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

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361

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

Mitsubishi Low-Speed

Engines

The dominance enjoyed for many years by MAN Diesel and Wärtsilä in

the low-speed two-stroke crosshead engine sector is challenged by Mitsubishi,

the only other contender in the international arena. The Japanese group has

widened its UEC portfolio in recent years to offer new and reﬁned models

across the full propulsion power spectrum. Mitsubishi has traditionally served

its home market, but a stronger direct-export thrust has earned installations in

ships built for European and other overseas owners in Japan and Europe.

Most Mitsubishi low-speed engines, unlike MAN B&W and Wärtsilä

engines, are built by the group itself rather than by overseas licensees. The

complete UEC programme can be handled by its Kobe factory while engines

of up to 600-mm bore can be built by its Yokohama works and the Japanese

licensees Akasaka and Kobe Diesel. UEC engine technology was extended to

new licensees in China and Vietnam in 2005. The Mitsubishi Heavy Industries

(MHI) group also has a 23 per cent stake in the major new Qingdao Marine

Diesel joint venture in China, which is primed to build Wärtsilä and Mitsubishi

low-speed engines. A business alliance has also been formed at home with IHI

enginebuilding subsidiary Diesel United on production and after-sales services.

An experimental UEC model built in 1952 yielded a power output 50 per

cent higher than its predecessor, the MS engine, which had been developed

from the early 1930s. The ﬁrst commercial UEC engine, a nine-cylinder UEC

75/150 model developing 8830 kW, was introduced in 1955. The UEC engine

now shares a common design philosophy with MAN Diesel’s MAN B&W

MC and Wärtsilä’s RTA series, exploiting constant pressure turbocharging and

uniﬂow scavenging through a central hydraulically opened/pneumatically

closed exhaust valve in the cylinder cover. (The UEC designation refers to

uniﬂow-scavenged, exhaust gas turbocharged and crosshead-type engine.)

The original UEC engines—progressing from A to E type—featured

impulse turbocharging, three mechanically actuated exhaust valves per cylinder

and a single centre-injection fuel valve. This arrangement fostered high per-

formance with a smooth exhaust gas outlet and good combustion, but exhaust

valve lifetime was sometimes short.

362 Mitsubishi Low-Speed Engines

A two-stage turbocharging system (unique in contemporary production

low-speed engine practice) was introduced for the UEC-E type in 1977 to

boost speciﬁc output and achieve a mean effective pressure of 15 bar. Low-

and high-pressure turbochargers were linked in series (Figure 11.1). To match

this increase in maximum cylinder pressures, the engine frame, bearings and

combustion chamber components were made more rigid than those of the

UEC-D type predecessor while still retaining the monobloc cast iron con-

struction (three elements held by tie bolts). A new piston crown of specially

reinforced molybdenum cast steel was speciﬁed with a combination of radial

and circular ribs behind the frame plate to withstand the rise in maximum pres-

sure (increased from 90 bar to 110 bar).

The UEC-E engine was comparatively short-lived in the programme, how-

ever, as the steep rise in bunker prices in the late 1970s dictated a stronger focus

on fuel economy rather than sheer output. Mitsubishi therefore introduced the

Low-press

exhaust pipe

Moisture

separator

To atmosphere

Low press

turbocharger

Low-press

turbine

High-press

exhaust pipe

High-press

turbine

Low-press

blower

From atmosphere

First-stage

air cooler

Moisture

separator

High-press

blower

High press

turbocharger

Scavenging air trunk

Combustion chamber

Exhaust gas

Air

Second-stage

air cooler

FigurE 11.1 Two-stage turbocharging system for uEC-E engines

long-stroke UEC-H type model in 1979, abandoning impulse turbocharging in

favour of constant pressure turbocharging based on the group’s own MET-S tur-

bocharger (see Chapter 7). The H-type also broke with previous UEC engine

practice in using one large diameter exhaust valve in the centre of the cylinder

cover and twin side-mounted fuel injection valves (Figure 11.2). A signiﬁcant

reduction in fuel consumption was yielded. The H-type is detailed later in this

chapter. The HA-type, which followed in 1982, adopted the higher efﬁciency

non-water-cooled MET-SB turbocharger and an improved combustion system,

further enhancing fuel economy. Output was also raised by 11–14 per cent.

