Jurnal Teknologi, 48(A) Jun 2008: 1 - 18
© Universiti Teknologi Malaysia
INHERENT FUEL CONSUMPTION AND EXHAUST
EMISSION OF THE CNG-PETROL BI-FUEL ENGINE BASED AT
NON-LOADED OPERATION
RAHMAT MOHSIN1*, ZULKEFLI YAACOB2, ZULKIFLI ABDUL
MAJID3 & SHAMEED ASHRAF4
Abstract. Compressed natural gas (CNG) is the most successful and widely used alternative
fuel for vehicles in the market today. Petrol fuelled vehicles are fitted with natural gas vehicle
(NGV) conversion kit to enable bi-fuel operation between CNG and petrol. This experimental
approach is focused on the fuel consumption, exhaust emission and fuel cost between natural
gas and petrol operations. The specially constructed test rig comprises of the bi-fuel fuel system
employed in the 1500 cc 12 valves carburettor engine NGV taxis. The inherent fuel consumption
and corresponding exhaust emission are acquired at different engine revolution per minute (rpm)
during petrol and CNG operation separately. The engine rpm operating without load is varied
from idle to more than 5000 rpm to acquire the fuel consumption and exhaust emission profile.
These two acquired data are then used to calculate the engine’s air fuel ratio. All three parameters
acquired are used to conduct comparisons between petrol and natural gas operation. It is seen
that the bi-fuel system operates with air fuel ratio ranging from 19 to 16.3 for petrol operation and
ranges from 40 to 18.7 for natural gas operations. The emission during CNG operation clearly
shows significant decrease in hydrocarbon (HC), carbon monoxide (CO), carbon dioxide (CO2)
and nitrogen oxide (NOx) over the use of petrol. In terms of cost, the use of CNG provides savings
exceeding 50% through all engine rpm compared to petrol non-loaded operations.
Keywords: Abstrak. Gas asli termampat (CNG) merupakan bahan api alternatif yang paling berjaya dan
digunakan dengan meluas bagi kenderaan terkini yang berada di pasaran. Kenderaan pacuan
petrol bagi tujuan ini biasanya dilengkapkan dengan kit penukar gas asli bagi membolehkan
operasian dwi-bahan api di antara CNG dan petrol. Pendekatan secara uji kaji ini difokuskan
ke atas penggunaan bahan api, emisi ekzos dan kos bahan api di antara operasian gas asli
dan petrol. Rig ujian terdiri dari sebuah sistem enjin teksi dwi-bahan api menggunakan 1500
cc dengan 12 injap sistem karburetor adalah dibina khusus. Penggunaan bahan api dan emisi
ekzos yang setara diperolehi pada kelajuan putaran seminit (rpm) enjin yang berbeza ketika
operasian menggunakan bahan api CNG dan petrol secara berasingan. Pengoperasian rpm enjin
tanpa bebanan diubahsuai dari kedudukan pegun kepada kedudukan melebihi 5000 rpm untuk
memperolehi profil penggunaan bahan api dan emisi ekzos. Kedua-dua data yang diperolehi
ini kemudiannya digunakan bagi mengira kadar udara bahan api enjin. Kesemua ketiga-tiga
1-4
*
1_inherent.indd 1
NGV, bi-fuel engine, fuel consumption, exhaust emission, CNG, natural gas
Gas Technology Centre (GASTEG), Faculty of Chemical and Natural Resources Engineering,
Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Darul Takzim, Malaysia
Corresponding author: Tel: 07-5535653, Fax: 07-5545667, Email: rahmat@fkkksa.utm.my
9/22/08 12:10:11 PM
RAHMAT, ZULKEFLI, ZULKIFLI & SHAMEED
parameter yang diperolehi digunakan untuk membuat perbandingan terhadap operasian
gas asli dan petrol. Pemerhatian yang dibuat menunjukkan kadar udara bahan api bermula
dari 19 ke 16.3 bagi operasian petrol dan dari 40 ke 18.7 untuk operasian menggunakan gas
asli. Emisi ketika operasian menggunakan CNG jelas menunjukkan penurunan ketara ke atas
keluaran hidrokarbon (HC), karbon monoksida (CO), karbon dioksida (CO2) dan nitrogen oksida
(NOx) dibandingkan dengan operasian menggunakan petrol. Dari segi kos, penggunaan CNG
memberikan keuntungan melebihi 50% terhadap kesemua kelajuan rpm enjin jika dibandingkan
dengan operasian menggunakan petrol.
Kata kunci: NGV, enjin dwi-bahan api, pengunaan bahan api, emisi ekzos, CNG, gas asli
1.0 INTRODUCTION
Natural gas can be used as motor vehicles fuel in compressed form known as
compressed natural gas (CNG). Natural gas is currently the most successful
alternative fuel in the world. CNG vehicles emit much less carbon monoxide than
