Swift 94 fuel fails trial at UND

[DXB]

This is interesting - they switched back to 100LL due to excess valve seat recession.   Particularly disappointing is the lack of reduction on plug fouling.  I wonder how 100UL will fare…

https://www.avweb.com/aviation-news/citing-valve-damage-und-drops-unleaded-fuel-and-returns-to-100ll/?fbclid=IwAR13GIS5xzSIEX9W1ewJoa9La-ZAsYdXP5rJPVylJa2aHbKHWrjfDuPYWNk

[hammdo]

Wow, I have Swift STC too…

[Pinecone]

A couple of things.

1)  As the comments point out, they did not have a control group still running 100LL.  And they had not been doing the extensive monitoring when they were running 100LL.   It will be interesting if they continue the monitoring once they switch back.

2)  Not sure what they teach about leaning.  Many big schools teach to never touch the red knob.

[EricJ]

29 minutes ago, Pinecone said:

A couple of things.

1)  As the comments point out, they did not have a control group still running 100LL.  And they had not been doing the extensive monitoring when they were running 100LL.   It will be interesting if they continue the monitoring once they switch back.

That's an enormous school that flies many thousands of hours a year, so their previous history was an effective control group.

[GeeBee]

There is some question about the carb UND uses vs the original Marvel carb which is worth investigation. The Avatar carbs operate leaner than the Marvels at high power settings.

 

[Shadrach]

Can someone enlighten me as to the relationship between leaning and valve recession? 

[GeeBee]

10 minutes ago, Shadrach said:

Can someone enlighten me as to the relationship between leaning and valve recession? 

As the mixture leans, approaching stoichiometric proportions the exhaust temperature gets higher. The valve returning to position, pounds the seat. The supposition is without lead as an intermediator and consequently conductor of heat at the valve to valve seat interface, erosion occurs.  At lower power setting not a problem. At climb or take off power it can be a big problem, especially if the engine turns high rpms where the exhaust valve spends less time against the seat cooling. One of the reasons why when going for LOP you should be at 65% or less and do not linger in high EGT regions during lean. I.E. "Big pull". 

[Andy95W]

I wonder if the additives in G100UL serve as an acceptable intermediator in lieu of the lead?

[Shadrach]

1 hour ago, GeeBee said:

As the mixture leans, approaching stoichiometric proportions the exhaust temperature gets higher. The valve returning to position, pounds the seat. The supposition is without lead as an intermediator and consequently conductor of heat at the valve to valve seat interface, erosion occurs.  At lower power setting not a problem. At climb or take off power it can be a big problem, especially if the engine turns high rpms where the exhaust valve spends less time against the seat cooling. One of the reasons why when going for LOP you should be at 65% or less and do not linger in high EGT regions during lean. I.E. "Big pull". 

Thanks for the explanation. The theory seems plausible with regard to recession.  As a reason for the “big mixture pull” on the other hand, seems more myth than reality.

[GeeBee]

1 hour ago, Shadrach said:

Thanks for the explanation. The theory seems plausible with regard to recession.  As a reason for the “big mixture pull” on the other hand, seems more myth than reality.

Well, none other than Mike Busch noted that people who spent a lot of time leaning rather than going straight to a given FF (big pull) had more cylinder problems because you spend to much time in the red box.

[wombat]

How can the plugs get fouled if there is no lead in the fuel?  I've been told that the fouling was lead from the fuel.

 

@GeeBee How do you figure that the exhaust valve spends less time against the seat at high RPMs?   I would think it would spend the exact same percentage of time no matter the RPM.

 

The idea that there is liquid or solid lead passing through (or condensing on)  the exhaust valve and providing a cushioning effect and as a thermal conductor  is interesting.   The boiling temp for lead is 3.180F, but coming out of the fuel solution and tetra-ethyl compound it  could easily be vapor that condenses on the valve.

Edited November 10, 2023 by wombat

[GeeBee]

 

"Another factor is RPMs. The lower the RPM operating range, the better the heat transfer rate to the seat."

https://generalaviationnews.com/2022/11/23/what-causes-exhaust-valve-recession-in-an-engine/

[wombat]

@GeeBeeThat may be true, but I don't think it's because it spends more time against the seat.

