This presentation was prepared by Mechanical Engineering students during their Internal Combustion Course. Students belong to a very prestigious Engineering institute of Pakistan "University of Engineering and Technology Lahore"

1. 1
Thermochemistry of fuel air mixtures
Presented to:
Dr. Shahid Imran
Presented by:
Sana Naz (2013-ME-301)
Wajiha Rehman (2013-ME-304)
Usman Sikandar (2013-ME-308)
M.Awais (2013-ME-332)
Salman Ayub (2013-ME-337)

2. 2
Outline
• Combustion Process
• What is the Flame?
• Organic Compounds
• Combustion Stoichiometry
• 1st
Law of Thermodynamics
• 2nd
Law of Thermodynamics
• Problems

3. 3
Combustion Process
• Thermodynamic aspects of particular type process
involving chemical reactions, is called combustion
• Usually occurs between fuel and an oxygen carrier (air)
• Energy stored in the bonds between constituent atoms of
fuel and air (form of internal energy) and in the
combustion process it will transformed to new molecules
of lower energy level combustion products plus release
heat (exothermic reaction)

4. 4
Combustion Process
• Controls the engine power
• Efficiency
• Controls the emissions
• Different for SI and CI engines

5. 5
Flames
• A flame is a combustion reaction propagate subsonically
through space
• The existence of flame motion implies that the reaction is
confined to a zone which is small in thickness compared to
the engine combustion chamber.
• The reaction zone is usually called the flame front
• Flames can be categorized as premixed and diffusion
flame
• Flames also categorized as laminar (mixing and transport
done by molecular process) and turbulent (enhanced by
eddies and lumps)
• Flames also categorized by whether the flow is steady or
unsteady

6. 6
Classes of Organic Compound
• Paraffins
• Olefins
• Acetylenes
• Aromatics
• Alcohols

7. 7
Combustion Stoichiometry
• Following equation defines the stoichiometric (or
chemically correct or theoretical) proportions of fuel
and air; i.e., there is just enough oxygen for conversion
of all the fuel into completely oxidized products.
• Above equation is general equation that helps to write
equation for other hydrocarbons
2 2 2 2 2( 3.773 ) 3.773
4 2 4
a b
b b b
C H a O N aCO H O a N
   
+ + + = + + + ÷  ÷
   

8. 8
First Law of Thermodynamics
• Conservation of energy is the fundamental principle in the
first law of thermodynamics.
• For a fixed mass system, energy conservation is expressed
for a finite change between two states, 1 and 2,as

9. 9
a) Energy and Enthalpy Balance
Systems changes from reactants to products (since mass is
constant, apply first law for a closed system)
RPPRPR UUWQ −=− −→
)( R
P
R
PPR VVPPdVW −== ∫−
pressureconstantatreactionofheat,TP
RPRRPPRPRPPR
H
HHUPVUPVUUPVPVQ
′
→
∆=
−=−−+=−+−=

10. 10
b) Enthalpies of formation
• The enthalpy increase associated with the reaction of forming
one mole of the given compound from its elements, with each
substance in its thermodynamic standard state at the given
temperature.
• The enthalpy of the products at the standard state relative to the
enthalpy datum is then given by
• The enthalpy of the reactants is given by
• The enthalpy increase, is then obtained from the difference
,
o
P t f i
PRODUCT
H n h= ∆∑
,
o
R t f i
REACTANT
H n h= ∆∑

11. 11
c) Heating Values
• The calorific value of a fuel is the magnitude of heat of
reaction at constant pressure or at constant volume at a
standard temperature (usually 25°C) for the complete
combustion of unit mass of fuel
• The two heating values at constant pressure are related
by
• Calorimeter
,( )P OHV P TQ H= − ∆
0
0
,
,
( )
( )
p
V
HV p T
HV V T
Q H
Q U
= − ∆
= − ∆
2
2P P
H o
HHV LHV fgH O
f
m
Q Q h
m
 
