Page 53 - Introduction to Marine Engineering

40 Diesel engines

The slow-speed two-stroke cycle diesel is used for main propulsion
units since it can be directly coupled to the propeller and shafting. It
provides high powers, can burn low-grade fuels and has a high thermal
efficiency. The cylinders and crankcase are isolated, which reduces
contamination and permits the use of specialised lubricating oils in each
area. The use of the two-stroke cycle usually means there are no inlet
and exhaust valves. This reduces maintenance and simplifies engine
construction.
Medium-speed four-stroke engines provide a better power-to-weight
ratio and power-to-size ratio and there is also a lower initial cost for
equivalent power. The higher speed, however, requires the use of a
gearbox and flexible couplings for main propulsion use. Cylinder sizes
are smaller, requiring more units and therefore more maintenance, but
the increased speed partly offsets this. Cylinder liners are of simple
construction since there are no ports, but cylinder heads are more
complicated and valve operating gear is required. Scavenging is a
positive operation without use of scavenge trunking, thus there can be
no scavenge Fires. Better quality fuel is necessary because of the higher
engine speed, and lubricating oil consumption is higher than for a
slow-speed diesel. Engine height is reduced with trunk piston design and
there are fewer moving parts per cylinder. There are, however, in total
more parts for maintenance, although they are smaller and more
manageable.
The Vee engine configuration is used with some medium-speed
engine designs to further reduce the size and weight for a particular
power.

Couplings, clutches and gearboxes

Where the shaft speed of a medium-speed diesel is not suitable for its
application, e.g. where a low speed drive for a propeller is required, a
gearbox must be provided. Between the engine and gearbox it is usual
to fit some form of flexible coupling to dampen out vibrations. There is
also often a need for a clutch to disconnect the engine from the gearbox.

Couplings

Elastic or flexible couplings allow slight misalignment and damp out or
remove torque variations from the engine. The coupling may in
addition function as a clutch or disconnecting device. Couplings may be
mechanical, electrical, hydraulic or pneumatic in operation. It is usual to
combine the function of clutch with a coupling and this is not readily
possible with the mechanical coupling.