2012-01-0366Published 04/16/2012

Copyright © 2012 SAE Internationaldoi:10.4271/2012-01-0366

saefuel.saejournals.org

Performance of Particle Oxidation Catalyst and ParticleFormation Studies with Sulphur Containing Fuels

Piotr BielaczycBOSMAL Automotive R&D Institute

Jorma KeskinenTampere Univ of Technology

Jakub Dzida and Rafal SalaBOSMAL Automotive R&D Institute

Topi RonkkoTampere Univ of Technology

Toni Kinnunen and Pekka MatilainenEcocat Oy

Panu KarjalainenTampere Univ of Technology

Matti Juhani HapponenTampere Univ. of Technology

ABSTRACTThe aim of this paper is to analyse the quantitative impact of fuel sulphur content on particulate oxidation catalyst

(POC) functionality, focusing on soot emission reduction and the ability to regenerate. Studies were conducted on fuelscontaining three different levels of sulphur, covering the range of 6 to 340 parts per million, for a light-duty application.The data presented in this paper provide further insights into the specific issues associated with usage of a POC with fuelsof higher sulphur content. A 48-hour loading phase was performed for each fuel, during which filter smoke number,temperature and back-pressure were all observed to vary depending on the fuel sulphur level. The Fuel Sulphur Content(FSC) affected also soot particle size distributions (particle number and size) so that with FSC 6 ppm the soot particleconcentration was lower than with FSC 65 and 340, both upstream and downstream of the POC. Conversely, FSC did nothave major effects on the soot particle number reduction efficiency of the POC. Soot and other exhaust compoundsaccumulated within the POC during this phase, gradually built a pressure drop across the POC. The final mass of collectedmatter in the POC differed significantly according to the sulphur content. The efficiency of removal of gaseous pollutantsby the POC was found to be markedly worse for the fuels with higher sulphur content, although this deterioration wasobserved to be non-linear. Following the accumulation phase, a duty cycle was applied that caused the POC to commencepassive regeneration. The time taken for the POC to cleanse itself of accumulated matter and thereby eliminate the pressuredrop was observed to increase with increasing fuel sulphur content. The proportion of NOx leaving the POC in the form ofNO2 was also found to vary as a strong function of fuel sulphur content.

CITATION: Bielaczyc, P., Keskinen, J., Dzida, J., Sala, R. et al., "Performance of Particle Oxidation Catalyst and ParticleFormation Studies with Sulphur Containing Fuels," SAE Int. J. Fuels Lubr. 5(2):2012, doi:10.4271/2012-01-0366.

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http://dx.doi.org/10.4271/2012-01-0366http://saefuel.saejournals.org/

INTRODUCTIONParticulate matter in ambient air, including the pollutants

emitted from internal combustion engines, can adverselyaffect human health [1,2,3,4,5] and the environment byinterfering with the climate and ecosystems. Among others,the particulate matter (PM) present in diesel exhaust is ofspecial concern because, due to their respirable size, they canpenetrate deep into human lungs affecting health; see e.g.[6,7,8]. On the other hand, the vehicle particle emissions aregenerated in our immediate environment meaning thatmillions of people get exposed daily to these emissions.However, harmful characteristics and affecting mechanismsof the pollutants are unclear. Fuel composition, its physicaland chemical parameters have very important influences onthe quantity and constitution of emissions. Fuelcharacteristics can affect emissions in three key areas[9,10,11]:

• by influencing injection parameters,

• by connection with the creation of pollutant matter in theengine or in the atmosphere,

• by influencing the efficiency of emission control systems.Particulate matter emitted in diesel exhaust is a complex

