This article was downloaded by: [Kean University] On: 25 April 2014, At: 00:57 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Aerosol Science and Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/uast20 Characterization of Exhaust Particulates from a Dual Fuel Engine by TGA, XPS, and Raman Techniques Nirendra N. Mustafi a , Robert R. Raine b & Bryony James c a Department of Mechanical Engineering , Rajshahi University of Engineering and Technology , Rajshahi, Bangladesh b Department of Mechanical Engineering , University of Auckland , Auckland, New Zealand c Department of Chemical and Materials Engineering , University of Auckland , Auckland, New Zealand Published online: 23 Aug 2010. To cite this article: Nirendra N. Mustafi , Robert R. Raine & Bryony James (2010) Characterization of Exhaust Particulates from a Dual Fuel Engine by TGA, XPS, and Raman Techniques, Aerosol Science and Technology, 44:11, 954-963, DOI: 10.1080/02786826.2010.503668 To link to this article: http://dx.doi.org/10.1080/02786826.2010.503668 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
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This article was downloaded by: [Kean University]On: 25 April 2014, At: 00:57Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
Aerosol Science and TechnologyPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/uast20
Characterization of Exhaust Particulates from a DualFuel Engine by TGA, XPS, and Raman TechniquesNirendra N. Mustafi a , Robert R. Raine b & Bryony James ca Department of Mechanical Engineering , Rajshahi University of Engineering andTechnology , Rajshahi, Bangladeshb Department of Mechanical Engineering , University of Auckland , Auckland, New Zealandc Department of Chemical and Materials Engineering , University of Auckland , Auckland,New ZealandPublished online: 23 Aug 2010.
To cite this article: Nirendra N. Mustafi , Robert R. Raine & Bryony James (2010) Characterization of Exhaust Particulatesfrom a Dual Fuel Engine by TGA, XPS, and Raman Techniques, Aerosol Science and Technology, 44:11, 954-963, DOI:10.1080/02786826.2010.503668
To link to this article: http://dx.doi.org/10.1080/02786826.2010.503668
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
Characterization of Exhaust Particulates from a Dual FuelEngine by TGA, XPS, and Raman Techniques
Nirendra N. Mustafi,1 Robert R. Raine,2 and Bryony James3
1Department of Mechanical Engineering, Rajshahi University of Engineering and Technology, Rajshahi,Bangladesh2Department of Mechanical Engineering, University of Auckland, Auckland, New Zealand3Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand
Particulate matter (PM) emitted from a dual fuel engine ischaracterized using thermogravimetry, X-ray photoelectron spec-troscopy (XPS) and Raman spectroscopy. Thermogravimetricanalysis (TGA) provides the mass fractions of elemental carbonand volatile materials in PM; XPS provides the possible chemi-cal compositions in the topmost layer of PM surface and Ramananalysis provides the possible structure of the carbon presented inPM. Dual fuel engine uses both liquid (diesel) and gaseous fuelssimultaneously to produce mechanical power and can be switchedto only diesel fueling under load. The dual fuel engine is operatedwith natural gas and simulated biogases (laboratory prepared) andresults are compared between the dual fueling and diesel fuelingunder the same engine operating conditions. Significantly highervolatile fractions in PM are obtained for dual fueling comparedto diesel fueling complementing the gravimetric results. The maxi-mum contribution of the graphitic carbon or aliphatic carbon suchas hydrocarbons and paraffins (C C or C C) are found in the top-most atomic layers of both the diesel and dual fuel PM samples.The other chemical states are found to be the carbon-oxygen func-tional groups indicating significant oxidation behavior in the PMsurface. Lesser aromatic content is noticed in the case of dual fuelPM than diesel PM. The carbon in dual fuel PM is found to bemore amorphous compared to diesel PM. These characterizationsprovide us new information how the PM from a diesel engine canbe different from that from a dual fuel engine.
[Supplementary materials are available for this article. Go to thepublisher’s online edition of Aerosol Science and Technology toview the free supplementary files.]
1. INTRODUCTIONDiesel engines are inherently more efficient than gasoline
engines and can be operated at higher compression ratios, whichpermits to use alternative low energy content fuel gases. They
Received 19 February 2009; accepted 10 April 2010Address correspondence to Nirendra N. Mustafi, Department of
Mechanical Engineering, Rajshahi University of Engineering & Tech-nology, Rajshahi, 6204, Bangladesh. E-mail: [email protected]
are widely used in both stationary and mobile applications,especially where high power output is needed. While oftenresulting in smaller amounts of carbon monoxide, and hydro-carbons emissions in comparison with gasoline engines, dieselengines emit significantly higher PM. Particulate emissionscan be classified as potential occupational carcinogen and canhave a number of other negative health impacts associated withexposure (Zhu et al. 2005). It is generally agreed that dieselengines used in transport systems represent an important sourceof ambient particulate matter (Vouitsis et al. 2003). Dieselengines emit fine and ultrafine (particles having diameter ofless than 100 nm) particles that can easily penetrate deep intothe respiratory system (Kennedy et al. 2009). There might bea causal relationship between exposure to diesel emissionsfrom mobile sources and the incidence of cancer, respiratorysymptoms, and respiratory diseases (El-Zein et al. 2007).Diesel PM have therefore potential environmental impacts,including health effects, climate change, ecological effects,and visibility. Current regulations concern PM concentrationsonly, i.e., the mass of all particulates that can be collected fromthe exhaust. Therefore, the regulation seems to be insufficientwith respect to the present health and environmental concerns.Modern engines are meeting the PM mass concentration targetsbut there is increasing concern about the emission of fineparticles in the exhaust (Burtscher 2005). Hence, in additionto the concentration measurements, PM characterization is alsoimportant to provide better understanding to address the issues.
Among different approaches to the challenge of minimizingthe emissions, one can be considered as the use of gaseous fuelsin diesel engines. Gaseous fuels in diesel engines operate indual fuel mode where the main energy comes from the gaseousfuel and the minimum amount of diesel fuel acts as the igni-tion source. Fuel gas, which is referred to as the primary fuel,is normally inducted into the diesel engine along with air andcompressed as usual. A minimum amount of diesel fuel spray,termed as pilot fuel (minimum quantity equivalent to 5–10%of the total full load fuel flow), is injected through the conven-tional fuel injection system near the end of compression stroke to
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initiate combustion. The pilot diesel fuel first self ignites andthen acts as a reliable source of ignition for the surroundinggas-air mixture. Since the engines can be operated at high com-pression ratios, they are suitable for the use of low energy contentgaseous fuels. Dual fueling therefore offers opportunities to usealternative (natural gas) and renewable (biogas) fuels in engines.
