Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 UDK - UDC 621.436:621.43.06 Izvirni znanstveni članek - Original scientific paper (1.01) Raziskava učinkov različnih stopenj vračanja izpušnih plinov na temperaturo plamena in nastanek saj pri uporabi dizelskega goriva z različnimi T90 temperaturami destilacije Experimental Study of the Effects of Different Exhaust Gas Recirculation Ratios on the Flame Temperature and Soot Formation when Using Diesel Fuels With Different T90 Distillation Temperatures Danilo Nikolič1 - Radoje Vujadinovič1 - Norimasa Iida2 (1University of Montenegro, Podgorica, Montenegro; 2 KEIO University, Yokohama, Japan) V študiji, opisani v tem prispevku, smo preizkušali krmiljenje dušikovega oksida (NOx) in nastanek saj. Kot razredčilo, pri simulaciji kroženja izpušnih plinov, smo uporabili ogljikov dioksid (CO 2 ) vsebnosti 4,3%, 9,5 in 14,3%, kar pomeni vsebnosti kisika (O2 ) 20%, 19% in 18%. V nadaljevanju smo uporabili tri različna dizelska goriva z različnimi T90 temperaturami destilacije. Lastnosti goriva smo zavarovali pred vplivi vsebnosti aromatov, žvepla in cetanskega števila. Za simulacijo dizelskega zgorevanja smo uporabili hitrokompresijski motor z enim cilindrom. Postopek vžiga in zgorevanje pri vbrizgavanju dizelskega goriva smo opazovali s pomočjo hitrega neposrednega fotografiranja. Temperaturo plamena (kazalnik nastanka NO) in faktor KL (kazalnik vsebnosti saj v vbrizgu dizelskega goriva) smo analizirali z uporabo dvobarvne metode. Preizkus je pokazal, da se s povečanjem vsebnosti CO2 v dovodu zmanjšata najvišja temperatura plamena in nastajanje saj. Prav tako so rezultati pokazali, da pri vsebnosti CO2 = 4,3% v dovodu, T90 temperatura destilacije nima posebnega vpliva na najvišjo temperaturo plamena in nastanek saj. © 2006 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: gorivo dizelsko, plini izpušni, nastanek saj, temperatura destilacije) In this paper the diesel in-cylinder control of nitrogen oxide (NOx) and soot formation was tested. Carbon dioxide (CO2 ) was used as a diluent to simulate the exhaust-gas recirculation (EGR) process at ratios of 4.3%, 9.5% and 14.3%, thus making oxygen (O2 ) concentrations of 20%, 19% and 18% respectively. In addition, three diesel fuels with different T90 distillation temperatures were used. The fuel parameters were isolated from the influence of the aromatics content, sulfur content, and cetane number. A single-cylinder rapid compression machine (RCM) was used to simulate the diesel-type combustion. The ignition and combustion processes of the diesel-fuel spray were observed using high-speed direct photography. The flame temperature (an indication of NO formation) and KL factor (an indication of the soot concentration inside the diesel-fuel spray) were analyzed using the two-color method. The study demonstrated that with an increase of the CO2 concentration in the intake charge, the maximum flame temperature and the soot formation decrease. Also, when there was a CO =4.3% concentration in the intake charge, the results showed no significant influence of the diesel-fuel T90 distillation temperature on the maximum flame temperature and the soot formation. © 2006 Journal of Mechanical Engineering. All rights reserved. (Keywords: diesel fuels, exhaust gas recirculation (EGR), distillation temperature, rapid compression machine) 0 INTRODUCTION The environmental impact of motor vehicles is of great concern worldwide. In particular, the con- tribution to atmospheric pollution from motor vehicles points to the need to reduce vehicular emissions. Relative to the spark-ignited internal combustion engine, the diesel engine emits large quantities 863 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 of particulate matter (PM) and nitrogen oxide (NOx). It is difficult to reduce both of these pollutants at the same time because of their trade-off. Diesel emission-reduction strategies can be divided into two main types: in-cylinder control and after-treatment. In-cylinder control implies changes to the design of the engine and diesel-fuel reformulation, with the former providing more opportunities to reduce emissions [1]. Exhaust-gas recirculation (EGR) is an effective in-cylinder technique that reduces NOx because it lowers the maximum flame temperature. But