• This fuel is not suitable for burning directly in the
diesel engine because it has some solids and
water as impurity, which may cause damage to
the engine parts and also has a very high
viscosity, which makes it difficult for atomization
of fuel in the combustion process. To make this
fuel suitable for burning, it has to go through a
conditioning process consisting of settling,
centrifuging, boosting of pressure, filtering and
heating.
Fuel Oil Systems
Heavy residual fuel consists of
residues left after lighter and costlier
distillates fuels and gases are removed
from petroleum crude oil in an oil
refinery. Marine diesel engines are
designed to burn heavy residual fuel
blended with distillate gasoil to meet
the specification of fuel oil ordered,
especially viscosity and density. This is
popularly known as “heavy fuel oil”.
Fuel oil supply system of a ship.
Specification Data for Fuel Oil
• (a) Density: It is the relationship between mass
and volume at 15C and is measured by
hydrometer. This value changes with
temperature, depending upon the coefficient of
expansion of the substance. For marine fuels the
values are 800-1010 kg/m3.Knowledge of
density is needed for quantity calculations and to
select the optimum size of gravity disc for
purifiers. 991 kg/m3 is the accepted limit for
normal centrifugal purification and 1010 kg/m3 in
ALCAP purifier.
• As already mentioned density is the ratio of the
mass of a substance to its volume, but not its
weight to volume ratio and therefore, density by
definition is in vacuo. The term ‘density in air”,
although often used, is incorrect and should be
referred to as “weight factor”. This is due to the
fact that a substance weighed in air is
supported, to a small extent, by the buoyancy of
the air acting on it. In effect therefore, the weight
of a liquid in air is slightly less than its weight in
vacuo.
• Viscosity : Viscosity can be termed as resistance to
flow. Viscosity is measured in a viscosimeter. The
kinematic viscosity is obtained by dividing dynamic
viscosity by density of the fluid and its unit of
measurement is stoke or centistoke and is quoted with a
reference temperature. For distillate fuels the reference
temperature is 40C and for residual fuels the reference
temperature is usually 50C or 100C. Each fuel has its
own temperature viscosity relationship and although oil
suppliers publish temperature/viscosity charts, it should
be understood that these charts are based on average
data of large number of representative fuels. Precise
relationship would depend upon crude oil source and
refining process. In general, for lower viscosity fuels the
difference is small, but it becomes wider as viscosity of
the fuel increases. A knowledge of viscosity is necessary
for the determination of the heating required for a fuel for
transfer purpose and the temperature range required for
satisfactory injection and combustion at the fuel
atomiser.
• In order to ensure efficient atomization of the charge,
when burning residual fuels it is essential to inject the
fuel at the most suitable viscosity. Despite wide
differences in engine and fuel system designs there is
considerable agreement that the most suitable viscosity
of the fuel leaving the injector nozzle lies between 12.5 18.0 cSt. In well designed systems, the viscosity is
controlled automatically within fairly close limits by
means of viscosity controllers.
• Pressure/viscosity characteristics : The viscosity of
hydrocarbon oils increases under pressure. The very
high fuel injection pressures now employed will increase
the fuel viscosity markedly. This should be allowed for
when preheating the fuel.
• Cloud and pour points : The cloud point
of a distillate fuel is the temperature at
which wax starts to crystallise out, and this
is seen when the clear fuel becomes
opaque. For marine fuels this
characteristic is only applicable to some
light grades.
• The pour point of an oil is the lowest
temperature at which the oil remains fluid.
It is determined by cooling the oil in a test
tube having a diameter of approx 30 mm.
The pour point is 3C higher than the
temperature at which the glass can be
held in the horizontal position for 5 second
without any visible signs of movement of
the oil surface.(Solidifying temperature is
3C below pour point). The pour point
result will give guidance regarding the
lowest temperature at which a fuel may be
stored. If fuels are held at temperatures
below pour point, wax will begin to
separate out.
