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Experimental Study of Biodiesel Blends’ Effects on Diesel Injection Processes Jose´ M. Desantes, Rau´l Payri,* Antonio Garcı´a, and Julien Manin CMT - Motores Te´rmicos, UniVersidad Polite´cnica de Valencia, Spain ReceiVed December 16, 2008. ReVised Manuscript ReceiVed February 12, 2009

This paper presents an experimental comparison of three biodiesel blends on DI diesel injection process using a standard injection system derived from a 4-stroke DI commercial diesel engine. Specifically, this work relies on several experiments carried out over a commercial diesel fuel, with 5.75% of rape methyl ester called B5, another with 30% of RME (B30), and a pure rape methyl ester fuel (called RME). Analyses on injection rate shape, spray force, spray tip penetration, and cone angle in nonevaporative conditions have been performed and studied to compare the effect of those different fuels over the injection process. A better understanding of injection process will be useful to achieve a better comprehension of mixing process which determines combustion and emissions in a compression ignition engine. It is well-known that fuel properties like density or kinematic viscosity are higher in vegetable oils and strongly affect how injection system operates. An increase in the hydraulic delay and a higher mass flow rate have been observed in injection rate measurements when using biodiesel; macrovisualization showed that the biodiesel fuel has slightly longer penetration and narrower cone angle that leads to smaller spray volumes.

1. Introduction In addition to the traditional pollutant emission problem associated to diesel engines, nowadays the CO2 emissions are starting to be an important concern because of its direct impact on the greenhouse effect.1 Most of the new combustion strategies developed in the last years (MK, PCCI, Smokeless Rich, ...) are based on the use of new injection strategies and high EGR rates 2,3 to simultaneously reduce soot and NOx emissions. These strategies, however, have a penalty in indicated efficiency in such a way that they do not contribute to solve the CO2 emission problem. More recently the interest of the use of highly oxygenated fuels to be combined with the combustion strategies mentioned in the previous paragraph has been outlined in some publications.1,4 This is because of their high potential to avoid soot formation, which improves the soot-NOx trade-off and prevents the reduction in engine efficiency, thus not worsening CO2 emissions. Nevertheless the production of such synthetic fuels is absolutely not easy. On the other hand, many publications underline the potential of using alkyl monoesters of vegetable origin, often referred as biodiesel, as a possible solution to CO2 emissions in conventional combustions.4 The main reason of this potential is because they close the CO2 cycle, thus avoiding its accumulation in the * To whom correspondence should be addressed. E-mail: rpayri@ mot.upv.es. (1) Cheng, A. S.; Upatnieks, A.; Mueller, C. J. Investigation of the impact of biodiesel fuelling on NOx emissions using an optical direct injection diesel engine. Int. J. Engine Res. 2006, 7, 297–318. (2) Pickett, L. and Siebers, D. Non-sooting, low flame temperature mixing-controlled DI diesel combustion, SAE Paper 2004-01-1399, 2004. (3) Cheng, A. S.; Upatnieks, A.; Mueller, C. J. Investigation of fuel effects on dilute, mixing-controlled combustion in an optical direct-injection diesel engine. Energy Fuels 2007, 21, 1989–2002. (4) Upatnieks, A. and Mueller, C. Clean, controlled DI diesel combustion using dilute, cool charge gas and a short-ignition-delay, oxygenated fuel, SAE Paper 2005-01-0363, 2005.

atmosphere. Besides, the use of such fuels reduces the dependency on the crude oil, which constitutes an additional advantage. Generally speaking, the use of biodiesel has been tested in compression ignition engines under conditions with no or low dilutions with exhaust gases in the past years. Those tests show that CO,5,6 UHC7,8 and soot emissions7,9-11 are reduced compared to the same engine running on standard diesel fuel, while the engine efficiency remains unaltered.7,12-15 The NOx emissions, however, significantly increase. The reason for this is currently under study, but it is not clear yet.16-18 It can be said that a lot of studies performed with biodiesel are mere emissions and performances comparisons against a standard diesel fuel using multicylinder engines and fundamental (5) Lapuerta, M.; Armas, O.; Rodrı´guez-Ferna´ndez, J. Effect of biodiesel fuel on diesel engine emissions. Prog. Energy Combust. Sci. 2008, 34, 198– 223. (6) Hansen, K. F. and Jensen, M. G. Chemical and biological characteristics of exhaust emissions from a DI diesel engine fuelled with rapeseed oil methyl ester (RME), SAE Paper 971689, 1997. (7) Shi, X.; Yu, Y.; He, H.; Shuai, S.; Wang, J.; Li, R. Emissions characteristics using methyl soyate-ethanol-diesel fuel blends on a diesel engine. Fuel 2005, 84, 1543–1549. (8) Monyem, A.; Van Gerpen, J. H. The effect of biodiesel oxidation on engine performance and emissions. Biomass Bioenergy 2001, 20, 317– 325. (9) Schmidt, K. and Van Gerpen, J. H. The effect of biodiesel fuel composition on diesel combustion and emissions, SAE Paper 961086, 1996. (10) Lapuerta, M.; Armas, O. and Ballesteros, R. Diesel particulate emissions from biofuels derived from Spanish vegetable oils, SAE Paper 2002-01-1657, 2002. (11) Gabroski, M. S.; McCormick, R. L. Combustion of fat vegetable oil derived fuels in diesel engines. Prog. Energy Combust. Sci. 1998, 24, 125–165. (12) Wang, W. G.; Lyons, D. W.; Clark, N. N.; Gautman, M.; Norton, P. M. Emissions from nine heavy duty trucks fuelled by diesel and biodiesel blend without engine modification. EnViron. Sci. Technol. 2000, 34, 933– 939. (13) Shaheed, A.; Swain, E. Combustion analysis of coconut oil and its methyl esters in a diesel engine. Proc. Inst. Mech. Engr. Part A: J. Power Energy 1999, 213, 417–425.

