Cylinder Barrel Coatings, Alloy Castings -- Heat Build Up??

Jim Comstock posted a You-Tube vid about testing iron cylinders with different coatings -- the result of this testing showed no difference in the temperature of several test barrels with either no paint or with a few other test coatings. Painting the barrels did not interfere with heat dissipation in iron barrels.



Is there any info about the heat performance of an alloy barrel, considering a paint coat versus no coating? I'd like to paint one black to match the "Combat" look. If it's OK to paint, suggestions for a good paint type?

Thanks, BY

prep is important for lasting good looks.

Blewdy Yaink said:

Jim Comstock posted a You-Tube vid about testing iron cylinders with different coatings -- the result of this testing showed no difference in the temperature of several test barrels with either no paint or with a few other test coatings. Painting the barrels did not interfere with heat dissipation in iron barrels.

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The final temperature measured (about 110 'F or 43 deg C) is far too low to differentiate the heat transfer of various coatings. Heat load should have been doubled, at least. In doing so, I think there will be a noticeable difference between silver and black paint. Jim recorded the rear side temperature to be 143 'F (62 'C). Keeping the ambient conditions constant is a challenge in all kinds of testing. Air speed of the cooling fan wasn't recorded. Another issue is the heat flux. In reality, most of the heat is generated in the upper part of the bore, or transferred from the cylinder head. Evidently, the heat conduction of cast iron is low, thus application of the heat flux is important. The test does not replicate this fact.

- Knut

No hard answer but I read a study one time on the value of painting Triumph alloy cylinders black. The study found no significant difference between naked and painted black. It was in a magazine, and I haven't found it online.

the main benefit is lack of rust, IMHO.

Black maximizes RADIATIVE heat transfer. This is a scientific law: The Stefan-Boltzman law of radiation. The heat transfer from a cylinder is composed of three functions Conduction, Convection, and Radiation. Radiation becomes an increasing factor at higher temperatures.

Slick

Although black paint is therefore the best paint… is it better than no paint (on alloy)?

I can see your radiation point, but exactly how much is the gain, vs how much insulating effect the coat of paint will provide ?

texasSlick said:

Black maximizes RADIATIVE heat transfer. This is a scientific law: The Stefan-Boltzman law of radiation. The heat transfer from a cylinder is composed of three functions Conduction, Convection, and Radiation. Radiation becomes an increasing factor at higher temperatures.

Slick

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Not an expert - just reading a lot of articles on this. In general:
1) Painting increases radiation with the dark colors doing slightly more. https://www.avweb.com/ownership/improving-engine-cooling-by-painting/
2) Radiant cooling is minimal compared to convection cooling, especially in a finned, moving, air-cooled engine. Sitting still changes things. Multiple non-authoritative sources.
3) Anodizing a dark color improves radiation cooling without any insulative effect (aluminum alloy can be anodized). Multiple non-authoritative sources.
4) Powder coat or thick smooth paint is bad as it smooths out the rough surface and reduces convection cooling. Multiple non-authoritative sources mostly related to Harley's

I prefer the look of black cylinders and have always painted them black even on bikes with alloy cylinders. Today, I use VHT Barrell Paint. On alloy cylinders, just enough to make them black. On Norton cylinders, enough to keep them from rusting.

Fast Eddie said:

I can see your radiation point, but exactly how much is the gain, vs how much insulating effect the coat of paint will provide ?

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"Insulation" is a matter in the heat-up phase only as trhere will be a temparature lag. Once a steady thermal state has been achieved, a paint layer does not offer a temperature drop vs. bare metal.
Radiative heat transfer will probably play a major role at the rear of the cylinder barrel.

- Knut

mdt-son said:

"Insulation" is a matter in the heat-up phase only as trhere will be a temparature lag. Once a steady thermal state has been achieved, a paint layer does not offer a temperature drop vs. bare metal.
Radiative heat transfer will probably play a major role at the rear of the cylinder barrel.

- Knut

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interesting point about the steady state situation regarding insulation, I wasn’t aware of that.

However, I still wonder ref the benefits of increased radiation vs increased insulation as an air cooled engine when in use, in most situations, is very seldom at a steady state, accelerating and decelerating all the time with heat build up rising and falling. I such cases I can still visualise the added insulation causing an average increase in latent heat?

When I had my motor built, the builder (same guy who built that fast SBR racing Commando a few years ago) recommended HPC for the barrels. So had that done.

