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Marine auxiliary diesel engine general construction
Over recent years, many owners have elected to burn low grade residual fuels
in medium-speed auxiliary engines, sometimes with disastrous results. Fuels
bunkered for slow-speed main engines may be of too poor a quality for use in
auxiliaries even where the engines have been designed for heavy fuel
operation.
Major problems have been experienced on large slow-speed
engines with some of the poor quality bunkers such as those containing
catalytic fines. Fuel should conform to the specification given in the instruction
book for the engine.
Typical of the medium-speed engines designed to be capable of running on
heavy fuel are the Allen S12 series in-line engines having four, six, eight
designated VS12 which are produced with twelve or sixteen cylinders.
General construction (based on the Allen S12 engine)
The in-line S12 engine structure (Figure 7.1) is based on a deep section cast iron
bedplate and a cast iron A-frame of monobloc construction which are flanged
and bolted together. The bedplate carries thin wall, steel-backed, white-metal
or aluminium-tin lined main bearings.
An additional bearing is incorporated to
carry the combined loads of the flywheel and part of the weight of the
generator. Access doors are provided at the front and back. Those on the back
of the engine are fitted with crankcase explosion relief valves.
In this style of construction, which is common to many medium-speed
engines, it is necessary to lift the A-frame if the crankshaft is to be removed.
Some designs incorporate a C-frame arrangement which permits side removal
of the crankshaft.
The one-piece alloy steel crankshaft is slab-forged, oil-hardened and
tempered. A solid half coupling forged integrally carries the flywheel to which
the generator is coupled. The main coupling bolts pass through the crankshaft
half coupling, the flywheel and the generator half coupling; two additional
bolts are incorporated to retain the flywheel on the crankshaft when the
generator is uncoupled.
Additional machines such as an air compressor or a
bilge pump may be driven from the free end of the crankshaft, through a clutch.
Axial location of the crankshaft is maintained by renewable thrust rings. Drilled
passages in the crankshaft feed oil from the main to the connecting rod
bearings. In the S12-F engine these oil passages are arranged to provide a
continuous supply of oil to the cooled pistons. Where necessary, balance
weights are bolted to the crank-webs. In four-cylinder engines having cranks at
180°, secondary balancing gear is required (Figure 7.2).
Figure 7,1 Six cylinder S12-F engine (APE-Allen Ltd)
Figure 7,2 Secondary balancing gear (APE-Allen Ltd)
The connecting rods are H-section steel forgings, bored to carry oil to the
gudgeon pin bush, which is a light interference fit in the rod. Crankpin bearings
are thin-walled steel, lined with aluminium-tin, split horizontally and secured
by four fitted bolts. Connecting rods of differing designs will be found in some
engines. Thus the bottom end assembly for the Allen VS12 (vee engine) series
(Figure 7.3) is made simple by axially displacing both banks of cylinders and
having two connecting rod bottom end bearings on each crankpin.
The large
end bearing is split diagonally to enable the connecting rod to be withdrawn
through the cylinder. Bearing butts are serrated for location. Bronze bushes are
used for the top ends; the large ends have steel backed, aluminium-tin bearings
with an overlay which is deposited by plating.
Figure 7.3 Valve timing diagram for turbo charged four stroke engine
(APE-Atlen Limited)
The fully-floating gudgeon pin of the Allen S12 is steel, case-hardened and
ground, retained in the aluminium alloy piston by circlips. The piston has an
integrally cast alloy iron carrier for the top piston ring, two additional pressure
rings and one slotted oil scraper ring, all above the gudgeon pin. Pistons for the
S12-F are one-piece aluminium-alloy castings incorporating oil cooling
cavities.
The oil provides intensive cooling of the piston particularly in the
region of the ring belt. The wet-type close grained, cast iron cylinder liners, are
supported by a flange at the top which is sandwiched between the cylinder
head and a spigot with ring gasket, on the engine frame. To permit vertical
expansion of the liner it is free to move at its lower end, a seal being effected by
two synthetic compound O rings carried in grooves in the liner wall
Camshaft and cylinder head
The camshaft, is driven by a roller chain (some engines have a gear train). To
allow accurate phasing of crankshaft and camshaft during initial set up and if
timing has to be reset, elongated holes are provided at the coupling between
the camshaft drive wheel and the camshaft. Adjustable packing pieces inserted
into the elongated holes ensure that correct timing is maintained.