the application of EGR can also adversely affect the quality of the lubricating oil, the engine durability and produce higher unburned hydrocarbon (HC) and PM exhaust emissions, resulting from the lower oxygen (O2) concentration. The influence of EGR on exhaust emissions can be efficiently simulated by the addition of carbon dioxide (CO2) to the intake charge. Ladommatos et al. [2] identified three major effects of introducing CO2 into the intake charge of a diesel engine: the dilution effect, the chemical effect, and the thermal effect. The dilution effect, which represents the reduction in O2 and nitrogen (N2) fractions in the intake charge due to the replacement with CO2, was shown to be the most significant, having an influence on both the combustion process and exhaust emissions. With the increase of CO2 in the intake charge, the ignition delay periods increase ([3] to [7]), the NOx formation decreases ([4] to [7]), while soot formation in some cases increases ([8] and [5]) and in other cases decreases ([4] and [6]). Many studies have been carried out to assess the effect of the fuel properties on diesel emissions. These studies showed that the fuel properties, such as the cetane number, the aromatic content and type, the distillation temperature, the density, and the viscosity, are the most influential on the combustion process and exhaust emissions. Many papers ([9] to [13]) have investigated the influence of the T90 Driving Pistone Piston Cylinder Injection Nozzle 1 Bore : 145mm Stroke : 692mm Compression Ratio : 15.5 Combustion Chamber f 145 x 48 mm Fig. 1. The Rapid Compression Machine distillation temperature of diesel fuels on exhaust emissions. Most of them reported an increase of NOx and PM emissions with the increase in the T90 distillation temperature. The aim of this study was to show the combined effects of the EGR and T90 distillation temperatures of diesel fuel on the formation of NOx and soot inside the combustion chamber. A rapid compression machine (RCM) was used to simulate diesel combustion, having a single fuel-spray injection in the high-temperature and high-pressure atmosphere of the surrounding gas. The RCM is capable of a diesel-fuel spray-combustion investigation with minimized influences of some parameters specific to a high-speed diesel engine. The ignition and the combustion processes of diesel-fuel spray were observed using high-speed direct photography. The flame temperature (an indication of NO formation) and KL factor (an indication of soot formation) were analyzed with the two-color method. The two-color method is based on the continuous radiation of soot particles during the burning diesel-fuel spray. 1 TEST EQUIPMENT AND CONDITIONS The RCM, Figure 1, used in this study for the simulation of the diesel-combustion process is a duplicated single-diesel-type compression cycle after the combustion process, which is carried out in the environment of a constant volume with high temperature and high pressure. The RCM is a pancake-type combustion chamber with a diameter of 145 mm and a thickness of 48 mm, as illustrated in Figure 2. The single fuel spray from one nozzle hole was injected straight down and did not collide with the walls of the combustion chamber. The piston stroke during compression is 692mm. The ratio and the time of compression are 15.5 and 200ms, respectively. Fuel Injection Nozzle Pressure Pick up '(strain gage) Pressure Pick up (piezo) Optical Probe Fig. 2. The RCM combustion chamber 864 Nikolič D. - Vujadinovič R. - Iida N. Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 Fuel Injection Combustion Nozzle -^Chamber Safety Enclosure High Speed Camera Mirror - Fig. 3. Optical arrangement The in-cylinder pressure inside the RCM combustion chamber was measured with a piezoelectric sensor. Observations and optical measurements were performed via the quartz windows installed on both the piston and the cylinder head. The image of the flame shape passed through the quartz window was reflected from two plane mirrors (one in the safety box and the other outside) and was caught by the high-speed camera. The speed of the high-speed camera is 5000 flashes per second (FPS) (Model: NAC 16 HD; Shutter constant: 5). The film used was Kodak VISION 500T 7279 (16 mm). Figure 3 shows the optical