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Jul 13, 2004 - for all land-based military aircraft, vehicles, and equipment during war and peace times. This is referred to as the single fuel forwar...
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Energy & Fuels 2004, 18, 1302-1308

Comparative Environmental Evaluation of JP-8 and Diesel Fuels Burned in Direct Injection (DI) or Indirect Injection (IDI) Diesel Engines and in a Laboratory Furnace Constantinos D. Rakopoulos, Dimitrios T. Hountalas, and Dimitrios C. Rakopoulos Mechanical Engineering, National Technical University of Athens, Athens 15780, Greece

Yiannis A. Levendis* Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts 02115 Received March 18, 2004. Revised Manuscript Received June 2, 2004

In recent years, NATO and U.S. military forces have decided to implement a single fuel (JP-8) for all land-based military aircraft, vehicles, and equipment during war and peace times. This is referred to as the single fuel forward concept. However, possible environmental pollution problems have been identified in the process of implementing this single fuel concept. This work presents data on the operational and, especially, environmental evaluation of the JP-8 fuel that is burned in diesel engines and furnaces. The investigation is conducted in a comparative manner between the JP-8 fuel and the proverbial Diesel Oil No. 2, which is being substituted. The two fuels were burned under identical conditions in disparate types of combustors, including direct injection (DI) and indirect injection (IDI) diesel engines as well as a laminar-flow muffle furnace. The primary goal was to contrast the emissions of the two fuels, because JP-8 is a substitute for diesel oil in military applications, such as engines of ground vehicles and power generators, as well as other burners. Regulated emissions of CO, particulate matter (soot), oxides of nitrogen, and total unburned hydrocarbons have been examined. In the case of the combustion of the two fuels in the furnace, unregulated organic compounds have also been monitored. Moreover, the operational behaviors of the two fuels in the engines have been discussed. Results demonstrated that, for both fuels, the magnitude of the emissions, as well as the trends with operating conditions, were influenced by the type of combustor. However, the emissions from the two fuels, when burned in the same combustor, were comparable overall, both qualitatively and quantitatively.

Introduction JP-8 is a kerosene-based fuel that is blended with specific additives to make it suitable for use in military applications. Although, initially, it was used as an aviation fuel, its applications have been expanded to include ground vehicles as well as all types of dieselengine-powered systems and furnaces. This is because both the U.S. and the NATO military forces have decided to simplify their fuel transport and distribution operations through the use of a single fuel.1 JP-8 is approximately 99.8% kerosene by weight1,2 and is a complex mixture of higher-order hydrocarbons, including alkanes, cyclo-alkanes, and aromatic molecules. JP-8 contains three mandatory additives: a fuel system icing inhibitor, a corrosion inhibitor, and a static dissipator * Author to whom correspondence should be addressed. Telephone: 617-373-3806. Fax: 617-373-2921. E-mail address: [email protected]. (1) JP-8, the Single Fuel Forward, an Information Compendium. U.S. Army Tank Automotive and Armaments Command, Warren, MI, May 2001. (2) Aviation Fuels Technical Review (FTR-3), Chevron Products Company, 2000.

additive.3 Because JP-8 fuel is replacing the familiar Diesel Oil No. 2 in a variety of combustors that are operating in the proximity of military personnel and civilians, there have been concerns in regard to the toxicity of the fuel itself,4-10 as well as about the nature and toxicity of its combustion products.8 Published results from exposure and toxicity studies have been summarized by Topal et al.11 (3) Annual Energy Outlook 2002 with Projections to 2020. U.S. Department of Energy and National Energy Information Center, DOE/ EIA 0383, December 2001. (4) http://www.cjskincover.com/army_times.asp. (5) http://www.cbsnews.com/stories/2002/01/09/60II/main323782. shtml. (6) http://www.jp8.org/JPoverview.htm. (7) Ritchie, G. D.; Bekkedal, M. Y. V.; Bobb, A. J.; Still, K. R. Naval Health Research Center, Report No. TOXDET 01-01, 2001. (8) Pleil, J. D.; Smith, L. B.; Zelnick, S. D. Environ. Health Perspect. 2000, 108, 183. (9) Dudley, A. C.; Peden-Adams, M. M.; EuDaly, J.; Pollenz, R. S.; Keil, D. E. Toxicol. Sci. 2001, 59, 251. (10) Material Safety Data Sheet, Hovensa L.L.C., St. Croix, Virgin Islands, 1999. (11) Topal, M. H.; Wang, J.; Levendis, Y. A.; Carlson, J. B.; Jordan, J. PAH and Other Emissions from Burning of JP-8 and Diesel Fuels in Diffusion Flames. Fuel 2004, in press.

