Automobile Air Conditioning System
Introduction :
● Air conditioning is the process of altering the property of air (primarily temperature and humidity) to more favorable conditions to the human.
● Air conditioning means the science of providing and maintaining the temperature irrespective of the atmospheric condition or the simultaneous control of temperature, relative humidity, purity and air motion.
● Because of the rapid growth of industrialization, there will be rapid change in atmospheric and environmental condition.
● Factors such as wind resistance, aerodynamics and engine power and weight finds the true variance between uses of the air conditioning system.
● In an automobile air conditioning certain new design principles are required. Hence air-conditioning is necessary for the automobile for providing comfort and luxurious life.
● Automobile air conditioner use significant power, on the other hand the drag of a car with closed windows is less than if the windows are open to cool occupant evaporatively.
Requirements of Air Conditioning :
● The automobile compartment is designed by considering several factors to achieve the required condition for all weather icons. Such as:
(i) Higher outside temperature.
(ii) Dissipation of heat by engine.
(iii) Solar radiation through glass, windows and roof depending upon the position of sun de intensity of solar radiation, cleanness of sky.
(iv) Vehicle colour
(v) Vehicle speed and wind direction and velocity of air.
(vi) Infiltration of air and ventilation.
● Faster is the car motion, greater is the amount of infiltration into the car and better heat transfer coefficient betwe outdoor air and car surface. The variation in car speed from idle to high speed, complicate unit is design to match the cooling load.
Air Condition System :
● The air conditioning system of most of the car consists of a compressor which is operated by the engine power from the crankshaft through belt.
● The evaporator and blower unit were located in the trunk compartment, while the air cooled fin type condenser is placed in front of the radiator and a throttle or expansion value is located between condenser and evaporator.
● The operation should include heating as well as cooling, defogging and remove dust smoke and odour.
● The system run on the principle of simple vapour compression refrigeration system with four type of controls low pressure side control, high pressure side control, solenoid operated bypass and combination of all three.
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| Automobile air conditioning system |
Operation :
● This system works in the same manner as the cooling system of an engine when the A/C is turned ON. The Wobble plate type compressor is used driven by engine power through crankshaft by using 'V' belt drive.
● The compressor has an electromagnetic clutch that can engage or disengage the compressor pulley when the engine started.
● And the compressor speed will be varying according to engine speed.
● The heat carrying medium is refrigerant in the system which is a liquid that has an extremely low boiling point, first the refrigerant comes in contact with dashboard where the evaporator is located.
● The refrigerant pick up the heat from the passenger compartment and is vaporized. This heat removal or evaporation or cooling cools the air entering the passenger compartment.
● This low pressure vapour refrigerant is sucked by the compressor and it compressed to high pressure and temperature and is easily discharged in condenser which is located in front of engine radiator, behind the grill.
● The condenser rejects the heat to the air flowing over it and converts its phase from vapour to liquid. This liquid refrigerant stored in receiver.
● The liquid refrigerant again flow to passenger compartment which acts as a evaporator through expansion valve where it pick up the heat and convert to vapour form and the cycle is repeated.
● The function of expansion or throttle valve is to control the mass flow rate of refrigerant according to need in compartment. With this rapid cooling going on, the fins of the evaporator get extremely cold.
● The air that passes by these fins on its way into the passenger compartment will also be cooled. Also any moisture or water vapour (humidity) in that air will condense on the evaporator fins and stick to those fins.
● This removes the humidity or moisture from the incoming air. The air leaving the evaporator is now very dry.
● As the moisture collects on the evaporator, dust and other contaminates stick to the water droplets that have formed on the fins. This has an advantage of cleaning those particles out of the air and drain outside.
● A refrigerant used in the system is R-22 or R-12. Now a day the refrigerant used is R134A) have a boiling point of - 26.3°C. This refrigerant has a good heat transfer property and non-flammable and non explosive, non corrosive and as an alternative to CFC refrigerant.
● This refrigerant has positive pressure both in evaporator and condenser and hence no necessity of purge unit and vacuum prevention system also it is eco-friendly with nature.
● By continuous removal of heat, the temperature in passenger compartment is reaches to required temperature which is controlled by driver by controlling the knob on dashboard.
Heating system:
● For heating the vehicle passenger compartment the control are set.
● The hot water from the engine block water Jacket circulating through heater coil by mean of water pump.
● The mass flow rate of water is controlled by valve located in the inlet hose of the heating coil and operated by a knob on the dashboard.
● This valve control the volume of water (coolant) and control the temperature of air passing through it.
● The position of temperature valve determine the flow rate of air or no air will pass through heating coil or atmospheric air is directly circulated or mix air is circulated in the passenger compartment through grill. Forcing air with the help of blower.
