Experimental Study of Diesel Fuel Effects on Direct Injection (DI

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Energy & Fuels 2007, 21, 2642-2654

Experimental Study of Diesel Fuel Effects on Direct Injection (DI) Diesel Engine Performance and Pollutant Emissions Theodoros C. Zannis,* and Dimitrios T. Hountalas Thermal Engineering Section, School of Mechanical Engineering, National Technical UniVersity of Athens, 9 Heroon Polytechniou Street, Zografou Campus, 157 80 Athens, Greece

Roussos G. Papagiannakis Thermodynamic & Propulsion Systems Section, Department of Aeronautical Sciences, Hellenic Air Force Academy, Dekelia Air Force Base, 1010 Dekelia, Attiki, Greece ReceiVed March 26, 2007. ReVised Manuscript ReceiVed June 11, 2007

An experimental investigation was conducted to specify the effect of diesel fuel composition and its physical and chemical properties on direct injection (DI) diesel engine performance characteristics and pollutant emissions. Engine tests were made on a single-cylinder naturally aspirated DI diesel engine (Lister LV1), which is located at the laboratory of the authors, at various operating conditions using seven conventional diesel fuels. The test fuels indicate variable hydrocarbon composition and physical and chemical properties, and they were prepared under a European Union research program aiming to identify future fuel formulations for use in modern DI diesel engines. Emphasis was given to the examination of the effect of diesel fuel density, viscosity, and compressibility factor on the fuel injection system, engine combustion characteristics, and diesel-emitted pollutants. Having observed that many diesel fuel parameters are strongly interrelated, we carried out a multivariable statistical analysis to determine the fuel property pairs that are statistically independent. Using experimental findings for pollutant emissions and independent fuel property pairs, a linear multiple regression analysis was conducted to identify potential correlations between diesel-emitted pollutant emissions and the fuel parameters. The sensitivity analysis revealed that soot emissions depend upon fuel viscosity, cetane number, and fuel volatility, whereas NO, CO, and HC emissions mainly depend upon fuel aromatic content. The evaluation of experimental results showed that reductions of soot, NO, and CO emissions can be attained with the reduction of the distillation temperature and the increase of paraffinics/napthenics ratio, which mainly provide the reduction of fuel viscosity and, secondarily, the decrease of the density and increase of the compressibility factor.

Introduction Global warming concerns in conjunction with recent high oil prices require drastic measures for the further reduction of brakespecific fuel consumption (bsfc) and pollutants emitted from stationary diesel engines and diesel vehicles. One promising technique for the reduction of gaseous and particulate emissions is the optimization of the chemical composition and the physical properties of conventional diesel fuels.1,3 Using this technique, significant improvements can be achieved in the environmental behavior not only of new diesel engines but also of existing fleet of diesel vehicles. According to the literature,1,2,4-23 diesel fuel physical and chemical properties affect in a different way each one of the * To whom correspondence should be addressed. Telephone: (+30210) 772-1253. Fax: (+30210) 772-3475. E-mail: [email protected]. (1) Lepperhoff, G.; Baecker, H.; Pungs, A.; Petters, K-D. 9th International Symposium for Transport and Air Pollution, Avignon, France, 2000. (2) Baecker, H.; Pungs, A.; Pischinger, S.; Petters, K-D.; Lepperhoff, G. 3rd International Fuels Colloquium, Esslingen, Germany, 2001. (3) Zannis, T. C.; Hountalas, D. T. Energy Fuels 2004, 18 (3), 659666. (4) Martin, B.; Aakko, P.; Beckman, D.; Del Giacomo, N.; Giavazzi, F. Soc. Automot. Eng. 1997, 972966. (5) Nakakita, K.; Ban, H.; Takasu, S.; Hotta, Y.; Inagaki, K.; Weissman, W.; Farrell, J. T. Soc. Automot. Eng. 2003, 2003-01-1914. (6) Nakakita, K.; Takasu, S.; Ban, H.; Ogawa, T.; Naruse, H.; Tsukasaki, Y.; Yeh, L. I. Soc. Automot. Eng. 1998, 982494.

diesel-emitted gaseous and particulate emissions. For example, it is reported that CO, HC, and aldehyde emissions depend upon cetane number, NOx emissions are mainly controlled by fuel (7) Takatori, Y.; Mandokoro, Y.; Akihama, K.; Nakakita, K.; Tsukasaki, Y.; Igushi, S.; Yeh, L. I.; Dean, A. M. Soc. Automot. Eng. 1998, 982495. (8) Kobayashi, S.; Nakajima, T.; Hori, M. Soc. Automot. Eng. 1994, 945121. (9) Hublin, M.; Gadd, P. G.; Hall, D. E.; Schindler, K. P. Soc. Automot. Eng. 1996, 961073. (10) Den Ouden, C. J. J.; Lange, W. W.; Maillard, C.; Clark, R. H.; Cowley, L. T.; Strandling R. J. Soc. Automot. Eng. 1994, 942022. (11) Beatrice, C.; Bertoli, C.; Del Giacomo, N.; na Migliaccio, M.; Guido, C. Soc. Automot. Eng. 2002, 2002-01-2826. (12) Morley, C.; Price, R. J.; Tait, N. P.; McDonald, C. R. 4th International Symposium COMODIA, 1998; pp 17-24. (13) Ullman, T. L.; Mason, R. L.; Montalvo, D. A. Soc. Automot. Eng. 1990, 902171. (14) Miyamoto, N.; Ogawa, H.; Shibuya, M.; Arai, K.; Esmilaire, O. Soc. Automot. Eng. 1994, 940676. (15) Kidoguchi, Y.; Yang, C.; Miwa, K. Soc. Automot. Eng. 2000, 200001-1851. (16) Kidoguchi, Y.; Yang, C.; Kato, R.; Miwa, K. JSAE ReV. 2000, 21, 469-475. (17) Sienicki, E. J.; Jass, R. E.; Slodowske, W. J.; McCarthy, C. I.; Krodel, A. L. Soc. Automot. Eng. 1990, 902172. (18) Miyamoto, N.; Ogawa, H.; Shibuya, M.; Suda, T. Soc. Automot. Eng. 1992, 922221. (19) Arai, M. Soc. Automot. Eng. 1992, 920556. (20) Kobayashi, S.; Akiyama, K.; Nakajima, T.; Sasaki, S. Jpn. Soc. Automot. Eng. 1994, 9433588.

