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

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be considered in order to deﬁne the required CMCR: for example, propulsive

power, propeller efﬁciency, operational ﬂexibility, power and speed margins,

possibility of a main engine-driven generator and the ship’s trading patterns.

The layout ﬁeld (Figure 12.5) is the area of power and engine speed within

which the CMCR of an engine can be positioned individually to give the

desired combination of propulsive power and rotational speed. Engines within

this layout ﬁeld will be tuned for maximum ﬁring pressure and best fuel efﬁ-

ciency. The engine speed is given on the horizontal axis and the engine power

on the vertical axis of the layout ﬁeld; both are expressed as a percentage of the

respective engine’s nominal R1 parameters. Percentage values are used so that

the same diagram can be applied to all engine models. The scales are logarith-

mic so that exponential curves, such as propeller characteristics (cubic power)

and mean effective pressure (mep) curves, are straight lines. The layout ﬁeld

serves to determine the speciﬁc fuel oil consumption, exhaust gas ﬂow and

temperature, fuel injection parameters, turbocharger and scavenge air cooler

speciﬁcations for a given engine.

The rating points R1, R2, R3 and R4 are the corner points of the engine lay-

out ﬁeld. R1 represents the nominal maximum continuous rating (MCR). This

is the maximum power/speed combination available for a particular engine.

Wärtsilä (Sulzer) low-speed engines 391

Engine power

[% R1]

100

70 75 80 85 90 95 100

Engine speed

[% R1]

Rating line fulfilling a

ship’s power

requirement for a

constant speed

Nominal propeller characteristic 1

Nominal propeller characteristic 2

FigurE 12.5 Layout eld applicable to all Wärtsilä rTA models. The contracted MCr

(rx) may be freely positioned within the layout eld

392 Wärtsilä (Sulzer) Low-Speed Engines

A 10 per cent overload of this ﬁgure is permissible for 1 h during sea trials in

the presence of authorized representatives of the enginebuilder. The point R2

deﬁnes 100 per cent speed and 55 per cent power. The point R3 is at 72 per cent

power and speed. The line R2–R4 is a line of 55 per cent power between 72

per cent and 100 per cent speed. Points such as Rx are power/speed ratios for

the selection of CMCRs required for individual applications. Rating points Rx

can be selected within the entire layout ﬁeld.

Sulzer smoothed the path for selecting the optimum model for a given pro-

pulsion project with its PC computer-based EnSel engine selection program,

which presented a list of all the models that fulﬁl the power and speed require-

ments, along with their main data. The input data call for the user to specify

the power and speed required, and whether or not an efﬁciency-booster power

turbine system is wanted. The output data then list the engines with the appro-

priate layout ﬁelds and detail their MCR power, speed and speciﬁc fuel con-

sumption, main dimensions and weight and other relevant information.

rTA dESign FEATurES

The RTA design beneﬁted from principles proven in earlier generations of

Sulzer R-type engines. The key elements are as follows:

l A sturdy engine structure designed for low stresses and small deﬂections

comprises a bedplate, columns and cylinder block pretensioned by vertical

tie rods. The single-wall bedplate has an integrated thrust block and incor-

porates standardized large surface main bearing shells. The robust

A-shaped columns are assembled with stiffening plates or are of monobloc

design. The single cast iron cylinder jackets are bolted together to form a

rigid cylinder block (multi-cylinder jacket units for smaller bore engines).

l Lamellar cast iron, bore-cooled cylinder liners with back-pressure

timed, load-dependent cylinder lubrication.

l Solid, forged bore-cooled cylinder covers with one large central exhaust

valve arranged in a bolted-on valve cage; the valve is made from a heat-

and corrosion-resistant material and its seat ring is bore cooled.

l Semi-built crankshaft divided into two parts for larger bore engines with

a large number of cylinders.

l Running gear comprising connecting rod, crosshead pin with very large

surface crosshead bearing shells (with high-pressure lubrication) and

double-guided slippers, piston rod and bore-cooled piston crown using

oil cooling. All have short piston skirts.

