Question about car AC workings

[atgcpaul]

When you turn on the AC in the house, the blower fan moves at one speed
and the AC stays on for as long as it takes to reach the desired temp.

In most cars there is no thermostat, just an adjustment for fan speed.
Putting the fan at a higher speed doesn't make the AC work harder, right? It
just blows the air faster, right?

[Tom Slick]

all the cars I've owned had a thermostat, it doesn't have numbers just graduated colors from blue to red. similar function as a house thermostat.
edit: higher end cars have numbers and dual or quad controls.

higher speed does make it work "harder" you are moving more air past the evaporator which in turn is absorbing more heat. More heat has to be pumped out so the system works "harder" to keep up.

[dbhost]

all the cars I've owned had a thermostat, it doesn't have numbers just graduated colors from blue to red. similar function as a house thermostat.
edit: higher end cars have numbers and dual or quad controls.

higher speed does make it work "harder" you are moving more air past the evaporator which in turn is absorbing more heat. More heat has to be pumped out so the system works "harder" to keep up.

That red / blue slider / knob whatnot isn't exactly a thermostat in the traditional sense, but rather an air mix control, your auto air conditioner runs full on when it runs, or full off when it is off. When it is on, the red / blue lever controls how much air gets blown over the heater core, and mixed with the air blown over the evaporator core creating a hot / cold mix. On the Max AC setting, NO air gets blown over the heater core.

The various fan speed settings control just that, control the fan speed.

More expensive cars will actually have a numeric indicator, and cycle the compressor, and the hot / cold mixture through a computer controlled program to achieve the desired temperature in the zone the operator desires. (Driver, front passenger, left rear passengers, right rear passengers etc...).

Mind you, my recollection is a bit hazy, I quite spinning wrenches professionally in '95. But FWIW back in the day I was IMACA (International Mobile Air Conditioning Association) and ASE Auto Air Conditioning Certified. (Although I much rather preferred suspension and drive train work over AC and drivability).

[LinuxRandal]

In addition the cycling the ac compressor on and off more, as well as the hot air going through the coil, will cause the radiator fluid to heat up quicker (more noticalble on smaller engines), and cause electric (if equipped) fans to kick on either more often or for longer periods (to keep the radiator fluid in temp range).

I wonder if this is why the original poster asked?

[LCHIEN]

yes, running the fan faster will transfer more heat from the circulating air to the evap coils. The compressor works harder and also some of the heat is transfered to the radiator.

a large number of cars have manual controls you adjust the air mixture and fan speed for comfort. The A/C system in these does have a thermostat to cycle the compressor on and off as needed and keep from freezing the evaporator coils. Once found only on luxury models, many more cars are now having cabin thermostatic controls, sometimes with two zones. This is computer controlled and controls the air mix and fan speed to regulate the cabin temperature. Of course on really hot days, the cold air blowing directly on your face and skin are what makes you feel cooler.

[docrowan]

I would further add that in most cars the MAX A/C recirculates the cabin air. The regular A/C setting allows outside air to mix in, which results in warmer temperatures (and more humidity here in the South).

[LCHIEN]

I would further add that in most cars the MAX A/C recirculates the cabin air. The regular A/C setting allows outside air to mix in, which results in warmer temperatures (and more humidity here in the South).

Yeah, that Max A/C or Recirculate button was a momentary pushbutton with an indicator on my Dodge Caravan. When the car was turned off and then restarted the power on condition was for the Max or recirc to be off. I always thought whoever designed that must not have lived on the Gulf coast, where the summer time heat and humidity usually requires to have the dang thing on, esp. when starting the car that has been parked outside in the daytime, to cool down as fast as possible.

Most other cars it was a latching pushbutton or a knob position where you could set it and leave it there.

[pierhogunn]

what drives the compressor in most cars?
how many horsepower ( on average) do you think it would take to run one of those compressors?

for most sedans how would you get one out of a junk car?

[LinuxRandal]

Pier, Nowdays, the serpentine belt drives it. In older cars, they had seperate belts.

I don't know on the horsepower issue, and I think that would involve a lot to figure.

The compressor bolts to the motor via a bracket of somekind (where and bolt sizes depend on vehicle). You would have to remove the serpentine belt, unplug the electrical connector, evacuate the system, then unbolt and remove the compressor.

[pierhogunn]

hmmm, wonder if a 2 - 3 hp stirling engine would be enough to drive some small thing like from a geo metro, hmmm

[tommyt654]

See if this link will help explain things better,http://www.familycar.com/ac1.htm

[pelligrini]

hmmm, wonder if a 2 - 3 hp stirling engine would be enough to drive some small thing like from a geo metro, hmmm

I seriously doubt it.

I seem to remember something like 10-15 hp loss at least for AC (I think it was more). That was in regards to older vehicles, Pre computerized fuel injected engines.

[master53yoda]

Automotive AC compressors are between 1 and 2 ton medium temp refrigeration compressors. The capacity varies based on RPM and airflow over the evaporator. True horsepower required is about 1 HP per ton. They all have a low pressure switch to shut off the compressor if the pressure drops below the freezing point. This will happen either if the fan is on low at higher engine RPM, the air temp over the evaporator is low ( as on defrost mode), or the system is low on refrigerant.

The air temperature delivered is controlled by mixing unconditioned air and cold air together.

Max cooling recirculates the air from inside the cab. This works best if you live in a high humidity area as it can strip the water out of the air inside the car and cool it colder easier then striping the water and cooling the outside air stream.

Not mentioned before is that the compressor also operates in the defrost cycle. the reason for this is that the compressor shaft seal is an oil seal. It will leak refrigerant when the seal becomes dry when not operated over and extended period of time. If you live in a n area that never need defrost operation I would suggest operating the defrost at least once per month for ten minutes or so.

[Kristofor]

Every extra system you add to the process will waste energy to friction and other inefficiencies. Support and maint. requirements will also be increased. There's a reason you don't see rube goldberg chains of contraptions in regular use...

I don't know what a Geo Metro AC needs, but just looking at the math.

1hp = 746 Watts
3hp = 2238W = ~20A @ 115V

There are small AC units that use this much power or less.