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Heating and Air Conditioning, Part 1
AUTOMOTIVE HEATING AND ENGINE COOLING SYSTEMS
The Automotive Engine Creates Heat
An automobile or truck engine is basically an air pump. The fuel
that’s fed to the engine is burned in a compressed form to maintain
the rotation of the motor and the motion of air through the motor.
Each time a cylinder ignites, the burning fuel and air mixture passes
some of its heat to the engine block. The rest of the heat of the burning
air/fuel mixture is passed on down the exhaust system to exit the
vehicle.
If this heat were allowed to accumulate, the engine would get so hot
that the expansion of the cylinder block and the pistons would finally
cause one or more pistons to seize in their bores. This often occurs
when someone unknowingly drives a vehicle with a cooling system
leak. Another cause of pistons seizing is when someone drives a vehicle
with a broken fan belt. When this happens, the water pump can’t
circulate coolant through the cooling system components. (You should
know that an engine is designed to operate with its built in metal-tometal surface tolerances when the engine is at about 195°F. A higher
temperature will decrease the range of these tolerances, while too low
a temperature will provide engine tolerances that are too high.)
A second source of engine heat is friction. Many people think of engines
as devices that turn easily. However, if you were to remove the spark
plugs from a motor and try to turn the crankshaft, you would find the
engine very difficult to turn. The many metal-to-metal contact points
within the assembly cause this friction. As a result, many engines are
now designed to run on low-viscosity oils to help decrease friction
and increase fuel mileage.
Methods of Removing Heat
Figure 1 displays a modern vehicle’s engine and engine cooling system.
This system is called a water-cooled system, since water, mixed with a
suitable antifreeze, is circulated through the system to lower engine
heat. As you can see in this figure, a radiator and a series of cooling
hoses are hooked to the engine.
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Heating and Air Conditioning, Part 1
FIGURE 1—An engine’s
cooling system is comprised largely of the
radiator and cooling
hoses.
A side view of the same type of system is shown in Figure 2. This
view more clearly displays the flow of coolant through the water
pump, the engine block, the thermostat, the upper radiator hose, the
radiator, and finally the lower radiator hose, at which point it flows
back to the engine block. This is known as a closed-loop system.
FIGURE 2—This drawing
illustrates the path of
coolant through an
engine’s cooling system.
At the engine block, the water/antifreeze mixture picks up engine heat
and transfers the heat to the radiator. The airflow through the radiator
then transfers this heat outward into the environment. During normal
operation, the same coolant circulates from the engine block to the
radiator again and again, first picking up, then releasing heat at various
points of the system.
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A second method of cooling an engine is known as air-cooling. In this
type of system, the cylinders of the engine are deeply finned; this allows
a large part of the cylinders’ surface area to be exposed to the flow of
air. The air-cooling system is very common in airplane engines and
motorcycles, and it was the sole type of system used to cool early
Volkswagen and General Motors Corvair engines. This system typically
employs an engine-driven fan to blow air over the engine cylinders.
But unlike water-cooled engines, air-cooled engines aren’t able to
regulate cylinder and engine temperature very easily.
A third method of removing engine heat is by oil cooling. In all engines,
a certain amount of heat is transferred from hot spots within the engine
to the oil and then the oil pan. The main engine hot spots are the
crankshaft and rod assemblies and the piston and wrist-pin assemblies.
In some engines, a small stream of oil is pumped under the pistons to
help cool the piston crowns and the wrist pins. Some motorcycle engines
actually use two oil pumps. One pump delivers a low-volume, highpressure flow for standard engine lubrication. The second pump is a
low-pressure, high-volume pump that’s used to spray the pistons,
especially the rear piston in a V-twin engine.
In any type of cooling system, the idea is to transfer heat. Heat is
transferred from the engine to the surrounding air to prevent the
damage that can occur from overheating; this transferral also insures
the engine is operating at its most efficient temperature. In almost all
automobile and truck engines, the three methods of cooling are used
together to cool the engine. However, the water cooling section is
responsible for removing most of the engine heat and for precisely
regulating the engine’s temperature.
A Basic Heating/Cooling System
Let’s take a more detailed look at the heating and cooling system
within a typical engine. In this “tour,” we’ll begin at the front of the
vehicle and progress towards the rear.
The Radiator, Pressure Cap, and Expansion Tank
The radiator is mounted in the front of the vehicle where cool air can
pass through it to pick up heat. The parts of a typical radiator are
shown in Figure 3.
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Heating and Air Conditioning, Part 1
FIGURE 3—Elements of an Engine Radiator
(Reprinted with permission from Toyota Motor Corporation. Ó Toyota Motor Corporation.
