Originally posted by techinspector1
Vegas, if you have no transmission and cannot switch the pitch on the propeller, I guess you'd have to run the engine in reverse to back the ship up.

Direct Reversing Engines
In order to manoeuvre a ship, the propeller thrust must be reversible, by means of reversing the propeller drive or by altering propeller pitch. Controllable pitch propellers, or diesel-electric drive, allow the use of unidirectional (non-reversible) engines. In systems of limited power using medium or high speed engines through gearboxes, arrangements of clutches and reverse gears may be used. In many ship, however, it is necessary for the main engine to be reversible and able to operate efficiently in both ahead and astern directions.

To run in the astern direction, all the operations in the engine cycle may need retiming. The starting air must first rotate the engine in the reverse direction and this will require retiming the distributor to supply compressed air to the appropriate cylinders in the correct order. The retiming may be carried out by altering the position of the distributor cam with respect to its drive from the main camshaft.

The number of the readjustments to be made and the methods used depend upon the engine cycle and type. Four-stroke engines will require a change in timing of fuel pumps, a different change for the air inlet valves, and yet another for the exhaust valves. To obtain all these changes on the same camshaft, a separate set of astern cams is fitted. Each astern cam is fitted to the camshaft adjacent to its corresponding ahead cam (below). The reversing procedure is then carried out by moving the whole camshaft axially, which moves the ahead cams clear of their followers, which now engage the astern cams. Ramps (sloping sides) fitted between corresponding ahead and astern cams cause the follower’s roller to slide smoothly from one to the other. The axial movement is carried out by a hydraulic cylinder fitted to the camshaft; locking devices and safety cut-outs ensure that the camshaft has carried out its full axial movement and is in the correct position before the engine can be restarted. To maintain alignment of the camshaft drive, a spline coupling may be necessary.



Turbo chargers are of course unaffected by reversal of the engine, but engine driven pumps must be reversible.

Large two-stroke engines have scavenge ports which control scavenge timing. This must therefore be symmetrical and will thus be unchanged when reversed. Engines operating with constant pressure turbocharge have almost symmetrical exhaust valve timing. Consequently no change in timing is necessary for exhaust cams.

Fuel pump timing must be readjusted since it will be the opposite flank of the cam, which will now raise the pump plunger to deliver fuel. There are alternative methods employed to change the fuel pump timing without altering the main camshaft. Two such systems are illustrated and described.

In both cases the main camshaft drive timing is not altered and therefore any other drives taken from this remain synchronised with the engine while running astern. This is important if a balancer system is fitted.





Sulzer RTA engines have oil pressure operated hydraulic ‘lost motion’ servomotors on the camshaft, which rotate the fuel pump cams to their astern positions. Fuel pumps and their cams are grouped in pairs along the camshaft and servomotor is fitted for each pair of adjacent cams. Oil pressure located and secures each servomotor in its correct position while the engine is running. A similar servomotor is used to re-time the starting air distributor and its drive from the camshaft. ‘Lost motion’ is the term used to indicate that the timing has been retarded, or moved back, through a given angle with respect to the ‘new’ direction of rotation.



MAN – B & W MC engines have their fuel pump cams fixed directly to the camshaft but the follower rollers can be displaced to alter the pump timing as shown below. The link, which displaces each follower, is actuated by the pneumatic cylinder and piston, powered by compressed air from the starting system. The link is self-locking in either position and a sensor is fitted to each pump to cut off fuel delivery if the link is not locked in its correct position.

Earlier models of two-stroke engines use elaborate systems of servomotors to rotate their whole camshaft to reverse the timing.