internally and/or externally mixed mixture of carbonaceoussoot, unburned fuel and lubricants and perhaps fuel pyrolysisproducts. Particulate matter can be solid and liquid; typicallyPM includes four fractions of diesel particulates: (1) solidsoot, (2) SOF (heavy hydrocarbons), (3) sulphur compoundsand (4) ash. Sulphur content is one of the fuel propertieswhich has the most influence on particulates, for modernengines [9,12]. The particulate emissions of diesel engineexhaust are a function of number of parameters, for example:engine type, engine operating conditions, fuel and lubricantoil composition [9]. The size distribution of submicron dieselexhaust particles can be divided typically into two separatemodes called accumulation mode (AM) and nucleation mode(NM) [13]. The accumulation mode consists of solidagglomerated soot particles which can carry volatile orsemivolatile components (e.g. sulfur compounds, water,hydrocarbons) on the surface [14]. Nucleation mode particleshave been commonly reported to consist of water, sulfuricacid and hydrocarbons. Several studies have indicated thatthe nucleation mode particles are semi-volatile and theparticle formation occurs when the exhaust gas is cooled anddiluted in the atmosphere [10,15,16]. In addition, the studieshave indicated that the sulphur from fuel and lubricant oilhave a driving role in the particle formation process.However, some of recent studies [17,18] have indicated thatthe nucleation mode particle formation can take place alsowithout the sulphur driven process so that the nuclei ofnucleation mode are formed already at high temperatureconditions.

Diesel vehicles are typically equipped with oxidativeexhaust aftertreatment like diesel oxidation catalysts (DOCs)or diesel particle filters (DPFs) in order to reduce

hydrocarbon, carbon monoxide and soot emissions. Theoxidative aftertreatment can affect the semivolatile exhaustparticle fraction both because of hydrocarbon oxidation andbecause usually it promotes the conversion of SO2 to SO3.SO3 reacts with water molecules, forming sulfuric acid.Further, based on modeling studies [19,20], the sulfuric acidand water can nucleate when exhaust gas cools and dilutes inthe atmosphere or it can condensate on the surfaces of sootparticles. With the DOC the semi-volatile nucleation modeexists at high load and high FSC even though no nucleationmode was measured without after-treatment [10,16,21,22].

Developing countries with growing economies, such asChina or India, have become significant and growingcontributors to sulphur containing particulate emissions asthere are many high-FSC (high fuel sulphur content) fuels inuse. It is well-known that normal, high-efficiency DieselParticulate Filter (DPF) with a wall flow design, cannot beused in conditions with high sulphur level fuels. Moreover,its application is less cost efficient than a ParticulateOxidation Catalyst's (POC) [23,24]. A POC, combined with aDiesel Oxidation Catalyst (DOC) placed upstream, is asignificant emissions control technology used to reduce thePM in those applications, and also typically to meet Euro 3and Euro 4 emission limits. The POC system is a flow-through passive self-regenerating and maintenance-freepartial Diesel particulate filter.

In the worst case scenario, there is a high sulphur fuel inuse and the engine is running at low load (low exhausttemperature) for a long time without phases allowing thepassive system to work. This scenario was targeted forsimulation in this study, illustrating the differences betweenthree fuel sulphur content levels and low driving speeds, likecity driving conditions in major cities in India and China withrural high-FSC fuels (as high as 2500 ppm).

TEST BED FACILITIESThe experimental setup is shown in Figure 1 (photos

Figure 2). Studies were performed on an engine test bed withan eddy-current dynamometer, equipped with a number ofmeasuring channels with a 1 Hz acquisition rate. Pressuresensors installed both upstream and downstream of the POCsystem permitted determination of the soot loading as well asthe rate of POC regeneration during the test. In turn, thetemperature sensors, installed close to the pressure sensors,were very closely observed for any gas temperaturesfluctuation which could strongly interrupt soot collectioninside the POC system. Also embedded into the test standwas a Horiba gas analyser to measure gaseous compounds,and AVL Smoke Meter for particulate emission monitoring.

In particle size distribution measurements the exhaustsample was diluted in two different ways. In order to measuresoot particle size distributions and particle reductionefficiency of the POC system, the sample, taken upstream ordownstream of the aftertreatment system, was diluted usingtwo stage dilution (ejector diluters). In the first stage the

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dilution air temperature was 250 °C and the dilution ratio(DR) was around 10. In the second stage the dilution airtemperature was equal to room temperature (25 °C) and theDR was around 8. To study the effects of FSC on volatileparticle fraction and to mimic real-world exhaust particleformation [25], the exhaust sample was diluted using aporous tube diluter (PTD) [26,27]. The PTD dilution systemhas been used widely in nucleation mode particle studies[10,17,25,27]. In the PTD, the dilution air temperature was 30°C, the DR was set to 12 and the relative humidity of thedilution air was below 5%. After the PTD, the diluted samplewas led into an ageing chamber and secondary dilutionconducted by an ejector (DR 8, room temperature). Thecarbon dioxide concentration was measured in the rawexhaust and at two other locations in the sampling setup.Final dilution ratio corrections were performed according tothe measured CO2 concentrations.

