Effects of Injection Pressure on Diesel Engine Particle Physico-Chemical Properties

Abstract

The effects of injection pressure on diesel particle physical and chemical properties were investigated on a heavy-duty diesel engine. Three injection pressures (600 bar, 800 bar, and 1000 bar) were selected at two engine loads (0.3 MPa BMEP and 0.9 MPa BMEP). The exhaust particle size distribution was measured by a scanning mobility particle sizer (SMPS). Consistent with previous studies, increasing injection pressure effectively removes accumulation mode particles, which results in a significant decrease in particle total mass concentration. The elemental carbon emission factors were then tested through organic carbon/element carbon (OC/EC) analysis. The emitted EC is decreased by 64% and 50% with increasing injection pressure from 600 bar to 1000 bar at the low and high engine loads, respectively. Particle morphology and oxidation reactivity were investigated by means of transmission electron microscope (TEM) imaging and thermogravimetric analysis (TGA) technology, respectively. Smaller primary particles with shorter and flatter graphene layer segments are observed at higher injection pressure conditions, and the particle oxidation reactivity is increased with injection pressure.

Copyright 2014 American Association for Aerosol Research

1. INTRODUCTION

Diesel engines, which are identified as a major source of ambient particles, contribute seriously to air quality deterioration, so that increasingly stringent particulate emission regulations for diesel engines have been implemented over the past decades. Diesel particulate filters (DPFs) have been extensively employed in modern vehicles and have been shown to be effective (Johnson 2006). However, since particles collected in a DPF increase exhaust backpressure, a periodical regeneration process is additionally needed to remove the trapped particles. The regeneration efficiency and frequency as two central factors are highly affected by engine exhaust particulate characteristics. High particulate emissions can increase the loading inside the DPF, and therefore may require more frequent DPF regeneration. Further, particles with low oxidation reactivity can

decrease the particle burnout rate, which leads to a longer regeneration time. Therefore, engine exhaust particle emission control is still the subject of many studies.

FIG. 1 Schematic diagram of the experimental setup.

With engine technology evolution, common rail injection systems allowing a thorough control of the combustion process have been considered to be a superior enabler to reduce engine exhaust particle emissions, and hence decrease the particle deposition rate in the DPF (Bertola et al. 2001; Dodge et al. 2002; Han and Bae 2012). Experimental results have indicated that particle formation in diesel engines was highly dependent on the mixing and combustion of injected fuel. With optical diagnostic methods, Bruneaux (2005) indicated that increasing injection pressure could accelerate fuel–air mixing and enhance air entrainment. Higher fuel–air mixing rates lead to leaner conditions upstream of the reaction zone, so higher heat release rates and peak temperatures could be reached in the combustion chamber, which favors the soot oxidation process. Siebers and Higgins (2001) suggested that increasing injection pressure increased the lift-off length which is defined as the distance between nozzle tip and stabilized near-nozzle flame front, and the amount of fuel–air premixing upstream of the lift-off length was increased. When the average equivalence ratio at the lift-off length was reduced to a value less than 2, there was no significant soot incandescence observed (Siebers et al. 2002). Fischer and Stein (2009) found out that there was a critical injection pressure indicating a maximum feasible reduction of soot-NOx emissions and no further reduction could be achieved beyond that critical pressure. Jaeger et al. (1999) stated that higher injection pressure injected more fuel during a given ignition delay, which increased the fuel fraction for premixed combustion, and resulted in a particle mass reduction. With the help of a validated simulation model, Hountalas et al. (2003) pointed out that increasing the injection pressure had a more pronounced effect on particle emission reduction at partial load and low engine speed, and a higher increase of injection pressure could be applied as engine speed and load were reduced.

