SHROFF S. R. ROTARY INSTITUTE OF CHEMICAL TECHNOLOGY (SRICT)
DEPARTMENT OF MECHANICAL ENGINEERING.
Subject: Internal Combustion Engine
Chapter 3. Combustion
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Chapter 3. Combustion
3.1 Combustion Equation
3.2 Stoichiometric air fuel ratio
3.3 Stoichiometric air per kg of fuel (Fuel contain C, H, S, O)
3.4 Enthalpy of formation
3.5 Adiabatic flame temperature
3.6 Calorific Value or Heating Value of fuel
3.7 The Bomb calorimeter
3.8 Junkers gas calorimeter
Outline
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3.1 Combustion Equation
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Principle constituents of dry air
GasMolecular
Weight
% by Volume or
Mole% by Weight
O2 31.998 20.95 23.20
N2 28.012 78.09 75.47
Ar 39.948 0.93 1.28
CO2 44.009 0.030 0.062
3.1 Combustion Equation
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Combustion Equation
Air contain N2 by 78%. So, Overall combustion equation
πΆππ»π + π +π
4π2 + 3.77π2 β ππΆπ2 +
π
2π»2π + 3.77 π +
π
4π2
Fuel composition could have been written CHy, Where y=b/a
3.2 Stoichiometric air fuel ratio
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Stoichiometric air fuel ratio
The theoretical amount of air required for complete combustion of unit
quantity of fuel is called stoichiometric air. A mixture of theoretical air
and fuel is called stoichiometric or chemically correct mixture.
For above combustion equation,
Stoichiometric air fuel ratio is,
π΄
πΉπ
=π +π4 32 + 3.77 Γ 28
π Γ 12 + π
πΆππ»π + π +π
4π2 + 3.77π2 β ππΆπ2 +
π
2π»2π + 3.77 π +
π
4π2
Equivalence ratio or Mixture Strength
=π΄ππ‘π’ππ πΉ π΄ πππ‘ππ
ππ‘πππβπππππ‘πππ πΉ π΄ πππ‘ππ
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Stoichiometric air per kg of fuel (Fuel contain C, H, S, O)
1) πΆ + π2 β πΆπ212 ππ + 32 ππ β 44 ππ
1 ππ +8
3ππ β11
8ππ
3) π + π2 β ππ232 ππ + 32 ππ β 64 ππ
1 ππ + 1 ππ β 2 ππ
2) 2π»2 + π2 β 2π»2π
4 ππ + 32 ππ β 36 ππ
1 ππ + 8 ππ β 9 ππ
If Fuel contain C kg, H kg, S kg and O kg respectively C, H, S, and O
then amount of air required
=100
23[8
3πΆ + 8π» + π β π]
=100
23[8
3πΆ + 8 π» β
π
8π» + π]
3.3 Stoichiometric air per kg of fuel (Fuel contain C, H, S, O)
3.4 Enthalpy of formation
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Enthalpy of formation (πππ)
In order to determine the energy before and after chemical reaction, it is
necessary to determine the energy of different substance before and after
chemical reaction with reference to a certain standard state so that no
ambiguity exist.
Enthalpy of formation (βπ0) of a chemical compound is defined as the
change in enthalpy when a compound is formed from its constituents in
an isothermal reaction from its natural stable elements at standard
reference state (25Β° C and 1 atm. Indicated by superscript β0β).
3.4 Enthalpy of formation
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Enthalpy of formation
Enthalpy datum for all naturally occurring stable elements are assigned
zero. (Example O2, N2, H2, Graphite)
Enthalpy at compound at (P,T)
βππ = βπ0 + βπ β β25
0 = βπ0 + ββ
3.5 Adiabatic flame temperature
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Adiabatic flame temperature
Adiabatic flame temperature or theoretical flame temperature is defined
as the theoretical temperature attained by the products of combustion in
an adiabatic process assuming complete combustion.
