Thermal barrier reflective coating?

[Andrew Tinsley]

Hello All,

  Over on SSW, there has been a gentleman offering thermal barrier reflective coatings. One claim is that it cures the OS 40 FP runaway situation, because it is caused by overheating.
  Anyone any idea what this special coating is. I have been in the business of thick and thin film manufacture for many years. As a physicist, I know that there is no better way of radiating any form of electromagnetic radiation than the classic Plankian black body radiator. That being so, then the best way of cooling an engine (strictly by emitted radiation) is to paint it matt black with one of the high temperature paints. Seems a cheaper way to do the job than paying $40 US for this "thermal barrier reflective coating" for a Brodack 40 cylinder head.
  Maybe I am missing something here, or maybe I should just ask the gentleman who is advertising this service?

Regards,

Andrew.

[Brett Buck]

  Maybe I am missing something here, or maybe I should just ask the gentleman who is advertising this service?

    I asked for a picture. And in any case, I would think anything like a "thermal barrier" is likely to make it run hotter, not cooler. I would also be very surprised if any appreciable fraction of the cooling is done by radiation. So I share your puzzelement.

    Brett

[Marvin Denny]

  I agree with Brett!!  Any "reflective" barrier will simply "reflect" the heat back into the engine rather than radiating it out into the cooler atmosphere.
  As to the "merely coating the engine" with a high temperature paint, if the engine already is painted with something which is NOt a thermal absorbant type, the addition of another coating on top of it will do little if any good.   First remove the old coating, deoxidize the bare aluminum parts to be painted, chemically treat with an alodine,iridite, or similar chemical conversion coating  THEN paint with Flat Black high temp ENGINE enamel.
  I know --- that ia a lot of work and that still mighe not cure any "run-away" problem.

  Bigiron

[Howard Rush]

Perhaps it actually works, and perhaps it's because it makes the head warmer.  Coming up with erroneous theoretical explanations for real phenomena is not unknown at the highest levels of stunt.

[Andrew Tinsley]

Marvin,
You are quite correct in pointing out that any previous coating on the engine may have low thermal conductivity and hence may limit heat transfer. I was merely pointing out that the best surface to radiate heat is a genuine "black body" black. Perhaps wrongly assuming that the finish was an aluminium casting (very good heat conductivity).
  I am not sure if Howard is supporting the concept of "Thermal barrier reflective coating" or simply poking a little fun at the comments so far! You never can tell with Howard!
  If you want to go really technical, I could design a multilayer thin film stack that would give near 100% emissivity from say 0.8 to 5 micron in the infra red spectrum. About as good as you can get and better than black paint! It would however be a very complex film stack and I can't see how I could do it for $40 US per part, unless I coated at least a 1000 items in one go!
  Brett's observation that very little of the cooling of our engines takes place by radiation. I agree, the temperatures involved are nowhere near high enough to provide much radiation cooling. The whole thing smells strongly of something and it isn't infra red!
  To my mind there is no way that this Thermal barrier reflective coating can be of any use to our model engines. If you know any different Howard, let us know!

Regards,

Andrew.

[frank williams]

When we moved to ABC and AAC sleeves, I've always worried that our heat transfer was suspect.  The ABC fit of sleeve to case is not the interference fit of the steel sleeve and aluminum case.  The ABC/AAC fit is loose when cold and we are to believe that it is a good tighter fit when warm, snug enough to provide a good transfer of heat from sleeve to the case fins.  The only sure transfer contact point is at the head where all three are bolted together, sleeve, head, and case.  The head probably carries most of the transfer to the atmosphere for an ABC.

The Johnson and McCoy integral sleeve - fins was for sure a good transfer of heat from the engine to the atmosphere.  The PA does this a little with a couple of rows of fins on the sleeve. 

[CircuitFlyer]

I have worked with the thermal sprayed coatings for several years.  I've applied a ceramic type of thermal barrier coatings to aircraft turbine blades using a plasma flame spray system.  I think this gent is applying a high temp water based ceramic-metallic type of paint to the inside of the cylinder head to reflect the heat away from the head and back into the combustion chamber.  Works in real aircraft and race car engines.  It might be worth a try.

[Howard Rush]

Aha!  We were all assuming that the coating was on the outside. 

[FLOYD CARTER]

The REAL answer is GLYCOL!  Build a jacket around the cylinder and pour in the glycol.  The Browning 30 cal LMG used a water jacket, but glycol is better.

Floyd

[Howard Rush]

I asked Matthew on which side of the head the coating goes.  He answered, "coating is applied to entire head insie and out, execpt plug threads."

[Brett Buck]

  I agree with Brett!!  Any "reflective" barrier will simply "reflect" the heat back into the engine rather than radiating it out into the cooler atmosphere.

  Maybe that's why it works, he could just be seeing a real phenomenon and guessing wrong on the mechanism.

