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

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Based on these simulations, it was decided to retain the bore size at

320 mm and increase the stroke to 410 mm; this would enable the engine to run

at full rating at 720 rev/min at the standard injection timing without exceeding

the P

max

limit of 210 bar. Having ﬁxed the bore and stroke dimensions, varia-

tions in compression ratio and start of combustion were investigated for their

effect on fuel consumption and maximum cylinder pressure (see Figure 16.7).

Finally, the effect of valve size on performance was assessed before a 96-mm

diameter inlet valve and an 89-mm diameter exhaust valve were selected as the

optimum choice.

Acknowledgements: MAN Diesel, Rolls-Royce and Wärtsilä.

Designing medium-speed engines 523

CR 13.5

CR 14.5

CR 15.5

CR 16.5

−3 −2 −1 0 1 2

Start of combustion timing (Deg-ATDC)

200

196

192

188

184

180

BSFC (g/kW h)

−3 −2 −1 0 1 2

Start of combustion timing (Deg-ATDC)

CR 13.5

CR 14.5

CR 15.5

CR 16.5

225

215

205

195

185

175

165

155

max

(bar)

FIgurE 16.7 Development of Allen 5000 series engine: eect of variations in com-

pression ratio and start of combustion on maximum cylinder pressure (left) and fuel

consumption (right

) at a power rating of 525 kW/cylinder

525

C h a p t e r | s e v e n t e e n

Allen (Rolls-Royce)

The British designer Allen, Rolls-Royce company, ﬁelded the S12/VS12 and

the S37/VS37 medium-speed engine series which were evolved over many

years but are no longer produced. The S12 was a 241 mm bore/305 mm stroke

design produced in four-, six-, eight- and nine-cylinder in-line versions cover-

ing a power band from 507 kW to 1584 kW at 720–1000 rev/min. The 325 mm

bore/370 mm stroke S37/VS37 design (Figure 17.1) was available in six, eight

and nine in-line and V12- and V16-cylinder forms to deliver outputs up to

5255 kW. The S12, VS12 and S37/VS37 designs were respectively rebranded

as the 2000 series, 3000 series and 4000 series.

The VS12 engine (which became the 3000 series) was ﬁrst introduced in

the 1950s but beneﬁted from a major redesign in 1990 to enhance compact-

ness and raise the power-to-weight and power-to-space ratios. V12- and V16-

cylinder versions of the 240 mm bore/300 mm stroke design covered a power

band from 1584 kW to 4160 kW at 900–1000 rev/min for propulsion and auxil-

iary power duties. Its rigid spheroidal graphite cast iron crankcase, designed to

carry the underslung forged steel crankshaft and main bearing caps, has inte-

gral mounting feet. The assembly is enclosed by a fabricated steel sump and a

cast iron gearcase. The single camshaft is carried in bearings centrally located

in the 60° vee between the cylinder banks and is driven by a gear train from the

crankshaft.

The one-piece crankshaft is arranged with a solid or ﬂexible coupling

through the ﬂywheel that carries a gear ring for air motor starting. The steel

shell main bearings have a lead–bronze lining with overlay plating. Thrust

rings on either side of the ﬂywheel-end main bearing locate the crankshaft.

The main bearing studs pass deep into the crankcase and are hydraulically ten-

sioned; bolts also pass through the crankcase into the sides of the main bearing

caps to maximize rigidity. The camshaft gear drive at the free end of the crank-

shaft and the auxiliary drive hub are ﬁtted on a taper using the oil injection

method.

Individual wet-type cylinder liners feature a thick top collar, drilled to pro-

vide bore cooling of the upper section. Three Viton O-rings make a watertight

joint between the lower liner land and the crankcase. A steel ring, sealed by

a Viton O-ring, is ﬁtted around the top collar of the liner to direct water from

drillings to the six cylinder head transfer holes, each of which is also sealed by

526 Allen (Rolls-Royce)

O-rings. A thick section mild steel ring makes the gas joint between the liner

and cylinder head.

Pistons are of composite construction, with a heat-resistant steel crown and

forged aluminium body. Three pressure rings are ﬁtted in the crown (the top

two featuring hardened ﬂanks) and an oil ring in the body. The pressure rings

and spring-loaded conformable oil scraper ring all have chrome-plated running

surfaces. The pistons are intensively cooled by lubricating oil supplied from

ﬁxed jets in the crankcase; this oil is also being used to lubricate the small end

bearings.