Advances in fuel economy were pursued with the development of longer

stroke/lower speed L-type engines from 1983 with successive reﬁnements

embodied in the LA-type (1985), the LS-type (1986) and the LSII-type (1987).

Stroke–bore ratios were increased to achieve lower direct-coupled propeller

speeds and hence higher propulsive efﬁciency. Efforts to improve speciﬁc fuel

consumption were reﬂected in rises in the maximum cylinder pressure.

For most of the 1980s, Mitsubishi was content to concentrate on the small-

to-medium size ship propulsion sector with just four bores (370 mm, 450 mm,

520 mm and 600 mm). Four- to eight-cylinder models covered outputs of up to

around 15 000 kW. The UEC-LA models, with a stroke–bore ratio of 3.17:1,

were supplemented in 1985 by LS options for the 520 mm and 600 mm models

(3.55 and 3.66:1 ratios, respectively).

A return to the large bore arena, long surrendered to MAN Diesel and

Sulzer, was signalled in 1987 by the launch of the UEC 75LSII series with a

maximum rating of 2940 kW/cylinder. The 750-mm bore/2800-mm stroke

design retained the main features of the LS engines but exploited an even

higher ratio (3.73:1) and lower running speeds (63–84 rev/min). Mitsubishi

stepped up its challenge in the VLCC and large bulk carrier propulsion markets

in 1990 with the introduction of an 850-mm bore/3150-mm stroke version. This

UEC 85LSII series is now available in ﬁve- to nine-cylinder versions, offering a

maximum rating of 3860 kW/cylinder and hence an upper output of 34 740 kW.

The propulsion market created by a new generation of large Panamax and

post-Panamax container ships requiring service speeds of up to 25 knots was

Mitsubishi low-speed engines 363

FigurE 11.2 The uEC-A to E series engines featured three exhaust valves and a

centre injection fuel valve (left). Later series (H to LS) are equipped with one exhaust

valve and two side-injection fuel valves (right)

364 Mitsubishi Low-Speed Engines

addressed by Mitsubishi in 1992 with its UEC 85LSC series, a shorter stroke

(2360 mm) faster running derivative of the UEC 85LSII ((Figure 11.19). The. The

design is the highest rated engine in the group’s programme, with an output

of 3900 kW/cylinder at 102 rev/min. The series covers a power band of up to

46 800 kW from ﬁve- to 12-cylinder models.

In 1991, a determination to secure small-ship propulsion business led to

the introduction of a 330-mm bore/1050-mm stroke LSII design (Figure 11.3).

The series succeeded the 370-mm bore LA models, which pioneered the

modern ‘mini-bore’ two-stroke engine from the late 1970s for small general

cargo tonnage, tankers, colliers, feeder container ships and even large ﬁsh-

ing vessels. The LS and more advanced UEC 33LSII engine now develops

566 kW/cylinder at 215 rev/min and covers an output band from 1230 kW to

4530 kW at 162–215 rev/min with four- to eight-cylinder models.

Non-cooled

MET-SC turbocharger

Non-cooled

fuel valve

Monobloc exh.

gas receiver

Exh. gas diffuser

Valve cage with non-cooled

exh. gas passage

Hydraulically actuated

exh. valve with rotator

Water-cooled exh. valve

seat

Oil-cooled piston

Timely cyl. liner

lubrication

CSS port

Casted monobloc

frame

Gear-driven

camshaft

Casted monobloc

bedplate

Cylinder jacket

with scav. trunk

Crosspin bearing

with shell metal

Crankpin bearing

with shell metal

FigurE 11.3 Construction of uEC 33LSii engine

Mitsubishi enhanced its medium-power programme in 1992 by upgrading the

500-mm and 600-mm bore models to LSII status. Later power upratings of 3–5

per cent for these UEC 50LSII/60LSII models, without major design changes,

were introduced to meet the demands of larger ships and higher service speeds.

The speciﬁc outputs, respectively, rose to 1445 kW and 2045 kW per cylinder.

With a stroke–bore ratio of 3.9:1, the 50LSII series embraces four-to-eight-

cylinder versions covering an output range of up to 11 560 kW at 127 rev/min.

