The learning objective of this chapter is that the learner will realize that
not all of the quality issues are covered by the current fuel standard
we have looked at the contents of the iso eighty-one seven standards table for
both distillate and residual fuels in some detail we have also considered the
quality issues that affect marine fuels it should now be obvious to you that not
all of the quality issues are covered by these tables what is missing why are
some of the quality issues not covered we will now look at some of these
missing quality issues and see why they are not included in the tables fuel as
we have already defined is an energy source marine fuels are a source of
chemical energy which we can convert to mechanical energy through the combustion
process the amount of energy that can be released during combustion depends on
the chemical makeup of the fuel and the efficiency of the combustion process you
are now aware of the fact that fuel oil is a mixture of different hydrocarbon
compounds the actual content of hydrogen and carbon varies depending on the
molecular structure of these compounds you also now know that fuel contains
other elements such as sulfur and ash the basic unit for heat energy is the
Joule this is a very small amount of heat energy and so when we talk about
the heat released during combustion of a fuel we normally use the mega Joule
which is one million joules you may recall from an earlier chapter that the
distillate fuel table from the ISO eight to one seven standard contained a
parameter called calculated cetane index this was described as a key performance
indicator of the ignition quality of the fuel the time delay between the start of
fuel injection and the fuel igniting it was stated that this indicator was not
suitable for residual fuels you may have noticed that there was no ignition
quality indicator in the table for the residual fuels
if ignition quality is a key performance parameter then surely it should be there
in a table which defines fuel quality the reason it is not included in the
residual table is that it is difficult to reliably measure ignition quality due
to the mixture of compounds within the residual fuel there have been a number
of indicators devised to identify ignition quality of residual fuel two of
these are the calculated carbon aromatisse ax t index
or CCAI for short and the calculated ignition index or CII CCAI
is the indicator most often used the calculated carbon aromatisse ax t index
is obtained from an empirical formula which has been developed from
experimental data the formula relating the density and viscosity of the fuel is
as shown and usually gives values in the range 800 to 950 with values above 870
indicating problems with ignition the index has no units unfortunately the
results have limited value due to the fact that the viscosity and density
relationship can be affected by the chemical makeup of the fuel due to the
unreliability of the results it has not been accepted for inclusion in the
standard long ignition delays can lead to extremely high pressures in the
cylinder of diesel engines in certain circumstances this can happen due to the
almost instant combustion of the increased amount of fuel which will
accumulate in the cylinder during the delay period once the ignition occurs
the pressure rise of the cylinder is caused by the heat being released during
combustion this however depends not only on the ignition quality but also the
combustion quality of the fuel there is evidence to suggest that the ignition
delay is more of a problem with medium and high speed diesel engines due to the
relatively shorter time available for combustion in the cycle the combustion
quality of a fuel may be considered as the rate at which heat is released
during the combustion process the two main factors affecting this are the
chemical composition of the fuel and the condition of the fuel as it is injected
into the cylinder the fuel density and the micro carbon residue value of the
fuel are considered by some to give an indication of potential combustion
performance but this is not always the case
fuel with a high proportion of aromatic hydrocarbons or other high carbon
content compounds would have a high density and micro carbon residue value
and could be expected to present combustion difficulties there are
currently no reliable indicators for combustion quality and therefore it is
not included in the standard poor combustion quality can lead to heavy
fouling of the Bastien chamber and the exhaust system
with increased emissions and fuel consumption combustion can also be
affected by the amount and quality of combustion air available and the
condition of the engine cylinder there have been many instances of unexpected
problems either during storage or or when handling fuel oil these problems
are generally due to the fuel either not being stable or compatible if you have
not come across these two terms before then consider them for our purpose as
being stable when the components that make up the fuel remain as a homogenous
mixture during storage and handling and compatible when the fuel remains stable
when mixed with another fuel as we have already discussed residual fuels are
complex mixtures different fuels may also be mixed together or blended to
bring the parameter values in particular density and viscosity to within the
limits of the ISO eight to one seven standard blending should be done in a
controlled manner which results in a stable product where fuels are
potentially unstable then separation of the different components may occur over
a period of time due to high storage or treatment temperatures this may result
in either the heavier component of the blend settling out at the bottom of
storage or service tanks or heavy fouling of purifiers both resulting in
operating problems this problem can be more likely with high asphaltene content
in the fuel to stable fuels from different sources may become unstable
when mixed together in storage tanks this is due to incompatibility between
the fuels ideally new bunkers should be loaded into a clean tank and not mixed
with existing bunkers to avoid the risk of compatibility problems it is now
recognized as bad practice to mix fuels from different sources in storage and
this should be avoided whenever possible obviously as you will be aware it is
difficult to have completely empty and clean fuel tanks ready each time you
take bunkers however every effort should be made to have minimum residues in a
tank before filling with new bunkers the problem moving compatibility can be
linked to the asphaltene content of the fuel where there are high levels of
asphalt in the fuel then there is a greater risk
of compatibility problems you may recall from earlier discussion that some of the
unsaturated hydrocarbon content of residual fuel may be as 14 molecules
these are heavy complex structures which may account for between 4 and 12 percent
of the content of the fuel the actual quantity within the fuel depends on the
crude source and the refining processes employed in production of the fuel for
fuels which are inherently unstable high asphaltene content may lead to serious
smudging problems in both storage and treatment facilities onboard high
asphaltene content is also associated with poor combustion performance
measuring the asphaltene content would give an indication of sludge potential
but will not indicate directly if the fuel is stable it should be obvious that
when dealing with fuels which are naturally flammable there will be a
possible risk of explosion if proper care is not taken
although the fuel standard gives an indication of the flashpoint of the fuel
it does not indicate the range of flammability this is important when
considering the vapor space of a fuel tank or when the tank is empty there is
an upper flammable limit UFL and a lower flammable limits
LFL and a minimum oxygen limit the actual values do vary depending on the
exact nature of the fuel but are generally taken as 10% gas by volume for
the upper limit 1% gas by volume for the lower limit and approximately 11% oxygen
by volume for minimum oxygen bunker tanks are normally in an over rich
condition when in use care must be taken when ventilating empty fuel tanks before
entry as the atmosphere may pass through the flammable range during venting