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Impact of aggressive drive cycles on motor vehicle exhaust PM emissions
Author’s Accepted Manuscript Impact of aggressive drive cycles on motor vehicle exhaust PM emissions M. Matti Maricq, Joseph J. Szente, Amy L. Harwell, Michael J. Loos www.elsevier.com/locate/jaerosci
PII: DOI: Reference:
S0021-8502(17)30154-4 http://dx.doi.org/10.1016/j.jaerosci.2017.07.005 AS5145
To appear in: Journal of Aerosol Science Received date: 21 April 2017 Revised date: 26 June 2017 Accepted date: 21 July 2017 Cite this article as: M. Matti Maricq, Joseph J. Szente, Amy L. Harwell and Michael J. Loos, Impact of aggressive drive cycles on motor vehicle exhaust PM e m i s s i o n s , Journal of Aerosol Science, http://dx.doi.org/10.1016/j.jaerosci.2017.07.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Impact of aggressive drive cycles on motor vehicle exhaust PM emissions M. Matti Maricq*, Joseph J. Szente, Amy L. Harwell, Michael J. Loos Research & Advanced Engineering, Ford Motor Company, P.O. Box 2053, MD 3179, Dearborn, MI 48121 * Author for correspondence
Abstract Aggressive drive cycles can cause substantial discrepancies between particulate matter (PM) measurement methods and increased emissions variability. Previous work demonstrated good agreement, within ~10%, between aerosol instruments and gravimetric determinations of PM mass emissions from gasoline direct injection engine (GDI) vehicles run over the Federal Test Procedure (FTP). In contrast, the present study reveals discrepancies of a factor of three or more for these vehicles run over the US06 portion of the supplemental FTP test. Two aspects of this are examined: 1) Changes in particle composition and morphology and 2) variability associated with vehicle history and test preparation. PM emissions during the US06 cycle are often accompanied by a strong nucleation mode. The organic to elemental carbon ratio increases relative to the FTP cycle. Also, the black carbon to elemental carbon ratio decreases, suggesting the formation of brown soot. The origins of these changes are not entirely clear, but are likely associated with the high engine exhaust temperatures during the US06 cycle. This also contributes to the higher test to test variability observed for the US06 versus FTP cycle. High temperatures can thermally desorb and pyrolyze various materials, including heavy hydrocarbons, ash, and inorganic salts deposited in the exhaust pipe, catalytic converter, and muffler that provide an additional PM source not present during FTP driving. The first day’s PM mass emissions are often substantially higher than on following days for the US06 cycle, but not the FTP. There is also a distinct drop in PM emissions between first and subsequent US06 cycles run in series.
Keywords: Particulate emissions, soot, US06 drive cycle, elemental carbon, black carbon
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Introduction New emissions regulations world-wide are introducing high speed / high acceleration drive cycles into their particulate matter (PM) emissions standards. The United States Environmental Protection Agency (EPA) Tier 3 and California Air Resources Board (CARB) LEV III regulations have both established a PM standard of 6 mg/mi for the US06 portion of the Supplemental Federal Test Procedure (EPA 2016a, CARB 2016). Europe, China, and much of the rest of the world are moving to adopt the Worldwide harmonized Light vehicles Test Procedure (WLTP) into their emissions testing, in which includes a “extra high” speed portion with vehicle speeds up to 130 km/h (DieselNet 2016). These high speed cycles are intended to address the increased amount of real world driving that occurs on highways, where speed limits can be in excess of 70 mph (110 km/h), and the emissions that may occur under such driving conditions. However, as the present work shows, engine operation at high speed and load and the resulting high exhaust temperatures produce changes in PM emissions relative to the better understood US Federal Test Procedure (FTP) and EU New European Drive Cycle (NEDC). Specifically, this impacts the relationship between gravimetric PM mass, as required by US regulations, and a variety of aerosol instruments that have been developed over the past couple of decades to provide real time, on-line, PM measurement capability. These include the TSI Engine Exhaust Particle sizer (EEPS) (Johnson et al. 2004), Cambustion DMS500 (Reavell et al. 2002), Dekati Mass Monitor 230A (DMM) (Lehmann et al.
2004), Dekati Electrical Low Pressure Impactor (ELPI) (Marjamäki et al. 2000), AVL Micro Soot Sensor 483 (MSS) (Schindler et al. 2004), and Artium LII300 (Snelling et al. 2005). Because these instruments register particles based on various aerosol properties, namely electrical mobility size, aerodynamic size, photoacoustic light absorption, and laser induced incandescence, their relative responses to exhaust PM will vary if the nature of the particles changes. While some change in particle characteristics is inevitable during a transient drive cycle, previous work has shown that consistent correlations are found between the
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instruments and gravimetric PM mass for moderate drive cycles, such as the FTP and NEDC (Maricq et al. 2016). These correlations result from the fact that modern gasoline direct injection (GDI) and diesel vehicle exhaust PM is dominated by an accumulation mode that is largely soot. The single mode size distribution fulfills the operational requirement of the DMM. The existence of published soot effective density data (Park et al. 2003, Maricq and Xu 2004, Quiros et al. 2015) enables reasonable approximations of PM mass from integrated EEPS and DMS500 size distributions (Liu et al. 2012, Li et al. 2014). And the soot optical absorption remains sufficiently consistent that the MSS and LII regularly record a soot mass which is approximately 80% of the total PM (Khalek et al. 2010, Forestieri et al. 2013, Maricq et al. 2016, Chang & Shields 2017). The US06 drive cycle PM emissions measurements presented below stand in stark contrast to this. The instruments underestimate PM mass on average by a factor of three and overestimate solid PN by a similar margin. The results below further show that the instruments are no longer consistent with each other; for example, the MSS substantially underestimates soot for the US06 cycle relative to thermosoptical measurements of EC.
One aim of the present paper is to examine what changes in PM
morphology and composition occur between FTP and US06 drive conditions that account for these discrepancies. The second issue examined in this work is the higher measurement variability and, hence, more elaborate vehicle preparation requirements associated with aggressive drive cycles. The high exhaust temperatures can lead to storage / release of material from the vehicle exhaust system and exhaust sampling system that interfere with PM measurement (Maricq et al. 1999). This can appear as test to test variations of a single vehicle, where its exhaust system is progressively cleaned by a series of US06 tests. Or it can manifest between different vehicles where contamination of the sampling system by one vehicle appears as PM in a subsequent vehicle’s tests. Below we examine two aspects of this: Day to day differences in US06 PM emissions and variations over successive US06 cycles. Overall, the results of
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this study demonstrate that aggressive drive cycles pose important challenges to aerosol measurement and to vehicle emissions testing.
Experimental Method Test Vehicles and Procedures The four test vehicles examined in detail are described in Table 1. Vehicle 2 was used in the previous study (Maricq et al. 2016), but the other 3 are new. All are powered by current technology turbocharged gasoline direct injection engines. One utilizes centrally mounted fuel injectors, whereas the others employ side mounted injectors. These placements are often associated with spray guided and wall guided fuel sprays, respectively; however, in actuality both types of spray motion are present in the two geometries, although to different extents (Fiengo et al. 2013). The vehicles are all equipped with 3-way catalysts.
They were fueled with regular grade E10 certification fuel and used manufacturer
recommended lube oil. Three of them are high mileage because they were undergoing a significant amount of testing by developers and were conveniently available for the additional US06 testing we required. Their emissions characteristics are consistent with current GTDI vehicles, but being under development are not identical to those of the commercial product.
Table 1. Test vehicle descriptions
1 2
Vehicle
Technology1
Year
1 2 3 4
GTDI GTDI GTDI GTDI
2015 2012 2015 2015
Displacement L 1.6 3.5 2.7 2.7
Injector Mounting Central Side Side Side
Odometer 103 miles 11.5 140 150 60 & 1502
CVS flow m3/min 15.5 10 – 20 20 20
GTDI = turbocharged gasoline direct injection; Using two different aged aftertreatment systems
Testing was conducted in Ford’s Vehicle Emissions Research Laboratory chassis dynamometer facility. The vehicles are run on a 48” diameter single roll electric dynamometer. Vehicle exhaust is 4
conducted through a 1 meter long, 10 cm diameter, corrugated metal hose to a remote mix tee. At the tee it is mixed with filtered (>90%) and conditioned room air (38 oC, -9 oC dewpoint). The mixture is then transported through a 7 meter long, 25 cm diameter, conductive coated Teflon hose to a 30.4 cm diameter stainless steel tunnel from which measurements are made. Drive Cycles The vehicles in the original test campaign, and vehicle 4, were run over the three phase FTP drive cycle followed by a short, under a minute, pause and then one or two US06 cycles. To investigate how vehicle operation prior to a US06 test affects emissions, vehicles 1 – 3 in Table 1 were tested using various warmup schemes: No warmup, 6 minutes at 40 mph, 15 minutes at 60 mph, and the cold start phase of the FTP. Each of these warmup schemes was followed by three or four successive US06s. The US06 is an aggressive high speed drive cycle. Figure 1 compares the US06 speed trace to that of the FTP cold start phase (the FTP hot start has the same speed trace). The main differences are a number of steep accelerations from 0 to >50 mph and a ~350 second stretch of 60 – 80 mph driving. One major impact of this for light duty gasoline vehicles is the increase in exhaust temperature from the range of about 550 oC in the FTP to temperatures approaching 800 oC during the US06.
