Switch off A/C at altitude?

[cctdiag]

Hi

Could anybody please explain why there is a sign at the top of Yosimite Pass which asks drivers to switch off their cars air conditioning. I am presuming it has something to do with the effect of air prssure at altitude which could damge the A/C unit - but not being fridge minded can somebody help

many thanks

Jeff

 

[BRT549]

That's a good one. I would guess that the A/C unit's heat exchanger isn't able to transfer as much heat to the thinner air. Also, by climbing the hill, your engine is putting out more heat which is also having trouble transfering out of the radiator fins to the thinner fluid (air). So with the loading of the engine by climbing the hill and running the additional load of the A/C unit, maybe the intent of the sign is to keep people from overheating their cars and clogging up the highway.

I don't think it has to do with any quirks of running A/C at elevation.

 

[IRstuff]

Most such signs warn about switching off A/C to prevent overheating

TTFN

 

[cbiber]

If the aim is to prevent overheating, shouldn't the sign be near the *bottom* of the pass?!

Maybe it's just cooler up there, making the A/C unnecessary!

In all seriousness, though, the altitude does significantly affect the air pressure. The density of air is about 30% less at 10,000 feet than at sea level, and since cooling capacity of the heat exchanger scales as the air density. Also, if I remember correctly, the A/C heat exchangers are usually mounted behind the engine radiator, so they're seeing air preheated more than usual for the same reason.

Cathy

Biber Thermal Design

http://www.biberthermal.com

 

[IRstuff]

Valid point, but I've never gone over a 10,000 ft pass. but there are lots of those signs for passes less than 5,000 ft.

TTFN

 

[JOM]

Hi.

Just a stab in the dark, but are there bears in Yosemite? I seem to recall they can be a problem to campers there. (Or are the campers the problem?)

Perhaps the park rangers are particularly concerned about tourists breaking down on lonely mountain roads. Just a thought.



Cheers,
John.

 

[IRstuff]

They simply don't want to be rescuing stranded motorists all the time ;-)

Don't forget that it's also dependent on mean temperature. The pass that leads into Las Vegas is not much more than 4000 ft, but it's pretty steep and hot and there's a warning at the bottom about A/C.

TTFN

 

[ivymike]

re Also, if I remember correctly, the A/C heat exchangers are usually mounted behind the engine radiator

my experience has been the opposite.

 

[pennpoint]

cbiber&ivymike;

Sorry to contradict but the A/C condenser needs to see the coolest air first so its usually located in "front" of the engine radiator. Its heat rejection plus the mechanical load on the engine at that altitude is why "they" are advising you to shut off the A/C system.

pennpoint

 

[RUFUS2K]

If the refrigerant in the condenser is not cooled suficiently, this will lead to higher pressures in the system and an increase in the horse power required from the engine to continue to operate the A/C. The cooling comes from the air moving over the condenser coils. The majority of the cooling effect in automotive A/C systems comes from the ram-air as the vehicle is moving. This is only supplimented by the action of the fan.

My speculation would be that during mountain driving, the low vehicle speed would cause the refrigerant pressures to rise, requiring the fan to run continually (even with modern electric fans, the electricity is still being generated by the engine); the A/C compressor would draw more HP from the engine; and the heat that IS removed from the refrigerant next travels through the radiator, reducing its capacity to cool the engine. All of these things are adding strain to the engine which is working extra hard at high altitude.

A posible answer for why the warnings might be at the peak rather than the base would be that while decending, the engine RPM's are not high enough to produce the additional HP required for these extra functions.

Anyway, that's my 2 cents.

Rufus2k

 

[JOM]

http://www.nps.gov/yose/now/conditions.htm

That's the web address of the Yosemite National Park. Why not write and ask them? It's a whacky idea, I know, but it might just work.

Cheers,
John.

 

[farraj]

idont no why no body think about the refrgirant r11, r22
inside the refrigration cycle as you go up the temp. will come down then the ref. will get high density to move inside the cycle throw the comp. that will cause high temp to it then it will dameg the comp.

 

[Montemayor]

cctdiag:

Where is everybody at? A automobile's A/C unit depends on successfully condensing the compressed refrigerant vapor with external air.

If the auto is at an altitude, the air is considered "rarefied" - very low density (high specific volume). The air cools with the heat transfered to its MASS, not its volume. Therefore, with less air mass, the A/C contributes to the car's overheating.

The cooling system does best at the bottom of the pass, not the top. The air density at the bottom favors better cooling because it is denser.


Art Montemayor
Spring, TX

 

[ClydeMule]

*Less Air at altitude
*car is working harder going up a hill, during a relatively warm day with less dense air. Running the AC increases the temp of the air 20 F or so above ambient entering the radiator. This is OK at lower elevations or level grades, but it is problem going up Tioga.
*AC adds additional mechanical load to the car. I worked for a pack outfit up in Yosemite for 4 years. A cheap way to add braking on the way down Tioga with 20 horses is to crank up the AC. Every litle bit helps.

If you have a relatively new, well functioning car it should not be a problem. Some sloppy jalopy that barely runs downtown in January will have problems.

They play it safe because it is the only way out of the eastern end of Yosemite, and it is a 2 lane higway, with STEEEP sides down the canyon with not many guard rails.

The reason for putting the sign at the peak vs. the base is probably due to funding. A lot of other grades in CA have the same sign at the base (Tejon, Cajon, Sherwin, etc.)

Have fun camping.