Why engines need cooling

An engine is a heat machine, and not a very efficient one. Of all the energy in the fuel it burns, only about a third pushes the car along. About a third goes out of the exhaust pipe as hot gas. The last third soaks into the engine itself.

That’s a lot of heat. The burning gas in a cylinder reaches over 2,000°C (3,632°F). The pistons and cylinder head are often made of aluminium, which melts at about 660°C (1,220°F), and engine oil starts to break down above about 150°C (302°F). Without a way to carry the heat away, an engine would wreck itself within minutes.

The cooling system does that job. Most cars are liquid-cooled: a liquid called coolant flows round the engine, picks up the heat and carries it to the radiator at the front of the car, where the air takes it away.

But it isn’t just about keeping the engine cool. An engine works best at about 90°C (194°F). Too cold, and it wears faster, burns more fuel and makes dirtier exhaust. So the cooling system’s real job is to keep the engine at the right temperature, whether you’re stuck in traffic or on the motorway, in winter or summer.

Step through the parts with the numbered buttons. Then try different speeds, start the engine cold, and break the fan.

1. Heat. Every time the fuel burns, it heats the cylinder walls and the head around it. The faster you drive, the more fuel the engine burns and the more heat it makes.
2. Pump. The water pump is a small spinning impeller, usually turned by the engine’s belt. It pushes coolant through passages cast into the engine round every cylinder, called the water jacket. The coolant soaks up the heat as it goes.
3. Thermostat. The hot coolant leaves the engine through the thermostat, a valve with a plug of wax inside. When the engine is cold, it stays shut, and the coolant just goes back round the engine, so the engine warms up quickly. As the coolant nears 90°C (194°F), the wax melts and swells, pushing the valve open just far enough to hold that temperature.
4. Radiator. Coolant that gets past the thermostat runs through the radiator: dozens of thin tubes with thin metal fins between them. Air rushing in through the grille passes between the fins and carries the heat away. The cooler coolant goes back to the pump to collect more.
5. Fan. Standing still, no air rushes in, so an electric fan behind the radiator switches on when the coolant gets too hot and pulls air through. That’s the fan you sometimes hear whirring after you park.

Notice that on the move, the thermostat is only open a little. A radiator is made big enough to cope with towing a heavy trailer up a mountain pass on a hot summer day, so most of the time it has plenty to spare.

Try Fan broken. Driving along at 30 mph (48 km/h) or faster, nothing changes: the air rushing in does the job. Then try Traffic. With no air going through the radiator, the coolant climbs past boiling and the engine overheats.

Coolant is roughly half water and half antifreeze. Plain water would freeze in winter, and as ice it swells hard enough to crack an engine block. Antifreeze stops the mix freezing down to about -35°C (-31°F). It also stops the inside of the engine rusting and corroding, which is why it stays in all year round.

The system is sealed and runs under pressure. As coolant warms up, it expands, and the pressure cap holds it at about 1 bar (15 psi) above the air outside. Under pressure, the coolant doesn’t boil until about 125°C (257°F), rather than 100°C (212°F) in an open pan. The extra coolant pushed out as it expands goes into the expansion tank, the see-through bottle under the bonnet with MIN and MAX marks on its side.

Never open the cap on a hot engine. The coolant is under pressure and far above boiling point; with the cap off, it boils instantly and sprays out.

Your car’s heater uses the engine’s spare heat. Some of the hot coolant runs through a small radiator behind the dashboard, called the heater matrix, and a fan blows air through it into the cabin. That’s why the heater blows cold for the first few minutes on a winter morning: the thermostat is keeping all the heat in the engine until it’s warm.

A few engines are air-cooled instead, with thin fins on the outside of the cylinders and a big fan blowing air over them. The original Volkswagen Beetle and Porsche 911s up to 1998 worked this way, as do many motorbikes. Liquid cooling holds the temperature much more steadily, which is why almost every car uses it today.

• Leaks. Hoses perish, radiators corrode and water pump seals wear. Look out for a sweet smell, drips under the front of the car or the expansion tank level dropping. Check the level with the engine cold, and top up with the coolant the maker recommends.
• A stuck thermostat. Stuck open, the engine takes ages to warm up, the temperature gauge sits low, the heater stays lukewarm and the car uses more fuel. Stuck shut, coolant can’t reach the radiator, and the engine overheats within minutes.
• A broken fan. As in the demo, the car is fine on the move but overheats in traffic.
• A failing water pump. Its bearing can whine or its seal can leak. On many engines the timing belt drives it, so it’s often replaced at the same time as the belt.
• Overheating. If the temperature gauge climbs into the red or a warning light comes on, pull over when it’s safe and switch the engine off. Driving on can warp the cylinder head and blow the head gasket, the seal between the head and the block. A blown head gasket lets coolant into the cylinders, so you get sweet-smelling white smoke, coolant going missing and sometimes a creamy sludge under the oil filler cap. Fixing it is a big job.
• Coolant is poisonous. It tastes sweet, which makes it especially dangerous to children and pets. Mop up any spills.