Thermal advantage to coatings?

[Anonymous]

After reading a question posted in the 240/300 forum regarding ceramic coating of headers, I started looking for data to backup that somehow paying extra money to coat headers is going to improve performance.

Most sites I visited didn't have much for scientific explanation or data to backup their claims. Sure I understand the Main advantage is corrosion resistance, but placing a coating of a few mils thick doesn't seem to me to pan out when it comes to a REAL difference in performance.

Silicon Carbide has a TC of 71 W/m*K(sintered) while say SS 316 is at 16.2 W/m*K. So, the ceramic coating has less thermal resistance than SS. How exactly does this reduce under hood temps? If the concept is to make the header's thermal resistance high, why place a ceramic over it? Why not a material with a HIGHER thermal resistance than SS? Like something insulating...like wraps? Ceramic materials seem counter productive.

Another question that came up in the post is about coating the internal passage of the header. A couple of sites quoted increased exhaust velocity due to the smoother surface. Granted I understand that turbulence leads to greater frictional forces, but what kind of entry length are we talking about before the flow is developed and transitionining to turbulent given the assumption that flow starts as laminar?

On a different note, I can see the advantages to coating valves, chambers and pistons simply for better surface finish and dissapation of heat and less "hot" spots.

I suppose my question is: does ceramic coating of headers actually offer any kind of performance improvement? The exchange of heat within the working fluid and temperature wells seems to take place further upstream or am I missing something here?

 

[addo]

It's years since I did any thermofluid type thinking. That said - if the ceramic coating has a smoother finish inside the pipes than plain metal, won't the boundary layer be thinner? This should be most advantageous where the exhaust system commenced having a scavenging effect.

Added to that, the toughness of the coating material under thermal stress cycles, may mean that surface defects develop at a much slower rate than an uncoated header, prolonging effective life of the unit.

Is the coating actually silicon carbide? Just curious.

Also, you lost me with the assumption of flow beginning as laminar - I would have thought that as the gases are still "burning" slightly and expanding as they leave the ports, they are anything but laminar.

Final thought on the coating - cosmetic appearances are a high value item to many people. Lipstick may have a slight beneficial effect in reducing skin dryness - but it's main appeal is visual!

Looking forward to SR's thoughts on this (header coatings, not wearing lipstick ).

 

[rbohm]

8) ceramic coatings do reduce underhood temps. while heat transfer is high with ceramic coatings compared to what you mentioned, it isnt as high as say high temp paint, no where near in fact.

as far as exhaust flow goes, when it comes out of the cylinder, it is actually tumbling, and really doesnt straighten out for several inches, but it never becomes laminar in its flow characteristics. because of this tumbling, large tube headers allow maximum flow, BUT because of the pressure pulse reversion effect, same as on the intake side, you would lose flow, unless you used an anti-reversion cone at the beginning of the header. it reflects the pressure wave back down the pipe to the collector where it then helps pull exhaust out of the next cylinder.

 

[Anonymous]

Turbulent flow out of the cylinder is along the lines that I was thinking, so boundry layers are out the window.

I did read a little more after posting this question last night and realized that given the high temps radiation heat Xfer is the primary mode and ceramic coatings lower the emissivity. That said, yes under hood temps decrease. It doesn't appear to be significant on a daily driver, but maybe on a dragster sitting idling?

I quoted silicon carbide as just a generic ceramic to get a ball park idea of the thermal resistance. Most of the values out of a text I was looking at for ceramics were in the same region of values.

I'm thinking Addo is on about this, ceramic coating of headers is a balance between corrosion resistance and aesthetics. Performance value seems to be minimal unless you're running a turbo and want to minimize heat loss prior to the flow reaching the turbine.

 

[StrangeRanger]

FWIW at an SCCA Competition Clinic a few years ago, one of the engine builders noted that the addition of ceramic coating to the headers will increase exhaust gas velocity possibly requiring longer headers to keep the same tuning frequency.

I don't know anything technical about the thermal conductivity of coatings yet, except this bit of anecdotal evidence. I can put my hand on a Jet-Hot coated header. It's not exactly comfortable, but I didn't leave any skin behind either. The coating definitely reduces the amount of heat lost to atmosphere. As has been pointed out, this is way more important on a turbocharged engine than on a N/A one. The reduction of underhood heat should lead to an improvement in the life of underhood components, especially rubber hoses and electronic components. The increased ability to work underhood with a running (or recently shut off) engine is also a plus.

On a non-crossflow head with the intake and exhaust runners adjacent to one another, the ceramics should also reduce the heat added to the intake air. That should give a noticable performance benefit, even on the street.

I need to research a bit. Hopefully, I'll be back with more.

 

[rbohm]

8) i believe the engine builder is right. since oyu are keeping more heat in the pipe, exhaust velocity will increase. how much i dont know, but it adds another element to think about when tuning exhaust systems for a particular engine.

 

[StrangeRanger]

Temperature data here:
http://www.hpcoatings.com/technology/default.aspx

 

[Anonymous]

been something of an info hog on ceramics the last few days, mostly for air-cooled/avation purposes...

check out swain tech, too. looks like some seriously impressive coatings, wish i had something worth trying them on.
Ern

 

[Twinscrew]

The exhaust as it passes the valve is most certainly very turbulent, but quickly tries to stabilize and go laminer. How quickly and how laminer depends on several factors. Port shape and taper, surface roughness, misalignment between head and manifold, as well as continued combustion all play a large part. A boundary layer forms as soon as laminer flow develops; the thinner the higher the velocity. The more heat energy retained the higher the velocity as well. Ceramic coating the inside of mild steel or cast iron manifolds will greatly improve velocity if conditions will allow the exhaust wave to develop laminer flow before exiting into the exhaust pipe. My personal belief is that one of the biggest improvements realized from the coating is it's ability to return the resonant wave to the valve and promote scavanging. However, the resonant wave is a reflection of the original positive wave and if the positive wave is turbulent and distorted the ceramic coating will do little or nothing to improve performance. It does however, greatly lighten the wallet of the driver thereby improving the horsepower to weight ratio.

