Emissivity of Aluminium (Engines)

[Guest Andys@coffs]

Guys

 

I was playing around with an IR thermometer the other day and looking at a characteristic that needs to be set on the instrument and that is emissivity.

 

So the theory......

 

A perfectly matt black item has an emissivity co-efficient of 1.0. that means it has a reflectivity co-efficient of 0.0 since the sum of the emissivity and reflectivity co-efficient always total 1.0. Many engines are painted Matt black for a reason, you want to emit as much heat as you can.

 

Aluminium has varying emissivity co-efficients, using the table I found here (http://www.monarchserver.com/TableofEmissivity.pdf and http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html ) I can see that highly polished Aluminium has a E of 0.03 to 0.06 depending on the temperature of the material. heavily oxidised between 0.2 and 0.3 and Anodized (which our engines aren't) 0.77

 

In the end Aluminium as a radiator of heat is pretty well about the worst material you could have chosen......

 

Weathered Stainless steel (thinking exhaust here....) is 0.85 and cast iron around 0.7, but with blacking higher.....

 

So I wonder why our engines which have problems of heating aren't treated so that they are not as they come out of the CNC machine.... Apparently (Bottom of Page 13 of http://core.materials.ac.uk/repository/eaa/talat/1501.pdf ) treatment with Electoplated black raises the E to 0.95, which would seem to be a much better alternate to what we have.....

 

Anyone else looked at this or tried treating their engine (19 Rego only) to see what impact on cooling it has?

 

In the last PDF link I provided reading pages 12 (Bottom Para) and all of 13, gives a good understanding of why aluminium, but suggests that there was more that J needed to do to make it a good option....

 

Andy

 

P.S Im no engineer and I may well be completely missing something.....forced air cooling may not rely on Emissivity.....but I think it does

 

 

[Guest Andys@coffs]

When we land and turn everything off the heat that is in the engine slowly is dissipated, and the cowls can get hot to touch but not enough to burn....If emissivity was higher then the cooldown might happen a lot faster (and shock cooling might be more of an issue......) which might mean that the glass cowls get too hot........

 

 

[Powerin]

I haven't read the links yet, but wouldn't emissivity only account for the heat actually radiated away from the surface? Whereas an air or water cooled engine is relying mostly on conduction...ie air or water actually touching the surface and absorbing heat energy and taking it away with it. Does conduction rely on emissivity?

 

 

[facthunter]

If radiation was relied on more to cool the engine it would be a bigger factor. The engine is cooled by forced airflow.so is contact + a radiation factor. The point is made about heat within the cowl. If the cylinder and heads were anodised / painted black matt, they would certainly cool better. If the fins were not so squared off at the top where the airflow comes from there would be a better airflow and better cooling, achieved. Nev.

 

 

[Guest Andys@coffs]

Hey Good catch Powerin...Your right, and another post (which wouldn't cut it for an academic reference!) is this one which seems to pretty much address this issue. http://www.newton.dep.anl.gov/askasci/eng99/eng99494.htm

 

Thanks for the pointer....

 

Andy

 

 

[damkia]

I would like to take the contrary view that it is in fact a very good material to use in engine manufacture. That low emissivity means that the heat of combustion is actually held in the cylinder as increased gas pressure, improving efficiency. By all means, after the combustion process is complete (exhaust) then dissipate the heat as needed, and be aware of the maximum working temperature of the aluminium in the cylinder head/components.

 

 

[facthunter]

Of course keeping the engine hot will improve efficiency. It does not however allow it to continue running reliably IF the temperature gets too high, destructive things occur. Like lubrication problems, detonation, ( a long claimed problem with these engines) Distortion and the need for large running clearances and piston tops may even melt and valve tops come off because the excess heat reduces their strength. Theory has to apply reality as a limiting factor. All hot engines run into metallurgical and lubing problems . Through the development over the years these limits are pushed up, but at any time are the main limiting factor. Nev

 

 

[eightyknots]

Something that hasn't yet come up in this discussion is the conductivity of the material itself. Aluminium conducts heat away from where the combustion takes place. Aluminium is one of the best heat conductors: without looking it up, copper and silver are common elements which are better but only marginally so. Once the heat is conducted away from the source, the surface area of the cooling fins take over and the greater the airflow, the better the heat dissipation.

 

I agree with Andy's post No 2. If aluminium engines were anodised to radiate heat more efficiently, the inside of the cowls would receive the additional radiation and could become too hot for the fibre reinforced plastic to handle.