Figure 1. Comparison of vehicle speed traces for the US06 and FTP cold start drive cycles.
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PM Measurement Methods Following the EPA method of CFR40 Part 1065 (EPA 2016b), diluted exhaust PM is collected onto 47 mm Teflon membrane filters (MTL PT47) via a Horiba HF-47 sampler operating at 475 oC and 100 cm/s filter face velocity (2 scfm). The filters are robotically weighed with a 0.1 g balance in a regulatory compliant humidity, temperature, and air flow controlled weigh room. They undergo static removal and buoyancy correction. An estimated 4 g gaseous artifact correction is applied to the raw PM masses prior to calculating PM emission rates. PM is collected in parallel onto 47 mm diameter precleaned quartz filters at 2 scfm for thermal EC/OC analysis by a Sunset model 5L thermo-optical analyzer using NIOSH method 5040. A 1.5 cm2 punch is taken from the quartz filter for analysis. The organic carbon mass is multiplied by a factor of 1.2 to approximate the hydrogen content expected for primary combustion PM. Generally a larger factor of ~1.5 is used to account for both oxygen and hydrogen in ambient organic PM, because it is expected to have undergone some extent of atmospheric oxidation (Turpin and Lim 2001). Quartz filters are collected for sulfate and nitrate analysis. Multiple US06 tests were sampled by a single filter to improve detection limits. Filters are extracted with 10 mL deionized water and analyzed by capillary electrophoresis with a quantification limit of 2.5 g/filter. Solid particle number emissions >23 nm are recorded with a UNECE-R83 compliant AVL APC 489. Per EU regulations, particles are sampled from the dilution tunnel and pass through a ~350 oC evaporation tube to remove volatile particles. The aerosol instruments sampled from the dilution tunnel to ensure they analyze exhaust aerosol that experiences the same dilution and temperature history as recorded by the filters. When needed, secondary dilution by an ejector pump (dilution factor = 8.5) was used to avoid overloading the instruments. EEPS raw currents are inverted into size distributions using the newly available soot inversion (Wang et al. 2016b). The size distributions are fitted second by second to bimodal lognormal distributions. PM mass is calculated by ascribing 0.85 g/cm3 oil-like density spherical particles to the
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nucleation mode and a size dependent effective density for the accumulation mode. The latter assumes a fractal like soot geometry described by a 2.3 mass-mobility exponent and ~20 nm primary particles with a material density of ~2 gm/cm3 (Park et al. 2003, Maricq and Xu 2004, Quiros et al. 2015). The DMM calculates total PM mass from charging particles and passing them through a rudimentary electrical mobility classifier followed by a cascade impactor. The mass calculation relies on factory calibration of the charging efficiency and the mobility and impactor cutpoint diameters. These functions of the DMM were regularly cleaned to maintain proper operation. The MSS reports soot mass based on a factory calibration for its optical absorption at 808 nm, which is chosen to minimize interference from NO2 (Schindler et al. 2004). It was generally run without secondary dilution, but the cell windows were cleaned as prescribed. We periodically verified proper operation of these instruments by comparison against filter collected soot produced by a laboratory flame generator (miniCAST) (Mamakos et al. 2013, Moore et al. 2014).
Results US06 Cycle Impact on PM Measurement Figure 2 displays the marked contrast between the correlations of gravimetric to instrument derived PM mass observed for the FTP drive cycle versus those for the US06 cycle. It includes data from the 6 GDI vehicles tested in our previous study (Maricq et al. 2016) in addition to the vehicles in Table 1. The discrepancy between FTP and US06 results is particularly notable since it occurs in the middle of the emissions test; namely, the US06 is simply a continuation of the overall FTP + US06 cycle. The FTP data behave as expected. The DMM is sensitive to all aerosol components and yields total PM mass that reproduces within experimental variability the gravimetric values, whereas the MSS records the soot component, which is ~80% of the total. However, in the US06 portion of the test the situation becomes very different. The DMM underestimates total PM mass by roughly a factor of 3. At emissions rates of tens of milligrams per mile this cannot be caused by filter artifact, so there is clearly a discrepancy. The
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MSS value for soot mass is unexpectedly low, about 30% of total PM, but in principle possible. As discussed below, however, EC/OC measurements show that these MSS US06 results too are incorrect.
Figure 2. Contrast in correlations between instrument and gravimetric PM mass for the US06 versus FTP cold start drive cycles. Data are from 35 FTP and 48 US06 tests of 10 GDI vehicles.
Figure 3 demonstrates that PM mass calculated from EEPS measured size distributions suffers a similar fate. As described above the mass is the sum of the soot accumulation mode scaled by a size dependent effective density and a nucleation mode with assumed 0.85 g/cm3 oil-like density. This yields reasonable agreement with gravimetric PM mass during the FTP cycle; however, it likewise severely underestimates the US06 mass emissions.
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Figure 3. Contrast in correlations between EEPS derived and gravimetric PM mass for the US06 versus FTP cold start drive cycles.
Changes in PM Characteristics To understand why the instruments have difficulty assessing PM mass, consider the transient size distributions exhibited in Figure 4. They illustrate a number of changes in the nature of the particles between FTP and US06 drive cycles. These changes are vehicle dependent, so two examples are provided. The FTP cold start particle emissions shown in the top panels are very similar for the two vehicles; the differences are primarily quantitative. PM emissions peaks predominantly occur at two times, engine start and the accelerations during the initial drive away, and then decrease substantially as the engine warms up. The size distributions exhibit little if any nucleation mode. They are lognormal with geometric mean in the 70 – 90 nm range during cold start, which sometimes decreases to 40 – 60 nm after the engine warms up.
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Figure 4. Comparison of transient exhaust PM size distributions for two vehicles run over the cold start phase of the FTP (top panels) versus the US06 cycle (bottom panels). Note that the FTP graphs employ identical concentration scales, whereas those for the US06 data differ by a factor of ~40.
The PM size distributions during the US06 exhibit substantial differences both compared to the FTP cycle and between the two vehicles (note the changes in concentration ranges).
One major
difference is the appearance of a nucleation mode. For vehicle 2 this is mostly unresolved from the accumulation mode, but one intense nucleation burst can be seen.
Other tests show this to occur
sporadically throughout the US06. A distinct nucleation peak is more prevalent for vehicle 3, occurring nearly throughout the US06. Note that for vehicle 3 the nucleation mode does not begin at the start of the US06 cycle, but rather about 200 seconds into the test. Similar behavior is observed for other test vehicles and suggests that heating of the exhaust and aftertreatment systems plays a role.
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The second major difference is the shift of the accumulation mode to smaller size. For vehicle 2 the mode shifts to a geometric mean of about 50 nm, which although a significant decrease remains consistent with the mode comprising soot agglomerates. For vehicle 3 the accumulation mode either shifts much more, to about 20 nm, which is a size typical of soot primary particles, or it disappears leaving a bimodal nucleation mode. Thus, to the extent solid carbon is present, it is mostly not in the form of fractal-like agglomerates. These changes impact the suitability of the DMM for PM mass measurement. This instrument interprets the single mobility and six aerodynamic size dependent signals to estimate the particle number, mean size, and effective density it requires to calculate PM mass. This determination is based on the assumption of a unimodal size distribution (Lehmann et al. 2004). The presence of a nucleation mode causes the DMM to underestimate accumulation mode size, which leads to the low values of PM mass seen in Figure 2. Filter based PM analyses are performed to measure sulfate emissions for vehicles 3 and 4 and EC/OC composition for vehicles 2 – 4. The appearance of significant nucleation mode emissions during US06 driving prompts the interest in sulfate, since it is often associated with this mode. However, the measured levels over the US06 cycle are quite low: 0.240.02 mg/mi for vehicle 3 and below the detection limit of 0.06 mg/mi for vehicle 4. Nitrate levels were also below detection limit. Such low levels are consistent with previous results from port fuel injected gasoline vehicles, where negligible sulfate emissions were found even with fuel sulfur concentrations up to 1000 ppm [Maricq et al. 2002]. This was attributed to the lack of oxygen under stoichiometric combustion for conversion of SO2 to sulfate. Nevertheless, minute amounts of sulfate below the detection limit could still play a role in promoting semi volatile hydrocarbons to nucleate instead of condensing onto soot particles. Total carbon from thermo-optical analysis is converted to PM mass as EC + 1.2 OC to account approximately for the hydrogen mass expected in primary combustion emitted organic matter. The absolute EC + OC emission rates fall short of their gravimetric counterparts by about 20% for the FTP cold start and 30% or more for the US06 cycle. In contrast, calibration measurements using a propane flame CAST soot generator indicate that the EC + OC analysis reproduces gravimetric values within a 11
couple of percent. We subsequently determined that this inconsistency arises from a non-uniform PM deposition associated with our 47 mm filter holders. This caused variability in the fraction of mass on the 1.5 cm2 punch taken for EC/OC analysis. This should not affect the EC/OC ratios presented below, but we nevertheless include an additional 10% uncertainty in the reported error bars. Figure 5 compares the organic PM fraction measured during the FTP cold start versus two to four subsequent US06 cycles. Each vehicle repeated its test sequence over 3 to 4 days, vehicle 3 on two occasions.