Many experiments were also conducted to determine the effects of injection pressure on engine exhaust particle characteristics. Kittelson (1998) found high fuel injection pressure led to more effective reduction of carbonaceous material than soluble organic fraction (SOF). Nucleation was then facilitated as there would not be enough surface available for volatiles adsorption (Johnson et al. 1994). Baumgard and Johnson (1996) found the emission particle size shifted to a smaller size with higher SOF content when testing a modern engine equipped with a high pressure injection system. The similar particle size distribution pattern was observed by other researchers (Pagán 1999; Desantes et al. 2005; Lähde et al. 2011). Besides these particle size distribution measurements, few recent studies have concerned particle morphology changes under different injection pressure conditions. Zhang and Kook (2012) and Mathis et al. (2005) sampled particles from the in-cylinder reacting fuel jet and exhaust gas, respectively, and then photographed the aggregated soot particles with TEM. Both the amount and size of soot particles were found to be decreased with increasing injection pressure. Ryser et al. (2009) made the similar finding when observing particle size development inside a combustion cell during unsteady diesel combustion processes through laser-induced incandescence technique.

Although the consensus that increasing injection pressure can significantly decrease particle mass emissions but increase the number concentration has been reached from the previous studies, the information about injection pressure influence on particle physico-chemical properties, especially the particle nanostructure and oxidation reactivity, which is highly related to the regeneration efficiency, is still limited. The present work is not to find an optimal strategy for diesel engine particle reduction, but to study both the physical and chemical properties of particles from different injection pressures that include particle size distribution, OC/EC composition, primary particle size, particle nanostructure and oxidation reactivity, and to explore the impacts of injection pressure on DPF regeneration behavior.

2. MATERIALS AND METHODS

2.1. Test Engine and Fuel

The schematic diagram of the experimental setup is shown in Figure 1. This study was conducted on a 2002 model-year diesel engine, which meets the Chinese national stage III emission standards. The main specifications of the engine are described in Table 1. The engine was coupled with an eddy-current dynamometer, which was available for measuring and adjusting the engine speed and torque. In-cylinder pressure was acquired by a Kistler 6125B piezoelectric pressure transducer with a resolution of 0.5 crank angle, and was recorded by a combustion analyzer (Osiris Model EVOL3). For each measuring point, the pressure trace was obtained by averaging the recorded pressure data of 200 consecutive cycles. Heat release rate, bulk gas temperature, and burned mass fraction were calculated from that in-cylinder pressure data using a zero-dimensional ideal-gas combustion model. An open access electronic control unit (ECU) was used to precisely control injection timing and injection pressure. The EGR valve was kept closed in order to isolate the EGR effects during different operation conditions. The fuel used in this study was the commercial diesel fuel that met the 50 ppm fuel sulfur limit of China IV emissions regulation for heavy duty diesel vehicles. The fuel properties are listed in Table 2. The lubricating oil used was a 15W-40 full mineral oil, with a measured sulfur content of 2687 ppm. Three injection pressures were selected for two different engine load conditions, and the injection timing was kept constant at TDC. All the tested conditions are listed in Table 3.

TABLE 1 Engine specifications

Number of cylinders	4
Fuel injection system	Common-rail injection system
Injection strategy	One main injection
Suction type	Turbo intercooler
Displacement	4.751 L
Bore×stroke	110 mm × 125 mm
Compression ratio	17.8:1
Maximum power	96 kW/2500 rpm
Maximum torque	450 Nm/1450 rpm

TABLE 2 Properties of diesel fuel

Property	Diesel fuel
Cetane number	51
Lower heating value (MJ/kg)	42.5
Density (kg/m3) at 20°C	837
Viscosity (mPa s) at 20°C	3.03
Heat of evaporation (kJ/kg)	276
Carbon content (% wt)	88.6
Sulfur content (mg/kg)	<50
Aromatic content (% wt)	28.4

TABLE 3 Operating conditions

Speed	BMEP^1	Injection timing	Injection
(r/min)	(MPa)	(°CA ATDC)	pressure (bar)
1450	0.3	0	600, 800, 1000
1450	0.9	0	600, 800, 1000
^1BMEP refers to brake mean effective pressure.