Lubrication of the camshaft bearings is by a forced feed system; an oiiway
bored through the full length of the camshaft conducting the oil to the
bearings. An extension of the camshaft at the driving end, is provided with a
flexible coupling for the hydraulic governor and tachometer drive.
The individual alloy cast iron cylinder heads have totally enclosed valve
gear which is lubricated from the engine oil system. The S12-D engine has one
inlet and one exhaust valve, the S12-F, because of its higher running speed, is
fitted with heads having two inlet and two exhaust valves.
The valve pairs are
parallel and operated by rocking levers and guided bridges. Each valve has two
springs and Is fitted with a rotator. The valves seat in the cylinder heads on
renewable inserts of iron alloy. The centrally placed fuel injector is situated
between the valve covers so that fuel oil contamination of lubricating oil is
avoided. This also enables the injectors to be withdrawn for servicing without
disturbing the valve covers.
Fuel pump and timing diagrams
Separate camshaft-actuated helix-type fuel pumps are employed for each
cylinder. These deliver fuel to the injectors which are set to lift at a pressure of
176 kg/cm2 on the S12-D and 211 kg/cm2 on the S12-F, Fuel pump delivery
volume is controlled by a rack which alters the cut-off or spill point. The racks
are linked through a control shaft to the engine governor which thus regulates
the end of the fuel delivery period and hence the quantity of fuel delivered
according to the power required. Fuel injection commences at approximately
15° before top-dead-centre and takes place over a period of about 35° of crank
angle. Combustion should be completed within this period.
A typical valve
timing diagram (Figure 7.3) shows the large overlap between the opening of
the air inlet and closure of the exhaust compared with the previous normally
aspirated engine (Figure 7.4) which is no longer produced. The overlap allows a
through flow of charge air which is essential for exhaust valve cooling,
particularly for an engine which operates using residual fuel.
Vanadium and
sodium ash from the fuel tends to adhere to valves and seats if their
temperature exceeds the melting point of the ash. The deposited ash itself
causes surface damage in the form of pitting and also tends to prevent closure.
Valve surface temperature should ideally not exceed 420°C if ash deposit is to
be avoided. Localized high surface temperature can be prevented on that part
of the valve adjacent to the fuel injector by the rotator. Valve surfaces can be
protected by a stellite deposit or, alternatively, valves can be made from nimonic.
The turbo-blower, mounted at the free end of the engine, has a filter/silencer
fitted on the air intake. The charge air cooler is similar to that described in
Chapter I,
Turbochargers
The turbocharger is driven by the exhaust gas leaving the cylinders of the
diesel engine it serves. The gas has sufficient pressure and heat when released
from the cylinder at exhaust opening, to drive the turbocharger. It is directed
on to turbine blades by nozzles which are built into a nozzle ring in the axial
flow type or into a radial turbine from the peripheral volute casing of smaller
turbochargers.
Figure 7.4 Valve timing diagram for normally-aspirated four-stroke engine
(APE-Allen Limited)
A small turbocharger for a generator diesel prime mover may be driven by a
radial flow gas turbine, which closely resembles the impeller it drives. This type
of machine costs less to manufacture than the axial flow turbine, and has a
simpler construction.
Turbocharger blades are rotated, partly by the impact of jets of gas from the
nozzles and partly by the reaction, as gases leave the blades. Correct nozzle and
blade shape is vital. Performance of turbocharger and engine can deteriorate
seriously with sometimes very moderate surface marking due to erosion or
corrosion.
Nozzle shape can be altered in service, by:
1 deposit build up;
2 corrosion of surfaces;
3 erosion, by solids entrained in the exhaust gas.
Oil refinery residuals are used in a blend with clean distillate fuels for
economy in some engines. Deposits are common when heavy residual fuel is
used. Regular cleaning is necessary to prevent nozzle blockage. Water washing
rather than dismantling and cleaning is used for deposit removal. Fittings
should be provided for water washing if an engine is to be operated using
residual fuel.
