4.Fuels.pptx for engineering technology fifth year

4. Fuel for IC Engines
Fuels are those materials which can be burned to release
thermal (heat) energy.
Most of the commonly used fuels consists primarily of
hydrogen and carbon, hence they are hydrocarbon fuels,
denoted by CnHm or CaHb.
These fuels exist in all states; for instance, coal (solid),
gasoline or diesel oil (liquid) and propane or butane (gas).

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How to obtain Gasoline and Diesel as a fuel for IC engines ?
Conventional fuels for IC engines can be classified as:
Primary …….and …... Secondary fuels.
•The fuels which exist in nature and used directly to produce heat are called
primary or natural or raw fuels (eg:- Natural gas, petroleum, etc.).
•The fuels which are obtained from the primary fuels are called secondary or
processed or artificial fuels (eg:- Gasoline, Diesel, etc.).
•For instance, Gasoline fuel (C8H17) can be obtained by mixing or blending
the two secondary fuels, n-heptane (C7H16) and Iso-Octane (C8H18), using
Cracking, Bonding or other types of refinery processes and adding proper
additives.
•One way of getting secondary fuels is by cracking as shown,
CH3(CH2)8CH3  C8H18+CH2=CH2
Primary fuel Secondary fuel

Useful products of Petroleum
i) Petroleum Gas:– It is a mixture of methane, ethane,
propane, and butane. Its molecular composition varies
from C1 to C4 hydrocarbons with boiling point below 313 K.
Hence, it is a ‘gas’.
ii) Gasoline:– It is a mixture or blend of mainly n-Heptane
(C7H16) having an ‘Octane Number (ON)’ of ‘0’ and Iso-
Octane (C8H18) having an ‘Octane Number (ON)’ of ‘100’
and also other additives. It is ‘commonly ‘ represented by
C8H17 . Its molecular composition, in general, varies from C5
to C12 hydrocarbons with boiling point range of 313 K to 443
K. Hence, it is a ‘liquid’.
iii) Kerosene:– Its molecular composition varies from C10 to
C12 hydrocarbons with boiling point range of 443 K to 623
K.

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iv) Diesel Oil:–
• It is a mixture or blend of mainly n-cetane or n-Hexadecane
(C16H34) having a ‘Cetane Number (CN)’ of ‘100’ and Iso-Cetane or
heptamethylnonane (C16H34) having a ‘Cetane Number (CN)’ of ’15’
and also other additives.
• It is ‘commonly‘ represented by C12H26 ‘for Light Diesel Oil’, by C13H28
‘for Medium Diesel Oil’ and by C14H20 ‘for Heavy Diesel Oil’.
• Its molecular composition, in general, varies from C13 to C15
hydrocarbons with boiling point range of 523 K to 623 K.
• Hence, it is a ‘liquid’.
v) Fuel Oil:–
• Its molecular composition varies from C13 to C15 hydrocarbons with
boiling point range of 623 K to 723 K.
vi) Lubricating Oil:–
• Its molecular composition varies from C17 to C20 hydrocarbons
with boiling point above 723 K.
• Hence, it is a ‘liquid with high viscosity’.

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IC engines use different kinds of Conventional and Alternative fuels.
Conventional fuels are derivatives of crude petroleum (crude oil).
•Crude petroleum is a dark oily liquid known as rock oil (In Greek,
petra means rock and oleum means oil).
•It is formed from bacterial decomposition of the remains of animals
and plants buried under the sea or the crust of the earth over millions
of years ago.
•When these organisms (animals and plants) died, they sank to the
bottom and got covered by sand and clay. Over a period of millions of
years, gradually converted into hydrocarbons due to heat, pressure
and catalytic action.
•Natural gas is found above petroleum. The crude petroleum is
obtained by drilling a hole into the earth’s crust and sinking a pipe
into it. When the pipe reaches the oil deposit, natural gas comes out
first with a high pressure. After the pressure has subsided, the crude
oil is pumped out of the oil well. This process of obtaining crude oil
from its source is called mining.

