Carbonaceous composition changes of heavy-duty diesel
engine particles in relation to biodiesels, aftertreatments and
engine loads
Man-Ting Cheng1, Hsun-Jung Chen1, Li-Hao Young2, Hsi-Hsien Yang3, Ying I.
Tsai4, Lin-Chi Wang5, Jau-Huai Lu6, Chung-Bang Cheng7
1
Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-Kuang
Road, Taichung 40254, Taiwan
2
Department of Occupational Safety and Health, China Medical University, 91 Hsueh-Shih
Road, Taichung 40402, Taiwan.
3
Department of Environmental Engineering and Management, Chaoyang University of
Technology, 168, Jifeng E. Road, Wufeng District, Taichung 41349, Taiwan
4
Department of Environmental Engineering and Science, Chia Nan University of Pharmacy and
Science, 60, Sec. 1, Erh-Jen Road, Jen-Te, Tainan 71710, Taiwan
5
Department of Chemical and Materials Engineering, Cheng Shiu University, 840, Chengcing
Road, Niaosong District, Kaohsiung 83347, Taiwan
6
Department of Mechanical Engineering, National Chung Hsing University, 250 Kuo-Kuang
Road, Taichung 40254, Taiwan
7
Fuel Quality and Engine Performance Research, Refining and Manufacturing Research Institute,
Chinese Petroleum Corporation, 217, Minsheng S. Road, West District, Chiayi City 600, Taiwan
Corresponding author. Tel.: +886422053366 Ext.6219; Fax.: +886422075711; E-mail address:
lhy@mail.cmu.edu.tw (L.-H. Young)

1
Abstract
Three biodiesels and two aftertreatments were tested on a heavy-duty diesel engine under the US
FTP transient cycle and additional four steady engine loads. The objective was to examine their
effects on the gaseous and particulate emissions, with emphasis given to the organic and elemental
carbon (OC and EC) in the total particulate matter. Negligible differences were observed between
the low-sulfur (B1S50) and ultralow-sulfur (B1S10) biodiesels, whereas small reductions of OC
were identified with the 10% biodiesel blend (B10). The use of diesel oxidation catalyst (DOC1)
showed moderate reductions of EC and particularly OC, resulting in the OC/EC ratio well below
unity. The use of DOC plus diesel particulate filter (DOC2+DPF) yielded substantial reductions
of OC and particularly EC, resulting in the OC/EC ratio well above unity. The OC/EC ratios were
substantially above unity at idle and low load, whereas below unity at medium and high load. The
above changes in particulate OC and EC are discussed with respect to the fuel content, pollutant
removal mechanisms and engine combustion conditions. Overall, the present study shows that the
carbonaceous composition of PM could change drastically with engine load and aftertreatments,
and to a lesser extent with the biodiesels under study.
Keywords: Heavy-duty diesel engine, Diesel engine particles, Organic carbon, Elemental carbon,
Control technology
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1. Introduction
Diesel engine exhaust is a complex mixture that contains gaseous pollutants and particulate
matter (PM) with diameters primarily less than 2.5 m (PM2.5). The key gaseous pollutants consist
of nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide, sulfur dioxide (SO2), and light
hydrocarbons (HC). The PM components are composed predominantly of carbonaceous material
in the form of organic carbon (OC) and elemental carbon (EC) [1]. The OC has recently been
found to be highly associated with the oxidative potential of PM, which is an indicator of cell
toxicity [2,3]. Diesel engine particles (DEP) are considered as the major source of EC in the
atmosphere [4,5]. Collectively, the diesel engine exhaust contributes to adverse human health
effects, restricted visibility, acid rain, and global climate change [5,6,7]; more recently, it has been
reclassified as “carcinogenic to humans” [8].
The EC in DEP is formed by the pyrolysis of the unburnt fuels at temperatures above 1300 K,
while the OC is produced as a result of incomplete combustion of fuel and lubricant oil [9,10]. The
EC and metal ashes make up the agglomerated nearly spherical solid particles, with their surface
coated with semivolatile OC and sulfate. The emission rates of particulate EC and OC from engines
are shown to vary considerably, depending on the sampling conditions, engine type, mode of
engine operation, fuel type, and the presence of aftertreatment [11,12,13]. Unlike the nonvolatile
EC that forms in the engine, the gas/particle partitioning of semivolatile OC varies with dilution
ratio, temperature and residence time [14,15].
