A fuel is a combustible substance, containing carbon as the main constituent, which on burning gives large amount of heat.

1. BY:
Dr. A. RAVIKRISHNAN,
M.Sc., M.Phil., M.Ed., Ph.D., MISTE., MILCS.,
Asst.Prof of Chemistry

2. FUELS
5.1 Introduction
 A fuel is a combustible substance, containing carbon as the main
constituent, which on burning gives large amount of heat.
 During the process of combustion of a fuel, the atoms of carbon,
hydrogen, etc., combine with oxygen with simultaneous liberation of
heat.
C + O2 ------- > CO2 + 94 k cals.
2H2 + O2 ------- > 2H2O + 68.5 k cals.
 The main source of fuel is coal and crude petroleum oil.
 These are stored fuels available in earth’s crust and are generally
called fossil fuels, because they were formed from fossilised remains
of plants and animals.

3. 5.2 CHARACTERISTICS (or) REQUIRMENTS OF A GOOD
FUELS
 It should be cheap and readily available.
 It should be safe and economical for storage and transport.
 It should not undergo spontaneous combustion.
 It should have higher calorific value.
 It should have moderate ignition temperature.
 The combustion should be easily controllable.
 It should have low moisture content, because the moisture content
reduces the calorific value.
 The products of combustion should not be harmful.
 It should have low non-combustible matter or ash content.

4. CLASSIFICATION OF FUELS
 Fuels are classified based on occurance and physical state
as follows.
I. Classification based on occurrence
 Fuels are classified into two types.
1. Primary fuels: It occurs in nature as such.
eg., coal, petroleum, natural gas.
2. Secondary fuels: It is derived from primary fuels.
e.g., coke, gasoline, coal gas.

5. II. Classification based on their physical state
 Both primary and secondary fuels may be further sub-
classified into three types.
 Solid fuels, eg., coal, coke.
 Liquid fuels eg., gasoline, diesel.
 Gaseous fuels eg., coal gas, natural gas

6. Thus, the classification is summarised as follows.

7. I SOLID FUELS
5.4 Goal
 Coal is an important primary solid fuel, that has been formed as a
result of alteration of vegetable matter under some favourable
conditions.
Coalification (or) Metamorphism
The process of conversion (or alteration) of vegetable matter to
anthracite (coal) is called coalification or metamorphism of coal.
5.4.1 Classification of Coal
Coal is classified on the basis of its rank.
The rank of coal indicates its degree of maturity.
Various types of coal are:

8. The progressive transformation of wood to anthracite results in
 decrease in moisture content,
 decrease in volatile content,
 decrease in hydrogen, oxygen, nitrogen and
sulphur contents,
 increase in carbon content,
 increase in hardness,
 increase in calorific value.

9. 5.5 ANALYSIS OF GOAL
5.5.1 Proximate Analysis
 It is the analysis involving the determination of physical
constituents like percentage of
 Moisture content.
 Volatile matter.
 Ash content.
 Fixed carbon in coal.

10. 1. Moisture content
 About 1 gm of powdered air-dried coal sample is taken in a
crusible, and is heated at 100 ± 105 C in an electric hot-air oven
for 1 hour.
 The loss in weight of the sample is found out and the % of
moisture is calculated as
0

11. 2. Volatile matter
 After the analysis of moisture content the crusible with residual
coal sample is covered with a lid, and is heated at 950 ± 20 C for
7 minutes in a muffle furnace.
 The loss in weight of the sample is found out and the % of
volatile matter is calculated as
0

12. 3. Ash content
 After the analysis of volatile matter, the crusible with residual
coal sample is heated without lid at 700 ± 50 C for 1/2 an hour in
a muffle furnace.
 The loss in weight of the sample is found out and the % of ash
content is calculated as
0

13. 4. Fixed carbon
 It is determined by subtracting the sum total of moisture,
volatile and ash contents from 100.
% of fixed carbon in coal
1. Moisture content
 High percentage of moisture is undesirable because
 it reduces the calorific value of coal,
 moisture in coal consumes more heat in the form of latent
heat of evaporation and hence more heat is to be supplied to
the coal,
 it increases the transport cost.

