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heat retention, chrome VS powdercoat

77ironhead

Titanium
Joined
Jan 22, 2006
Location
maryland
wondering if anyone has any thoughts on or knows where I can see comparisons on heat retention of chrome (true 3-layer copper/nickel/chrome) versus powdercoating (real powdercoating, not the cheapo do-it-yourself harry homeowner kind)
 
Are you talking about something like headers? From what I've gathered there is a difference. The coating and color you use apparently is the same as if you were coating a car to protect the inside from heat. Chrome or white will reflect the heat regardless of which side it comes from, and black radiates heat. I've never seen the research though. I'ld love to hear from the experts though, as I've always had a little trouble getting my head around the idea of white paint reflecting heat back through the substrate it's coating.
 
What do you mean by heat retention?

Larry

heat retention as in headers, engine parts, etc. For instance, will a chrome-plated valve cover shed heat faster or slower than powdercoat?

Forestgnome-not so much worried about the heat absorbed from outside sources (if I'm reading your post correctly), more concerned with shedding the heat generated by an engine (altho anything that generates heat would be subject to the same Q)
 
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heat retention as in headers, engine parts, etc. For instance, will a chrome-plated valve cover shed heat faster or slower than powdercoat?

Forestgnome-not so much worried about the heat absorbed from outside sources (if I'm reading your post correctly), more concerned with shedding the heat generated by an engine (altho anything that generates heat would be subject to the same Q)

That's what I was saying. From what I've read, it doesn't matter whether the heat's coming from the inside or outside, the reflective or absorbitive properties are the same. Not to say I havn't had my share of chrome burns!:bawling:
 
I have stood next to glowing hot exhaust manifolds and turbochargers on trucks running on a chassis dynomometer many times. It is better for the sake of the tender rubber and plastic stuff around the engine (hoses and wires, for instance) if the exhaust heat goes out the tail pipe. But if you want to increase the temperatures under the hood, you could increase the surface area of the manifolds, like with fins.

Larry
 
Powder coat is a thermal insulator. That is, the polyester or whatever polymer it is made
from, is a poor heat conductivity. The metal in the plating is thinner and very high
thermal conductivity.

So the thermal impedance from the interior of the valve cover, to the surface, will
be lower for the plated one compared to the powder coated one. By how much,
hard to say.

The next issue as mentioned, and probably the more important one (given the relatively
thin powder coating) is the surface area and emissivity, in the IR as mentioned.
Probably the best would be black oxide, with fins to increase surface area. Think of
a heat sink for an IC chip. Black anodize, lots of fins.

You could probably look up the surface emissivity (measure of how easily a surface
gives up or accepts photons) for various materials and coatings. Often things that
look white, or shiny, in visible light, are actually quite 'dark' in the IR.
 
Is there any sort of airflow over the surface area of the part? If there is I would probably use a coating that would increase surface area and has high thermal conductivity. I personally would stay away from the polyester based powder coats for anything high temp.
 
As is so often the case in these questions, you won't give enough information. Do you want to keep the heat or get rid of it? If you want heat radiation with rust protection, use black chrome.

Bill
 
OK. Lessee if I can supply a few concepts. The problem is: heat transfers from warm places to cool places. Thermo-dynamisists have studied this topic to death producing acres of incomprehensible paper only they can understand. I'm not a physisit nor have I ever protereyed on on TV.

Heat (energy) flows from warm to cool (entropy).

Heat flow is obtructed by a barrier (radiator tube, cylinder wall or exhaust header.)

Heat can be transfer via radiation, convection, or conduction.

A barrier has several properties: specific heat, conductivity, emissivity, to name a few. All affect one way or another the passing of heat from one side of the barrier to the other.

Then there is heat itself and how it's manifested: Radiance (as from the sun), heated fluid (exhaust gas, coolant), latent heat (evaporation /condenation of liquids), heat of compression (air),

Phyical dimensions of the barrier. (400 miles of rock in the Earth's mantle is as effective a barrier to heat transfer as the walls of a cryogenic flask.)

Flow make a difference. If the heat containing fluid has low viscosity and velocity sufficient to produce turbulent flow more heat will be transfered per unit than high vicosity low velocity fluid and all permutations between.

With all these variable to master, quantify, and corelate, I guess thermo-dynamics can be pardoned for baffling the likes of me. I can repect it but I don't like it. I want undertanding NOW without the intervening study.

So - maximizing heat transfer from the metal parts of an engine to ambent air: look for very thin, high conductivity coatings having broad spectrim, high emmsivity/aborbtion characteristics. I understand "black" chrome is superior in this category. It's used in thermal solar arrays.
 
Thermo Lesson

The heat flux from the object is dependent on geometry, temperature of object and environment. so:
total heat transfer from object= Convected+conducted+radiated

I dont know what your object is (did I miss this?) but unless it is glowing you can ignore the radiated as it is insignificant.

so for greater heat transfer the surface should be rough, as large as possible and have air flowing past. It should also be made from a good conductor - usually a metal (unless we are talking exotic)

I am an engineer so I do these sort of sums fairly often but it is a bit dull to stick in here and pointless unless you can fill in the specific variables.
 
All I have ever read and heard related to heat and engine exhaust says that you want to keep the exhaust as hot as possible until it exits the exhaust pipe. Hot air is less dense than cold air, so it moves easier and faster. Also keeps engine bay temps low.
 
hey all, thanks for all the replies, got my answer.

To satisfy the inherent curiosity in a bunch of machinist/engineer types (who can't help themselves any more than I can ;) ):

I'm working on several different 'new' projects mostly aimed at the air-cooled V-twin market. Without going in to specifics, it seems to me that offering the most complete cost-effective package possible (finished product as opposed to semi-finished) will increase sales potential- but I don't want to offer options that will decrease longevity to my customer's engine life....so, powder-coated engine parts are effectively a non-starter
 
Have you Googled "Powder Coat"?????

