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

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temperatures, and the low ports give a longer effective expansion stroke in the

cycle (Figure 12.23).

The RTA84T-B engine was uprated at the end of 1998 from 3880 kW/cyl-

inder at 74 rev/min to 4100 kW/cylinder at 76 rev/min to create the Version

D through tuning and turbocharging matching measures. The power avail-

able from a seven-cylinder engine was thereby increased from 27 160 kW to

28 700 kW, better addressing the propulsive demands of VLCCs with higher

service speeds. Nine-cylinder RTA84T-D models, each developing 36 900 kW,

were speciﬁed to power a series of 442 500 dwt tankers with a service speed of

16.5 knots.

rTA design developments 411

with VIT/VEC/IPDLC

without VIT/VEC/IPDLC

140

130

120

110

100

t Valve centre

Engine speed

Load

BSFC

Temperatures

before turbine b.T.

after turbine a.T.

t Liner at TDC

Top-ring

45 50 55 60 65 70 75 min

–1

25 50 65 75 90 100 %

comp

Smoke

BSFC

135

130

125

120

(g/kW h) (g/bhp h)

450

400

350

300

250

(C)

350

300

250

t Valve seat

Max. cylinder pressure pz

Compression pressure pc

(%)

Smoke

(C)

240

220

200

180

160

140

(C)

600

550

500

cyl.

t Valve centre

t Valve seat

t Liner at TDC

t after Turbine

(bar)

t before Turbine

(C)

180

170

160

FigurE 12.21 inuence of the viT/vEC combination (solid line) compared with

results without viT/vEC (dashed line) on the performance characteristics of a

7rTA84T engine. The engine has an ri rating of 27 160 kW at 74 rev/min and exploits

an exhaust gas power turbine

412 Wärtsilä (Sulzer) Low-Speed Engines

Structural, running gear, combustion chamber, fuel injection, turbocharging

and scavenge air system design details are similar to those of the RTA84C and

RTA96C engines described later. The large stroke-to-bore ratio of the RTA84T,

however, allows a relatively deeper combustion chamber with more freedom

in the layout of the fuel spray pattern. The semi-built crankshaft had to cater

for the longest stroke ever applied in a Sulzer engine; to limit the crankshaft

weight for production, assembly and transport, the main journals and crankpins

are bored. The design of the crank has a good transverse width at the upper

part of the web, allowing the latter to be slim longitudinally.

The favourable torsional vibration characteristics allow six-cylinder engines

to use a viscous damper in many cases instead of a Geislinger damper.

Preheating pump

Expansion tank

Water

suppy

Compressed

air

Fresh-water

generator

Cooler

Preheater

RTA84T

Main & standby

circulating pumps

Engine

supply

FigurE 12.22 Schematic layout of the rTA84T cylinder jacket cooling water system

with a load-dependent controlled bypass past the jacket to the cylinder cover

RTA84T-B and RTA84T-D engine data

RTA84T-D RTA84T-B

Bore (mm) 840 840

Stroke (mm) 3150 3150

Stroke–bore ratio 3.75 3.75

power (mcr) (kW/cyl) 4100 3880

Cylinders 5–9 5–9

Speed range (rev/min) 61–76 59–74

Mean eective pressure (bar) 18.5 18

Mean piston speed (m/s) 8 7.77

Maximum cylinder pressure (bar) 144 140

Specic fuel consumption (g/kW h) 160–168 160–168

rTA design developments 413

0°C

300

200

0°C

300

400

500

300

200

°C

100

200

100 200 300 °C

TDC-

Top ring

FigurE 12.23 Surface temperatures measured on the combustion chamber compo-

nents of the rTA84T-B engine at full load r1 rating. The thickness of the lines repre-

sents the circumferential variation in temperature

414 Wärtsilä (Sulzer) Low-Speed Engines

rTA68T-B Engine

A further development of the RTA-T programme resulted in the 680-mm bore

RTA68T-B engine, the ﬁrst example of which entered service in November

2000. The design beneﬁted from all the reliability improvements incorporated

in the earlier RTA-T engines, with parameters similar to those of the smaller

bore RTA48T-B and RTA58T-B models. TriboPack design measures (see later)

contributed to piston-running behaviour, and a new design of piston rod gland

featured oil scraper rings with grey cast iron lips: the system oil is fully recir-

culated and a dry neutral space effected.

RTA68T-B engine data

Bore

680 mm

Stroke 2720 mm

Stroke–bore ratio 4

power (mcr) 2940 kW/cyl

Cylinders 5–8

Speed range

75–94 rev/min

Mean eective pressure 19 bar

Mean piston speed 8.5 m/s

Maximum cylinder pressure 150 bar

Specic fuel consumption 161–169 g/kW h

rTA84C and rTA96C Engines

Introduced in September 1988 for propelling the coming generation of larger

and faster container ships, the RTA84C engine was developed from the RTA84,

already established in that market. In 1993 its power output was increased by 6

per cent, the cylinder cover modiﬁed and the number of fuel nozzles increased

from two to three; these measures contributed to a reduction in the thermal

load of the combustion chamber. The performance of the series, shown in the

following table, equated to a maximum output of 48 600 kW at 102 rev/min

from the 12-cylinder model.