2.2. Particle Dilution and Sampling System

A partial flow dilution system was used in this study. The sampling probe was placed first so that sampling line purging of the gas analyzer would not affect particle size distributions or dilution ratio measurements. Diesel exhaust gas was drawn through an insulated and heated stainless steel tube into a primary dilution tunnel where the exhaust gas was diluted with a steady flow of clean cooled air filtered by a high efficiency particulate air filter (HEPA). The insulated sampling line was about 1 m and heated by a temperature controller to maintain a surface temperature of 190°C to prevent thermophoretic deposition of solid particles and condensation of water and volatile organic carbon on the tube wall. The particulate matter was diluted and cooled to 52°C based on the EPA regulation (SAE 1993). A TSP sampler and a MOUDI (MSP Model 110-R) were settled at the end of the primary dilution tunnel in order to collect total suspended and size-fractionated particles, respectively. Both samplers had a sampling flow of 30 L/min. In this experiment, a 47 mm quartz filter (Whatman Model 1851-047) was selected as the collection substrate because of its thermal stability and flexibility that minimize particle bounce without affecting the cutpoint of the cascade impactor (Miguel et al. 2004). However, it is well known that quartz filters may give rise to both positive (adsorption of organic gases) and negative (evaporation from the filter and collected particles) OC artifact (Kirchstetter et al. 1999). It may not be possible to generalize which is the dominant artifact because many variables, such as sample volume, filter face velocity, air temperature, and sample chemical composition (Turpin et al. 1994; Chase et al. 2004). Therefore, to minimize the OC artifact, all sampling procedures were completed within the same day, which provided consistent ambient conditions, and meanwhile the sampling time, the sampling flow rate, and the dilution ratio were all kept constant in this experiment. For particle size distribution measurement, a secondary dilution tunnel was employed to further dilute the exhaust gas. The dilution ratio of each stage was calculated based on the comparison of CO₂ concentration at the tunnel inlet and the sampling location, correcting for background CO₂ concentration (Phuleria et al. 2007). During the whole experiment, the primary dilution ratio was adjusted to about 5 and a fixed value of 12 was used for the secondary dilution ratio. An SMPS (TSI Model 3034) was used to measure the size distribution of the exhaust particles. For each steady state operating condition, three SMPS scans were recorded. The instrumental difference was within 2 nm for the modal diameter and within 5% for the averaged values of the total number concentration. The average size distribution results corrected with the overall dilution ratio are plotted in the following section.

FIG. 2 Particle size distributions for three different injection pressures at 0.3 MPa and 0.9 MPa engine load conditions.

2.3. Analysis Methods

All quartz filters were pre-baked at 550°C for 8 h and stored in baked aluminum foil prior to deployment. All the quartz filters from both samplers were analyzed for OC/EC composition. Only the TSP samples were further analyzed by TEM for particle morphology and TGA for particle oxidation reactivity.

For OC/EC analysis, the IMPROVE TOR protocol was implemented on a thermo/optical carbon analyzer (DRI Model 2001) (Han et al. 2007). The oven temperature was stepwise heated to 140°C, 280°C, 480°C, and 580°C within the helium environment, which could produce OC1, OC2, OC3, and OC4 thermo-carbon fractions. After that, 2% oxygen was introduced into the system, and the oven was further heated to 580°C, 740°C, and 840°C for producing EC1, EC2, and EC3. Therefore, OC could be calculated as OC1 + OC2 + OC3 + OC4 + POC (pyrolyzed organic carbon decreasing the reflected laser light intensity) while EC was composed of EC1-POC, EC2, and EC3.

Particulate morphology was analyzed with TEM spectroscopy. A portion of the filter loaded with particulate sample was soaked in ethanol, and extracted in an ultrasonic bath for 10 min. Immediately following the extraction, the solution was dripped onto a copper grid with a lacey carbon film. The substrate was then dried in an infrared drying oven. The similar sample preparation methods have been used by Vander Wal and Mueller (2006) and Yehliu et al. (2012).

