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Applied Thermal Engineering 30 (2010) 2219e2226

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Comparative engine performance and emission analysis of CNG and gasolinein a retrofitted car engine

M.I. Jahirul a,*, H.H. Masjuki b, R. Saidur b, M.A. Kalam b, M.H. Jayed b, M.A. Wazed c,d

a School of Engineering and Built Environment, CQUniversity, Rockhampton, QLD 4702, AustraliabDepartment of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, MalaysiacDepartment of Engineering Design and Manufacture, University of Malaya, 50603 Kuala Lumpur, MalaysiadDepartment of Mechanical Engineering, Chittagong University of Engineering & Technology, 4349 Chittagong, Bangladesh

a r t i c l e i n f o

Article history:Received 7 January 2010Accepted 28 May 2010Available online 14 June 2010

Keywords:Retrofitted carCNGEngine performanceEmission

* Corresponding author at: School of Engineering anof sciences, Engineering & Health. Central QueenslanRockhampton, QLD 4702, Australia. Tel.: þ61 (0)4138

E-mail addresses: md_jahirul@yahoo.com, m.j.islam

1359-4311/$ e see front matter Crown Copyright � 2doi:10.1016/j.applthermaleng.2010.05.037

a b s t r a c t

A comparative analysis is being performed of the engine performance and exhaust emission on a gaso-line and compressed natural gas (CNG) fueled retrofitted spark ignition car engine. A new 1.6 L, 4-cylinder petrol engine was converted to the computer incorporated bi-fuel system which operated witheither gasoline or CNG using an electronically controlled solenoid actuated valve mechanism. The enginebrake power, brake specific fuel consumption, brake thermal efficiency, exhaust gas temperature andexhaust emissions (unburnt hydrocarbon, carbon mono-oxide, oxygen and carbon dioxides) weremeasured over a range of speed variations at 50% and 80% throttle positions through a computer baseddata acquisition and control system. Comparative analysis of the experimental results showed 19.25%and 10.86% reduction in brake power and 15.96% and 14.68% reduction in brake specific fuelconsumption (BSFC) at 50% and 80% throttle positions respectively while the engine was fueled with CNGcompared to that with the gasoline. Whereas, the retrofitted engine produced 1.6% higher brake thermalefficiency and 24.21% higher exhaust gas temperature at 80% throttle had produced an average of 40.84%higher NOx emission over the speed range of 1500e5500 rpm at 80% throttle. Other emission contents(unburnt HC, CO, O2 and CO2) were significantly lower than those of the gasoline emissions.

Crown Copyright � 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction

The scarcity of petroleum fuel resources and turmoil in the oilmarket along with the acutely growing demand of oil threatens thesecurity of energy production. The necessity of fuel has gained theground for adaptation of suitable energy policy for the trans-portation sector in order to balance the demand and supply of oiland to contain the overall release of the greenhouse gases with theeventual undesirable environmental impacts. The drive created bythe energy security, climate change and the rapidly growingdemand of transport fuel lead to a quest for clean burning fuel.

Energy policy and planning with the related orientation havebecome a very important public agendum of most developed anddeveloping countries nowadays, as a result of which, the govern-ments are encouraging the use of alternative fuels of petroleum oilin the automotive engines. When evaluating different alternativefuels one has to take into account many aspects [1]:

d Built Environment, Facultyd University (CQUniversity),09227.@cqu.edu.au (M.I. Jahirul).

010 Published by Elsevier Ltd. All

- Adequacy of fuel supply,- Process efficiency,- Ease of transport and safety of storage,- Modifications needed in the distribution/refueling network in

the vehicle,- Fuel compatibility with vehicle engine (power, emissions, ease

of use, and durability of engine).

