Exhaust Valve Inspection

[Area-51]

Interested on what people may think of these two exhaust valve images? Is this possibly lead fusion from running avgas? Combustion chambers and piston crowns all present well.

[facthunter]

Liquid cooled heads on a Rotax keep the parts too cool to properly handle 100 LL. Uneven build up on the valve SEATS can affect compression. Sometimes  that's temporary and will recover when you go back to Mogas.  Nev

[onetrack]

I've never heard of "lead fusion" on valve faces, and I can find no technical references, or online discussions about it. The generally accepted principle is that the lead compounds in avgas (primarily TEL or Tetra Ethyl Lead) change under the heat and chemical reactions going on during combustion, to form Lead Oxide deposits on combustion chamber and valve surfaces.

I have never seen pure lead deposits in a combustion chamber, because combustion process temperatures are normally high enough to vaporise metallic lead, and because there are a wide range of chemical interactions going on in the combustion process, metallic lead will rapidly form other lead compounds, which are often erroneously referred to as "lead deposits". They are lead compounds, and other chemical compounds, deposits; not metallic lead.

 

Ethylene Dibromide is added to Avgas to scavenge undesirable deposits of Lead Oxide, and when Ethylene Dibromide reacts with Lead Oxide, it forms Lead Bromide, which produces the light brown/ash-coloured deposits we see in exhaust system components.

 

It's a little hard to tell from the photo what the irregularities are on the flat part of the valve face. Are they removable deposits, or are they deposit build-ups or disruptions of the valve face metal? The recessed section of the valve face shows distinct metallic erosion and pitting, which is normally caused by contaminants in the fuel combustion process, such as water.

 

An engine ingesting very moist intake air, takes in a lot of water in vapour form, and erosion and pitting on combustion chamber components is created when the water is heated into steam, and the tiny steam bubbles then implode, eroding the metal. It's essentially cavitation taking place during combustion.

The water and steam in the combustion chamber also cleans off any protective deposits on combustion chamber components, thus exposing the raw metal surface to chemical and cavitation attack.

Acids are also formed during the combustion process and these also adversely affect the metal surfaces exposed to them.

 

Below is a link to a Shell company article, discussing lead fouling in aviation engines. I believe it's a worthy read to add to ones knowledge bank.

 

Lead Fouling | Preventing Lead Fouling in Aircraft | Shell Global

WWW.SHELL.COM

---

This article discusses lead fouling, what causes it and the most efficient way to prevent it in aviation engines. Read the full details here.

 

[facthunter]

Lead works like a flux and if it's not hot enough on the surface it doesn't lubricate the SEATs. It does NOTHING for the valve guides.  Nev

[Area-51]

Thanks for the technical information there. I did not factor in the water aspect regarding pitting. Would this not display across all four cylinders though?

 

The deposits do not scrape off; at least not with light pressure. They are resembling crows foot aspects which I would associate with thermal shock.

 

The more I get into this engine the less I am confident it has only run for the 147hrs logged. The valve throats are consistent with normal oil ingestion from the turbo shaft seals to induction side; nothing too extreme but not what would be expected at 150hrs TT. And then there is the lifter face condition previously found to consider as well. But the exhaust system appearance is consistent with 150hrs usage. Maybe some rascal got to it and swapped out a couple of the heads at some stage; who knows. At least its getting a thorough scrutiny and attention before spinning up again.

[facthunter]

The seats are what counts and if the stem has stretched.   The valve TEL was developed in the Late 20s do enable more power to be obtained by enabling higher boost or higher compression to be used with out detonation, Rotax Valve material and the seats (Inserted) should be hard enough to last ok.. Nev

[Area-51]

Thanks, all other aspects are within specification and condition.

 

Will put a new set of exhaust valves in for insurance.

