Article pubs.acs.org/est
Black Carbon Emissions in Gasoline Exhaust and a Reduction Alternative with a Gasoline Particulate Filter Tak W. Chan,*,† Eric Meloche,† Joseph Kubsh,‡ and Rasto Brezny‡ †
Emissions Research and Measurement Section, Air Quality Research Division, Environment Canada, 335 River Road, Ottawa, Ontario K1A 0H3, Canada ‡ Manufacturers of Emission Controls Association, 2200 Wilson Boulevard, Suite 310, Arlington, Virginia 22201, United States S Supporting Information *
ABSTRACT: Black carbon (BC) mass and solid particle number emissions were obtained from two pairs of gasoline direct injection (GDI) vehicles and port fuel injection (PFI) vehicles over the U.S. Federal Test Procedure 75 (FTP75) and US06 Supplemental Federal Test Procedure (US06) drive cycles on gasoline and 10% by volume blended ethanol (E10). BC solid particles were emitted mostly during cold-start from all GDI and PFI vehicles. The reduction in ambient temperature had significant impacts on BC mass and solid particle number emissions, but larger impacts were observed on the PFI vehicles than the GDI vehicles. Over the FTP-75 phase 1 (cold-start) drive cycle, the BC mass emissions from the two GDI vehicles at 0 °F (−18 °C) varied from 57 to 143 mg/mi, which was higher than the emissions at 72 °F (22 °C; 12−29 mg/mi) by a factor of 5. For the two PFI vehicles, the BC mass emissions over the FTP-75 phase 1 drive cycle at 0 °F varied from 111 to 162 mg/mi, higher by a factor of 44−72 when compared to the BC emissions of 2−4 mg/mi at 72 °F. The use of a gasoline particulate filter (GPF) reduced BC emissions from the selected GDI vehicle by 73−88% at various ambient temperatures over the FTP-75 phase 1 drive cycle. The ambient temperature had less of an impact on particle emissions for a warmed-up engine. Over the US06 drive cycle, the GPF reduced BC mass emissions from the GDI vehicle by 59−80% at various temperatures. E10 had limited impact on BC emissions from the selected GDI and PFI vehicles during hot-starts. E10 was found to reduce BC emissions from the GDI vehicle by 15% at standard temperature and by 75% at 19 °F (−7 °C).
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increase. In North America, the current light-duty vehicle fleet is dominated by gasoline port fuel injection (PFI) vehicles. Gasoline direct injection (GDI) vehicles are one of the latest additions to the light-duty fleet. The concept of GDI is not new, and the first automotive application of this technology can be traced back to the 1950s, but the popularity of the GDI vehicle has traditionally been limited due to high production costs and technological barriers. The first mass-produced, modern GDI vehicles were introduced to the marketplace in the late 1990s. At first, the concept quickly gained popularity among Japanese and European manufacturers, but since then, technological breakthroughs and mass production furthered the deployment of this technology in the North American and all other major global markets. Compared to the traditional PFI engines, GDI engines offer many advantages, such as better fuel injection control, lower fuel consumption, less fuel pumping loss, higher compression ratio, and charge air cooling. All these advantages allow a downsized GDI engine to deliver equal or better performance than a larger PFI counterpart.6,7,26,35 All of the above characteristics position GDI engines as a favorable technology for reducing CO2 emissions to help gasoline vehicles meet more
INTRODUCTION Atmospheric black carbon (BC) particles are generated from incomplete combustion processes (e.g., internal combustion engines). When these particles are emitted to the atmosphere, they absorb solar radiation, influence cloud processes, and alter the melting of snow and ice surfaces.1 BC particles are relatively inert in the atmosphere but typically have atmospheric lifetimes from days to a week1−3 and are considered short-lived pollutants. Atmospheric BC particles are removed by dry and wet deposition. Exposure assessment studies have shown that BC particle exposure is linked to various human health issues, such as lung function and blood pressure.4,5 Current research suggests that atmospheric BC particles have an overall positive climate forcing only second to carbon dioxide. Reducing the emissions of short-lived BC particles could lead to an immediate reduction of localized heating in environmentally sensitive areas, such as the Arctic, and also bring additional health benefit by reducing particulate matter (PM) exposure by commuters.1 Modern diesel vehicles are getting cleaner due to the deployment of efficient emission controls such as diesel particulate filters to comply with the more stringent regulations on PM.40 While the heavy-duty fleet is progressively getting cleaner with the new diesel trucks replacing older trucks, the relative BC emissions attributed to the light-duty gasoline vehicles, to the overall BC emissions, is expected to gradually © 2014 American Chemical Society
Received: December 9, 2013 Accepted: April 23, 2014 Published: April 23, 2014 6027
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10% ethanol. This could have an impact on affecting the BC emissions from on-road GDI and PFI vehicles across different regions of Canada. By using concurrent solid particle number and BC mass measurements, this study provides a systematic way of evaluating emissions from two pairs of GDI and PFI vehicles over two different drive cycles and at different ambient temperatures to understand how solid particle number and BC mass relationships vary under the influence of different factors. In this study, an aethalometer was used to measure real-time BC emissions while the solid particles were measured using a European Union Particle Measurement Programme compliant system. In anticipation that GPFs could become a strategy to reduce BC emissions from future GDI vehicles, this study also presented the performance of a prototype GPF on removing BC particles from a selected GDI vehicle at various conditions.
