Post on 17-Nov-2015
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IC Engine Fuels
Their Properties and Tests
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Fuels for IC Engines
The main fuel for IC engines is derived from crude petroleum which is a mixture of many hydrocarbon compounds
Crude oil is made up almost entirely of carbon and hydrogen with some traces of other species
It varies from 83% to 87% carbon and 11% to 14% hydrogen by weight
The crude oil mixture which is taken from the ground is separated into component products by cracking and/or distillation using thermal or catalytic methods
It is entirely possible, but not economically feasible, to convert almost completely the crude petroleum into gasoline and diesel fuel
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Fuels for IC Engines
Fuels for IC Engines
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A: No. 2D diesel fuel (good grade)
B: No. 1D light distillate (high-
speed diesel engine and JP-5 jet
fuel)
C: Kerosene
D: JP-4 jet fuel
E: Automotive gasoline
F: Aviation gasoline
Families of IC Engine Fuels
The Paraffin family (Alkanes) open-chain structure
(aliphatic hydrocarbons) with general formula CnH2n+2 The critical compression ratio (CR) for audible knock in an
SI engine decreases rapidly as the length of the chain of
the normal members is increased
Thus the normal paraffins in the volatility range of gasoline
are poor SI fuels
The reference scale to measure SI knock has been
established by arbitrarily selecting two primary reference
fuels iso-octane (2,2,4 trimethyl pentane) has been
arbitrarily assigned an "octane rating" of 100 while n-
heptane has been assigned an "octane rating" of zero
Families of IC Engine Fuels
The knock ratings of the fuels are in rough proportion to
the self-ignition temperatures
In progressing downward with higher carbons, the
suitability of the fuels for SIE progressively decreases,
and for CI engines, progressively increases thus,
hexadecane (cetane) has a low self-ignition temperature
and therefore it is a good fuel to prevent knock in a CIE
The reference scale for measuring CI knock is based
upon hexadecane and heptamethylnonane as primary
fuels with assigned values of 100 cetane and 15 cetane
respectively
Families of IC Engine Fuels
Interestingly, the air-fuel ratio (AFR) for the chemically
correct mixture is essentially constant even though the
fuel structure and phase change
Because of this constancy, various fuels can be supplied
to SIE without changing the carburettor adjustment
Also that the energy content per unit volume of mixture is
essentially constant and therefore the power output of
the engine is not affected by a change in fuels (unless
knock is present)
The fuels of this family are stable, clean burning and do
not attack gaskets or metals
NeonTypewriterFamilies of IC Engine Fuels
The Olefin family (Alkenes) the mono-olefins have
open-chain structures (aliphatic olefins) with genral
formula CnH2n
The physical properties correspond closely to similar
compounds in the paraffin family and are also clean
burning and a higher octane rating
Because of the free bond, the olefins are chemically
active and unite readily with hydrogen to form the
corresponding paraffin or naphthene
When they unite with oxygen, they form an undesirable
residue, gum which can cause smog
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The Di-olefin family diolefins have two double bonds
with a general formula CnH2n-2
These are undesirable fuel components because, upon
storage, reactions take place that lead to coloring of the
fuel and, also, to the formation of a cloudy gum
The Naphthene or Cycloparaffin family (Cyclanes)
have the same general formula as the olefins but are
saturated, ring-structure compounds
These are desirable components of motor gasoline
The Aromatic family (Benzene derivatives) ring-
structured hydrocarbons with general formula CnH2n-6
NeonTypewriterFamilies of IC Engine Fuels
These are excellent gasoline fuels and can be selectively
produced by catalytic cracking or by thermal cracking at
high temperatures (1200 F)
Benzene or benzol is an excellent blending agent to
raise the octane ratings of low-grade fuels
The aromatics have the highest densities of the HC and
therefore have the highest heating values per unit
volume
These fuels are stable in storage, smoky in burning with
high solvency powers
Families of IC Engine Fuels
The Alcohols are a partial oxidation product of petroleum with a chain structure of the general formula ROH, where R is the paraffin group