The major contributor to the EC in PM2.5 is heavy-duty diesel engines (HDDE), followed by
light-duty diesel engines, and then gasoline engines; on average, the EC contributed to 75% of the
DEP and ~ 25% of gasoline engine particles [5]. From in-use heavy heavy-duty diesel trucks, Shah
et al. [11] showed that the OC/EC ratios were below unity under cruise and transient operation,
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whereas above unity under creep and cold-start/idle operation. Sharma et al. [16] reported the
fraction of EC in DEP increased from 25% to 48% with increasing load from idling to 70% load.
Ålander et al. [12] showed that reformulated low-sulfur, low-aromatic fuel as well as diesel
oxidation catalyst (DOC) reduced the particulate OC in DEP; additionally, the OC contributions
to the total carbon (TC) from an indirect injection engine was significantly greater than that from
a direct injection engine. Tsai et al. [17] tested soy-biodiesel blends of 0%-50% (B0, B10, B20
and B50) on a diesel generator and showed that the OC and EC decreased with increasing biodiesel
blend to a minimum with B20 and then sharply increased to a maximum with B50. Similarly, Lin
et al. [18] reported that the soluble organic fraction decreased to a minimum with palm-biodiesel
blend of 20% (P20) and reached a maximum with neat P100. Biswas et al. [19] have shown that
significant reductions (> 90%) of PM, EC, OC and water-soluble OC were achieved with catalyzed
or uncatalyzed diesel particulate filters (DPF), between which only catalyzed aftertreatments were
found to decrease the OC solubility. In addition, the DEP was dominated by TC in which a higher
fraction of OC was observed for HDDE vehicles retrofitted with selected catalyzed continuously
regenerating traps or electric particle filter that emits less or insignificant nucleation mode particles.
With increasing stringent PM emission standards, the development and application of advanced
emission control technologies are an area of active research in recent years. Although new vehicles
are required to meet the new emission standards, emission reductions from in-use vehicles are an
integral part of an effective reduction strategy. Among those in-use vehicles, HDDE buses and
trucks are of special interest as they contributed to ~50% of the total PM10 emitted from mobile
sources, despite making up only a minute fraction of the total fleet in Taiwan [20]. In addition,
diesel engine commonly has a life expectancy of 20-30 years or more. Therefore, retrofitting inuse vehicles with high-efficiency aftertreatments as well as the use of biofuels have become
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attracting options for air pollution abatement and energy conservation. However, the impact of
control strategies on the emissions needs to be evaluated for changes in human and environmental
health implications. Although there are numerous publications on similar subjects, the study
focuses vary substantially with respect to engine types, pollutants, fuels, aftertreatments and engine
operating conditions. This study therefore supplement earlier studies by focusing on how the
carbonaceous composition of diesel engine particles changes in relation to three biofuels and two
aftertreatments under transient cycle and during steady engine loads. In addition, this study
addresses practical applications as to how the selected biofuels and aftertreatments perform when
retrofitted to old, high-emission in-use engines.
2. Experimental methods
2.1 Test engine, fuels and aftertreatments
This study was carried out in the testing facility of the Refining and Manufacturing Research
Institute, Chinese Petroleum Corporation in Taiwan. The test engine was a 6-cylinder, 6 L,
turbocharged air aspiration, water-cooled, direct-injection 1996 Cummins HDDE (Table S1). At
the time of study, the market diesel fuels all contain ~1% waste cooking oil biodiesel (B1) supplied
by the Chinese Petroleum Corporation in Taiwan. Three test fuels under study included a premium
low-sulfur (< 50 ppmw) diesel (B1S50), an ultralow-sulfur (< 10 ppmw) diesel (B1S10), and a
biodiesel blend containing 10% volume (B10) of neat waste cooking oil (B100). The fuel
properties of the B1S50, B1S10 and B100 are given in Table S2. The biodiesel fuel quality and
diesel fuel quality standards for use in mobile vehicles in Taiwan, the Chinese National Standards
(CNS) 15072 and CNS 1471, are comparable to the European standards EN 14214 and EN 590,
respectively. The above fuel selection was due to our national policy changes in 2010 and 2011 to
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further increase the biofuel content to above 2% and to lower the diesel sulfur contents,
respectively. As a result, B1S50 and B1S10 could serve as the base cases for evaluating how policy
changes might impact mobile pollutant emissions. The two aftertreatments under study included a
DOC (denoted as DOC1) and a prototype DOC+DPF (denoted as DOC2+DPF). The DOC1 is of
length (L) 40 cm and outer diameter (o.d.) 21 cm, whereas the DOC2 is of L 10 cm and o.d. 24
cm, made of a ceramic straight-through honeycomb catalyzed with a mixture of platinum and
palladium. The DPF is an uncatalyzed ceramic wall-flow honeycomb (L=30.5 cm, o.d.=24 cm)
with porosity of 45-50% and pore size of 2-3 m.
