Page 27 - Introduction to Marine Engineering

Diesel engines 15

also joined to the crosshead bearing. Ports are arranged in the cylinder
liner for air inlet and a valve in the cylinder head enables the release of
exhaust gases. The incoming air is pressurised by a turbo-blower which
is driven by the outgoing exhaust gases. The crankshaft is supported
within the engine bedplate by the main bearings. A-frames are mounted
on the bedplate and house guides in which the crosshead travels up and
down. The entablature is mounted above the frames and is made up of
the cylinders, cylinder heads and the scavenge trunking.

Comparison of two-stroke and four-stroke cycles
The main difference between the two cycles is the power developed. The
two-stroke cycle engine, with one working or power stroke every
revolution, will, theoretically, develop twice the power of a four-stroke
engine of the same swept volume. Inefficient scavenging however and
other losses, reduce the power advantage to about 1.8. For a particular
engine power the two-stroke engine will be considerably lighter—an
important consideration for ships. Nor does the two-stroke engine
require the complicated valve operating mechanism of the four-stroke.
The four-stroke engine however can operate efficiently at high speeds
which offsets its power disadvantage; it also consumes less lubricating
oil.
Each type of engine has its applications which on board ship have
resulted in the slow speed (i.e. 80— 100 rev/min) main propulsion diesel
operating on the two-stroke cycle. At this low speed the engine requires
no reduction gearbox between it and the propeller. The four-stroke
engine (usually rotating at medium speed, between 250 and 750 rev/
min) is used for auxiliaries such as alternators and sometimes for main
propulsion with a gearbox to provide a propeller speed of between 80
and 100 rev/min.

There are two possible measurements of engine power: the indicated
power and the shaft power. The indicated power is the power developed
within the engine cylinder and can be measured by an engine indicator.
The shaft power is the power available at the output shaft of the engine
and can be measured using a torsionmeter or with a brake.

The engine indicator
An engine indicator is shown in Figure 2.8. It is made up of a small
piston of known size which operates in a cylinder against a specially