All combustion chamber components are bore cooled, a traditional fea-

ture of Sulzer engines fostering optimum surface temperatures and preventing

high temperature corrosion due to high temperatures on one side and sulphuric

acid corrosion due to too low temperatures on the other side. At the same time,

rigidity and mechanical strength are provided by the cooler material behind the

cooling bores (Figure 12.6).

Comfortable working conditions for the exhaust valve are promoted by:

hydraulic operation with controlled valve landing speed; air spring; full rota-

tional symmetry of the valve seat, yielding well-balanced thermal and mechani-

cal stresses and deformations of valve and valve seat, as well as uniform seating;

extremely low and even temperatures in valve seat areas due to efﬁcient bore

cooling; valve rotation by simple vane impeller; valve actuation free from lateral

forces, with axial symmetry and simple guide bushes sealed by pressurized air.

rTA design features 393

(a)

(b)

FigurE 12.6 The combustion chamber of the rTA series engines is fully bore cooled.

Cooling oil spray nozzles on top of the piston rod direct oil into the bores of the pis-

ton crown

394 Wärtsilä (Sulzer) Low-Speed Engines

The low exhaust valve seating face temperature reportedly secures an

ample safety margin to avoid corrosive attack from vanadium/sodium com-

pounds under all conditions. Efﬁcient valve cooling is given by intimate con-

tact with the bore-cooled seat, together with the appropriate excess air ratio in

the cylinder. The speciﬁc design features of the valve assembly are also said to

deter the build-up of seat deposits, seat distortion, misalignment and other fac-

tors which may accelerate seat damage.

l Camshaft gear drive housed in a special double column or integrated

into a monobloc column, placed at the driving end or in the centre of the

engine for larger bore models with a large number of cylinders.

l Balancer gear can be mounted on larger bore engines, when required, to

counter second-order couples for four-, ﬁve- and six-cylinder models,

and combined ﬁrst- and second-order couples for four-cylinder models.

l A compact integral axial detuner can be incorporated, if required, in the

free end of the engine bedplate.

l The fuel injection pump and exhaust valve actuator are combined in

common units for each of the two cylinders. The camshaft-driven injec-

tion pump with double valve-controlled variable injection timing (VIT)

delivers fuel to multiple uncooled injectors. The camshaft-driven actua-

tors impart hydraulic drive to the single central exhaust valve working

against an air spring.

l Constant pressure turbocharging is based on high-efﬁciency uncooled

turbochargers; auxiliary blowers support uniﬂow scavenging during low

load operation. In-service cleaning of the charge air coolers is possible.

A standard optional three-stage charge air cooler unit can be speciﬁed

for heat recovery.

l A standard pneumatic engine control system is based on a remote

manoeuvring stand in the control room and an emergency manoeuvring

stand on the engine itself. The optional SBC-7.1 electro-pneumatic

bridge control system is matched to the engine and arranged for engine

control and manoeuvring from the wheelhouse or bridge wings.

l Standard optional power take-off drives can be arranged either at the

side or at the free end for connecting an alternator to the main engine.

l Sulzer’s efﬁciency-booster system (see Chapter 7) could be speciﬁed

for the RTA84C, RTA84T and RTA84M engines, as well as for the

RTA72U, RTA62U and RTA52U models, to exploit surplus exhaust gas

energy in a power recovery turbine.

l RTA engines can satisfy the speed-dependent IMO limits on NOx emis-

sions from exhaust gases. Tuning is facilitated by the electronic variable

fuel injection timing (VIT) system.

rTA dESign dEvELopMEnTS

The basic RTA engine design has been reﬁned over the years and the range

expanded to address changing market requirements. The improvements have

yielded wider power/speed ﬁelds, increased power outputs, reduced fuel con-

sumption, lower wear rates and longer times-between-overhauls (TBOs).

