Automobile - AC SYSTEM

Air COnditioning system of automotive vehicles. A part of "Automobile Engineering" course.
Author: Prof. Dr. Md. Ehsan (BUET)

1. ME 467 : Automobile Engineering Dr. Md. Ehsan © 2015 ME, BUET
Automotive Air Conditioning System

2. ME 467 : Automobile Engineering Dr. Md. Ehsan © 2015 ME, BUET
Automotive Air Conditioner: The automotive air conditioner is an accessory system that
cleans, cools, dries, and circulates passenger-compartment air. This may also provide some
indirect noise control. The air conditioner is basically a mobile refrigeration system. It includes an
air-delivery system and a temperature-control system. The parts are connected by wiring, hoses,
and tubing. The compressor in the automotive air conditioner is powered by the engine. Running the
air conditioner may result in reduced engine performance and fuel economy. In a vehicle with
factory-installed air conditioning, the heater and air conditioner are integrated into a heater-air-
conditioner.

The automotive air conditioning system high pressure circuit consists of the – Compressor, Condenser
and Receiver-Drier; while the low pressure part of the circuit consists of – the Thermal Expansion
Valve and the Evaporator.

The unit may be manually-controlled or automatically-controlled. In manually controlled ones the
driver selects the desired mode by moving a mode or selector lever. Its position shuts off or allows
air delivery through the vents, and controls heater and air-conditioner operation. Moving the
temperature lever on the control panel controls the temperature of the air entering the passenger
compartment. Turning the blower switch or fan control determines the speed of the blower motor.
The automatic system is basically the same but allows the driver to select automatic control and the
desired temperature. The system will then maintain that temperature by providing heat or cooling as
required. Many systems also automatically adjust the blower speed. The instrument-panel controls
usually allow the driver to override the automatic operation.


VENTILATING THE PASSENGER COMPARTMENT

For health and comfort, some fresh air must pass through the passenger compartment. This replaces
the stale and sometimes smoke-filled air inside the vehicle. The process is called ventilation. There are
two methods: uncontrolled ventilation and controlled ventilation.

Uncontrolled ventilation occurs when windows are opened. Controlled ventilation is either ram-air or
power. In the ram-air system, opening vents or ducts admits air to the passenger compartment.
Forward movement then forces or rams air into the vehicle. However, when the vehicle stops or
moves slowly, little fresh air enters. This is one reason most vehicles have a power ventilating
system. An electric blower motor and fan provide air circulation regardless of vehicle speed. The
blower is also part of the heater and the air conditioner.



It is important that the cols air from the air conditioning outlets are well circulated all over the vehicle.
The passengers can choose to have fresh air in the car or use recirculation of the air inside the

3. ME 467 : Automobile Engineering Dr. Md. Ehsan © 2015 ME, BUET
passenger cabin. Switches can be used to direct the air flow to particular set of vents/outlets for
comfort. The speed of the blowers can also be changed to regulate the air circulation rates.

Although the cool air is supplied by the blower passing air across the evaporator section of the air
conditioner, hot air is supplied by the blower passing air across a heater core which uses hot coolant
engaged in engine cooling as the heat source. In some vehicle the hot air flow is supplemented by an
electric heater attached to the heating core to supply hot air, while the engine is still cold.






AUXILIARY REAR HEATING AND COOLING

Some vehicles have a large interior space to heat and cool (eg. microbus, minibus). These vehicles
may have an auxiliary rear heating-and-cooling system. It keeps the rear passengers more
comfortable without disturbing air distribution to the front seat. The auxiliary system may be in the
trunk of a car. It may be on the roof or in one side of a van. The system may include a heater core,
evaporator core, orifice tube or expansion valve, blower motor and controls, and ducts. The auxiliary
system does not have its own compressor or condenser. Hose and tubing carry the refrigerant
between the front of the vehicle and the rear evaporator. Hot engine coolant is also piped to and
from the rear heater core.