• This wax may cause blocking of filters and can
deposit on heat exchangers. In severe cases the
wax will build up in storage tank bottom and on
heating coils, which can restrict the coils from
heating the fuel. When dealing with heavy
marine fuels, both the pour point and the
viscosity of the fuel need to be considered, if the
fuel is to be maintained at a temperature to
prevent wax formation and allow pumping. For
efficient pumping the viscosity of the fuel should
not be above approximately 600 cSt. If the
suction line from the pump to the tank is very
long the viscosity should be lower.
• Flash Point: The flash point of a fuel is the lowest
temperature at which sufficient vapour is given off to
produce a flash on application of flame under specified
test condition. The flash point may be measured as a
closed or open cup figure and for marine fuels the closed
cup figure is used. The test method uses the PenskyMarten apparatus. The minimum flash point for fuel in
the machinery space of a merchant ship is 60C. For
fuels used for emergency purposes, external to the
machinery space, for example the lifeboats, the flash
point must be greater than 43C. The purpose of defining
a minimum flash point is to minimise fire risk during
normal storage and handling. The general rule is that
fuels should not be heated above 10C below the flash
point, unless specific requirements are met. (Solas
Chapter II-2, Regulation 15)
• Fire Point : It is the lowest temperature at which
vapour is generated at a rate sufficient to sustain
combustion for 5 second. The same equipment
which is used for determining flash point is used
for this test also.
• (f) Auto-ignition temperature or Self ignition
temperature : It is the lowest temperature at
which the generated vapour will ignite
spontaneously without any source of ignition.
• Calorific Value or Heat of Combustion or
Specific Energy : Heat of combustion of a fuel
is the amount of heat released during
combustion of a unit mass under following
circumstances:
• (a) The temperature of fuel before combustion
and that of the combustion products after
combustion is 20C.
• (b) The combustion products from carbon and
sulphur are solely gaseous carbon dioxide and
sulphur dioxide and no oxidation of nitrogen has
occurred.
• In gross heat of combustion, the water existing before
combustion as well as the water generated by the
combustion process is to be found in the combustion
products in liquid state. In net heat of combustion the
above mentioned water is to be found in the form of
vapour at 20C. The gross heat of combustion can be
determined by Berthlot-Mahler calorimeter. The net heat
of combustion(hi) is calculated if the gross heat of
combustion(hs) is known.
hi = hs - 25 (f+w) kJ/kg
where water content of fuel is f% by mass, and that w%
by mass of water is generated by combustion of
hydrogen in fuel.
• Heat of combustion can be calculated with a degree of
accuracy sufficient for normal purposes from the density
of the fuel and the application of corrections for any
sulphur, water and ash that are present. On a world-wide
basis the heat of combustion does vary slightly,
depending mainly on density and sulphur content of the
fuel.
• Water - Normally the water content in the
fuel oil is very low and 0.1-0.2% by volume
is typical. Ingress of water can come from
tank condensation, tank leakage and
heating coil leakage. Water is normally
removed by gravitational separation in fuel
oil tanks and centrifugal purification
system.
• Ash : Nickel, Aluminium, Silicon, Sodium and
Vanadium
• The ash content is defined as the residue left after all the
combustible components of the oil has been burnt. In
distillate fuel this quantity is negligible. The ash
constituents are concentrated in residual fuels. The ash
consists generally of oxides and/or sulphates of nickel,
aluminium, silicon, sodium and vanadium. The sources
of these are (a) inorganic material naturally present in
the crude oil, (b) Catalytic fines picked during refining
process ( Catalytic fines are particles arising from the
catalytic cracking process in the refinery and are in the
form of complex alumino-silicates) (c) Contamination by
sand, dirt, rust scale and sea water subsequent to
refining process.
• Sodium and Vanadium - Fuels leaving refinery have
sodium level below 50 mg/kg. If contaminated with
sea water subsequently, sodium level will increase. A
1% sea water contamination represents a potential
100 mg/kg increase. Normally sea water can be
removed by gravitation separation in settling tank
and centrifugal separation. Vanadium is present in
all crude oils in an oil soluble form and the levels
found in residual fuels depends mainly on the crude
oil source, with those from Venezuela and Mexico
having the highest levels. The actual level is also
related to the concentrating effect of the refining
processes used in the production of the residual
fuel. There is no economic process for removing
vanadium from either the crude oil or residue.