Advances in key performance parameters are illustrated in Figure 12.7. Operating

economy has also beneﬁted from the reduced auxiliary power demand of current

RTA engines—imposed by electric pumps for cooling water, lubricating oil and

fuel supply—which is some 40 per cent less than that of the RL engines.

rTA-2u Series

Sulzer’s introduction of upgraded RTA-2 engine designs called for some modi-

ﬁcations to match the higher power outputs and maximum combustion pres-

sures involved (summarized in Figure 12.8). The shrinkﬁt of the crankshaft was

strengthened to suit the increased torque values; the main bearing was adapted

rTA design developments 395

Max. cylinder pressure, P

max

(bar)

RTA-2U

RTA96C

RTA58T

RTA48T

RTA84T

4RTX54

RTA-2

RTA-8

4RTX54

RTA84T

RTA-2

RTA-8

4RTX54

RTA-2

RTA-8

RTA-2U

Brake mean effective pressure (BMEP)

RTA96C

RTA58T/RTA48T

1980 85 90 95

RTA-2U/RTA96C

RTA58T/RTA48T

Mean piston speed (m/s)

180

120

140

160

bar

m/s

RTA84C

FigurE 12.7 Advances in key parameters of rTA series engines. (The rTX54 was a

Sulzer research engine replaced in 2008 by the rTX-4 research engine)

396 Wärtsilä (Sulzer) Low-Speed Engines

to accommodate the higher loads and ensure optimum oil ﬂow; and the cylin-

der cover material was changed to take advantage of better fatigue strength at

higher loads. Some of the design ideas tested on the 4RTX54 research engine

(see Chapter 9) and already applied to the RTA84T engine were also adopted

for the RTA-2U series.

Three fuel injection valves per cylinder were speciﬁed for the RTA62U and

RTA72U models (though not, because of its smaller bore size, for the RTA52U

model, which retained two valves per cylinder). The reported beneﬁts of the

triple-valve conﬁguration are a more uniform temperature distribution around

the principal combustion space components (cylinder cover, liner and pis-

ton crown) at the increased maximum combustion pressures, along with even

lower temperatures despite the higher loads. Three fuel valves also foster sig-

niﬁcantly lower exhaust valve and valve seat temperatures.

Other spin-offs from the research engine included a modiﬁed cylinder liner

bore-cooling geometry whose tangential outlets of the bores aim for optimum

distribution of wall temperatures and thermal strains at higher speciﬁc loads.

The geometry of the oil cooling arrangements of the piston crown was also

modiﬁed to maintain an optimum temperature distribution. The good piston

running behaviour was maintained by retaining established features of the

Cylinder cover new

material

3 injection valves

(simplified) even

temperature

distribution

High-efficiency

condensate

separator

Improved drain for

condensate

Redesigned column

opening

Main bearing cover

enlarged

Stronger shrinkfit

Unchanged

bearing loads

Cylinder liner

–New shape

–New bore cooling

–Optimum temperature

distribution

Multi-level

lubrication

New damper

adaption

New gland box

Adapted spray

cooling nozzles of

piston

High-pressure pipes

–Improved coupling

–Enlarged diameter

Top ring

–Thicker

–Preprofiled

–Plasma coated

FigurE 12.8 Main improvements introduced in rTA-2u series engines

RTA design: multi-level cylinder lubrication, die-casting technology for cyl-

inder liners and temperature-optimized cylinder liners. Advances in materials

technology in terms of wear resistance have permitted engines to run at higher

liner surface temperatures. This, in turn, allows a safe margin to be maintained

above the increased dew point temperature and thus avoiding corrosive wear.

Some reﬁnements were introduced, however, to match the new running

conditions. Four piston rings are speciﬁed for the RTA-2U engines instead of

the ﬁve previously used. The top ring is now thicker than the other rings and

has a pre-proﬁled running face, as on the RTA84C container ship engine. The

top ring is also plasma coated. The plasma-coated, pre-proﬁled thicker rings

have demonstrated excellent wear results. The radial wear rates were measured

at 

0.04 mm/1000 h in trials of up to and exceeding 13 000 h of operation.