14. 2. Volatile matter
 High percentage of volatile matter is undesirable
because
 it reduces the calorific value of coal,
 large proportion of fuel on heating will distill over as
vapour, which escapes out unburnt,
 coal with high percentage of volatile matter burns
with a long flame with high smoke,
 the coal containing high percentage of volatile matter
do not coke well.
3. Ash content
High percentage of ash content is undesirable because
 it reduces the calorific value of coal,
 ash causes hindrance to heat flow as well as
produces clinkers, which blocks the air supply through
the fuel,

15.  it increases the transporting, handling and storage
costs,
 it involves additional cost in ash disposal.
4. Fixed carbon
 High percentage of fixed carbon is desirable because
higher the percentage of fixed carbon in a coal, greater
is its calorific value,
 the percentage of fixed carbon helps in designing
the furnace and the shape of the fire-box.

16. 5.5.2 Ultimate Analysis
 It is the analysis involving the determination of chemical
constituents like percentage of
 carbon and hydrogen contents
 nitrogen content
 sulphur content
 ash content
 oxygen content.
1. Carbon and Hydrogen contents
A known amount of the coal sample is burnt in a current of O2
in a combustion apparatus.
 The carbon and hydrogen, present in the coal sample, are
converted into CO2 and H2O respectively according to the
following equations.
C + O2 ------ > CO2
H2 + 1/2O2 ------ > H2O

17.  The liberated CO2 and H2O vapours are absorbed respectively in
KOH and anhydrous CaCl2 tubes of known weights.
 The increase in weight of KOH tube is due to the formation of
CO2 while increase in weight of CaCl2 tube is due to the formation
of H2O.
 From the weights of CO2 and H2O formed, the % of carbon and
hydrogen present in the coal can be calculated as follows.
Calculations
2KOH + CO2 ----- > K2CO3 + H2O
CaCl2 + 7H2O ------ > CaCl2 . 7H2O
Let
m = weight of the coal sample taken.
x = increase in weight of KOH tube.
y = increase in weight of CaCl2 tube.

18. (a) % of carbon

19. (b) % of hydrogen

20. 2. Nitrogen content
 The determination of nitrogen content is carried out by
Kjeldahl’s method.
 A known amount of powdered coal sample is heated with con.
H2SO4 in presence of K2SO4 (catalyst) in a long necked flask
(called Kjeldahl’s flask).
 Nitrogen in the coal is converted into ammonium sulphate and
a clear solution is obtained.
2N + 3H2 + H2SO4 ------ > (NH4)2SO4
 The clear solution is then heated with excess of NaOH and the
liberated ammonia is distilled over and is absorbed in a known
volume of standard N/10 HCl.
(NH4)2SO4 + 2NaOH ----- > 2NH3 + Na2SO4 + 2H2O
NH3 + HCl ------ > NH4Cl

21.  The volume of unused N/10 HCl is then determined by titrating it
against standard N/10 NaOH.
 Thus the amount of acid neutralised by liberated ammonia from coal
is determined.
 From this the percentage of nitrogen is calculated as follows.
Calculation:

23. 3. Sulphur content
 A known amount of coal sample is burnt completely in a bomb
calorimeter. During this process sulphur is converted into sulphate,
which is extracted with water.
 The extract is then treated with BaCl2 solution so that sulphates
are precipitated as BaSO4.
 The precipitate is filtered, dried and weighed. From the weight of
BaSO4 obtained, the sulphur present in the coal is calculated as
follows.

25. 4. Ash content
 Determination of ash content is carried out as in proximate
analysis
5. Oxygen content
 The percentage of oxygen is calculated as follows.
% of oxygen in coal = 100 - % of (C + H + N + S + ash)
Significance (or) Importance of Ultimates Analysis
1. Carbon and hydrogen contents
 Higher the % of carbon and hydrogen, better is the
quality of coal and higher is its calorific value.
 The % of carbon is helpful in the classification of coal.
 Higher % of carbon in coal reduces the size of
combustion chamber required.

