Impact of Intake CO2 Addition and Exhaust Gas Recirculation on NOx

Aug 28, 2012 - The engine is equipped with a Bosch common-rail fuel injection system, ... data, the zero-dimensional single-zone model was used to cal...
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Impact of Intake CO2 Addition and Exhaust Gas Recirculation on NOx Emissions and Soot Reactivity in a Common Rail Diesel Engine Khalid Al-Qurashi,† Yu Zhang, and André L. Boehman* EMS Energy Institute, The Pennsylvania State University, 405 Academic Activities Building, University Park, Pennsylvania 16802, United States ABSTRACT: The impact of intake CO2 addition and exhaust gas recirculation (EGR) on engine combustion characteristics, NOx emissions, and soot oxidative reactivity was studied in a common rail diesel engine equipped with a cooled EGR system. The engine test results and the heat release analysis show that the reduced flame temperature, induced by the reduction of the oxygen concentration (dilution effect) is the dominant mechanism via which CO2 and EGR lower NOx emissions in diesel engines. On the other hand, the collected soot from the engine tests was examined for its oxidative reactivity using a thermogravimetric analyzer (TGA). Results show that EGR has a significant effect on soot reactivity and results in higher initial active sites compared to the CO2 case. We conclude that the reduced flame temperature (thermal effect) which is a consequence of the dilution effect is responsible for the observed increase in soot reactivity. These results confirm observations from our past work on flame soot, which showed that the peak adiabatic flame temperature is the governing factor affecting soot reactivity. These findings imply that driving the combustion concepts toward low temperature is favorable to effectively control engine pollutants, including soot reactivity.

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INTRODUCTION Exhaust gas recirculation (EGR) has become an essential technique in modern diesel engines due to its high effectiveness in reducing NOx emissions. For high temperature combustion systems, such as diesel engines, the Zeldovich mechanism is widely considered as the primary mechanism responsible for the formation of nitric oxide (NO), which shows strong dependence of NO formation on flame temperature. Researchers have extensively investigated the means by which EGR lowers the flame temperature and leads to the reduction of NOx emissions in diesel engines.1−4 Three major effects have been suggested to explain the impact of EGR. (1) The dilution effect: the intake oxygen concentration is reduced by replacing the intake air with recirculated exhaust gas, which lowers the oxygen availability in the engine cylinder, increases the mixing time, and results in lower flame temperature.1,5,6 (2) The thermal effect: the addition of carbon dioxide (CO2) and water vapor from the recirculated exhaust gas into the engine cylinder increases the heat capacity of the in-cylinder charge, leading to lower flame temperature during the combustion process.7−10 (3) The chemical effect: the endothermic dissociation of carbon dioxide and water vapor lowers the flame temperature and hence reduces NOx emissions.9,11 Thus far, there is still no consensus as to which one of the three effects is the primary means by which EGR lowers the NOx emissions. Ladommatos et al.1,8,9,12 conducted a series of engine tests in a naturally aspirated diesel engine and suggested that the dilution effect is the dominant effect that influences the NOx emissions and the chemical effect is insignificant in affecting the overall combustion characteristics and lowering NOx emissions compared with the dilution and thermal effects. However, Jacobs et al.11 argued that the thermal effect of EGR is more significant than the dilution effect based on their observation of very small influence of the dilution effect on ignition delay and the strong influence of EGR on flame © 2012 American Chemical Society