10.1021/ef801102w CCC: $40.75  2009 American Chemical Society Published on Web 05/13/2009

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combustion and overall injections aspects are not being completely addressed yet. The main objective of this study is to investigate the influence of using biodiesel blends (with conventional injection system) on the characteristics of the injection process. So, to achieve this main objective injection rate and spray force has been measured, additionally, spray visualization has been performed to obtain spray pattern (cone angle and spray tip penetration). For each case, injection parameters have been conscientiously chosen to match the different working points of a real diesel engine. The outline of this paper is the following one: A first section will describe the facilities and materials used. Then, test procedures and their results will be shown. The next section will present an analysis of the results and some explanations, giving other information like effective velocity of the spray at the exit and the global fuel concentration in the spray. Finally, the conclusions of this work will be made. 2. Experimental Setup In this section, the different experimental devices are presented. First, the three fuels and their properties are shown, followed by a short description of the injection rate meter, the spray momentum test rig, and finally, the spray visualization facility used. A brief explanation of the working of all these test rigs is provided in this part. 2.1. Injection System and Fuels Tested. To perform this study, a modified commercial common rail system was used to generate the sprays (including a high pressure volumetric pump driven by an electric motor). The injector is driven by a solenoid coil and equipped with an 8 conical orifices nozzle that means that the flow should be noncavitating.19 The nozzle has an outlet nominal diameter of 115 µm with k-factor of 1.5 and HE value of 10%. As it has been commented in the introduction, the three fuels used are: one commercial diesel, which is daily, delivered at the petrol station, with 5.75% of rape methyl ester (called B5), another is a 30% mixture of the same vegetable oil coming from rape and standard diesel fuel (called B30), and the last one is a pure biodiesel fuel made from rape methyl ester (called RME in the article, B100 can be seen in some other works1,3,20,21). During the last 5 years, a lot of papers5-16,20-24 can be found reporting that the use of biodiesel fuels influences the combustion and therefore engine performances. Biodiesel is known to have a lower heating value than a standard diesel fuel. But it is also known that the fluid characteristics involved during the injection process are somewhat different than the parameters which affect the combustion. The fuel properties would hardly explain the differences that could be observed when testing. In Table 1, some of the fluid properties are presented; these values will be used further when the analysis will be done. Only fluid (14) Canakci, M. Performance and emissions characteristics of biodiesel from soybean oil. Proc. Inst. Mech. Engr. Part D: J. Automobile Eng. 2005, 219, 915–922. (15) Lapuerta, M.; Rodriguez-Ferna´ndez, J. and Agudelo, J. R. Diesel particulate emissions from used cooking oil biodiesel. Bioresour. Technol. 2008, 99 (4), 731–740. (16) Senatore, A. U.; Cardone, A.; Buono, M.; Rocco, V. Combustion study of a common rail diesel engine optimized to be fueled with biodiesel. Energy Fuels 2008, 22, 1405–1410. (17) Gabroski, M. S.; Ross, J. D. and McCormick, R. L. Transient emissions from no. 2 diesel and biodiesel blends in a DDC series 60 engine, SAE Paper 2004-01-2924, 2004. (18) Yuan, W.; Hansen, A. C.; Tat, M. E.; Van Germen, J. H.; Tan, Z. Spray, ignition and combustion modeling of biodiesel fuels for investigating NOx emissions. Trans. ASAE 2005, 48, 933–939. (19) Desantes, J. M.; Payri, R.; Pastor, J. M.; Gimeno, J. Experimental characterization of internal nozzle flow and diesel spray behavior. Part I: Nonevaporative conditions. Atomization Sprays 2005, 15, 489–516.

Desantes et al. Table 1. Fluid Properties fuels density at 15 °C [kg/m3] viscosity at 40 °C [mm2/s] surface tension at 20 °C [N/m]

