Comparison of the Impact of Intake Oxygen Enrichment and Fuel

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Energy & Fuels 2004, 18, 1282-1290

Comparison of the Impact of Intake Oxygen Enrichment and Fuel Oxygenation on Diesel Combustion and Emissions Juhun Song, Vince Zello, and Andre´ L. Boehman* The Energy Institute, The Pennsylvania State University, 405 Academic Activities Building, University Park, Pennsylvania 16802

Francis J. Waller Air Products and Chemicals, Inc., Allentown, Pennsylvania 18195 Received December 16, 2003. Revised Manuscript Received May 27, 2004

Among continuing efforts to develop low-emission combustion engines, oxygen-enhanced combustion has long been considered a promising approach. A number of investigations have focused on the effects of oxygen addition on soot formation and oxidation by using various oxygen introduction techniques, such as blending different oxygen-containing fuels or direct oxygen addition into the intake air stream. The present study of oxygen addition was performed on a Volkswagen 1.9 L “TDI” turbodiesel engine to investigate and compare the relative effect of two oxygen addition methods on diesel emission and combustion: oxygen enrichment of the intake air and oxygenation of the fuel. The oxygen enrichment was accomplished by connecting an oxygen generator to the intake air surge tank, while fuel oxygenation was accomplished using two compounds with different cetane number and molecular structure. The key observations are that both intake oxygen enrichment and fuel oxygenation via linear structure oxygenated molecules are effective for reduction of diesel particulate matter, yielding even greater reductions in PM emissions than for fuel oxygenation via ring-structured oxygenated molecules. However, NOx emissions are greatly increased with intake oxygen enrichment, owing to either increased availability of atomic oxygen or attainment of a higher temperature during leaner combustion, which enhances the kinetics for thermal NOx formation. Comparison between the addition of two substantially different oxygenated fuels, a mixture of glycol ethers and 1,3-dioxolane, has also shed light on the mechanisms of soot reduction via oxygen addition. With their linear structure, the glycol ethers were shown to be far more effective for soot reduction than an equivalent oxygen addition via dioxolane, which has a ring structure, despite no significant difference in heat release rate.

Introduction Addition of oxygen to assist the diesel combustion process is by no means a new idea. Many researchers have been actively investigating the effects of oxygenates on diesel combustion, and reviews of many relevant papers can be found in refs 1 and 2. In ongoing work by Boehman and co-workers, a number of studies have considered oxygenation of diesel fuel through additives and alternative fuels.1,3-5 Oxygen enrichment of intake air has also been considered as a measure to control the * Corresponding author. E-mail: [email protected]. (1) Litzinger, T.; Stoner, M.; Hess, H.; Boehman, A. Effects of Oxygenated Blending Compounds on Emissions from a Turbo-charged Direct Injection Diesel Engine. Int. J. Engine Res. 2000, 1, 57-70. (2) Natarajan, M.; Frame, E.; Naegli, D. W.; Asmus, T.; Clark, W.; Garbak, J.; Gonzalez, M. A.; Liney, E.; Piel, W.; Wallace, J. P. Oxygenates for Advanced Petroleum-Based Diesel Fuels: Part 1. Screening and Selection Methodology for the Oxygenates. Society of Automotive Engineers Technical Paper No. 2001-01-3631, 2001. (3) Hess, H. S.; Roan, M. A.; Bhalla, S.; Butnark, S.; Zarnescu, V.; Boehman, A. L.; Tijm, P. J. A.; Waller, F. J. Reduction of Particulate Emissions from a Single-Cylinder IDI Engine with Oxygenated Diesel Fuels. In Chemistry of Diesel Fuels; Song, C., Hsu, C. S., Mochida, I., Eds.; Taylor & Francis: New York, 2000; Chapter 14, pp 255-288.

PM emissions, to improve thermal efficiency, and to improve the ignition quality of diesel fuels. An example of such work is the efforts at Argonne National Laboratory where researchers have developed a membrane that efficiently separates standard air into oxygen and nitrogen.6 Some discussion of the relevant literature on these two subjects follows. The consensus in the literature is that incorporation of oxygen into diesel fuel reduces particulate emissions. Where there has been disagreement is on the role of oxygenate structure in controlling the amount of PM reduction and the mechanism by which oxygenated fuel (4) Hess, H. S.; Boehman, A. L.; Tijm, P. J. A.; Waller, F. J. Experimental Studies of the Impact of CETANERTM on Diesel Combustion and Emissions. Society of Automotive Engineers Technical Paper No. 2000-01-2886, 2000. (5) Chapman, E. M.; Bhide, S. V.; Boehman, A. L.; Tijm, P. J. A.; Waller, F. J. Emission Characteristics of a Navistar 7.3L Turbodiesel Fueled with Blends of Oxygenates and Diesel. Society of Automotive Engineers Technical Paper No. 2000-01-2887, 2000. (6) Poola, R. B.; Longman, D. E.; Stork, K. C.; Sekar, R.; Nemser, S.; Callaghan, K. Membrane-Based Nitrogen-Enriched Air for NOx Reduction in Light-Duty Diesel Engine. Society of Automotive Engineers Technical Paper No. 2000-01-0228, 2000.

