Sodium Filled Valves
Old wive's tales: A lot has been said about the valve problems in Lycoming engines. If valve problems were limited to just Lycoming engines, I'd say, "Hey, there's the problem." Problem is, Teledyne Continental engines have valve problems too. I talked with Ken Tunnell from Lycon Aircraft Engines and Steve Mehalek, formerly with Lycon, about the valve problems. A summary would be: Fuel formulations have changed (more toluene has been added) which makes the fuel burn longer and hotter going past the valve.
Do your homework! If you do your homework, this is what you'll find on sodium filled valves. (Return to "Technical")
Gotta Love Engineers: Using the dynamometers at McCook Field for measuring power, speed, torque, fuel and oil consumption, engineers experimented with thermodynamic improvements, such as greater compression ratio, higher temperatures, and more effective combustion. To achieve these improvements, they sponsored the technology for higher octane fuels, higher temperature coolants, and better exhaust valve design. For example, exhaust valves were a limiting component since they experienced the highest temperatures. Extensive research in high temp steel alloys offered a partial solution, but the breakthrough in 1923 was the sodium-cooled valve invented by Sam Heron (1891-1963), a civilian engineer in the Power Plant Section at the Ethyl Corporation. Heron's exhaust valves contained a cavity filled with liquid sodium that conducted heat away from the valve face to the cooler stem. Sodium-cooled valves opened the way for higher compression ratios that, in turn, required higher octane fuels. (Creating the Military Plane, 2001)
Initial studies: In Sodium filled valves (the type of valve used in Lycoming engines) the valve stem runs hot at the expense of the valve face running cooler. The heat path from the valve stem to the valve guide becomes critical. If the valve guides wear prematurely (or they are honed too large) the heat path is breached and valve cooling suffers. But, you can't run them too tight either due to thermal expansion - Samuel Heron, Circa 1919 (re-printed in Light Plane Maintenance, January 1990)
Valve Sticking: Valve sticking is more of a problem in Sodium filled valves versus solid valve stems. It was found that valve guides that protruded into the exhaust port (some valve guides do not have a step or flange which determines the depth the guide will protrude into the port) were more likely to cause valve sticking when using Sodium filled valves. It was further determined that increasing the flow of oil to the valves and guides tended to cause valve sticking with sodium filled valves. According to Robert V. Kerley, "Sodium filled valves and valve guides should be run dry." (SAE Quarterly Transactions, Vol 1, No. 2, 1947, pp 253-263)
The valve guide, for sodium filled valves, must run with a minimum of lubrication and at remarkedly close tolerances; typically, valve stem to guide clearance is 0.004 inches. If the clearance is too large, excessive combustion byproducts and lubricating oil would eventually migrate into the guide and coke onto the stem in the intense heat. (AOPA Pilot, February 1997)
Lycoming fix to sticking valves: Remember the Lycoming fix for sticking valves on the Mooney? Yea. They added squirters to the rockers to squirt oil onto the valve. This is the exact opposite of what is needed. All of that extra oil just cokes onto the valve stem.
Potential problem: Another problem with the Sodium filled valve is the potential for corrosion on the INSIDE of the valve stem (scale buildup in the Sodium chamber) and loss of cooling ability due to inadequate heat transfer from the valve stem to the Sodium. Sodium chamber corrosion is a result of trapped moisture within the stem during the manufacture of the valve. "... the most important path for the heat flow, since the valve spends two-thirds of its time in the closed position (contacting the valve seat) is from the valve face through to the valve seat." Studies by Thompson (SAE Paper 650484) have shown that as much as 75 percent of an exhaust valve's heat dissipation is through the seat area. (Light Plane Maintenance, April 1988) (this is true on about .05% of the valves)
Lycoming: The percentage of cooling accomplished by the oil flow over the valve stem is approximately 20 percent. The larger percentage of cooling, 80 percent, is accomplished by heat transfer to the valve seat, the cylinder head, and ultimately to the cooling air. ... The parallel valve Lycoming engines have a limited number of cooling fins cast into the head in the area adjacent to the valve seat. (Light Plane Maintenance, August 1996)
Personally, I think this is total bullshit: Design limitations: Cylinder head temperatures, which appear normal during climb and cruise, do not necessarily mean that localized valve face, valve stem, and valve guide temperatures do not exceed prudent values. ... Normally the installed valve guide inside diameter is 0.4985 to 0.4995 inches. This tight clearance causes the valve to run exceptionally hot during breakin and forms spots of coked oil on the guides inner surface. Engines with guides fitted in this manner have a greater potential for valve sticking, metal transfer, and accelerated wear. ... If the guide is sized to 0.5005 inches during assembly, the additional clearance allows more oil to flow through the guide placing an effective layer of oil between two very hot parts. Much of this layer of oil will burn itself to the wall of the valve guide forming a very smooth glazed surface. This glazing of the guide wall will act to inhibit the production of carbon buildup and seems to allow for greater heat transfer away from the valve stem. (Light Plane Maintenance, August 1996)
Light Plane Maintenance, August 1996: It's obvious that whoever wrote this has not done his homework. Either that, or he isn't talking about sodium filled valves. Oil that burns itself onto the valve guide is called "Coking" and is the source for future valve sticking. It's crap journalism like this that leads to a complete misunderstanding of any relevant topic.