stringent emission standards that target fuel consumption and/or vehicle greenhouse gases. At the same time, several recent studies have shown that GDI vehicles could emit more PM than traditional PFI vehicles and heavy-duty trucks equipped with diesel particulate filters,6,10,11 challenging current GDI vehicles to comply with the recently finalized California LEV III and U.S. EPA Tier 3 particulate emissions standards without the use of new emission control strategies, such as a gasoline particulate filter (GPF). The major difference between the PFI and GDI engines lies in the fuel injection method and mixture preparation. This leads to the potential for wetting the cylinder wall in the GDI engines. In addition, the inhomogeneity of the fuel mixture under stratified operation in GDI engines during the compression stroke tends to contribute to higher particulate emissions compared to the exhaust emissions from traditional PFI engines.6,8−11,35,38 Summarizing available literature provides some general trends with respect to particulate emissions. For example, cold-start at low ambient temperature is found to significantly increase particle number emissions from a PFI vehicle compared to a GDI vehicle primarily due to the difference in fuel injection and mixture preparation strategies between GDI and PFI engines. This difference typically causes PFI engines to inject significantly more fuel during cold-start at low ambient temperatures to compensate for the poor volatility of the fuel.6,9,32 Laboratory and on-road measurements have also shown that nanoparticle emissions from gasoline spark ignition vehicles can be strongly dependent on vehicle speed and engine load.21−23 The impact of fuel ethanol content on BC formation is also not straightforward. A detailed chemical kinetic model suggested that oxygenated compounds, such as ethanol, could reduce BC formation because the oxygenated carbon is more strongly bonded than nonoxygenated carbon atoms, making the oxygenated carbon unavailable to form BC precursors during combustion, leading to overall less BC formation.24 However, observations from flame studies suggest that ethanol can both increase or decrease BC formation depending on whether a nonpremixed or premixed flame is involved during the combustion process.25−28 This situation is further complicated when splash blending ethanol with gasoline as this changes several important fuel properties, such as the aromatic content, vapor pressure, and distillation profile of the resulting fuel.36,37 These changes in fuel properties affect particle formation during combustion as well as the cold and hot weather driveability of the vehicle. The result is mixed observations of particle emissions on ethanol blended gasoline depending on the type of fuel injection system and the operating conditions of the vehicle.8,9,11,29−31,38,39 A brief review of some recent observations is summarized in Chan et al.8 With the anticipation of more GDI vehicles on the road in the near future,12 it is important to get a better understanding of the emission characteristics, such as BC mass and solid particle number emissions, from modern GDI engines compared to traditional PFI engines because the relative contribution to particle emissions from GDI vehicles is expected to increase gradually over time. Also, understanding how various factors, such as ambient temperature, driving pattern, and fuel ethanol content, influence the BC mass emissions from different engine technologies also provides useful information for better assessing future emissions in various scenarios. As of September 1, 2010, the Federal Government of Canada has regulated that motor gasoline sold in Canada must contain an annual pool average of 5% ethanol while different mandates also exist among different provinces. As a result, gasoline sold in Canada may contain up to
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EXPERIMENTAL SECTION Vehicles, Test Details, and Ambient Conditions. Two midsize sedans and two compact vehicles were used in this study. The midsize sedans were a 2011 2.4 L Hyundai Sonata wallguided, stoichiometric GDI vehicle (GDI#1) and a 2010 2.4 L Volvo S40 PFI vehicle (PFI#1). The GDI#1 vehicle represents one of the early versions of available GDI vehicles that were introduced into the North American market during the start of this test program in 2011 while the PFI#1 vehicle is a comparable PFI vehicle in terms of vehicle weight, engine size, and category of emission compliance. The two compact vehicles were a 2012 2.0 L Ford Focus wall-guided, stoichiometric GDI vehicle (GDI#2) and a 2013 2.0 L Ford Transit Connect PFI vehicle (PFI#2). The two compact vehicles are examples of some more recent vehicles with more advanced improvements but with smaller engine displacements compared to the GDI#1 and PFI#1 vehicles. All vehicles were mileage accumulated on-road prior to conducting all the emission measurements.8 All vehicles were equipped with three-way catalytic converters (TWCs). A summary of the specifications of all tested vehicles is given in Table S1 (Supporting Information). The two fuels tested were a Tier 2 certification gasoline (E0) and a splash blended 10% by volume blend of ethanol with certification gasoline (E10). The fuel