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

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452 Burmeister & Wain Low-Speed Engines

Bedplate and Frame

The standard K-GF engine bedplate is of the high and fabricated design with

cast steel main bearing housings welded to the longitudinal side frames. The

standard thrust bearing is of the built-in short type and separated from the crank-

case by a partition wall (Figure 13.5).

The frame or entablature section consists of three units: a frame box of a

height corresponding to the length of the crosshead guides, bolted together in

the longitudinal direction in the chain drive section only, and two longitudinal

girders, also bolted together in this section. This method of construction gives

rigid units designed for easy handling and mounting, both on the testbed and

when erecting the engine in the ship. The rigid design with few joints ensures

oil tightness, and large hinged doors provide easy access to the crankcase.

Air inlet

Exhaust valve

Exhaust outlet

Air receiver

Scavenge port

FiGurE 13.1 B&W uniow scavenging

K-GF type engines 453

FiGurE 13.2 B&W uniow scavenged engines of 1950 (left) and 1970 (right) with

impulse turbocharging

Exhaust closed

(Constant pressure)

Exhaust closed

(Impulse)

Scavenging

Constant pressure

turbocharging

Exhaust open (Constant pressure)

Exhaust open (Impulse)

Impulse turbocharging

Cylinder

pressure

Piston stroke

ATA

FiGurE 13.3 Working diagrams for impulse and constant pressure turbocharging

454 Burmeister & Wain Low-Speed Engines

The crosshead guides, consisting of heavy I sections, are attached at top

and bottom in the frame boxes, with the lower attachment ﬂexible in the longi-

tudinal direction.

Cylinder Jackets

Cast iron one-piece cylinder sections connected in the vertical plane by ﬁtted

bolts are held to the frame section by long tie-bolts, which are secured in the

main bearing saddles.

The cylinder liner is provided with a water-cooled ﬂange low down to

secure low operating temperatures in the most heavily loaded area. The liners

FiGurE 13.4 Cross-section of K90GF engine

are made of alloyed cast iron with ports for scavenge air and bores for the cyl-

inder lubrication system.

Crankshaft

The crankshaft is semi-built for all cylinder numbers with cast steel throws for

six- to ten-cylinder engines. Balancing of the engine is undertaken by varying

the crankpin bore holes, thus entirely eliminating bolted-on counterweights.

Lubrication of the crank bearing is from the crosshead through the connect-

ing rod, thus eliminating stress-raising bores in the crankshaft.

K-GF type engines 455

FiGurE 13.5 K-GF engine structural arrangement

456 Burmeister & Wain Low-Speed Engines

Crosshead

The crosshead (Figure 13.6) is short and rigid, and the bearings are so con-

structed that the bearing pressure between journal and bearing is distributed

evenly over the entire length of the bearing. The bearing pressure is smaller

than before, and the peripheral speed is higher, which improves the working

conditions of the bearings.

Interchangeable bearing shells of steel with a 1-mm white-metal lining are

ﬁtted to the crosshead bearings. The shells are identical halves, precision bored

to ﬁnish size and can be reversed in position during emergencies when a dam-

aged or worn lower half can be temporarily used as an upper shell.

piston

The piston is oil cooled and consists of the crown, cooling insert and the skirt.

The cooling insert is fastened to the upper end of the piston rod and transfers the

combustion forces from the crown to the rod. The crown and skirt are held

together by screws while a heavy Belleville spring is used to press the crown

and the cooling insert against the piston rod. The crown has chromium-plated

grooves for ﬁve piston rings while the rod has a longitudinal bore in which the

cooling oil outlet pipe is mounted. The oil inlet is through a telescopic pipe

fastened to the crosshead, and the oil passes through a bore in the foot of

the piston rod to the cooling insert (Figure 13.7).

FiGurE 13.6 Crosshead arrangement of K-GF engine

Cylinder Cover

The cylinder cover is in two parts, a solid steel plate with radial cooling water

bores, to which a forged steel ring with bevelled cooling water bores is bolted.

The insert has a central bore for the exhaust valve cage and mountings for fuel

valves, a safety valve and an indicator valve.

The cover is held against the cylinder liner top collar by studs screwed into

the cylinder block, and the nuts are tightened by a special hydraulic tool to

allow simultaneous tightening and correct tension of the studs.

Exhaust Valve

The valve is similar to earlier types but with hydraulic actuation, a feature orig-

inally unique to the K-GF series (Figure 13.8). The valve consists of a cast iron

cage and a spindle, with steel seats and the valve mushroom faced with Stellite

for hard wearing. Studs secure the valve housing to the cylinder cover.