● The quantity of air heated is further control by air valve providing with three position.
● In some system evaporator is situated at front or at Rear of the car. There is a pair of intake air grill at suitable location and beneath of these there is a blower.
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| Flow diagram of air conditioning system |
● The blower takes in air through the air grills and pass the same over the evaporator.
● As ar is passes over the tubes and fins of evaporator it gets cooled. This air again directed into the passenger compartment as desired by adjustment of louvers or grills.
● In car the lowers are located at the front, as area of cooling is small but in case of bus or heavy duty vehicle a duct layout is provided at the top or at the side of the roof of the vehicle with few opening as area for cooling is larger.
Components of Air Conditioning of Automobile with its Functions
The basic automobile air conditioning system consist of:
i) Compressor
ii) Condenser
iii) Receiver / Drier
iv) Expansion valve
v) Solenoid valve expansion valve
vi) Evaporator
vii) Blower
viii) Control panel
(1) Compressor :
● It acts as a human heart which suck vapour refrigerant and increase its pressure and temperature and supply in the system
● It is fitted in the front portion of the engine and driven by 'V' belt. It is operated by magnetic clutch when air conditioner is switch off.
(2) Condenser :
● It is located in front of radiator. The vapour refrigerant reject heat and changes its phase from vapour to liquid.
● It is fiu type Air cooled. The fan forces the air to flow over it which increase heat transfer rate.
(iii) Receiver / Drier :
● This is a small reservoir vessel for the storage of liquid refrigerant, and removes any moisture if present in the system or that may have leaked into the refrigerant.
● This drier operates on the principle that cooling of the compressed air which causes condensation of water carried by the compressed air in vapour form.
● Moisture in the system causes havoc, with ice crystals causing blockages and mechanical damage.(iv) Expansion valve :
● It is a thermostatic valve fitted in the system. It regulates the mass flow rate and keep the total heat enthalpy) constant.
● Its regulating action is controlled by the temperature sensing bulb place on the outlet line of evaporator or suction to compressor.
(v) Solenoid valve expansion valve :
● Solenoid valve is the controlling valve in the refrigeration systems when it is energized (or closed) refrigeration takes place i.e. refrigerant circulate through the system.
● The refrigerant in the liquid form passes from the receiver through the sight glass and dehydrator filter to the expansion valve.
● The dehydrator filter removes dirt and moisture from the refrigerant.
● The expansion valve holds back to the high pressure from the condenser and admits liquid refrigerant to the evaporator in a relatively small stream and at low pressure.
● As the liquid refrigerant passes into the evaporator at low pressure, it begins to evaporate. This evaporation soaks up i.e. abstracts the heat. The evaporated refrigerant is then pump back through the compressor.
● When the solenoid valve is opens i.e. de-energized by being disconnected from the vehicle battery.
● The bypass line between the condenser and the compressor is opened, and then the pressure in the system relived.
● Now the refrigerant can circulate freely between the condenser and the compressor. Since no pressure can be built up and since no refrigerant is sent the long way around, hence no cooling action takes place.
● Some refrigeration system do not use a bypass valve, instead, the refrigeration system is controlled by a solenoid operated clutch in the compressor drive pulley.
● In this case when the system is not to be refrigerated, the pulley is declutched so that the compressor is not operating.
● On the other hand when the control system calls for cooling the pulley clutch is engaged so that the compressor is driven and the system thus undergoes into operation.
(vi) Evaporator :
● The evaporator consists of a tubing and radiating fins. In some system evaporator is located at back of the car, just behind the car seat. In some other system, the evaporator is located at the front of the car.
● There is a pair of intake air grill at suitable location and bencath these there is a blower. The blower takes the air through the grill and passes the same through the evaporator.
● As air passes around the tubes and fins it get cooled. This cooled air is then directed into the car cabin as per the desirable adjustment of lowers.
vii) Blower
● It is a small fan drive by electric motor and directs the air the compartment/cab.
viii) Control panel :
● The control head is the "control panel“ mounted in the dash board, that allows selection of the various system functions.
(a) Thermostat : Temperature Settings knob is provided on control panel. The temperature inside the cabin can be set by using the thermostat knob or the remote control. When the desired temperature is attained inside the trunk, the thermostat stops the compressor of the AC and restarts the compressor to produce the cooling effect hence saving the fuel.
(b) Speed setting knob: The Speed of the air can be set by the fan motor button provided on the control panel.
(c) Heating and cooling Switches and knob : The system is provided with the dials and buttons used to regulate heating as well as cooling requirement in vehicle cabinet.
