The learning objective of this chapter is to introduce the learner to alternate
fuel systems for marine diesel engines
in the previous chapter we have looked at the conventional fuel systems for
diesel engines and considered some of the more common variations that have
resolved such as variable injection timing now we
will have a brief look at some of the other variations that you may come
across on the vessels that you say long we will not go into detail about these
systems only give an overview you should study the manufacturers instruction book
for details of the actual systems when you sail with them many auxilary engines
are designed to operate on fuels other than clean distillate fuels residual
fuels are however not always suitable particularly when running on low load
due to ignition and combustion quality issues a number of inline fuel blending
systems have been developed which allow diesel engines to operate on a mixture
of distillate and residual fuel while avoiding the storage problems sometimes
encountered with fuel mixtures the temperature controller controls the
supply of steam to the heater to maintain a constant temperature
independent of the viscosity or blend ratio the viscosity controller output is
used to control the speed of the distillate supply pump to maintain
constant viscosity by varying the ratio of the two fuels to be blended if the
viscosity rises the pump speed is increased to earn more distillate and if
it falls the pump speed is reduced unless distillate is added another
method of control is to give the blend component ratio a load dependent preset
value which varies throughout the load range at any given load the blend ratio
is fixed and the viscosity is maintained at a preset value by varying the
temperature of the mixture the temperature would be low for blends with
a high percentage of distillate when the engine operates on low load and high
when it operates on fine loads with only a small percentage of distillate in the
blend in line blending can also be used to control the fuels sulphur content by
mixing high and low sulphur fuels to obtain the desired sulphur content of
the blended product in order to overcome ignition delay
problems with fuels that also have poor combustion quality that may result in a
low and late peak pressure some systems are designed with pilot injection in
these systems a small amount of fuel the pilot is injected earlier than normal in
the cycle and before the main fuel quantity this allows the pilot fuel more
time to heat up to auto ignition temperature once ignition takes place
and a flame is established the cylinder temperature rises and it is easier to
ignite and combust the main fuel charge with minimum ignition delay most pilot
injection systems use an electronically controlled single fuel injector with two
distinct injection periods pilot injection is also used for dual fuel
diesel engines and is described later in this chapter
fuel injectors operate at the optimum at a particular load of fuel setting as the
load increases or decreases about the optimum point then injector performance
will fall off in twin injection systems one of the injectors operates at full
capacity at all engine loads giving an amount of fuel just less than that
required for minimum load running the fuel quantity passing through the second
injector is varied depending on the load on the engine the fixed fuel quantity is
injected into the cylinder by the first injector followed by the varied quantity
from the second injector the system is sometimes considered as a form of the
pilot injection system as it has two injection periods however it operates
with normal fuel timing and optimized injector performance from the first
injector rather than early injection of the pilot fuel to improve ignition and
combustion
the rapid increase in LNG trade by sea has brought with it an urgency to
develop dual fuel systems for diesel engines which allow the boil off gas
from the cargo to be used as fuel these systems have now been introduced and
there are an increasing number of dual fueled diesel engines now in operation
the dual fuel system is electronically controlled with the facility to operate
on either gas from the cargo boil off or fuel oil inputs from the engine for
speed power combustion quality and the air fuel ratio are fed to the engine
control unit the output from this unit controls the fuelling rate to give
optimum conditions including efficiency and exhaust emissions in gas mode the
combustion process is initiated by injecting a light fuel oil as a pilot
fuel followed by the gas which is admitted to the air inlet in fuel mode
the fuel system operates as a conventional system with electronic
control the dual fuel injection valve serves for pilot fuel in the gas mode
and as the main fuel valve in the fuel oil mode
the use of fuel emulsions have been in some recent engine designs has one
method of reducing the NOx level in exhaust emissions NOx production from a
diesel engine is dependent to a large degree on the maximum cycle temperature
and the length of time that it is maintained during combustion by
introducing water into the combustion process the maximum temperatures are
reduced so reducing the amount of NOx produced this however can reduce engine
efficiency and lead to an increase in specific fuel consumption fuel emulsions
can also reduce the effectiveness of cylinder lubrication one side effect of
using emotions with very poor quality heavy fuel oils is that the water in the
emulsion flashes off into steam in the engine cylinder the rapid increase in
volume during this change of state can aid atomization and improve combustion
efficiency we have seen in this chapter that developments in diesel engine fuel
systems are usually driven by commercial pressures the quality of the fuel and/or
by environmental concerns we have looked briefly at a number of developments all
of which you may come across on vessels that you sail on in the future you
should always use the engine manuals provided on board to get specific
details about the systems that you sail with try the questions in the following
self-test to check your understanding so far