Numerous researches are going on worldwide in alternativefuels/sources of energy, such as, biodiesel, bioethanol, hydrogen cell,solar energy and compressed natural gas have so far been mostcommon approaches in this arena. Solar powered car are still notmarket adaptive as it requires more dedicated design features.Hydrogen fuel has low volumetric efficiencies and frequent pre-ignition combustion event because the power densities of premixedor port-fuel-injected hydrogen engines is significantly lower thangasoline [2]. Many academic researchers on the hydrogen economyhave queried the rationale on why hydrogen might not be the bestalternative transport fuel, including safety, cost and overall effi-ciency [3,4]. On the contrary, biodiesel and bioethanol require noengine modification for smooth operation, but they create variousproblems in the long term operation and in the higher percentage

rights reserved.

Table 1Combustion related properties of gasoline & CNG [12].

Properties Gasoline CNG

Motor octane number 86 119Molar mass (kg/mol) 108 17.2Carbon weight fraction (mass %) 86 73Stoichiometric air fuel ratio (A/F)s 12.5 14.3Stoichiometric mixture density (kg/m3) 1.4 1.7Lower heating value (MJ/kg) 42.5 46.9Lower heating value of stoic. mixture (MJ/kg) 2.9 2.3Flammability limits (vol% in air) 5.2 15.6Spontaneous ignition temperature (�C) 512 633

M.I. Jahirul et al. / Applied Thermal Engineering 30 (2010) 2219e22262220

usage, especially when biofuels are mostly derived from vegetableoils and crops-seeds. These alternatives are strongly criticized for itsenvironmental impact and phenomenal threat to food security[5e7]. Apart from experimental investigations, several theoreticalresearches are proceeding in the quest for alternative fuels. Saiduret al. [8] evaluated the effect of partial substitution of diesel fuel bythe natural gas on performance parameters of a four-cylinder dieselengine. Other types of alternative fuels, such as, methyl and ethylalcohol, boron, liquefied petroleum gas, biomass, electricity solarenergy, etc., are also potential alternative sources of energy in theinternal combustion engine [9]. Artificial neural network has beenapplied to predict the gasoline engines emission and performance[10]. As the consequence of these studies, researches on CNG fueledengine are alsoprogressing throughout theworlddue to its potentialas an alternative fuel for the spark ignition (S. I.) engine. The differ-ence between the operation of the conventional gasoline fueled andthe CNG-engine system arises from the physical and chemicalproperties of these two fuels. It is a well known fact that petroleumfuels are liquid at room temperature and CNG remains in a gaseousstate at a much lower temperature (�161 �C). CNG has a lowerdensity but higher octane number then gasoline. It can easilyoperate in a high compression ratio and higher self/spontaneousignition temperature makes it a safer fuel in case of leakage [11].Table 1 represents the comparison between the physiochemicalproperties of CNG and that of the gasoline.

As a gas, CNG requires a different approach of fuel inductionmechanism at all normal temperatures and pressures. This hasresulted in an increased interest in the use of CNG as fuel for theinternal combustion engines and hence CNG has now been used topower vehicles of various ranges, starting from light delivery trucksto full size urban buses and other varieties of applications [13,14].But most of the CNG-engine vehicles used today are retrofitted fromthe gasoline engine. This type of engines cannot advantageouslyperform on CNG as an engine fuel. However, the research has some-what succeeded tominimize the drawbacks of the CNG in retrofittedcars andharvests themaximumobtainable fromtheCNG the result ofwhich concludes that a dedicated CNG engine is a must. In thisexperimental study, a comparative evaluation of the performance ofgasolineandCNGfueledretrofittedspark ignitioncarenginehadbeenperformed. The enginewas converted to a computer incorporated bi-fuel system and operated with either gasoline or CNG using an elec-tronically controlled solenoid actuated valve system. The engine was

Fig. 1. Schematic diagram of

operated at constant throttle positions with a variable speed toevaluate the performance and exhaust emission for both the fuels.

2. Experimental study

2.1. Experimental setup

The layout of experimental setup is as shown in Fig. 1. The testengine was converted into a bi-fuel natural gas engine from an SIengine. The specification of this SI engine is shown in Table 2.

