Effect of Oxygenated Fuel on Combustion and ... - ACS Publications

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Energy & Fuels 2002, 16, 294-301

Effect of Oxygenated Fuel on Combustion and Emissions in a Light-Duty Turbo Diesel Engine Juhun Song, Kraipat Cheenkachorn, Jinguo Wang, Joseph Perez, and Andre´ L. Boehman* The Pennsylvania State University, University Park, Pennsylvania 16802

Philip John Young and Francis J. Waller Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195-1501 Received July 17, 2001. Revised Manuscript Received November 19, 2001

In previous studies on a single-cylinder IDI diesel engine and a V-8 DI turbo diesel engine, significant reductions in particulate matter emissions were observed with the blends of glycol ethers in diesel fuel. In this study, experiments on the effects of oxygenated fuels on emissions and combustion were performed in a 4-cylinder TDI diesel engine. A blend of 20 wt % monoglyme and 80 wt % diglyme, referred to as CETANER, has been examined as a diesel reformulating agent. Blend ratios were considered to provide approximately 2, 4, and 6 wt % oxygen to lowsulfur diesel fuel. Gaseous and particulate emission measurements, as well as heat release rate analysis, have been used to address how emissions and combustion scale with increasing weight percent oxygen in the fuel. The results demonstrate that the oxygenated fuel provides significant reduction in particulate matter with a small penalty on NOx emission, especially at high load. This oxygenated fuel effect may result from an enhanced concentration of oxygen atoms in the over-rich mixture thereby contributing to soot suppression and thermal NOx formation through a shift to a leaner mixture. Low load results imply that the combined effect of relatively high exhaust gas recirculation (EGR) ratio and oxygen addition contributes to both NOx and soot reduction through a combination of flame temperature decrease and suppression of soot precursors. The combined effects on thermal NOx reduction at low load appear to be confirmed by heat release analysis, which indicates a small reduction in premixed burn peak and in-cylinder pressure. The slight reduction in HC and CO emissions under most conditions indicates an improvement of combustion efficiency with the use of oxygen addition. This result also represents the potential of diesel reformulation coupled with high EGR ratio for a better particulate/NOx tradeoff. Particulate morphology, as seen in transmission electron microscopy (TEM) micrographs, shows that enhanced oxidation of unburned hydrocarbon due to oxygen addition leaves a less agglomerated particulate structure especially at low mode, leading to a higher number density of smaller particles and a lower particulate mass.

Introduction The trend toward cleaner burning diesel fuels is occurring worldwide. In the United States, future engine regulations will require a further improvement of both combustion chamber and injection system design.1 It is commonly accepted that these goals can be achieved only if engine development is coupled with diesel fuel reformulation. As a consequence, much research has focused on screening of oxygenated fuel additives for reformulated diesel fuels. The influence of fuel oxygen content on soot reduction in diesel engines is wellknown.2-13 It has been demonstrated that certain alco* Author to whom correspondence should be addressed at The Pennsylvania State University, 405 Academic Activities Building, University Park, PA 16802. Tel: +1-814-865-7839. Fax: +1-814-8653248. E-mail: [email protected]. (1) Beatrice, C.; Bertoli, C.; Del Giacomo, N. Society of Automotive Engineers, 1999; No. 993595. (2) Miyamoto, N.; Ogawa, H.; Anma, T.; Miyakawa, K. Society of Automotive Engineers, 1996; No. 962115.

