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

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and without disturbing the rest of the engine. The exhaust manifolds feature

sliding joints and, like the turbochargers, are housed in water-cooled gas-tight

aluminium casings; together with the turbocharger enclosure, they form a sin-

gle, cool gas-tight shell around the hot parts of the engine. The casings foster

low radiant and convective heat losses from the engine.

The gear train is mounted at the engine’s free end, the camshaft gearwheel

meshing directly with the crankshaft gearwheel. Auxiliary drives are provided

for the externally mounted lubricating oil and jacket water pumps, the gover-

nor, overspeed governor and fuel lift pump; further auxiliary power take-off

capability is incorporated for different applications. The PTO at the free end

accepts either gear-driven or belt-driven pumps and alternators. The VP185

was supplied as a complete assembly with engine-mounted jacket water and

lubricating oil heat exchangers to simplify installation procedures. A choice of

air, electric or hydraulic starting systems was available.

Marine engine versions are controlled by a Regulateurs Europa digital

Viking 2200 governor and 2231 actuator with the option of ball head back-up.

Multiple Schwitzer turbochargers in a passive two-stage conﬁguration with

intercooling and aftercooling were retained for the V18-cylinder model, albeit

with an increased number of sets. Three HP turbochargers each expand into a

pair of similar LP turbochargers, giving a total of nine turbochargers. Unlike the

12VP185 engine, the turbochargers are packaged in groups of three (one HP

and two LP), each group having its own water-cooled gas-tight housing. The hot

turbine volutes are ﬁxed to the inside of each housing, with the rest of the tur-

bocharger, the ‘cartridge’, plugged in from the outside: an arrangement which

paxman (man Diesel) 811

A1 : High-speed craft, 4000 kWb, 1950 r/min at air 25°C,

sea water 25°C

A2 : High-speed commercial craft, 3300 kWb, 1830 r/min

B : Long-range patrol craft, 3000 kWb at 1770 r/min at

air 45°C, sea water 25°C

Torque limit A1

200

205

210

215

225

240

260

280

Specific fuel consumption

g/kW h (…0%, +5%) at

Air 25°C, seawater 25°C

195

Torque limit A2

Cube law

600 800 1000 1200 1400 1600 1800 2000 2200

Engine speed (r/min)

4500

4000

3500

3000

2500

2000

1500

1000

500

Engine power (kW)

FigurE 30.32 performance map for paxman 18vp185 engine

812 High-Speed Engines

enables the turbochargers to be replaced quickly. The hot exhaust manifold pipes

are similarly housed in their own water-cooled gas-tight casings, so eliminating

unwanted heat and gas around the engine (an important safety aspect).

SEmt-piElStiCK

The Paris-based designer SEMT-Pielstick ﬁelded three long-established high-

speed programmes, the PA4, PA5 and PA6 series, before and after its acquisi-

tion ﬁrst by MTU and MAN B&W Diesel and then wholly by MAN Diesel.

Some engines are still produced for special land-based and submarine applica-

tions but not for mainstream marine markets.

The PA4 series, produced in 185 mm and 200 mm bore versions (both hav-

ing a stroke of 210 mm), featured a variable geometry (VG) pre-combustion

chamber; while the PA5 is a 255 mm bore/270 mm stroke design.

Development beneﬁted the 280 mm bore PA6 design, introduced in 1971

with a stroke of 290 mm and a maximum continuous rating of 258 kW/cylin-

der at 1000 rev/min. The output was raised to 295 kW/cylinder in 1974 and

to 315 kW/cylinder in 1980. Extensive service experience in naval vessels

moulded progressive reﬁnements over the years, including the creation of a

BTC version in 1980 which yielded 405 kW/cylinder at 1050 rev/min through

the adoption of a reduced compression ratio in conjunction with two-stage tur-

bocharging. This PA6 BTC model was released in 1985 with a higher output of

445 kW/cylinder. The series was extended in 1983 by a longer stroke (350 mm),

slower speed PA6 CL variant with a rating of 295 kW/ cylinder at 750 rev/min.

A sequential turbocharging system (STC) was introduced to the 290 mm

stroke engine in 1989, this PA6 STC model developing 324 kW/cylinder at

1050 rev/min. Performance was enhanced in a 330 mm stroke B-version from

1994, an STC system and a cylinder head with improved air and gas ﬂow rates

contributing to a nominal maximum continuous rating of 405 kW/cylinder at

1050 rev/min. A maximum sprint rating of 445 kW/cylinder at 1084 rev/min

equated to an output of 8910 kW from the V20-cylinder PA6B STC engine

which targeted high-performance vessels such as fast ferries.

