How car AC works
The
average person requires a comfort zone of approximately 21 C to 26 C, with a
relative humidity of 45 to 50%. In this temperature and humidity range, we feel
most comfortable. As the temperature of anything goes above or below this
range, we think of it as HOT or COLD.
How
does heat get inside a vehicle?
When a car
is driven or parked in the sun, heat enters the vehicle from many sources.
- Ambient
air
- Sunlight
- Engine heat
- Road
heat
-
Transmission
- Exhaust
heat
All of
these and other miscellaneous heat sources, increase the air temperature within
the
vehicle.
In a high ambient temperature situation, the interior of vehicle left standing
in the sun with windows closed could reach 65®C - 70®C.
Your A/C
system is nothing more than a heat exchanger.
There are only 6 major parts to the system.
1)
Compressor
2)
Condenser
3)
Receiver/Dryer or Accumulator
4)
Thermal Expansion Valve or Orifice
Tube
5)
Evaporator
6)
Refrigerant
1)
Compressor-variable displacement pump.
The compressor is the power unit of the
air-conditioning system that puts the refrigerant under high pressure before it
pumps it into the condenser, where it changes from a gas to a liquid.
A fully functioning compressor is necessary for the
air-conditioning system to provide peak performance. On most cars, A/C
compressors are driven by an engine-accessory belt. If the belt is worn and
slips, the compressor won’t operate at full strength. Compressors can also leak
refrigerant, resulting in less cold air going into the interior.
Compressor (variable displacement pump) In
Detail
Clutch less variable displacement A/C compressor
due to ever increasing fight for high efficiency & Fuel economy Automobile
manufactures Have develop the clutch less A/C compressor sanding being one of
these compressor manufactures has develop a PXE Series variable displacement
A/C Compressor which will use as a study example of these technology on standard
compressor the belt turns a pulley with an electric clutch when you turn on A/C
the clutch engages & the piston start pumping to compress the refrigerator.
There two problem with this traditional
approach.
1)
A compressor needs a fair amount of
Power. In car with small engines the engagement of clutch to active the
compressor puts big load on the engine especially if it is idling.
2)
The compressor will produce low flow
or not enough when the engine idling and high flow when the engine is the high
RPM & Possibly too much refrigerant flow.
Harrison developed the v5 variable
displacement compressor in early 2003. Sandin launched the PXE variable
displacement compressor for the Volkswagen Golf & Volkswagen polo. The E
stands for external control valve later it was given to many Volkswagen and
Audi models. The PXE uses a variable angle swash plate which function as a form
of camshaft to convert rotary motion shaft motion in to the axial motion of the
pistons which located on one end of the swash plate the swash plate inclination
angle also called stork is controlled by an electrical duty cycle PWM (Pulse Width
Modulation) signal. The signal is driven by an air control computer from
an analysis of ambient interior temperature engine RPM & other parameters.
A control solenoid in the PXE compressor regulate
the flow of refrigerator from the discharge pressor side or presser discharge
to the crank case pressure or PC in Response to the pollo with modulated PWM
electrical signal the control device consists of solenoid valve its position is
the relationship of the solenoid acting against a suction release spring.
Maximum stroke mode of the compressor happens
as follows a high duty cycle to a control valve solenoid causes the valve to
close the passage of Pressure discharge in to the crank case a variable of
orifice allows gas pass the piston ring to flow into the suction chamber
pressure crank case then reaches the
same as the suction as the PS or suction pressure or pressure crankcase come
closer to the same value the average gas pressure force on the top and bottom
of the piston approach the same value and the swash plate inclination angle
increase to the maximum setting a cooler refrigerator is the goal here.
The final result of the wider duty cycle
control is a long stroke, higher horse power recrement from engine and
increasing of refrigerant mass flow. Then there also the modulated stroke
control mode.
The control valve has a mechanism which sense
the suction pressure.
The change in capacity is achieved by the
valve adjusting the flow of Pressure discharge refrigerant into the crankcase.
A change in the pulse width modulation PWM
value of the signal to the solenoid various the force on the plunger in the control
valve. A higher inclination angle can then be achieved. Finally is minimum
stroke mode or near off with solenoid signal off the spring force oppose the
solenoid force open the valve and a specific amount pressure discharge flow
into the crankcase regardless of suction pressure the inclination angle force
to minimum hence acceleration has higher priority due to less engine load then
cooling need as indication by the PS Or suction pressure this allows an easy
& Ignite start optimum power during acceleration at prevention of
evaporator freezing the compressor shaft always turns at engine speeds so no
electric magnetic clutch required. Clutch failure is not an issue in this
system.
The compressor pully has a built-in torque
limiter slip clutch to prevent drive belt loss if the compressor has an
internal seizure. If the torque limiter clutch slip during a compressor failure
there no belt friction and a lower compressor noise.
