Reasons: Exhaust valve temperature in modern engine is as high as 750˚C. Thus cooling of exhaust gas becomes very important. Cooling water jackets are arranged near the valves for valve cooling. In many cases nozzles are directed towards hot spot caused by the exhaust valve.
Working: In heavy duty engine, sodium cooled valves are used, the working of this valve is stated below – A sodium cooled valve has a hollow stem, which is partly filled by metallic sodium. Sodium melts at 97.5˚C. Thus at operating temperature sodium is in liquid state. When engine runs, valve moves up and down, thus sodium is thrown upward in hotter part of valve. There it absorbs heat, which is later given to cooler stem as it falls back to stem again. This keeps the valve head cool.