Figure 1. Experimental setup for the test bedmeasurements.

Particle size distributions were measured with a SMPS(Scanning Mobility Particle Sizer, DMA 3081 and CPC 3025,TSI inc.) [28] Nano-SMPS (DMA 3085 and CPC 3025, TSIinc.) [29] and ELPI (Electrical Low Pressure Impactor,Dekati Ltd.) [30] equipped with a filter stage. Themeasurement ranges of the Nano-SMPS, the SMPS and theELPI were 3-60 nm, 10-420 nm and 7 nm-10 μm,respectively. In all the particle measurements there was anoption to use a thermodenuder (TD) with low nanoparticlelosses to study particle volatility characteristics. The particlesdownstream of the TD (operating at 265 °C) are regarded asnonvolatile. All size distributions presented in this paper have

been corrected with particle losses in the TD [31] and withthe dilution ratio.

Figure 2. Test bed layout.

DESCRIPTION OF MEASUREMENTSSeparate DOC+POC systems were used to test three

different fuels containing 6 ppm, 65 ppm and 340 ppm ofsulphur, respectively named as follows: S6, S65 and S340.The engine chosen was a passenger car diesel enginecomplying with the Euro 4 emission standard with 1.9 l ofdisplacement and maximum output power of 150 hp. The testprofile used for the comparison study lasted 49 h andconsisted of two phases, determined according to the smokenumber [FSN] and the inlet DOC+POC exhaust gastemperature as crucial factors:• Soot loading phase - 48 h of engine running at a low loadpoint with a maximum exhaust temperature of 250 °C (tosimulate city-driving conditions in major cities of India andChina)• Regeneration phase - 1 h of engine running at a mediumload point with a maximum exhaust temperature of 400°C (tosimulate conditions where regeneration initiates after a longloading period)

The test cycle was run for each fuel of various sulphurcontent. The loading phase was performed in order to

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compare the amount of soot collected in the POC as afunction of fuel sulphur content. Loading of the POC systemwas possible due to the low exhaust temperature (below 250°C) to avoid regeneration and the filter smoke number from0.35 to 0.65 (depending on fuel type) which were reasonablevalues for such a low engine load. Moreover, gaseousemission measurement permitted evaluation of the influenceof the sulphur on catalyst efficiency. Each loading phasestarted with the POC empty of soot, which was weighed, andcontinued with the soot loading. The continuous growth ofaccumulated soot was determined by observation ofbackpressure increase over 48h of the loading phase. Thefinal evaluation of collected soot was performed directly byre-weighing the loaded POC after 48 h of soot loading, andthus the collected mass was calculated. The loaded POCswere then tested under the regeneration phase, which - due tothe high temperature of the exhaust gas - allowed the POC toregenerate. The phenomenon of regeneration was observedwith continuous acquisition of gaseous emission, exhaustbackpressure and temperature. The phase lasted 1 h andstabilization of backpressure decrease was observed uponcompletion.

RESULTSCONTINUOUS BACKPRESSURE ANDTEMPERATURE MONITORING

Continuous acquisition of gas pressures and temperaturestook place for each of the three cycles. The pressure dropover the DOC+POC was determined by two pressure sensorsplaced upstream and downstream of the sample. Togetherwith pressure sensors, thermocouples were installed in orderto measure both temperatures. Figure 3 shows resultsobtained for all three fuels.

The 48 hours of soot loading resulted in a significantincrease in backpressure upstream of the POC, as shown inFigure 3. Although engine load and speed remained the samefor the 48h loading phase, the exhaust gas temperature wasaffected by changes in backpressure. Temperatures increaseswere observed both upstream and downstream of the POC.Figure 3a shows unexpected peaks in temperature upstreamof the POC after 30 h of soot loading. The temperature riseswere caused by uncontrolled behaviour of the engine EGRvalve. The peaks exceeded 250 °C, which resulted in therapid burning of accumulated soot and rapid growth oftemperature downstream of the POC system and a reductionin backpressure. However, the clear correlation betweenbackpressure and temperature can be observed, apart from inthe area of the deviation. The types of selected fuelssignificantly influenced the backpressure growth rate andconsequently the POC pressure drop. An increase of 30 mbarin pressure drop was observed for S6 fuel and consequently40 mbar for S65 and 70 mbar for S340, Figure 3. Thepressure drop increase corresponds with the amount of

accumulated soot, which was proved by direct weighing ofthe POC system.