• During combustion of the fuel, vanadium and
sodium constituents form a mixture of sodium
sulphate and vanadium pentoxide. This mixture has
a low melting point (approx 500-600C)
corresponding to the temperature of the exhaust
valve seating. The semi-fluid particles of ash adhere
firmly to the surfaces they touch, gradually forming a
very hard, thin layer of slag which, after having
reached a certain thickness, allows the hot
combustion gases to leak out, the result being that
the slag melts forming a narrow channel. If the layer
of slag is of sufficient thickness, the channel grows
and the combustion gases heat up the seating
material, causing what is known as high-temperature
oxidation, which in turn results in the seating
material melting in the vicinity of the channel. The
most critical sodium to vanadium ratio is about 1 to
3.
• Silicon and aluminium :Silicon may be present in the fuel
in form of sand and aluminium may also be present in
very small quantities, having been picked up by the
crude oil in sub-surface rocks. However presence of
aluminium and silicon is mainly due to catalytic fines
discussed earlier. Catalyst is an expensive material for
the oil refiner and stringent methods are taken for its
retention but some still find their way in residual fuel.
Excessive catalytic fines can lead to high wear of piston
rings and liners, fuel pump barrels and plungers, and fuel
injector nozzle needle and guide. The level of catalytic
fines in delivered fuels can be significantly reduced by
efficient centrifugal purification prior to combustion in the
engine.
• Carbon Residue: The carbon residue of a fuel
is the tendency to form carbon deposits under
high temperature conditions in an inert
atmosphere, and may be expressed as either
Ramsbottom carbon residue, Conradson carbon
residue (CCR) or micro carbon residue (MCR).
This parameter is considered by some to give an
approximate indication of the
combustibility/deposit forming tendency of the
fuel.
• Sulphur: Sulphur is naturally occurring element
in crude oil which is concentrated in the residual
component. The amount of sulphur in fuel oil
depends mainly on the source of crude oil and to
a lesser extent on the refining process. Sulphur
content is typically 1.5-4% wt in residual fuel
world wide. In the combustion process in a
diesel engine the presence of sulphur in the fuel
can give rise to corrosive wear. This can be
minimised by suitable operating conditions, and
suitable lubrication of the cylinder liner with
alkaline lubricant. MARPOL Annex VI limits the
sulphur content of marine oil to reduce
atmospheric pollution, in the form of sulphur
dioxide, from international shipping.
Amendment to Marpol Annex VI
• Regulation 14 of MARPOL Annex VI has
been significantly revised. For the Global
Cap, the sulphur content limits are as
follows:
• 4.5% prior to 1 January 2012
• 3.50% on and after 1 January 2012
• 0.50% on and after 1 January 2020
Special Emission Control Areas
• For the Special Emission Control Areas,
the sulphur content will be as follows:
• 1.50% prior to 1 March 2010
• 1.00% on and after 1 March 2010
• 0.10% on and after 1 January 2015
• The existing Emission Control Areas
(ECAs) are the North Sea and the Baltic
Sea.
Review provision
• The amended Regulation 14 has a “review
provision” which requires the IMO to
complete by 2018 a review of the
availability of the 0.50% sulphur content
fuel. Based on the results of such a
review, the Parties to MARPOL Annex VI
will decide whether the global cap of
0.50% can be enforced from 1 January
2020. If not, the 0.50% sulphur global cap
will be enforced on 1 January 2025 without
any additional review.
• Ignition Quality : Cetane number - The cetane
number for any fuel is a measure of the oil’s
readiness to ignite, under conditions prevailing
in the diesel engine. This number is determined
by comparing the oil with a mixture of cetane
and heptamethylnanone. Cetane, which has a
very high spontaneous combustion ability is
rated at 100 and the corresponding cetane
number for heptamethylnonane is 15. The oil for
which cetane number is to be determined is
used as fuel in a so-called CFR (Co-operation
Fuel Research) engine, which is a single
cylinder diesel engine with a variable and
controllable compression ratio.
