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Energy & Fuels 2009, 23, 1406–1411
Analysis of Flame and OH* Natural Emissions of n-Heptane Combustion in a Homogeneous Charge Compression Ignition (HCCI) Engine: Effect of Burnt Gas Dilution Anthony Dubreuil, Fabrice Foucher, and Christine Mounaïm-Rousselle* Institut PRISME, UniVersite´ d’Orle´ans, 8 rue Le´onard de Vinci, 45072 Orle´ans Cedex 2, France ReceiVed July 11, 2008. ReVised Manuscript ReceiVed December 9, 2008
The present study examines the global effect of a laboratory-simulated exhaust gas recirculation (EGR) on the homogeneous charge compression ignition (HCCI) combustion of n-heptane in a transparent monocylinder diesel engine. The investigations were carried out during the engine cycle for two EGR rates, 0 and 30%, at a constant 0.3 equivalence ratio. The analysis is focused on the relation between natural combustion emissions during the cold and main combustion phases and the OH* chemiluminescence and the processes involved inside the combustion chamber of a compression engine. By observing cool and main flame emissions, it was observed that the dilution by burnt gas delays and degrades the combustion phenomenon. For both cases, the natural flame emission is not uniformly distributed, because of the nonhomogeneity of the gas temperature. By coupling kinetic analysis, provided by a zero-dimensional model, it was shown that the natural emissions of combustion are sufficiently sensitive to yield combustion process analysis.
Introduction Lean premixed combustion applied to diesel engines [homogeneous charge compression ignition (HCCI) or low-temperature combustion] is considered to be one of the most promising combustion regimes, which has the potential to reduce both nitrogen oxides (NOx) and particle matter (PM).1 The HCCI combustion mode is based on the auto-ignition of a homogeneous air-fuel mixture (with or without burnt gas dilution) by compression heating. The diluted homogeneous mixture autoignites and burns with a macroscopic homogeneity throughout the cylinder,2 generating low-temperature regions. Many HCCI combustion-mode studies have been published and are references in the HCCI domain.3-9 The combustion is controlled by chemical kinetics processes, which induce difficulties to control ignition timing during the cycle. The control of autoignition and combustion development is particularly difficult and still requires basic studies about physical and chemical processes to improve this concept and to optimize future engine efficiency in terms of pollutant emissions, combustion control, and fuel consumption reduction.10-13 To perform experiments * To whom correspondence should be addressed. Telephone: +33-238-49-43-62. Fax: +33-2-38-41-73-83. E-mail: christine.rousselle@ univ-orleans.fr. (1) Christensen, M.; Hultqvist, A.; Johansson, B. Demonstrating the multi-fuel capability of a homogeneous charge compression ignition engine with variable compression ratio. SAE Tech. Pap. 1999-01-3679, 1999. (2) Hultqvist, A.; Christensen, M.; Johansson, B.; Franke, A.; Richter, M.; Alde´n, M. A study of the homogeneous charge compression ignition combustion process by chemiluminescence imaging. SAE Tech. Pap. 199901-3680, 1999. (3) Onishi, S.; Jo, S. H.; Shoda, K.; Jo, P. D.; Kato, S. Active thermoatmosphere combustion (ATAC)sA new combustion process for internal combustion engines. SAE Tech. Pap. 790501, 1979. (4) Ishibashi, Y.; Asai, M. Improving the exhaust emissions of twostroke engines by applying the activated radical combustion. SAE Tech. Pap. 960742, 1996. (5) Takeda, Y.; Keiichi, N.; Keiichi, N. Emission characteristics of premixed lean diesel combustion with extremely early staged fuel injection. SAE Tech. Pap. 961163, 1996.
Figure 1. Global scheme of the experimental setup.