An electronic control unit (ECU) was used with the CP 128control and managing system which was compatible with anycomputer having a serial interface. This system was designed toperform engine tests either under an automatic or a manualcontrol. The ECU system was incorporated with the “CADET10”software. The CADET10 system was fed by parameters, such as,engine speed (rpm), engine torque (kW) and throttle (%) valveposition as input. The set of parameters was programmed accordingto the experimental condition and stages required. Each stagerequired either two settings, such as, engine speed and throttleposition. The results recorded in the CADET systemwas transferredto a spreadsheet for further analysis. An eddy-current dynamom-eter (Model AG 150, Froude Consine) was used for engine loading.Each engine test started with idle running for engine heating upand stability in power generation.

2.2. Test plan

All equipments were calibrated according to the manufacturer’srecommendations before starting the test. The engine was

the experimental setup.

Table 2Specification of the SI Engine.

Engine Type Gasoline

Displacement, cc 1594Number of cylinder 4Compression ratio 9.5:1Bore, mm 78Stroke, mm 83.4Max. power, kW/rpm 79.43/5700Max. torque, Nm/rpm 143.42/4500Maximum speed, rpm 6500

Table 3Typical composition (vol. %) of CNG (source: PETRONAS).

Component Symbol Volumetric %

Methane CH4 94.42Ethane C2H6 2.29Propane C3H8 0.03Butane C4H10 0.25Carbon dioxide CO2 0.57Nitrogen N2 0.44Others (H2O þ) 2

Table 4Standard error in measurements.

Item 50% Throttle(CNG)

50% Throttle(Petrol)

80% ThrottleCNG)

80% Throttle(Petrol)

SFC(kg/kWh) 0.0132 0.0207 0.0064 0.0105Engine speed

(rev/min)0.5131 0.6439 0.7201 0.7669

Power (kW) 0.0248 0.0188 0.0680 0.1634Temperature 0.0272 0.0775 0.06271 0.0561HC (ppm) 0.0156 0.0285 0.06364 0.0892CO (%) 0.0159 0.0123 0.01127 0.0292O2(%) 0.0266 0.0593 0.01659 0.08834CO2(%) 0.02483 0.0293 0.01869 0.03174NOx(%) 0.09783 0.0459 0.07231 0.08125

M.I. Jahirul et al. / Applied Thermal Engineering 30 (2010) 2219e2226 2221

operated according to SAE J1349 standard and tested for the bestsetting for each fuel type as well as at the stoichiometric condition.The ignition timing was 23� before TDC and 28�btdc for gasolineand CNG, respectively. The data were saved for analyzing afteraveraging each test for three times repeatedly. The engine wasoperated in constant throttle positions and variable speed modesfor both the gasoline and the CNG to test the exhaust emissionperformance. The relevant data were collected from each enginetest to calculate the performance parameters, such as, brake power(kW), brake specific fuel consumption (BSFC, kg/kWh), breakthermal efficiency (%) and exhaust gas temperature (�C). The testsettings were as follows-

- 50% throttle position with a speed range from 1500 to5500 rpm at a constant increment of 500 rpm.

- 80% throttle position with a speed range of 1500e5500 rpm ata constant increment of 500 rpm.

Over the speed range, the loadwas varied from 25% to 65% of fullload (122 Nm). Engine run at full throttle position with CNG wasavoided due to safety measures. An attempt to run the engine at fullthrottle resulted in the burning of the engine exhaust manifold andthe tail pipe insulators with the emission of unusual sound whichmight be due to the high exhaust gas temperature being producedby continuous operation on the CNG.

A computerized data acquisition and control system was usedfor controlling all the operations regarding the tests where everystage was allowed to run around 6e8 min duration providing datawhich were captured for every 30 s. All measurements wererepeated at least three times for each test setting while the testsequences were repeated for four times.

