shroff s. r. rotary institute of chemical...

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

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


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Combustion Equation

Air contain N2 by 78%. So, Overall combustion equation

𝐶𝑎𝐻𝑏 + 𝑎 +𝑏

4𝑂2 + 3.77𝑁2 → 𝑎𝐶𝑂2 +

𝑏

2𝐻2𝑂 + 3.77 𝑎 +

𝑏

4𝑁2

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,

𝐴

𝐹𝑠

=𝑎 +𝑏4 32 + 3.77 × 28

𝑎 × 12 + 𝑏

𝐶𝑎𝐻𝑏 + 𝑎 +𝑏

4𝑂2 + 3.77𝑁2 → 𝑎𝐶𝑂2 +

𝑏

2𝐻2𝑂 + 3.77 𝑎 +

𝑏

4𝑁2

Equivalence ratio or Mixture Strength

=𝐴𝑐𝑡𝑢𝑎𝑙 𝐹 𝐴 𝑟𝑎𝑡𝑖𝑜

𝑆𝑡𝑜𝑖𝑐ℎ𝑖𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝐹 𝐴 𝑟𝑎𝑡𝑖𝑜

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Stoichiometric air per kg of fuel (Fuel contain C, H, S, O)

1) 𝐶 + 𝑂2 → 𝐶𝑂212 𝑘𝑔 + 32 𝑘𝑔 → 44 𝑘𝑔

1 𝑘𝑔 +8

3𝑘𝑔 →11

8𝑘𝑔

3) 𝑆 + 𝑂2 → 𝑆𝑂232 𝑘𝑔 + 32 𝑘𝑔 → 64 𝑘𝑔

1 𝑘𝑔 + 1 𝑘𝑔 → 2 𝑘𝑔

2) 2𝐻2 + 𝑂2 → 2𝐻2𝑂

4 𝑘𝑔 + 32 𝑘𝑔 → 36 𝑘𝑔

1 𝑘𝑔 + 8 𝑘𝑔 → 9 𝑘𝑔

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

3𝐶 + 8𝐻 + 𝑆 − 𝑂]

=100

23[8

3𝐶 + 8 𝐻 −

𝑂

8𝐻 + 𝑆]

3.3 Stoichiometric air per kg of fuel (Fuel contain C, H, S, O)


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Enthalpy of formation (𝒉𝒇𝟎)

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 (ℎ𝑓0) 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 (25° C and 1 atm. Indicated by superscript ‘0’).


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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)

ℎ𝑖𝑇 = ℎ𝑓0 + ℎ𝑇 − ℎ25

0 = ℎ𝑓0 + ∆ℎ


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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) Can’t make perfect insulation

2) Complete combustion is not possible

3) At high temperature the gases may dissociate and reduce the flame

temperature


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Adiabatic flame temperature

By 1st law of thermodynamic

𝑄𝑐 −𝑊𝑐 = 𝐻𝑝 −𝐻𝑅

𝐻𝑝 = 𝐻𝑅

𝑝

𝑛𝑗 ℎ𝑓0 + ℎ𝑇2 − ℎ25

0 =

𝑅

𝑛𝑖 ℎ𝑓0 + ℎ𝑇1 − ℎ25

0

(∵ 𝑄𝑐 = 0,𝑊𝑐 = 0)


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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.


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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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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 ◦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.


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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 ΔH0 =−3227 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.


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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.


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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 100◦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.

𝑚𝑓𝐻𝑉 = 𝑚𝑔 𝐶𝑝𝑝𝑇𝑔,𝑜𝑢𝑡 − 𝐶𝑝𝑚𝑇𝑔,𝑖𝑛 +𝑚𝑤𝐶𝑤[𝑇𝑤,𝑜𝑢𝑡 − 𝑇𝑤,𝑖𝑛]

Energy balance requires that the following hold:

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