10.1021/ef034103p CCC: $27.50 © 2004 American Chemical Society Published on Web 07/10/2004

Oxygen Enrichment and Diesel Combustion/Emission

reduces PM emissions. In some recent work by researchers at Sandia National Laboratory and co-workers, both of these issues have been clarified. Dec and co-workers have considered the impact of oxygenated fuels on the diesel spray combustion process, through a combination of analytical and experiment information.7 Chemical kinetic analysis and consideration of the fuel-air mixing process during combustion of n-heptane/methanol and n-heptane/dimethyl ether mixtures showed that the percentage mass of carbon existing as soot precursors falls off with increasing O/C atomic ratio and that CO concentrations will be elevated. With more of the fuel carbon bound up as CO, less carbon is available to form particulate precursors. While this CO formation alone cannot account for the reductions in soot observed for many oxygenated fuels, the combination of the CO formation from oxygen in the fuels, CO liberated through bond cleavage in the oxygenate (e.g., for the glycol ethers), the scavenging of H by CO and CO2, and the attack of soot precursors by OH that is produced by the H + CO2 reaction, all contribute to the suppression of soot. Recently, Mueller and co-workers have used an optically accessible DI diesel engine to examine the impact of oxygenate structure on the soot formation process, providing additional support for the model of soot suppression by oxygenates and demonstrating that oxygenate structure does affect soot suppression.8,9 In this work, Mueller and co-workers compared di-butyl maleate and tri-propylene glycol methyl ether at fixed “oxygen ratio,” defined as the amount of oxygen available in the reactants divided by the amount required for stoichiometric combustion.9,10 The oxygen ratio is shown to be a more effective basis for comparison than equivalence ratio, O/C ratio, and oxygen mass fraction. In the work presented here, however, the more traditional O/C ratio will be used as the basis for comparison. From laminar flame studies where gaseous oxygen is added to the fuel stream, oxygen addition effects on soot formation and oxidation were identified to provide some implications for diesel engine combustion: (1) a dilution effect resulting from the change in the amount of carbon per unit mass of gaseous fuel mixture that will later involve soot precursor formation, (2) a thermal effect due to flame temperature change upon diluent addition, and (3) a direct chemical suppression or soot formation increase.11 In some experiments, thermal and dilution effects by certain oxygen-containing additives are believed to be major factors leading to suppression (7) Flynn, P. F.; Durrett, R. P.; Hunter, G. L.; zur Loye, A. O.; Akinyemi, O. C.; Dec, J. E.; Westbrook, C. K. Diesel Combustion: An Integrated View Combining Laser Diagnostics, Chemical Kinetics, and Empirical Validation. Society of Automotive Engineers Technical Paper No. 1999-01-0509, 1999. (8) Mueller, C. J.; Martin, G. C. Effects of Oxygenated Compounds on Combustion and Soot Evolution in a DI Diesel Engine: Broadband Natural Luminosity Imaging. Society of Automotive Engineers Technical Paper No. 2002-01-1631, 2002. (9) Mueller, C. J.; Pitz, W. J.; Pickett, L. M.; Martin, G. C.; Siebers, D. L.; Westbrook, C. K. Effects of Oxygenates on Soot Processes in DI Diesel Engines: Experiments and Numerical Simulation. Society of Automotive Engineers Technical Paper No. 2003-01-1791, 2003. (Also, JSAE 20030193). (10) Upatnieks, A.; Mueller, C. J. Investigation of the Relationships Between DI Diesel Combustion Processes and Engine-Out Soot Using An Oxygenated Fuel. Society of Automotive Engineers Technical Paper No. 2004-01-1400, 2004. (11) Glassman, I. Sooting Laminar Diffusion Flames: Effect of Dilution, Additives, Pressure, and Microgravity. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1998; pp 1589-1596.

