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

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650 Sulzer (Wärtsilä)

two-stroke design was succeeded in 1972 by a four-stroke version (the Z40) which

was in turn completely redesigned and replaced in 1982 by the ZA40 engine. A

longer stroke (560 mm instead of 480 mm) ZA40S engine was introduced in 1986

to succeed the Z40 and ZA40 designs. The performance development steps over

the years are illustrated in Figure 26.2.

In autumn 1988, 3 years after its announcement with a cylinder output of

660 kW, the ZA40S was uprated to 720 kW/cylinder following successful serv-

ice experience and heavy fuel endurance tests. The new rating was associated

with an increased maximum cylinder pressure of 165 bar and mean effective

pressure of 24.1 bar, underwritten by thermodynamic and mechanical optimi-

zations. Some design modiﬁcations were also introduced to improve product

reliability and durability in general (Figure 26.3):

MCR

B.S.F.C

ratio: p

max

/b.m.e.p

°C

1972 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91

Year

ER II

ER I

MCR

ER I

ER II

MCR

ER I

ER II

Cylinder output

Valve seat temperature at MCR

700

600

500

400

210

g/kW h

200

190

180

170

500

450

400

350

ZA 40

Z 40

ZA 40S

FIgure 26.2 development steps of successive Sulzer Z40, ZA40 and ZA40S designs

illustrated by key performance parameters

l An exhaust valve with a 45° seat angle, this reﬁnement achieving: better

cooling without reduction of mechanical and thermal safety at the higher

engine load and maximum cylinder pressure; and more efﬁcient seating

due to a wider seat area.

l Sulzer’s new combined ‘jet-shaker’ piston cooling principle in which the

conventional shaker effect in the piston crown is supplemented by oil

jets sprayed through nozzles. This offers advantages in lowering piston

crown temperatures, thereby preventing carbon formation and gaining

an improved washing effect.

l A new version of the waste gate in the exhaust gas manifold before the

turbine, securing a better match between the engine and turbocharger at

the increased power rating.

rotating Piston

A classic feature of Sulzer Z-type engines retained throughout successive gen-

erations is the rotating piston. Sulzer patented the concept in 1937 after tests

on a number of research engines with cylinder bores from 90 mm to 420 mm

which conﬁrmed the special beneﬁts offered for highly loaded trunk piston

Introduction 651

Introduced for all engine power steps (600–720 kW/cyl)

Jet-shaker cooled

piston

Corrosion-free

injection nozzle

Exhaust

waste gate

Aluminium

bearings

Chromium-plated oil

scraper ring

Improved piston

skirt shape

Wear-resistant inlet

valves

Continuous grain

flow forged

crankshaft

Reinforced

regulating

linkage

45° Nimonic

exhaust valve

Improved relief

valve

Electronic speed-

governor

Reinforced governor

drive and overspeed

cut-off device

FIgure 26.3 Main modications for uprated ZA40S engines

652 Sulzer (Wärtsilä)

engines. A ratchet mechanism transforms the swinging motion of the connect-

ing rod into a smooth rotation of the piston (Figure 26.4). The connecting rod

has a spherical small end which allows some 40 per cent more bearing area

than a gudgeon pin bearing.

The rotating piston was eventually adopted for the Z-type design, ﬁrst intro-

duced in two-stroke form in 1964. The concept remained unique to Z-engines

until its adoption in 1995 for GMT’s upgraded 550 mm bore VA55 medium-

speed model. (The Italian designer beneﬁted from a technology transfer arrange-

ment with Sulzer; both later became members of the Wärtsilä Corporation.) For

rotating the piston, the connecting rod is provided with two pawls positioned

slightly out of the centre of the spherical small end bearing. When the connect-

ing rod performs its swinging movement relative to the piston, the pawls impart

an intermittent rotating motion to a toothed rim, from which it is transmitted to

the piston by an annular spring. The ﬂexible connection maintains the forces

necessary for rotating the piston at a constant low level.

Sulzer cites the following merits of the rotating piston:

l Even distribution of temperature around the piston crown, as there are

no particular inlet and outlet zones.

l Small and symmetrical deformation from a top end bearing with a rela-

tively large area and of spherical design (Figure 26.5).

l Optimum sealing and working conditions for the piston rings because

the small, symmetrical deformations of the piston allow the use of the

smallest running clearance between piston and cylinder liner.

n = 530 rpm

0° 90° 180° 270° 360°

rad/s

1.0

0.5

Spring

Ratchet-ring

Ratchet

= angular velocity of the piston

= angular velocity of the ratchet-ring

FIgure 26.4 drive principle of rotating piston

l Low and stable lubricating oil consumption because the small piston

running clearances minimize piston slap and obviate the need for the tra-

ditional oil cushion, thereby allowing the oil scraper ring to be located at

the lower end of the piston skirt.

l Good margin for unfavourable running conditions with the smallest risk

of seizure because the grey-iron piston skirt is always turning to a fresh

part of the cylinder liner surface.