Brooking 850 also has his Maney repro exhausts HPC coated,

Such coatings are very good indeed. But you gotta be careful with what you ask for…

The coatings applied to exhaust pipes are designed to insulate! The coatings on engine parts need to be designed to dissipate.

Make sure the person applying the stuff knows what’s what !!

hi all,
I’m extremely dubious that adding a layer of paint of any colour would increase cooling.
as the vast majority of cooling occurs from the physical contact of the air flow, I suspect that applying paint is just puting an extra layer between your motor and the air that cools it. Even when stationary I believe the majority of the cooling occurs through convection. To prove the point, deny your engine airflow and see how long it takes to reduce it to a molten lump of metal.
Temperature is interesting as the Lycoming engines I operate have a maximum recommended cylinder head temperature of 450F and an absolute red line of 500F which to me is insanely hot. I prefer to keep them below 400F if possible.
As the temperature increases, valve guide wear becomes significant, followed by valve tuliping and failure and eventually metallurgical changes.
regards
alan

Alan L said:

hi all,
I’m extremely dubious that adding a layer of paint of any colour would increase cooling.
as the vast majority of cooling occurs from the physical contact of the air flow, I suspect that applying paint is just puting an extra layer between your motor and the air that cools it. Even when stationary I believe the majority of the cooling occurs through convection. To prove the point, deny your engine airflow and see how long it takes to reduce it to a molten lump of metal.
Temperature is interesting as the Lycoming engines I operate have a maximum recommended cylinder head temperature of 450F and an absolute red line of 500F which to me is insanely hot. I prefer to keep them below 400F if possible.
As the temperature increases, valve guide wear becomes significant, followed by valve tuliping and failure and eventually metallurgical changes.
regards
alan

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Alan,

See the link I gave in post #8 - right up your alley, I think.

If you are painting it yourself this is proven, brush or spray.

POR-15 Rust Preventive Coating- Gloss Black -1 pint ... Amazon​

Amazon product ASIN B00H2VVM8E

marshg246 said:

Not an expert - just reading a lot of articles on this. In general:
1) Painting increases radiation with the dark colors doing slightly more. https://www.avweb.com/ownership/improving-engine-cooling-by-painting/
2) Radiant cooling is minimal compared to convection cooling, especially in a finned, moving, air-cooled engine. Sitting still changes things. Multiple non-authoritative sources.
3) Anodizing a dark color improves radiation cooling without any insulative effect (aluminum alloy can be anodized). Multiple non-authoritative sources.
4) Powder coat or thick smooth paint is bad as it smooths out the rough surface and reduces convection cooling. Multiple non-authoritative sources mostly related to Harley's

I prefer the look of black cylinders and have always painted them black even on bikes with alloy cylinders. Today, I use VHT Barrell Paint. On alloy cylinders, just enough to make them black. On Norton cylinders, enough to keep them from rusting.

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I concur fully with Greg on points 2 to 4, point 4 being especially bad. All these points have their basis in scientific law. With regard to point 1), dark colors have an emissivity of about 0.9; silver and gold about 0.3. Thus the radiation heat transfer of a black color to silver will be 0.9/0.3 or 3X. Just to cover all possibilities, some modern super paints get emissivities over 0.9 regardless of color, but unless you are a NASA engineer looking for a specialty paint, do not expect to find it at a neighborhood store, or a price you can afford.

All very scientific.

Slick

marshg246 said:

4) Powder coat or thick smooth paint is bad as it smooths out the rough surface and reduces convection cooling. Multiple non-authoritative sources mostly related to Harley's

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This is correct.

Out of curiosity, I made a simplifed calculation of the loss by having a green sand iron casting surface (e.g. , a cooling rib) coated. Assuming an average roughness of 300 um (which is about as fine as it gets) and a sinusoidal roughness distribution in an x-y planar frame with a vawe length of 3mm (which is a coarse approximation), the area difference to a completely level surface is 19.3 %. Reducing the average roughness to 200 um reduces the area difference to 8.7 % .

