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

Подождите немного. Документ загружается.

690 Wärtsilä

optimized constant pressure valve design for short injection period and high

brake mean effective pressure; no mixing of lubricating oil and fuel between

plunger and barrel to avoid lacquering and sticking; separation of leakage fuel

from the oil sump to avoid lubricating oil contamination; integrated low-pressure

fuel supply lines; fuel injection valves with uncooled nozzles; and shielded high-

pressure lines and connections. The fuel injection system is fully covered by the

hot box, and possible low- and high-pressure leakages are detected by alarms.

The fuel pump has an integrated low-pressure channel which means there

is no low-pressure pipe to ﬁt to the pump because the fuel pipes are ﬁtted

directly to the fuel pump foundation. Integrated fuel pulse dampers in the fuel

supply lines ensure low-pressure pulses on the system side. Each fuel pump

is equipped with a ﬂow distribution throttle. The high-pressure fuel pipe is

shielded and ﬁtted with a leak alarm. Maintenance is facilitated by a locking

device ﬂange on the cylinder head for the high-pressure pipe.

The engine starting system incorporates a starting fuel-limiter to secure a

smooth and safe start. Each fuel pump is equipped with an overspeed trip cyl-

inder to ensure that the fuel rack is pushed to zero in the event of overspeed.

Additional safety is provided by an extra stop cylinder turning the regulating

shaft to zero in the event of overspeed. Overall compactness and lightness were

prime targets for the W38 engine designers. Component integration resulted in

40 per cent fewer parts than the older generation TM410 engine, with many of

the traditional bolted-on elements now incorporated in the main block casting

(for example, lines and piping for lubricating oil, cooling water and fuel).

Simplicity of maintenance and extended overhaul intervals were other

design goals, with only a modest envelope required for servicing and dis-

mantling. Engine-driven cooling and lubricating oil pumps as well as engine-

mounted coolers and ﬁlters simplify installation and release machinery space.

Integrated brackets are provided for resilient mounting systems. The bending

stiffness and torsional rigidity of the engine block and crankshaft allow savings

in the speciﬁcation of ﬂexible couplings, torsional vibration dampers and elas-

tic elements for resilient mounting.

W38A Specication

Engine block: underslung crankshaft; integrated air receiver; single-piece cast-

ing for all cylinder numbers; and arranged for resilient mounting with screwed-

on feet.

Crankshaft: fully balanced and designed for space saving, maximum over-

all rigidity and moderate bearing loads. The crankshaft of the V-cylinder mod-

els has undercuts for both the main and pin ﬁllet, the design combining a short

cylinder distance and maximum bearing length. Normal ﬁllets are applied in

the in-line cylinder engines.

Connecting rod: marine head design, completely machined and dimen-

sioned to achieve the lowest dismantling height (Figure 27.18).

Bearings: Thick-pad bearing technology and moderate main bearing load.

Cylinder liner: high-collar design with optimized operational roundness

and straightness; bore cooling for effective control of liner temperature; and

anti-polishing ring.

Piston and rings: composite piston with forged steel crown and nodular

cast iron skirt; pressurized skirt lubrication; two ceramic chromium-plated

directional barrel-shaped compression rings in the crown; a ceramic chromium-

plated taper-faced compression ring and an oil distributor ring in the skirt.

Cylinder head: stiff box with four screws; ﬂame plate supported by inter-

mediate deck, allowing no deformation of valve seat environment.

Camshaft: single-cylinder shaft segments with integrated cams; and built-in

valve tappet modules screwed onto the engine block.

Turbocharger/charge air cooling: modular exhaust gas pipes in insulated

hot box; two-stage air cooler; and integrated, rigid turbocharger support/charge

air cooler.

Figure 27.18 Wärtsilä 38 engine connecting rod

Wärtsilä 38 691

692 Wärtsilä

W38B Version

The popularity of the Wärtsilä 38 encouraged the development of a B-version

offering higher powers with lower fuel consumption and reduced emissions.