All rights reserved.)
The two basic types of radiators are down-flow and cross-flow radiators.
Their names reflect how the coolant passes through the radiator on its
way back to the engine. Most of today’s radiators are of the cross-flow
design.
The main section of the radiator is normally made of aluminum tubes
that are “finned” to offer more surface area of metal to the passing airflow. The top and bottom sections of the radiator are called the tanks.
These tanks can be made up of many different materials. Older vehicles
used copper and aluminum tanks that were soldered to the radiator’s
tubes. But many of today’s vehicles use plastic tanks that are glued or
pressed onto the tubes.
Each tank contains a fitting that connects to a radiator hose. Usually,
one fitting is located on the upper tank while another attaches to the
lower tank. This configuration allows for the cross-flow radiator to work
more efficiently and for the engine to pull in coolant of a reduced
temperature. Radiators normally fail if they’re leaking coolant. Leaks
often start small but worsen as time passes.
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At the top of the upper tank is a fitting known as a pressure cap. A
typical pressure cap is shown in Figure 4.
FIGURE 4—A pressure
cap, such as the one
shown here, is used to
keep the coolant under
pressure and therefore
from boiling in the
radiator, hoses, and
engine. (Photo courtesy of
the Gates Rubber Company)
The pressure cap is responsible for maintaining a pressure within the
cooling system. This pressure can be as little as a few pounds per
square inch, or psi, to a maximum of about 15 psi. The cooling system
is pressurized because water typically boils at 212 °F. If and when the
engine’s coolant reached that temperature, steam would be created;
and steam is a very poor coolant. Instead, when the coolant is placed
under pressure, the boiling point is raised and the coolant remains in
a liquid form within the system.
The pressure cap is simply a cap containing a spring-loaded plunger,
or seal. As long as the pressure within the radiator remains at or
below the rated pressure of the cap, the plunger will remain seated in
the fitting. If the pressure exceeds the rating of the pressure cap, the
pressure will lift the plunger from its seat and release some of the
pressure in the form of pressurized coolant. A hose that connects to
the bottom of the pressure cap fitting is used to collect this pressurized
coolant, after which it’s sent to an expansion tank.
The expansion tank is responsible for collecting the coolant that has
escaped from the radiator due to excess pressure. This tank is normally
made of a semi-clear plastic and is mounted on one of the inside fender
wells. The expansion tank often has markings on its side. These markings are used to define the upper and lower coolant levels for the
heating and cooling system.
The Thermostat
The thermostat is a component of the heating and cooling system that’s
responsible for maintaining the engine’s temperature. A typical
thermostat is shown in Figure 5.
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Heating and Air Conditioning, Part 1
FIGURE 5—A thermostat
is used to control the
temperature of the
vehicle’s coolant. (Photo
courtesy of the Gates Rubber
Company)
The thermostat, mounted in the engine’s water outlet, works as a
temperature-activated fluid-control valve. When the temperature of
the engine’s coolant lies below the preset value of the thermostat, the
thermostat remains closed and won’t allow coolant to flow through
the radiator. Instead, the coolant will bypass the radiator and circulate
through the engine block. When the temperature of the coolant is
equal to or exceeds the value of the thermostat, the thermostat opens
and allows coolant to circulate through the radiator.
Typical thermostat temperature values are 180, 185, 190, and 195°F.
Most thermostats operate by means of the controlled expansion of a
pellet. This pellet resides in the lower chamber of the thermostat and is
normally made of wax. When cold, the pellet shrinks, pulling downward on the bellows and allowing the thermostat to close. When the
passing coolant heats the thermostat, the wax pellet expands and
presses upward on the bellows, which in turn opens the thermostat.
A thermostat will normally fail in one of two ways. First, it can stick
when closed and cause the engine to overheat. Secondly, the thermostat can stick when open; in a case such as this, the engine warms up
too slowly and the engine never reaches its designed operating
temperature since coolant is constantly flowing to the radiator.
The Water Pump
The water pump is mounted on the side of the engine where the beltdriven alternator, power steering pump, and air conditioning compressor are located. Like these other accessories, the water pump is
belt-driven by the pulley that’s located on the crankshaft. A side view
of a typical water pump is shown in Figure 6.
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FIGURE 6—A side view of
a typical water pump is
shown here.