• Fuel injection and combustion timing are controlled by
electronic equipment. When these have been
determined then engine is run with different mixtures of
of cetane and heptamethylnanone until a mixture gives
same results. Cetane number = a -0.15 x b, where a is
the volume % of cetane and b the volume % of
heptamethylnanone. For high speed diesel engines, a
cetane number of over 50 is desirable. Medium speed
diesel engines require a fuel with a cetane number of
around 40-50. Large, slow speed diesel engines operate
satisfactorily with fuels having a cetane number of
approx 30. However slow speed engines are said to be
not so sensitive to with regard to the cetane number and
it is not normally specified for these engine types.
• (b) Calculated Ignition Index (CII) and Calculated Carbon
Aromatic Index(CCAI) :These are calculated by
empirical equations ,where use is made of the density
and viscosity of the residual fuel.
Standards of Fuels - Need for
quality control in bunker fuel
• The cost of bunker fuel is one of the most
significant components of a ships operating cost.
Ship owners in their effort to limit this cost have
preferentially turned to the use of heavier and
thus less expensive bunker fuels. Technology
developments in petroleum refining, such as in
vacuum distillation, catalytic cracking etc, often
result in a deterioration of the characteristics of
heavy fuels as lesser volumes of residues are
left after petroleum refining.
• These residuals may contain elevated levels of
undesirable constituents such as Aluminium and
Silicon, compounds that could result to
significant engine wear and damage. In addition
to the above the supply of marine bunker fuels is
nowadays often the result of a complex
sequence of buying, selling and mixing of fuels
of different origins. The use of poor quality fuel is
known to result to the serious damage of boilers,
fuel pumps springs, pistons and cylinders.
ISO 8217
• During 2005 and 2006 the set of regulations included in
MARPOL Annex VI that relate to the use of the marine
bunker fuels came into effect. The sampling of the
bunkered fuels became mandatory, following a detailed
list of requirements listed in the above document and in
the MEPC.96(47) IMO document.
• To obviate dispute between ship owners and bunker
suppliers and also to meet MARPOL Annex VI
requirements, International Organisation for Standards
published the first edition of International fuel
specification ISO 8217 known as “Petroleum products Fuels (class F) - Specifications of marine fuels” in 1987.
It was revised in 1996 and again in 2005. ISO 8217-2005
defines four distillate grades (DMX, DMA, DMB, DMC)
and ten residual grades.
• Distillate grades remain same and the main changes are
in marine residual fuels:
• Reduction of residual fuel grades from 15 to 10 - With
the viscosity classification of residual fuel grades being
measured at 50 °C (instead of 100°C as under ISO
8217-1996), the names of the 10 residual fuel grades
have been changed as follows – RMA30, RMB30,
RMD80, RME180, RMF180, RMG380RMH380,
RMK380, RMH700 AND RMK700.
• Maximum sulphur limit reduced to 4.5% - for all the
residual fuel grades with viscosity higher than that of
RMD 80. For RMA, RMB and RMD grades the previous
lower sulphur limits have been retained.
• Fuel to be free of ULO (used lubricating oil)
• Reduced water content - from 1.0%v to 0.5%v.
• Reduced ash content
CATEGORY ISO - F
LIMI
T
CHARACTERISTIC
DM
X
DM
A
DMB
DMC(a)
Density at 15°C
(Kg/m3)
max.
---
890,0
900,0
920,0
Viscosity at 40°C
(mm2/s
b)
min.
max.
1,40
5,50
1,50
6,00
--11,0
--14,0
Flash Point
(°C)
min.
43
60
60
60
Sulfur
(% m/m)
max.
1,00
1,50
2,00
(e)
2,00 (e)
min.
45
40
35
---
Cetane index
Carbon residue
(%m/m)
max.
---
---
0,30
2,50
Carbon res. on 10% (V/V) distillation
bottoms
(% m/m)
max.
0,30
0,30
---
---
Ash
(% m/m)
max.
0,01
0,01
0,01
0,05
Clear and
bright
(f)
---
Appearance (f)
Total Sediment Existent
(% m/m)
max.
---
---
0,10
(f)
0,10
Water
(% V/V)
max.