Biogas can be produced from many different organic wastesby anaerobic fermentation. Developing countries produce enor-mous amount of fermentable biomass such as agricultural wastesand hence have huge potential to produce and use biogas in en-gines. The use of stationary small capacity diesel engines inpower generation and in agriculture and construction works isquite common in these developing countries. Biogas can pro-vide a good alternative source for those engines and thus cancontribute to the national economy and to the environment. Themain component of biogas is methane (CH4), which is treatedas a very harmful green house gas. In many cases this CH4 canjust be escaped to the atmosphere and thus may have harm-ful effects to the environment. Therefore, the use of biogas hastwofold benefits: provides an alternative and renewable energysource and protects the environment.
Significant research gaps are noted in the case of biogasoperated dual fuel engines in terms of both the gaseous and PMemissions. Although numerous works have been published overthe last decades on the regulated gaseous emissions from naturalgas (NG) operated dual fuel engines (Boisvert et al. 1988; Wonget al. 1991; Weaver and Turner 1994; Abd Alla et al. 2000; Galalet al. 2002; Papagiannakis and Hountalas 2004; Patterson et al.2006), only a very few of them are found to investigate eitherthe smoke or PM emissions for the NG-diesel dual fuel engines(e.g., Boisvert et al. 1988; Wong et al. 1991; Papagiannakisand Hountalas 2004). On the other hand, a very limited numberof published research works are found for biogas-diesel dualfuel engines especially from an emissions perspective such asKarim and Amoozegar (1982), Karim and Weirzba (1992), andHenam and Makkar (1998). Virtually no published researchwork is found during this research project that deals with thePM emissions from a biogas-diesel dual fuel engine.
Although the use of gaseous fuels in diesel engines may helpto reduce PM emissions, the basics are still not clearly under-stood. It is therefore important to carry on detailed experimentalinvestigations in terms of PM measurements and characteriza-tion for the dual fuel applications in order to achieve increasedunderstanding.
In this study, thermogravimetric analysis (TGA) is employedto quantify the mass fractions of the elemental carbon andvolatile materials in the sampled PM. The surface characteriza-tion technique, X-ray photoelectron spectroscopy (XPS), is usedto characterize the PM samples collected on filters. The inter-nal microstructure of the carbon in PM samples is investigatedand quantitatively analyzed using Raman spectroscopy. Thisleads to a better understanding of how the diesel PM changesin terms of chemical composition with different engine oper-ating conditions and fuel qualities, especially the comparative
picture between the diesel PM and dual fuel PM under the sameoperating conditions.
2. EXPERIMENTAL DESCRIPTIONS
2.1. Test Engine Facility and MethodA Lister Petter, single cylinder, direct injection diesel engine
is used for the present study. The engine is modified to run inboth diesel and dual fuel modes. The original injection systemof the engine is maintained for the dual fuel operations. Furtherdetail of the engine and PM sampling system are in Mustafi andRaine (2008).
New Zealand low sulfur diesel fuel, limiting sulfur to a max-imum of 50 ppm, is used for the experiments. Natural gas (NG)is obtained from the pipeline supply and simulated biogas isprepared in the laboratory by mixing NG with CO2 with de-sired proportions to obtain different biogas mixtures: biogas-1(80% CH4 and 20% CO2); biogas-2 (67% CH4 and 33% CO2);biogas-3A (59% CH4, 41% CO2 and about 820 ppm H2S); andbiogas-3B (58% CH4 and 42% CO2).
Tests are performed at a constant engine speed of 1750 rpm.Two modes of steady state operation are chosen for diesel fu-eling: low load (∼3 Nm) and high load (∼28 Nm), which areapproximately 8% and 75% of the rated output of the enginerespectively for the mentioned speed. Under dual fuel operation,the amount of pilot diesel is always kept constant and the outputtorque of the engine is raised to 28 Nm by increasing the flowrate of gaseous fuel. About 62 percent diesel fuel by volume isreplaced during the dual fuel operations.
2.2. Exhaust Dilution and PM Sampling SystemsA single stage partial flow dilution system (PFDS) is used in
this study to dilute the representative exhaust gas sample drawnfrom the engine. The details of the system are described inMustafi and Raine (2008). The dilution ratio (DR) is maintainedat approximately 10 to 1 for the whole experimental program.The typical values of the flow rates used in the experimentwere: exhaust sample flow rate ∼14 L/min; dilution airflowrate ∼126 L/min; and the total flow rate ∼140 L/min. Thesample transfer tube (about 1 m of total length and about 8 mmin diameter) between the engine exhaust pipe and the dilutiontunnel is insulated and heated, maintaining a wall temperatureof 190◦C to minimize thermophoretic deposition. Though thePFDS does not reflect the true atmospheric dilution because ofthe limitations of using higher DR, its use in PM measurementsis certified by the standards and is treated as a substitute for fullflow constant volume sampling system at steady-state condition(Khalek et al. 2002). It has been established that DR has effectson volatile materials of the measured PM but has little effect onsolid carbonaceous materials (Lapuerta et al. 1999).
2.3. Thermogravimetric Analysis (TGA)PM samples are collected on glass fiber filters without Teflon
coating (Whatman GF/C 1822/047), as they are suitable for TGA
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TABLE 1TGA heating program
1 Initial atmosphere: nitrogen2 Isothermal for 10 minutes3 Ramp 3◦C per minute up to 450◦C4 Changing atmosphere: air5 Ramp 5◦C per minute up to 500◦C6 Isothermal for 70 minutes
(Price et al. 2007; Lapuerta et al. 2007). After collection, theloaded filters are kept in sealed Petri dishes and are conditionedin an environmental chamber before TGA.