In actual, flame temperature are less than adiabatic flame temperature
due to
1) Canβt make perfect insulation
2) Complete combustion is not possible
3) At high temperature the gases may dissociate and reduce the flame
temperature
3.5 Adiabatic flame temperature
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Adiabatic flame temperature
By 1st law of thermodynamic
ππ βππ = π»π βπ»π
π»π = π»π
π
ππ βπ0 + βπ2 β β25
0 =
π
ππ βπ0 + βπ1 β β25
0
(β΅ ππ = 0,ππ = 0)
3.6 Calorific Value or Heating Value of fuel
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Calorific Value or Heating Value of fuel
LCV = HCV - mwhfg
Higher Calorific Value (HCV) is defined as the amount of heat energy
released due to complete combustion of unit quantity of fuel when the
products of combustion are cooled back to STP and water vapour is
condensed.
Lower Calorific Value (LCV) of the fuel is a fictitious quantity of heat
that would be obtained due to combustion of unit quantity of fuel if the
water vapour formed in the products of combustion are cooled back to
STP and still water remains in gaseous state.
3.6 Calorific Value or Heating Value of fuel
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Measurement of C.V.
The basic principle used in determining C.V. of fuel is that the known
quantity of fuel is burned and heat energy liberated is transferred to a
medium of known mass and specific heat and the rise in temperature of
medium is measured.
Bomb calorimeter is used for the measurement of heating value of a
solid fuel, while Junkers Gas Calorimeter (Continuous flow calorimeter)
is useful for the measurement of heating value of gaseous and liquid fuel.
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3.7 The Bomb calorimeter
3.7 The Bomb calorimeter
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The bomb is a heavy walled pressure vessel within which the combustion
reaction will take place at constant volume.
At the bottom of the bomb is placed sufficient amount of water such that
the atmosphere within the bomb remains saturated with water vapor
throughout the experiment. This guarantees that the water that may be
formed during the combustion reaction will remain in the liquid state.
The bomb is immersed within a can of water fitted with a precision
thermometer capable of a resolution of 0.01 β¦C. This assembly is placed
within an outer water filled jacket. The jacket water temperature remains
the same both before and after the combustion within the bomb. There is
no heat gain or loss to the bomb from outside and the process may be
considered to be adiabatic.
3.7 The Bomb calorimeter
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The fuel is taken in the form of a pellet (about 1 g) and the combustion is
accomplished by initiating it by an electrically heated fuse wire in
contact with the pellet. The bomb is filled with oxygen under high
pressure (25 bar) such that there is more than enough oxygen to
guarantee complete combustion. The heating value is estimated after
accounting for the heat generated by the fuse wire consumed to initiate
combustion.
The bomb calorimeter has approximately a diameter of 25 cm and a
height of 30 cm. Benzoic acid (C7H6O2 - solid) is used as a standard
reference material of known heat of reaction ΞH0 =β3227 kJ/mol.
Benzoic acid is taken in the form of a pellet and burnt in a bomb
calorimeter to provide the data regarding the heat capacity of calorimeter.
3.8 Junkers gas calorimeter
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3.8 Junkers gas calorimeter
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A continuous flow calorimeter is useful for determining the heating value
of gaseous fuels.
We assume that all the processes that take place on the gas side are at a
mean pressure equal to the atmospheric pressure.
The processes that take place in the calorimeter are in the steady state
with continuous flow of the gas air mixture (air provides oxygen for
combustion) and the coolant (water) through the cooling coils.
As indicated temperatures and flow rates are measured using appropriate
devices and the enthalpy fluxes involved in the apparatus are given
below.
3.8 Junkers gas calorimeter
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In the above equation HV is the heating value of the fuel, Cpp is the
specific heat of products of combustion and Cpm is the specific heat of
gas air mixture. The determination of Cpp will certainly require
knowledge of the composition of the products formed during the
combustion process.
If the exit temperature of the products is above 100β¦C the water will be
in the form of steam or water vapour. The estimated heating value is
referred to as the lower heating value (LHV) as opposed to the higher
heating value (HHV) that is obtained if the water vapour is made to
condense by recovering its latent heat.
πππ»π = ππ πΆππππ,ππ’π‘ β πΆππππ,ππ +ππ€πΆπ€[ππ€,ππ’π‘ β ππ€,ππ]
Energy balance requires that the following hold:
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