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NEW CHANGES MADE ON 11-AUG-2023
For direct propulsion marine engines, there are three basic types.
A sliding camshaft
A reversing latch
A rotating camshaft
The rocker arm roller is in constant contact with the cam, either the ahead cam or the astern cam. When the engine stops, the camshaft moves in a lengthwise direction. A ramp between the head and the astern cams allows for constant rocker arm roller contact. There may be cam followers with pushrods that lead to the rocker arms. This type of reversing system is used on two and four-stroke cycle engines.
Starting And Reversing Four Stroke Medium Speed Engine Reversing Principle
Most medium speed engines are unidirectional (i.e they only rotate one way).
This is because they either are driving an alternator, or
Because if they are used as direct main propulsion they tend to be driving a controllable pitch propeller.
In the case where the engine is reversing, then the camshaft has two sets of cams, one for ahead operation, and one for astern.
The fuel pump, inlet and exhaust valves must all be retimed for astern running
To reverse the direction of the engine, pressure oil is led to one side of a hydraulic piston which is coupled to the camshaft. The whole camshaft is moved axially and the cam followers slide up or down ramps which connect the ahead and astern cams
Reversing latch
The side view in the figure shows that there are two rollers on the reversing latch cart. The camshaft does not move, but the reversing latch, as it is called, does. By moving the reversing latch, the roller is allowed to engage with the cam.
Rotating camshaft
In a two-stroke cycle engine, the same fuel cams may be used for ahead and astern operations of the engine. This is accomplished by rotating the camshaft by means of a reversing servomotor, actuated by lubricating oil pressure.
Lost motion cam diagram
Consider the engine position to be dead centre Ahead with the cam peak centre line to be 55° after this position, anti-clockwise ahead rotation, for correct injection timing ahead. If now the engine is to run astern (clockwise) the cam is 55 + 55 = 110° out of phase. Either the cam itself must be moved by 110° or while the engine rotates 360° the cam must only rotate 250° (110° of lost motion). Note the symmetrical cam 75° each side of the cam peak centre line made up of 35° rising flank and 40° of dwell.
The flank of the cam is shown on an enlarged scale in Fig.. It will be noted that the 35° of cam flank is utilised for building up pressure by the pumping action of the rising fuel pump plunger (14°) for delivery at injection 10° before firing dead centre to 8° after firing dead centre, and 3° surplus rise of flank for later surplus spill variation.
Hydraulic control system ( sulzer RD )
Consider a reversing action from ahead to astern.
Oil pressure from left of reversing valves to right of clutches and under relay valves A and B and air block valve.
The telegraph reply on the engine telegraph is first moved to stop and the fuel lever moved back to about notch 3.5, the starting lever is mechanically blocked by the linkage.
The telegraph linkage to the reversing valve moves this valve and releases oil pressure from both lost motion clutches. This drop in pressure causes both relay valves A and B to move down by spring action, which relieves pressure on the block piston (fuel) so cutting off fuel injection. The pressure on the block valve (air) is also relieved which serves to also lock the starting lever.
Safety lock for correct rotation ( sulzer RD )
Hydraulic control system ( sulzer RD )..
When engine speed reduces the telegraph lever can be moved to astern. This allows pressure oil to flow from the right through the reversing valve, as shown on the sketch, to the left of the lost motion clutches to re-position them for astern.
When the servos have almost reached the end of their travel pressure oil is admitted to relay valve B and the block valve (air) releases the lock on the air start lever. (The mechanical lock on the air lever with the telegraph had been released when the telegraph lever was moved to the astern running position.) Pressure oil also acts on relay valve A admitting oil to block piston (fuel) so allowing the fuel control linkages to the fuel pumps to assume a position corresponding to the load setting of the fuel lever.
If the pressure trips act in the event of low oil pressure (supply and bearings) or low water pressure (jacket or piston) then a trip piston moves up under preset spring pressure so connecting the oil pressure connection to drain. This pressure drop causes the block piston (fuel) to rise up under its spring force and shut off fuel injection.
Connections 1 and 3 from the running direction safety interlock to the reversing valve only allow fuel to the engine if the rotation agrees with the telegraph position. If not, the block piston (fuel) is relieved of pressure via block valve (air) and relay valve A.
Movement of the air starting lever can now be carried out as both locks have been cleared and subject to no trip action and satisfactory correspondence between rotation direction and telegraph reply lever indication fuel can be admitted following the full sequence of air starting. It is obvious that this system has a large amount of auto-control and is easily adjusted for bridge control.
The function is to withhold fuel supply during manoeuvering if the running direction of the engine is not coincident with the setting of the engine telegraph lever.
At the camshaft forward end the shaft is coupled to the camshaft and carries round with it , due to the key , a flanged bush and spring plates which cause an adjustable friction pressure axially due to the springs and nut.
This pressure acts on the coupling disc which rotates through an angular travel T until the stop pin prevents further rotation. This causes angular rotation of a fork lever and the re-positioning of a control valve plug in a new position within the sleeve. Oil pressure from the reversing valve can only pass to the block valve ( air ) and unlock the start lever and the fuel control if the rotation of the direction interlock is correct. If the stop pin were to break the fork lever would swing to position M and the fuel supply would be blocked.
If the engine direction is reversed at this point, then air will enter the pneumatic cylinder as shown and will move the piston to the right. The cam follower will be moved across and would finish in the position shown which would be at the correct fuel pump timing for running astern.
It should be noted that the reversal of the follower only takes place while the engine is rotating. If the engine had been stopped from running ahead, and then started astern, the fuel pump followers would move across as the engine starts to rotate, and before the fuel is admitted by venting the fuel pump puncture valves
A micro switch shown on the LHS detects whether the follower has moved across. If not, an indicator light is lit in the control room, However the engine will still start if a follower fails to move, perhaps due to corrosion in the servo cylinder. A high exhaust temperature deviation alarm would operate within a short time. Allowing the engine to start in this situation could be useful during manoeuvring in confined waters.
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