Chemistry fuels and combustion

 Knocking
 Octane and Cetane numbers
Unit 3: Fuels and Combustion
1
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CY2161-engineering chemistry-II
year/semester:I/II

 Suction Stroke: The mouth of the cylinder
opens up letting the fuel vapours with air from
carburettor to enter into the cylinder and
moves the piston down.
 Compression Stroke: The piston moves up
again and compresses the air into a much
smaller volume before igniting it with the spark
plug. The amount of compression is called the
compression ratio of the engine. A typical
engine might have compression ratio 8 to 1.
2
Knocking

 Power Stroke: When a gasoline-air mixture is
ignited by electric spark in the cylinder, it
produces a flame that rapidly and
homogeneously spread throughout the gasoline
mixture. The hot gases produced due to
combustion increases the volume and pressure
and pushes the piston down.
 Exhaust stroke: When combustion is
completed, the pressure decreases. This drives
the piston upwards again and pushes out the
exhaust gases from the cylinder through the
exhaust valve.
3
Knocking

 In certain situations, the smooth burning is
interrupted by the un-burnt mixture in the
combustion chamber.
 The first portion of the fuel burns in a normal
way, while the last portion of the charge almost
ignite instantaneously in the form of an
explosion or detonation.
 The cylinder pressure rises dramatically beyond
its design limits and damages the engine parts.
 This results in a characteristic metallic sound
called the knocking.
 Such a knocking results in loss of efficiency.
4
Knocking - Explanation

 The ratio of the gaseous volume in the cylinder
at the end of the suction-stroke to the volume at
the end of the compression stroke of the piston
is known as the “Compression Ratio”.
5
Compression Ratio - Definition

 In certain circumstances, the rate of oxidation
becomes so great that the last portion of the
fuel-air mixture gets ignited instantaneously,
producing an explosive violence, known as
“knocking”.
 Knocking is also known as “Pinking” or
“Pinging”.
 Knocking results in loss of efficiency.
6
Knocking - Definition

 The main causes of knocking are:
 Poor quality of fuel (nature and composition)
 Poor conditions of engine such as:
 Poor design of engine
 Poor mechanical condition of engine
 Poor operational manners of engine such
as:
 Incorrect combustion process
 Improper cooling of engine
 Improper exhaust gas re-circulation
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Knocking - Causes

 Knocking can be prevented by –
 Using good quality fuel with higher octane rating
 Adding anti-knocking agents
 Increasing the amount of fuel injected or
lowering the air to fuel ratio
 Reducing the cylinder pressure
 Improving the combustion chamber design
 Correct ignition timings are necessary for better
engine performance and fuel efficiency.
 Now-a-days, modern automotives have sensors that
can detect knock and retard ignition (spark plug
firing) to reduce knocking and protect the engine.
8
Knocking - Prevention

 The tendency of fuel constituents to knock is
given in the following order:
 Straight-chain paraffins > branched-chain
paraffins (i.e., iso-paraffins) > olefins >
cycloparaffins (i.e., naphthalenes) > aromatics
 Hence, olefins of the same carbon chain length
possess better anti-knocking properties than the
corresponding paraffins.
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Knocking and Chemical Structure

 In the year 1972, Edger introduced a system of
ranking of fuel based on the efficiency of fuel.
 It has been found that n-heptane knocks very badly
and hence anti-knocking value of heptane has
arbitrarily been fixed as zero.
 On the other hand, iso-octane (2,2,4-trimethy
pentane) was found to give very little knocking and
hence its anti-knocking value was fixed as 100.
CH3-(CH2)5 – CH3
n-heptane
Octane No.=0
10
Octane Rating

 The octane Number (or octane rating) of a
gasoline ( or any other IC engine fuel) is the
percentage of iso-octane in a mixture of iso-
octane and n-heptane, which matches the fuel
under test in knocking characteristics.
 Hence, if a fuel’s octane number is 80: It means
that the fuel is one which has the same
combustion characteristics as a 80:20 mixture of
iso-octane and n-heptane has.
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Octane Number - Definition

 The most common way of expressing the
knocking characteristics of a fuel is by octane
rating.
 It is a system of grading gasoline.
 It is a measure of suitability of fuel (petrol) for
high compression engines.
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Octane Number - Significance

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Octane Number of some Hydrocarbons
Hydrocarbon Octane No.
N-Octane -10
N-Heptane 0
Diesel fuel 15-25
2-Methyl Heptane 23
N-Hexane 29
1-heptene 60
1-pentene 84
Cyclohexane 97
Iso-octane (2,2,4-trimethyl pentane) 100
Benzene 101
Toluene 112
The Octane Number :
Increases with increase in compactness, double bonds and cyclic structures.

