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

3 System description 28
Figure 3.5: System and fan characteristics in a car for different fan voltages. [1]
through flap position changes could be included. One factor that can be at least estimated
is the inlet pressure change due to the cars speed [1].
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Pressure coefficient [-]
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(3.23)
The pressure coefficient is given by Großmann with a value between 0.1 and 0.3.
This pressure rise is only valid for full fresh air. For a partial recirculation a lot depends
on the geometry of the flap at the inlet. For this reason no general relation can be
given. Some cars have a so-called “ram door”, a flap at the fresh air inlet, to reduce the
influence of the car speed and prevent air flow into the cabin through the recirculation
path. For a good air flow estimation without any flow or pressure sensors in the car a lot
of experiments and curves are needed.
The fan’s power consumption is governed by the desired air mass flow and by its
motor control. In manual controlled HVAC-systems series resistors lowering the voltage
to the fan are often used. This method is very cheap and sufficient for switching between
three or four speeds. The resistance also consumes power, though, making this control
not very energy efficient. The modern version is realising the motor control through pulse