The new model also beneﬁted from an advanced control and monitoring sys-

tem, and full modularization to ease assembly. The B-series was fully released

in 2001 with in-line six, eight and nine and V12-, 16- and 18-cylinder models

covering a power band from 4350 kW to 13 050 kW at 600 rev/min. The V18-

cylinder model was subsequently dropped, lowering the upper output of the

programme to 11 600 kW. Two speciﬁc ratings are now offered: 675 kW/cyl-

inder (25.1 bar mep) and 725 kW/cylinder (26.9 bar mep). The original engine,

re-designated the Wärtsilä 38A, remained in the programme for a while, both

versions assigned for production at Wärtsilä Italia in Trieste.

Careful selection of thermodynamic processes and progress in component

design contributed to the achievement of the development goals. The reﬁnements

included a higher maximum cylinder pressure, compression ratio and mean effec-

tive pressure, and advanced fuel injection timing (see p. 693). Special attention

was paid to optimizing the combustion chamber characteristics, the fuel injection

proﬁles and the piston shape. The air ﬂow system was fully revised (inlet bends

to the cylinder head, ﬂow channels at the head, port design, exhaust bends and

turbocharger). State-of-the-art valve timing (including application of the Miller

cycle) was also introduced. New generation turbochargers were adopted to fur-

ther boost overall performance and economy, with integration of the bypass and

waste gate valves. The position of the valves and their controls was selected to

secure low vibration levels, low local temperatures and short pipework.

These measures resulted in a signiﬁcant reduction in speciﬁc fuel con-

sumption (6 g/kW h below the 38A engine) and opened the way to further

improvements in fuel economy. The limits on pollutant emissions were also

comfortably met; the NOx emission levels can be lowered further by up to 50–

60 per cent by incorporating direct water injection (see Chapter 3).

Critical components were given greater strength to support the higher

power levels and raise the safety margins for further rating growth; examples

include additional strengthening of the connecting rod and geometrical modiﬁ-

cations at the interface of the cylinder liner with the cylinder head (area of the

gas sealing ring). All bearing loads are kept low and an ample oil ﬁlm thick-

ness ensures safe bearing operation.

A new three-piston ring pack and an anti-polishing ring incorporated at the

top of the cylinder liner foster low lube oil consumption. The ‘hot box’ envi-

ronment was completely redesigned to secure high reliability and functional

integration; externally, there are swinging hot box covers and new cylinder

head covers enclosing the hot box area; internally a new LP and HP fuel sys-

tem is based on a new generation pump.

The lubrication module incorporates a cooler, an automatic back-ﬂush ﬁl-

ter, a centrifugal ﬁlter in the back-ﬂush line and thermostatic valves. The over-

all system includes main and pre-lubrication pumps and the regulating valve.

Lubricating oil ﬂow through the engine was revised and based on optimized

bearing clearances to allow large safety margins. Running-in ﬁlters of the spin-

on type can be mounted on the non-operating side of the engine and removed

after the factory approval test. The LT and HT cooling water systems embrace

pumps and thermostatic valves built on the engine.

Compactness and easier installation were promoted by ‘all on’ engine sup-

port systems, with complete lubrication and cooling water modules mounted

on the engine (Figure 27.19). All piping connections are located at the free end

to allow simpliﬁed interfacing with external systems. Among the advanced

automation features is Wärtsilä’s WECS 7000 engine-integrated system, which

is tailored for the group’s medium-speed designs.