2.2 Duty cycles and emission factor calculation
The HDDE was tested with the US Federal Test Procedure (FTP) heavy-duty transient cycle and
four steady cycles that include idle, low, medium and high load. The engine torque and speed were
controlled by an engine dynamometer (Schenck GS-350). Prior to each transient cycle emission
test, the engine was given an overnight cold soak, after which the experiments were conducted in
the order of a cold-start emission test and then 3 to 5 hot-start emission tests, with a 20-minute hot
soak in between. In the steady cycle emission tests, the engine was maintained steady for 20
minutes with four sets of torque and speed given in Table 1, denoted as idle, low, medium, and
high load. As shown, with the exception of idle, the engine loads differed mainly in the speed
(1200-2065 rpm), while the torque was similar (210-255 Nm). In each steady cycle, the emission
tests were repeated three times.
With the transient cycle, the emission tests were conducted with the engine running on the three
biofuels without any aftertreatments (denoted as B1S50, B1S10 and B10) and with the B1S50 in
combination with one of the two aftertreatments (denoted as DOC1 and DOC2+DPF). With the
steady cycle, all the emission tests were conducted with the engine running on B1S50 without any
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aftertreatments under four different loads (Table 1). This resulted in a total of 5 transient-cycle and
4 steady-cycle emission test conditions, where the former aims to investigate the effects of fuels
and aftertreatments and in the latter the effects of engine loads on the engine emissions.
In the transient cycle, the weighted emission factors (EFw, mg/bhp-hr) were derived from the
cold-start and hot-start according to the US Title 40 CFR Part 86 Subpart N, for which the B1S50
with no aftertreatment is the baseline case, as follows:
𝐸𝐹𝑤 =
𝑔𝑐 + 6𝑔ℎ
(𝑏ℎ𝑝-ℎ𝑟𝑐 ) + 6(𝑏ℎ𝑝-ℎ𝑟ℎ )
where the gc and bhp-hrc are the measured pollutant mass and the brake horsepower-hour under
cold-start, respectively, whereas the subscript h denotes the hot-start. In the steady cycle, the
average emission factors (EFa, mg/hr) were directly computed from the measured pollutant mass
and sampling duration, for which the B1S50 at idle is the baseline case.
2.3 Sampling and analytical
Fig. 1 shows the schematic diagram of the experimental setup, consisting of a constant-volume
sampling (CVS) dilution tunnel and PM/gas sampling systems. Both the transient and steady cycle
emission tests were conducted with the CVS tunnel. Using filtered air, the full-flow engine exhaust
was first diluted in the CVS primary tunnel (i.d. = 35.56 cm) with the dilution ratios (DR) between
17.5 and 21.5. A small portion of the diluted engine exhaust was then further diluted inside the
secondary tunnel (i.d. = 10.16 cm) with the DR between 5.9 and 9.9. The average residence time
in the primary and secondary tunnel were 0.88 and 3.04 s, respectively. The diluted exhaust
temperatures at the sampling zone of the secondary dilution tunnel was <51.7 °C, from which the
PM2.5 samples were taken by a cyclone (URG-2000-30EN) at 10 liters per minute (lpm) and the
total particulate matter (TPM) samples taken directly by filtration at 80 lpm. For each emission
test condition, a PM2.5 sample on a 47-mm Zefluor filter plus a pair of TPM samples on 70-mm
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quartz filters were taken for gravimetric and carbon analysis. The quartz filters (2500QAO-UP,
Pall Life Sciences) were pre-baked at 900 C for 4 hours to remove carbonaceous contaminants.