10.1021/ef049931c CCC: $27.50 © 2004 American Chemical Society Published on Web 07/13/2004

Environmental Evaluation of JP-8 and Diesel Fuels

Only a limited number of studies on the emissions from the combustion of JP-8 can be found in the literature. Because this fuel is composed of hundreds of compounds, which vary from purchase to purchase,6-9 researchers have often studied the combustion of JP-8 surrogates. Emissions from kerosene-based aviation fuels have been investigated in gas turbines and other combustors,12-17 and blends of pure hydrocarbons have been burned as surrogates to JP-8 to facilitate combustion modeling.18-20 Specifically in regard to JP-8 combustion and emissions, Kobayashi and Kikukawa21 found higher concentrations of formaldehyde, which is a severe eye and respiratory irritant, in the exhaust of jet engines after their fuel was changed from JP-4 to JP-8. Conkle et al.22 detected and identified 273 organic compounds at the exhaust of a turbine combustor that was burning JP-8 fuel. Zhu and Cheng23 investigated human exposure to aerosols from unvented heaters (burning JP-8, JA-1, and 1-K kerosene) in tents, in an effort to assess the contribution of this exposure to the “Gulf War Syndrome”. Particulate emissions were sampled, characterized for particle size distributions, and chemically analyzed. Elemental and organic carbons were detected, as well as large amounts of sulfur. Amounts of fine particles that can be deposited in the lungs were reported. Tichenor et al.24 investigated the effects of using JP-8 in heating-plant boilers, as a substitute for Diesel Oil No. 2. They reported that the use of JP-8 was associated with lower stack emissions of SOx as well as NOx and particulates. There was negligible difference between the organic emission measurements in full-scale burn tests of the two fuels. Yost et al.25 reported on diesel engine exhaust emissions from burning JP-8 fuels with various amounts of sulfur and a commercial higher-sulfur-content diesel oil, whereas Arkoudeas et al.26 reported on diesel engine exhaust emissions from burning JP-8 fuel blended with various amounts of bio-diesel fuels (sunflower oil and olive oil). Kouremenos et al.27 conducted a comprehensive experimental investigation on the performance and exhaust emissions of a swirl chamber indirect injection (12) Treynor, G. W.; Apte, M. G.; Sokol, H. A. Environ. Sci. Technol. 1990, 24 (8), 1265. (13) Vovelle, C.; Delfau J.-L.; Reuillon, M. Springer Ser. Chem. Phys. 1994, 59, 50. (14) Dagaut, P.; Reuillon, M.; Boettner J.-C.; Cathonnet, M. Proc. Combust. Inst. 1994, 24, 919. (15) Patterson, P. M.; Kyne, A. G.; Pourkashanian, M.; Williams, A.; Wilson, C. W. J. Propul. Power 2001, 16 (2), 453. (16) Zhou, Y.; Cheng, Y.-S. Aerosol Sci. Technol. 2000, 33, 510. (17) Geldermann, J.; Gabriel, R.; Rentz, O. Environ. Sci. Pollut. Res. Int. 1999, 6, 115. (18) Shultz, W. D. Presented at the Symposium on Structure of Jet Fuels III, The Division of Petroleum Chemistry, American Chemical Society, San Francisco, CA, April 5-10, 1991. (19) Edwards, T.; Maurice, L. Q. J. Propul. Power 2001, 17 (2), 461. (20) Violi, A.; Yan, S.; Eddings, E. G.; Sarofim, A. F.; Granata, S.; Faravelli, T.; Ranzi, E. Combust. Sci. Technol. 2002, 174, 399. (21) Kobayashi, A.; Kikukawa, A. Aviat., Space Environ. Med. 2000, 71 (4), 396. (22) Concle, J. P.; Lackey, W. W.; Martin, C. L.; Miller, R. L. Proceedings of the International Conference on Environmental Sensing Assessment; IEEE: New York, 1975; Vol. 2, p 11. (23) Zhu, Y.; Cheng, Y.-S. Aerosol Sci. Technol. 2000, 33 (6), 510. (24) Tichenor, L. B.; Shaaban, A. H.; Mayfield, H. T. NTIS Report No. AFESC/ESL-TR91-46, 1991. (25) Yost, D. M.; Montalvo, D. A.; Frame, E. A. SAE Tech. Pap. Ser. 1996, 961981. (26) Arkoudeas, P.; Kalligeros, S.; Zannikos, F.; Anastopoulos, G.; Karonis, D.; Korres, D.; Lois, E. Int. J. Energy Convers. Manage. 2003, 44 (7), 1013. (27) Kouremenos, D. A.; Rakopoulos, C. D.; Hountalas, D. T. Int. J. Energy Res. 1997, 21 (12), 1173-1185.