Energy & Fuels, Vol. 18, No. 5, 2004 1283

of soot formation. However, there is still controversy about the presence of the chemical suppression effect, as well as its variation in effectiveness of oxygenates with different structure at reducing soot even though other experimental studies have thoroughly demonstrated the suppression of soot formation by alcohol addition and provided information on the apparent influence on the chemical kinetic pathways by which soot is suppressed.12,13 With increasing oxygen content in the oxidizer stream, increase of soot formation together with modest reductions in soot yields have been reported in laminar premixed diffusion flame studies.14,15 They attributed this soot suppression effect of increasing the oxygen concentration on the oxidizer side entirely to a thermal effect. The corresponding dilution and chemical effects were found to be negligible. Instead, examination of oxygen addition via fuel oxygenation in a practical diesel engine presents experimental difficulties in isolating such effects. Since changes in fuel properties such as volatility and cetane number can cause the additional thermal and dilution effects through air entrainment during the ignition delay period, it is hard to attribute the observed emission changes purely to the chemical effect of oxygen addition without a careful consideration of fuel property effects at the same time. Furthermore, the typically lower heating value and higher density of the oxygenated fuel might change the timing of the combustion process as the engine’s control system makes injection timing adjustments in response to fuel property changes which compounds the oxygen addition effect, unless fuel injection timing can be held fixed. In this regard, the recent work of Mueller and co-workers provides invaluable insights into the mechanism of soot suppression by fuel oxygenation through their ability to perform precisely controlled experiments and to apply sophisticated laser diagnostic techniques to examine the incylinder processes.8,9 A key insight reported by Mueller and Martin is that differences of a factor of 2 in the spatially integrated natural luminosity of the diesel spray flame (a relative measurement of soot volume fraction) were observed between di-butyl maleate (a diester compound) and tri-propylene glycol methyl ether, despite using fuel blends containing the same amount of oxygen at the lift-off position.8 The follow-up studies by Mueller et al., which included experiments in a constant-volume combustion vessel and numerical simulation, further support the assertion that the effect of oxygen addition to the fuel spray plume is to reduce soot-precursor levels.9 Furthermore, their calculations and experiments show that for some oxygenates, specifically di-butyl maleate, not all of the oxygen is available (12) Frenklach, M.; Yuan, T. Proceedings of the 16th International Symposium on Shock Tubes and Waves; VCH: Weinheim, 1987; pp 487-493. (13) Ni, T.; Gupta, S.; Santoro, R. Suppression of Soot Formation in Ethene Laminar Diffusion Flames by Chemical Additives. TwentyFifth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1994; pp 585-592. (14) Du, D. X.; Axelbaum, R. L.; Law, C. K. The Influence of Carbon Dioxide and Oxygen as Additives on Soot Formation in Diffusion Flames. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1990; pp 1501-1507. (15) Lee, K.-O.; Megaridis, C. M.; Zelepouga, S.; Saveliev, A. V.; Kennedy, L. A.; Charon, O.; Ammouri, F. Soot Formation Effects of Oxygen Concentration in the Oxidizer Stream of Laminar Coannular Nonpremixed Methane/Air Flames. Combust. Flame 2000, 121, 323333.

1284 Energy & Fuels, Vol. 18, No. 5, 2004

for soot precursor reduction, and for others, specifically tri-propylene glycol methyl ether, all of the oxygen is available for soot precursor reduction. Some review of the literature on oxygen enrichment is provided by Ghojel et al.16 and Desai et al.17 In an early paper on oxygen enrichment, Karim and Ward used the variation of intake composition to study the relationship between the physical and chemical time scales in the direct injection compression ignition engine.18 They also observed greatly reduced smoke emissions, higher peak cylinder pressure, increased power out, and significantly shortened ignition delay with oxygen enrichment up to 55 vol % O2 in the intake air. They observed increased smoke emissions as the oxygen content in the intake air was decreased, with degraded combustion occurring at 7-8 vol % O2. The impact of both nitrogen (N2) and oxygen (O2) enrichment have been considered in recent work. At Inha University, the impact of N2, carbon dioxide (CO2), and argon (Ar) enrichment have been shown to significantly affect PM and NOx emissions, with nitrogen enrichment leading to a reduction in NOx emissions but a moderate increase in PM.19 In studies by researchers at Argonne National Laboratory, nitrogen enrichment has been shown to reduce NOx emissions but increase PM emissions,6 although the PM emissions increase is similar to that observed with EGR. However, across a range of engine platforms, oxygen enrichment has been observed to have beneficial effects on PM but mixed effects on NOx. With significant optimization, Poola, Sekar and co-workers have shown that oxygen enrichment can yield reduction in both NOx and PM in locomotive diesel engines through use of modest oxygen enrichment and retarded fuel injection timing.20,21 Others have made similar observations that retarded injection timing can counteract the adverse impact on NOx emissions from oxygen enrichment.22-26 In an investigation by Schmidt and (16) Ghojel, J.; Hilliard, J. C.; Levendis, J. A. Effect of Oxygen Enrichment on the Performance and Emissions of I.I.I Diesel Engines. Society of Automotive Engineers Technical Paper No. 830245, 1983. (17) Desai, R. R.; Gaynor, E.; Watson, H. C.; Rigby, G. R. Giving Standard Diesel Fuels Premium Performance Using Oxygen-Enriched Air in Diesel Engines. Society of Automotive Engineers Technical Paper No. 932806, 1993. (18) Karim, G. A.; Ward, G. The Examination of the Combustion Processes in a Compression-Ignition Engine by Changing the Partial Pressure of Oxygen in the Intake Charge. Society of Automotive Engineers Technical Paper No. 680767, 1968. (19) Li, J.; Chae, J. O.; Park, S. B.; Paik, H. J.; Park, J. K.; Jeong, Y. S.; Lee, S. M.; Choi, Y. J. Effect of Intake Composition on Combustion and Emission Characteristics of DI Diesel Engine at High Intake Pressure. Society of Automotive Engineers Technical Paper No. 970322, 1997. (20) Assanis, D. N.; Poola, R. B.; Sekar, R.; Cataldi, G. R. Study of Oxygen-Enriched Combustion Air in Locomotive Diesel Engines. J. Eng. Gas Turbines Power 2001, 123, 157-166. (21) Poola, R. B.; Sekar, R. Reduction of NOx and Particulate Emissions by Using Oxygen-Enriched Combustion Air in a Locomotive Diesel Engine. J. Eng. Gas Turbines Power 2003, 125, 524-533. (22) Iida, N.; Suzuki, Y.; Sato, G. T.; Sawada, T. Effects of Intake Oxygen Concentration on the Characteristics of Particulate Emissions from a D. I. Diesel Engine. Society of Automotive Engineers Technical Paper No. 861233, 1986. (23) Watson, H. C.; Milkins, E. E.; Rigby, G. R. A New Look at Oxygen Enrichment (1) The Diesel Engine. Society of Automotive Engineers Technical Paper No. 900344, 1990. (24) Sekar, R. R.; Marr, W. W.; Schaus, J. E.; Cole, R. L.; Marciniak, T. J.; Eustis, J. N. Cylinder Pressure Analysis of a Diesel Engine Using Oxygen-Enriched Air and Emulsified Fuels. Society of Automotive Engineers Technical Paper No. 901565, 1990. (25) Virk, K. S.; Kokturk, U.; Bartels, C. R. Effects of OxygenEnriched Air on Diesel Engine Exhaust Emissions and Engine Performance. Society of Automotive Engineers Technical Paper No. 931004, 1993.

