Derating Engines

Derating: a solution for high fuel savings and lower emissions Rudolf Wettstein1 & David Brown2 Wärtsilä Switzerland Switzerland Ltd, Winterthur Winterthur -

Summary is paper sets out ways to achieve worthwhile reductions in the fuel consumption of Wärtsilä low-speed engines when designing newbuildin newbuildings. gs. e key approac approach h is to use the flexibilit flexibilityy offered by the full power/ power/speed speed layout field to select a better layout point at a derated power with a lower BSFC and also possibly a higher propeller efficiency. efficiency.

Introduction Fuel efficiency and environmental friendliness are high on the list of requirements for ship propulsion engines from today’s shipping- and shipbuilding industries. us Wärtsilä is committed to creating better technology in these areas that will benefit both the customers and the environment. Yet Y et it is often forgotten forgotten by many ship designers designers and those specifying low-speed main engines that advantage can be taken of the power/speed layout field of Wärtsilä low-speed engines to select an engine rating point with a still lower fuel consumption. e concept of the power/speed layout field for low-speed marine diesel engines originated in the 1970s. e layout options were step-by-step widened until, in 1984, our low-speed low-speed engines began beg an to be offered with a broad power/speed power/speed layout field. An engine’ss contracted maximum engine’ ma ximum continuous rating (CMCR) can be selected at any point in the power/ speed field defined by the four corner points: R1, R2,, R3 and R4 (Fig. 1). R2 1). e rating point R1 is the maximum continuous rating (MCR) of the engine. Most recently, the layout fields for certain engines, the RT-flex82C, RTA82C, RT-flex82T and RTA82T, are extended to increased speeds for the R1+ and R2+ points (Fig. 2). e extended fields offer widened flexibility to select the most efficient propeller speed for lowest daily fuel consumption, and the most economic propulsion equipment, 1 -

Rudolf Wettstein is Director, Marketing & Application Applica tion Development, Development, Ship Power Power,, Wärtsilä Wärtsilä Switzerland Ltd. 2 David Brown is Manager, Marketing Support, Wärtsilä W ärtsilä Switzerland Switzerland Ltd. Ltd.

Engine power, %R1

100

Higher propulsive efficiency

e i n l e u q o r t t n Rx t a s n o C

90

80

R1

0 -1 ∆BSFC -2 g/kWh -3 -4 -5 -6

R3 -7

70

60 70

R2

R4

80

Lower specific fuel consumption

90

100

Engine speed, %R1

Fig. 1: Typical layout field for RTA and RT-flex engines. e contracted maximum continuous rating (CMCR) can be selected at any point, such as Rx, within the layout field. e ∆BSFC is the reduction in full-load BSFC for any rating point Rx relative to that that at the R1 rating. rating. [08#044]

namely the propeller, shafting, etc. One basic principle of the engine layout field is that the same maximum cylinder pressure (Pmax) is employed at all CMCR points within the layout field. us the reduced brake mean effective pressure (BMEP) obtained at the reduced power outputs in the field results in an increased ratio of Pmax/BMEP and thus lower brake specific fuel consumption (BSFC). e other principle behind the layout field is

—1—

© Wärtsi Wärtsilä lä Corporation, June 2008 2008 -

Engine power, %R1 Engine power, %R1

R1

100 R1

100

R1+

90

Rx2

Rating line slope = α

Rx1 90

R3 80

70 80

R4

80

R2

R3

R2

R4

R2+

90

100

60 70

Engine speed, %R1

80

90

100

Engine speed, %R1

Fig. 2: For the RT-flex82C, RTA82C, RT-flex82T and RTA82T engines the layout fields are extended to the ratings R1+ and R2+ at the same powers as R1 and R2 respectively but with increased shaft speed. [08#049]

Fig. 3: For a given ship, a rating line (slope α ) can be applied to the layout field so that all rating points on that line would give the same ship speed with a suitably optimized propeller. Rating points at lower speeds on the rating line require a larger propeller diameter and give a greater propulsive efficiency.

