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Reversing gears of a marine diesel engine
How the reversing gears works ? :
The diesel engine is a type of internal combustion engine which ignites
the fuel by injecting it into hot, high-pressure air in a combustion
chamber. In common with all internal combustion engines the diesel
engine operates with a fixed sequence of events, which may be achieved
either in four strokes or two, a stroke being the travel of the piston
between its extreme points. Each stroke is accomplished in half a
revolution of the crankshaft.
The gearing arrangement used to reduce the medium-speed engine
drive down to suitable propeller revolutions . Where a gearbox is used with a diesel engine, reversing gears may be
incorporated so that the engine itself is not reversed. Where a
controllable pitch propeller is in use there is no requirement to reverse
the main engine.
However, when it is necessary to run the engine in
reverse it must be started in reverse and the fuel injection timing must be
changed. Where exhaust timing or poppet valves are used they also must
be retimed. With jerk-type fuel pumps the fuel cams on the camshaft
must be repositioned. This can be done by having a separate reversing
cam and moving the camshaft axially to bring it into position.
Alternatively a lost-motion clutch may be used in conjunction with the
ahead pump-timing cam.

Fig: Reversing arrangements
The fuel pump cam and lost-motion clutch arrangement is shown in
Figure . The shaping of the cam results in a period of pumping first
then about 10° of fuel injection before top dead centre and about 5° after
top dead centre. A period of dwell then occurs when the fuel pump
plunger does not move.
A fully reversible cam will be symmetrical about
this point, as shown. The angular period between the top dead centre
points for ahead and astern running will be the 'lost motion' required for
astern running. The lost-motion clutch or servo motor uses a rotating
vane which is attached to the camshaft but can move in relation to the
camshaft drive from the crankshaft. The vane is shown held in the
ahead operating position by oil pressure.
When oil is supplied under
pressure through the drain, the vane will rotate through the lost-motion
angular distance to change the fuel timing for astern operation. The
starting air system is retimed, either by this camshaft movement or by a
directional air supply being admitted to the starting air distributor, to
reposition the cams. Exhaust timing or poppet valves will have their own
lost-motion clutch or servo motor for astern timing.
Related Information:
Starting air system for diesel engine - how it works
Couplings, clutches and gearboxes of a marine diesel engine
Difference between two stroke and four stroke cycle diesel engines
Guideline for work in machinery spaces on board cargo ship
Four stroke cycle diesel engines
The fuel oil system for a diesel engine
Function of fuel injector
Lubricating oil system for a marine diesel engine
Fresh water & sea water cooling system
Two stroke cycle diesel engines
Scavenging methods
Dealing scavange fires
Heat exchanger for running machinery
Starting air system for diesel engine
Power measurement -Engine indicator
Sea water circulation of coolers
Gearing arrangement
Function of governors
MAN B&W diesel engine
Sulzer RTA72U diesel engine
Couplings, clutches and gearboxes of a marine diesel engine
Explosion relief valve
Cylinder relief valve
Diesel engine Turning gear
Crankcase oil mist detector
Marine machineries - Useful tags
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