4. ME 467 : Automobile Engineering Dr. Md. Ehsan © 2015 ME, BUET
Car Air Conditioning Panels








Components of Automotive AC Panel

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Compressor: The compressor is a pump driven by a belt attached to the engine's crankshaft.
When the refrigerant is drawn into the compressor, it is in a low-pressure gaseous form. Once the gas
is inside the pump, the compressor lives up to its name. The belt drives the pump, which puts the gas
under pressure and forces it out to the condenser. Compressors cannot compress liquids, only
gasses. Compressors can be fixed displacement or variable displacement(wobble/swash plate) types.





COMPRESSOR CLUTCH

The compressor clutch is a solenoid-type magnetic clutch located in the compressor pulley. The clutch
engages and disengages so the compressor shaft turns only as needed. When the clutch engages,
the compressor runs and cooling takes places. When compressor operation is not needed, the clutch
disengages. Then only the pulley turns. The pulley is driven by a belt from the engine crankshaft. The
clutch has a stationary coil that becomes magnetized when voltage is applied. A clutch hub or
armature attaches to the compressor shaft. When no voltage is applied to the coil, the armature moves
forward slightly. This disengages the compressor by opening a slight air gap between the armature
and the front face of the pulley. The pulley mounts on ball bearings so it can rotate freely when the
clutch is disengaged. Applying voltage magnetizes the coil. This pulls the armature back and locks the
pulley to the armature. Now the compressor shaft rotates with the pulley.

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CYCLING AND NON-CYCLING COMPRESSORS

Many automotive air conditioners have a compressor with a cycling clutch. It automatically engages
and disengages to cycle the compressor on and off. This regulates the temperature of the conditioned
air. Clutch cycling is often controlled typically by evaporator temperature or pressure, sensed by an
electric switch.

Some compressors run continuously while the engine is running and the air conditioner is ON. These
compressors have a non-cycling clutch. Such systems often have a variable-displacement
compressor. It runs continuously and varies the amount of refrigerant pumped according to need.


Condenser: The condenser is basically a heat exchanger with lot of surface area, and it serves the
same purpose as the one in your car: to radiate heat out of the system. The refrigerant enters the
condenser as a pressurized gas from the compressor. The process of pressurizing the gas and
moving it to the condenser creates heat, but air flowing around the twisting tubes of the condenser
cool the refrigerant down until it forms a liquid again. The liquid refrigerant is now a high-pressure
liquid and nearly ready to cool the car. This is typically located at the most front end of the car, parallel
to the engine coolant radiator, to get the assistance of incoming air when the car runs forward.

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Receiver-Dryer: But first, the refrigerant needs to be prepared for the evaporator. As it moves out
of the condenser, the liquid goes through a little reservoir installed in the line. This receiver-dryer
contains desiccants, small granules that attract water. In the receiver-dryer, they remove any water
that has entered the system. If the water is allowed to remain and possibly form ice crystals, it can
damage the air conditioning system. Some automotive air-conditioning systems have a sight glass. It
is usually in the top of the receiver or in the high-pressure liquid line. When the air conditioner runs, the
sight glass allows you to see the condition of the refrigerant as it leaves the condenser. For example,
gas bubbles (white foam) may indicate loss of refrigerant.


Thermostatic Expansion Valve (TXV): Here, the system changes from the high-pressure
side to the low-pressure side. If you touch this part of the system, you'd feel it change from hot to cold.
The high-pressure liquid refrigerant flows from the receiver-dryer through the expansion valve, where it
is allowed to expand. This expansion reduces the pressure on the refrigerant, so it can move into the
evaporator. The thermostatic expansion valve (TXV) is a precision device, which is designed to
regulate the rate at which liquid refrigerant flows into the evaporator. The valve senses pressure and
regulates the flow of refrigerant, which allows the system to operate steadily, but the moving parts of
the valve can wear out and sometimes require replacement. Some vehicles may have an orifice
tube rather than an expansion valve, but it serves the same purpose in allowing the refrigerant to
expand and the pressure to be lowered before the liquid enters the evaporator.



