Post Number: 140
Votes: 0 (Vote!)
|Posted on Thursday, 18 September, 2008 - 02:35 pm: ||
I like this project in EPE Oct 08 but the authorís analysis of the way car aircons work is too simplistic. Maybe in Australia their cars work as described, but in Europe there are further variations which may limit the type of cars the project can be used on. My knowledge on this is not great but I have repaired a couple of car aircons so had to research it.
On many non climate controlled cars the thermostat shown in the article regulates the air to say 15C by cutting the compressor in and out. It does not, as implied in the article set the cabin temperature. If the driver wants his car hotter than 15C, the facia ďtemperatureĒ control blends hot air from the normal car heater with the 15C air to get a higher temperature. The blending is not thermostatically controlled so the driver needs to regularly adjust the knob for comfort.
The advantage of this system is that you get low humidity and itís cheap to build. The disadvantage is that itís wasteful of fuel and not temperature controlled.
I suggest a more fuel efficient way to use this project on these cars is to insert a simple cabin thermostat (electromechanical or electronic type) in series between the existing thermostat and the compressor flag input. Then the outlet air will be regulated to what the cabin thermostat is set to and the facia knob should be left on its coldest setting when the aircon is used. But I canít guarantee there won't be side effects like excessive humidity and conceivably icing up of the evaporator. Also will this circuit upset the way the PIC logs the flag signal?
Some cars, e.g. previous model Skoda Fabias and no doubt others especially by VAG (Volkswagen Audi Group), use a variable swashplate compressor. Superficially the wiring diagram looks as in the article but a 0-5V analogue signal is applied to this, instead of the 12V one, which varies the swashplate angle and hence the flowrate/pressure. The temperature sensor in the evaporator air outlet is a thermistor which inputs into a processor which then controls the swashplate angle to achieve the steady 15C air temperature. The compressor is always rotating, even when the aircon is off and so there is no electromechanical clutch between engine and compressor for this project to operate on. Thus the project would need some modification to work on this sort of system and trying to use the projectís 12V output on the compressorís 0-5V input may damage it. Possibly reducing the voltage to a fixed 5V may do the trick.
One way of telling the type of compressor is the listen under the bonnet when the aircon is turned on. If there is loud click and a drop in revs itís the electromechanical clutch cutting in. Swashplate ones change from small output to high with no real change in noise noticed. But many modern clutch ones are however quite quiet and the revs very stable so may not be distinguishable from swashplate ones by listening. Measuring the voltage on the input to see if itís analogue is the best way to tell them apart.
(Message edited by echase on 18 September, 2008)