Advances in Chemical Engineering and Science, 2011, 1, 102-117
doi:10.4236/aces.2011.13017 Published Online July 2011 (http://www.SciRP.org/journal/aces)
Detailed Kinetic Mechanism of CNG
Combustion in an IC Engine
Muhammad Mansha1*, Javed Syed Hassan1, Anwar Rashid Saleemi1, Badar M. Ghauri2
1Department of Chemical Engineering, University of Engineering and Technology, Lahore, Pakistan
2Institute of Space Technology, Islamabad, Pakistan
Received March 25, 2011; revised April 29, 2011; accepted May 10, 2011
Abstract
In this study, the four kinetic reaction mechanisms were developed to simulate the formation of pollutant
species in CNG fired IC engine. The reactions were generated using EXGAS and coupled with Leed’s NOx
reactions to develop four kinetic mechanisms. These reaction mechanisms described the combustion of
natural gas at low (below 800 K) to high (above 1000 K) temperature in combustion chamber. The simula-
tion studies predicted that the maximum cylinder pressure was achieved up to 18.0 atm & 40.0 atm under
fuel leaner conditions (φ ≈ 0.6) and fuel rich conditions (φ = 1.13 to 1.3) respectively. The simulation based
data was compared with the experimental data (when engine was operated at 3000 rpm, φ = 1.0, Pinlet = 0.67
atm). For fuel rich conditions, high concentrations of CO were observed while NOx levels were lowered
while the fuel leaner mixture produced the lower CO emissions and moderate levels of NOx emissions. The
NOx and CO profiles depicted that Mechanism-I, Mechanism-II and Mechanism III seemed to be inappropri-
ate for predicting the emissions in due to CNG combustion IC engine. The molded data for Mechanism-IV
exhibited closer agreement with experimental measurements. The rate of production analysis identified the
important reactions in each mechanism which were contributing in the formation of emission concentrations
of pollutant species. Although each proposed mechanism illustrated some discrepancies among the profiles,
Mechanism-IV (consisting of 208 reactions and 78 species) showed good agreement with experimental data
for pressure, temperature and pollutant species profiles.
Keywords: Simulation, EXGAS, Mechanism, Oxidation, Species Profile
1. Introduction
Combustion is a key process of converting chemical en-
ergy to heat which is ultimately converted to mechanical
energy to derive in IC engine. Due this combustion, pre-
ssure and temperature suddenly rise within the engine
chamber which provides the driving force to push the
piston back to produce the motion in the automobile [1].
Zheng et al. [2] discussed the application of Compressed
Natural Gas (CNG) in light-duty engines. The authors
concluded that the direct injection of CNG has shown
better results in the thermal efficiencies comparable to
those attained by high compression ratio. It was also con-
cluded that the CNG fired engines produced lower NOx
emissions while maintaining the smoke-free operation of
Spark Ignition (SI) engines. To meet more stringent emi-
ssion regulations, there are still many technical obstacles
are required to be resolved in reducing the emissions
both NO and CO simultaneously emitted from the inter-
nal combustion engine as discussed by Aslam et al. [3].
In the recent decade, CNG has been promoted as a pro-
mising clean fuel alternative to spark ignition engines
because of its relatively high octane level. Lean burning
of CNG in SI engines has the potential to improve ther-
mal efficiency and reduce emissions compared with the
burning of gasoline. Recently, an in-depth understanding
of the physical and chemical processes is required to speed
up the greener engine designs.
In literature, it is reported that many research groups
recently have been working on natural gas-fuelled en-
gines worldwide via experimental work or through simu-
lation studies. For example; Shiga et al. [4] have inves-
tigated the characteristics of emissions from CNG direct
injection combustion using experiment on a rapid com-

M. MANSHA ET AL. 103
pression-machine. In study cited at [5], Computational
Fluid Dynamics (CFD) codes have been utilized to study
the fuel combustion. Kusaka et al. [6] have investigated
the dual fuel engine for both diesel and natural gas en-
gine. Ando et al. [7] has carried out the computational
analysis of combustion with self-ignited natural gas en-
gine. Zheng et al. [2] have studied combustion process
and fluid flow in a compression ignition natural gas en-
gine with separated chamber using the CFD code. These
CFD based studies required the reaction mechanisms to
study the turbulence, flame development, flame propaga-
tion etc in combustion of various fuels using Fluent,
ANSYS etc. In the literature, several detailed mecha-
nisms [8-13] and reduced mechanisms [14-20] for meth-
ane (CH4) combustion exist. The reported studies dis-
cussed the reaction mechanisms of single component
hydrocarbon fuels such as methane, butane, octane, hy-
drogen etc. [21-26].
