Automotive air conditioning system chapter 1

Introduction

• A liquid like water or alcohol absorbs the heat in its vicinity as it evaporates.

• When alcohol is put on your skin, you feel chilly.

• This happens because, when the alcohol evaporates, it removes heat from your skin.

• On the air conditioner cycle, by using the above principle and refrigerant which is easier to evaporate than alcohol, takes place.

• An application of this principle is refrigeration.

• The higher the vaporizing capacity, the larger the heat absorbing power and the higher the refrigerating capacity.

Function

• The function of air conditioner is to maintain the life environment in a comfortable condition. Therefore, complete air conditioning involves:

i. Temperature control

ii. Air circulation control

iii. Humidity control

iv. Air purification

The air conditioner for an automobile generally

consists of a:

i. heating unit

ii. cooling unit

iii. moisture remover

iv. ventilator

Basic Cooling System

Refrigeration System

• A device that cools or dehumidifies the air in the car of fresh outside air drawn into the car so as to produce comfortable air.

Basic Theory of Cooling

• A container provided with a tap is placed in a well-insulated box.

• A liquid that will vaporize readily at

atmospheric temperature is placed in the

container.

• When the tap is opened, the liquid in the

container will take away the heat necessary for vaporization from the air inside the box, turn into a gas and escape outside.

• At this time, the temperature of the air inside the box will become lower than that before the tap was opened.

• We can cool objects in this way

Refrigerant

• The refrigerant is used to absorb, transfer and release heat in the air-conditioning circuit. A refrigerant must be :

i. Non-flammable

ii. Non-explosive

iii. Non-poisonous

iv. Non-corrosive

v. Odorless

R-12 • Substance known as Dichlorodifluoromethane commonly referred to as

Freon or R-12.

• R-12 has the following properties:

i. non-flammable

ii. non-toxic

iii. stable at all temperatures

iv. does not react with aluminium, steel or copper,

v. soluble in mineral oils

vi. low boiling point

vii. vaporizes at minus 30°C

viii. harmful to the environment

R-13-a

• It is an HFC or Hydrofluorocarbon.

• R-134a has a boiling point of minus 26.2°C.

• All its other refrigerant characteristics are similar to R-12 EXCEPT

i. Its operating pressures and temperatures in the evaporator and condenser are slightly higher than for R-12.

ii. R-134a is not soluble in mineral oils (so new compressor lubricating oils have been developed)

Water cycle, also known as the hydrologic cycle

Principle Operation

Receiver Expansion

valve Evaporator

Compressor Condenser

Liquid refrigerant

High Pressure, High Temperature

Low Pressure Low Temperature

Liquid refrigerant

Gaseous refrigerant

Low Pressure Low Temperature

High Pressure High Temperature

Gaseous refrigerant

Liquid refrigerant

Refrigeration Cycle

Types of Air Conditioner

Car air conditioners are classified according to

the air conditioner unit mounting location

and by the functions of the air conditioner.

Dash Type

The air conditioner unit in this type is normally

installed under the dash panel.

Trunk Type

The air conditioner units is installed in the trunk.

Mounting Location

Dual Type

The cold air is blown out from the front and rear

of the car interior.

Function

Simple Type

This type consists of ventilator connected either

to heater OR to refrigeration system, used

simply for heating or cooling.

All Season Type

• This type combines a ventilator with heater and refrigeration system.

• An all-season air conditioner can be used on a cold, wet day to dehumidify (dry) the air.

Ventilator

A device for leading the fresh outside air into the car, that also

serves for ventilating the car.

Types of Ventilator

Natural flow-through ventilator

Intake of external air into the vehicle’s interior due to the air pressure generated by the vehicle’s movement

Forced air ventilator (Boost Ventilator)

An electric fan or similar device is used to force air through the vehicle

Intake vents are located in places where the air pressure is

positive and the exhaust vents are located in places where

the air pressure is negative.

Operation of Dampers

Two types of damper operation used are:

1. Lever type

A lever on the control panel is connected to a cable which

moves the damper when the lever is moved.

2. Push-button type

Pressing a push-button on the control panel causes a servo

motor to operate, moving the damper.

Blower Operation

• Fresh air or re circulated air drawn in by the

blower.

• In figure below, blower speed control lever on the control panel can change the blower speed in four steps, from LO to HI.

• The blower speed is controlled by passing current through resistors with different resistance values to change the voltage to the blower motor, thus changing the blower speed.

Blower Operation

Air Conditioning Electrical Circuit

Magnetic Clutch Control

When the ignition switch is turned ON with blower

switch in any position except OFF, the heater relay goes

ON. If the air conditioner switch is then turned

ON, the magnetic clutch relay is turned ON by the A/C

amplifier. This causes the magnetic clutch to be

engaged. However, the magnetic clutch is disengaged,

and the compressor goes off, under the following

condition.

1. Blower Switch OFF

When the blower switch is turned OFF, the heater relay goes OFF, and power is no longer supply to

the air conditioner.

2. A/C Switch OFF

The main power of the A/C amplifier, which is controls the magnetic clutch relay, is cut off.

3. Evaporator Temperature Too Low

If the temperature at the surface of the evaporator falls to 3°C or below, the A/C amplifier shut off

the magnetic clutch relay

4. Dual Pressure Switch OFF

If the pressure in the high-pressure side of the refrigeration circuit is extremely high or extremely

low, this switch goes OFF. The A/C amplifier senses this, and shuts off the magnetic clutch relay.

5. Compressor Lock-up

If the difference between the compressor speed and the engine speed rises above a certain value,

the A/C amplifier judges that the compressor has locked up, and shuts off the magnetic clutch

relay.

6. Refrigerant Temperature Too High

If the refrigerant temperature in the compressor rises above certain specified level, the temperature

switch goes OFF.