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Conventional fuels…
Crude oil is a mixture of hydrocarbons, such as: Alkanes(Paraffins),
Alkenes(Olefins), Naphtenes (Cycloparaffins), Aromatics(Benzene). The
actual composition of petroleum depends upon its place of origin. The crude
petroleum obtained by mining is a dark viscous liquid called Crude oil. Before
using a crude oil (crude petroleum) it must be separated into various
components. The process of separating petroleum into useful fractions or
components and removal of undesirable impurities is called refining. The
refining of petroleum is carried out by the process of fractional distillation as
shown in the figure.

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The refining of petroleum is done in big
refineries. The first step during refining
is neutralization of crude oil by washing
with acidic or basic solution as needed.
Then, the oil is heated in a furnace to
about 675 K and the vapor so obtained is
entered into a fractionating tower. The
tower is divided into a number of
compartments by means of shelves
(trays) having holes as shown in the
figure. The holes are covered by caps
which allow the lighter (more volatile)
components to pass up the column while
the heavier (less volatile) components
condense and flow onto trays below.
Each shelf is provided with an overflow
pipe which keeps the liquid to a certain
level and allows the rest to flow down to
the lower shelf.
Refining of petroleum

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Chemical Composition of Petroleum
i) Alkanes(Paraffins):- have a chemical formula CnH2n+2. They can be
‘Straight chain or Normal Paraffins (n-Paraffins)’ and ‘Branched chain or Iso-
Paraffins’.
a) Straight chain or n-Paraffins consist of a single bond, straight chain
molecular structure and are saturated compounds as the valency of
carbon is fully utilized. Hence they are stable but have poor knock-
resistant quality.
Some of the hydrocarbons in this series are shown in the figure.
Structure of Alkanes (Straight chain or n-Paraffins)

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Chemical Composition of Petroleum…
i) Alkanes(Paraffins):- …
b) Branched chain or Iso-Paraffins consist of a single bond, branched
molecular structure and are saturated compounds as the valency of
carbon is fully utilized. Hence they are stable and also have good knock-
resistant quality.
Structure of Alkanes (Branched chain or Iso-Paraffins)

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ii) Alkenes(Olefins):- have a chemical formula CnH2n for ‘mono-olefins’ and
CnH2n-2 for ‘di-olefins’.
They consist of one or more double bonded carbon atoms with a straight
chain molecular structure and are unsaturated compounds as the valency of
carbon is not fully utilized. Hence they are unstable but have good knock-
resistant quality. These hydrocarbons can readily oxidize when they present
in a cracked fuel. This results in ‘gummy deposit’ formation in the storage. As
a result, the percentage of these hydrocarbons should be kept lower (less
than about 3%).
Structure of Alkenes (Olefins)

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iii) Naphtenes(Cyloparaffins):- have a chemical formula CnH2n.
They consist of a single bond with a ring molecular structure and are
saturated compounds as the valency of carbon is fully utilized. Hence they
are stable and also have good knock-resistant quality.
Structure of Naphtenes (Cycloparaffins)

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iv) Aromatics( Benzene) :- have a chemical formula CnH2n-6.
They consist of one or more double bonded carbon atoms with a ring
molecular structure and are unsaturated compounds as the valency of carbon
is not fully utilized. Hence they are unstable but have good knock-resistant
quality.
Structure of Aromatics (Benzene)

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Refinery processes
• Fractional distillation:- is the basic refining process used to separate
the crude oil (the crude petroleum) into more desirable fractions or
products.
• Cracking:- is the process of breaking down large and complex molecules
into lighter and simpler molecules with lower boiling points. Thermal
cracking is the process of braking down larger molecules under high
temperature and pressure. Catalytic cracking is the process of breaking
down larger molecule at a relatively lower temperature and pressure but
in the presence of a catalyst. It is normally used in the production of good
quality gasoline. For instance,
CH3(CH2)8CH3  C8H18+CH2=CH2
This cracking processes is taking place at a temperature of about 770 K
and hence, one of the major component of gasoline fuel C8H18 (Octane)
is obtained.
Note:- C8H18 is ‘stable’ whereas CH2=CH2 is ‘unstable’. Why ?