Vehicle 2 was warmed up for six minutes at 40 mph, vehicle 3 with the cold start phase of
the FTP, and vehicle 4 with a full three phase FTP. The organic component comprises ~18% of the total PM emissions during the cold start FTP cycle, consistent with the soot fraction of ~80% recorded by MSS in Figure 2. All three vehicles exhibit increases in OC fraction for the US06 cycle by 30% to near 50%. Even with this increase, the US06 cycle EC fraction of 50% to 70% remains about double the ~30% soot fraction measured by MSS in Figure 2.
Figure 5. Comparison of the organic PM fraction between FTP cold start phase and 2 - 4 subsequent US06 cycles. Error bars represent the 1 scatter over 3-4 repeat tests.
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This discrepancy between elemental and black carbon is examined in more detail in Figure 6. It compares the BC to EC ratio and the geometric mean particle size of the accumulation mode recorded for the FTP versus US06 cycles. As pointed out previously (Petzold et al. 2013), these quantities are method dependent; here BC refers to light absorption at 808 nm recorded photoacoustically by MSS and EC to thermal evolution of CO2 according to the NIOSH method. For the FTP cold start phase, the BC/EC ratio is ~1. This value is consistent with the MSS being calibrated to measure diesel engine soot. The displayed error bars represent test to test measurement variability plus a 10% uncertainty to account for the mass discrepancy between thermo-optical and gravimetric measurements.
Figure 6. Comparisons of black carbon / elemental carbon ratio (top panel) and geometric mean particle size (bottom panel) between FTP cold start phase and 2 - 4 subsequent US06 cycles. Error bars represent 1 scatter over 2-4 repeat tests. Vehicle 3 was tested on two separate occasions. 13
In contrast to the FTP cycle, soot emitted by these vehicles during US06 driving has an apparently lower optical absorption at 808 nm; in effect the soot appears brown, which leads to the underestimation of soot mass relative to EC. The US06 values for BC fall only slightly, to about 90% of EC for vehicle 2, but decrease substantially to 30 - 60% of EC for vehicles 3 and 4. A similar drive cycle dependence on BC to EC ratio was observed by Kamboures et al. (2013) and hypothesized to result from differences in the physiochemical properties of the PM generated between US06 versus FTP driving. In Figure 6 (lower panel) we see that this decrease in BC is accompanied by a reduction in accumulation mode particle size, from about 75 nm geometric mean diameter to 45, 30, and 25 nm, respectively, for vehicles 2 – 4. Particularly in the latter two cases this is close to the size of soot primary particles, not agglomerates. This suggests that these may represent “young” soot particles, which have not yet carbonized to a graphitic structure (Dobbins 2002) and, thus, appear less black to the MSS. US06 PM measurement variability In addition to its impacts on PM characteristics, the US06 cycle is more susceptible to vehicle and sampling system history. We examine here two manifestations of this: one involves day to day trends and the other successive US06 tests. Emissions testing often proceeds in a weekly sequence: test, overnight soak, test, etc. The first test is used to prep the sequence, so emissions are typically not measured. The subsequent tests are then labeled “day 1”, “day 2”, and so on. Figure 7 contrasts the day 1 to day 2 correlation in PM emissions rates observed for the US06 versus FTP cold start drives. In all tests the full three phase FTP cycle precedes the US06. All four PM methods show reasonable correlations between the first and second day’s FTP tests (only cold start data are plotted). The data scatter is proportional to emissions rates, which is expected for day to day variability. Other vehicles are run daily in the facility, but they are not observed to impact the FTP PM measurements. In contrast, day 1 US06 gravimetric PM mass emissions often grossly exceed subsequent days' values. This occurs even when the prep drive includes the US06 cycle (solid symbols), but is worse when
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only the FTP cycle is used for vehicle preparation (open symbols). DMM and MSS data for the US06 test are omitted because of the measurement issues discussed above (Figure 2). Interestingly solid particle number emissions are better behaved from day 1 to 2. We suspect that the excess PM mass in day 1 originates largely from desorption and pyrolysis of material deposited on exhaust system and sampling system surfaces during previous vehicle operation and not adequately removed during the prep cycle. Gasoline vehicle exhaust temperatures during the US06 cycle are 200 – 250 oC higher than the FTP cycle, and can reach 800 oC. These temperatures are in the range used to regenerate diesel particulate filters (Guan et al. 2015). Thus, they can similarly “regenerate” the vehicle exhaust system (exhaust pipe, muffler, etc.) as well as the transfer hose used to conduct exhaust from the vehicle to the dilution tunnel mixing point. An added observation is that the day 1 excess emissions seem to occur with vehicles that have not been tested for some time, a few weeks or more and, thus, have an unknown driving history. If the vehicle is run again in the subsequent week the day 1 versus 2 difference is diminished.
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Figure 7. Correlation between PM measurements from six vehicles made day 1 versus day 2 of an emissions test sequence. Top panel: US06 cycle. Bottom panel: FTP cycle.
Differences in PM emissions rates are also observed between successive US06 cycles run during a single day’s test, as displayed in Figure 8. The two US06 cycles follow less than a minute after a 3 phase FTP cycle to warm up the vehicle. Solid particle number emissions and gravimetric mass both decline on average by about 10 - 25% from the first to second US06. At the same time Figure 5 shows that OC increases. This can reflect a “cleaning” of the exhaust and sampling systems by the hot exhaust, but the
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similar decrease of solid particle number suggests that it could also arise from stabilization of the engine and aftertreatment system to the increased operating temperatures incurred during US06 driving.
Figure 8. Correlation between first and second of two consecutive US06 tests following a three phase FTP cycle.
To explore this further, four sets of tests were conducted using various warm-up schemes followed by three to four successive US06 cycles (vehicle 2 was tested twice).
Figure 9 displays the resulting PM
mass and solid number emissions rates. The data are normalized by the emissions during the first US06. The drop after the first US06 depends on the warmup strategy employed: 15 minute 60 mph drive for V1, 6 minute 40 mph drive for V2a, no warmup for V2b, and the FTP cold start phase for V3. As can be expected, PM emissions during a cold start US06 are rather high, so the subsequent US06 emissions for V2b experience the largest decrease. The 6 minute 40 mph, 15 minute 60 mph, and FTP cold start provide progressively better warm-ups; thus, the emissions drop for subsequent cycles decreases. Vehicle 3 appears to show no PM mass decrease from first to second US06, but we believe this to be anomalous. For unknown reasons this vehicle exhibited large deviations, by about a factor of 3, in gravimetric PM emissions over its three tests, resulting in a 1 error of 30% in the second to first US06 PM mass ratio. 17
In contrast vehicle 3’s solid PN number emissions are much more consistent day to day and exhibit the same first to second US06 emissions decrease as the other vehicles
After the decrease from first to
second US06, PM emissions during subsequent US06s remain stable within test variability. This suggests that the critical aspect of stabilizing US06 emissions is to run a US06 cycle as part of the vehicle preparation.
Figure 9. Normalized PM mass and solid number emissions rates over successive US06 cycles. The mass and number data are displaced horizontally for clarity. Error bars represent the 1 scatter over 3-4 repeat tests.