There are four major components to a water pump—the shaft, the
seal, the bearing, and the impeller. The bearing at the front of the
housing supports the pump’s shaft. The seal is also present at this location to prevent coolant from leaking. The impeller is pressed onto this
shaft and is located within the housing next to the engine. By turning
the impeller within the pump housing, the water is pulled into the
inlet and compressed before being pushed through the outlet. The
coolant leaving the water pump flows through the engine, while the
coolant entering the water pump comes from the radiator.
The seal and bearing can both fail on a water pump, either individually
or at the same time. In the case of either occurrence, the water pump
should be replaced.
The Heater Core
Another major component of the heating and cooling system is the
heater core. The heater core is mounted either inside the vehicle’s
dashboard or just behind the dashboard on the engine side. It’s
normally located near the glove compartment.
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Heating and Air Conditioning, Part 1
The purpose of the heater core is to transfer heat from the circulating
coolant to the passing air stream, which in turn warms the vehicle’s
passenger compartment. A typical heater core is shown in Figure 7.
FIGURE 7—A heater
core contains tubes
and fins similar to a
radiator.
A heater core is basically just a small radiator. Like a radiator, it contains tubes and fins. Unlike a radiator, though, a heater core contains
only two fittings and generally no side tanks.
Heater cores normally fail when they become blocked, and coolant
isn’t able to flow through the radiator. A blocked heater core can
often be repaired by back-flushing the core. In addition to blocking up,
though, cores also leak. A leaky heater core produces a sticky-sweet
smell within the vehicle and can fog the insides of the windows in
colder climates. You may even notice drops or a puddle of coolant on
the passenger floor when the core is leaky.
Hoses
There are two sets of hoses used in automotive vehicles—radiator hoses
and heater hoses. Both of these may appear to be simple rubber tubes.
However, the typical radiator or heater hose is made up of a complex
structure of layers. A typical automotive hose is shown in Figure 8.
FIGURE 8—Section of
Automotive Hose
Replacement radiator hoses are specially made to fit particular vehicles.
Heater hoses are easier to replace—a new hose is simply matched to
the length and diameter of the old hose.
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Belts
The automotive accessory drive belt is also a device of complex construction. A typical drive belt is made up of layers of cords that are
covered by rubber and/or synthetic materials. When these layers
form a flat loop, the belt is called a serpentine belt, while those forming
a “V” shape create a V belt. The serpentine belt is shown in Figure 9A
and the V belt is installed as shown in Figure 9B.
FIGURE 9A—The Serpentine Belt
FIGURE 9B—The V Belt
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Heating and Air Conditioning, Part 1
In either case, the belt is normally used to drive the water pump, the
alternator, the air pump, the power steering pump, and the air conditioning compressor off of a pulley on the crankshaft. In the case of a
serpentine belt, a single belt drives all of the accessories. In the case of
a V belt, two or three belts are normally used to drive the accessories.
Fan Assemblies
There are two different methods of using a fan to pull cool air through
a vehicle’s radiator. Both methods involve the use of either an enginedriven or electrically-powered fan.
When the vehicle is a rear-wheel drive vehicle with a longitudinally
mounted engine, a fan can be connected to the front of the engine to
pull outside air through the radiator. In most cases, the fan is mounted
to a flange at the front of the water pump shaft and is driven by a belt
off of the crankshaft. At low engine speeds, this arrangement allows
the engine to turn the fan at the proper speed. However, at high engine
speeds, the vehicle is usually traveling quickly enough to move enough
air through the radiator naturally. At this time, the fan is typically
stealing horsepower from the engine and providing no useful purpose.
Two different types of fan designs are used to counteract these problems. The first design utilizes flexible fan blade tips that flatten out, or
feather, at high speeds. The second design involves the placement of a
clutch between the fan and its mounting flange, as shown in Figure 10.
FIGURE 10—This type
of engine-cooling fan
clutch only drives the
fan blades when it’s
necessary to circulate cooling air.
(Courtesy of Chrysler Corporation)
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This clutch contains an internal fluid that’s temperature-sensitive.
Below about 170°F, the fluid passes the motion of the water pump shaft
along to the fan. At higher temperatures, the fan is uncoupled from
the water pump shaft and flange and freewheels, or rotates naturally.
When the vehicle is a front-wheel drive vehicle containing a transversemounted engine, it’s impossible to use an engine-driven fan. An
electrically powered fan must therefore be used. This fan is mounted
to a housing located behind the radiator. The electric motor, when
energized, will rotate a fan assembly to pull air through the radiator
(Figure 11). This motor is normally controlled by a relay. The relay is
in turn controlled by various sensors on the engine.
FIGURE 11—An electric fan motor is shown in (A),while its position in the fan housing is shown in (B).
