Introduction
This report will investigate and discuss the importance of air conditioning system in the vehicle. The report will also cover the use of climate control, heating and cooling, negative and positive aspects as well as sustainability, social and ethical issues regarding the system.
History
The Packard was the first car to introduce the factory installed air conditioning in 1940. 30 years later, more than half of the vehicles sold were fitted with air conditioning. In 1971 the Freon that was used in the air con units were blamed for being responsible of reducing the ozone layer, manufacturers were then forced to use R134a refrigerant gas from R12 gas because it was less harmful compared to another, this was in 1996.
Now days some passenger cars have separate temperature settings both driver and passenger, some cars are fitted with rear additional zone for rear passenger, this is known as dual zone automatic climate control. Company like Volkswagen uses climatic system which is manual and Climatronic as an automatic air conditioning. Most of the new cars are in fitted with air conditioning system. The Consumer Reports revealed that the using of the air conditioning at highway speeds can cause the loss of up to 3 mpg while leaving the windows open has no fuel effect on economy (Automobile Magazine, 2011)
Vehicle Ventilation and Heating System.
To increase passenger safety and comfort, prevent fogging or reducing fatigue to the driver, fresh air is very important to be supplied in the car, it is taken from outside through ducts at the base of the windscreen that goes straight to the passenger compartment. Ducts and flaps are used and well positioned to direct and distribute the air inside the car. Cables, vacuum servos or electric servos are designed to control the flaps. By using this method the air is able to be straight delivered to the rear passenger compartment ducts, window demisting ducts, floor ducts and face ducts. The electric motor is used to drive the fan to support the air flow inside the car when vehicle travels in low speed or when demand is high.
airflow
The heat that is generated by the internal combustion engine is usually lost in the air, this can be avoided by using this as an incoming warm air during the winter for passengers comfort. Liquid cooled engine uses heated coolant through the heater core to heat the air. The heater core is a mini radiator that is made of tubes, fins and tanks. It is located under the dash panel inside the air plenum section where air can be passed across the core fins inside the car. Flexible rubber hoses are better option for movement, less vibration and noise reduction to use as the connection of the engine cooling system and the heater core. The rubber hoses are fabrics that are designed to withstand temperature and cooling system pressure. Water pump creates the coolant flow, through the heater core and back to the engine, the amount of the coolant flow through the heater core is adjusted by the heat controlled valve that is located in one of the coolant hoses. A cable and lever system is used to operate the heater valve to allow the maximum coolant to circulate through the heater core. At normal temperature, the heat is transferred to the atmosphere passing over them by the heater fins, it can then be taken directly into the passenger compartment as raised temperature. cool temperature is available by moving the lever to decrease the opening of the valve that limits the flow of the coolant into the core of the heater which causes less heat over the heater core (CDX Online eTextbook)
Vapor compression refrigeration
Generally, the vapor compression cycle contain the refrigerant circulation, which is evaporating that is to absorb the heat and gives it up when condensing. Latent heat of vaporization is the product of the heat that is lost or gained during the process, this can be as little as one degree temperature heat. This evaporating and condensing process always depend on the application, type of refrigerant as well as the space to be cooled. The following is an example simple compression circuit for an air conditioning system.
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It begins with the high pressure liquid that goes through a restrictor device, using an expansion valve that allows the flow regulation and expansion. it then goes into the evaporator for the reduction of the pressure, this is done by the absorbance of the heat from the metal coil. The suction line is used to direct the vaporized refrigerant from the evaporator to the compressor. The compressor is then compresses the vapor and causes the pressure to raise and condense the temperature. The high pressure refrigerant vapor travels inside condenser where the fan is used to draw ambient air over it and this causes it to be condensed back into a liquid form. The high pressure refrigerant goes through the condenser coil or into liquid receiver where the whole cycle starts all over again. With the help of the sensor that is normally in the evaporator, the thermostat or special controller is used to monitor the temperature condition in the system (MacTechinical bulletin, 2009)
Air conditioning refrigerant
R-12 or Freon which is the chemical substance dichlorodifluoromethane (CCI2F2,) is the refrigerant gas that was used in the earlier automotive air conditioning system. This gas has the ability to carry the heat away and still remaining on its normal working temperature range, however water can do this but R-12 is better because it stays at much lower temperature range.
