Textos de Apoio (pdf)

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.
Thus 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.
There 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. The overall dimensions of the D
version are identical to those of the B and original
versions of the RT-flex50. There would, however, be
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.
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.
The concept would be to set up the engine for
the derated output at the chosen reduced service
speed. Then 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. The 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: 6RT-flex50 6RT-flex50-D
Cylinder bore, mm: 500 500
Piston stroke, mm: 2050 2050
S/B ratio: 4.1:1 4.1:1
MCR, kW / rpm: 9720/124 10,470/124
CMCR, kW / rpm: 9720/124 9720/124
BMEP at CMCR, bar: 19.5 19.5
CSR at 90% CMCR, kW / rpm: 8748/119.7 8748/119.7
BSFC at CMCR, g/kWh:
– 100% load: 171 165.7
– 90% load: 167.6 163.0
Daily fuel consumption, tonnes/day:
– ISO fuel, LCV 42.7 MJ/kg: 35.2 34.2
– LCV 40.5 MJ/kg: 37.1 36.2
– As percentage, %: 100 97.3 – 2.7%
Annual fuel costs, US$: 4,637,000 4,513,000
Fuel saving, US$: 0 – 124,000
— 10 — © Wärtsilä Corporation, June 2008