FiguRe 17.1 Cutaway of Allen 12VS37g engine

The connecting rods are arranged in pairs, ﬁtting side-by-side on the crank-

pins. Each rod has a detachable cap secured by four set-screws and located by

dowels; the joint is inclined at 45° to enable the rod to be withdrawn through

the cylinder bore. Steel shell bearings lined with lead–bronze and overlay plat-

ing are ﬁtted at the big end; a steel-backed lead–bronze-lined bush is ﬁtted at

the small end. Individual four-valve cylinder heads are secured to the crankcase

by four studs and hydraulically tensioned nuts. The two inlet and two exhaust

valves seat on renewable inserts in the head, those for exhaust service being

intensively cooled and sealed with Viton O-rings. The inlet valves are manu-

factured from EN 52 and the exhaust valves from Nimonic. Each valve has two

valve springs, a rotator ﬁtted below them to ensure even-wear distribution. The

thick ﬂame plate of the cylinder head is cooled by drilled water passages close

to the combustion face. A combined fuel pump/injector is ﬁtted centrally in the

cylinder head, the fuel inlet and return connections being via drilled passages

in the cylinder head sealed by Viton O-rings (Figure 17.2).

A cast iron rocker housing supports the fulcrum for the fuel and valve

levers, encloses the valve gear and also forms the cylinder head water outlet

connection and a part of the water outlet main. An aluminium cover is ﬁt-

ted over the rocker housing. Lubricating oil is fed to the fuel pump and valve

levers through hollow pushrods and is drained to the sump via an external pipe.

A drilled passage is provided in the cylinder head to allow an adaptor and indi-

cator cock to be ﬁtted for performance checks.

Allen (Rolls-Royce) 527

FiguRe 17.2 VS12 unit fuel pump/injector

528 Allen (Rolls-Royce)

The camshaft is built up from two-cylinder sections, each machined from a

solid forging and complete with two inlet valve, two exhaust valve and two fuel

pump cams. The sections are rigidly bolted together and registered by dowels

to ensure precise alignment. The ﬁnger-type cam followers are pivoted from a

central fulcrum shaft. Lubricating oil ﬂows from each camshaft bearing into

the fulcrum through a drilled hole in the follower lever and then to the hollow

pushrod. An oil spray from the lever lubricates the roller and pin.

The cast aluminium housing encloses the camshaft and cam followers, and

also carries brackets for the two control shafts. The cover on the housing incor-

porates a heat shield.

A unit fuel pump injector with removable nozzle is arranged in each cyl-

inder head and operated from the camshaft by the fuel cam via cam follower,

pushrod and lever in a similar manner to the inlet and exhaust valves. It is sealed

into the head by four O-rings. An engine-driven fuel-pressurizing pump supplies

fuel via a ﬁlter to the fuel supply main and is controlled conventionally using an

electronic governor and electric actuator. A relief valve in the fuel return main

regulates the system pressure, with excess fuel returning to the pump suction.

Engine lubrication is effected by a wet sump system. A gear-type lubricat-

ing oil pump, mounted on the gearcase and driven from the crankshaft gear-

wheel, draws oil from the sump through a ﬁlter strainer. A relief valve on the

pump delivery safeguards the system against excessive pressure. Oil under

pressure is cooled and ﬁltered before being fed into oil mains bolted to the

sides of the crankcase. The mains supply oil to the piston cooling jets and to

crankcase drillings feeding the crankshaft and camshaft bearings and the inter-

mediate gear.

External pipes supply oil to the turbochargers, water pumps and intermedi-

ate gears mounted in the gearcase. The oil ﬁlters are of the replaceable cartridge

type. A motor-driven priming pump is incorporated in the system to ensure that

all parts are supplied with oil before the engine is started ((Figure 17.3)..

Cooling water from the primary system circulates through the engine jack-

ets, turbochargers and heat exchanger or radiator. A secondary water system

supplies the charge air coolers and lubricating oil cooler. Primary and second-

ary water pumps are gear driven from an auxiliary gear train connected to the

camshaft by a quill shaft.

