Page 32 - Introduction to Marine Engineering

20 Diesel engines

arrangements have low temperature air and high temperature exhaust
gas passing through adjacent ports, causing temperature differential
problems for the liner material. Uniflow is the most efficient scavenging
system but requires either an opposed piston arrangement or an exhaust
valve in the cylinder head. All three systems have the ports angled to
swirl the incoming air and direct it in the appropriate path.

Scavenge fires
Cylinder oil can collect in the scavenge space of an engine. Unburned
fuel and carbon may also be blown into the scavenge space as a result of
defective piston rings, faulty timing, a defective injector, etc. A build-up
of this flammable mixture presents a danger as a blow past of hot gases
from the cylinder may ignite the mixture, and cause a scavenge fire.
A loss of engine power will result, with high exhaust temperatures at
the affected cylinders. The affected turbo-chargers may surge and sparks
will be seen at the scavenge drains. Once a fire is detected the engine
should be slowed down, fuel shut off from the affected cylinders and
cylinder lubrication increased. All the scavenge drains should be closed.
A small fire will quickly burn out, but where the fire persists the engine
must be stopped. A fire extinguishing medium should then be injected
through the fittings provided in the scavenge trunking. On no account
should the trunking be opened up.
To avoid scavenge fires occurring the engine timing and equipment
maintenance should be correctly carried out. The scavenge trunking
should be regularly inspected and cleaned if necessary. Where carbon or
oil build up is found in the scavenge, its source should be detected and
the fault remedied. Scavenge drains should be regularly blown and any
oil discharges investigated at the first opportunity.

Fuel oil system

The fuel oil system for a diesel engine can be considered in two
parts—the fuel supply and the fuel injection systems. Fuel supply deals with
the provision of fuel oil suitable for use by the injection system.

Fuel oil supply for a two-stroke diesel
A slow-speed two-stroke diesel is usually arranged to operate con-
tinuously on heavy fuel and have available a diesel oil supply for
manoeuvring conditions.
In the system shown in Figure 2.11, the oil is stored in tanks in the
double bottom from which it is pumped to a settling tank and heated.