Fuel Effect on Particulate Matter Composition and Soot Oxidation in a

Feb 10, 2014 - (5, 6) In the wall-guided DISI engine, fuel is directed to the spark plug using a .... It is expected that, in the real world, the PM e...
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Fuel Effect on Particulate Matter Composition and Soot Oxidation in a Direct-Injection Spark Ignition (DISI) Engine Chongming Wang,† Hongming Xu,†,‡,* Jose Martin Herreros,† Thomas Lattimore,† and Shijin Shuai‡ †

University of Birmingham, Birmingham, B15 2TT, United Kingdom State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Particulate matter (PM) composition and soot oxidation were investigated in a single-cylinder spray-guided direct-injection spark ignition (DISI) research engine using the thermogravimetric analysis (TGA) technique. Fuels including gasoline, ethanol, 25% volumetric blend of ethanol in gasoline (E25), and a new biofuel candidate (2,5-dimethylfuran, DMF) were studied. The engine was operated at 1500 rpm with a rich fuel/air ratio (λ = 0.9) and late fuel injection strategy, representing one of the worst scenarios of PM emissions from DISI engines. A TGA method featuring devolatilization and soot oxidization functions was developed and a kinetic model was used to analyze the soot oxidation process. The results show that volatile components are the main contributor to the PM produced from gasoline, E25, and DMF, and elemental soot accounts only up to 35% of PM mass at 8.5 bar IMEP. Ethanol combustion is so clean that only 6.3% of PM mass comes from elemental soot. The reaction rate of the soot oxidation is highly dependent on fuel and is sensitive to engine load. Soot from ethanol combustion is the most easily oxidized, indicated by the lowest temperature and activation energies (83 kJ/mol) required for oxidization. Soot from gasoline combustion is the most difficult to be oxidized, requiring the highest temperature and activation energy. It is found that the activation energy required for the soot from gasoline combustion increases with the engine load; however, the increase for soot from DMF combustion is very small.

1. INTRODUCTION Significant reduction of hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx) emissions from spark ignition (SI) engines has been achieved using various advanced technologies such as direct injection (DI) and highly efficient three-way catalysts (TWCs).1,2 In recent years, the PM emissions from DISI engines have attracted much attention since research evidence shows that they are similar in level or even higher than those of diesel engines equipped with diesel particulate filters (DPFs).3,4 Euro 5+ regulations limit the PM mass-based emission from vehicles equipped with DISI engines, and the proposed Euro 6 regulations, for the first time, limit the particle number (PN) emissions. The PM emissions from the wall-guided and spray-guided DISI engines are significantly different.5,6 In the wall-guided DISI engine, fuel is directed to the spark plug using a specially designed piston with a curve on the crown. Andersson et al. found that PM emissions from the wall-guided DISI engine were similar to those from diesel engines, and elemental soot dominated the PM mass emissions (72%).5 In the spray-guided DISI system, the injector is located close to the spark plug whereby fuel evaporates. Price et al. investigated PM emissions from the spray-guided DISI engine and concluded that the PM composition was dominated by volatile components while the elemental soot fraction accounted for, at most, 2%−29%, depending on the injection pressure, air/fuel ratio, and start of injection.6 PM emissions from DISI engines will become complicated when various fuel properties and qualities are involved. Aromatic content, vapor pressure, volatility, and oxygen content are the fuel properties commonly studied.7−10 The © 2014 American Chemical Society

