Get extra cooling on your bike with black paint.

[Guest]

Radiation Details (more than you want to know)

I thought this could use its own thread, so it didn't get lost. (And not just because I wrote it)

Quote:
Originally Posted by ZOMBIEEATER
do you have any overheating problems w/the painted engine?


In fact, with flat black being the ideal and polished aluminum being the worst, he should be getting very good cooling.

According to my uncle, who's been dealing with bikes much longer than I have, when they first started selling blacked out engines it was for looks, however it turned out they cooled better than normal. I'm not sure which came first, but either way, I know why.

When something is really hot (like jugs on an air-cooled motorcycle), thermal radiation becomes a significant portion of the heat loss(because radiation heat loss is dependent on the temperature difference to the 4th power, not the 1st like conduction), with convection (due to the moving air) still being dominant. At a standstill radiation can actually be quite more than convection if the engine's already warmed up.

While the thin layer of paint reduces the convection very slightly, the radiation heat loss is actually improved significantly.

Now let's see if I can get this right...

Polished aluminum is almost a perfect "white body". This means that It is a poor emitter and a poor absorber of radiation. When it's cooler than it's surroundings, the radiation from it's surroundings tries to get in by is reflected away. When it is hotter than it's surroundings, it doesn't emit very much radiation because it's a poor emitter. Thus, a hot polished engine does a bad job of losing heat through radiation.

Flat black is almost a perfect "black body," meaning it is a good emitter and a good absorber. When it is colder than its surroundings ( think black t-shirt in the sun) the radiation from the environment is absorbed very easily. When it is hotter than its environment, it does a very good job of emitting radiation to its surroundings.

I don't know wher eshiny black lies between these two extremities, but there you have it.

Now if you'll excuse me, I have to go invest in flat black spraypaint before everyone starts buying it.

 

[Jethro]

That's interesting, but you didn't explian why the black engine... "When it is hotter than its environment, it does a very good job of emitting radiation to its surroundings."

With my simple mind I would imagine that anything surrounding an item-basically insulating it- reduces heat transfer. That's how vapor barriors work (the biggest buzz word in construction).

However, engines are designed with cooling fins. The size of these fins aid in the transmission of heat to the surounding air. A layer of paint would effectively make the fins larger in size.

I would imagine that as long as you don't powder coat your engine in a 4mil coating, the heat difference would be minimal. Your choice in oil weight, and the health of your engine probably make more of a difference in overall engine temps than anything.

 

[Nerobro]

the color of an object makes a very large difference. Black objects both warm up faster, and cool down faster than reflective objects. I need to dig up the data I actually remember it from mr wizzard.

 

[bonanzadave]

Seems as tho all electronic heat sinks are anodized black. I think your on to something !! \\/

 

[Jethro]

Mr. Wizard!! Cool stuff! I'm curious why though if you find the data.

I love my black engine...

 

[Guest]

Traco Engineering used this small detail starting back in the sixties in TransAm racing Camaro engines. One of the users was somewhat successful, the car driven by Mark Donahue of Roger Penske team. It's a small detail but it does work, I suspect it may be one of the reasons GM went to flat black on their engines back in the early eighties. Ray

 

[Guest]

That's interesting, but you didn't explian why the black engine... "When it is hotter than its environment, it does a very good job of emitting radiation to its surroundings."

With my simple mind I would imagine that anything surrounding an item-basically insulating it- reduces heat transfer. That's how vapor barriors work (the biggest buzz word in construction).

The key is, things IN GENERAL that are good absorbers are also good emitters. therefor when the black is hotter than its surroundings it does a better job emitting than a poor emitter would.



Another critical point is the way radiation works, compared to conduction and convection.

(the vapor barrier essentially traps air so it can't move, a very poor conductor). Convection is just conduction, except the fluid is moving so it can remove heat faster than a stationary fluid (air, in this case).

Convection is dependent on the temperature difference (constant *(T_hot-T_cold)). Radiation is dependent on the difference between the ABSOLUTE temperature (kelvin) difference to the 4th power. (constant *(T_hotabs^4-Tcoldabs^4)). In radiation, the constant is much smaller than the convection constant. However, once the temperature differences become large, radiation can become much more dominant. Once example is a large bonfire. the blast of heat near the fire on your face is from radiation more than convection.

I found a neat chart in my heat transfer book that mentions how different finished affect radiation.

 

[drhach]

Heat Transfer

Heat Transfer

I think it is important to be careful of a couple of things. First of all, if your engine overheats, I would doubt very much that painting it black will change that. My point is that, conceptually you are right. There is a difference. In reality, I think it is probably a small difference. The other thing to consider is that a polished engine is being compared to a painted engine. There is a third possibility; an engine that has not been polished or painted.

First of all, polishing lowers the high spots and evens everything out. In essence you are reducing the available surface area for cooling. So, another question to ask is, how does a black engine compare to a polished or an un-polished engine.

Secondly, black is an emitter and an absorber. There is a third mode to consider though, and that is emmisivity. Heat travels in, out and through a body. If the paint has a low factor of emmisivity, it will simply be hot. Yes it absorbs great, but it may not emit well and it may also hold the heat that it absorbs. Furthermore, anytime there is a wall, in this case paint, there will be a reduction in temperature from one side to the other. In other words, the heat is held in, always.

What's the moral of the story? I think if you have three engines, one polished one untouched and one painted black. If all other factors were equal, you proabably wouldn't notice a big difference. Especially in the empirical sense. If painting your engine black knocked a half of a second off of your 1/4 mile e.t, nobody would race unpainted engines again.