Mansha et al. has implemented the oxidation mecha-
nisms of methane in IC engine and predicted the com-
bustion temperature and pressure in the combustion
chamber of the IC engine [27]. In this study, authors suc-
cessfully predicted the emission profiles of CO using the
GRI 3.0 mechanism and concluded that the kinetic me-
chanisms can be implemented in IC engine to simulation
combustion process. In another study, the authors devel-
oped the kinetic mechanisms of CNG and simulation
based investigation of CNG combustion was carried out
[28].
CNG is multi-component (mixture of CH4, C2H6, C3H8,
C4H10, N2, CO2) fuel and the combustion mechanism of
this mixture is needed to develop for the understanding
the formation of various products gases especially pol-
lutants (CO, NO. NO2, NH3 etc.).
In present study, reaction mechanisms (models) of
CNG combustion in IC engine have been developed to
predict the emission profiles of pollutants including
Carbon monoxide (CO) and Oxides of Nitrogen (as NO
and NO2). The model based results were validated
through experimental measurements of the in-cylinder
measurements of pressure, temperature and pollutant spe-
cies on the dedicated experimental setup developed for
this study.
2. Experimental (Material and Methods)
2.1. Development of Kinetic Reaction
Mechanisms of Natural Gas Combustion
In this study, four kinetic reaction mechanisms were
generated using EXGAS (an automatic mechanism gen-
eration tool). The EXGAS has three types of input (reac-
tants, kinetic data and thermo-chemical data) to generate
the reaction mechanisms of natural gas combustion. The
reacting mixture species (as input to the EXGAS) have
special one dimensional (1D) notation [29]. The software
has built-in-notations for methane, ethane species as base
species. These 1D notation were linked by the computer
to an internal representation, which allows species reac-
tants and products) to be stored in a unique format by
using a canonicity algorithm. Further details of mecha-
nism generation of EXGAS exist in User’s Guide of the
EXGAS [23]. The mechanisms generated were com-
posed of a) C0-C1-C2 reaction base b) primary reaction
mechanism in which the species in the initial gas mixture
are taken as the reactants and c) secondary mechanism
which contain reactions of inputs are the molecular
products of the primary mechanism. The types of con-
sidered in each of each of the four kinetic reaction me-
chanisms are given in Tabl e 1 . The types of primary mo-
lecules formed during the combustion of low hydrocar-
bons include Monohydroperoxides (OOH), Dihydroper-
oxydes (OOH)2, Allylic molecules YH, Ethers (O), Ke-
tones (CO), aldehydes (CHO) etc are given in Table 2.
Each of the four EXGAS generated reaction mechanisms
were coupled with Leeds NOx Mechanism 2.0
(http://www.chem.leeds.ac.uk/Combustion/nox.htm).
Mechanism-I is a comprehensive reaction mechanism
containing reactions feasible at range of temperature con-
ditions (below 800 K and above 1000 K). This mecha-
nism is composed of 935 elementary reactions and 185
species. Mechanism-II is a high temperature (above
1000 K) reaction mechanism and consists of 124 species
and 792 elementary reactions. This mechanism com-
posed of that type of reaction feasibly at high tempera-
ture during the combustion of natural gas. Mechanism-
III is a low tmperature (below 800 K) reaction mecha-
nism and consists of 152 species and 864 elementary
reaction. Mechanism-IV is developed by the simplifica-
tion of Mechanism-I by the chemical lumping technique.
By this technique, the reaction steps with similar reac-
tants and products have been eliminated. This reaction
model is consisting of only 72 species and 208 elemen-
tary reactions. The proposed reaction mechanisms were
used to simulate the combustion of natural gas under
various temperature conditions and to predict the emis-
sions.
2.2. Simulation of Natural Gas (as CNG)
Combustion Using Kinetic Reaction
Mechanisms in IC Engine
The combustion of CNG in IC engine was simulated
using CHEMKIN 4.1.1 simulation package. It is a pow-
erful set of software tools used to study reacting flows,
such as those found in combustion, catalysis, chemical

M. MANSHA ET AL. 104
Table 1. Primary reactions in different reaction mechanisms (generated by EXGAS) for natural gas oxidation.
