Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines
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270 Low-Speed Engines—Introduction
and cooling water were used to raise steam, which was then supplied to the
underside of the working piston.
Evolution decreed that the surviving trio of low-speed crosshead engine
designers should exploit a common basic configuration: two-stroke engines with
constant pressure turbocharging and uniflow scavenging via a single hydrauli-
cally operated exhaust valve in the cylinder head.
All the three companies now offer electronically controlled (MAN Diesel
ME, Mitsubishi Eco-Engine and Wärtsilä RT-flex) versions alongside their tra-
ditional camshaft-controlled designs (respectively, the MC/MC-C, LSII/LSE
and RTA series). The ME and RT-flex engines with electronic control of fuel
injection and exhaust valve operation are increasingly popular and well estab-
lished in service, but some operators retain a preference for the mechanically
controlled models.
Current programmes embrace mini-to-large bore models with short-, long-
and ultra-long-stroke variations to match the propulsive power demands and
characteristics of most deep sea (and even some coastal/short sea) cargo ton-
nage. Installations can be near-optimized for a given duty from a permutation
involving the engine bore size, number of cylinders, selected output rating and
running speed.
Bore sizes range from 260 mm to 1080 mm, stroke/bore ratios up to 4.4:1,
in-line cylinder numbers from 4 to 14 and rated speeds from around 55 rev/min
to 250 rev/min. Specific fuel consumptions as low as 154 g/kW h are quoted
for the larger bore models whose economy can be enhanced by optional Turbo
Compound Systems in which power gas turbines exploit exhaust energy sur-
plus to the requirements of modern high-efficiency turbochargers.
Progress in the performance development of low-speed engines in the
popular ~600 mm bore class is shown in Figure 9.1.
Recent years have seen the addition of intermediate bore sizes to enhance
coverage of the power/speed spectrum and further optimize engine selection.
Both MAN Diesel and Wärtsilä (Sulzer) also extended their upper power limits
in the mid-1990s with the introduction of super-large bore models—respectively,
of 980 mm and 960 mm bore sizes—dedicated to the single-screw propulsion of
new generations of 6000 TEU-plus container ships with service speeds of 25 knots
or more. The 12-cylinder version of the current MAN B&W K98ME design,
delivering 74 760 kW, highlights the advance in specific output achieved since
the 1970s when the equivalent 12-cylinder B&W K98GF model yielded just
under 36 800 kW. Large bore models tailored to the demands of new generation
VLCC and ULCC propulsion have also been introduced.
Successively larger and faster generations of post-Panamax container ships
(now with a maximum capacity of 14 000 TEU) have driven the development
of specific output and upper power limits by low-speed engine designers. To
ensure that container ship designs with capacities exceeding 10 000 TEU could
continue to be specified with single engines, both MAN Diesel and Wärtsilä
extended their respective MC/ME and RTA/RT-flex programmes. Wärtsilä raised
the specific rating of the RTA96C design by 4 per cent to 5720 kW/cylinder
at 102 rev/min, and introduced an in-line 14-cylinder model delivering up to
80 080 kW. (The previous power ceiling had been 65 880 kW from a 12-cylinder
model.) The debut 14RT-flex96C engine (80 080 kW at 102 rev/min) was com-
pleted by Wärtsilä licensee Hyundai Heavy Industries in 2007 to power the
first of a series of 8600 TEU container ships.
MAN Diesel responded to the challenge with a 14-cylinder variant of the
K98MC and MC-C series, offering outputs up to 80 080 kW at 94 rev/min or
104 rev/min, which have since been superseded by the electronically control-
led 14K98ME model with ratings up to 87 220 kW at 97 rev/min on a mean
effective pressure of 19.2 bar. The first such engine (a Mark 7 version rated
for 84 280 kW at 104 rpm)—the most powerful diesel engines ever commis-
sioned—was due for delivery in 2008 to power a container ship.