specifications and distillation profiles for the test fuels are summarized in TableS S2 and S3 (Supporting Information). Most of the E0 measurements were conducted at three ambient temperatures (72 °F/22 °C, 19 °F/−7 °C, 0 °F/−18 °C). A temperature of 19 °F was chosen to be consistent with the temperature used for cold CO compliant emission testing while 0 °F was chosen to represent one commonly encountered winter temperature in many parts of Canada. Due to availability of the test cell, some of the US06 and E10 measurements were only conducted at selected temperatures. Table S4 (Supporting Information) summarizes the test cycle and different test conditions for all four vehicles used in this study. Typically three or four repeats were conducted for all test conditions (unless noted otherwise). The drive cycles used in this study were the U.S. Federal Test Procedure (FTP-75) and US06 Supplemental Federal Test Procedure (US06). The FTP-75 is a city driving cycle, designed to broadly represent the driving conditions in the U.S. The cycle consists of three phases (i.e., cold-start, urban, and hot-start phases). Phases 1 and 3 of the FTP-75 drive cycle are identical with phase 1 being a cold-start. Phase 2 of the FTP-75 drive cycle represents a typical urban driving pattern that includes moderate accelerations and decelerations. The US06 drive cycle is an aggressive driving cycle aimed to simulate aggressive driving 6028
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conditions.20,35 Additional details regarding the drive cycles and test procedure are given in the Supporting Information. A figure summarizing the speed trace for the test sequence of a typical test day is given in Figure S2 (Supporting Information). Gasoline Particulate Filter. The GPF used on the GDI#1 vehicle was a custom design device provided by the Manufacturers of Emission Controls Association (MECA). The noncatalyzed, wall-flow GPF was made using a cordierite ceramic material and was 5.66 in. (14.4 cm) in diameter and 6 in. (15.2 cm) in length. The GPF was optimized to lower the effects on back pressure and did not increase the fuel consumption over the test cycles used in this study.9 The cell density of the GPF is 200 cells per square inch (cpsi) with 12 mil (0.3 mm) walls and approximately 50% wall porosity. When in use, the GPF was installed in the underfloor position about 18 in. (46 cm) downstream of the original underfloor TWC converter, replacing the original resonator. The particle filtration principle of the test GPF is similar to a noncatalyzed wall-flow diesel particulate filter. Starting from a clean filter, filtration efficiency is typically low, and exhaust particles are expected to deposit inside the porous wall of the GPF, resulting in a change in wall microstructure (i.e., deep-bed filtration). When the pore space becomes filled, the additional particles begin to deposit on the channel walls (inside the channels) forming a thin soot layer (i.e., soot-cake filtration). Gradually, the soot layer further develops, and this significantly improves the filtration efficiency, particularly for ultrafine particles, as the soot layer also acts as an additional filtration medium.13,14,41 Typically, filtration efficiency is the lowest for particles with diameters of about 100−200 nm because these particles are too large to be effectively removed by diffusion and too small to be removed by impaction.8,9 As solid particles continue to be accumulated in the GPF, the filter gradually becomes overloaded and eventually leads to filter clogging, reduces the collection capacity, and increases back pressure. This problem is resolved by thermal regeneration of the filter, which in this case, is triggered by the hot exhaust gas temperature (typically >1022 °F/550 °C). During regeneration, the accumulated PM will be oxidized (by NO2 and O2) and accompanied by the release of a large number of ultrafine particles.8,9,41 After regeneration, the filtration efficiency drops, and the filtration mechanism restarts all over again. The vehicle testing and prepping procedure ensured the emission tests to begin with a clean GPF at the beginning of the test day. Previous measurements showed that regeneration did not occur over the FTP-75 drive cycle except during 0 °F (−18 °C) testing while the high exhaust gas temperature generated during the US06 drive cycle was able to caused periodic regeneration during the course of the cycle.8,9 Sampling Setup. The complete sampling setup is given in Figure S3 (Supporting Information). Descriptions of the individual components and all gaseous emissions results were documented previously8,9 and will not be discussed here. For the purpose of this paper, descriptions here are only limited to the solid particle and BC analytical methods. During the testing of the GDI#1 and PFI#1 vehicles, diluted exhaust was extracted directly from the constant volume sampling (CVS) dilution tunnel and introduced into a European Union Particle Measurement Programme (PMP) compliant system for solid particle detection (Supporting Information). A Magee Scientific AE51 microaethalometer was also included downstream of the PMP volatile particle remover system to measure BC mass concentration in parallel with the solid particle measurements.