The filters were then conditioned at 233 C and 405% RH for 24 hours and weighed before and
after sampling with a microbalance of 10 g precision (XR105DR, Mettler Toledo). Quartz filter
samples were further analyzed for total carbon (TC) and elemental carbon (EC) with an elemental
analyzer (W.C. Heraeus CHN-O-Rapid) [21]. In brief, the TC was determined by combusting the
“non-preheated” quartz filter samples, along with a Sn foil wrap, under pure O2 to temperature up
to 1800 C. The resulting combustion product CO2 was then purged by a He flow into the thermal
conductivity detector (TCD) for carbon determination. The EC was determined by the same
analytical process except that the quartz filters were “preheated” to 340 C under the presence of
O2 flow of 0.5 lpm for 80 minutes. This heat treatment basically removes the vast majority of
volatile and semivolatile organic carbon (OC) fraction off the particulate matter. The carbon
contents were then determined by the calibration curves between the TCD signals and varying
concentrations of acetanilide (C6H5NH(COCH3)) standards of known elemental composition (N:
10.36%, C: 71.09%, H: 6.71% and O: 11.80%). Finally, the OC was obtained by subtracting the
EC from the TC. The above thermal method for OC/EC determination was modified from the
French two-step protocol [22], except that our method did not include the decarbonation and the
charring correction processes, and that we used a higher final combustion temperature. According
to a number of inter-comparison studies, we suggest that the diesel particulate OC and EC
concentration differences between our method and the TOR and TOT methods are less than 25%,
whereas the TC concentration differences are insignificant [23-27]. The gaseous total hydrocarbon
(THC), carbon monoxide (CO), and nitrogen oxides (NOx) were monitored by means of flame
8
ionization detector, non-dispersive infrared analyzer, and chemiluminescence, respectively
(Rosemount Analytical Inc., CA, USA).
3. Results and discussion
In the following, EFw (mg/bhp-hr) is used to evaluate the transient cycle test results, in which
the B1S50 with no aftertreatment is the baseline case, while EFa (mg/hr) is used for the steady
cycle test results, in which the B1S50 at idle is the baseline case. Idle represents a unique condition
during which the engine typically emits the highest PM and CO, hence its results have important
environmental and health implications.
3.1 Regulated gaseous and PM emissions
The EFw and EFa of gaseous (THC, CO and NOx) and particulate pollutants (TPM and PM2.5)
under the US FTP transient cycle and four steady cycles are given in Tables S3 and S4, respectively.
The pollutant reduction efficiencies with respect to the baseline cases are shown in Fig. 2. The
transient cycle results show that the reduction of fuel sulfur content from <50 ppmw to <10 ppmw
yielded almost negligible reductions (<7%) with respect to regulated pollutants (HC, CO, NOx
and TPM) and PM2.5, but keep in mind that SO2 and sulfate were not measured here. On the other
hand, the B10 biodiesel blended with waste cooking oil showed small to moderate reductions (847%) of all gaseous and particulate pollutants, except for NOx which increased moderately (49%).
Such emission characteristics are consistent with the majority of studies reviewed by Lapuerta et
al., Xue et al. and Pullen and Saeed [28-30], and could be attributed to the fuel properties of
biodiesel that favor a more complete combustion, advance combustion or injection and
subsequently higher combustion temperature. The DOC1 and DOC2+DPF both showed small to
moderate reductions (25-45%) of THC and CO, whereas small increases of NOx were observed
9
with the DOC2+DPF. The small increase (24%) of NOx with DOC2+DPF, but not DOC1, was
somewhat unexpected. Previous studies have shown little changes of NOx emission with the use
of DOC+DPF or catalyzed DPF [31,32], in which the DOC is commonly used to convert NO to
NO2 to assist DPF regeneration [33]. Although the two aftertreatments utilize catalysts to enhance
oxidation but they differed primarily in their reduction efficiencies of TPM and PM2.5; moderate
reductions (35-43%) with the DOC1, whereas significant reductions (79-94%) with the
DOC2+DPF. Clearly, the wall-flow structure of the DPF is much more effective in trapping the
particles than the flow-through structure of the DOC. The above results suggest that the two
aftertreatments under study are more effective in reducing TPM and PM2.5 than the changes in fuel
sulfur and biodiesel content. Nevertheless, the high-efficiency DPF may not be enough for highPM emitting diesel engines to meet the current standards, particularly those uncontrolled or poorly
maintained engines.