A US gas station pump offering five
different octane ratings
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 By definition, the maximum octane number is
100, i.e., 100% of iso-octane characteristics.
 But, some fuels are more knock resistant than
iso-octane, and hence the octane number is
extended to allow for numbers greater than
100.
 The octane performance of certain substances
have higher shock resistance and has been
assigned with numbers greater than 100.
 Octane Boosters such as tetra ethyl lead (TEL),
toluene, etc, also increases the octane number
of gasoline.
15
Octane Number – Above 100

 Aviation gasoline has hydrocarbons in low
boiling range (40-170oC), which shows high
octane number.
 During ‘take-off’, an aircraft engine develops
increased power which is provided by an
increase of fuel supply to give a rich mixture of
air and fuel.
 Using higher octane number fuel in aircraft
engines, increases the power to take-off.
 A change in fuel from 90 octane no. to 100
octane no. leads to 10-30% increase in power
available for ‘take-off’, due to increase in CR.
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Octane Number – Aviation Gasoline

 Efficiency of engine cycle may be defined by an
expression:
 E = 1 - (1/CR)0.258
 Consider the octane number of an aviation fuel
is increased from 90 to 100 and the
compression ratio changed from 7.8 to 9.5, let
us calculate the increase in efficiency of the
engine.
 Initial efficiency, E1 = 1 – (1/7.8)0.258 = 0.411
 Subsequent efficiency, E2= 1 – (1/9.5)0.258 = 0.441
 Increase in efficiency = 0.441 – 0.411 = 0.030
 % increase in efficiency = (0.030/0.411) = 7.29%
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Octane Number – Aviation Gasoline

 Ignition quality order among hydrocarbon
constituents of a diesel fuel is as follow:
n-alkanes > naphthalenes > alkenes > branched
alkanes > aromatics
 Thus, hydrocarbons which are poor gasoline fuels
are good diesel fuels.
18
Diesel Engine Fuels

 The suitability of a diesel fuel is determined by
cetane number or cetane value.
 Definition of Cetane number:
Cetane number is the percentage of hexadecane in a
mixture of hexadecane and 2-methyl naphthalene,
which has the same ignition characteristics as the
diesel fuel under test.
CH3-(CH2)14-CH3
Hexadecane or cetane
Cetane no.: 100
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Cetane Rating

 The cetane number of a diesel fuel can be raised by
the addition of small quantity of certain compounds
like ethyl nitrate, acetone peroxide, iso-amyl nitrite,
etc. Such compounds are called “pre-ignition
dopes”.
20
Cetane Number

 CN is a significant expression for diesel fuel quality
 It is a measure of fuel’s ignition delay, the time
period between the start of injection and start of
combustion (ignition) of the fuel.
 In a particular diesel engine, higher cetane fuels will
have shorter ignition delay periods than lower
cetane fuels.
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Cetane Number - Significance

Summary
Knocking
Octane Number
Cetane number
22

Gaseous Fuels
23

 Gas fuels are the most convenient - because
 they require the least amount of handling and
 are used in the simplest and most
maintenance-free burner systems.
 Large individual consumers do have gasholders
and some produce their own gas.
 Water Gas
 Producer Gas
 CNG
 LPG
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Gaseous Fuels

Types of gaseous fuel
(A) Fuels naturally found in nature
Natural gas (NG)
Methane from coal mines
(B) Fuel gases made from solid fuel
Gases derived from coal
Gases derived from waste and biomass
From other industrial processes
(C) Gases made from petroleum
Liquefied Petroleum gas (LPG)
Refinery gases
Gases from oil gasification
(D) Gases from some fermentation
25

Advantages of Gaseous Fuels
1. Can be produced at a central location and clean
gas can be distributed over a wide area.
2. Nuisance of smoke production and ash disposal
eliminated at point of fuel utilization.
3. Greater control of variation in demand,
conditions of combustion and nature of flame
and heating atmosphere possible.
4. Greater economy by use of efficient heat
exchange methods possible.
5. Gaseous fuels require far less excess air for
complete combustion.
26

Drawbacks in using gaseous fuel
1. Its high specific volume results in displacement
of air in premixed combustion systems.
2. Hence power produced with gaseous fuels is
less when compared to solid and liquid fuels.
3. Due to its high specific volume, gaseous fuel
containers are much larger than those for
liquid fuels.
27

Natural Gas
Natural gas may be used as
1. Liquefied Natural Gas (LNG).
2. Compressed Natural Gas (CNG).
“Natural” gas when made artificially it is called
substitute, or synthetic or supplemental
natural gas (SNG).
28

CNG compared to LNG
CNG is often confused with liquefied natural
gas (LNG). While both are stored forms of natural
gas, the key difference is that CNG is gas that is
stored (as a gas) at high pressure, while LNG is in
uncompressed liquid form.
CNG has a lower cost of production and storage
compared to LNG as it does not require an
expensive cooling process and cryogenic tanks.
29

CNG compared to LNG
CNG requires a much larger volume to store the
same mass of gasoline or petrol and the use of
very high pressures (3000 to 4000 psi, or 205 to
275 bar).
CNG can be stored at lower pressure in a form
known as an ANG (Adsorbed Natural Gas) tank, at
35 bar (500 psi, the pressure of gas in natural gas
pipelines) in various sponge like materials, such
as activated carbon and metal-organic
frameworks (MOFs).
30