RTA84C engine data

Bore

840 mm

Stroke 2400 mm

Stroke–bore ratio 2.86

power (mcr) 4050 kW/cyl

Cylinders 6–12

Speed range 82–102 rev/min

Mean eective pressure 17.9 bar

Mean piston speed 8.2 m/s

Maximum cylinder pressure 140 bar

Specic fuel consumption 166–171 g/kW h

Increasing sizes of post-Panamax container ships dictated more powerful

engines and stimulated the introduction of the RTA96C series in December

1994. The 960-mm bore design was fully based on the RTA84C (Figures 12.24

and 12.25), which it supplemented and later replaced in the programme. The

RTA84C series was further superseded by the launch at end-2005 of the 82-bore

design (see p. 437).

The selection of a stroke (2500 mm) some 100 mm longer than that of the

RTA84C engine for the RTA96C design (Figure 12.26) was inﬂuenced by

demands for the highest reliability. By adopting a longer stroke, the absolute

depth of the combustion chamber could be proportionally increased to give

more room for securing the best combustion and fuel injection parameters,

and better control of temperatures in the combustion chamber components.

Additionally, the slightly longer stroke fosters a simpliﬁed crankshaft design

with enhanced reliability, since the shrunk-in main journals do not cut the jour-

nal ﬁllets at the inner sides of the crank webs.

A TBO of 3 years from the RTA96C engine’s key components was sought,

a goal underwritten by a cast iron (preferably die cast) cylinder liner with the

necessary amount of wear-resistant hard-phase particles and a smoothly machined

rTA design developments 415

FigurE 12.24 A 12-cylinder rTA84C engine on test

416 Wärtsilä (Sulzer) Low-Speed Engines

and honed surface for quick and trouble-free running-in; bore cooling of all the

main combustion chamber components; three fuel injection valves symmetrically

distributed in the cylinder cover to contribute to evenness of temperature distribu-

tion; cylinder oil lubrication of the liner surface via two levels of quills to achieve

effective and economical distribution; and a top piston ring pre-proﬁled and

plasma coated to secure the lowest wear rate to reach the 3-year TBO goal with

sufﬁcient margin. Diametral cylinder liner wear rates of around 0.03 mm/1000 h

are reported for the RTA96C design, in service. All new engines further beneﬁt

from TriboPack measures to enhance piston-running behaviour (see p. 423).

The semi-built crankshaft comprises combined crankpin/web elements forged

from a solid ingot, with the journal pins then shrunk into the crank web. The main

bearings have white-metal shells, and the main bearing caps are held down by a

pair of jack bolts in the RTA84C and by a pair of elastic holding-down studs in the

RTA96C. The crosshead bearing is designed to the same principles as for all other

RTA engines; it also features a full-width lower half bearing. The crosshead bear-

ings have thin-walled shells of white metal to yield a high load-bearing capacity.

Wärtsilä low-speed engines retain a separate elevated-pressure lube oil supply to

the crosshead; this provides hydrostatic lubrication, which lifts the crosshead pin

off the shell during every revolution to ensure that sufﬁcient oil ﬁlm thickness is

maintained under the gas load: crucial to long-term bearing security.

FigurE 12.25 Crosshead and connecting rod assembly for rTA84C engine

The combustion chamber was recognized as the most important design area

because of its key inﬂuence on engine reliability and the high power concentra-

tion. Component design was based on established practice and beneﬁted from

work carried out for the medium-bore RTA48T and RTA58T engines. Wärtsilä

suggests that bore-cooling technology provides an escape from the rule that

larger components (resulting from a larger bore) when subjected to thermal

loading will also have higher thermal strains. With bore cooling, the thermal

strains in the cylinder cover, liner and piston crown of the RTA96C can be

kept fully within the values of earlier generations of RTA engines, as can the

mechanical stresses in those components.

rTA design developments 417

FigurE 12.26 Cross-section of rTA96C engine, designed for propelling ultra-large

container ships

418 Wärtsilä (Sulzer) Low-Speed Engines

The solid forged steel, bore-cooled cylinder cover is secured by eight elas-

tic studs, and the central exhaust valve of Nimonic 80A material is housed in a

bolted-on valve cage. Anti-corrosion cladding is applied to the cylinder covers

downstream of the injection nozzles to protect the covers from hot corrosive or

erosive attack. The pistons feature a forged steel crown with a short skirt; the

crown is cooled by combined jet-shaker oil cooling achieving moderate tem-

peratures on the crown and a fairly even temperature distribution across the

crown surface. No coatings are necessary.

A high structural rigidity with low stresses and high stiffness is important for

low-speed engines. The RTA84C and RTA96C designs exploit a well-proven struc-

ture with a ‘gondola’ type bedplate surmounted by very rigid A-shaped double-

walled columns and cylinder blocks, all secured by pre-tensioned vertical tie

rods. Both bedplate and columns are welded fabrications designed for minimum

machining. The cylinder jacket is assembled from individual cast iron cylinder

blocks bolted together to form a rigid whole. The fuel pumps are carried on sup-

ports on one side of the column and the scavenge air receiver on the other side of

the cylinder jacket.