Gasoline consumption was measured on a volumetric basisusing a pipette and the gasoline delivery system was accordinglyconfigured so that the spillback from the fuel injector was returnedto a downstream position of the measuring pipette. CNGconsumption was measured by means of a high sensitive digitalweighing machine. The CNG cylinder was placed on the platform ofthe weighing machine which recorded the weight of the cylinderwith the CNG. While running the engine, CNG was consumed andthis resulted in the reduction of weight of the cylinder which couldbe monitored through the weighing machine digital display. Afterthe completion of each testing stage, the weight reductions of CNGcylinder and operation time were recorded and used to estimatethe CNG consumption rate thereafter. The stoichiometric airefuelratio of the CNG was also calculated from its composition as shownin Table 3. This composition of CNG was provided by the suppliercompany, PETRONAS.

2.3. Retrofitting equipments

Retrofitting is themodificationsof theengine to runonadifferentfuel type instead of the base fuel. Retrofitting was required on theconventional gasoline fueled engine for running with the CNGbecause of different ignition and burning characteristics of the CNG

from that of the gasoline. A conversion kit (model “LROMEGASe 3rdGeneration”) was installed on the engine test bed and the CNG wasstored in a cylinder at a maximum pressure of 200 bar (approxi-mately). The gas regulator used with the conversion kit wasa compensated, two-stage diaphragm type regulator, together withthe wateregas heat exchanger, filter, gas solenoid valve and safetyvalve which was duly calibrated for a supply pressure of 2 bar(200 kPa) above the pressure of the intake manifold. Electronicswitch regulator was used to monitor the current fuel (CNG or,gasoline) by using two illuminated LED and the pressure of the CNGin the tank was monitored by 5 illuminated LEDs. The system wascapable of controllingother functions, suchas, fuel gauge, actuator ofthe solenoid valve, automatic switch of the solenoid valve, automaticswitch from the gasoline to the CNG and vice versa. The systemperformance and diagnosis during the installation andmaintenancephases could be done by connecting the ECU with a computer. ECUcouldalsomanage theautomatic switch intogasolinemode incaseoffailure. The CNG fuel stored in the rail was injected by the CNGinjectors into the intakemanifoldwhile the injectorswere driven bythe CNG ECU. The rail was installed with 4 injectors which weredrivenwith a “peak and hold” actuation.

2.4. Emission analyzer

A BACHARACH exhaust gas analyzer is used to measure theconcentration of NOx (ppm) emission and Bosch gas analyzer(model ETT 00.36) is used to measure CO (vol.%), CO2 (vol.%) andanother O2 (vol.%) concentrations. The technology of this analyzerconsisted of automatic measurements with microprocessor controland self-test, auto calibration before every analysis and a highdegree of accuracy in the analysis of low concentrations of gasesfound in the engine. The exhaust gas for the analysis was tappedfrom the exhaust pipe, approximately 1 m from the exhaust valve.

3. Results and discussions

Standard errors of this experiment are shown in Table 4 to showrepeatability of it.

Fig. 2. Brake power vs. engine speed (a) at 50% throttle condition (b) at 80% throttle condition.

M.I. Jahirul et al. / Applied Thermal Engineering 30 (2010) 2219e22262222

3.1. Brake power

The brake power output versus engine speed for both the gaso-line and the CNG fuel was measured. Fig. 2 shows the brake poweroutput for the 50% and 80% throttle positions respectively. The brakepower of the enginewas lower than that of the gasoline throughoutthe speed range for the CNG operation. Displacement of air bynatural gas and by the slower flame velocity of CNG were the mainreasons of the lower brake power as compared to that of the gaso-line, as a result of which both the air volumetric efficiency and thecharge energydensity per injection into the engine cylinder reducedtheCNGcontent. In the case of liquid fuels, itwas considered that thefuel didnot reduce the amountof air sucked into the cylinder.Hence,a gasoline-fuel-designed engine which was converted to CNGoperation would significantly produce the low peak power.