 

I have another question about cooling which I have often wondered about. Going back many years, some aircraft designers used the plane's surface for cooling. Imagine cooling hoses being piped to the aluminum fuselage which has quite a bit of surface area and then a matrix of cooling pipes were fitted to the inside of the fuselage. This may help cool the engine and, at the same time, take away the need for a cooling radiator at the front which introduces quite a lot of drag on an aeroplane. The lowered drag should help the additional weight required to take cooling to (and on) the plane's surface.

 

What do you people think?

 

 

[Zibi]

I have another question about cooling which I have often wondered about. Going back many years, some aircraft designers used the plane's surface for cooling. Imagine cooling hoses being piped to the aluminum fuselage which has quite a bit of surface area and then a matrix of cooling pipes were fitted to the inside of the fuselage. This may help cool the engine and, at the same time, take away the need for a cooling radiator at the front which introduces quite a lot of drag on an aeroplane. The lowered drag should help the additional weight required to take cooling to (and on) the plane's surface.

What do you people think?

Unless you restrict that cooling system to the wings, you're going to be heating up the cabin, which gets hot enough in here anyway.

Also unless you use very thin pipes, you're looking at quite a bit of coolant and that also means quite a bit of weight.

 

Another thing you'd have to add is some good pump, to push the liquid through those pipes, as otherwise you'd end up with a lot of heat near the engine, and cool coolant in the pipes.

 

 

[facthunter]

Surface radiators were used for some liquid cooled engines but I don't think they were considered a howling success. A well designed cowl can utilise some of the heat for thrust but this is not achieved often. The need to cool on the ground is a factor in the design. Adjustable cowl flaps are a consideration as you can keep the engine warm on descents and get the minimum drag at other times. Air cooling suits aeroplanes better than most other vehicles for obvious reasons. You don't need a dedicated fan to force the air over the engine. ( generally). Nev

 

 

[Yenn]

It is not heat that caused detonation, but detonation causes the heat. The high pressures occur too early in the cycle and build up the temps.

 

Painting is not necessarily a good way to remove heat from aluminium. I know black gives off more radiation, but the heat has to get from the aluminium to the paint before it is radiated. The paint is another layer between the heat and the cooling air.

 

 

[facthunter]

It has to be the right paint or anodising. Tests have been done along these line years ago at universities. The smoothness of the surface has an effect too . Shiny surfaces do not radiate heat out well. Recall the polished teapot holds the heat longer Dull and dark is the go for max radiation of heat.

 

.Yenn. re heat caused by detonation. By the time that happened the engine would be long gone. Detonation does destructive damage as soon as it happens. Aircooled engines running at higher temps are much more inclined to experience it than liquid cooled motors that generally run a little cool of what is desired due to the coolant type. ( Normally contains water + glycol) with the boiling point not raised that much when you go to a higher altitude the pressure cap lowers the boiling point there because it operates relating to the ambient pressure. Nev

 

 

[cooperplace]

GuysI was playing around with an IR thermometer the other day and looking at a characteristic that needs to be set on the instrument and that is emissivity.

 

So the theory......

 

A perfectly matt black item has an emissivity co-efficient of 1.0. that means it has a reflectivity co-efficient of 0.0 since the sum of the emissivity and reflectivity co-efficient always total 1.0. Many engines are painted Matt black for a reason, you want to emit as much heat as you can.

 

Aluminium has varying emissivity co-efficients, using the table I found here (http://www.monarchserver.com/TableofEmissivity.pdf and http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html ) I can see that highly polished Aluminium has a E of 0.03 to 0.06 depending on the temperature of the material. heavily oxidised between 0.2 and 0.3 and Anodized (which our engines aren't) 0.77

 

In the end Aluminium as a radiator of heat is pretty well about the worst material you could have chosen......

 

Weathered Stainless steel (thinking exhaust here....) is 0.85 and cast iron around 0.7, but with blacking higher.....

 

So I wonder why our engines which have problems of heating aren't treated so that they are not as they come out of the CNC machine.... Apparently (Bottom of Page 13 of http://core.materials.ac.uk/repository/eaa/talat/1501.pdf ) treatment with Electoplated black raises the E to 0.95, which would seem to be a much better alternate to what we have.....

 

Anyone else looked at this or tried treating their engine (19 Rego only) to see what impact on cooling it has?

 

In the last PDF link I provided reading pages 12 (Bottom Para) and all of 13, gives a good understanding of why aluminium, but suggests that there was more that J needed to do to make it a good option....

 

Andy

 

P.S Im no engineer and I may well be completely missing something.....forced air cooling may not rely on Emissivity.....but I think it does

what is needed is numbers re the proportion of heat removed by air molecules taking heat away from the engine compared with the heat loss by radiation. I suspect that by far the former is greater. if not, wouldn't every maker of air-cooled engines use black anodising? Putting it another way, did Wright bother to black anodise the R3350, which was so beset with overheating problems?