arrangement of the high-speed photography. The image of a halogen lamp, with a known luminous temperature, was also recorded on each frame as a standard light source for the two-color method analysis. This lamp was positioned at the optical distance from the camera that was equal to the fuel-spray flame. This study involved high-speed direct photography of the luminous flames, a combustion analysis Table 1. Composition and state of the intake charge 400 350 300 r T10 T1 > / T9 K yy ----- 250 200 0 20 40 60 80 90100 Evaporated Fraction % Fig. 4. Distillation curves of the test fuels from pressure diagrams, and flame-temperature and KL-factor analyses by the two-color method applied to the color image of the luminous flames. CO2 was used as a diluent to simulate the EGR process at the ratios of 4.3%, 9.5% and 14.3%, thus making O2 concentrations of 20%, 19% and 18%, respectively. Table 1 shows the composition and the state of the intake charge (gas). Table 2 shows the experimental conditions. In the experiment, three JCAP (Japanese Clean Air Project) fuels were used, T1, T9, and T10, each with a different T90 distillation temperature. The influence of the T90 was isolated from the influences of the aromatics content (0%), the sulfur content (0%), and the cetane number. Some types of n-paraffins and i-paraffins were mixed to keep the cetane number constant. Table 3 shows the main properties of the test fuels, while Figure 4 shows their distillation curves. Figure 5 shows the chromatogram data analyzed by Miwa [14]. The fuels with a higher T90 contain heavier elements. Air + CO2 O2 vol % CO2 vol % N2 vol % Ar vol % Tin K T0 K Pin MPa P0 MPa 21 0 78 1 353 905+5 0.1 3.0+0.1 20 4.3 74.2 0.95 353 895+5 0.1 3.0+0.1 19 9.5 70.5 0.91 353 885+5 0.1 3.0+0.1 18 14.3 66.8 0.86 353 875+5 0.1 3.0+0.1 Table 2. Experimental conditions Max. Inj. Pressure Pinj 70 MPa Nozzle Hole Diameters d 0.18 mm Fuel-Injection Period Tinj 4.0+0.1 MPa Injection Equipment JERK type Fuel Pump Injection-valve Opening pressure Pop 23 MPa Raziskava učinkov različnih stopenj - Experimental Study of the Effects 865 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 Table 3. Properties of the test fuels Fuel properties __________ 0.7880___________0.7852___________0.7916 Viscosity@30°C | mm2/s | 3.630 | 3.322 | 4.946 Density @15°C unit g/cm3 T1 T9 T10 Cetane Number 48.8 48.6 48.5 Distillation IBP °C 213.5 213.0 211.0 10%____________^C____________229.5____________2305____________233.0 50%___________^C__________249.0___________247.5___________254.5 90%____________^C____________307.0____________275.0____________387.5 EP °C 326.0 295.5 396.5 Sulfur mass % Mean Molecular Weight Lower Calorific Value C/H Ratio J/g 206 n-Paraffin i-Paraffin (atom/ atom) % (v/v) 43910 202 0.476 43920 219 Naphthene % (v/v) 36 0.479 43970 Total-Aromatics % (v/v) 57 35 0.479 % (v/v) 7 59 24 0 6 70 0 6 0 Fuel T9 Fuel T1 5 10 15 20 25 30 Retention Time min Fig. 5. Chromatograms of the test fuels Figure 6 shows the time histories of the in-cylinder pressure and temperature when only air was compressed. The fuel injection was set to begin at 60 ms after the RCM drive piston reached its top position. After the in-cylinder pressure reaches its maximum at the top dead center (TDC) and constant-volume conditions begin, the in-cylinder pressure gradually declines due to heat losses and gas leakages. To and po are the in-cylinder temperature and pressure at the time of the fuel injection. 1.1 Definition of the ignition delay time and the characteristic combustion times Figure 7 shows a typical example of the in-cylinder pressure rise at the time of the fuel injection and during combustion, and the fuel-injection pressure and the needle-lift histories. When the fuel was injected, mixture cooling occurred and the in-cylinder pressure decreased more rapidly. The time from the start of the fuel injection to the start of the heat release was defined as the ignition delay time 866 Nikolič D. - Vujadinovič R. - Iida N. Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 I Fuel Injection Timing *—----------------------¦ 1200 Compression T0=908 + 5K Dura tion P0=3.0±0.1MPa Air Tin=353K Pin=0.1MPa 1000 800 600 400 200 0 100 200 300 400 500 Time after Compression Start t ms Fig. 6. Typical in-cylinder pressure and temperature histories for the RCM 0 tid t10 t50 t90 Fuel Inj. Period tinj h-----------H tpmax Time after Fuel Injection Start t Fig. 7. Model of in-cylinder pressure, fuel-injection pressure and needle lift tid, while the time from the start of the fuel injection to the peak of in-cylinder pressure was defined as t. Pmax The in-cylinder pressure increase was defined as DPmax, and the times to reach 10%, 50% and 90% of the peak pressures (DPmax) were defined as t10, t50 and t90, respectively. Because the equivalence ratio was rather small, the specific heat during combustion was assumed to be nearly constant, and thus the burnt-fuel fraction and the pressure increase were considered almost proportional. Therefore, t10, t50 and t90 could be considered as 10%, 50% and 90% of the burned-fuel timings. The time from the start of the fuel injection to the first appearance of the luminous flame was defined as tfa, and the time of the luminous flame’s disappearance was defined as tfd. CO2 = 0 %vol. O2 =21 % vol. T = 905±5 K CO2 = 4.3 %vol. O2 =20 % vol. T = 895±5 K CO2 = 9.5 %vol. O2 = 19 % vol. T =885±5 K CO2 =14.3 %vol O2 = 18 % vol. T = 875±5 K 2 RESULTS AND DISCUSSION Figure 8 shows examples of two-dimensional direct photograph images of the T10 fuel-spray combustion for all the test conditions. With the increase of the CO2 concentration in the intake charge, the luminous flame area and the intensity of the flame decrease. A similar trend appears for the T1 and T9 fuels. Figure 9 shows pressure diagrams for all the test conditions and the test fuels. During compression and at the time of the fuel injection, the pressure of the intake charge (gas mixture) decreases as the CO2 concentration increases, maintaining lower pressures during the whole combustion period. This pressure decrease with the increase of the CO2 concentration in the intake charge is the reason for 3ms 4ms 5ms 6ms 7ms Time after fuel injection start, t ms Fig. 8. Examples of direct photograph images of diesel-fuel spray combustion (fuel T10) Raziskava učinkov različnih stopenj - Experimental Study of the Effects 867 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 Fuel injection period Fuel injection period 500 8 10 12 Time after fuel injection start t, ms Fig. 9. In-cylinder pressure histories the changed composition of the combustion air and could be associated with the increase of the specific heat of such a mixture. Figure 10 shows the rate-of-heat-release (RHR) diagrams for all the test conditions and the test fuels. With the increase of the CO2 concentration in the intake charge the high peaks of the RHR increase and are delayed. This is caused by the increased ignition delay, which means that at the time of the ignition there is more fuel available in the cylinder, well mixed, and with a faster burning rate after the ignition. Figure 11 shows the relationships between the ignition delays, the luminous-flame appearance times, the luminous-flame periods and the combustion periods with characteristic times. The ignition-delay periods are longer with the increase of CO2 concentrations. This is caused by the decrease of O2 concentrations in the intake charge -2 0 2 4 6 8 10 12 Time after fuel injection start t, ms Fig. 10. Rate of the heat-release-time histories as well as the decrease of the intake gas temperature and pressure at the moment of the diesel-fuel injection. This fact makes the periods of fuel/gas-mixture preparation longer. The luminous-flame appearance times (the start of soot radiation) follow the same trend as the ignition delay periods. The luminous-flame periods and combustion periods decrease with the increase of the CO2 concentration in the intake charge. This could be due to a prolonged ignition delay, the decrease of the O2 concentrations from 21% to 18%, the lower in-cylinder temperatures and pressures, which all contribute to incomplete combustion. Figure 12 shows an example of two-dimensional images of the flame temperature and the KL value distribution inside the diesel-fuel spray flame determined by the two-color method. In the case of CO2=14.3%, due to the low luminosity of the flame, the two-color method was not applicable. 868 Nikolič D. - Vujadinovič R. - Iida N. Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 Luminous Flame Period tfa t10 t50 t90 0 tid ¦^--------------------------------------------------------------------------------------------------------------------------------- Combustion Period Injection period tpmax CO =14.3% CO =9.5% T90 CO2=4.3% CO2=0% T1 CO =14.3% CO =9.5% T9 T90=275°C CO =4.3% CO =0% T10 02 6 8 1012 T i me af t e r f ue l i nj ectio n start, ms Fig. 11. Ignition delays, characteristic combustion periods and luminous-flame periods Figure 13 (at the end of the paper) shows the time histories of the luminous-flame area and the flame-temperature distribution inside the fuel-spray flames. The flame temperatures obtained from the image analysis using the two-color method were hierarchies at 100K intervals, starting from 1750K. There are decreases in the luminous-flame areas and in the high-temperature areas with the increase of the CO2 concentrations in the intake charge. Figure 13 also shows the area-averaged flame temperature, which is defined by the ratio of SAiTi / SAi, in which Ti is the median value of each hierarchy, Ai is the area having temperature Ti, and SAi is the total flame area. The maximum values of this temperature decrease with the increase of the CO2 concentration in the intake charge. This decrease is more Flame Temp. Flame Temp. K KL Value 2425 «0.375 2175 0.283 Fuel T10; CO2 =0%,; t =5ms Fig. 12. Examples of direct photograph images of luminous fuel spray flame, temperature and KL factor distribution in the fuel spray flame obvious for CO2 concentrations of 4.3% and higher. This decrease is a consequence of the O2 concentration decrease in the combustion chamber and the increase of the inert gases, the decrease of the intake-charge temperatures and the pressures at the time of the fuel injection as well as during the whole combustion period. Regarding the T90 distillation temperature of the diesel fuel, there is a decrease in the maximum area-averaged flame temperature as well as the duration of high temperatures with a decrease of the T90 distillation temperature in the cases of CO2=0% and CO2=9.5%. In the case of CO2=4.3% there is no significant difference in the maximum area-averaged flame temperatures as well as in the duration of the high temperatures. Figure 14 shows the time history of the area-integrated KL value. The KL value is a multiple of the absorption coefficient K, which is nearly proportional to the soot-particle number density in the flame (Beers law) and the optical path length L in the soot region. The KL value is an index of the total number of soot particles along the optical path. The area-integrated KL value SAi(KL)i was defined as the product of the median value of each hierarchy (KL)i and its area, Ai. With the increase of the CO2 concentration in the intake charge, soot starts forming later due to increased ignition delays. The maximum area-integrated KL values decrease with the increase of the CO2 concentration. It was expected that there would be more soot formed with the increase of CO2 concentration due to a lower O2 concentration. Lower values of the area-integrated KL value with the increase of CO2 concentration could be the result of a more homogeneous mixture at the time of the ignition due to more time being available for mixture Raziskava učinkov različnih stopenj - Experimental Study of the Effects 869 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 To = 905 ± 5K O2 = 21% CO2 =0% To = 895 ± 5K O2 = 20% CO2 =4.3% To = 885 ± 5K O2 = 19% CO2 =9.5% 2300 2000 2300 2000 2300 Flame Temp. 2350K 2250K 2150K 2350K 2250K 2150K 2050K 1950K 1850K 1750K 2000 1700 80 80 T i me af te r inj e c tio n start, ms Fig. 13. Time histories of temperature distribution inside diesel fuel spray flame and of area averaged flame temperature Fuel injection period 1000 800 600 400 200 0 800 600 400 200 0 800 600 400 200 0 CO T1 T90=307°C 0 1 2 3 4 5 6 7 8 910 Time after injection start, ms Fig. 14. Time history of the area-integrated KL values preparation, due to lower temperatures inside combustion chamber at the time of the ignition, as well during the whole combustion period, and due to the fact that there is still enough air in such a big combustion chamber, which supports soot oxidation. Iida SAE950213 and Mitchell SAE932798 published similar results. The period of the KL existence decreases with the increase of