 

 

Also, a note about that article..... I don't know any tractors that produce significant power (> 20HP) that are gasoline..  I grew up on a farm and while I don't know everything about agriculture these days, the fact that he's talking about gasoline tractors makes me suspect something isn't right.  Also, most tractors run at 1,800 RPM, not 1,200.   So that is a bit weird too....

Edited November 10, 2023 by wombat

[McMooney]

wow, just wow.  70's and 80's all over again.

[ArtVandelay]

Well, none other than Mike Busch noted that people who spent a lot of time leaning rather than going straight to a given FF (big pull) had more cylinder problems because you spend to much time in the red box.

Only if you are leaning at high power (>65%) and ignoring high cylinder temperatures.

[Utah20Gflyer]

I thought that Lycoming switching to hardened valve seats a long time ago made lead unnecessarily for anything but octane rating.  I’ve never heard about people having this issue from running Mogas in planes that are approved for it.  I’d like to see more details about this problem.  

[EricJ]

30 minutes ago, Utah20Gflyer said:

I thought that Lycoming switching to hardened valve seats a long time ago made lead unnecessarily for anything but octane rating.  I’ve never heard about people having this issue from running Mogas in planes that are approved for it.  I’d like to see more details about this problem.  

I think it says that in the article, that the seats had been hardened, but there's not been a lot of experience with unleaded fuel.   Aviatable mogas is nearly unobtainium due to the ethanol content in most automotive pump gas.    So the UND experiment is a good data point for seeing what sorts of things can happen.   I'm sure it's a solvable issue, and much better to find things out from early deployments like this than from the populace at large.

[GeeBee]

14 hours ago, wombat said:

That may be true, but I don't think it's because it spends more time against the seat.

 

What every VW bug owner and in fact the factory manual says. Increase the #3 exhaust valve gap an extra .002 on the number 3 cylinder to increase its time against the seat for cooling because #3 runs hot. Yes, those few milliseconds make a difference. If you've ever run a high performance engine with solid tappets, you know how critical time against the valve seat is to exhaust valve longevity. Too much time and performance is lost, too little and the valve overheats. I used to set valve lash with a dial indicator.

[GeeBee]

6 hours ago, ArtVandelay said:

Only if you are leaning at high power (>65%) and ignoring high cylinder temperatures.

I will quote directly (I will also note you can be inside the red fin on some engines as low as 60%):

When we reach top-of-climb, level off, and commence the cruise phase of the flight, we perform a “big mixture pull” (BMP) to transition from ROP to LOP. This should be done quickly to minimize the amount of time spent inside the red fin (and especially the ultra-abusive purple zone). About 2 or 3 seconds is about right for the BMP. Note that we lose a bit of power as we transition from ROP to LOP; that’s normal and expected, and will be reflected by a small loss of airspeed. I recommend NOT using the “lean-find mode” of your engine monitor when doing this, because this requires you to lean very slowly to locate peak EGT. That results in spending a considerable amount time inside the red fin (and the dreaded purple zone), which is exactly what you don’t want to do. If you feel compelled to locate peak EGT, it’s much better to perform a quick BMP to get into the LOP zone below the fin, and then slowly richen to locate peak EGT from the lean side. Personally, I don’t care about locating peak EGT, so I skip this step altogether.”

— Mike Busch on Engines: What every aircraft owner needs to know about the design, operation, condition monitoring, maintenance and troubleshooting of piston ... Airplane Maintenance and Ownership Book 2) by Mike Busch

 

[A64Pilot]

Disregarding Mike Busch and leaning, but lead has been known forever to prevent valve recession (fancy name for wear)

The cure for valve recession is hard seats and Sodium filled exhaust valves (in automotive engines), of course we have run unleaded fuel in Autos for over 50 years, it’s just not an issue in autos.

I’m theorizing that maybe it didn’t work on aircraft for two reasons, Air cooled heads likely therefore higher cyl head temps, and secondly higher continuous power settings (higher cyl head temps)

I have no idea what the valve seat temp is on a water cooled engine is, does anyone know?

I do know that classic autos without hardened seats do suffer from valve recession, but Stellite valve seats can be fitted and that cures the problem.

However having said that many a C-85 and other small aircraft engines have been operating for decades on unleaded Auto fuel with no issues, but maybe they are lower stressed motors?

