when it comes to transportation of cargo and ship
it is not just about loading and discharging
there are several other operations involved such as
navigation, mooring, maneuvering, anchoring, channel crossing and so on but at the
heart of the ship operations is the most important machinery, that is the
propulsion plant of the ship or marine engine.
The marine engine allows the vessel to maneuver
accelerate and stop whenever required
now as you all must be aware of, ships do not have brakes
which can be used whenever they want to stop. So how do ships actually
stop? let's understand in this video, but before that please do subscribe to
our youtube channel and press the bell icon to view all our
future videos. Back to the topic, the halting of the
ship is done using the marine engine. Whenever a ship
needs to be stopped, reversing of the marine engine or the
propeller direction is carried out to generate an opposite
thrust which ultimately reduces the ship's
speed and makes it stop as required. This whole process
might sound very simple but the engine is a complex machinery and so is the
reversing operation it undergoes. Let's understand how the reversing
process is carried out. The two-stroke marine engine used in
ships is made up of several parts which help it to operate
efficiently and safely. some of these components work together
in order to reverse the engine.
unlike medium or small sized internal combustion engines
the two-stroke marine engine does not have a clutch and
gear systems to reverse the direction of the propeller.
The entire engine rotation needs to be reversed
for change of thrust direction which in turn will reverse
the ship motion. In large marine diesel engines the exhaust and
scavenging timing are almost symmetrical in reverse and ahead direction, hence
in order to carry out reverse direction, the two important components
which needs to be changed are fuel pump timing
and air injection timing. Now let's understand how each of these
can be done. Fuel pump comprises a camshaft a
negative cam and a cam follower which operates the
plunger of the pump. The fuel pump will deliver the fuel when
the cam follower operates on top of the cam. Here a
pneumatic valve is installed which shifts the cam follower roller to
alter the timing when the reverse or astern command is given.
Compressed air from the starting system is used to actuate the pneumatic
cylinder and piston which displaces each follower
unit. If the position is not properly locked
in its place the fuel delivery will be stopped.
This is done by a sensor fitted to each pump
now once the engine is reversed and cam follower is shifted
the timing when the injection happens will change
when the fuel pump timing is altered for reversing of the engine
that is the firing order is altered it is important
to supply the air into the cylinder as per the new reverse firing order
else the reversal of engine will not take place
for this purpose the large mc engines use
a radial air start distributor operated by two cams
fixed on the end of the camshaft
The pilot walls arranged radially in housing
are either pushed by pilot air onto the inner cam profile when starting
ahead or onto the outer profile when starting
astern. When not in use a spring holds the pilot
valve follower clear of both profiles thus reducing
wear. If the engine needs to be moved in
forward direction the telegraph is moved to the ahead
position. Till now the starting air is not
supplied to the air distributor
once the ahead signal is given, the air signal goes to the starting air cutoff
valve, making it inactive. This allows the
starting air to go to the air distributor
and all the air starting valves via the air control valve.
Now the air from the air distributor will be supplied to different cylinders
as per the firing order. As the ahead cam rotates
the plunger which falls on the cam will move
allowing the air to pass through, delivering starting air to the
combustion chamber. When the astern command is given, the
astern air signal will shift the distributor cam and the
second cam which is responsible for astern firing
order will come into play. The astern air signal will make the air
cutoff valve inactive supplying the starting air from the air
manifold to the air distributor.
Now the air from the air distributor will be supplied to different cylinders
as per the reverse firing order, because the astern cam will operate
each unit plunger in aseron firing sequence as it rotates
allowing the starting air to go to the combustion chamber.
In other engines like mitsubishi, the camshaft contains
two cams one for ahead and one for astern direction for each unit.
The entire camshaft is axially moved to change the cam and hence the firing
order
in Sulzer engines, the cam alone is moved with respect to the shaft by use of
servo motor. Cam shaft are an integral part of any two stroke
engine as without them there will not be any fuel injection