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Low-speed engine lay-out.ppt / TN
Low speed engines propulsion layout
7056-03/F100484
Principal propulsion layout(Ref. Wartsila Marine Propulsion
Guide Jan. 2000)
Propulsion layout of low speed engines driving FP-propellers
must be given special attention Once a FP-propeller has been
designed, it is very limited possibilities to control the torque
and power absorbed
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Low-speed engine lay-out.ppt / TN
Low speed engines propulsion layout
Low speed engines normally drives the propeller directly. The
engine should therefore turn at a speed close to the optimum
propeller speed
Engine Layout fields have been introduced in order to optimise
the engine speed according to the envisaged propeller diameter
The Contract Maximum Continuos Rating (CMCR) can be freely
selected within the layout field
Engine Load range is the admissible area of operation once the
CMCR has been determined
General remarks
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Low-speed engine lay-out.ppt / TN
Selection of the CMCR point
Various parameters should be considered
Required propulsive power Propeller efficiency (rpm and diameter
influence) Operational flexibility Power and speed margins (weather
and ageing reserves) Ship trading pattern Possibility of a shaft
generator
Selecting the most suitable engine is vital to achieve an
efficient cost/benefit relation for the actual vessel
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Low-speed engine lay-out.ppt / TN
Selection of CMCR point
Why derating ?
Normally engines are delivered with CMCR close to the maximum R1
rating (Commercial pressure)
Derating may be applied for the following reasons: Reduction of
engine speed at constant mep (mean effective
pressure) to improve propeller efficiency Reduction of engine
power at full engine speed to reduce specific
fuel oil consumption General derating to increase power and
torque margins, and to
reduce engine wear/maintenance cost
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Low-speed engine lay-out.ppt / TN
Engine layout field - RTA engines
Engine speed
Note : logarithmic scales
*) 90% for RTA96C
Rating line for constant vessel speed
CMCR (Rx) point (example)100%
100%
Engine Power
70%
80% *)
R1
R2
R3
R4
Nominal 100% constant mean effective pressure
80% *)Reduced specific fuel oil consumption
Improved propeller efficiency
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Low-speed engine lay-out.ppt / TN
Propeller speed influence on power requirement
Lower propeller revolutions combined with larger propeller
diameter increases propulsion efficiency. Less power will be needed
to propel the vessel at a given speed
The relative change in required power can be approximated by the
following relation:
=
1
2
1
2
NN
PP
Pj = Propulsive power at propeller revolution NjNj = Propeller
speed corresponding with propulsive power Pj = 0,15 for tankers and
general cargo ships up to 10000 dwt
0,20 for tankers, bulk carriers from 10000 to 30000 dwt0,25 for
tankers, bulk carriers larger than 30000 dwt0,17 for reefers and
container ships up to 3000 TEU0,22 for container ships larger than
3000 TEU
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Low-speed engine lay-out.ppt / TN
Propeller efficiency task
A VLCC is fitted with a 7 cylinder Sulzer RTA84TCMCR is 28700
kWService rating is 25830 kW at 76 rpm (90% CMCR)
Estimate the reduction in required service rating if the
propeller speed is reduced to 61 rpm (assuming a larger optimum
propeller diameter)
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Low-speed engine lay-out.ppt / TN
Hull roughness effect on resistance
Increased hull roughness effects
10 m increased roughness 1 % power increase up to 250 m10 m
increased roughness 0,5 % power increase over 250 m
1) Permanent roughness: Due to quality of steel work, surface
treatment, type of paint, age etc.2) Temporary roughness: Due to
marine growth
Permanent roughness increase / longtime antifouling (5 years):
125 m 300 m 15 % powerPermanent roughness increase / selfpolishing
antifouling (5 years):125 m 145 m 2 % power
Total roughness increase / longtime antifouling (4 years): 125 m
300 m 15 % powerTotal roughness increase / selfpolishing
antifouling (4 years): 125 m 175 m 5 % power
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Low-speed engine lay-out.ppt / TN
Definitions of margins
Sea Margin (SM)Power reserve to maintain a given ship speed,