testing method

B5

B30

RME

ASTM D4052

831

851

879

ASTM D445

2.38

3.12

4.47

ASTM D971

0.023

0.025

0.028

properties that are interesting for this study are presented; boiling point or heating values are not relevant in nonreactive conditions. Similar values of fluid physical properties either for biodiesel or for standard diesel fuel can be found in the literature.24-26 The biodiesel has higher density, viscosity, and surface tension than the diesel fuel, and as expected, the properties of the mixture of 30% of biodiesel are between those of regular diesel and rape methyl ester fuels (RME). In addition, since these three fuels have been made to work in real engines recently, they are in accordance with the international fuel standards requirements (EN 590 for Diesel fuels and EN 14214 for biodiesel fuel types). 2.2. Injection Rate Meter. Injection rate measurements were carried out with a commercial injection discharge rate curve indicator (IRDCI). This device makes possible to display and record the data that describe the chronological sequence of an individual fuel injection event. The measuring principle used is the Bosch method,27 which consists of a fuel injector that injects into a fuelfilled measuring tube. The back pressure is held with a cavity filled with nitrogen; this avoids back pressure oscillations. The fuel discharge produces a pressure increase inside the tube, which is proportional to the increase in fuel mass. The shape of this pressure increase corresponds to the injection rate.28 A pressure sensor detects this pressure drop, and then an acquisition and display system processes the recorded data for further use. To obtain a good estimation of the experimental errors, several repetitive measurements were carried out at the same test point (energizing time, rail pressure, and backpressure). The standard deviation for these tests is generally around 0.6% with proper calibration of the equipment. 2.3. Spray Momentum Test Rig. With this experimental equipment, it is possible to determine the impingement force of a spray on a surface. This force is equivalent to the spray momentum flux. This force can be determined with the use of the spray momentum test rig designed in our facilities. Sprays are injected into a chamber that can be pressurized with nitrogen up to 10 MPa in order to simulate pressure discharge conditions that are representative of real pressure values inside the engine combustion chamber during the injection process. (20) Nagaraju, V.; Henein, N.; Quader, A.; Wu, M. and Bryzik, W. Effect of Biodiesel (B-20) on performance and emissions in a single cylinder HSDI diesel engine, SAE Paper 2008-01-1401, 2008. (21) Ejim, C. E.; Fleck, B. A.; Amarfazli, A. Analytical study for atomization of biodiesels and their blends in a typical injector: Surface tension and viscosity effects. Fuel 2007, 86, 1534–1544. (22) Murillo, S.; Mı´guez, J. L.; Portiro, J.; Granada, E.; Mora´n, J. C. Performance and exhaust emissions in the use of biodiesel in outboard diesel engines. Fuel 2007, 86, 1765–1771. (23) Patterson, J.; Hassan, M. G.; Clarke, A.; Shama, G.; Hellgardt, K. and Chen, R. Experimental study of DI diesel engine performance using three different biodiesel fuels, SAE Paper 2006-01-0234, 2006. (24) Lujan, J. M.; Tormos, B.; Salvador, F. J. and Gargar, K. Comparative analysis of a DI diesel engine fuelled with biodiesel blends during the European MVEG-A cycle: Preliminary study (I). Biomass Bioenergy 2009, 33, 950–956. (25) Kegl, B.; Kegl, M.; Pehan, S. Optimization of an injection system for diesel and biodiesel usage. Energy Fuels 2008, 22, 1046–1054. (26) Rosca, R.; Rakosi, E.; Manolache, G. and Niculaua, M. Fuel and injection characteristics for a biodiesel type fuel from waste cooking oil, SAE Paper 2005-01-3674, 2005. (27) Bosch, W. The fuel rate indicator: a new instrument for display of the characteristic of individual injection, SAE Paper 660749, 1966. (28) Payri, R.; Salvador, F. J.; Gimeno, J.; Bracho, G. A new methodology for correcting the signal cumulative phenomenon on injection rate measurements. Exp. Tech. 2008, 15, 46–49.

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Figure 1. Spray momentum measurement principle.

Figure 1 shows a sketch of the spray momentum measuring principle. The impact force is measured with a piezo-electric pressure sensor calibrated to measure force and placed at 5 mm from the nozzle orifice exit. The sensor frontal area and position are selected so that the spray impingement area is much smaller than that of the sensor. The pressure inside the chamber is constant and surrounds the entire spray, and the fuel is deflected perpendicularly to the axis direction. Under this assumption, and due to the conservation of momentum, the force measured by the sensor is the same as the axial momentum flux at the orifice outlet or at any other axial location29 when the injection is in its stationary part, that is why long injections are performed; only the stationary part of the signal is taken to analyze the spray momentum flux. 2.4. Macroscopic Spray Visualization. To obtain the tip penetration and the cone angle of the spray, the visualization of the sprays has been performed in a nitrogen test rig. It basically consists of a steel cube with a chamber and various connecting flanges machined into it. The design is modular, and ancillaries can be added depending on the required experiment.30 The assembly is designed for a maximum pressure of 70 bar. It is necessary to circulate the nitrogen through the rig because otherwise the injected diesel would obscure the windows and severely degrade the quality of the images. Furthermore, it is important to keep rig pressure (Pb) and nitrogen temperature constant during each experiment. Two filters collect the fuel injected to keep the gas stream clean. The temperature of the nitrogen can be set to values between 15 and 50 °C to obtain the desired density inside the chamber. The test rig operates at ambient temperatures thus avoiding fuel evaporation. In Figure 2, a photograph of the N2 test rig is shown. The images are taken with a 12-bit color CCD camera (Pixel Fly by PCO) with a spatial resolution of 1280 × 1024 pixels, a minimum exposure time of 20 microseconds with a jitter of (3 microseconds. Lighting is ensured by four high power xenon flash lamps that illuminates the spray from four sides judiciously orientated allowing a homogeneous view of the sprays avoiding background reflections, the flash duration is around 7 µs. All the experimental equipment (camera-flash-injection) has been synchronized with a purpose-built electronic system, using the injector trigger signal as a reference to take the image sequences. A very low injection frequency is used (0.25 Hz); this high time interval between injections is required for the N2 flow in the rig to be able to remove the fuel droplets from the previous injection and, hence, to maintain similar conditions and a good optical access to the spray. The injector is mounted so that all the spray axes are visualized simultaneously through the frontal window as could be seen in Figure 3. The images are digitally processed using purpose-developed software. The segmentation algorithm, based on the log-likelihood (29) Payri, R.; Garcı´a, J. M.; Salvador, F. J.; Gimeno, J. Using spray momentum flux measurements to understand the influence of diesel nozzle geometry on spray characteristics. Fuel 2005, 84, 551–561. (30) Desantes, J. M.; Payri, R.; Salvador, F. J.; Soare, V. Determination of diesel sprays characteristics in real engine in-cylinder air density and pressure conditions. J Mech. Sci. Technol. 2005, 19, 2040–2052.