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(IDI) diesel engine that was burning JP-8. Nitrogen oxides, unburned light hydrocarbons, carbon monoxide (CO), and smoke emissions were monitored under a variety of engine operating conditions. They contrasted such emissions against those from baseline engine operation using Diesel Oil No. 2. Moreover, Kouremenos et al.28 presented preliminary results on the operational characteristics and emissions of direct injection (DI) diesel engines that were burning JP-8 fuel. This work extends that of Kouremenos et al.28 on the combustion characteristics and emissions of JP-8 aviation fuel in DI diesel engines. It provides a comprehensive comparison of the emissions of JP-8 with those of Diesel Oil No. 2, based on experiments emphasizing DI and, to a lesser extent, IDI single-cylinder laboratory diesel engines. New data collected in this laboratory (NTUA), as well as previously published data, are used. Insight into emission trends with operating parameters of the DI diesel engine is obtained using a theoretical engine combustion model (described below). Comments on the operating parameters of the engines with the two fuels are also made. Results are contrasted with those of Yost et al.25 in larger-scale multicylinder engines using JP8 fuels. Moreover, to examine the emissions of the two fuels burned in other types of combustors, results are qualitatively contrasted with those from a recent study by Topal et al.11 They burned pools of the same two fuels in a laboratory muffle furnace under strongly sooting laminar diffusion-flame conditions, and they measured both regulated and unregulated pollutants (air toxics). By presenting and discussing the emissions from the two fuels burned under such disparate conditions, it is intended to comment and partially conclude on the environmental impact of the ongoing substitution of the proverbial Diesel Oil No. 2 with the JP-8 fuel in engines of military ground vehicles, power generators, and other burners and furnaces. Experimental Apparatus and Procedure Fuel Specifications. The fuels used in this study were JP-8 aviation fuel (average molecular formula C11H21) and Diesel Oil No. 2 (average molecular formula C12H26). The JP-8 aviation fuel (kerosene) had a specific gravity of 0.79, a lower heating value of 42 500 kJ/kg, a sulfur content of 0.15 wt %, a cetane index of 47, and a kinematic viscosity of 0.85 cSt at 100 °C and 1.65 cSt at 40 °C. The Diesel Oil No. 2 had a specific gravity of 0.83, a lower heating value of 42 500 kJ/kg, a sulfur content of 0.30 wt %, a cetane index of 52, and a kinematic viscosity of 1.1 cSt at 100 °C and 2.7 cSt at 40 °C. Direct Injection (DI) Diesel Engine. A single-cylinder, naturally aspirated, four-stroke, air-cooled, direct-injection Lister LV1 diesel engine was run on a test bed. The engine has a bore of 85.73 mm, a stroke of 82.55 mm, and a rod length of 180 mm. The compression ratio is 18 and the normal operation speed range is 1000-3500 rpm. A Bryce highpressure fuel pump, with a 6.5-mm-diameter plunger, is connected to the three-hole injector nozzle (each hole having a diameter of 0.25 mm), which is located in the middle of the combustion chamber head. The injector nozzle opening pressure is 190 bar. The combustion chamber of the engine is of the bowl-in-piston design. The engine was coupled to a Heenan & Froude hydraulic dynamometer. Further details of the (28) Kouremenos, D. A.; Rakopoulos, C. D.; Hountalas, D. T. Performance and Emissions of a High-Speed Direct Injection Diesel Engine Operating with JP-8 Fuel. Presented at the 1st European Conference on Clean Cars, May 15-17, 1997.