hols, esters, glycol ethers, and carbonates can have a beneficial effect on particulate emission from diesel engines. In particular, glycol ethers have been shown to be very effective in suppressing particulate emission, (3) Tsurutani, K.; Takei, Y.; Fujimoto, Y.; Matsudaria, J.; Kumamoto, M. Society of Automotive Engineers, 1995; No. 952349. (4) Spreen, K. B.; Ullman, T. L.; Mason, R. L. Society of Automotive Engineers, 1995; No. 950250. (5) Liotta, F. J., Jr.; Montalvo, D. M. Society of Automotive Engineers, 1993; No. 932734. (6) Hess, H. S.; Roan, M. A.; Bhalla, S.; Butnark, S.; Zarnescu, V.; Boehman, A. L.; Tijm, P. J. A.; Waller, F. J. ACS Reprints, Div. Pet. Chem. 1998, 43, 593-596. (7) Litzinger, T.; Stoner, M.; Hess, H. S.; Boehman, A. L. Int. J. Engine Res. 2000, 1, 57-70. (8) Donahue, R.; Foster, D. Society of Automotive Engineers, 2000; No. 2000-01-0512. (9) Marr, W. W.; Sekar, R. R.; Cole, R. L.; Marciniak, T. J.; Longman, D. E. Society of Automotive Engineers, 1993; No. 932805. (10) Hallgren, B. E.; Heywood, J. B. Society of Automotive Engineers, 2001; No. 2001-01-0648. (11) Hess, H. S.; Boehman, A. L.; Tijm, P. J. A.; Waller, F. J. Society of Automotive Engineers, 2000; No. 2000-01-2886.

10.1021/ef010167t CCC: $22.00 © 2002 American Chemical Society Published on Web 01/15/2002

Combustion and Emissions in a Turbo Diesel Engine

as well as overall pollutant emission. Prior studies have suggested that particulate matter correlates with the weight percent oxygen content in the fuel blend and not with other properties such as chemical structure or volatility.2-4 However, the reduction can be engine and test mode specific. Other studies have shown a definite dependence on the type of oxygenate.5-7 Similar PM reductions have been obtained by enriching the intake air with a higher molar concentration of oxygen.8-9 The use of oxygenates in diesel fuel has been shown to increase NOx and reduce HC and CO emissions by most work found in the literature. However, NOx emissions are also influenced by diesel fuel composition. Lowering aromatics, increasing cetane number, and decreasing fuel density can reduce NOx emissions.10 Although the reductions are engine specific, the impact is more pronounced on CI engines utilizing older fuel injection technologies and engine design.11 Murayama’s work has shown that with the only use of their oxygenated fuel, simultaneous reduction of PM and NOx cannot be obtained directly.12 However, their studies have demonstrated the possibility of simultaneous reduction of PM and NOx with the combined use of high EGR ratio and oxygenated fuel. This technique originated from the fact that while EGR had a strong influence on the thermal NOx reduction, available oxygen to inhibit soot and combustion temperatures needed to oxidize soot became reduced due to recirculated exhaust gas, resulting in PM increase. This potential negative impact of EGR motivates study of the means to provide more oxygen efficiently, such as through the use of oxygenated fuel, which can substantially inhibit soot formation. For this reason, the use of oxygenated fuel with high rates of EGR represents a potential route to a clean diesel engine. The objectives of the experimental work reported here are to evaluate the impact of glycol ethers, specifically CETANER, on overall emissions, as well as to examine how EGR effects interact with oxygenated fuel. CETANER is an oxygenated diesel fuel additive that can be produced through oxidative coupling of dimethyl ether. Using Air Products and Chemicals, Inc.’s Liquid-Phase Dimethyl Ether (LPDME) technology, dimethyl ether can be produced from syngas. So, CETANER represents a potential synthetic fuel additive for diesel fuel reformulation. Three blends of CETANER in the baseline fuel (5, 10, 15% in weight) were evaluated in a 4-cylinder 1.9 L Volkswagen TDI diesel engine. The engine was also equipped with a cylinder pressure transducer to provide information about the impact of the oxygenated fuel on the combustion process. Emissions of particulate matter, NOx, CO, CO2, HC were measured and particulate morphology was examined via a thermophoretic sampling technique and transmission electron microscopy. Experimental Setup Test Engine. For this study of the impact of oxygenated fuel on light-duty diesel combustion, a 4-cylinder, 1.9 L turbocharged direct injection (TDI) Volkswagen diesel engine (12) Murayama, T.; Zheng, M.; Chikahisa, T.; Oh, Y. T.; Fujiwara, Y.; Tosaka, S.; Yamashita, M.; Yoshitake, H. Society of Automotive Engineers, 1995; No. 952518. (13) Chapman, E. M.; Bhide, S. V.; Boehman, A. L.; Tijm, P. J. A.; Waller, F. J. Society of Automotive Engineers, 2000; No. 2000-01-2887.