Song et al.

Van Gerpen of the mechanism of PM reduction with biodiesel, an oxygen enrichment test was conducted to simulate the oxygen content associated with the esters in biodiesel. Similar soot reduction from oxygen enrichment was observed along with an increase on the NOx emission.27 They also compared methyl soyate blends with straight chain alkane blends with the intention of isolating the effect of the cetane number and long chain hydrocarbons on particulate emissions. At the University of Wisconsin-Madison, Donahue and Foster compared the impact of fuel oxygenation and intake oxygen enrichment.28 They found that effectiveness for particulate matter reduction depended on the local concentration of oxygen in the fuel plume, indicated by the local equivalence ratio. In contrast, they found that the global thermal and dilution effects from oxygen enrichment led to a far more significant effect on NOx emissions than did fuel oxygenation. In subsequent work by Foster and co-workers, the use of “auxiliary gas injection” for late cycle oxygen enrichment was simulated in a heavy duty engine and was shown to provide 80% soot emission reduction with negligible effect on NOx through controlled enhancement of incylinder mixing. This recent work and the earlier work on oxygen enrichment has shed some light on the effects of intake charge composition, but left many unanswered questions that served as motivation for the work presented here. The objective of the present study was to pursue studies on the impact of oxygen enrichment on combustion and emissions in a Volkswagen 1.9L “TDI” turbodiesel engine to clarify the relative effects of O2 enrichment and fuel oxygenation. To that end, a vacuum swing absorption (VSA) oxygen generator (Air Products’ VSA type A-040/120L Oxygen Generator) was attached to the intake air management system of a diesel engine to compare oxygen enrichment with oxygenation of the fuel. Clarification of the impact of oxygenation and oxygen enrichment can be accomplished by addition of a cetane improver (ethyl hexyl nitrate, EHN) to match the ignition delay of the baseline diesel fuel and the oxygenated fuel whenever necessary, so as to clarify the impacts of oxygen addition on pollutant formation. Experimental Section The experimental system consists of an engine test cell (dynamometer, controller), an engine, combustion analysis instrumentation, and emissions analyzers. Figure 1 presents a schematic diagram of the VW TDI 1.9 L turbodiesel engine and test cell instrumentation. Table 1 presents specifications for the TDI test engine. The facility and experimental procedures for engine operation and emissions tests have been described earlier.29 Steady-state engine operation at 75% load (140 ft-lbs torque) and 1900 rpm was chosen for this study because the effect of oxygenates was found to be more pronounced at medium and high load than at low load. A (26) Marr, W. W.; Sekar, R. R.; Cole. R. L.; Marciniak, T. J.; Longman, D. E. Oxygen-Enriched Diesel Engine Experiments with a Low-Grade Fuel. Society of Automotive Engineers Technical Paper No. 932805, 1993. (27) Schmidt, K.; Gerpen, J. V. The effect of Biodiesel Fuel Composition on Diesel Combustion and Emissions. Society of Automotive Engineers Technical Paper No. 961086, 1996. (28) Donahue, R. J.; Foster, D. E. Effects of Oxygen Enhancement on the Emissions from a DI Diesel via Manipulation of Fuels and Combustion Chamber Gas Composition. Society of Automotive Engineers Technical Paper No. 2000-01-0512, 2000.



