Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines
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relation to the desired mean effective pressure for the engine, the Miller cycle
results in a significant derating. Even when the achievable boost pressure is
high enough, the fuel consumption benefits of the Miller cycle are partially
neutralized when the efficiency of the compressor and turbine of the turbo-
charger decreases too rapidly at high compression ratios. The practical imple-
mentation of the Miller cycle requires a turbocharger or turbocharging system
(see the next section) capable of achieving very high compressor pressure
ratios and offering excellent efficiency at high pressure ratios.
Two-sTage TurBoCharging
Lower NOx and carbon dioxide emissions are among the key drivers of marine
diesel engine development. The former can be efficiently reduced by cooling
down the combustion process using a Miller cycle, whereby inlet valve clo-
sure (IVC) timings are normally affected before BDC. (The Miller cycle was
originally intended to increase the indicated mean effective pressure of petrol
engines, and based on shortening the compression stroke and lowering the
charge temperature in the cylinder.)
In the Miller cycle the combustion air is compressed to a much higher pres-
sure than is needed to fill the cylinder for the desired air/fuel ratio. Closure of
the inlet valve is timed so that just the right amount of air is sucked into the
cylinders. The charge air thus expands, resulting in a lower temperature at the
beginning of the cycle.
High degrees of early IVC timings, however, demand high boost pressures
for which a powerful solution is a two-stage turbocharging system capable of
delivering pressure ratios of up to 10. Engine efficiency is improved by such a
system, which further results in lower carbon dioxide emissions. Higher engine
efficiency is yielded by higher turbocharger efficiencies from two-stage turbo-
charging along with a more optimum division between compression and expan-
sion strokes from the Miller cycle. Introducing a Miller cycle in combination
with the two-stage turbocharging thus efficiently reduces both NOx and CO
2
emissions. (It is one of the few measures that can be applied to an internal com-
bustion engine to simultaneously cut NOx emissions and fuel consumption.)
A major difference with the Miller cycle compared with a normal diesel
cycle is the decreased compression due to the additional expansion of the cylin-
der load after IVC before BDC. The total compression work is thereby reduced
compared with a standard cycle. Since the total expansion work remains the
same, there is a positive effect on engine efficiency, which results in lower
overall CO
2
emissions. This is also ensured by an increased air receiver pres-
sure guaranteeing that the pressure inside the cylinder remains the same.
Another major difference is seen in the overall temperature level inside the
cylinder. The additional expansion of the cylinder load after IVC before BDC
ensures that a considerably lower temperature is attained in the cylinder. This
difference is maintained and even increased during the high-pressure cycle.
Lower NOx emissions, lower exhaust/component temperatures and lower
Two-stage turbocharging 189
190 Pressure Charging
cooling losses (i.e., better efficiency) are among the positive effects of lower
temperatures inside the cylinder.
Intercooling between the low- and high-pressure turbocharger stages is a
key factor because, by allowing an approximation of isothermal compression,
it improves the air compression process and consequently reduces the required
compression energy. The efficiency gain depends on the intercooling tempera-
ture and on the overall pressure ratio. Intercooling is not the only reason for
the higher efficiency with two-stage turbocharging. Another is found on the
gas side: the expansion energy available for the two turbines is higher than that
with a single stage because the losses in the high-pressure stage increase the
energy at the low stage inlet. Furthermore, the efficiencies of the compressor
and turbine can be generally higher due to the lower specific loading.
ABB Turbo Systems reports that although the switch from single- to
two-stage high-pressure turbocharging is a major step, the argument for
change and a significant increase in pressure ratio is strengthened by these
considerations:
l Two relatively small turbochargers can be used as the turbine area is the
controlling factor and decreases with an increasing pressure ratio.
l Smaller turbochargers exhibit better acceleration and vibration
behaviour.
l Intercooling results in a considerable rise in turbocharging efficiency,
the benefit growing as the pressure ratio increases.
l System matching is generally not restricted by the map width and design
conditions.
l Two-stage turbocharging offers good control flexibility.
l Reliability and durability are generally better because of the moderate
pressure ratio at each stage.
Several years of development work on both engine and turbocharger tech-
nologies were nevertheless foreseen by Wärtsilä in 2007 before two-stage tur-
bocharging systems became ready for commercial production installations.