Energy & Fuels, Vol. 16, No. 2, 2002 295 Table 1. Engine Specification engine bore × stroke displacement compression ratio rated power peak torque injection system low idle speed configuration

1997 VW Passat 1Z 79.5 × 95.5 mm 1.9 liter 19.5:1 66 kW at 4000 rpm 202 N-m at 1900 rpm direct injection, EDC 903 rpm turbocharged, intercooler (air-air), electronically controlled EGR with actuator

Table 2. Engine Test Condition mode

speed (RPM)

torque (lbf-ft)

1 2 3 4 5 6 7 8

903 1243 1553 1894 4000 3845 3845 3659

0 30 86 121 21 48 83 118

was coupled to a 250 horsepower eddy-current dynamometer. The engine specifications are shown in Table 1. A Leeds & Northrup Micromax data system logged the real-time engine speed, torque, and power, as well as exhaust, motor oil, and coolant temperatures. A duty cycle of exhaust gas recirculation valve which would infer the EGR ratio was also monitored on the PC by communicating with ECM via the onboard diagnostics (OBD) software. The intake air flow rate was directly measured via an electronic mass air flow sensor. Fuel consumption was determined by weighing the fuel at the beginning and end of each test mode or each fuel blend through a Sartorius precision scale, with an accuracy of (2 g. For each fuel blend and test mode, fuel consumption was recorded over five minute periods as the difference in the fuel tank weight before advancing to the next set of test conditions. On the basis of the power output for each test, brake specific fuel and energy consumption (BSFC and BSEC) were calculated. To observe the impact of oxygenated fuel on the diesel combustion process, a pressure transducer (Kistler 6053C) was mounted in the glow-plug hole in the first cylinder. This pressure sensor was used with a shaft encoder (Kistler 2611) to provide time-resolved pressure traces for heat-release rate calculation.14-17 The pressure traces were analyzed with PTrAn, a software product designed by the Optimum Power, to perform heat release and statistical analyses. Test Conditions. In these experiments, an AVL 8-mode test procedure was chosen as a model for diesel emission tests.18 The AVL 8-mode tests were designed to correlate to the U.S. federal Heavy-Duty Transient Test procedure through a weighted 8-mode steady-state test procedure. The 8 modes are a combination of speeds and loads, to produce the same emissions output as would be recorded for a transient cycle. The AVL 8-mode test protocol for this engine is reported in Table 2. During the completion of the engine test matrix, no adjustments were made to the engine operating parameters. (e.g., fuel injection timing and EGR ratio were not optimized for each fuel blend, but were left at the factory settings). Emissions Measurement. An extended warm-up period was used to prepare the engine for testing. The emission (14) Krieger, R. B.; Borman, G. L. ASME, 1966; No. 99-WA/DGP-4. (15) Lancaster, D. R.; Krieger, R. B.; Lienesch, J. H. Society of Automotive Engineers, 1975; No. 750026. (16) Gatowski, J. A.; Balles, E. N.; Chun, K. M.; Nelson, F. E.; Ekchian, J. A.; Heywood, J. B. Society of Automotive Engineers, 1984; No. 841359. (17) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1988. (18) Fleisch, T.; McCarthy, C.; Basu, A.; Udovich, C.; Charbonneau, P.; Slodowske, W.; Mikkelsen, S. E.; McCandless, J. Society of Automotive Engineers, 1995; No. 950061.

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Song et al. Table 3. Fuel Blend Properties

base

5% CETANER

10% CETANER

15% CETANER

fuel composition

Mobil low-sulfur diesel

cetane number heating value (MJ/kg) API gravity and density (g/cm3) Saybolt viscosity (SUS) @38 °C sulfur (ppm) oxygen (wt %)

