chap48 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 49, the reader should
be able to:
•

Prepare for ASE Heating and Air-conditioning (A7) certification test
content area “A” (Air conditioning System Diagnosis and Repair) and
content area “C” (Heating and Engine Cooling Systems Diagnosis and
Repair).

•

•

Describe how the heater functions.
Describe how the refrigeration cycle functions.

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OBJECTIVES:
After studying Chapter 49, the reader should
be able to:
•
•

List the parts of a typical air-conditioning system.
Explain how the air-conditioning system removes heat from the
passenger compartment.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
absolute humidity • axial compressor
barrier hose • blower motor • boiling point
calorie (c) • capillary tube • CFC-12 (R-12) • compressor •
condensate line • condensation point • cycling clutch orifice tube
(CCOT)

ester oil • electromagnetic clutch • evaporator • evaporator
pressure regulator (EPR) valve
freon
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KEY TERMS:
heat • heater core • heater hoses • heating, ventilation, and airconditioning system (HVAC) • HFC-134a (R-134a) • humidity •
H-valve • hygrometer
kinetic energy • latent heat of vaporization • liquid • miscible
pilot operated absolute (POA) valve • pintle valve • positivedisplacement compressor • psychrometer
reed valve • relative humidity

Continued
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KEY TERMS:
section 609 (clean air act) • solid • superheat • swash plate
thermostatic expansion valve (TEV or TXV) system • thermo
switch (icing switch or defrost switch)
vapor

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PRINCIPLES OF HEATING AND
REFRIGERATION
Driver and passenger comfort is the primary purpose of the heating, ventilation, and airconditioning system, abbreviated HVAC.

Matter is found in three different states: solid, liquid, or vapor (gas).
The state depends upon the nature of the substance, the temperature,
and the pressure or force exerted on it.
Figure 48–1 Water is a substance found naturally in solid, liquid, and vapor states.

Water occurs naturally in all
three states: solid ice, liquid
water, and water vapor,
depending upon temperature
and pressure of the location.
Continued

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Changes of State A solid cannot be compressed and has strong
resistance to flow. Molecules of a solid attract each other
strongly, and resist changes in volume and shape.
A substance is solid at any temperature below its melting point.
The melting point is a characteristic of a substance, and the
temperature at which a solid turns liquid. For water, melting
point is 32°F (0°C),
A liquid is a substance that cannot be compressed. A substance
in a liquid state has a fixed volume, but no definite shape.
The boiling point is the temperature at which a solid substance
turns to a vapor. For water at normal seal level conditions, the

boiling point is 212°F (100°C).
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Vapor can be easily compressed, has no resistance to flow, and no
fixed volume. It is considered a fluid just like liquids are.
A substance changes to a vapor if the temperature rises above its
boiling point. A vapor condenses to liquid if the temperature falls
below it. Like melting and freezing, boiling point and condensation
point are the same temperature, and vary with pressure.

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Heat and Temperature Molecules vibrate rapidly in all directions;
disorganized energy is called heat. Intensity of vibration depends
kinetic energy, or energy of motion, the atom or molecule contains.
We measure the level of this energy as temperature.
Heat and temperature are not the same. Heat is measured in calories
(c). The calorie is a metric unit for the amount of heat needed to
raise the temperature of one gram of water one degree Celsius.
Heat is also measured in British Thermal Units (BTU). One BTU is
the heat required to raise the temperature of one pound of water 1°F
at sea level. One BTU equals 252 calories.

Continued

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Why is Liquid Sprayed From a Can Cold?
If you spray a can of liquid continuously, the can becomes cold, and the
liquid being sprayed becomes cold. The can becomes cold because the
pressure in the can is reduced while spraying, allowing the liquid propellant
inside the can to boil and absorb heat. The liquid being sprayed has also
been cooled by the liquid propellant. The propellant vapor is further cooled
as it decompresses when it hits the open air. Rapid decompression results
in a rapid temperature drop.

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Latent Heat The “extra” heat that needed to transform a substance
from one state to another. When a solid reaches melting point, or
liquid reaches boiling point, their temperatures stop rising. The
solid begins to melt, and the liquid begins to boil. This occurs
without any change in temperature, even though heat is still being
added.
This extra, hidden amount of energy necessary to change the state
of a substance is called latent heat. See Figures 48–2 and 48–3.
Latent heat is important in air-conditioning system operation
because the cooling effect is derived from changing the state of
liquid refrigerant to a vapor.
The refrigerant absorbs latent heat of vaporization, cooling the air
blown into the passenger compartment. You take away the heat to

cool the air.
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Figure 48–2 The extra heat required to change a standard amount of water at its boiling point to
a vapor is called latent heat of vaporization.