Sr. No Types of Reactions Mechanism-I Mechanism-II Mechanism-III Mechanism-IV
1 Unimolecular Initiations √ √ √ √
2 Bimolecular Initiations √ √ √ √
3 Additions √ X X X
4 Additions with Oxygen √ X √ X
5 Isomerizations √ √ √ X
6 Decomposition of OOQOOH
into Branching Agents √ X √ X
7 Beta-scissions √ √ √ √
8 Decompositions to O-rings √ X √ X
9 Oxidations √ √ √ √
10 Branching √ √ √ √
11 Metatheses √ √ √ √
12 Combinations √ √ √ √
13 Dismutations √ X √ X
Table 2. Different primary molecules classes in four reaction mechanisms (generated by EXGAS) of natural gas oxidation.
Sr.No PRIMARY MOLECULES CLASSES Mechanism-I Mechanism-II Mechanism-III Mechanism-IV
1 Monohydroperoxides (ooh) 26 4 6 4
2 Dihydroperoxydes (ooh)2 X X X X
3 O-rings X X X X
4 Allylic molecules YH 6 3 2 2
5 Alkohols (oh) 5 5 5 5
6 Ethers (o) 1 1 1 1
7 Ketones (co) 10 5 5 5
8 Aldehydes (cho) 11 5 5 5
vapour deposition, and plasma etching. It consists of
rigorous gas-phase and gas-surface chemical kinetics in a
variety of reactor models that can be used to represent
the specific set of systems of interest [30].
The IC engine module (of CHEMKIN 4.1.1) is a mo-
del developed by supposing that the combustion chamber
is a 0-D closed system which assumes the complete ho-
mogenous gas mixture as working fluid. The simulation
is only valid within the time period when both intake and
exhaust valves are closed. Conventionally, the engine
cylinder events are expressed in crank rotation angle
relative to the top dead center (TDC). The intake valve
close (IVC) time of our test engine is –142 degrees
(crank angle) before TDC (BTDC). The general inputs of
the simulation (Table 3) are:
 Chemical reaction mechanisms with Arrhenius Coef-
ficients,
 Thermodynamic data,
 Molar fractions of reactive species (Natural Gas: CH4,
C2H6,C3H8, and air—O2, N2) for various equivalence
ratios and
Geometrical parameters of the IC Engine (cylinder
displaced volume, clearance volume, crank to connecting
rod ratio etc.).
The outputs extracted from the simulation studies
were:
 Temperatures profiles,
 pressures profiles,
 Main species profiles
2.3. Experimental Setup
An experimental set up was designed and developed to
measure the engine in-cylinder pressure, temperature and
concentration of carbon monoxide (CO) and oxides of
nitrogen (as NO and NO2) when the engine is fully oper-
ated on CNG. The experimental results were compared
with those obtained by the simulation studies using the
proposed kinetic reaction mechanisms.
The engine used in the present study is a spark ignition
IC engine that is widely used in three-wheeled automo-
bile rickshaw in Pakistan (called a CNG rickshaw). The
engine is having bi-fuel system operating on both petrol
and CNG. The schematic diagram of this bi-fuel system

M. MANSHA ET AL. 105
Table 3. Typical engine geometrical input parameters and initial gas (feed) mixture composition.
Sr. No Engine Geometrical Input Parameters Initial Gas Mixture (CNG Composition), Mole Fraction
Parameter (unit) Value Component Mole Fraction
1 Cylinder Volume (cm3) 63 Methane (CH4) 0.8903
2 Displaced Volume (cm3) 56.52 Ethane (C2H6) 0.0105
3 Clearance Volume (cm3) 6.48 Propane (C3H8) 0.027
4 Cylinder Diameters (cm) 14.67 Butane (C4H10) 0.0017
Nitrogen (N2) 0.072
5 Crank to Connecting Rod Ratio 1.632 Carbon Dioxide (CO2) 0.026
is shown in Figure 1 and lab view is shown in Figure 2.
The other details of this experimental setup and speci-
fications of engine are given in Tables 4 and 5.