Wärtsilä 38A and 38B dierences

38A 38B

Bore

380 mm 380 mm

Stroke 475 mm 475 mm

Output/cylinder 660 kW/cyl 725 kW/cyl

Cylinders 6, 8, 9L/12, 16, 18V Same

Speed

600 rev/min 600 rev/min

Compression ratio 12.7 15

Maximum combustion

pressure

180 bar 210 bar

Mean eective pressure 24.5 bar 27 bar

Piston speed 9.5 m/s 9.5 m/s

injector holes 11 10

injector position ref 6 mm higher

Fuel cam ref Low speed

Cylinder head

ref Optimized air ow

Piston shape

ref Optimized

inlet valve timing ref Miller cycle

injection timing ref Advanced

exhaust system ref Optimized ow

Turbocharger eciency 65 per cent 70 per cent

Specic fuel consumption

ref 6 g/kW h

NOx emissions iMO iMO-50 per cent

Prepared for direct water injection

Wärtsilä 38 693

694 Wärtsilä

WärTSiLä 46

Several innovative features were introduced by Wärtsilä Diesel with the launch

of the W46 engine in 1988 which became popular in diverse passenger and

cargo ship propulsion sectors (Figures 27.20 and 27.21). The designer’s prime

development goal in creating a large bore medium-speed trunk piston engine

to compete with low-speed crosshead machinery was the highest reliability. A

signiﬁcant contribution was sought from thick-pad bearing technology, exploit-

ing large bearings with thick oil ﬁlms, and pressure-lubricated piston skirts.

During its ﬁrst 4 years of production the 460 mm bore/580 mm stroke

design was offered with an output of 905 kW/cylinder at 450/500/514 rev/min

as the Wärtsilä 46A. Experience with pilot installations encouraged the release

in 1992 of a B-version with an output of 975 kW/cylinder at 500/514 rev/min.

The C-output version followed in autumn 1995, yielding 1050 kW/cylinder

at 500/514 rev/min with reduced thermal load and lower emissions thanks to

advances in turbocharging and fuel injection systems and the adoption of low

NOx combustion principles.

The three ratings allow the 46 series to cover a power band from 5850 kW

to 15 600 kW with six, eight and nine in-line and V12- and V16-cylinder mod-

els. The C-output versions operate with mean effective pressures up to 28.8 bar

and the engine is designed for a maximum combustion pressure of 210 bar.

A nodular cast iron engine block (Figure 27.22) is conﬁgured to achieve a

rigid and durable construction for ﬂexible mounting (the W46 engine was per-

haps the ﬁrst of its size designed for elastic and super-elastic mounting, Figure

27.23). The main bearings are of the underslung type with hydraulically tight-

ened bolts; side bolts add further rigidity to the main bearing housing. In-line

cylinder engines are equipped with an integrated air receiver fostering rigidity,

CAC HT

CAC LT

HT pump

HT standby

HT in

LT standby

LT in

LT out

HT out

Lub. oil

standby

LT pump

Lub. pump

Pre-lub. pump

Centrifugal

filter

Lub. oil

filter

Lub. oil

cooler

Figure 27.19 integrated cooling water and lubrication oil systems of W38B engine

simplicity and cleanliness. A welded block of steel castings and plates is

offered as an alternative.

A rigid box-like cylinder head design aims for even circumferential contact

pressure between the head and cylinder liner, with four ﬁxing bolts simplify-

ing maintenance procedures. No valve cages are used, improving reliability

and increasing the scope to optimize exhaust port ﬂow characteristics. Water-

cooled exhaust valve seat rings are speciﬁed. Both inlet and exhaust valves

receive a forced rotation from Rotocaps during every opening cycle, fostering

an even temperature distribution and wear of the valves, and keeping the seal-

ing surface free from deposits, resulting in good heat conduction.

Cylinder liner deformations are normally caused by cylinder head clamping

and thermal and mechanical loads. A special liner design with a high collar-to-

stroke ratio for the W46 engine minimizes deformation, the round liner bore in

combination with efﬁcient lubrication enhancing conditions for the piston rings

and reducing wear. The liner material is a special grey cast iron alloy developed

for wear resistance and high strength. An anti-polishing ring in the upper part of

the liner prevents the bore polishing that leads to local liner wear and increased

lubricating oil consumption. The simple yet highly effective device was intro-

duced as standard on later engines and retroﬁtted to existing installations.

A composite low-friction piston with a nodular cast iron skirt and steel

crown incorporates a special cooling gallery conﬁguration to secure efﬁcient

cooling and high rigidity for the piston top. It is designed to handle combus-

tion pressures beyond 200 bar. A long lifetime is sought from hardened top ring

grooves. The three-ring set includes a top ring with a special wear-resistant

coating; all the rings are dimensioned and proﬁled for maximum sealing and

Figure 27.20 Six-cylinder version of the Wärtsilä 46 engine

Wärtsilä 46 695

696 Wärtsilä

pressure balance. Low friction is addressed by a skirt lubrication system deliver-

ing a well-distributed clean oil ﬁlm that eliminates the risk of piston ring scuff-

ing and reduces the wear rate, and fostering cleaner rings and grooves free from

corrosive combustion products. Noise and wear are reduced by hydraulically

damped tilting movements provided by an oil pad between liner and piston.

