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

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pipe connections are designed to support this type of modular assembly and

achieve the shortest possible assembly time in the works or, if necessary, in the

ship itself.

Piston-running technology was based on experience gained and reﬁned

from the RTA-2U and RTA84C engines. The cylinder liner is made of cast iron

with sufﬁcient hard-phase content and a smooth machined running surface.

Bore cooling and three fuel injection valves underwrite favourable temperature

levels and distributions. The top piston ring is pre-proﬁled for easy running-in

and plasma coated for low wear rates over extended periods. The multi-level

cylinder lubrication system fosters optimum distribution of the lubricating oil

and adapts to the longer stroke; and condensate water is taken out of the scav-

enge air efﬁciently to avoid contamination of the oil ﬁlm.

rTA60C Engine

A new type added to the programme in mid-1999, the RTA60C engine was

designed to serve faster cargo tonnage such as medium-sized container ships,

rTA design developments 401

FigurE 12.14 The main bearings of the rTA48T and rTA58T engines use holding-

down studs instead of the previous hydraulic jack bolts

402 Wärtsilä (Sulzer) Low-Speed Engines

car carriers, RoRo ships and reefer vessels (Figure 12.15). A desire for com-

pactness and economical production costs moulded the design, resulting in

an engine with a shorter length and lighter weight (5–10 per cent lower) for a

given power than others in its class.

A higher rotational speed to suit the intended ship types allowed a reduced

piston stroke, which, together with the short connecting rod, fostered a signiﬁcant

reduction in engine height: the engine is 8.52 m tall above the shaft centreline

and requires a hook height of only 10.4 m for withdrawing pistons (or less by

using special tools). The length was minimized by applying measures from the

FigurE 12.15 Cross section of rTA60C engine, the rst rTA model designed from

the start to accept electronic rT-ex systems (camshaft-controlled version illustrated)

RTA48T and RTA58T engines, a cylinder distance of 1040 mm being achieved.

The six-cylinder RTA60C model has an overall length of 7.62 m, including the

ﬂywheel, and weighs 330 tonnes.

The need to keep engine length to a minimum called for particularly

good bearing design; thin-walled white-metal shells are used for crosshead,

main and bottom end bearings. The main bearing housing was speciﬁcally

designed to accommodate and support the thin-walled bearing shell, with four

elastic holding-down bolts for each main bearing cap. Two pairs of studs are

said to give the most even distribution of holding-down load, and also allow

the tie rods to be located close to the bearing for efﬁcient transfer of ﬁring

pressure loads.

Cylinder covers are secured by eight elastic holding-down studs arranged

in four pairs, promoting compactness and contributing to a shorter engine

length, and also simplifying manufacture. A support ring between the cylinder

block and the collar of the cylinder liner carries both liner and cylinder cover;

it also passes cooling water to the cooling bores and to the cover. A simple

water jacket provides the necessary cooling for the portion of the liner length

immediately below the bore-cooled top ﬂange of the liner.

The cylinder jacket is a single-piece iron casting, its height determined by

the space required for the scavenge air receiver. Access to the piston underside

is possible from the receiver side of the engine to allow maintenance of the

piston rod gland and piston ring inspection. On the fuel side, one door per cyl-

inder can be opened for inspection and to support in-engine work from outside.

The tilting-pad thrust bearing is integrated in the bedplate, the pads arranged to

ensure a safe and uniform load distribution. The thrust-bearing girder consists

of only two steel cast pieces, omitting welding seams in critical corners; the

girder is clearly stiffer than in previous designs.

The piston comprises a forged steel crown with a very short skirt; the

four piston rings are all 16-mm thick and of the same geometry. Piston run-

ning behaviour beneﬁts from an anti-polishing ring (APR) incorporated at the

top of the liner, preventing deposits on the piston top land from damaging the

liner running surface and its lubrication ﬁlm. Keeping the top land clean also

ensures a good spread of lubricant over the liner surface while using a lower

cylinder oil feed rate. (More details of the APR are provided in the section

TriboPack for Extended TBO.)

An exhaust system featuring tangential gas inlet and outlet in the manifold

allows a smooth ﬂow of gases from the exhaust valve to the turbocharger inlet

with an energy-saving swirl along the manifold. The combined turbocharger

and scavenge air cooler module is designed to accommodate one or two tur-

bochargers, and allows for different sizes of coolers. The ﬂexibility to locate

the turbocharger at the aft (driving end) of the engine is particularly suited to

modern all-aft ship designs.