---
---
0,3 (f)
0,3
Vanadium
(mg/kg)
max.
---
---
---
100
Aluminum plus silicon
(mg/kg)
max.
---
---
---
25
Aluminum plus silicon
(mg/kg)
max.
---
---
---
25
_____
The fuel shall be free of ULO
(g) 15
15
30
Used lubricating oil (ULO)
1.Zinc
2.Phosphorus
3.Calcium
mg/kg
max.
____
_
____
_
• Fuel Testing: Analysis of particular
characteristics of the fuel delivered may be
carried out by some independent shore based
laboratory or by tests carried out on board.
Testing of fuel on board may range from one or
two tests to fully automated online monitors
where direct read out of viscosity, density and
elemental analysis (e.g. sulphur, silicon,
vanadium) as well as derived parameters such
as ‘ignition index’ expressed as CII or CCAI are
available.
• Storage and Transfer : The pump for fuel transfer is of
the positive displacement type and are usually of screw
or gear design. The temperature of fuel in the storage
should be maintained 5C above its pour point otherwise
there is a possibility of wax formation and in case of high
wax content, if left to cool, it may be difficult to reheat the
fuel to a temperature above the pour point. Also the
temperature has to be raised for higher viscosity fuel to
45C to bring it below 500 cSt for pumping it. Fuel oil is
heated in storage tanks by low pressure steam, but in
some ships thermal fluid heating is used.
STORAGE
• Bunkering: Marine heavy fuel oils are blends of
viscous residues from various refinery operations,
cut back with distillate cutter stock. The growing
trend is towards cracked residues of a highly
aromatic/asphaltic nature to be cut back to an
acceptable viscosity with cracked aromatic
distillates. Both components have a high
carbon/hydrogen ratio, cracked distillates having
good solvency properties for large-asphaltene
hydrocarbons. In a stable fuel the asphaltenes are
carried in a colloidal dispersion in the lighter phase.
If the equilibrium between the two phases is
disturbed the asphaltene will agglomerate to a size
which can no longer be maintained in suspension,
and they will tend to separate out as ‘sludge’. If
sludge deposition does occur this is made worse,
not better, by the addition of more distillate,
• This is particularly true if a high-quality straight-run
paraffinic distillate is added to a cracked, high
asphaltenic content, residual fuel. It is possible that
two residual fuels, each stable by themselves, when
mixed together can prove to be incompatible and
throw down objectionable sludge or sediment. If
compatibility tests have not been carried out
beforehand, when bunkering, every effort should be
made to segregate bunkers from different source in
different tanks to avoid potential problems of
incompatibility. In such a case an unstable blend
may occur in the ship’s tanks, which could result in
precipitation of asphaltenic deposits as sludge in the
tanks, pipes, filters and centrifuges.
TREATMENT OF FUEL OIL
• Before the fuel is burnt in diesel engine or a
boiler, a shipboard treatment takes place.
Distillate fuels are generally filtered through a
coalescer type filter to remove water and solid
impurities. For boilers burning residual fuels, in
addition to settling tanks, cold and hot filters are
installed in the system prior to boiler. In case of
diesel engines burning residual fuel, in addition
to settling tanks and filters, centrifuges are
installed to clean the fuel to take account of the
fine clearances which exist in fuel system of
diesel engine.
• Treatment of High Density Fuel: As the density
of fuel oil increases and exceeds 991kg/m3, the
density difference between the fuel oil and fresh
water is so small that any change in oil
temperature, viscosity or flow rate will cause the
oil/water interface to fluctuate leading to a
potential failure of water seal. For residual fuel
having density above 991 kg/m3, alternative
arrangements to traditional purifier are used.
One such arrangement called ALCAP system is
used, where fuels of density upto 1010 kg/m3
can be treated. The centrifuge operates as a
clarifier and clean oil is continuously discharged
from clean oil outlet, and any free water and
separated sludge accumulate at the periphery of
the bowl.