The in-cylinder pressure and temperature were meas-
ured using a special spark plug with a built-in pressure
sensor and thermocouple. The spark plug used in this
study is a diode laser based electric spark plug and has a
fast transient pressure transducer (Kistler, Model 603B1).
This sensor was combined with a charge amplifier (Ki-
stler, Model 5010. A high precision angle displacement
sensor with a one degree resolution was added to the
crankshaft. The data acquisition for the pressure sensor
was triggered by the crank angle sensor such that a pres-
sure measurement was taken every crank angle degree.
During the pressure measurements, substantial variation
was observed in consecutive operating cycles (One en-
gine cycle consist of 4-strokes and completed in 0.0077
sec and swap 720˚ crank rotation angle). The in-cylinder
CO and NOx (as NO plus NO2) were determined through
fast sampling valve. On the exhaust side, an emission
analyzer (Test 350 XL) was used to determine the com-
position of the gases. The data was acquired at 100 Hz
through a 12 bit data acquisition system. Eight channels
of data collected are the time histories of the trigger sig-
nal (used as time reference) and cylinder pressure. Data
for all individual tests are normalized based on the initial
time reference signal. The data collection is performed
with a personal computer equipped with an AJD board
(Data Translation, Model DT2837).
3. Results and Discussion
An IC engine performance was characterized by meas-
uring the profiles of in-cylinder pressure and temperature
variations during engine cycle. The formation of carbon
monoxide (CO) and oxides of nitrogen (as NO, NO2)
was studied.
The measured data of cylinder temperature, pressure
and pollutants emission profiles (for CO, NO, NO2) were
compared with the simulation (modeled) outcomes. Dur-
ing the measurements, significant variations in the se-
lected parameters were observed within the combustion
chamber. This comparison provided the baseline infor-
mation for the selection of an appropriate kinetic model
(mechanism) of oxidation of natural gas (as CNG) in IC
engine.
3.1. The In-Cylinder Pressure and Temperature
The recorded pressure data was plotted versus crank an-
gle (the time of complete cycle duration which four-
stroke was converted to crank angle position with simu-
lation model (Chemkin 4.1.1). This variation in meas-
urements is plotted for the results obtained when engine
operating conditions were under stoichiometric (φ = 1.0)
conditions and at 3000 rpm. In Figure 3, each plot
showed the similar pattern of variations of chamber pres-
sure for each cycle while the major variations were ob-
served for peak pressures during the combustion process
occurring from –20˚ to 0˚ crank angle. The plot has not
smooth variation in the each profile (both model based
and experimental based data) which indicate the occur-
rence of unpredicted reactions and sudden release of en-
ergy as discussed in reference cited at [1]. The average
measured cylinder pressure varied from 0.61 atm to
32.62 atm for six (06) consecutive engine cycles. Under
fuel-lean conditions (φ ≈ 0.6), the maximum cylinder
pressure was reached up to 18.0 atm. Similarly, for
fuel-rich conditions, a maximum pressure of about 40.0
atm was achieved for equivalence ratio of φ = 1.13 and
1.3. Unexpectedly, there was lower maximum cylinder
pressure (≈38 atm) was observed at φ ≈ 1.4.
This high cylinder pressure indicates the degree of
completion of the combustion which depicts the maximum
release of the energy from the fuel contents during com-
bustion. Similarly, the low pressure indicates the in-com-
plete combustion which means that net heat of reaction
released is low [1]. The cylinder pressure is studied by a
number of researchers [31-35] and obtained the pressure
profiles as shown in Figure 3. For example, Ridha et al.
[31] in the study of three chemical kinetic mechanisms of
methane combustion obtained the similar pressure pro-
files under stoichiometric conditions. Li Cao et al. [30]
simulated the engine flow using the turbulence model

M. MANSHA ET AL. 106
Figure 1. Schematic diagram of experime ntal setup.
Figure 2. Lab view of experimental setup.
and correlated the engine pressure variation with the heat
released during the engine cycle. Ibrahim et al. [33] in-
vestigated the cylinder pressure for various engine speeds
and obtained the pressure profiles experimentally for

M. MANSHA ET AL. 107
Table 4. Experimental setup components.