None of these alternative fuels have really been fully tested, and likely can’t be due to the huge cost in doing so, there are too many variables, soaking fuel lines and O-rings etc in a jar at room temp only tells you that the fuel in a jar at room temp doesn’t hurt them, but the darnedest things happen in an aircraft in flight, things that weren’t tested.

The only “real” test (in my opinion) is putting the fuel in fleets of aircraft in different operating conditions and putting hours and years on them, so it’s likely in my opinion that we will in truth be the testers.

[N201MKTurbo]

I just read a patent from Chevron concerning industrial engines run on natural gas. They operate at high power settings for long periods of time and have no deposit forming additives in the fuel. They suffer from exhaust valve recession. They cured it with oil additives that would put enough ash in the cylinders to lubricate the valves.

It seems the best additive for this is ZDDP, but they said it would foul the catalytic converters that all the engines had. They came up with a formula that would lubricate the valves and not foul the converter.

It seems from reading this that either a new oil formula that wasn't ashless, or a fuel additive would solve the problem.

I have a feeling that putting one quart of automotive oil in your crankcase every oil change would probably solve the problem. Even though I'm sure a more aviation friendly solution would be better.

https://patents.google.com/patent/EP2398878A2/en

 

[A64Pilot]

ZDDP of course is THE additive that would most likely solve our cam problems too.

Unfortunately it’s not ashless and an engine that burns oil it will leave deposits, so we shouldn’t burn it. Classic hopped up cars add ZDDP to their oil or their cams would be eaten up, modern Auto engines almost exclusively run roller tappets because ZDDP and many other additives have been eliminated from oil in order to protect pollution control devices and of course O2 sensors etc.

ZDDP is a type of zinc and I think Phosporous?, Zinc among other things neutralizes acid in oil as well as an excellent high pressure lubricant, I think from memory it deposits a layer on metal that is sacrificial in wear?

https://www.speedwaymotors.com/the-toolbox/zddp-content-chart-high-zinc-oil-list-brand-breakdown/32479

[A64Pilot]

12 minutes ago, N201MKTurbo said:

It seems from reading this that either a new oil formula that wasn't ashless, or a fuel additive would solve the problem.

Since the 70’s there have been lead replacement additives marketed, I suspect most are snake oil though, no idea how many if any work.

I remember one I think was called “instead o lead” or similar.

Redline is usually a respected brand, who knows?

https://www.redlineoil.com/lead-substitute

[A64Pilot]

I’m not saying we owners should add anything, perhaps the fuel supplier should though

[1980Mooney]

17 hours ago, wombat said:

@GeeBee How do you figure that the exhaust valve spends less time against the seat at high RPMs?   I would think it would spend the exact same percentage of time no matter the RPM.

 

17 hours ago, wombat said:

@GeeBeeThat may be true, but I don't think it's because it spends more time against the seat.

@wombat Your statement is correct. 

• Take an engine operating at 2,100 RPM.  Every 2 revolutions, a given valve will open, close and then spend a certain amount of time closed and against the valve seat : call it T seconds
  • In one minute the seat and valve will touch for 1,050 x T seconds
• Increase RPM 20% to 2,520 and that same valve will open, close and then touch 20% more times or 1,260 times.  However the amount of time that it remains closed and touching the seat will be reduced to 1/1.2 % : hence it will only touch for 0.8333 x T seconds
• At 2,520 RPM total time touching (excluding inertial effects of spring closing at higher velocity)  will be approx. 1,260 X 0.8333 x T = 1,050 x T second : i.e. EXACTLY THE SAME total time as at 2,100 RPM

Assuming that both engine RPM's are with WOT, at 2,520 RPM there will be approximately 20% more fuel/air passing through the valves with 20% more energy that has to be dissipated by shaft HP and residual heat in the 20% greater exhaust flow (i.e. higher power setting).  Therefore, the valves and valve seats experience a higher "heat transfer rate to the seat" at higher RPM.  Otherwise the heat will stay in the valve resulting in melting.  In order to increase the transfer rate, the valve will have to get hotter. Rate of transfer (conduction rate) has nothing to do with how long it touches.

"Heat transfer rate to the seat." (assuming identical physical valve and seat design and dimensions) is driven by the temperature of the valve and the thermodynamic property of the valve and seat materials".  Air/fuel ratios are generally similar in a narrow range for a given operating regime ROP or LOP (not wanting to debate the merits of either right now) - that means at higher power settings exhaust valves run hotter.