A field emission HRTEM (Joel Model JEM-2100F) operated on 200 kV with a point-to-point resolution of 0.23 nm was employed to acquire HRTEM and TEM images. Different magnifications were used in the TEM and HRTEM inspections: 20,000× to image the soot particle morphology and above 500,000× to observe the primary particle nanostructure. The images were then analyzed using a commercial image processing software Image-Pro Plus 6.0 (Media Cybernetics). For the TEM images, about 150 primary particles with clear boundaries were randomly chosen for the primary particle diameter measurement. For the HRTEM images, particle nanostructure parameters including fringe length and tortuosity were quantified to describe the nanostructures of particles from different operating conditions. The digitizing and measuring method was adopted according to the work of Lu et al. (2012).

For particle oxidation reactivity tests, a thermogravimetric analysis instrument (TA Model Q5000IR) was used to measure weight changes of each sample as a function of temperature, and obtain the ignition temperature which is defined as the temperature where soot oxidation rate is maximized (Stratakis and Stamatelos 2003). A filter was cut into small pieces and about 15 mg of that sample was deposited into a platinum pan. All the samples were heated at 500°C for 20 min under high-purity nitrogen to drive off volatile matter, after a heating rate of 50°C/min. The volatile organic fraction (VOF) could be determined by measuring the weight loss at the end of this step. Subsequently, the furnace temperature was reduced to 400°C and air was then introduced into the system to start the oxidation process. The air-flow rate was 100 mL/min and the temperature was raised up to 800°C with a heating rate of 20°C/min.

FIG. 3 Particle total mass concentrations (TMC) with the nucleation/

accumulation particle partitioning for all tested conditions. The bright areas refer to the nucleation mode particle mass, and the dark areas refer to the accumulation mode particle mass.

3. RESULTS AND DISCUSSIONS

3.1. Particle Size Distribution

Figure 2 shows the particle size distribution patterns for different injection pressures at both engine loads. Clearly, increasing injection pressure reduces accumulation mode particles (particle diameter >50 nm), especially those particles larger than 100 nm. The total mass concentration (TMC) is calculated assuming that all particles are spherical with a constant density of 1 g/cm³ (Kittelson 1998). Figure 3 shows the particle's TMCs for all the conditions. The TMC is monotonically decreased with increasing injection pressure. Increasing injection pressure from 600 bar to 1000 bar results in 43% and 34% reductions in particle mass for the low and high engine load conditions, respectively, which is attributed to the accumulation mode particle reduction. Accumulation mode particles are mainly composed by solid carbon agglomerates formed during the in-cylinder combustion process, and account for the majority of exhaust particle mass as shown in Figure 3 (Desantes et al. 2005). Increasing injection pressure improves the fuel mixing and atomization processes and lowers the equivalence ratio, which encourages fuel complete combustion and reduces carbonaceous soot formation. Meanwhile, as indicated by Figures S1 and S2 (detailed combustion analysis is provided in the online supplemental information [SI]), higher in-cylinder peak pressure and temperature with elevated injection pressure enhance the soot oxidation process (Tree and Svensson 2007). Therefore, the net engine exhaust accumulation mode particles are reduced, which means less solid particles can be captured when the engine is equipped with emission control devices. Along with this decrease, a significant increase in nucleation mode particles (particle diameter <50 nm) is also observed for both engine loads, which is consistent with the studies mentioned above. In this study, the nucleation mode particles are increased to 10.4-fold and 7.5-fold for the low and the high engine