   Brett

[Andrew Tinsley]

Hello Silver Dart,

  I have also worked on coating jet turbine blades, but used electron beam gun evaporation system in vacuum conditions. The whole object of that exercise was to reflect heat away from the turbine blade.
  If you apply the type of coating that you used to say a model engine cylinder head on both the outside and the inside, then you will reflect heat back into the cylinder (from the coating on the inside). The coating on the outside will reduce the emissivity of the surface and prevent heat from being radiated away. The result will be increased temperature in the top of the engine. Usually this is what you don't want!
  As Brett observed, the amount of cooling via radiation at the relatively low cylinder head temperatures that we use, is likely to be small compared to the forced air cooling that will predominate.
  Looks to me like a good technical solution for some applications, but actually achieves the reverse of what we require for our toy engines. It should increase running temperature very slightly rather than reducing it.

Andrew.

[Brett Buck]

  If you apply the type of coating that you used to say a model engine cylinder head on both the outside and the inside, then you will reflect heat back into the cylinder (from the coating on the inside). The coating on the outside will reduce the emissivity of the surface and prevent heat from being radiated away. The result will be increased temperature in the top of the engine. Usually this is what you don't want!
  Looks to me like a good technical solution for some applications, but actually achieves the reverse of what we require for our toy engines. It should increase running temperature very slightly rather than reducing it.

<<some snips for brevity>>

     To be honest with you, I am not so sure we need to reduce the temperatures, or that is necessarily beneficial. Particularly in the head. One of the biggest common issues with the current engines is not that they overheat (we are running AAC 75s at <.5 hp) but that the ignition is inconsistent. We seek out the hottest possible plugs to keep the thing going in inside corners. And you get more power per unit fuel consumption if you burn it at a higher temperature. So in general I can't see a lot of good reason to improve the cooling.

     Brett

[CircuitFlyer]

I find it odd that the coating is applied to the outside a well.   I would think the thermal conductivity of bare aluminum should be ideal to transfer the heat to the cooling airflow.

[Steve Helmick]

A (very) fine point of traditional ABC/AAC/ABN engines is that the head protrudes down into the cylinder bore, and is a snug, piston-like fit into that bore. The head being hottest (and aluminum), it will expand more than the cylinder, and this forces the cylinder bore to stay round. It also stretches the bore at the top, which allows for that tight "pinch" in the last 1/4" of stroke. If the head is too loose, or the cylinder taper a bit much, then the engine will tend to flame-out, usually as it cools in the down-strokes of the RWO, Hourglass, OH8, etc.

This has been a problem with new ABC/AAC/ABN engines with good "pinch", almost always cured with a more running, a head with a tighter fit in the cylinder, or retaining more heat in the head. The Brodak .40 is an engine that frequently has this "problem", and it can usually be solved by reducing the cooling on the head. Therefore, I think the coating/probably reduces cooling through the head, making it expand more at running temp, thus stretching the top of the cylinder more.

The one thing I can't explain is why some newer designs have the head sitting on top the cylinder, not sticking down into it, and thus not keeping the cylinder round.  I've used only two of this sort, the Magnum XLS .36, and the ST G.51 (which is not A/?/? of course...sad, that!).  So why do they run so very, very well? 

One of my first concerns (as a machinist for 35 years), is how much is this "2 mills"?  It should probably be "mils", but most commonly (plastic sheet or film), one mil is .001". If the head is already a good fit, and you put .001" (on each side, it seems?), then it probably wouldn't fit into the cylinder without a very big hammer. If he means 2 millionths, or even 2 tenths (.0002"), then it may still fit, and I think that's the intended definition. Recent issue RO-Jetts have black cylinder heads, and quite a bit of "pinch", OBTW.  Steve

[Doug Moon]

uh...what???  The runaway is caused by the timing.  Cut it and doesnt runaway.  Or Block the boost port. 

[Andrew Tinsley]

Hello Doug,

  Presume you mean runaway on the OS 40 FP? If so then I couldn't agree more. If this reflective thermal barrier coating works as claimed, it would simply heat up the cylinder head a little more. If anything, that would help make the runaway somewhat worse in my humble opinion.

Regards,

Andrew.

[Howard Rush]

If this reflective thermal barrier coating works as claimed, it would simply heat up the cylinder head a little more.

I am a little confused by Andrew's claims.  I hope he posts his heat transfer calculations for us.  In his classified ad, he asserts, "ceramic coat is like thermal sponge".  My interpretation of this is that the coating has a heat capacity not dreamt of in your philosophy, although one not too far removed from that commonly attributed to castor oil. 

[Howard Rush]

One of my first concerns (as a machinist for 35 years), is how much is this "2 mills"? 

Two Bridgeports, for example.

Hope this helps.

[Brett Buck]

Two Bridgeports, for example.

    He said that it was 2 millionths of an inch. I am skeptical for a home-applied material.

     I really don't know what to make of this. My detector, however, is going off like a fire alarm

     Brett

[Andrew Tinsley]

Hi Howard!

  You seem to have got me confused with the gentleman selling the thermal barrier reflective coating! His name is Matthew Jeffery Mutz. The coating seems to be a low tech spray coating. If so then thickness control would be very difficult to achieve. The only way that such a coating can work is simply that it alters the emissivity of the surface that is coated. There is nothing magic about it. I assume that the emissivity for infra red would be low hence infra red would be difficult to be both emitted and absorbed. As others have said, it would make very little difference for our stunt applications.
  I assume that this technique has probably been developed and used for some form of competitive motor sport. It may well give a measurable benefit in such applications.

Regards,

Andrew.