Thermal exhaust wrap

[jrthomas]

Do any of you have your exhaust wrapped? Any thoughts pro or con? James Thomas Mod 4/912ul.

[Dave S]

James,

Haven't done it; and, not likely that I ever will on an airplane - but that's just me.

I know this is done on cars in some cases (hot rods, etc W/ Headers) however, the pipe materials are much heavier and most cars don't continuously operate at high power settings...even rolling down the freeway, few car engines are producing more than 15% of rated power - meaning a whole lot less heat out the exhaust compared to what we see on an airplane.

I'd be leery of doing this. Here's why. The wrapped exhaust pipe will operate at a much higher temperature - unless we have empirical data as to what that temperature will be and what effect it will have on the pipe regarding strength and integrity; also, considering the intensity and frequency of vibration the pipe is exposed to....and then know the limits of the metal the pipe is made of under those conditions (it's thinner that car pipes for weight saving), it's dealing with a lot of unknowns.

If a person could view the pipe where it is coming off the engine when cruising with 75% or 85% power you'd see that it glows - it's red. the pipe holding together is in part dependent on the pipe shedding heat as well as the nature of the metal it is made of.
Quite a lot of heat is shed off the exhaust - the exhaust gas cools quite a bit on its rather short trip through the pipes and muffler due to heat loss through the pipes.

If there is a need to insulate other under cowl components from the exhaust pipe heat, another strategy is to insulate the other components with ceramic fiber wrap, a metal shield, or similar.

On our aircraft, in one case, an aluminum shield was fabricated to reflect the exhaust heat away from a nearby component.

Sincerely,

Dave S
Kitfox 7 Trigear
912US Warp Drive

St Paul, MN

[Esser]

My exhaust is ceramic coated which does the same thing just a little bit better and a lot more money (I didn't pay for it, it came with the engine). I don't know how it performs as I have yet to fire it up.

Why do people want to heat wrap or coat their exhaust? (BTW I am not expert, this is the word on the street)

When parts get red hot (like turbo housings) and then cool down they go through a thermal cycle which over time can degrade the metal. Ceramic coatings help reduce the thermal cycle and reduce surface temperatures. It does this in a variety of ways.

Exhaust gases that flow through your system are extremely corrosive and can eat away at your exhaust system from the inside out. A good coating can be applied to the inside of pipes to help make sure the exhaust gases and moisture either exit quickly or are burned off

On turbo components the thermal cycle process can go from glowing red to cool. Over time this can weaken the cast housing. A high heat ceramic coating can provide protection, keeping the heat internal to the system.

On headers and exhaust piping the bends and curves where heat can concentrate at can thermally degrade the metal over time. These hot spots can also develop pin holes as exhaust gases erode the inner substrate.
So a good high heat coating over 1600 degrees F can protect your exhaust and turbo components from the problems associated with thermal cycling.

Ceramic coatings help increase the speed and maintain the quality of exhaust gas flow. It also helps with thermal tumbling which happens when hot spots develop in the metal due to thermal cycling. By coating your headers you can make sure that you have a smooth laminar flow of exhaust.

Finally you get rid of heat soak, which can in turn affect other parts of the engine. By keeping the heat in the pipes you lessen the risk of excess heat damaging other engine parts like plastic parts, wiring harnesses, and spark plug wires. My engine is fuel injected so this can help with vapour lock issues.

[jiott]

Go to the Rotax-Owner.com website and click on forums. Do search for wrapping the exhaust and you will find quite a bit of discussion on it. My recollection is that the general opinion is that it is a good thing if done properly for many of the reasons given by Esser.

[jrevens]

Specifically regarding wrapping, I'm with Dave S. There are situations where it can be helpful under the cowl, but my considered opinion is that it is not particularly good for your expensive exhaust system. The pipes are actually going to have a greater range of thermal cycling if wrapped - the pipe itself will get hotter. With the pipes being hidden under the wrap, cracks or corrosion that are forming can be hidden, etc.

As far as ceramic coatings go, there is some controversy about the brittleness of the coating actually causing a greater possibility of cracks developing. One thing is certain - if you do have to repair the system by welding, it's a real pain to try to remove the coating... many welders will not even consider doing it. I know this from experience. Exhaust gas downstream of a turbocharger is no hotter than it is without a turbocharger... in fact it's possibly cooler.

I prefer lightweight thermal shielding if necessary, as Dave suggested.

[Av8r3400]

To pull back just a bit, what is the benefit you would be looking for to wrap the exhaust?

Personally, I don't see the need for it. The risks of damage is more than any benefit potential. I would say the same for ceramic coating as well. Other than looking pretty (very pretty for sure!) the risk reward ratio is not there.

Just my opinion.

[Esser]

I think the theory behind is it it will cool slower and more evenly. But like I said, I am no expert.