In 2003 MAN Diesel opened another route to higher powers: a 1080-mm
bore version of the MC engine (now the ME-C) with a rating of 6950 kW/
cylinder at 94 rev/min, the 14-cylinder K108ME-C model thus offering
97 300 kW at 94 rev/min on a mean effective pressure of 18.2 bar. No K108ME-C
engine—the most powerful design in the world—had been ordered by 2009,
the need obviated by further uprating of the K98ME/ME-C models, whose
Mark 9 version would be capable of delivering the same output as its stand-
ard larger bore derivative. (V-cylinder configurations of existing low-speed
engine designs have also been proposed in the past by MAN Diesel to propel
mega-container ships, promising significant savings in weight and length per
unit power over traditional in-line cylinder models. These engines would allow
Low-Speed Engines—Introduction 271
P
max
Mep
P
scav
η
TC
η
e
C
m
kW/t
kW/cyl
kW/m
20.5 mep (bar)
17.5
15.0
12.5
10.0
70 η
60
50
40
40 kW/t
35
30
25
20
P
max
(bar)
150
100
50
P
scav
3.5
2.5
1.5
C
m
(m/s)
9
8
7
6
kW/cyl., kW/m
2,500
1,500
500
1970 1975 1980 1985 1990 1995 2000
6K62EF
6K62EF
6L55GF
6L55GFC
6L55GFCA
6L55GB
6L60MC
6L/S60MC
6S60MC
6S60MC-C
FIgurE 9.1 Development of key performance parameters for low-speed engines
(~600-mm bore) over a 30-year period (MAN Diesel)
272 Low-Speed Engines—Introduction
the higher number of cylinders to be accommodated within existing machinery
room designs [see Figure 9.2].)
Recent years have also seen renewed activity by all designers at the other
end of the power spectrum, with significant smaller bore engine develop-
ments (350–500 mm) targeting market opportunities from small ship propul-
sion. Electronic control, initially confined to larger bore designs, has also
been extended to smaller bore models, such as MAN Diesel’s new ME-B pro-
gramme, which exploits a stroke/bore ratio of 4.4.
Parallel development by the designer/licensors seeks to refine existing mod-
els and lay the groundwork for the creation of new generations of low-speed
engines. Emphasis in the past has been on optimizing fuel economy and rais-
ing specific outputs, but reliability, durability and overall economy are now
priorities in R&D programmes, operators valuing longer component lifetimes,
extended periods between overhauls and easier servicing.
Lower production costs through more simple manufacture and easier instal-
lation procedures are also targeted, reflecting the concerns of enginebuilder/
licensees and shipyards. More compact and lighter weight engines are appre-
ciated by naval architects seeking to maximize cargo space and deadweight
capacity within the given overall ship dimensions.
In addition, new regulatory challenges—such as noxious exhaust emission
and noise controls—must be anticipated and niche market trends addressed if
the low-speed engine is to retain its traditional territory (e.g., the propulsion
demands of increasingly larger and faster container ships, which might other-
wise have to be met by multiple medium-speed engines or gas turbines).
FIgurE 9.2 V-cylinder versions of larger bore MAN B&W engines have been
proposed by MAN Diesel
A number of features have further improved the cylinder condition and
extended the time between overhauls through refinements in piston design
and piston ring configurations. A piston cleaning ring incorporated in the top
of the cylinder liner now controls ash and carbon deposits on the piston top-
land, preventing contact between the liner and these deposits, which would
otherwise remove part of the cylinder lube oil from the liner wall.
Computer software has smoothed the design, development and testing of
engine refinements, and new concepts, but the low-speed engine groups also
exploit full-scale advanced hardware to evaluate innovations in components
and systems.