Experimental investigations were carried out by the company on a two-stage
system in which an ABB TPS48 turbocharger served as the high-pressure
stage, receiving compressed air from an ABB TPS52 turbocharger-driven low-
pressure stage via an intercooler (Figure 7.11). Tests carried out on a W20
medium-speed engine demonstrated that reductions of up to 50 per cent in
NOx emissions were achievable with extreme Miller timings in combination
with two-stage turbocharging and that both full-load brake specific fuel con-
sumption and thermal load were somewhat improved, respectively, thanks to
the increased turbocharger efficiencies and higher boost pressures.
Difficult engine start-up behaviour and low load running due to the very
low in-cylinder temperatures were among the challenges noted by Wärtsilä.
The most efficient solutions were considered to be a fully flexible variable
inlet valve closure (VIC) system for larger bore engines, and a simple on/off
VIC system or other options (such as an external blower and cooling water
temperature heating system) for small bore engines, where investment in a
fully flexible VIC system was not commercially viable.
ABB Turbo Systems, which is also a developing two-stage system technol-
ogy, suggests that it is reasonable first to fully exploit the limits of single-stage
turbocharging. However, significant benefits are promised in the future from
two-stage turbocharging for the enginebuilder (with more freedom to explore
an engine’s full design potential), the ship operator (reduced fuel consumption)
and the environment (lower NOx and CO
2
emissions).
TurBoCharger Cleaning
Fouling on the compressor wheel and diffuser as well as on the turbine noz-
zle ring and turbine blades degrades turbocharger performance. The result can
be increased exhaust temperatures, higher engine component temperatures and
increased fuel consumption or, in extreme cases, an increased danger of surg-
ing. Dirt deposited on the compressor side is the usual dust from the environ-
ment along with oil mist from the engineroom. On the turbine side the deposits
are normally non-combustible components of the heavy fuel oil and surplus
cylinder lubricating oil.
The solution is to clean the turbocharger components at regular intervals,
usually carried out while the engine is running but at reduced speed. Dry clean-
ing of the turbine needs to be executed every 24–48 h using a granular medium,
with turbine wet cleaning every 48–500 h using water alone. Wet cleaning of
the compressor is performed every 25–100 h.
BursT TesTs
The rise in bmep ratings of engines in recent years has dictated a considerable
increase in the power density of the turbocharger rotors. Burst tests are per-
formed to ensure that even in the worst case scenario (e.g., a fire in the exhaust
system)—in which the compressor or turbine wheel can burst—all parts remain
within the turbocharger casings and do not endanger personnel. The turbine
casing design, which includes inner and outer burst protection rings, seeks
optimum containment.
Burst tests 191
Exh. wastegate
Air by-
pass
Inter
cooler
After
cooler
LP stageHP stage
Figure 7.11 schematic diagram of the two-stage turbocharging system based on
aBB TPs turbochargers used by wärtsilä in conjunction with Miller cycle early inlet
valve closure timings
192 Pressure Charging
TurBoCharger designers
aBB Turbo systems
Switzerland-based ABB Turbo Systems, through Brown Boveri & Cie (BBC),
has strong links with the pioneering days of turbocharger development by
Alfred Büchi and marine engine applications of turbocharging. Büchi applied
for his first patent in 1905, but it was not until after World War I (during which
the first turbochargers were applied to aero engines) that turbochargers entered
seagoing service. The 1925-commissioned liners Preussen and Hansestadt
Danzig were the first ships with exhaust gas turbocharged machinery, the out-
put of the MAN 10-cylinder four-stroke engines being raised from 1250 kW
(naturally aspirated) to 1840 kW by BBC turbochargers.
BBC’s launch of the VTR..0 series in 1944 marked the end of an era in
which each turbocharger was custom built for the application, and made avail-
able a range of volume-produced standard turbochargers for serving engines
with power outputs from 370 kW to 14 700 kW. Until then, turbochargers had
been applied commercially to four-stroke engines, but it now became possible
to turbocharge two-stroke engines equipped with engine-driven scavenging air
pumps. In order to eliminate the scavenging pumps and reduce fuel consump-
tion, however, the turbocharging system had to be a pulse system. This feature
was successfully introduced in 1952 on a B&W engine powering the Danish
tanker Dorthe Maersk, the first ship to be equipped with a turbocharged two-
stroke engine. The two VTR 630 turbochargers boosted engine output by some
35 per cent to 5520 kW.