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Refinery processes…
• Hydrogenation:– differs from the cracking process is that hydrogen
atoms are added to a certain hydrocarbon under high pressure and
temperature to produce more desirable compounds. This process is often
used to convert ‘unstable’ compounds to ‘stable’.
• Polymerization:- is a process of combining light, unsaturated gases of
one family to produce liquid in the presence of a catalyst.
• Alkylation:- is a process of combining light gases of different families in
the presence of a catalyst. In general an olefin is combined with paraffin
to give branch chain paraffins. This process increases or improves the
anti-knock quality of a fuel.
• Isomerization:- is the process of changing the relative positions of atoms
within the molecule without affecting its molecular formula. Which means,
it forms ‘isomers’ of the original hydrocarbon. This process increases or
improves the anti-knock quality of a fuel. eg:- n-Octane to Iso-Octane.
• Cyclization:- is the process of joining together the ends of a straight
chain molecules to form a ring molecules of the naphthene family. This
process increases or improves the anti-knock quality of a fuel.

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Refinery processes…
• Aromatization:– differs from the cyclization process is that the ring
product is an aromatic family.
• Reforming:- is a kind of cracking process in which naphtha or straight
gasoline is converted into gasoline of higher anti-knock quality.
• Bonding:- is a process of mixing refinery products to obtain a
commercial products of desired quality. eg:- n-Heptane + Iso-Octane = ?.
• Desulfurization:- is a process of removing sulfur content from a fuel.
Useful products of Petroleum
i) Petroleum Gas:– It is a mixture of methane, ethane, propane, and
butane. Its molecular composition varies from C1 to C4 hydrocarbons
with boiling point below 313 K. Hence, it is a ‘gas’.
ii) Gasoline:– It is a mixture or blend of mainly n-Heptane (C7H16) having
an ‘Octane Number (ON)’ of ‘0’ and Iso-Octane (C8H18) having an
‘Octane Number (ON)’ of ‘100’ and also other additives. It is ‘commonly ‘
represented by C8H17 . Its molecular composition, in general, varies from
C5 to C12 hydrocarbons with boiling point range of 313 K to 443 K.
Hence, it is a ‘liquid’.

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Conventional fuels for SI engines ( Gasoline )
Gasoline is one of the major fuels used for SI engines which is a mixture of
various hydrocarbons, such as: paraffins, olefins, napthenes, and aromatics. The
composition depends upon the source of crude oil and the nature of refining
process.
Main desirable properties of ‘Gasoline’
1. Heating (Calorific) Value (Higher (HHV) or Lower (LHV)).
2. Volatility and Vapor pressure.
3. Knock resistance (Octane Number) and Self Ignition Temperature (SIT).
4. Ease of handling or storage (Safety).
5. Availability and Cost.
6. Clean burning (minimum deposit or residue).
Note:- The Heating (Calorific) Value of carbon (C) is 32.8 MJ/kg and that of
molecular hydrogen (H2) is 120.9 MJ/kg.

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Conventional fuels for SI engines ( Gasoline )…
Heating Value (HV):- refers to the energy content of the fuel. The higher
the heating value, the more energy it releases during combustion and this
depends on the H/C ratio. Higher H/C ratio mean, more HV.
Note:- Heating Value (HV) can be of Higher Heating Value (HHV) or
Lower Heating Value (LHV) depending on the state of H2O.
Volatility:- is commonly known as the evaporating tendency of a liquid
fuel. Higher volatility mean higher vapor pressure.
Advantage
• If the fuel is more volatile, there will be good or homogeneous air-fuel
mixture formation during intake and compression strokes (for SI
engines) and promotes cold staring behavior of the engine (for both SI
and CI engines).
Disadvantage
• If the fuel is more volatile, the self ignition temperature (SIT) or the
fuel resistance to knock will be lower. Hence, there will be self- or
auto-ignition of the air-fuel mixture. (for SI engines).