The peak brake power of 27.7 kW was obtained by the gasolinefuel at 50% throttle position and 22.67 kW for the CNG, both at4000 rpm. For 80% throttle position themaximumbrake powerwas54.97 kW and 50.44 kW for the gasoline and the CNG used,respectively. The CNG fuel produced an average (overall the speedranges) of 19.25% and 10.86% less brake power than that of thegasoline at 50% and 80% throttle positions, respectively. These

Fig. 3. Specific fuel consumption vs. engine speed (a) at 5

power differences among the throttle positions were attributed tothe variations in the difference of the fueleair equivalent ratio withthe decrease of the throttle opening between the gasoline opera-tion and the CNG operation. As the friction loss was constant, thepercentage of friction power loss with the increase of the throttleopeningwas less for the CNG fuel. Therefore, the difference in brakepower through the gasoline and the CNG operations decreasedwith the increase of throttle opening.

3.2. Brake specific fuel consumption

Fig. 3 shows the variation of brake specific fuel consumptionover the speed range of 1500e5500 rpm. Specific fuel consumption(SFC), when the engine was running using the CNG, was alwayslower than that for the gasoline throughout the speed range. Thiswas mainly due to the higher heating value of the CNG (47.669 MJ/kg) as compared to that of the gasoline (44 MJ/kg) and the slowburning of CNG as compared to that of the gasoline.

At low throttle operation the SFC increased at high rpm becauseof the rapid increase of friction power as compared that as dis-played by the indicated power. The average SFC differences

0% throttles condition (b) at 80% throttles condition.

Fig. 4. Exhaust gas temperature vs. engine speed (a) at 50% throttle condition (b) at 80% throttle condition.

Table 5Air Fuel Ratio.

Engine Speed(rpm)

Air fuel ratio

CNG 50%throttle

Petrol 50%throttle

CNG 80%throttle

Petrol 80%throttle

1500 15.04 17.88 15.85 19.772000 14.98 17.03 15.76 18.602500 15.00 15.97 15.65 16.213000 14.99 15.24 15.34 15.293500 14.99 14.97 15.13 15.124000 14.96 14.90 15.07 14.934500 15.02 14.83 14.99 14.915000 14.99 14.80 15.03 14.845500 14.93 14.82 14.91 14.87

M.I. Jahirul et al. / Applied Thermal Engineering 30 (2010) 2219e2226 2223

between the gasoline and the CNG operations were around 15.96%and 14.68% at 50% and 80% throttle conditions, respectively.

SFC rapidly dropped in the low speed range and nearly leveledoff at medium speeds and finally spurted in the high speed range(Fig. 3). At low speeds, the heat lost to the combustion chamberwalls was proportionately greater, resulting in higher fuelconsumption for the power produced. At high speeds, the frictionpower was rapidly increasing, resulting in a slower increase in thebrake power than the rate in fuel consumption, with a consequentincrease in the SFC. At 50% constant throttle position the lowest SFCwas found to be at 2000 rpm for both fuels and it was 0.45 kg/kWhfor the gasoline and 0.37 kg/kWh for the CNG, respectively. At 80%constant throttle position, the lowest SFC was at 4500 rpm for bothfuels and it was 0.32 kg/kWh for the gasoline and 0.29 kg/kWh forthe CNG, respectively.

However, for the 50% throttle position, the average SFC of theengine for the gasoline and the CNG were found to be 0.448 and0.376 kg/kWh, respectively, while for the 80% throttle position theaverage BSFC of the engine for the gasoline and the CNGwere foundto be 0.326 and 0.28 kg/kWh, respectively. The percentage differ-ences of SFC were 16.07 and 14.11 kg/kWh respectively for the 50%and the 80% throttle positions.

3.3. Exhaust gas temperature

The exhaust gas temperature comparison at the 50% and the 80%throttle condition with the variable speeds of 1500 rpme5500 rpmare as shown in Fig. 4. The exhaust gas temperature of the CNG wasalways higher than that of the gasoline throughout the speed range.On the average, the exhaust gas temperature was around5.91e24.21.6% more than the gasoline for the 50% and the 80%throttle conditions, respectively, due to the higher heating valueand ignition temperature of the CNG than that of the gasoline.Slower flame propagation speed of the CNG than that of thegasoline allowed the combustion to proceed until the end of theexpansion stroke which increased the exhaust gas temperature forthe CNG operation.