temperature in an electronically controlled common rail heavyduty diesel engine. Hence, it is of great interest to further investigate the relative importance between the dilution and thermal effects of EGR in a modern common rail diesel engine equipped with a cooled EGR system. Since there is generally more CO2 present in the exhaust gas than water vapor on a mass basis, especially under the circumstance of cooled EGR, the impact of CO2 is considered more significant than that of water vapor.9,11 Therefore, to study the thermal effect of EGR, the effect of CO2 addition can be singled out and studied first. Then, by comparing the impact of CO2 addition and the impact of actual EGR, a conclusion can be drawn as to which effect has the most significant influence on NOx emissions. Despite the large body of work that has been conducted to investigate the impact of EGR on NOx and particulate matter (PM) emissions, the impact of EGR on soot reactivity has only recently been investigated.13−15 It is known that changing soot formation conditions by changing the combustion conditions or using different parent fuels results in soot with different chemical and physical properties.13,14,16−19 Before discussing the EGR effects on soot reactivity, it is noteworthy to briefly shed light on some physicochemical properties that control the oxidation characteristics of the PM. A major constituent of the PM is the soluble organic fraction (SOF), which stems from the unburned hydrocarbons during the combustion process. The SOF composition and mass fraction depend mainly on the fuel composition and combustion condition. However, its impact on soot reactivity is controversial and there have been no systematic analyses carried out to determine the influence of Received: July 28, 2011 Revised: August 18, 2012 Published: August 28, 2012 6098

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SOF on soot reactivity. For instance, De Soete20 found that the combustion kinetics of the DPM is different with and without the presence of the SOF. Lee et al.21 and Neer and Koylu22 found that soot with a high volatile fraction oxidized faster than low volatile soot. On the other hand, Ahlström et al.23 and Collura et al.24 showed that the SOF adsorbed on the PM tends to vaporize before soot reaches its ignition temperature and therefore has no impact on soot reactivity. The impact of soot surface chemistry on soot reactivity was demonstrated by Boehman et al.19 The authors illustrated that the inclusion of biodiesel in the fuel lowers the ignition temperature of soot and consequently lowers the temperature required for regeneration of the DPF, which was attributed to the high surface oxygen content of biodiesel soot. Vander wal et al.16,17 conducted experiments to understand the effect of soot physical properties on the reactivity and demonstrated that lowering the temperature and residence time during soot formation modifies soot nanostructure and enhances the reactivity of the soot. EGR can also alter the soot formation route through its dilution, thermal, and chemical effects. Al-Qurashi and Boehman13−15 examined the impact of applying different levels of air diluents on the oxidative reactivity of soot. The authors demonstrated that the simulated EGR (SEGR)13,14 enhances the soot reactivity in the same manner as engine EGR.15 The soot oxidation rate was found to increase with increased level of the diluents. These diluents altered the physical properties of the soot and resulted in soot that is inherently more prone to oxidation. The authors inferred that soot becomes more oxidatively reactive mostly due to the thermal effect of EGR.15 Seong and Boehman18 carried out research on a four-cylinder turbo-charged common rail diesel engine to examine the impact of intake oxygen enrichment on soot reactivity. They showed that oxygen enrichment increases soot reactivity, especially at high engine load, and reasoned that the soot oxidation was catalyzed by lubricating oil-derived metals incorporated into the soot. Interestingly, reduction in the oxygen concentration in the intake charge of the diesel engine also enhances soot reactivity but through a different mechanism. Lowering the content of oxygen of the intake charge results in an increase in flame liftoff.25,26 Lowering the oxygen concentration in the oxidizer stream of a diffusion flame delays the formation of soot precursors and shortens the residence time of soot formation.27 Due to the higher heat capacity of the EGR gas relative to intake air (the thermal effect of EGR), introducing EGR into the combustion process results in a lower flame temperature. However, the thermal effect of EGR is offset by the rise of inlet charge temperature. The chemical effect of EGR stems from the presence of the CO2 which is a chemically active species. The chemical impact of CO2 on soot reduction is known from the flame literature.27−29 The dissociation of CO2 leads to an increase in O atoms and the reaction of CO2 with H atoms results in increasing the OH and decreasing the H concentration.27,28 However, the CO2 concentration in engine EGR is relatively low, under low EGR rate, and its contribution to soot formation conditions may be masked by the significant influence of the thermal and dilution effects. In this work, for example, the intake charge of the baseline case has 20.3 vol % oxygen and negligible concentration of CO2 from the background. The 26% EGR contains 1.8 vol % CO2 and 18.3 vol % oxygen. With the EGR addition, it is seen that the oxygen availability was reduced by more than 9%, which in turns, significantly reduces the flame temperature. Similar observations were shown by Kook et al.30

The objectives of this study are twofold. First, it is of our interest to examine whether the thermal effect is the dominant effect via which EGR lowers NOx emissions, which can possibly be achieved by comparing the effectiveness in reducing NOx emissions between intake CO2 addition and the use of EGR. Second, we would like to determine the relative importance of the chemical effect of EGR on soot oxidative reactivity.