Each cylinder bank is served by its own turbocharger, charge air cooler

and inlet and exhaust manifolds. The turbocharger compressor discharges via

the air shut-off butterﬂy valve into the air inlet manifold through the charge

air cooler. A single exhaust manifold leads to the turbocharger inlet, and a

restricting throat is incorporated in the exhaust branch at each cylinder head.

Bellows units and V-clamps connect the individual exhaust branches to form

the exhaust manifold.

Engine effected by a compressed air-powered starter motor which engages

automatically with the gear ring on the ﬂywheel. An electronic governor is ﬁt-

ted, the standard system including a crankshaft speed-sensing pick-up installed

near the ﬂywheel gear ring, a control box mounted off-engine, and an actuator

unit mounted on the engine gearcase connected through a collapsible link to

the fuel injection pump control shaft. A stop/run handle is connected to the fuel

control shafts to ensure that manual shutdown is always possible. The levers

operating the control racks of the unit injectors are spring loaded so that an

engine can always be shut down even if a control rack should stick.

Overspeed protection on the engine comprises two air shut-off butterﬂy

valves ﬁtted between each turbocharger compressor discharge and air mani-

fold. The valves close on overspeed and cut off the combustion air supply to

the engine. They are held in the open (running) position by an air-operated

actuator equipped with a spring return arranged to close when the air supply

is lost. The air supply is fed via a solenoid valve energized to pass air to the

butterﬂy actuators. The solenoid valve is fed from a 24 V dc supply control-

led by an Amot speed monitor, which takes its speed signal from the magnetic

pick-up mounted over the engine ﬂywheel gear ring. Other contacts in the

speed monitor are connected to the electronic governor so that the actuator also

moves to zero fuel on overspeed. The overspeed trip is set to operate when the

engine speed reaches 15 per cent above normal.

Explosion relief valves are ﬁtted to the crankcase to relieve any build-up

of excess pressure inside the engine; they are also designed to prevent the

admission of air. The valves open if crankcase pressure rises, and the oil mist

mixture directed to atmosphere via a ﬂame trap and deﬂector cowl. As soon as

the crankcase pressure returns to normal a coil spring closes the valve.

FiguRe 17.3 Allen S12 engine in-line cylinder form

Allen (Rolls-Royce) 529

531

C h a p t e r | e i g h t e e n

Alpha Diesel

(MAN Diesel)

From their introduction in the early 1970s, MAN B&W Alpha Diesel’s kin-

dred 23- and 28-type medium-speed engines were reﬁned and uprated a

number of times to address changing requirements from small-ship propulsion

and genset drive markets. Upgradings by the Frederikshavn, Denmark-based

member of the MAN Diesel group tapped extensive service experience with

both designs which respectively featured 225/300 mm and 280/320 mm bore/

stroke dimensions. The last 28/32A engine programme covered a power range

from 1470 kW to 3920 kW at 775 rev/min with in-line 6- to V16-cylinder mod-

els, optimized for service intervals of 24–30 months based on an annual run-

ning time of 6000–7000 h. Outputs from 800 kW to 1920 kW at 825 rev/min

or 900 rev/min were developed by 6L, 8L and V12-cylinder variants of the

23/30A series.

Early experience with small medium-speed engines burning heavy fuel oil

found that exhaust valves could function for around 2000 h (although often

less) and had to be scrapped after 3000 h of service. In a few engines, whose

fuel was less aggressive and the load moderate, the interval could be extended

to 6000 h. Generally, however, the valve’s lifetimes were considered too short

in relation to the cost savings made from using heavy fuel and owners opted

for marine diesel oil instead.

Subsequent 28/32A engine developments raised the maintenance interval

to at least 15 000 h, representing a time-between-overhaul of 2 years. Operation

on heavy fuel was thus cost-effective for almost all ships, according to the

designer, the exceptions being tonnage whose load proﬁles dictated frequent

starting and stopping and low annual running hours.

Advances in performance were sought over the years along with improve-

ments in durability and reliability. The speciﬁc power rating rose by almost 40

per cent while the brake mean effective pressure remained at around 19 bar and

the speciﬁc fuel consumption at 190 g/kW h.