55.4 45.8 42.5 34.3 39 N/A

5 wt % CETANER, 95 wt % baseline 59 45.0 42.5 32.3 37 1.79



measurements during each mode were begun when the exhaust temperature reached steady-state. During this time, RPM and torque were maintained within 2% of target test conditions. For particulate sampling, a portion of the exhaust gas was passed through a Sierra Instruments BG-1 minidilution tunnel with constant dilution air/sample flow ratio of 10:1 and total flow of 100(L/min). These settings were chosen in order to maintain the filter temperature below the EPA specification of 52 °C. Actual temperatures never exceeded 45 °C. Particulate collection occurred on the Pallflex 90 mm filters, conditioned in an environmental chamber at 25 °C and 45% relative humidity before and after sampling. Five particulate samples were taken for each fuel at each test mode. In addition to PM mass measurement, particulate samples were extracted directly from the exhaust stream using a thermophoretic sampling approach patterned after the work of Megaridis19 and Koylu,20 as described by Rudnik et al.21 The thermophoretic samples were collected on 3 mm diameter grids, constructed of 1-µm thick copper and coated on one side with silicon oxide, using a pneumatically controlled probe to inject the sampling grid into the undiluted exhaust near where the other emissions samples were obtained in the exhaust pipe. Transmission electron microscopy (TEM) was used to analyze the microstructure of particulate samples. The transmission electron microscopy observations were conducted in a Philips EM420T microscope operated at 120 kV. The high-resolution transmission electron microscopy (HRTEM) observations were performed on a field emission HF-2000 instrument operated at 200 kV, with a point-to-point resolution of approximately 0.23 nm. Gaseous emissions were measured using a Nicolet Magna 550 Fourier transform infrared (FTIR) spectrometer. At each mode, five gas samples were analyzed for CO2, CO, NO, and NO2 emissions. The carbon dioxide results were used in conjunction with a mass air flow sensor to determine and verify the air-fuel ratio. A Horiba model OPE-115 NDIR CO2 analyzer was also used as a method of verifying the CO2 concentration from the FTIR measurement. In addition, online hydrocarbon analysis was completed using a California Analytical Instruments Model 300 HFID Heated Total Hydrocarbon Gas Analyzer. Undiluted exhaust gas was collected via a heated sample line, which was maintained at 190 °C. Calibration of the HFID was completed prior to each run on the basis of C1, using propane as the span gas. Figure 1 shows the test cell setup and additional apparatus for emission measurement. Fuel Blends. The baseline fuel was a high cetane number diesel with low aromatic and sulfur content. The choice of a glycol ether blend as the oxygenated additive was due to its performance in terms of combustion behavior and sootreducing tendency, observed in previous tests.1-2 It has been observed that this performance stems from both high cetane number and high oxygen content. For this experimental work, (19) Megaridis, C. M. Ph.D. Thesis. Division of Engineering: Brown University, 1987. (20) Koylu, U. O. Ph.D. Thesis. Aerospace Engineering: University of Michigan, 1992. (21) Rudnik, M. K.; Perez, J. M.; Boehman, A. L. ACS Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 2001, 46 (2), 597-598.

Figure 1. Schematic of experimental setup. diesel fuel blends at the various levels of oxygen addition were evaluated in this engine. A simplified mixture of 20% ethylene glycol dimethyl ether (monoglyme, CAS 110-71-4, C4H10O2) and 80% diethylene glycol dimethyl ether (diglyme, CAS 11-96-6, C6H14O3), referred to as CETANER, was chosen as the reformulating agent. CETANER is an oxygenated diesel fuel additive developed as a coal-derived syngas product by Air Products and Chemicals, Inc. As a diesel fuel additive, CETANER has been shown to exhibit high cetane number, roughly 125.22 Blend ratios to base diesel fuel of 5, 10, 15 wt % were considered to provide approximately 2, 4, 6 wt % oxygen to the base diesel fuel. Previous work has reported physical and thermodynamic properties of these fuel blends.23 Table 3 lists several properties for the baseline diesel and the fuel blends. Although this study is focused on the effect of oxygen addition to diesel fuel on combustion and emissions, knowledge of the impact on fuel properties is useful in evaluation of the combustion and emissions results.