The exhaust gas temperature increased with the increase ofengine speed, as shown in Fig. 4. At high speed, the heat remainedtrapped as heat transfer took time from the engine cylinder to thewater jacket, coolant while the lube oil was reduced. At 80%throttle, the average exhaust gas temperature was 602.36 �C and747.05 �C for the gasoline and the CNG, respectively. The maximumexhaust gas temperature was found to be 747.05 �C and 893.1 �C at

5000 rpm for the gasoline and the CNG, respectively when thethrottle position was at 80%.

At 50% throttle position the average exhaust gas temperaturewas 573 �C and 607 �C while running on the gasoline and the CNG,respectively. The maximum exhaust gas temperature was 723 �Cand 769 �C at 4500 and 5000 rpm for the gasoline and the CNG,respectively for 80% throttle positions.

3.4. Unburnt hydrocarbon (HC) emission

The rate of HC release is influenced by the molecular weight ofthe respective fuel. During expansion, drop in the pressure incylinder draws compressed unburnt fuel from crevice volume tocreate reverse blowby. At the end of this reverse blowby, flamereaction quenched and some unreacted fuel particle remains in theexhaust. Rich airefuel ratio with insufficient oxygen prompts theincomplete combustion of fuel as a misfire produces the unburnthydrocarbons. The airefuel ratio of this experiment is presented inTable 5. The airefuel ratio was calculated based on exhaust emis-sion data using reference. The optimized ignition timing was23�btdc and 28�btdc for gasoline and CNG respectively.

The molecular weight of gasoline (114) is much higher than NG(16.04) [12]. Being light weight fuel, NG can form much betterhomogeneous airefuel mixture. On the other hand, liquid fuelrequires time for complete atomization and vaporization toproduce a homogeneous mixture. Fig. 5 shows the HC emissioncomparison at the 50% and the 80% throttle conditions with thevariable speed from 1500 rpm to 5500 rpm. The HC emission ofCNG was lower than that of the gasoline throughout the speed

Fig. 5. Hydrocarbon (HC) emission over a speed range at 50% and 80% throttlecondition for gasoline and CNG.

Fig. 7. Oxygen emission over a speed range at 50% and 80% throttle condition forgasoline and CNG.

M.I. Jahirul et al. / Applied Thermal Engineering 30 (2010) 2219e22262224

range, and on an average of 22.14% and 29.71% lower than gasolinefor the 50% and the 80% throttle conditions, respectively.

The average HC emission found for 80% throttle position was355.1 ppm and 249.6 ppm for running the engine on gasoline andCNG, respectively, and for the 50% throttle condition those valuesare 484.35 ppm and 377.1 ppm, respectively.

3.5. Carbon mono-oxide (CO) emission

Poor mixing of air and fuel, local rich regions and incompletecombustion produces CO. Fig. 6 shows the CO emission at 50% and80% throttle conditions with the variable speed from 1500 rpm to5500 rpm for both the gasoline and the CNG, respectively. The COemission of the CNG was significantly lower than that of thegasoline throughout the speed range. On the average, 45.5% and29.87% less CO emission occurred for the CNG at the 50% and the80% throttle conditions, respectively. Thus, the CNG is morecombustible than the gasoline fuel. Higher combustion tempera-ture was another reason of the low CO emission of the CNG fueledengine. At high combustion temperatures, the CO converts to CO2during combustion.

3.6. Oxygen (O2) concentration

At low speed the oxygen concentration is very high in theexhaust gas and decreases rapidly with the increase of speed.

Fig. 6. Carbon monoxide (CO) emission over a speed range at 50% and 80% throttlecondition for gasoline and CNG.

Reason for the reduction is due to the complete combustion effectwith increasing speed engine.

O2 concentration in the CNG emission was lower than that ofgasoline throughout the speed range, and on the average, it was73% and 64% less at the 50% and the 80% throttle conditions,respectively. The average O2 concentration for the 80% throttleposition was 2.49% and 0.9% while running on the gasoline andthe CNG, respectively. Whereas, at 50% throttle opening thosevalues were 1.95% and 0.52% for the gasoline and the CNG,respectively, as shown in Fig. 7. The throttle opening increases O2concentration.

