Car coolant fluid pressue?

Question

why is the coolant fluid in an automobile kept under high pressure?

Why do radiator caps have safety valves, allowing you to reduce the pressure before removing the cap?

Answer 1

Its not necissarily that the coolant is kept at a high pressure, it actually has no pressure (gauge pressure) when cold.  As it heats up, it expands and any gases in the system compress, building up the pressure.  The typical maximum pressure is set to 15 psig.  This 15 psig limit was likely set so that the system would not need to be treated as a ASME section VIII pressure vessel (which would force the automakers to follow more stringent codes/testing).  There are relief valves (basically a diaphragm with a spring behind it) in the system that are set to discharge if this pressure is exceeded for safety reasons.  The discharge goes to an overflow tank that is vented to atmosphere to instantly remove any built up pressure.  If you pull the cap on a hot radiator, there is a chance that the hot fluid could shoot out and scald the person removing the cap.  This actually has happened to many people in the past and is still a major concern for anyone driving older vehicles.

Answer 2

The cooling system carries heat from combustion away from the combustion chambers, and as the water absorbs heat, it expands, pressurizing the system. The pressure relief is a safety device that purges excess system pressure caused by an overtemp condition. Without pressure relief the system pressure could rise to dangerous, even explosive levels.

Answer 3

Sealing a radiator keeps the fluid from evaporating away.  As the fluid heats, the radiator becomes pressurized from expanding gas and some water evaporating.   The radiator becomes pressurized as a natural result of the fluid/air getting hotter and the radiator being sealed.

The safety valve is for safety.  If you did not reduce the pressure, the water/vapor mixture in the radiator would explode out at you when you removed the cap.  It can be at boiling temperature.  People get burnt this way.