Energy Fuels 2010, 24, 4341–4350 Published on Web 07/15/2010
: DOI:10.1021/ef1003914
Effect of Fuel Properties on Spray Breakup and Evaporation Studied for a Multihole Direct Injection Spark Ignition Injector Lars Zigan,*,† Ingo Schmitz,† Alexandre Fl€ ugel,† Tobias Knorsch,‡ Michael Wensing,†,‡ and Alfred Leipertz†,‡ †
Lehrstuhl f€ ur Technische Thermodynamik and Erlangen Graduate School in Advanced Optical Technologies (SAOT), Universit€ at Erlangen-N€ urnberg, Am Weichselgarten 8, D-91058 Erlangen, Germany, and ‡ ESYTEC Energie-und Systemtechnik GmbH, Am Weichselgarten 6, D-91058 Erlangen, Germany Received March 30, 2010. Revised Manuscript Received June 25, 2010
Mixture formation in spray-guided direct injection spark ignition engines (SG-DISI) with late injection timing is mainly controlled by spray atomization and evaporation and strongly depends on fuel properties. The influence of fuel composition on the liquid spray structure was determined for a 12-hole solenoid injector with integral and light sheet Mie scattering as well as phase Doppler anemometry (PDA). Late injection timing in a high pressure atmosphere was simulated in an injection chamber (1.5 MPa, 293 and 673 K) to characterize the spray propagation and evaporation of alkanes with high and low volatility (n-hexane, n-heptane, n-decane) and a 3-component mixture of these alkanes with similar fuel properties like gasoline fuel. Under high chamber pressure and low ambient temperature, the spray propagation and the resulting droplet sizes are similar for all fuels. However, for n-decane, the droplet size distribution is shifted to smaller droplets and the spray appeared to be less dense because of fuel-dependent internal nozzle flow which results in a reduced injected fuel mass. In contrast, under high ambient temperature conditions for more volatile fuel components, the liquid spray length is reduced and droplet size as well as droplet momenta are decreased. Small amounts of high boiling fractions delay the evaporation and support the overall spray stability also for multicomponent mixtures which is indicated by increased spray length as well as larger droplet sizes and momenta. Moreover, the droplet size distributions and the small liquid Peclet numbers (PeL ≈ 1) of the 3-component fuel indicate a demixing of light and heavy boiling components in the 3-component fuel under conditions which are typical for DISI strategies with late injection.
spray. Details of the optical engine and the Tracer-LIF (laserinduced fluorescence) technique for determination of the λ-distribution are described elsewhere.1 So far, piezoelectric actuated injectors with outwardly opening pintle nozzles were applied for SG-DISI engines. This injector type is advantageous since a reproducible spray is formed with a stable spray cone angle and a short penetration also under increased back pressure. The characteristic outer vortex structures with small droplets are preferred for ignition. However, those injectors are very complex, and the improvement of sprays of less-expensive multihole injectors is aspired. The main challenge is to ensure an ignitable mixture at the spark plug for all engine operating points and for the usage of different worldwide available fuel compositions. To optimize combustion concepts, optical measurement techniques and spray modeling have become important tools. Nevertheless, most of the process steps and their interactions are still not well understood. Current CFD (computational fluid dynamics) models are limited in their application as they still need simplified correlations and measurement data for model calibration. Therefore, predictions of the whole spray process starting from the turbulent nozzle flow until the end of the combustion is hardly possible. On that account, for modern fuels like bioethanol, vegetable oil, and biodiesel, numerous experiments are conducted
1. Introduction Among the different gasoline engine combustion concepts with direct injection, the spray-guided technologies with charge stratification offer the highest potential to reduce fuel consumption. In spray-guided direct injection spark ignition (SG-DISI) engine concepts, the central position of the injector and a relatively small distance to the spark plug is preferred. For full load operating points, an early injection during the inlet stroke leads to a homogeneous mixture in the cylinder at the ignition time point. For partial load conditions, charge stratification is preferable to reduce fuel consumption. For those operating points with late injection, the fuel is injected in pressurized charge and only close to the spark plug an ignitable mixture is positioned whereas the cylinder volume is filled with excess air or recirculated exhaust gas (EGR). Therefore, a fast and reproducible fuel atomization and evaporation are required to ensure a stable ignition of the fuel-air mixture. Those extremely short mixing time scales dominate the whole combustion process and the pollutant formation. In Figure 1, typical injection frames are presented for such a spray guided concept in an optical accessible engine. The setup includes a research injector (12-hole solenoid injector, which is described in section 2.1) close to the spark plug. The fuel distribution is given by the fuel-air ratio λ for the gas phase. At the late injection time point, still, liquid fuel is present that marks the hollow cone
(1) Ipp, W.; Egermann, J.; Schmitz, I.; Wagner, V.; Leipertz, A.; Hartmann, M.; Schenk, M. SAE Technical Paper Series 2001-01-1977, 2001.