Figure 3. Backpressure and temperature changes overloading and regeneration phases: a) S6; b) S65; c) S340.

Immediately after the loading phase, the engine operatingpoint was changed to a higher load and speed resulting ingrowth of exhaust temperature and mass flow. The exhaustgas temperature was nearly 400 °C, causing passiveregeneration of the POC. The temperature remained stableduring the entire regeneration phase, whilst the pressure dropchanged as the POC was being cleansed of soot. The shape ofthe backpressure trace was observed to differ, depending onthe accumulated soot and the regenration temperature. Thestabilization occurred first after 0.5 h on S6 fuel (visible inFigure 3a). A similar stabilization was observed on S65 fuelwhilst the backpressure fall was still visible even after 1 h onS340, as shown in Figure 3c. During the one hourregeneration, the highest average regeneration rate of 0.53mbar/min occurred in the POC loaded by S340 fuel. The S6and S65 regeneration rates reached respectively 0.40mbar/min and 0.23 mbar/min.

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SOOT ACCUMULATIONAll three samples of the DOC+POC were empty before

testing. Each unit was weighed before the loading phase inorder to establish a reference weight for further weighing.After 48 h of soot accumulation, the samples were removedfrom the engine exhaust line and weighed hot on a portablebalance with a resolution of 0.5 g. After weighing, thesamples were remounted on the exhaust line. Before startingthe regeneration phase, samples were conditioned at the sameengine load as the loading phase until all parameters werestabilized. The calculated difference was 6 g for S6 fuel, 10 gfor S65 and 38 g for S340. Results are shown in Table 1. Thefilter smoke number [FSN] (measured downstream of thePOC at the beginning of the loading phase) increasedproportionally with fuel sulphur content (Table 1).

Table 1. PM mass in POC and FSN for three types offuel.

Soot particle size distributions upstream of the DOC+POC are presented in Figure 4. The particle sizedistributions consisted of the soot mode only. The GMD ofthe soot mode was 64-68 nm with all the fuels. However,particle concentrations were higher with S65 and S340 fuelsthan with S6 fuel. The particle concentrations were notdependent on the pressure drop caused by the aftertreatmentdevice. With all the fuels, somewhat larger particleconcentrations were observed after 40 h than after 20 h.Overall, the changes in the soot mode concentration wereminor, both due to the changes in fuel and the POC systemageing, and the soot particle emissions were relatively stablethroughout the measurements.

The shapes of the particle reduction efficiency curves(Figure 5) were similar to the ones in heavy duty dieselengine applications [31,32]; the highest reduction efficiencieswere observed in the smallest particle sizes and the reductionefficiency decreased from ∼60% at 15 nm to ∼30% at100-200 nm. After 20 hours of driving, the reductionefficiency curves were similar for each fuel. However, after40 hours, the reduction efficiency was slightly higher with theS6 fuel compared to S65 and S340 fuels.

Figure 4. Engine out soot particle size distributions after(a) 20 hours and (b) 40 hours from the start of the low

load mode.

Figure 5. Soot particle reduction efficiencies of the DOC+POC (a) 20 hours and (b) 40 hours from the start of thelow load mode. Pressure drops over the DOC+POC (inmbar) were 18.1, 20.5, and 21.7 (a) and 17.9, 27.0 and31.2 (b), with the S6, S65 and S340 fuels, respectively.

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With all fuels, the change in the engine load increased thesoot particle number concentration by a factor of 3-4 whilethe mean particle diameter decreased 3-11 nm. The total sootparticle number reduction efficiency, measured only with S65fuel, was about 50% and thus higher than during the low loadphase (27-38%). Using PTD sampling and the TD, thenumber emissions of soot particles were nearly identicalbetween the fuels. The FSC did not significantly affect thesoot particle emissions. In contrast, the fuel sulphur contentaffected the formation of semivolatile nucleation modeparticles (Figure 6); with S6 fuel the nucleation mode was notobserved at all and with S65 the nucleation mode existed onlyat the very beginning of the regeneration phase. With S340fuel, the nucleation mode was observed during whole the 1 hperiod. In general, the results linked with FSC and nucleationmode are consistent and qualitatively similar with Vogt et al.[16] and Maricq et al. [21] who studied the effect of FSC onnucleation mode with a DOC equipped passenger car;nucleation mode formed only at high load and with high FSCfuels.