that the lower CMCR speeds allow flexibility in selection of the optimum propeller with consequent benefits in propulsion efficiency and thus lower fuel consumption in terms of tonnes per day. One feature to be borne in mind when selecting the rating point for the derated engine is the rating

line (Fig. 3). is is the line through a CMCR rating point such that any point on the line represents a new power/speed combination that will give the same ship speed in knots. e points on the rating line all require the same propeller type but with different adaptations to suit the power/speed combination. In general, lower speeds of rotation require larger propeller diameters and thereby increase the total propulsive efficiency. Usually the selected propeller speed depends on the maximum permissible propeller diameter. e maximum diameter is often determined by operational requirements, such as design draught and ballast draught limitations, as well as class recommendations concerning propeller–hull clearance (pressure impulse induced by the propeller on the hull). e slope of the rating line ( α) depends broadly upon the ship type. It can range from 0.15 for tankers, bulk carriers and general cargo ships up to about 10,000 tdw to 0.22 for container ships larger than 3000 TEU and 0.25 for tankers and bulk carriers larger than 30,000 tdw.

Fig. 4: Since the 1980s engine ratings have been selected over a steadily smaller area of the layout field. [08#051] Engine power, %R1 100

R1

Area of recent CMCR selection

90

80

70

60 70

R3

Area of CMCR selection in the 1980s

R2

R4

80

90

100

Engine speed, %R1

Changing engine selection strategies When the broad layout field was introduced in RTA engines in 1984 it was widely welcomed by shipowners and shipbuilders. Afterwards RTA engines were frequently selected at ratings in the lower part of the layout field to gain the benefits of —2—

© Wärtsilä Corporation, June 2008 -

Bunker price, US$/tonne 380cSt HFO 500

400

300

200

100 2004

2005

2006

2007

2008

Fig. 5: Bunker prices have considerably increased in recent times. e chart shows the average price of 380 cSt heavy fuel oil (HFO) from various ports around the world from 2004 to 2008. e green bars indicate the mean price for each year. [08#045] -

lower fuel consumption. However, under the pressure of first costs and softening bunker prices the strategy was changed and the selected power/speed combination has, during the past 15 years or so, been selected to be closer to the R1 rating (Fig. 4). Yet, more recently, bunker prices have steadily climbed, rising by some 85 per cent in the course of 2007 from US$ 270 to US$ 500 per tonne (Fig. 5). e result is that bunkers are now the dominant part of ship operating costs. Such drastic increases in bunker prices give a strong impetus to reduce fuel costs. ey can also justify additional investment cost such as selecting an engine with an extra cylinder. e consequent fuel saving may make for an acceptable payback time on the additional investment cost. It would justify any efforts to increase the overall efficiency of the complete propulsion system. Further impetus to implementing such changes in engine selection strategy will come from a future need to cut CO2 emissions. If a carbon trading

scheme is imposed on shipping it would give further economic advantage to reducing fuel consumption and further help to pay for any necessary extra investment costs. In addition it is important to bear in mind that the fuel savings measures discussed here will also result in lower NO X emissions in absolute terms.

Derating engines for greater fuel savings In the following pages are some case studies for ship installations for which an engine is selected with an extra cylinder without increasing the engine’s power. ese cases demonstrate that such engine derating can be an advantageous solution with remarkable saving potential. Depending on bunker costs, such a strategy can have a very attractive pay-back time. e four case studies are for a Suezmax tanker, a Capesize bulk carrier, a Panamax container ship and a Post-Panamax container ship. ey include estimations of the respective pay-back times for the additional engine costs.

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Case 1: Suezmax tanker & Capesize bulk carrier In this case, a typical Suezmax tanker might be specified with a six-cylinder Wärtsilä RT-flex68-D main engine. However, if a seven-cylinder engine is employed instead, the daily fuel consumption can be reduced by some 3.4 per cent. In the engine/propeller layout for this ship as shown in figure 6, the CMCR points for the two alternative engines are on the same rating line (α = 0.3) through a common design point for the same ship service speed (knots). e calculation of annual fuel costs given in table 2 is based on 6000 hours running with heavy fuel oil

costing US$ 500 per tonne. e resulting payback time for the extra cost associated with the additional engine cylinder is estimated to be between 3.5 and six years depending on the bunker price of US$ 600–400 per tonne respectively (Fig. 7). e calculations of the payback are based on an interest rate of eight per cent. A similar case may be made for a Capesize bulk carrier as it would be similar in size and speed to a Suezmax tanker and would thus require a similar engine.