 

 

[ianboag]

Air going past the fins picks up heat by forced convection. The faster the air is moving, the thinner the boundary layer (resistance to flow) so the more heat gets transferred. The heat gets from the engine to the fin surface by conduction then into the air by convection. Radiation in a confined space goes from a source to a sink - source cools by losing heat and sink warms up. If there was significant heat loss by radiation it would make the baffles hot.

 

I would guess that if you let the fins "see" the outside world and paint them black then there would be more heat loss by radiation BUT the airflow round the fins would be less because it is no longer confined in a duct. So you can have (a) black paint, motor in the outside world (no ducts) or (b) silver with motor in a duct (baffles). The world seems to go with (b).

 

 

[aj_richo]

VW operators have long reported the stock dull black steel rocker covers are much better at dissipating excess heat than shiny after market aluminum jobs, fins or no fins

 

 

[facthunter]

I think you would find that having black fins would dissipate more heat cowled or out in the open. Rougher surface with very thin coat works best. Polished is worst. Silver copper and aluminium are good conductors. Normally when something conducts electricity well, it conducts heat well too. High thermal gradient gives better heat dissipation Nev

 

 

[Bruce Tuncks]

Yes you would be better off with everything matt black, but not by that much. And you could do more harm than good if the paint acted even slightly as an insulation layer.

 

In a Jabiru engine at 170 degrees C, if you were to paint it black, there would be a gain in radiated energy from about 200 watts (at emissivity = 0.2) to about 1000 watts (at emissivity = 1) . Now at 12kg/hour avgas usage, there is 147,000 watts of energy input. Some of this energy goes out as work and some out the exhaust pipe as hot gas, but a lot goes into heating the engine, which is mainly cooled by transfer of heat from the metal surface to the flowing air at the junction between the air and the cooling fins. The cooling gain from the black paint would only be 1 or 2 percent of this airflow cooling effect, so if you want to improve your cooling, the airflow past the fins is the place to look at.

 

I wrote an article about this for the magazine last year asking the editor how it could be improved for publication, but didn't get a reply. It was obviously too technical and therefore boring. You get better technical stuff here anyway.

 

regards, Bruce

 

 

[gregrobertson]

Builders of hot rods discovered many years ago that chromed diffs were much more difficult to cool and could easily boil the diff oil. There is a very good reason why many car parts eg radiators and diffs are painted black.

 

Greg.

 

 

[facthunter]

I think Adelaide university did a fair amount of work on this years ago. The finish and colour have a lot to do with it. Surfaces that reflect seem to emit least. A polished teapot stays hot much longer. Black is where no light is reflected. White is where all colours are reflected. Infra red (heat) is a wave motion like light. Nev

 

 

[Bruce Tuncks]

Yes that's right about shiny polished and matt black things. I once ran a practical class where 2 identical urns (except for the surfaces) cooled and the poor students had to calculate the cooling rates by theory and then show this agreed with their measurements.

 

One thing about diffs and teapots is worth noting though, and that is they are not enclosed. If they were completely enclosed, the radiant heat would warm up the insides of the enclosure and then some would be re-radiated back.

 

regards, Bruce

 

 

[facthunter]

That is true Bruce, but in a cowl (Which may have some silver paper near exhausts to stop the cowl getting hot), the cowl generally will not be very hot, unless the engine is stopped on the ground ( Probably not much above ambient) The rate of heat transfer is related to the temperature difference in the system considered. This helps aircooled engines as they run much hotter than liquid cooled ones.. The situation of air flowing over engine parts and cooling them is complex but Jabiru are the only engine I know where the surface finish is ex machined left"shiny". The small Continental and Lycoming engines normally operate without an oil cooler quite satisfactorily. Nev

 

 

[sain]

wouldn't you get a higher gain in heat dissapation from having a rough finish on the engine, rather than a smooth, shiny (painted black or otherwise) finish?

 

seems to me the higher surface area would be much more effective.

 

I don't recommend taking a belt sander to your shiny new jabiru engine though...

 

 

[facthunter]

Dull rather than "rough" the rough surface more area sounds good but it's probably a marginal thing. An as cast finish is common . I'm going by memory but the wavelength of the heat( infra red) and the finish has some relationship. Nev

 

 

[Bruce Tuncks]

Good point about those engines with no oil coolers Nev, that helps explain why Jabiru didn't have an oil cooler at first.

 

And yes my cowl has heavy aluminium foil ( the stuff you get in those disposable baking pans) stuck on near the exhaust.

 

regards, Bruce

 

 

[facthunter]

It's only the smaller ones Bruce and certainly not the turbocharged ones. Nev