Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines

Подождите немного. Документ загружается.

472 Burmeister & Wain Low-Speed Engines

Cylinder Lubricators

The cylinder lubricators are mounted on the cylinder frame, one per cylinder,

and interconnected with shaft pieces. The lubricators have built-in adjustment of

the oil quantity. They are of the ‘sight feed lubricator’ type, and each lubricating

point has a glass. The oil is led to the lubricator through a pipe system from an

elevated tank. A heating element rated at 75 W is built into the lubricator.

Manoeuvring System (Without Bridge Control)

The engine is provided with a pneumatic manoeuvring and fuel oil regulating

system, which transmits orders from the separate manoeuvring console to the

engine.

By means of the regulating system, it is possible to start, stop, reverse

and control the engine. The speed control handle in the manoeuvring console

activates a control valve, which gives a pneumatic speed-setting signal to the

governor depending on the desired number of revolutions. The start and stop

functions are controlled pneumatically. At a shutdown function, the fuel pumps

are moved to zero position independent of the speed control handle.

Reversing of the engine is controlled pneumatically through the engine tel-

egraph and is effected via the telegraph handle.

Reversing takes place by moving the telegraph handle from ‘ahead’ to

‘astern’ and by moving the speed control handle from ‘stop’ to ‘start’ position.

Control air then moves the starting air distributor and, through the pressurizer,

the reversing mechanism to the ‘astern’ position.

turning Gear and turning Wheel

The turning wheel has cylindrical teeth and is ﬁtted to the thrust shaft; it is

driven by a pinion on the terminal shaft of the turning gear, which is mounted

on the bedplate. The turning gear is driven by an electric motor with built-

in gear and brake. Further, the gear is provided with a blocking device that

prevents the main engine from starting when the turning gear is engaged.

Engagement and disengagement of the turning gear is executed by axial trans-

fer of the pinion.

Gallery Brackets

The engine is provided with gallery brackets placed at such a height that the

best possible overhaul and inspection conditions are obtained. The main pipes

of the engine are suspended in the gallery brackets.

A crane beam is placed on the brackets below the centre gallery manoeu-

vring side.

Scavenging air System

The air intake to the turbocharger takes place directly from the engineroom

through the intake silencer of the turbocharger. From the turbocharger the air is

led via charging air pipe, air cooler and scavenging air pipe to the scavenging

ports of the cylinder liner. The charging air pipe between turbocharger and air

cooler is provided with a compensator and insulated.

Exhaust turbocharger

The engine is as standard arranged with MAN or BBC (now ABB) turbocharg-

ers. The turbochargers are provided with a connection for Disatac electronic

tachometers, and prepared for signal equipment to indicate excessive vibration

of the turbochargers. For water cleaning of the turbine blades and the nozzle

ring during operation, the engine is provided with connecting branches on the

exhaust receiver in front of the protection grid.

Exhaust Gas System

From the exhaust valves the gas is led to the exhaust gas receiver where the

ﬂuctuating pressure will be equalized and the gas led further on to the tur-

bochargers with a constant pressure. After the turbochargers, the gas is led

through an outlet pipe and out in the exhaust pipe system.

The exhaust gas receiver is made in one piece for every cylinder and con-

nected to compensators. There are also inserted compensators between the

receiver and the exhaust valves and between the receiver and the turbocharger.

For quick assembling and dismantling of the joints between the exhaust

gas receiver and the exhaust valves, a clamping band is ﬁtted. The exhaust gas

receiver and exhaust pipe are provided with insulation covered by a galvanized

steel plate.

There is a protection grid between the exhaust gas receiver and each

turbocharger.

auxiliary Blower

The engine is provided with two electrically driven blowers, which are

mounted in each end of the scavenging air receiver as standard. The suction

sides of the blowers are connected to the pipes from the air coolers, and the

non-return valves on the top of the outlet pipes from the air coolers are closed

as long as the auxiliary blowers can give a supplement to the scavenging air

pressure.

The auxiliary blowers will start operating before the engine is started and

will ensure complete scavenging of the cylinders in the starting phase, which

gives the best conditions for a safe start.

During operation of the engine, the auxiliary blowers will start automati-

cally every time the engine load is reduced to about 30–40 per cent, and they

will continue operating until the load is again increased to over approximately

40–50 per cent.

In cases when one of the auxiliary blowers is out of service, the other aux-

iliary blower will automatically function correctly in the system without any

manual readjustment of the valves being necessary. This is secured by auto-

matic non-return valves in the suction pipe of the blowers.