A strip of the loaded filter, about 5 mm wide, is cut and thencut again into small squares which are carefully stacked into thesample pan of the thermogravimetric analyzer. The PM masscollected on the whole filter used for TGA is in the range of2.6 to 3 mg (as suggested by Lapuerta et al. 2007). The massof a whole clean filter is typically 25 mg. The initial weightof the selected portion of the loaded filter that is put into thesample pan ranged from 2.5 to 3.8 mg. The initial mass gainfound in TGA plots (online Supplemental Information, FiguresS1–S3) was not investigated during the experiments. However,this symptom was observed for all the samples analyzed andtherefore had consistency in the results obtained.
2.3.1. TGA ParametersA Shimadzu TGA-50 thermogravimetric analyzer facility has
been used for the present study. All the PM samples are heatedin the TGA according to the heating program listed in Table 1which has been adapted from those recommended by Lapuertaet al. (2007) and Price et al. (2007).
TGA has limitations that some of the organic material can bepyrolysed during the first part of the test when the environmentis inert. This would appear as non-volatile matters and thuslead to that material being classified as elemental carbon. Theappropriate term may therefore be solid or non-volatile fraction.However, like other researchers (Price et al. 2007), elementalcarbon (EC) term has been used in this article, as the major partof the non-volatile fraction would be carbonaceous materials.
2.4. XPS AnalysisPM samples are collected onto glass fiber filters without
Teflon coating (the same as for TGA). The collection time isusually longer than for gravimetric analysis in order to collectenough sample material on the filter.
XPS examination of the collected PM samples is performedon a Kratos Axis Ultra DLD, using monochromatic Al Kα X-rays (1486.69 eV) having a spectral resolution of 0.1–0.2 eV.The X-ray power used is 150 W (10 mA, 15 kV). Two types ofXPS spectra are recorded; survey (or wide) and narrow scans.Survey scans are performed with 160 eV pass energy, while thenarrow scans are performed with pass energy of 20 eV. The XPSsystem included a charge neutralization system, which is used
for non-conducting samples. Calibration of the spectrometer isperiodically performed using the Au 4f and Cu 2p lines at 84and 932.67 eV, respectively. The identification of the elementspresent in a sample specimen is performed by recording a sur-vey scan over the range of 1000-0 eV binding energy (BE), assuggested by Moulder et al. (1992). The carbon (C 1s) peakwith the binding energy of 285 eV is commonly used as the ref-erence for calibration of the spectra as suggested by Beamsonand Briggs (1992).
Narrow scans are performed for carbon and oxygen elementsto provide information regarding the chemical states of the ele-ment under examination. Each elemental peak in the spectrumis usually fitted with a few other component peaks. Each ofthese assigned peaks provide information regarding the corre-sponding chemical state or environment of the element basedon their BE positions. All the spectra of the narrow scans arealso calibrated against C 1s (BE: 285 eV) position. A mixed lineshape GL(50) (ratio of Gaussian to Lorentzian) is used to fit theindividual component peaks and the backgrounds of the spectraare subtracted using a Shirley background.
2.5. Raman AnalysesThe microstructures of the sampled PM are quantitatively an-
alyzed by using a Raman spectroscopy system (Renishaw, Sys-tem 1000). The particulate filters and the dilution and samplingsystem used here are similar to those used for the XPS studies.The Raman system is coupled with a Lecia optical microscopehas two available excitation lasers. The excitation laser used isa Renishaw NIR solid-state diode laser emitting a line at ∼785nm (source power 26 mW). The spectral resolution is about 2cm−1. The spectrometer is controlled by the Renishaw WiRE(Windows-based Raman Environment) software package.
Raman spectra are collected over the range of 900 to 2000cm–1. The Raman spectrometer is operated in continuous scan-ning mode. Laser power, spot diameter and beam exposure timeare optimized for the PM samples as 25% of the maximum laserpower, fully defocused laser beam and exposure time of 20 s.GRAMS/32 software (Graphic Relational Array ManagementSystem) is used to perform the curve fitting of the spectra andto determine the spectral parameters. The spectra are fitted toconvergence with four bands without fixing or restricting anyspectral parameter in the iteration process. The goodness of thefits is indicated by the reduced values of χ2 where values be-tween 1 and 3 imply the convergence of the curve-fit towardsthe observed spectrum. In this study the value of χ2 varied from1.36 to 1.67.
The in-plane crystalline dimension, La , is determined usingthe Equation (1) (Zhu et al. 2005):
ID
IG
= C1
La
[1]
where C is a prefactor which depends on the laser excitationwavelength λ0, ID , and IG are the intensities of “D” band and
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“G” band peaks, respectively, and La is the in-plane crystallinedimension. The value of C has been found to change for dif-ferent λ0. Thus, the intensity ratio, R (= ID/IG) and C bothare functions of λ0. Matthews et al. (1999) observed that fora number of different carbon samples such as polyparapheny-lene (heat treated to 2400◦C), glassy carbons and carbon blacks(heat treated to 2000◦C), R increased linearly with λ0 (400< λ0 < 700 nm). The investigators then estimated C(λ0) in alinear regime as C(λ0) ≈ C0 + λ0C1; where C0 and C1 wereestimated to be –12.6 nm and 0.033, respectively. The findingsof Matthews’s work are used in the present study to estimatethe value of the prefactor C for λ0 = 785 nm (assuming linearrelationship of R with λ0 up to λ0 = 785 nm) and the calculatedvalue of C is obtained as 13.3 nm. Whelan (2001) used a C
value of 12 nm for the HeNe 633 nm excitation source.
3. RESULTS AND DISCUSSION
3.1. TGATGA is used to distinguish between the volatile fraction (VF)
and solid fraction of the collected PM samples. Typical TGAplots for diesel low load, diesel high load and diesel-NG PMsamples are presented in the online Supplemental Information(Figures S1–S3). The total weight loss is considered to deter-mine the mass fractions of the elemental carbon and volatilefraction of the PM. The volatile fraction is further divided intohigh and low volatility fractions (Price et al. 2007). The fractionsare defined based on the temperature ranges:
High Volatility: Mass/weight loss up to temperature, T ≤ 200◦C(nitrogen environment)
Low Volatility: Mass/weight loss when 200 < T ≤ 450◦C (ni-trogen environment)
Elemental Carbon: Mass/weight loss when 450 < T ≤ 500◦C(air environment)
In addition to these fractions, a non-combustible residualfraction, ash, may remain on the filter after the completion ofTGA. However, the ash fraction is not resolved in the presentstudy.