No Component No Component
1 Petrol Tank 7 CNG Solenoid Valve
2 Petrol Solenoid 8 GAS (CNG, Methane, Fuel B) Storage Tank
3 Petrol Flow Meter 9 Spark Timing Advancer
4 CNG Mixer 10 Spark Plug
5 Carburetor 11 Fuel Selector switch
6 CNG Pressure Regulator
Table 5. Specification tested engine used in 4-stoke automobile rickshaws.
Parameter Typical Value
Engine Capacity 200 CC
Compression Ratio 10.51
Bore, mm 33.5
Stroke, mm 35.5
Cylinder Volume (cm3) 63
Displaced Volume (cm3) 56.52
Clearance Volume (cm3) 6.48
Cylinder Diameters (cm) 14.67
Crank to Connecting rod ratio 1.632
Carburetor 2-barreldown-draft
Specification of CNG carburetion system
Regulator LANDI RENZO TN1-B-SIC
Mixer Remote extractor
Spark advancer STAP 51
Crank Rotation Angle Crank Rotation Angle
Figure 3. Pressure profiles for four proposed kinetic models (simulation with IC modules of Chemkin 4.1.1 and experimental
cylinder pressure data.

M. MANSHA ET AL. 108
CNG fuel in IC engine.
It clear from Figure 3 that the curves for proposed ki-
netic Mechanism-I and Mechanism-IV shows similar
behavior throughout the cycle but the profile of Mecha-
nism-IV are much closer agreement with the experimental
data plot. Although, the both the experimental and simu-
lation profiles for Mechanism-IV do not follow the nor-
mal trend but the Mechanism-IV predicted the closer
trend with the experimental trend. On the basis of pres-
sure profiles (both simulation data and experimental data)
for Mechanism-IV, it can be conclude that Mechanism-IV
is an appropriate mechanism of CNG combustion in IC
engine as shown experimental setup (Figure 1).
Figure 4 shows the variation of average cylinder tem-
perature measured for six successive engine cycles (each
cycle consists of 4-strokes). Mechanism-I over predict
the chamber temperature (up to about 5000 K) during the
combustion of reacting mixture (CNG-Air) while Me-
chanism-II and Mechanism-III indicate the early start of
the combustion which do not coincide with the experi-
mental data.
According to Figure 4, the experimental data showed
the good agreement with simulation results were obtained
using Mechanism-IV. The average peaks cylinder tem-
perature reached up to 4000 K while the engine was op-
erating at 3000 rpm, φ = 1.0, Pinilet = 0.67 atm for Mecha-
nism-IV. The initial temperature rises (up to 1600 K) due
to the compression stroke from –100˚ to –20˚ of crank
angle position and after this there is substantial increase
in the temperature which is due to start of combustion
process (occurring between –20˚ and 2˚). When expan-
sion stroke starts, then fall of temperature was observed
and it continued with higher gradient for exhaust stroke.
The effect of fuel/air equivalence ratio on the cylinder
temperature has been investigated in the engine operating
at 3000 rpm, Pinilet = 0.67 atm. The plots in Figure 5
shows that a smooth trend of variation in cylinder tem-
perature was observed for fuel rich conditions for
equivalence ratios of φ = 1.1, 1.3 and 1.4.
The maximum peak temperature (about 4100 K) was
observed when the engine was achieved equivalence ra-
tio of 1.3. The peak temperatures and pressure observed
in the combustion cylinder are given in Table 6 along
with mean values and St. Dev. in the data for various
equivalence ratios.
3.2. Investigation of Formation of Pollutant
Species
The formation of two criteria gaseous pollutants (CO and
NOx) were studied in present research work. A fast re-
sponse NO detector, based on the chemiluminescence
Figure 4. Cylinder temperature of predicted for four mechanisms and average of six (06) consecutive operating cycles of en-
gine operating at 300 rpm, φ = 1.0, Pinilet = 0.67 atm.

M. MANSHA ET AL. 109
Figure 5. Measured cylinder temperatures at various equivalence ratios of CNG fired IC Engine operating at 3000 rpm, Pinilet
= 0.67 atm.
Table 6. Peak cylinder temperature and pressure at various equivalence ratios.
