Page 43 - Introduction to Marine Engineering

30 Diesel engines

arranged in duplicate with one as standby. The fine filters will be
arranged so that one can be cleaned while the other is operating. After
use in the engine the lubricating oil drains back to the sump or drain
tank for re-use. A level gauge gives a local read-out of the drain tank
contents. A centrifuge is arranged for cleaning the lubricating oil in the
system and clean oil can be provided from a storage tank.
The oil cooler is circulated by sea water, which is at a lower pressure
than the oil. As a result any leak in the cooler will mean a loss of oil and
not contamination of the oil by sea water.
Where the engine has oil-cooled pistons they will be supplied from the
lubricating oil system, possibly at a higher pressure produced by booster
pumps, e.g. Sulzer RTA engine. An appropriate type of lubricating oil
must be used for oil-lubricated pistons in order to avoid carbon deposits
on the hotter parts of the system.

Cylinder lubrication

Large slow-speed diesei engines are provided with a separate lubrication
system for the cylinder liners. Oil is injected between the liner and the
piston by mechanical lubricators which supply their individual cylinder,
A special type of oil is used which is not recovered. As well as lubricating,
it assists in forming a gas seal and contains additives which clean the
cylinder liner.

Cooling

Cooling of engines is achieved by circulating a cooling liquid around
internal passages within the engine. The cooling liquid is thus heated up
and is in turn cooled by a sea water circulated cooler. Without adequate
cooling certain parts of the engine which are exposed to very high
temperatures, as a result of burning fuel, would soon fail. Cooling
enables the engine metals to retain their mechanical properties. The
usual coolant used is fresh water: sea water is not used directly as a
coolant because of its corrosive action. Lubricating oil is sometimes used
for piston cooling since leaks into the crankcase would not cause
problems. As a result of its lower specific heat however about twice the
quantity of oil compared to water would be required.

Fresh water cooling system
A water cooling system for a slow-speed diesei engine is shown in Figure
2.18. It is divided into two separate systems: one for cooling the cylinder
jackets, cylinder heads and turbo-blowers; the other for piston cooling.