CO2 concentration in the intake charge, except for the fuel T9 with the lowest T90 distillation temperature. The KL existence period could be related to the soot exhaust emissions from a real diesel engine. By lowering the intake-gas temperature it is possible to use higher EGR rates, which will significantly lower NOx and, at the same time, not influence significantly the soot-formation and soot-extinction periods. Regarding the effect of the fuel on soot formation and emission, for cases of CO2=0% and 9.5%, the soot-extinction periods decrease with the decrease of the T90 distillation temperature. For the case of CO2=4.3%, there is a slight influence of the T90 distillation temperature on the soot-formation and soot-existence periods. This means that by using this CO2 concentration there is a possibility to use heavier fuels for the same soot exhaust emission. 870 Nikolič D. - Vujadinovič R. - Iida N. Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 3 CONCLUSION In this study exhaust gas recirculation (EGR) was simulated by the introduction of carbon dioxide (CO2) into the intake air for four different concentrations, 0%, 4.3%, 9.5%, and 14.3%, using three diesel fuels with different T90 distillation temperatures of 275°C, 307°C, and 387.5°C, in order to determine the influence on the nitrogen oxide (NOx) and soot formation inside the combustion chamber of a rapid compression machine (RCM). The main conclusions from this study are as follows: Maximum flame temperature (NOx formation) decreases with CO2 concentration increase in the intake charge. Soot formation and existence periods decrease with CO2 concentration increase in the intake charge. The fuel with the lowest T90 distillation temperature, T9, showed a good soot-NOx trade-off in the case without any CO2 addition to the intake charge. The fuel with the highest T90 distillation temperature, T10, showed a good soot-NOx trade-off in the case of CO2=4.3% in the intake charge. 4 REFERENCES [I] K.Mitchell (2000) Effects of fuel properties and source on emission from five different heavy duty Diesel engines, SAE Paper No. 2000-01-2890. [2] N.Ladommatos, S.M.Abdelhalim, H.Zhao and Z.Hu (1996) The dilution, chemical and thermal effects of exhaust gas recirculation on Diesel engine emission – Part1: Effects of reducing inlet charge Oxygen, SAE Paper No. 961165. [3] D. Nikolic, K. Wakimoto, S. Takahashi and N. Iida (2001) Effect of nozzle diameter and EGR ratio on the flame temperature and soot formation for various fuels, SAE Paper No. 2001-01-1939. [4] D.L.Mitchell, J.A.Pinson and T.A.Litzinger (1993) The effects of simulated EGR via intake air dilution on combustion in an optically accessible DI Diesel engine, SAE Paper No. 932798. 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Ogawa, M.Shibuya, K.Arai, and O.Esmilaire (1994) Influence of the Hydrocarbon fuels on Diesel exhaust emissions, SAE Paper No. 940676. [II] M.Tamanouchi, H.Morihisa, S.Yamada, J.Iida, T.Sasaki and H.Sue (1997) Effects of fuel properties on exhaust emissions for Diesel engines with and without oxidation catalyst and high pressure injection, SAE Paper No. 970758. [12] W.S.Neill, W. L.Chippior, O.L.Gulder, J.Cooley, E.K.Richardson, K.Mitchell and C.Fairbridge (2000) Influence of fuel aromatics type on the particulate matter and NOx emissions of a H.D. Diesel engine, SAE Paper No. 2000-01-1856. [13] K.Tsurutani, Y.Takei, Y. Fujimoto, J. Matsudaira and M. Kumamoto (1995). The effects of fuel properties and oxygenates on Diesel exhaust emissions, SAE Paper No. 952349. [14] Consignment research report concerning influence of Diesel engine technology and fuel technology on exhaust emission, Petroleum Energy Center PEC-1999JC-25 (1999). • • • • Raziskava učinkov različnih stopenj - Experimental Study of the Effects 871 Strojniški vestnik - Journal of Mechanical Engineering 52(2006)12, 863-872 Authors’ Addresses: Prof. Dr. Danilo Nikolič Doc. Dr. Radoje Vujadinovič University of Montenegro Cetinjski put bb 81000 Podgorica, Montenegro dannikol@cg.yu Prof. Dr. Norimasa Iida KEIO University 3-14-1 Hiyoshi, Kohoku-ku Yokohama 223-8522, Japan iida@sd.keio.ac.jp Prejeto: Sprejeto: Odprto za diskusijo: 1 leto 13.3.2006 25.10.2006 Received: Accepted: Open for discussion: 1 year 872 Nikolič D. - Vujadinovič R. - Iida N.