Discussion and Conclusions The differences between PM measurements over the FTP versus US06 cycles remind us that engine exhaust PM is a complex substance both in terms of composition and morphology. Although exceptions arise, it is typically composed of two size modes: an accumulation mode comprised mostly of soot with condensed sulfate and organics, and a nucleation mode of the latter semivolatile species. Exactly which species and modes are present depends on engine technology, fuel, type of aftertreatment, 18
and as the present work shows, the drive cycle. Whereas the data presented here compare US06 and FTP cycle PM emissions, we expect the same effects to be present in the “extra high” portion of the WLTP cycle. Various aerosol instruments look at this PM in different ways. The MSS measures soot mass via optical absorption. Its underestimation of soot emissions has been observed previously in US06 vehicle tests (Kamboures et al. 2013), as well as in flame studies and CAST generated soot (Mamakos et al. 2013, Maricq 2014). In flame generated soot, a low BC/EC is also accompanied by small soot size and associated with soot sampled earlier in the flame. Soot inception is still a subject of research, and both aromatic and aliphatic mechanisms are thought to participate (Wang 2011). Flame studies show that nascent soot particles retain some extent of hydrogen atoms (Dobbins 2002, 2007). As they grow and coagulate in the flame the hydrogen atoms are lost through carbonization, and soot matures into a more graphitic structure (Vander Wal 1998; Vander Wal and Tomasek 2004). This expands the regions of conjugated aromatic rings and, thereby, produces an increasingly delocalized electronic structure that extends optical absorption to progressively longer wavelengths and makes the soot appear blacker. What possibly happens in GDI engines is that the high speed / load operation demanded by the US06 cycle curtails this graphitic development. The high rpms reduce the time available for soot maturation. And at high load the engine typically runs very slightly rich, not enough to produce soot, but it eliminates any residual oxygen. Directionally these reduce carbonization and lower optical absorption. Interestingly, the issue of brown carbon is also of current interest to the aircraft emissions community, where the question of which soot source to use to calibrate instruments such as the MSS and LII for PM measurement is under investigation (Saffaripour et al. 2017). The DMM and EEPS measure particles by their physical properties and are, therefore, less affected by composition. Their difficulties with the US06 cycle arise primarily from interference from the nucleation mode that is substantially more prevalent here than during the FTP cycle.
The DMM’s
analysis for PM mass is predicated on the assumption of a unimodal exhaust PM; thus, while it can
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tolerate a minor nucleation mode, such that sometimes occurs during the FTP cycle, it is not expected to work under US06 conditions. The changes in size distribution between FTP and US06 may impact the EEPS’s ability to estimate PM mass. We know from the recent work on developing EEPS inversion algorithms for compact versus soot particles that this instrument is sensitive to the morphology of the aerosol (Wang et al. 2016a, 2016b). Over the FTP cycle the size and shape of soot agglomerates emitted by GDI vehicles (Barone et al. 2012) are similar to the diesel soot used to develop the soot inversion, whereby EEPS estimations of particle number and mass compare well to the regulatory methods. However, more work is needed to understand how well this inversion works when the emissions are bimodal, and especially if the inversion has sufficient dynamic range to recover the accumulation mode in the presence of an order of magnitude larger nucleation mode. A second question related to calculating PM mass is what density to use for immature soot particles in range of 20-40 nm. Another difficulty with US06 PM emissions testing is its greater sensitivity to vehicle and sampling system history. Previous work has shown that high exhaust temperatures during high speed / load vehicle operation can lead to PM artifacts associated with storage and release of material from sampling system surfaces (Maricq et al. 1999). This could just as well occur from the vehicle’s exhaust system walls (Hall and Dickens, 2000). Both effects increase the uncertainty in US06 PM measurement relative to the FTP cycle. Sampling system artifacts can be reduced by moving the dilution point as close as possible to the tailpipe, since these arise primarily from portions of the sampling system that contact hot raw exhaust. The use of partial flow diluters to sample directly at the tailpipe, as allowed under CFR 40 Part 1066, provides one option to achieve this, since this eliminates the transfer hose normally used to transport undiluted exhaust from tailpipe to dilution tunnel. Dealing with vehicle related storage release effects is more problematic. There appear to be two aspects to this: one is related to the initial US06 test after a vehicle has remained unused for some period, and the other concerns daily effects. For FTP testing, one typically runs a prep test prior to the sequence of overnight soaks and daily tests. With the US06 test, sometimes two prep days of FTP + US06 cycle 20
are needed to achieve a stable series of subsequent tests. And then two US06 cycles are needed in the emissions test with data recorded during the second US06 to allow the vehicle to stabilize to US06 operating conditions.
This raisesa dilemma between the reduction of test variability and the
representativeness of how vehicles are operated in the real world. To test if a vehicle meets emissions standards one wants standardized test conditions that reduce variability in the results. But an elaborate preparation procedure is unlikely to leave the vehicle in a condition that is reasonably similar to a real world vehicle prior to its being driven under US06 –like conditions, .e.g., high speed highway driving. This risks a divergence between what the test achieves relative to its policy goals. Finally, how does this impact engine and aftertreatment development? For FTP testing, the EEPS, DMM, and MSS are valuable because they correlate well to the regulatory methods, but provide much more detailed data as to the nature of the emissions and when and under what conditions they occur. The accuracy is sufficiently good that gravimetric mass and solid number data are not routinely needed, periodic checks suffice. This is unfortunately not true for US06 testing. The instruments can still qualitatively indicate when emissions occur, but not the absolute levels. For example, the MSS will indicate when soot is emitted, but not necessarily how much and the EEPS will show when nucleation occurs. Therefore, engine and aftertreatment development to meet US06 PM emissions targets will need to rely more heavily on gravimetric mass and solid number methods until new or improved instruments become available.
References Barone, T. L., Storey, J. M., Youngquist, A. D., & Szybist, J. P. (2012). An analysis of direct-injection spark-ignition (DISI) soot morphology. Atmospheric Environment, 49:268-274. California Air Resources Board (2016). The California Low-Emission Vehicle Regulations. LEV III ammendments, § 1961.2. Exhaust Emission Standards and Test Procedures - 2015 and Subsequent Model Passenger Cars, Light-Duty Trucks, and Medium-Duty Vehicles. https://www.arb.ca.gov/msprog/levprog/cleandoc/cleancomplete%20lev-ghg%20regs%207-16.pdf
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Chang, M.-C. O., Shields, J. E. (2017). Evaluation of solid particle number and black carbon for very low particulate matter emissions standards in light-duty vehicles. J. Air & Waste Manag. Assoc. in press. DieselNet (2016). Worldwide Harmonized Light https://www.dieselnet.com/standards/cycles/wltp.php
Vehicles
Test
Procedure
(WLTP)
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Liu, Z., Swanson, J., Kittelson, D. B., Pui, D. Y. H. (2012). Comparison of methods for online measurement of diesel particulate matter. Environ Sci Technol, 46:6127-6133. Mamakos, A., Khalek, I., Giannelli, R., and Spears, M. (2013). Characterization of Combustion Aerosol Produced by a Mini-CAST and Treated in a Catalytic Stripper, Aerosol Sci. and Technol., 47:927–936. Maricq, M. M. (2014). Examining the relationship between black carbon and soot in flames and engine exhaust, Aerosol Sci. Technol. 48:620-629. Maricq, M. M., Chase, R. E., Podsiadlik, D. H., and Vogt, R. (1999). Vehicle exhaust particle size distributions: A comparison of tailpipe and dilution tunnel measurements. SAE Technical Paper 1999-011461. Maricq, M. M., Chase, R. E., Podsiadlik, D. H., and Xu, N. (2002). The effects of the catalytic converter and fuel sulfur level on motor vehicle particulate matter emissions: Gasoline vehicles, Environ. Sci. Technol. 36:276-282. Maricq, M. M., Szente, J. J., Harwell, A. L. and Loos, M. J. (2016). How well can aerosol instruments measure particulate mass and solid particle number in engine exhaust? Aerosol Sci. Technol. 50:605-614. Maricq, M. M. and Xu, N. (2004), The effective density and fractal dimension of soot particles from premixed flames and motor vehicle exhaust, J. Aerosol Sci. 35:1251-1274. Marjamäki, M., Keskinen, J., Chen, D.-R., and Pui, D. Y. H. (2000). Performance evaluation of the electrical low-pressure impactor (ELPI). J. Aerosol Sci. 31:249–261 Moore, R.H., Ziemba, L.D., Dutcher, D., Beyersdorf, A.J., Chan, K., Crumeyrolle, S., Raymond, T.M., Thornhill, K.L., Winstead, E.L., and Anderson, B.E. (2014). Mapping the operation of the miniature combustion aerosol standard (Mini-CAST) soot generator. Aerosol Sci. Technol., 48:467-479. Park, K., Cao, F., Kittelson, D. B., & McMurry, P. H. (2003). Relationship between particle mass and mobility for diesel exhaust particles. Environ. Sci..Technol., 37:577–583. Petzold, A., Ogren, J. A., Fiebig, M., Laj, P., Li, S.-M., Baltensperger, U., Holzer-Popp, T., Kinne, S., Pappalardo, G., Sugimoto, N., Wehrli, C., Wiedensohler, A. and Zhang, X.-Y. (2013). Recommendations for reporting “black carbon" measurements, Atmos. Chem. Phys., 13:8365-8379. Quiros, D. C., Hu, S., Hu, S., Lee, E. S., Sardar, S., Wang, X., Olfert, J. S., Jung, H. S., Zhu, Y. and Huai, T. (2015). Particle effective density and mass during steady-state operation of GDI, PFI, and diesel passenger cars. J. Aerosol Sci., 83:39-54. Reavell, K., Hands, T., and Collings, N. (2002). A fast response particulate spectrometer for combustion aerosols. SAE Technical Paper 2002-01-2714. Saffaripour, M., Tay, L.-L., Thomson, K. A., Smallwood, G. J., Brem, B. T., and Durdina, L. (2017). Raman spectroscopy and TEM characterization of solid particlulate matter emitted from soot generators and aircraft turbine engines. Aerosol Sci. Technol. xx:yy-yyy.