Table 1: Typical ship parameters for a Suezmax tanker

Length overall: Beam: Design draught: Scantling draught: Sea margin: Engine service load:

about 274 46–50 16 17 15 90

m m m m % %

Table 2: Main engine options -

Alternative engines: Cylinder bore, mm: Piston stroke, mm: Stroke/bore ratio: MCR, kW / rpm: CMCR, kW / rpm: BMEP at CMCR, bar: CSR at 90% CMCR, kW/rpm: BSFC at CMCR, g/kWh: – 100% load: – 90% load: Daily fuel consumption, tonnes/day: – ISO fuel, LCV 42.7 MJ/kg: – LCV 40.5 MJ/kg: – As percentage, %: Annual fuel costs, US$: Fuel saving, US$: Engine length, mm: Engine mass, tonnes:

6RT-flex68-D 680 2720 4:1 18,780/95 18,780/95 20.0 16,902/91.7

7RT-flex68-D 680 2720 4:1 21,910/95 18,460/89.7 17.9 16,614/86.6

169.0 165.6

164.8 162.6

67.2 70.8 100 8,853,000 0

64.8 68.4 96.6 8,544,000 – 309,000

8690 472

9870 533

—4—

–3.4%


Case 1: Suezmax tanker & Capesize bulk carrier -

Engine power, kW 22,000 7RT-flex68-D

20,000 Constant ship speed α = 0.3

Fig. 6: Engine/propeller layouts for a typical Suezmax tanker with a derated seven-cylinder RT-flex68-D engine compared with a six-cylinder engine at the full MCR power and speed. [08#052]

CMCR 18,460 kW 89.7 rpm

Design point CMCR = R1 18,780 kW, 95 rpm 6RT-flex68-D

18,000

CSR 16,902 kW 91.7 rpm

CSR 16,614 kW 86.6 rpm

16,000

75

80

85

90

95

100

Engine speed, rpm

Millions US$ Bunker price, HFO: $600/tonne $500/tonne

3.0

$400/tonne

2.0

Fig. 7: Variation of payback times from fuel savings according to bunker costs for the derated engine with an extra cylinder for a typical Suezmax tanker. [08#144]

Investment approx. ($)

1.0

0 2

4

—5—

6

8

10

12

14 Years


Case 2: Panamax container ship In this case, a typical Panamax container ship with a container capacity of up to 5000 TEU might be specified with an eight-cylinder Wärtsilä RT-flex82C main engine. However, if a nine-cylinder engine is employed instead, the daily fuel consumption can be reduced by some two per cent. In the engine/propeller layout for this ship as shown in figure 8, the CMCR points for the two alternative engines are on the same rating line (α = 0.2) through a common design point for the same ship service speed (knots).

e calculation of annual fuel costs given in table 4 is based on 6000 hours running with heavy fuel oil costing US$ 500 per tonne. e resulting payback time for the extra cost associated with the additional engine cylinder is estimated to be between four and seven years depending on the bunker price of US$ 600–400 per tonne respectively (Fig. 9). e calculations of the payback are based on an interest rate of eight per cent.

Table 3: Typical ship parameters for a Panamax container ship


about 295 32.2 12 13.5 15 90

m m m m % %

Table 4: Main engine options

Alternative engines: Cylinder bore, mm: Piston stroke, mm: Stroke/bore ratio: MCR, kW / rpm: CMCR, kW / rpm: BMEP at CMCR, bar: CSR at 90% CMCR, kW / rpm: BSFC at CMCR, g/kWh: – 100% load: – 90% load: Daily fuel consumption, tonnes/day: – ISO fuel, LCV 42.7 MJ/kg: – LCV 40.5 MJ/kg: – As percentage, %: Annual fuel costs, US$: Fuel saving, US$: Engine length, mm: Engine mass, tonnes:

8RT-flex82C 820 2646 3.2:1 36,160/102 36,160/102 19.0 32,544/98.5

9RT-flex82C 820 2646 3.2:1 40,680/102 35,480/97.5 17.5 32,250/94.3

169.0 166.5

166.6 164.6

130.0 137.1 100 17,138,000 0 14,055 1020

—6—

127.4 134.3 98 – 2.0% 16,790,000 – 348,000 16,500 1140


Case 2: Panamax container ship -

Engine power, kW 42,000

40,000

9RT-flex82C

38,000

Fig. 8: Engine/propeller layouts for a typical Panamax container ship with a derated nine-cylinder RT-flex82C engine compared with an eightcylinder engine at the full MCR power and speed. [08#062]