The steady cycle results show that the EFa of the gaseous and particulate pollutants at idle were
comparable to that reported for HDDE equipped with electronic fuel injection [34]. In this study,
the EFa of TPM and PM2.5 at idle were highest, followed by low load and then more or less leveled
off at moderate and high load. Shi et al. [35] reasoned that air-to-fuel ratios at low load were
highest and hence lower combustion temperature is favorable for higher PM emissions. In this
study, the medium and high load differed primarily in engine speed, not engine torque (Table 1),
thus indicating higher engine speed led to higher TPM and PM2.5 emissions. Di et al. [36] reported
similar results and suggested that the increase in mechanical friction loss, the more incomplete
combustion and the reduction in volumetric efficiency at the higher engine speed resulted in higher
PM emissions. In an internal combustion engine, the volumetric efficiency is the ratio between the
inducted air volume to the displaced air volume. This ratio is affected by the fuel, engine design
10
and operating conditions. Typically, the volumetric efficiency is lower at low and high engine
speed, whereas its maximum lies in between the two ends. Overall, the THC and NOx increased
with increasing engine load or speed, whereas the CO and PM decreased from idle to medium load
and then leveled off at or increased from medium to high load, possibly related to the reduced
engine performance at high engine speed. Combining all the transient and steady cycle emission
data, a strong linear relationship (R2=0.84) was observed between the TPM and PM2.5, indicating
that 70% of the TPM in diesel engine exhaust on average are composed of PM2.5 (Fig. 3).
3.2 Effects of biofuels and aftertreatments on carbonaceous PM
The EFw of OC and EC and their respective OC/EC ratios in TPM with the engine running on
different fuels and aftertreatments under transient cycle are given in Table 2. Using B1S50 as the
baseline case, the OC, EC and TC (=OC+EC) reductions in TPM are shown in Fig. 4. Similar to
the gaseous pollutants, the change from B1S50 to B1S10 showed minor reductions (<8%) of OC
and EC, with an overall TC reduction of only 5%. Again, this is anticipated as the sulfur content
(34.5 vs. 7.3 ppm) as well as other fuel properties are very similar between B1S50 and B1S10
(Table S2). Nevertheless, a notable reduction of OC but little reductions of EC when the fuel sulfur
content was reduced substantially from ~400 ppm to <50 ppm has been reported elsewhere [12,37].
With the B10, the OC and EC showed reductions of 21% and 7%, respectively, with an overall TC
reduction of 14%. The higher reductions of OC than EC indicate that the higher oxygen content in
B10 is more favorable for complete combustion of OC. Studies have shown that high blends of
biofuels could result in elevated OC emissions, e.g., above 20% and neat biofuels [17,18,38],
indicating that the OC reductions do not monotonically increase with increasing biodiesel blend.
On the other hand, the OC and EC reductions increased considerably with the use of
aftertreatments. With the DOC1, the OC and EC showed moderate reductions of 53% and 36%,
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respectively, with an overall TC reduction of 45%. This observation is consistent with earlier
studies that suggested the oxidation catalyst reduces the PM by oxidizing and removing the
toluene-soluble organic fraction of PM [39]. It is notable that the DOC1 also caused the reduction
of EC, consistent with the moderate reductions of TPM and PM2.5 shown previously. A substantial
improvement in the EC reduction was observed with the DOC2+DPF, where the OC and EC
reductions increased to 71% and 83%, respectively, with an overall TC reduction of 77%. Biswas
et al. [19] also observed significant reductions in PM, EC and OC emissions from various types of
DPF. This agrees with the fact that the DPF is designed to filter out the nonvolatile fraction of PM,
i.e., the EC component.
Fig. 5 presents the scatter plots of the measured EFw of TPM with TC, OC and EC under transient
cycle. As shown, the TPM showed strong positive linear relationships with TC, EC and then OC,
in the order of decreasing strength where the R2 were between 0.84-0.95. It is important to note
that those relationships span across various fuels and aftertreatments, indicating that the major
carbonaceous make-up of the TPM are rather stable and hence predictable across the fuels and
aftertreatments under study. Across a variety of fuels and aftertreatments, Noll et al. [40] have
shown that the diesel PM and TC can be accurately predicted using EC as a surrogate when the
concentrations of diesel PM and TC are above 0.16 mg/m3. In this study, the measured TC
concentrations were typically above 0.5 mg/m3. The slopes of the linear fit indicate that, the TC,
EC and OC on average contributed approximately 83%, 45% and 38% of TPM, respectively.