[jrthomas]

I did a post on www.rotaxowner.com (http://www.rotaxowner.com) aimed at Roger Lee. I'm anxious to hear what he has to say. I don't have an opinion either way so I'm open to leaving the wraps on or taking them off. I'm flying the plane now with no issues. Thanks for your comments. James Thomas

[jrthomas]

I got an answer from Roger Lee. I consider him one of the best Rotax guys out there. He says that he knows a hundred others using thermal wraps with no issues and he has run wrap for over 8 years himself. He says that the advantage is that it cuts down on under the cowl heal, which I believe is a good thing and as long as it's done correctly shouldn't be a problem. James Thomas

[KFfan]

Are you able to expand regarding "done correctly"?
Thanks

[tx_swordguy]

I do not know the effects on airplanes, however on motorcycles wrapped exhaust was/is a style kind of thing. Problems are more related to moisture being either held or retained on the exhaust pipe and then cooked off during heat cycle. (motorcycle exhaust is exposed to water on the road and during washing much more so than an airplane) Not 100% sure of the how, but what is happening is the pipes end up rusting from the outside much quicker (even when chromed). Vibration + moisture+ heat all rubbing the outside of the pipe = rusted pipes. It seems much worse on the header part where the most heat is. I think if you can shield parts you want shielded you would be better off. That being said if you don't wash or get the wraps wet, it may not even be an issue.
Mark

[jrthomas]

Answer for KFfan. Quoting from Roger: "Bottom line, it's up to you but the wrap shouldn't hurt anything if it's installed correctly. By that I mean that the edges are not over wrapped by (more than) 3/8". If it gets over wrapped by half way then that's too thick and heavy and it will retain too much heat." James Thomas

[Kurt.A]

The very first time that I seen exhaust wraps was in a turbocharged automotive application. I discussed the theory behind it with the owner and was told that a Turbocharger is actually a heat pump. By that statement he meant that it was not the flow of exhaust gas that spins the turbine wheel but the expansion of the exhaust gasses that spins the turbine. I have since read over and over that this is the true theory behind turbocharging. With that in mind I can certainly see that holding the heat inside the pipe prior to the turbo would increase the efficiency of a turbocharged application. With regards to a naturally aspirated eng the proposed merits is that the exhaust gasses have a higher velocity which may assist with better scavenging of the exhaust from the cylinders. In my opinion an insulated exhaust system has eng performance merits to be considered along with reduced under cowl temperatures thereby allowing your cooling system to be more efficient. With regards to the negative aspects of reduced exhaust system life that is for each to evaluate. My 912 came to me with exhaust wraps, I see no signs of abnormal degradation in the 700 hrs on the clock.

[jrevens]

Kurt,
With all due respect to you and the person you talked to, it certainly is the flow of exhaust gases that spin the turbine. It is the pressure differential between the inlet and outlet of the turbo that increases the velocity of the gases flowing through it and increases the kinetic energy of the gases that are impacting the rotor blades and driving them. If the exhaust gases leaving the engine can be kept from cooling off before they reach the turbocharger by wrapping, then they will remain at a higher pressure than if they were allowed to cool (all things being equal), which will result in a greater pressure differential and more potential kinetic energy driving the rotor. That being said, I believe that the difference in performance is probably tiny in this instance. You'd have to test the engine on a dynamometer to see. On the other hand, wrapping the pipes downstream of the turbo will increase the temperature and potentially the pressure of the exhaust gases in that part of the system, which could decrease the differential across the turbocharger & decrease efficiency. There are many other factors that come into play of course. IMHO, the only really practical reason to wrap would be to decrease temperatures in the engine compartment.
700 hours is a good long time and a testament to the probability that there is no harm being done to your system. I'm curious though - have you un-wrapped it to see if any corrosion is occurring on your pipes? Stainless steel is much less vulnerable to it in this instance of course. I wouldn't even consider wrapping if I had a mild steel exhaust system, but that's just my opinion.

[KFfan]

Answer for KFfan. Quoting from Roger: "Bottom line, it's up to you but the wrap shouldn't hurt anything if it's installed correctly. By that I mean that the edges are not over wrapped by (more than) 3/8". If it gets over wrapped by half way then that's too thick and heavy and it will retain too much heat." James Thomas
James
Thanks for the clarification

[jmodguy]

Theres a guy in FL that has a Rotec radial on a RV-8. He wrapped his exhaust and somehow it got some oil on it. Nearly caught fire so he pulled it all off. Yikes!

Jeff

[Kurt.A]

Kurt,
With all due respect to you and the person you talked to, it certainly is the flow of exhaust gases that spin the turbine...

John you typed what I wanted to say far more eloquently. I definitely used the word flow in poor context. I did unwrap the exhaust and took a pic of the results for you. One part of the equation to take note of is I am the third owner. The person I bought from did say that the exhaust was wrapped when he bought the plane at 400 hrs.

[jrevens]

Thank you Kurt, although I don't think I did a very good job and I'm sure there are some inaccuracies in what I said, but I appreciate your kind words, and being able to see the unwrapped pipe... it looks pretty good, doesn't it?