engines have improved a lot, they are still not very
efficient at turning chemical energy into mechanical power.
Most of the energy in the gasoline (perhaps 70%) is converted
into heat, and it is the job of the cooling system to
take care of that heat. In fact, the cooling system on a car
driving down the freeway dissipates enough heat to heat two
average-sized houses! The primary job of the cooling system is
to keep the engine from overheating by transferring this heat
to the air, but the cooling system also has several other
The engine in
your car runs best at a fairly high temperature. When the
engine is cold, components wear out faster, and the engine is
less efficient and emits more pollution. So another important
job of the cooling system is to allow the engine to heat up as
quickly as possible, and then to keep the engine at a constant
Diagram of a cooling system: how the plumbing
In this edition of HowStuffWorks,
we'll learn about the parts of a car cooling system and how
they work. First, let's look at some basics.
The Basics Inside your car's engine, fuel is
constantly burning. A lot of the heat from this combustion
goes right out the exhaust system, but some of it soaks into
the engine, heating it up. The engine runs best when its
coolant is about 200 degrees Fahrenheit (93 degrees Celsius).
At this temperature:
The combustion chamber is hot enough to completely
vaporize the fuel, providing better combustion and reducing
The oil used to lubricate the engine has a lower
viscosity (it is thinner), so the engine parts move more
freely and the engine wastes less power moving its own
Metal parts wear less.
There are two types of cooling systems found on cars:
liquid-cooled and air-cooled.
Liquid Cooling The
cooling system on liquid-cooled cars circulates a fluid
through pipes and passageways in the engine. As this liquid
passes through the hot engine it absorbs heat, cooling the
engine. After the fluid leaves the engine, it passes through a
heat exchanger, or radiator, which transfers the heat from the
fluid to the air blowing through the exchanger.
Air Cooling Some older
cars, and very few modern cars, are air-cooled. Instead of
circulating fluid through the engine, the engine block is
covered in aluminum fins that conduct the heat away from the
cylinder. A powerful fan forces air over these fins, which
cools the engine by transferring the heat to the air.
Since most cars are liquid-cooled, we will focus on that
system in this article.
Plumbing The cooling system in your car has
a lot of plumbing. We'll start at the pump and work our way
through the system, and in the next sections we'll talk about
each part of the system in more detail.
The pump sends the fluid into the engine
block, where it makes its way through passages in the
engine around the cylinders. Then it returns through the
cylinder head of the engine. The thermostat is
located where the fluid leaves the engine. The plumbing around
the thermostat sends the fluid back to the pump directly if
the thermostat is closed. If it is open, the fluid goes
through the radiator first and then back to the pump.
There is also a separate circuit for the heating system.
This circuit takes fluid from the cylinder head and passes it
through a heater core and then back to the pump.
Click on "Start" to see the fluid
flow through the engine as the engine warms
On cars with automatic
transmissions, there is normally also a separate circuit
for cooling the transmission fluid built into the radiator.
The oil from the transmission is pumped by the transmission
through a second heat exchanger inside the radiator.
Fluid Cars operate in a wide variety of
temperatures, from well below freezing to well over 100 F (38
C). So whatever fluid is used to cool the engine has to have a
very low freezing point, a high boiling point, and it has to
have the capacity to hold a lot of heat.
Water is one of the most effective fluids for holding heat,
but water freezes at too high a temperature to be used in car
engines. The fluid that most cars use is a mixture of water
and ethylene glycol (C2H6O2),
also known as antifreeze. By adding ethylene glycol to water,
the boiling and freezing points are improved significantly.
0 C / 32
-37 C / -35
-55 C / -67
100 C / 212
106 C / 223
113 C / 235
The temperature of the coolant can sometimes reach 250 to
275 F (121 to 135 C). Even with ethylene glycol added, these
temperatures would boil the coolant, so something additional
must be done to raise its boiling point.
The cooling system uses pressure to further raise
the boiling point of the coolant. Just as the boiling
temperature of water is higher in a pressure cooker, the
boiling temperature of coolant is higher if you pressurize the
system. Most cars have a pressure limit of 14 to 15 pounds per
square inch (psi), which raises the boiling point another 45 F
(25 C) so the coolant can withstand the high temperatures.
Antifreeze also contains additives to resist corrosion.
Water Pump The water pump is a simple
centrifugal pump driven by a belt connected to the crankshaft
of the engine. The pump circulates fluid whenever the engine
A centrifugal pump like the
one used in your car
The water pump uses centrifugal force to send fluid to the
outside while it spins, causing fluid to be drawn from the
center continuously. The inlet to the pump is located near the
center so that fluid returning from the radiator hits the pump
vanes. The pump vanes fling the fluid to the outside of the
pump, where it can enter the engine.
The fluid leaving the pump flows first through the engine
block and cylinder head, then into the radiator and finally
back to the pump.
Engine The engine block and cylinder head
have many passageways cast or machined in them to allow for
fluid flow. These passageways direct the coolant to the most
critical areas of the engine.
Note that the walls of the cylinder are quite
thin, and that the engine block is mostly
Temperatures in the combustion chamber of the engine can
reach 4,500 F (2,500 C), so cooling the area around the
cylinders is critical. Areas around the exhaust valves are
especially crucial, and almost all of the space inside the
cylinder head around the valves that is not needed for
structure is filled with coolant. If the engine goes without
cooling for very long, it can seize. When this happens, the
metal has actually gotten hot enough for the piston to weld
itself to the cylinder. This usually means the complete
destruction of the engine.