TXV Pressure Balance Equation
P1+P4 = P2+P3
P1 = Bulb Pressure (Opening Force)
P2 = Evaporator Pressure (Closing Force)
P3 = Spring Pressure (Closing Force)
P4 = Liquid Pressure (Opening Force)
THERMOSTATIC EXPANSION VALVE

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Evaporator: This is where the real cooling happens. While all the other parts of the system are
located in the engine compartment, this one is in the cabin, usually above the foot-well on the
passenger side. It also looks like a radiator, with its coil of tubes and fins, but its job is to absorb heat
rather than dissipate it.
Refrigerant enters the evaporator coil as a cold, low-pressure liquid, ideally at about 0°C, which is why
you don't want any water in the system. The refrigerant doesn't freeze at this temperature, but it does
have a very low boiling point. The heat in the cabin of the car is enough to make the R-134a in the
evaporator boil and become a gas again, just like water turning back to steam. In its gaseous form,
refrigerant can absorb a lot of heat.
The gas moves out of the evaporator -- and out of the passenger compartment of the car, taking the
heat with it. A fan blowing over the outside of the evaporator coil blows cool air (at about 10°C) into the
passenger compartment. The refrigerant in gas form then enters the compressor, where it is
pressurized and the whole process starts all over again. The evaporator also takes humidity out of the
air in the car, which helps you feel cool. Water in the air condenses on the evaporator coil, along with
dirt and pollen and anything else floating around in the cabin. When you stop the car and see water
dripping underneath, it's probably the water from the AC evaporator and nothing to worry about



REFRIGERANTS :
Most automotive air conditioners have used a chlorofluorocarbon (CFC)
refrigerant that is often referred to by its brand name Freon. It is also called refrigerant-12 or simply R-
12. However, R-12 is being phased out and replaced by a Non-CFC refrigerant called R-134a. The
release of chlorofluorocarbons such as R-12 into the atmosphere damages the earth's ozone layer.
This is the protective layer 10 to 30 miles [16 to 48 km] above the earth's surface. The ozone layer
shields the earth from the sun's ultraviolet radiation. R-134a is chlorine free and not damaging to the
ozone layer. The refrigerants R-12 and R-134a have similar physical properties, but they are not
interchangeable. R-12 boils at —22°F [-30°C] at atmospheric pressure. R-134a boils at —15°F [-
26°C]. The higher boiling point makes R-134a a slightly less efficient refrigerant than R-12. This lower
efficiency requires higher pressures and a greater air flow across the condenser. R-12 and R-134a
require different refrigerant oils and a different desiccant.


REFRIGERANT OIL :
The refrigeration system needs oil to lubricate the compressor and to keep
other moving parts and seals lubricated. This lubricating oil is the refrigerant oil or compressor oil. It is
a non foaming and highly refined oil. The compressor oil needs to be compatible with the refrigerant
used. Some compressors have no way to add oil to them. Oil for compressor lubrication is added to
the system during assembly. Then the oil circulates with the refrigerant. Now-a-days some refrigerant
manufacturers add oil with the refrigerant, so they do not need additional refrigerant oil for them.

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Recharging a Car's AC
Recharging the AC is not an unusual thing. It simply means that fresh refrigerant is added to the
system. If the refrigerant is a bit low, it can be topped off, the same as if you were a bit low on oil in the
engine. If it's really low, though, whatever refrigerant is still in the system needs to be drained out and
replaced. This process of clearing out the system and adding new fluid is called recharging.
In either case, you've lost some refrigerant, which isn't so great. Even though R-134a is better for the
environment than Freon, we should rather not have any air conditioning refrigerant leaking into the soil
and rivers. If you're going to have the system drained and recharged, it is recommended to have the
technician take a look at the system to find the source of the leak and fix it first. The leak may occur at
the seals of the joints of the AC components, micro-fractures at tube bents or at the moving pistons of
the compressor. The leakage points should be detected first by pressurizing system for some time with
an air compressor after removal of the refrigerant.
The lack of precious cold air could be caused by:
• A refrigerant leak caused by a failed o-ring, seal, joints, hose or component.
• A clogged expansion valve or receiver drier.
• Failed compressor.
• Failed compressor clutch.
• Failed blower motor or blower motor resistor.
• Damaged or failed condenser or evaporator.
• Failed switch, fuse, relay, control module, blend door or solenoid.


