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 Posted: Thu Feb 23, 2012 10:18 am Post subject: Sodium filled valves |
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"I disassembled a IO-520 Continental engine with 1700 hrs on it (TBO) and found the worst exhaust valve guide had only about .008" wear. Cylinder bore wear was barely .003" at the top in the choke area. The choke was gone and the cylinder was now straight, which according to Chuck Ney is better. No cylinder work or any other work besides spark plugs and magneto inspection was ever done on this engine. The engine in my Bonanza that was replaced by this one after overhaul went 700 hrs beyond TBO with very few cylinder problems."
Cliff, I have a customer with an engine coming up on 1900 hours. It's never been apart. It's already schedule for a LyCon overhaul next year. Should be interesting.
When I bought my Tiger, it had 1100 hours on it. I wanted new cylinders and was going to have Lycon overhaul the 1100 hr cylinders. The guides were fine. The exhaust ports were full of cracks. There was so much varnish on the inside of the engine from short flights and sitting long periods that I could scrape it out with my finger nails. Think 1956 Ford 312 that has never had the oil changed. It was awful.
I removed the sump and used a brass wire brush to scrape all the crap out. I then flushed 5 gallons of solvent under high pressure through all the galleys. 3 years later the oil pick up finger screen still had crud in it. Runs clean now. But, wow, not a good way to treat an engine.
Sodium filled valves have a long history. Solid stem valves even loner. Either method is suitable when setup correctly. Either method is terrible if not setup correctly. I like the sodium filled valves. They transfer more heat away from the valve head than does the solid valve. Sodium filled valves are lighter and less light to cause valve float (not really a problem in our engine when operated less than 3400 rpm). Some production Chevy engines use sodium filled valves.
From: flyv35b <flyv35b(at)minetfiber.com>
To: teamgrumman-list(at)matronics.com
Sent: Thursday, February 23, 2012 6:48 AM
Subject: Re: TeamGrumman-List: Oil Consumption
--> TeamGrumman-List message posted by: flyv35b <flyv35b(at)minetfiber.com (flyv35b(at)minetfiber.com)>
On 2/22/2012 6:59 PM, FLYaDIVE wrote:
| Quote: | > Gary said:
> They are wrong. Spraying extra oil on the valve stem causes coking on
> the valves. Oil flow to the rocker boxes on an engine with sodium
> valves is intentionally limited.
>
> The reality is, sodium filled valves do their work by direct metal to
> metal contact. Or as close to that as possible.
Gary:
Now that does not make sense. I don't think anyone is around that was
involved in the original design of the Lycoming engine. The idea of a
sodium filled valve and the extra transfer of heat because of the sodium
is feasible but has proven short of goal. Our problem has always been
over heating of the exhaust valve and coking on the stem of that valve.
Continental uses solid stem valves and their failure mode is the same as
Lycoming. There is no way of collecting data as to how many Lycoming
and Continental valve failures there are.
BUT! Isn't it COKING on the valve that causes the problem? AND... What
is COKING? Coking is the BURNING of OIL with the subsequent build up
that causes the valve to stick. When they stick in the DOWN position
the piston comes up to smash the valve head and bend the valve shank.
SO! HOW WOULD ONE PREVENT THE COKING?
BY LOWERING THE TEMPERATURE THAT CAUSES THE COKING.
AND HOW WOULD ONE DO THAT?
BY INCREASING THE COOLING BY INCREASING THE OIL FLOW OVER THE VALVE
STEM. No, I'm not yelling, just emphasing the point.
Now, if this does not make sense, please explain why. I cannot accept
the idea that minimal oil for heat transfer is the way to go or the intent.
Barry
Here are my thoughts:
No mater how good or bad the heat transfer is between metal to meatal
(something OIL is the medium that carries the heat away.
"I looked up Sam Heron and could not find anything. Can you post a link?
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IF - IF - The cooling is kept to a minimum to allow heat transfer someone has been feeding the industry a HUGE BLIVIT. It makes better sense to keep the valve and guide clearance to a minimum to control valve and guide wear. YOU DO NOT WANT THE VALVE SLOPPY AND FLOPPING AROUND. Think of SB-388!!! AND if you want to take the path that the heat is mainly or REQUIRED to be transferred through the Valve Head and Seat- That is true and all well and good, BUT... What do you think would happen if the valve head LOOSES contact surface because of SLOP between the valve guide and stem due to a larger SLOPPY fit?