FIG. 1. Elemental carbon and volatile PM mass fractions as determined byTGA for different engine fueling (diesel low: 3 Nm; diesel high and dual fuel:28 Nm; 1750 rpm; 28◦bTDC; 0.6 kg/h pilot for dual fueling).
TGA has been performed at least twice for every samplecollected and the results are presented in Figure 1. The samplefor biogas2 fueling was not ready during the TGA experimentand is not included in this study. Two filters were found tobe stacked together during the sample collection for biogas1fueling from the engine, which was noticed only at the time ofTGA and therefore, the results for this particular sample mighthave some artifacts due to a proportionally smaller amount ofcollected mass. In the TGA plots (Figures S1–S3) there are twocurves: one for the mass loss and the other for the temperatureincrease. The third one is the manual selection in mass losscurve for which range the mass loss is counted.
3.1.1. Volatile Fraction (VF)Figure 1 shows that the mass fraction in the range of high
volatility varies between 19 and 78% and that for low volatilityvaries from 16 to 27% for different engine fueling. The highestvalue of VF is obtained in the case of diesel (low) fueling andthe lowest is for diesel (high) fueling. Higher volatile organicfractions (VOFs) and soluble organic fractions (SOFs) for dieselfueling under low to medium loads compared to high loads arealready reported (Kweon et al. 2002, 2003; Ning et al. 2004;Lapuerta et al. 2007). The TGA results obtained here for dieselPM therefore agree well with these previous studies.
When we compare between diesel and dual fuel conditionsat high load (Figure 1), a significant difference in the values ofVF is measured. VF is always much higher in the case of dualfueling, even though the engine operating condition is the same.VF is between 78 and 87% for dual fueling compared with 35%for diesel fueling. The high volatility fraction varied between51 and 70% for different dual fueling compared with only 19%for diesel fueling. This indicates that the fuel composition hasa great influence on the volatile/EC composition of the emittedPM. The difference is also noticed in filter appearances, as filtersfor dual fueling appeared to be less black than that for dieselfueling. Higher SOF or ratio of organic carbon to elementalcarbon (OC/EC) for NG fueling has been reported in the liter-ature (such as Barbour et al. 1986; Wong et al. 1991; Lev-Onet al. 2002). Barbour et al. (1986) observed about 46% SOF(volatile aldehydes especially formaldehyde) for NG fuelingcompared to about 20% for diesel fueling for 75% engine loadcondition. This result is qualitatively similar to ours (87% and35%, respectively) as VF possibly includes the water fractionand is known to give higher readings than SOF (Lapuerta et al.2007).
According to Kweon et al. (2002, 2003) the premixed com-bustion is a primary controlling factor for the particle-phaseorganic compounds in diesel PM. As the stoichiometric A/Fratio decreases with the introduction of gaseous fuels in theengine, theoretically richer mixture are in existence in dualfueling condition than diesel (high load) condition. Experi-mentally, the CO emissions for dual fueling are found to beslightly higher than for diesel fueling, indicating rich mixturecombustion in the first case. The higher the proportion of pre-mixed combustion, the higher the tendency to produce organic
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TABLE 2The actual mass-loss of the samples during TGA for different
engine fueling conditions
High volatile Low volatile ECmass loss mass loss mass loss
compounds. Since ignition delay is increased with the additionof gaseous fuels, this causes a higher proportion of premixedcombustion and therefore an increase in particle-phase organiccompounds.
3.1.2. Elemental Carbon (EC) FractionAccording to Figure 1, measured elemental carbon mass frac-
tion varies from about 5 (diesel low load) to 65% (diesel highload) indicating that the elemental carbon mass fraction is afunction of the amount of diesel injection. Although the amountof diesel injection is the same, higher mass fractions of EC areobtained for the dual fuel conditions compared to diesel lowload condition (Figure 1). This indicates that there is a signifi-cant contribution of gaseous fuels on the total elemental carbonmass fractions during dual fueling under the engine operatingconditions. However, the net result is far below the level ofemissions of diesel (high) load, as gaseous fuels tend to producemuch less amount of PM (Mustafi and Raine 2008). Table 2presents the actual mass of the samples lost during TGA fordifferent engine fueling conditions.
3.2. XPS Analysis3.2.1. Survey Scans
The XPS survey scans provided the identification of the ele-ments present in the surface of the PM samples. A survey scan
is also taken for the blank filter to know the elements present inthe filter material. As there is no easy way to subtract the filterelements to get the net quantification for the detected elementsin a loaded filter, only a proportionate picture of the quantityof the elements present in PM is obtained. However, the majorelements to consider are carbon and oxygen. When comparingbetween a blank filter and a loaded (with diesel (high) PM)filter, the blank filter has significantly lower amount of carboncompared to the loaded filter. Since the diesel (high) sample ismainly composed of carbon it can be assumed that the majorityof carbon appearing in the XPS spectra has originated from thecollected diesel particulate matter on the filter. These carbonsappearing on the sampling filters may not reflect the true amountof carbons that originated from the combustion processes in en-gine. Some of the exhaust gas phase organic compounds mayadsorb onto the filter during sampling (Lipsky and Robinson2006). Nearly all of the PM found in the tailpipe before dilutionis present as solid carbonaceous agglomerates including a smallamount of metallic ash. However there also may be a signifi-cant quantity of volatile organic and sulfur compounds in thegas phase at exhaust temperatures that are transformed to dieselparticulate matter by nucleation, adsorption, and condensationas the exhaust dilutes and cools (Kittelson and Abdul-Khalek1999).