Results and Discussion An uncertainty analysis was completed for the experimental results, based on methods reported by Moffat.24 In the figures presented, error bars indicate 95% confidence intervals. The addition of the oxygenate to the baseline fuel was expected to produce an increase in the fuel consumption. Figure 2 shows that brake specific fuel consumption increases with oxygenate concentration due to reduced heating value. When fuel consumption is viewed on an energy basis, specific energy consumption is unchanged with the fuel blends. Particulate matter emissions (grams of pollutant per kilogram of fuel) follow the well-known effect of increasing fuel oxygen content, as observed in Figure 3. Using the AVL 8-mode weighting factors, net particulate emissions reductions were 13%, 21%, and 24% with respect to the baseline fuel for 5%, 10%, and 15% (22) Tijm, P. J. A.; Waller, F. J.; Toseland, B. A.; Peng, X. D. Energy Frontiers International Conference, 1997. (23) Hess, H. S.; Szybist, J.; Boehman, A. L.; Tijm, P. J. A.; Waller, F. J. ASME; Proceedings of the 35th National Heat Transfer Conference, 2001; No. NHTC01-11462. (24) Moffat, R. J. Exp. Thermal Fluid Sci. 1988, 1, 3-17.


Figure 2. Fuel consumption, brake specific basis, g/kW-h.

Figure 3. Particulate matter emissions, g/kg fuel basis.

Figure 4. AVL 8-mode weighted particulate matter emissions, oxygen basis.

additive concentration. Figure 4 shows that the percent PM reduction on an oxygen addition basis. While the particulate emissions were consistently reduced with increasing quantity of oxygen added, particulate emission reduction was not linear with oxygen content and additional oxygen resulted in diminished PM reduction. A similar trend has also been observed previously for oxygenated diesel fuels by Tsurutani et al.3 and in previous work by the authors.6,11,23 The influence of oxygen addition also varies with engine conditions. As shown in Figure 5, at higher load test modes where higher soot emissions occur, suppression of particulate emissions is stronger, which the steeper trend line indicates. At light loads where the overall mixtures are much leaner, the oxygenated fuel has only a slight effect on particulate emissions. Choi et al.25 have attributed the contrast in the effect of oxygenated fuel on both PM and NOx emission at

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different load conditions to different types of combustion. They observed that at high loads, the greater the oxygen concentration in the fuel blend, the greater the reduction in particulate emission. At this load, a very rich fuel core exists because the fuel injection pulse is relatively long. With the use of oxygenated fuels, an oxidizer is more effectively being introduced and made available in the fuel-rich regions. The oxygen atoms, through the formation of hydroxyl radical (OH), consume the soot precursors, yielding a reduction in soot formation.26 In this high-temperature rich flame region, NOx emissions are more likely to be controlled by thermal NOx kinetics.27 According to simple rate equations for the thermal NOx mechanism, the NOx formation rate is directly proportional to equilibrium concentration of oxygen atom.28 So, available oxygen might shift this locally rich mixture toward a slightly lean mixture where the maximum equilibrium O-atom mole fraction lies. Consequently, the higher equilibrium concentration of oxygen atom results in higher kinetically formed NOx. In addition, the higher NOx may result from super-equilibrium of O atoms, up to several orders of magnitude greater than equilibrium in fast post-flame zone. On the other hand, low load combustion is known to be dominated by the premixed burn. One reason for the small effect of the oxygenated fuel on particulate emissions may be the reduced flame temperature that arises from the reduced heat of combustion of the fuel blend and overall leaner mixture. The lower flame temperature reduced the rate of oxidative attack on soot precursors, resulting in less soot suppression in the premixed flame region, which dominates the low load case. This lower temperature can reduce NOx emissions at the same time. Finally, Choi et al.25 suggested the influence of the oxygenated fuel on PM reduction in the premixed region may be limited by the flame temperature effect. This trend that oxygenated fuel would have a weaker effect on both PM and NOx at low load can clearly be confirmed in this study as shown in Figure 5. Nitrogen oxide emissions previously reported by many researchers have shown that diglyme addition to base diesel fuel results in higher NOx production.5,12 In other words, the reductions of particulate emissions are accompanied by small increases in NOx emissions as oxygen content increases - typically 2%. This slight penalty in NOx emissions was more evidently observed with the use of oxygenates, especially at high mode with advanced injection timing.25 The work reported by Murayama and co-workers has shown that the NOx increase was very significant with oxygen addition, in the range of 10-20% change from the base diesel fuel. However, their studies have demonstrated the possibility of simultaneous reduction of PM and NOx when they adopted the use of high EGR ratio in conjunction with an oxygenated fuel.12 Figure 6 presents the tradeoff between particulate matter and NOx as oxygen content increases using the AVL 8-mode weight factors. As oxygen content in(25) Choi, C. Y.; Reitz, R. D. Fuel 1999, 78, 1303-1317. (26) Dec, J. E. Society of Automotive Engineers, 1997; No. 970873. (27) Turns, S. R. An Introduction to Combustion; McGraw-Hill: New York, 1996. (28) Glassman, I. Combustion, 2nd ed.; Academic Press: Orlando, FL, 1987.

