OT How does my car's AC work?

[Micky]

OT This is ot but I figure I have a better chance of getting a good
answer here than most other places.

After only 4 days over 90 in Baltimore last summer, so far this summer
we've had about 33 with more on the way.

And after 49 years with a convertible, in the last three or four years
I've finally been willing to use the AC even when the temp is below
90 or 95 (I forget.)

So it seems to me that when the fan is on speed 1, the air coming out
of the dash will be quite cold (and it is), but on speed 2, which is
faster, it seems to me the air go through the evaporator faster and
would have less time to cool off and it woudn 't be quite so cold, and
on speed 3, I'd expect it to be noticeably not as cold. (I don't use
speeds 4 or 5 because they make too much noise, but I could make tests
if anyone wants me to.)

But in practice, they all seem to be the same temperature. How can
that be? Is it just that I'm right but the difference is too small
to sense?

 

[root]

Micky <NONONObobbyburns1111@gmail.com> wrote:

OT This is ot but I figure I have a better chance of getting a good
answer here than most other places.

After only 4 days over 90 in Baltimore last summer, so far this summer
we've had about 33 with more on the way.

And after 49 years with a convertible, in the last three or four years
I've finally been willing to use the AC even when the temp is below
90 or 95 (I forget.)

So it seems to me that when the fan is on speed 1, the air coming out
of the dash will be quite cold (and it is), but on speed 2, which is
faster, it seems to me the air go through the evaporator faster and
would have less time to cool off and it woudn 't be quite so cold, and
on speed 3, I'd expect it to be noticeably not as cold. (I don't use
speeds 4 or 5 because they make too much noise, but I could make tests
if anyone wants me to.)

But in practice, they all seem to be the same temperature. How can
that be? Is it just that I'm right but the difference is too small
to sense?

One possible explanation is that the heat exchanger is able to
equilibrate the air temperature at the different speeds you
chose. This would mean the air passages within the heat exchanger
are sufficiently long.

 

[pfjw@aol.com]

On Friday, August 12, 2016 at 3:15:21 PM UTC-4, root wrote:

But in practice, they all seem to be the same temperature. How can
that be? Is it just that I'm right but the difference is too small
to sense?

One possible explanation is that the heat exchanger is able to
equilibrate the air temperature at the different speeds you
chose. This would mean the air passages within the heat exchanger
are sufficiently long.

Exactly this. most auto AC units have an evaporator sensor to help prevent the coil from freezing. At a higher fan speed, more refrigerant is permitted through the coil to maintain an even discharge temperature. I had one fail recently (under warranty), it takes about 30 minutes at 95F/70RH to freeze a coil to a block of ice if the refrigerant discharge is not regulated.

Peter Wieck
Melrose Park, PA

 

>"Exactly this. most auto AC units have an evaporator sensor to help >prevent the coil from freezing. At a higher fan speed, more >refrigerant is permitted through the coil to maintain an even >discharge temperature. I had one fail recently (under warranty), it >takes about 30 minutes at 95F/70RH to freeze a coil to a block of >ice if the refrigerant discharge is not regulated. "

They have apparently changed again then. All though the 1980s it was a cap tube/critical charge system. Cheap, compared to the old ones that had a real thermostatically controlled expansion valve and an accumulator.

The ones I saw simply worked on low side pressure. The lower the pressure, the lower the temperature. It also served the purpose of keeping the compressor from sucking in air in case of a low side leak. But of course most leaks are on the high side.

I haven't worked on alot of newer cars, but I do know they are doing alot of things to improve efficiency, like variable valve timing which is pretty expensive to implement. But so were roller lifters at one time. I am not sure how much they're getting out of the direct cylinder injection but if it raises the power to weight ratio that is a plus. Both of these things were unheard of in the 1970s.

It was said the AC takes about 12 HP in a car. People have told me also that the performance is off when they have the air on. Some cars had a switch to temporarily disengage the compressor at wide open throttle. Not sure if they still have that or just use the computer to do it.