 

[Guest]

I think it is important to be careful of a couple of things. First of all, if your engine overheats, I would doubt very much that painting it black will change that. My point is that, conceptually you are right. There is a difference. In reality, I think it is probably a small difference. The other thing to consider is that a polished engine is being compared to a painted engine. There is a third possibility; an engine that has not been polished or painted.

First of all, polishing lowers the high spots and evens everything out. In essence you are reducing the available surface area for cooling. So, another question to ask is, how does a black engine compare to a polished or an un-polished engine.

Secondly, black is an emitter and an absorber. There is a third mode to consider though, and that is emmisivity. Heat travels in, out and through a body. If the paint has a low factor of emmisivity, it will simply be hot. Yes it absorbs great, but it may not emit well and it may also hold the heat that it absorbs. Furthermore, anytime there is a wall, in this case paint, there will be a reduction in temperature from one side to the other. In other words, the heat is held in, always.

What's the moral of the story? I think if you have three engines, one polished one untouched and one painted black. If all other factors were equal, you proabably wouldn't notice a big difference. Especially in the empirical sense. If painting your engine black knocked a half of a second off of your 1/4 mile e.t, nobody would race unpainted engines again.

See my attached chart on the previous email for some details on emmisivity on different finishes. Emmisivity is a measurement of how good of an emitter something is.

I agree, that the heat transfer coefficient (conduction through the coating) of the coating is probably very low. However at a few thousandths thick it would make little difference.

 

[jm_foote]

Interesting idea. I will put in what I remember from thermodynamics class (although it was only a second year class, as thermo was not my major).

Emissivity is how much thermal energy is passed through and object. To make a dictionary quote: "is the ratio of energy radiated to energy radiated by a black body (mmm halle berry) at the same temperature" Theoretically, a true "black" object has an emissivity of 1. This means that all thermal energy will go right through it, it will reflect none.

Reflectivity is inversly related to emissivity. This also ranges from 0 to 1. If something has an emissivity of 0.5, it has a reflectivity of 0.5 and will reflect 50% of the thermal energy.

In order for thermal energy (heat) to dissipate without a state change, you must go from small surface area to large surface area. For instance, the inside of a heat sink vs the outside of a heat sink (hence the "fins"), or the outside of a heat sink vs the (relatively) infinite atmosphere (this is a simple model assuming a homogeneous medium, whatever, close enough). Why then do heat sinks work? Because the thermal conductivity of steel or aluminum is sigificantly greater than that of air.

Assuming that the emissivity of the paint is a perfect 1, then all the thermal energy coming from the engine will be absorbed by the paint.

Our paint can only radiate as much energy as it is given (by the metal).

Energy * 1 = Energy

Unless matter is being changed to energy (for all you star trek fans)... well, we won't go there, I'm not a particle physicist (although I could ask one...).

In order for the paint to cool the metal faster than air it must either:

a) Increase the surface area in contact with the air, while having a thermal conductivity greater than that of air; or

b) Be cooler than the air.

We have, for the sake of discussion eliminated (a).

The paint cannot be cooler than the air unless something is activly cooling it (involving a change of state, or other energy absorbing process). "Other energy absorbing proces"? Back to the particle physics, I won't go there.

Once again, though, my knowledge of thermodynamics is limited, so I may be wrong.

 

[jm_foote]

This does not however, change the fact that a black engine looks really cool!!

 

[Guest]

You are mixing conduction and radiation. Assuming radiative heat transfer was zero for a moment, you are correct, adding an insulative body around an object can only make the convective heat transfer worse.

However, when an object can only emit radiation with an emmisivity of 0.05 (polished aluminum), hurting the convective heat transfer by adding a thin coating that improves the emmisivity to ~0.4-0.6 can increase heat transfer via radiation significantly, enough to counter the negative effect of having a thin coating.

I will finish by saying that "I don't know" definitively that the improvement in radiation counters the harm done to the conduction to the surface and the convection at normal operating temperatures. However, THIS WILL improve the radiative heat transfer off the engine. Also there is evidence that this is used as a functional addition. Example: Big twin cylinders, which suffer far more than our motors when at a standstill.

PS - I only use this as a reason not to polish my jugs, not as a reason to paint the motor. I am far too lazy for that.

 

[jm_foote]

And i'll use this as an excuse to keep the grease and grime on my bike. Washing is for chumps!

...and to drink beer.

 

[Rock]

So if I wear a black shirt and pants in the sun I wil stay cooler than wearing light clothing?

 

[mbelan]

Guys ,guys, start up a bike with a black header,and one with a chrome header ,let cook for two minutes ,turn off.Wait one minute,, touch headers. The fingers on the chrome header will still be on the chrome header,and the black header will have dissipated the heat,leaving one good hand.This is the idea behind,wood-burning stoves(always black),radiators,(also always black).Black is not for holding heat ,but releasing it

 

[dpep]

There is something not right here. I am thinking there is a difference between black metal and metal painted black.

 

[gs54]

You guys are getting way to deep.

 

[Nerobro]

Um... Air cooled race motors ARE painted black. Race headers ARE painted black. It really does make a huge difference.

 

[beby99]

Black is Cool....or Hot...depending on which slang you want to use.

 

[crag antler]

Mr. Wizard!! Cool stuff! I'm curious why though if you find the data.

I love my black engine...

See. Now you got it figured out\\/ \\/ \\/