Why doesn’t a car engine melt?
Every bang leaves heat behind. Follow where it goes, then stop the pump and watch what happens.
Extra heat in the engine
Steady
Step 1 of 3 · Following the heat
Pick up heat
Coolant flows through passages around the cylinder. Heat crosses the metal wall into the liquid. The coolant never enters the space where fuel burns.
Heat leaves as fast as the engine makes it, so the metal stays at a steady temperature.
Follow the big dot, or pick a step to hold the drawing still.
This is sped up to show the idea. It isn’t a timer for a real car. The engine starts already warm, with the radiator route open. Restart the pump early and it recovers. Once it seizes, press Start over. The tiny gap around the piston is drawn much bigger so you can watch it close. Seizing is one way a badly overheated engine can fail. It doesn’t predict when or how a real engine would.
Wait, what?
The fire and the metal aren’t at the same temperature.
Each burst of burning gas is very hot, but it’s over in a flash. Heat takes time to soak into the metal, and the cooling system keeps carrying it away. The trick isn’t tough metal. It’s giving the heat a steady way out.
The heat ends up in the air.
Coolant is a delivery service. It picks up heat from the engine, carries it to the radiator, then goes back for more.
The radiator’s thin fins give the air a huge area of warm metal to brush past. Heat crosses from liquid to metal to air. The coolant comes back cooler, not cold.
When the car is moving, air rushes through the radiator by itself. When it’s standing still, a fan can switch on to keep air moving. That whirr isn’t making cold. It’s carrying heat away.
Hot enough. Not too hot.
Too cold isn’t good either. When the engine is cold, a thermostat keeps most of the coolant on a short loop around the engine, skipping the radiator.
As the engine warms up, the thermostat opens the longer route through the radiator. That’s how it controls how much heat can leave. The playable drawing at the top starts with a warm engine, so that route is already open.
We’re following a typical gasoline engine cooled by liquid. Some engines skip the liquid. Fins on the engine itself pass heat straight to the air.
It isn’t just water.
Coolant is water mixed with additives that help stop it freezing and protect the metal from corrosion. The mix, plus the pressure sealed inside the system, raises its boiling point. That helps it stay liquid while it carries heat.
Oil has a second job.
Oil keeps moving metal surfaces apart. It also carries heat away. Some engines even spray oil at the underside of the pistons, right where there’s a lot of heat to collect.
If an engine gets far too hot, its thin film of oil can break down. Then metal rubs on metal until it grabs and the engine seizes. That can happen long before anything melts. So moving the heat out is what keeps the engine moving.
Talk like you’ve followed the loop
Coolant
The water-and-additive mix that carries heat from the engine to the radiator.
“The coolant just goes around and around.”Water jacket
Passages in the engine metal where coolant flows around the hot parts.
“Coolant runs through the water jacket.”Radiator
Tubes and thin metal fins that pass heat from the coolant to the air.
“The radiator dumps the heat into the air.”Thermostat
A valve that opens as the engine warms up, letting more coolant through the radiator.
“The thermostat stays mostly shut until it’s warm.”Go down the rabbit hole
ReadFollow the heat through a radiatorDENSO follows heat from the coolant into the air, and explains why a car that isn’t moving still needs air flowing through.ReadMeet the valve that decides the routeDENSO explains how a thermostat controls how much coolant reaches the radiator as the engine warms up.
TryNext time you’re stuck in traffic on a hot day, listenAfter a while you may hear a whirr from the front of the car. That’s the radiator fan. When the car is barely moving, hardly any air gets pushed through the radiator. So the fan pulls it through instead.