From Wikipedia:
Powder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is typically applied electrostatically and is then cured under heat to allow it to flow and form a "skin." The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for coating of metals, such as "whiteware", aluminium extrusions, and automobile and bicycle parts. Newer technologies allow other materials, such as MDF (medium-density fibreboard), to be powder coated using different methods.

You don't mean putting a plastic coat on anything going above 250 degrees, do you?


My guess is you mean mean "Plasma Coating".

Since I don't know much about it, here is a shameless copy and paste, from:

http://www.strappe.com/plasma.html

- When is plasma coating useful? IMO, ceramic/plasma coating is most useful in aircooled 2stroke motors, nitro-burning 4strokers and anytime you MUST get every last ounce of long-term performance out of an engine. The parameter I look for is very very high piston temps and long run times - the more of either or both of these, the greater the benefit from plasma coating. I have not seen enough of a performance increase to make it worth the expense and trouble in a properly watercooled 2stroke engine not used in world GP racing, such as privateer Honda RS125s or RS250s. RZ350's are borderline cases - if they're really pumped (70 hp and up) I think it may be worthwhile, maybe. Use in 2stroke GAS dragsters is debatable as the motors generally don't overheat in a single 1/4 mile run, though they will overheat if you do several runs very quickly back-to-back. We've all seen the pics of the Kawi triple dragger from the mid-70's that had five gallons of water sprayed on it between runs to cool it down... I have seen very positive results in aircooled RD350s and RD400s on roadrace tracks (heh heh), and old Kawi triples where the center cylinder is in a poor thermal position. So contrary to what some folks have said on the Internet (always beware your sources!), plasma coating is not something I recommend all the time, but it sure is helpful when you need it.

To give you another benchmark/useful mental reference, every single jet airplane engine I know about uses ceramic plasma coatings in several high-temp places, such as combustion chamber coatings, compression fins and turbine. Why? These parts run while exposed to very high temps for very long periods of time. The plasma coating reduces the heat soak into the part and thus allows it to maintain tolerance for a much longer time. Less heat in with same cooling equals lower part temperature. It's all physics, folks.

- What exactly is plasma coating and where does it go? Plasma coating is really an abbreviated name, like Kawasaki is short for Kawasaki Heavy Industries Limited, Inc. One possible more proper name is "ceramic coating applied in plasma form". The word "plasma" actually means "superheated material" or "really friggin' hot!!!" as in 3000 degrees and up. (For a few years I played with rare gas plasmas in the range of 10,000 degrees F.) As ceramic is a material that becomes solid at a very high temperature, it must be heated to very high temp, usually 3-5000 degrees F and sprayed on. For use in internal combustion engines there are four useful places for ceramic/plasma coatings - the piston crown (NOT the skirts or undercrown!!!), combustion chamber, inside of the exhaust port and outside of the header pipes.

Also, doing ceramic plasma coating is a batch process, so you should expect per-unit prices to decrease with large lots. "Back in the day" we had our pistons batched in with jet engine parts, which cut the per-unit price in half compared to just doing a pair of pistons. Of course, a smart coater will know this and will have figured out how to batch and price to keep their business alive, so don't be surprised to hear "we only triple coat one day per week, and it's $X per part". Get them talking about their business, what they do and who they do it for, and you can tell if they're a good smart operator. If so, KEEP THEM!!!

- What benefit(s) do plasma coating(s) offer? In engine use ceramics are an insulator, meaning they do not absorb heat as quickly as metals, in fact several orders of magnitude slower than metals such as aluminum or iron. To our problems, ceramics offer two simple answers that reduce to "ceramic/plasma coating helps reduce engine metal heating from combustion". First: Reduce the heating of engine parts. Uncontrolled heat is the enemy of internal combustion engines. We want ONLY the combustion gasses to be very hot, and we want everything else to run at "design temperature" or "not too hot". The ideal behind plasma coating is to surround combustion gasses with an insulated shell. In uncoated engines this shell is the aluminum or iron of the cylinder head, piston crown, cylinder and exhaust port. In an ideal plasma coated engine, all of these except the cylinder would be coated with one to three coats (depending on the part) of ceramic/plasma coating to contain the heat and keep it from heating the metal of the engine. This is good because when metal gets hot it expands. Aluminum expands a lot when it gets hot. Pistons are made from a variety of silicon-aluminum alloys. The more it expands the tighter clearances get. Motor parts are designed to run at a specific temperature which results in clearances becoming "as-designed". When you work to get more power out of an engine it runs hotter and these clearances shrink. If they get small enough (thinner than the film tolerance of the oil being used), the adjacent parts begin to rub against each other as metal-on-metal instead of metal. In 2stroke land we affectionately call this "seizing", which I don't think needs further explanation.

Second: Reduce the heating of the incoming air-fuel mix (intake charge). Every engine builder eventually runs up against the problem of intake charge heating. The cooler the intake charge, the more dense it is. More dense means it carries more oxygen and gas molecules per cubic centimeter, or per cubic inch if you're working on American 2strokes. Intake charge is heated by compression in the intake path (airbox, carb, port and crankcase) - we can't do anything substantial about this short of running refrigerant lines in these areas. What we can affect is the heating of the intake charge in the crankcase, which is VERY significant. The hottest thing in the "box" surrounding the intake charge in the crankcase is the piston crown. We've all seen and diagnosed overheating by looking at the color of intake charge that is burned against the BOTTOM of the piston crown. Doesn't burning the intake charge before it gets to the combustion chamber sound like a bad idea?! Reducing piston crown temperature reduces the heating of the intake charge and thus maintain a higher intake charge density in the crankcase.
 








 
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