The hydraulic actuation system is based on a piston pump driven by a cam

on the camshaft, and the pressure oil is led to a working cylinder placed on top

of the exhaust valve housing. The oil pressure is used to open the valve and

closing is accomplished by a ring of helical springs.

Camshaft

The camshaft is divided into sections, one for each cylinder, enclosed and sus-

pended in roller guide housings with replaceable ready-bored bearing shells.

K-GF type engines 457

FiGurE 13.7 Combustion chamber and piston showing bore-cooling arrangement

458 Burmeister & Wain Low-Speed Engines

The cams and couplings are ﬁtted to the shaft by the SKF oil-injection pressure

method, and the complete camshaft unit is driven by chain from the crankshaft.

For reversing of the engine, the chain wheel ﬂoats in relation to the camshaft,

which is driven through a self-locking crank gear; reversing crankpins are

turned by built-on hydraulic motors.

turbocharging

Until 1978 all B&W engines were equipped with turbochargers operating on

the impulse system, as was the case for the K-GF when ﬁrst introduced. All

1. Cam

2. Cam follower

3. Retaining washer

4. Spring collar

5. Spring guide

6. Bleed valve

7. Tapered collar

8. Bleed pipe

9. Spring cover

10. Pump plunger

11. Valve body

12. Actuating cylinder casing

13. Collar

FiGurE 13.8 Components of hydraulically actuated exhaust valves

B&W engines (including the K-GF) were subsequently speciﬁed with a con-

stant pressure charging system in which the exhaust pulses from the individual

cylinders are smoothed out in a large volume gas receiver before entering the

turbine at a constant pressure. This system gives a fuel consumption which is

some 5 per cent lower. For part-load operation and starting, electrically driven

auxiliary blowers are necessary. The arrangement of the constant pressure tur-

bocharged K-GF engine is shown in Figure 13.4.

L-GF and L-GB EnGinES

The oil crisis in 1973 and the resulting massive increase in fuel prices stimu-

lated enginebuilders to develop newer engines with reduced speciﬁc fuel

L-GF and L-GB engines 459

FiGurE 13.9 Fuel pump

460 Burmeister & Wain Low-Speed Engines

consumption. B&W’s answer was the L-GF series combining constant pres-

sure turbocharging with an increase in piston stroke: an increase in stroke of

about 22 per cent results in a lowering of the shaft speed by around 18 per cent,

leading to greater propulsive efﬁciency when using larger diameter propellers

and around 7 per cent increase in thermal efﬁciency.

The design of the L-GF series engine was heavily based on that of the K-GF,

and the necessary changes were mainly those relating to the increase in piston

stroke and modiﬁcations to components to embody thermodynamic improve-

ments (Figure 13.11). A major component design change was the cylinder liner,

made longer for the increased stroke, and featuring cooling bore drillings form-

ing generatrices on a hyperboloid to ensure efﬁcient cooling of the high liner

collar without cross borings with high stress concentration factors.

FiGurE 13.10 Fuel injector

The cylinder covers are of the solid plate type used for the K-GF, while the

pistons are also of the original oil-cooled type but with somewhat improved

cooling caused by the stronger ‘cocktail shaker’ effect resulting from the greater

quantity of oil in the elongated piston rod. The designs of the crankshaft, cross-

head, bearings and exhaust valve actuating gear are, in the main, similar to the

components introduced with the K-GF series.

Constant pressure turbocharging

The increased stroke and thus reduced revolutions per minute of L-GF engines

compared with K-GF engines was not aimed at developing more power but

rather an improvement in ship propulsive efﬁciency. The uniﬂow scavenging

system has the advantage of a good separation between air and gas during the

scavenging process, and the rotating ﬂow of air along the cylinder contributes

to the high scavenging efﬁciency and clean air charge.

As an engine’s mean indicated pressure increases the amount of exhaust

gas energy supplied during the scavenging period, relative to the impulse

energy during the blow-down period, constant pressure turbocharging is advan-

tageous; also for uniﬂow scavenged engines with unsymmetrical exhaust valve

timing. Theoretical calculations for improved fuel economy showed a possible

gain of 5–7 per cent in speciﬁc fuel consumption by using constant pressure

turbocharging.

The most obvious change with constant pressure turbocharging is that the

exhaust pipes from each valve body led to a common large exhaust gas receiver

L-GF and L-GB engines 461

FiGurE 13.11 Short- and long-stroke 67GF engines (left and right)