Sulzer (since absorbed into Wärtsilä) began operating its first technology
demonstrator in 1990, an advanced two-stroke development and test engine
designated the 4RTX54 whose operating parameters well exceeded those of
any production engine (Figure 9.3). Until then, this group had used computer-
based predictions to try to calculate the next development stage. Extrapolations
were applied, sometimes with less than desirable results. The 4RTX54 engine,
installed at the Swiss designer’s Winterthur headquarters, allowed practical
tests with new parameters, components and systems to be carried out instead
Low-Speed Engines—Introduction 273
max.cyl.pressure
mean effective pressure
1978 80 82 84 86 88 90 92 1978 80 82 84 86 88 90 92
180
160
140
120
100
8
10
12
14
16
18
20
4,5
4,0
3,5
3,0
2,5
2,0
8,5
8,0
7,5
7,0
6,5
6,0
m/s
bar
bar
RLA
RLB
RTA.2U
RTA.2/84C
RTA 84T
RTA.8
RLA
RLB
RTA.8
RTA.2U
RLB
RTA.8/84C
RTA.2U
RLA
RLB
RTA.8
RTA.2
RTA 84T
RTA 84C
1978 80 82 84 86 88 90 92
1978 80 82 84 86 88 90 92
‘Technology
demonstrator’
RTA.2/84C
RTA 84T
RTA.2U
RTA 84T
RTA.2
mean piston speedstroke / bore
FIgurE 9.3 Evolution of Sulzer low-speed engine parameters from the late 1970s in
comparison with those of the rTX 54 two-stroke technology demonstrator engine
274 Low-Speed Engines—Introduction
of just theory and calculations. Operating data gathered in the field could be
assessed alongside results derived from the test engine.
The four-cylinder 540-mm bore/2150-mm stroke engine had a stroke/bore
ratio approaching 4:1 and could operate with mean effective pressures of up to
20 bar, maximum cylinder pressures up to 180 bar and mean piston speeds up
to 8.5 m/s. For operating without a camshaft—reportedly the first large two-
stroke engine to do so—the RTX54 was equipped with combined mechanical,
hydraulic and electronic (mechatronics) systems for fuel injection, exhaust
valve lift, cylinder lubrication and starting, as well as controllable cooling
water flow. The systems underwrote full flexibility in engine settings during
test runs.
Sulzer’s main objectives from the technology demonstrator engine were to
explore the potential of thermal efficiency and power concentration, to increase
the lifetime and improve the reliability of components, to investigate the mer-
its of microprocessor technology and to explore improvements in propulsion
efficiency. A number of concepts first tested and confirmed on the 4RTX54
engine were subsequently applied to production designs. The upgraded RTA-
2U series and RTA84T, RTA84C and RTA96C engines, for example, benefit
from a triple-fuel injection valve system in place of two valves. This configura-
tion fosters a more uniform temperature distribution around the main combus-
tion chamber components and lower overall temperatures despite higher loads.
Significantly lower exhaust valve and valve seat temperatures are also yielded.
An enhanced piston ring package for the RTA-2U series was also proven
under severe running conditions on the 4RTX54 engine. Four rings are now
used instead of five, the plasma-coated top ring being thicker than the others
and featuring a pre-profiled running face. Excellent wear results are reported.
The merits of variable exhaust valve closing (VEC) were also investigated on
the research engine whose fully electronic systems offered complete flexibility.
Significant fuel savings in the part-load range were realized from the
RTA84T ‘Tanker’ engine, which further exploits load-dependent cylinder liner
cooling and cylinder lubrication systems refined on the 4RTX54. The 4RTX54
was replaced as a research and testing tool in 1995 by the prototype 4RTA58T
engine adapted to serve as Wärtsilä’s next two-stroke technology demonstra-
tor. This RTX-3 research engine, installed in the Diesel Technology Centre in
Winterthur, subsequently played a key role in developing the electronically
controlled common rail RT-flex design in Wärtsilä’s programme.
