With reference to a particular make of main propulsion unit, describe how the engine is reversed manually and discuss the problems involved and the safety precautions which would be recurred if the control were operated remote from the machinery space (16)
Structured for DG Shipping MEO Class II examination scoring criteria.
For Sulzer and MAN B&W (MC-C) two-stroke engines, the engine is reversed by shifting the roller position on the fuel cams. This is achieved using pneumatic cylinders, with one cylinder attached to each fuel pump roller. The position of these rollers determines whether the engine operates in the ahead or astern direction.
To manually reverse the engine, the control must first be switched from remote (bridge control) to local (engine room control). This supplies control air to the local maneuvering station and all associated valves. At the local station, a selector switch for 'Ahead' and 'Astern' is used. This switch sends an air signal to a main reversing valve, which in turn directs air to all the pneumatic cylinders, shifting the fuel pump rollers to the desired position.
The most significant problems with this type of reversing system are related to the control air quality. If the control air is not properly dried and contains moisture or lubricating oil carryover, it can cause the control valves to become sticky, faulty, or even stuck in position. This could lead to a complete failure of the reversing system, which is a major safety concern, especially during critical manoeuvres like docking or navigating in confined waters.
To ensure safe and reliable operation, the following safety precautions are crucial:
In contrast, when the engine is operated from a remote location, solenoid valves automatically control the reversing process based on the position of the bridge telegraph lever. If these solenoids fail, it would also result in a reversing problem, highlighting the importance of regular checks on all components of the control system.