For the testing of GDI#2 and PFI#2 vehicles, the microaethalometer was set up to extract diluted exhaust directly from the CVS dilution tunnel with secondary dilution. BC measurements from the GDI#1 and PFI#1 vehicles obtained using both configurations were compared to verify the consistency of the BC measurements between the two methods. Black Carbon Detection with the Microaethalometer. The AE51 microaethalometer has the same operating principle as a conventional aethalometer.15 Detailed descriptions and derivations of the BC mass concentration from the aethalometer measurements are documented elsewhere16,17 (Supporting Information). Briefly, in the microaethalometer, incoming particles are deposited continuously onto a Teflon-coated borosilicate glass fiber filter ticket where the light intensities, derived from a LED source at 880 nm, transmitted through the sampled spot and a reference blank spot are compared continuously to determine the light attenuation. The change in light attenuation at any given time interval is related to the light absorption coefficient that can be used to infer BC mass concentration using the predetermined mass attenuation cross section from the manufacturer. Several studies have shown the need to apply postmeasurement artifact corrections to aethalometer measurements due to the multiple scattering corrections (caused by the filter fibers) and the filter loading corrections16−18 (Supporting Information). All the microaethalometer measurements reported in this study were sampled in 1 Hz time resolution. A noise reduction procedure19 was first applied to the measurements before applying the filter loading and multiple scattering corrections17 to the data. The artifact corrected microaethalometer BC measurements were shown to be linearly correlated with the thermally determined elemental carbon (R = 0.83) and the refractory carbon mass measured by laser-induced incandescence (R = 0.91) over the mass concentration range from 0.1 to 10 mg/ mi (Supporting Information).
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RESULTS AND DISCUSSION Impact of Cold-/Hot-Start and Ambient Temperature on Black Carbon Emissions. In order to compare the impact of cold- and hot-starts on emissions, results from phase 1 and phase 3 portions of the FTP-75 drive cycle (both share the same vehicle speed trace with phase 1 being a cold-start) are compared. Figure 1 shows how the average BC mass (mg/mi; typical number of repeats = 3−4) (left panel) and the solid particle number (particles/mi) (right panel) emissions associated with the FTP-75, phase 1 drive cycle (cold-start) vary with ambient temperature. Solid markers correspond to measurements from the GDI#1 and PFI#1 vehicles while open markers correspond to the GDI#2 and PFI#2 vehicles. Corresponding results for the FTP-75 phase 3 drive cycle (hot-start) are given in Figure 2. All the emission rates are summarized in Tables S6 and S7 (Supporting Information). In both Figures 1 and 2, the trends for the BC mass and solid particle number emissions were consistent suggesting that the solid particles emitted from both the GDI and PFI vehicles are light absorbing, consistent with a number of studies that reported a linear relationship between solid particle number and BC or PM mass.33−35,42 In all cases, cold-start emissions increased with decreasing ambient temperatures. At standard temperature, cold-start BC mass emissions from the two GDI vehicles represented 65−84% of the total BC mass emitted over the entire FTP-75 drive cycle. For the two PFI vehicles, cold-start BC mass attributed to 86−100% of the total BC mass emissions over the entire FTP-75 drive cycle. These 6029