*Corresponding author. Fax: þ49-9131-85-29901. Phone: þ49-913185-29770. E-mail:
[email protected]. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 4341–4350
: DOI:10.1021/ef1003914
Zigan et al.
Figure 1. Optical accessible engine with arrangement of the injector close to the spark plug and typical injection frames for partial and full load operating points. BDC=bottom dead center of the piston, TDC=top dead center, SOI=start of injection, EOI=end of injection. Recording time point is 18 °CA (crank angle) BTDC.
to examine atomization and spray formation2-4 as well as pollutant formation.5 To reduce the complexity of the spray processes of common gasoline fuels, most often single-component substitute fuels are used which are not capable to model realistic fuel behavior. Contrariwise, optical detection of different fuel fractions in a realistic multicomponent mixture is complex. Especially, the prediction and measurement of fuel evaporation under engine-like conditions are still challenges. The evaporation of single- and multicomponent fuels was studied for several decades. Numerous experiments for single droplets were conducted to describe the fuel evaporation behavior.6 However, engine-related processes of dense sprays under high pressure and turbulence conditions are much more complex compared to the evaporation of isolated single droplets. In general, for direct injection engines with early injection at low cylinder pressure and near ambient gas temperatures, the release of light and heavy ends may potentially take place at different times and locations in the cylinder. In this case, evaporation is suppressed by comparative low temperature. Fuel components with high volatility evaporate faster, and components with high boiling points remain in the droplet and determine the droplet breakup and evaporation at later points in time. However, long residence time and charge motion enhances mixing of light and heavy boiling ends in the vapor phase with air. Consequently, almost no vapor componentstratification at the ignition point was detected in transparent engines.7-11
For late injection strategies with high ambient pressure, the evaporation behavior is expected to be different compared to low ambient pressures and temperatures. The effect of the diffusion resistance on the evaporation is described by a modified liquid Peclet number PeL,12 which considers the ratio of the surface regression rate K (or evaporation rate) and the liquid diffusion coefficient DL (which reflects internal mixing of the components inside the droplet): 1 K ð1Þ PeL ¼ 8 DL For fast evaporation or suppressed internal diffusion (PeL . 1),12,13 coevaporation of the components is assumed and the droplet composition is not changed during the evaporation process. This can be assumed for fuel mixtures under high pressure and high temperature conditions.12 For PeL , 1, equilibrium evaporation takes place following the fuel distillation curve and a change of the spatial droplet composition occurs. However, this approach does not consider internal recirculations inside the droplet which also enhance internal mixing of the components in the liquid phase. This is caused by high relative velocities between the droplet and the surrounding gas and could lead to a fast mixing of the components, and therefore, it could support the coevaporation of the components especially in regions close to the injector.14 For late injection into high pressure atmosphere, only few measurement data are available which describe the evaporation behavior of multicomponent gasoline fuels. Egermann and Leipertz15 analyzed the spray evaporation with the Raman scattering technique in an injection chamber. Demixing of the high and low boiling components was detected inside the gas phase of the spray also for an ambient pressure of 3.0 MPa, a rail pressure of 10 MPa, and an ambient temperature of 523 K. This is expressed by a distinct larger vapor concentration of the fuel component with high volatility at early observation times. The chosen measurement location was 41 mm
(2) Kim, H. J.; Suh, H. K.; Park, S. H.; Lee, C. S. Energy Fuels 2008, 22, 2091–2098. (3) Park, S. H.; Suh, H. K.; Lee, C. S. Energy Fuels 2008, 22, 605–613. (4) Park, S. W.; Kim, S.; Lee, C. S. Energy Fuels 2006, 20, 1709–1715. (5) Tsolakis, A. Energy Fuels 2006, 20, 1418–1424. (6) Sirignano, W. A. Fluid Dynamics and Transport of Droplets and Sprays; Cambridge University Press, Cambridge, U.K.: 1999. (7) Han, D.; Steeper, R. R. Proc. Combust. Inst. 2002, 727–734. (8) Kr€ amer, H.; Einecke, S.; Schulz, C.; Sick, V.; Nattrass, S. R.; Kitching, J. S. SAE paper No. 982467, 1998. (9) Tonini, S.; Gavaises, M.; Arcoumanis, C.; Theodorakakos, A.; Kometani, S.; Proc. IMechE 221 Part D: J. Automobile Engineering, 2007. (10) Vanderwege, B. A.; Hochgreb, S. Proc. Comust. Inst. 1998, 1865– 1871. (11) Williams, B.; Ewart, P.; Stone, R.; Ma, H.; Walmsley, H.; Cracknell, R.; Stevens, R.; Richardson, D.; Qiao, J.; Wallace, S. SAE Technical Paper Series 2008-01-1073, 2008.
