during heating.

   Fig11_13

FIGURE 11-13: Increasing the pressure of the cooling system increases the boiling point of the coolant. Adding antifreeze to the coolant further increases the boiling point.

 

  Technicians Tip  
  Diesel engine cooling systems have traditionally operated at pressures between 7–10 psi (48–69 kPa), which is lower than gasoline engines. The primary reason for this lower pressure is to prevent overheating the oil in the oil cooler. At higher coolant temperatures, the lubricating oil loses its viscosity, which can potentially cause a loss in lubricating abilities. At a lower boiling point, coolant escaping as steam carries heat away from the engine and keeps the oil temperature in the oil coolers lower. Higher operating temperatures allow the oil to potentially overheat. Newer engines using cooled EGR have higher cooling system pressures to prevent coolant from boiling in the EGR cooler. Some manufacturers, such as Volvo, pressurize the cooling system using chassis air pressure.  

 

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