Table 3 lists all the detected elements for PM samples andfor the blank filter. Since zinc and calcium are still detectable indiesel (high) sample, it seems likely that these two elements arepresent in PM and may originate from lubrication oil. The high-est amount of PM was collected on the filter for the diesel highload condition among all the samples due to the nature of PMemission for this case. Thus a thicker layer of PM covered thefilter surface compared to the other cases. It was expected thatthe elements of the blank filters would not be detected as XPShas limitations in detectable depth of the layer under focus. Withthis thicker layer, XPS could only detect the elements present inPM and could not reach unto the filter surface. For this reason,the other elements of the blank filter were not detected for thiscase and lower elemental signals were observed in the XPS spec-tra than those obtained for the blank filter. These elements havepreviously been observed in XPS analysis on vehicle exhaust
TABLE 3Elemental composition of different PM samples detected by XPS survey scans
Elements and their atomic concentration (%)
PM samples O 1s C 1s Si 2p P 2p Zn 2p Co 2p Ca 2p S 2p Na 1s
TABLE 4Assignment of various functional groups for carbon element according to the binding energy
Functional group Binding energy (eV) Source
C C 284.5–284.8 Beamson and Briggs 1992graphitic carbon 284.6–285.1 Biniak et al. 1997; Braun
2005aliphatic/aromatic 284.5–285.3 Collura et al. 2005C C/C H 284.6 Hutton and Williams 2000;
Muller et al. 2006aliphatic contributions such as Albers et al. 2000hydrocarbons and paraffins Albers et al. 2000(-C C-)n 285.0–285.94 Beamson and Briggs 1992C H 286.0 Muller et al. 2006
C O-C; C OH; O/OH C- 286.30–287.02 Beamson and Briggs 1992carbon present in alcohol Biniak et al. 1997; Hutton
and Williams 2000;or ether groups Wild et al. 1997-C O; O C O 287.81–288.36 Beamson and Briggs 1992carbonyl groups Biniak et al. 1997
287.3 Wild et al. 1997288.6 Hutton and Williams 2000
288.85–290.44 Beamson and Briggs 1992carboxyl or ester groups Biniak et al. 1997; Kirchener
et al. 2003; Collura et al.2005
Carbon with florin 291.2–293.8 Beamson and Briggs 1992shake-up satellite peaks due to Biniak et al. 1997; Albers
et al. 2000π − π* transitions in aromatic rings(polyaromatic or graphite-like basic structure)
PM samples (Hutton and Williams 2000). A sample was alsocollected for diesel (high) PM on a silver membrane metallicfilter to avoid the interference of filter elements such as car-bon with the PM elements in XPS analysis. In this case, onlythree elements were detected: C1s (97.48%), O1s (2.49%), andZn 2p (0.04%), showing good agreement with the quartz filtersamples. In TGA we denote elemental carbon (EC) mass frac-tion for different PM samples under investigation. In XPS, wealso obtain carbon as one of the major elements present in PMsurface. Thus, in diesel high load case, we obtained the highestamount of EC by TGA and the highest amount of carbon byXPS. Carbon content on PM surfaces by XPS thus complementthe results obtained by TGA.
3.2.2. Narrow ScansFor all the PM samples, carbon and oxygen are found to
be the main components and therefore the narrow scans havebeen done for these two elements. These elements are mainlyobserved in diesel engine/vehicle exhaust PM and have been
analyzed by various researchers (Albers et al. 2000; Hutton andWilliams 2000; Collura et al. 2005; Muller et al. 2006). Curvefits are performed for individual elements for each PM sam-ple using Gaussian/Lorentzian (G/L ratio = 0.5) line shapesas mentioned earlier. The computer program, Computer AidedSurface Analysis for XPS (CasaXPS), was used in this studyto analyze the XPS spectra. Spectra collected in the standardformat were selected, viewed, and processed using CasaXPS.The peaks were selected basically looking at the general prac-tices that have been observed in the literature for the similarapplications as cited here (such as Albers et al. 2000; Muller etal. 2006). The assignment gave a perfect fit between the originalspectrum and the resultant curve.
3.2.2.1. Carbon (C1s) spectra. The elemental spectra areresolved into several component peaks, each corresponding toa different surface functionality. Literature has been reviewedto assign appropriate carbon functional groups for these peaksand these are summarized in Table 4. Carbon (C1s) spectra ofdifferent PM samples are presented in the online Supplemen-tal Information (Figures S4–S8). The C1s spectra have been
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resolved into five individual component peaks for all PM (Fig-ures S5–S8) except the diesel (low) PM sample where onlythree individual peaks are fitted (Figure S4). The peaks at higherbinding energies refer to oxygenated carbon compounds. Thisindicates that as the load increases from low to high, differ-ent carbon-oxygenated groups are detected implying significantoxidation in the case of diesel (high) and dual fuel PM.
The highest peak of the C1s signals for all the PM samplesranged between 284.9 and 285.0 eV, indicating the presence ofgraphitic carbon or aliphatic contributions such as hydrocarbonsand paraffins (C C or C C) in the topmost atomic layers of thePM. Similar observations have been noted previously for dieselexhaust particulates or urban aerosols, and for activated carbon(Biniak et al. 1997; Albers et al. 2000; Hutton and Williams2000; Collura et al. 2005; Muller et al. 2006). When comparingbetween diesel (high) and dual fuel PM samples, the peak isbroadened in the latter case (FWHM = 1.0–1.2 compared to0.78) indicating a less developed graphitic structure in dual fuelPM as suggested by Muller et al. (2006). The next dominantpeak at BE = 285.47–285.79 eV, is obtained for all PM exceptthe diesel (low) PM sample. According to the literature, thispeak refers to the n-paraffin functional group and might be afunction of combustion temperature.
The next peaks are for higher binding energies indicatingthe presence of different carbon-oxygenated functional groupsin PM. According to the results, all the PM samples have apeak for carbon with alcohol or ether groups except for diesel(high) PM. Quantitatively it is observed that this particular typeof carbon structure is in higher proportion (6.3–16.2%) for thedual fuel PM samples in comparison to diesel PM samples (0–3.9%). The C1s peak at 287.34 and 288.15 eV for diesel (high)and (low) PM, respectively, refers to carbonyl functional group( C O), which is not seen in dual fuel PM.
The next carbon-oxygen functional group, which refers to thecarboxyl, or ester functional group ( C OOH), is a commonoxidized state of carbon for all PM samples. It is observed thatthis particular carbon structure is approximately constant forall the cases indicating a basic oxygenated carbon structure ofdiesel PM independent of fuel quality and engine operation.