The head of the engine also has large coolant
One interesting way to reduce the demands on the cooling
system is to reduce the amount of heat that is transferred
from the combustion chamber to the metal parts of the engine.
Some engines do this by coating the inside of the top of the
cylinder head with a thin layer of ceramic. Ceramic is
a poor conductor of heat, so less heat is conducted through to
the metal and more passes out of the exhaust.
Radiator A radiator is a type of heat
exchanger. It is designed to transfer heat from the hot
coolant that flows through it to the air blown through it by
Most modern cars use aluminum radiators. These radiators
are made by brazing thin aluminum fins to flattened aluminum
tubes. The coolant flows from the inlet to the outlet through
many tubes mounted in a parallel arrangement. The fins conduct
the heat from the tubes and transfer it to the air flowing
through the radiator.
The tubes sometimes have a type of fin inserted into them
called a turbulator, which increases the turbulence of
the fluid flowing through the tubes. If the fluid flowed very
smoothly through the tubes, only the fluid actually touching
the tubes would be cooled directly. The amount of heat
transferred to the tubes from the fluid running through them
depends on the difference in temperature between the tube and
the fluid touching it. So if the fluid that is in contact with
the tube cools down quickly, less heat will be transferred. By
creating turbulence inside the tube, all of the fluid mixes
together, keeping the temperature of the fluid touching the
tubes up so that more heat can be extracted, and all of the
fluid inside the tube is used effectively.
Picture of radiator showing side tank with
Radiators usually have a tank on each side, and inside tank
is a transmission cooler. In the picture above, you can see
the inlet and outlet where the oil from the transmission
enters the cooler. The transmission cooler is like a radiator
within a radiator, except instead of exchanging heat with the
air, the oil exchanges heat with the coolant in the radiator.
Pressure Cap The radiator cap actually
increases the boiling point of your coolant by about 45 F (25
C). How does this simple cap do this? The same way a pressure
cooker increases the boiling temperature of water. The cap is
actually a pressure release valve, and on cars it is usually
set to 15 psi. The boiling point of water increases when the
water is placed under pressure.
Cutaway of radiator cap and
When the fluid in the cooling system heats up, it expands,
causing the pressure to build up. The cap is the only place
where this pressure can escape, so the setting of the spring
on the cap determines the maximum pressure in the cooling
system. When the pressure reaches 15 psi, the pressure pushes
the valve open, allowing coolant to escape from the cooling
system. This coolant flows through the overflow tube into the
bottom of the overflow tank. This arrangement keeps air out of
the system. When the radiator cools back down, a vacuum is
created in the cooling system that pulls open another spring
loaded valve, sucking water back in from the bottom of the
overflow tank to replace the water that was expelled.
Thermostat The thermostat's main job is to
allow the engine to heat up quickly, and then to keep the
engine at a constant temperature. It does this by regulating
the amount of water that goes through the radiator. At low
temperatures, the outlet to the radiator is completely blocked
-- all of the coolant is recirculated back through the engine.
Once the temperature of the coolant rises to between 180
and 195 F (82 - 91 C), the thermostat starts to open, allowing
fluid to flow through the radiator. By the time the coolant
reaches 200 to 218 F (93 - 103 C), the thermostat is open all
The open and closed positions of a
If you ever have the chance to test one, a thermostat is an
amazing thing to watch because what it does seems impossible.
You can put one in a pot of boiling water on the stove. As it
heats up, its valve opens about an inch, apparently by magic!
If you'd like to try this yourself, go to a car parts store
and buy one for a couple of bucks.
The secret of the thermostat lies in the small cylinder
located on the engine-side of the device. This cylinder is
filled with a wax that begins to melt at around 180 F
(different thermostats open at different temperatures, but 180
F is a common one). A rod connected to the valve presses into
this wax. When the wax melts, it expands significantly,
pushing the rod out of the cylinder and opening the valve. If
you have read How Thermometers
Work and done the experiment with the bottle and the
straw, you have seen this process in action -- the wax just
expands a good bit more because it is changing from a solid to
a liquid in addition to expanding from the heat.
This same technique is used in automatic openers for
greenhouse vents and skylights. See this
page for an example. In these devices, the wax melts at a
Fan Like the thermostat, the cooling fan has
to be controlled so that it allows the engine to maintain a
Front-wheel drive cars have electric fans because
the engine is usually mounted transversely, meaning the output
of the engine points toward the side of the car. The fans are
controlled either with a thermostatic switch or by the engine
computer, and they turn on when the temperature of the coolant
goes above a set point. They turn back off when the
temperature drops below that point.
Rear-wheel drive cars with longitudinal engines usually
have engine-driven cooling fans. These fans have a
thermostatically controlled viscous clutch.
This clutch is positioned at the hub of the fan, in the
airflow coming through the radiator. This special viscous
clutch is much like the viscous
coupling sometimes found in all-wheel
Heating System You may have heard the advice
that if you car is overheating, open all the windows and run
the heater with the fan going at full blast. This is because
the heating system is actually a secondary cooling system that
mirrors the main cooling system on your car.
The heater core, which is located in the dashboard of your
car, is really a small radiator. The heater fan blows air
through the heater core and into the passenger compartment of
A heater core looks like a small
The heater core draws its hot coolant from the cylinder
head and returns it to the pump -- so the heater works
regardless of whether the thermostat is open or closed.
For more information, check out the links on the next page.