Temperature, (K) Pressure , (atm) Equivalence Ratio (φ)
Peak Mean St. Dev Peak Mean St. Dev
0.6 3976.07 3587.14 676.14 31.06 8.27 8.75
1.0 3994.09 3613.25 597.88 28.37 7.55 7.97
1.13 4053.30 3753.77 656.50 30.30 8.06 8.52
1.3 4102.4 3808.46 686.05 33.76 8.45 8.94
1.4 4100.56 3783.11 711.84 32.87 8.68 9.27

M. MANSHA ET AL. 110
(CLD) method has been used to measure continuous,
real-time levels of NO in the cylinder. The NOx concen-
trations data (as calculated as mole fraction of sum of
NO and NO2 data) is plotted versus crank angle position
as shown in Figure 6. The data was collected when en-
gine was operated at 3000 rpm under stoichiometric con-
ditions (when φ = 1.0).
The fluctuations in the NOx levels in the combustion
chamber were observed after combustion process during
expansion and exhaust phases. The NOx profile showed
that maximum NOx concentrations were produced when
engine was fired by the CNG. The increase or decrease
of the concentrations of NOx after combustion indicates
occurring of the reactions during the expansion of the
burned gas mixture but reactions consuming NOx mole-
cules are seems to dominant the producer reactions.
During the exhaust phase, the constant levels indicate the
freezing of the NOx chemistry at the low temperature [1].
It can be concluded that higher NOx levels were observed
at higher temperatures in the combustion chamber. The
curves of Figures 6 and 7 illustrated that lower emis-
sions were observed under lean mixture conditions (φ <
1.0). The shapes of these curves showed the complexities
emission controls. When engine operation becomes sta-
ble (usually when fuel flow increased and combustible
mixture become fuel-rich), high concentrations of CO
were observed while NOx levels were lowered. During
the measurements, it was also observed that when engine
was operated under load condition, leaner mixture pro-
duced lower CO emissions and moderate levels of NOx
emissions.
Figures 7 and 8 also indicate that highest concentra-
tion of NOx were near the stoichiometric conditions and
then become lower while CO level shows increasing
trend. Maximum variation in the concentrations of these
pollutants (NO and NO2) is clearly shown in Figures 7
and 8.
There are two major sources of nitrogen to in fuel
Figure 6. Measured NOx concentrations (in-cylinder) for three fuel (3000 rev/min, φ = 1.0 and Pinlet = 0.67 atm).

M. MANSHA ET AL. 111
Figure 7. Variation of CO and NOx concentration in exhaust of CNG fired IC engine (200 CC) at various equivalence ratio
(fuel/ratio).
Figure 8. Variation in concentrations of NO and NO2 in engine exhaust as function of equivalence ratio.

M. MANSHA ET AL. 112
mixture (fuel nitrogen and air nitrogen) in pre-mixed
combustion of CNG in IC engine. Studies [1] revealed
that the fuel nitrogen is source of NO via different
mechanisms.
Figure 9 shows the comparison of results of experi-
mental data of in-cylinder NOx data of IC engine fired
with CNG operating at 3000 rpm and equivalence ratio
of ≈ 1.0 and simulation data of NOx (as mole fraction)
obtained for four kinetic mechanisms using IC engine
module of Chemkin 4.1.1.
During combustion lower hydrocarbons, lower mo-
lecular weight nitrogen-containing compounds such as
ammonia (NH3), hydrogen cyanide (HCN) and cyanide
(CN) are formed [1,29]. Simulation of CNG combustion
in IC engine (Chemkin 4.1.1) predicts the existence and
formation of these precursor compounds on NO forma-
tion.
Rate of Production (ROP) analysis of each of mecha-
nism was carried out to know which type of reaction
contribute the NOx formation at typical 3000 K under
stoichiomteric conditions (φ = 1.0) at moderate speed of
3000 rpm. ROP analysis shows the how much a particu-
lar reaction contributes in production or consumption of
a specie/product. Table 7 shows the important plausible
reactions involved in NOx formation due to CNG com-
bustion in IC engine and normalized rate of production
coefficients. The positive (+ve) value of Normalized
ROP coefficient indicate the production or forward rate
is dominant while negative (-ve) value means the con-
sumption or backward rate of reaction is dominate.
In Table 7 , it is clear that NO2 is converted to NO via
reaction; NO2+M<=>NO+O+M where M is the third
body molecular specie or radical (which may be OH).
Figure 8 shows the emission data NO and NO2 from the
tested engine data measured at various equivalence ratios.