26. 2. Nitrogen content
 Nitrogen does not have any calorific value, and its presence in
coal is undesirable.
 Good quality coal should have very little nitrogen content.
3. Sulphur content
Though sulphur increases the calorific value, its presence in
coal is undesirable because
 The combustion products of sulphur, i.e., SO2 and SO3 are
harmful and have corrosion effects on equipments.
 The coal containing sulphur is not suitable for the preparation
of metallurgical coke as it affects the properties of the metal.
4. Oxygen content
 Lower the % of oxygen higher is its calorific value.
 As the oxygen content increases its moisture holding capacity
increases, and the calorific value of the fuel is reduced.

27. Table 5.1 Differences between proximate analysis and ultimate
analysis
S.No. Proximate analysis Ultimate analysis
(i) It involves the
determinations of physical
constituents like moisture,
volatile, ash and fixed
carbon contents in coal.
It involves the
determination of chemical
constituents like carbon,
hydrogen, nitrogen and
sulphur and oxygen contents
in coal.
(ii) It gives the approximate
composition of the main
constituents of coal.
It gives the exact
composition of the
elementary constituents of
coal.

28. 5.6 CARBONISATION
 When coal is heated strongly in the absence of air (called
destructive distillation) it is converted into lustrous, dense, porous and
coherent mass known as coke.
 This process of converting coal into coke is known as
Carbonisation.
 Caking coals and coking coals When coals are heated strongly, the
mass becomes soft, plastic and fuses to give a coherent mass.
 Such type of coals are called Caking Coals.
 But if the mass so produced is hard, porous and strong then the
coals are called Coking Coals.
 Coking coals possess lower volatile matter and are used for the
manufacture of metallurgical coke.
 Thus all coking coals are caking coals but all caking coals are not
coking coals.

29. 5.6.1 Types of carbonisation
 Based on the temperature, used for carbonisation,
carbonisation is classified into two types.
1. Low Temperature carbonisation (LTC)
In LTC, carbonisation is carried out at 500 – 700 C.
2. High Temperature carbonisation (HTC)
 In HTC, carbonisation is carried out at 900 – 1300 C.
0
0

30. S.No. Property LTC HTC
1. Temperature 500 – 700 C 900 – 1200 C
2. Nature of the coke formed soft Hard
3. Yield 75 - 85% 65 - 75%
4. Percentage of volatile
matter
5 - 15% 1 - 3%
5. Yield of gaseous
Products (m /tonne)
130 - 150 300 - 390
6. Mechanical strength of
coke obtained
less strong
7. Calorific value of gaseous
products
Low High
8. Use of the coke
obtained
Domestic
purposes
Metallurgical
purposes
0 0
3

31. 5.7 Metallurgical Coke
 When bituminous coal is heated strongly in the absence of air,
the volatile matter escapes out and the mass becomes hard, strong,
porous and coherent which is called Metallurgical Coke.
5.7.1 Requisites (or) characteristics of good metallurgical coke
1. Purity
 The moisture, ash, sulphur and phosphorus contents in
metallurgical coke should be low, because moisture and ash reduce
the calorific value.
 Sulphur and phosphorus may contaminate the metal.
2. Porosity
 Coke should be highly porous so that oxygen will have
intimate contact with carbon and combustion will be complete and
uniform.

32. 3. Strength
 The coke should have very high mechanical strength inorder
to withstand high pressure of the overlying material in the
furnace.
4. Calorific value
 The calorific value of coke should be very high.
5. Combustibility
 The coke should burn easily.
6. Reactivity
 The reactivity of the coke should be low because low reactive
cokes produce high temperature on combustion.
7. Cost
 It should be cheap and readily available.