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EXPERIMENTAL SECTION

Fuel. An ultralow sulfur diesel fuel with 15 ppm sulfur content obtained from BP was used in this study. Some fuel properties of the ultralow sulfur diesel fuel are provided in Table 1.

Table 1. Properties of Test Fuels ultralow sulfur diesel density (g/cm at 15 °C) lower heating value (MJ/kg) kinematic viscosity (cSt at 40 °C) 90% recovered distillation temperature (°C) cetane number sulfur content (ppm) 3

0.837 42.1 2.48 322.3 50.5 15

Engine Setup. This study was conducted in a DDC/VM Motori 2.5 L, 4-cylinder, turbocharged, common-rail, direct injection lightduty diesel engine. Detailed engine specifications are listed in Table 2.

Table 2. Test Engine Specifications engine displacement bore stroke compression ratio connecting rod length rated power peak torque injection system valve train

DDC 2.5 L TD DI-4 V automotive diesel engine 2.5 L 92 mm 94 mm 17.5 159 mm 103 kW at 4000 rpm 340 N m at 1800 rpm Bosch electronically controlled common-rail injection system 4 valves/cylinder

The engine is equipped with a Bosch common-rail fuel injection system, which enables up to three injections per cycle and provides a 1600 bar maximum rail pressure. Exhaust gas recirculation was achieved by directing a fraction of exhaust gases upstream of the exhaust manifold back to the intake manifold after passing through a heat exchanger to reduce the recirculated gas temperature. The intake CO2 was achieved by injecting the CO2 gas into the intake air before the intake charge passed through the turbocharger. An ETAS hardware and INCA software interface were used to access the engine electronic control unit (ECU) and adjust the ECU calibration to adjust the injection timing and EGR rate. Diagnostics. Engine cylinder pressure was measured using AVL GU12P pressure transducers. An AVL 365C angle encoder along with an AVL Indimodul 621 high speed data acquisition system provided a 0.1 crank angle resolution data acquisition of cylinder pressure. On the basis of the cylinder pressure data, the zero-dimensional single-zone model was used to calculate the apparent heat release rate in this study, as described by Heywood.31 The intake oxygen concentration, CO2 concentration, and the NOx emissions were measured using an AVL CEBII emissions bench. The exhaust gas was collected via a heated sample line that was maintained at 190 °C. Soot Characterization. Diesel particulate matter samples were collected from the raw exhaust of the engine on Teflon filters. The diesel particulate matter was subsequently removed from the filters and thermally treated under ultrahigh-purity nitrogen (UHP-N2) at 6099

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500 °C for 60 min to remove volatile compounds. Thus, the soot considered in this work is the volatile-free fraction of the diesel particulate. A simultaneous differential thermogravimetric analyzer (Model No. SDT-Q600) was used in this study to evaluate the oxidative reactivity of the soot samples and to measure the oxygen chemisorption capacity. Isothermal experiments in which the soot was heated in air (100 cc/min) at 450° were conducted to study the oxidative reactivity of the soot samples. For these experiments, 4 mg of the soot was first heated at 500 °C under UHP-N2 for 60 min to remove the volatile fraction. Subsequently, the temperature was reduced to 450 °C and the air was then introduced into the system to start the oxidation process. The soot conversion is defined as m X=1− m0 (1) where m is the sample mass at a given time and m0 is the initial sample mass. The initial active sites were determined by a direct measurement of oxygen uptake. Low-temperature oxygen chemisorption was used in this work. The soot samples were heated in the TGA instrument at 200 °C in UHP air for 10 h. Then, UHP N2 was introduced to desorb the physisorbed oxygen. The initial and final weight of each sample was recorded and converted into active surface area by using the following equation:32 ASA =

NOσONA mi

Figure 1. Brake specific NOx emissions under the three engine tests conditions.