The small bore programme was strengthened in 1995 with 370 mm and

430 mm UEC LSII designs to attract business from Handy-sized bulk carri-

ers, feeder container ships, small tankers and gas carriers. The engines plugged

a gap between the established 330 mm and 500-mm bore LSII series. The

UEC 37LSII engine offers an output of 772 kW/cylinder at 186 rev/min

from ﬁve-to-eight-cylinder versions, while the UEC 43LSII engine delivers

1050 kW/cylinder at 160 rev/min from four-to-eight-cylinder models. The ﬁrst

UEC 37LSII engine, a six-cylinder model, completed testing in early 2000,

and the debut order for a 43LSII engine, a seven-cylinder model, was logged in

2002 (Table 11.1).

Development of the LSII design from 1998 resulted in the LSE series,

headed by a 520-mm bore model (UEC 52LSE) targeting feeder container

ships and Panamax bulk carriers. The series was extended by a 680-mm bore

model (UEC 68LSE) for propelling tonnage including Aframax and Suezmax

tankers, Cape-size bulk carriers and container ships. The LSE models share

a more compact conﬁguration and a 25 per cent reduction in overall number

of components compared with the LSII engines. Beneﬁts were also gained in

production costs, reliability and maintainability. The mean effective pressure

Mitsubishi low-speed engines 365

Table 11.1 uEC37LSii engine data

Bore 370 mm

Stroke 1290 mm

Stroke–bore ratio 3.49

Output

772 kW/cyl

Cylinders 5–8

Power range

2095–6180 kW

Speed 186 rev/min

Mean piston speed 8 m/s

Mean eective pressure 18 bar

Maximum cylinder pressure 147 bar

Specic fuel consumption 175 g/kW h

366 Mitsubishi Low-Speed Engines

rating and mean piston speed were, respectively, raised by 5 per cent and 6

per cent to 19 bar and around 8.5 m/s. The ﬁrst 450 mm-bore LSE engine, a

six-cylinder model, was completed in summer 2008. A successor to the UEC

52LA engine, this 45LSE has proven particularly popular for powering Handy-

size bulk carriers built in Japan (Table 11.2).

An extension of the same ‘total economy’ LSE concept later resulted in

a 600-mm bore version. The current UEC programme ((Table 11.3) embracesembraces

models with bore sizes of 330 mm, 370 mm, 430 mm, 450 mm, 500 mm,

520 mm, 600 mm, 750 mm and 850 mm in LA, LS, LSII and LSE versions,

covering an output band from 1120 kW to 46 800 kW at 215 rev/min to 54 rev/

min (Figures 11.4 and 11.5). Typical performance curves and layout diagrams

are shown in Figures 11.6 and 11.7.

Development progress since 1955 is underlined by the LS-type whose spe-

ciﬁc output is three times that of the original UEC engine and whose weight/

power and length/power ratios are reduced by one-third. The advances in key

design and performance parameters over the years are illustrated graphically in

Figures 11.8 and 11.9.

uEC dESign dETAiLS

Structural and design details are exempliﬁed by the UEC 75LSII and 85LSII

engines (Figure 11.10). The bedplate and column are of simple welded mono-

block construction, and the partition for each cylinder consists of a single

welded plate. Side forces are supported by horizontal steel ribs arranged behind

Table 11.2 uEC-LSE engine data

52LSE 60LSE 68LSE

Bore (mm) 520 600 680

Stroke (mm) 2000 2400 2690

Stroke–bore ratio 3.85 4 3.96

Output (kW/cyl) 1705 2255 2940

Cylinders 4–12 5–8 5–8

Power range (kW)

3700–20 460 7650–18 040 10 050–23 520

Speed (rev/min) 127 105 95

Mean piston speed (m/s) 8.47 8.4 8.52

Mean eective pressure (bar) 19 19 19

Specic fuel consumption (g/kW h) 160–167 160–166 159–165

See also Table 11.4

the guide bar. The pistons are of the well-proven central support type and are

ﬁtted with four rings (Figure 11.11). Hard chromium plating applied to the

running surfaces of the rings improves wear resistance under high pressure and

corrosion resistance in a low-temperature corrosive environment. Chromium

plating the ring grooves has the same purpose, as well as improving the dura-

bility of the rings by ensuring normal movement in the grooves.