At the nominal mcr level the V12-, 16- and 20-cylinder models respec-

tively offered 4860 kW, 6480 kW and 8100 kW for fast ferry propulsion, being

released for sustained operation at these ratings. The weight/power ratio of the

20PA6B STC model at 8100 kW is 5.2 kg/kW (Figure 30.33).

pa6B StC Design

High rigidity without compromising overall engine weight was achieved by a

stiff one-piece crankcase (Figure 30.34) of nodular cast iron specially treated

for shock resistance, with transverse bolt connections between both crankcase

sides through the underslung-type bearing caps. Integrated longitudinal steel

piping supplies lubricating oil to the main bearings.

Large dimension main journals yield a particularly large bearing surface

and conservative pressures which underwrite prolonged bearing life, as do

the large surface area connecting rod bearing shells. The alloy steel one-piece

forged crankshaft (Figure 30.35) has high frequency hardened crankpins and

journals, and is bored to feed lubricating oil to the connecting rods.

A nodular cast iron cylinder head was conﬁgured to achieve improved

air and gas ﬂow rates for the PA6B STC version. The composite piston (steel

crown and aluminium skirt) is ﬁtted with ﬁve rings. The forged steel connect-

ing rod is ﬁtted with large surface bearing shells to yield high durability under

high ﬁring pressures. Special low-wear characteristics were sought from the

centrifuged cast iron material speciﬁed for the cylinder liner (Figure 30.36).

The camshaft is formed from several sections for ease of dismantling.

SEmt-pielstick 813

FigurE 30.33 SEmt-pielstick v20-cylinder pa6B StC engine with sequential turbo-

charging system; a rating of 8100 kW was oered for fast ferry propulsion

FigurE 30.34 SEmt-pielstick pa6B StC crankcase

814 High-Speed Engines

Operational ﬂexibility and power output are fostered by the single-

stage sequential turbocharging system which is based on two turbochargers

and delivers a large combustion air excess at partial loads. Supercharging is

effected by one turbocharger for engine loads up to 50 per cent of the nomi-

nal power, its effort boosted by the second identical turbocharger at higher

loads. Switching from single to twin turbocharger mode is performed auto-

matically by opening two ﬂap valves. Engine performance at prolonged low

load is thereby improved with respect to fuel consumption, smoke emissions,

fouling resistance and transient performance. Additionally, engine utilization is

expanded towards the high torque/low revolutions per minute area.

Developing the High-performance Derivative

Reﬁning an established engine for high-speed vessel propulsion duty must

focus on achieving a lighter and more powerful package within a more com-

pact envelope, while also enhancing operating ﬂexibility. The resulting

FigurE 30.35 SEmt-pielstick pa6B StC crankshaft

FigurE 30.36 SEmt-pielstick pa6B StC cylinder liner

PA6B STC design was released with a nominal maximum continuous rating

of 405 kW/cylinder at 1050 rev/min and a sprint rating of 445 kW/cylinder at

1084 rev/min (Figure 30.37).

SEMT-Pielstick’s development goals aimed to:

l Increase the power rating by 25 per cent.

l Use an operating ﬁeld allowing operation following a double propeller

law.

SEmt-pielstick 815

FigurE 30.37 Cross-section of SEmt-pielstick pa6B StC engine

816 High-Speed Engines

l Maintain, and if possible reduce, the speciﬁc fuel consumption.

l Reduce the weight/power ratio and enhance engine compactness.

l Reduce installation costs.

l Retain the maximum number of existing components.

Power can be raised in two ways: by increasing the brake mean effec-

tive pressure and by increasing the piston speed. Increasing piston speed can

be effected by increasing the revolutions per minute and by lengthening the

stroke. Increasing the revolutions per minute was not attractive because of a

need to maintain an acceptable synchronized speed for land power plant appli-

cations but also because of these drawbacks: increased speciﬁc fuel consump-

tion; higher wear rate; and higher noise levels.

Raising piston speed by lengthening the stroke is considered preferable

because it reduces speciﬁc fuel consumption and fosters optimum performance

at starting and low load (the compression ratio is secured without resorting to a

combustion chamber that is too ﬂat). This route was therefore selected but with

the piston speed limited to 11.5 m/s (representing an increase of 14 per cent

with a stroke of 330 mm). To reach the required power rating, the brake mean

effective pressure had to be increased by 10 per cent; this was achieved by

adopting high-performance turbochargers. Attaining the desired speciﬁc fuel

consumption called for the maximum cylinder pressure to be raised to 160 bar.