ZA40 And ZA40S engIneS

The last ZA40S programme was released with a rating of 750 kW/cylinder at

510 rev/min on a mean effective pressure of 25.1 bar and with a mean piston

speed of 9.5 m/s. Six, eight and nine in-line models and V12-, 14-, 16- and

18-cylinder models covered a power band from 4500 kW to 13 500 kW (Figure

26.6). The crankcase is fabricated for in-line cylinder engines while V-form

versions feature a cast iron monobloc. The originally fabricated two-part design

of the Z-engine frame was superseded by a monobloc casting for the ZA40

and ZA40S in-line models, as had always been standard for the V-engines

(Figure 26.7).

Uneven cylinder liner wear was initially encountered in the mid-1970s by

the ﬁrst Z40 engines burning blended or heavy fuel oils with increased sul-

phur content, at a height equal to the top dead centre of the piston rings. This

wear could be deﬁned as local low-temperature corrosion which was coun-

tered by a four-step solution: (1) ensuring adequate alkalinity of the lubricat-

ing oil, (2) modiﬁed cylinder lubrication, (3) higher cooling water temperature

and (4) increased cylinder liner wall temperature from a modiﬁed bore-cooling

ZA40 and ZA40S engines 653

FIgure 26.5 Symmetric design, with rotation, ensures that Sulzer Z-engine pistons

(right) keep their circular form. gas and mass forces are transmitted centrally through

the spherical bearing with no load on the piston skirt. Comparison is with a conven-

tional gudgeon pin arrangement (left)

654 Sulzer (Wärtsilä)

arrangement with local insulating tubes. (Operators of earlier Z40 engines were

offered insulating tube sets, together with appropriate ﬁtting devices, for ret-

roﬁt to original cylinder liners during a routine overhaul.) Subsequent service

results demonstrated that these measures reduced low-temperature corrosion to

a negligible (if any) amount.

FIgure 26.6 Sulzer ZA40S engine showing the rotating piston, fully bore-cooled

combustion space and single-pipe exhaust system

FIgure 26.7 engine blocks of ZA40S vee-form and in-line cylinder versions

A modiﬁed O-ring material (changed to a silicon quality and an appropriate

ﬂuor elastomer quality, such as Viton) was adopted for the cooling water space

of the cylinder liner after leakage was experienced with some units. The dry

lower half of the liner almost entirely eliminates the risk of water leakage into

the crankcase and contamination of the lubricating oil.

The double-bottomed water-cooled cylinder heads of Z40 engines, made

of cast iron, performed well. Some cracks resulted, however, either from cast-

ing and machining faults or from the supply of spares not intended for engines

with the later, higher outputs. The material for the O-rings of the valve seats

and guides was also changed to an appropriate ﬂuor elastomer quality, both

with good results.

The cylinder heads of the ZA40 and ZA40S engines are bore cooled

(Figure 26.8). The measured bottom deformations under gas load are only

one-third those of the conventional double-bottom head, and the mechanical

ZA40 and ZA40S engines 655

FIgure 26.8 Fully bore-cooled combustion space of Sulzer ZA40S engine

656 Sulzer (Wärtsilä)

stresses were also found to be low in the same proportion. The drilled cooling

passages permitted a uniform temperature adjustment within a few degrees at

the desired low level. The resulting low thermal distortion as well as the high

mechanical stiffness fosters good valve seat sealing with optimum heat transfer

to the exhaust valve seats.

The well-proven rotating piston concept was retained for the ZA40 and

ZA40S engines but beneﬁted from simpliﬁcation; bore cooling of the crown

was also introduced. An optimized arrangement of the cooling bores reduced

stresses by around 10 per cent with practically no change in crown temper-

atures. The substantially stiffened periphery secured by bore cooling also

resulted in reduced thermal deformation of the crown and hence improved

working conditions for both piston and rings (Figure 26.9).

The top ring of the ZA40S piston features a Sulzer plasma-coated running

face, while the running faces of the other two rings are chromium plated. All

three rings are barrel shaped to promote initial running in and to avoid a need

to hone smoothened cylinder liners at routine overhauls. (The complete set

of piston rings and scraper rings should therefore be replaced during a piston

overhaul.) The ﬂanks of the rings and their grooves are also chromium plated.

Sulzer reports that a combination of bore-cooled rotating pistons and plasma-

coated piston rings promotes low wear rates and long piston life. Rotation and

negligible distortion due to the spherical top end, as well as no transmission of

gas forces through the cast iron piston skirt, underwrite a small skirt clearance.