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similar compared to the GDI#1 vehicle and were higher than the PFI#2 vehicle. The difference in BC emission trends between the two pairs of GDI and PFI vehicles could be related to differences in how engines with differing displacements respond to the same engine load during the two drive cycles. It could also be related to the different optimized emission control strategies adopted by the different vehicle manufacturers during cold-start conditions. Future studies are planned to determine if the morphology of the BC particles may have a role in altering the light absorbing property of the solid particles that are emitted between the two different GDI vehicles. After the GPF was installed on the GDI#1 vehicle, BC mass emissions were reduced significantly. At standard temperature and 19 °F (−7 °C), the GDI#1 post-GPF BC mass emissions were lower compared to the stock GDI configuration by 73% and 88%, respectively. Despite the occurrence of soot regeneration at 0 °F (−18 °C),9 the BC mass emission reduction was still maintained at 85%. These reductions were consistent with the solid particle number emission reductions of 75%, 85%, and 68% at standard temperature, 19 °F, and 0 °F, respectively. The lower solid particle number filtration efficiency at 0 °F compared to BC mass emission reduction was caused by the presence of a large number of ultrafine particles emitted during the soot regeneration process, which generated a large number of ultrafine particles, ranging from 6 nm to almost 50 nm in diameter, downstream of the GPF when sampling from the CVS tunnel (Supporting Information). Some of these ultrafine particles contributed to the solid particle number measurements leading to reduced solid particle filtration efficiency at 0 °F. However, these particles did not significantly contribute to the total light absorption detected by the microaethalometer, likely due to their small particle mass and possibly nonabsorbing nature, and therefore, the BC mass filtration efficiency at 0 °F was still high. Compared to the cold-start results (Figure 1), the hot-start emission results (Figure 2) were lower by orders of magnitude. In general, both the stock GDI#1 and GDI#2 BC mass emissions were comparable, varying from about 1.6−2.5 mg/mi at standard temperature to 4.3−4.5 mg/mi at 0 °F. Although much lower than the cold-start results, these values were still higher compared to both the PFI#1 and PFI#2 BC mass emissions, which varied from nondetectable to 0.6 mg/mi at standard temperature to 0.01−0.5 mg/mi at cold ambient temperatures. The GDI#1 post-GPF BC mass emissions exhibited a slightly different trend: they varied from about 0.1 mg/mi at standard temperature to nondetectable at 0 °F, consistent with the solid particle number emissions trend. The different post-GPF BC emission trends over various ambient temperatures for phases 1 and 3 can be explained by the different GPF filtration stages. A clean GPF (prior to the start of phase 1) has low particle filtration efficiency. Thus, the postclean GPF BC emissions at various ambient temperatures followed the stock GDI BC emission trend. It takes a finite amount of time for the soot layer to develop in the GPF before effective filtration begins, and the higher BC emissions at low ambient temperatures reduced the transition time from deep-bed filtration to soot-cake filtration, resulting in the apparent improvement in the particle filtration efficiency in the GPF (over phase 3) at colder ambient temperatures.9 The importance of a well-developed soot layer in improving filtration efficiency was also demonstrated by the continued reduction in post-GPF particle number emissions over subsequent cold-start Los Angeles Route Four drive cycles (i.e., phases 1 and 2 of FTP-
Figure 1. BC mass and solid particle number emissions as a function of ambient temperature over the FTP-75 phase 1 drive cycle. Solid markers represent measurements from the GDI#1 (circle and square) and PFI#1 (triangle) vehicles while open markers represent data from the GDI#2 (circle) and PFI#2 (triangle) vehicles.
Figure 2. BC mass and solid particle number emissions as a function of ambient temperature over the FTP-75 phase 3 drive cycle. Solid markers represent measurements from the GDI#1 (circle and square) and PFI#1 (triangle) vehicles while open markers represent data from the GDI#2 (circle) and PFI#2 (triangle) vehicles.
