Definition
A poppet-type exhaust valve used in high-performance reciprocating aircraft engines, manufactured with a hollow stem partially filled with metallic sodium. As the valve operates, the heat from the valve head melts the sodium, and the motion of the valve causes the liquid sodium to slosh between the head and the stem. This action transfers heat away from the valve head and into the cooler valve guide area, where it can be carried off by the engine's cooling system, allowing the valve to run significantly cooler than a solid valve.
Plain English
An exhaust valve with a hollow stem partly filled with sodium metal. When the engine runs, the sodium melts and sloshes around inside the valve, carrying heat away from the hot valve head so the valve doesn't burn out.
Context Anchor
Seen in aircraft piston-engine maintenance, engine overhaul, and discussions of exhaust valve cooling.
Derivation
Named for its construction. Sodium is used because it melts at a low temperature (about 208°F) and is an excellent conductor of heat in liquid form, making it ideal for shuttling heat from one end of the valve to the other.
Why Pilots Care
Prevents exhaust-valve overheating and failure in high-power or high-temperature operations, directly affecting engine reliability and safety.
Grounding Statement
When the engine is running, the sodium inside the valve becomes liquid and helps move heat away from the valve face that is exposed to very hot exhaust gas.
Intuition Check
Do not read sodium here as table salt. In a sodium-filled exhaust valve, it means metallic sodium sealed inside the valve to help carry heat.
Example Sentence 1
The mechanic explained that the engine's sodium-filled exhaust valves help prevent valve burning during prolonged high-power operation.
Example Sentence 2
High-compression engines often use sodium-filled exhaust valves to keep valve temperatures within limits during prolonged climb.