gasoline or methanol vehicles, because CNG mixes better with air than do liquid
fuels and requires less enrichment for engine start-up, but the extent of the reduction
in pollutants will depend upon the emission control system. Because natural gas is
mostly methane, NGVs will emit fewer non-methane hydrocarbons (NMHCs) than
gasoline and methanol vehicles. NGVs will emit essentially no unregulated pollutants
(e.g. benzene), smoke or sulphur oxides, and slightly less formaldehyde than gasoline
vehicles, and their use would be expected to reduce ozone levels relative to the levels
resulting from gasoline vehicles use [1].
There are approximately 3 317 036 NGV in the world, and Malaysia has 8300
NGV as up to January 2004. The NGV in Malaysia mostly consist of bi-fuel taxis
having a 1500 cc carburetor engine. A typical petrol powered vehicle is fitted with a
conversion kit to enable the bi-fuel operation that provides the user with fuel options
of petrol and natural gas with the ease of a switch. The bi-fuel NGV conversion
kit used on the 1500 cc carburettor taxis consist of a fuel selector switch, a spark
advancer, air fuel mixer, pressure regulator, CNG solenoid valve, petrol solenoid valve
and a storage cylinder. A typical configuration of CNG system is shown in Figure
1. When the fuel sector switch is turned to natural gas, the petrol solenoid valve will
block the flow of the petrol to the carburettor while the current is supplied to the
NGV solenoid valve that permits natural gas to flow into the pressure regulator. The
pressure regulator regulates the CNG from the storage cylinder according to the
engine requirement and sends it to the mixer (placed between the air filter and the
carburettor). The natural gas supplied to the engine is proportional to the vacuum
pressure generated by the air flow through the mixer. This vacuum signal generated
by the mixer geometry acts on the 3rd stage diaphragm of the pressure regulator
to ensure suitable supply of natural gas according to the engine speed. The spark
advancer is activated during natural gas operation to alter the spark angle.
1_inherent.indd 2
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INHERENT FUEL CONSUMPTION AND EXHAUST EMISSION OF THE CNG-PETROL
Due to various differences between petrol and natural gas operation, this study
on 1500 cc carburettor engine is focused on the inherent fuel consumption and
exhaust emission at various engine speeds. Both parameters are used to calculate
the air fuel ratio of the carburetion system during petrol and natural gas operations.
This will develop a relation between the inherent fuel consumption, air fuel ratio
and the corresponding exhaust emission according to engine speed. This is used to
compare fuel consumption, operational cost and exhaust emission between fuels.
2.0 METHODOLOGY
The sole dependency on conventional fuel for vehicles and its polluting emission
have caused the growth of alternative fuel vehicles. By virtue of its abundance
reserves, the natural gas is the most widely used alternative fuel. The commercially
available fuel system serves as a basis for the new fuel system design. A schematic
diagram of the bi-fuel engine rig set up is shown in Figure 1 whilst the flow diagram
in Figure 2 provides the frame work for the entire study.
A bi-fuel engine test skid is designed and fabricated to form strong theoretical and
practical understanding on its functionality and operation. A carburettor engine is
fitted with an NGV conversion kit to form the bi-fuel engine test skid. The skid is
also fitted with wheels to ease the maneuvering of the skid.
The engine rpm gauged using an infrared tachometer installed at the flywheel
is varied and held constant by manipulating the throttle at the control panel. The
fuel consumption and emission are obtained while the engine rpm is held constant.
The petrol consumption is measured using a Gilmont variable area flow meter
whereas the exhaust emission is analysed using an autocheck emission analyser.
Three runs were conducted for each engine rpm to ensure the repeatability of the
result obtained. The engine rpm is then increased from idle to values greater then
5000 in order to obtain the fuel consumption and emission profile of the bi-fuel
engine operating on petrol.