33. 5.8 MANUFACTURE OF METALLURGICAL COKE
 There are so many types of ovens used for the manufacture of
metallurgical coke.
 But the important one is Otto-Hoffman’s by product oven.
5.8.1 Otto-Hoffman’s by-product oven
Inorder to
 increase the thermal efficiency of the carbonisation process
and,
 recover the valuable by products (like coal gas, ammonia,
benzol oil, etc.) Otto-Hoffman developed modern by product
coke oven.
 The oven consists of a number of silica chambers. Each
chamber is about 10 - 12 m long, 3 - 4 m height and 0.4 - 0.45 m
wide. Each chamber is provided with a charging hole at the top.
 It is also provided with a gas off take valve and iron door at
each end for discharging coke (Fig 5.1).

34.  Coal is introduced into the silica chamber and the chambers are
closed.
 The chambers are heated to 1200 C by burning the preheated air
and the producer gas mixture in the interspaces between the
chambers.
 The air and gas are preheated by sending them through 2nd and
3rd hot regenerators.
 Hot flue gases produced during combustion are allowed to pass
through 1st and 4th regenerators until the temperature has been raised
to 1000 C.
 While 1st and 4th regenerators are being heated by hot flue gases,
the 2nd and 3rd regenerators are used for heating the incoming air
and gas mixture.
0
0

36.  For economical heating, the direction of inlet gases and flue
gases are changed frequently.
 The above system of recycling the flue gases to produce heat
energy is known as regenerative system of heat economy.
When the process is complete, the coke is removed and
quenched with water.
 Time taken for complete carbonisation is about 12-20 hours.
The yield of coke is about 70%. The valuable by products like
tar, ammonia, H2S and benzol, etc. can be recovered from coal
gas.

37. Recovery of by product
1. Tar
 The coal gases are first passed through a tower in which liquor
ammonia is sprayed.
 Tar and dust get dissolved and collected in a tank below, which is
heated by steam coils to recover back the ammonia sprayed.
2. Ammonia
 The gases are then passed through another tower in which water
is sprayed.
 Here ammonia gets converted to NH4OH.
3. Naphthalene
 The gases are again passed through a tower, in which cooled
water is sprayed.
 Here naphthalene gets condensed.

38. 4. Benzene
 The gases are passed through another tower, where petroleum is
sprayed.
 Here benzene gets condensed to liquid.
5. Hydrogen Sulphide
 The remaining gases are then passed through a purifier packed
with moist Fe2O3.
 Here H2S is retained.
 The final gas left out is pure coal gas, which is used as a gaseous
fuel.
Advantages of Otto Hoffman’s Process
 Valuable by products like ammonia, coal gas, naphthalene
etc., are recovered.
 The carbonisation time is less.
 Heating is done externally by producer gas.

39. II LIQUID FUELS
5.9 Petroleum
 Petroleum or crude oil is naturally occuring liquid fuel.
 It is a dark brown or black coloured viscous oil found deep in
earth’s crust.
 The oil is usually floating over a brine solution and above the
oil, natural gas is present.
 Crude oil is a mixture of paraffinic, olefinic and aromatic
hydrocarbons with small amounts of organic compounds like N,
O and S.

40. The average composition of crude oil is as follows
Constituents Percentage (%)
C 80 - 87
H 11 - 15
S 0.1 - 3.5
N + O 0.1 - 0.5

41. 5.9.1 Classification of petroleum
 Petroleum is classified into three types.
1. Paraffinic-Base type crude oil
 It contains saturated hydrocarbons from CH4 to C35 H72 with a
smaller amount of naphthenes and aromatics.
2. Naphthenic (or) Asphaltic Base type crude oil
 It contains cycloparaffins or naphthenes with a smaller amount
of paraffins and aromatics.
3. Mixed Base type crude oil
 It contains both paraffinic and asphaltic hydrocarbons.

42. 5.9.2 Refining of Petroleum or Crude Oil
 The crude oil obtained from the earth is a mixture of oil, water and
unwanted impurities.
 After the removal of water and other impurities, the crude oil is
subjected to fractional distillation.
 During fractional distillation, the crude oil is separated into various
fractions.
 Thus, the process of removing impuritaies and separating the
crude oil into various fractions having different boiling points is
called Refining of Petroleum.
 The process of refining involves the following steps.