These figures indicate that the NOx formation is greatly
affected by Fuel/Air Equivalence Ratios. During the ex-
periments, it was observed that as equivalence ratio is
increased, the oxygen concentrations were lowered and
in parallel burned gas temperature decreased. When
mixture was leaned, increasing oxygen concentrations
offset the lowered gas temperature and NO emissions get
peak concentrations nearly at φ ≈ 0.97 while highest con-
centrations reached at φ ≈ 0.85.
The NOx concentrations showed different character
both under fuel lean and fuel rich conditions. Based upon
both simulations results and experimental data, it can be
concludes that;
Figure 9. Comparison of NOx profiles (with modeled data and experimental data).

M. MANSHA ET AL. 113
Table 7. Reactions involved in formation and consumption of NOx (NO & NO2) in proposed four kinetic mechanisms with
calculated normalized rate of production coefficients.
Mechanism Reaction
Normalized Rate of Pro-
duction Coefficient
Mechanism-I
1. NO REACTION
R-790 NO+H=>N+OH
R-811 NO+M<=>N+O+M
R-932 HNO+M<=>H+NO+M
2. NO2 REACTION
R-799. NO+HO2<=>NO2+OH
R-814. NO2+O<=>NO+O2
R-818. NO2+NH<=>N2O+OH
R-819. NO2+NH2<=>N2O+H2O
R-821. NO2+M<=>NO+O+M
(–0.078)
(1.000)
(0.916)
(0.082)
(0.014)
(–0.038)
(–0.617)
(0.904)
Mechanism-II
1) NO-Reaction
R-648. NO+H=>N+OH
R-649 N+OH=>NO+H
R-660 NO+N=>N2+O
R-661 N2+O=>NO+N
R-746 O+NH<=>NO+H
R-789 HNO+M<=>H+NO+M
2) NO2 Reactions
R-657 NO+HO2<=>NO2+OH
R-671 NO2+H<=>NO+OH
R-672 NO2+O<=>NO+O2
R-679 NO2+M<=>NO+O+M
(–0.642)
(0.870)
(–0.349)
(0.334)
(0.024)
(0.033)
(–0.064)
(–0.460)
(–0.475)
(1.000)
Mechanism-III
a) NO Reactions
R-704 O2+N<=>NO+O
R-719 NO+H=>N+OH
R-720 N+OH=>NO+H
R-731. NO+N=>N2+O
R-732 N2+O=>NO+N
R-743 NO2+O<=>NO+O2
R-818 O+NH<=>NO+H
b) NO2 Reactions
R-728 NO+HO2<=>NO2+OH
R-742 NO2+H<=>NO+OH
R-743 NO2+O<=>NO+O2
R-750 NO2+M<=>NO+O+M
(0.022)
(–0.267)
(0.626)
(–0.210)
(0.410)
(0.014)
(0.012)
(–0.086)
(0.548)
(–0.913)
(0.452)
Mechanism-IV
3. REACTION NO
R-61 NO+H=>N+OH
R-62 N+OH=>NO+H
R-73 NO+N=>N
2+O
R-74 N
2+O=>NO+N
R-82 NO+M<=>N+O+M
R-84 NO
2+H<=>NO+OH
4. REACTION NO2
R-84 NO2+H<=>NO+OH
R-92 NO2+M<=>NO+O+M
(–0.742)
(0.063)
(–0.239)
(0.028)
(0.905)
(–0.015)
(0.789)
(1.000)
 NO concentration become stable little early in the
expansion process and a little conversion of NO may
occur in lean fuel mixture conditions.
 In fuel rich mixture, the sharp decrease in NO con-
centrations occurs from peak values when maximum
cylinder pressure is achieved.
The in-cylinder CO measurements were done through
Rapid Sampling Valve. Figure 10 shows the measure-
ments of CO levels (measured in term of % while it is
then converted to mole fraction and ppm units). The data
is plotted against crank angle position. The CO data was
recorded at nearly –8˚ (7.8˚ actual). The varying levels of
CO after combustion process shows the occurring of
some reactions CO consumption formed during the com-

M. MANSHA ET AL. 114
Figure 10. Measured CO concentrations (in-cylinder) for 3000 rpm, φ = 1.0 and Pinlet = 0.67 atm.