10.1021/ef070149x CCC: $37.00 © 2007 American Chemical Society Published on Web 07/19/2007

Diesel Engine Performance and Pollutant Emissions

density, whereas polyaromatic hydrocarbons (PAHs) and soot are directly related with fuel aromatic content.4,9 Diesel oil composition in single-, double-, and triple-bonded hydrocarbons as well as in aromatic rings directly affects its physical and chemical properties and thus formulates its tendency to form gaseous and particulate emissions when burned in diesel engines.1,2,4-9,13,14,16,17,24-26 In general, there is an interrelation between the molecular structure (paraffins, olefins, napthenes, and aromatic hydrocarbons), the chemical properties (cetane number, ignition point, etc.), and the physical properties (density, viscosity, surface tension, etc.) of the diesel fuel.1,2,4-9,13,14,16,17,24-27 Consequently, it is quite difficult, if not impossible, to ascribe the variations observed in diesel engine performance and exhaust emissions when replacing diesel oil with another, to the change of a single fuel parameter. In the past, various theoretical and experimental investigations were conducted to examine the effect of the physical and chemical properties of conventional fuels on diesel pollutants, which were emitted from various types of diesel engines and vehicles.1,2,4-26 The examination of these studies derived the following general findings concerning the effect of diesel fuel properties on pollutant emissions: (a) Reduction of sulfur and aromatic contents (especially of PAHs) resulted in a noticeable decrease of particulate emissions (PM). (b) NOx emissions indicated low sensitivity to the variation of diesel fuel properties. According to the literature,1,2,4-26 the kind and magnitude of diesel fuel effects on engine performance characteristics and exhaust emissions vary significantly with the type of engine used in the experiments, the experimental procedure followed, and the method used for the preparation of the test fuels. In addition, in previous studies, different methods were employed to statistically correlate diesel-emitted pollutants and fuel chemical and physical properties.24,27 However, these approaches failed to describe the underlying statistical dependencies between fuel hydrocarbon composition and its physical properties and, thus, their pertinent correlation with measured pollutant emissions. For this reason, an experimental investigation was conducted to examine the effect of diesel fuel composition and properties on direct injection (DI) diesel engine performance parameters and pollutant emissions. Various engine tests were carried out in a single-cylinder naturally aspirated DI diesel engine (Lister LV1) at 2500 rpm and at various engine loads using seven diesel fuels. The test fuels were prepared under a European research program focusing on the examination of diesel fuel physical properties on DI diesel engine performance characteristics and exhaust emissions. The seven diesel fuels indicate variable fuel density, viscosity, and compressibility factor. A multivariable analysis was conducted to specify the fuel property pairs, which are statistically irrelevant. Using the independent fuel parameter pairs and the measured values of pollutant emissions, a linear multiple regression analysis was carried out to identify possible correlations between diesel-emitted pollutants and fuel properties. The evaluation of experimental findings and statistical (21) Ogawa, T.; Araga, T.; Okada, M.; Fujimoto, Y. Soc. Automot. Eng. 1995, 952351. (22) Kwon, Y.; Mann, N.; Rickeard, D. J.; Haugland, R.; Ulvund, K. A.; Kvinge, F.; Wilson, G. Soc. Automot. Eng. 2001, 2001-01-3522. (23) Ullman, T. L. Soc. Automot. Eng. 1989, 892072. (24) Ladommatos, N.; Xiao, Z.; Zhao, H. Proc. Inst. Mech. Eng., Part D 2000, 214, 779-794. (25) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26, 565-608. (26) Zannis, T. C.; Hountalas, D. T. J. Inst. Energy 2004, 77, 16-25. (27) Karonis, D.; Lois, E.; Stournas, S.; Zannikos, F. Energy Fuels 1998, 12, 230-238.

Energy & Fuels, Vol. 21, No. 5, 2007 2643 Table 1. Design specifications of “base” fuel DS1 density viscosity cetane number total aromatic content polyaromatic content sulfur content distillation temperature at 50% (v/v) distillation temperature at 95% (v/v)