load conditions, respectively, as the injection pressure is increased from 600 bar to 1000 bar. Nucleation mode particles originate from high-weight hydrocarbons coming from unburned fuel and oil (Kittelson 1998). Since the carbonaceous agglomerates are reduced, it is more difficult for those gaseous hydrocarbons to find available particle surface to adsorb or condense, and hence the nucleation process is enhanced to form more ultrafine particles during the dilution and cooling process (Johnson et al. 1994; Kittelson 1998). The values of geometric mean diameter (GMD) as an indicator of particle size distribution pattern are listed in Table 4. In general, the GMD values become smaller when the injection pressure is increased, which means the size distributions are shifted to smaller sizes. Nevertheless, the GMD values for the high load condition cannot be well correlated to the injection pressure increment. This may be caused by the extremely high concentration of nucleation mode particles at 1000 bar may favor the aggregation process during the dilution and cooling processes, particularly with the relatively low primary dilution ratio used in this study. Further investigation should be conducted to validate this speculation. However, for engines with oxidation catalyst, engine exhaust hydrocarbons can be effectively removed, which may decrease the condensates for nucleation mode particle formation (Maricq et al. 2002). Furthermore, the nucleation process is not favorable inside a DPF where the temperature is much higher than that of the ambient environment, so the nucleation mode particle increase is believed to be not so drastic as measured in a dilution system.

TABLE 4 Geometric mean diameter (GMD) values for all the tested conditions

BMEP (Mpa)	0.3			0.9
Injection pressure (bar)	600	800	1000	600	800	1000
GMD (nm)	62	42	34	60	29	43

3.2. OC/EC Composition

All the MOUDI and TSP filters were analyzed to investigate the organic and elemental carbon composition. Most of the elemental carbon formed during in-cylinder combustion can be filtered by a DPF, so the emission factor of EC can be considered as an indicator of particulate loading rate for the DPF. As shown in Figure 4, the emitted EC is decreased by 64% and 50% from 600 bar to 1000 bar at the low and high engine loads, respectively. The reduction is more significant at the 0.3 MPa BMEP condition than at the 0.9 MPa BMEP condition in this study. The limited oxygen content and shortened combustion duration (SI) at the high load condition may lessen the particle oxidation process and hence reduce the effectiveness of the impact of injection pressure on particle reduction (Stanmore et al. 2001). Since the particle sampling condition in this study is very much different with the situation inside the DPF, the hydrocarbon gas to particle process is completely altered as mentioned above, which means that the OC composition results here have limited values for understanding the composition variations of the particles filtered by the DPF. Therefore, the detailed OC information will not be discussed in the text but can be found in the SI for those interested in the effect of injection pressure on raw particle chemical composition without posttreatment.

FIG. 4 Elemental carbon emission factors versus injection pressures at 0.3 MPa and 0.9 MPa engine load conditions.

FIG. 5 Representative HRTEM images of the samples generated from the 600 bar and 800 bar injection pressures at both two engine loads.

3.3. Soot Particle Morphology

Effects of injection pressure on soot particle morphology were also investigated. Here, only the images from the 600 bar and 800 bar samples for both engine loads were statistically analyzed while the 1000 bar samples showed an anomalous nanostructure whose correctness needs to be further verified. The average primary particle diameters are 28.3 nm for 600 bar, 0.3 MPa BMEP, 25.4 nm for 800 bar, 0.3 MPa BMEP, 22.9 nm for 600 bar, 0.9 MPa BMEP and 22.3 nm for 800 bar, 0.9 MPa BMEP, respectively. The primary particle size distributions and standard deviations for all tested conditions are included in the SI. The decreasing trend of primary particle size with increasing injection pressure is consistent with other researches cited in the introduction. Enhanced fuel–air mixing with high injection pressure produces less soot precursor, which inhibits particle formation and growth processes (Zhang and Kook 2012). Moreover, high injection pressure increases the maximum adiabatic flame temperature (Figure S2) and offers more oxidants such as OH radial, which increases the soot oxidation rate

(Leidenberger et al. 2012). Therefore, the smaller primary particle size at higher injection pressure becomes reasonable.