The final peak which ranges from 291 to 291.6 eV BE maybe due to enhanced plasmon-loss features (graphite) or π − π*shake-up satellite signals (aromatics) indicating the presence ofuncovered polyaromatic or graphite-like basic structural unitsin the surface regions of the PM samples (according to Biniak etal. 1997; Albers et al. 2000). This feature of carbon structure ispresent in less proportion for dual fuel PM compared to diesel(high) sample indicating less aromatic content in dual fuel PMstructures. This is to be expected since the amount of diesel fuel,which can be regarded as the source of aromatics, is minimizedin the case of dual fueling conditions.
3.2.2.2. Oxygen (O1s) spectra. In all cases the narrowscan of O1s spectra have been resolved into two individualcomponent peaks except diesel (high) PM sample where threeindividual peaks are fitted. The maxima of the O1s signals for
all the PM samples ranged from 533.3 to 533.8 eV, suggestingthe dominant contributions of aliphatic carbon-oxygen func-tional group, O-C (Beamson and Briggs 1992) in PM surfaces.However, the maximum of the O1s signal for diesel (low) PMappears at 532.75 eV suggesting the dominant contribution ofcarbon-oxygen functional group, aliphatic C O C or C OHor aromatic O C (Beamson and Briggs 1992). It is found thatthe amount of this aliphatic carbon-oxygen functional group O-C is higher for dual fuel PM compared to diesel (high) PM. Asthe load increased from low to high the peaks of the O1s spectrashifted to higher binding energies (see O1s spectra in the onlineSupplemental Information; Figures S9–S11).
The other peak in the O1s signals ranged from 532.0 to532.45 eV, indicating the presence of aliphatic O C carbon-oxygen functional group (Beamson and Briggs 1992) in thetopmost layers of the PM samples. Quantitatively the amount ofthis carbon-oxygen functional group in dual fuel PM structureis found to be lower than that of diesel (high) PM that is inagreement with the assigned carbon-oxygen functional groupsfor C1s spectra. The presence of similar carbon-oxygen func-tional groups in diesel exhaust particles and for urban aerosolsand activated carbon particulates is reported (Biniak et al. 1997;Albers et al. 2000; Hutton and Williams 2000; Muller et al.2006). The component peaks for O1s thus complement the find-ings of similar carbon-oxygen functional groups that have beenobserved in the carbon spectra.
In addition to the two major peaks in the O1s spectra, there isa small shoulder at about 535.9 eV for diesel (high) PM samplethat has not been observed for the other samples. This peak mayappear due to the presence of chemisorbed oxygen and/or waterin the diesel (high) PM surface as suggested by Biniak et al.(1997).
3.3. Raman AnalysesThe well-known G (or “Graphite”) and D (or “Defect”) bands
of graphite are the main features observed, with varying de-grees of intensity and width (FWHM) near 1330 cm–1 and 1600cm–1, respectively, for all PM samples. Typical spectra for diesel(high) and diesel-NG PM are shown in the online SupplementalInformation (Figures S12 and S13). Raman signal depends onthe size of the particle under focus. A large coagulated particlewas easily available on the filter to focus for diesel (high) PMsample. On the other hand, for diesel-NG PM sample, it wasdifficult to find such a large coagulated particle on the filter tofocus, as PM emission in this case was lower. Hence, a poorersignal was obtained for the latter case. Three to five band com-binations exist in the literature to fit the visible Raman spectraof amorphous (combustion/diesel PM) carbons (Mustafi 2008).However, in the present work, four band combinations (G, D1,D3, and D4) show better fit to the Raman spectra compared tofive band combinations (G, D1, D2, D3, and D4).
In the present study, the G band included the D2 band and theother bands were separately considered. The results presented
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TABLE 5Peak intensity ratios and in-plane crystalline size, La for
in Table 5 show that the intensity ratio (ID1/IG) is similar inthe case of diesel (high) and diesel (low) load PM, but the ratiois significantly lower in the case of dual fuel PM. Uncertaintiesprovided in Table 5 are the variations observed from differentruns for a particular PM sample under investigation. This mayprovide an indication of increasing degree of graphitization ofcarbonaceous materials in dual fuel PM. The crystallite sizeincreased and FWHM decreased in the case of diesel (high) PMcompared to diesel (low) PM. This implies that the disorderof graphite structures decreases with increasing engine load(according to Zhu et al. 2005). Although the crystallite size,La , determined from Equation (1), increases for dual fuel PMcompared to diesel PM (Table 5).
Previous studies (Cuesta et al. 1994; Jawhari et al. 1995)have reported an increase of ID3/IG with the proportion ofamorphous carbon in PM. In the present study this intensityratio varied from 0.5 for diesel (light) to 1.3 for dual fuel PMsamples (Table 5) indicating the more amorphous nature of thecarbon in dual fuel PM compared to diesel (high) samples.
Ivleva et al. (2007) correlated the intensity ratio, ID3/IG, tothe ratio of elemental carbon (EC) to total carbon (TC) present inPM. According to the authors the decrease in the intensity ratiowith increasing EC/TC ratio implied a decrease in amorphousorganic carbon fraction. In the present work, it has been observedthat the EC/TC ratio (from TGA analysis) is the highest in thecase of diesel (high) PM compared to others and also it has thelowest ID3/IG ratio, which implies that carbon in diesel (high)PM is less amorphous compared to the others.
Ferrari and Robertson (2000) proposed that the visibleRaman spectra in many experiments can be interpreted bythe three-stage model (graphite → nanocrystalline graphite→ a − C,amorphous carbon → ta-C) explaining the amor-phization trajectory ranging from graphite to ta-C (tetrahedralamorphous or diamond). According to the authors the maineffects in the evolution of the Raman spectrum in:
Stage 1: From graphite to nanocrystalline graphite
• the G band moves from 1581 to ∼ 1600 cm–1
• the D band appears and ID/IG increases• no dispersion of the G band
Stage 2: From nanocrystalline graphite to a − C
• the G band decreases from 1600 to ∼1510 cm–1
• ID/IG approaches to zero• increasing dispersion of the G band
Based on this model, it can be assumed that the carbon struc-ture of the diesel PM samples in this study falls in the firststage where the main structural change is passing from graphiteto nanocrystalline graphite with virtually no sp3 sites as the Gband is near 1600 cm–1 and the ID/IG increases. The dual fuelPM on the other hand can be assumed to fall in stage-2, i.e., fromnanocrystalline graphite to a − C with small sp3 sites as the Gband decreases from 1600 cm–1 and the ID/IG ratio decreasesand the FWHM of G band increases.