Figure 6. Particle size distribution measured with anSMPS w/o TD during the regeneration phase (PTD

dilution).

GASEOUS EMISSIONSEngine-out and post-DOC+POC gaseous emissions of

THC, CO, NOx and NO2 were measured during the loadingphase, with the exhaust gas temperature below 250 °C. Theimportance of temperature on catalyst behaviour is widelyrecognised; low temperatures enhance the influence of thefuel on catalyst efficiency and make conclusions clearer.

Figure 7. DOC+POC conversion efficiency against fuel.

Engine-out and downstream DOC+POC gaseousemissions of THC, CO, NOx and NO2 were measured duringthe loading phase. Calculated values of catalyst conversionefficiency are shown in Figure 7. The engine-out emission ofNOx was found not to be affected by FSC; therefore all pre-DOC emissions of NOx remained at an approximate level of115 ppm. Nevertheless, an influence of FSC on NO2/NOx inthe raw exhaust was observed. The measured ratios were36%, 33% and 22% for S6, S65 and S350, respectively. Theexperiment confirmed the minor influence of DOC+POC onNOx emission, however NO2 emission is significantlyaffected.

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Figure 8. Gaseous emission of THC, NO and NOxduring regeneration phase.

All measurements confirmed the decrease of NO2downstream of the DOC+POC, which was named NO2consumption. The intensity of this phenomenon wascalculated according to the same formula as for conversionefficiency of other pollutants:

During the regeneration phase, the research focused onemission of nitrogen oxides and hydrocarbons, as shown inFigure 8. The plain black trace follows THC emission and forall three tests emission decreases, with some fluctuation forS340 at the beginning. Emission of NO - and presumably theconsequential NOx emission - increases for the first 0.1 h (6min) and soon stabilises as temperature in the combustionchamber achieves a certain level. The dark grey thin line is adirect NOx measurement and is to be considered thebackground for NO2 emission.

DISCUSSIONThe negative impact of high-sulphur fuels on emission

was determined some years ago. Sulphur harms theenvironment in several ways. The direct products of burntsulphur are sulphur dioxide (SO2) and sulphur-basedparticulate matter. Moreover, its poisonous influence oncatalysts decreases the conversion efficiency and tailpipeemissions of THC, CO and NOx (for gasoline engines)subsequently increase. Reduced oxidation ability of thecatalyst also decreases conversion of NO to NO2, whichrequires a higher regeneration temperature of the particulatefilter. However, as sulphur is a naturally occurringcomponent of crude oil, it remains in diesel and gasolinefuels. Western countries decided to gradually decrease fuelsulphur content by applying further technological efforts infuel production. The current limit in the EU is 10 ppm. In thedeveloping countries the fuel quality and emission limits aretightening all the time, but there are still relatively highcontents of sulphur in diesel fuel. Although most countriesdeclare their intention to reduce the amount of sulphur in fuelin the future, in the meantime, emissions can be reduced bye.g. aftertreatment devices and engine control development.

In this case the effort of research work must be focused ona simple and reliable solution that copes with sulphured fuelsand can be accepted by developing countries, especially interms of cost. The fulfilment of these requirements is the aimof the POC. The S340 exhaust gas contains significantlymore PM than S6 exhaust, which can be indirectly read fromTable 1. This study has confirmed the POC's capability interms of collecting PM and, as a result, decreasing tailpipePM emissions, and Table 1. As predicted, the sulphurdecreased the effectiveness of THC and CO conversion attemperatures below 250 °C, Figure 7. However, conversion athigher temperatures was not strongly affected. Weightingmeasurements indicated strong effect of the FSC on theaccumulated material in the DOC+POC. However there wasno significant difference in the engine out soot particleemissions and soot reduction efficiency between the fuels.This indicates that also sulphur based material is accumulatedin the DOC+POC, by adsorption on the washcoat or theaccumulated particles. At least the sulphur adsorption on thewashcoat of the DOC affects the gaseous pollutant reduction.