The use of VTR-type turbochargers on two- and four-stroke engines prolifer-
ated rapidly from the 1950s, the series subsequently benefiting from continual
design refinements to raise overall efficiencies and compressor pressure ratios
(Figure 7.12). A turbocharger efficiency factor of 74.7 per cent was achieved
VTR..4E
VTR..4
VTR..1
VTR..0
1970 1975 1980 1985 1990 1994
75
70
65
60
55
50
η
TL
Figure 7.12 The total eciency of successive aBB VTr turbocharger generations,
which has increased signicantly
by a VTR 714E model in 1989 (contrasting with the 50–55 per cent efficiency
of BBC turbochargers in the early 1950s), and a VTR 304P model attained a
pressure ratio of 5:1 in 1991. Apart from performance improvements, R&D has
sought enhanced capability to serve engines burning low-grade fuels and operat-
ing under severe loading conditions, with reliability and minimal maintenance.
ABB Turbo Systems’ current programme embraces the following series:
l VTR series: designed for operation with low-speed two-stroke and
medium-speed four-stroke heavy duty engines with outputs from around
700 kW to 18 500 kW per turbocharger.
l VTC series: for engines with outputs from 1500 kW to 3200 kW per tur-
bocharger; particularly favoured where compactness is required, which
is achieved by using internal plain bearings lubricated by the engine
lube oil system.
l RR series: a lightweight, low-cost turbocharger of robust design offering
high efficiency, high gas inlet temperature capability and good compres-
sor characteristics; mainly applied to high-speed engines and smaller
medium-speed engines with ratings between 500 kW and 1800 kW per
turbocharger.
l TPS-D/E series: for modern high-speed diesel engines, small medium-
speed diesel and gas engines with power outputs ranging from 500 kW
to 3000 kW.
l TPS-F series: a compact turbocharger for small medium-speed die-
sel engines, large high-speed diesel and gas engines in the 400 kW to
3300 kW power range; three series, respectively, offer full-load pressure
ratios of up to 4.75, 5.0 and 5.2.
l TPL-A series: designed for modern four-stroke medium-to-large diesel
and gas engines with outputs from 2500 kW to 12 500 kW.
l TPL-B series: designed primarily for large modern two-stroke diesel
engines, six frame sizes covering an engine output range from 3000 kW
to 28 000 kW per turbocharger.
l TPL-C series: designed for advanced medium-speed four-stroke diesel
and gas engines in the power range from 3000 kW to 10 000 kW per
turbocharger.
l A100 generation: embraces three series (-H, -M and -L) covering the
requirements of high- and medium-speed diesel and gas engines (com-
pressor ratios of up to 5.8) and of low-speed engines (pressure ratio of
up to 4.7).
VTR..4 series turbochargers are fitted with single-stage axial turbines and
radial compressors. The combination of various turbine blade heights with a
large number of guide vane variants (nozzle rings)—as well as different com-
pressor wheel widths and various corresponding diffusers—fosters optimum
matching of engines and turbocharger operating characteristics.
External bearings result in lower bearing forces and allow the use of self-
lubricating anti-friction bearings. Sleeve bearings with external lubrication
Turbocharger designers 193
194 Pressure Charging
(engine lubrication) are available as an alternative for larger turbocharger types.
Any imbalance of the rotor shaft caused by the contamination of foreign parti-
cles is absorbed by damping spring assemblies in the bearings, the lower bear-
ing forces associated with external bearings being an advantage in this context.
External bearings are also accessible for servicing and can be removed without
dismantling the compressor wheel, and the rotor shafts can be removed without
dismantling the gas pipes.
ABB Turbo Systems recommends the use of self-lubricating anti-fric-
tion bearings whose reduced friction promotes a higher mechanical efficiency
and other advantages when starting up the engine and during manoeuvring.
Another benefit cited is the ability of this type of bearing to withstand short-
term oil failure. Oil centrifuges in the closed lubrication system of the anti-
friction bearings separate out any dirt particles and ensure constant lubrication
with pure lube oil, even in an inclined position. The turbine oil used has a posi-
tive effect on service life. Sleeve bearings with external lubrication are availa-
ble, but these normally involve substantially higher costs, particularly for large
turbochargers. Separate oil filtering systems and a standby tank for emergency
lubrication must be provided.
The gas inlet, gas outlet and compressor casings of the VTR turbocharger
are split vertically and bolted together. The three casings and the supports can
be rotated with respect to one another in increments of 15° or 30° to give the
engine designer freedom in mounting the turbocharger to the engine. A large
selection of gas inlet casing variants is available with different numbers and
arrangements of inlets. Flexibility in the matching of the turbocharger to vari-
ous charging systems, engine types (vee or in-line) and numbers of cylinders is
thereby fostered.
