Understanding your car’s cooling system

What does a car cooling system look like?

Prop open your bonnet and at the front of the engine bay sits the car radiator  – a slim, rectangular honeycomb panel with hoses attached to it.

Shift your gaze back a little and you’ll see a small, clear plastic tank with a cap on it, usually filled with pink, green or blue fluid. That’s the car radiator expansion tank. Just to be sure, look for a couple of narrow hoses leading from it, one of them to the radiator.

By the way, if you’ve just been for a drive, don't open this cap – you may be scalded by hot, pressurised coolant. Wait until the engine is cold before unscrewing that cap.

Look behind the radiator and you’ll see a large fan, either mounted on the engine or separate from it. This pulls air through the radiator to help take away the heat.

Nearby, you’ll also see the long rubber auxiliary belt that drives various ancillary systems of the engine including (on some engines) a strange thing with hoses coming off it. That’s the water pump that sends the coolant fluid around the system. On some other engines, the pump is driven by the timing belt, whilst on very new vehicles, the pump is driven by an electric motor.

How does the cooling system work?

Here’s what happens when the engine and coolant are cold

A cold engine isn't good since fuel doesn't vaporise easily at cold temperatures, while the engine oil is cold and sluggish, and doesn't do a good job of lubricating the engine’s moving parts. So it needs to heat up –­ fast.

Bizarrely, the cooling system can help. The moment you start the car, the water pump begins to propel the cold coolant from the lower tank in the radiator (basically, the bottom section) to the cold engine block. From here it travels through channels in the casting to the cylinder head, and then back towards the pump.

Now here’s the clever bit. Located close to the pump is a thermostatically controlled valve. If the coolant is too cold, the valve remains closed, preventing it reaching the radiator and causing it to be pumped straight back to the engine uncooled, as well as around the cabin heater.

The coolant begins to warm up very quickly, helping to carry heat around the engine and accelerating the warming-up process, so improving the engine’s efficiency.

Here's what happens when the engine and coolant are hot

An engine’s maximum operating temperature is around 120C but when the coolant reaches around 90C a magical thing happens: the thermostatically controlled valve opens, diverting the hot coolant to the radiator via the upper radiator hose and into the upper radiator tank.

Of course, it’s not magic. In fact, wax inside the thermostat melts and expands, forcing the valve to open. Incidentally, this change in temperature is being monitored by a sensor that relays the data to the car’s engine control unit, which makes small running adjustments to the fuel and ignition systems as necessary.

On the latest cars, the operation of the thermostat is controlled completely by the engine management system. This allows precise control of the coolant temperature, further reducing emissions and increasing efficiency.