employing actual EGR apparently leads to less NOx (48% reduction) emissions than using intake CO2 addition (13% reduction). To unravel the causes of the observed difference of NOx emissions difference between the above-mentioned two conditions, combustion analysis was performed. Figure 2 shows the pressure, heat release rate, and temperature traces from the three test cases. As seen in Figure 2, compared to the baseline, intake CO2 addition results in lower peak cylinder pressure and lower bulk cylinder temperature. As a result, the adiabatic flame temperature was reduced and thus the NO x emissions were reduced. Furthermore, Figure 3 shows that the addition of CO2 to the intake charge only slightly decreases the intake oxygen concentration compared to the baseline condition, implying that the dilution effect is not significant when using this approach. Therefore, the thermal effect (reduced flame temperature) should be the dominant mechanism responsible for the lowered NOx emissions when implementing intake CO2 addition. When comparing the heat release rate profiles between the baseline condition and the actual EGR condition, an earlier start of pilot ignition and a larger amount of pilot heat release are observed. As shown in Table 3, the intake manifold temperature was increased when using actual EGR, indicating that less charge mass are trapped in the engine cylinder when using EGR. Due to the reduced cylinder charge mass, the bulk cylinder gas temperature is higher under the EGR condition than at the baseline condition, as evidenced in Figure 2c. This argument is further confirmed by the observed higher exhaust temperature under the EGR condition, as seen in Table 3. During the pilot combustion process, there is enough oxygen available for the combustion reactions, even under high EGR rate conditions, thus cylinder temperature becomes a more important factor affecting the magnitude of pilot heat release and the start of pilot combustion. Hence, the observed earlier onset of pilot combustion and larger amount of pilot heat release under the EGR condition can mainly be attributed to the increased bulk cylinder temperature when using EGR. In contrast, during the main combustion process, it is observed that the amount of main heat release is evidently less for the EGR condition than for the baseline condition. Since the magnitude of pilot heat release is considerably smaller than that of main heat release and the pilot heat release occurs at relatively low temperatures, the amount of NO formed during

(2)

where NO is the number of moles of chemisorbed oxygen, σO is the cross-sectional area of the oxygen atom (0.083 nm2), NA is Avogadro’s number, and mi is the initial mass of soot. Engine Test Conditions. The engine tests were conducted at conditions of 1500 rpm and 68 N m, which is a representative of road load vehicle driving condition. Detailed operating parameters are listed in Table 3. The intake CO2 concentration is 1.8 vol % under the intake

Table 3. Engine Operating Conditions and Intake Charge Properties engine speed: 1500 rpm engine torque: 68 N m start of pilot injection: 31° BTDC start of main injection: −3° BTDC baseline (0% EGR)

CO2 addition

intake manifold temperature 37.5 (°C) exhaust temperature (°C) 215.2 air−fuel ratio 44 Intake Charge Composition CO2 (%) O2 (%) 20.3 CO (%) THC (%) specific heat of intake charge 33.13 (kJ/kmol·K) adiabatic flame temperature (K) 2603

EGR (26% EGR)

35.6

78.2

212.3 45

241.4 38

1.8 19.8 33.15

1.8 18.4 0.02 0.01 34.13

2558

2495

CO2 addition test condition, which corresponds to an equal amount of CO2 concentration to that observed at an EGR rate of 26%. The 0% EGR condition was set as the baseline condition. The injection timing was held constant throughout the three engine testing conditions.

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RESULTS AND DISCUSSION NOx Emissions. Figure 1 shows the comparison of NOx emissions between the condition of using intake CO2 addition and the condition of applying actual EGR. Compared to the baseline case (no EGR or CO2 addition), it can be seen that 6100