 

[StrangeRanger]

...It does however, greatly lighten the wallet of the driver thereby improving the horsepower to weight ratio.

Hey! That's my line!

So the biggest gains with the ceramic coatings (and virtually everything else) may very well not be in the coatings themselves but in the prep work you do before sending the manifolds/headers off to the coater? If the ports are as clean and as straight as possible and the port to manifold/header transition is a smooth as possible with no protrusions/obstructions in the direction of flow, yuo maximize the benefits of the ceramics.

 

[Twinscrew]

That's the reality of it StrangeRanger. The coatings no doubt lower under hood temps and minimize heat soak of the intake and so on and so forth. But to realize any measurable increase in performance everything else must be optimized first. Ceramic coatings are like the icing on the cake. If the cake itself is no good, all the icing in the world will not make it taste good. I hope you don't mind that I borrowed your line. It's actually what my father used to tell me every time I asked him if sometning would make my car go faster.

 

[Anonymous]

Keep the responses coming guys, definitetly a stimulating discussion. I like the icing on the cake analogy...like lipstick on a pig.

It appears for the cost, coating chamber parts and bumping up compression is a more solid way to go to improve performance when it comes to ceramic coating.

 

[Twinscrew]

So it would seem. However, coating the chambers must be done only after much calculation. I'll explain. When combustion is initiated a flame front develops which travels across the chamber space until it reaches the chamber wall which is made up of the head surface, valves, piston surface, and to a small degree the cylinder wall. Upon reaching the chamber wall, a pecentage of the heat energy from the flame front is absorbed and the balance of the energy is reflected. If the chamber is coated, the amount of energy that is absorbed goes way down, and coversely the amount of energy that is reflected goes way up. This is the desired affect with regard to minimizing the loss of the brake mean effective pressure that is created by the combustion. However the detonation threshold becomes a fine line that is very easy to cross. Here is an example: You decide to build an aluminum headed engine because the use of the aluminum heads will allow you to run higher compession and more advanced timing than if you had used iron heads. This is possible because the aluminum will absorb much more of the flame front energy and relfect much less than the iron, reducing the tendancy for detonation. If you coat the aluminum head surface you change the absorption and reflection properties and the same high compression engine would then be very prone to detonation.

 

[jamyers]

Twinscrew,

I'm considering coating the chambers in another engine project...got some links to more good reading about that topic?

 

[Twinscrew]

I do not have any links. However, the late John Lingenfelter did much testing of thermal barriers and has adressed the isssue in a few of his books. Same goes for Corky Bell. Do a web search and see what comes up.

 

[Anonymous]

I don't know anything technical about the thermal conductivity of coatings yet, except this bit of anecdotal evidence. I can put my hand on a Jet-Hot coated header. It's not exactly comfortable, but I didn't leave any skin behind either. The coating definitely reduces the amount of heat lost to atmosphere.

Agreed completely mate. I had a custom set of pipes made for my Crossflow Cortina a few years ago, and thought, what the hell, I'll get em coated, to see what its like. The HPC coating cost me something like 200$ at the time. I noticed a really significant reduction of underbonnet temps, so much so, that I could take the alfoil I had ziptied to my clutch cable off, and never had any further problems with them going bad.

 

[Thad]

The builders competing in the "Engine Master Challenge" seem to beleive in the various thermal barrier and anti friction coatings available, combustion chambers, piston tops/ skirts, exhaust ports, headers.

Smoky Yunic's work on containing/ controlling heat should be another source of related info

 

[Twinscrew]

I think that anyone building a maximum effort engine believes in coatings. I certainly do. This does not apply to most builds. And certainly not to builds that cannot be not considered high performance due to design limitations.

 

[Thad]

Reducing heat loss back to the engine will inrease power, keeping heat away from most engine parts and under hood conponents will increase reliability.

Keeping heat in the exhaust stream, keeps it from soaking back into the engine. We think of heat as a killer of engine parts when extreme but bear in mind heat lowers the strength and durability of parts at what is considered normal. Maybe not causing parts failure but it will increase rate of wear. Example: engine runs cooler, higher compression ratio possible, engine runs cooler oil is cooler, last longer and lubes better.

Note today the many things that started out as a perfomance market developement that wind up in factory engines. The list is very long. One lately is the beehive type valve springs couple years ago it was "hot rod", today 80% cars engines are made with.

We will see thermal and anti-friction coatings used by the car manufactures very soon.

 

[Anonymous]

"the aluminum heads will allow you to run higher compession and more advanced timing"

This might be true, but its not necessarily desirable. The auto manufactures went to aluminum heads to save weight. The aluminum transfers heat much better than cast iron. But heat is power. Heat is what makes the IC engine go. An aluminum head transfers power (heat) away from the combustion chamber, therefor, compression ratio and timing increase just to compensate for the loss of heat. At the same time, combustion chamber, port and valve design have improved for a net increase in power.

At least thats what I believe.

dean