Volvo D11 series, Installation Manual

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2
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
output 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.
Tolerances according to ISO standards are usually ±
5%, which is a reality that must be accepted for line
produced 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
engine 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 0 kPa. If all
engine manufacturers followed the same test proce-
dure it would be easier for a boat producer to com-
pare products from various suppliers.
Engine performance
Engine output is affected by a number of different
factors. Among the more essential are barometric
pressure, ambient temperature, humidity, fuel thermal
value, fuel temperature (not EDC engines) and back-
pressure. 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 per-
formance 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
power curve of the engine will cross at point B, mak-
ing 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.