Access to the piston under-side is normally from the fuel pump side but is also

possible from the receiver side of the engine, to facilitate maintenance of the pis-

ton rod gland and also for inspecting piston rings. The tilting-pad thrust bearing is

integrated in the bedplate. The use of gear wheels for the camshaft drive allows the

thrust bearing to be very short and stiff, and to be carried in a closed rigid housing.

The three uncooled fuel injection valves in each cylinder cover have nozzle

tips sufﬁciently long for the cap nut to be shielded by the cylinder cover and

not exposed to the combustion space. The camshaft-driven fuel injection pumps

are of the double-valve controlled type, traditional in Sulzer low-speed engines.

Injection timing is controlled by separate suction and spill valves regulated

through eccentrics on hydraulically actuated lay shafts. Flexibility in timing is

possible through the VIT system for improved part-load consumption, while the

FQS lever can adjust timing according to the fuel oil quality. The valve-controlled

fuel injection pump, in comparison with a helix type, has a plunger with a signiﬁ-

cantly greater sealing length. The higher volumetric efﬁciency reduces the torque in

the camshaft; additionally, injection from a valve-controlled pump is far more

stable at very low loads, and rotational shaft speeds down to 15 per cent of the

rated speed are achieved. Valve control also has the beneﬁts of less deterioration

of timing over the years owing to reduced wear and freedom from cavitation.

The camshaft is assembled from a number of segments, one for each fuel

pump housing. The segments are connected through SKF sleeve couplings,

each segment having an integral hydraulic reversing servomotor located within

the pump housing. The camshaft drive is a traditional Sulzer arrangement,

effected in this case by three gearwheels housed in a double column located

at the driving end or in the centre of the engine. The main gearwheel on the

crankshaft is in one piece and ﬂange mounted.

Scavenge air is delivered by a constant pressure turbocharging system based on

one or more turbochargers, depending on the number of engine cylinders. For start-

ing and during slow running, scavenge air delivery is augmented by electrically

driven auxiliary blowers. The scavenge air receiver incorporates non-return ﬂaps,

an air cooler and the auxiliary blowers; the turbochargers are mounted on the

receiver, which also carries the ﬁxed foot for the exhaust manifold. Immediately

after the cooler, the scavenge air passes through a water separator comprising a row

of vanes that divert the airﬂow and collect the water. Ample drainage is provided to

completely remove the condensed water collected at the bottom of the air cooler

and separator. Effective separation of condensed water from the stream of scavenge

air is thus accomplished, a necessity for satisfactory piston-running behaviour.

RTA engines have simple seating arrangements with a modest number of

holding-down bolts and side stoppers (14 side stoppers are needed for a 12-

cylinder RTA96C engine). No end stoppers or thrust brackets are needed as thrust

transmission is provided by ﬁtted bolts or thrust sleeves applied to a number of

the holding-down bolts (Figure 12.27). The holes in the tanktop for the thrust

sleeves can be made by drilling or even ﬂame cutting. After alignment of the

bedplate, epoxy resin chocking material is poured around the sleeves. The engine

is equipped with an integrated axial detuner at the free end of the crankshaft, and

a detuner monitoring system developed by Wärtsilä is standard equipment.

rTA96C uprating and programme Expansion

The ﬁrst RTA96C engine, an 11-cylinder model, started tests in March 1997

and was followed into service by eight-, nine- and ten-cylinder models and

numerous twelve-cylinder versions. The design was originally offered with an

output of 5490 kW/cylinder and up to 12 cylinders, but the rating was increased

by 4 per cent in the year 2000 to 5720 kW/cylinder at 102 rev/min (see the fol-

lowing table). At the same time, the programme was extended to embrace an

in-line 14-cylinder model delivering 80 080 kW (the ﬁrst time such a low-speed

engine conﬁguration had featured in a production programme) and suitable for

powering single-screw post-Panamax container ships with capacities of 10 000

TEU-plus and service speeds of up to 25 knots (Figure 12.28).

The ﬁrst-ever 14-cylinder low-speed engines (RT-ﬂex96C models) were

ordered in early 2005 to provide a series of 8600 TEU container ships with

a service speed of 27 knots. The original RT-ﬂex96C crankshaft had sufﬁcient

rTA design developments 419

Thrust Thrust

Thrust bracket

Side stopper

Side stoppe

FigurE 12.27 Arrangements for transmitting propeller thrust to the rTA84T, rTA84C

and rTA96C engine seatings; the inset shows the thrust sleeve for the thrust bolts

420 Wärtsilä (Sulzer) Low-Speed Engines

208.5

*min.637 1194.5 2215.5 *min.500

3150

2950

(13000)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

265 411 1944 1680 1680 1680 1680 1680 1295 1295 1680 1680 1680 1680 1680 1680 1680 1220 723

2620

22750

1943

27313

FigurE 12.28 Side elevation of a 14-cylinder rTA96C engine