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I live in Oklahoma. HOT in summer.

I drive a small car. Honda Fit.

Seems to me, a smaller cabin would cool faster than a larger one.

If I "made my car cabin smaller," would the ac work better?

I was thinking empty pop bottles, but figure need to assert care they don't explode.

My car is mostly empty most of the time.

Would big blocks of scrap styrofoam (sealed in cordura duffle-bags) help with this?

I could always take the bags out when I need the space for passengers or cargo, which isn't often.

Thoughts?

Rob Dye
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  • But adding material will also store and liberate heat… if the filler material has a lower heat capacity than the air it replaces… a bigger ac system… – Solar Mike Aug 17 '21 at 19:02
  • I imagine darkening your windows would have a larger effect. It might actually get worse with your approach. Sure, there is less air to cool but there is also less air that needs to be heated to get warm and you have not reduced the surface area with which your car transmits heat through. Remember, a smaller ice cube melts faster and your scenario is kind of like hollowing out a larger ice cube large surface area to melt through without the advantage of lots of ice on the inside as a buffer. – DKNguyen Aug 17 '21 at 19:34
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    Install a plastic shield diving the car into two-compartment. – r13 Aug 17 '21 at 20:34
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    having to carry more mass -> a bit more fuel burned. try just shutting the vents to areas you don't care about. I suspect heat rejection - tint, oiled dash, covers when not in use would help most. the duffel bags have the potential to be another item retaining heat to cool later. air in car gets hotter than the outside air. there's a gain from being able to just remove that air at the start of a short trip. – Abel Aug 17 '21 at 21:11
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    I added foam insulation between the root liner and roof , there was very little insulation there as manufactured. Most heat is coming from roof and windows; do you have tinted glass? – blacksmith37 Aug 17 '21 at 21:17
  • If you hung a blanket across the backs of the front seats you would have less air to cool. But the best thing to do is use a windshield shade, tint your windows, & leave windows cracked in the sun. It's the sun load that's the biggest impact. Drive your car into a shaded street and you will see the change in temp immediately. – Tiger Guy Aug 17 '21 at 21:25
  • park car in the shade? – Pete W Aug 18 '21 at 00:35
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    Switch to full recirc so that you're not replacing the conditioned air all the time. That way on each pass through the chiller you're cooling the same air a little bit more. – Transistor Aug 18 '21 at 06:16
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    Are you asking how to deal with the heat after it's been parked, or getting it cold during an extended drive? – Carl Witthoft Aug 18 '21 at 12:43

3 Answers3

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First of all, smaller car means smaller cabin but it also mean smaller AC unit. So the question is if the AC unit (and the system in general) is adequate for the task at hand. If you live in a very hot climate, then the best solution might be to look for an upgrade of your AC unit (however this is costly, and you might create more problems to your car if the installation is not done properly).

Regarding your proposal about styrofoam, in a way makes sense in certain conditions. However, the problem is that the air-ducts and stream are -usually- designed to work is a specific manner. Adding blocks in the flow (even though its styrofoam and has little conductivity), it will:

  • block the air ways
  • in general the styrofoam will act as a thermal battery.

Regarding the last point, I will provide the following comparison:

styrofoam air
density $kg/m^3$ 40 1.225
heat capacity $\frac{J}{kg.K}$ 1131 1012
energy required for 1 $m^3$ to raise 1 $K$ in $\frac{J}{K}$ 45 1.239

That means that if you have left also the car in the sun for a few hours, the styrofoam will heat up, and it will act a thermal battery, so it will work against the AC unit. It will store about 30-40 times more than the air, which will be released slowly while the AC is operating.

My advice to you - I live in a place, where last month we had daily max over 38 for 10 consecutive days (some times 40s)-, is:

  • when you get into the car,
    1. open up the windows and let the air cool (the temperature in a car left in the sun can exceed 50 C) for a few minutes as you drive.
    2. switch on the AC. It will take a minute or so to produce cool air.
  • As soon as the AC unit produces cool air, close the windows, and wait for it.
  • Air ReCirc: This is probably something very important: You should have air recirculation active, when in those conditions (symbol below)

enter image description here

Other things you can do to help yourself, o get into the car in a hot summer day is:

  • park the car in a shaded place
  • use windshield sun shades.
  • leave the window a fraction open (it will create a chimney effect, that will let the very hot air to escape, and reduce the overheating).

Regarding the styrofoam I would suggest to forego it. Mostly due to the logistics, because even in the once a week that you need to carry something else you will have the problem where do I store the styrofoam). If you really don't use the car for other reasons, carrying other things, consider a bike.

NMech
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There is a balance between how well your car is insulated and how powerful is your car air conditioner.

my understanding is the car AC is designed to maintain an stream of conditioned air around your upper body, not to chill all of the interior of the car.

So ideally you need to focus all the AC vents on yourself.

Another thing to consider in very hot and humid days, keep one of the windows a crack open. Dark windows help too.

i have seen in some hot climates (India) the taxi drivers, use a bamboo roof floating above the roof. which makes sense because it protects the roof from direct sun.

i am not sure about adding insulation in the car, it is hard to maintain its integrity. seams and voids encourage leak.

As far as the size it has to do with your AC performance. if the AC is good larger size makes sense. otherwise it will be extra load on AC. There is an optimal size.

DKNguyen
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kamran
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One trick you could try is to duct the cold air straight to your body with a flexible tube (like a 3" diameter corrugated shop vacuum cleaner hose) and blow the cold air directly in under your clothes. This will very strongly chill your skin, and not waste any "cold" on the rest of the car. You'll need loose clothing to furnish an exhaust path for the air after it has cooled you. Please report back here after you have tried some experiments on this! (PS Note that this is how spacesuits prevent astronauts from getting cooked when things get hot.)

niels nielsen
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