• When the sludge space is filled up, the
separated water approaches the disc and traces
water start to escape with clean oil. Increased
water content in clean oil is sensed by the water
transducer in the clean oil outlet side. The
electrical signal from water transducer are
continuously transmitted to and interpreted by
the control unit. When the water content in clean
oil reaches a specific ‘trigger’ point, the control
unit determines, based on the time elapsed
since the last sludge sequence, which of the two
methods it will use to empty the bowl. This can
either be through a water drain valve or with the
sludge through the sludge ports at the periphery
of the bowl.
• Fuel Heating : Residual fuels have to be heated
to reduce the viscosity to that required for
atomisation. In case of boilers this is in the range
of 15-65 cSt, whilst for diesel engines the
injection viscosity is usually 12.5 -18 cSt. Fuel
heaters may be operated by low pressure
saturated steam, a thermal fluid or electrical
elements. It is important to maintain correct
viscosity range under all conditions. Local
overheating may cause cracking of fuel, which
may lay down deposits on the heating surface,
impairing efficient operation of the heater.
• Viscosity Controller : A viscosity controller
is often installed downstream of a fuel oil
heater so that a constant injection
viscosity can be maintained. There are
various types of these. One of these
measures the differential pressure
resulting from laminar flow through a
capillary tube and compares this value to a
set point, generating a signal to control the
temperature of fuel oil heater.
• Additives :There are two types of additives: (a) Which
reduce problem in pre-combustion phase
• (b)Which react during post combustion phase
• (a)With normal fuel handling procedures, with respect to
correct heating, and avoidance of mixing of fuels from
different bunkering, no problem should occur. In the
event of problems, an effective additive should
contribute as follows :
• (1) Dispersion of possible sludge in fuel oil tanks.
• (2)Promotion of separation of any dispersed water.
• (3) Prevention of sludge formation.
• (b)An additive which has the effect of an ash modifier
(ability to raise the melting temperature of ash) may be
beneficial. Slagging and high temperature corrosion
occurs when molten ash adheres to the metal surface.
By increasing the melting temperature the ash is not in
molten form and less likely to stick to metal surfaces.
Combustion In Diesel Engine
• For combustion of fuel in a diesel engine, the air charge
is highly compressed to a temperature well above the
spontaneous ignition temperature (SIT) of the fuel. As
the piston approaches TDC fuel is injected at high
pressure and suitable viscosity. This continues for 14-28
degrees of crankshaft rotation, depending upon engine
speed and design. The fuel passes through the following
phases :
1. A delay period between the commencement of injection
of the very finely divided fuel droplets and the
commencement of ignition.
2. Rapid combustion of the fuel accumulated in the cylinder
during the initial delay period, accompanied by a rise in
pressure.
3. Steady combustion of the remainder of the fuel charge
as it is mixed.
4. An after burning period during which remaining unburnt
fuel finds oxygen and combustion is completed.
Factors Influencing ignition
1. Exactly when ignition commences is dependent upon
several factors, the most important being:
2. The size of the droplets injected into the cylinder;
3. The pressure of the fuel at the injector tip;
4. The velocity of the droplets entering the dense air mass;
5. The air pressure and temperature in the cylinder;
6. The air turbulence in the cylinder;
7. The ignition delay properties of the fuel;
8. The surface tension of the fuel;
9. The chemical composition of the fuel;
10. The engine design.
Droplet formation and size
• The size of the droplets in the injected fuel spray is
controlled primarily by the size, shape and number of
holes in the injector tip, their position and the fuel
injection pressure and the viscosity of the fuel leaving
the injector. The higher the viscosity, the larger will be
the droplet size. As the fuel leaves the small injector
orifices at pressures in modern pressure-charged
engines of upto 1500 bar the pressure falls sharply as it
enters the cylinder, in which the charge-air pressure is
much lower. The pressure energy is converted into
kinetic energy, so that there is a sharp rise in velocity.
Both the fall in pressure and the shearing action as the
fuel passes through the dense air charge at high velocity
break up the liquid stream, while its viscosity and surface
tension form the mechanically disrupted liquid into small
droplets.