However, a closer examination of Table 2 shows that the effects of biofuels and aftertreatments on
the TC reductions were not equally apportioned to the OC and EC, and thus causing changes in
the OC/EC ratio. The reduction of fuel sulfur content yielded almost negligible reductions (<6%)
of OC and EC, consistent with the reductions for other pollutants under study. The minor TC
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reductions (8%) with the B10 were mainly a result of OC reductions (18%), possibly due to its
higher oxygen content, whereas the EC practically stayed constant. As a result, the average OC/EC
ratio dropped from 1.00 with the B1S50, and 0.95 with the B1S10, and to 0.81 with the B10. The
TC reductions increased notably to 45% and 77% with the use of aftertreatments DOC1 and
DOC2+DPF, respectively. In the case of DOC1, the TC reductions were mainly attributed to the
moderate OC reductions (53%), while the EC reductions were relatively smaller (36%). The
effectiveness of the DOC1 to preferentially remove the OC fraction via catalytic oxidation is
revealed in its smallest OC/EC ratio of 0.72. On the other hand, the DOC2+DPF was considerable
more effective in removing both the OC and EC (71% and 83%, respectively), and particularly the
less volatile EC fraction. Such relatively higher EC reduction efficiency therefore resulted in the
largest OC/EC ratio of 1.73. This also highlights the increasing importance of semi-volatile organic
materials over the nonvolatile soot particles, as the DPF is one of the primary aftertreatments
applied to meet stringent PM emission standards.
3.3 Effects of engine load on carbonaceous PM
The EFa of OC and EC and their respective OC/EC ratios in TPM with the engine running on
B1S50 and without any aftertreatments under steady cycle are given in Table 3. Using the idle as
the baseline case, the OC, EC and TC emission reductions in TPM under varying engine load are
shown in Fig. 6. The emissions of OC and EC at idle were the highest and about 2 to 10 times
higher than other engine loads. The air-to-fuel ratio at idle is the highest and the resulting lower
combustion temperature is favorable for elevated PM emissions; in this study, the highest CO
emissions were observed at idle, indicating the strongest degree of incomplete combustion. From
low, medium to high engine load, the emissions of OC and EC showed different trends.
Specifically, the reduction of OC (32%) was notable when increasing from low to medium load,
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but then became negligible (~2%) from medium to high load. More fuel is combusted at higher
load and thus leads to a higher combustion temperature. The OC is therefore more easily oxidized
under higher temperature. Unlike the OC, the emissions of EC showed an increasing trend with
increasing load. This can be explained by the decreases of the volumetric efficiency and hence
post-flame soot oxidation at higher engine speed, such that the DEP contain more EC [30]. The
reductions of EC therefore decreased with increasing load, from 79% to 54% for TPM and from
53% to 24% for PM2.5. The OC/EC ratios in TPM were well above unity at idle and low engine
load, but then dropped sharply to <0.75 at medium load and to <0.46 at high load, consistent with
earlier studies [11,32]. The above results clearly show a trend of decreasing OC/EC ratio with
increasing load.
4. Conclusions
Three biodiesels (B1S50, B1S10 and B10) and two aftertreatments (DOC1 and DOC2+DPF)
were tested on a HDDE to examine their effects on the gaseous and particulate emissions under
the US FTP transient cycle and four steady engine loads, with emphasis given to the carbonaceous
composition (OC and EC) in the TPM. The transient cycle results show that the use of DOC2+DPF
yielded the highest reductions (79-94%) of TPM and PM2.5, followed by the DOC1 (35-43%),
whereas small reductions (<25%) were obtained from the three biodiesels. With its higher oxygen
content, the B10 biodiesel blended with waste cooking oil showed the highest reduction (47%) of
CO, but highest increase (49%) of NOx. On the other hand, the reduction of the fuel sulfur content
from B1S50 to B1S10 yielded almost negligible reductions (<8%) with respect to all the pollutants
under study, but keep in mind that SO2 and sulfate were not evaluated here. The steady cycle
results show that the THC and NOx increased with increasing engine load or speed, whereas the
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CO and PM decreased from idle to medium load and then leveled off at or increased from medium
to high load, possibly related to the reduced engine performance at high engine speed. On average,
PM2.5 contributed to 70% of the TPM.
Under the transient cycle, the emissions of particulate TC (=OC+EC) increased proportionally
with that of the TPM. However, a closer examination shows that the effects of fuels and
aftertreatments on the TC reductions were not equally apportioned to the OC and EC, and are thus
causing changes in the OC/EC ratio. Unlike the two other biofuels, the small reduction preference
of B10 for OC, but not EC, resulted in a decrease of the OC/EC ratio from about unity to 0.81. The
TC reductions increased notably to 45% and 77% with the use of aftertreatments DOC1 and
DOC2+DPF, respectively. The effectiveness of the DOC1 to preferentially remove the OC fraction
via catalytic oxidation is revealed in its smallest OC/EC ratio of 0.72. On the other hand, the
DOC2+DPF was considerably effective in removing both the OC and EC (71% and 83%,
respectively), particularly the nonvolatile EC fraction. Such a preference therefore resulted in the
largest OC/EC ratio of 1.73. Under the steady cycle, the emissions of OC and EC at idle were
about 2 to 10 times higher than for other engine loads because of the strongest level of incomplete
combustion at idle, evident by its highest CO emissions. From low, medium to high engine load,
the emission of OC decreased while the EC increased with increasing engine load. As a result, the
OC reductions (32%) increased from low to medium load, but then leveled off (~2%) from medium
to high load. On the other hand, the reductions of EC decreased with increasing load, from 79% to
54%. The above opposing trends can be attributed to the higher combustion temperature but
decreased post-flame soot oxidation at higher engine speed, respectively.