The key operating parameters of the RTA60C (see table) are only slightly

higher than those of the B versions of the RTA48T, RTA58T and RTA68T

models, from whose service experience the new engine beneﬁted.

rTA design developments 403

404 Wärtsilä (Sulzer) Low-Speed Engines

RTA60C engine data

Bore

600 mm

Stroke 2250 mm

Stroke–bore ratio 3.75:1

output, r1 (mcr) 2360 kW/cyl

Speed range (r1–r3) 91–114 rev/min

Mean eective pressure at r1 19.5 bar

Mean piston speed at r1 8.55 m/s

Maximum cylinder pressure 155 bar

number of cylinders 5–8

power range 8250–18 880 kW

Specic fuel consumption 164–170 g/kW h

The RTA60C engine was designed from the outset to smooth its accept-

ance of Sulzer RT-ﬂex fully electronically controlled fuel injection and exhaust

valve systems, which eliminate the camshaft and individual fuel pumps (see

the next section). The debut orders for RTA60C engines—in December 2000—

speciﬁed RT-ﬂex versions.

rTA50C and rT-ex 50C Engines

At the end of 2002 Wärtsilä Corporation announced a joint venture with

Mitsubishi to design and develop a new small bore low-speed engine. The

Japanese group, which designs and builds its own UEC two-stroke engines

(Chapter 11), had been a proliﬁc licensed builder of Sulzer engines since 1925.

The new 500-mm bore/2050-mm stroke design was planned for release

in two versions: the conventional ‘mechanical’ RTA50C and the electroni-

cally controlled RT-ﬂex 50C. Each type offers an MCR of 1620 kW/cylinder

at 124 rev/min, ﬁve-to-eight-cylinder models covering a power range from

5650 kW to 12 960 kW at 99 rev/min to 124 rev/min. Output and speed ratings

target Handymax and Panamax bulk carriers, large product tankers, feeder

container ships and medium-sized reefer vessels. Both versions incorporate

TriboPack measures for enhancing piston-running behaviour, underwriting low

cylinder wear rates for 3 years between overhauls, and minimizing cylinder

lube oil consumption (see p. 423). High efﬁciency, compactness and environ-

mental friendliness were set as design goals for a joint working group of engi-

neers from Wärtsilä and Mitsubishi. The ﬁrst examples of the RT-ﬂex50 engine

were tested and type-approved in mid-2005, the design subsequently proving

popular in the market.

RTA50C/RT-ex 50C engine data

Bore

500 mm

Stroke 2050 mm

Stroke–bore ratio 4:1

output (mcr) 1620 kW/cyl

Mean eective pressure at r1 19.5 bar

max

155 bar

Mean piston speed at r1 8.5 m/s

Cylinders 5–8

Speed range (r1–r3) 124–99 rev/min

power range 5650–12 960 kW

Specic fuel consumption, full load (r1–r2) 165–171 g/kW h

rTA84T Engine renements

The piston of the RTA84T engine is similar to that of the earlier RTA84M

engine but simpliﬁed in design. The skirt is bolted directly to the crown, and

the increased rigidity of the piston rod can accept higher loads. The crown is oil

cooled by the ‘jet/shaker’ cooling principle, with the oil spray nozzles matched

to secure optimum piston temperatures. The piston rod gland design (Figure

12.16) features bronze rings and a modiﬁed scraper/gas sealing package on top

compared with previous RTA-series engines. This gland principle conﬁrmed

the expected appropriate oil leakage rates in the neutral space. The well-proven

RTA exhaust valve design was adopted for the RTA84T, embracing an efﬁciently

bore-cooled valve seat, Nimonic valve, hydraulic actuation and a rotation device.

A deeper cylinder cover, with its lower joint between cover and liner,

reduces the portion of the liner exposed to gas pressure and high tempera-

ture. The bore cooling principle was adapted to control the temperatures and

mechanical and thermal stresses in the components adjacent to the combustion

space. The cylinder cover is bolted down with eight studs. Compact uncooled

fuel injection valves make it possible to place three nozzles symmetrically in

the cover despite the very short cylinder pitch. The nozzle tips are sufﬁciently

long for the cap nut to be shielded by the cylinder cover and hence not exposed

to the combustion space (Figure 12.17).

The camshaft was raised to be close to the cylinder covers and is driven

via gear wheels. The hydraulic reversing mechanism of the RTA series was

retained unchanged. The decision to move the camshaft up towards the engine

top was taken after evaluating such factors as engine cost, fuel injection system

performance, hydraulic valve actuation and torsional vibration. In particular,

with an elongated stroke–bore ratio of 3.75, the advantage of shorter hydraulic

rTA design developments 405

406 Wärtsilä (Sulzer) Low-Speed Engines

connections for injection and valve actuating pipes was signiﬁcant in avoid-

ing high pressure losses. The cost of an additional gearwheel (Figure 12.18)

is compensated by the slimmer camshaft, the fuel pump arrangement and the

shorter high-pressure injection pipes. Higher injection pressures, and hence

more efﬁcient and cleaner combustion, are thus facilitated.

The camshaft-driven fuel pump is of the valve-controlled design used in the

longer stroke RTA-2 engines and located at a high level, bolted directly to the

side of the cylinder block (Figure 12.19). The double valve-controlled pump

principle, traditional in Sulzer low-speed engines, offers high ﬂexibility and

stability over time in controlling the injection timing. Timing is controlled by

separate suction and spill valves regulated through eccentrics on hydraulically

actuated lay shafts. The pump housing incorporates the compact RTA-series

arrangement, with fuel injection pump and exhaust valve actuator modules pro-

vided to serve one pair of cylinders.

