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Sample Questions

Second Engineers Motor EKs
 

 

July 2002 Question 1.

With reference to rotating diesel exhaust valves:

  1. sketch the complete valve assembly; (8)

  2. explain how the device sketched in (a) functions. (8)

a.

b.

The sketch shows a caged exhaust valve fitted with a rotating device as found on large 4 stroke medium speed diesel engines burning residual fuel oil.

The valve spinner fitted on the stem causes the valve to rotate as the exhaust valve passes across the vanes; i.e. a change in direction of the gas causes a force to be imparted on the vanes in a similar way to gas acting on the turbine blades in a turbocharger. To allow this rotation to take place, a bearing is fitted between the spring carrier and the valve cover, which allows the valve to float on the bearing whilst it is open.

Because the rotation of the valve is effective over the opening period of the valve, it means the valve is rotating as it reseats. This provides a grinding action which removes deposits from the valve seat, which may otherwise interfere with the correct seating of the valve. Rotation of the valve also prevents localised overheating and distortion, both of which are causes of early failure.

 

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