Operation of Marine Engines with Cutting off Cylinder Unit on Ships
Running Marine Engines with Cylinders out of Operation
The engine is designed and balanced to run with all cylinders as well as all turbochargers working. If a breakdown occurs which disables one or more cylinders, or turbochargers, repair should preferably be carried out immediately.
One unit may need to be disabled when any of these conditions exist:
- Damage to a main component of the combustion chamber
- Damage to a crosshead or bottom end bearing on that cylinder
Precautions Before Cutting off Cylinder Unit
If immediate repair is not possible, the engine can be operated with one or more cylinders or turbochargers out of operation, but with reduced speed owing to the following:
- As, in such cases, the air supply is no longer optimal, the thermal load will be higher. Therefore, depending upon the actual circumstances, the engine will have to be operated according to the restrictions mentioned in operating manual of the engine.
- Pressure pulsations may occur in the scavenge and exhaust receivers, which can give a reduced air supply to any one of the cylinders, and as a result cause the respective exhaust temperatures to increase. The fuel pump index for these cylinders must therefore be reduced to keep the exhaust temperatures (after valves) within prescribed limits.
- Since the turbochargers will be working outside their normal range, surging may occur. This can generally be remedied by “blowing off” from the scavenge air receiver.
- If more than one cylinder must be cut out of operation, and the engine has two or more turbochargers, it may be advantageous to cut out one of the turbochargers.
- When cylinders are out of operation, governor hunting may occur. When this happens, the fuel pump index must be limited by operating the electric governor on “index control”.
- With one or more cylinders out of operation, torsional vibrations, as well as other mechanical vibrations, may occur at certain engine speeds. Should unusual noise or extreme vibrations occur at the chosen speed, the speed must be further reduced.
- If the engine is to be run for a prolonged period with cylinders out of operation, the engine builder should always be contacted in order to obtain advice concerning possible recommended barred speed ranges.
Procedure for Cutting off Cylinder Unit
The following sequence would be carried out for ships which use the MAN B&W slow speed engine:
- Stop engine, isolate systems and allow to cool
- Ensure a procedure is written that minimises the risk to personnel during the operation.
- Discuss the task and written procedure with the engine room personnel to ensure they are familiar with the risks, and the methods to be used to minimise these risks.
- Ensure the fuel pump is de-activated by lifting roller and locking.
- Lift exhaust valve actuators so exhaust valve remains closed during running. (Note: the air spring supply to be left open)
- Dismantle air start supply line, and blank with suitable steel plates, the main and control air pipes
- Dismantle bottom end bearing, and turn engine to suspend piston, crosshead and connecting rod from supplied crosshead supports.
- Secure big end of connecting rod in crankcase.
- Blank off main lube oil inlet to crosshead within the crankcase with a blanking plate.
- Isolate the cylinder lubricator for that cylinder by placing all lubricators on no stroke.
Difficulties During Manoeuvring
When one engine cylinder is isolated, then one problem that may occur is a “dead spot” during manoeuvring. This is due to the air start valve being isolated for that unit, and is more likely when a smaller number of cylinders are present. The Master must be informed that this could occur, and the remedy would be to kick the engine in the opposite direction, and then restart in the required direction.