CMCR 35,850 kW 97.5 rpm

Constant ship speed α = 0.2

36,000

Design point CMCR = R1+ 36,160 kW, 102 rpm

8RT-flex82C

34,000

CSR 32,544 kW 98.5 rpm

CSR 32,250 kW 94.3 rpm 32,000 85

90

95

100

105

Engine speed, rpm

Millions US$ 4.0 Bunker price, HFO: $600/tonne

3.0

$500/tonne $400/tonne

Fig. 9: Variation of payback times from fuel savings according to bunker costs for the derated engine with an extra cylinder for a typical Panamax container ship. [08#145]

2.0 Investment approx. ($)

1.0

0 2

4

—7—

6

8

10

12

14 Years


Case 3: Post-Panamax container ship In this case, a typical Post-Panamax container ship with a container capacity of around 7000 TEU might be specified with an eleven-cylinder Wärtsilä RT-flex96C main engine. However, if a 12-cylinder engine is employed instead, the daily fuel consumption can be reduced by some 2.4 per cent. In the engine/propeller layout for this ship as shown in figure 10, the CMCR points for the two alternative engines are on the same rating line (α = 0.2) through a common design point for the same ship service speed (knots).

e calculation of annual fuel costs given in table 6 is based on 6000 hours running with heavy fuel oil costing US$ 500 per tonne. e resulting payback time for the extra cost associated with the additional engine cylinder is estimated to be between two-and-a-half and four years depending on the bunker price of US$ 600– 400 per tonne respectively (Fig. 11). e calculations of the payback are based on an interest rate of eight per cent.

Table 5: Typical ship parameters for a Post-Panamax container ship


about 325 42.8 13 14.5 15 90

m m m m % %

Table 6: Main engine options -

Alternative engines: Cylinder bore, mm: Piston stroke, mm: Stroke/bore ratio: MCR, kW / rpm: CMCR, kW / rpm: BMEP at CMCR, bar: CSR at 90% CMCR, kW / rpm: BSFC at CMCR, g/kWh: – 100% load: – 90% load: Daily fuel consumption, tonnes/day: – ISO fuel, LCV 42.7 MJ/kg: – LCV 40.5 MJ/kg: – As percentage, %: Annual fuel costs, US$: Fuel saving, US$: Engine length, mm: Engine mass, tonnes:

11RT-flex96C 960 2500 2.6:1 66,330/102 66,330/102 19.6 59,697/98.5

12RT-flex96C 960 2500 2.6:1 72,360/102 65,919/98.9 18.4 59,327/95.5

171.0 166.8

168.0 163.8

239 252 100 31,500,000 0 21,550 1910

—8—

233.2 245.9 97.6 – 2.4% 30,738,000 – 762,000 23,230 2050


Case 3: Post-Panamax container ship -

Engine power, kW

72,000 12RT-flex96C

70,000

68,000 Constant ship speed α = 0.2 66,000

Fig. 10: Engine/propeller layouts for a typical Post-Panamax container ship with a derated 12-cylinder RT flex96C engine compared with an 11-cylinder engine at the full MCR power and speed. [08#127]

Design point CMCR = R1 66,330 kW, 102 rpm

CMCR 65,919 kW 98.9 rpm

11RT-flex96C

64,000

62,000

60,000

CSR 59,327 kW 95.5 rpm

CSR 59,697 kW 98.5 rpm

58,000 90

95

100

105

Engine speed, rpm

Millions US$ Bunker price, HFO: $600/tonne

8.0

$500/tonne

6.0

$400/tonne

Fig. 11: Variation of payback times from fuel savings according to bunker costs for the derated engine with an extra cylinder for the typical Post-Panamax container ship. [08#146]

4.0 Investment approx. ($)

2.0 0 2

4

—9—

6

8

10

12

14 Years


a modest increase in cost of the D version for the higher-efficiency turbochargers used, but the extra cost would soon be repaid by the fuel cost savings.