(12) Makino, A.; Law, C. K. Combust. Flame 1988, 73 (3), 331–336. (13) Burger, M.; Schmehl, R.; Prommersberger, K.; Sch€afer, O.; Koch, R.; Wittig, S. Int. J. Heat Mass Transfer 2003, 46 (23), 4403–4412. (14) Stegemann, D.; Merker, G. P. ILASS-Europe 2005, 20th Annual Conference on Liquid Atomization and Spray Systems, Orleans (France), 2005. (15) Egermann, J.; Leipertz, A. SAE Technical Paper Series 2000-012863, 2000.
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downstream from the nozzle, and the measurements were conducted between 5 and 15 ms after start of injection. This implies that at late time points after injection the liquid Peclet numbers PeL must be very small due to a relatively low evaporation rate. Myong et al.16 presented numerical results of a 3-component fuel which show a stratification of less and more volatile components in the gas phase of the spray at an ambient temperature of up to 700 K, a chamber pressure of 3.0 MPa, and an injection pressure of 72 MPa. The more volatile fuel isooctane evaporates earlier and close to the nozzle and is mainly distributed in the central spray region. The vapor concentration of the less volatile n-hexadecane is observed mainly around the spray tip region, and for n-dodecane, an intermediate trend of the vapor spatial distribution is obtained. In contrast to this, Hoffmann et al.17 detected a very similar vapor propagation of different boiling point fuel components for sprays under Diesel engine conditions by means of laserinduced fluorescence (prail = 60 MPa, pchamber = 5 MPa, Τchamber = 800 K) indicating a very rapid evaporation, i.e., coevaporation of the components. From this, it can be concluded that the multicomponent fuel spray evaporation under a high pressure atmosphere is still a research topic with many open questions. Especially, there is still a lack of quantitative information about the evaporation of the fuel components at the time point of ignition at the spark plug. A stratification of fuel components with different ignition behavior in the region of the spark plug can effect combustion and pollutant formation. The spark plug is usually positioned at 10-15 mm from the injector nozzle for spray-guided concepts and ignition starts approximately 1 to 2 ms after start of injection for stratified engine operating conditions and occurs potentially in the presence of evaporating droplets. In commercial CFD codes, common implemented evaporation models assume “well mixed” conditions within the droplet, i.e., there is a uniform temperature and fuel distribution of the liquid phase throughout the droplet evaporation process. Therefore, this model is limited to a fast heat and mass transport;6 however, the evaporation of multicomponent fuels generally lies in between the well-mixed and the diffusionlimit cases (i.e., distillation-like evaporation), especially under high pressure conditions.18 These effects are considered in recently developed more complex evaporation models,13,18,19 but still, a lack of experimental data for validation and verification under DISI engine-like conditions exists. For a deeper insight into evaporation processes, this study observes a 3-component fuel in comparison with several singlecomponent model fuels in an optical accessible pressurized and heated injection chamber. Therefore, the spray processes are decoupled from the flow and turbulence pattern occurring in an internal combustion engine. A simplified 3-component fuel was chosen to describe effects of main fractions in the fuel. This fuel mixture was proved and tested to model the realistic spray behavior of a piezo-actuated pintle nozzle with a multicomponent gasoline fuel under charge stratification condi-
tions very precisely. Also, a numerical implementation of this fuel can reduce the computational effort. Additionally, different single-component fuels were used which provide a more comprehensive insight in the spray subprocesses. To study the fuel dependent atomization and evaporation behavior, the sprays were investigated under high pressure as well as low and high ambient temperature conditions. To describe the liquid spray structures and evaporating droplets, Mie imaging and phase Doppler anemometry (PDA) measurements were applied for the investigation of a 12-hole solenoid DISI-injector. 