Page 56 - Introduction to Marine Engineering

Diesel engines 43

and usually single helical. Reduction ratios range from about 2:1 to 4:1
on modern installations.
Pinion and gearwheel arrangements will be similar to those for steam
turbines as described in Chapter 3, except that they will be single helical
or epicyclic.

Reversing

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.
The fuel pump cam and lost-motion clutch arrangement is shown in
Figure 2.27. 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.

Some typical marine diesel engines

Sulzer
The RTA72U is a single-acting, low-speed, two-stroke reversible marine
diesel engine manufactured by New Sulzer Diesel Ltd. It is one of the
RTA series engines which were introduced in 1981 and in addition to a