They have good antiknock qualities with octane ratings in excess of 100
Alcohols absorb water from atmosphere
They also require a different AFR than gasoline
Gas or natural gas can be associated or unassociated
The composition of gas varies widely with methane usually predominating from 60 to 98% and with percentages of ethane and other paraffins along with carbon dioxide, helium, and nitrogen
Families of IC Engine Fuels
A sour gas is one which contains hydrogen sulphide,
otherwise, it is called sweet gas
Before the gas enters the pipeline it must be sweetened,
dehydrated, and liquid HC removed
Synthetic or substitute natural gas (SNG) are produced
from solid wastes, petroleum crudes or fractions, or coal
Liquefied petroleum gas (LPG) may be propane, butane,
or a mixture of the two
Natural gas and LPG are excellent fuels for the SIE,
home, and gas-diesel with low emission pollutants
LNG and CNG are the two other versions of natural gas
Characteristics of Modern Gasoline
Modern gasoline may be made up of straight-run
gasoline (from fractional distillation), cracked gasoline
(from catalytic cracking), reformate (from catalytic
reforming), alkylate and polymerized gasolines
(produced from gases), with some butane or propane to
achieve the desired Reid vapour pressure
The two most important characteristics of gasoline are its
volatility and octane number
The gasoline sold on the market is a blend of a number
of products produced in several processes
The gasoline, irrespective of its origin, should have the
properties listed as follows:
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Desirable Properties of Gasoline
Knock characteristics the octane rating the fuel
should have an octane rating to fit the engine
requirements
The tests for determining octane number are performed
using the ASTM-CFR (Cooperative Fuel Research)
engine, a variable compression ratio engine
Two octane numbers are: research octane number (RON)
and motor octane number (MON), the former is generally
higher
High octane fuels enable high compression ratios to be used
give increased power output and improved economy
Typically, a CR of 7.5 requires 85 octane fuel, while a CR of
10.0 would require 100 octane fuel
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Desirable Properties of Gasoline
Volatility expressed in terms of the volume percentage
that is distilled at or below fixed temperatures
If the fuel is too volatile
It will start the engine readily
When it is used at high ambient temperatures, the fuel is
liable to vaporise in the fuel lines and form vapour locks
If the fuel is not suffieciently volatile
The engine will be difficult to start, especially at low ambient
temperatures
In general, the fuel with the lowest distillation temperatures
is the best
Dilution of the lube oil may occur when the fuel condenses
of fails to vaporise in the engine
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NeonTypewriterDesirable Properties of Gasoline
The volatility also influences the cold start fuel economy
SIE are started on very rich mixtures, and continue to
operate on rich mixtures until they reach their normal
operating temperature increasing the volatility of the
gasoline at low temperatures will evidently improve the fuel
economy during and after starting
Gasoline stored for a long time in vented tanks is said to go
stale, the loss of more volatile components that are necessary
for easy engine starting
Gum and Varnish Deposits the fuel should not deposit either gum
or varnish in the engine
Corrosion the fuel and the products of combustion should be non-
corrosive
Cost the fuel should be inexpensive
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NeonTypewriterAdditives Used in Gasoline
In order to achieve the required properties of gasoline,
as well as for other purposes, various additives are used:
Antiknock to reduce or eliminate the knock in SIE
Previously TEL or TML were used along with some
scavengers; now-a-days, MTBE or TBA are used
Deposit modifiers to alter the chemical character of
combustion chamber deposits and so reduce surface
ignition and spark plug fouling usually, phosphorous and
boron compounds are used
Antioxidants are used to prevent gum formation
amines of amount 0.5 to 6.5 kg per 1000 bbl
Detergents are used to prevent deposits in carburettor
and manifold alkyl amine phosphates, of amount, 5 kg
per 1000 bbl17
NeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterAdditives Used in Gasoline