820-840 kg/m3 2-4 mm2/s around 55 around 20 wt % less than 5 wt % around 20 ppm around 265 °C around 360 °C

analysis results revealed that the simultaneous reduction of soot, NO, and CO emissions can be achieved mainly with the reduction of fuel viscosity and the simultaneous increase of the compressibility factor. Experimental Section Experimental Installation. Facilities to monitor and control engine variables were installed on a single-cylinder test-bed Lister LV1 experimental engine. This is a four-stroke, naturally aspirated, air-cooled engine with a “bowl-in-piston” combustion chamber having a bore of 85.73 mm, a stroke of 82.55 mm, and a rod length to crank radius of 3.6. The compression ratio is 18:1, and the nominal speed range is between 1000 and 3000 rpm. The engine is equipped with a pump-line-nozzle injection system. A Bryce highpressure fuel pump, having a 6.5 mm diameter plunger, is connected to the three-hole injector nozzle (each hole having a diameter of 0.23 mm), which is located in the middle of the combustion chamber head. The injector nozzle opening pressure is 180 bar. The engine is coupled to a Heenan and Froude hydraulic dynamometer.3,26 The main measuring instruments were an Alcock (viscous type) air flowmeter; tanks and flowmeters for fuel; temperature sensors for the exhaust gas, inlet air, lubricating oil, and cooling water; a top dead center (TDC) marker (magnetic pick-up); a rpm indicator; and a Kistler piezoelectric transducer for the combustion chamber pressure.3,26 Another similar piezoelectric transducer was fitted to the high-pressure fuel pipe (from pump to injector) near the injector. A fast data-acquisition and recording system was used to record the pressure diagrams obtained by the piezoelectric transducers. Exhaust gas analyzers were used to measure smoke, nitrogen oxide (NO), total unburned hydrocarbons (HC) (equivalent propane), and carbon monoxide (CO) at the tailpipe. A Bosch RTT100 smokemeter was used to measure smoke levels in the exhaust gases; NO emissions were measured with a Signal chemiluminescent analyzer; and the HC emissions were measured with a Signal flameionization detector. The last two devices were fitted with thermostatically controlled heated lines. Finally, CO was measured with a Signal nondispersive infrared analyzer. Description of Test Fuels. The fuels used in this study were developed under a European research program aiming to identify diesel fuel formulations, which will be used in future diesel engines.28 The main objective of this program was to specify the “optimum” fuel physical properties for further decreasing diesel exhaust emissions without affecting negatively or, if possible, decreasing further bsfc in modern diesel engines. Hence, seven test fuels with different molecular types were prepared to study mainly the effect of fuel density, viscosity, and compressibility factor on DI diesel engine performance and pollutant emissions. Initially, the specifications of a reference fuel named as “base” were selected. This fuel mixture would be used for the preparation of the other test fuels. The characteristics of the “base” fuel, which was selected to be a Finnish summer-grade city diesel oil are given in Table 1. Then, a series of six conventional diesel fuels was prepared with values of density ranging from 800 to 850 kg/m 3, viscosity varying between 1.5 and 5 mm2/s, and the compressibility factor ranging between 6.6 and 7.05 × 10-5 bar-1. The composition and the chemical and physical properties of the test fuels are shown in Table 2. At this point, it is necessary to address the basic differences in (28) NEDENEF. New Diesel Engines and New Diesel Fuels. GROWTH Programme, Final Technical Report, 2003.

DS7

14.4 1.7 1.9 16.3 40.8 44.6 14.6 830.0 210.5 261.3 344.1 54.7 2.74 28.9 43.22 6.80 6.07 14.8 1.7 1.8 16.6 46.2 39.1 14.7 827.7 210.8 273.4 340.5 54.8 2.97 28.8 43.31 6.81 6.04 14.4 1.3 1.7 16.1 37.5 48.6 13.9 837.7 227.2 319.4 380.2 57.5 4.81 29.7 43.22 6.60 6.14 15.7 0.7 0.8 16.5 40.7 45.3 14.0 818.8 194.1 223.2 301.1 50.9 1.80 28.1 42.92 7.04 6.14 14.6 1.3 1.5 16.0 33.5 52.7 13.8 839.3 219.9 299.1 347.2 56.1 3.88 29.4 43.00 6.62 6.14 15.3 1.1 1.2 16.5 53.0 32.2 14.8 817.2 208.7 246.0 311.8 53.3 2.22 28.1 43.41 7.04 6.0 Mod.IP 391/95 Mod.IP 391/95 Mod.IP 391/95 Mod.IP 391/95 ASTM D2425 ASTM D2425 ASTM D2549 ISO 3675 ISO 3405 ISO 3405 ISO 3405 ISO 5165 ISO 3104 ASTM D971 ISO 1928 FEV ASTM D5291 monoaromatics (wt %) diaromatics (wt %) polyaromatics (wt %) total aromatics (wt %) paraffines (wt %) naphtenics (wt %) aromatics (wt %) density, 15 °C (kg/m3) distillation temperature at 5% (v/v) (°C) distillation temperature at 50% (v/v) (°C) distillation temperature at 95% (v/v) (°C) cetane number viscosity, 40 °C (mm2/s) surface tension, 20 °C (mN/m) net heating value (MJ/kg) compressibility, 60 bar, 20 °C (×10-5 bar-1) C/H mass ratio