Similar test are conducted to obtain the natural gas consumption and the emission
generated at different engine rpm. The method used to measure the natural gas
consumption is based on the fact that the storage cylinder pressure will deplete
proportionally to the moles of gas consumed at constant engine rpm. Higher engine
rpm consumes more fuel causing greater rate of pressure depletion within storage
cylinder. The storage cylinder pressure at constant engine rpm was monitored using
a Beamex MC5 Calibrator for accurate pressure reading. The time taken for the
acquired pressure depletion during constant rpm operation will be used to attain
the mol flowrate of natural gas to the engine.
1_inherent.indd 3
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RAHMAT, ZULKEFLI, ZULKIFLI & SHAMEED
Air intake
4
M
9
7
1
3
5
6
2
Engine
CNG fuel line
Petrol fuel line
8
NGV wiring
10
No
Component
No
Bi-fuel system pressure
regulation study
1
CNG storage cylinder
2
3
CNG�������������������
pressure regulator
4
Parameter selection
&
Component
CNG solenoid valve
Natural gas mixer
boundary conditions
5
Carburettor
7
Petrol solenoid valve
9
Petrol tank
Figure 1
6
8
Regulator prototype
design
10
Petrol flow meter
Spark timing advancer
Fuel selector switch
Schematic diagram of the bi-fuel engine rig set up
Restriction design
2.1 Experimental Rig Setup
Computational fluid
dynamic flow study
A Proton Magma 1.5 litre 12 valve carburettor engine as described in Table 1
was fitted
with an
NGV conversion kit to form the bi-fuel engine test rig. All three
Actuator
design
Diaphragm
Leverage
Spring
mounting points are made adjustable
in both horizontal
and vertical
direction to
aid the alignment of the engine and suit other similar engines. The rig has closure
panels to safe guard the operator from moving component hazard and fitted with
wheels to ease the manoeuvring of the
rig.
Threaded
Pressure
Regulator body design
fastener
vessel code
Regulator prototype
fabrication
1_inherent.indd 4
9/22/08 12:10:12 PM
7
5
6
Engine
CNG fuel line
Petrol fuel line
8
NGV wiring
10
INHERENT FUEL CONSUMPTION AND EXHAUST EMISSION OF THE CNG-PETROL
Bi-fuel system pressure
regulation study
Parameter selection &
boundary conditions
Regulator prototype
design
Computational fluid
dynamic flow study
Restriction design
Actuator design
Spring
Diaphragm
Pressure
vessel code
Regulator body design
Leverage
Threaded
fastener
Regulator prototype
fabrication
Regulator prototype
testing
Findings and
recommendation
Figure 2
1_inherent.indd 5
Research methodology
9/22/08 12:10:12 PM
RAHMAT, ZULKEFLI, ZULKIFLI & SHAMEED
The rig metal structure is coated with epoxy (powder coating) to avoid corrosion
caused by extreme working conditions. The installations of the NGV fuel system were
based on the MS 1096 [2], MS 1024 [3] and NFPA 52 [4] to ensure safe operation.
Drawing of the main structures used to support the engine and the NGV storage
cylinder are shown in Figures 3 and 4 that display the control panel structure used
to hold controlling devices such as the throttle control, ignition key, emergency shutoff, pressure sensor, pressure display unit and also the petrol tank. Figure 5 depicts
the bi-fuel system suspended from the rig structure. Figure 6 presents the entire rig
equipped with closure panels and extended exhaust pipe for safe operation.
Table 1
Test engine specifications
Characteristics
Proton Magma -12 valves
Displacement, cc
1468
Compression ratio
9.2
Bore, mm
75.5
Stroke, mm
82
Max output (DIN) PS/rpm net (kW/rpm)
87/6000/(64/6000)
Max torque (DIN) kg.m/rpm net (Nm/rpm)
12.5/3500/(122/3500)
Carburettor
Down-draft 2-barrel
Specification of NGV Carburetion System Tested:
Regulator
Landi Renzo TN1-B-SIC
Mixer
Remote extractor
Spark advancer
STAP 51
1_inherent.indd 6
Figure 3
Main engine frame
Figure 4
Bi-fuel fuel systems
9/22/08 12:10:21 PM
INHERENT FUEL CONSUMPTION AND EXHAUST EMISSION OF THE CNG-PETROL
Figure 5
Test rig with control panel
Figure 6
Engine test rig for operation
(MPa)
2.2 31.03
Calculations Relating to Cylinder Pressure and Mol Content
Due 27.58
to the energy equivalent factor, the energy within the natural gas priced at
RM 0.68
24.13 per litre is the same as the energy content within 1 litre of petrol priced at
RM 2.70.