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Disadvantage…
• If the fuel is more volatile, during warm or ‘hot’ ambient condition there
will be evaporative emission (this is the cause of lower fuel economy and
higher atmospheric pollution) and vapor lock in the fuel system (this may
cause hard ‘hot’ starting and engine stalling). This also makes storage or
handling of the fuel difficult, as the fuel gradually escape from the
container and not safe as it readily catch fire. (for SI engines).
• If the fuel is less volatile, during cold engine operation the fuel may not
readily vaporize. As a result, the fuel enters into the engine cylinder as a fine
droplets which tend to wash away the lubricating oil around the piston and
cylinder wall. The effect of this will lead to engine wear and oil
contamination. (for SI engines).

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Volatility (Distillation) curves of various fuels.

Knock resistance and Self Ignition Temperature (SIT)
 When the temperature of an air-fuel mixture is raised high enough, the mixture will
tend to ignite by itself without the assistance of the igniter (spark plug). The
temperature at which ‘self-ignition’ occur is called ‘self ignition temperature (SIT)’.
 The SIT is big greater than the boiling point of the fuel. This is the basic principle of
ignition in CI (Diesel) engines, where compression ratio is high enough so that the
temperature rises above self ignition temperature (SIT) at the end of compression
stroke.
 Self-ignition then occurs when the fuel is injected into the combustion chamber. On
the other hand, self-ignition or auto-ignition is not desirable in SI (Gasoline)
engines, where spark plug is used to ignite the air-fuel mixture at the proper time
during the engine cycle.
 If self-ignition occurs in SI engine, undesirable high pressure pulses or waves are
generated and this may cause an audible sound and vibration and also engine
damage. It is, therefore, recommended to use a fuel with a lower Self Ignition
Temperature (SIT) or higher Knock-resistant quality (higher Octane Number).
(for SI engines).

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Knock resistance and Self Ignition Temperature (SIT)…
If the temperature of the fuel is raised above self ignition temperature (SIT), the
fuel spontaneously ignites after a short ignition delay (ID). The higher the fuel is
heated above SIT the shorter will be ID as shown.
Self Ignition Temperature (SIT) and Ignition Delay (ID).

Ignition Delay (ID)
period is a very small fraction of a second ( 0.001 sec. at a
particular intermediate engine speed). During this short period of
time, pre-ignition reaction occurs including oxidation of some fuel
components and even cracking of some large hydrocarbon
components into smaller hydrocarbon molecules. These pre-ignition
reactions raise the temperature at local spots, which then promotes
additional reactions until the actual combustion reaction occurs.
Octane Number (ON)
The property of a fuel which describes how a fuel will or will not
self ignite is called the Octane Number (ON). This is a numerical
scale generated by comparing the self-ignition characteristics of the
given fuel to that of a standard reference fuels. The two standard
reference fuels are:
 Normal heptane or n-heptane (C7H16):- this fuel has lower
resistance to knock and is given an octane number of ‘0’.
 Iso-octane (C8H18):- this fuel has higher resistance to knock and is
given an octane number of ‘100’.

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Blends of these reference fuels define the knock resistance qualities of
intermediate octane number fuels. A blend fuel of 10% n-heptane and 90% iso-
octane has an Octane Number (ON) of 90 (eg:- Gasoline (C8H17)).
Note:- Octane Number (ON) can be of Motor Octane Number (MON) or
Research Octane Number (RON). In this context ON mean, the average value of
the two.
As shown in the figure, high compression
engines must use high octane fuel to avoid
self-ignition and knock.
The higher the octane number of a
fuel, the lesser it will self-ignite.
ON versus Compression ratio (r).

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Conventional fuels for CI engines ( Diesel )
Diesel oil is one of the major fuels used for CI engines which can be
obtained from crude oil (crude petroleum) in a similar way as
gasoline is obtained.
Main desirable properties of ‘Diesel’
1. Heating (Calorific) Value (Higher (HHV) or Lower (LHV)).
2. Volatility and Vapor pressure.
3. Diesel knock (Cetane Number) and Ignition Delay (ID).
4. Sulfur content.
5. Flash point (Ease of handling or storage or safety).
6. Viscosity.
7. Cloud point.
8. Water and Sediments.
9. Smoke and Particulates.
10. Availability and Cost.
11. Clean burning (minimum deposit or residue).

