Improving ac efficiency in car

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?

• Install a plastic shield diving the car into two-compartment.
– r13
Aug 17 at 20:34
• 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 at 21:11
• 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? Aug 17 at 21:17
• 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. Aug 18 at 6:16
• Are you asking how to deal with the heat after it's been parked, or getting it cold during an extended drive? Aug 18 at 12:43

3 Answers

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)

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.

• @Andrew thanks you are right. I will update it. Thanks for that. I just noticed, I had the columns also labeled wrong. Now the numbers should add up. Aug 18 at 15:54
• It's probably best to switch off the air recirculation every now again, and especially if feeling drowsy, to let some more oxygen into the cabin. At least some of interior will have cooled a bit, so it won't take so long to cool down when the recirc is re-enabled. Aug 18 at 15:57
• @AndrewMorton No argue with that. However for me its only a 10 minute commute at worst, so I never get to do it. Aug 18 at 16:01
• You seem to be reading your table wrong. A block of styrofoam is mostly air and the amount of heat energy you can store in it, is therefore almost the same as for air. (If the styrofoam seems to be lighter than air, that is because the weight of air isn't included in its weight.) And otherwise your numbers would say that it stores about 30 times less energy. (Which would obviously also be excluding the air inside the styrofoam.) Aug 18 at 18:43
• @ingenørd I might be wrong (I've never tested styrofoam this way), but at the same time I am not sure I get your point. I think that the the density is between 30-40 kg/m3, compared to polystyrene's (which is the parent material of styrofoam is) 1050 kg/m^3, and the heat capacity is close enough to air, while for polystyrene's is 1200 J/kg.K. All in all, I think these are reasonable estimates. I doubt that the heat properties or density for styrofoam are quoted for without air. Also, I'm not sure I follow "If the styrofoam seems to be lighter than air". I don't think its lighter than air. Aug 18 at 19:41

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.

• Ohhh is that what that bamboo thing is for? I guess I never gave it so much as a second thought when I saw it in movies haha. I thought it was just a cultural thing I guess. Aug 17 at 23:07
• Most recent cars (15 years?) tie the A/C to a thermostat, so they don't free-run Aug 18 at 12:42
• @DKNguyen it is called shading and works well... Aug 20 at 7:23

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.)