(A: Provided by JIDECO; B: Courtesy SCS/Frigette)
An electrically operated fan offers many advantages. First, since sensors
control the fan motor, the motor is only energized when the engine
temperature exceeds a preset limit; this saves energy. Second, the fan
doesn’t require horsepower from the engine, saving further energy.
The Blower
A second type of electrically powered fan is used in vehicles to pull
air through the under-dash ductwork and into the passenger compartment. This fan-and-motor assembly is often referred to as, simply,
a blower. A typical blower motor assembly is shown in Figure 12.
Many blower motors are mounted to the cowling under the hood of
the vehicle. This location allows for simple replacement. However,
many blower motors are located within the ductwork and are somewhat difficult to replace.
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Heating and Air Conditioning, Part 1
FIGURE 12—A blower
motor is used to force
air through the heater
core or evaporator to
either warm or cool
the vehicle. (Provided
by JIDECO)
Most blowers operate at three or more speeds. In order to provide
these speeds, the switch on the dashboard of the vehicle connects the
motor to the power source through series resistors of different values.
These resistors are mounted inside the ductwork to keep them cool
while they provide their function in dropping the voltage to the motor.
Ductwork
Below the top surface of the dashboard is a complex system of ductwork that’s used to move outside, heated, or air-conditioned air into
the passenger compartment. A typical ductwork system is shown in
Figure 13.
FIGURE 13—This illustration displays the paths air can take through the ductwork that resides under a
vehicle’s dash.
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Note that there are two components of the heating/air conditioning
system mounted inside the engine compartment. These are the air
conditioning system’s evaporator and the blower motor assembly. The
major component mounted in the ductwork within the vehicle is the
heater core.
The ductwork contains a series of doors that direct the fresh, heated,
or air-conditioned air throughout the vehicle. These doors can be
cable-operated from a dashboard control assembly. In addition, this
control assembly usually contains a switch for the blower motor speed
control and a switch that turns the air conditioning system on and off.
In other cases, the doors are controlled by vacuum actuators, called
pots, that move into one of two possible positions via computer
controlled vacuum solenoids. The former system is a manually
controlled environmental system, while the vacuum-controlled system is used more often as an automatic climate control system.
Another type of modern ductwork door-control system uses small,
DC-powered motors called servo motors. When these motors are
energized, they’re able to either open or close a particular door in the
ductwork. In addition, the servo motor contains a feedback device
that allows the computer-controlled system to position a door at any
position—from fully open to fully closed. Servo motors have become
more common in recent years, as computers are now used to precisely
control the interior climates of vehicles.
Control Valves
There are two main types of control valves in a typical automotive
heating system. The first type, which we just discussed, is the ductwork door-control valve, or vacuum pot, shown in Figure 14.
FIGURE 14—A vacuum
actuator, or vacuum
pot, can be used to
open or close doors
inside the vehicle’s
ductwork.
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Heating and Air Conditioning, Part 1
The heart of this valve is the bellows. When vacuum action is applied
from a solenoid, the bellows are pulled back to the left. This action in
turn pulls on the attached rod, opening or closing the door within the
ductwork. During normal operation, these vacuum pots rarely fail.
When failures occur, it’s usually due to a stuck door that the valve
can’t move, a leaky bellows, or a stuck rod at the exit of the pot.
The second type of control valve is used to control the flow of coolant
from the engine to the heater core. This valve is placed on one of the
heater hoses and can be controlled manually, using a cable, or by way
of a vacuum. Heater control valves are normally located under the
hood on the passenger side of the vehicle, near the firewall. A heater
control valve will sometimes fail by getting stuck in either an open or
closed position after being set in one position for a long period of
time. This type of valve can also get blocked if the coolant is circulating dirt and scale along with coolant.
Power Check 1
In each section of Heating and Air Conditioning, Part 1, you’ll be asked to check your
understanding of what you’ve just read by completing a “Power Check.” Writing the answers
to these questions will help you review what you’ve learned so far. Please complete
Power Check 1 now.
1. A water pump is leaking coolant. This means that the _______ has failed.
a. bearing
c. impeller
b. seal
d. belt
2. Which one of the following components has upper and lower tanks?
a. The radiator
c. The expansion tank
b. The heater core
d. The water pump
3. The _______ radiator is the type most commonly used in today’s vehicles.
a. front-flow
c. V-belt
b. cross-flow
d. serpentine-belt
4. What part of a thermostat responds to the temperature of the coolant by expanding and
contracting?
a. The bellows
c. The tower
b. The wax pellet
d. The pressure cap
Check your answers with those on page 65.