It is known from reference that 1 litre of petrol has approximately 30.676
20.68
MJ of energy [5]. The energy content of Malaysian natural gas is 1053 Btu per
cubic17.24
ft which translates to 0.9574 MJ per mol of natural gas. From experiments
BWR 350 K
13.79
conducted, one 55 litre water capacity tank would accommodate
approximately
BWR 300 K
RM 10.34
8.43 worth of natural gas. The energy content Friend
of this
fully
charged
tank is
300 K
calculated
as 380 MJ by the energy content calculations
(shown
in Appendix A).
Friend 350
K
6.89
The value is then divided with the energy content perPitzer
mol300ofK natural gas to obtain
3.45
PitzerThe
350 K petrol cost for similar
397 moles of natural gas per charge of a 55 litre cylinder.
0.00
amount of energy would sum up to RM 33.45, giving savings up to RM 25.00 for
0
200
400
600
800
1000
every charge of the NGV
tank.
Mol content within 55 L storage cylinder
Referring to MS 1096, the cylinder temperature would rise up to 800°C (353 K)
during charging [2]. This internal temperature increase during charging depends on
the cylinder’s material of construction that has different heat dissipation abilities. The
storage capacity of a cylinder is inversely related to the internal temperature during
fast fill. CNG cylinders constructed primarily from steel (NGV Type 2) employed
on the system understudy provides the most cost effective storage capabilities [6].
Currently the most commercially available CNG dispensers rely on a mathematical
model or algorithm utilizing the measured temperature of the gas flowing in the
dispenser to determine full charge pressure [6]. This temperature compensating
1_inherent.indd 7
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RAHMAT, ZULKEFLI, ZULKIFLI & SHAMEED
mechanism included in the CNG dispenser causes the full charge pressure of the
storage cylinder to be below the NGV fuel system designed operating pressure of
20.68 MPa during charging. This safety mechanism helps avoid extreme internal
pressure build up caused by heat within the vehicle during extreme weather.
The amount of moles within the NGV cylinder as predicted by the energy
equivalent calculation shown in Appendix A and described above is validated using
correlations based on storage cylinder internal pressure. The generalized correlation
for gases by Pitzer [7], Benedict /Webb/Rubin [8] is compared in Figure 7 to obtain
the best prediction of mol content within the 55 litre storage cylinder. The finding
by Benedict/Webb/Rubin validates the cylinder content of 400 moles at 350 K
[2] which would bring about 18.62 MPa of pressure. This value corresponds to the
18.62 MPa value obtained during the experimental charging done with an empty
cylinder. The Pitzer correlation deviates greatly at 350 K in the vicinity of 18 616
kPa region, proving its deviation from the actual value.
(MPa)
31.03
27.58
24.13
20.68
17.24
BWR 350 K
13.79
BWR 300 K
10.34
Friend 300 K
6.89
Friend 350 K
Pitzer 300 K
3.45
Pitzer 350 K
0.00
0
200
400
600
800
1000
Mol content within 55 L storage cylinder
Figure 7 Comparisons between Benedict/Webb/Rubin, Pitzer and Friend relations of
mol content within the storage cylinder at 300 K and 350 K
1_inherent.indd 8
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INHERENT FUEL CONSUMPTION AND EXHAUST EMISSION OF THE CNG-PETROL
Table 2 Typical natural gas compositions [9]
Component
Mole (%)
Methane
83.44
Ethane
10.55
Propane
1.31
Isobutane
0.13
Normal-butane
0.07
Isopentane
0.01
Hexane
0.01
Carbon dioxide
4.17
Nitrogen
Density (kg/cm3)
0.31
0.81
Gross calorific value (MJ/kg)
49.00
Molecular weight
19.19
Specific gravity
0.64
2.3 Calculations Relating to Air-Fuel Ratio, Fuel Consumption and
Emission
The fuel consumption together with the emission results were used to conduct
carbon, hydrogen and oxygen balanced to determine the air fuel ratio [9]. The
calorific
value and specific gravity of the used gasoline fuel (C8H18) are 45 kg/
Cα Hgross
β Oγ N
δ
m3 and 0.692 respectively [9]. The physicochemical properties of CNG used in this
experiment are shown in Table 2. From this table, a chemical formula for CNG was
α = derived
1.136, βas= C4.098
.0834 α = 1.136, βδ == 4.098,
0.0062γ. = 0.0834 and δ = 0.0062.
H O, γN= ,0where
α β γ δ
 Air 


