In any internal
combustion engine, fuel and oxygen are combined in a combustion process to
produce the power to turn the crankshaft of the engine. The job of the
cooling system is to prevent damage to the engine parts which could
result from high temperatures and to remove excess heat from the engine, to
keep the engine operating at its most efficient temperature, and to get the
engine up to the correct temperature as soon as possible after starting.
Ideally, the cooling system keeps the engine running at its most efficient
temperature no matter what the operating conditions are.
WHY A COOLING SYSTEM IS NECESSARY IN THE
ENGINES?
Although gasoline 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 important jobs.
As fuel
is burned in the engine, about one-third of the energy in the fuel is
converted into power. Another third goes out the exhaust pipe unused,
and the remaining third becomes heat energy.
A cooling system of some kind is necessary in any internal combustion
engine. If no cooling system were provided, parts would melt from the
heat of the burning fuel, and the pistons would expand so much they
could not move in the cylinders (called "seize").
The cooling system of
a water-cooled engine consists of: the engine's water jacket, a
thermostat, a water pump, a radiator and radiator cap, a cooling fan
(electric or belt-driven), hoses, the heater core, and usually an
expansion (overflow) tank.
Fuel burning engines
produce enormous amounts of heat; temperatures can reach up to 4,000 degrees
F when the air-fuel mixture burns. However, normal operating temperature is
about 2,000 degrees F. The cooling system removes about one-third of the
heat produced in the combustion chamber.
The exhaust system takes away much
of the heat, but parts of the engine, such as the cylinder walls, pistons,
and cylinder head, absorb large amounts of the heat. If a part of the engine
gets too hot, the oil film fails to protect it. This lack of lubrication can
ruin the engine.
On the other hand, if an engine
runs at too low a temperature, it is inefficient, the oil gets dirty (adding
wear and subtracting horsepower), deposits form, and fuel mileage is poor--
not to mention exhaust emissions! For these reasons, the cooling system is
designed to stay out of the action until the engine is warmed up.
Most auto engines are cooled by the liquid type; air
cooling is used more frequently for airplanes, motorcycles and lawnmowers.
AIR COOLING SYSTEMS
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.
Air cooling systems are mostly used in the case of the
aircrafts and motorcycle engines. In the motor cycles, heat is taken away
from the cylinder walls by the cooling fins.
The cooling fins are metallic plates with projected strips
that increase the surface area very much. Since the engine of the
motorcycles is open to atmosphere, air, rapidly moving past the engine
cylinder, takes the heat away from the fins efficiently.
Liquid cooled
engines have passages for the liquid, or coolant, through the cylinder block
and head. The coolant has to have indirect contact with such engine parts as
the combustion chamber, the cylinder walls, and the valve seats and guides.
Running through the passages in the engine heats the coolant (it absorbs the
heat from the engine parts), and going through the radiator cools it. After
getting "cool" again in the radiator, the coolant comes back through the
engine. This business continues as long as the engine is running, with the
coolant absorbing and removing the engine's heat, and the radiator cooling
the coolant.
A cooling
system pressure tester is used to check the pressure in the cooling system,
which allows the mechanic to determine if the system has any slow leaks. The
leak can then be found and fixed before it causes a major problem.
HOW THE COOLING IS DONE
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 up.
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.
Pure Water
50/50
C2H6O2/Water
70/30
C2H6O2/Water
Freezing Point
0 C / 32 F
-37 C / -35 F
-55 C / -67 F
Boiling Point
100 C / 212 F
106 C / 223 F
113 C / 235 F
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 is running.
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 hollow.
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
passageways.
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 the fan.
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
cooler
Radiators usually have a tank on each side, and inside the 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 reservoir
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.