A commitment to sustained development was underlined by investment in
a new RTX-4 research and laboratory engine, a four-cylinder 600-mm-bore
facility, which is based on an RT-flex60 design but evolved from a clean sheet
via CAD models and simulations. This engine started running in April 2008
(Figures 9.4 and 9.5).
Initially developing 10 160 kW at a nominal speed of 114 rev/min, the RTX-4
was designed to allow its power output to be increased in refining technologies
for future market requirements. The engine exploits all the RT-flex common
rail features with integrated electronic control of all the key processes: fuel
injection, exhaust valve operation, cylinder lubrication and air starting. It is
used as a tool for developing and testing future technological steps, the main
areas of research including engine efficiency, emissions reduction, component
reliability, easier manufacture and lower maintenance. Design ideas can be ver-
ified before field testing and commercial application, test engines often running
at specific loads far above those of production engines to identify risks quickly
and investigate higher limits.
The importance of curbing exhaust gas emissions to progressively lower
limits was reflected in the RTX-4 engine’s after-treatment systems support,
which includes a selective catalytic reduction (SCR) unit capable of reducing
NOx emissions discharge to the atmosphere by more than 90 per cent from the
level at the engine outlet. In addition, the installation is prepared for investigat-
ing particulate filters and scrubbers. Scavenge air humidification and exhaust
Low-Speed Engines—Introduction 275
FIgurE 9.4 Wärtsilä’s rTX-4 research engine in the Diesel Technology Centre at
Winterthur in Switzerland
276 Low-Speed Engines—Introduction
gas recirculation (EGR) systems for NOx emissions reduction can also be
examined.
Advanced turbocharging arrangements, including two-stage systems, were
among the projected R&D tasks, some extending the European Union-funded
Hercules environmental research projects in which Wärtsilä and MAN Diesel
have participated as leaders. A fully enclosed control room is provided for
controlling and monitoring engine and support system operation from an inte-
grated automation platform.
FIgurE 9.5 Development of Wärtsilä low-speed engine parameters since 1940
Wärtsilä research engines
RTX-3 RTX-4
Model
4rT-ex58T 4rT-ex60
Year installed 1995 2008
Bore (mm) 580 600
Stroke (mm) 2416 2250
Stroke/bore ratio 4.16 3.75
Cylinders 4 4
Speed, nominal (rev/min) 105 114
Mean piston speed (m/s) 8.5 8.6
Mean eective pressure (bar) 19.5 21
Maximum cylinder pressure (bar) 150 168
Output/cylinder (kW) 2125 2540
Power output (kW) 8500
10 160
The widest flexibility in operating modes and the highest degree of reliabil-
ity are cited by Copenhagen-based MAN Diesel as prime R&D goals under-
writing future MAN B&W two-stroke engine generations, along with ease of
maintenance, production cost reductions, low specific fuel consumption and
high plant efficiency over a wide load spectrum, high tolerance towards var-
ied heavy fuel qualities, easy installation, continual adjustments to the engine
programme in line with the evolving power and speed requirements of the
market, compliance with emission controls and integrated intelligent electronic
systems.
Continuing refinement of the MAN B&W MC and ME low-speed engine
programmes and the development of intelligent engines (see section Intelligent
Engines) are supported by an R&D centre adjacent to the group’s Teglholmen
office and factory in Copenhagen. At its heart is the 4T50MX research engine,
an advanced testing facility, which exploits an unprecedented 4.4:1 stroke/bore
ratio. Although the four-cylinder 500-mm bore/2200-mm stroke engine is based
on the current MC series, it is designed to operate at substantially higher ratings
and firing pressures than any production two-stroke engine available today.
An
output of 7500 kW at 123 rev/min was selected as an initial reference
level for carrying out extensive measurements of performance, component
temperatures and stresses, combustion and exhaust emission characteristics,
and noise and vibration. The key operating parameters at this output equate to
180 bar firing pressure, 21 bar mean effective pressure and 9 m/s mean piston
speed. Considerable potential was reserved for higher ratings in later test run-
ning programmes.