 Air 
 Fuel 

 stoichiometric
 Fuel  actual
= air fuel equivalence ratio = λ
(1)
l is the ratio between actual air fuel ratio against the stoichiometric air fuel

 H 
ratio given
by ×Equation
(0.5)Equation
4.321
15.999 × (1)
×   (2) provides complete derivation of the
2 +[10].
 Cfor
 the gasoline C H [7]. The stoichiometric
 A stoichiometric
Petrol
 calculations
air fuel ratio
8 18
=
 
 for natural gas C1.136 H4.098
H O
 0.0834 N0.0062 is carried out based on Equation
 F air
S fuel ratio
(12.011 × 1)+ (1.0079)×  C 
 

 A  Petrol 4.3211 × 15.999 × [2 + (0.5)× (2.25 )]
=
 
[(12.011 × 1)+ (1.0079 )× (2.25)]
 F S
 A  Petrol
= 15.1303
 
F S

1_inherent.indd 9
9/22/08 12:10:24 PM
Cα H β Oγ Nδ
Cα H β Oγ Nδ
α = 1.136, β = 4.098, γ = 0.0834
δ = 0.0062.
α = 1.136, β = 4.098, γ = 0.0834
δ = 0.0062.
 Air 

 RAHMAT, ZULKEFLI, ZULKIFLI & SHAMEED
 Air 
 Fuel  actual

= air fuel equivalenc
e ratio = λ
(3) [10]. The
actual
air
fuel
ratio
is calculated
based by conducting mass balance
 Fuel
actual
 Air 
=
air
fuel equivalence ratio = λ
on C, O and H [9]. The ratioAirbetween actual and
stoichiometric air fuel ratio is

 Fuel  stoichiometric 

used to calculate the inherent lambda value of the bi-fuel operation of both fuels
 Fuel  stoichiometric
10
separately.

 H 
4.321 × 15.999 × 2 + (0.5)×  
 C.999
 × 2 + (0.5)×  H 
 A  Petrol
 4.321 × 15
 
=
 

 C 
 A  Petrol

H 

 F S

 × 1)+ (1.=0079)×  
(12.011
  C 
 F S
 H 

(12.011 × 1)+ (1.0079)×  C 
 
 A  Petrol 4.3211 × 15.999 × [2 + (0.5)× (2.25 )]
=
 
4.3211
[(12.011
)+ (1=.0079
)× (×2.15
25.)999
] × [2 + (0.5)× (2.25)]
 A × 1Petrol
 F S
 
[(12.011 × 1)+ (1.0079 )× (2.25)]
 F S
 A  Petrol
=
15
.
1303
 
 A  Petrol
 F S
= 15.1303
 
F S

(2)
 A  CNG
 
 F S
 A  CNG
 
 F S
 A  CNG
 
 F  S

 H   O   N 
4.321 × 15.999 × 2 + (0.5)×   −   + 1 
   C   H   O   N 
 C
 4.321 ×15C.999
× 2 + (0.5)×  −   + 1 
=

H
O
 C N C   C 
 A  CNG
= )×   + (15.999 )×   + (14.007 )×  
(12.011×F1)+ (1.0079
 C 
 S
 N 
 CH 
 C O

 
(12.011 × 1)+ (1.0079)×  C  + (15.999 )×  C  + (14.007 )×  C 
 
 
 


 4.098   0.0834   0.0062 
4.3211 × 15.999 × 2 + (0.5)× 
−
+
1
 
 

1.136    1.136   4.098
1.136
  0.0834   0.0062 
 4.3211 ×15
.999 × 2 + (0.5)× 
=
−
 + 1