In comparison with a helix type, the valve-controlled fuel injection pump

has a plunger with a signiﬁcantly greater sealing length. The higher volumetric

efﬁciency reduces the torque in the camshaft. Wärtsilä says additionally that

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 offers beneﬁts in reduced deterioration of timing over the

years owing to less wear and to freedom from cavitation.

Scraper rings for

contaminated oil

Gas sealing

rings

Oil scraper rings

Neutral space

FigurE 12.16 piston rod gland of rTA84T engine; all rings are made of bronze

A combination of variable exhaust valve closing (VEC) and VIT devices

provides an improved degree of setting ﬂexibility. In the upper load range, the

speciﬁc fuel consumption is optimized through the electronically controlled

VIT system, maintaining the maximum cylinder pressure by injection timing

advance. The fuel quality setting (FQS) function is incorporated in the VIT

system. Engine efﬁciency is additionally improved in the lower load range

(between 80 per cent and 65 per cent load) via the VEC system (Figure 12.20).

The compression ratio is thereby effectively increased by early closing of the

exhaust valve.

rTA design developments 407

FigurE 12.17 rTA84T fuel injection valve. The nozzle is not exposed to the combus-

tion space, thereby avoiding material burning o

408 Wärtsilä (Sulzer) Low-Speed Engines

The working process of the engine can thus be ‘shaped’ for optimum per-

formance over the whole load range, facilitating its adaptation to a particular

ship sailing mode. The resulting beneﬁts promised are the lowest possible fuel

consumption in the most common load range, and higher exhaust gas tempera-

tures within that load range compared with other solutions, such as variable

turbocharger geometry. (Higher exhaust gas temperatures are valued for waste

heat recovery.) The VEC, VIT and FQS devices are electronically regulated

from the engine control system. The inﬂuence of the VIT/VEC combination on

the RTA84T engine’s performance characteristics are shown in Figure 12.21.

A load-dependent cylinder liner cooling system (Figure 12.22) helps to

avoid cold corrosion of the liner surface over the whole load range and hence

to reduce wear rates further. The liner temperature is maintained above the pre-

vailing dew point throughout the load range: the liner is efﬁciently cooled at

full load, while over-cooling at part load is avoided. This is achieved by split-

ting the cooling water supply to the engine into two lines. Only a restricted

FigurE 12.18 Camshaft drive gear train for rTA84T engine

amount of water is led through the cylinder liner jacket at part load while the

main water ﬂow is led directly to the cylinder cover. The water ﬂow is elec-

tronically controlled as a function of the engine load.

Cylinder liner lubrication is effected by Wärtsilä’s multi-level accumula-

tor system. This allows the cylinder lubricating oil to be distributed in very

small amounts at each engine stroke, thereby creating an optimum oil ﬁlm dis-

tribution. The main improvement for the RTA84T was the introduction of an

electronically controlled ﬂexible oil dosage promoting low wear rates and low

lubricating oil consumption. The oil feed rate is controlled according to engine

load and is adjustable as a function of engine condition. A simpliﬁed dual-line

distributor eliminates the mechanical drive shaft.

Piston-running experience with RTA84T engines in service demonstrated

that engines with very long strokes need higher liner wall temperatures than

rTA design developments 409

FigurE 12.19 rTA84T fuel injection pump with double control valves

410 Wärtsilä (Sulzer) Low-Speed Engines

had previously been required by the RTA-2 series (with stroke–bore ratios

of around 3.5) to overcome corrosive wear problems when operating on

high sulphur fuels. This experience was applied to beneﬁt later RTA84T

production models as well as the design of the smaller RTA-T engines. A

number of improvements introduced for the latter series have also been

applied to the RTA84T, the reﬁnements including design simpliﬁcations to

make the engine more manufacturing friendly. A signiﬁcant saving in weight

also resulted, the seven-cylinder model being trimmed by some 6.5 per cent to

960 tonnes.

rTA84T-B and rTA84T-d versions

Following the introduction of the RTA48T and RTA58T models in 1995, the

same design concepts were applied to the RTA84T ‘Tanker’ engine—originally

introduced in May 1991—resulting in a Version B in 1996. Easier manufactur-

ing and enhanced service behaviour were realized, with no change in power

output. In July 1998 a lower speciﬁc fuel consumption (down by 2 g/kW h) was

gained by applying ‘low port’ cylinder liners (scavenging air inlet ports with

a reduced height) in combination with higher efﬁciency turbochargers. There

is no penalty in either higher component temperatures or too low exhaust gas

Oil supply

Control valve

Compressed ai

FigurE 12.20 rTA84T engine exhaust valve actuating arrangement with variable

closing (vEC) system