Figure 2. Photograph of the N2 test rig injection chamber.

Figure 3. Sample image of an injection recorded by the CCD.

ratio test (LRT), has the advantage of using the three channels of RGB images for a proper determination of boundaries that are not well-defined, as in the case of sprays. This method proved to be almost completely insensitive to intensity fluctuations between pictures for the tested cases and provided better results than some other algorithms checked. Prior to the systematic use of the algorithm for parametric studies, the influence of the illumination quality on the results was evaluated in specific tests. Results demonstrated that the algorithm properly detects the estimated spray boundaries even in case of comparatively poor illumination. Details of the image processing software are available in ref 31.

3. Results In this section, the procedures of the tests carried out with the installations described before are explained. Then, the results coming from all the experiments are presented. Table 2 shows the tests matrix used for injection rate and spray momentum. Five injection pressure (300, 500, 800, 1200, and 1600 bar) and 3 discharge pressure values (Pback ) 20, 50, and 80 bar) have been measured for each fuel. Points 1-3 are short to long injection times where energizing time is the parameter; point 4 is a short injection, the goal being to inject 1 mg of each fuel. Finally, points 5 and 6 are long injections at (31) Pastor, J. V.; Arre`gle, J.; Palomares, A. Diesel spray image segmentation with a likelihood ratio test. Appl. Opt. 2001, 40 (No. 17), 2876–2885.

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Figure 4. Mass flow rate at 1600 bar (BP ) 50 bar, ET ) 500, 1000, and 2000 µs).

Figure 5. Injection rate (short injection: injected mass ) 1 mg) at 300 bar, BP ) 50 bar.

Table 2. Operating Conditions for Injection Rate and Spray Momentum points (for all pressures)

back pressure [bar]

energizing time [µs]

1 2 3 4 5 6

50 50 50 50 20 80

500 1000 2000

injected mass [mg]

1 2000 2000

different back pressures to see the effect of cavitation; these long injections are also used to process spray momentum and then to obtain spray effective velocity. This has led to 30 measured points for each fuel type, as a total of 90 measurements either in injection rate or in spray momentum measurement. For spray tip penetration visualization, only a long injection (1000 µs) has been performed. In fact, the spray penetration does not depend on injection time if the injection is long enough. Two discharge pressures have been tested (20 and 50 bar). This leads to an amount of 30 experimental points to obtain spray tip penetration and cone angle. 3.1. Injection Rate Measurements. To compare fuels, a distinction has to be made between stationary and transient part of the injection rate shape. The fluid properties act differently depending on the conditions and the results should show it. 3.1.1. Stationary Conditions. The mass flow rate for the three fluids can be seen on Figure 4. On this graph, results corresponding to three different energizing times are presented: 500, 1000, and 2000 µs. The time scale is the time ASOE (after start of energizing) because it allows knowledge of the hydraulic delay of each fluid. The pure biodiesel fuel (RME) has a higher injection rate than the two others. The difference appears at the top part of the curves, when the needle is fully open. However, differences are not clear between B5 and B30 on this graph; the curves have no particular tendency (one compared to the other). 3.1.2. Transient Conditions. Short injections have been made in the purpose of seeing the influence of biofuels on the injector’s needle dynamic. In fact, short injections are strongly influenced by injector’s dynamic. An injected mass (1 mg, constant for all the short injections) instead of an energizing time as been chosen to get a similar injected mass, the energizing time always starts at the same instant, referenced by “0” in the time scale. For these tests, the energizing time has been adjusted to get the targeted injected mass. The results shown in Figure 5 clearly show that RME has a slower opening than the other fuels and slower closing. First, at the beginning of the curves, a time difference can be appreciated; this means that the hydraulic delay is not the same with the three fluids. The difference is not only on the

Figure 6. Spray momentum at 1600 bar (BP ) 50 bar, ET ) 500, 1000, and 2000 µs).

