Some Facts About Sodium-Filled Valves

One reader wrote in to propose that perhaps valve springs might be cooled by making them hollow and filling them with sodium. Actually, all that liquid sodium can do (it is liquid above 208 degrees Fahrenheit) is carry heat from one place to another. It cannot cool anything by simply filling a hollow space within it.

Always ask questions! I once asked a valve spring engineer why hollow wire was not used to make them. He replied with a question: “How would you polish that interior surface so that defects on it did not quickly lead to cracking?” I felt a bit silly but I had learned something.

In the aviation of the between-the-wars era (1919–1939) a major problem was that the exhaust valves of large air-cooled aircraft engines ran so hot that valve seal deteriorated rapidly in service and valves frequently broke right at the point where the valve’s stem began to flare out to form the valve head.

Normally, exhaust valves are cooled mainly by intimate contact with their valve seats, with only a small amount of heat being conducted along the valve stem. This is why exhaust valve seats are made wider than those of intake valves. But large valves gather so much heat through their exposed area that it can be difficult to remove it fast enough by seat contact, particularly in hot-running air-cooled engines.

The duty cycle of aircraft engines was a special problem: Takeoffs at 100-percent power followed by hours of cruise at 50-percent power. Cars on the freeway typically operate at 10- to 15-percent power, and even racing engines are on full throttle for only seconds at a time, followed by closed-throttle braking and part-throttle acceleration.

The basic idea of the internally cooled valve is to make valve head and stem hollow and then to partially (about two-thirds) fill that cavity with a heat transfer liquid that would, by sloshing back and forth as the valve opened and closed, move heat from the very hot (possibly glowing) valve head into the much cooler valve stem, where that heat could be transferred to the valve guide and cylinder head mass.

Water was tried first, and some experimental sealed valves exploded from steam pressure. Other schemes proposed to circulate water through the valves—that was complex and too much for existing technologies. Mercedes made and used some hollow exhaust valves partially filled with mercury, but that metal did a poor job of wetting steel. Some Mercedes racing cars of the 1930s used such mercury-filled valves.

The clever, persistent aerospace engineer Sam Heron next tried partial filling with salts of appropriate melting point, but the best heat transfer fluid turned out to be liquid sodium. Such valves became the standard solution for the survival of valves in large aircraft engines, certain heavy-duty truck engines, and some racing applications.

Sodium-filled exhaust valves are today unnecessary in motorcycle engines because 1) motorcycle valves are quite small (especially when four valves per cylinder is the norm) and cool well by normal seat contact, and 2) motorcycle engines are today generally liquid-cooled, greatly reducing the temperature of exhaust valve seats.

On the other hand, air-cooled two-valve racing motorcycle engines had their share of exhaust valve problems and have on occasion been given sodium-filled valves.

Here on my windowsill is a valve from a Pratt & Whitney R-2800 aircraft engine (each of its 18 cylinders has a displacement of 155ci, or 2,540cc). The valve weighs almost a pound, has a head diameter of 2.6 inches, and its fat hollow stem has a diameter of 0.675 inch. By contrast, one of a Yamaha FZ750’s two exhaust valves weighs 1/17 as much, has a diameter of 0.910 inch and a stem of 0.195 inch. Making things even tougher for the 2800’s valves was that it was supercharged.

When the French built their futuristic experimental nuclear breeder reactor “Superphénix,” its internal coolant was circulating liquid sodium.