impact of alcohol blends on PM emissions from a DISI engines was previously studied, and the results showed that alcohol blends led to lower PM emissions than gasoline.11 The fuel volatility or boiling range is directly linked to air/fuel mixture preparation.12 The combustion of liquid fuel such as iso-octane produces more PM emissions than that of gas-phase fuels such as propane.11 Honda R&D Co., Ltd. suggested that fuel vapor pressure and fuel structure (double bond and aromatic ring) played important roles in the PM formation, and thus a “PM index” was formulated for predicting PM emissions in gasoline vehicles.7 Ethanol is widely used as a gasoline alternative, because of its high octane number, enthalpy of vaporization, and oxygen content.13−15 However, ethanol has several limitations: low energy density, high volatility, and energy consumption in its production phase. Therefore, the search for superior alternatives to ethanol is an important area of energy development. DMF has become an attractive biofuel candidate since a new method for its production was developed using fructose as the raw material.16−18 Its physicochemical properties are similar to those of ethanol and gasoline. First, its lower heating value (LHV = 29.3 MJ/L) is much higher than that of ethanol (21.3 MJ/L) and closer to that of gasoline (31 MJ/L). Second, its octane number is higher than gasoline and close to ethanol. Third, unlike ethanol, DMF is insoluble in water, making it stable in storage and transportation. The authors’ group was the first one that has researched DMF as an engine fuel.19,20 The Received: November 29, 2013 Revised: January 30, 2014 Published: February 10, 2014 2003

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2. EXPERIMENTAL SETUP

results indicate that DMF has similar combustion characteristics and emissions to gasoline, making it easily adoptable to current DISI technologies. Research on DMF by other groups is also reported.21−25 The upcoming PM emission regulations may not be met by optimized engine calibrations and/or using cleaner fuels alone, necessitating the use of gasoline particulate filters (GPFs). There are many studies by original equipment manufacturers (OEMs) and research organizations focusing on the GPFs,26−28 particularly since the year 2010. To ensure the durability of GPFs and thermal management in the regeneration process, a profound understanding of the soot oxidation process is needed. There are several techniques available for PM characterization. DMS500 supplied by Cambustion and SMPS/EEPS supplied by TSI are commonly used for PM size distribution measurements.12,29,30 Transmission electron microscopy (TEM) is utilized for structural characteristics of particles in the size range of 1−1000 nm. For the soot reactivity and the oxidation behavior study, some techniques such as TGA, differential scanning calorimetry (DSC), and X-ray diffraction (XRD) are widely used. There are many other methods used to study the PM composition such as energy-dispersive X-ray (EDX),31 infrared spectroscopy (IR),32−34 X-ray fluorescence analysis (XRF),34 and gas chromatography−mass spectrometry (GC-MS). TGA is a widely used instrument in the study of the soot produced by diesel engines,35−39 where the PM sample is oxidized inside a temperature-programed furnace while the weight loss is constantly monitored. There is very limited information in the literature about the oxidation of soot from DISI engines, especially when using TGA for the study. To the best knowledge of the authors, there are only a few publications available in this area,6 because the particles from gasoline engines have not received the full attention of the automotive industry until recently and, indeed, they are very difficult to collect. There are several key factors concerning the TGA measurement reliability and repeatability with respect to the instrument and sample: balance, furnace, temperature calibration, sample mass, sample atmosphere gas flow rate, and heating ramps. The calibration of the balance, furnace, and temperature are independent to samples and can be completed following standard operation procedures. When TGA is used for the study of DISI engine PM, there is a need to investigate the effect of sample mass on the soot oxidization process. It is also believed that the heating ramp affects the accuracy of the results.39 A lower heating ramp gives a more accurate and reliable result but the test can be unacceptably long; therefore, a comprised and optimized process for TGA application in the study of gasoline PM must be identified. In this study, PM composition and soot oxidation were investigated in a DISI research engine. The engine was operated at 1500 rpm with a rich fuel/air ratio (λ = 0.9) and late fuel injection strategy. In the first part of this study, the key factors such as sample mass and heating ramp were studied to develop an optimized TGA method which was then applied to the study of the effects of fuel and engine load on PM from the combustion of gasoline, ethanol, E25, and DMF.

2.1. Engine and Instrumentation. The specifications of the DISI research engine are listed in Table 1, and its experimental system is

Table 1. Experimental Single-Cylinder Engine Specifications specification

value/comment

engine type combustion system swept volume bore × stroke compression ratio engine speed DI pressure and injection timing intake valve opening exhaust valve closing

4-stroke, 4-valve spray-guided GDI 565.6 cc 90 × 88.9 mm 11.5:1 1500 rpm 15 MPa, 280° bTDCa 16.5° bTDCb 36.7° aTDCb

a TDC refers to TDC of combustion stroke. bTDC refers to TDC of expansion stroke.