injector opening delay but also the slope of the injection rate during the needle lift. The RME injection rate curve is not as steep as the ones from B5 and B30, which have really similar shapes. All the results are commented and analyzed in section 4, where an explanation for this different needle dynamic will be presented. 3.2. Spray Momentum Measurements. The measurements made in the spray momentum test rig should present similar results for those three fluids. It is known that the spray momentum, at the outlet, is not affected by fluid properties, the theoretical analysis developed by Payri et al. in ref 29 shows that it is only affected by geometrical parameters and operating conditions; some interesting part of this demonstration are presented in section 4. As it can be seen on Figure 6, the spray momentum measurements are really close for the three fuels tested and agree with the expectations. For each injection pressure, no particular influence has been seen changing the back pressure. The spray momentum being always proportional to the pressure drop for any injection and discharge pressure.28 This behavior of the spray momentum flux agrees with results obtained previously on this test rig. Only minor differences can be observed between the fluids, and they are attributed to test to test deviation. 3.3. Macrovisualization of the Sprays. The macroscopic parameters that have been analyzed through these tests are the penetration (S) and the spray angle (θ). Both of these parameters are sketched in Figure 7 on a generic spray. The penetration is the distance between the orifice exit and the furthest point reached by the front of the spray in the axial direction. The spray angle is generally considered as the cone angle formed by the spray considering a 60% of penetration.30 The standard deviation over the results was around 1.2 mm in spray length and 1.8° for the cone angle when the spray is long enough; this standard deviation, represented by the error bars in Figure 8, comes from shot to shot and hole to hole dispersion

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Mass flow rate and spray momentum are two interesting data for characterization of the spray, but when the results are put together, these two test rigs are even more interesting. Payri et al. 29 analyzed the theoretical definitions of injection rate (1) and momentum (2) to see influences of nozzle geometry on spray characteristics. Figure 7. Parameters to characterize the spray.

m ˙f )

∫

uFfdAeff

(1)

˙f ) M

∫

u2FfdAeff

(2)

A0

A0

˙ f are the fuel mass flow rate and the momentum flux, m ˙ f and M respectively. u is the spray velocity at the orifice exit, and Ff the fuel density. uB is the theoretical velocity of Bernoulli (eq 3), according to Bernoulli’s equations, the theoretical velocity at the orifice exit only depends on the pressure difference (∆P) and fuel density uB )



2∆P Ff

(3)

The theoretical velocity allows obtaining a theoretical mass flow, with this, a dimensionless parameter is obtained, the discharge coefficient that show the efficiency of the nozzle fuel release. The discharge coefficient (Cd) is the actual mass flow divided by the theoretical mass flow, Ageo being the geometric area of the orifice: Cd )

Figure 8. Results of penetration and cone angle (Pinj ) 1600 bar and Pback ) 20 bar).

since the nozzle has 8 plumes and the experiments were made with 5 repetitions for each time step. Figure 8 shows a sample for spray penetrations and angles for the three fuels tested visualized at 1600 bar. On this figure, it can be appreciated that the penetration is similar for all the fuels. Only slight differences can be observed between RME and the two others, as seen all along the experiments RME sprays seem to penetrate a little bit more and their cone angle is narrower than with the other fuels. The hydraulic delay noticed in mass flow rate is not depicted on the graphs because they are presented according to the time after start of injection (ASOI). The goal is to show the difference in penetration length and the hydraulic delay might interfere with the real spray tip penetration, which is why the figures are time-adjusted so that the first droplets appear at the same time for each fuel. In section 4, the relation between physical properties of the fluids and spray characteristics will be shown.

m ˙f m ˙f ) AgeoFf uB Ageo√Ff 2∆P

This parameter includes two losses that could be divided in two coefficients; one is due to velocity and the other to area contraction. This leads to decompose the discharge coefficient as a coefficient of velocity (Cv, see eq 6) and a coefficient of area (Ca, see eq 7) Cd ) CvCa

(5)

These two dimensionless parameters are defined by the effective value divided by the theoretical (or geometric) one CV )

ueff uB

(6)

Ca )

Aeff Ageo

(7)

with ueff being the effective velocity, uB being Bernoulli theoretical velocity, Aeff being the effective area, and Ageo being the geometrical area. This analysis leads to the calculation of the effective velocity of the jet at the orifice exit (eq 8), as well as the area of this orifice (eq 9)

4. Analysis of the Results This section presents a global analysis of the results; the aim is to merge the data altogether and make conclusions of the results obtained. The set of all the results of the experiments carried out in this study can help to understand why such results have been obtained independently. This analysis should provide a better understanding over all the effects that produce the fluid properties, and explain how they affect the injection process.

(4)

˙f M ) CVuB m ˙f

(8)

m ˙ 2f ) Ca Ageo ˙f FfM

(9)

ueff ) and Aeff )

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Figure 10. Stabilized spray momentum vs pressure drop. Figure 9. Mass flow rate vs square root of the pressure drop.