L-GB type engines 473

474 Burmeister & Wain Low-Speed Engines

Starting air System

The starting air system contains a main starting valve (two ball valves with

actuators), a non-return valve, a starting air distributor and starting valves. The

main starting valve is combined with the manoeuvring system, which controls

start and ‘slow turning’ of the engine. The slow turning function is actuated

manually from the manoeuvring stand.

The starting air distributor regulates the control air to the starting valves so

that these supply the engine with air in the ﬁring order. The distributor has one

set of starting cams for ‘ahead’ and ‘astern’, respectively, and one control valve

for each cylinder.

475

C h a p t e r | f o u r t e e n

Doxford Low-Speed

Engines

The last British-designed low-speed two-stroke engine—the distinctive

Doxford opposed-piston design—was withdrawn from production in 1980, but

a few of the J-type (Figure 14.1) remain in service and merit description. In its

ﬁnal years the company also designed and produced the unusual three-cylinder

58JS3C model, which developed 4050 kW at 220 rev/min and was speciﬁed

to power several small container ships. The 58JS3C design (Figure 14.2) was

FigurE 14.1 A Doxford 76J4 engine on the testbed

476 Doxford Low-Speed Engines

based on the J-type but with reﬁnements addressing the higher rotational speed

and relatively short piston stroke.

DoxForD J-typE

Doxford J-type engines were built in long- and short-stroke versions in bore

sizes of 580 mm, 670 mm and 760 mm, and with three to nine cylinders deliv-

ering up to around 20 000 kW (Table 14.1). The engine is a single-acting two-

stroke opposed-piston type with each cylinder having two pistons, which move

in opposite directions from a central combustion chamber.

The pistons in each cylinder are connected to a three-throw section of the

crankshaft, the lower piston being coupled to the centre throw by a single con-

necting rod, crosshead and piston rod. Each pair of the side cranks is connected

to the upper piston by two connecting rods, crossheads and side rods. As the

pistons move towards each other, air is compressed in the cylinder, and, shortly

before reaching the point of minimum volume between the pistons (inner dead

centre), fuel at high pressure is injected through the injector nozzles into the

combustion space (Figure 14.3).

FigurE 14.2 Cutaway drawing of a 58JS3C engine

Doxford J-type 477

table 14.1 Doxford low-speed engines

Engine type 58J4 67J4 67J5 67J6 76J3 76J4 76J5 76J6 76J7 76J8 76J9

Number of cylinders 4 4 5 6 3 4 5 6 7 8 9

Cylinder bore (mm) 580 670 670 670 760 760 760 760 760 760 760

Stroke upper piston (mm) 480 500 500 500 520 520 520 520 520 520 520

Stroke lower piston (mm)

1 370 1 640 1 640 1 640 1 660 1 660 1 660 1 660 1 660 1 660 1 660

Combined piston stroke (mm) 1 850 2 140 2 140 2 140 2 180 2 180 2 180 2 180 2 180 2 180 2 180

Number of turbochargers 1 1 2 2 1 1 2 2 3 3 4

Weight (excluding oil and water,

tonnes)

175 270 320 370 270 335 400 470 540 600 680

(Continued)

478 Doxford Low-Speed Engines

table 14.1 (Continued)

Engine type 58J4 67J4 67J5 67J6 76J3 76J4 76J5 76J6 76J7 76J8 76J9

Ratings

Maximum continuous rating

(mcr) (bhp metric)

7 500 9 000 11 000 13 500 9 000 12 000 15 000 18 000 21 000 24 000 27 000

Engine speed (rev/min) 160 127 127 127 123 123 123 123 123 123 123

Brake mean eective pressure

(bmep) (bar)

10.57 10.34 10.34 10.34 10.81 10.81 10.81 10.81 10.81 10.81 10.81

recommended max service

power (90% mcr)

6 750 8 100 9 900 12 200 8 100 10 800 13 500 16 200 18 900 21 600 24 300

Engine speed (rev/min) 155 123 123 123 119 119 119 119 119 119 119

Brake mean eective pressure

(bmep) (bar)

9.82 9.61 9.61 9.61 10.06 10.06 10.06 10.06 10.06 10.06 10.06

Consumption

Fuel consumption

150–153 g/bhp h with HV fuel of 10 000 kcal/kg

Cylinder lubricating oil 0.45 g/bhp h

During the ﬁrst stages of combustion, the pressure in the cylinder contin-

ues to rise until a maximum value is reached soon after the pistons begin to

move apart. After combustion is completed, the hot gases continue to expand,

thereby forcing the pistons apart until the exhaust ports in the upper liner are

uncovered by the upper piston. As the exhaust ports open, the hot gases in the

cylinder, now at a reduced pressure, are discharged to the turbine of the turbo-

chargers, so causing the pressure in the cylinder to drop to a level just below

that of the scavenge air. At this point the air inlet ports in the lower liner are

uncovered by the lower piston, so allowing air under pressure, which is deliv-

ered to the scavenge space by the turbocharger, to ﬂow through the cylinder

and expel the remaining burnt gases (Figure 14.4a).