Finnish city diesel summer grade measurement method recipe

19.9 2.2 2.4 22.3 40.1 40.1 19.8 833.7 208.0 270.2 347.9 53.3 2.94 28.0 43.03 6.81 6.1

high distillation temperature fuel

base fuel plus paraffinic compound

DS6 DS5 DS4

base fuel plus low distillation temperature additive

DS3

napthenics plus high distillation temperature additive

DS2

paraffinics plus low distillation temperature additive

DS1 “base” fuel

Table 2. Salient features of test fuels DS1-DS7

base fuel plus napthenic compound

2644 Energy & Fuels, Vol. 21, No. 5, 2007

Zannis et al. chemical synthesis and physical properties between fuels DS2 and DS3 and DS4 and DS5. (a) According to the initial planning of the research program, fuels DS2 and DS3 are called “density fuels” because they were prepared to have different densities, whereas all other fuel parameters were constant. However, as evidenced from Table 2, the increase of the density is accompanied by a significant increase of fuel viscosity (almost 75%), an increase of the cetane number, and a decrease of the compressibility factor. This is attributed mainly to the partial replacement of fuel paraffinic content of fuel DS2 by napthenics and, secondarily, to the increase of the distillation temperature because of the addition of a high distillation temperature additive. (b) Fuels DS4 and DS5 were initially named “viscosity fuels” because they were prepared to have different viscosities, with all other properties to be constant. However, in this case also, the drastic increase of viscosity (almost 170%) was accompanied by an increase of the density, an increase of the cetane number, and a decrease of the compressibility factor. In this case, these changes can be ascribed mainly to the increase of the distillation temperature and to the partial replacement of paraffines by napthenics. Hence, both fuel pairs DS2 and DS3 and DS4 and DS5 can be used to examine the combined effect of fuel density, viscosity, and compressibility factor mainly because of the increase of the distillation temperature on diesel engine performance characteristics and pollutant emissions using pertinent measurements from the Lister LV1 engine. According to research program planning, fuels DS6 and DS7 were prepared to examine the effect of a variable compressibility factor on diesel engine performance and exhaust emissions. For this reason, they were initially named “compressibility fuels”. However, as observed from Table 2, their compressibility and density values are almost the same. The transition from fuel DS6 to DS7 is accompanied by a reduction of fuel viscosity because of a small decrease of distillation temperatures. Thus, the examination of experimental findings for fuel pair DS6 and DS7 will assist to the comprehension of the effect of fuel viscosity on diesel engine performance and exhaust emissions. As evidenced from Table 2, there is an interrelation between the chemical and physical properties of a conventional diesel fuel, which becomes very strong between certain parameters. Hence, it is quite difficult to attribute to one fuel property alone the changes observed in diesel engine performance and pollutant emissions when replacing one fuel with another. This is ascribed to the fact that the change of one fuel parameter results in the change of one or more properties. Thus, a statistical analysis is necessary to determine which fuel properties are statistically intertwined. A multivariable statistical analysis was conducted to specify the correlation coefficients between all possible pairs of fuel parameters. According to this analysis, fuel property pairs with a correlation coefficient higher or equal to 75% are characterized as statistically related, whereas values of a correlation coefficient lower than 75% indicate that the statistical relation between fuel parameters is statistically unimportant.24 The results of the multivariable statistical analysis are presented in Table 3. Experimental Procedure. Engine tests were conducted at a constant engine speed of 2500 rpm, and three different engine loads, namely, 20, 60, and 80% of full load were considered. It must be noted that, in a previous experimental study,29 which was performed in the Lister LV1 engine using diesel oils with varying chemical composition and physical properties, engine tests were conducted at three different engine speeds, namely, 1500, 2000, and 2500 rpm. As evidenced from the assessment of the experimental findings of this investigation, the effect of fuel properties on diesel engine performance characteristics and exhaust emissions did not vary significantly with the engine speed. For this reason, in the present study, engine tests were conducted only at 2500 rpm. According to the assembled experience from previous experimental investigations, which were conducted at the laboratory of the authors, the (29) Hountalas, D. T.; Kouremenos, D. A. Soc. Automot. Eng. 1999, 1999-01-0189.

0.89 0.93 0.77 0.10 -0.92 0.34 0.95 0.99 0.85 0.10 -0.95 0.35 0.79 0.87 0.74 -0.26 -0.97 0.55 -0.29 -0.12 -0.53 -0.28 0.06 0.02 -0.43 0.26 0.81 0.41 -0.24 0.22 0.90 0.39 -0.64 -0.02 0.72 0.13 -0.36 0.12 0.12 0.11 0.20 -0.38 -0.79 -0.52 0.04 -0.19 0.39 -0.07

-0.40 -0.58 -0.58 0.73 0.73 -0.91

0.46 0.59 0.71 -0.63 -0.70 0.88

-0.27 -0.15 -0.55 -0.17 0.08 -0.11

-0.57 0.80 0.57 0.70 -0.48 0.88 0.60 0.78 -0.30 0.87 0.64 0.79 -0.44 -0.31 -0.34 -0.27 0.40 -0.96 -0.69 -0.77 0.31 0.69 0.49 0.59

0.98 0.95 0.86 0.30 -0.87 0.18

0.94 0.88 0.89

0.96 0.89 0.70

0.38

0.54

0.80

0.85 -0.35

-0.57 -0.08

0.09 0.47

0.53 -0.54

-0.53 0.09

-0.08 0.48

0.67 0.93 0.51 -0.07

0.80 0.33 -0.21

1.00 0.69 0.79 0.94 0.76 1.00 -0.18 -0.53 -0.49 -0.36 -0.66 1.00 -0.06 -0.16 -0.29 -0.39 0.12 -0.19 1.00 -0.37 0.13 0.86 0.71 0.77 0.70 0.41 1.00 -0.95 0.06 -0.12 -0.87 -0.67 -0.70 -0.79 -0.37 -0.03 -0.28 0.99 -0.01 -0.12 -0.25 -0.33 0.15 -0.21 -0.09 -0.45 -0.20 -0.15 0.42 -0.03 0.75 0.01 0.69 -0.24 -0.24 -0.26 0.23 0.76 0.38 -0.07 0.22 0.00 -0.18 0.49 -0.05 0.49 0.73 0.62 0.24 0.83 0.39 0.00 -0.30 0.96 0.04 -0.22 -0.27 -0.36 0.02 -0.34