43. Step 1: Separation of water (Cottrell’s process)
 The crude oil from oil well is an extremely stable emulsion
of oil and salt water.
 The crude oil is allowed to flow between two highly charged
electrodes, where colloidal water droplets combine to form large
drops, which is then separated out from the oil.
Step 2: Removal of harmful sulphur compounds
 Sulphur compounds are removed by treating the crude oil
with copper oxide.
 The copper sulphide formed is separated out by filtration.

44. Step 3: Fractional distillation
 The purified crude oil is then heated to about 400 C in an iron
retort, where the oil gets vapourised.
 The hot vapours are then passed into the bottom of a
“fractionating column” (Fig 5.2).
 The fractionating column is a tall cylindrical tower containing a
number of horizontal stainless steel trays at short distances.
 Each tray is provided with small chimney covered with a loose
cap.
0

46.  When the vapours of the oil go up in the fractionating column,
they become cooler and get condensed at different trays.
 The fractions having higher boiling points condense at lower trays
whereas the fractions having lower boiling points condense at higher
trays.
 The gasoline obtained by this fractional distillation is called
straight-run gasoline.
 Various fractions obtained at different trays are given in table 5.3.

47. Table 5.3 Various fractions, compositions and their uses
S.
No
Name of
the fraction
Boiling
Range C
Range of
C-Atoms
Uses
1. Uncondensed
gases
Below 30 C1 - C4 As a fuel under
the name of
LPG.
2. Petroleum ether 30 - 70 C5 - C7 As a solvent.
3. Gasoline or petrol 40 - 120 C5 - C9 Fuel for IC
engines.
4. Naphtha or solvent
spirit
120 - 180 C9 - C10 As a solvent
in paints and
in dry
cleaning.
0

48. S.
No
Name of
the fraction
Boiling
Range C
Range of
C-Atoms
Uses
5 Kerosene oil 180 - 250 C10 - C16 Fuel for stoves
and jet engines.
6. Diesel oil 250 - 320 C15 - C18 Diesel engine
fuel.
7. Heavy oil 320 - 400 C17 - C30 Fuel for ships
and for
production of
gasoline by
cracking.
0

49. Table 5.4 Refractionation of Heavy oils
Heavy oils on refractionation gives
S.No. Name of the Fraction Uses
1. Lubricating oils As lubricants.
2. Petroleum jelly or
vaseline
Used in medicines and
cosmetics.
3. Grease Used as lubricant.
4. Paraffin wax Used in candles, boot
polishes etc.
5. Pitch at above 400 C Used for making roads,
water proof roofing etc.

50. 5.10 SYNTHETIC PETROL
 The gasoline, obtained from the fractional distillation of
crude petroleum oil, is called straight run petrol.
 As the use of gasoline is increased, the amount of straight
run gasoline is not enough to meet the requirement of the
present community.
 Hence, we are in need of finding out a method of
synthesizing petrol.
5.10.1 Hydrogenation of coal (or) Manufacture of synthetic
petrol
 Coal contains about 4.5% hydrogen compared to about 18%
in petroleum. So, coal is a hydrogen deficient compound.
 If coal is heated with hydrogen to high temperature under
high pressure, it is converted to gasoline.

51. The preparation of liquid fuels from solid coal is called
hydrogenation of coal (or) synthetic petrol.
 An important method, available for the hydrogenation of coal
is Bergius process (or direct method).
Bergius process (or ) (Direct method)
 In this process, (fig. 5.3) the finely powdered coal is made into
a paste with heavy oil and a catalyst powder (tin or nickel oleate) is
mixed with it.
 The paste is pumped along with hydrogen gas into the
converter, where the paste is heated to 400 – 450 C under a
pressure of 200 - 250 atm.

52.  During this process hydrogen combines with coal to form
saturated higher hydrocarbons, which undergo further
decomposition at higher temperature to yield mixture of lower
hydrocarbons.
 The mixture is led to a condenser, where the crude oil is
obtained.
 The crude oil is then fractionated to yield
 Gasoline
 Middle oil
 Heavy oil.