1304 Energy & Fuels, Vol. 18, No. 5, 2004 experimental setup are given by Rakopoulos.29 The static injection timing was kept constant at 26° CA BTDC (degrees crank angle, before top dead center), which, on average, gives the best performance over the speed range of interest, at least when using diesel fuel. To have comparable results when using either diesel fuel or JP-8 fuel, the air inlet temperature was constant for all experiments at 23 °C. The engine was operated at combinations of four loads corresponding to 20%, 40%, 60%, and 80% of full load, and three speeds (1500, 2000, and 2500 rpm), i.e., a total of 12 runs for each fuel. During the tests, cylinder pressure diagrams, high-pressure fuel pipe pressures, exhaust gas temperatures, fuel consumption, exhaust smokiness, and exhaust gas emissions were recorded. Indirect Injection (IDI) Diesel Engine. A single-cylinder, water-cooled, Ricardo E-6 experimental engine was installed on a test bed. The engine was operated as a naturally aspirated four-stroke diesel engine fitted with a Comet MK.V turbulence combustion chamber head. The engine bore was 76.2 mm, and the stroke was 111.1 mm. The normal speed range is 10003000 rpm. In the tests described herein, the compression ratio was 20. The CAV fuel injection pump was fitted with a 6-mmdiameter plunger. A CAV injector body with a pintle-type nozzle, which opens at a pressure of 110 bar, was used for the fuel injection into the pre-chamber. The engine was coupled to a Laurence-Scott NS-type swinging-field ac dynamometer. The static injection timing was kept constant at 38° CA BTDC, which, on average, gives the best performance over the speed range of interest, at least when using diesel fuel. To have comparable results when using either diesel fuel or JP-8 fuel, the air inlet temperature was kept constant for all experiments at 23 °C, the lubricating oil temperature at 65 °C, and the exhaust water temperature at 67 °C. Further experimental details are given by Rakopoulos29 and Kouremenos et al.27 The engine was run at combinations of four loads (50%, 66%, 83%, and 100% of full load) and three speeds (1000, 1500, and 2000 rpm), i.e., a total of 12 runs for each fuel. Again, during the tests, cylinder pressure diagrams, high-pressure fuel pipe pressures, exhaust gas temperatures, fuel consumption, exhaust smokiness, and exhaust gas emissions were recorded. Engine Exhaust Gases Monitoring Apparatus. For both the aforementioned DI and IDI diesel engines, exhaust gas analyzers were used to measure, at the tail pipe, smoke, nitrogen oxides (NOx), total unburned hydrocarbons (HC) (equivalent propane), and CO emissions. A Bosch EFAW65 smoke meter was used to measure smoke levels in the exhaust gases, NOx emissions were measured with a Signal chemiluminescent analyzer, and the HC emissions were measured with a Signal flame-ionization detector (FID). The last two devices were fitted with thermostatically controlled heated lines. The CO emission was measured with a Signal nondispersive, infrared analyzer. Laboratory Muffle Furnace. A two-stage laboratory setup was also used to investigate emissions from batch combustion of these two fuels (see Topal et al.11). A pool of the liquid fuel (0.5 g), contained in a porcelain boat, was inserted at midlength of a 1-kW horizontal, split-cell, electrically heated muffle furnace (4 cm inner diameter and 87 cm long) fitted with a quartz tube. Upon ignition, combustion of the fuel occurred inside this primary furnace in a transient and sooty diffusion flame. Nonluminous combustion reactions continued in a secondary muffle furnace (afterburner, 2 cm inner diameter and 38 cm long) that was connected in series to the primary furnace, as described by Topal et al.11 The effluents of both furnaces were monitored simultaneously and detailed results are presented in ref 11; herein, average values are presented. The air flow rate in this primary furnace was 4 L/min, and its temperature was varied between 600 °C and 1000 °C. The secondary furnace was operated at 1000 °C. Furnace Emissions Monitoring. Regulated and unregu(29) Rakopoulos, C. D. Renew. Energy 1992, 2 (3), 327-331.

Rakopoulos et al. lated emissions including CO, carbon dioxide (CO2), NOx, particulate matter, and light hydrocarbon species, as well as semivolatile polycyclic aromatic hydrocarbons (PAH) from the combustion of JP-8 and Diesel Oil No. 2, were monitored at the exits of the two furnaces. PAH and particulates were collected using Graseby sampling heads, each equipped with a filter stage and a glass cartridge that contained Supelco XAD-4 adsorbent, as the effluent passed through. A Beckman 951A chemiluminescent NO/NOx analyzer was used to monitor NOx; a Rosemount Analytical 590 UV was used to monitor SO2; Horiba infrared analyzers were used to monitor CO/CO2; and a Beckman 350 paramagnetic analyzer was used to monitor O2. Particulate matter collected on cellulose filter papers was measured gravimetrically. Afterward, a Dionex ASE 200 accelerated solvent extractor was used to extract the semivolatile organic compounds from the XAD-4 resins and from the filter papers using methylene chloride. The samples were then analyzed by a Hewlett-Packard (HP) Model 6890 GC equipped with a HP Model 5973 mass selective detector. Details of the rather involved extraction procedure and of the conditions of the gas chromatography-mass spectroscopy (GC-MS) analysis are given by Topal et al.11 Samples were also withdrawn from the effluent with glass syringes, fitted with needles, and analyzed for volatile hydrocarbons via GC, using a Hewlett-Packard instrument (Model HP 6890) that was equipped with an FID.