It also has a low boiling point. R-12 changes it states when the temperature is above -21.6 F, this allows it to vaporize and absorbs heat which causes the cooling effects inside the vehicle. R-12 is a non-toxic chemical as long as it is nowhere close to the flame, odorless, non-corrosive, soluble in oil, does not affect the rubber components and easy to use when it is handled carefully. R-12 has also some few bad characteristics. It can discolor stainless steel and chrome parts and it contains chlorofluorocarbons (CFCs). Researches have shown that this chemical absorbs more than 99% of the harmful ultraviolet rays produced by the sun. Once the R-12 leaks from the car as a result of using air conditioning system, this chemical moves to the stratosphere which is 9 - 30 miles above the earth surface and the home of ozone layer which protects the surface from ultraviolet. The chlorine from CFC contributes to breaks off the stratosphere and causes the ozone molecules to be destroyed and let in more ultraviolet on the world surface which causes skin cancer and kills plankton in the ocean's surface (Stubblefield & Haynes, 2006, p.15)
Due to the world pollution concern, the European Union has forced the manufactures for different technologies of refrigerants HVAC units from R134a which is a hydro fluorocarbon (HFC)-based substance. This chemical is 95% less harmful to stratospheric ozone than R-12. However, an improvement is needed to make it as leak free system or supersede it. There are few possible alternatives to replace R134a are considered, these are CO2 based system (R744) which has been used since 2008, absorption refrigeration, secondary loop system (HFC152), gas refrigeration (R729), evaporative cooling and thermo electric cooling (Peltier effect) (Daly, 2006, p.60 - 61)
Air conditioning system
Air conditioning system has been designed to facilitate the heat removal from inside the car. convection and conduction method are used to remove the heat. An evaporator that is cold absorbs the hot air that goes through it and pushes it out as cold air through the vents inside the car by a blower motor. This is achieved by forcing and compressing the refrigerant (134a) and releases it inside the air conditioner evaporator.
Car Air Conditioner System
Some manufactures fit their vehicle with an automatic climate control system to automatically control or adjust the temperature inside the vehicle. The computer is used to monitor the temperature that is required or set by the user. This temperature is a mixture of hot air from the heater and cold air from the air conditioner. Solid state speed controller controls the blower motor speed electrically and also replaces the conventional blower motor resistor system. The heating unit and air conditioning system offers thermal comfort to passengers. The air inside the car can be, heated, cooled ventilated or disinfected. The climate control system is installed in order to maintain the required temperature inside the car, this system is also known as the HVAC system. Fluid mechanics, thermodynamics and heat transfer is the principle that work together which climate control system uses it to deliver heat and cold, good air quality, optimal pressure and thermal comfort inside the car (2CarPros, 2010)
Air Conditioning Components:
AC System
The Compressor
This is known as the major part of the air conditioning system, it is driven by an engine belt. It compresses the refrigerant (R134a) and transfers it. The air conditioning system consists of low pressure and high pressure side, also known as discharge and suction. The compressor is built with an intake and discharge side. The suction side draws in refrigerant from the evaporator's outlet. An accumulator can also do this in different circumstances. Once the refrigerant is in intake side, it is compressed and taken to the condenser where it transfers the absorbed heat from the inside the car.
A reciprocating compressor also contains pistons that are pushed by spring loaded plates that with the pressure created on exhaust port (discharge port) and intake stroke (suction port). This type of compressor was used in bigger engines, it can transfer a lot of refrigerant but it can also cause a pulsating of the pressure in the system.
Small engines use a vane pump configuration compressors while the latest development scroll compressor contain less moving parts and no produces a balanced refrigerant flow with no pulse.