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Figure 5. The effect of oxygenate concentration on PM and NOx emissions for different load conditions.

Figure 6. The tradeoff between PM vs NOx emissions, weighted averages from AVL 8-mode test, brake specific basis, g/kW-h.

Figure 8. HRR diagram with and without oxygen addition at light load.

Figure 9. HC emissions, g/kg fuel basis. Figure 7. NOx emissions, g/kg fuel basis.

creases, the rate of NOx increase is not negligible compared to the rate of PM decrease. The net NOx emissions increase is 12%, 14%, and 26% with respect to the baseline for 5%, 10%, and 15% additive concentration. Figure 7 clearly shows that the oxygenated fuel leads to substantially higher NOx at the highest load. At this test condition, the engine management system uses advanced injection timing and a small EGR rate since all available oxygen is needed for the diffusioncontrolled combustion. It means that the effects of both EGR and injection timing on NOx emissions combine with the oxygenated fuel effect to contribute to NOx emissions. As stated earlier, the available oxygen in the high-temperature flame region from the oxygenated fuel may shift this locally rich mixture toward a slightly lean

Figure 10. CO emissions, g/kg fuel basis.

mixture where the maximum equilibrium O-atom mole fraction lies. Consequently, the high equilibrium con-



Figure 11. TEM micrograph of particulate matter from baseline and oxygenated fuel (6 wt %), at 105 000× magnification.

centration of oxygen atom along with super-equilibrium O-atom results in higher kinetically formed NOx. In contrast, the NOx emissions slightly decreased at low load as more oxygenated additive was blended with the base fuel. The reduced NOx emissions reflect the lower in-cylinder temperature with the oxygenated fuel blends due to the reduced heat of combustion and leaner overall mixture in this premixed-dominated combustion. In addition, at this test condition, a high EGR rate was

expected and observed. Therefore, the EGR effect, which is more beneficial at the low load condition compared to high load case, simultaneously might contribute to NOx reduction through a decrease in specific heat capacity of the cylinder charge making it difficult to separate the oxygenate effect on NOx emissions from the EGR effect. Despite this confounding mixing of influences on emissions, the test results imply that the combined effect of relatively high EGR ratio and oxygen

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Figure 12. TEM micrograph of particulate matter from baseline and oxygenated fuel (6 wt %), at 500 000× magnification.

addition contribute to both NOx and soot reduction through flame temperature decrease and enhanced availability of oxygen in the fuel spray. The light load heat release in Figure 8 exhibits the general shape that is dominant in the premixed burn region due to a large amount of the fuel being injected during the ignition delay. At light load, the effect of the oxygenated fuel can be noted as a small reduction in premixed burn peak and in-cylinder pressure, which may explain the decrease in thermal NOx formation. Oxygen addition did cause a small change in ignition

delay because the cetane number increases with the glycol ether content. The increase of fuel cetane number reduces the peak of the premixed combustion rate and, as a consequence, increases the diffusion-controlled phase of combustion, as reported for high cetane number oxygenated fuels.1 Figure 9 shows that hydrocarbon emissions decrease with higher engine loads, as the combustion efficiency increases. It would be expected that increased oxidation of combustion intermediates by the oxygen present in the additive would result in a decrease of HC and CO


