A ship fitted with a Fixed Pitch Propeller (FPP) cannot change the direction of thrust by altering the propeller blade angle. To move the vessel astern, the main engine itself must rotate in the opposite direction. This operation is known as main engine reversing.
Among low-speed two-stroke marine diesel engines, the MAN B&W MC engine uses a reliable mechanical camshaft reversing system. Instead of electronically changing fuel injection timing, the engine mechanically shifts the fuel pump cam followers while simultaneously changing the starting air sequence.
Main engine reversing is one of the most critical manoeuvring operations on board, enabling safe berthing, unberthing, anchoring, emergency stopping, canal transit, and navigation in confined waters.
When an Astern command is received from the bridge, the engine control system first reduces the engine speed to zero RPM. Once the engine has completely stopped, the reversing mechanism changes the timing of the engine components to suit reverse rotation.
The sequence is as follows:
Note: on conventional camshaft-type engines (MC, RTA), the exhaust valve cam profile is symmetrical about bottom dead centre, so it does not need to be repositioned when reversing — only the fuel pump cam/follower and the starting-air distributor timing change. Electronically controlled engines (RT-flex, ME) do adjust exhaust valve timing electronically for each direction, since they have no fixed mechanical cam at all.
Although every reversible two-stroke engine follows this basic principle, the reversing mechanism differs between engine manufacturers.
The MAN B&W MC engine retains a conventional camshaft carrying separate ahead and astern cam profiles for each fuel pump. Reversing is carried out not by moving the cam itself, but by shifting the fuel pump cam follower (roller) sideways from the ahead cam track onto the astern cam track. On most MC-series engines this shift is performed by an air-operated (pneumatic) cylinder fitted to each follower unit, triggered by the starting-air/control-air system once the engine has stopped; the roller only transfers across when the engine is turning, since the incline between the two cam tracks needs cam motion to carry it over.
At the same time, the engine-driven starting-air distributor is switched to the reverse firing sequence so that starting air is admitted to the correct cylinders in the correct order for astern rotation.
Reversing Sequence
Advantages
The engine control system will not permit fuel admission unless the follower position and starting-air distributor position are correctly verified by the monitoring system.
The Sulzer RTA engine uses a hydraulic “lost-motion” reversing servomotor mounted directly on the camshaft to change fuel injection timing. The fuel pump cams are pressed onto sleeves that surround the camshaft; the servomotor (typically shared between a pair of adjacent cylinders) rotates these cam sleeves relative to the camshaft through a defined “lost motion” angle, re-timing the same physical cam for astern running instead of fitting a separate cam. As with the MC engine, the exhaust valve cam profile is symmetrical and does not need to be moved.
Reversing Sequence
Advantages
The Sulzer RT-flex engine represents a later generation of electronically controlled low-speed marine diesel engines. Unlike conventional camshaft engines, the RT-flex uses a common-rail system controlled by an Electronic Control Unit (ECU).
There are no mechanical reversing cams or hydraulic reversing servomotors.
Reversing Sequence
Advantages
Electronic control removes many mechanical components (camshaft, cam followers, reversing servomotors, mechanical starting-air distributor), making the reversing process simpler, quicker, and more reliable.
Before the engine is permitted to start astern, the control system verifies several critical conditions:
Only after all these conditions are successfully verified does the control system permit fuel injection and engine starting.
Routine inspection and maintenance ensure reliable reversing performance.
Key maintenance points include:
Any sticking mechanism, hydraulic leakage, worn components or incorrect timing can lead to failed reversing, poor combustion, excessive exhaust temperatures and unsafe manoeuvring.
Main engine reversing is essential for ships equipped with Fixed Pitch Propellers, as the propeller itself cannot reverse thrust. Reversing is achieved by changing the timing of fuel injection, starting air and exhaust valve events.
Ships with a Fixed Pitch Propeller (FPP) must reverse the main engine to generate astern thrust.
The engine stops, fuel injection and starting-air timing are adjusted, and compressed air starts the engine in the opposite direction.
It uses a hydraulic servomotor to rotate the fuel pump cam sleeves and change fuel injection timing.
Hydraulic servo cylinders reposition the cam followers onto the ahead or astern cam profile.
An Electronic Control Unit (ECU) electronically controls fuel injection, exhaust valve timing, and starting-air sequencing.
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