Coping with these new parameters dictated redesigning some key compo-

nents. The existing cylinder head, for example, was incompatible with the tar-

geted speciﬁc fuel consumption level. This was partly because its mechanical

strength was insufﬁcient in relation to the necessary peak combustion pressure,

and partly because the pressure loss through the inlet and exhaust ports would

be unacceptable in relation to the intervening gas ﬂow dictated by the increased

power. A new cylinder head was therefore designed with a reinforced bore-

cooled ﬁre plate and incorporating inlet and exhaust ports with large dimen-

sions. In addition, cooled seats were speciﬁed for the inlet valves as well as the

exhaust valves to maximize seat reliability and avoid risks of burning.

A redesigned connecting rod was also necessary to address the increased

peak compression pressures and the inertia efforts linked to the lengthening of

the stroke. The bevel-cut design of the original component was abandoned in

favour of a straight-cut rod to avoid weak points, such as the bevel-cut serra-

tions and the threading anchorage in the shank. Piston cooling by jet replaced

the traditional oil supply through the connecting rod from the crankpin, simpli-

fying machining and allowing the bearing shell grooves to be eliminated and

yield these beneﬁts: increasing capacity of the bearing shells; stopping cam

wear of the crankpin (differential wearing of the pins between the side areas

of the plain bearing shell and the central area; that is, the area including the

groove); and cutting out the risk of cavitation erosion on the bearing shells at

the end of the grooves. The new connecting rod is 10 per cent lighter than its

bevel-cut forerunner. The weight reduction, along with an improved bearing

shell capacity, partly compensates for the increased inertia efforts.

Temperature measurements on the jet-cooled piston head indicated simi-

lar levels to those of the original piston, and even lower in some areas. Critical

points of the piston were modiﬁed to cope with the higher peak combustion

pressure: a spherical shape was given to the support spot face used for the piston

head/skirt tightening spacers; and the radius under the skirt vault was increased.

Crankshaft dimensions were modiﬁed to target the same reliability from

the component as before, despite the increased stroke and peak combustion

pressure. Finite element analyses of the crank webs and hydrodynamic calcula-

tions led to an increase in the journal diameter from 230 mm to 250 mm, and in

the crankpin diameter from 210 mm to 230 mm.

High-performance turbochargers were necessary to secure the brake mean

effective pressure increase with the required efﬁciency, a model from MAN

B&W’s then new NA series—the NA 34S—being selected to meet the perform-

ance and compactness parameters. Sequential turbocharging was applied, based on

the principle of reducing the number of turbochargers in operation as the engine

speed and load fall. The speed of the turbochargers still operating consequently

rises and signiﬁcantly larger quantities of air are thus delivered to the engine.

A simple system was adopted using only two turbochargers, one being

switched off at below approximately 50 per cent of the nominal engine power rat-

ing. This was effected by closing two ﬂap valves located at the compressor outlet

and at the turbine inlet of one of the turbochargers. The designer cited the follow-

ing beneﬁts from the PA6 engine’s sequential turbocharging (STC) system:

l High torque and power ability at reduced engine speed.

l A gain in fuel economy at low and part loads.

l Capability to run the engine at very low loads for extended periods with

minimal fouling (the light deposits can be cleaned out by running for

half an hour at 50 per cent load).

l Invisible smoke emissions over a wide working range.

l Reduced exhaust temperature.

l Lower thermal stresses in the combustion chamber components at part

loads.

A higher output rating naturally reduced the engine’s weight/power ratio

but other measures were pursued to trim overall weight. The scope for using

aluminium alloy was explored for all components where operating stresses

(particularly thermal) and class rules allowed, leading to the engine supports,

turbocharger support, air manifolds (after the air cooler), lube oil and water

cooler support, and lube oil ﬁlter support being designed in cast aluminium.

Studies assessed other components for which aluminium could not be consid-

ered, either to use alternative materials with higher mechanical properties and

so reduce thickness, or simply to optimize existing shapes and thicknesses. As

an example, specifying high yield point steel sheet for the manifolds connect-

ing the turbocharger to the air cooler allowed a reduction in thickness from

10 mm to 4 mm. The lube oil sump plate was also modiﬁed by reducing the

material thickness.