shown in Figure 1. The engine was coupled to a direct current (DC) dynamometer and maintained at a constant speed (±1 rpm), regardless of the engine torque output. The in-cylinder pressure was measured using a Kistler Model 6041A water-cooled pressure transducer. Coolant and oil temperatures were precisely controlled at 358 and 368 K (±3 K), respectively, using a proportional integral differential (PID) controller and heat exchangers. A 100-L intake plenum tank was used to stabilize the intake airflow. The engine was controlled by an in-house program written in LabVIEW. All the engine operating data, pressure, and temperature data were acquired using another in-house LabVIEW program. 2.2. Experimental Procedure. The properties of the fuels tested in this work are shown in Table 2. Both gasoline and ethanol were supplied by Shell Global Solutions, U.K. DMF was supplied by Beijing LYS Chemicals Co., Ltd. at 99.8% purity. The engine test conditions are listed in Table 3. In DISI engines, soot formation is far less, compared with that in diesel engines. Therefore, a late injection and rich combustion engine condition was chosen in order to collect sufficient PM on filters. It is expected that, in the real world, the PM emissions under stoichiometric conditions will be less compared with the rich and late injection engine running conditions. The results presented in the study serve to give a general idea of the soot oxidation behavior of DISI engines. In the engine experiment, baseline-operating points were monitored to check the consistency and repeatability of the data; these include the in-cylinder pressure trace, exhaust temperature, and fuel consumption. The engine was considered warm when the coolant and oil temperatures were stabilized at 358 and 368 K, respectively. The exhaust temperature and peak in-cylinder pressure were constantly monitored as important indicators of the stability of an engine operating condition. All the tests were carried out at ambient air intake conditions (∼298 K). After the engine is stabilized under predetermined running conditions, the PM collection starts. For PM collection, the exhaust was sampled by a heating line (464 K) 300 mm downstream of the exhaust valves, and then diluted by hydrocarbonfree air (air/exhaust = 8:1). The diluted sample was then pumped and collected by glass microfiber filters, maintaining the collection temperature between 308 K and 318 K. The sampling flow (after dilution) was controlled at 10 L/min. The loaded filter was cut into small pieces using a purposely made cutter and then transferred into the TGA unit. The TGA method is described in Figure 2. Because the collected PM is wet and contains adsorbed/condensed volatile compounds, it was critical for a devolatilization process to be designed to remove all of the volatile compounds. Steps 1−3 represent the devolatilization treatment, consisting of heating the sample in a nitrogen atmosphere up to 773 K and maintaining this temperature for long enough to ensure a complete removal of volatile components. Previous experimental tests have been carried out to determine the temperature 2004

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Figure 1. Schematic of the engine and instrumentation setup. weight loss during the devolatilization treatment. Step 4 was used to cool the sample back down to 373 K, to ensure a safe margin to record any soot oxidation that may occur at lower temperatures. In step 5, the sample atmosphere was switched to air. Step 6 was designed to ensure that all soot was fully oxidized. The mass of elemental soot in the sample was calculated by identifying the sample weight loss during the oxidization process. 2.3. Arrhenius-type Reaction Model. The soot oxidation process was calculated through an Arrhenius-type reaction model, which is used in many other publications.39−43 The model is described as below:

Table 2. Properties of the Fuels Studied chemical formula H/C ratio O/C ratio gravimetric oxygen content (%) density @ 20 °C (kg/m3) research octane number, RON motor octane number, MON stoichiometric air-fuel ratio lower heating value, LHVd (MJ/kg) (MJ/L) heat of vaporization (kJ/kg) initial boiling point (°C)

gasoline

DMF

ethanol

C2−C14 1.881 0.017 0.02 733.2 96.8 85.4 14.23

C6H8O 1.333 0.167 16.67 889.7a 101.3b 88.1b 10.72

C2H6O 3 0.5 34.78 790.9a 107c 89c 8.95

42.26 31.0 591 25.4

32.89a 29.3a 332 92

26.90a 21.3a 840c 78.4

−

⎛ −E ⎞ dm = kcmnpO r = A exp⎜ a ⎟mnpO r 2 2 ⎝ RT ⎠ dt

(1)

where m is the sample mass, t the time, kc the reaction rate constant, A the pre-exponential factor, Ea the activation energy of the reaction, and pO2 the partial pressure of oxygen. The parameters n and r are the reaction orders of sample and oxygen, respectively; R is the universal gas constant and T is the temperature. The reaction orders are simplified to unity.39,40

a

Measured at the University of Birmingham, 2010. bAPI Research Project 45, 1956. cData taken from ref 46. dCalculated from higher heating value.