How much do peaks and dales contribute to convection? The air passing by is assumed to be turbulent, unfortunately the turbulent flow field close to a rough surfaces isn't fully understood by the scientific community.
Roughly speaking, the horizontal turbulent air flow induces further turbulence in the dales. It's well known that turbulent flow enhances heat transfer due to the high circumferential speed of swirls. This is captured by the heat transfer coefficient (h), which is part of Newton's law of cooling. Isaac Newton took (h) to be a constant but it's not. Even for smooth surfaces, the coefficient (h) shows a dependancy of the air speed. For rough surfaces, the dependency (h) of v (the horizontal air speed) is very strong. An example: A fin with moderate roughness (Ra=22 um maximum roughness) was exposed to vinds ranging from 3.4 and 15 m/s (ok, modest vind speeds for a motorcycle). The computed heat transfer coefficient rose from 72 to 230 W/m2/K.

In Newton's law of cooling, the area (A) is the projected area.

Downside of painting cast cooling ribs isn't the diminished area, it's the reduced secondary turbulence. Hence, it's advisable to apply a thin paint layer.

In summary, to obtain the highest possible convective heat transfer of an air cooled engine, you should

* maximize the finned area A
* use separate barrels which are better for cooling than joint castings
* keep or enhance roughness of cooling ribs
* keep the barrels unpainted (which is possible when using certain aluminum alloys)
* duct the engine, ensuring even rearward ribs receive fresh air
* use enforced cooling by fan (as VW, Porsche did).
* use a casting material with a high thermal conductivity coefficient k which evens out the difference between well cooled and less well cooled areas
* operate the engine at low ambient temperatures (moist seaside air cools better than hot desert air).

- Knut

Knut states:
The air passing by is assumed to be turbulent, unfortunately the turbulent flow field close to a rough surfaces isn't fully understood by the scientific community.

No so! It is resonably well understood by aerodynamicists, as I am.

The overal heat ransfer rate is governed by conduction in series with convection. Conduction heat transfer is what is happening within the material itself. Within a solid material such as a cylinder barrel, heat is conducted by the intermolecular transfer of energy from molecule to molecule. In effect, molecules nearest the high temperature region vibrate at higher frequency, this vibration excites the neighboring molecules which vibrate at a higher frequency than before, but not as high as the first exciter molecule. The excited molecules then excite their neighbors to vibrate, and again the excited never become as energetic as the excitor molecule. This results in a linear temperature drop within the material moving from the hottest to the coolest part.

Now consider the air actually touching the exterior surface of a hot material such as a cylinder fin. This air is comprised of a very thin laminar sublayer that is for the most part unmoving, or for practical reasons "stuck" to the fin. Heat is conducted thru this thin layer of air by conduction, or a process of one molecule exciting its neighbor, just as within the body of a solid material. Unmoving air is actually a good thermal insulator; thus this laminar sublayer is undesirable from a heat transfer perspective.
At some small height, the sublayer begins to move but remains laminar. Laminar means the fluid is in layers, the layers higher (more distant from the solid material base surface) move faster than the layer below, but no mass transfer occurs between layers, that is, air within a layer stays in that layer. Heat transfer in the layers occurs via conduction from molecule to molecule. As long as the sublayer remains laminar, heat transfer from layer to layer is governed by conduction.

Moving more distant from the base material, the laminar sublayer becomes unstable and mass transfer begins to occur, that is, air begins to mix between layers. This begins the turbulent flow region of the boundary layer. Mixing of hot air with neighboring air is many times more rapid heat ransfer than that by conduction alone. Thus, transition from a laminar to a turbulent boundary layer is to be encouraged, and the greater the scale of turbulence (the size of the mixing vortices), the greater the mixing effect, and the greater the heat transfer.

A rough surface breaks up the laminar sublayer and starts the turbulent boundary layer closer to the base surface, greatly enhancing heat transfer by reducing the thickness of the conductive sublayer region. Therefore do not polish cylinder fins, or apply a thick paint which tends to smooth the surface.

Slick

marshg246 said:

Not an expert - just reading a lot of articles on this. In general:

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Thank you, Greg. This is very clearly explained and helpful. Excellent info (with references).

This is an interesting thread to which I have a couple of questions:

Quote from @mdt-son "In reality, most of the heat is generated in the upper part of the bore, or transferred from the cylinder head. Evidently, the heat conduction of cast iron is low"

• Presumably a copper head gasket better in terms of thermal transfer in each direction than it's composite equivalent. In a real world everyday bike could this be the reason for any common issues we see such as blown head gaskets, soft heads or loose guides etc.?
• How is the laminar layer affected when you have a hot surface dissipating heat but in reasonably close proximity to another trying to do the same? Is it better to have less but larger fins for example?

Cheers,

cliffa.