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that employing EGR leads to appreciable decrease in the amount of heat release during the main combustion process. As shown above, the addition of CO2 reduced the peak flame temperature but has no impact on the magnitude of the main heat release, as can be seen in Figure 2. As a result of EGR addition, on the other hand, considerably less heat release during the main combustion process is evident. This change in the heat release characteristics can be attributed mainly to the less oxygen availability (compared to the baseline case and CO2 case), namely the dilution effect. Therefore, based on the comparison in engine emissions results and the bulk combustion characteristics among the three cases, the reduced flame temperature, induced by the dilution effect, significantly suppress NOx emissions. It should be noted that the specific heat capacities of all cases studied in this work are equivalent, as shown in Table 3, implying that the impact of heat capacity on NOx emissions is negligible. Soot Reactivity. The soot samples generated under the aforementioned conditions are designated as soot1 (baseline), soot2 (generated under CO2 condition), and soot3 (generated under EGR condition). Figure 4 compares the oxidation

Figure 4. Conversion−time profiles of the soot samples. The soot was oxidized at 450 °C in air (21% oxygen).

Figure 2. Pressure, heat release rate, and temperature results for the three test cases.

behavior of these samples. The figure shows the overall effect of EGR and CO2 (i.e., dilution, thermal, and chemical). It is seen from Figure 4 that soot conversion starts earlier for soot3 than soot1 and soot2. This implies that the concentration of the initial active sites of soot3 is greater than that of soot2 and soot1. Thus, the oxygen atoms require shorter time to reside on the carbon free (active) sites. It is clear that both diluents (EGR and CO2) enhance the reactivity of the soot. The effect of CO2, however, is less pronounced than the effect of EGR. Table 3 shows the specific heat (Cp) of the intake charge of the three cases. The Cp values were calculated at the start of combustion (SOC) temperature. It is seen from Table 3 that soot1 and soot2 have similar Cp values. It can be concluded, therefore, that the heat capacity effect on the observed change of soot reactivity between soot1 and soot2 can be discounted; leaving the dilution and chemical effects as the governing factors enhancing the soot reactivity. Comparing soot2 and soot3, where the CO2 concentration is fixed, it can be concluded that the observed change in reactivity between the two cases is attributed to the thermal and dilution effect. This observation leads to the conclusion that the chemical effect is not

Figure 3. Intake oxygen concentrations under the three tests conditions.

the pilot combustion should be far less than the amount of NO formed during the main combustion process. Figure 2b shows 6101

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Considering carbon oxidation takes place at the active sites, the total carbon can be expressed as

responsible for the enhanced reactivity of soot2 and soot3. The Cp of the soot3 case, on the other hand, is about 3% higher than that of the other two cases. Therefore, the Cp effect may contribute marginally to the increased reactivity of soot3. Since the oxygen availability during the formation of soot3 is much lower than that of soot2, the dilution effect is thought to be the origin of the observed change on soot reactivity. It is wellknown that with decreasing oxygen mole fraction in the oxidizer stream, soot inception is delayed and soot formation is retarded.33 Consequently, soot with less-developed nanostructure and thus more oxidatively reactive is formed. It should be realized that the dilution effect also induces a thermal effect as oxygen reduction in the mixture lowers the adiabatic flame temperature. As we showed previously,15 the effect of lower oxygen concentration on soot reactivity is of secondary importance to the reduced peak adiabatic flame temperature. It should be noted from Figure 2 that the bulk cylinder temperature of soot3 case is greater than the baseline soot. However, the soot of the former case is more reactive than the soot of the latter case. This is explained by the fact that the corresponding adiabatic flame temperature, which is one of the governing factors of soot reactivity, is less for the former case. It is worth pointing out that the differences in reactivity between the soot samples seen in Figure 4 are statistically significant but error bars are not shown in these figures as uncertainties are relatively small. The fixed error contributions to these results were found to be insignificant and repeat trials of the isothermal TGA results on the soot samples were repeatable to within 1.3% of the reported specific oxidation rate. It is not an easy task to use an engine to determine the relative importance of the dilution and thermal effects on soot. A simplified approach using a coflow diffusion flame was considered in our laboratory to elucidate the roles of these effects on soot reactivity and recently we have shown that the thermal effect (reduced flame temperature) is the governing factor to enhance soot reactivity. The thermal effect reduces the degree of carbonization/graphitization of the soot and results in less-developed and immature soot particles.14 The results of this work support our previous findings that the chemical effect of EGR is less important than the other two effects of EGR. To better understand the difference in reactivity between the three samples, we carried out experiments to estimate the concentration of active sites of each sample. The concept of active sites was introduced in the coal literature to determine the impact of heat treatment on charcoal reactivity.34 These active sites possess high binding energy with oxygen atoms.35,36 Oxygen atoms reside preferentially on the prismatic edges or on the defects of the basal planes of the soot.36−38 The concentration of active sites can also be transformed into active surface area (ASA) by using eq 2. The method to estimate the active sites of the soot samples is adopted from the coal literature and the absence of metal impurities in our samples permits us to compare the reactivity of the soot on the ASA basis. However, the method to determine the total active surface area (ASAt) involves heating the samples to a relatively high temperature (800−1000 °C) to remove the oxygen complexes that occupy the active sites.34,36 In the current experiments, the samples were not heated to remove the oxygen complexes. This way, the initial nanostructure of the soot samples is retained. Accordingly, the active sites determined in these experiments represent the initial active surface area (ASAi) that is available at the time of initial reaction.