136   1.136 

 A  CNG
 4.098 
 0.0834   1.136   01..0062
= )× 
 + (15.999)× 
 + (14.007 )× 

(12.011×F1)+ (1.0079
.136 
1.136 
  1.136 
 S
41.098
 0.0834

(
12
.
011
×
1
)
+
(
1
.
0079
)
×
+
(
15
.
999
)
×
+
(14.007 )×  0.0062 





 1.136 
 1.136 
 1.136 

= 14.6858
(3)
 A  CNG
= 14.6858
 
F
 S
3.0 RESULTS AND DISCUSSION
The natural gas and petrol consumption at the corresponding engine crankshaft
revolutions (rpm) are displayed in Figures 8 and 9 respectively. The engine would
draw similar amount of air irrespective of natural gas or petrol operation. The
petrol supply system is designed by the engine manufacturer to maintain the desired
air fuel ratio to ensure proper power extraction and emission control. The NGV
conversion kit does this through the mixer that converts the air flow to the carburettor
into vacuum signal that act on the third stage diaphragm of the pressure regulator
1_inherent.indd 10
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INHERENT FUEL CONSUMPTION AND EXHAUST EMISSION OF THE CNG-PETROL
11
to provide corresponding amount of natural gas to maintain the desired air fuel
ratio. This validates the similar increment pattern of both the curves indicating
the engines intrinsic fuel requirement at the desired engine rpm. Since the fuel is
in different phases, comparisons are done by converting the fuel in terms of energy
content (similarly done in Appendix A) as shown in Figure 10.
3.5 3.5
Mols per minute
Mols per minute
3 3
2.5 2.5
2 2
1.5 1.5
1 1
0.5 0.5
0 0
0 0
1000
1000
2000
2000
3000
3000
4000
4000
5000
5000
6000
6000
RPM
RPM
Figure 8 Moles of natural gas consumed by engine at varying rpm
Liters per minute
Liters per minute
0 .12
0 .12
0 .10 .1
0 .08
0 .08
0 .06
0 .06
0 .04
0 .04
0 .02
0 .02
0 0
0 0
1000
1000
2000
2000
3000
3000
4000
4000
5000
5000
6000
6000
RPM
RPM
Figure 9
Litres of petrol required by engine at varying engine rpm
16 16
NGNG
12 12
Petrol
Petrol
Cents per minute
Cents per minute
14 14
10 10
1_inherent.indd 11
8 8
6 6
9/22/08 12:10:26 PM
0 .02
0
0
1000
2000
3000
4000
5000
6000
5000
6000
RPM
RAHMAT, ZULKEFLI, ZULKIFLI & SHAMEED
12
Cents per minute
16
14
NG
12
Petrol
10
8
6
4
2
0
0
1000
2000
3000
RPM
4000
Figure 10 Cost of fuel in cents per minute at varying engine rpm
The fuel consumption rate combined with the price of fuel has lead to the
generation of the graph in Figure 10 which compares the cost of fuel in terms of
cents per minute at the relevant engine rpm. It clearly shows that the use of natural
gas as fuel provides more then 50% savings on fuel cost over petrol operation
regardless of the engine rpm.
Table 3 Emission data for gasoline (C8H18) and CNG fuels
Fuels
RPM
CO (%)
CO2 (%)
O2 (%)
NOx (ppm)
HC (ppm)
Gasoline
1239
0.7
11.33
3.19
129
142
2037
1.33
11.86
1.73
159
82
2928
0.41
13.33
0.48
364
47
4219
2.3
11.98
0.4
701
174
5264
0.55
12.37
1.67
1158
388
863
0.04
4.91
11.48
26
776
2082
0.11
8.74
4.33
84
62
3430
0.12
10.03
1.9
194
29
4257
0.29
10.48
0.59
373
50
5073
0.44
10
1.1
805
133
CNG
1_inherent.indd 12
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INHERENT FUEL CONSUMPTION AND EXHAUST EMISSION OF THE CNG-PETROL
13
(%)
Oxygen Oxygen
(%)
The exhaust emission shown in Table 3 for both natural gas and petrol operations
are presented in the graphs shown from Figures 11 to 15 to ease analysis of the
engines inherent emission pattern during petrol and natural gas operation. It is
clear that the use of natural gas is favourable in terms of emission quality as all the
polluting components have been reduced throughout the engine rpm value from idle
to values above 5000 rpm. These emissions are the result of the engine’s intrinsic
behaviour coupled with the provided air fuel ratio (Figure 15) by the designated fuel
system. 14
12
NG
14
10
Petrol
128
NG
106
Petrol
48
26
04
2 0
1000
2000
3000
4000
5000
6000
4000
5000
6000
RPM
0
0
1000
2000
3000
RPM
Figure 11
Percentage oxygen content in exhaust gas at varying engine rpm
1400
NG
1400
1000
Petrol
1200
800
NG
1000
600