Results and Discussion Direct Injection (DI) Diesel Engine Operating Characteristics and Performance. As the load (torque output) of the engine was increased from 20% to 80% of the full engine load, the brake specific fuel consumption (BSFC, i.e., the fuel consumption normalized by the power output) of this engine decreased monotonically at all speeds (low, medium, high) for both fuels. The BSFC is inversely proportional to thermal efficiency; therefore, this trend reflects an equivalent monotonic increase in efficiency with load. Little, if any, difference in the BSFC values for the two fuels was observed. Moreover, the maximum cylinder pressure (peak combustion pressure) and the exhaust temperature of the engine increased (almost linearly) with load. Maximum pressures and exhaust temperatures were, overall, commensurate for both fuels, whereas at high speeds, the former was slightly lower and the latter slightly higher for JP-8. The ignition delay decreased as the load increased, as higher equivalence ratios rendered the mixture easier to ignite. JP-8 fuel exhibited lengthier ignition delays in most cases, which was attributed to its lower cetane number. Apart from a noticeable increase in the residual pressure in the fuel pipe at low engine speeds with JP-8, caused by its lower density and viscosity, no other discernible differences in engine operation with the two fuels were found. Direct Injection (DI) Diesel Engine Emissions. Exhaust particulate matter (mostly soot) emissions increased as the engine load increased, because both the local and global equivalence ratios {φ ) (mf/mair)actual/ (mf/mair)stoichiometric} increased; overall, they also increased with speed, but this increase was less pronounced and was not monotonic (see Figure 1). In many cases (power settings), particulate matter emissions from JP-8 were higher, especially at high speeds. NOx emissions consistently increased with load, as combustion temperatures increased with more fuel injected in the combustion chamber (see Figure 1); they decreased

Environmental Evaluation of JP-8 and Diesel Fuels

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Figure 2. Results from the simulation model, showing the calculated local equivalence ratio in the spray zone (based on the current fuel burning and the corresponding available air), against the crank angle, during the combustion event. The DI diesel engine operation was simulated at a speed of 2000 rpm and four loads of 20%, 40%, 60%, and 80%.

Figure 1. Exhaust emissions of soot, oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) from the combustion of JP-8 and Diesel Oil No. 2 in an experimental single-cylinder direct injection (DI) diesel engine. The engine was operated at 20%, 40%, 60%, and 80% of full load, and speeds of 1500, 2000, and 2500 rpm.

as the engine speed increased. NOx emissions were comparable for both fuels, except at high speeds, where JP-8 resulted in somewhat lower values, because its late ignition resulted in lower combustion temperatures. Unburned hydrocarbon emissions increased mildly as the load increased (see Figure 1), whereas they were largely unaffected by engine speed. CO exhibited mixed trends with engine load (see Figure 1) and speed. Combustion of JP-8 fuel generated higher unburned hydrocarbon emissions than diesel oil but, in most cases, lower CO emissions. Theoretical Calculations for the Direct Injection (DI) Diesel Engine. A two-zone model for the approximation of the closed cycle of a DI diesel engine30,31 was used to calculate engine temperatures, as well as O2 and CO2 mole fractions, which were not measured experimentally. CO and nitric oxide (NO) concentrations were also calculated, to compare with the experimental results. Most importantly, the model was used to elucidate the observed NOx emission trends with load and especially with speed. The cylinder contents were taken to comprise a nonburning zone of air, and another homogeneous zone in which fuel is continuously supplied from the injector holes during injection and burned with entrained air from the air zone. The growth of the (30) Rakopoulos, C. D.; Rakopoulos, D. C.; Kyritsis, D. C. Int. J. Energy Res. 2003, 27 (14), 1221. (31) Rakopoulos, C. D.; Rakopoulos, D. C.; Giakoumis, E. G.; Kyritsis, D. C. Int. J. Energy Convers. Manage. 2004, 45 (9-10), 1471.