CONCLUSIONSThe purpose and target of this study was the deepening of

current understanding of particle formation phenomena andits effects on particle emission characteristics and collectionefficiency and regeneration. Furthermore, this new andvaluable information will (and already has been) utilized forhigher sulphur areas wherein POC-type partial filtration is themajor aftertreatment technology to reduce PM emissionsfrom mobile sources. The importance of low temperatureloading and evaluations of regeneration rates are especiallylinked to ensure proper performance of DOC+POC in

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passenger cars in urban driving conditions in metropolitancities, wherein high sulphur fuel and very low exhaust gastemperatures simultaneously cause additional challenges.

A comparison of the DOC+POC pressure drop linesshowed that the pressure drop growth rate increasedsimultaneously with sulphur content in fuel. The pressuredrop strictly corresponded to the POC particulate materialloading level, as confirmed by the mass collected. Itincreased by 6 g for the S6 fuel up to by 38 g for the S340.The growth of accumulated material was caused by anincrease of engine emission depending on the fuel sulphurcontent. Consequently, the larger the amount of accumulatedmaterial, the longer the time required for regeneration.

The dry soot particle emissions of the engine were notaffected by the FSC during neither the loading phase nor theregeneration phase. With all fuels, the change in the engineload, from low load to medium load, increased the sootparticle number concentration by a factor of 3-4 while themean particle diameter decreased 3-11 nm. During theloading phase, the highest soot particle reduction efficienciesof the DOC+POC were observed in the smallest particle sizesand the reduction efficiency decreased from ∼60% at 15 nmto ∼30% at 100-200 nm. After 20 hours of driving, thereduction efficiency curves were similar for each fuel.However, after 40 hours, the reduction efficiency was slightlyhigher with the S6 fuel compared to the S65 and S340 fuels.During the regeneration phase, the total dry particle reductionefficiency was ∼50% (27-38% during the loading phase).There were no nucleation mode particles during the loadingphase at any fuel, however with the S340 fuel, a continuousnucleation mode was observed during the regeneration phase.Therefore the nucleation mode existence was linked to thehigh sulphur level fuel and high enough exhaust temperatureallowing the formation of nucleation precursors.

Sulphur-bearing fuel strongly affected the performance ofoxidation catalysts. The conversion efficiency of CO droppedfrom nearly 100% to 54% respectively for S6 and S340. THCconversion behaved similarly with a drop from 83% for S6 to34% for S340. Fuel sulphur content above 65 ppm did notstrongly affect the performance of the catalyst. The differencein efficiency between S65 and S340 varied only by 8% forCO and 4% for THC.

A stronger influence of the FSC was found in the field ofthe relative NO2/NOx ratio. While all the results show adecrease of NO2 after the DOC+POC, its value is influencedby the type of fuel. Fuel with the lowest amount of sulphurresulted in the lowest consumption of NO2 to NO, achievinga level of 81%. S65 and S340 fuels had peaks up to 100%with nearly no emission of NO2 downstream of the DOC+POC.

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CONTACT INFORMATIONCorresponding authors:

Piotr Bielaczycpiotr.bielaczyc@bosmal.com.pl

Toni KinnunenToni.Kinnunen@ecocat.com

Karjalainen Panupanu.karjalainen@tut.fi

DEFINITIONS/ABBREVIATIONSAM

accumulation modeCO

Carbon monoxideCO2

Carbon dioxideDOC

Diesel Oxidation CatalystDPF

Diesel Particulate FilterDR

dilution ratioELPI

Electrical Low Pressure ImpactorFSC

Fuel Sulphur ContentFSN

Filter Smoke NumberNM

nucleation modeNO

Nitrogen monoxide, nitricNO2

Nitrogen dioxideNOx

Oxides of nitrogen (NO + NO2)PM

Particulate matterPOC

Particle Oxidation Catalyst

PTDporous tube diluter

SMPSScanning Mobility Particle Sizer

SOFheavy hydrocarbons

SO2Sulphur dioxide

SO3Sulphur trioxide

TDthermodenuder

THCTotal hydrocarbons

Bielaczyc et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 2(May 2012) 619

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