Car engines cannot convert the chemical energy in fuel into mechanical power entirely. Some of the energy is converted into heat. The exhaust system takes away much of the heat, but parts of the engine also absorb large amounts of it.

The components include a radiator, fan, water pump, thermostat, an overflow tank, water, coolant, and a series of connecting pipes and passageways. The system works by directing fluid past the hottest parts of your engine (the cylinder heads and valves) and redirecting the heated fluid out to the radiator, where the heat gets dissipated into the cooler atmosphere. The whole process then repeats.

The fluid within your car’s cooling system contains water and coolant. The coolant consists of a green fluid that contains ethylene glycol, also known as antifreeze. This chemical extends the freezing and boiling point of water and also inhibits rust. However, even with ethylene glycol added, the coolant would still boil, so something more must be done to raise its boiling point.

The boiling temperature of coolant is raised by pressurising the system, in the same way that the boiling point of water is increased in a pressure cooker. The radiator cap increases the boiling point of the coolant by about 25 degrees Celsius. The cap is actually a pressure release valve, and on cars it is usually set to 15psi.

When the pressure reaches 15psi, the pressure pushes the valve open, allowing coolant to escape from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank, which serves as a reservoir for the excess coolant that results from the water’s expansion when heated.