⎛ 1 dm ⎞ Ea ⎛ 1 ⎞ ⎜ ⎟ + ln(Ap ⎟ = − ln⎜− ) O2 ⎝ m dt ⎠ R ⎝T ⎠

(773 K) at which most of the volatile organic material is vaporized (i.e., the weight loss of PM was zero when the temperature was increased under an inert atmosphere). High-quality oxygen-free nitrogen with a purity of 99.999% was used in this study. The mass of volatility in the sample was calculated by identifying the sample

(2)

When logarithms are taken from eq 1, a line (given by eq 2) can be derived with 1/T plotted on the x-axis and ln (−(1/m)(dm/dt)) plotted on the y-axis. The slope is a function of the activation energy.

Table 3. Engine Test Conditions scope

fuel

IMEP (bar)

λ

engine speed (rpm)

start of injection (SOI) (°bTDC)

TGA method development effect of fuel effect of engine load

gasoline gasoline, DMF, E25, ethanol gasoline, DMF

8.5 8.5 5.5 and 8.5

0.9 0.9 0.9

1500 1500 1500

100 100 100

2005

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Figure 2. TGA method. In the calculation of the activation energy, the temperature range that leads to the most linearity was used, corresponding to the soot weight loss of 10%−50%.

3. RESULTS AND DISCUSSION The results are divided into two parts. The first one covers the development of the TGA method. In the second part, PM from the combustion of gasoline, ethanol, E25, and DMF was investigated. 3.1. TGA Method Development. The function designed for soot oxidization was optimized in this section. PM samples have been processed for devolatilization prior to the soot oxidation. PM used in the TGA method development was collected from the gasoline-fuelled DISI engine, operated under rich combustion (λ = 0.9), late injection strategy, and 8.5 bar IMEP (see Table 3). 3.1.1. Effect of Heating Ramp. In the study of the effect of heating ramp on soot oxidation, soot with a mass of ∼0.1 mg was analyzed. Figure 3a shows that the normalized soot weight profiles during the oxidization process were highly dependent on the heating ramps. It can be seen that heating ramps higher than 10 K/min led to poor soot weight profiles which moved toward a higher temperature range. It appears that elemental carbon, which can be completely oxidized at lower temperatures, was only partially oxidized due to the overhigh heating rate with the remainder oxidized within the higher temperature range. As the heating ramp was reduced to 5 K/min or lower, the differences between the oxidization profiles were diminished. Figure 3b shows the effect of heating ramp on activation energy of soot oxidation. The slope of the lines was used to calculate the activation energies. Table 4 lists the effect of heating ramp on the activation energy. It can be seen that, for the 3−5 K/min heating ramp, the linearity of the results as indicated by the correlation coefficient (R2) is good (the average is between 145 kJ/mol and 153 kJ/mol). The activation energies for heating ramp higher than 10 K/min all had a much higher deviation from the average, ranging from 67.1 kJ/mol to 114.5 kJ/mol. 3.1.2. Effect of Soot Mass. Five soot samples ranging from 0.025 mg to 0.111 mg were tested. The heating ramp used in here is 3 K/min. In theory, a higher sample mass increases the data accuracy; however, a sample with an excessive mass would lead to diffusive reactions because the deeper layers of the sample would be inaccessible to the oxidation.39 Figure 4a

Figure 3. Effect of heating ramp on (a) soot oxidization profiles and (b) kinetic model (PM collected from gasoline combustion with the engine condition of 8.5 bar IMEP, 1500 rpm, λ = 0.9, SOI = 100°bTDC).