Lubricants to lubricate valve giudes and upper cylinder regions 0.1 to 0.5 percent light mineral oils
Metal deactivators to destroy the catalytic activity of traces of copper amine derivatives, of amount, 0.5 kg per 1000 bbl
Anti-rust agents to prevent rust and corrosion arising from water (and air) fatty-acid amines, sulfonates or alkyl phosphates, of amount 0.5 to 6.5 kg per 1000 bbl
Anti-icing agents to prevent "gasoline freeze" from water in fuel and throttle-plate icing from water in air methyl alcohol and isopropyl alcohols of about 1% each are used for the purpose; sometimes, a surface-action agent, such as ammonia salts or phosphates (about 0.005%) is also used
NeonTypewriterCharacteristics of CI Engine Fuels
The most important characteristic of diesel fuel is the
cetane number which indicates how readily the fuel self-
ignites usually, the self-ignition temperatures of the
normal paraffins decrease as the length of chain
increases
Cetane (hexadecane, C16H34) is the primary standard of
the cetane scale, with an arbitrary rating of 100 (isocetane,
heptamethylnonane (HMN) is at the bottom of the scale
with a cetane number 15) while other paraffins have
cetane ratings which vary almost linearly with the length of
the chain thus the straight-run gasoline fuels which might
be poor SI fuels, are desirable for CIE
NeonHighlightNeonTypewriterNeonTypewriterCharacteristics of CI Engine Fuels
The other important characteristic is the viscosity,
especially for lower-grade fuels used in the larger
engines, may need to have heated fuel lines
Another problem with diesel fuels is that, at low
temperatures, the high molecular weight components
can precipitate to form waxy deposit (cold filter plugging
point)
The requirements for a good CI fuel cannot be as simply
stated as those for gasoline
Because of the added complexity of the CIE from its
heterogeneous combustion process, which is strongly
affected by injection characteristics
Some of the desirable characteristics are listed below:20
Characteristics of CI Engine Fuels
Knock characteristics if an engine runs on a fuel with too
low cetane number, there will be diesel knock
Diesel knock is caused by too rapid combustion and is the
result of a long ignition delay period, since during this period
fuel is injected and mixes with air to form a combustible
mixture
Ignition occurs only after the pressure and temperature have
been above certain limits for sufficient time, and fuels with
high cetane numbers are those that self-ignite readily
The minimum cetane number of the good grade diesel fuel is
50 and for the other is 45
Starting characteristics the fuel should start the engine
easily requires high volatility to form readily combustible
mixture, a high cetane rating so that the self-ignition
temperature is low21
Characteristics of CI Engine Fuels
Smoke and odour the fuel should not produce smoke or
odour after combustion
Corrosion and wear should not cause corrosion before
combustion, or corrosion and wear after combustion
directly related to sulphur, ash, and residue contents of the
fuel
Handling ease should be liquid that will readily flow
under all conditions measured by the pour point and
viscosity of fuel; the fuel should also have a high flash
point since an advantage of the CIE is its use of fuels with
low fire hazards
Flash point of diesel fuel is at least 55C, while that for petrol
and kerosene are about 40C and 30C respectively
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Additives in diesel fuel to improve the cetane number are
referred to as ignition accelerators their
concentrations are greater than those of antiknock
additives used in petrol
By adding 1% v/v of amyl nitrate, C5H11ONO2 increases
the cetane number by about 6; ethyl nitrate, C2H5ONO2and ethyl nitrite, C2H5ONO are other effective substances
Ignition delay is most pronounced at slow speeds
because of the reduced temperature and pressure
during compression
Cold-starting can be a problem, and under severe
conditions, heaters may be needed; also excess fuel may
be injected
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Characteristics of CI Engine Fuels
Sometimes, volatile fuels with high cetane numbers, such
as ether, can be added to the intake air
Sometimes a cetane index is used, as the only
information needed is fuel viscosity and density with no
need for engine tests
The cetane index can be used only for straight petroleum
distillates without additives
Other fuels that are suitable for diesel engines are derived
from coal and vegetable oils