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Schindler, W., Haisch, C., Beck, H. A., Niessner, R., Jacob, E., and Rothe, D. (2004). A photoacoustic sensor system for time resolved quantification of diesel soot emissions. SAE Technical Paper 2004-010968. Snelling, D. R., Smallwood, G. J., Liu, F., Gülder, Ö. L., and Bachalo, W. D. (2005). A calibrationindependent laser-induced incandescence technique for soot measurement by detecting absolute light intensity. Appl. Optics, 44:6773-6785. Turpin, B. J. and Lim. H. (2001). Species contributions to PM 2.5 mass concentrations: Revisiting common assumptions for estimating organic mass. Aerosol Sci. Technol. 35:602-610. Vander Wal, R. L. (1998). Soot precursor carbonization: Visualization using LIF and LII and comparison using bright and dark field TEM. Combust. Flame, 112:607-616. Vander Wal, R. L. and Tomasek, A. J. (2004). Soot nanostructure: dependence upon synthesis conditions. Combust. Flame 136:129-140. Wang, H. (2011). Formation of nascent soot and other condensed-phase materials in flames. Proc. Combust. Inst. 33:41-67. Wang, X.L., Grose, M.A., Avenido, A., Stolzenburg, M.R., Caldow, R., Osmondson, B.L., Chow, J.C., and Watson, J.G. (2016a). Improvement of Engine Exhaust Particle Sizer (EEPS) Size Distribution Measurement - I. Algorithm and Applications to Compact Aerosols. J. Aerosol Sci. 92:95-108. Wang, X.L., Grose, M.A., Caldow, R., Osmondson, B.L., Swanson, J.J., Chow, J.C., Watson, J.G., Kittelson, D.B., Li, Y., Xue, J., Jung, H., and Hu, S. (2016b). Improvement of Engine Exhaust Particle Sizer (EEPS) Size Distribution Measurement – II. Engine Exhaust Aerosols. J. Aerosol Sci. 92:83-94.
Highlights
GDI vehicle PM can change dramatically from moderate to aggressive driving PM size distribution can be bimodal for US06 driving, with a large nucleation mode Soot accumulation mode can shift to small size representative of immature soot Soot absorption decreases, producing brown soot and a low BC/EC ratio Storage / release and contamination increase PM measurement variability
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PII: DOI: Reference:
S0021-8502(17)30154-4 http://dx.doi.org/10.1016/j.jaerosci.2017.07.005 AS5145
To appear in: Journal of Aerosol Science Received date: 21 April 2017 Revised date: 26 June 2017 Accepted date: 21 July 2017 Cite this article as: M. Matti Maricq, Joseph J. Szente, Amy L. Harwell and Michael J. Loos, Impact of aggressive drive cycles on motor vehicle exhaust PM e m i s s i o n s , Journal of Aerosol Science, http://dx.doi.org/10.1016/j.jaerosci.2017.07.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Impact of aggressive drive cycles on motor vehicle exhaust PM emissions M. Matti Maricq*, Joseph J. Szente, Amy L. Harwell, Michael J. Loos Research & Advanced Engineering, Ford Motor Company, P.O. Box 2053, MD 3179, Dearborn, MI 48121 * Author for correspondence
Abstract Aggressive drive cycles can cause substantial discrepancies between particulate matter (PM) measurement methods and increased emissions variability. Previous work demonstrated good agreement, within ~10%, between aerosol instruments and gravimetric determinations of PM mass emissions from gasoline direct injection engine (GDI) vehicles run over the Federal Test Procedure (FTP). In contrast, the present study reveals discrepancies of a factor of three or more for these vehicles run over the US06 portion of the supplemental FTP test. Two aspects of this are examined: 1) Changes in particle composition and morphology and 2) variability associated with vehicle history and test preparation. PM emissions during the US06 cycle are often accompanied by a strong nucleation mode. The organic to elemental carbon ratio increases relative to the FTP cycle. Also, the black carbon to elemental carbon ratio decreases, suggesting the formation of brown soot. The origins of these changes are not entirely clear, but are likely associated with the high engine exhaust temperatures during the US06 cycle. This also contributes to the higher test to test variability observed for the US06 versus FTP cycle. High temperatures can thermally desorb and pyrolyze various materials, including heavy hydrocarbons, ash, and inorganic salts deposited in the exhaust pipe, catalytic converter, and muffler that provide an additional PM source not present during FTP driving. The first day’s PM mass emissions are often substantially higher than on following days for the US06 cycle, but not the FTP. There is also a distinct drop in PM emissions between first and subsequent US06 cycles run in series.
Keywords: Particulate emissions, soot, US06 drive cycle, elemental carbon, black carbon
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Introduction New emissions regulations world-wide are introducing high speed / high acceleration drive cycles into their particulate matter (PM) emissions standards. The United States Environmental Protection Agency (EPA) Tier 3 and California Air Resources Board (CARB) LEV III regulations have both established a PM standard of 6 mg/mi for the US06 portion of the Supplemental Federal Test Procedure (EPA 2016a, CARB 2016). Europe, China, and much of the rest of the world are moving to adopt the Worldwide harmonized Light vehicles Test Procedure (WLTP) into their emissions testing, in which includes a “extra high” speed portion with vehicle speeds up to 130 km/h (DieselNet 2016). These high speed cycles are intended to address the increased amount of real world driving that occurs on highways, where speed limits can be in excess of 70 mph (110 km/h), and the emissions that may occur under such driving conditions. However, as the present work shows, engine operation at high speed and load and the resulting high exhaust temperatures produce changes in PM emissions relative to the better understood US Federal Test Procedure (FTP) and EU New European Drive Cycle (NEDC). Specifically, this impacts the relationship between gravimetric PM mass, as required by US regulations, and a variety of aerosol instruments that have been developed over the past couple of decades to provide real time, on-line, PM measurement capability. These include the TSI Engine Exhaust Particle sizer (EEPS) (Johnson et al. 2004), Cambustion DMS500 (Reavell et al. 2002), Dekati Mass Monitor 230A (DMM) (Lehmann et al.
2004), Dekati Electrical Low Pressure Impactor (ELPI) (Marjamäki et al. 2000), AVL Micro Soot Sensor 483 (MSS) (Schindler et al. 2004), and Artium LII300 (Snelling et al. 2005). Because these instruments register particles based on various aerosol properties, namely electrical mobility size, aerodynamic size, photoacoustic light absorption, and laser induced incandescence, their relative responses to exhaust PM will vary if the nature of the particles changes. While some change in particle characteristics is inevitable during a transient drive cycle, previous work has shown that consistent correlations are found between the
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instruments and gravimetric PM mass for moderate drive cycles, such as the FTP and NEDC (Maricq et al. 2016). These correlations result from the fact that modern gasoline direct injection (GDI) and diesel vehicle exhaust PM is dominated by an accumulation mode that is largely soot. The single mode size distribution fulfills the operational requirement of the DMM. The existence of published soot effective density data (Park et al. 2003, Maricq and Xu 2004, Quiros et al. 2015) enables reasonable approximations of PM mass from integrated EEPS and DMS500 size distributions (Liu et al. 2012, Li et al. 2014). And the soot optical absorption remains sufficiently consistent that the MSS and LII regularly record a soot mass which is approximately 80% of the total PM (Khalek et al. 2010, Forestieri et al. 2013, Maricq et al. 2016, Chang & Shields 2017). The US06 drive cycle PM emissions measurements presented below stand in stark contrast to this. The instruments underestimate PM mass on average by a factor of three and overestimate solid PN by a similar margin. The results below further show that the instruments are no longer consistent with each other; for example, the MSS substantially underestimates soot for the US06 cycle relative to thermosoptical measurements of EC.