observations suggest that GDI BC emissions are related to the method of fuel injection caused by inhomogeneous mixing between air and fuel.35 For PFI vehicles, BC emissions are mostly due to cold-starting and are related to the incomplete vaporization of the fuel and wall wetting because PFI engines tend to overfuel during cold start to compensate for the poor volatility of the fuel.6,32 When comparing the different pairs of GDI and PFI vehicles, some interesting trends emerge. Over the cold-start, the ratio of the stock GDI#1 BC mass to PFI#1 BC mass emissions varied from 18 at standard temperature to 1.3 at 0 °F (−18 °C). The corresponding ratio of stock GDI#1 solid particle number to PFI#1 solid particle number emissions varied from 7.8 at standard temperature to 1.4 at 0 °F. When switching to the smaller displacement GDI#2 and PFI#2 vehicles, increasing BC emissions at low ambient temperatures from both vehicles were still observed. Interestingly, the BC mass emission trends from the GDI#2 vehicle at cold ambient temperatures were significantly lower than from the PFI#2 vehicle at the same conditions, although the solid particle emission trends remained 6030
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The drive pattern for the phase 2 portion of the FTP-75 drive cycle is more transient in nature compared to the phase 3 portion of the FTP-75 drive cycle and for the US06 drive cycle. In addition, the average and maximum vehicle speeds for the phase 2 portion are also the lowest (Supporting Information). The observations on the particle number and BC mass emissions from the different drive patterns for both GDI vehicles suggest that the additional transient driving nature, even though at relatively low speeds, led to increased particle emissions from the warmed GDI vehicles compared to their PFI counterparts. This could be related to the different modes of operation of the GDI engine. At low and medium engine load and speed conditions, the GDI engine typically operates under stratified-charge or homogeneous lean operation mode, when a compact spray with a reduced penetration rate is injected during the compression stroke, to improve fuel economy.6 BC particle formation is possible at localized fuel-rich zones during stratified-charge mode operation. At high engine load and speed, particularly with the need for fast acceleration, a GDI engine typically operates in a homogeneous-charge mode using early fuel injection. The better mixing associated with this type of operation helps to suppress the formation of BC.6 The exact transition between the different fuel injection modes varies depending on the engine design thus resulting in different emission trends for different GDI vehicles. In the PFI vehicle case, it would appear that the need to inject more fuel during high engine load and speed conditions (e.g., US06) may lead to possible incomplete vaporization of the fuel that then contributed to the increased BC particle emissions from the two PFI vehicles. The BC mass emissions from the GDI#1 post-GPF configuration were higher during the US06 drive cycle than those from the phases 2 and 3 portions of the FTP-75 drive cycle (Table S6, Supporting Information). However, post-GPF BC measurements from the GDI#1 vehicle over the US06 drive cycle were still much lower than the BC emissions from the stock GDI configuration at all ambient temperatures by 59−80%. This is consistent with the 60−83% reduction in solid particle number emission over the same test conditions (Table S7, Supporting Information). These observations are consistent with the multiple spontaneous soot regenerations during the US06 drive cycle that impacted the integrity of the soot layer, leading to decreased particle filtration efficiency, compared to the filtration efficiency observed over the FTP-75 drive cycle.8,9 Impact of Fuel Ethanol Content on Black Carbon Emissions. Tables S8 and S9 (Supporting Information) summarize the BC mass and solid particle number emissions for all four GDI and PFI vehicles (typical number of repeats = 3− 4) when operated on E10 fuel. Measurements from the GDI#1 vehicle with the GPF installed are also included. These measurements are presented graphically in Figure 4. The uncertainties in the figure are the standard deviation and represent approximately the margin of error in a 95% confidence interval (Supporting Information). Comparing the E10 data with the E0 results (Tables S6 and S7, Supporting Information) shows that E10 has minimum impact on both BC mass and solid particle number emissions from the PFI vehicles in general. The PFI#1 vehicle appeared to have lower BC mass emissions when operated on E10 at 0 °F (−18 °C) although these changes are not statistically significant. For the GDI#1 vehicle, E10 has a mixed impact on BC emissions over the US06 and the phases 2 and 3 portions of the FTP-75 drive cycles, suggesting that ambient temperature, drive conditions, and the physical fuel properties of E10 all have
75) that did not destroy or regenerate the deposited materials in the GPF between these test cycles.8 Impact of Drive Conditions on Black Carbon Emissions. Observations have shown that nonsteady state and aggressive driving conditions could have large influences on particle emissions.21−23 To understand how the different drive patterns may influence particle emissions, hot-start emission results from the FTP-75 phases 2 and 3 as well as US06 drive cycles were compared. As illustrated in Figure S2 (Supporting Information), the phases 2 and 3 portions of the FTP-75 and the US06 drive cycles represent three very different drive patterns in the order of increasing driving speed and aggressiveness (Supporting Information). Figure 3 shows the comparison of the (a) solid
Figure 3. (a) Solid particle number and (b) BC mass emissions for all four GDI and PFI vehicles over the three different driving patterns. Solid bars represent low ambient temperature measurements whereas open, dashed bars represent standard temperature measurements.