An important contribution to low wear rates of liners and pistons results

from improving the separation and draining of water borne in the cooled scav-

enge air before it enters the cylinders, particularly at higher engine loads. The

RTA-2U engines were speciﬁed with a more efﬁcient condensate water separa-

tor in the scavenge air ﬂow after the cooler, along with a more effective drain.

rTA-T Series

A number of design modiﬁcations to the RTA series were introduced for the

ultra-long stroke RTA48T, RTA58T and RTA68T engines to achieve more

compact, lower weight models offering reduced production, installation and

maintenance costs (Figures 12.9 and 12.10). A TBO for the main components

of 15 000 h was sought. Cutting the manufacturing cost, despite the greater

stroke–bore ratio (4.17) than previous RTA series engines, was addressed by a

number of measures: reducing the size and weight of components, simplifying

the designs of components and sub-assemblies and making them easier to pro-

duce, reducing the number of parts and designing to save assembly time.

An example of the design changes made to reduce the sizes and weights

of components is provided by the cylinder cover, along with its exhaust valve,

housing and valve actuator. An overall weight saving of 30 per cent was

achieved on each new cover, largely due to the smaller dimensions. Reducing

the distance between cylinder centres by around 9 per cent allowed compo-

nents such as the bedplate, monobloc columns and cylinder block to be reduced

in size, resulting in weight savings of 13–14 per cent (Figures 12.11–12.13).

The cylinder block is lower in overall height and thus lighter than in equivalent

RTA-2U engines. The freedom for ship designers to create short enginerooms

was enhanced by a degree of ﬂexibility in the fore and aft location of the turbo-

charger and scavenge air cooler module.

The hydraulic jack bolts on the main bearings were eliminated and replaced by

simple holding-down bolts that ﬁx the bearing cap directly to the bedplate (Figure

12.14). In addition, it was possible to simplify the column structure in the region

where the jack bolts had needed support and thereby omit machining operations.

The scavenge air receiver was simpliﬁed by using a ‘half-pipe’ design, which is

welded to the module incorporating the turbocharger and scavenge air cooler.

rTA design developments 397

398 Wärtsilä (Sulzer) Low-Speed Engines

The result was an easier manufacturing process and signiﬁcant weight sav-

ings compared with equivalent RTA-2U series engines. The single-piece col-

umn structure can be manufactured without the need for machining inclined

surfaces: all machined surfaces on the columns are either vertical or horizontal

by design. Eliminating the jack bolts on the main bearings also allowed the

design of the columns to be made more tolerant to welding quality.

The exhaust valve actuator was redesigned for the RTA-T engines. The

bush of the actuating piston was eliminated and its function integrated directly

FigurE 12.9 Cross-section of rTA58T engine. note the high camshaft level allowing

the use of shorter high-pressure fuel injection pipes for better injection control

into the housing, thus reducing the number of components to be produced and

the assembly time. Smaller dimensions and a lighter weight (by 27 per cent)

were also achieved. A substantial cost saving can be realized if an electrically

driven balancer at the forward end of the engine is speciﬁed, allowing the pre-

viously necessary coupling between balancer and camshaft to be omitted.

Valve-controlled fuel injection pumps are located at the height of the cylin-

der blocks. The fuel pump blocks can be directly bolted to the cylinder blocks

without using any shims. The intermediate gear wheels of the camshaft drive

can be aligned more quickly than before by means of a built-in device to move

the wheels vertically and horizontally into position.

Engines can be built up from modules, starting with the bedplate and crank-

shaft, and working up with the columns and cylinder blocks. The modules

can be pre-assembled at convenient, separate locations in the workshop. All

rTA design developments 399

FigurE 12.10 Sulzer 4rTA58 engine on test

400 Wärtsilä (Sulzer) Low-Speed Engines

FigurE 12.11 Bedplate of rTA58T engine

FigurE 12.12 The monobloc columns of the rTA48T and rTA58T engines were

designed for simpler welding and machining

FigurE 12.13 The monobloc dry cylinder blocks of the rTA48T and rTA58T engines

were designed for reduced machining, compactness and lower weight