A conventional camshaft system was used during the initial testing period
of the 4T50MX engine. After reference test-running, however, this was
replaced by electronically controlled fuel injection pumps and exhaust valve
actuators driven by a hydraulic servo-system (Figure 9.6). The engine was
prepared to facilitate extensive tests on primary methods of exhaust emission
reduction, anticipating increasingly tougher regional and international controls
in the future. Space was allocated in the R&D centre for the installation of a
large NOx-reducing SCR facility for assessing the dynamics of SCR-equipped
engines and catalyst investigations.
MAN Diesel reports that the research engine, with its electronically con-
trolled exhaust valve and injection system, has fully lived up to expectations
as a development tool for components and systems. A vast number of possi-
ble combinations of injection pattern, valve opening characteristics and other
parameters can be permutated. The results from testing intelligent engine
concepts are being tapped for adoption as single mechanical units as well as
stand-alone systems for application on current engine types. To verify the lay-
out of the present standard mechanical camshaft system, the 4T50MX engine
was rebuilt with a conventional mechanical camshaft unit on one cylinder. The
results showed that the continuous development of the conventional system
seems to have brought it close to the optimum, and the comparison gave no
reason for modifying the basic design.
Low-Speed Engines—Introduction 277
278 Low-Speed Engines—Introduction
An example of the degrees of freedom available is shown by a comparison
between the general engine performance with the firing pressure kept constant
in the upper load range by means of variable injection timing (VIT) and vari-
able compression ratio (VCR). The latter is obtained by varying the exhaust
valve closing time. This functional principle has been transferred to the present
exhaust valve operation with the patented system illustrated in Figure 9.7. The
uppermost figure shows the design of the hydraulic part of the exhaust valve;
below is the valve opening diagram. The fully drawn line represents control
by the cam while the dotted line shows the delay in closing, thus reducing the
compression ratio at high loads so as to maintain a constant compression pres-
sure in the upper load range. The delay is simply obtained by the oil being
trapped in the lower chamber, and the valve closing is determined by the open-
ing of the throttle valve, which is controlled by the engine load.
Traditionally, the liner cooling system has been arranged to match the
maximum continuous rating load. Today, however, it seems advantageous to
control the inside liner surface temperature in relation to the load. Various pos-
sibilities for securing load-dependent cylinder liner cooling have therefore been
investigated. One system exploits different sets of cooling ducts in the bore-
cooled liner, the water supplied to the different sets depending on the engine
FIgurE 9.6 MAN Diesel’s 4T50MX low-speed research engine arranged with a
conventional camshaft (a) and with electronically controlled fuel injection pumps
and exhaust value actuating pumps (b)
load. Tests with the system have shown that the optimum liner temperature can
be maintained over a very wide load range. The system is considered perfectly
feasible, but the added complexity has to be carefully weighed against the serv-
ice advantages.
The fuel valve used on MC engines operates without any external con-
trol of its function. The design has worked well for many years but could be
challenged by the desire for maintaining an effective performance at very low
loads. MAN Diesel therefore investigated a number of new designs with the
basic aim of retaining a simple and reliable fuel valve without external con-
trols. Various solutions were tested on the 4T50MX engine, among them a
design whose opening pressure is controlled by the fuel oil injection pressure
level (which is a function of the engine load). At low load, the opening pres-
sure is controlled by the spring alone. When the injection pressure increases at
higher load, this higher pressure adds to the spring force and the opening pres-
sure increases.
Another example of fuel valve development is aimed at reduced emis-
sions. This type incorporates a conventional conical spindle seat as well as
a slide valve inside the fuel nozzle, minimizing the sac volume and thus the
Low-Speed Engines—Introduction 279
h
t
Maximum
Nominal
Inlet
Throttle
valve
FIgurE 9.7 Mechanical/hydraulic variable compression ratio system (MAN Diesel)