Which statement about exhaust valves is correct?

The statement regarding exhaust valves that emphasizes the use of stems partly filled with sodium for cooling is accurate. In high-performance and high-temperature applications, such as in some reciprocating engines, exhaust valves endure significantly greater thermal stresses than inlet valves due to the direct exposure to combustion gases. To mitigate this heat, some exhaust valves are designed with stems that contain sodium. Sodium is effective in enhancing heat dissipation due to its high thermal conductivity and low melting point. As the valve heats up, the sodium moves within the stem, helping to transfer heat away from the valve head and into the cooler areas of the valve assembly. This design effectively prolongs the life of the exhaust valves by preventing overheating and thermal degradation.

In contrast, the other statements do not accurately characterize exhaust valves. For example, the opening mechanism hinted at in the first statement refers to a more conventional system and does not specifically address the specialized designs that enhance thermal management in high-performance engines. The second statement underestimates the harsh conditions under which exhaust valves operate, as they are indeed more affected by the heat of combustion compared to inlet valves. Lastly, while rocker gear does play a role in valve operation, it is misrepresented in the third statement as a means to open exhaust valves, which is