1.00 1.00 0.65 1.00 0.48 0.02 1.00 0.29 0.98 0.69

monoaromatics diaromatics triaromatics polyaromatics total aromatic content paraffines napthenes aromatics mononapthenes dinapthenes trinapthenes tetranapthenes density distillation temperature at 5% distillation temperature at 50% distillation temperature at 95% cetane number viscosity surface tension heating value compressibility C/H ratio

1.00 0.54 -0.10 0.49 0.98

0..2

1.00 0.78 1.00 0.72 0.73 1.00 0.93 0.68 0.58

density

at 5%

1.00

1.00

1.00

1.00 0.94 0.91 0.30 -0.90 0.18

1.00 0.87 0.05 -0.94 0.45

1.00 0.08 -0.87 0.41

1.00 0.10 -0.83

1.00 -0.49

1.00

cetane surface heating compress- C/H at 50% at 95% number viscosity tension value ibility ratio

Energy & Fuels, Vol. 21, No. 5, 2007 2645

tetra tri

napthenes

di mono

total aromatic content paraffines napthenes aromatics poly tri

aromatics

di mono

Table 3. Multivariable statistical analysis of diesel fuel properties

distillation


“optimum” value of dynamic injection timing is evidenced in the Lister LV1 engine at 2500 rpm in terms of engine performance characteristics and pollutant emissions. All measurements were taken at constant static injection timing. An attempt was made to conduct all experiments without significant fluctuations in air inlet temperature and lubricating oil temperature as a method to prevent possible discrepancies in engine operation during the tests and mainly to avoid variations in engine loading. The experimental procedure consisted of the following two steps: (a) Initially, engine tests were conducted at 2500 rpm and at all engine loads examined herein using “base” fuel only. At each operating condition, the values of engine performance parameters, such as fuel consumption, cylinder pressure and injection pressure histories, as well as pollutant emissions, were recorded. This way was constituted the “engine baseline operation”. (b) The aforementioned procedure was repeated at the same operating conditions with the engine fueled consecutively with fuels DS2-DS7. During the engine tests, the following parameters were measured at each operating condition considered: (i) in-cylinder pressure, (ii) high-pressure fuel pipe pressure, (iii) exhaust gas temperature, (iv) fuel consumption, and (v) soot, NO, CO, and HC emissions. Cylinder pressure measurements (100 cycles) as well those from the high-pressure fuel pipe were analyzed using a processing code of experimental data, which was developed at the laboratory of the author.3,26 The analysis of the aforementioned pressure data provided information concerning peak combustion pressure, heat release rate, ignition angle, and duration of combustion.

Results and Discussion Combined Effect of Fuel Density, Viscosity, and CompressibilitysFuels DS2 and DS3. A comparison of measured cylinder pressure and net heat release rate histories for test fuels DS2 and DS3 is presented in parts a-c of Figure 1 at 2500 rpm and at 20, 60, and 80% of full load. As observed, the replacement of fuel DS2 by DS3 is accompanied by slightly higher values of cylinder pressure around TDC after commencement of combustion. An earlier initiation of combustion is evidenced for the fuel with a higher density (DS3) because of its higher cetane number compared to DS2. At low load, the increase of the density and viscosity is accompanied by a more intense premixed combustion and a small increase of the cylinder pressure around TDC. Oppositely, at 60 and 80% of full load, the corresponding effect on the cylinder pressure as shown is negligible. The effect of increasing fuel density and viscosity on the measured injection pressure profile is given in parts a-c of Figure 2, where pertinent results for test fuels DS2 and DS3 are presented at 2500 rpm and at all loads examined in this study. At 60 and 80% of full load, the significant increase of viscosity (75%) mainly results in an abrupt injection pressure rise, leading to an increase of the peak injection pressure. At low load, the effect of fuel properties on the injection pressure is rather imperceptible. This can be attributed to the fact that the effect of viscosity on injection pressure is intensified with an increasing engine load. The combined effect of fuel density, viscosity on bsfc, ignition delay, and NO, soot, CO, and HC emissions is shown in parts a-f of Figure 3 at 2500 rpm and at all engine loads examined herein. A small decrease of bsfc is observed with an increasing density and viscosity. The reduction of the ignition delay is also evidenced when replacing fuel DS2 with DS3 because of the increase of the cetane number. NO and soot emissions increase whereas HC emissions decrease with an increasing density and viscosity (DS2 f DS3). In addition, the increase of the density and viscosity results in a small increase of CO emissions at


Figure 1. Combined effect of fuel density, viscosity, and compressibility on the cylinder pressure and net heat release rate profiles. Experimental results from the Lister LV1 engine are given for test fuels DS2 and DS3 at 2500 rpm and at (a) 20%, (b) 60%, and (c) 80% of full load.

high engine loads. However, the overall effect of physical properties on CO emissions is rather imperceptible for this pair of fuels. Combined Effect of Fuel Density, Viscosity, and CompressibilitysFuels DS4 and DS5. In parts a-c of Figure 4,

Zannis et al.

