Page 52 - Introduction to Marine Engineering

Diesel engines 39

Cover. _
Spring retaining _
plate
Spring

Valve carrier 1
Valve guide / / Gauze assembly
Valve' 0<rin 9
Figure 2.24 Crankcase explosion relief valve

holds the valve closed against its seat and a seal ring completes the joint.
A deflector is fitted on the outside of the engine to safeguard personnel
from the outflowing gases, and inside the engine, over the valve
opening, an oil wetted gauze acts as a flame trap to stop any flames
leaving the crankcase. After operation the valve will close automatically
under the action of the spring.

Turning gear

The turning gear or turning engine is a reversible electric motor which
drives a worm gear which can be connected with the toothed flywheel to
turn a large diesel. A slow-speed drive is thus provided to enable
positioning of the engine parts for overhaul purposes. The turning gear
is also used to turn the engine one or two revolutions prior to starting.
This is a safety check to ensure that the engine is free to turn and that no
water has collected in the cylinders. The indicator cocks must always be
open when the turning gear is operated.

Medium- and slow-speed diesels

Medium-speed diesels, e.g. 250 to 750 rev/min, and slow-speed diesels,
e.g. 100 to 120 rev/min, each have their various advantages and
disadvantages for various duties on board ship.