= +  ÷ ÷
 

12. 12
d) Adiabatic Combustion Process
•For an adiabatic constant volume combustion process
•For an adiabatic constant pressure combustion process
•The final temperature of the products in an adiabatic
combustion process is called the adiabatic flame
temperature
•Adiabatic flame temperature is the maximum temperature
that can be achieved
0P RU U− =
0P RH H− =

13. 13
Combustion Efficiency of ICE
Combustion efficiency is simply a measure of how efficient
is combustion process , it is given by
Where
1.Net chemical energy released within engine
2.Maximum fuel energy
( ) ( )R A P A c
t
f HV
H T H T
m Q
η
−
=
f HVm Q
, ,
,Re tan ,Pr
( ) ( ) o o
R A P A i f i i f i
i ac t i oducts
H T H T m n h n h
 
− = ∆ − ∆ ÷
 
∑ ∑

14. 14
Second Law of Thermodynamics
• No process is possible whose sole result is the absorption of heat
from a reservoir and the conversion of this heat into work
• There exists for every system in equilibrium a property called
entropy, which is a thermodynamic property of a system
• The entropy change of any system and its surroundings,
considered together, is positive
• Entropy approaches zero for any process which approaches
reversibility
• Entropy is a function of the state of the system and can be found
if any two properties of the system are known, for example

15. 15
Max. Work from an ICE & Efficiency
A fundamental measure of the effectiveness of any practical
internal combustion engine is availability conversion efficiency
Another is fuel conversion efficiency
Thermal combustion efficiency is given by
The fuel conversion, thermal conversion, and combustion efficiencies
are related by
max
a
U
W
W
η
∆
=
∆
c
f
f HV
W
m Q
η =
( ) ( )
c
t
R A P A
W
H T H T
η =
−
f c tη η η= ×

16. 16
Chemical Equilibrium
• Products of combustion are not a simple mixture of ideal
products
• We used ideal products approach to determine sto-
ichiometry
• Ideal combustion products for a hydrocarbon fuel is

17. 17
Homework Set # 2
Problem # 1
In problem 5.3 we need to find the equivalence ratio
for a turbo charged diesel engine that was∅
originally designed as naturally aspirated with
equivalence ratio = 0.75. The equivalence ratio of∅
turbocharged engine should be reduced to keep the
maximum pressure constant.

18. 18

19. 19

20. 20
Using the initial conditions, the values of temperature, pressure and
volume at different points are:

21. 21

22. 22
Problem # 2
Isooctane is supplied to a four-cylinder spark-ignition engine
at 2 gls. Calculate the air flow rate for stoichiometric
combustion. If the engine is operating at 1500 rev/min,
estimate the mass of fuel and air entering each cylinder per
cycle. The engine displaced volume is 2.4 liters. What is the
volumetric efficiency?. Also calculate the power output,
assume fuel conversion efficiency is 0.3.

23. 23
Solution:

24. 24

25. 25

26. 26
Problem # 3
The molar composition of dry exhaust gas of a
propane-fueled SI engine is given below (water was
removed before the measurement). Calculate the
equivalence ratio and then do the exhaust gas
analysis.
CO2 = 10.8%, O2 = 4.5%, CO = 0%, H2 = 0%

27. 27
Solution:

28. 28

29. 29
Problem # 4
A spark-ignition engine is operated on isooctane fuel
(CSH18). The exhaust gases are cooled, dried to remove
water, and then analyzed for CO2 , CO, H2 , O2 , Using the
overall combustion reaction for a range of equivalence ratios
from 0.5 to 1.5, calculate the mole fractions of CO2 , CO, H2 ,
and O2 in the dry exhaust gas, and plot the results as a
function of equivalence ratio. Assume:
• that all the fuel is burnt inside the engine (almost true) and
that the ratio of moles CO to moles H2 in the exhaust is 3:1,
and
• that there is no hydrogen in the exhaust for lean mixtures.

30. 30
Solution:

31. 31

32. 32
MATLab Sol.(a):

33. 33
Output

34. 34
MATLab Sol.(b):

35. 35
Output:

36. 36
Thank You
Any Question?