4. CONCLUSIONSCharacterizations have been done of the sampled PM emitted
from a dual fuel engine operated on natural gas and biogas.Based on the TGA, XPS, and Raman analysis the followingconclusions can be made:
• Engine load and the quality of gaseous fuel are found tohave significant effects on volatile mass fractions of themeasured PM. Significantly higher volatile mass frac-tions but lower elemental carbon mass fractions are ob-tained for dual fuel PM compared to diesel (high) PM.This implies that the elemental carbon mass fractionin PM is approximately proportional to the quantity ofdiesel fuel injected.
• Carbon and oxygen are the main elements found inthe PM structure by XPS studies. Quantitatively thehighest carbon fraction is found in diesel (high) PMthat has reflected in TGA results. The maximum con-tribution of the graphitic carbon or aliphatic carbonsuch as hydrocarbons and paraffins (C C or C C) areobtained in the topmost atomic layers of the PM. Theother chemical states are mainly the different oxidizedstates of carbon such as carbon with alcohol (C-OH),carbonyl (-C O) or ether functional group, carboxylor ester functional group (-O-C O or HO-C O). Itcan be concluded that the chemical states of carbonand oxygen in dual fuel PM structure are similar tothose observed for diesel (high) PM. However, a rel-atively less amount of aromatic carbon compounds isobserved in dual fuel PM surface.
• Raman spectra for different PM samples are mainlycomposed of “G” (“graphite”) and “D” (“defects”)bands of graphite carbon. There might be an indica-tion from the band intensity ratio (ID1/IG) that thecarbon in dual fuel PM structure has more graphiti-zation compared to diesel (high) PM. The disorderof graphite structures in PM improves with increas-ing engine load for diesel fueling. The crystallinesize La , increases for dual fuel PM as compared todiesel (high) PM. There might be an indication that
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the carbon presented in dual fuel PM is of moreamorphous nature compared to diesel (high) PM. Itmight be possible that the main structural change isfrom graphite to nanocrystalline graphite with vir-tually no sp3 sites in the carbon structure of dieselPM. The dual fuel PM, on the other hand, can be as-sumed to have a path from nanocrystalline graphite toa − C (amorphous carbon) with a small sp3 site in thestructure.
• These results show that PM from dual fueling are verydifferent from diesel fueling in terms of volatile mattersbut similar in terms of PM topmost surface character-istics. The carbon structure is also found to be differentfor dual fuel PM compared to diesel PM.
REFERENCESAbd Alla, G. H., Soliman, H. A., Badr, O. A., Abd Rabbo, M. F. (2000). Effect
of Pilot Fuel Quantity on the Performance of a Dual Fuel Engine, EnergyConversion and Management, 41:559–572.
Albers, P. W., Klein, H., Lox, E. S., Seibold, K., Prescher, G., and Parker,S. F. (2000). INS-, SIMS- and XPS-Investigations of Diesel Engine ExhaustParticles, Physical Chemistry Chemical Physics. 2(5):1051–1058.
Barbour, T. R., Crouse, M. E., and Lestz, S. S. (1986). Gaseous Fuel Utilizationin a Light-Duty Diesel Engine, SAE Paper 860070.
Beamson, G., and Briggs, D. (1992). High Resolution XPS of Organic Polymers:The Scienta ESCA300 Database. John Wiley, Chichester, England.
Biniak, S., Szyman’ski, G., Siedlewski, J., and S’wiatkoski, A. (1997). TheCharacterization of Activated Carbons with Oxygen and Nitrogen SurfaceGroups, Carbon. 35(12):1799–1810.
Boisvert, J., Gettel, L. E., and Perry, G. C. (1988). Particulate Emis-sions for a Dual-Fuel Caterpillar 3208 Engine, ASME Paper 88-ICE-18:277–285.
Burtscher, H. (2005). Physical Characterization of Particulate Emissions FromDiesel Engines: A Review, J. Aerosol Sci. 36:896–932.
Collura, S., Chaoui, N., Azambre, B., Finqueneisel, G., Heintz, O., Krzton, A.,Koch, A., and Weber, J. V. (2005). Influence of the Soluble Organic Fractionon the Thermal Behavior, Texture and Surface Chemistry of Diesel ExhaustSoot, Carbon. 43(3):605–613.
Cuesta, A., Dhamelincourt, P., Laureyns, J., Martinez-Alonso, A., and Tascon,J. M. D. (1994). Raman Microprobe Studies on Carbon Materials, Carbon.32(8):1523–1532.
El-Zein, A., Nuwayhid, I., El-Fadel, M., and Mroueh, S. (2007). Did a Ban onDiesel-Fuel Reduce Emergency Respiratory Admissions for Children? Sci.Total Environ. 384 (1–3):134–140.
Ferrari, A. C., and Robertson, J. (2000). Interpretation of Raman Spectra ofDisordered and Amorphous Carbon, Physical Review. B 61:14095–14107.
Galal, M. G., Abdel Aal, M. M., and El Kady, M. A. (2002). A ComparativeStudy between Diesel and Dual-Fuel Engines: Performance and Emissions,Combustion Sci. Technol. 174:241–256.
Henam, A., and Makkar, M. K. (1998). Combustion of Simulated Biogas ina Dual-Fuel Diesel Engine, Energy Conversion and Management, 39:2001–2009.
Hutton, B. M., and Williams, D. E. (2000). Assessment of X-ray PhotoelectronSpectroscopy for Analysis of Particulate Pollutants in Urban Air, Analyst.125(10):1703–1706.
Ivleva, N. P., McKeon, U., Niessner, R., and Poschl, U. (2007). Raman Mi-crospectroscopic Analysis of Size-Resolved Atmospheric Aerosol ParticleSamples Collected with an ELPI: Soot, Humic-Like Substances, and Inor-ganic Compounds, Aerosol Sci. Technol. 41:655–671.
Jawhari, T., Roid, A., and Casado, J. (1995). Raman Scattering and the Charac-terization of Some Commercially Available Carbon Black Materials, Carbon.33:1561–1565.