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Figure 48–3 The latent heat of vaporization that water vapor stores is given off when the vapor
condenses to a liquid. The temperature stays the same.

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Temperature, Volume, and Pressure of a Vapor Unlike a solid, a
vapor has no fixed volume. Increasing temperature of a vapor, while
keeping volume confined in the same space, increases pressure.
This relationship between temperature and pressure in vapor is why
a can of nonflammable refrigerant can explode when heated by a
flame—pressure buildup inside the can will eventually exceed the
can’s ability to contain the pressure.
Increasing pressure by compressing
a vapor increases temperature.
Decreasing pressure by permitting expansion
of the vapor decreases temperature.
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Pressure-Temperature Relationships Two aspects are
important to understanding the operating of an HVAC system:
The temperature at which a liquid boils (and vapor
condenses) rises and falls with the pressure.
Pressure in a sealed system that contains both liquid and
vapor rises and falls with the temperature.

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Humidity refers to water vapor present in air. Level of humidity
depends upon water vapor present and temperature of the air.
Absolute humidity is the measurement of the weight of the water
vapor in a given volume of air. Relative humidity is the percentage
of how much moisture is present in the air compared to how much
moisture the air is capable of holding at that temperature.
Relative humidity is commonly measured with a hygrometer or a
psychrometer.
A hygrometer uses sensitive element that expands and contracts,
based on the humidity. Hygrometers resemble a clock, with a scale
reading from 0% to 100% relative humidity.

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A psychrometer uses two thermometers, one with the bulb covered in a
cotton wick soaked in distilled water from a built-in reservoir.

The wick keeps the bulb of the
“wet thermometer” wet so that it
can be cooled by evaporation.
Figure 48–4 A sling psychrometer is used to
measure relative humidity.

As the evaporator blows air, the
wet bulb’s temperature drops, and
the dry bulb reads the
temperature of the airflow.
Sling psychrometers are spun
round in the air a certain number
of times.
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Water evaporates from the wick at a rate inversely proportional
to the relative humidity of the air; faster if the humidity is low,
and slower if the humidity is high.
The “dry thermometer” registers ordinary air temperature.
The higher the relative humidity, the closer the readings of the
two thermometers; the lower the humidity, the greater the
difference. The different temperatures are compared to a chart,
which gives relative humidity.

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HEATING SYSTEM
All automotive and light-truck heater systems use the hot coolant from the engine to produce heat.
Coolant (antifreeze and water) flows through heater hoses and a heater core. The water pump
supplies force necessary to circulate the coolant through the heater core, a small radiator with
tubes and fins that transfer heat from the coolant to air flowing through core. See Figure 48–5.
A blower motor with a squirrel cage-type fan is used to force air through the heater core and into
the passenger compartment. Before the heater can function correctly, the cooling system has to be
functioning correctly. See Figure 48–6.

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Figure 48–5 Typical flow of air through an automotive heat, ventilation, and air-conditioning
system when placed in the heat position.

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Figure 48–6 A typical heater core as installed in an HVAC housing.

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What is an Auxiliary Electric Water Pump?
Some vehicles are equipped with an auxiliary electric water pump. The
purpose and function of this pump is to help warm the interior of the
vehicle by circulating coolant from the engine through the heater core
when the engine is at idle speed. At idle speed, the water pump does not
pump a sufficient quantity of coolant through the heater core to warm the

interior in freezing weather.

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AIR-CONDITIONING REFRIGERATION CYCLE
All automotive air-conditioning systems are closed and sealed. A
refrigerant is circulated through the system by a compressor that
is powered by the engine through an accessory drive belt.
Older systems used refrigerant CFC-12, commonly referred to by
its Dupont trade name of Freon® or R-12. Manufacturers now use
HFC-134a, less harmful to the atmosphere.
The basic principle of the refrigeration cycle is that as a liquid
changes into a gas, heat is absorbed. The heat absorbed by an

automotive air-conditioning system is heat from inside the vehicle.

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Automotive air-conditioning function in this manner:
1. Liquid refrigerant evaporates in a small radiator-type unit
called the evaporator. As the refrigerant evaporates, it
absorbs heat as it changes from a liquid to a gas. As heat is
absorbed by the refrigerant, the evaporator becomes cold.

Figure 48–7
The evaporator removes heat from the

air that enters a vehicle by transferring
it to the vaporizing refrigerant.

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