Repainting a car for Aerodynamics?

[Southcross]

gawd.. are you a Troll? If so, I'll remember to plain ignore your arguments in the future. otherwise, I'm done because you don't seem to understand or want to understand

[Christ]

Not trolling at all, but you still seem to be avoiding the original argument, that you said you wanted to paint your car a lighter color so the interior would stay cooler. As many times as you veer from that subject, I just brought it back up.

What exactly don't I understand? That you don't want to listen to fact, because you have a preconception of what "must" be happening with your vehicle? If that's what you're referring to, then you're right, I don't understand.

Other than that, I'm sure this discussion has outlived it's usefulness, so I guess we agree.

[cfg83]

NachtRitter -

Originally Posted by NachtRitter

Would be interested in seeing how this was done, as well as the results... Was temp taken only twice, once at the start and once at the end? Or was it taken "continuously" over the 4 hours (like every 5 or 10 mins)?

Curious if the rate of temp increase is equal between the two colors... does make sense that after some amount of time they'd equalize.

Of course, perception is sometimes good enough too.

That's what I was thinking. Each car would reach the same equilibrium temp, but the black car would get there first.

I think the best way to reduce the heat gain is to stop the light from getting inside in the first place. There are some window covers that are installed externally, but those are problematic (weather and thievery).

MetroMPG -

Regarding your golf ball picture. Would a racing disk gain benefit from having "dimples"? The racing disk is obviously not a sphere, but it does rotate. (Maybe what we really need are sphere-shaped tires with dimple-tread)

CarloSW2

[Christ]

Originally Posted by cfg83

I think the best way to reduce the heat gain is to stop the light from getting inside in the first place. There are some window covers that are installed externally, but those are problematic (weather and thievery).

CarloSW2

Ya know those stupid foil windshield things? They actually help. Of course, if you have the option, you could always just park inside, in or shade.

Tinting the windows helps tremendously. There are also anti-UV coatings that are virtually clear, and can be applied to windows, but I don't know how effective they are or how long they last. (I just heard about them on the Green channel the other day, haven't looked into it fully yet.)

[Christ]

Oh - there are also those window coverings which turn white when power is applied... you could use the power of the sun (solar panel to power them) to block out the power of the sun!

[evolutionmovement]

SPD Smartglass. I just wonder what it costs. I'd love to use it, but doubt it will be financially practical.

[NachtRitter]

OK, here's what I was looking for...

Christ, I think what I understand you to be saying is that over time (you tested with 8hrs, EPA says 4 hrs, and it may be as little as 1 hour), the interior temp of a car is going to be identical regardless of the outside color when sitting in direct sun. Makes perfect sense, especially since the car isn't very well insulated to begin with.

I think what Edward is trying to say is that the rate of heating within the car would be higher with darker colors vs a lighter color... so if he is parked in the shade (and the car is cool inside) and then drives out in the direct sun, the interior will get hotter faster if it is dark colored vs a lighter colored car. Eventually though (in 1 hour, 4 hours, whatever), it will still reach the same temp. Likewise, in order to keep the interior cooler, the AC (if used) would need to work harder to counteract the rate of heating in a darker colored car.

One study done by Florida Power & Light on houses indicates that a lighter colored roof helps reduce electric bills... (see Residential | FPL | Keep the Heat Out and the Savings In) not directly applicable to the discussion at hand, I know... but a similar idea. Obviously not the only thing that'll help... I agree that window tinting and those foil "sun shades" would help a lot as well.

[Christ]

NachtRitter - My primary concern in saying that the paint really doesn't matter is that the glass transmits more heat energy into the car than the metal ever would. Glass transmits light into the cabin of the vehicle, and since there is a large surface area of clear glass to act as an intensifier for the light, the vehicle's interior surface will heat much faster as a result.

It's the interior surfaces absorbing the heat from light energy, not heat radiance from metal panels.

If the color of your car makes any difference at all, it would be an extremely negligible difference, at best. Possibly a few degrees over a 2 or 3 hour period, hardly something worth painting your car over.

Think about how efficient heat transfer can be:

Glass transmits light, into the surface area of the vehicle's interior,
The surface of the interior upholstery absorbs the light energy as heat.

Light hits metal painted panels, which absorb that energy as heat.
While the temperature below those panels is different from the temperature of the panel, heat radiates slowly into the vehicle.
As the temperature increases, the differential decreases, and heat transfer slows. Eventually, the interior temperature is equal to the temp of the metal surfaces, and the surfaces begin dissipating heat into the open air.

Obviously, black absorbs more heat than white, but I don't think the light absorption occurs any faster. (I'm not sure, correct me if I'm wrong, and show a source.)

In any event, the windows are the primary source of heat being absorbed into the car, indirectly via the larger surface area of the upholstery inside the car.

That was the whole argument in a nutshell, by the way.

[Concrete]

Southcross,

paint it light - it makes a difference
did this on a an 1988 S10 - dark brown to a light tan pewter - it helped
(neither were pretty)

just changed from forest green to silver on a chrysler mini van - it helped

not arguing it was hugh difference
- obviously it does not keep people from buy black cars


there are lots of good thoughts but really bad thermo data in this thread
not going to challenge it, except to say - check this out as a data point

http://www.sae.org/altrefrigerant/presentations/sun.pdf

[NachtRitter]

Christ,

I'm with you, in general. Like most things, there is really is no one "right" or "complete" answer.

For the OP, who was planning to repaint anyway, or for anyone that has the opportunity to choose the color of their vehicle (e.g. new vehicle buyer), evidence provided in this thread seems to indicate that a lighter color can help reduce the rate of heat build-up within the car. True, it may be negligible in some cases, especially if the windows are large and the interior is a dark color.

Certainly would not advise anyone running out and rattle-canning their new car white if they currently have a dark exterior color.

I would expect a light colored interior will help a little as well. And tinted windows would help even more (or instead of).

In the end, though, a car sitting out in direct sun for several hours will be friggin hot no matter what. Best bet, as you said, is to park in shade or indoors, if that's an option. Maybe something like those 'bikini' car covers that just go over the passenger area with UV protection...