The overall results indicate that the relative importance of particulate OC and EC varies largely
with engine load and the use of aftertreatments such as the DOC and DPF and, to a lesser extent,
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for low-percentage (10%) biodiesel. Specifically, the semivolatile OC in PM is significantly more
important at idle and low load or when the DPF is in place, whereas nonvolatile EC is more
prominent at medium and high load or when aftertreatments are not in place. It is notable that the
OC in PM are strongly associated with cell toxicity. On the other hand, the reduction of fuel sulfur
content from <50 ppmw to <10 ppmw has negligible effects on the carbonaceous composition in
diesel engine particles, though its relation with SO2 and sulfate emissions was not evaluated in the
present work. Finally, the high-efficiency DPF may not be enough for high-PM emitting diesel
engines to meet the current standards, particularly for those uncontrolled or poorly maintained
engines. As diesel engine, emission control technology and biodiesel are finding increasing
worldwide application in abating air pollution and global warming, such carbonaceous
composition changes would impact the roles of PM in adverse human health and atmospheric
chemistry, and its use in source apportionment.
In the present study design, the effects of aftertreatments on exhaust emissions can only be
evaluated under transient test results, whereas the effects of engine load on exhaust emissions can
only be evaluated under steady-cycle test results. Our study therefore could not straightforwardly
elucidate how aftertreatments would impact exhaust emissions at specific engine operation
conditions. Nevertheless, the use of DOC or DPF under idle would change the PM mass and
OC/EC fraction simply because of the catalytic and filtration processes. In the future, the potential
interactions between aftertreatments and engine load should be included for an improved, refined
understanding of the factors influencing diesel engine exhaust. The present work has focused on
the changes of particulate “mass” in relation to varying control measures. However, it is important
to note that mass alone is not sufficient to fully address the adverse human health effects from
diesel engine particles. Other possible exposure metrics, e.g., particle surface chemistry,
16
morphology and toxic chemical constituents, should also be taken into consideration. Unless more
detailed information is available, most risk assessment still uses mass-based exposure metric as a
surrogate for evaluating dose-response relationships.
Supplementary data
Table S1 shows the test engine specifications. Table S2 gives the fuel properties. Table S3 gives
the weighted emission factor (EFw, mg/bhp-hr) of THC, CO, NOx, TPM and PM2.5 under transient
cycle. Table S4 gives the average emission factor (EFa, mg/hr) of THC, CO, NOx, TPM and PM2.5
under steady cycle.
Acknowledgments
The authors acknowledge the financial and administrative support from the Taiwan Ministry of
Science and Technology and the Environmental Protection Administration under the contract #
NSC 98-EPA-M003-002 and NSC-99-EPA-M-002-002.
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22
Table Captions
Table 1. Engine test modes under steady cycle.
Table 2. The weighted emission factor (EFw, mg/bhp-hr) of OC and EC in TPM under transient
cycle.
Table 3. The average emission factor (EFa, mg/hr) of OC and EC in TPM under steady cycle.
23
Table1. Engine test modes under steady cycle.
Load
Torque
Speed
(Nm)
(rpm)
Idle
-
810
Low
210
1200
Medium
255
1635
High
255
2065
Table 2. The weighted emission factor (EFw, mg/bhp-hr) of OC and EC in TPM under transient
cycle.
OC
Fuel/Aftertreatment
EC
OC/EC ratio
Mean
SD
Mean
SD
Mean
SD
B1S50
66
12
67
3
0.99
0.21
B1S10
61
15
66
1
0.91
0.21
B10
52
3
62
3
0.84
0.09
DOC1
31
3
43
3
0.73
0.12
DOC2+DPF
20
1
11
2
1.77
0.35
SD: standard deviation.
