My understanding is that an air conditioning unit it?s self can only be either on or off. So, how does automatic climate control modulate the coolth produced?
As the temperature inside the car approaches the set point does the climate controller combine hot or warm air with the cold air to produce the desired temperature? Or does it just turn the chiller on and off?
SGB
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"As the temperature inside the car approaches the set point does the climate controller combine hot or warm air with the cold air to produce the desired temperature?"
Yes.
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And stand by for a full blown explanation of closed feedback loops from our resident rocket scientist, number cruncher.
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TourVanMan TM < Ex RF >
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Thanks for the big build up RF!
Alas, I must disappoint, because, I think Dave N has answered the question.
Without joyous devices like Laplace Transforms, Z Transforms, Nyquist plots, and Root Locus plots explaining system response in terms of open loop pole placement in the complex plane, "explanations" of closed loop controllers are so much more arm waving than real explanation.
Having said that, one aspect of this type of controller which always intrigues me is the aspect of transport lag, or time delay between the a change in the heat output of the heater and its transferral into the air, conveyance along the pipework, and finally registration by the sensor - it's very easy to make time delayed systems unstable!
Number_Cruncher
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So, climate control is no more efficient than plain air-con, when the set point is below ambient?
sgb
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Sorry meant to say, climate control is no more FUEL efficient than plain air-con, when the set point is below ambient?
sgb
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The exact control system will vary from one vehicle make to another.
It could be as simple as the 'limit cycle' control described above, where there is a feedback signal taken from a cabin temperature sensor and the A/C is cycled on and off (with hysteresis) about set point.
A more sophisticated system would include a 'sunlight sensor' (a photodiode) to determine the amount of solar radiation entering the car and so provide some kind of 'feedforward' control (i.e. you drive into bright sunlight and cooling starts immediately - prior to cabin temp actually rising). Then there could be some sort of 'proportional' control of the air mix flap. I guess you could go to a full PID controller - but that might be overkill for a car.
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Aplogies if this is a statement of the bloomin obvious but generally they also vary the fan speed and, I infer from the way mine has operated on the last 3 cars, the flap positions. All three cars have also had a sunlight sensor as well as an air temperature sniffer.
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Yes, my Mazda has climate control and a sunlight sensor on top of the dash (and the owner's manual contains dire warnings about covering up the sunlight sensor - presumably at risk of thinking the climate control system is faulty).
This car also adjusts fan speed according to how much temperature change is required, and has an ECO mode for the A?C - but I haven't worked out what that means, really.
Annoyingly, if I choose to change the air direction, by directing it to my feet instead of the face vents, it switches off the climate control's Auto Mode. That's my only complaint about the system.
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