How a Car Cooling System Keeps an Engine at Temperature
A pump, radiator, thermostat and pressurised coolant work together to shed engine heat. Here is what each part does and why the mix and pressure matter.
In this article
Burning fuel makes a petrol or diesel engine very hot, and a car's cooling system carries away enough of that heat to keep the metal within safe limits. It does not try to make the engine as cold as possible. A thermostat holds the engine near a working temperature, because an overcooled engine wastes fuel. The system is a closed loop: coolant picks up heat in the engine, gives it to the air in a radiator, and returns.
The loop in order
YourMechanic describes the circuit in six steps. A pump pushes coolant through the radiator, where airflow cools it. The thermostat stays shut while the engine warms up and opens at operating temperature. Coolant then passes through channels around the engine, called the coolant jacket, and absorbs heat. It also flows through the heater core, a small radiator behind the dashboard, which warms the cabin. Then it returns to the pump.
The water pump
Wikipedia's article on engine radiators explains that a centrifugal pump circulates the coolant, because natural circulation caused by hot liquid rising is far too slow for a car. YourMechanic says the pump is driven by a belt or timing chain, and that flow rises roughly in step with engine speed.
The radiator and fans
The radiator is a network of thin tubes with a large surface area. Heat passes from the coolant into the air moving past. When the car is driving, air is forced through the front. When it is slow or stopped, a fan draws air through. Wikipedia notes that engine-driven fans often use a clutch that slips when cool to save power, while electric fans are switched on by a temperature switch or the engine control unit. Both types cool well at low engine speed or when stationary, and the article observes that airflow speed has a strong effect on how much heat is shed.
The thermostat
Modern thermostats use a wax pellet. When the engine is cold the valve stays nearly closed, so coolant circulates only inside the engine and warms it up faster. As the coolant warms, the wax expands and pushes the valve open, sending flow to the radiator. According to Wikipedia, this keeps the engine at its optimum temperature, and overcooling reduces fuel efficiency and raises emissions.
Why coolant is not plain water
Water boils at 100 degrees Celsius and freezes at 0, and it can corrode metal parts. Wikipedia says coolant is normally water mixed with antifreeze, usually ethylene glycol or propylene glycol, plus a small amount of corrosion inhibitor, in proportions suited to the climate. It adds that corrosion protection became more important with aluminium engines. YourMechanic gives a 50/50 mix of ethylene glycol and water as the typical coolant.
Pressure raises the boiling point
The system is sealed and runs above atmospheric pressure, which raises the temperature at which coolant boils. A calibrated relief valve sits in the filler cap. Wikipedia gives a typical range of 4 to 30 psi (30 to 200 kPa) and says modern vehicles typically run around 15 psi, a figure YourMechanic also quotes. If pressure goes above the cap's rating, the valve releases a little coolant.
Wikipedia also warns of why a hot system should be treated with care: opening the cap of a hot radiator can cause the coolant to boil and be ejected. Caps are designed to relieve internal pressure before they fully open, but the safest approach is general caution around hot cooling systems.
Common causes of overheating
Sources on this subject note that a coolant leak, a blocked radiator, a thermostat stuck shut or a failing pump can all stop heat from leaving the engine. A fan that does not switch on matters mostly in slow traffic, when there is little natural airflow. Because the parts work as a loop, a fault in one place can show up as a high temperature reading somewhere else.
A short summary
- Pump: moves coolant around the loop
- Coolant jacket: carries heat out of the engine block
- Thermostat: stays closed when cold, opens when warm
- Radiator and fan: pass heat to the air
- Pressure cap: raises the boiling point
- Heater core: lends warmth to the cabin
Electric vehicles and hybrids also use liquid cooling loops for batteries and power electronics, but the details differ and are not covered by the sources used here. This article is general information about conventional liquid-cooled engines; a vehicle's own manual has the correct coolant type and service steps.
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