Table 4. Reactive Energy for the Effect of Heating Ramp heating ramp (K/min)

activation energy, Ea (kJ/mol)

correlation coefficient, R2

3 5 10 20 50

153.0 145.0 84.0 67.1 114.5

0.9901 0.993 0.8993 0.8903 0.9754

shows the effect of soot mass on soot derivative weight profiles. Figure 4b shows the effect of sample mass on the activation energy and maximum mass loss rate temperature (MMLRT). MMLRT is the temperature in the soot derivative weight profile that leads to the highest sample weight loss rate. It can 2006

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The abbreviations “ULG” and “ETH” represent unleaded gasoline and ethanol, respectively. PM samples from the engine exhaust are mainly composed of particles of different nature: (i) Hydrocarbon (HC) which could either form the nucleation mode or be adsorbed/condensed on the soot surface, and (ii) elemental carbon forming the soot agglomerates.44 According to the literature,6 the volatile components (HCs, lubricants, etc.) in the PM were classified into two ranges: low volatility (313−473 K) and high volatility (473−773 K). It is obvious that the majority of the PM consisted of volatile components. The elemental soot fraction is strongly affected by the fuel properties. For PM from ethanol combustion, elemental soot only accounted for 6.3% of the total PM. The reason is that ethanol has a 34.8% gravimetric oxygen content, which has an advantage of lower soot formation, compared with gasoline, because of more-complete combustion and more post-flame oxidation.8,9 For DMF, elemental soot accounted for 29%, which was much higher than that of ethanol, because of lower gravimetric oxygen content (16.7%). If the error bar is considered, the elemental soot in PM for gasoline accounted for 35%, which is almost the same as for E25 (36%). It is known that ethanol combustion leads to much lower HC and soot emissions, compared to gasoline.9,45 Therefore, it is highly possible that gasoline in the blended fuel E25 is the main contributor to the HC and soot emissions. In other words, there are no significant differences in the particulate matter composition of E25, with respect to gasoline. While wall wetting by fuel on the piston, valve, and cylinder liner is the main source for PM formation,46 ethanol and light hydrocarbons of gasoline in the blended fuel E25 form an almostazeotropic mixture that evaporates easily and produces little PM emissions.9,47 The heavy compounds of gasoline left on the piston and liner creates extremely favorable conditions for the formation of soot, which is possibly why that elemental soot fraction for PM from E25 is slightly higher than for gasoline. This is in agreement with existing publications in the literature.9,12,47 Figure 6a shows the normalized volatility weight profiles during the devolatilization process. There were no significant differences in the volatility weight profiles among the four tested fuels. For DMF and ethanol, the low volatility fraction was ∼60%, while for gasoline and E25, the fraction was ∼55%. The normalized soot weight profiles during the oxidization process are presented in Figure 6b. Unlike the volatility profiles, the soot oxidization profiles are highly fuel-dependent. The oxidization of soot from DMF and ethanol combustion started at a lower temperature than soot from gasoline and E25. Figure 7a shows the soot derivative weight profiles for soot produced from gasoline, ethanol, E25, and DMF. Based in Figure 7a, it is possible to calculate the activation energy and MMLRT, which are presented in Figure 7b. The area integrated on the temperature and soot derivative weight diagram is proportional to the soot mass. From Figure 7a, it is concluded that E25 combustion produces slightly less soot, compared to gasoline. DMF leads to less soot formation, compared to gasoline and E25, but not as low as neat ethanol. Figure 7b presents activation energies and MMLRTs. It reveals that the activation energy for the soot from gasoline combustion is the highest (153 kJ/mol), whereas for soot from ethanol combustion, it is the lowest (83 kJ/mol). By adding 25% ethanol into the blend (E25), the activation energy is reduced from 153 kJ/mol to 124 kJ/mol. For PM from DMF combustion, the activation energy (109 kJ/mol) is between

Figure 4. Effect of heating ramp on (a) soot oxidization derivative weight profiles and (b) activation energy and MMLRT (PM collected from gasoline combustion with the engine condition of 8.5 bar IMEP, 1500 rpm, λ = 0.9, SOI = 100°bTDC).

be seen that, for the samples larger than 0.04 mg, the calculated activation energies are in the range of 146−152.4 kJ/mol and the MMLRTs are in the range of 490−494 K, both of which show good repeatability. Thus, a soot mass larger than 0.04 mg was chosen to ensure an accurate sample analysis. 3.2. TGA Method Application. 3.2.1. Effect of Fuel. In this subsection, the four fuels (gasoline, DMF, ethanol, and E25) were tested at 8.5 bar IMEP with rich combustion (λ = 0.9) and late injection strategy (see Table 3). Figure 5 shows the PM composition including volatile components and soot.