Figure 2. Combined effect of fuel density, viscosity, and compressibility on injection pressure histories. Experimental results from the Lister LV1 engine are given for test fuels DS2 and DS3 at 2500 rpm and at (a) 20%, (b) 60%, and (c) 80% of full load.

cylinder pressure and net heat release rate profiles are compared for test fuels DS4 and DS5 at 2500 rpm and at 20, 60, and 80% of full load. As observed, the increase of the distillation temperature (DS4 f DS5), which results in a noticeable increase of the cetane number (13%), leads to an earlier initiation of combustion. As evidenced from Figure 4a, the earlier com-



Figure 3. Combined effect of fuel density, viscosity, and compressibility on (a) bsfc, (b) ignition delay, (c) NO emissions, (d) exhaust soot, (e) CO emissions, and (f) HC emissions. Experimental results from the Lister LV1 engine are given for test fuels DS2 and DS3 at 2500 rpm and at all loads examined.

mencement of combustion results in slightly higher peak combustion pressures. However, the effect of the distillation temperature and cetane number on the cylinder pressure is imperceptible at high engine loads. At high engine load also the transition from fuel DS4 to DS5 results in a less intense premixed combustion (lower peak value of released heat).

The effect of viscosity on the measured injection pressure profile at 2500 rpm and at 20, 60, and 80% of full load is presented in parts a-c of Figure 5. At high engine loads, the considerable increase of viscosity and the consequent decrease of the compressibility factor (DS4 f DS5) result in a steeper injection pressure rise and, thus, to higher peak values. However,


Figure 4. Combined effect of fuel density, viscosity, and compressibility on the cylinder pressure and net heat release rate profiles. Experimental results from the Lister LV1 engine are given for test fuels DS4 and DS5 at 2500 rpm and at (a) 20%, (b) 60%, and (c) 80% of full load.

this is not evident at 20% of full load, where the effect of fuel viscosity and compressibility is negligible. In parts a-f of Figure 6, experimental data for bsfc, ignition delay, and NO, soot, CO, and HC emissions are presented for

Zannis et al.

Figure 5. Combined effect of fuel density, viscosity, and compressibility on injection pressure profiles. Experimental results from the Lister LV1 engine are given for test fuels DS4 and DS5 at 2500 rpm and at (a) 20%, (b) 60%, and (c) 80% of full load.

test fuels DS4 and DS5 at 2500 rpm for all loads examined herein. According to Figure 6a, bsfc does not vary significantly with increasing viscosity. The replacement of fuel DS4 with DS5 results in the reduction of the ignition delay at all operating cases because of the increase of the fuel cetane number. As



Figure 6. Combined effect of fuel density, viscosity, and compressibility on (a) bsfc, (b) ignition delay, (c) NO emissions, (d) exhaust soot, (e) CO emissions, and (f) HC emissions. Experimental results from the Lister LV1 engine are given for test fuels DS4 and DS5 at 2500 rpm and at all loads examined.

evidenced from parts c and d of Figure 6, NO and soot emissions increase with an increasing fuel viscosity. Oppositely, according to parts e and f of Figure 6, CO emissions remain almost unaffected with the change of the fuel distillation temperature, whereas HC emissions indicate a slight recession with an increasing viscosity and cetane number at all test cases examined, except from the one at 20% of full load. The last is possibly attributed to the different behavior of the fuel injection system at this point as shown in parts a-c of Figure 5.

Effect of Fuel ViscositysFuels DS6 and DS7. As mentioned, fuels DS6 and DS7 were initially planned to be used for the examination of the effect of the compressibility factor on engine performance and exhaust emissions. However, as evidenced from measurements conducted in two European research institutes, fuels DS6 and DS7 indicate almost the same compressibility factor. Fuels indicating the higher difference in the compressibility factor were fuels DS4 and DS5. Hence, the observed differences in experimental results for engine performance parameters and pollutant emissions when replacing fuel


Figure 7. Effect of fuel viscosity on the cylinder pressure and net heat release rate histories. Experimental results from the Lister LV1 engine are given for test fuels DS6 and DS7 at 2500 rpm and at (a) 20%, (b) 60%, and (c) 80% of full load.

DS6 by DS7 can be attributed to the small reduction of viscosity because, according to Table 1, all other fuel properties remain almost constant. Figures 7-9, where experimental results for cylinder pressure history, neat heat release rate profile, injection pressure traces,

Zannis et al.

Figure 8. Effect of fuel viscosity on the injection pressure history. Experimental results from the Lister LV1 engine are given for test fuels DS6 and DS7 at 2500 rpm and at (a) 20%, (b) 60%, and (c) 80% of full load.

bsfc, ignition delay, and NO, soot, CO and HC emissions are presented, provide evidence that the small decrease of fuel viscosity because of the decrease of the distillation temperatures (DS6 f DS7) results in (i) an insignificant variation of the cylinder pressure, heat release rate, and injection pressure, (ii) a slight increase of bsfc, (iii) a reduction of ignition delay, (iv)



Figure 9. Effect of fuel viscosity on (a) bsfc, (b) ignition delay, (c) NO emissions, (d) exhaust soot, (e) CO emissions, and (f) HC emissions. Experimental results from the Lister LV1 engine are given for test fuels DS6 and DS7 at 2500 rpm and at all loads examined.

an increasing tendency of NO emissions, (v) a decreasing tendency of soot and CO emissions, and (vi) a reduction of unburned hydrocarbons at all loads examined. Effect of Fuel Physical PropertiessPercentage Changes of bsfc and Measured Pollutants. In parts a-e of Figure 10, the percentage changes of bsfc and soot, NO, CO, and HC emissions are given for fuel pairs DS2 and DS3, DS4 and DS5, and DS6 and DS7. Observing relative changes for test fuels

DS2 and DS3 and DS4 and DS5, it can be remarked that the partial replacement of paraffines by napthenes and the increase of the distillation temperature (increase of the density and viscosity and decrease of the compressibility factor) result overall in (i) a decrease of bsfc, (ii) an increase of soot, especially at 20% of full load, (iii) an increase of NO emissions, and (iv) a reduction of HC emissions.