The condenser
This looks similar to radiator and is usually located in front of the radiator or it can be elsewhere because of the aerodynamic improvements of the body. It dissipates and radiates the heat. When the condenser is operating, it requires a good airflow while engine's cooling fan helps the rear wheel drive cars. Electric cooling fan is used to supplement the condenser air flow in front wheel drive cars. The condenser than receives and cools off the hot compressed refrigerant, the gas is condensed and goes out as high pressure liquid via the bottom of the condenser (Heath, 2005; TheFreezerGeezer, 2006)
The evaporator
The main job of this component is to absorb the heat. It also helps to remove the heat from inside the car and dehumidification. When the warm air passes through the aluminum fins of the cooled evaporator coil, the evaporator surface is condensed by the air moisture. Dust that is stuck in the wet surface drains off to outside of the car and this can be seen as water dripping during the humid days. The normal temperature of the evaporator is 32 degrees. A low pressure liquid that is in form of refrigerant enters the bottom of the evaporator. The refrigerant which has a very low boiling point is caused to boil by the warm air that passes through the evaporator, it than absorbs so much heat which is sent outside the car together with refrigerant. controllers are used to control and keep out the evaporator from freezing because it will not absorb too much heat when it is frozen (SmartTrac Computer Systems, 2005)
Orifice tube
An orifice tube is designed to restrict the refrigerant's flow into the evaporator, and it is located slightly ahead of the evaporator and at the high pressure side of an air conditioning system. The orifice tube is there to control the refrigerant gas that is going through the system. It causes the pressure to drop and causes the refrigerant to start evaporating inside the evaporator. Some manufacturers attach the orifice tubes in the condenser's outlet, because when the system shuts down it helps to reduce the hissing noises. Well machined small brass tube that has perfect diameter is designed to measure the refrigerant flow. It is also covered by a plastic carrier that contains the mesh filter screen to stop dust and dirt from entering the orifice. In order to block the refrigerant from flowing around, the seals are kept to make sure the liquid line is safe and secure (Miller)
The thermal expansion valve
This is used to control and regulate a cold refrigerant flow to the evaporator so that the inside of the car can receive a cool air and not cold one. Few different types of expansion valves are available now days but they all do the same job.
The drier or Accumulator
This works as the safety device in the air conditioning system. When the compressor compresses the refrigerant, there is always a slightly chance of the liquid to come back where it is from, this is where the drier comes and stop the liquid from damaging the compressor. No matter how small the leak is or bad installation, it can always lead to having the water moisture in the system. The drier uses chemical that is known as dessicant type XH-7 to absorb this. This dessicant is compatible with both R12 and R134a refrigerants. The drier also contains a filter that stops and catches any mess or dirt that might be around (Wright, M. 2011).
Control switches
According to (Daly, S, 2006, p. 111 - 112), states that there are few control switches in the air conditioning that control the pressure in the system. These switches are explained below.
Low pressure switch: when the pressure drops below 1.5 bars in the intake side, this switch reacts and causes the accumulator to disengage the clutch of the compressor. Very often the same device is used can be used for cycling switch and pressure switch in an FOV system.
Dual pressure (high pressure switch and condenser switch): both of the switches are sensitive and they are in the high pressure side of the system. One switch is for safety to avoid excessive or unwanted system pressure. The second switch is for turning on the condenser auxiliary fan when the pressure is approximately 20.7 bars and switches it off at 17.2 bar, this helps to increase the system performance by monitoring it all the time.
High pressure switch: this is a closed pressure switch that is located at the high pressure side, it is designed to slow down the compressor due to the high pressure in the system. It can be between 30- 35 bars.
Trinary switch: three sensitive pressure switches that combined together to form a trinary switch. A high pressure switch which eliminates the current of the air conditioning compressor in case of a blockage in the system. The second type is a low pressure switch that generates an open circuit to eliminate the compressor current when pressure drops below 1.4 bars, a natural discharge or a leak can cause this problem. The third switch is designed to get rid of the heat by improving the speed of the condenser fan. It is at the high pressure side of TXV system and operates around 18 bars. A single pressure sensor is used in new system.
Pressure sensor: a metal plate ceramic disc is used to build this sensor, it is attached next to proximity. The thinner disc that is close to the pressure connection can bent due to the pressure. This is the reason why the metal plate is changed due to the pressure force. A circuit integrated
Results from a pressure sensor
Bar
voltage
2
0.4
10
1.6
16
2.35
20
3.1
21
3.85
Sensors and actuators
Sustainability
Recycling: since we know that the Freon or any other refrigerant gas can be very harmful to the environment. Freon must be recovered, recycled or reclaimed when the system is serviced or vehicle is sent to the scrap yard. Freon is recovered by eliminating it from the system into a temporary storage system. After testing or repair the air conditioning system the refrigerant can be put back into the system.
Recycling method involves a special machine that evacuates the refrigerant from the air conditioning system, a machine cleans it by eliminating particles, moisture, oil and air from the Freon and once it is done it can either be used in the same vehicle or store it for another vehicle.
Reclaiming Freon to return the refrigeration back into a high level of purity than just recycling, this is done by a certified reclaimer.
Other air conditioning system components that are metal, plastic and rubber can be used again by following a proper procedure or simply recycled at the scrap yard after the above methods have been completed (Evans, 2010)
Conclusion