Figure 5. Effect of fuel on PM composition for PM from gasoline, DMF, E25, and ethanol combustion (PM collected from the engine condition of 8.5 bar IMEP, 1500 rpm, λ = 0.9, SOI = 100°bTDC). 2007

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Figure 6. Effect of fuel on (a) devolatilization profiles and (b) soot oxidization profiles for PM from gasoline, DMF, E25, and ethanol combustion (PM collected from the engine condition of 8.5 bar IMEP, 1500 rpm, λ = 0.9, SOI = 100°bTDC).

(i) Soot produced from ethanol was the most easily oxidized, indicating that it is easy to regenerate the particle filters in DISI engines fuelled with ethanol. (ii) Soot produced from gasoline combustion was difficult to oxidize, while adding 25% ethanol into gasoline could change the soot oxidization reaction significantly. (iii) The reactivity of soot produced from DMF is higher than soot from gasoline combustion; however, it is lower than the reactivity of soot produced from ethanol. Based on the literature on diesel soot reactivity, it could be hypothesized that the easier oxidization process of DMF and ethanol generated soot could be due to four main factors: carbon structure, size of primary particles, size of agglomerated particles, and soot oxidization mode. (i) Soot with smaller primary particles and smaller agglomerated particles has a tendency to be oxidized more easily. Smaller size particles have a higher surfaceto-volume ratio and, therefore, are oxidized more easily. Supported by evidence from previous studies, the combustion of oxygenated fuels produces smaller primary particles.48 Research evidence also confirmed that DMF and ethanol combustion yields smaller particles20 This partially explains the higher oxidization rate of DMF and ethanol generated soot, compared to the soot produced from gasoline combustion. (ii) Research on the influence of different functional groups on soot reactivity has been published,37,50,51 even though the findings are not conclusive. It was reported that soot from the combustion of oxygenated fuel has oxygenated functional groups associated with internal burning leading to a higher oxidization rate.36 Therefore, it is envisaged that DMF and ethanol generated soot will experience this type of oxidization. However, it was also reported the relationship between the presence of oxygenated surface functional groups and soot reactivity was not clear, suggesting that the relative amount of aliphatic C−H groups on the soot surface govern the soot oxidation reactivity.49 Therefore, some further studies of the soot oxidation and reactivity of oxygenated fuels should be carried out. 3.2.2. Effect of Engine Load. In this subsection, gasoline and DMF were tested at 5.5 and 8.5 bar IMEP under the engine condition of rich combustion (λ = 0.9) and late injection (see Table 3). The PM emissions with ethanol at 5.5 bar IMEP were very low and very difficult to collect by using the filters; therefore, no data are available for this engine condition. The

Figure 7. Effect of fuel on (a) soot oxidization derivative weight profiles and (b) activation energy and MMLRT for PM from gasoline, DMF, E25, and ethanol combustion (PM collected from the engine condition of 8.5 bar IMEP, 1500 rpm, λ = 0.9, SOI = 100°bTDC).

that of gasoline and ethanol. The differences in activation energies between fuels are statistically significant. The MMLRT for gasoline is close to that for E25, both of which are higher than those for DMF and ethanol. Comparing MMLRT and soot activation energy for the different fuels, it can be concluded that: 2008