particle number and (b) BC mass emissions (typical number of repeats = 3−4) from the different GDI and PFI vehicles over the three different driving patterns. In all cases, only standard temperature and 0 °F (−18 °C) results are shown for clarity. In general, the solid particle number and BC mass emission trends are similar and suggest that emissions from the PFI vehicles decreased with decreasing drive speed and aggressiveness. The two GDI vehicles show much different patterns. Both GDI vehicles had either similar or much higher emissions during the less aggressive FTP-75 phases 2 and 3 drive cycles than for the US06 drive cycle, compared to their corresponding PFI counterparts. 6031
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Figure 4. BC mass emissions from all four GDI and PFI vehicles as a function of ambient temperature over the phase 1 portion of the FTP-75 drive cycle when operated on E0 (solid color) and E10 (hashed area).
need for a more sensitive method to detect particle emissions from very low emitting vehicles. A number of studies have reported a positive relationship between particle number and BC (or PM) mass suggesting particle number measurements could be used to infer refractory (or particle) mass. Giechaskiel et al.33 summarized results from more than 20 heavy-duty engines and more than 150 light-duty vehicles and observed that the solid particle number to total PM mass ratio varied from 1 to 4 × 1012 particles/mg. When using refractory BC mass instead of PM mass, variability of the measurements was improved, resulting in an increased ratio. Their measurements showed that solid particle number to BC mass ratio approached 6 × 1012 particles/ mg for particles with a count median diameter (CMD) of 50 nm and the ratio decreased to 1 × 1012 particles/mg for a CMD of 75 nm. Khalek et al.42 reported a solid particle number to PM mass ratio of 2.3 × 1012 particles/mg and 3.2−3.9 × 1012 particles/mg for solid particle number to BC mass for a GDI vehicle (particle diameter ∼45−60 nm). In two studies by Maricq et al.,34,35 a solid particle number to PM mass ratio of 1.9−3.0 × 1012 particles/mg were observed for various GDI vehicles (particle diameters ∼60−90 nm). Figure 5 shows the correlation between solid particle number and BC mass measurements at all ambient conditions observed from this study. The three straight lines on the figure represent
different degrees of impacts on the BC mass emissions from this GDI vehicle at different ambient temperatures. In comparison, E10 was observed to have a much larger impact on BC mass emissions from both GDI vehicles over certain conditions of the phase 1 portion of the FTP-75 drive cycle (i.e., during cold-start; Figure 4). For example, BC emissions from the GDI#1 vehicle on E10 at standard temperature and 19 °F (−7 °C) were lower than the corresponding emissions on E0 by 52% and 75%, respectively (Figure 4). At 0 °F (−18 °C), E10 was found to increase BC emissions from the GDI#1 vehicle by 21% although results were not statistical significant within a 95% confidence interval. For the GDI#2 vehicle, E10 BC emissions over the phase 1 portion of the FTP-75 drive cycle at standard and 0 °F were lower than the E0 emissions by 66% and 30%, respectively (no measurements at 19 °F for E10 for comparison). The observations from this study suggest that E10 may have some benefit for GDI engines. Comparing between E0 and E10, E10 has a lower heating value and a larger volume of E10 fuel is needed to provide the same amount of energy as for E0.29,43 However, the distillation curve also shows that splash blending ethanol will raise the Reid Vapor Pressure (RVP) and increase the percent fuel evaporated in the 140−200 °F (60−93 °C) range44 (Supporting Information). Compounds in the midrange of the distillation curve (i.e., the percentage of fuel evaporated up to about 212 °F (100 °C)) are known to have an influence on the warm-up and the cold and hot weather driveability of the vehicle.37,43 As measurements between the PFI E0 and E10 measurements in this study were not statistically different, this suggests that all the PFI test vehicles adapted well to the E10 fuel, and the increased E10 fuel usage,29 even at low ambient temperature with cold-starting, is largely compensated for by the increased RVP of the fuel and did not lead to significant degradation of the vehicle driveability. Injecting fuel directly into the combustion chamber helps to reduce the over fueling issue during cold-start and limits the opportunity of excessive liquid fuel impingement on the piston and cylinder wall and reduces BC emissions.11,38 As GDI engines typically use late injection during part load operation, the leaning effect of the E10 fuel could further help to limit BC emissions.31 Black Carbon Mass and Solid Particle Number Relationship. A particle number standard has been adopted in Europe as part of the Euro 5/6 light-duty vehicle and Euro VI heavy-duty vehicle emission requirements in order to address the
Figure 5. Variations of the solid particle number emissions versus BC mass emissions over the US06 and individual phases of the FTP-75 drive cycles. 6032
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BC mass and solid particle number emissions from future GDI vehicles without compromising fuel economy. Over the FTP-75 phase 1 drive cycle, the GPF reduced BC emissions by 73−88% at various ambient temperatures. Over the US06 drive cycle, the BC emissions were reduced by 59−80%.
