Operation of a Governor

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

(a)     With the aid of a block diagram describe the operation of an electronic governor fitted to a main engine                                                                                                      (6)

(b)     An engine fitted with an electronic governor behaves erratically during load changes. Explain the possible causes                                                                                       (5)

(c)    Describe a device fitted in order to safeguard an engine in the event of sudden and complete removal of its load                                                                          (5)

a)         The diagram shows the main components of the electronic or digital governor.

·         Bridge command unit. This will indicate both the required direction, and the required speed. This speed signal is sent to the comparator as the RPM command signal

·         Engine speed is measured by a toothed wheel fitted at the flywheel. The proximity probe detects the number of teeth per second and sends this signal as the Measured RPM to the comparator.

·         The comparator compares the measured and desired values and its output signal is sent to the fuel actuator to minimise this deviation.

·        

The Fuel limiter’s prevent damage to the engine by limiting the fuel during the following conditions

·         Insufficient scavenge air

·         Excess output torque

·         Operation at the critical speed

·         Excess fuel command request

·         The output from the fuel limiter filter is passed to the actuator fitted at the engine side to control the position of the fuel rack.

·         The position of the fuel rack is measured and fed back to the actuator control to prevent excess fuel being requested, which would lead to hunting and instability of the engine speed.

b)      Erratic speed control could be due to

·         Fluctuating load, due to external weather conditions

·         Poor power balance between cylinder, causing the speed to fluctuate even though a stable fuel rack position is present

·         Sticking fuel rack, causing the governor input to the engine to be unstable

·         Excess gain on the comparators, producing instability. This gain can be adjusted, but is usual a function of speed or external switch position (calm or rough sea)

·         Unstable/non-true measurement of the engine speed by the induction pick-ups

·         Signal faults between components due to cable damage (earth faults) causing the electrical signals to drift.

·         Poor electrical contacts between components due to the effects of vibration.

c)      The governor of the engine is fitted to control the engine speed. To enable the engine to be shut down if the load is suddenly removed, then some provision is required to quickly stop fuel injection. This system must be independent of the speed governing system. To enable this to occur, then speed detection and fuel shutoff provision must be made.

·         For slow speed engines the speed can be sensed from a digital pick-up similar to the induction pick-up. This speed is measured by the overspeed control system, and if it exceeds the trip value then the fuel rack is shut down. For NSD this is by a collapsible link fitted between the governor and the fuel rack, whereas for MAN B&W this occurs by the puncture valves on the top of each fuel pump opening to spill the high pressure fuel oil.

·         For medium speed engines, offset flyweights utilising centrifugal force sense the speed. At the trip speed, the flyweight will move out activating the stop cylinder. The stop cylinder will move the fuel racks to the no fuel position.

Note the students answer should contain either slow or medium speed. Mentioned could also be made that these overspeed trip devices must be manually reset following activation.