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to higher temperatures as the engine load increased. At 5.5 bar IMEP, 80% of the soot produced from gasoline combustion was oxidized up to the temperature of 733 K. However, a higher temperature (803 K) was required to oxidize 80% of the soot at 8.5 bar IMEP. For DMF, the shift in soot oxidization profiles was not as much as that observed in gasoline combustion. The effect of engine load on soot derivative weight profiles for the PM from DMF and gasoline combustion is represented in Figure 10a. From the soot derivative weight profiles, it can be seen that, in the case of gasoline combustion, as the engine load was increased, the net soot production (soot formation + soot growth − soot oxidation) was increased, indicated by the area covered by the soot derivative weight profiles. Soot from DMF combustion also increased with the engine load, but to a lower extent. Figure 10b shows the effect of engine load on MMLRT and activation energies for the PM produced from gasoline and DMF combustion. For DMF, there was a 20 K difference in MMLRT between 5.5 and 8.5 bar IMEP, while the difference was 70 K in the case of gasoline. The activation energy for the DMF-generated soot was 109 kJ/mol and 114 kJ/mol at 5.5 and 8.5 bar IMEP, respectively, which means that 4.6% more energy was needed to activate the oxidization process as engine load was increased. However for the gasoline generated soot, the reaction energy increased from 131 kJ/mol to 153 kJ/mol as the load was increased from 5.5 bar IMEP to 8.5 bar IMEP. As a result, the oxidization process for soot from DMF and gasoline became difficult as the engine load was increased. As in the case of the fuel effect, several factors have been used to explain this result: soot structure, size of primary particles, and size of agglomerated particles. (i). Soot Nanostructure. It has been previously reported that the soot nanostructure influences the ratio of edge carbon atoms (active) to basal carbon atoms (inactive) and, therefore, soot reactivity. Different crystalline parameters of the soot structure have been studied using experimental techniques such as XRD and HR-TEM, respectively.51,52,50 It is concluded that particles with a less ordered nanostructure (lower crystalline length, crystalline height and fringe length, and higher tortuosity, resulting in lower number of carbon atoms at the edge sites than the ones in the basal plane of a graphene layer,50,51) have a higher reactivity. It has been also reported that higher temperatures in diesel combustion led to a higher ordered structure of soot.52,53 Therefore, the present work seems to support the results of previous research that the soot

effect of engine load was not studied for the E25 blend, since PM characteristics are similar to the baseline gasoline. Figure 8 shows the effect of engine load on PM composition. It can be seen that, for PM produced from gasoline and DMF

Figure 8. Effect of engine load on PM composition for PM from gasoline and DMF (PM collected from the engine condition of 5.5 and 8.5 bar IMEP, 1500 rpm, λ = 0.9, SOI = 100°bTDC).

combustion, the soot fraction increased and volatility fraction decreased with engine load. For DMF-generated PM, the low volatility fraction was decreased from 54% at 5.5 bar IMEP to 42% at 8.5 bar IMEP. This trend is much clearer for gasoline generated PM in which the low volatility fraction decreased from 57% at 5.5 bar IMEP to 36% at 8.5 bar IMEP. The decreased fraction of low volatility HC in the PM composition is due to the fact that more HC was post-oxidized as the incylinder temperature increased with load. As for the soot fraction, it formed 15.4% and 28.9% of the DMF-generated PM at 5.5 and 8.5 bar IMEP, respectively. In gasoline combustion, the soot accounted for 9.8% and 35.1% at 5.5 and 8.5 bar IMEP, respectively. DMF is an oxygenated fuel and therefore has less PM emissions, compared to gasoline. Therefore, as the engine load is increased, the soot formation rate from gasoline combustion increases more rapidly compared with DMF combustion. Figure 9a represents the effect of engine load on the PM devolatilization profiles. For both fuels, the sensitivity of the PM devolatilization process to engine load was low. Figure 9b represents the effect of load on the soot oxidization profiles. The soot oxidization profile for gasoline-generated soot shifted

Figure 9. Effect of engine load on (a) devolatilization and (b) oxidization profiles for PM from gasoline and DMF combustion (PM collected from the engine condition of 5.5 and 8.5 bar IMEP, 1500 rpm, λ = 0.9, SOI = 100°bTDC). 2009