˙ f), divided by the mass flow rate (m The spray momentum (M ˙ f), gives the effective velocity (ueff) that can be also expressed as the theoretical velocity (from Bernoulli, uB) and the velocity coefficient (Cv). Actually, the effective area (Aeff) is not relevant in this work as the first goal is to compare different fuels; this area is obtained by dividing the square of the mass flow rate (m ˙ f) by the fuel ˙ f). In addition, the test density (Ff) and the spray force (M conditions should not produce any coking on the nozzle because all the experiments are carried out at low temperature. Finally, joining all the theoretical expressions, the following equations are obtained; they give the mass flow rate (eq 10) as can be seen in ref 32 and the spray momentum (eq 11): m ˙ f ) CvCa Ageo√2Ff√∆P

(10)

˙ f ) Cv2Ca Ageo2∆P M

(11)

where Cv, velocity coefficient; Ca, area coefficient; Ageo, geometrical area of the nozzle orifices; Ff, fuel density; ∆P, injection pressure - back pressure (Pinj - Pback). After examination of these two expressions, it seems obvious to compare the data coming from injection rate and spray momentum. The difference lies in the velocity coefficient that does not have the same influence in both equations and in the fuel density that only influences the mass flow rate. So, the density affects mass flow rate but not spray momentum (see eqs 10 and 11), it has been seen a higher mass flow rate with RME, but the spray momentum flux was really similar for all fuels. In the same way, RME density is higher than the one of the other fluids (see Table 1). This could explain the difference in maximum mass flow rate when the conditions are stationary. Figure 9 shows the maximum mass flow rate according to the square root of the pressure difference between injection pressure and back pressure. Experimental results are displayed as five groups of three points; a single group of points represents a characteristic injection pressure and each point in a group corresponds to a different back pressure (20, 50, and 80 bar). These tests have been done to see the stabilized part of the injection; it is a better way to analyze the stationary conditions results. Only the top part of the longer injection is taking into account to plot these curves. As seen before, the mass flow rate in stabilized conditions is higher with RME, it can be appreciated on this graph that when the rail pressure increases, the difference becomes larger. This behavior is due to the type of plot; nevertheless, the density has the same effect at all injection (32) Siebers, D. Liquid-phase fuel penetration in diesel sprays, SAE Paper 980809, 1998.

pressures. The relative difference in term of maximum mass flow rate (when the injector is fully open) has been calculated for all tests and found to be around 5% between B5 and RME and 2% between B5 and B30. (Such a difference (1-2%) could be hidden by the experimental errors made during the measurements.) For transient conditions, it is really hard to compare mass flow rate and spray momentum for short injections (injected mass ) 1 mg) because the dynamic of the spray momentum test rig does not allow obtaining such a signal to appreciate the delay. In fact, the sensor is not placed directly at the orifice exit but at around 5 mm, and the interaction with N2 along this distance affects the spray momentum at the front part of the spray during the transient part of the injection; this confirms that only the stabilized part of spray momentum can be used. We assume that for really short injections, the effect of density is minor. The dynamic of the injector needle is affected by fluid viscosity that leads to these results: The hydraulic delay is higher when the kinematic viscosity increases and the injection rate shape, in the first instants, is smoother with the biodiesels than with the standard diesel fuel (values of viscosity are presented in Table 1). The biodiesel being stickier, the needle will not work as it does with regular diesel; the movement will take more time and the result is a longer time for the injector to get totally open. In addition, a part of the hydraulic delay is also attributed to the working of the injector holder. In fact, the injector holder used for the experiments is electro-hydraulically driven by the pressure difference in a small volume at the top part of the body. The fluid has to pass through an orifice in order to move the needle. This process might be affected by fluid characteristics as the flow of the liquid through the hole is influenced by viscosity. It has been also noticed during the tests that the biodiesels require a longer energizing time to inject the same mass (short injections). Energizing times to inject 1 mg with B5, B30, and RME are 242, 246, and 260 µs, respectively (Pinj ) 1600 bar). This means that the higher viscosity of biofuels affects the injection rate shape on the opposite direction to that of the effect of density (biodiesels are denser than regular diesel fuels, see Table 1). Actually, the injected mass is lower with biodiesels for short injection times and higher when the injection time becomes longer, when the injection is small, the injected mass is influenced by viscosity and when the injected mass is higher (long injection times), the density effect is more important. Figure 10 shows the results of spray momentum for the stationary condition. As for injection rate, only the stabilized part of the signal has been taken into account to plot this graph. There are no significant differences in spray momentum with the fluids tested, even changing the injection pressure.

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π ˙0 ) m M ˙ fu0 ) Ff φ20u20 4

(12)

where φ0 is the outlet diameter of the nozzle’s orifices and u0 the orifice outlet velocity. In the results obtained before, with the spray momentum test rig, almost no differences have been observed. It is assumed that spray momentum remains constant along the spray travel. That is why fuel density has almost no influence in those tests, but when the spray travels inside the chamber, both fluid and gas affect tip penetration and cone angle. When the Buckingham π theorem38is applied, the theoretical spray tip penetration obeys the following proportionality: 1

Figure 11. Effective velocity of the spray vs square root of the pressure drop.