Another reason they need some sort of regulation is that most cars run the compressor when you use the defrosters. It allows it to start clearing the window long before the car is warmed up. In the 1980s GMs at least you could hear the compressor kick in and out in the winter as if you are low on Freon, but it is because the temperature is so low. Obviously the heater core goes before the evaporator but if the engine is not warmed that means nothing.

 

[Tim R]

I haven't checked lately, but the last time I looked an automotive air conditioner was about 3 tons capacity.

That will cool the average house.

But a car needs more, because of the solar gain from all the glass, and the fact that you have to cool it down in ten minutes rather than 24 hours.

 

On Friday, August 12, 2016 at 9:17:43 PM UTC-5, Tim R wrote:
> I haven't checked lately, but the last time I looked an automotive air conditioner was about 3 tons capacity.

That jibes, 36,000 BTU converts to 14.something HP. Makes sense as it would unload somewhat when the evaporator gets nice and cold and it is pulling cold air in from the cabin.

http://www.endmemo.com/sconvert/btu_hhp.php


> That will cool the average house.

Wish it would cool this one better, and actually this is bigger than three tons.

But a car needs more, because of the solar gain from all the glass, and the fact that you have to cool it down in ten minutes rather than 24 hours.

I have to wait ten minutes ? No way.

 

in a Ford Econoline, SWFL/AZ .... higher fan flow gives colder temps as the higher flow fills the cockpit ...there's a partition behind seats.

The coldness increases with each of 3 speeds and with Hi speed there is surplus cold flow overcoming heat gain from engine/large windshield...the roof is overlain with a rack floor. Paint is WHITE...there is a 30+ degree difference between WHITE and Safety Blue at the outside surface midmorning here in SWFL

The surplus coldness ...I forget the temp...hot is 150 ..... prob around 42-45..
is too cold unless the outside is too hot.

If your system is constant temp at varying speeds then change the coolant....inspect the system. Design satisfaction would spec excess available heat removal for all speeds. Or, why have variable speeds ?

Ford's test track is down the road below Naples.

 

avag...@gmail.com:

Coldness "increases" at higher fan speeds because the higher fan speed moves a higher volume of air per minute from plenum into the cabin. The air itself doesn't get any colder - that is dictated by the compressor and the user temperature setting.

 

[Ron D.]

One reason that would happen is if the AC is set to take in outside air. To get the coldest AC, you want to use recirculate. Also depending on humidity don;t expect a change of more that 15-20 degrees F of the intake and discharged air.

If outside air is used and say the temp is 90 outside, you only get say 75 deg degree air no matter what. If you were using recirculate, eventually, the cabin temp would be 70F and then further cooling it would lower it to say 65 and even further yet and you will have to add a bit of heat.

One thing that's useful is to use the AC and heat at the same time when it's raining and cold. The AC will de-humidify and the heater will heat making the air actually feel warmer.

AC On and pointed to the front window will defog while Heat pointed at the window will defrost.

 

[Tim R]

On Saturday, August 13, 2016 at 7:59:15 PM UTC-4, thekma...@gmail.com wrote:

avag...@gmail.com:

Coldness "increases" at higher fan speeds because the higher fan speed moves a higher volume of air per minute from plenum into the cabin. The air itself doesn't get any colder - that is dictated by the compressor and the user temperature setting.

I'm speculating rashly here because I don't know how the automotive AC works.

But it's the internet so I'll think out loud based on a building AC setup.

There is no such thing as cold. There is only greater and lesser heat.

Dumping cold air into a space does not cool it UNLESS you are sucking warm air out.

In a building, there is a supply and a return. The return sucks out hot air and then the supply lets some cooler air move in and mix. No return, no cooling.

So MAYBE in a car, a higher speed will suck more hot air out of the space and cool it better even if the faster air over the coil is not cooled quite as much. At some point air must go over the coil too fast to cool all the way down to coil temperature (or heat up to coil temperature in the winter).. But that might not be the limiting factor.