Petrol
NOx (ppm)
NOx (ppm)
1200
800
400
600
200
4000
200 0
1000
2000
3000
4000
5000
6000
RPM
0
0
2000
3000 gas at varying
4000
5000
Figure1000
12 NOx content
in exhaust
engine
rpm
6000
RPM
1_inherent.indd 13
HC (ppm)
ppm)
900
800
NG
700
900
600
800
500
700
400
600
300
500
Petrol
NG
Petrol
9/22/08 12:10:27 PM
0
0
1000
2000
3000
4000
5000
6000
RPM
RAHMAT, ZULKEFLI, ZULKIFLI & SHAMEED
14
HC (ppm)
900
800
NG
700
Petrol
600
500
400
300
200
100
0
0
1000
2000
3000
4000
5000
6000
RPM
Figure 13
Unburned hydrocarbon content in exhaust gas at varying engine rpm
CO2 (%)
16
14
NG
12
Petrol
10
8
6
4
2
0
0
1000
2000
3000
4000
5000
6000
RPM
Figure 14
CO2 content in exhaust gas at varying engine rpm
CO (%)
As stated in the previous sections, the fuel consumption and emission are used to
calculate3the air fuel ratio as shown in Figure 15. It is seen that the engine operates
with air2.5fuel ratios NG
in the range of 16.2 to 18.8 for petrol operation and 17.0 to
87.3 for natural
gas
operations. For petrol operations, the air fuel ratio within 16 to
Petrol
2
17 provides the highest thermal efficiency [11]. This validates the inherent air fuel
ratio of1.518.8, designed to save petrol during engine idle while air fuel ratio of 16
to 17 is experienced
for engine speeds between 2000 and 5000. This ensures the
1
vehicle 0.5
will mostly operate at the highest thermal efficiency to obtain the highest
attainable work from fuel to minimise fuel consumption which is directly related
0
to cost. As
stated earlier, the natural gas operates at very lean air fuel ratio of 87.3
0
1000
2000
3000
4000
5000
6000
RPM
7
1_inherent.indd 14
6
NG
9/22/08 12:10:28 PM
16
14
14
NG NG
12
12
PetrolPetrol
10
10
8
8
6
6
CO2 (%)
CO2 (%)
16
4
INHERENT
FUEL CONSUMPTION AND EXHAUST EMISSION OF THE CNG-PETROL
4
15
2
2
during idle.
This is to minimise the use of natural gas as an NGV has limited CNG
0
0
supply onboard.
The 1000
inherent natural
gas air fuel 4000
ratio increases
with6000
the engine
0
0
1000
2000 2000
3000 3000
4000
5000 5000
6000
rpm to provide richer blend that provides
the engine with more combustion power.
RPMRPM
The supplied air fuel ratio is next compared with the stoichiometric air fuel ratio to
form lambda value as shown in Figure 16.
3
3
2
2
CO (%)
CO (%)
2.5 2.5
1
NG NG
PetrolPetrol
1.5 1.5
1
0.5 0.5
0
0
0
0
1000 1000
2000 2000
3000 3000
4000 4000
5000 5000
6000 6000
RPMRPM
CO content in exhaust gas at varying engine rpm
7
7
6
6
NG NG
5
5
P etrol
P etrol
4
4
3
3
2
2
1
1
Lambda ratio
Lambda ratio
Figure 15
0
0
0
0
10001000
20002000
30003000
40004000
50005000
60006000
RPMRPM
Figure 16
Lambda ratio of the air fuel ratio corresponding to engine rpm
Petrol operated engines in general have air fuel ratio of approximately between
λ = 0.85 and λ = 1.2 if no lambda control is present [12]. The engine understudy
has an inherent design that operates with lambda ratio of 1.2 to 1.0. These values
are at the upper limit of the suggested range, provide high thermal efficiency which
in return reduces fuel consumption [11].
1_inherent.indd 15
9/22/08 12:10:28 PM
16
RAHMAT, ZULKEFLI, ZULKIFLI & SHAMEED
4.0 CONCLUSION
The bi-fuel engine test rig has successfully provided the inherent behaviour of
1500 cc bi-fuel NGV system in terms of fuel consumption and exhaust emission.
Most NGV operating in Malaysia will have similar characteristics with the findings
obtained. It is clear that the use of natural gas release less polluting exhaust emission
compared to petrol operation. The cost of natural gas operation provides over 50%
savings over the use of petrol. All this factors proves the benefits of natural gas over
petrol operations.
ACKNOWLEDGEMENTS
The authors would like to extend their gratitude to the Ministry of Science
Technology and Innovation (MOSTI) for the financial support under vot. 74169.
Special thanks is dedicated to the Research Management Centre (RMC), UTM for
the kind assistance and support during the tenure of research.The authors are also