bustion phase and freezing of CO chemistry when used
CNG
The experiments were conducted for various equiva-
lence ratios and data was recorded when engine was op-
erated at 3000 rpm and exit temperature was nearly 494
K. The molded data of CO data was also determined for
the four proposed kinetic mechanisms using IC engine
module of Chemkin 4.1.1 software. The simulation con-
ditions were set as given in Table 4 and fuels fractions
were used as given for CNG in Table 5. The modeled
data was compared with the experimental data (collected
when engine was operated at 3000 rpm, φ = 1.0, Pinlet =
0.67 atm fired with CNG and Figure 11 shows the com-
parison of modeled data for CO levels obtained for four
proposed kinetic mechanisms. The CO profile for Me-
chanism-I, Mechanism-II and Mechanism III indicate
that these mechanisms seem to be inappropriate for pre-
dicting the CO in IC engine. According to Figure 11, the
molded data for Mechanism-IV exhibits closer agree-
ment with experimental measurements.
The major reactions involved in formation and con-
sumption of CO in the combustion chamber of the tested
automobile (IC) engine for proposed kinetic mechanisms
are given in Table 8. The reactions are listed according
to the normalized rate of production coefficient deter-
mined by the ROP analysis of each mechanism.
According to Table 8, the following reaction were
proposed to take part in the formation of CO and shows
more contribution) during the combustion of CNG in
tested IC engine;
HNCO+M<=>NH+CO+M,
NCO+M<=>N+CO+M
4. Conclusions
The proposed mechanisms were successfully imple-
mented to predict the profiles of temperature, pressure
and pollutant species in Chemkin 4.1.1. The average mea-
sured cylinder pressure varied from 0.61 atm to 32.62 atm
for six consecutive engine cycles. The highest concentra-
tions of NOx were near the stoichiometric conditions and
then become lower while CO level shows increasing
trend. The modeled data was compared with the experi-
mental data (measured when engine was operated at
3000 rpm, φ = 1.0, Pinlet = 0.67 atm) for each proposed
mechanisms. The profile for Mechanism-I and Mecha-
nism-II showed very early start of the combustion, which
does not agree with the experimental data. Mecha-
nism-III also shows far away behavior from the actual
monitored data of the actual engine operation data. The
predicted CO profiles for Mechanism-IV exhibits closer
agreement with experimental measurements. Although
there are some discrepancies exist in the CO profiles for
Mechanism-IV and experimental data but on whole,

M. MANSHA ET AL. 115
Figure 11. Comparison of CO profiles (with modeled data and expe rime ntal data).
Table 8. Reactions in four kinetic mec hanisms (proposed) in CO for mation and consumption.
Mechanism Reactions
Normalized Rate of
Production Coefficient
Mechanism-I
CO+N2O<=>CO2+N2
CO2+N<=>NO+CO
HNCO+M<=>NH+CO+M
NCO+M<=>N+CO+M
–0.139
0.740
0.322
1.000
Mechanism-II
CO+N2O<=>CO2+N2
CO2+N<=>NO+CO
HCN+O<=>NH+CO
NCO+H<=>NH2+CO
HNCO+M<=>NH+CO+M
H+NCO<=>NH+CO
O+CN<=>CO+N
O+NCO<=>NO+CO
OH+NCO<=>NO+CO+H
NCO+M<=>N+CO+M
–0.013
0.094
0.044
0.119
–0.025
0.304
–0.017
0.290
0.138
–0.945
Mechanism-III
CO+N2O<=>CO2+N2
CO2+N<=>NO+CO
HCN+O<=>NH+CO
HNCO+H<=>NH2+CO
H+NCO<=>NH+CO
O+NCO<=>NO+CO
OH+NCO<=>NO+CO+H
NCO+M<=>N+CO+M
–0.032
–0.713
0.020
0.056
0.250
0.383
0.285
–0.246
Mechanism-IV
HNCO+M<=>NH+CO+M
OH+NCO<=>NO+CO+H
NCO+M<=>N+CO+M
0.975
0.022
1.000

M. MANSHA ET AL. 116
we can conclude the Mechanism-IV is representative
scheme of reactions predicting the combustion in CNG
fired IC engine.
5. Acknowledgements
Authors are thankful to Mrs Warth, NANCR, France for
providing copy of EXGAS software free of cost. Authors
appreciate the technical support of Mr. Ahmad Din of
Rafae Engineering for development of experimental
setup. We also extend our gratitude to the Higher Educa-
tion Commission of Pakistan for sponsoring this research
work.
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