Figure 1. Schematic of test engine setup. Table 1. Test Engine Specifications engine

1997 VW Passat 1Z

bore × stroke displacement compression ratio rated power peak torque injection system low idle speed configuration

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

pressure transducer (Kistler 6053C) with 130 kHz frequency response, mounted in the glow-plug hole in the first cylinder, was used with a shaft encoder (Kistler 2611) to provide timeresolved pressure traces for heat-release rate calculations. Pressure data were measured and processed at increments of 0.5 crank angle degrees over 100 consecutive cycles. The variable error in the pressure measurement from 100 cycles during the combustion period is less than 8%, which propagates into the subsequent heat release rate calculations creating a similar level of uncertainty. Therefore, the associated experimental uncertainty has little impact on comparison of heat release rate between the different fuels and resolution of the premixed burn spike. Within the heat release rate program, an ensemble-averaged pressure was used to calculate the apparent heat release rate through a high frequency rejection filtering algorithm, referred to as a Butterworth filter that removes frequencies greater than the threshold frequency. Since this filtering can slightly shift the combustion phase, such as the start of combustion (SOC), an additional correction was made to return to the phase of the original unfiltered data. Determination of injection timing such as dynamic start of injection (SOI) was made on the PC monitor by communicating with the ECM via the onboard diagnostics (OBD) software. Emissions of particulate matter, NOx, CO, CO2, and HC, were measured, respectively, via Sierra Instruments BG1 dilution tunnel, a Nicolet Magna 550 FTIR spectrometer, and a California Analytical HFID hydrocarbon analyzer. (29) Song, J.; Cheenkakorn, K.; Wang, J.; Perez, J.; Boehman, A. L.; Young, P. J.; Waller, F. J. Effect of Oxygenated Fuel on Combustion and Emissions in a Light-Duty Turbo Diesel Engine. Energy Fuels 2002, 16, 294-301.

The intake air flow rate was directly measured via an electronic mass air flow sensor. Fuel consumption rate was recorded via fuel tank weight loss using a precision weighing scale, and real-time fuel consumption rate was also measured using two separate flowmeters (Micromotion D06) on the fuel flow to the engine and fuel return from the engine. For oxygen enrichment of the intake air, modification of the intake air supply line was required. The oxygen was supplied by an oxygen generator (Air Products’ VSA type A-040/120L) into the intake air surge tank through a pressure regulator, a metering valve, and flowmeter. Fumigation into the intake surge tank was chosen to allow thorough mixing of the oxygen-air mixture through the subsequent components of the intake system, such as the air filter, turbocharger, and intercooler, before entering the intake manifold. With known air flow rates at the desired engine operation condition, the required oxygen flow rate was adjusted to obtain the desired volume percent oxygen enrichment. To verify the level of oxygen enrichment, the oxygen-air mixture was sampled at the air filter by using a Rosemount O2 analyzer (Model 755R). Fuel oxygenation was accomplished using two compounds with widely different cetane number and molecular structure. A mixture of glycol ethers, 20 vol % monoglyme and 80 vol % diglyme (referred to as CETANER in several publications3-5,29), with a cetane number of 100 represented a cetane-improving oxygenate. The glycol ethers tested in this study are reported to be potential teratogens, raising health concerns about genotoxicity that causes birth defects.30 In contrast, 1,3dioxolane with a cetane number of 30 represented a cetanesuppressing oxygenate. Both compounds were soluble in diesel fuel and provided a means of distinguishing between cetane number and oxygen addition effects. Figure 2 shows the molecular structures of the oxygenates used in this study. Tables 2-4 show the properties of the reference fuels and their blends.

Results and Discussion Effect of Different Oxygen Addition Methods. The experiments presented here were intended to investigate and compare the relative effect of two oxygen addition methods on diesel combustion and emissions, (30) McGregor, D. B. Genotoxicity of Glycol Ethers. Environ. Health Perspect. 1984, 57, 97-103.


Song et al.

Table 2. Properties of the Oxygenated and Alkane Compounds formula cetane number boiling point (°C) oxygen (wt %) specific density (g/cm3) heating value (MJ/kg) a

base diesel

monoglyme

diglyme

dioxolane

n-heptane

n-dodecane

C15H27.66a

C4H10O2 100 85 36.6 0.867 31.6

C6H14O3 112 162 35.8 0.943 30.3

C3H6O2 30 78 43 1.060 35

C7H16 56 98.4 0 0.684 44.9

C12H26 87.6 216 0 0.749 44.5

55.4 180/370 0 0.833 45.8

This chemical formula is used to calculate oxygen-to-carbon ratio.

Figure 2. Molecular structures of the oxygenates used in the present study: (a) monoglyme, (b) diglyme, and (c) 1,3dioxolane.

Figure 3. Particulate matter emissions for different means of oxygen addition, g/kWh basis. Table 3. Properties of Base Test Fuels base diesel cetane number heating value (MJ/kg) specific density (g/cm3) Saybolt viscosity (SUS) @38 °C sulfur (wt ppm) aromatics (wt %) oxygen (wt %)

55.4 45.8 0.833 34.3 39.0 24.3 0.00

via intake oxygen enrichment and fuel oxygenation. To directly compare both oxygen enrichment of the intake air and oxygenation of the fuel under the same extent of oxygen addition, the oxygen-to-carbon ratio (O/C) in the overall mixture was kept constant. An elementary analysis was performed to match the oxygen-to-carbon ratio on the basis of the overall in-cylinder gas compositions as shown in Table 4, in the same manner as used by Donahue et al.28 As seen in Figure 3, whether it comes from intake oxygen enrichment or via oxygenated fuel, oxygen addition reduces soot, while the extent of PM reduction

Figure 4. Cylinder pressure and heat release rates for different means of oxygen addition at O/C ) 5.1 condition.