Figure 5 shows the representative HRTEM image for each tested condition. Soot particles generated at the low engine load condition show a disordered configuration with randomly oriented graphene segments in the center, as the relatively low

combustion temperature at 0.3 MPa BMEP condition may facilitate the different PAH species addition to the soot surface and inhibits thermodynamically favorable orientations (Vander Wal and Tomasek 2004). No significant difference can be confirmed by means of direct observation, so that fringe length and tortuosity were quantified through an image processing method. Figures 6 and 7 show the histograms of the fringe length and the tortuosity for each sample, respectively. It is quantitatively testified that with a smaller fringe length and a higher tortuosity, particles generated from the low engine load are less structurally ordered than those from the high engine load. For different injection pressures, particles from 800 bar are composed of smaller but flatter graphene layer segments, compared to those from 600 bar conditions. The median values of the fringe length for the samples from 800 bar are 0.95 nm at 0.3 MPa BMEP and 1.08 nm at 0.9 MPa BMEP, shorter than those for the corresponding 600 bar samples (1.02 nm at 0.3 MPa and 1.12 nm at 0.9 MPa). In addition, the 800 bar samples also have smaller median fringe tortuosity values (1.16 at 0.3 MPa BMEP and 1.11 at 0.9 MPa BMEP), compared to those 600 bar samples (1.22 at 0.3 MPa BMEP and 1.14 at 0.9 MPa BMEP). The graphene layer segment is formed during the soot nucleation and growth processes. With the addition of soot precursors to the graphene layer, the segment can be extended and then be grouped together within stacks. Based on the model developed by Dec (1997), soot is first formed in the fuel-rich premixed zone where the fuel–air equivalence ratio is in the range of 2–4, and then grows within the high-temperature fuel-rich combustion environment. As suggested by Pickett and Siebers (2004), increasing injection pressure reduces the residence time of the fuel fluid element within the soot-forming region, resulting in less time for soot formation and growth. On the other hand, the reduced soot precursor with high injection pressure as mentioned above may decrease the soot nucleation and growth rate. The combination of these two effects may illustrate the relatively small fringe lengths at the 800 bar conditions. With regard to fringe tortuosity, as stated by Vander Wal and Tomasek (2004), curvature or tortuosity arises from 5-membered rings within the aromatic framework. The amount of curvature reflects the competitive pathways producing C₅-containing PAHs relative to those based on C₆. Because of the higher thermodynamic stability of six-membered ring PAHs, higher combustion temperature at the 800 bar injection pressure conditions may favor the six-membered ring PAHs production through fuel pyrolysis, and hence results in less curvature in the soot nanostructure. Although only limited soot samples have been investigated in this study, injection pressure variation does affect the soot particle nanostructure formation. Based on the experimental results, it is speculated that the segment growth is dependent on the residence time in the soot-forming region and the reaction rate of soot precursor addition to the segment, while the segment curvature is more sensitive to the combustion temperature.

FIG. 6 Fringe length histograms of the soot particles from the 600 bar and 800 bar conditions.

FIG. 7 Fringe tortuosity histograms of the soot particles from the 600 bar and 800 bar conditions.

TABLE 5 Ignition temperatures of the samples from all the tested conditions

Operating condition
BMEP (MPa)	Injection pressure (bar)	Tig (°C)
0.3	600	586.18 ± 5.14
	800	587.13 ± 4.39
	1000	535.03 ± 7.58
0.9	600	639.03 ± 3.54
	800	628.75 ± 5.89
	1000	554.89 ± 5.76

FIG. 8 Particulate nonvolatile mass loss curves (a) and the corresponding DTG curves (b) for all the tested conditions.

FIG. 9 Correlation between the TGA derived VOF and the ignition temperature.

