Volvo Penta KAMD300, Installation Manual

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12
Marine engine environment
Power losses due to atmospheric conditions
Losses due to large propeller
Critical
area
Rated
rpm
rpm
A
Power
B
C
The marine engine and its environment
Marine engines, like engines for cars and trucks, are
rated according to one or more power norms. The out-
put is indicated in kW, usually at maximum engine
speed.
Most engines will produce their rated power provided
they have been tested under the conditions specified
by the power norm and have been properly run in. Tol-
erances according to ISO standards are usually ± 5%,
which is a reality that must be accepted for line pro-
duced engines.
Measuring output
Engine manufacturers normally assign an engine’s
output to the flywheel, but before the power reaches
the propeller, losses occur in the transmission and in
the propeller shaft bearings. The amounts of these
losses are 4-6%.
All major marine engine manufacturers indicate en-
gine power according to ISO 8665 (supplement to ISO
3046 for leisure boats), based on ISO 3046, which
means that the propeller shaft power will be given. If
an exhaust system is optional, engine tests are
conducted with a backpressure of 10 kPa. If all engine
manufacturers followed the same test procedure it
would be easier for a boat producer to compare prod-
ucts from various suppliers
Engine performance
Engine output is affected by a number of different fac-
tors. Among the more essential are barometric pres-
sure, ambient temperature, humidity, fuel thermal val-
ue, fuel temperature (not EDC engines) and backpres-
sure. Deviation from normal values affects diesel and
petrol engines differently.
Diesel engines use a large amount of air for combus-
tion. If the mass flow of the air is reduced, the first
sign is an increase in black smoke. The effect of this
is especially noticeable at planing threshold speed,
where the engine must produce maximum torque.
If the deviation from normal mass flow is substantial,
even a diesel engine will lose power. In the worse
case the reduction could be so large that the torque is
not sufficient to overcome the planing threshold.
The above figure illustrates the consequences of climate
variation.
Point A is where rated power from the engine is equal
with the power absorbed by the propeller. Selection of
the propeller size at this point is correctly located for
utilising max. rated power at a certain weather and
load condition.
If atmospheric conditions cause the power to drop to
point B, the propeller curve will cross the output curve
from the engine at point C. A secondary performance
loss has occurred because the propeller is too large.
The propeller reduces the rpm from the engine.
By replacing the propeller with a smaller one, the pow-
er curve of the engine will cross at point B, making it
possible to regain previous rpm, but at reduced power.
For planing or semi-planing boats, the planing thresh-
old (“hump” speed), which mostly occurs at 50 - 60%
of max. speed, is the critical area. In this section it is
important that the distance between the engine max.
power curve and the propeller curve is large enough.