SEmt-pielstick 817

818 High-Speed Engines

A 10 per cent reduction in the original engine weight, along with the

increased power output, contributed to a weight/power ratio of 4.8 kg/kW, includ-

ing all ancillaries. In parallel with the weight trimming studies, SEMT-Pielstick

focused on reducing the overall dimensions of both engine and ancillaries.

A key element in this regard was the combi-cooler, integrating one lube

oil and one freshwater plate cooler circulated by a common sea water sys-

tem (Figure 30.38). The combi-cooler’s support is used as the rear plate of

the cooler and includes as-cast part of the connections to the HT freshwater

and lube oil systems. Its front plate incorporates as-cast the connections to the

LT freshwater loop and the water thermostatic valve. The main self-cleaning

lube oil ﬁlter is incorporated axially in a cast support located under the combi-

cooler, and includes the lube oil thermostatic valve as well as the centrifugal

oil ﬁlters.

Such solutions fostered compactness, a simple pipeless conﬁguration and

good access to the main subjects of maintenance. Integration of the ancillar-

ies on the engine further eases shipboard installation procedures. All pumps

(water, oil, fuel make-up), as before, are driven by the engine upon which the

fuel ﬁlter is also mounted.

A potential for burning an intermediate fuel oil grade such as IF30 was

addressed in the development programme, and reﬂected in the speciﬁcation of

cooled valve seats and exhaust valve rotators. The 75°C temperature necessary to

reach an adequate viscosity for its injection could be derived by taking heat from

the engine’s HT freshwater system. In such installations, however, the engine

was derated to 360 kW/cylinder and the time-between-overhauls reduced.

WärtSilä

The medium-speed specialist Wärtsilä added high-speed designs to its four-

stroke engine portfolio with the acquisition in 1989 of SACM Diesel of France.

Engine outlet

Engine

inlet

Freshwater

drive pump

Freshwater circuit

Air cooler

Sea water circuit

Sea water-

driven pump

Oil cooler

Freshwater

cooler

Combi-cooler

FigurE 30.38 Fresh and sea water circuits integrated on the pa6B StC engine

The Finnish parent group saw the market potential for a new generation engine

blending the best features of high- and medium-speed designs for continu-

ous duty applications, resulting in the 1994 launch of the Wärtsilä 200 series

(Figures 30.39 and 30.40).

The 200 mm bore/240 mm stroke engine was produced by Wärtsilä

France for propulsion and genset drive applications, with an output band from

2100 kW to 3600 kW at 1200 rev/min or 1500 rev/min covered by V12-, 16-

and 18-cylinder models. High reliability in continuous duty (deﬁned as 24 h a

day operation with an annual running period of over 6000 h) was sought from

an engine structure and main components designed for a maximum cylinder

pressure of 200 bar. Wärtsilä’s medium-speed engine technology was exploited

to achieve a high power density, low emissions and fuel consumption, and ease

of maintenance. Although W200 engine production was comparatively short

lived, its innovative features are worthy of study by engine designers.

A multi-functional connection piece or ‘multi-duct’ located between the

cylinder head, engine block and exhaust manifold has the following duties:

l Combustion air transfer from charge-air receiver to cylinder head.

l Introduction of an initial swirl to the inlet air for optimal part-load

combustion.

l Exhaust gas transfer to the exhaust system.

l Cooling water transfer from the cylinder head to the return channel in

the engine block.

Wärtsilä 819

FigurE 30.39 Wärtsilä W200 engine

820 High-Speed Engines

l Insulation/cooling of the exhaust transfer port.

l Support for the exhaust manifold and insulation.

l Inclined face towards the cylinder head facilitates easy removal/

remounting of the cylinder head. The exhaust gas piping and insula-

tion box, supported by the multi-duct, stay in place when the head is

removed.

The nodular cast iron engine block was designed for maximum stiffness

with a V60° conﬁguration optimizing balancing and hence limiting vibra-

tion. Bolted supports dictate only four to six ﬁxing points, and provision is

made for elastic mounting. The locations of the camshaft drive (integrated

in the ﬂywheel end of the block) and the air receiver channel (in the middle

of the vee-bank and also integral with the block) reﬂect solutions adopted in

Wärtsilä medium-speed engines. Arranging the oil lubrication distribution

through a channel in the middle of the vee, however, was a new idea. The areas

of the block that support the camshafts and fuel injection pumps are designed

for high strength to accept the forces created by the high injection pressures.

Large openings on both sides of the block facilitate access for inspection and

maintenance.

FigurE 30.40 Wärtsilä W200 engine. the original automotive-type turbochargers

shown here were later replaced by aBB or Holset models