Hard skirt contact against the liner experienced with the ﬁrst seagoing W46

engine installation was solved by introducing a piston with increased compres-

sion height, a slightly increased skirt length and an optimized skirt form.

A three-piece marine design connecting rod was designed for distribution of

the combustion forces over a maximum bearing area with relative movements

Figure 27.21 Cross-section of Wärtsilä 46 engine

Figure 27.22 engine block of Wärtsilä 46 engine

Figure 27.23 Flexible mounting system for Wärtsilä 46 engine

Wärtsilä 46 697

698 Wärtsilä

between mating surfaces minimized. Piston overhaul is facilitated without

touching the big end bearing, and the bearing can be inspected without remov-

ing the piston. The three-piece design also reduces the piston overhauling

height.

The one-piece forged crankshaft is designed to accept a high combustion

pressure while maintaining a conservative bearing load; rigidity is under-

written by a moderate bore/stroke ratio and large pin and journal diameters.

Counterweights ﬁtted on every crank web provide 95 per cent balancing.

Reliability is sought from the thick-pad bearing design and bearing loads

reduced by increasing crankshaft journal and pin diameters as well as length.

Low bearing loads allow for softer bearing materials with greater conformabil-

ity and adaptability, underwriting a virtually seizure-free bearing, according to

Wärtsilä. A thick corrosion-resistant overlay of tin–antimony is used in the big

end bearings.

The key elements of Wärtsilä’s thick-pad bearing technology are: ample oil

ﬁlm thicknesses achieved by proper overall dimensioning; corrosion-resistant

bearing materials with excellent embeddability properties; radially rigid bear-

ing assemblies during both mass and gas force loading conditions; conform-

able bearing materials and axially conformable housings, which minimize edge

pressure; and oil grooves and oil holes located where no harm can be caused to

the oil ﬁlm.

The camshaft is built of single-cylinder sections with integrated cams. The

shaft sections are connected through separate bearing journals which make

it possible to remove the sections sideways from the camshaft compartment.

The valve follower is of the roller tappet type, the roller proﬁle being slightly

convex for good load distribution. The valve mechanism includes rocker arms

working on yokes guided by pins.

W46 Fuel injection

An innovation on the original W46 engine was a twin injection fuel system

exploiting a pilot valve and a main injection valve (Figures 27.24 and 27.25).

The initial aim of the system was to secure safe combustion with the poorest

fuels on the market; the very fast main injection process facilitated by the sys-

tem also contributed to a low fuel consumption. It became evident too that the

twin injection conﬁguration yielded a positive effect on NOx emissions, stimu-

lating further reﬁnement of the system.

The ﬁrst seagoing W46 engine installation was later optimized with a

reduction of the pilot fuel amount to below 3 per cent. Both pilot and main

injection valves were designed with a small heat absorbing surface facing the

combustion space and arranged for efﬁcient heat transfer to the cooling water,

eliminating the need for a separate nozzle temperature control system. Fuel

was transported over the shortest distance from the pump to the valve via a

high-pressure pipe in the cylinder head.

Wärtsilä cited the following merits of twin injection through individually opti-

mized nozzles: excellent pilot fuel atomization and combustion over a wide load

and speed range; reasonable injection rates even under idling conditions when the

combustion process is often a poor compromise; facility to burn fuels of low igni-

tion quality with a minimum of ignition delay; low combustion noise level thanks

to controlled pre-burning, resulting in a lower rate of increase in cylinder pressure

Figure 27.24 Wärtsilä 46 twin injection system, showing the pilot fuel valve (left)

and the main fuel valve (centre)

Pilot

injection

Main

injection

Liquid fuel jet

Atomized fuel

Injection

nozzle

Premixed combustion after

ignition delay

Air entrainment nozzle-

induced recirculation

Diffusion combustion following

premixed combustion

Figure 27.25 Twin injection system principle for original Wärtsilä 46 engines

Wärtsilä 46 699