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Conventional fuels for SI engines ( Diesel )…
Heating Value (HV):-
• Affects power output and fuel economy.
• The heat of combustion (MJ per kg or per liter) is a measure of the amount
of energy available to produce work.
• In general, a fuel with a higher volumetric heating value (MJ per liter) will
produce more power or provide better fuel economy.
Note:- Diesel fuel is more denser than Gasoline fuel. Hence, for the same
volume diesel has more energy content than gasoline.
Volatility:-
• Less volatile fuel causes: hard cold starting, deposit formation, wear, and
exhaust smoke.
• Less volatile fuels have higher boiling points and higher self ignition
temperature (SIT).
Note:- Diesel fuel is less volatile than Gasoline fuel. Hence, it has poor cold
starting behavior, but suitable for storage or handling and safe.

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Conventional fuels for SI engines (Diesel )…
Diesel knock (Cetane Number) and Ignition Delay (ID)
The cetane number (CN) is a measure of the ignition delay of a diesel
fuel.
When a diesel fuel is injected into the hot compressed air within the
engine cylinder, atomization, vaporization, physical mixing of fuel and
air, molecular interaction and ignition (combustion) at various locations
taking place. If the ignition delay is longer due to lower cetane number
(CN) or due to accumulation of large quantity of fuel as a result of large
quantity of fuel injection, a bulk of charge or mixture catch fire almost
instantaneously and causing violent combustion called ‘diesel knock’ or
‘detonation’. In order to avoid or minimize ignition delay (ID), a diesel
fuel with the proper or higher cetane number (CN) should be used.
Note:- As the combustion mechanism of diesel engine is based on the
auto-ignition of the charge, mild (soft) knock always present.

Cetane Number (CN)
 Cetane Number is an indicator of the ignition delay or ignition
quality of a diesel fuel. This is a numerical scale generated by
comparing the ignition delay (ID) characteristics of the given fuel
to that of a standard reference fuels. The two standard reference
fuels are: n-cetane or n-Hexadecane (C16H34) and Iso-cetane or
heptamethylnonane (C16H34) .
Normal cetane or n-cetane or n-Hexadecane (C16H34) :- this
fuel has a short ignition delay (high ignition quality) and is given a
cetane number of ‘100’.
Iso-cetane or heptamethylnonane (C16H34) :- this fuel has a long
ignition delay (low ignition quality) and is given a cetane number
of ‘15’.
 Blends of these reference fuels define the ignition qualities of
intermediate cetane number fuels (eg:- Light Diesel (C12H26) has
a Cetane Number (CN) of about 45).

Sulfur content
• The sulfur content depends on the crude oil source and the refining steps it
undergoes. As the fractions temperature increases, the content of sulfur also
increases. Hence, the content of sulfur in diesel is more than gasoline. It
causes wear, deposits, and particulate emissions.
• Sulfur oxides (SO2 and SO3) are produced during combustion of fuel. SO3
combines with water in the exhaust to form sulfuric acid (H2SO4). Sulfuric
acid causes corrosive wear and contributes to engine deposits.
Flash point
• It is the temperature at which fuel vapors can be ignited when exposed to a
flame.
• It is related to volatility and hence, fire hazard (safety) and handling. Or
storage are taken into consideration. In general, diesel fuel has higher flash
point as compared to gasoline. Hence, it will not easily catch fire. This
mean, it is safe to store such fuels.

Viscosity
• Affects injector lubrication and atomization. Low viscosity fuels
may not provide sufficient lubrication in close-fit pumps and
injectors. Hence, this may cause abnormal wear, loss of power
and smoke. On the other hand, high viscosity fuel will not
atomize properly. This mean, as droplets are relatively larger
(more surface area to volume ratio) may not readily vaporize.
This will lead to more ignition delay and hence ‘diesel knock’.
High viscosity also increases wear on fuel pump and injectors
due to higher injection pressure.
Cloud point
• Cloud Point is the temperature where a cloud or fog appears in
the fuel. This happens when the temperature falls below the
melting point of the wax in the fuel. Hence, it must be below the
lowest outside operating temperature to prevent filter plugging
or it is recommended to use a fuel heater or blended fuel or
proper additives.