[facthunter]

Good idea but they aren't cheap.  Check no seats  have moved in the heads. Nev

[Area-51]

2 hours ago, facthunter said:

Good idea but they aren't cheap.  Check no seats  have moved in the heads. Nev

Cheaper than a destroyed engine. Seats are good 👍

[Ian]

On 25/10/2024 at 12:02 PM, onetrack said:

An engine ingesting very moist intake air, takes in a lot of water in vapour form, and erosion and pitting on combustion chamber components is created when the water is heated into steam, and the tiny steam bubbles then implode, eroding the metal. It's essentially cavitation taking place during combustion.

The water and steam in the combustion chamber also cleans off any protective deposits on combustion chamber components, thus exposing the raw metal surface to chemical and cavitation attack.

I'm not too sure about the cavitation explanation and physics given here. Steam bubbles don't implode in gaseous mediums.

Cavitation is a process that only occurs in liquid phase and the collapse of vapor bubbles creates extremely high energies which damage materials.

https://en.wikipedia.org/wiki/Cavitation

 

Water or Water/Methonol injection is commonly used in engines and only in exceptional circumstances will it cause erosion of metallic components.

What is more common is erosion of turbine components due to water droplets impacting high speed compressor components or with extremely high usage remove lubricating oils.

 

[Area-51]

From experience another aspect of engines running water injection is the combustion chambers are clean and free of carbon deposits upon removal of the cylinder head.

 

Maybe rust from sitting idle for extended period that burns off leaving an indentation as the hours go on? 

[facthunter]

Water or water Meth certainly cleans the combustion chambers.  Exhaust valves are usually pretty good stuff these days and sometimes the inlets are aluminised. Your biggest worry is a crack starting at the edge of the Head (RARE these days) or the stem stretching and getting  Cracks when it's too hot. Retarded, lean, over boosted or muffler part blocked due baffle detaching etc.  ALL  our mogas fuels have too much sulphur  and with a bit of moisture after burning will form sulphuric acid which is very corrosive. Nev

Edited October 29 by facthunter

[Freizeitpilot]

“all our Mogas fuels have too much sulphur”

 

Hopefully that will change on December 15 this year.

[onetrack]

I bow to Ian's superior knowledge re cavitation, that is news to me. However, I have seen similar valve head corrosion previously, and always blamed it on a cavitation process - but as he says, and as many other sources advise me, cavitation is only possible in a liquid state - so the only other potential reason has to be corrosion caused by some form of acid attack.

 

Sulphur certainly is high on the suspect list, but I was under the impression sulphur level in fuels had dropped substantially in recent years. 

 

Another area to consider is, if the engine was operated with a small internal coolant leak (via the head gasket), that is also a potential source of chemicals that could create valve head corrosion.

[Freizeitpilot]

Maybe a little drift from the thread, but here is the anticipated change in fuel quality for Australia. Of course Australia now imports the majority of our finished fuels which may or may not exceed this standard already.

Federal Register of Legislation - Fuel Quality Standards (Petrol) Determination 2024.pdf

[BurnieM]

15 hours ago, Freizeitpilot said:

“all our Mogas fuels have too much sulphur”

 

Hopefully that will change on December 15 this year.

0r 15 Dec 2025

[F10]

Dec 15...? Is this when we might start seeing lead free AVGAS? Rotax engines don't like LL AVGAS, as I understand it leaves deposits on the reduction gearbox overload clutch, never mind the plugs and valves. In E Vic, only two airfields offer MOGAS on tap as it were. However, as has been mentioned on this forum, MOGAS is not as stable as LL AVGAS with reference to shelf life and the petrol tanker delivering to your local car supply, might have carried a load of diesel just before! Being able to fill up with lead free at most airfields, will open up a whole new world to Rotax owners! 

[Area-51]

Corrosion from acid is definitely plausible; there is no head gasket on the 9XX engines or water jacket in the cylinders. Other than a crack in the head itself, that leaves atmospheric vapour/humidity as the source of moisture to create sulphuric acid, or previous owner running an ethanol fuel.

 

I found only one other similar question from 2013 regarding this valve head pitting on a 9XX; which received no responses. The engine had only 120hrs on it with a single exhaust valve displaying corrosion pitting and degradation of the face. Perhaps a valve coating issue at production?

