So a car sits in the sun, and the interior temperature rises to, oh, say, 120º-130ºF (outside temp, maybe 90 or so). I get into it, and want to start cooling it off. Now what I would do was be drive for a “short” (not more than a mile) while with the windows down to ventilate the superheated air, then roll up windows, and crank up the AC.
I’ve probably removed the superhot interior air at this point, but the surfaces are still pretty hot, and radiating heat.
The conundrum:
Would cranking up the AC (recycling interior air) at that point cool the surfaces quicker, or continuing driving with the windows down—in either case to cool the interior more efficiently. The AC will blow out much cooler air, but the windows will blow a much heavier volume of warm/hot air that is still cooler than the surfaces.
The overwhelming amount of air from a moving vehicle would have the biggest effect. You can quickly knock 5-10 degrees off the interior temp, but if you ever watched a climate control system work, you can see how hard it has to work to knock just 1 degree off.
For comfort, after the initial 2-minute windows down cool off, you are still going to be battling humidity, which is best handled with the windows up and the A/C on Max w/ recycle chosen.
I am under the impression that the quickest way to get cool is to turn the AC fan speed to the SLOWEST setting. I haven’t done any actual scientific experiments, but it feels to me that the air coming out the vents gets colder sooner with the lower fan speed. After the AC coil is allowed to reach it’s lowest temp (~42F), then crank up the fan.
Thanks—and just so that the actual question doesn’t get lost:
I’m not asking about displacing the interior air. I’m wondering about the still-hotter-than-outside-temp surfaces (dash, seats, etc) and their heat radiation. They would seem to be still heating up the interior after the hot air has been removed. The AC is blowing a low volume of cool air, but the windows would move a high volume of hot, but less hot, outside air. Sounds like a job for Mythbusters!
double checking my thermal systems book, which just happens to be sitting right next to me
Considering that convection is calculated with the difference of the two temperatures in question, while the radiative effect is calculated by raising the temperatures to the fourth power and then taking that difference, a cursory review would suggest that having a significantly cooler interior after you’ve brought up the interior temperature down to the bulk ambient temperature would be the most efficient for removing heat from the interior surfaces.
Part of this is probably simply mitigating how fast the hot air stagnant in the pipes gets to you. If that air weren’t there, we’d get more efficient cooling with a larger volume of slightly cooled air than a smaller volume of chilled air (Qdot = h A (Ts - Tf), so it’s only true up to a point, of course.).
Which one is preferable in a subjective sense would depend pretty significantly on what the temperature of the outside air is like. If it’s 105 deg. in the car and the outside shade temperature is 90 deg., opening the windows isn’t going to help much.
I don’t know who thought it would be a good idea to provide us with cars in the Sonora desert that were painted black with all black interiors, but it’s what a lot of us had. Worry about 120 degrees on the inside? It got a heck of a lot hotter than that on the outside. My commute “home” was sometimes as long as 45 minutes, and the air conditioner wasn’t effective at all until just about the time I pulled into our driveway. All of that heat radiating off of every black surface just completely overpowered what the poor air conditioner could compete with. The only alternative was to blast the air conditioner, and ride with the windows down and the moonroof open.
On the other hand, the driveway was covered (i.e., shade), and so when the only heat was ambient heat, all of the black wouldn’t radiate much. It was only ambient heat, and not collected solar radiation. A quick couple of minutes with the window down got rid of the stale air, and then the air conditioner worked excellently, despite the 120 degree temperature outside.
Any warm object heats the air near itself. Humans, for example, can stay relatively warm while standing naked outside in the winter if there is zero wind. The body heats up the air around it and provides you with a layer of warm insulation. Once the wind hits, you get cold. Sure, the warmer air around you is constantly cooling and after a while you just can’t heat the air anymore and freeze to death, but the wind will do it a lot quicker.
Looking at the interior of a car, I think the best bet is to move massive amounts of air over the surface to keep the interior dissipating heat.
As anecdotal evidence, if my body is hot from being outside (as opposed to getting in the car after chilling out indoors), it takes forever to cool down even with a cold A/C. But with the windows down, I am comfortable a lot sooner.
yeah try leave your door open as long as possible and keep the windows open with just the fan on for the first minute or two that you drive, the a/c will cope better. One of those sun-visor things that covers the windscreen while parked can help a lot too.
That’s the first step. The air in your car is 130-130 degrees F.
Let’s say the outside air is 90.
First, open all the doors, and the sunroof, if you have one.
Reach inside, start the engine and turn up the AC to high, but not recirculate.
When the air inside is close to the exterior temp, get in and close the doors.
The best your AC can ever do is a reduction by 40 degrees F. If you were to get in right away and set it to recirculate, the vents will blow no cooler than 90. If you let the superheated air blow out (mostly) before getting in, the vents will blow 50 degrees.
The seats, carpet, dash, etc., will still be over 100 degrees for quite awhile, no matter what you do.
Face it, though, if you’re driving only a mile, you will not get comfortable by the time you get there. Having a folding shade in the windshield will often keep the steering wheel from burning your fingers.
Yes, that is key. The best way is to prevent your car’s interior from reaching 120-130º in the first place. Much more effective.
Folding cardboard shades that you put just inside your windshield to reflect the sunlight help. Sometimes you can find them for free, with advertising printed on them. You can also get them for the back window, too, if needed.
There are also small fans that fit inside the side window opening, solar powered, and temp controlled so that whenever the interior temp is warmer than the outside, they start pumping air out of the car.
And these are much cheaper than running the air conditioner, given nearly $4/gallon gasoline. And they are effective as right when you get into the car – you don’t have to wait for the air conditioner to cool it down.
Of course if you live in a spot where there’s zero chance that some young punk won’t mess with your car (these are easy to remove externally), then you also live in a spot where you can probably just leave your windows down when you leave your car. It’s a lot cheaper still. Personally, it seems rather easier to just wait thirty seconds if you can’t bear the temperature inside, since you’ll be spending at least that time futzing with your windows to get the thing out and put it away, not to mention the time putting it back in the window next time you park.
Au contraire. I bought two of those, solar powered ones, and use a tobacco shed thermometer to test their effectiveness. They dropped the temp from 120 degrees to 118 degrees F.
I agree. Most cars won’t turn the compressor on unless the fan is on so I set it on the lowest speed and crack the window for 15-30 seconds. By that time I’ve exhausted a fair amount of hot interior air and the vent is producing cold air.
Not necessarily. ::sigh:: Florida afternoon thunderstorms…no way I’d leave my windows down, but I might be able to use one of those fan things, depending on how large an opening in the window they require.
In all seriousness your best bet is to park the car some place where it will be in the shade when you get off work. I used to do this when I worked at a mall, it was certainly worth the extra walk to get into a car that wasnt an oven.