AND what about the change in material of the valve seat years ago to better handle the heat?
AND there was a change in the valve guide material as well. Maybe not to handle the heat transfer - But, to handle the ware. And where does ware come from?
No, I would not change the tolerance between valve stem and guide. I would just REMOVE as much heat as possible while allowing two actions to take place:
1 - The material coefficient to keep the valve and guide tolerances.
2 - The oil to remain in its optimum range.
Remember COKING is the burning of oil. Keep the oil from burning and there is no coking.
No coking equals no valve failure - At least not from coking."
Barry
Gary is referring to an old SAE paper on sodium exhaust valve stems, which may have had an influence on Lycoming changing to their half inch sodium cooled exhaust valves. I don't know that they intentionally limited the oil to the exhaust guide as that may have been dictated by the old automotive lifter design they chose to use years before.
At any rate there is precious little oil flow to the exhaust rocker area, especially on the RH side of the engine. Certainly not enough to carry away any significant amount of heat, but just engouh to coke up the valve stem/guide under the right operating conditions.
In all the Tiger engines I have worked on and disassembled the valves from I don't ever recall seeing a stuck exhaust valve due to coking. I think this happens much more often on the 150 hp engines and planes that are operated at low throttle settings for considerable time and at full rich mixture settings. I think this is a faily uncommon failure more for a Tiger that is operated at high power and high rpm on x-country flights. These engines tend to wear out the guides and looses up the clearance before they can coke up.
Since the sodium valves rely on transferring a good deal (maybe a majority) of the heat up the stem and through the guide to the head for cooling, then this interface is pretty important. Continental engines transfer heat to the valve seat as the primary heat path. Take a look at SKy Ranch Engineering Manual by John Schwaner for a detailed discussion of this.
The problem seems to be that guide wear is rapid under these conditions of high heat and material compatibility (hence the HC content guide change) and as the clearance increases the ability to transfer heat to the guide decreases, causing the valve to run hotter and wear the guide even quicker and fatigue the valve. Eventually the valve head will break off as the valve is weakened and hits the seat cockeyed. Happens all the time if you ignor the wear and let it go long enough. Lycoming knows this, hence SB388C. They have done little to really solve the problem.
I suspect that they may be some point where enough oil could be introduced and continuously flowed around the guide area to extract a significant amount of heat and lower the temperature to the point it could not coke up. How much that would take, who knows. I thought that was what Lycoming and Mooney were attempting to do on the parallel valve IO-540 engines in the Mooneys which were having serious valve problems on planes that only had 250 hours on them. Owners were not happy. Gary says that didn't work. I don't know if that is so or not. It would be nice to see the service reports and results after the change but that info probably is not available.
One thing is interesting is that Bill Marvel has loss of compression, exhaust valve leakage and wear problems with his old Tiger in time intervals as low as 250 hrs to 400 hrs on a routine basis. He flew the plane entirely on x-country trips at altitude at 2700 rpm. Since building his RV-8A with the same engine but with a CS prop operating at 2300 rpm and full throttle he has had no valve problems in over 1200 hrs (or maybe more).
Personally, I'm not so sure that the sodium cooled exhaust valves have not aggravated the guide wear problem. With the HC guides and SB388C the liability risk of an exhaust valve failure is reduced and the owner is just left with a low grain headache rather than a migraine one!
I disassembled a IO-520 Continental engine with 1700 hrs on it (TBO) and found the worst exhaust valve guide had only about .008" wear. Cylinder bore wear was barely .003" at the top in the choke area. The choke was gone and the cylinder was now straight, which according to Chuck Ney is better. No cylinder work or any other work besides spark plugs and magneto inspection was ever done on this engine. The engine in my Bonanza that was replaced by this one after overhaul went 700 hrs beyond TBO with veronics.com/Navigator?TeamGrumman-List" target="_blank">http://www.matronic http://forums.ma &nbs//www.matronics.com/contribution" target="_blank">http://www.matronics.co==================
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