The connecting rod with marine-type bottom end features large crankpin

diameters and hydraulically tightened studs for easy maintenance; no interfer-

ence with the bottom end bearing is necessary at piston overhauls. Shims can

be applied to achieve the different compression ratios for three engine tuning

options.

209°C 220°C

310°C

136°C

FIgure 26.9 Measured temperatures in a ZA40S engine’s rotating piston

The well-proven exhaust valve arrangement reﬁned over many years in the

Z40 and ZA40 engines was retained for the ZA40S: Nimonic exhaust valves with

rotators and water-cooled valve seat inserts but no valve cages (Figure 26.10). The

higher rigidity of the bore-cooled cylinder head and the generally lower tempera-

ture levels from the enhanced turbocharging system further improved the sealing

conditions of the exhaust valves in the ZA40S. Longer times-between-overhauls

were thus secured, the intervals coinciding with those for the piston.

A new generation of aluminium/tin alloy bimetal-type bearings was intro-

duced for both main and bottom end bearings of ZA40 and ZA40S engines

(Figure 26.11). These ‘Rillenlager’ and bimetal types dispensed with the tra-

ditional overlay of the previous tri-metal type to give the following reported

beneﬁts: sustained fatigue resistance and embeddability; better wear resistance

for heavy fuel operation; improved local self-healing and emergency running

behaviour; and better adaptability after inspections.

The loading of the spherical top end bearing remained some 15–20 per cent

lower than that of a conventional gudgeon pin design. The big end of the ZA40

and ZA40S connecting rod was also enlarged from the Z40 design, resulting

ZA40 and ZA40S engines 657

FIgure 26.10 Inlet valve (left) and exhaust valve (right) of ZA40S engine

658 Sulzer (Wärtsilä)

in a reduction of the dynamic stresses of around 20 per cent. The bolts are

hydraulically tightened for easier and faster maintenance procedures.

Helix-controlled fuel injection pumps (Figure 26.12) are provided for each

cylinder, with fuel leakage into the lubricating oil prevented by an oil barrier at

the lower end of the plunger sleeve. Depending on the engine application, helix

control edges can be chosen for constant or variable injection timing (CIT or

VIT). The simple arrangement of short high-pressure fuel pipes to the injection

valves, together with the circulation of preheated fuel oil up to the level of the

pumps, underwrites routine pier-to-pier operation on heavy fuel (with appro-

priate circulation and preheating).

The injection valves (Figure 26.13) are equipped with sleeve-type noz-

zles cooled by freshwater and provided with rounded-off inner edges of the

spray holes (Figure 26.14). Rounded-off spray holes are traditional for Sulzer

nozzles and reportedly result in a more stable spray pattern over longer service

periods with a narrower scatter of injection rates between individual cylinders.

A more corrosion-resistant sleeve material was introduced to replace the origi-

nal material whose punishment threshold was not large enough to cope with

corrosion attacks by insufﬁciently treated cooling water (as a result, some tip

breakages occurred).

Plunger seizures initially occurred on Z40 engines due to insufﬁcient clear-

ance (either through manufacturing with too small a clearance or reduction of

clearance in heavy fuel service as a result of the tendency of the material used

earlier to grow at higher temperature levels). An increased plunger clearance

and a changeover to a new material speciﬁcation for the plungers, supported

by stricter quality control, proved to be design remedies. Operationally, ship-

yards and operators must take care that no undue impurities are present in the

Barrier layer : Ni

Shell back: Steel

Running layer 2: Al with 6% Sn

Running layer 1: Pb Cu Sn-(or Sn Sb-) overlay as groove

fillings

FIgure 26.11 Features of ‘rillenlager’-type bearings: thickness of the Al/Sn running

layer no. 2 is a few tenths of a millimetre; depth of grooves is a few hundredths of a

millimetre; grooves are laid as a helical thread; shape and dimension of grooves may

vary, depending on the manufacturer

fuel and lubricating oil systems which may lead to avoidable plunger seizures.

Slow heating up of the fuel system and the maintenance of the correct tempera-

ture level for the heavy fuel oil used are also advisable.

Some sticking of fuel pump plungers was reported from a growing lac-

quer formation on the guide part side, depending on the combination of heavy

fuel oil and lubricating oil used. As a solution, the lubrication and oil barrier

arrangement was redesigned to increase the oil ﬂow and thus reduce the ten-

dency for oil ageing, if any.

With regard to the hydraulically shrunk-on fuel cams and rollers, some fail-

ures initially occurred on Z40 engines as a result of misaligned drive housings

or insufﬁcient case hardening of the cam surfaces. Stricter speciﬁcations and

quality control, together with wider cams and rollers, remedied the problem.

Two-piece replacement cams were introduced for non-reversible engines to

simplify renewal.

ZA40 and ZA40S engines 659

FIgure 26.12 Fuel injection pump for ZA40S engine