3.4. Particle Oxidation Reactivity

In order to investigate the effect of the morphology changes resulting from different injection pressures on particle oxidation reactivity, TGA test was implemented in this study. Figure 8 shows the normalized particulate nonvolatile mass loss curves and the corresponding DTG curves for all the tested conditions, and Table 5 lists the ignition temperature for each tested condition. In general, particles are prone to be burned at a lower temperature, as indicated by the decreasing ignition temperature, with increasing injection pressure. According to the morphology changes observed in this study, several factors are considered to contribute to the particle oxidation reactivity increase. As well explained by other studies (Vander Wal and Tomasek, 2003; Al-Qurashi and Boehman 2008), small graphene layer segments at high injection pressure increase edge site carbon atoms whose reactivity is 10–100-fold higher than those of carbon atoms within basal plane. Therefore, high injection pressure increases the ratio of edge site versus basal plane carbon atoms, which provides more opportunities for oxidants to react with the high reactivity edge site carbon atoms. In addition, a relationship between the VOF and the particle oxidation reactivity is observed. Figure 9 shows a good correlation between VOF and ignition temperature. A similar result was found by Yehliu et al. (2013). Although the VOF does not participate in the soot oxidation process, increased VOF is thought to be caused by the larger particle surface porosity, which is more likely to contain organic materials. When more organic content condensed on the particle surface and the micropores inside or between the primary particles is evaporated during the inert gas heating period, larger porous surface is exposed into the oxidation environment, which in turn increases the accessibility of active sites in carbon layers (Collura et al. 2005). Further, the relatively small primary particle size at high injection pressure increases the specific surface area and provides more surface available for oxidation reaction (Leidenberger et al. 2012). Previous studies also indicated that fringe tortuosity was another indicator for the soot overall reactivity. A high tortuosity value is always associated with high oxidation reactivity. However, in this study, the effect of segment curvature on the soot reactivity seems to be masked by the other factors based on the analytical results.

4. CONCLUSIONS

Three injection pressures (600 bar, 800 bar, and 1000 bar) were selected at two tested engine loads (0.3 MPa BMEP and 0.9 MPa BMEP). An SMPS was used to investigate particle size distribution, and a MOUDI and a TSP sampler were employed to collect particles from diluted exhaust gas which were used for further off-line analysis.

With respect to particle size distribution, increasing injection pressure can decrease the accumulation mode particles, while increasing the nucleation mode particles. Therefore, the total mass concentration of the exhaust particles is decreased with a significant increase in the particle's total number. The elemental carbon emission factors were also investigated by OC/EC test. The emitted EC is decreased with increasing injection pressure more efficiently at the low engine load.

Transmission electron microscope (TEM) imaging was employed to observe the soot particle morphology changes with different injection pressure. The primary particle size is decreased with increasing injection pressure, which is attributed to the variations in the strength of soot formation and oxidation effects. Increasing injection pressure decreases soot formation and growth rates but increases the oxidation rate. Injection pressure also affects the soot particle nanostructures. The shorter but flatter graphene layer segments exist in the samples from higher injection pressure conditions. The reduced residence time for fluid elements in the soot-forming region together with the slower soot growth rate may be the main cause of the segment size reduction, while the elevated combustion temperature is responsible for less curvature.

Finally, a TGA test was conducted to investigate the particle oxidation reactivity. As indicated by the ignition temperature for each tested condition, the particle oxidation reactivity is increased with injection pressure. Three key factors are considered to contribute to this particle oxidation reactivity increase based on the analytical results in this study:

1.	Small graphene layer segments at high injection pressure increase the ratio of edge site versus basal plane carbon atoms, which provides more active sites for oxidation reaction.
2.	Larger pore area may be created through more VOF content evaporation, which increases the accessibility of active sites.
3.	Smaller primary particle size provides more surface for oxidation reaction.

Acknowledgments

The authors would like to thank the Projects of International Cooperation and Exchanges National Natural Science Foundation of China (No. 51210010) and the National Natural Science Foundation of China (No. 51006067) for financial support to this study.

[Supplementary materials are available for this article. Go to the publisher's online edition of Aerosol Science and Technology to view the free supplementary files.]