Karim, G. A., and Wierzba, I. (1992). Methane-Carbon Dioxide Mixtures as aFuel, SAE Paper 921557.
Karim, G. A., and Amoozegar, N. (1982). Examination of the Performance of aDual Fuel Diesel Engine with Particular Reference to the Presence of SomeInert Diluents in the Engine Intake Charge, SAE Paper 821222.
Kennedy I. M., Wilson D., and Barakat A. I. (2009). Uptake and InflammatoryEffects of Nanoparticles in a Human Vascular Endothelial Cell Line. ResearchReport 136, Health Effects Institute, USA.
Khalek, I. A., Ullman, T. L., Shimpi, S. A., Jackson, C. C., Dharmawardhana,B., Silvis, W. M., Kreft, N., Harvey, R. N., Munday, D., Yamagishi, Y.,Graze, R., Smitherman J., and Adkins, J. (2002). Performance of Partial FlowSampling Systems Relative to Full Flow CVS for Determination of ParticulateEmissions under Steady-State and Transient Diesel Engine Operation, SAEPaper 2002-01-1718.
Kittelson, D., and Abdul-Khalek, I. (1999). Formation of Nanoparticles dur-ing Exhaust Dilution; University of Minnesota, Department of MechanicalEngineering, EFI Members Conference: “Fuels, Lubricants Engines, andEmissions,” January 18–20.
Kweon, C. -B., Foster, D. E., Schauer, J. J., and Okada, S. (2002). DetailedChemical Composition and Particle Size Assessment of Diesel Engine Ex-haust, SAE Paper 2002-01-2670.
Kweon, C-B., Okada, S., Foster, D. E., Bae, M.-S., and Schauer, J. J. (2003).Effect of Engine Operating Conditions on Particle-Phase Organic CompoundsIn Engine Exhaust of a Heavy-Duty, Direct- Injection (D. I. ) Diesel Engine,SAE Paper 2003-01-0. 342.
Lapuerta, M., Armas, O., Ballesteros, R., and Duran, A. (1999). Influenceof Mini-Tunnel Operating Parameters and Ambient Conditions on DieselParticulate Measurement and Analysis, SAE Paper 1999-01-3531.
Lapuerta, M., Ballesteros, R., and Rodri’guez-Fernandez, J. (2007). Thermo-gravimetric Analysis of Diesel Particulate Matter, Measurement Sci. Technol.18 (3):650–658.
Lev-On, M., LeTavec, C., Uihlein, J., Alleman, T. L., Lawson, D. R., Vertin,K., Thompson, G. J., Wayne, W. S., Clark, N., Okamoto, R., Rieger, P.,Yee, G., Zielinska, B., Sagebiel, J., Chatterjee, S., and Hallstrom, K. (2002).Chemical Speciation Of Exhaust Emissions From Trucks and Buses Fueledon Ultra-Low-Sulfur Diesel and CNG, SAE Paper 2002-01-0432.
Lipsky, E. M., and Robinson, A. L. (2006). Effects of Dilution on Fine ParticleMass and Partitioning of Semivolatile Organics in Diesel Exhaust and WoodSmoke, Environ. Sci. Technol. 40:155–162.
Matthews, M. J., Pimenta, M. A., Dresselhaus, G., Dresselhaus, M. S., andEndo, M. (1999). Origin of Dispersive Effects of the Raman D Band inCarbon Materials. Physical Review B. 59:R6585–R6588.
Moulder, J. F., Stickle, W. F., Sobol, P. E., and Bombon, D. E. (1992). InChastain J. (Ed.), Handbook of X-Ray Photoelectron Spectroscopy, PerkinElmer Corporation, Eden Prairie, Minnesota, USA.
Muller, J.-O., Su, D. S., Jentoft, R. E., Wild, U., and Schlogl, R. (2006).Diesel Engine Exhaust Emission: Oxidative Behavior and Microstruc-ture of Black Smoke Soot Particulate, Environ. Sci. Technol. 40(4):1231–1236.
Mustafi N. N., and Raine R. R. (2008). A Study of the Emissions of a Dual FuelEngine Operating with Alternative Gaseous Fuels, SAE Paper 2008-01-1394.
Mustafi, N. N. (2008). Particulate Emissions from a Dual Fuel Engine. Doc-toral thesis, Department of Mechanical Engineering, University of Auckland,Auckland, New Zealand.
Ning, Z., Cheung, C. S., and Liu, S. X. (2004). Experimental Investigationof the Effect of Exhaust Gas Cooling on Diesel Particulate, J. Aerosol Sci.35(3):333–345.
Papagiannakis, R. G., and Hountalas, D. T. (2004). Combustion and ExhaustEmission Characteristics of a Dual Fuel Compression Ignition Engine Oper-ated with Pilot Diesel Fuel and Natural Gas, Energy Conversion and Man-agement, 45:2971–2987.
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Patterson, J., Clarke, A., and Chen, R. (2006). Experimental Study of the Per-formance and Emissions Characteristics of a Small Diesel Genset Operatingin Dual-Fuel Mode with Three Different Primary Fuels, SAE PAPER 2006-01-0050.
Price, P., Stone, R., OudeNijeweme, D., and Chen, X. (2007). Cold Start Partic-ulate Emissions from a Second Generation DI Gasoline Engine, SAE Paper2007-01-1931 (JSAE 20077213).
Vouitsis, E., Ntziachristos, L., and Samaras, Z. (2003). Particulate Matter MassMeasurements For Low Emitting Diesel Powered Vehicles: What’s Next?Progress in Energy and Combustion Sci. 29(6):635–672.
Weaver, C. S., and Turner, S. H. (1994). Dual Fuel Natural Gas/Diesel Engines:Technology, Performance, and Emissions, SAE Paper 940548.
Wong, W. Y., Midkiff, K. C., and Bell, S. R. (1991). Performance and Emissionsof a Natural Gas Dual-Fueled, Indirect Injected Diesel Engine, SAE Paper911766.
Zhu, J., Lee, K. O., Yozgatligil, A., and Choi, M. Y. (2005). Effects of EngineOperating Conditions on Morphology, Microstructure, and Fractal Geom-etry of Light-Duty Diesel Engine Particulates, Proc. Combustion Institute.30:2781–2789.