varies remarkably with method of oxygen addition. For the equivalent level of oxygen addition based on an oxygen-to-carbon ratio of 5.1, oxygen enrichment of the intake air was as effective for reducing particulate matter emissions as fuel-bound oxygen addition via the glycol ether blends. The difference in PM reduction was not statistically significant between intake air enrichment and the glycol ethers blend, as seen in Figure 3 where the error bars indicate the 90% confidence interval for the data by accumulating all error sources evolved in this test. However, it is evident that fuel oxygenation via the 1,3-dioxolane blends is the least effective for reductions in PM emissions. This significant effectiveness of intake oxygen enrichment might be explained by combined effects of the thermal and chemical interaction arising from intake oxygen addition. The heat release rate in Figure 4 indicates that oxygen enrichment lowered the peak premixed burn, which leads to lower soot precursor formation, without any shift in combustion phase from the baseline. The resulting increased heat release in the mixing-controlled combustion phase will also contribute to increased soot oxidation. A similar trend of heat release characteristics has been observed previously for intake oxygen enrichment by Iida et al.,22 Desai et al.,17 Donahue et al.28 and others. It is well-known that cetane improvers enhance ignition quality by generating a radical pool at a lower temperature than the components in the base fuel. Recent work by Yonei et al. has demonstrated through quantum mechanical calculations of radical decomposition and reaction that the best radical species for improving cetane number are those that promote the generation of an alkyl peroxy radical from an alkyl radical through an oxygen addition reaction.31 This oxygen addition reaction can presumably also be enhanced by having a greater concentration of O2 available. So oxygen enrichment can achieve the



Table 4. Calculation of Diluted Properties of Base Fuel and Blended Fuels at O/C ) 4.8 Condition fuel blends

blend ratio (wt %)

O/C

mol % of blend

C% of blend

aromatics (wt %)

sulfur (ppm)

Mair (g/s)

Mf (g/s)

base fuel glycol ethers n-heptane/n-dodecane dioxolane n-heptane

15 15 10 10

4.6 4.8 4.6 4.8 4.6

17.9 15.0 23.8 18.7

9.8 14.7 5.9 9.7

24.3 20.6 20.6 21.9 21.9

39 33 33 35 35

39.9 39.9 39.9 39.9 39.9

1.735 1.823 1.629 1.757 1.629

Table 5. Combustion Event Analysis for Only Different Oxygenate Blends at O/C ) 5.1 Condition fuel blends

dynamic SOI (ATDC)

SOC (ATDC)

ignition delay (CA)

base fuel 2 vol % O2 addition 30 wt % glycol ethers 20 wt % dioxolane

-4.7 -4.7 -5.9 -5.7

-0.8 -0.9 -1.9 -1.2

3.9 3.8 4.0 4.5

same end as a cetane improver, by shifting the explosion limits for the reactant mixture to lower temperature and lower pressure. Another possible explanation is that the presence of additional OH and O in the mixingcontrolled phase will more likely attack the soot precursors, which results in lower acetylene concentration as postulated by Gu¨lder.32 The free oxygen supplied by the intake air can be the key species affecting the soot formation chemistry, thereby reducing soot formation and growth and increasing the oxidation of the soot. Table 5 presents fuel injection timing changes that occurred with the different test fuels. The oxygenated fuels were seen to advance the start of injection timing in Figure 4 and Table 5 because the engine maintains power at the requested level through injection timing adjustment to compensate for the higher density and lower heating value of the oxygenated fuel as compared to the baseline fuel, as summarized in Table 3. Specifically, injection timing for the glycol ether blend was more advanced than that for 1,3-dioxolane, but the high cetane number of the glycol ether blend compensates this advanced injection timing so that ignition delay was similar to the baseline and 23% intake oxygen enrichment, as shown in Table 5. For the same ignition delay, the glycol ether blend was also observed to lower the peak premixed burn as with intake oxygen enrichment, which leads to the lower soot formation. The heat release characteristics of the glycol ethers shown in Figure 4 are consistent with the observations reported by Beatrice and co-workers, that the premixed burn rate decreases while the diffusion heat release rate increases at high load. The higher diffusion heat release was attributed to increased oxidation of the combustion intermediates by oxygen present in the additive.33 Litzinger et al. also reported the same trend of a lower premixed burn rate when diglyme is blended with diesel.1,34 (31) Yonei, T.; Hashimoto, K.; Arai, M.; Tamura, M. Quantum Chemical Study of Cetane Improvers. Energy Fuels 2003, 17, 725730. (32) Gu¨lder, O. L. Effects of Oxygen on Soot Formation in Methane, Propane and n-Butane Diffusion Flames. Combust. Flame 1995, 101, 302-310. (33) Bertoli, C.; Del Giacomo, N.; Beatrice, C. Diesel Combustion Improvements By the Use of Oxygenated Synthetic Fuels. Society of Automotive Engineers Technical Paper No. 972972, 1997. (34) Stoner, M.; Litzinger, T. Effects of Structure and Boiling Point of Oxygenated Blending Compounds in Reducing Diesel Emissions. Society of Automotive Engineers Technical Paper No.1999-01-1475, 1999.