three different solid particle number to BC mass ratios that vary from 0.2 × 1012 to 2 × 1012 particles/mg. Consistent with other studies, measurements in Figure 5 show that solid particle number and BC mass were generally linearly correlated. Most of the scattering occurs at low concentrations and was attributed mostly to the PFI#1 and PFI#2 vehicles measurements over the phases 2 and 3 portions of the FTP-75 drive cycle, when the BC mass measurements were often too low to be accurately determined. Measurements from all four vehicles were typically bounded between the ratios of 1−2 × 1012 particles/mg. In comparison, the cold temperature measurements over the FTP75 drive cycle for various vehicles appeared to approach the solid particle number to BC mass ratio of about 0.2 × 1012 particles/ mg (Supporting Information). Particle number size distributions (Supporting Information) showed that the geometric mean diameter of the particles shifted from 60 nm at standard temperature to as much as 90 nm at 0 °F (−18 °C). The size shift could be caused by particle coagulation due to the increased particle number emissions at cold ambient temperatures. A shift of the particle number size distribution may not have a significant impact on the solid particle number measurements as most particles are larger than 23 nm. Larger BC particles absorb significantly more light than smaller BC particles, and thus, increased particle diameter could increase the BC mass measurements and lower the particle number to BC mass ratio. The reduced particle number to BC mass ratio with increasing particle diameter trend in this case was consistent with the observation from Giechaskiel et al.33 Implications. Observations from this study revealed several important implications regarding BC emissions obtained at realworld, on-road ambient conditions. First of all, both BC mass and solid particle number emissions increased considerably with decreasing ambient temperature, particularly evident during cold-starts from both the GDI and PFI vehicles. In addition, this study also showed that driving conditions have different impacts on BC emissions between GDI and PFI vehicles. The two warmed-up PFI vehicles had very low emissions over the phases 2 and 3 portions of the FTP-75 drive cycle compared to emissions observed during the US06 drive cycle. However, the two warmed-up GDI vehicles still emitted considerable amounts of BC mass during phases 2 and 3 portions of the FTP-75 and US06 drive cycles. Although the change in BC mass emissions at less demanding driving conditions varies from one GDI vehicle to another, these emission levels were still considerably higher than their corresponding PFI counterparts. This implies that information such as the relative fleet mix of GDI and PFI vehicles could be important in projecting ambient BC emissions in areas with mixed amounts of highways and intracity roads where driving conditions vary considerably. Another observed finding was that the 10% by volume ethanol in gasoline had limited impact on BC mass emissions from both PFI test vehicles. However, a significant reduction in BC mass emissions, up to 52−66%, from the two GDI vehicles was observed during the initial engine start up and operation at 22 °C. The reduction in BC mass emissions at low ambient temperatures associated with the ethanol-containing fuel varied between the different GDI vehicles. Particles that are emitted from GDI engines are primarily solid in nature; therefore, a GPF with a similar operating principle as a diesel particulate filter was expected to be useful in removing these emission particles. The assessment of the BC mass emissions from the prototype GPF used in this study showed that an optimum designed GPF can provide one alternative to reduce
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ASSOCIATED CONTENT
S Supporting Information *
Further details on background of the current emission test program, vehicles, fuels, test cycles, test procedure, sampling setup, error analysis and margin of error determination, microaethalometer BC measurements, additional figures, and tabulated emissions data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (613) 998-7913; fax: (613) 952-1006; e-mail: tak.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the contribution of the ERMS staff for their assistance in conducting this vehicle emissions research project. Financial support was provided by the Government of Canada’s Program of Energy Research and Development (PERD) P&E, Project C11.006, AFTER10, and MECA. The prototype GPF for this work was provided by MECA. The authors would also like to thank the anonymous reviewers for their helpful comments.
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