 

Have ordered two new standard valves. It will be good to see this engine go back together. Glad it got stripped down for inspection.

 

 

[facthunter]

No exhaust valves have a coating that I know of except chrome on the valve stem sometimes. The valve is generally made of something fairly exotic to perform as a Eutectic metal. Early ones were silicon and chrome. (silchrome). to (run  at temps above red)  By comparison Inlet Valves don't have to be made of anything very special and some are aluminised on the surface. Some aero aircleaners are not so good and some dusts are corrosive when moist. Some of what's mistaken for just carbon has a fair bit of dust with it. That bakes onto EX valves with a light brick colour and some anti scuffing additives in the oil   might contribute to that also.   Nev

[Area-51]

5 hours ago, facthunter said:

No exhaust valves have a coating that I know of except chrome on the valve stem sometimes. The valve is generally made of something fairly exotic to perform as a Eutectic metal. Early ones were silicon and chrome. (silchrome). to (run  at temps above red)  By comparison Inlet Valves don't have to be made of anything very special and some are aluminised on the surface. Some aero aircleaners are not so good and some dusts are corrosive when moist. Some of what's mistaken for just carbon has a fair bit of dust with it. That bakes onto EX valves with a light brick colour and some anti scuffing additives in the oil   might contribute to that also.   Nev

I'm going to have to correct you on this. Contemporary high performance valves do come in a few different alloys these days along with coatings to mitigate a variety of combustion ed related badness... Maybe some water went back up the exhaust when the AC went into a ditch and caused thermal shock of the valve face (crazing) 🤷🏽‍♂️; which is puzzling to me.

[Area-51]

Got the Nikon out tonight for a closer look at the rocket science; definitely corrosion right across the board... so no thermal shock as thought...image of outer valve head surface thought to be crazed but it does appear to be raised rather than pitted in this area; something bonded, something eroded... It's all just electrical plasmodium in the end!

[Area-51]

The valve stem is magnetic. The valve head is not magnetic.

[Ian]

The environment that valves operate in is extremely harsh and the metallurgy complex. A quick summary of what they do to address this is https://www.intervalves-technologies.com/exhaust-valve/

The difference between nominal operating temperatures and temperatures high enough to damage the valves is fairly small. Modern car engines, being computer controlled have smaller temperature excursions than the more primitive tech used in airplane engines.

My guess, and it is a guess is that this valve has been running hot at some point so you're seeing pitting in some areas, ablation and deposition in others. 

Rotax introduced sodium filled exhaust valves, which assist in valve heat removal, in some engines which indicates that the margins in their designs were close enough to warrant design changes.

[facthunter]

Sodium wouldn't do much in small dia valve stems. It also reduces the strength of the stem because it's hollow.. Do you have good evidence of Rotax using these valves?   I would expect a bit of  EXTRA attention might be required with turbo  charged models but the others aren't like racing car engines.   Some valve stems are welded to the head  by pressure and friction, which may be of something like Inconel which was used in some turbine blades I've has seats built up with Inconel as well as Stellite (tm).   If you are interested , look up G&S Valves UK . They are specialists and will make anything you ask for. Some exhaust valves are nitrided but that is not deep and they usually appear black. Jus saying"I have stainless" valves doesn't MEAN anything. Stainless GALLS like no other metal  and when it does it usually seizes or resembles a round file and finishes off the guide in no time.  Good valves are usually SWIRL ground to reduce the likelihood of stem cracking  achieve  a good shape for gas flow. .Nev

[Ian]

Liquid sodium has extremely good head transfer properties which is one of the reasons why they've been trying to use it in Nuclear reactors for a long time.

You're right in that it weakens the stem, however it has a significant cooling advantage.

Like everything, its a tradeoff and it would appear that some rotax's were having enough of an issue to accept the compromise.

Below is a link to more information.

https://www.rotax-owner.com/en/support-topmenu/40-uncategorised/658-si914030-914