Zannis et al.

Figure 10. Percentage change of (a) bsfc, (b) exhaust soot, (c) NO, (d) CO, and (e) HC emissions between fuel pairs DS2 and DS3, DS4 and DS5, and DS6 and DS7. Experimental results are given for the Lister LV1 engine at 2500 rpm and at all loads examined.

Aforementioned variations of bsfc and pollutant emissions are inversed when the fuel DS6 is replaced by DS7 because of the reduction of viscosity as a result of the decrease of the distillation temperature. As observed from Figure 10d, a clear trend between CO emissions and fuel physical properties cannot be determined. At this point, a question arises whether the effects of fuel properties on diesel engine performance characteristics and

pollutant emissions will be similar at engine speeds higher to those considered in the present study. According to previous experimental studies conducted by our research group, the increase of the engine speed in the Lister LV1 engine from 2500 to 3000 rpm is accompanied by an increase of the fuel injection pressure. However, it has been observed that this increase was not so significant to vary considerably the influence of fuel parameters on engine performance characteristics and exhaust



emissions. In addition, the effects of fuel physical properties on diesel engine performance characteristics and exhaust emissions at injection pressures significantly higher than those measured during the present experimental investigation are expected to be similar with the ones observed in this study. This claim is supported by the experimental results, which were presented by Beatrice et al.11 They conducted engine tests in a Fiat four-cylinder common-rail diesel engine, which was installed on a Alfa Romeo 156, with the same set of fuels used in the present study. Measurements were taken at 1500 rpm and 5 bar of brake mean effective pressure (bmep) and at 2500 rpm and 8 bar of bmep. In this first case, the rail pressure was 600 bar, and in the second case, the rail pressure was 900 bar. These values of injection pressure are considerably higher compared to those evidenced in our investigation. The experimental findings from the study of Beatrice et al.11 are generally in agreement on a qualitative basis with those demonstrated in the present experimental study. Statistical Analysis between Measured Pollutants and Fuel Properties. In a preceding paragraph, a multivariable statistical analysis was presented aiming to determine the fuel property pairs, which are statistically independent. Here, a multiregression statistical analysis is described, which was conducted to examine whether and in which degree there is statistical relation between each measured pollutant (soot, NO, CO, and HC) and each one of the statistically independent fuel property pairs. Hence, the following linear multiregression model was used:

S ) a + bP1 + cP2

(1)

where S corresponds to the measured value of each pollutant and P1 and P2 correspond to the two parameters of each fuel property pair. The aforementioned mathematical expression was deliberately selected because coefficients b and c express the degree of dependence (sensitivity degree) of each pollutant from fuel properties 1 and 2, respectively, as can be shown from the following relation:

b (%) )

∂S ∂S × 100 c (%) ) × 100 ∂P1 ∂P2

(2)

The regression model was applied for measured values of pollutant emissions obtained from the Lister LV1 engine at 2500 rpm and at 80% of full load. In each case, the application of the statistical model provided results for (i) coefficients a, b, and c of the multiple regression model, (ii) Pearson product moment coefficients (P values) of regression model coefficients b and c [There is statistical relation (at a 95% level of confidence) between one pollutant and one or both fuel properties Pi (i ) 1 or 2) when the P value of the corresponding model coefficient b and/or c are lower than 0.05.], (iii) correlation coefficient R2, which expresses the correlation degree between one fuel property pair and one measured pollutant, and (iv) adjusted correlation coefficient R2, which is more suitable for multiple regression models with a different number of independent variables. A characteristic example from the application of the regression model is given in Table 3. P values of coefficients b and c of the regression model must be taken into account to determine whether the statistical model can be simplified or not. When the P value of one of the coefficients b and c is higher or equal to 0.1, then the corresponding independent variable (fuel parameter) P1 or P2, respectively, must be neglected from the statistical model (bold values in Table 3).

Table 4. Application example of the multiple regression model between two fuel property pairs and soot emissions for the Lister LV1 engine (2500 rpm and 80% of full load)

property 1 property 2 R2 property 1 property 2 R2

distillation temperature at 50% (v/v) C/H ratio 88.0% distillation temperature at 95% (v/v) heating value 80.9%

coefficients

P value

ln a b

-66.7 0.3

0.78 0.01

c adjusted R2 ln a b

3.2 82.0% -611.1 0.4

0.08

c adjusted R2

12.3 71.4%

0.39

0.33 0.02

Table 5. Average and variation range of the sensitivity degree of measured pollutant emissions from fuel properties fuel property

average sensitivity (%)

viscosity cetane number distillation temperature at 5% (v/v) distillation temperature at 50% (v/v) distillation temperature at 95% (v/v)

diaromatics polyaromatic hydrocarbons

Soot 10.2 4.4 0.9

9.08-12.2 4.1-4.75 0.88-0.98

0.4

0.35-0.42

0.3

0.3-0.36

Nitric Oxide 46.3 -35.7 -12.8 -11.6 -9.8 -9.6

35-51 30.4-38.7 12.1-13.4 10-12.8 8.4-10.8 8-10.7

Carbon Monoxide 20.1

16.4-23.15

Total Unburned Hydrocarbons 23.2 21.5

17.5-31.5 19-22.9

polyaromatics C/H ratio total aromatic content monoaromatics dinapthenics tetranapthenics monoaromatics

variation range of sensitivity degree (%)