During the inward compression stroke of the pistons, the air inlet ports are

closed just before the exhaust ports. The air in the cylinder is then compressed

and the cycle repeated. The opening and closing of the exhaust and air inlet

ports are shown diagrammatically in the typical timing diagram (Figure 14.4b).

This diagram is constructed with reference to the inner dead centre position;

the unsymmetrical timing of the ports is due to the fact that the side cranks are

at an angle of more than 180° from the centre crank. The angle of lead of the

side cranks varies with the size of the engine.

The upper piston of the engine requires its own running gear and crank

throws. The advantages obtained with this concept are considerable. For equal

Doxford J-type 479

Water-cooled

upper piston

Scavenge

air

Camshaft and

fuel pump

chaindrive

Fuel pump

Side bottom

end bearing

Centre bottom

end bearing

Oil-cooled

lower piston

Exhaust gas

turbocharger

Centre top

end bearing

Side top

end bearing

Fuel injection

timing valve

FigurE 14.3 Doxford J-type conguration

480 Doxford Low-Speed Engines

mean indicated pressure, mean piston speed and cylinder bore, the Doxford

opposed-piston engine will develop 30–40 per cent higher power per cylinder

than a single-piston engine. The ﬁrst-order inertia force from the lower piston

is balanced against the corresponding force from the upper pistons, and a well-

balanced engine is obtained. All of the forces from the upper piston are trans-

ferred through the running gear.

Long tension bolts, as used in single-piston engines to transfer the forces

from the cylinder covers to the bedplate, are not required in the opposed-

piston engine. The engine structure is therefore simple and relatively free from

stresses. No valves are required in the scavenge-exhaust system, and the scav-

enge efﬁciency of the cylinders is high. The ﬂow areas through the exhaust ports

Inner dead centre

Exhaust ports open

Air

opens

closes

Exhaust

Centre crank

angle

before

inner dead

centre

Centre

crank angle

after

inner dead

centre

Air

closes

180

Air inlet ports open

(b)

FigurE 14.4b port timing diagram for Doxford 76J4C engine

Exhaust

ports

Scavenge

ports

Scavenge and

exhaust ports fully

open

Scavenge ports

open scavenging

commences

Scavenge ports

closed exhaust

ports still open

Exhaust ports

closed

compression

commences

Inner dead

centre

combustion

taking place

Exhaust ports

open exhaust

gases escape

(a)

FigurE 14.4a Exhaust and scavenge events

are substantial, and fouling, which inevitably takes place in service, has little

effect. The large port areas also make the engine well suited for turbocharging.

Engine Construction

The bedplate is built up of two longitudinal box girders extending over the full

length of the engine. The transverse girders, which incorporate the main bear-

ing housings, are welded to these longitudinal members. A semi-circular white

metal–lined steel shell forms the lower half of each main bearing and is held in

place by a keep secured by studs to the bedplate ((Figure 14.5)..

The entablature is also a welded steel box construction, arranged to carry

the cylinders, which are bolted to the upper face. It is bolted to the tops of the

columns and to the crosshead guides. The entablature forms the air receiver

from which the cylinders are supplied with air, and its volume is supplemented

by the air receiver on the back of the engine, whose top face forms the back

platform. The intercoolers are mounted on this receiver and the air deliveries

from the turbochargers are connected to these coolers. Alternatively, in the

case of engines with end-mounted turbochargers, the air from the turbocharger

is supplied directly to the entablature after passing through the end-mounted

intercoolers.

Crankshaft

Each cylinder section of the crankshaft is made up of three throws, the two side

cranks being connected to the upper piston and the centre crank to the lower

piston. The side crankwebs are circular and form the main bearing journals.

Each centre crank is made of an integral steel casting or forging for semi-built

shafts. For fully built shafts these units are made of two slabs shrunk onto the

centre pin. The centre cranks are shrunk onto the side pins.

Doxford J-type 481

FigurE 14.5 Main bearing assembly