Water and sediments
 Water introduces during shipment and condensation during
storage.
 Water affects the life of fuel filters, injectors, particularly the
rotors of distributor (rotary) pumps. Hence, water separators are
critical to fuel systems.
 Sediments (rust, scale, dirt, etc.) should be removed by
straining or filtration.
Smoke and Particulates
White Smoke is the result of too low a temperature or water in
the combustion chamber.
Blue Smoke is the result of excess lubricating oil in the
combustion chamber.
Black Smoke and particulate matters are produced near or at
full load due to excess fuel.

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Conventional fuels for SI engines (Diesel)
Diesel ‘Additives’
• Detergents:- prevent deposit buildup and extend
injector, and filter life by cleaning.
• Cetane Improvers:- raise the cetane number (CN).
• Lubricity:- replaces natural lubricants.
• Anti-foam:- reduces foaming when pumping a fuel.

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Alternative fuels
Why alternative fuels ?
1. Petroleum (crude oil) becomes very scarce and costly from time
to time.
2. Petroleum derivative fuels release more emissions.
3. To save the money expended in importing conformist fuels.
Some of the Alternative fuels are:
i) Alcohol (Methanol and Ethanol).  (Renewable).
ii) Biogas.  (Renewable).
iii) Biodiesel (Jatropha, Castor,…).  (Renewable).
iv) Hydrogen.  (Renewable).
v) Liquefied Natural Gas (LNG).  (Non-Renewable).
vi) Liquefied Petroleum Gas (LPG). (Non-Renewable).

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Alternative fuels…
Advantages of alternative (renewable) fuels
1. Alcohol can be produced from a highly reliable and long-lasting raw
sources like sugarcane, starchy materials, corns, potatoes, etc. Hence,
they are renewable energy sources.
2. Biogas can be produced from byproducts and wastes, such as: cow-dung,
sewage, etc. Hence, it is a renewable energy source.
3. Biodiesel can be obtained from vegetable oils and animal fats. Hence, it
is a renewable energy source.
4. Hydrogen is a naturally available element and can also be obtained from
various chemical processes (eg:- electrolysis of water). Hence, it is a
renewable energy source.
5. While burning, such fuels release less emission.
Disadvantages of alternative (renewable) fuels
1. Social problems; as Alcohol and Biodiesel fuels need large hectares of
land for cultivation.
2. Need of some engine design modifications.
3. Relatively low heating (calorific) value, except Hydrogen; this affects the
performance of an engine.

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Alternative (renewable) fuels for IC engines
Alcohol
Alcohol is an attractive alternative fuel as it is obtained from a number of
sources. Methyl alcohol (Methanol) and Ethyl alcohol (Ethanol) are the
two main types of alcohols used in IC engines.
Advantages of Alcohol:
• It is a high octane fuel with ON over 100. Thus, Tetraethyl lead (TEL) can be
replaced as it forms deposits within the engine cylinder and damages the
catalytic converter.
• It can be obtained from a number of sources.
• It has high evaporative cooling which makes the intake and compression
process cooler. This raises the volumetric efficiency and reduces the work
input during the compression process.
• It produces relatively less emission.
• Its sulfur content is low.
• It forms more moles of combustion products. This mean, high cylinder
pressure during power stroke; and hence, high power output.
• Its effect on ecology or human health is relatively less.

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Alternative (renewable) fuels for IC engines…
Alcohol…
Disadvantages of Alcohol:
• Low calorific value (less energy content).This mean, the engine consumes
more fuel in order to develop the same power as that of gasoline under the
same operating conditions.
• The exhaust contains more aldehydes which cause atmospheric pollution.
Aldehydes are class of organic compounds containing ‘CHO’ group , which
yields acids when oxidized.
• As compared to gasoline, alcohols have more corrosive effect on:
aluminium, copper, brass, and rubber. This makes the design and
manufacturing process relatively difficult.
• It has, in general, poor ignition characteristics.
• Low flame temperature generates less NOx, but the resulting lower exhaust
temperature takes longer time to bring the catalytic converter to its
operating temperature.