Sensitivity Analysis between Measured Pollutants and Fuel Properties. Applying the regression model for all independent fuel property pairs and each one of four measured pollutants (soot, NO, CO, and HC emissions), groups of values were collected for regression coefficients b and c, which corresponded to fuel parameters, indicating high statistical interconnection with a specific pollutant. For each one of these fuel properties, the average, the highest, and the lowest values of recorded values of the pertinent regression coefficient were calculated, thus providing the average value and the variation range of the degree of dependence of one measured pollutant from a specific fuel parameter. In Table 4, the average values and the variation range of the sensitivity degree of each pollutant from the fuel properties are given, which were found to be strongly related. When the average sensitivity degree has a negative sign, then the specific fuel parameter affects the variation of the pollutant negatively. In other words, the increase of this fuel property results in the decrease of the pollutant and vice versa. According to the literature,9,24,27 several methods were adopted in the past to examine the potential statistical relation between diesel-emitted pollutants and one or more fuel parameters. In most of these investigations, the statistical correlations, which were derived, were misleading, concerning the dependence of a measured pollutant from a certain fuel property because these analyses did not take into account the inherent interrelation between fuel


composition and properties. Hence, the method of statistical analysis presented here does not only determine the group of fuel properties, which are statistically related with measured pollutant emissions, but also estimates the sensitivity degree of each pollutant from these fuel parameters. According to the results of the sensitivity analysis, which are presented in Table 5, pollutant emissions depend upon (in descending order) the following fuel properties (i) for soot emissions: viscosity, cetane number, and fuel volatility [distillation temperature at 5, 50, and 95% (v/v) respectively]; (ii) for ΝΟ emissions: polyaromatics, C/H ratio, total aromatic content, monoaromatics, dinapthenics, and tetranapthenics; (iii) for CO emissions: monoaromatics; and (iv) for HC emissions: diaromatics and polyaromatics. The results of the statistical analysis for each pollutant are in agreement with the general observations made above for the effect of fuel properties on diesel-emitted pollutants. It must be clarified that the results of the sensitivity analysis are indicative, providing an evaluation of the effect of modern fuel mixtures on diesel engine pollutant emissions, on more of a qualitative rather than quantitative basis. Conclusions A detailed experimental investigation was made to examine the effect of diesel fuel properties on diesel engine performance characteristics and pollutant emissions. Engine tests were conducted on a single-cylinder DI diesel engine (Lister LV1) at various engine-operating conditions with seven newly developed diesel fuels having variable composition and physical and chemical properties. Emphasis was given to the effect of fuel physical properties, such as density, viscosity, and compressibility factor, on diesel engine combustion characteristics and pollutant emissions. A multivariable statistical analysis was conducted to identify potential statistical interrelations between fuel parameters. Then, a multiple regression analysis was conducted to examine the sensitivity degree of each one of the measured pollutants from the fuel properties, which were found to be statistically independent. Using the experimental findings and the results of the sensitivity analysis, the following conclusions for the effects of diesel fuel properties on diesel engine performance characteristics and pollutant emissions were derived: (i) The increase of the distillation temperature and the partial replacement of paraffines with napthenes resulted mainly in the increase of fuel viscosity and cetane number and, secondarily, in the increase of the density and the decrease of the compressibility factor. (ii) The increase of fuel viscosity and density in conjunction with the reduction of the compressibility factor resulted in (a) an earlier initiation of combustion,

Zannis et al.

(b) a reduction of the ignition delay because of an increase of the cetane number, (c) an increase of the rate of injection pressure rise and slightly higher peak injection pressure values at high engine loads, (d) a small increase of the peak cylinder pressure at low engine load, (e) an increase of soot emissions, (f) a reduction of HC emissions, (g) a small increase of NO emissions, and (h) a small decrease of bsfc. (Overall, the effects of fuel physical properties on bsfc and NO and CO emissions were insignificant.) (iii) The sensitivity analysis between measured pollutants and statistically independent fuel properties revealed that soot emissions depend upon fuel viscosity, cetane number, and distillation curve, whereas NO, CO, and HC emissions are sensitive to the variation of fuel aromatic composition. The results of the statistical analysis agree on a qualitative basis with the experimental findings. This reveals that statistics can be used as a promising technique to identify potential causes for observed changes in diesel engine performance parameters and pollutant emissions between diesel fuels. Hence, with the reduction of soot, NO and HC emissions without significant penalties in bsfc can be attained in diesel engines with the reduction of the distillation temperature and the increase of the paraffinic/napthenic ratio in conventional diesel fuels, which, as evidenced, will result mainly in the reduction of fuel viscosity and, secondarily, in the decrease of the fuel density and the increase of the compressibility factor. Acknowledgment. The authors express their gratitude to the European Union for funding the research program “NeDeNeF” under which the experimental investigation was conducted and to the Institut Francais du Petrole (IFP) for coordinating it. We must thank Fortum Oil and Gas Oy for preparing all of the test fuels and supplying us with invaluable raw data.

Nomenclature bsfc ) BTDC ) CO ) DI ) HC ) NO ) NOx ) PAHs ) PM ) ppm ) rpm ) TDC )

Brake-specific fuel consumption Before top dead center Carbon monoxide Direct injection Total unburned hydrocarbons Nitric oxide Nitrogen oxides Polyaromatic hydrocarbons Particulate matter Parts per million Rotations per minute Top dead center

EF070149X

Experimental Study of Diesel Fuel Effects on Direct Injection (DI

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