allowing for weather, sea force, ageing and fouling of hull, as
well as roughening of propeller blades
Engine Margin (EM)Mechanical & thermodynamic power reserve
for economical operation of the engine, i.e. low fuel and
maintenance cost (also called operational margin OM)
Light Running Margin (LR)Margin in propeller revolutions with
new ship (i.e. under sea trial condition) to attain or maintain any
power up to 100% in future continuos service. Related to Sea
Margin
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Low-speed engine lay-out.ppt / TN
Light running and heavy running
Light running :- Same power is achieved at a higher rpmHeavy
running :- Same power is achieved at a lower rpm
Power
rpm
P
n1 n2
Heavy prop. curve
Light prop. curve
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Low-speed engine lay-out.ppt / TN
Sea margin & Light running margin
CSR
Engine/propeller speed
100%
Sea margin(10-25%)
Light running margin(3-7%)
Propeller curves
Sea trial condition New and clean ship Calm weather
Hull ageing and fouling
Service condition with fouled hull, wind and waves
resistance
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Low-speed engine lay-out.ppt / TN
Power versus ship speed in trial and service
Power
Vessel speed
CSR
Service Trial
Service condition
Sea trial condition
Difference between service speed and trial speed at CSR depends
on how much sea margin that has been selected
Sea margin
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Low-speed engine lay-out.ppt / TN
Light running margin
Example of light running margin of 5-6% to compensate for
expected future drop in revolution for constant power
1,5 - 2% influence from wind and weatherDifference between sea
trial (Beaufort 2) and average service condition (Beaufort 4-5)
1,5 - 2% influence from hull resistance and increased
wakeRippling of hull, fouling and increased hull roughness, change
in hull trim and immersion
1% frictional losses from increased propeller blade
roughnessSurface roughness increase from 12 to 40 microns
1% deterioration of engine efficiencyFouling of scavenge air
coolers and turbochargers, piston rings condition, fuel injection
system etc.
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Low-speed engine lay-out.ppt / TN
Load range starting point Rx with marginsEngine Power(% Rx)
100
100
Engine speed (% Rx)
90
76,5
10% engine margin (EM)
15% sea margin (SM)
Sea trial propeller curve
Nominal propeller curve
Rx
A
BD
5% light running margin (LR)
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Low-speed engine lay-out.ppt / TN
Load range limits - RTA engines - 1
100
100
Engine speed (% Rx)
Rx
A
BD
10495
95
70
45
Const. mepOperational range limit
Area reserved for acceleration and operational flexibilityIt is
recommended to operate below the nominal propeller curve through
the Rx point
Engine Power(% Rx)
90
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Low-speed engine lay-out.ppt / TN
Load range limits - RTA engines - 2
100
100
Engine speed (% Rx)
Rx
A
BD
108
110
93,8 103,2
Engine Power(% Rx)
Constant torque
Overload range limit>100% load only at shop test
90
Service range with operational time limitOnly to be used for
transient conditions i.e. fast acceleration
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Low-speed engine lay-out.ppt / TN
Load Range with shaft generator (PTO)Engine Power(% Rx)
100
100
Engine speed (% Rx)
85
72,3
Sea trial
Nominal propeller curves
Rx
A
BD
90D
15% sea margin
5% for shaft generator
10% engine margin
With PTO
Without PTO
Example with a shaft generator absorbing 5% of nominal engine
power
Constant speed operation range for shaft generator, typically
3,3%
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Low-speed engine lay-out.ppt / TN
FPP versus CPP layout
FPP
rpm
Power
rpm
PowerCPP
Outer non-controllable conditions moves load curve up/down.
Challenge: Correct design (P/D)
Adjustment of propeller pitch (P/D) moves load curve up/down.
Challenge: Correct operation (P/D-rpm)
Weather & age Captain
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Low-speed engine lay-out.ppt / TN
40 60 80 100 120 140 160 180Propeller speed [RPM]
Engine power [kW]5000 P/D=1.6 1.4 1.2
21 kn
1.0
19 kn
17 knP/D=0.8
15 kn210 g
195 g
187 g
183 g
181 g
0500
10001500
200025003000350040004500
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: combinator curve: nominal pr. curve: 100 % load curve
Cavitationarea
Over
load
Com
binat
or
Performance diagram - CP-propeller operation