fuel spray zone, consisting of several fuel-air conical jets equal to the injector nozzle holes, was carefully modeled by incorporating jet mixing to determine the amount of oxygen available for combustion. Application of the mass, energy, and state equations in each one of the two zones yielded local temperatures and cylinder pressure histories. To calculate the concentration of constituents in the exhaust gases, a chemical equilibrium scheme was adopted for the C-H-O-N system of the eleven species considered, together with chemical rate equations for the calculation of NO and CO. A model for the evaluation of soot formation and oxidation rates was incorporated.32 Satisfactory comparisons have been reported between the theoretical results from the computer program implementing this analysis and experimental results from a vast experimental investigation using a versatile, experimental direct-injection Ricardo Hydra diesel engine, following a multiparametric study of the constants incorporated in the various submodels.30,31 Sample results from these simulations are shown here, at a speed of 2000 rpm, for the four tested loads of 20%, 40%, 60%, and 80%. Figure 2 shows the calculated local equivalence ratio in the spray zone (based on the current fuel burning and the corresponding available air) against the crank angle during the combustion event. The shape in this figure is very similar to the heat release curve expected from a DI diesel engine, with the first peak corresponding to the (early) premixed mode combustion and the second one corresponding to the diffusion-mode combustion.33 This similarity is due to the uniform value of the actual fuel/ air ratio in the former mode and to the small changes in the available air, which do not appreciably distort the shape of the heat release curve. For the same conditions during this simulated closed engine cycle, Figure 3 depicts the corresponding calculated spray zone (flame) temperatures increasing with load, Figure 4 shows the calculated mole fractions of molecular oxygen (32) Hiroyasu, H.; Kadota, T.; Arai, M. Bull. JSME 1983, 26, 569. (33) Ferguson, C. R. Internal Combustion Engines; Wiley: New York, 1986.

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Figure 3. Results from the simulation model, showing the calculated flame temperature in the spray zone against the crank angle. The DI diesel engine operation was simulated at a speed of 2000 rpm and four loads of 20%, 40%, 60%, and 80%.

Figure 4. Results from the simulation model, showing the calculated average mole fractions of molecular oxygen (O2) and CO2 in the cylinder against the crank angle. The DI diesel engine operation was simulated at a speed of 2000 rpm and four loads of 20%, 40%, 60%, and 80%.

(O2) and CO2, and Figure 5 shows the CO mole fraction. Figure 5 illustrates the freezing of CO in the expansion stroke, which is due to the effect of chemical kinetics,33 whereas the corresponding exhaust values are fairly close to the measured values. On the other hand, Figure 6 shows results from the simulation model concerning calculated NO concentration against the crank angle, for the engine operated at (all) four loads of 20%, 40%, 60%, and 80% and (all) speeds of 1500, 2000, and 2500 rpm. Again, the freezing of NO in the expansion stroke is evident, because of the respective effect of chemical kinetics,33 whereas the increase of NO concentration with load is due to the corresponding temperature increases, with the calculated exhaust values being close to the measured values. Note that the model captures correctly the decrease of NO with increasing speed for the same load, at least inside the range examined, a fact that is attributed to the prevailing influence of the dynamic injection timing decreasing as speed increases.34,35 Note that, although the exhaust NOx (i.e., NO + NO2) emissions are measured, the NO emission

Rakopoulos et al.

Figure 5. Results from the simulation model showing the calculated CO average mole fraction in the cylinder against the crank angle. The DI diesel engine operation was simulated at a speed of 2000 rpm and four loads of 20%, 40%, 60%, and 80%.