54.  The middle oil is further hydrogenated in vapour phase to
yield more gasoline.
 The heavy oil is recycled for making paste with fresh coal
dust. The yield of gasoline is about 60% of the coal used.
5.11 KNOCKING
Definition
Knocking is a kind of explosion due to rapid pressure rise
occurring in an IC engine.
5.11.1 Causes of knocking in S.I (Spark Ignition) Engine
[Petrol engines]
 In a petrol engine, a mixture of gasoline vapour and air at
1:17 ratio is used as fuel.
 This mixture is compressed and ignited by an electric spark.
 The products of oxidation reaction (combustion) increases
the pressure and pushes the piston down the cylinder.

55.  If the combustion proceeds in a regular way, there is no problem
in knocking.
 But in some cases, the rate of combustion (oxidation) will not be
uniform due to unwanted chemical constituents of gasoline.
 The rate of ignition of the fuel gradually increases and the final
portion of the fuel-air mixture gets ignited instantaneously producing
an explosive sound known as “Knocking”.
 Knocking property of the fuel reduces the efficiency of engine. So
a good gasoline should resist knocking.
Chemical structure and knocking
 The knocking tendency of fuel hydrocarbons mainly depends on
their chemical structures.
 The knocking tendency decreases in the following order.

56. Straight chain paraffins > Branched chain paraffins >
Cycloparaffins > Olefins > Aromatics.
 Thus olefins of the same carbon-chain length possess better
anti-knock properties than the corresponding paraffins.
5.11.2 Improvement of antiknock characteristics
Prevention
The octane number of fuel can be prevented by
 blending petrol of high octane number with petrol of low
octane number, so that the octane number of the latter can be
improved,
 the addition of anti-knock agents like Tetra-Ethyl Lead
(TEL),
 now a days aromatic phosphates are used as antiknock
agent because it avoids lead pollution.

57. 5.12 OCTANCE NUMBER (or) OCTANCE RATING
 Octane number is introduced to express the knocking characteristics
of petrol.
 It has been found that n-heptane knocks very badly and hence, its
anti-knock value has been given zero.
 On the other hand, iso-octane gives very little knocking and so, its
anti-knock value has been given 100.
Definition
Thus, octane number is defined as ‘the percentage of iso-octane
present in a mixture of iso-octane and n-heptane.’

59. 5.13 LEADED PETROL (ANTI-KNOCK AGENT)
 The anti-knock properties of a gasoline can be improved by the
addition of suitable additives.
 Tetra Ethyl Lead (TEL) (C2H5)4 Pb is an important additive
added to petrol.
 Thus the petrol containing tetra ethyl lead is called leaded
petrol.
Mechanism of knocking
 TEL reduces the knocking tendency of hydrocarbon.
 Knocking follows a free radical mechanism, leading to a chain
 growth which results in an explosion.
 If the chains are terminated before their growth, knocking will
cease.
 TEL decomposes thermally to form ethyl free radicals which
combine with the growing free radicals of knocking process and thus
the chain growth is stopped.

60. Disadvantages of using TEL
When the leaded petrol is used as a fuel, the TEL is converted to lead
oxide and metallic lead.
This lead deposits on the spark plug and on cylinder walls which is
harmful to engine life.
 To avoid this, small amount of ethylene dibromide is added along with
TEL.
 This ethylene dibromide reacts with Pb and PbO to give volatile lead
bromide, which goes out along with exhaust gases.
But this creates atmospheric pollution.
So now a days aromatic phosphates are used instead of TEL.