The continue movement of internal compressor
component make it necessary to the compressor for lubrication of the
continuously moving parts.
This is normally accomplished by he uses of
cross changed TXV (Thermal expansion valve).
The cross charge consist of two types of
refrigerant in the sensing element of TXV that that are made to provide normal
control at normal condition when the TXV is set near the closed position and
refrigerant flow is small the valve switches from positive to negative there by
return of refrigerant within tap trap oil to the compressor is assured these
compressor were a derivative technology is making in road into the present and
future of AUTOMOTVE A/C system.
2)
Condenser
The air-conditioning condenser is a radiator
positioned between the car’s grille and the engine-cooling radiator in which
the gaseous refrigerant sheds heat and returns to a liquid state. The liquid
refrigerant flows to the evaporator inside the dashboard, where it cools the
cabin.
3)
Receiver Dryer or Accumulator
3 main function of receiver Dryer
1)
They act as a temporary storage
container for oil and refrigerant when neither are needed for system operation
such as during periods of low cooling demand.
2)
Receiver/driers contain a material
called desiccant. The desiccant is used to absorb moisture (water) that may
have gotten inside the A/C system during manufacture, assembly or service.
Moisture can get into the A/C components from humidity in the air.
3)
Removing any dust particle and any
other debris that could be inside of the refrigerant that could otherwise harm
other A/C Component.
Use of Desiccant in Receiver Dryer
Receiver/driers contain a material called
desiccant. The desiccant is used to absorb moisture (water) that may have
gotten inside the A/C system during manufacture, assembly or service. Moisture
can get into the A/C components from humidity in the air.
Accumulator
It serves similar to receiver/drier, but
slightly different functions. An accumulator is also a metal cylinder, but
differs from a receiver/drier in these three ways.
1)
An accumulator is considerably larger
than a receiver/drier, usually around twice the volume.
2)
The accumulator is connected to the
evaporator outlet, in the low-pressure section of the system.
3)
The accumulator’s primary function is
to store liquid refrigerant that is exiting the evaporator, to prevent it from
reaching the compressor. If liquid refrigerant were to enter the compressor, it
could cause damage, as the compressor is not designed to pump liquid, only
vapor.
Like receiver/driers, accumulators also serve as a temporary
storage container for oil when the oil is not needed by the system.
The accumulator contains a desiccant that absorbs moisture.
4)
Thermal expansion valve or orifice tube
Thermal expansion valve
A thermal expansion
valve or thermostatic expansion valve is a component in refrigeration and air
conditioning systems that controls the amount of refrigerant released into the
evaporator thereby keeping superheat, that is, the difference between the
current refrigerant temperature at the evaporator outlet and its saturation
temperature at the current pressure, at a stable value, ensuring that the only
phase in which the refrigerant leaves the evaporator is vapor, and, at the same
time, supplying the evaporator's coils with the optimal amount of liquid
refrigerant to achieve the optimal heat exchange rate allowed by that
evaporator.
Orifice Tube
Orifice tubes are used
in systems that don’t use expansion valves. Like an expansion valve, the
orifice tube is used to control the amount of refrigerant entering the
evaporator. This, of course, regulates the amount of refrigerant leaving the
compressor. Orifice tubes also serve as a dividing line between the high- and
low-pressure sections of the system.
5)
Evaporator
The evaporator works the
opposite of the condenser, here refrigerant liquid is converted to gas,
absorbing heat from the air in the compartment. When the liquid refrigerant
reaches the evaporator, its pressure has been reduced, dissipating its heat
content and making it much cooler than the fan air flowing around it.
6)
Refrigerant
Freon
is the cooling agent used in most air conditioning systems. Every air
conditioning system needs a refrigerant (also called a coolant) that actually
creates the cool air.
Now
we See how This component work together in your car A/C
HOW
CAR A/C WORKS
The A/c compressor initiates the
high-side of the system where it is compresses the freon/refrigerant into a
high-pressure state causing it to liquefy. It travels through the high-pressure
lines to the condenser. The condenser which is similar to a small radiator,
puts the liquid in contact with fresh air on the outside of the vehicle, which
absorbs the heat from the liquid. It then flows into the expansion valve or
orifice tube where it is restricted and becomes gaseous into the low-pressure
side of the a/c system. Then it flows into the receiver dryer/accumulator that
contains a desiccant bag to remove and collect unwanted moisture/water and
impurities. The clean gaseous freon/refrigerant then travels through the tubing
into the evaporator (that is usually located in the passenger compartment of
the dash). Freon/refrigerant in its gaseous state is now able to absorb heat
from the air passing through the evaporator fins, leaving behind the cooler
air. Fans blow this cooler dry air into the car’s cabin. The refrigerant
travels back to the compressor in the suction hose of the a/c system to get
compressed back into the high-pressure gas and begin the process again.
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