24
Table 3. The average emission factor (EFa, mg/hr) of OC and EC in TPM under
steady cycle.
B1S50
OC
EC
OC/EC ratio
Load
Mean
SD
Mean
SD
Mean
SD
Idle
219
-
61
-
3.60
-
Low
97
25
29
2
3.34
0.64
Medium
24
3
32
2
0.75
0.07
High
23
2
48
2
0.47
0.07
SD: standard deviation.
25
Figure Captions
Figure 1. The experimental setup consists of a constant-volume sampling dilution tunnel, an
engine dynamometer, a heavy duty diesel engine (HDDE) and PM/gas sampling systems, with
bold arrows indicating flow directions.
Figure 2. The reduction efficiencies of gaseous and particulate emissions under (a) transient cycle
with B1S50 as the baseline and (b) steady cycle with idle as the baseline.
Figure 3. A scatter plot of the weighted emission factors of TPM and PM2.5 under transient and
steady cycle.
Figure 4. The TC, OC and EC reduction efficiencies for TPM under transient cycle with B1S50
as the baseline. The error bar gives the standard deviation.
Figure 5. Scatter plots of TPM against TC, OC and EC weighted emission factors under transient
cycle.
Figure 6. The TC, OC and EC reduction efficiencies for TPM under steady cycle with idle as the
baseline. The error bar gives the standard deviation.
26
THC
NOxx
Load/speed
profile
CO
HDDE
Computer
Exhaust
Exhaust sample bag
Engine
dynamometer
Venturi Heat exchange
Pump
Pump
Engine
exhaust
Filter
1st dilution
TPM/PM2.5
sampling
2nd dilution
Ambient air
Fig. 1. The experimental setup consists of a constant volume sampling dilution tunnel, an engine
dynamometer, a heavy duty diesel engine (HDDE) and PM/gas sampling systems, with bold
arrows indicating flow directions.
27
100
(a) FTP transient cycle
Reduction efficiency (%)
80
60
40
THC
CO
NOx
TPM
PM2.5
20
0
-20
-40
-60
B1S10
B10
DOC1
DOC2+DPF
Type of fuel/aftertreatment
100
Reduction efficiency (%)
80
(b) Steady cycle
60
40
20
0
-100
-200
-300
-400
-500
-600
-700
Low
Medium
High
Engine load
Fig. 2. The reduction efficiencies of gaseous and particulate emissions under (a) transient cycle
with B1S50 as the baseline and (b) steady cycle with idle as the baseline. The error bar gives the
standard deviation.
28
250
1:1
FTP transient cycle
Steady cycle
PM2.5 (mg/bhp-hr)
200
150
100
y  0.70  0.07 x  14.5  9.4 
50
R2=0.84
0
0
50
100
150
200
250
TPM (mg/bhp-hr)
Fig. 3. A scatter plot of the weighted emission factors of TPM and PM2.5 under transient and steady
cycle. The uncertainty of the linear regression is given as (coefficient ± standard error).
29
100
Reduction efficiency (%)
FTP transient cycle
80
TC
OC
EC
60
40
20
0
B1S10
B10
DOC1
DOC2+DPF
Type of fuel/aftertreatment
Fig. 4. The TC, OC and EC reduction efficiencies for TPM under transient cycle with B1S50 as
the baseline. The error bar gives the standard deviation.
30
TC (mg/bhp-hr)
200
B1S50
B1S10
B10
DOC1
DOC2+DPF
150
100
1:1
FTP transient cycle
y  0.83  0.04 x  0.001  0.005
50
R2=0.95
0
0
50
OC (mg/bhp-hr)
200
100
150
TPM (g/bhp-hr)
200
1:1
y  0.38  0.04 x  0.003  0.004 
150
R2=0.84
100
50
0
0
50
EC (mg/bhp-hr)
200
100
150
TPM (g/bhp-hr)
200
1:1
y  0.45  0.03x  0.001  0.004 
150
R2=0.91
100
50
0
0
50
100
150
200
TPM (mg/bhp-hr)
Fig. 5. Scatter plots of TPM against TC, OC and EC weighted emission factors under transient
cycle. The uncertainty of the linear regression is given as (coefficient ± standard error).
31
100
TC
OC
EC
Reduction efficiency (%)
Steady cycle
80
60
40
20
0
Low
Medium
High
Engine load
Fig. 6. The TC, OC and EC reduction efficiencies for TPM under steady cycle with idle as the
baseline. The error bar gives the standard deviation.
32