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Modern air-conditioners are sold nowadays more from the safety relevant aspect and no longer as a comfort feature. Drivers who feel comfortable can react better in difficult situations. In addition the right temperature distribution from head to foot, the air cleansing- and de-humidification should be available. This, and the favorable price, have increased the sales (in Germany to 90%) so that even vehicles in the lower price range are equipped with air-conditioning. Cars without this accessory are increasingly more difficult to sell. Earlier the coolant was considered to be environmentally damaging. However, the coolants allowed nowadays are CFC-free.
How it works
Apart from the automation, all air-conditioners function, in principle, alike. As already demonstrated in the physical basic principles, a coolant with a boiling point of approx.-27°C is used for the air-conditioning. The coolant flows through the upper (shown in green) part of the above figure at higher pressure and through the lower (shown in blue) part at low pressure. In the compressor, the boiling point (and with it the condensation point), is increased, so that, in the condenser, at higher temperatures, the liquid state of the coolant can be achieved. It functions vice versa in the lower heat exchanger (evaporator). In the evaporator, heat is absorbed, and in the condenser, it is emitted. As long as in both heat exchangers the aggregate-change-state functions, the equipment works with a relatively high efficiency. However, the coolant, and particularly the oil, for the lubrication of the moving parts, through the intake of water, changes the boiling point and with it, decisively, the efficiency. Apart from that, through the absorbed humidity, corrosion can easily develop in the unit.
The axial piston pump, which drives the coolant in the circulation, can be switched on or off electrically and - depending on the unit, be regulated - in its transporting capacity. It is connected with the internal combustion engine through a fan-belt. In the belt-pulley, an electrically switchable magnetic clutch is installed. In the axial piston pump, depending on design, up to 10 pistons rotate in a drum, and in addition, every half a rotation a stroke movement is carried out. The stroke height, and with it the transport capacity is determined by the diagonal position of the swashplate with which the individual pistons are connected. If the shaft on which the swashplate runs, is not solidly connected with the casing, its incline, and thus the transport capacity can be altered.
In the above shown equipment, the swashplate is not adjustable. Therefore, an expansion valve (in the figure, above right) is necessary here. It regulates the pressure of approx. 1-12 bar in the liquid part depending on the temperature in the vapour part, which lies at only a little more than 1 bar. If the temperature rises here, the expansion valve increases the pressure difference between the liquid- and the gaseous part.
The air-conditioning is certainly no longer a very large expense factor nowadays. The coolant has also become more environment-friendly. Moreover, it can be switched off as much as possible when not in use. Nevertheless, the additional consumption through such equipment is high. In Central Europe, with approx. 15,000 kilometres per year, one can count on an average additional consumption of 0.6 l/100 km, thus, approx. 90 liters per year. Here the dead weight of the equipment and the electric energy consumption are included. Tests on the performance test bench have proved a momentary additional consumption of up to 1 litre and more.
Switch off, particularly an automatic unit, or use the economy button in winter. To ensure that the seals do not dry out too much, and that the lubrication of the components is guaranteed, switch it on briefly every now and then. To avoid long term bad smells from the remaining condensation in the evaporator, switch off the air-conditioning in summer about 5-10 minutes before the end of the journey, or turn it down. The blower then provides for drying out.
Some garages offer an extra service for air-conditioners. However, as a consumer, one should always orientate oneself on the manufacturer's data. If the servicing intervals are longer in the manual, then these are, as a rule, completely sufficient. In theory one could carry on driving, until two bad habits appear:
- The air conditioner stinks.
- The performance of the unit decreases.
If it stinks, the evaporator in the interior must be disinfected. If the performance of the unit decreases, there may be e.g., not, enough coolant. This can damage the air compressor, because with insufficient coolant, there is perhaps also too little lubricant in the unit. Testing of the equipment at regular intervals (also in winter) is worthwhile. 07/09
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