One aim of the present paper is to examine what changes in PM
morphology and composition occur between FTP and US06 drive conditions that account for these discrepancies. The second issue examined in this work is the higher measurement variability and, hence, more elaborate vehicle preparation requirements associated with aggressive drive cycles. The high exhaust temperatures can lead to storage / release of material from the vehicle exhaust system and exhaust sampling system that interfere with PM measurement (Maricq et al. 1999). This can appear as test to test variations of a single vehicle, where its exhaust system is progressively cleaned by a series of US06 tests. Or it can manifest between different vehicles where contamination of the sampling system by one vehicle appears as PM in a subsequent vehicle’s tests. Below we examine two aspects of this: Day to day differences in US06 PM emissions and variations over successive US06 cycles. Overall, the results of
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this study demonstrate that aggressive drive cycles pose important challenges to aerosol measurement and to vehicle emissions testing.
Experimental Method Test Vehicles and Procedures The four test vehicles examined in detail are described in Table 1. Vehicle 2 was used in the previous study (Maricq et al. 2016), but the other 3 are new. All are powered by current technology turbocharged gasoline direct injection engines. One utilizes centrally mounted fuel injectors, whereas the others employ side mounted injectors. These placements are often associated with spray guided and wall guided fuel sprays, respectively; however, in actuality both types of spray motion are present in the two geometries, although to different extents (Fiengo et al. 2013). The vehicles are all equipped with 3-way catalysts.
They were fueled with regular grade E10 certification fuel and used manufacturer
recommended lube oil. Three of them are high mileage because they were undergoing a significant amount of testing by developers and were conveniently available for the additional US06 testing we required. Their emissions characteristics are consistent with current GTDI vehicles, but being under development are not identical to those of the commercial product.
Table 1. Test vehicle descriptions
1 2
Vehicle
Technology1
Year
1 2 3 4
GTDI GTDI GTDI GTDI
2015 2012 2015 2015
Displacement L 1.6 3.5 2.7 2.7
Injector Mounting Central Side Side Side
Odometer 103 miles 11.5 140 150 60 & 1502
CVS flow m3/min 15.5 10 – 20 20 20
GTDI = turbocharged gasoline direct injection; Using two different aged aftertreatment systems
Testing was conducted in Ford’s Vehicle Emissions Research Laboratory chassis dynamometer facility. The vehicles are run on a 48” diameter single roll electric dynamometer. Vehicle exhaust is 4
conducted through a 1 meter long, 10 cm diameter, corrugated metal hose to a remote mix tee. At the tee it is mixed with filtered (>90%) and conditioned room air (38 oC, -9 oC dewpoint). The mixture is then transported through a 7 meter long, 25 cm diameter, conductive coated Teflon hose to a 30.4 cm diameter stainless steel tunnel from which measurements are made. Drive Cycles The vehicles in the original test campaign, and vehicle 4, were run over the three phase FTP drive cycle followed by a short, under a minute, pause and then one or two US06 cycles. To investigate how vehicle operation prior to a US06 test affects emissions, vehicles 1 – 3 in Table 1 were tested using various warmup schemes: No warmup, 6 minutes at 40 mph, 15 minutes at 60 mph, and the cold start phase of the FTP. Each of these warmup schemes was followed by three or four successive US06s. The US06 is an aggressive high speed drive cycle. Figure 1 compares the US06 speed trace to that of the FTP cold start phase (the FTP hot start has the same speed trace). The main differences are a number of steep accelerations from 0 to >50 mph and a ~350 second stretch of 60 – 80 mph driving. One major impact of this for light duty gasoline vehicles is the increase in exhaust temperature from the range of about 550 oC in the FTP to temperatures approaching 800 oC during the US06.
Figure 1. Comparison of vehicle speed traces for the US06 and FTP cold start drive cycles.
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PM Measurement Methods Following the EPA method of CFR40 Part 1065 (EPA 2016b), diluted exhaust PM is collected onto 47 mm Teflon membrane filters (MTL PT47) via a Horiba HF-47 sampler operating at 475 oC and 100 cm/s filter face velocity (2 scfm). The filters are robotically weighed with a 0.1 g balance in a regulatory compliant humidity, temperature, and air flow controlled weigh room. They undergo static removal and buoyancy correction. An estimated 4 g gaseous artifact correction is applied to the raw PM masses prior to calculating PM emission rates. PM is collected in parallel onto 47 mm diameter precleaned quartz filters at 2 scfm for thermal EC/OC analysis by a Sunset model 5L thermo-optical analyzer using NIOSH method 5040. A 1.5 cm2 punch is taken from the quartz filter for analysis. The organic carbon mass is multiplied by a factor of 1.2 to approximate the hydrogen content expected for primary combustion PM. Generally a larger factor of ~1.5 is used to account for both oxygen and hydrogen in ambient organic PM, because it is expected to have undergone some extent of atmospheric oxidation (Turpin and Lim 2001). Quartz filters are collected for sulfate and nitrate analysis. Multiple US06 tests were sampled by a single filter to improve detection limits. Filters are extracted with 10 mL deionized water and analyzed by capillary electrophoresis with a quantification limit of 2.5 g/filter. Solid particle number emissions >23 nm are recorded with a UNECE-R83 compliant AVL APC 489. Per EU regulations, particles are sampled from the dilution tunnel and pass through a ~350 oC evaporation tube to remove volatile particles. The aerosol instruments sampled from the dilution tunnel to ensure they analyze exhaust aerosol that experiences the same dilution and temperature history as recorded by the filters. When needed, secondary dilution by an ejector pump (dilution factor = 8.5) was used to avoid overloading the instruments. EEPS raw currents are inverted into size distributions using the newly available soot inversion (Wang et al. 2016b). The size distributions are fitted second by second to bimodal lognormal distributions. PM mass is calculated by ascribing 0.85 g/cm3 oil-like density spherical particles to the
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nucleation mode and a size dependent effective density for the accumulation mode. The latter assumes a fractal like soot geometry described by a 2.3 mass-mobility exponent and ~20 nm primary particles with a material density of ~2 gm/cm3 (Park et al. 2003, Maricq and Xu 2004, Quiros et al. 2015). The DMM calculates total PM mass from charging particles and passing them through a rudimentary electrical mobility classifier followed by a cascade impactor. The mass calculation relies on factory calibration of the charging efficiency and the mobility and impactor cutpoint diameters. These functions of the DMM were regularly cleaned to maintain proper operation. The MSS reports soot mass based on a factory calibration for its optical absorption at 808 nm, which is chosen to minimize interference from NO2 (Schindler et al. 2004). It was generally run without secondary dilution, but the cell windows were cleaned as prescribed. We periodically verified proper operation of these instruments by comparison against filter collected soot produced by a laboratory flame generator (miniCAST) (Mamakos et al. 2013, Moore et al. 2014).
Results US06 Cycle Impact on PM Measurement Figure 2 displays the marked contrast between the correlations of gravimetric to instrument derived PM mass observed for the FTP drive cycle versus those for the US06 cycle. It includes data from the 6 GDI vehicles tested in our previous study (Maricq et al. 2016) in addition to the vehicles in Table 1. The discrepancy between FTP and US06 results is particularly notable since it occurs in the middle of the emissions test; namely, the US06 is simply a continuation of the overall FTP + US06 cycle. The FTP data behave as expected. The DMM is sensitive to all aerosol components and yields total PM mass that reproduces within experimental variability the gravimetric values, whereas the MSS records the soot component, which is ~80% of the total. However, in the US06 portion of the test the situation becomes very different. The DMM underestimates total PM mass by roughly a factor of 3. At emissions rates of tens of milligrams per mile this cannot be caused by filter artifact, so there is clearly a discrepancy. The
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MSS value for soot mass is unexpectedly low, about 30% of total PM, but in principle possible. As discussed below, however, EC/OC measurements show that these MSS US06 results too are incorrect.
Figure 2. Contrast in correlations between instrument and gravimetric PM mass for the US06 versus FTP cold start drive cycles. Data are from 35 FTP and 48 US06 tests of 10 GDI vehicles.
Figure 3 demonstrates that PM mass calculated from EEPS measured size distributions suffers a similar fate. As described above the mass is the sum of the soot accumulation mode scaled by a size dependent effective density and a nucleation mode with assumed 0.85 g/cm3 oil-like density. This yields reasonable agreement with gravimetric PM mass during the FTP cycle; however, it likewise severely underestimates the US06 mass emissions.
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Figure 3. Contrast in correlations between EEPS derived and gravimetric PM mass for the US06 versus FTP cold start drive cycles.