Corrosion is a potential problem for the turbochargers of engines which
operate using residual fuels containing vanadium, sodium and sulphur as
impurities and in a marine environment where sodium chloride is present in the
intake air. The impurities listed burn to form a number of different ash products
which may adhere to surfaces at higher temperatures. Corrosion and surface
damage follows breakdown of the protective film on the metal surface by ash
compounds.
Remedies for these problems are based on designing for lower operating
temperatures and regular water washing to remove the accumulated slag (ash).
Corrosion problems are well known and documented in various technical
papers such as those of the Institute of Marine Engineers.
Erosion by solids entrained in the exhaust gas is another potential problem.
Catalytic fines (based on aluminium and silicon which are abrasive) will be
present in some fuels and could cause surface damage to nozzles. Purification of
fuels is necessary to remove solids and where catalytic fines are suspected, the
use of centrifuges arranged as two purifiers in parallel or a purifier and a clarifier
in series is recommended .
Serious damage by corrosion or erosion will finally require renewal of parts
if the efficiency of the turbocharger and diesel is to be maintained.
Summarized below some of the basic procedure of marine auxiliary machinery :
- Auxiliary engine general construction
Major problems have been experienced on large slow-speed
engines with some of the poor quality bunkers such as those containing
catalytic fines. Fuel should conform to the specification given in the instruction
book for the engine.
......
- Auxiliary engine back pressure turbine
Many ships have used an auxiliary steam turbine as a primary pressure reducing stage before passing the steam to other auxiliaries demanding steam at a substantially lower pressure than that available. Such an arrangement gives a heat balance which is far more favourable than that obtained with a pressure reducing valve......
- Auxiliary engine fuel pump
The most common fuel pump used on auxiliary diesel engines is the Bosch
type. This is a cam operated jerk pump with a helical groove on the plunger to
control the fuel cut-off and therefore the quantity of fuel delivered to the
cylinder for combustion.
......
- Auxiliary engine common fuel injector
Fuel is delivered to an annular space in the nozzle via a hole, drilled through
the nozzle body from the inlet. The nozzle valve is forced from its seat in the
nozzle body by the pressure of fuel from the pump, acting on the shoulder of
the needle valve.
......
- Auxiliary engine cooling system
A variety of cooling systems may be adopted for marine auxiliary engines but
the most commonly used is the simple closed circuit system . Sea
water is passed through the intercooler, the oil cooler and then the jacket water
cooler in series flow.
......
- Auxiliary engine hydraulic governor
When used for alternating current power generation, a diesel engine is normally fitted with a hydraulic governor. This incorporates a centrifugal speed sensing device (spring loaded flyweights) controlling a suitably damped oil operated servo-cylinder through a pilot valve.
......
- Auxiliary engine speed governing system
Unlike propulsion turbines, generator turbines work at constant speed and must be governed accordingly. Classification Society rules require that there must be only a 10% momentary and a 6% permanent variation in speed when full load is suddenly taken off or put on.
......
- Auxiliary engine tracing faults
The failure of an engine to start or problems while running may be traced to
faults with the fuel injection system or other possible causes. Instruction
manual guidance on fault finding and remedies will include some of the typical
problems
......
- Generators driven from the main propulsion
Generators can variously be driven from the propeller shaft, through a gearbox or by being mounted on the engine itself.
......
- Exhaust gas boilers
The original exhaust gas boilers or economizers were of simple construction and produced, from the low powered engines of the time, a very moderate amount of steam. As large slow speed engine powers increased, the larger quantity of steam that could be generated from otherwise wasted exhaust energy,
......
- Auxiliary engine Turbo generator construction
Turbo-generator construction-For electrical power generation, turbines are conventionally horizontal axial flow machines of the impulse reaction type. They may exhaust either to an integral condenser (invariably underslung) or to a separate central auxiliary condenser or the ship's main condenser.
......
- Caterpillar engine fuel system
The range of larger Caterpillar engines use helix-type fuel pumps driven from a
separate camshaft.......
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