emission. In this work, a similar trend was observed in that HC emissions decreased as compared to the baseline fuel although there is less change at higher mode. These results are consistent with other work.1,5,12 Figure 10 reports carbon monoxide emissions and shows no clear trends, with some modes showing an increase and other modes showing a decrease. However, there is a moderate decrease in CO especially at higher modes. While earlier work with this engine and oxygenated fuel additives showed increased CO emissions at light loads leading to speculation that CO was an important intermediate in soot suppression,7,11 CO was observed to decrease at light load under the test protocol in this study. The thermophoretic particulate sampling and TEM analysis at the 105 000× magnification show that particulate morphology distinctly changed with engine operating conditions, as seen in Figure 11. In Figures 11a and 11c for the baseline fuel, Mode 8 yields soot aggregates of large extent and with relatively small primary soot particles spherules, whereas for Mode 1 the aggregates are less extensive but have larger particle size. The primary particle size change is attributed to a greater amount of unburned hydrocarbon that condensed and adsorbed onto the aggregate in Mode 1, where lower combustion efficiency leads to greater hydrocarbon emissions. A modest change in particulate matter morphology is seen with the oxygenated fuel relative to the base diesel fuel, especially at Mode 1. Comparing Figures 11a and 11b, the oxygenated fuel leads to less condensation of hydrocarbon onto the soot aggregate than the conventional fuel. In Mode 8, there is not as distinct a difference between the particulate from the baseline fuel (Figure 11c) and oxygenated fuel (Figure 11d). This trend is supported by the hydrocarbon emission results presented in Figure 9, which shows a substantial decrease in Mode 1 with the oxygenated fuel, but no change in Mode 8. Enhanced oxidation of unburned hydrocarbon due to oxygen addition would lead to less hydrocarbon condensation on the particulate structure. High-resolution TEM results in Figure 12, which are at a higher magnification (500 000×) than the images in Figure 11, did not indicate a significant change in the microstructure of the primary soot particles, although evidence of graphitic structures is clearly visible in Mode-8 images (Figures 12c and 12d), may be visible in Mode 1 with oxygenated fuel (Figure 12b) and may be absent in Mode 1 with the baseline fuel (Figure 12a). The images in Figures 12a and 12b may show that the soot particles shift from consisting of predominantly amorphous carbon to a more graphitic inner structure with oxygenated fuel. The potential that soot microstructure can vary with engine operating conditions and fuel type has important implications for the oxidation of particulate matter once it is collected onto particulate traps. This issue warrants further examination, through


additional microscopic analysis and through study of particulate reactivity via thermogravimetric analysis. Conclusions Oxygenated fuel additives, such as the blend of glycol ethers studied in this work, can significantly reduce particulate matter emissions but can lead to slight increases in NOx emissions. This impact varies with engine operating conditions and additive concentration. Oxygen addition to the fuel has a stronger effect on particulate reduction and NOx increase at higher load conditions where high-temperature, rich combustion occurs. This oxygenated fuel effect may result from an enhanced concentration of oxygen atoms in the overrich mixture thereby contributing to soot suppression and thermal NOx formation through a shift to a leaner mixture. Low load results imply that the combined effect of relatively high EGR ratio and oxygen addition contributes to both NOx and soot reduction through a combination of flame temperature decrease and suppression of soot precursors. The combined effects on thermal NOx reduction at low load appear to be confirmed by heat release analysis, which indicates a small reduction in premixed burn peak and in-cylinder pressure. Therefore, to achieve simultaneous PM and NOx reduction with the use of oxygenated fuel, other measures such as EGR ratio adjustments need to be pursued at high load to improve the tradeoff between PM and NOx. The slight reduction in HC and CO emissions under most conditions indicated an improvement of combustion efficiency with the use of oxygen addition in this modern engine control system. Electron microscopy of the particulates and their microstructure shows that enhanced oxidation of unburned hydrocarbons due to oxygen addition leads to modest changes in the size and perhaps the inner structure of primary soot particles. Acknowledgment. The authors acknowledge financial support from Air Products and Chemical, Inc. The authors also acknowledge Glen Chatfield of Optimum Power for providing the PtrAn software. In addition, the authors acknowledge technical support from V. Zello, J. Szybist, E. Chapman, and L. I. Boehman of The Pennsylvania State University. Transmission electron microscopy was performed using the facilities of the Materials Research Institute at The Pennsylvania State University and the authors are grateful for access to these facilities.This paper was written with support of the U.S. Department of Energy under Contract DEFC22-95PC93052. The Government reserves for itself and others acting on its behalf a royalty-free, nonexclusive, irrevocable, worldwide license for Governmental purposes to publish, distribute, translate, duplicate, exhibit and perform this copyrighted paper. EF010167T

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