is calculated by the model and compared with experimental values. This type of comparison is certainly acceptable, because the NOx species emitted by reciprocating piston internal combustion engines are almost completely dominated by NO.33 Indirect Injection (IDI) Diesel Engine Operating Characteristics and Performance. Data for such an engine have been published in ref 27, where detailed explanations are given; herein, results are only summarized. Again, as the load (torque output) of the IDI engine was increased from 50% to 100% of the full engine load, BSFC decreased monotonically at all speeds (low, medium, high) for both fuels. No clear trend in BSFC was observed with the fuel type, as the BSFC of JP-8 was higher at low speeds and low loads, and at high speeds, but it was lower than that of diesel oil at medium speeds.27 The maximum cylinder pressures were almost independent of the load, and, in most cases, they decreased with engine speed. JP-8 combustion resulted in higher maximum pressures. However, unlike the smooth engine operation with diesel fuel, intense cylinder pressure cycle-to-cycle fluctuations were observed in the case of JP-8. They were attributed to pressure fluctuations in the fuel pipe, because the injection pump delivery was not optimized for the lower density and viscosity of the JP-8 fuel.27 Ignition delay decreased as the load increased, as higher equivalence ratios rendered the mixture easier to ignite. Again, the JP-8 fuel exhibited lengthier ignition delays in most cases. The exhaust temperature of the engine increased almost linearly with load and speed for both fuels. JP-8 combustion was associated with lower exhaust temperatures, because the more intense burning of this fuel resulted in quicker expansion of the burned gases.27 Indirect Injection (IDI) Diesel Engine Emissions. As in the case of the DI engine, exhaust soot emissions increased as the engine load increased, because both the local and global equivalence ratios increased; however, no clear trend with engine speed (34) Pischinger, R.; Cartellieri, W. SAE Tech. Pap. Ser. 1972, 720756. (35) Walder, C. J. SAE Tech. Pap. Ser. 1973, 730214.

Environmental Evaluation of JP-8 and Diesel Fuels

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Figure 6. Results from the simulation model, showing the calculated NO average mole fraction in the cylinder against the crank angle. The DI diesel engine operation was simulated at four loads of 20%, 40%, 60%, and 80% and speeds of 1500, 2000, and 2500 rpm.

was observed.27 At most power settings, the soot emissions from JP-8 were higher. In contrast to the DI engine, the IDI engine showed NOx emissions decreasing with load as is known in the mid- to high-load range,33,34 whereas mixed results were obtained, relative to engine speed (NOx mostly increased). The aforementioned decrease of NO concentration with load in the high-load range is due to the fact that NO is formed mainly in the pre-chamber.35 The present pre-chamber accounts for half of the total clearance volume and, thus, contains ∼50% of trapped air during combustion. At high loads, the mixture therein becomes too rich and leads to decreasing NO formation rates.33 NOx emissions were comparable for both fuels. Unburned hydrocarbon emissions were, overall, unaffected by engine load and speed. Higher unburned hydrocarbon values were generally observed for JP-8 fuel, most likely attributed to the aforementioned cycle-to-cycle combustion variability. CO emissions increased as the engine load increased, whereas mixed results were observed relative to engine speed. Generally, JP-8 generated higher CO emissions. It was shown, however, by Kouremenos et al.27 that, in the case of JP-8 combustion, retarding the static injection timing of the engine (from 38° CA to 33° CA) curtailed the CO, HC, and particulate (soot) emissions to the levels of Diesel Oil No. 2. General Comments on Direct Injection (DI) versus Indirect Injection (IDI) Diesel Engine Emissions. The experimental investigations involving the aforementioned DI and IDI diesel engines were made independent of each other, with the sole purpose of comparing the performance of diesel and JP-8 fuels burned in each one of them. If a comparison is to be effected in this study between DI and IDI engines concerning their emissions, a problem arises because of the different maximum values of smoke assigned to the full load, which is a subjective criterion. The maximum smoke value for the IDI was defined originally to be slightly higher than that for the DI engine when using diesel fuel (see Figure 7). This convention was also kept here and the comparison was attempted using the same load range of 50%-80% of each full load. The multiple diagrams of Figure 7 show that, as it is typically the case,34,35 the IDI engine had lower unburned hydrocarbon emissions and also lower NOx and CO emissions. Of course, if the comparison had been made on the grounds of the same maximum smoke values, the result would have been proved still better for the IDI engine. This type of agreement for all

Figure 7. Comparative results on exhaust emission ranges for particulate matter (soot), oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO), emanating from the combustion of JP-8 and Diesel Oil No. 2 in DI and indirect injection (IDI) diesel engines, operated in the 50%-80% load range and in a laboratory muffle furnace under all conditions examined.

emissions, with what is reported in the literature, must be considered to be very satisfactory, given that the present comparison encompasses various injection timings, speeds, and fuels, whose effects are probably engine-specific. Muffle Furnace Emissions. Batch combustion of pools of fuel in the muffle furnace was fuel-rich, overall, and the global (bulk) equivalence ratio (φglobal) therein was calculated to be in the range of 1-1.5. Particulate emissions from burning the two fuels in this furnace were in the range of 45-90 mg/g of fuel burned,11 which is on the order of several mg/L of effluent. Such