Changes in PM Characteristics To understand why the instruments have difficulty assessing PM mass, consider the transient size distributions exhibited in Figure 4. They illustrate a number of changes in the nature of the particles between FTP and US06 drive cycles. These changes are vehicle dependent, so two examples are provided. The FTP cold start particle emissions shown in the top panels are very similar for the two vehicles; the differences are primarily quantitative. PM emissions peaks predominantly occur at two times, engine start and the accelerations during the initial drive away, and then decrease substantially as the engine warms up. The size distributions exhibit little if any nucleation mode. They are lognormal with geometric mean in the 70 – 90 nm range during cold start, which sometimes decreases to 40 – 60 nm after the engine warms up.
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Figure 4. Comparison of transient exhaust PM size distributions for two vehicles run over the cold start phase of the FTP (top panels) versus the US06 cycle (bottom panels). Note that the FTP graphs employ identical concentration scales, whereas those for the US06 data differ by a factor of ~40.
The PM size distributions during the US06 exhibit substantial differences both compared to the FTP cycle and between the two vehicles (note the changes in concentration ranges).
One major
difference is the appearance of a nucleation mode. For vehicle 2 this is mostly unresolved from the accumulation mode, but one intense nucleation burst can be seen.
Other tests show this to occur
sporadically throughout the US06. A distinct nucleation peak is more prevalent for vehicle 3, occurring nearly throughout the US06. Note that for vehicle 3 the nucleation mode does not begin at the start of the US06 cycle, but rather about 200 seconds into the test. Similar behavior is observed for other test vehicles and suggests that heating of the exhaust and aftertreatment systems plays a role.
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The second major difference is the shift of the accumulation mode to smaller size. For vehicle 2 the mode shifts to a geometric mean of about 50 nm, which although a significant decrease remains consistent with the mode comprising soot agglomerates. For vehicle 3 the accumulation mode either shifts much more, to about 20 nm, which is a size typical of soot primary particles, or it disappears leaving a bimodal nucleation mode. Thus, to the extent solid carbon is present, it is mostly not in the form of fractal-like agglomerates. These changes impact the suitability of the DMM for PM mass measurement. This instrument interprets the single mobility and six aerodynamic size dependent signals to estimate the particle number, mean size, and effective density it requires to calculate PM mass. This determination is based on the assumption of a unimodal size distribution (Lehmann et al. 2004). The presence of a nucleation mode causes the DMM to underestimate accumulation mode size, which leads to the low values of PM mass seen in Figure 2. Filter based PM analyses are performed to measure sulfate emissions for vehicles 3 and 4 and EC/OC composition for vehicles 2 – 4. The appearance of significant nucleation mode emissions during US06 driving prompts the interest in sulfate, since it is often associated with this mode. However, the measured levels over the US06 cycle are quite low: 0.240.02 mg/mi for vehicle 3 and below the detection limit of 0.06 mg/mi for vehicle 4. Nitrate levels were also below detection limit. Such low levels are consistent with previous results from port fuel injected gasoline vehicles, where negligible sulfate emissions were found even with fuel sulfur concentrations up to 1000 ppm [Maricq et al. 2002]. This was attributed to the lack of oxygen under stoichiometric combustion for conversion of SO2 to sulfate. Nevertheless, minute amounts of sulfate below the detection limit could still play a role in promoting semi volatile hydrocarbons to nucleate instead of condensing onto soot particles. Total carbon from thermo-optical analysis is converted to PM mass as EC + 1.2 OC to account approximately for the hydrogen mass expected in primary combustion emitted organic matter. The absolute EC + OC emission rates fall short of their gravimetric counterparts by about 20% for the FTP cold start and 30% or more for the US06 cycle. In contrast, calibration measurements using a propane flame CAST soot generator indicate that the EC + OC analysis reproduces gravimetric values within a 11
couple of percent. We subsequently determined that this inconsistency arises from a non-uniform PM deposition associated with our 47 mm filter holders. This caused variability in the fraction of mass on the 1.5 cm2 punch taken for EC/OC analysis. This should not affect the EC/OC ratios presented below, but we nevertheless include an additional 10% uncertainty in the reported error bars. Figure 5 compares the organic PM fraction measured during the FTP cold start versus two to four subsequent US06 cycles. Each vehicle repeated its test sequence over 3 to 4 days, vehicle 3 on two occasions.
Vehicle 2 was warmed up for six minutes at 40 mph, vehicle 3 with the cold start phase of
the FTP, and vehicle 4 with a full three phase FTP. The organic component comprises ~18% of the total PM emissions during the cold start FTP cycle, consistent with the soot fraction of ~80% recorded by MSS in Figure 2. All three vehicles exhibit increases in OC fraction for the US06 cycle by 30% to near 50%. Even with this increase, the US06 cycle EC fraction of 50% to 70% remains about double the ~30% soot fraction measured by MSS in Figure 2.
Figure 5. Comparison of the organic PM fraction between FTP cold start phase and 2 - 4 subsequent US06 cycles. Error bars represent the 1 scatter over 3-4 repeat tests.
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This discrepancy between elemental and black carbon is examined in more detail in Figure 6. It compares the BC to EC ratio and the geometric mean particle size of the accumulation mode recorded for the FTP versus US06 cycles. As pointed out previously (Petzold et al. 2013), these quantities are method dependent; here BC refers to light absorption at 808 nm recorded photoacoustically by MSS and EC to thermal evolution of CO2 according to the NIOSH method. For the FTP cold start phase, the BC/EC ratio is ~1. This value is consistent with the MSS being calibrated to measure diesel engine soot. The displayed error bars represent test to test measurement variability plus a 10% uncertainty to account for the mass discrepancy between thermo-optical and gravimetric measurements.
Figure 6. Comparisons of black carbon / elemental carbon ratio (top panel) and geometric mean particle size (bottom panel) between FTP cold start phase and 2 - 4 subsequent US06 cycles. Error bars represent 1 scatter over 2-4 repeat tests. Vehicle 3 was tested on two separate occasions. 13
In contrast to the FTP cycle, soot emitted by these vehicles during US06 driving has an apparently lower optical absorption at 808 nm; in effect the soot appears brown, which leads to the underestimation of soot mass relative to EC. The US06 values for BC fall only slightly, to about 90% of EC for vehicle 2, but decrease substantially to 30 - 60% of EC for vehicles 3 and 4. A similar drive cycle dependence on BC to EC ratio was observed by Kamboures et al. (2013) and hypothesized to result from differences in the physiochemical properties of the PM generated between US06 versus FTP driving. In Figure 6 (lower panel) we see that this decrease in BC is accompanied by a reduction in accumulation mode particle size, from about 75 nm geometric mean diameter to 45, 30, and 25 nm, respectively, for vehicles 2 – 4. Particularly in the latter two cases this is close to the size of soot primary particles, not agglomerates. This suggests that these may represent “young” soot particles, which have not yet carbonized to a graphitic structure (Dobbins 2002) and, thus, appear less black to the MSS. US06 PM measurement variability In addition to its impacts on PM characteristics, the US06 cycle is more susceptible to vehicle and sampling system history. We examine here two manifestations of this: one involves day to day trends and the other successive US06 tests. Emissions testing often proceeds in a weekly sequence: test, overnight soak, test, etc. The first test is used to prep the sequence, so emissions are typically not measured. The subsequent tests are then labeled “day 1”, “day 2”, and so on. Figure 7 contrasts the day 1 to day 2 correlation in PM emissions rates observed for the US06 versus FTP cold start drives. In all tests the full three phase FTP cycle precedes the US06. All four PM methods show reasonable correlations between the first and second day’s FTP tests (only cold start data are plotted). The data scatter is proportional to emissions rates, which is expected for day to day variability. Other vehicles are run daily in the facility, but they are not observed to impact the FTP PM measurements. In contrast, day 1 US06 gravimetric PM mass emissions often grossly exceed subsequent days' values. This occurs even when the prep drive includes the US06 cycle (solid symbols), but is worse when
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only the FTP cycle is used for vehicle preparation (open symbols). DMM and MSS data for the US06 test are omitted because of the measurement issues discussed above (Figure 2). Interestingly solid particle number emissions are better behaved from day 1 to 2. We suspect that the excess PM mass in day 1 originates largely from desorption and pyrolysis of material deposited on exhaust system and sampling system surfaces during previous vehicle operation and not adequately removed during the prep cycle. Gasoline vehicle exhaust temperatures during the US06 cycle are 200 – 250 oC higher than the FTP cycle, and can reach 800 oC. These temperatures are in the range used to regenerate diesel particulate filters (Guan et al. 2015). Thus, they can similarly “regenerate” the vehicle exhaust system (exhaust pipe, muffler, etc.) as well as the transfer hose used to conduct exhaust from the vehicle to the dilution tunnel mixing point. An added observation is that the day 1 excess emissions seem to occur with vehicles that have not been tested for some time, a few weeks or more and, thus, have an unknown driving history. If the vehicle is run again in the subsequent week the day 1 versus 2 difference is diminished.