61. 5.14 DIESEL OIL
 It is a fraction obtained between 250 – 320 C during fractional
distillation of petroleum.
 It is a mixture of C15H32 to C18 H38 hydrocarbons. Its calorific
value is about 11000 kcal/kg.
 It is used as a very good diesel engine fuel.
Causes of knocking in CI engines (Diesel engines)
 In a diesel engine, first air is alone compressed.
 This compression raises the temperature of the cylinder to about
500 C.
 Then the oil is sprayed into the heated air.
 This further raises the temperature as well as pressure.
 The expanding gases push the piston and power stroke begins.
0
0

62.  The combustion of a fuel in a diesel engine is not instaneous and the
time between injection of the fuel and its ignition is called Ignition lag
or Ignition delay.
 This delay is due to the time taken for the vapourisation of oil
droplets and raising the temperature of vapour to its ignition
temperature.
 Long ignition lags lead to accumulation of more vapours in the
cylinder, which undergo explosion during ignition.
 This is responsible for diesel knock.
 If the ignition lag is short, diesel knock will not occur.
5.14.1 Diesel index
 The quality of a diesel oil is indicated by diesel index number using
the following formula.

63. Aniline point and specific gravity is noted from API (American
Petroleum Institute) Scale.
5.15 CERTANE NUMBER (or) CERTANE RATING
 Cetane number is introduced to express the knocking
characteristics of diesel.
 Cetane (hexa decane) (C16 H34) has a very short ignition lag and
hence its cetane number is taken as 100.
 On the other hand ɑ-methyl naphthalene has a long ignition lag and
hence its cetane number is taken as zero.

64. Definition
Thus, cetane number is defined as "the percentage of hexa decane
present in a mixture of hexa decane and ɑ-methyl naphthalene,
which has the same ignition lag as the fuel under test".

65. The cetane number decreases in the following order.
Straight chain paraffins > Cycloparaffins > Olefins > Branched
paraffins > Aromatics.
The cetane number of a diesel oil can be increased by adding
additives called dopes.
Important dopes: Ethyl nitrate, Iso-amyl nitrate.

66. 5.15.1 Comparison of gasoline oil and diesel oil
S.
No.
Gasoline Oil Diesel Oil
1. Low boiling fraction of
petroleum contains
C5 - C9 hydrocarbons.
High boiling fraction of
petroleum contains
C15 - C18 hydrocarbons.
2. Fuel for SI engine. Fuel for CI engine.
3. Knocking tendency is
measured in octane rating.
Knocking tendency is
measured in cetane rating.
4. Knocking is due to
premature ignition.
Knocking is due to ignition
lag.

67. S.
No.
Gasoline Oil Diesel Oil
5. Anti-knocking is improved
by the addition of TEL.
Anti-knocking is
improved
by doping with ethyl
nitrate.
6. Its exhaust gases contain
higher amount of
pollutants.
Its exhaust gases
contain
lesser amount of
pollutants.
7. More consumption, lower
thermal efficiency.
Less consumption,
higher
thermal efficiency.

68. 5.16 POWER ALCOHOL
 When ethyl alcohol is blended with petrol at concentration of
5-10%, it is called power alcohol.
 In other words absolute alcohol (100% ethyl alcohol) is also
called power alcohol.
 Ethyl alcohol is used in an internal combustion (IC) engine.
 The addition of ethyl alcohol to petrol increases its octane
number.
 When ethyl alcohol is blended with diesel it is called E diesel.
Manufacture
 Manufacture of power alcohol involves the following two
steps

69. Step I Manufacture of Ethyl alcohol
 Ethyl alcohol can be synthesised by fermentation of
carbohydrates (sugar material).
 Fermentation of molasses, which is the residue left after the
crystallization of sugar, with yeast generates alcohol.
 This fermentation yields only about 20% alcohol.
yeast
C6H12O6 ------- > 2 C2H5OH + 2 CO2
Glucose (sugar) Ethyl alcohol
 Concentration of alcohol can be increased up to 97.6% by
fractional distillation yields rectified spirit.
 The concentration of alcohol cannot be increased by
distillation above 97.6%, because it forms a constant boiling
mixture with water.
 The constant boiling mixture has a lower boiling point than
alcohol.