 

Cars are built with vents designed to always suck some air out the back to assure exhaust does not get in the cabin. Air is always drawn in from the front. Where the windshield wipers are is a gutter, and the fresh air intake is usually under there somewhere. Some higher end models actually have a filter there you have to replace or clean every once in a while.

Incidentally, if you smoke in the car do not use recirculate. As a non-smoker for five years I can smell it. It does not bother me ad I don't get pissy about it but if you go to sell the car it hurts the value and in fact some people simply won't buy it.

 

[Ron D.]

in one of my vehicles (circa 1982) that I put the AC in the car from a "box of parts", so I know that system intimately.

There was a switch that was set when the temp was set to "recirculate".

The outside air was "partially" controlled by the "AC module". 1/2 of the recirculate damper was controlled by the AC module. To prevent freezing of the evaporator, the module would open up 1/2 the damper to outside air. If that didn't work, it could shut off the compressor.

Idle typically increases when the AC is turned on.

Even with the fan off, you get "flow thru ventilation" in non-recirculate mode. So, the temperature was still modulated by how far the "water valve" was open. Not all cars have a water valve, but many have a way to turn the water off to the core under MAX cold.On Friday, August 12, 2016 at 2:51:36 PM UTC-4, Micky wrote:

Latest car has climate control.

 

[Micky]

On Sat, 13 Aug 2016 16:59:03 -0700 (PDT), thekmanrocks@gmail.com
wrote:

avag...@gmail.com:

Coldness "increases" at higher fan speeds because the higher fan speed moves a higher volume of air per minute from plenum into the cabin. The air itself doesn't get any colder - that is dictated by the compressor and the user temperature setting.

Exactly. I wasn't referring to the temp inside the car, but to the
air coming out of the vents. I held my finger right on a vent while I
changed the fan speed. The compressor and my temperature setting
stayed the same. I think my temp setting was "COLD" meaning as far
down as it goes. But the air didn't seem to get warmer as the fan
speed increased.

 

[Micky]

On Mon, 15 Aug 2016 15:46:02 -0700 (PDT), "Ron D."
<Ron.Dozier@gmail.com> wrote:

in one of my vehicles (circa 1982) that I put the AC in the car from a "box of parts", so I know that system intimately.

There was a switch that was set when the temp was set to "recirculate".

The outside air was "partially" controlled by the "AC module". 1/2 of the recirculate damper was controlled by the AC module. To prevent freezing of the evaporator, the module would open up 1/2 the damper to outside air. If that didn't work, it could shut off the compressor.

But the compressor never got shut off, and the AC is fully charged and
not subject to freezing, and most importantly I can run in any of
those 3 speeds for a long time and nothing changes.

Idle typically increases when the AC is turned on.

But the idle doesn't change speed by changing the AC fan speed.

Even with the fan off, you get "flow thru ventilation" in non-recirculate mode. So, the temperature was still modulated by how far the "water valve" was open. Not all cars have a water valve, but many have a way to turn the water off to the core under MAX cold.

Toyota doesn't call it max, and the fan wasn't on the high speed, but
the outside air vent was closed.

On Friday, August 12, 2016 at 2:51:36 PM UTC-4, Micky wrote:

Latest car has climate control.

 

[Micky]

On Sun, 14 Aug 2016 16:14:26 -0700 (PDT), "Ron D."
<Ron.Dozier@gmail.com> wrote:

One reason that would happen is if the AC is set to take in outside air. To get the coldest AC, you want to use recirculate. Also depending on humidity don;t expect a change of more that 15-20 degrees F of the intake and discharged air.

I was using recirculate.

If outside air is used and say the temp is 90 outside, you only get say 75 deg degree air no matter what.

Why would this affect what I said? The air would be 75 on low speed,
but on higher speeds it would blow by the evaporator so fast it
wouldn't cool as much.

If you were using recirculate, eventually, the cabin temp would be 70F and then further cooling it would lower it to say 65 and even further yet and you will have to add a bit of heat.