As explained before, the spray momentum joined with the mass flow rate give other parameters like effective velocity (ueff) or effective diameter (Aeff for effective area) of the orifices. Thus, the velocity of the spray is obtained by dividing the spray momentum flux by the mass flow rate in the stabilized part of the injection. Figure 11 shows the results obtained with the different fluids for all pressures (Pinj and Pback), it can be directly noticed that the pure RME has a lower velocity. Some authors33 studied the diesel spray in its first millimeters and found that the influence of chamber density on penetration is really weak in the region close to the nozzle. In addition, in the area close to the nozzle, the liquid concentration is really high and this leads to assume that the velocity calculated, which would be the penetration velocity, could be the speed of the droplets in the spray. Another study34 shows that the spray velocity is theoretically calculated by the Bernoulli’s equations; in this simple relation, only two parameters are involved: The pressure drop and the fluid density. This might explain the difference seen on Figure 11 by different densities between those three fuels and so, an increase in fuel density leads to a decrease in velocity. The spray tip penetration is assumed to be affected by other parameters and not only density and viscosity. Injection rate and spray momentum can help to understand the first millimeters of the spray. Then, droplets segregation also called break-up (first and second) is influenced by surface tension as well. Several theoretical approaches are available in the literature. The most used are certainly those proposed by Dent35 and Wakuri et al.36 Later, Hiroyasu and Arai37 proposed another correlation that includes different penetration law depending on the region, before and after break-up. In all these equations to predict spray penetration, the spray momentum flux is always used. The momentum could be assimilated as the energy of a spray delivered at the orifice exit, and if we combine eq 2 and a rectangular injection rate, then this theoretical spray momentum flux is obtained (33) Payri, R.; Salvador, F. J.; Gimeno, J. and De la Morena, J. Macroscopic behavior of diesel sprays in the near-nozzle field, SAE Paper 2008-01-0929, 2008. (34) Siebers, D. Scaling liquid-phase fuel penetration in diesel sprays based on mixing-limited vaporization,SAE Paper 1999-01-0528, 1999. (35) Dent, J. C. A basis for the comparison of various experimental methods for studying spray penetration, SAE Paper 710571, 1971. (36) Wakuri, Y.; Fujii, M.; Amitani, T.; Tsuneva, R. Studies of the penetration of a fuel spray in a diesel engine. J. Soc. Mech. Eng. 1960, 3 (9), 123–130. (37) Hiroyasu, H.; Arai, M. Structures of fuel sprays in diesel engines, SAE Paper 900475, 1990.

S(t) ) KF-1/4 M-1/4 t1/2 tan- 2 g 0

( θ2 )

(13)

K is a constant depending on the spray internal distribution; Desantes et al.39 found that for K ) 1.26, the penetration is correctly determined for a wide range of diesel sprays. In eq 13, it can be appreciated time, momentum flux, and spray angle (θ) dependencies. The gas density (Fg) is also included in this expression. Because the results of spray momentum flux are really similar, the differences observed either in penetration or in angle come from the spray development. This means that break-up, average droplets size and segregation is different for the three fuels. The pure biodiesel shows slightly longer penetration and narrower angle on the graphs presented in section 3.3. These tendencies appear in all the experiments and have been observed by other researchers.25,40,41 According to spray length and opening, both density and surface tension differences can explain these results. In the theoretical analysis made before, it has been shown that the density slightly affects the tip penetration so that the angle should be affected. The surface tension is not shown in the equations to predict spray penetration, but its influence becomes evident during the spray break-up. This should affect the segregation of the droplets in the spray and obviously, it influences spray length and cone angle. When the surface tension is higher, the “internal forces” in a droplet are higher and the break-up process appears later. A recent study by Lee et al. 42 indicates that at 0.5 ms after start of injection (ASOI) and ambient temperature (less than 300 K), SMD from biodiesel fuels was about twice that from diesel fuel. Ejim et al. 21 used the correlation presented by Elkotb43 to evaluate SMD of the droplets through viscosity, density, surface tension, and operating conditions in a diesel engine. They observed a worse atomization because of higher global SMD with biodiesel, especially with pure rapeseed vegetable oil. As the droplets are bigger, the inertial mass is higher and as a result, the droplets should travel further and affect the general shape of the sprays. The curves also show differences on spray penetration before and after break-up as proposed by Hiroyasu and Arai37 in their model. With biodiesel, (38) Buckingham, E. Model experiments and the forms of empirical equations. Trans. Am. Soc. Mech. Eng. 1915, 37, 263–296. (39) Desantes, J. M.; Payri, R.; Salvador, F. J.; Gil, A. Development and validation for a theoretical model for Diesel spray penetration. Fuel 2006, 85, 910–917. (40) Podorevc, P.; Kegl, B.; Skerget, L. Diesel and biodiesel fuel spray simulations. Energy Fuels 2008, 22. (41) Kim, H. J.; Suh, H. K.; Park, S. H.; Lee, C. S. An experimental and numerical investigation of atomization of biodiesel, dimethyl ether, and biodiesel-ethanol blended fuel. Energy Fuels 2008, 22. (42) Lee, C. S.; Park, S. W.; Kwon, S. I. An experimental study on the atomization and combustion of biodiesel-blended fuels. Energy Fuels 2005, 19, 2201–2208. (43) Elkotb, M. M. Fuel atomization for spray modeling. Prog. Energy Combust. Sci. 1982, 8, 61–91.

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Desantes et al.

Figure 12. Volume of a spray with the three fuels (Pinj ) 300 bar and Pback ) 20 bar).