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NeonHighlightTests for Diesel
Viscosity is exactly defined as the ratio of shearing
stress in a fluid to the rate of shear, and is a measure of
the resistance of the fluid to flow
Viscosity changes rapidly with temperature, thus the test
temperature must be specified
The viscosity of the fuel exerts a strong influence on the
shape of the fuel spray high viscosity causes low
atomization and high penetration of the spray jet
In small combustion chambers, the effect of viscosity may
be critical
The SU viscosity required for most high-speed engines
ranges between 35 and 70 sec (100F)
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Tests for Diesel
Gravity is an indication of the density or mass per unit
volume of the fuel defined as the mass of a unit
volume of fluid (at 60F) to that of the same volume of
water (at 60F)
The API gravity is defined in terms of the specific gravity
API Gravity = [(141.5/sp. Gr. At 60F/60F) 131.5]
In general, high API gravities imply high cetane fuels
Sulphur Corrosion and increased wear is caused in
the engine along with carbon deposits on the piston and
rings and deterioration of lub oil due to the presence of
sulphur in fuels
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Tests for Diesel
Sulphur trioxide is formed during diesel combustion which
may attack lub oil on the cylinder walls to form resinous
materials that harden to form varnish and carbon
SO3 may also form sulphuric acid
Sulphur content over 1.0% is detrimental, while amounts of
0.5% are economically feasible
Carbon Residue is the result of the fuel burned with a
limited amount of oxygen
In the Conradson carbon test, a sample of the fuel in a
crucible is heated to a high temperature for a relatively
long period of time; the percentage by mass of residue to
the original sample is the carbon residue
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NeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterNeonTypewriterTests for Diesel
High carbon residues contribute to deposits in the
combustion chamber and around nozzle tips, thus
interfering with the spray shape
Ash In testing for ash, the fuel is heated until the
vapours can be ignited when the flame dies away, any
carbonaceous material is oxidized by heating in a flame
or muffle furnace the unburned residue is called ash
The ash content is a measure of the abrasiveness of the
products of combustion that could cause wear in the
engine
Water and Sediment of all specifications for diesel, the cleanliness
factor is probably the most important because of the precisely fitted
parts in the fuel pump and nozzle
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Tests for Diesel
Flash Point is the lowest temperature of the fuel that
allows inflammable vapours to be formed
It is found by heating the fuel slowly and then sweeping a
flame across the surface of the liquid, a distinct flash is
obtained at the flash point
It is important for safety purposes and serves as a
measure of the fire hazard
Distillation range should be as low as possible without
unduly affecting the flash point, the burning quality, or
the viscosity of the fuel
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Tests for Diesel
Ignition Quality The ease of igniting the fuel in the
engine by autoignition is called the ignition quality of the
fuel; the diesel index number (DI) has been empirically
found to correlate, approximately, the cetane number of
most commercial fuels which is defined as:
DI =[aniline point (F) API Gravity (60F)]/100
The DI and the cetane rating of the fuel for most high-
speed diesels should be of the order of 50-60
Cetane ratings below 40 may cause exhaust smoke, with
increased fuel consumption and loss of power
Pour Point is determined by cooling a sample of oil in a test jar
until , when the jar is displaced from the vertical to horizontal
position, no perceptible movement of the fuel will occur (within 5
secs)30
NeonTypewriterNeonTypewriterNeonRectangleNeonTypewriterNeonTypewriterRelation between Cetane Rating and DI
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Tests for Diesel
Heating Value or the heat of combustion is detrmined
by burnign the fuel with oxygen in a bomb and noting the
temperature rise of a cooling bath
If all of the water vapour can be condensed, the HHV is
obtained, if none of the water vapour is condensed, the
LHV is obtained
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Tests for Diesel
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