Engineers Motor EKs
July 2002 Question 1.
With reference to rotating diesel
sketch the complete valve assembly; (8)
explain how the device sketched in (a) functions. (8)
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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