70. Step II Conversion of ethyl alcohol into power alcohol
 But, for the use in IC engines, 100% alcohol (absolute alcohol)
is prepared by removing last traces of water from rectified spirit.
 It can Alcohol, containing traces of water, is distilled with
benzene. When benzene passes over with a portion of alcohol
and water, it leaves behind absolute (power) alcohol.
 Alcohol is distilled in the presence of dehydrating agent,
which holds the water. Finally absolute alcohol is mixed with
petrol at concentration of 5-10% to get power alcohols.
be done by the following two methods.

71. Properties
 Power alcohol has a lower calorific values (7000
k.cal/kg).
 It has high octane number (90).
 Its anti-knocking properties are good.
 It generates 10% more power than the gasoline of
same quantity.
 Its compression ratio is also higher.
Uses
 It is used, as a very good fuel, in motors.

72. 5.16.1 Advantages and Disadvantages of power alcohol
Advantages
 It is cheaper than petrol.
 If any moisture is present, power alcohol absorbs it.
 As ethyl alcohol contains oxygen atoms, complete combustion
occurs, so emission of CO, hydrocarbon, particulates are reduced.
Disadvantages
 As the calorific value of power alcohol (7000 cal/gm) is lower
than petrol (11,500 cal/gm), specially designed engine is required.
 Output power is reduced upto 35%.
 Due to its high surface tension, atomization of power alcohol is
difficult, so it causes starting trouble.

73.  It may under go oxidation to give acetic acid, which
corrodes engine part.
 As it contains oxygen atoms, the amount of air required
for combustion is less therefore, the engine and carburetor
need to be modified.
5.17 BIO-DIESEL
 Vegetable oils comprise of 90-95% triglycerides with small
amount of diglycerides, free fatty acids, phospholipids, etc.,
Triglycerides are esters of long chain fatty acids, like stearic acid
and palmitic acid.
 The viscosity of vegetable oils are higher and their molecular
weights are in the range of 600 to 900, which are about 3 times
higher than those of the diesel fuels.

74. Problems in using vegetable oil directly
 As the viscosity of vegetable oils are high, atomization is very
poor and hence inefficient mixing of oil with air leads to
incomplete combustion.
 Oxidation and thermal polymerization of vegetable oils cause
deposit formation.
 Their high viscosity causes misfire and ignition delay.
 Their high volatility and consequent high flash point lead to
more deposit formation.
 The use of vegetable oils as direct fuel requires modification
of the conventional diesel engine design.

75. Manufacture :Trans – esterification (or) Alcoholysis
 The above problems are overcome by reducing the viscosity of the
vegetable oils by the process known as trans-esterification or
alcoholysis.
 Alcoholysis is nothing but displacement of alcohol from an ester by
another alcohol.
 It involves treatment of vegetable oil (sunflower oil, palm oil,
soyabean oil, mustard oil, etc) with excess of methanol in presence of
catalyst to give mono ethyl esters of long chain fatty acid and glycerine.
 It is allowed to stand for some time and glycerine is separated.

76. Alcoholysis reaction is represented as
 Methyl esters of fatty acids, thus formed, are called “Bio-diesel”.
 Bio-diesel is defined as mono-alkyl esters of long chain fatty
acids derived from vegetable oils or fats.
 It is a pure fuel before blending with conventional diesel fuel.
 Bio-diesel can be blended with petroleum diesel.

77. 5.17.1 Advantages and Disadvantages of Bio-diesel
Advantages
 Bio-diesel is bio-degradable.
 It is prepared from renewable resources.
 The gaseous pollutants are lesser as compared to the
conventional diesel fuel.
 Bio-diesel can be produced from different types of vegetable
oils.
 Best engine performance and less smoke emission are
achieved.

78. Disadvantages
 Bio-diesel gels in cold weather.
 As bio-materials are hygroscopic, bio-diesel can
absorb the water from atmosphere.
 Bio-diesel decreases the horse power of the engine.
 Bio-diesel degrades and soften the rubber and
plastics, that are used in some old cars.
 Bio-diesel has about 10% higher nitrogen-oxide
(NOx) emission than conventional petroleum.