Figure 13. Fuel mass fraction in the spray at 300 bar (Pback ) 20 bar).

the first part of the spray (before break-up, near the orifice exit) seems to travel slower than the regular diesel fuel. After the break-up, as the droplets are supposed to be bigger because of a higher surface tension, they have more inertia and might go further in the chamber. As it has been seen with the theoretical analysis that the spray momentum flux being similar, this implies a narrower cone angle if the penetration is longer with biodiesel; the results of spray angle obtained clearly show this tendency. When the conditions are evaporative, but nonreacting, the conclusions given here for sprays at nonevaporative conditions are not valid. Previous works published in this field44,45 show that the use of biodiesel fuels seem to provide sprays that are longer and narrower. Different boiling point and evaporation behavior might be additional parameters that do not affect the sprays in the present work. By merging mass flow rate and spray visualization, the global fuel concentration in the spray is obtained. This could give some information about which fuel has the worst atomization and so, this could explain the differences observed throughout the literature16,46 concerning ignition and combustion in diesel engines operating with biodiesel. For injection processes, these studies observed that a higher fuel mass has to be injected to compensate the lower heating value. During the image processing, the spray volume is calculated; the pattern of the spray is assumed to be revolution-shaped. The error on spray volume calculated by this method has been evaluated to be less than 4%. Integration of the mass flow rate curve gives the injected mass along the injection time. When these two data are put together, the overall fuel mass concentration in the spray is obtained. Figure 12 shows the volume of the spray calculated during the acquisition of the pictures. On this graph, it appears clearly that the pure biodiesel (RME) has sprays with smaller volumes. Knowing the volume of the spray and the injected mass of the three tested fuels, the fuel concentration is calculated as

Figure 13 gives the fuel concentration in the spray for the three fuels at low injection pressure (300 bar). These curves are explicit enough to understand that a spray has a higher fuel concentration when RME is injected. Narrower cone angles with the pure biodiesel lead to global smaller sprays, and because the injection rate is higher with this fuel, the result is a higher mass fraction. In addition, the fuel concentration decreases really slowly after some time ASOI and seems to have a similar tendency with the different fluids, this time depends on injection pressure, the spray development being different at low and high injection pressures. When RME is injected, a higher fuel mass fraction is present in the chamber than when regular diesel fuel is injected. This result leads to get a worse atomization and, in that way, a worse fuel/air mixing. Some works5,44 found that the use of biodiesel in real engines provides shorter sprays; they concluded that different boiling points and stoichiometric ratio have stronger influences on flame length than the evaporation and the fuel mass fraction in the spray. This means that the results revealed in this work about fuel mass fraction are secondary because this apparent worse fuel/air mixing is not relevant when the fuel is injected in a real engine.

xf )

mf ) mT

mf

(

mf mf + Fg VT Ff

)

(14)

where xf, fuel mass fraction; mf, accumulated injected mass; Ff: fuel density; Fg density of gas (N2) in chamber; VT, total spray volume. (44) Mueller, C. J. An introduction to biodiesel. For EST Article #104, 2007. (45) Higgins, B. S.; Mueller, C. J. and Siebers, D. Measurements on fuel effect on Liquid-phase penetration in DI sprays, SAE Paper 1999-010519, 1999. (46) Ramadhas, A. S.; Jarayaj, S.; Muraleedharan, C. Use of vegetable oils as I.C. engine fuels. A review. Renewable Energy 2004, 29.

5. Conclusions The paper presents a comparison of the effect of the use of three different fuels on injection process and spray development. All the tests performed indicate that the pure biodiesel (RME) has a different trend than the two others, which are really close. In this experimental study, it has been seen that the pure biodiesel, produced from rapeseed oil, has a higher mass flow rate and similar spray momentum flux. Injection rate is affected by fuel density and spray momentum is not. Information coming from injection rate measurements told us that the dynamic of the injector is also affected by using biodiesel; the higher viscosity of biodiesels makes the needle lift slower, this effect of viscosity is very important in small injections when the mass injected is controlled by needle dynamic. It has been noticed that RME has slightly longer penetration and as expected from spray momentum results, the cone angle is narrower with this fuel; this means smaller spray volumes when RME is injected. Theoretical comparisons have been made to verify the results obtained experimentally and to understand better what properties influence these results. The velocity calculation shows that the pure biodiesel has a slower effective velocity than the others, certainly due to its higher density. The surface tension strongly affects the spray development and as biodiesel has larger droplets, the inertial forces make the spray longer and narrower. This directly leads to spray with

Biodiesel Blends’ Effects

smaller volumes and as a consequence with higher mass flows and a higher fuel concentration in the spray when using biodiesel. This should affect negatively ignition timings and combustion process, however, it seems that biodiesel molecular composition (overall O2 content) is more relevant and therefore, changes produced in mixing process when a biodiesel is used are highly compensated.

Energy & Fuels, Vol. 23, 2009 3235 Acknowledgment. This research has been financially supported by the project TRA2006-15620-C02-02 provided by Spanish ministry of education and sciences. The authors thank Jose´ Enrique del Rey (from CMT-Motores Te´rmicos, Universidad Polite´cnica de Valencia) for his collaboration in the experimental measurements. EF801102W