Combustion inside a cylinder briefly reaches temperatures well above 2,000°C — hot enough to melt the aluminium or cast iron surrounding it several times over. An engine survives that only because a constant, carefully managed loop of liquid is pulling heat away from the metal every second it runs, fast enough to keep the block and head at a stable, safe working temperature. Get that loop wrong and the result isn’t a gradual problem: it’s a warped cylinder head, a blown gasket, or a seized engine within minutes.
Water jackets: coolant runs where the heat actually is
Cast into the cylinder block and cylinder head, wrapping around each cylinder bore and the combustion chambers above them, are hollow passages called water jackets. Coolant is pumped through these passages, in direct contact with the hottest metal in the engine, and carries heat away by simple convection before continuing on to be cooled itself. The jackets are shaped deliberately unevenly: more coolant flow is routed to the areas around the exhaust valves and the top of the cylinder bores, which run hottest, than to the relatively cooler lower cylinder walls.
The water pump: usually driven by the engine itself
Moving the coolant around that loop, continuously, is the job of the water pump — on most combustion engines, a simple centrifugal impeller pump spun directly off the engine via a belt or the timing chain, so its speed scales automatically with engine RPM: more heat generated, more coolant flow, without any electronics involved. That mechanical simplicity is also its main weakness, which is why a growing number of modern engines now use an electric water pump instead, decoupled from engine speed entirely, controlled by the ECU: it can keep pumping after the engine is switched off to prevent heat-soak damage, run at a precisely calculated speed rather than whatever the crankshaft happens to be doing, and stay off altogether during warm-up to help the engine reach operating temperature faster.
The thermostat: a valve that deliberately keeps the engine hot
Counterintuitively, the first job of the cooling system isn’t to cool the engine at all — it’s to stop it from cooling too much, too soon. The thermostat is a wax-pellet valve sitting between the engine and the radiator that stays fully closed while the engine is cold, forcing coolant to recirculate only within the engine itself rather than through the radiator, so the engine reaches its efficient operating temperature (typically 90-105°C) as quickly as possible. Only once the coolant reaches the thermostat’s calibrated opening temperature does the valve start letting coolant flow out to the radiator. A cold engine burns more fuel, wears faster and produces more emissions than a warm one, which is exactly why manufacturers deliberately delay full cooling rather than maximising it from the moment the key turns.
The radiator: getting rid of the heat for good
Once coolant is hot enough to need it, the thermostat routes it to the radiator: a network of thin tubes and fins mounted at the front of the car, designed to maximise surface area so that air passing through — from the car’s forward motion, an electric cooling fan, or both — can strip heat out of the coolant before it’s pumped back into the engine to absorb more. The electric fan, unlike the airflow generated just by driving, is switched on by the ECU whenever coolant temperature or vehicle speed alone isn’t providing enough airflow — which is why it’s the fan, not the radiator itself, that keeps running audibly for a minute or two after you park a hot car and switch off the engine.
Why the coolant isn’t just water
Pure water is actually a poor long-term coolant for an engine: it freezes at 0°C, which would crack the block in winter, it boils at 100°C at sea-level pressure, and it corrodes iron and aluminium engine components on its own. Automotive coolant is a mixture of water and ethylene or propylene glycol (antifreeze), typically in roughly a 50/50 ratio, which lowers the freezing point well below 0°C and, more importantly for a hot-running engine, raises the boiling point. That boiling point is pushed even higher by the fact that the entire system is pressurised, usually to around 1-1.5 bar above atmospheric, via a spring-loaded pressure cap: raising pressure raises the boiling point further still, letting the coolant run safely above 100°C without turning to steam and losing its ability to carry heat. The mixture also carries corrosion inhibitors specifically formulated for the mix of aluminium, iron, rubber hoses and gaskets it has to sit against for years without attacking any of them.
What actually fails, and why it’s rarely gradual
A cooling system failure is almost never a slow decline — a slipped drive belt, a stuck-closed thermostat, a leaking hose or a failed water pump can turn a normally-running engine into an overheating one within a couple of minutes, because the heat generated by combustion doesn’t pause while the fault is diagnosed. The most expensive failure mode is a blown head gasket: the thin sealing layer between the cylinder head and the block, sitting directly next to both the combustion chambers and the coolant jackets, which can fail from sustained overheating and let combustion gases into the coolant loop or coolant into the cylinders — either one turning a cooling system problem into an engine rebuild.
A different problem entirely on an EV
An electric car has no 2,000°C combustion event to manage, but it isn’t cooling-free: the battery pack, electric motor and power electronics all generate heat under load and, just as importantly, a lithium-ion battery pack ages faster and can lose usable range if it’s allowed to run too hot or too cold for extended periods, which is why most EVs run a separate, precisely regulated liquid cooling (and sometimes heating) loop dedicated to the battery pack alone, entirely independent of the cabin climate system — a genuinely different engineering problem from the one described above, built around protecting battery chemistry rather than protecting metal from combustion heat.
Photo: © Car-Shooters