deeply indebted to those directly and indirectly involved in this project
REFERENCES
[1] Wiederkehr, P. 1995. Motor Vehicle Pollution: Reduction Strategies Beyond 2010. Paris: Organisation for Economic
Co-operation and Development.
[2] MS 1096. 1997. Code of Practice for the Use of CNG in Internal Combustion Engines. Department of Standards
Malaysia.
[3] MS 1024. 1986. Specification for Wheel Nuts for Passenger Vehicles. Department of Standards Malaysia.
[4] NFPA 52-1984. 1984. Compressed Natural Gas (CNG) Vehicular Fuel Systems. National Fire Protection
Association.
[5] Perry, R. H. and D. W. Green 1997. Perry’s Chemical Engineers’ Handbook. 7th ed. USA: McGraw-Hill Inc.
2-195.
[6] Diggins, D. D. 1998. CNG Fuel Cylinder Storage Efficiency and Economy in Fast Fill Operations. SAE
Technical Paper 981398.
[7] Smith, J. M., H. C. Van Ness, and M. M. Abbott. 1996. Introduction to Chemical Engineering Thermodynamics.
5th ed. USA: McGraw-Hill Inc. 85-93.
[8] Chengal, Y. A. and M. A. Boles. 1998. Thermodynamics an Engineering Approach. 3rd ed. McGraw-Hill Inc. 85.
940.
[9] Kalam, M. A., H. H. Masjuki, M. A. Maleque, M. A. Amalina, H. Abdessalem, and T. M. I. Mahlia.
2004. Air-Fuel Ratio Calculation for a Natural Gas-Fuelled Spark Ignition Engine. SAE Technical Paper
2004-01-0640.
[10] Kett, P. W. 1982. Motor Vehicle Science Part 2. California: Chapman and Hall Ltd.
[11] SAE J1616. 1994. Recommended Practice for Compressed Natural Gas Vehicle Fuel.
[12] SAE J1829. 2002. Stoichiometric Air-Fuel Ratios of Automotive Fuels.
1_inherent.indd 16
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INHERENT FUEL CONSUMPTION AND EXHAUST EMISSION OF THE CNG-PETROL
17
APPENDIX A
i) From the ideal gas law, we know one mole of gas would occupy the following
volume at room temperature.
kPa litre
8.314
× 298 K
RT
mol ⋅ K
v=
=
= 24.45 litre
litrePa
P 8.314 kPa
101325
kPa
litre
× 298 K
kPa litre
kPaRT
litre
8.314
× 298 K
⋅KK
×mol
298
8.314v8.=314
× 298
K
RT
=
= 24.45 litre
mol
⋅
K
RT RT
mol=⋅ K 101325
v =P⋅ K
= 24.45 litre
Pa
= mol
=
24
.
45
litre
v = v ==
=
24
.
45
litre
PPa Pa 101325Pa
P
101325
P
101325
Btu gas. 1 cft
24.45 litre (at 298K )
1 MJ
MJ
ii) Energy content of1053
natural
×
×
×
= 0.957
cft 28.317 litre
1 mol
947.817 Btu
mol
Btu
1 cft
24.45 litre (at 298K )
1 MJ
MJ
1.45
cft
.45Klitre
(at 1298
1 MJ MJ
×
×
= 0.957 MJ
Btu 11053
cftBtu
24
litre
(×atK24
) 1 MJ
MJK×) × MJ
cft 1cft
24.×45.317
litre
(at
298
)298
= 0.957
1053 Btu
litre
1 mol
947
.817
Btu
mol
1053×
× 1053
×28.317
×
=
0
.
957
×cft 28
×
=
0
.
957
mol
.817 mol
Btu
mol
cft.31728litre
.317 litre
1 mol
947Btu
.817 Btu 947mol
cft 28
1 mollitre
9471.817
kg of1000
g
1 mol
5.07 × 10 6 J
1 MJ
30.625 MJ
iii) Energy content of0.169litre
× petrol.×
×
×
=
6
1 litre
1 kg
114 .23 g
1 mol6
1 litre
1 × 10 J
0.69 kg 1000 g
1 mol
5
.
07
×
10
J
1
MJ
30
.
625
MJ
6
0.69
kg
1000
16 Jmol
5.07
× 10
J 30MJ
1625
MJMJ
× 1 mol
×g ×510
×6 1J MJ
×625
= 30.625 MJ
0.691000
kg g1000
.
07
×
10
1
MJ
.
0.69 kg
1gmol
5
.
07
30
.
6
× .23 g× ××1 mol=
kg ×114
× 1×litre
× ×1 ×
=1×× 10 J 6 J = 1 litre
×
6
1 litre
1 kg
114
.231g× 10 61J×110
mol
1 litre
g1 mol
1 mol
itre
1 litre 1 litre1 kg 1 kg
114
.23114
g .23
J 1 litre11×l10
iv) Energy within a 55 litre NGV cylinder charged for RM 8.43
RM 8.43 30.625 MJ 379.660 MJ
based on RM 0.68 NGV
=×RM 1.34 = 30.625
MJ.
=
cylinder RM 0.68
cylinder
RM 8.43 30.625 MJ 379.660 MJ
RM
43
30
.625
MJ
×
=MJ 379.660 MJ
RM 8.RM
.43.62530MJ
.6258.MJ
379
.660
43 830
379×.660
MJ
= cylinder
RM
0.68
×cylinder
=
×
=
cylinder
RM
0
.
68
cylinder
cylinder
cylinder
cylinder
RM 0RM
.68 0.68 cylinder
v) Mol content within a cylinder based on energy per mol of natural gas.
379.660 MJ
1 mol
396.65 mol
×
=
cylinder
0.9573 MJ
cylinder
379.660 MJ
1 mol
396.65 mol
MJ
1mol
mol
×
= 396.65 mol
379MJ
.660 MJ1379
1 mol
.65 mol
379.660
mol.660
396.65396
=cylinder
MJ
× cylinder
× cylinder
= 0×.=9573
0
.
9573
MJ
cylinder
cylinder0.95730.MJ
9573 MJcylinder
cylinder
cylinder
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