HANSA 06-2019

Schiffstechnik | Ship Technology
© MAN
Figure 3: A layout A layout diagram diagram that indicates that indicates two SMCR two points. SMCR Moving points. the SMCR Moving closer the to the min. mep line Figure increases A layout diagram indicating two SMCR points. Moving the
4: A layout diagram indicating two SMCR points. Moving the SMCR parallel to lines of constant
engine efficiency; SMCR this closer is called to derating the minimum mep line increases engine efficiency;
increase the SMCR engine parallel efficiency to (but lines may of increase constant propeller mep does efficiency) not increase the engine
this is called derating
efficiency (but may increase propeller efficiency)
Referring to Figure 3, since a given power output from the engine is often required, derating Having is usually discussed how engine efficiency can be improved by careful engine selection
not achieved by reducing the SMCR power but rather by adding an extra cylinder to the relevant engine. With to return to the fuel equation and remember that the approach needs to be ho
an extra cylinder, the layout diagram moves vertically upwards and, for a given position successful: of the SMCR,
this results cal in a energy lower mep by and, the piston thereby, before derating. the combustion
gases are expelled to the exhaust, as high as possible to achieve the high-
the SMCR power RRRR TTTT ∙ VVVVbut rather by adding an
maximum pressure and mep should be rating is usually not achieved by reducing
Figure 4 illustrates compared a movement to the situation of the SMCR in Figure within 1, the resulting
in higher engine efficiency.
ing that a given engine type is designed to layout diagram moves vertically upwards
engine layout est possible diagram engine that will efficiency. not result in Consider-
PPPP ffffffffffffffff extra = cylinder ηηηη to the engine. Thus, the
HHHH ∙ ηηηη OOOO ∙ ηηηη RRRR ∙ ηηηη SSSS ∙ ηηηη
derating.
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Advances in engine design have made withstand a certain maximum pressure, and, for a given position of the SMCR,
it possible to design marine two-stroke
engines that are capable of withstanding
the objective is to reduce From mep the fuel if the equation, focus it is clear this that results although in a lower have mep now and, spent thereby, some time poring o
is more on high engine efficiency, η E , than the engine on efficiency derating. is just Figure one element 4 illustrates of vessel a movement fuel-efficiency. Chang
maximum cylinder pressures of 200bar. high engine power. efficiency may have implications of the on the SMCR other within elements the of engine the fuel layout equation, di-foagram examp
larger engine that will to develop not result a given in derating. power in order to achie
This makes it possible to reach unprecedented
engine efficiencies and lower engine, the degree of derating may can result be seen in a change of Having the aft-ship discussed hull lines how that engine may affect efficien-
R T , η H , η O and
When ordering a marine • Selecting two-stroke a physically
SFOC. By derating it is possible to achieve by the position of the specified negative maximum way, cy can be improved by careful engine
even lower fuel consumption.
continuous rating (SMCR) • Moving the engine the SMCR within selection the engine and layout, diagram, let’s return to achieve to derating, the in a w
layout diagram (Figures 3 and a change 4). Referring
of the propeller fuel curve equation may influence and remember η O and ηthat R in a the way ap-
that may can
to Figure 3, since a given effect power of the out-
derating. proach needs to be holistic to be success-
More on derating
put from the engine is often required, deful.
From the fuel equation it is clear that
These interactions complicate the vessel-design process and it is important to get them
The degree of derating can be identified
by how much the maximum engine
However, this is also what makes vessel the fuel-efficiency. life of the naval architect Changing interesting, engine so letʼs not
to build a vessel with the lowest engine possible efficiency fuel consumption η E is just and one the element smallest of possible c
torque is reduced. Although it is possible
difficulties but focus on the possibilities. efficiency may have implications on the
to look at the torque values, it is more
usual and typically easier to look at the
so-called »mep« values that are independent
of engine size (mep = mean effective
pressure, work done per engine
cycle per cubic meter of engine displacement
V d). Engine designers use mep instead
of torque almost exclusively.
mep = W cycle : V d , mep is proportional to
engine torque according to:
mep = 2 ∙ π ∙ Torque ∙ x :V d (x = 1 for a
2-stroke engine, x = 2 for a 4-stroke engine,
all values in SI units)
Derating an engine reduces maximum
torque – equivalent to maximum mep –
in order to create an engine with a lower
fuel consumption. A high maximum
other elements of the equation, e.g.:
••
Selecting a physically larger engine to
develop a given power in order to
achieve derating may result in a change
of the aft-ship hull lines that may negatively
affect R T, η H, η O and η R.
••
Moving the SMCR within the engine
layout diagram, to achieve derating, in
a way that forces a change of the propeller
curve may influence η O and η R in
a way that may cancel the positive derating
effect.
These interactions complicate the vessel-design
process. It is important to get
them right in order to build a vessel with
the lowest possible fuel consumption and
the smallest possible carbon footprint.
cylinder-pressure also reduces fuel consumption.
Combining the two concepts
leads to the old rule that the ratio between
Author: Kim Rene Hansen, Senior Research
Engineer, MAN Energy Solutions
© MAN
58 HANSA International Maritime Journal 06 | 2019