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Figure 7. Correlation between PM measurements from six vehicles made day 1 versus day 2 of an emissions test sequence. Top panel: US06 cycle. Bottom panel: FTP cycle.
Differences in PM emissions rates are also observed between successive US06 cycles run during a single day’s test, as displayed in Figure 8. The two US06 cycles follow less than a minute after a 3 phase FTP cycle to warm up the vehicle. Solid particle number emissions and gravimetric mass both decline on average by about 10 - 25% from the first to second US06. At the same time Figure 5 shows that OC increases. This can reflect a “cleaning” of the exhaust and sampling systems by the hot exhaust, but the
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similar decrease of solid particle number suggests that it could also arise from stabilization of the engine and aftertreatment system to the increased operating temperatures incurred during US06 driving.
Figure 8. Correlation between first and second of two consecutive US06 tests following a three phase FTP cycle.
To explore this further, four sets of tests were conducted using various warm-up schemes followed by three to four successive US06 cycles (vehicle 2 was tested twice).
Figure 9 displays the resulting PM
mass and solid number emissions rates. The data are normalized by the emissions during the first US06. The drop after the first US06 depends on the warmup strategy employed: 15 minute 60 mph drive for V1, 6 minute 40 mph drive for V2a, no warmup for V2b, and the FTP cold start phase for V3. As can be expected, PM emissions during a cold start US06 are rather high, so the subsequent US06 emissions for V2b experience the largest decrease. The 6 minute 40 mph, 15 minute 60 mph, and FTP cold start provide progressively better warm-ups; thus, the emissions drop for subsequent cycles decreases. Vehicle 3 appears to show no PM mass decrease from first to second US06, but we believe this to be anomalous. For unknown reasons this vehicle exhibited large deviations, by about a factor of 3, in gravimetric PM emissions over its three tests, resulting in a 1 error of 30% in the second to first US06 PM mass ratio. 17
In contrast vehicle 3’s solid PN number emissions are much more consistent day to day and exhibit the same first to second US06 emissions decrease as the other vehicles
After the decrease from first to
second US06, PM emissions during subsequent US06s remain stable within test variability. This suggests that the critical aspect of stabilizing US06 emissions is to run a US06 cycle as part of the vehicle preparation.
Figure 9. Normalized PM mass and solid number emissions rates over successive US06 cycles. The mass and number data are displaced horizontally for clarity. Error bars represent the 1 scatter over 3-4 repeat tests.
Discussion and Conclusions The differences between PM measurements over the FTP versus US06 cycles remind us that engine exhaust PM is a complex substance both in terms of composition and morphology. Although exceptions arise, it is typically composed of two size modes: an accumulation mode comprised mostly of soot with condensed sulfate and organics, and a nucleation mode of the latter semivolatile species. Exactly which species and modes are present depends on engine technology, fuel, type of aftertreatment, 18
and as the present work shows, the drive cycle. Whereas the data presented here compare US06 and FTP cycle PM emissions, we expect the same effects to be present in the “extra high” portion of the WLTP cycle. Various aerosol instruments look at this PM in different ways. The MSS measures soot mass via optical absorption. Its underestimation of soot emissions has been observed previously in US06 vehicle tests (Kamboures et al. 2013), as well as in flame studies and CAST generated soot (Mamakos et al. 2013, Maricq 2014). In flame generated soot, a low BC/EC is also accompanied by small soot size and associated with soot sampled earlier in the flame. Soot inception is still a subject of research, and both aromatic and aliphatic mechanisms are thought to participate (Wang 2011). Flame studies show that nascent soot particles retain some extent of hydrogen atoms (Dobbins 2002, 2007). As they grow and coagulate in the flame the hydrogen atoms are lost through carbonization, and soot matures into a more graphitic structure (Vander Wal 1998; Vander Wal and Tomasek 2004). This expands the regions of conjugated aromatic rings and, thereby, produces an increasingly delocalized electronic structure that extends optical absorption to progressively longer wavelengths and makes the soot appear blacker. What possibly happens in GDI engines is that the high speed / load operation demanded by the US06 cycle curtails this graphitic development. The high rpms reduce the time available for soot maturation. And at high load the engine typically runs very slightly rich, not enough to produce soot, but it eliminates any residual oxygen. Directionally these reduce carbonization and lower optical absorption. Interestingly, the issue of brown carbon is also of current interest to the aircraft emissions community, where the question of which soot source to use to calibrate instruments such as the MSS and LII for PM measurement is under investigation (Saffaripour et al. 2017). The DMM and EEPS measure particles by their physical properties and are, therefore, less affected by composition. Their difficulties with the US06 cycle arise primarily from interference from the nucleation mode that is substantially more prevalent here than during the FTP cycle.
The DMM’s
analysis for PM mass is predicated on the assumption of a unimodal exhaust PM; thus, while it can
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tolerate a minor nucleation mode, such that sometimes occurs during the FTP cycle, it is not expected to work under US06 conditions. The changes in size distribution between FTP and US06 may impact the EEPS’s ability to estimate PM mass. We know from the recent work on developing EEPS inversion algorithms for compact versus soot particles that this instrument is sensitive to the morphology of the aerosol (Wang et al. 2016a, 2016b). Over the FTP cycle the size and shape of soot agglomerates emitted by GDI vehicles (Barone et al. 2012) are similar to the diesel soot used to develop the soot inversion, whereby EEPS estimations of particle number and mass compare well to the regulatory methods. However, more work is needed to understand how well this inversion works when the emissions are bimodal, and especially if the inversion has sufficient dynamic range to recover the accumulation mode in the presence of an order of magnitude larger nucleation mode. A second question related to calculating PM mass is what density to use for immature soot particles in range of 20-40 nm. Another difficulty with US06 PM emissions testing is its greater sensitivity to vehicle and sampling system history. Previous work has shown that high exhaust temperatures during high speed / load vehicle operation can lead to PM artifacts associated with storage and release of material from sampling system surfaces (Maricq et al. 1999). This could just as well occur from the vehicle’s exhaust system walls (Hall and Dickens, 2000). Both effects increase the uncertainty in US06 PM measurement relative to the FTP cycle. Sampling system artifacts can be reduced by moving the dilution point as close as possible to the tailpipe, since these arise primarily from portions of the sampling system that contact hot raw exhaust. The use of partial flow diluters to sample directly at the tailpipe, as allowed under CFR 40 Part 1066, provides one option to achieve this, since this eliminates the transfer hose normally used to transport undiluted exhaust from tailpipe to dilution tunnel. Dealing with vehicle related storage release effects is more problematic. There appear to be two aspects to this: one is related to the initial US06 test after a vehicle has remained unused for some period, and the other concerns daily effects. For FTP testing, one typically runs a prep test prior to the sequence of overnight soaks and daily tests. With the US06 test, sometimes two prep days of FTP + US06 cycle 20
are needed to achieve a stable series of subsequent tests. And then two US06 cycles are needed in the emissions test with data recorded during the second US06 to allow the vehicle to stabilize to US06 operating conditions.
This raisesa dilemma between the reduction of test variability and the
representativeness of how vehicles are operated in the real world. To test if a vehicle meets emissions standards one wants standardized test conditions that reduce variability in the results. But an elaborate preparation procedure is unlikely to leave the vehicle in a condition that is reasonably similar to a real world vehicle prior to its being driven under US06 –like conditions, .e.g., high speed highway driving. This risks a divergence between what the test achieves relative to its policy goals. Finally, how does this impact engine and aftertreatment development? For FTP testing, the EEPS, DMM, and MSS are valuable because they correlate well to the regulatory methods, but provide much more detailed data as to the nature of the emissions and when and under what conditions they occur. The accuracy is sufficiently good that gravimetric mass and solid number data are not routinely needed, periodic checks suffice. This is unfortunately not true for US06 testing. The instruments can still qualitatively indicate when emissions occur, but not the absolute levels. For example, the MSS will indicate when soot is emitted, but not necessarily how much and the EEPS will show when nucleation occurs. Therefore, engine and aftertreatment development to meet US06 PM emissions targets will need to rely more heavily on gravimetric mass and solid number methods until new or improved instruments become available.
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Highlights
GDI vehicle PM can change dramatically from moderate to aggressive driving PM size distribution can be bimodal for US06 driving, with a large nucleation mode Soot accumulation mode can shift to small size representative of immature soot Soot absorption decreases, producing brown soot and a low BC/EC ratio Storage / release and contamination increase PM measurement variability
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