Figure 5. NOx emissions for different means of oxygen addition, g/kWh basis.

On the other hand, the dioxolane blend produces a longer ignition delay compared to the glycol ether blend so that a greater premixed burn occurs, which is one of the contributing factors for reducing soot. This longer ignition delay was expected since the dioxolane blend has a lower boiling point and lower cetane number, in addition to yielding advanced injection timing. An increased ignition delay presumably could account for a portion of the soot reduction observed with dioxolane, since the longer ignition delay gives the fuel and air time to mix, thereby resulting in leaner combustion during the fuel-rich premixed phase.11 Furthermore, the high volatility of the dioxolane blend may also contribute to the reduced soot formation since the formation of a fuelrich mixture may not be as likely in the fuel spray plumes due to enhanced air entrainment.9,34 Therefore, the soot reduction with the dioxolane blends may result directly from leaner combustion through fuel-air mixing, rather than through oxygen addition to the fuel. Consequently, for dioxolane the oxygen content is far less effective for reducing soot. In the recent oxygenated fuel study by Litzinger et al.,1 they postulated that a key issue in understanding possible effects of the oxygenated compounds is the chemical structure of parent fuel molecules from which the oxygen (or oxygenbearing fragments) will be released when these compounds decompose. So differences in the molecular structure of the oxygenated fuels can partly explain the differences in PM reduction between the glycol ether and dioxolane blends, as will be discussed in the next section. As shown in Figure 5, the unfavorable effect of oxygen addition on NOx emissions becomes greater in the order of 1,3-dioxolane, glycol ethers and intake oxygen enrichment. This trend follows the typical PM-NOx tradeoff behavior, i.e., the lower the PM, the higher the NOx.35 In other words, the increased oxygen concentration (35) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill: New York, 1988.


Figure 6. CO emissions for different means of oxygen addition, g/kWh basis.

causes a NOx increase in all cases, but the extent of the increase depends strongly on how the oxygen is added. Intake oxygen enrichment leads to increases in NOx emissions by 53 and 126% with respect to baseline for 22 vol % and 23 vol % oxygen concentration, respectively. Considering that NOx is more likely to form in a post flame zone where the mixing controlled regime dominates, heat release in the mixing controlled phase of combustion might be one of the key factors. Under the present engine operation where mixing controlled combustion occurs, the higher heat release rate of the mixing controlled combustion phase leads to higher temperatures for the intake oxygen enrichment case shown in Figure 4. This thermal effect contributes to the NOx increase in combination with the local φ (equivalence ratio) effect. According to local φ theory, the available oxygen in the high-temperature flame region may shift this locally rich mixture toward a slightly lean mixture where the maximum equilibrium O-atom mole fraction lies.28 Consequently, the high equilibrium concentration of oxygen atom along with super-equilibrium O-atom results in higher kinetically formed NOx. Van Gerpen et al. have also noticed a NOx increase with oxygen addition to the intake air.27 It has been speculated that the higher oxygen concentration may increase the flame temperature in otherwise fuelrich regions of the combustion chamber, which increases the rate of formation of NOx. The incorporation of the fuel-bonded oxygen mitigates the adverse impact of leaner combustion conditions compared to free oxygen atom from intake oxygen enrichment, causing a smaller NOx increase compared to oxygen-enriched intake air. The glycol ether blend yields a moderate NOx increase of 10 and 23% with respect to the baseline for 15 wt % and 30 wt % glycol ether addition. The dioxolane blends yield an even smaller NOx increase of 7 and 15% with respect to the baseline for 10 wt % and 20 wt % dioxolane addition. This less pronounced effect of dioxolane blends on NOx emissions may be due to a reduced temperature in the mixing controlled regime with oxygen addition and less oxygen being liberated due to differences in molecular structure compared to the glycol ether blends, as explained earlier. Figure 6 shows that trends in CO emissions correlated well with PM emissions. The effectiveness of CO reduction is similar to that of PM reduction, in the order of intake air enrichment, glycol ethers, dioxolane, although

Song et al.

Figure 7. Cylinder pressure and heat release rates for comparison of oxygen addition effects with glycol ether blend and the equivalent alkane blend (O/C)4.8).

