Main engine fixed pitch propeller reversing explained…

Modern massive two-stroke marine engines with fixed pitch propellers that are directly connected must be reversed, that is, the engine’s rotational direction must be changed to drive the ship ahead and astern. This is generally necessary during ship maneuvering, such as berthing, passing through canals, lock gates, and anchoring, among other things.

I have received many questions about main engine reversing mechanism and it is important to mention that there are different systems in place for reversing the large marine engines, which are typically specific for every engine manufacturer, despite the fact that every one of them lead to the same result…reversing the engine.

This post is about large marine two-stroke diesel main engines with fixed pitch propeller. The variable pitch propeller engines is another story and will be presented into another post.

When a two-stroke engine is required to reverse direction, the fuel pumps and the operating mechanism for starting air valves must be retimed (i.e. run astern). This is accomplished by shifting the locations of the fuel pump cams or fuel pump cam follower relative to the crankshaft. Here, in the link below you can find an excellent explanatory video about engine reversing process.

Reversing of marine diesel engines. Source and credit: marineinsight

On the Sulzer camshaft type engine there is a reversing servomotor of which duty is to move the fuel cams on the camshaft for forward or reverse engine rotation. Normally, one fuel cam is arranged on either side of the reversing servomotor for each pair of engine cylinders.

Sulzer RTA reversing mechanism

In engines with uneven number of cylinders the servomotor at the free end is fitted with a single fuel cam. The camshaft turns in the same direction as the crankshaft.

The cams can be reversed (turned relative to the camshaft) corresponding to the desired direction of rotation of the engine, to the position where the fuel injection is actuated at the right moment.

Sulzer reversing mechanism schematic

The fuel cam 5 and the tapered bushes 14 are fitted to the sleeves 13 by pressure bond. The sleeve 13 and the segment 7 are screwed together. Fuel cams 5, tapered bushes 14, sleeves 13, segment 7 and bushes 8 from individual units which can execute a turning movement limited by the wing 11.

When the camshaft rotates in the direction of the arrow “DR” the segments 7 are pushed by the wings 11 in the direction of the arrow. The connecting piece “VA” is subjected to control oil pressure. It fills the spaces “RA” through drillings in the bush 8 and ensures that the segment 7 lie firmly pressed against the wings 11. The correct end position of the reversing motor is transmitted to the valve group in the pneumatic logic unit via the connecting piece “VR”.

When reversing the connecting piece “VA” has zero pressure, and the spaces “RB” are filled with control oil through the connecting piece “VB. The segment 7 and therefore also the fuel cams 5 are turned on the camshaft, until they rest against the wings 11. As soon as the corrected reversal indicated by the connecting piece “VR” and the correct direction of rotation is attained , the fuel is injected at the correct timing for the new direction of rotation.

Sulzer reversing valve

On the Sulzer electronic engine, the reversing process is more simpler as the injection and starting air timing is computer controlled via solenoids valves and doesn’t imply complicated mechanisms.

On the old MAN B&W engines the reversing technique is carried out by axially shifting the camshaft. A separate astern cam is added to the camshaft for this purpose and each astern cam is fitted next to the corresponding ahead cam. The whole camshaft can be shifted axially with the help of hydraulic cylinders fitted to the camshaft. The hydraulic oil(sometimes pressurized air) gets pressurized and forces the piston inside the cylinder which moves the entire shaft from ahead to astern or from astern to ahead.

A clever method used by MAN B&W is used on the MC series engines which alters the position of the cam followers, instead of moving the entire camshaft. The MAN B&W MC series engine’s fuel pump cam is designed to raise the plunger on the injection stroke and then hold it at the top of its stroke while the follower stays on the peak of the cam until just before the next delivery stroke, when the follower returns to the base circle of the cam and the fuel pump plunger moves down on its suction stroke. The animation below shows the working principle for the above mentioned method.

MAN MC series reversing mechanism. Source and credit: Aayushman Singh

In the following video can be seen a live reversing on a vessel:

MAN MC Series engine live reversing. Source and credit: Emrah Yalci

It should be noted that the reversal of the follower only takes place while the engine is stopped. The engine has been stopped from running ahead, and then started astern, the fuel pump followers move across as the engine starts to rotate, and before the fuel is admitted by venting the fuel pump puncture valves.

A micro switch senses whether or not the follower has moved. If not, a warning indicator light in the control room will blink. However, if a follower fails to move, maybe owing to corrosion in the servo cylinder, the engine will still start and a high exhaust temperature deviation alarm will soon activate, but allowing the engine to start in this case may be beneficial while maneuvering in confined waters.

As the SULZER and MAN engines are the most common engines used onboard vessel as main propulsion engines, the above description about reversing methods are the most important to know as a marine engineer.

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