Operation Manual

Diesel fuel flow sensor kit Installation and Operation Manual
19
NAVMAN
Most engine manufacturers publish graphs
of the specific fuel consumption for their
engines.
Specific fuel consumption is a graph showing
how much fuel the engine burns to produce
one horsepower (or kW) for one hour. Each
engine has a particular RPM range where it
burns the least amount of fuel to produce one
unit of power. Some engines are designed to
work most efficiently at high RPM, others at low
RPM. Modern lightweight diesels commonly
have a most economical and efficient speed
of operation at around 70 to 80% of maximum
rated RPM.
The specific fuel consumption curve is worked
out for the engine working at its rated load,
at the RPM settings along the bottom of the
graph.
As you can see from the discussion on engine
and propeller curves (see section 5-1), the only
point where the engine is working at its rated
load should be at maximum RPM. At any speed
less than the maximum RPM, the engine is not
working to its maximum, and often well below
its maximum. So the specific fuel consumption
curve is of limited use to get overall boat fuel
consumption. It will give you an indication of
whether your engine has been designed for
high or low speed efficiency.
Sometimes a manufacturer will also give a
curve of fuel consumption for that engine when
matched to the theoretical ideal propeller. This
will give you a reasonable estimate of the fuel
consumption of your engine, provided you have
a well chosen propeller. It will not, of course,
take into account different loads carried or sea
state, or any other factor that affects the actual
fuel consumption.
One rule of thumb you can use to estimate fuel
is that a modern, large, high efficiency diesel,
when averaged out over most of its operating
range, will produce 23hp for one hour from one
US gallon of fuel (3.33 kW for one hour out of 1
litre of fuel). That is quite a good engine.
Smaller, older, less efficient engines might sink
as low as only 19 horsepower for one hour from
one US gallon of fuel (3.11 kW for one hour
from 1 litre of fuel)
Over all the engines available in the pleasure
craft market there is very little difference in
the fuel burning rate, when averaged out over
their normal working range. Some engines
will have an optimum at lower revs, others at
higher revs.
The important thing to remember is that the
quantity of fuel you actually use is determined
by your propeller and hull much more than by
your particular choice of engine.
5-2 Specific fuel consumption curve