• The droplets sprayed into the cylinder are of varying
sizes; the higher the injection pressure, the higher the
percentage of small droplets. With current trend towards
much higher injection pressures fuel droplet sizes will be
reduced correspondingly. The droplet size decreases as
the compression pressure increases. The increased
density of the air charge helps to break up the spray into
smaller droplets. This is beneficial, as the smaller the
droplets, the quicker they will vaporize as there is a
greater overall area of the oil charge exposed to the hot
compressed air. This reduces the ignition delay period,
measured either in milliseconds or degrees of crank
angle.
Importance of high fuel pressure
• If the droplets leaving the injector have a diameter of
about 20-40m, there is minimum delay in combustion.
Conversely, if the droplet diameter exceeds some 100120m, the combustion period is so long that even a
slow-speed, two-stroke engine runs the risk of some
particles remaining unburnt when the exhaust ports or
valves open. Below 20m droplet size there is
insufficient kinetic energy in the tiny droplet to penetrate
the dense air mass in the cylinder, resulting in poor
fuel/air admixture. In order to ensure the required fine
droplet size, an injection pressure exceeding 1200 bar is
being used by some engine manufacturers.
The effect of air temperature
• The temperature of the air compressed in the cylinder
has a major effect upon ignition delay. The higher the
temperature, the shorter the delay period, everything
else being equal. Several factors determine the air
compression temperature, the main ones being the
engine compression pressure, which, in turn, is
determined by the charge air pressure, the compression
ratio and the volumetric clearance, the temperature of
the induction air entering the cylinder, and the
temperature of the cylinder head, liner and piston crown.
In turn, the combustion chamber and piston
temperatures are controlled by the temperature of the
cooling water, or oil, and by the design of the combustion
chamber and piston components. Compression
temperatures in normally aspirated engines are in the
order of 500-600C, but in modern highly pressurecharged medium and large output engines, they may be
as high as 700C.
Compression Pressure
• Increased compression pressure (or
densities), which are now as high as 90110 bar in modern crosshead and trunk
piston engines, not only promote the
formation of more, smaller fuel droplets
but, equally important, reduce the
spontaneous ignition temperature of the
fuel appreciably.
Air Turbulence
• Turbulence or swirl, in the compressed air
charge promotes efficient distribution of
the fuel spray droplets throughout the
combustion chamber, ensuring thorough
mixing of the fuel and clean air (increasing
the rate of heating and vaporization) thus
tending to reduce ignition delay and
assisting in efficient burning of the fuel
charge.
Droplet combustion process
• If an individual fuel droplet is considered it will be found
to be very small, the size depending upon factors
discussed previously but, as compared with the physical
size of individual hydrocarbon molecules which form the
droplet, they are relatively large. Even the smallest
droplets in the fuel spray contain thousands of
hydrocarbon molecules having widely different chemical
structures. This is particularly true of heavy residual fuels
with high carbon-numbers. The molecules vary
appreciably in their volatility, ignition temperature, rate of
burning, the completeness of burning and their tendency
to release carbon and associated organometallic
compounds. The heating of the spherical droplet occurs
from the outer surface inwards to the centre, so that
evaporation and subsequent ignition commences at the
surface. The more volatile constituents with the lowest
ignition temperatures burn first, leaving the less
combustible hydrocarbon constituents to find clean air
and burn slowly.
Advanced injection timing
• During a long ignition delay, injection of fuel into the
cylinder continues, so that the longer the delay, the
greater is the amount injected before ignition
commences. When ignition finally occurs, the
accumulated fuel ignites violently with a very rapid, highpressure rise. The resultant high pressure causes shock
loading on the piston and running-gear bearings. With a
poor equivalent Cetane Number residual fuel, within
fairly narrow limits one way of reducing this harmful
effect is to advance the ignition timing. In case of lowspeed crosshead engines, upto 2 degrees crank angle
may be adequate, with a somewhat greater advance for
medium-speed engines - possibly 3 to 6 degrees,
depending upon engine design and, in particular, engine
speed. Advancing the injection timing enables ignition to
occur at maximum compression pressure and
temperature and smooth combustion to be completed
earlier in the stroke. The manufacturer's maximum firing
pressure, related to load conditions, should be
maintained.
Injector recirculation
From DO
Tank
Supply
Pumps