Uncooled casings are available as an alternative to the water-cooled casing
on VTR..4 turbochargers for applications where heat recovery is paramount
(Figure 7.13). All the gas ducts are entirely uncooled and not in contact with
cooling water at any point, making the greatest possible amount of exhaust
heat available for further exploitation (e.g., steam generation for electricity
production or ship’s services).
Functional reliability is fostered by arranging for the bearing housing on
the turbine end to be cooled with a small amount of water, helping to keep the
lubricating oil temperature low. The simultaneous cooling of the jacket of the
gas outlet casing makes it possible for the temperature of the entire surface of
the casing to be kept within the limits stipulated by classification societies for
the prevention of fire and protection against accidental contact.
Air to be compressed is drawn in either through air suction branches or
filter silencers. An integral part of every silencer is a filter which intercepts
coarse dirt particles from the intake air and counteracts compressor contamina-
tion. The filter can be cleaned during operation without having to dismantle the
silencer. The compressor noise is reduced in line with international regulations
by felt-covered, shaped plates.
Addressing the demands of low-speed two-stroke engine designers, ABB
Turbo Systems introduced the VTR..4D turbocharger series with a compressor
pressure ratio of up to 4. Three model sizes (VTR 454D, VTR 564D and VTR
714D) cover engine outputs from 5000 kW to 70 000 kW. Efficiency levels for
contemporary low-speed engines can be met by the established VTR..4 and
VTR..4E turbochargers (Figures 7.14 and 7.15). The significantly higher effi-
ciency of the VTR..4E series allows surplus exhaust energy to be exploited in
a power gas turbine connected mechanically to the engine crankshaft or a gen-
erator (see the section on Turbo Compound Systems). But neither series is able
to meet fully the demands of low-speed engines running with mean effective
pressures of up to 20 bar, requiring compressor pressure ratios of 3.9 at full-
load and turbocharger efficiencies of 67 per cent and higher. The VTR..4 does
Turbocharger designers 195
Section through water-cooled VTR turbocharger
Section through uncooled VTR turbocharger
Figure 7.13 water-cooled and uncooled aBB VTr turbochargers
196 Pressure Charging
not reach the required efficiency level, and the aluminium alloy compressor
wheel of the VTR..4E limits the achievable pressure ratio. The VTR..P turbo-
charger, developed for four-stroke engines (see next page), does not represent a
solution either, although it offers an efficiency improvement at higher pressure
ratios compared with the VTR..4.
Development goals for the VTR..4D series were defined as a compressor
pressure ratio of 4 while improving efficiency, the highest level of reliability
despite an increase in mechanical load and the retention of key elements of
Required full-load efficiency for large
2-stroke diesel engines as a function of
the compressor pressure ratio
TL
Turbocharger efficiency
v
Full-load compressor pressure ratio
Aluminium impeller
Titanium impeller
100
75
70
65
60
η
TL
η
TL
1.5 2.0 2.5 3.0 3.5 4.0 4.5 1.4
1.8 2.2
2.6
3.0 3.4
3.8
4.2
π
v
π
v
.75
.65
.55
VTR..4E
VTR..4
VTR..4P
η
TL
Turbocharger efficiency (ABB definition)
η
π
v
Compressor pressure ratio
π
1099
1599
VTR 454 D
(a) (b)
Figure 7.14 (a) Turbocharger eciency as a function of the compressor pressure
ratio for today’s aBB turbochargers (valid for applications on large two-stroke diesel
engines). (b) Turbocharger eciency as a function of the compressor pressure ratio
for the VTr 454d along a typical two-stroke diesel engine operating line
Compressor
pressure ratio
Engine rating (kW)
5.0 10.0 15.0 20.0
V
300
[m
3
/s]
*
4.0
3.0
2.0
1.0
240
π
v
4
3
2
1
5000 10000 20000
250
VTR 564E
n
300
[s
–1
]
220
200
180
160
120
140
π
0
5
0.87
η
sv
= 0.86
0.84
0.80
0.76
0.72
*
VTR 554 E
VTR 714 E
VTR 454 E
(b)(a)
Figure 7.15 (a) range of application of VTr..4e turbochargers and (b) the performance
map of a VTr 564e model
the proven VTR design and its external dimensions, and hence facility for full
interchangeability.
