Textos de Apoio (pdf)

Derating: a solution for
high fuel savings and lower emissions
Rudolf Wettstein 1 & David Brown 2
Wärtsilä Switzerland Ltd, Winterthur
Summary
This paper sets out ways to achieve worthwhile reductions in the fuel consumption of Wärtsilä low-speed engines
when designing newbuildings. The key approach is to use the flexibility offered by the full power/speed layout field to
select a better layout point at a derated power with a lower BSFC and also possibly a higher propeller 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. Thus Wärtsilä is committed to creating
better technology in these areas that will benefit both
the customers and the environment.
Yet it is often forgotten by many ship 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.
The concept of the power/speed layout field for
low-speed marine diesel engines originated in the
1970s. The layout options were step-by-step widened
until, in 1984, our low-speed engines began to be
offered with a broad power/speed layout field. An
engine’s contracted maximum 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). The 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). The 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 Development, Ship Power, Wärtsilä
Switzerland Ltd.
2
David Brown is Manager, Marketing Support,
Wärtsilä Switzerland Ltd.
Engine power, %R1
100
90
80
70
60
70
Higher propulsive
efficiency
R3
R4
80
Constant torque line
90
R1
0
-1 ∆BSFC
-2 g/kWh
-3
-4
-5
100
Engine speed, %R1
Fig. 1: Typical layout field for RTA and RT-flex engines. The
contracted maximum continuous rating (CMCR) can be
selected at any point, such as Rx, within the layout field. The
∆BSFC is the reduction in full-load BSFC for any rating
point Rx relative to that at the R1 rating.
[08#044]
Rx
R2
-6
-7
Lower
specific
fuel
consumption
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. Thus 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).
The other principle behind the layout field is
— 1 — © Wärtsilä Corporation, June 2008