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Methanol (CH3OH) (Methyl alcohol)
• Methanol can be obtained from both fossil and renewable sources. These
includes: biomass, wood, coal, natural gas, petroleum, etc.
• The specific heat consumption of methanol is about 50% less than gasoline.
• Methanol can be mixed or blended with gasoline in some proportion. This:
a) improves cold starting,
b) lowers engine emission,
c) reduces the total cost of the fuel,
d) increases resistance to knock,
e) prevents freezing of fuel in winter.
But, if the amount of methanol in the blend is increased considerably:
a) more aldehydes will be formed in exhaust,
b) the engine needs some modification based on the properties of
methanol.

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Performance of methanol as IC engine fuel
Effects of equivalence ratio and engine speed on brake power.

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Performance of methanol as IC engine fuel…
Effects of engine speed on specific heat consumption and volumetric efficiency.

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Performance of methanol as IC engine fuel…
Effects of equivalence ratio on emission (CO, HC and NOx).

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Ethanol (C2H5OH) (Ethyl alcohol)
• Ethanol can be obtained from fermentation of grains and sugarcane.
• Ethanol has less HC emission than gasoline but more than methanol.
• Ethanol is used to form E-10 and E-85 types of blend fuels.
E-10 (Gasohol) is a blend or mixture of 90% of gasoline and 10% of
ethanol.
Whereas, E-85 is a mixture of 85% alcohol (ethanol) and 15% of
gasoline. This is to minimize some of the problems of pure alcohol (
like:- cold starting, tank flammability, etc.).

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Performance of ethanol as IC engine fuel
Effects of engine speed on Pb, b,th, and brake specific heat consumption.

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Important properties of Gasoline, Methanol, and Ethanol

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Alcohol-Gasoline Blends
Normally pure alcohols are not used in IC engines (except methanol in racing
cars). If the engine is operated with pure alcohol, some major design
modifications are required. The design modifications include:
a) The materials to be used with alcohols have to be corrosive resistant.
b) Adjustments or improvements in carburetor and fuel injection system to
compensate for leaning effect.
c) Use of high energy ignition system with lean mixture.
d) Design of the fuel pump and fuel system to avoid vapor lock as
vaporization rate is very high.
e) Increase in compression ratio to make use of its better antiknock property.
f) Addition of detergents and additives to reduce engine deposits and to
assist cold starting.
g) Use of cooler running spark plugs to avoid pre-ignition.

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Some detail design modifications
a) The carburetor jet needs to be relatively wider to increase the flow about
1.56 times that of gasoline.
b) The float has to be adjusted (lower down) to match the higher specific
gravity.
c) Air inlet has to be modified to get less air as the blend requires less air for
complete combustion than pure gasoline.
d) Provisions and arrangements for heating the carburetor and intake
manifold as lower vapor pressure of alcohol makes engine starting
difficult below 700C.
Some specific advantages of blends over pure gasoline
a) improves cold starting,
b) lowers engine emission,
c) reduces the total cost of the fuel,
d) increases resistance to knock,
e) prevents freezing of fuel in winter.
What are the disadvantages ?

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Biogas
Biogas sources are: animal dung's, sewage, crop and vegetable wastes, etc. It is
produced by digestion or hydro-gasification process. Digestion is a biological
process which occurs in the absence of oxygen and in the presence of anaerobic
organisms at ambient pressure and temperature of about 350C to 750C. The
container in which digestion is taking place is known as ‘digester’.
Composition of biogas
The components are:
• CH4 (50 to 60% by volume), CO2 (30 to 45%), H2 and N2 (5 to 10%), and H2S
and O2 are traces. The octane rating is 110 with CO2 and 130 without CO2.