Vacuum Pump
Manifold Gauge Set
R134a

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Before working on a refrigeration system technical specifications like – type of refrigerant used, type of
compatible refrigerant oil, amount of refrigerant to be charged, pressure limits of high pressure and low
pressure sides need top be noted first. This typically comes on a sticker/plate attached some place
under the bonnet. The service manual can also be consulted.

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The Manifold gauge set is the standard tool for carrying out such operations. This tool allows attaching
air compressor, vacuum pump and refrigerant tank to the system while pressure on both high and low
pressure sides can be monitored. The high and the low pressures of a refrigeration system during
operation varies depending on the weather it is working against (ambient temp, relative humidity etc.).
High side pressure typically ranges from 200-300 psig and the low side pressure is typically 40-50
psig. The correct amount of refrigerant needs to be charged – just like undercharging may generate
low pressure and poor cooling performance, over charging can generate excessive pressure and
leakage/rupture. Beware that once the low side reaches around 50 psi the high side can continue to
increase to excessive pressure levels if not monitored - do not overcharge, fix leaks use a manifold
gauge set and be safe!
Leakage Detection in Refrigeration Systems
Leaks can be devastating. When an A/C system develops a leak, you have what’s called an “open
system.” If you or your technician discovers the leak early, your repair will be less expensive.
Unfortunately, if a leak has been affecting your cold air for a while, moisture will most likely have
entered your A/C system and may have damaged other vital and expensive parts.
There are four common ways for leakage detection -
• Physical inspection of the system components, especially the high pressure component joints
for traces of refrigerant leakages.
• Pressurizing the whole system, system part or individual component using compressed air.
Putting on soap solution of foam on the pressurized system can be used to detect the leakage.
• Using UV dies with the refrigerant cause it to leak out, which can be detected under UV lights.
• Refrigerant leakage detectors may be used to sniff traces of refrigerant.

After the leakage is detected the damaged component needs to be replaced/repaired. After that the
whole ac system needs to be reassembled and tested again for any leakage. Typically the system is
pressurized again using an air compressor and the pressure is monitored for a while (eg. 15-30 mins)
to detect any leakage in the new assembly.
Once you are sure of the system being air-tight, the air is released and a vacuum pump is used to fully
evacuate the system. Once the system is vacuumed at first some refrigeration oil is pulled in to the
system using system vacuum. Then the engine is started, the air conditioner is switched on and the
refrigerant tank is connected to the low pressure of the system. Soon the refrigerant charges in and we
start getting cool air out of the air conditioner.

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REAR-WINDOW DEFOGGER

Some vehicles have a heated rear window or rear-window defroster or defogger. It heats the rear
window to remove or prevent a buildup of fog, ice, or snow. An electric current flows through a grid of
resistance wire to provide the heat. The grid is baked onto the inside surface of the rear-window glass.

When the driver switches ON the rear-window defogger, a defogger relay closes. This sends current
through the grid. It also illuminates an instrument-panel indicator light and starts a timer. As electric
current flows through the grid, the wires get hot and heat the window. The timer automatically shuts off
the rear-window defogger after a preset time. This usually ranges from 10 to 25 minutes. The switch
can be turned ON again if the rear window needs additional heat.



Some vehicles have an electrically heated windshield. It melts ice and frost three to five times faster
than the defroster. The system is similar to the rear-window defogger. However, instead of wires, the
windshield heating element is a clear conductive coating. The coating is between the interior and
exterior layers of windshield glass.