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4. CONCLUSIONS An optimized TGA method has been developed and used to study PM composition and soot oxidation characteristics for gasoline type of fuels in a DISI engine. (i) It is found that that a slow heating ramp at 3−5 K/min and a minimum mass of soot sample of 0.04 mg are the favorable settings in the TGA experiment for gasoline type of fuels, with which the Arrhenius-type reaction model fits well and the results are repeatable. (ii) It is shown that particulate matter emitted from the DISI engine mainly consist of volatile components rather than soot, even at high engine load operation, in which a higher percentage of soot was obtained in comparison to low load. It is envisaged that the use of various strategies to reduce unburnt hydrocarbons could be an effective method to control/limit the formation of particulate matter emissions in DISI engines, because of reduced nuclear mode particles and absorption of volatiles by soot. (iii) Soot from oxygenated fuels (ethanol and DMF) are easier to oxidize compared to gasoline indicated by the lowest MMLRT and activation energies. Therefore, it is suggested that the use of oxygenated alternative fuels will alleviate the function of the after-treatment system due to the lower concentration of engine output particulate matter emissions and the lower energy demand to oxidize the formed soot. (iv) For gasoline-generated soot, oxidization behavior varies more with the engine load compared with soot from DMF combustion. The activation energies and temperature needed for soot oxidization increase with the engine load, indicating that soot is more difficult to be oxidized if it is formed under a higher temperature. Further work is needed to confirm the faster oxidation rates obtained with oxygenated fuels in gasoline DISI engines and to examine the influence of particulate matter characteristics such as particulate morphology, nanostructure, and soot surface functional groups on soot reactivity.

Figure 10. Effect of engine load on (a) soot oxidization derivative weight profiles and (b) activation energy and MMLRT for PM from gasoline and DMF (PM collected from the engine condition of 5.5 and 8.5 bar IMEP, 1500 rpm, λ = 0.9, SOI = 100°bTDC).

produced at higher engine load with higher in-cylinder temperature is more difficult to be oxidized. (ii). Size of Primary Particles and Agglomerates. Net soot formation was increased with engine load. It is hypothesized that this higher net soot production is a consequence of the higher temperature reached in the combustion chamber, which could increase the rate of reaction of soot production and growth. Furthermore, the higher quantity of fuel present in the cylinder and the higher probability of formation of locally rich areas in the combustion chamber where soot can be produced, even though the global lambda was maintained constant, enhances soot formation and growth at high load, which results in larger primary particles54 and agglomerates, reducing the particle surface/volume ratio. Therefore, the higher structural order, larger primary particles, and larger agglomerates produced under high load conditions explain why the soot produced at the high engine load was more difficult to be oxidized. As it can be seen in Figure 10a, the effect of engine load on the net production of soot in the case of DMF is lower than in the case of gasoline. Thus, it is expected that the change in the size of primary particles and agglomerates from DMF will be lower than in the case of those from gasoline, resulting in a smaller change in soot reactivity in the case of the DMF generated soot compared to gasoline-generated soot.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted in the Future Engines and Fuels Lab at the University of Birmingham, and it was financially supported by the Engineering and Physical Sciences Research Council (EPSRC) through the Research Grant No. EP/ F061692/1, and National Natural Science Foundation of China (Technical Communication and Cooperative Research, No. 51211130117). Thanks to Advantage West Midlands Science City and the European Regional Development Fund as part of the Science City Research Alliance Energy Efficiency Project. The authors thank Peter Thornton and Carl Hingley, for their 2010

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technical support of the engine testing facility, and Farshad Eslami, for his contribution to the measurement of the calorific values of DMF.

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NOMENCLATURE aTDC = after top dead center bTDC = before top dead center CO = carbon monoxide DC = direct current DISI = direct-injection spark-ignition DI = direct-injection DMF = 2,5-dimethylfuran DPF = diesel particulate filter GDI = gasoline direct injection GHG = greenhouse gas GPF = gasoline particulate filter E25 = volumetric 25% ethanol in ethanol/gasoline blend HC = hydrocarbon HV = heat of vaporization IMEP = indicated mean effective pressure LHV = lower heating value MMLRT = maximum mass loss rate temperature NOx = nitrogen oxide RPM = revolutions per minute PM = particulate matter PN = particle number PID = proportional integral differential SI = spark ignition SOI = start of injection TGA = thermogravimetric analyzer TWC = three-way catalyst VVT = variable valve timing

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