there is an anamolous result with the 10 wt % dioxolane blend. The reduced CO emissions also support the idea that there is a higher temperature in the mixingcontrolled regime, which will enhance the oxidation of any soot, CO, and unburned hydrocarbon. Isolating the Effect of Oxygen Addition. Tests with alkane addition at the same levels as the addition of glycol ethers and dioxolane were conducted to first isolate the dilution effect of base fuel components, such as the aromatic and sulfur content. The alkane blends were selected to secondarily match the boiling points of the oxygenated blend components, and thereby to examine the volatility effect on soot formation by comparison with the baseline results. Whenever matching of the combustion phase between the alkane blend and the oxygenate blend is necessary, an addition of EHN was employed. As summarized in Table 4, 15 wt % glycol ether blend addition and 10 wt % dioxolane addition were required for O/C ) 4.8 tests. n-heptane and n-dodecane were selected to match the volatility of the oxygenated blends. For example, a mixture of 20 wt % n-heptane and 80 wt % n-dodecane was blended into the base diesel fuel by 15 wt % to match the 15 wt % glycol ether blends, which is a mixture of 20 wt % monoglyme and 80 wt % diglyme. Thus, the observed change between the oxygen addition (glycol ethers or dioxolane blends) and no oxygen addition (alkane blends) might be attributed to the combined effects of thermal and chemical suppression resulting from the chemical structure of the oxygenated fuels. Furthermore, if there is no significant shift in premixed and mixing controlled phases of heat release between the oxygenated fuel blends and alkane blends, then any soot reduction is attributable to only the chemical effect from oxygenate addition. Figure 7 shows that there is little difference in combustion pressure and peak burn rate between the 15 wt % glycol ether blend and the equivalent alkane blend. Unlike the earlier heat release rate comparison at higher oxygen-to-carbon ratio (O/C ) 5.1), this lower oxygen-to-carbon ratio (O/C ) 4.8) caused no change in heat release rate between the oxygenated and nonoxygenated fuel blends, as summarized in Table 6. Therefore, changes in heat release are not expected to be major factors in the comparison of PM reduction between the glycol ether blends and the alkane blends.


Figure 8. Oxygen addition effect of glycol ethers blend (O/C)4.8) on PM, NOx and CO emissions, g/kWh basis.


Figure 10. Cylinder pressure and heat release rates for comparison of oxygen addition effects with 1,3-dioxolane blend and the equivalent alkane blend (O/C ) 4.8).

Figure 9. Oxygen addition effect of 1,3-dioxolane blend (O/C ) 4.8) on PM, NOx and CO emissions, g/kWh basis. Table 6. Combustion Event Analysis for Different Oxygenate and Alkane Blends at O/C ) 4.8 Condition

fuel blends base fuel 1 vol % O2 addition 15 wt % glycol ethers 10 wt % dioxolane 15 wt % n-heptane/n-dodecane 10 wt % n-heptane

ignition dynamic SOI SOC delay (ATDC) (ATDC) (CA) -4.7 -4.7 -5.3 -5.3 -5.3 -5.1

-0.8 -0.9 -1.4 -1.0 -1.4 -1.2

3.9 3.8 3.9 4.3 3.9 3.9

As a result, the reduction of PM emissions by 12% shown in Figure 8 between the baseline fuel and the alkane blend can be attributed to overall effects of heat release and dilution by fuel property changes from the alkane addition. The reduction of soot by alkane addition compared to the baseline fuel also may be caused by lowering of the boiling range which alters the fuelair mixture preparation as discussed earlier. Finally, the additional 20% PM reduction for the glycol ether blend can be attributed to the chemical effect of oxygen addition to the fuel, where again it should be noted that there were no distinct changes in heat release between the alkane and glycol ether blends. NOx and CO emission do not vary significantly with oxygen addition via the glycol ethers when compared to the alkane blend. The above reasoning can be applied to explain the chemical effect of oxygen addition for the dioxolane blends as well. As seen in Figure 9, PM and CO emissions with alkane addition (n-heptane) differ very little from the baseline fuel, whereas NOx increases by

Figure 11. Cylinder pressure and heat release rates for different means of oxygen addition at O/C ) 4.8 condition.

8%. Considering the higher volatility of dioxolane, less PM emissions are expected compared to the base fuel as in the case of the glycol ether blends, but the result is the opposite. With no distinct difference in heat release between the dioxolane blend and the alkane blend as shown in Figure 10, the PM emission results indicate that chemical effect of oxygen addition via dioxolane blends is modest, yielding only 5% PM reduction and only a slight change in NOx and CO emissions. For a direct comparison, Figure 11 shows that there is no distinct difference in heat release rate between the 15 wt % glycol ether and 10 wt % dioxolane blends. Recalling that both blends dilute the base fuel an equivalent amount in terms of oxygen-to-carbon ratio allows us to directly compare PM emission effects on the basis of the chemical structure of oxygenate. Therefore, the effect on the soot reduction by oxygenates depends on the chemical structure of the oxygenate. In this study, the linear glycol ethers were more effective for PM reduction than dioxolane, which contains a ring structure. This result is consistent with the recent result of Mueller and co-workers where DBM (ester group) can directly produce CO2, thereby wasting some of its oxygen, while TPGME(ether group) does not directly produce CO2, and its chemistry does not produce as many unsaturated species that can lead to soot.8,9


Conclusions While oxygen enrichment of intake air reduces diesel PM significantly, fuel oxygenation can provide PM reduction with only a modest affect on NOxemissions. With their linear structure, the glycol ether mixture considered here was shown to be far more effective for soot reduction than equivalent oxygen addition via dioxolane with its ring structure. The observations related to oxygenated fuels and oxygen enrichment reported here are seen to be consistent with much of

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the literature. On one point of continuing debate, the present work provides further evidence that fuel oxygenation by certain oxygenates with particular structures can provide larger soot suppression. Acknowledgment. We thank Air Products and Chemicals, Inc., and especially Tarik Nahieri and P. John Young for supporting this research project. EF034103P

Comparison of the Impact of Intake Oxygen Enrichment and Fuel

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