One thing that's useful is to use the AC and heat at the same time when it's raining and cold. The AC will de-humidify and the heater will heat making the air actually feel warmer.

AC On and pointed to the front window will defog while Heat pointed at the window will defrost.

 

[Ron D.]

If outside air is used and say the temp is 90 outside, you only get say 75 deg degree air no matter what.

Why would this affect what I said? The air would be 75 on low speed,
but on higher speeds it would blow by the evaporator so fast it
wouldn't cool as much.
- hide quoted text -

Air temperature should really stay the same, BUT the perceived coldness should change. Take a 70 degree day with no breeze vs a 70 degree day with a 15 mph breeze. In the latter, people are cooled by the increased evaporation on their skin.

Fan speed MIGHT make a difference depending on the design, but you really want the evaporator to be just above freezing. With some range of air velocity, the outlet temp won't really change.

 

[Ron D.]

But the compressor never got shut off, and the AC is fully charged and
not subject to freezing, and most importantly I can run in any of
those 3 speeds for a long time and nothing changes.

--

You really need to do a "performance test" which is basically open the doors, run on outside air and measure the incoming temp/humidity (Use weather bureau for humidity) and discharge temps.

You should get between 15 and 20 degree difference in temperature. A lower difference, if the humidity is high.

The compressor really should never shut off in a more modern vehicle unless the evaporator is in danger of freezing. Freezing is possible if your using the AC when it's 40 degrees outside, which I frequently do.

 

[Tim R]

On Wednesday, August 17, 2016 at 12:58:54 PM UTC-4, Ron D. wrote:

But the compressor never got shut off, and the AC is fully charged and
not subject to freezing, and most importantly I can run in any of
those 3 speeds for a long time and nothing changes.

--

You really need to do a "performance test" which is basically open the doors, run on outside air and measure the incoming temp/humidity (Use weather bureau for humidity) and discharge temps.

You should get between 15 and 20 degree difference in temperature. A lower difference, if the humidity is high.

The compressor really should never shut off in a more modern vehicle unless the evaporator is in danger of freezing. Freezing is possible if your using the AC when it's 40 degrees outside, which I frequently do.

Defrost always runs the compressor though, so in the winter everybody uses it. At least in cars I've had.

 

NO RECIRCULATE

recirculate is for a load of rotting fish.

https://www.google.com/search?site=imghp&tbm=isch&source=hp&biw=1791&bih=900&q=toyota+corolla+air+conditioning+system&oq=toyota+air+conditioning+system&gs_l=img.1.1.0i8i30k1l5j0i24k1l2.1933.11761.0.17238.30.28..0.2.2.0.323.3123.7j19j0j1.27.0....0...1ac.1.64.img..1.29.3129...0j0i30k1.gjR2pH0hJY0

more air moving over an operating exchanger gives cooler temps...otherwise the system needs repair.

try wiring the fan directly to the battery with a relay. prob the ground side.

first check for leaks in the output side n close down vents not needed.

 

On Wednesday, August 17, 2016 at 7:53:17 PM UTC-4, avag...@gmail.com wrote:

NO RECIRCULATE

recirculate is for a load of rotting fish.

https://www.google.com/search?site=imghp&tbm=isch&source=hp&biw=1791&bih=900&q=toyota+corolla+air+conditioning+system&oq=toyota+air+conditioning+system&gs_l=img.1.1.0i8i30k1l5j0i24k1l2.1933.11761.0.17238.30.28.0.2.2.0.323.3123.7j19j0j1.27.0....0...1ac.1.64.img..1.29.3129...0j0i30k1..gjR2pH0hJY0

more air moving over an operating exchanger gives cooler temps...otherwise the system needs repair.

try wiring the fan directly to the battery with a relay. prob the ground side.

first check for leaks in the output side n close down vents not needed.

Bing has the word

https://en.wikipedia.org/wiki/Zeotropic_mixture