The general design features of the last 28/32A engine dated from 1990. The

engine’s overall appearance was much the same as it was when launched in

1974, with closed-in fuel piping and pumps. But the cross-section (Figure 18.1)

532 Alpha Diesel (MAN Diesel)

shows that many of the key elements—such as the frame, cylinder liner, piston,

cylinder cover and valves—were improved.

FrAMe

The frame was reinforced and cast in nodular cast iron. The support and cool-

ing of the cylinder liner were improved, and the staybolts for the cylinder head

given a deeper and enhanced grip in the frame to carry the load from higher

combustion pressures.

Figure 18.1 Cross-section of MAN B&W Alpha V28/32A engine

CoMBustioN

Contributions to reduced fuel consumption and lower exhaust gas emissions

were made by (a) accurate start of fuel injection timing, (b) precision-made

injection nozzles, (c) fuel pumps with precise fuel metering, (d) modiﬁed com-

bustion chambers yielding a compression ratio of 13.9, (e) precisely deﬁned

fuel spray geometry and (f) increased injection pressure. Also inﬂuential was

the MAN B&W turbocharger, introduced in the 1980s and matched to the

28/32A engine.

Fuel VAlVes AND puMps

Good interaction between the engine and its fuel injection system determined

the engine’s torque and power output as well as its emissions and noise gen-

eration. Single-plunger injection operated with fuels of diverse viscosities and

with peak pressures up to around 1100 bar. The plunger and barrel assembly

was provided with a leakage oil return and an extra oil block for leaked fuel.

A ring-shaped groove was machined into the pump barrel. Lube oil was forced

into the fuel pump housing at the pump foot, preventing the spring guide from

sticking in asphaltenes. The very small leakage quantity involved was drawn

off through a separate outlet and led into a collecting tank.

CoMBustioN ChAMBer

Development of the 28/32A engine during 1988/1990 yielded an improved

combustion chamber in which the fuel spray mixed better with the air and

avoided spraying at the valves and piston top. In combination with an adjust-

ment of the pressures, the combustion process fostered reduced fuel consump-

tion and lower exhaust gas temperatures, along with minimized exhaust smoke.

CyliNDer heAD, VAlVes AND DuCts

Nodular cast iron was speciﬁed for the cylinder head to secure high strength

and stiffness. The head featured a bore-cooled ﬂame deck and was ﬁtted with

four rotating valves and cooled valve seats. Air and gas ducts were dimen-

sioned to ensure minimum resistance to an optimized air ﬂow and to achieve

exhaust valve temperatures of below 400°C. Overhaul intervals exceeding

18 000 h on heavy fuel oil were conﬁrmed in service. The performance of valves

and seats was closely monitored. Well-cooled seats, a rotating mechanism and

the construction material mean that it is not the valves that dictate the time for

maintenance. Regrinding can be carried out twice to give a valve lifetime of up

to 40 000 h (but sometimes shorter if the exhaust gases have attacked the valve

stem with sulphuric acid).

CyliNDer liNer

The liner is centrifugally cast from ﬁne-grained perlitic iron. The improved

combustion chamber made it possible to introduce a step in the bore diameter

at the top of the cylinder with an uncooled ﬂame ring. The inside diameter of

Cylinder liner 533

534 Alpha Diesel (MAN Diesel)

the ring is somewhat smaller than the liner bore. The piston crown is therefore

made correspondingly smaller, allowing carbon coke on the piston top to be

kept well clear of the running surface of the cylinder liner. The ﬂame ring—

introduced on all new engines from 1992—has proved to be beneﬁcial (Figure

18.2). It avoids liner polishing and micro-seizure, and helps to maintain the

piston rings in a good condition; a sound seal against the gas and combustion

forces results in a low and stable lube oil consumption.

pistoN

There were two suppliers of pistons for the 28/32A engine. The designs are

different but have both generally performed well. One type is a monobloc

piston of nodular cast iron with a splash-cooled piston top and a thin-walled

skirt to keep the weight down. The other type is a composite piston with a

steel top and a nodular cast iron skirt. Each has an intensively cooled top to

keep the rings in good condition and foster a low wear rate. The ring grooves

in the pistons are hardened and ground. The top piston ring was a focus of

development, the aim being to ﬁnd the best compromise between low lube oil

consumption and good lubrication. All the rings have a chrome layer on the

running surface.

Figure 18.2 section through 28/32A cylinder, highlighting the ame ring