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Design of an Automatic Control Algorithm for Energy-Efficient ...

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3 System description 27<br />

Figure 3.4: A schematic drawing <strong>of</strong> <strong>an</strong> air mix type HVAC-unit. [2]<br />

the evaporator <strong>an</strong>d is - if the evaporator is activated - cooled down <strong>an</strong>d dehumidified.<br />

A part <strong>of</strong> it is then, depending on the air mix flap, heated by the engine’s waste heat<br />

<strong>an</strong>d/or <strong>an</strong>other heating source (typically a PTC). Finally the air is (partly) mixed <strong>an</strong>d<br />

distributed to the different outlets. Different temperatures <strong>for</strong> the outlets c<strong>an</strong> be achieved<br />

by a good duct design <strong>an</strong>d not fully mixing the cold <strong>an</strong>d hot air. The foot area is usually<br />

tempered by hotter air th<strong>an</strong> the face outlets which blow directly to the passengers. To<br />

defrost <strong>an</strong>d defog the front windscreen, the defrost outlets are used.<br />

Even if the layout in <strong>an</strong> electric car may ch<strong>an</strong>ge, some basic equations c<strong>an</strong> be given.<br />

3.2.1 The f<strong>an</strong><br />

Each f<strong>an</strong> has a characteristic curve determining the pressure drop depending on the air<br />

flow. This curve varies according to the applied voltage. A characteristic curve relating<br />

the pressure drop <strong>an</strong>d the air flow c<strong>an</strong> also be found <strong>for</strong> the HVAC system including the<br />

air ducts, the filter, the heat exch<strong>an</strong>ger <strong>an</strong>d the vents. The intersection <strong>of</strong> both curves<br />

determines the air flow into the car. An example is seen in Figure 3.5.<br />

Several “disturb<strong>an</strong>ces” ch<strong>an</strong>ge the characteristic curve <strong>of</strong> the HVAC system. Some<br />

<strong>of</strong> the causes are very hard to take into account, such as blocked out- or inlets or other<br />

unpredictable events. With measurements in numbers the influence <strong>of</strong> the different ducts

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