The lower part of the cylinder liners feature controlled swirl scavenging

(CSS) ports, which promote improved scavenging efﬁciency and hence favour-

ably inﬂuence fuel consumption and thermal load. The angle of the lower

scavenging port is directed towards the centre of the cylinder to discharge

combustion gas on the piston top; and the angle of the upper port is directed

uEC design details 367

85LSC

68LSE

60LSE

52LSE

75LSII

85LSII

50LSII

43LSII

37LSII

33LSII

37LA

45LA

52LA

52LS

60LA

60LS

Engine speed (rpm)

220200180160140120105100908060

Output (kW × 10

)

FigurE 11.4 Mitsubishi’s uEC engine programme (see also Table 11.3); 35- and

40-bore LSE designs co-developed with Wärtsilä were released in 2008

368 Mitsubishi Low-Speed Engines

sideways to discharge gas around the inner surface of the liner. The CSS

conﬁguration (Figure 11.12) reportedly raises scavenging efﬁciency by 5–7 per

cent over the conventional port arrangement. Two-row bore liner cooling is

incorporated, and cylinder lubrication is timed so as to reduce oil consumption.

LSII

LSC

LSII

FigurE 11.5 Proportional design in uEC LSii/LSC series

Corrosive liner wear, caused by low temperatures, is inhibited by controlled

positioning of the cooling bores.

The single exhaust valve of each cylinder has a hydraulic rotation system,

which rotates the valve to improve running control conditions. The rotation

wings are ﬁtted on the actuator piston on the exhaust valve side, an arrange-

ment that is said to offer less obstruction to exhaust gas passage than with the

uEC design details 369

Te1, Te2 (°C)

be (g/ps-h)

noitpmusnoc lio leuf cificepS

be (ISO)

Te2

(kg/cm

)

nT×10

(rpm)

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

erusserp ria gnignevacS

500

400

300

200

100

erutarepmet sag tsuahxE

regrahcobrut )2eT( retfa & )1eT( erofeb

max

Te1

deeps regrahcobruT

120 150

200 220

Engine speed (rpm)

% Load

150

145

140

135

130

125

120

,erusserp rednilyc mumixaM

erusserp noisserpmoc

max,

(kg/cm

)

150

140

130

120

110

100

110

FigurE 11.6 Performance curves of 8uEC 33LSii engine (4316 kW at 210 rev/min)

370 Mitsubishi Low-Speed Engines

traditional rotator wing mounted on the valve spindle. A water-cooled valve

seat is speciﬁed.

The Bosch fuel pumps have control racks to adjust the quantity of fuel sup-

plied. The controllable injection timing (CIT) pumps also ensure that engine

performance is improved at the service load (Figure 11.13). Two side-injection

non-water-cooled fuel injection valves (Figure 11.14) are arranged on each

cylinder cover; a steam passage is provided for warming the valve body when

the engine is stopped.

An optional automatic ﬂexible rating (AFR) control system can be speci-

ﬁed to reduce fuel consumption during part-load operation by coordinating

the engine output with the operational conditions of the ship. The system

(Figure 11.15) controls the fuel delivery timing of the injection pump by using

the CIT system to produce maximum cylinder pressure, and also controls the

turbine nozzle area of the variable geometry (VG) turbocharger to secure an

optimum scavenging air pressure. The CIT fuel injection pumps, offered as

standard with LSII engines, are operated by a governor when the AFR control

system is not installed.

A mean time-between-overhaul of 4 years was targeted for the LSII

engines, Mitsubishi citing contributions from the following key elements:

l Piston rings and cylinder liners, yielding respective wear rates of

0.2 mm/1000 h and 0.02 mm/1000 h or less.

l Reliable hydraulically rotated exhaust valves with stellite-coated stems

and seats.

Torque ratio

70 75 80 85 90 95 100 105

Engine speed (%)

S.F.C. Reduction (g/PS-h)

Only for 85LSII

– 1

– 5

– 4

– 3

– 2

Calculation of fuel consumption at maximum

continuous output of a derated engine

100

)%( tuptuo enignE

1.00

0.95

0.90

0.85

0.80

0.75

0.723

FigurE 11.7 improvement in fuel economy by derating a uEC 85LSii engine