 Liquefied Petroleum Gas (LPG) is obtained as a by-
product, during the cracking of heavy oils or from
natural gas.
 Major constituents of LPG are n-butane, iso-butane,
butylene and propene.
 LPG is dehydrated, desulphurized and traces of
odorous organic sulphides (mercaptans) are added
to give warning of gas leak.
 LPG is supplied under pressure in containers under
the trade names like Indane, Bharat gas, etc.
 Its calorific value is about 27,800-28,000 kcals/m3.
31
LPG (or bottled gas or refinery gas)

 Water gas is essentially a mixture of combustible
gases, CO and H2, with a little non-combustible
gases, CO2 and N2.
 It is a synthesis gas, containing CO and hydrogen.
32
Water Gas (or Blue Gas)

Gas Producer
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1. ASH ZONE
2. COMBUSTION OR
OXIDATION ZONE
3. REDUCTION ZONE
4. DISTILLATION
ZONE
GAS PRODUCER

Summary
Gaseous Fuels
 Water Gas
 Producer Gas
 CNG
 LPG
34

 Flue Gas Analysis
Orsat’s Apparatus
Unit 3: Fuels and Combustion
Part 11
35u

Flue Gas
• What is Flue Gas ?
• …. gases and vapors resulting from the
combustion process
36

Flue Gas Components
• Nitrogen (N2)
• Carbon Dioxide (CO2)
• Oxygen (O2)
• Carbon Monoxide (CO)
• Nitrogen Oxides (NOx)
• Sulfur Dioxide (SO2)
• Unburned Hydrobarbons (CxHy)
• Soot and Particulate Matter
37

Analysis Of Flue Gases
• The object of a flue gas analysis is the
determination of the completeness of the
combustion of the carbon in the fuel, and the
amount and distribution of the heat losses due to
incomplete combustion.
• The quantities actually determined by an analysis
are the relative proportions by volume, of
• carbon dioxide (CO2),
• oxygen (O2), and
• carbon monoxide (CO); the determinations being
made in this order.
38

ORSAT APPARATUS
39

Theoretical air for combustion
• Principles of Combustion
• Combustion: rapid oxidation of a fuel
• Complete combustion: total oxidation of fuel
(adequate supply of oxygen needed)
• Air: 20.9% oxygen, 79% nitrogen and other
• Nitrogen: (a) reduces the combustion efficiency
(b) forms NOx at high temperatures
• Carbon forms (a) CO2 (b) CO resulting in less heat
production
40

Theoretical air for combustion
• Principle of Combustion
• Oxygen is the key to combustion
41

COMBUSTION
• Combustion is an exothermic chemical reaction,
which is accompanied by development of heat and
light.
• For proper combustion, substance must be
brought to its kindling or ignition temperature.
• Definition:
• Ignition temperature is “the minimum temperature
at which the substance ignites and burns without
further addition of heat from outside”.
42

CALCULATION OF AIR QUALITIES
• Following elementary principles are applied, to find the
amount of oxygen or air required for combustion of a unit
quantity of a fuel.
1. Substances always combine in definite
proportions and these proportions are
determined by the molecular masses of the
substances involved and the products formed.
Eg., C (s) + O2 (g)  CO2 (g)
Mass proportions 12 32 44
Similarly, 2H2 (g) + O2 (g)  2 H2O (g)
2 x 2 = 4 32 2 x 18 = 36
43

2. 22.4 L of any gas at STP (0oC and 760 mm
pressure) has a mass equal to its 1 mol. Thus,
22.4 l of CO2 at STP has a mass of 44 g (molar
mass of CO2).
3. Air contains 21% of oxygen by volume and 23%
by mass.
i.e., 1 kg of oxygen is supplied by:
= (1 x 100)/23 = 4.35 kg of air
similarly, 1 m3 of oxygen is supplied by:
= (1 x 100)/21 = 4.76 m3 of air
44

4. Molecular mass of air is taken as 28.94 g/mol
5. Minimum oxygen required = theoretical oxygen
required – oxygen present in the fuel
6. Minimum oxygen required should be calculated
on the basis of complete combustion
7. Mass of dry flue gases formed should be
calculated by balancing the carbon in the fuel
and carbon in the flue gases.
8. The mass of any gas can be converted to its
volume at certain T and P by using the gas
equation, PV = nRT
45

9. Hydrogen, present in the combined form (as
H2O) is a non-combustible substance and does
not take part in combustion. The rest of
hydrogen, called available hydrogen only takes
part in the combustion reaction.
As, 1 part of hydrogen combines chemically with
8 parts by mass of oxygen to form water, the
available hydrogen = mass of hydrogen – (mass
of oxygen/8)
46

• Theoretical amount of oxygen required for the
complete combustion of 1 kg solid or liquid fuel:
= [(32/12) x C] + 8[H – (O/8)] + S kg
• Theoretical amount of air required for the
complete combustion of 1 kg fuel:
= (100/23) [(32/12) x C] + 8[H – (O/8)] + S kg
(since, % of oxygen in air by mass is 23)
where, C, H, S and O are the masses of C, H, S
and O respectively per kg of the fuel.
47

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