What Does a Radiator Do?

By: HowStuffWorks.com Contributors & Talon Homer  | 
Do you know how to locate the cap to your car's cooling system? Kenneth Cheung / Getty Images

There's absolutely no shame in digging around the internet and asking yourself, "What does a radiator do?" The entire cooling system around it, are some of the most vital components to keeping internal combustion engines running reliably and efficiently.


Why Does a Car Need a Radiator?

Without a cooling system, your engine would surely overheat within just a few minutes as the vehicle burns fuel. Early cars were "air-cooled" using no liquid coolant, but in pursuit of performance, engineers realized a robust cooling system was needed.

The car’s radiator acts as a heat exchanger, transferring excess heat from the engine’s liquid coolant into the air. The radiator is composed of tubes through which hot coolant flows, a protective cap that’s actually a pressure valve and a tank on each side to catch the coolant fluid overflow.


In addition, the tubes carrying the coolant fluid usually contain a turbulator, which agitates the fluid inside.

This way, the coolant fluid is mixed together, cooling all the fluid evenly, and not just cooling the fluid that touches the sides of the tubes. By creating turbulence inside the tubes, the fluid can be used more effectively.


What Is a Heat Exchanger?

A heat exchanger is a device which is used to dissipate heat from a piece of machinery, and in turn heat up the air around itself.

Simple, air-cooled engines and many electronics today rely on thin metal fins known as heat sinks in order to draw air to cool themselves. On engines, the heat sinks are typically wrapped around the cylinder head near where the combustion chambers are located.


Air cooling is accomplished because the heat sink has a high surface area relative to the components underneath when it comes to making contact with surrounding air. Sufficient airflow is also necessary to keep the system working efficiently, and it may also need a cooling fan which blows cool air over the fins and regulates temperature.

Modern Engine Cooling Systems

An air-cooled engine block is very simple to produce but does not cool efficiently enough to keep up with modern performance demands. This is why the typical automotive cooling system today uses a water cooled system, containing a car radiator, engine coolant and a water pump which pressurizes the hot liquid through a series of radiator hoses and tubes.

The radiator in a car works on the exact same heat transfer principles as a simple heat sink; the radiator contains hundreds of those fins.


The radiator inlet tank receives hot liquid from the engine block, which passes through these fins where cooling fans force air between them. As a result, the radiator cools hot liquid and returns it to proper operating temperature before sending it back to the engine.

Regulating Hot Coolant

The engine's cooling system is under constant stress to achieve proper cooling and eliminate excess heat while the engine is running. High coolant temperature — or worse, low coolant level — can be immediately catastrophic for an engine block. Such an engine requires a robust and maintained water cooling system to control temperature.

In order to perform their best, cooling systems need all of their components — the car radiator, hoses, tubes, the water pump and reservoir tank — to be regularly inspected and replaced if necessary. One bad seal or small crack in the engine's cooling system can prevent coolant flowing properly, building up excess heat and eventually causing big problems for your engine.

The Water Pump

Second only to the car radiator, the water pump is one of the most important cooling components in an engine (and a likely failure point). It is driven by the timing system, pressurizing engine coolant and keeping it flowing through the entire system at a regulated pace.

As the water pump is always running alongside the engine, it is prone to wear and will eventually malfunction. A worn-out pump can obstruct coolant from flowing properly through the radiator, and it can also begin to leak coolant itself.

The water pump should always be inspected during regular vehicle maintenance and replaced at the first sign of potential failure.

The Radiator Cap

When coolant fluid overheats, it expands, causing the fluid to become highly pressurized. When it enters the radiator, the pressure of circulating liquid increases even more because it’s in an enclosed space.

The radiator cap (or pressure cap) acts as a release valve set to open at the maximum pressure point. Usually this is set at a density of 15 pounds per square inch (psi).

When the fluid pressure inside the radiator exceeds 15 psi, it forces the valve open, allowing heat to escape and excess coolant fluid to overflow into the tanks on either side of the radiator. Once the radiator cools down, the coolant fluid in the overflow tanks gets sucked back into the pump, continuing its route through the cooling system.

Secondary Cooling System

Cars with automatic transmissions also require transmission cooling in the same way, with a separate heat-exchange circuit built into the car radiator. This two-step process of cooling the transmission fluid is equivalent to a radiator within a radiator.

As the heated transmission fluid enters the transmission cooler, the oil’s heat is exchanged with the coolant fluid in the car radiator, making the transmission fluid cooler while heating the coolant fluid instead. Then the coolant fluid’s heat is transferred to air in the radiator itself.

In addition to the main cooling system on the engine, cars also typically contain an air conditioner cooling system which uses a smaller radiator and a liquid refrigerant to regulate temperature inside the vehicle's cabin.


Radiator FAQ

How much does it cost to replace a radiator cap?
It only costs around $10 to replace a radiator cap.
What is a pressure cap?
The cap acts as a release valve set to open at the maximum pressure point.
How many PSI is a pressure cap?
Usually this is set at a density of 15 pounds per square inch (psi). When the fluid pressure inside the radiator exceeds 15 psi, it forces the valve open, allowing heat to escape and the excess pressure of coolant fluid to overflow into the tanks on either side of the radiator.
Are all radiator caps the same size?
No, the fit will depend on the size of your vehicle's radiator.
How often should you change your radiator cap?
You shouldn't have to change it at all. Radiator caps typically last as long as the car is running, but make sure to inspect it every so often.