2. Experimental Section 2.1. Injection Chamber and Tested Fuels. A chamber pressure of 1.5 MPa and temperatures of 673 K were chosen to model late injection timing. These conditions refer to the end of injection close to the ignition time point; see Figure 1. Additionally, the experiments were also conducted at 293 K and 1.5 MPa to study the fuel dependent atomization. A constant air flow through the injection chamber heats the chamber and scavenges it from one injection to another (flow velocity 30, the resulting effec(27) Moore, J. W.; Wellek, R. M. J. Chem. Eng. Data 1974, 19 (2), 136–140. (28) Matthews, M. A.; Akgerman, A. AIChE 1987, 33 (No. 6), 881-885. (29) Dean, J. A. Lange’s Handbook of Chemistry; McGraw-Hill, Inc.: New York, 1999; pp 5.91-5.104. (30) Pandey, J. D.; Vyas, V.; Jain, P.; Dubey, G. P.; Tripathi, N.; Dey, R. J. Mol. Liq. 1999, 81, 123–133. (31) Wagner, V.; Goldl€ ucke, J.; Seelig, O.; Leipertz, A. In Engine Combustion Processes (VI. Congress); Leipertz, A., Ed.; ESYTEC: Erlangen, 2003; p. 254.
4. Conclusion A detailed spray characterization has been performed in a pressurized and heated injection chamber under late injection timing conditions (1.5 MPa, 293 and 673 K) for a 12-hole 4349
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solenoid injector which was designed for the application in a DISI engine. The influence of the fuel properties on the liquid spray and evaporation behavior has been demonstrated by the application of the integral and light sheet Mie imaging technique and the phase Doppler anemometry. The PDA measurement locations were chosen to represent typical positions of the spark plug for spray guided combustion concepts. A 3-component fuel which was designed to meet the same boiling behavior like gasoline (35% n-hexane, 45% n-heptane, 20% n-decane) showed a different spray structure compared to the high and low boiling point single-component gasoline substitute fuels. For low ambient temperature and high chamber pressure, the atomization behavior was similar for all fuels; however, for n-decane, the spray appeared less dense. This results are due to a reduced injected liquid mass because of higher liquid viscosity of n-decane at the same injection quantities. Therefore, the droplet size distribution is shifted to smaller diameters. Also, the subsequent secondary breakup of n-decane droplets is delayed which was indicated by reduced Weber numbers and lead to increased liquid spray penetration and droplet momentum. For high ambient temperature conditions, the droplet size and momentum is mainly controlled by evaporation. The droplet sizes and numbers as well as droplet Weber numbers of the 3-component fuel lied between those of n-heptane and n-decane which indicated that also the droplet evaporation was delayed by a small amount of less volatile fuels. From the resulting droplet size distributions and the droplet lifetimes as well as the estimated local evaporation rates, it can be concluded that in the spray a demixing of fuel components in the 3-component fuel occurs. However, the evaporation behavior is complex. The estimated liquid Peclet numbers (PeL), which
describe the ratio of evaporation rate to the mass diffusion of the components inside the droplet, lie between the limiting cases of distillation-like evaporation (PeL , 1) and coevaporation of the fuel components (PeL . 1) and increase during injection time (PeL ≈ 0.83. . .1.26). However, the evaporation of the 3-component fuel close to the spark plug position is more similar to batch distillation in comparison to numerical results of single droplet and spray calculation in the literature. This leads to the conclusion that also under high chamber pressure and temperature a stratification of the fuel vapor components can occur similar to early injection strategies under suppressed evaporation at low atmospheric pressure and temperatures. This results in a larger droplet momentum of the 3-component fuel and an increased spray penetration similar to fuels with low volatility. The droplet Reynolds numbers were found to be high enough (25-70) to support the heat and mass transport inside the droplet by flow induced internal recirculations. For an accurate simulation of the droplet evaporation in spray guided DISI engines with late injection timing, enhanced models are necessary which account for the distillation-like evaporation and the coevaporation of multicomponent fuels. Additionally, the transition evaporation regions between the two limitations have to be described. The data is applicable for the validation and verification of spray breakup and evaporation models within CFD. Acknowledgment. The authors gratefully acknowledge the financial support for parts of this work from the Erlangen Graduate School in Advanced Optical Technologies within the framework of the German Excellence Initiative by the German National Science Foundation (DFG).
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