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Properties of biogas
• It has excellent antiknock property.
• It has higher auto-ignition temperature than gasoline.
• As it is a gas, it mixes readily with air even at low temperature.
• Although its calorific value is lower than gasoline, it can be compensated
by increasing the compression ratio.
Use of biogas in SI engines
SI engines can operate on biogas after starting the engine by using gasoline.
Biogas can be used in these engines in two forms:
• To run the engine entirely on biogas.
• Using dual fuel engines where the engine run on both fuels.
Advantages of biogas as fuel for SI engines
• A uniform air-fuel mixture can be obtained during all engine operation.
• There is virtually no smoke and CO emission in exhaust.
• NOx emissions are reduced by about 60%.

50
Biodiesel
Jatropha is derived from vegetable oils and animal fats; and is known as a
clean and renewable fuel for CI (diesel) engines.
Advantages of jatropha as fuel for CI engines
• Higher cetane numbers of biodiesel compared to the petroleum diesel
indicates potential for higher engine performance.
• Superior lubricating properties of biodiesel increases functional engine
efficiency.
• Higher flash point makes it safer to store.
• Biodiesel is oxygenated, so its use dramatically reduces toxic air emissions
compared to petroleum diesel.
Biodiesel…
Disadvantages of jatropha as fuel for CI engines…
• Engine modifications are required.
• It may cause solidification and clogging of the fuel system at low
temperatures (around 0°C).

51
Fuel ‘Additives’
Additives are mainly chemicals mixed in a very small proportions to
the fuel to improve engine performance (combustion) and to reduce
undesirable effects, such as: emission, corrosion, freezing, gum
formation, etc. Some of the main additives are:
• Antioxidants:- prevent gum formation due to oxidation with air.
• Corrosion inhibitors:- prevent corrosion effect on parts.
• Demulsifiers:- promote water separation, inhibit water-fuel
emulsion formation.
• Detergents (Deposit control additives):- prevent deposits in
carburetors, fuel injectors, intake valves and ports, and combustion
chamber.
• Anti-icing:- prevents freezing of moisture in carburetor and
induction system.
• Lead Compounds,(C2H5)4Pb and (CH3)4Pb:- increases the anti-
knock quality of the fuel. But, now-a-days the use of this
compound is limited due to its damaging effect on the ‘catalytic
converter’ and formation of deposit and emission. Instead, a bend
of other fuels with high octane number (like alcohols) are used.

52
• Saturated hydrocarbon molecules will have only single carbon-to-
carbon bonds, which means they have maximum number of
hydrogen atoms. Hence, their heating value and ignitability will be
higher as the heating or calorific value of hydrogen (H) is very
high. But, their resistance to knock is ‘very poor’. In order to
increase their resistance to knock, one of the solution is to change
their molecular structure. For instance, normal octane (n-octane)
can be changed to its ‘isomer’ iso-octane as shown in the figure.
Both have the same chemical properties (That means: the same
chemical formula, the same molecular weight, etc.) but different
physical properties (That means: different knock resistance,
different boiling point or self ignition temperature (SIT), etc.).
Normal Octane (n-Octane) and Iso-Octane

53
Summary …
• Unsaturated hydrocarbon molecules will have one or more double or
triple carbon-to-carbon bonds, which means they have relatively less
number of hydrogen atoms. Hence, their heating value and ignitability
will be lower. But, their resistance to knock is ‘very high’. For
instance, ‘Benzene’ as shown in the figure. Some of the physical
properties of ‘Benzene’ (eg:- Heating value, etc.) can be changed by
replacing one or more of hydrogen atoms with an organic radicals,
such as: paraffins, naphtenes, and olefins or by adding methyl group
(CH3). In doing so, a new compound called ‘Toluene (C6H6CH3 or
C7H8)’ can be obtained.
Structure of unsaturated hydrocarbon (Benzene) and Toluene

54
Summary …
• The heating values of various hydrocarbons should be compared from a Hydrogen
to Carbon ratio (H/C). Higher H/C mean, higher Heating value as the heating
(Calorific) value of hydrogen (H) is very high.
• As the number of atoms in a molecular structure increase, the boiling point
temperature and self ignition temperature (SIT) also increases.
• Comparison of Gasoline and Light Diesel Oil,
Some properties of Gasoline and Light Diesel Oil

4.Fuels.pptx for engineering technology fifth year

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