A turbocharger combining the pressure ratio of the VTR..4/4P and the high
efficiency of the VTR..4E was targeted. The thermodynamic potential of exist-
ing compressors and turbine designs led to a strategy for developing a new series
optimally blending well-proven components and matching the fitting dimensions
of the VTR..4E. The high-pressure compressor of the VTR..4P turbocharger was
selected for thermodynamic and economic reasons, the designer citing its:
l High efficiency level for the whole range of competitive volume flow
rate.
l Greater margin for further increases in mean effective pressure and over-
load margin.
l Use of aluminium as the impeller material instead of the signifi-
cantly more expensive titanium; a full-load pressure ratio of 4.2 is
underwritten.
Two compressors of different diameters are available to cover a competi-
tive volume range at the required high efficiency level within the same overall
turbocharger envelope.
The turbine of the VTR..4E turbocharger was chosen for the VTR..4D
series since it offers efficiency advances of up to around 3.5 points over the
VTR..4/4P turbine. This is due to better recovery of kinetic energy at the tur-
bine wheel outlet, resulting from an optimally adapted outlet diffuser in combi-
nation with a new enlarged exhaust gas casing. The latest anti-friction bearing
technology was tapped to benefit the VTR..4D series, particularly on the tur-
bine side. Improved mechanical reliability is promised from a fully machined
steel cage with surface treatment, an optimized oil feed for better lubrication,
optimized smaller rollers for less slip and a larger cage guiding area.
Like other ABB turbochargers, the VTR..4D is designed to ensure that
in the worst case scenario—bursting of the impeller or turbine—all the parts
remain within the casings. Casing material was changed from grey (laminar)
cast iron to ductile (nodular) cast iron and the turbine enclosed in a protective
shroud. Burst tests on the compressor and turbine confirmed the containment
capability.
ABB Turbo Systems’ VTR..4P series addresses the needs of enginebuild-
ers seeking to raise the output of medium-speed designs. Such engines may
run with meps of up to 24–25 bar, requiring a turbocharger compressor ratio of
3.8–4.0 and an overall turbocharger efficiency exceeding 60 per cent. Meps of
up to 28 bar, required by some four-stroke engine designs, dictate a compressor
ratio of around 4.2–4.5 while maintaining or even improving the overall turbo-
charger efficiency. The VTR..4P series offers ratios of up to 5:1 from a single-
stage turbocharging system whose simplicity is favoured over more complex
two-stage arrangements.
Advanced flow calculations, stress analyses, CAD/CAM methods and lab-
oratory tests resulted in a one-piece compressor wheel with backward-curving
Turbocharger designers 197
198 Pressure Charging
main and splitter blades discharging through an airfoil diffuser. An aluminium
alloy impeller will suit most needs, but titanium can be specified for extreme
applications. The turbine disc is of a high-grade nickel alloy, with new con-
figurations applied for the blade roots and damping wires.
The VTR..4P programme was evolved to cover the demands of engines
with outputs ranging from around 1000 kW to 10 000 kW. The high perform-
ance units are designed to be physically interchangeable with VTR..4 models
for convenient retrofitting as well as for application to new engines. The tur-
bocharger can be flanged directly onto existing air and exhaust gas ducting to
improve engine performance by increasing the boost pressure. The pressure-
optimized series complemented the established efficiency-optimized VTR..4E
series in the ABB Turbo Systems’ portfolio.
TPs series
Advances in small medium-speed and large high-speed diesel engine technol-
ogy called for more compact turbochargers with increased pressure ratios at
higher overall efficiency levels. ABB Turbo Systems addressed the market in
the mid-1990s with the TPS series, designed to serve four-stroke engines with
outputs from 500 kW to 3200 kW per turbocharger (Figure 7.16).
To ensure the full range of pressure ratios required by such engines, the
TPS..D/E turbocharger is available with two completely different compressor
stages. One stage is designed for pressure ratios of up to 4.2 (TPS..D), and the
other for pressure ratios of up to 4.7 (TPS..E). Thanks to mechanically opti-
mized single-piece designs incorporating bore-less compressor wheels, pressure
ratios of up to 4.5 can be reached for a wide range of applications using alumin-
ium alloy as the compressor material. Both compressors feature a single-piece
Figure 7.16 aBB Turbo systems’ TPs turbocharger for small medium-speed and
large high-speed engines