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emissions are 2 orders of magnitude higher than the corresponding emissions of the engines. Particulate emissions from batch combustion of JP-8 were generally lower than those from the Diesel Oil No. 2 (see Figure 7). NOx emissions from this furnace were 2 orders of magnitude lower than those from the engines, as local values of the equivalence ratio (φlocal) in this laminar flow furnace were very fuel-rich and mixing was absent in the combustion zone (laminar diffusion flame). NOx emissions were only a few parts per million (ppm). NOx emissions from the two fuels were comparable (see Figure 7). In contrast to the NOx emissions, the CO emissions from the laminar flow furnace were 2 orders of magnitude higher than those from the engines (reaching tens of thousands of ppm), because of the aforementioned issues. Again, CO emissions from the combustion of the two fuels were comparable. Unburned hydrocarbon emissions from the combustion of diesel oil in the furnace were somewhat higher than those from JP-8 (see Figure 7). In these experiments, both volatile and semivolatile emissions were monitored, i.e., both in the gaseous and condensed phases. Thus, a direct comparison of magnitudes with the emissions from the engines is not feasible, because, therein, only gas-phase hydrocarbons were monitored. Detailed presentation of unburned hydrocarbon species is given in ref 11. The global oxygen mole fraction never fell below 3%; hence, globally there was sufficient oxygen in the furnace. This implies that the local equivalence ratio (φlocal) values were much higher than the aforementioned range of calculated global equivalence ratios (1 < φglobal < 1.5). Both oxygen consumptions and CO2 emissions were comparable for both fuels. The latter were in the range of 200-900 mg/g, i.e., up to a mole fraction of 10%. These mole fractions may be compared with those calculated at the exhaust of the diesel engine (see Figure 4). Conclusions An investigation has been conducted to determine the environmental effects of using JP-8 (kerosene-based) aviation jet fuel in various combustors. The reason for this comparison is that, in recent years, the military forces of NATO and the United States have made the decision to implement the “single fuel concept”, which calls for using JP-8 in such diverse machinery as landbased aircrafts, vehicles, power generators, furnaces, stoves, etc. In many cases, this concept calls for the substitution of Diesel Oil No. 2 with JP-8 fuel. Although this is expected to simplify the logistics and the costs of many operations, it has generated environmental concerns. To investigate the validity of these concerns,

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the present work has put together new and past results on the emissions from the combustion of JP-8 in various diesel engines. Such engines are used in ground vehicles and in power-generation systems. Results are presented in relation to the emissions from burning Diesel Oil No. 2 in the same engines, under similar operating conditions. Regulated pollutants, such as carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (as soot), and total unburned hydrocarbons, are presented. Moreover, a summary of results published elsewhere11 on unregulated organic emissions from the combustion of the two fuels in a laboratory furnace is also included, for comparison. The examined diesel engines were single-cylinder units (both of direct injection (DI) and indirect injection (IDI)) that have been operated in this laboratory, as well as multicylinder industrial diesel engines (again, both DI and IDI) that have been operated elsewhere.25 The most important observation from data analysis of the aforementioned investigations is that combustion of the two fuels (JP-8 and Diesel Oil No. 2) resulted in overall comparable emissions of pollutants. Some dependencies were observed, based on the type of combustor. However, no clear overall trend would emerge. Comparable emissions were determined to be CO, particulate matter, and NOx. There were more pronounced differences in the detected total unburned hydrocarbons. The engines emitted larger amounts of total volatile hydrocarbons in the case of diesel oil; however, there has been proof that this may be remedied by engine operation optimization.27 Lending support to this argument, the case of the combustion of the two fuels in the furnace showed that the types of semivolatile and volatile hydrocarbons detected were similar and quantities were not only comparable, but most often were slightly lower in the case of JP-8 fuel. In general agreement with these results, a recent investigation conducted elsewhere36 found no major differences in composition between organic compounds from jet engines (burning Jet-A1 and JP-8 fuels) and diesel vehicle engines (burning diesel oil). Further work is needed on the analysis of unburned hydrocarbon emissions from the two fuels to detect and quantify species, such as aldehydes, as well as other oxygenates and ozone precursors. Acknowledgment. Technical assistance from Mr. Murat Topal, Dr. Joel Carlson, and Mr. Jude Jordan is acknowledged. EF049931C (36) Tesseraux, I. Toxicol. Lett. 2004, 149, 295.