C tot = C + Cs + Cs(O)

(3)

where C is the core carbon, Cs is the carbon with available active sites, and Cs(O) is the carbon with occupied active sites.39 The surface oxygen complexes Cs(O) are mainly responsible for soot surface chemistry and include different oxygen functional groups. Haydar et al.40 showed that the decomposition of these complexes, which results in the release of CO or CO2, is multiple heterogeneous steps and concluded that the decomposition kinetics depend on the nature of species involved in desorption steps. In these experiments, the samples were not heated to remove the oxygen complexes. This way, the initial nanostructure and surface chemical composition of the soot samples is retained. Accordingly, the active sites determined in these experiments correspond to the Cs term in eq 3, which represents the initial active surface area (ASAi) that is available at the time of initial reaction. Table 4 summarizes the results of oxygen chemisorption of the three soot samples. Table 4. Summary of Oxygen Chemisorption Experiments amount of chemisorbed oxygen case

oxygen uptake (goxygen/gsoot)

ASAi m2/g

1 2 3

0.00485 0.00508 0.00835

15.2 15.86 26.1

The results in Table 4 show a clear relationship between soot reactivity and ASAi. As expected, soot3, which exhibits the highest reactivity among the other soot samples, demonstrates the highest ASAi. In addition, there is a small increase in ASAi in soot2 compared to soot1. This partly explains the slight difference in reactivity between soot1 and soot2. This result implies that the EGR increases the ASAi of the soot, which can be defined as the part total surface area (TSA) which has high affinity for oxidation reaction.36,38,41 Soot reactivity is related to ASAi, and the ASA evolves during the oxidation process. To more accurately analyze the oxidation reactivity, the specific rate (Rsp) is plotted against conversion (X) as shown in Figure 5. The specific rate can be expressed as41

Figure 5. Specific rate (Rsp) vs conversion for the soot samples. 6102

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1 dm t m t dt

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Another factor affecting soot reactivity is the carbonization rate. Less carbonized soot possesses higher reactivity. According to Dobbins et al.,45 the time required for the carbonization of soot precursor materials can be expressed as

(4)

where mt is the residual mass (fraction of unburned soot). A monotonically increasing rate is observed for all samples, as shown in Figure 5. This result suggests that the ASA available for reaction is continuously renewed as soot oxidation proceeds. It can be seen that, at any conversion level, Rsp and thus ASA is always higher in the order of soot3 > soot2 > soot1. This observation confirms our recent results.14 Although the TSA may change as well with conversion level, it was confirmed that TSA is not a reactivity parameter and the oxidation rate may be expressed in the basis of ASA.36,41 Impact of Adiabatic Flame Temperature on NOx Emissions and Soot Reactivity. To support the above discussion that the flame temperature is responsible for NOx reduction and improved soot reactivity, the peak adiabatic flame temperature was calculated for a stoichiometric mixture of vapor fuel and intake gases. The methodology of this calculation is based on the approaches proposed and used by many authors.30,42,43 The fuel used in the calculation was nheptane (C7H16). This calculation requires an estimate of the thermodynamic state of the in-cylinder reactant mixture, which includes fuel vapor, air, and EGR. For DI engines, the cylinder pressure increases significantly after ignition, so the authors evaluated the adiabatic flame temperature at the peak firing pressure. The peak firing pressure is defined as the peak cylinder pressure that occurs after the peak in the heat release rate,43 Figure 2. The unburned gas temperature at this point was estimated using an isentropic compression from the intake manifold conditions to the peak firing pressure. The compression process accounts for the gas composition and the temperature dependent specific heats. The calculated adiabatic flame temperature is shown in Table 3. It should be noted that the bulk gas temperature is not directly correlated to the NOx emissions. It is the peak adiabatic flame temperature that effects the engine-out NOx emissions. Previous studies have shown a good correlation between NOx emissions and the peak adiabatic flame temperature. The correlation can be expressed as30,42,43 XNOx = Ae