Case 4: Derating without adding an engine cylinder It is also feasible to apply a derated engine to obtain fuel savings in such a way that an additional engine cylinder is not required. An example of this can be seen with the Wärtsilä RT-flex50 engine. In October 2007, the D version of this engine was announced, in which the engine power was increased by 5.1 per cent and the BSFC at full-load was reduced by 2 g/kWh compared with the B version. us if a ‘-D’ engine is derated to the same cylinder power output as the original version of the RT-flex50, then the BSFC at full load is reduced by 4.5 g/kWh, or 2.7 per cent (see Table 7). For a typical bulk carrier with a six-cylinder RT-flex50 engine this can translate into annual savings of US$ 124,000 when operating for 6000 running hours a year with heavy fuel oil costing US$ 500 per tonne. Even greater savings are possible if the engine is derated to a lower running speed (rpm) at the derated power to gain the benefits of a better propulsion efficiency. ere are already a number of standard ship designs delivered and on order with RT-flex50-B or even the original RT-flex50 engine. So it would be perfectly feasible to install a derated RT-flex50-D in further newbuildings to the same ship designs and obtain the benefit of the substantial savings in operating costs. e overall dimensions of the D version are identical to those of the B and original versions of the RT-flex50. ere would, however, be

Derating with flexibility to full rating Although derating offers attractive economics, it can be frustrating to buy more ‘engine’ than seems necessary. Yet there is an interesting option to retain an ability to utilise the full available installed engine power, even up to the full R1 rating for future use to obtain higher ship service speeds. e concept would be to set up the engine for the derated output at the chosen reduced service speed. en for a later date, the engine could be re-adapted to the higher output. However, this needs corresponding provisions in the selection and design of the propeller, shafting and ancillary equipment to meet the requirements of the envisaged higher power. Furthermore the engine would need to be tested and approved by the Classification Society for both ratings with all the necessary emissions certification.

RT-flex technology as an important contribution to fuel saving Wärtsilä RT-flex technology plays an important role in fuel saving. Wärtsilä RT-flex low-speed engines incorporate the latest electronically-controlled common-rail technology for fuel injection and valve actuation. e result is great flexibility in engine setting, bringing benefits in lower fuel consumption, lower minimum running speeds, smokeless operation

Table 7: Options for the Wärtsilä RT-flex50 engine type

Alternative engines: Cylinder bore, mm: Piston stroke, mm: S/B ratio: MCR, kW / rpm: CMCR, kW / rpm: BMEP at CMCR, bar: CSR at 90% CMCR, kW / rpm: BSFC at CMCR, g/kWh: – 100% load: – 90% load: Daily fuel consumption, tonnes/day: – ISO fuel, LCV 42.7 MJ/kg: – LCV 40.5 MJ/kg: – As percentage, %: Annual fuel costs, US$: Fuel saving, US$:

6RT-flex50 500 2050 4.1:1 9720/124 9720/124 19.5 8748/119.7

6RT-flex50-D 500 2050 4.1:1 10,470/124 9720/124 19.5 8748/119.7

171 167.6

165.7 163.0

35.2 37.1 100 4,637,000 0 — 10 —

34.2 36.2 97.3 – 2.7% 4,513,000 – 124,000 © Wärtsilä Corporation, June 2008 -

at all running speeds, and better control of other exhaust emissions. Not only do RT-flex engines have a lower partload fuel consumption than RTA engines but they can be adapted through Delta Tuning so that their part-load fuel consumtion is even lower. [1] Owing to the interaction between fuel economy and NO X emissions, there is always the possibility that fuel saving measures will have an impact on NO X emissions. As with all new marine engines nowadays, Wärtsilä RTA and RT-flex engines are all fully compliant with the NO X emission regulation of Annexe VI of the MARPOL 1973/78 convention. Moreover, the engines in the Wärtsilä portfolio will be adapted to meet the coming IMO NO X reduction level Tier II.

also possibly a higher propeller efficiency. It must also not be forgotten that any fuel savings achieved at the ship design stage will have benefits in also reducing exhaust emissions. If you have a project for which you wish to explore the fuel-saving possibilities through derating as set out in this paper, then please contact your nearest Wärtsilä office. Our experts will be delighted to calculate various alternatives for your evaluation.

References 1. German Weisser, ‘Fuel saving with RT-flex’, Wärtsilä Switzerland Ltd, July 2004.

Conclusion e paper shows that there are techniques to achieve worthwhile reductions in the fuel consumption of Wärtsilä low-speed engines when designing newbuildings. e key approach is to use the flexibility offered by the full power/speed layout field to select a better layout point with a lower BSFC and

Published June 2008 by: Wärtsilä Switzerland Ltd PO Box 414 CH-8401 Winterthur Tel: +41 52 262 49 22 Fax: +41 52 262 07 18 www.wartsila.com — 11 —


Derating Engines

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