3.7. Carbon dioxide (CO2) emission

Fig. 8 shows that CO2 emission at the 50% and the 80% throttleconditions over the speed range. The CO2 emission of the CNG wasfound to be lower than that of the gasoline throughout the speedrange, and on the average, it was around 30.88% and 34.97% lowerthan that of the gasoline for the 50% and the 80% throttle condition,respectively. The average CO2 concentration for the 50% throttleopening was 10.95% and 7.57% for the engine running on thegasoline and the CNG, respectively, while for the 80% throttlecondition these average values were 12.42% and 8.08%, respectively.

The composition of gas showed that the CNG consistedmostly ofmethane (CH4) whereas the gasoline (C8H18) compound packedless hydrogen per carbon (2.5). Thus, the percentage of carbon inthe methane, i.e., the CNG was lower than that of the gasoline. This

Fig. 8. Carbon dioxide (CO2) emission over a speed range at 50% and 80% throttlecondition for gasoline and CNG.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

1500 2000 2500 3000 3500 4000 4500 5000 5500

)mpp(

xO

N

Engine Speed(rpm)

Petrol _80% throttle CNG _80% throttle Petrol_ 50% throttle CNG_50% throttle

Fig. 9. Nitrogen oxides (NOx) emission at 50% and 80% throttle condition for gasoline and CNG.

M.I. Jahirul et al. / Applied Thermal Engineering 30 (2010) 2219e2226 2225

led to the lower emission of CO2 for the CNG than the gasoline fuel.The CO2 emission increased with the increase of engine speed forboth the CNG and the gasoline fuels. This was due to the increase offuel conversion efficiency.

3.8. Nitrogen oxides (NOx) emission

NO is produced more in the post-flame gases than in the flame-front. The mixture which burned early in the combustion processwas being compressed to a higher temperature, thus increasing theNO formation rate, as the combustion proceeded and the cylinderpressure increased. Comparative emission of the oxides of nitrogen(NOx) by the CNG and the gasoline are shown in Fig. 9. The NOx

emission was strongly related to the lean fuel with the highcylinder temperature or high peak combustion temperature. A fuelwith high heat release rate at premix or rapid combustion phaseand lower heat release rate at mixing controlled combustion phasewould produce the NOx [15]. For this reason, the CNG emitted moreNOx than the gasoline both in the 50% and the 80% throttle position,as shown in the figure.

Fig. 10. Engine performance and emission change in CNG over gasoline in percentageat 50% and 80% throttle position.

4. Conclusion

A number of conclusions are comprehensible from the results ofthis experimental study.

- The CNG produces lower brake power than the gasolinethroughout the speed range.

- Retrofitted car engine runs on lower BSFC when using CNGthan on gasoline.

- The CNG has an advantage of higher brake thermal efficiencyon an average of 1.1% and 1.6% than that of gasoline.

- The engine exhaust gas temperature produced by the CNGburning is always higher as comparedwith that of the gasoline.

- CNG fueled retrofitted car engine produced lower HC, CO, O2emission throughout the speed range than gasoline.

- Higher NOx emission is the main emission concern for CNG asautomotive fuel. 41% and 38% higher NOx emissions have beenrecorded at 50% and 80% throttle position respectively,compared to that of gasoline. Such a huge emission rangeshould be a major environmental concern as CNG retrofittedautomotives are now mass produced and used.

Based on the performance and the emission test results, thepresent study indicates that the CNG is a better choice as auto-mobile fuel than the gasoline both economically and environ-mentally. An overall view of the experiment is shown in Fig. 10 bypercentage change in all the engine performance parameters andthe emission components.

Acknowledgement

The authors would like to thank Ministry of Science, Technologyand Innovation (MOSTI) for the project (IRPA 33-02-03-3011) for thefinancial support and University of Malaya excellent research envi-ronment. The authors would also like to express their gratitude towhosoeverhadcontributed to theirworkeitherdirectlyor indirectly.

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