E / RTf

tc =

E c RT e f A

(6)

where tc is the characteristic carbonization time, c is constant, and A and E are the pre-exponential factor and activation energy of the carbonization rate, respectively. It can be seen from eq 6 that the carbonization time increases exponentially with decreasing Tf. Thus, tc can be much longer than the residence time of the soot in the flame. As the EGR lowers Tf, it is concluded that less carbonized and more oxidatively reactive soot forms.

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CONCLUSIONS 1. After comparing the impact of intake CO2 addition and actual EGR on engine combustion and NOx emissions, the dilution effect is found to be the dominant effect by which CO2 and EGR lower the peak adiabatic flame temperature and reduce the NOx emissions in common rail diesel engines. 2. EGR has more a significant impact on soot reactivity than the effect of CO2 addition. The EGR dilution effect reduces the peak adiabatic flame temperature and consequently enhances the reactivity of diesel soot. This effect reduces the degree of carbonization/ graphitization, which subsequently results in less developed and less mature soot. EGR also yields soot with a high initial concentration of active sites, which are regenerated during the course of oxidation.

AUTHOR INFORMATION

Corresponding Author

*Present contact information: 1231 Beal Avenue, 2007 WE Lay Auto Lab, Ann Arbor, MI 48109-2133. Telephone: 734-7646995. E-mail: [email protected]. Present Address †

Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia.

(5)

Notes

where XNOx is the NOx mole fraction, A is constant, E is an overall activation energy, R is the universal gas constant, and Tf is the stoichiometric peak adiabatic flame temperature. This correlation shows the exponential relationship between NOx emissions and the adiabatic flame temperature. Plee et al.42 estimated the slope of the above correlation (E/R) to be −38 700 K, which is in good agreement with the values obtained by Kook et al.30 and Xin et al.43 Therefore, it can be concluded from this correlation that the effect of charge dilution (e.g., EGR) on NOx emissions is primarily due to a lowering flame temperature. Concerning soot reactivity, EGR has two effects on soot oxidative reactivity. First, the onset of in-cylinder soot is delayed with increasing EGR.44 Second, the EGR lowers the Tf, which has a strong effect on the gas phase chemistry of soot formation.14,16,17 The Tf affects both the species available to add to the soot particle and the manner in which they add.14,16,17 Lower Tf yields more reactive soot, and therefore, the ultimate soot nanostructure is flame temperature dependent.17

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Department of Energy for support under Grant DE-FC26-03NT41828, the National Science Foundation for support under Grant CTS-0553339, and the Saudi Ministry of Higher Education for fellowship support for Dr. Al-Qurashi. Thanks are also due to Dr. Allen Kimel in the Penn State Department of Materials Science and Engineering for access to the SDT instrument.

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REFERENCES

(1) Ladommatos, N.; Abdelhalim, S. M.; Zhao, H.; Hu, Z. The Dilution, Chemical, and Thermal Effects of Exhaust Gas Recirculation on Diesel Engine Emissions - Part 1: Effect of Reducing Inlet Charge Oxygen; Society of Automotive Engineers: Warrendale, PA, 1996; SAE Technical Paper 961165. (2) Ladommatos, N.; Abdelhalim, S. M.; Zhao, H.; Hu, Z. The Effects on Diesel Combustion and Emissions of Reducing Inlet Charge Mass Due 6103

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