in transparent engines by means of optical diagnostics techniques provides valuable information about thermodynamics, fluid mechanics, and kinetics processes. The global flame emission information, coupled with OH* emission, contributes to better investigation.14 In the present study, experiments were performed on an optical-access HCCI engine equipped with specific devices to simulate dilution by burnt gases. The use of this dilution [called exhaust gas recirculation (EGR)] is one way to limit cylinder pressure gradients and to adjust the ignition timing in the cycle.15,16 This dilution is thus a major factor, and its effect on HCCI development must be investigated. During the HCCI combustion of pure n-heptane, as a function of the dilution rate (0 and 30%), the natural and OH* chemiluminescence emissions of cool and main flames were recorded. Simultaneous to these experiments, a computational zero-dimensional model was used to supply information on chemical processes brought into play in the combustion development. Experimental and Numerical Procedures Experimental Setup. The optical-access diesel engine used during the experiments was a 499 cm3 single-cylinder, four-stroke engine. The engine specifications are reported Table 1. The cylinder pressure was measured by a Kistler 6143A model pressure
10.1021/ef800533c CCC: $40.75 2009 American Chemical Society Published on Web 01/28/2009
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Figure 2. Average images of cool flame light emissions during the cycle for the 0 and 30% EGR rate. Table 1. Engine Specifications item
specifications
type displacement of the active cylinder (cm3) bore (mm) stroke (mm) compression ratio
one cylinder, four stroke 499 85 88 16:1
Table 2. Inlet Conditions species (in moles)
0% EGR
30% EGR
air CO2 H20 N2 C7H16
176 0 0 0 1
153 3 0 21.2 1
water was replaced by a N2 quantity, to avoid the difficulty in controlling the quantity of water added. The equivalence ratio was maintained at 0.3. Then, the gaseous mixture, regulated in pressure, is heated by an electrical system. The inlet air-fuel-EGR mixture is maintained at a constant temperature before being sucked into the engine. The concentrations of all species for both conditions are specified in Table 2. For both cases, the heat capacity is equal to 1.02 kJ kg-1 K-1. The design of the transparent engine used a classic extended piston with a piston crown window, providing a full view of the combustion bowl by means of an UV-vis (45°) mirror arranged inside the extended piston (Figure 1).The images of combustion
Figure 3. Temporal evolution of the heat release rate and the average emission intensity during the cool flame for (a) 0% and (b) 30% EGR.
transducer. A shaft encoder on the crankshaft (7200 points per cycle) is used to clock pressure data acquisition. The pressure trace was obtained by averaging 100 sampled pressure data, and the net heat release rate (HRR) was then deduced. The fuel is introduced through a heating tube via an electromagnetic injector (at 0.25 MPa injection pressure) to control perfectly the air-fuel mixture distribution inside the cylinder. To study the dilution effect on the HCCI combustion development, a laboratory-simulated EGR system was developed. Three flowmeters, for air, CO2, and N2, are used to create the EGR.16 The
(6) Lavy, J.; Dabadie, J.-C.; Angelberger, C.; Duret, P.; Willand, J.; Juretzka, A.; Scha¨flein, J.; Ma, T.; Lendresse, Y.; Satre, A.; Schulz, C.; Kra¨mer, H.; Zhao, H.; Damiano, L. Innovative ultra-low NOx controlled auto-ignition combustion process for gasoline engines: The 4-SPACE project. SAE Tech. Pap. 2000-01-1837, 2000. (7) Thring, R. H. Homogeneous-charge compression-ignition (HCCI) engines. SAE Tech. Pap. 892068, 1989. (8) Zhao, F.; Asmus, T.; Assanis, D.; Dec, J.; Eng, J.; Najt, P. Homogeneous charge compression ignition (HCCI) engines: Key research and development issues. SAE, PT-94, 2003. (9) Dec, J.; Sjoberg, M. An investigation into lowest acceptable combustion temperatures for hydrocarbon fuels in HCCI engines. Proc. Combust. Inst. 2005, 30, 2719–2726. (10) Lu, X.; Chen, W.; Huang, Z. A fundamental study on the control of the HCCI combustion and emissions by fuel design concept combined with controllable EGR. Fuel 2005, 84, 1084–1092. (11) Andrae, J. C. G.; Brinck, T.; Kalghatgi, G. T. HCCI experiments with toluene referencn fuels modeled by a semidetailed chemical kinetic model. Combust. Flame 2008, 155, 696–712. (12) Machrafi, H.; Cavadias, S.; Guibert, P. An experimental and numerical investigation of external gas recirculation on the HCCI autoignition process in an engine: Thermal, dilution and chemical effects. Combust. Flame 2008, 155, 476–489. (13) Mital, G.; Sung, C. Homogeneous charge compression ignition of binary fuel blends? Combust. Flame 2008, 155, 431–439. (14) Mancaruso, E.; Merola, S. S.; Vaglieco, B. M. Extinction and chemiluminescence measurements in CR DI diesel engine operating in HCCI mode. SAE Tech. Pap. 2007-01-0192, 2007.
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Figure 4. Averaged emission images during main combustion phase for 0 and 30% EGR.
Figure 6. Modeling of the main combustion phase: heat release rate and chemical species involved in CO oxidation for (a) 0% and (b) 30% EGR.
Figure 5. Heat release rate and average image intensity evolutions during the main combustion phase for (a) 0% and (b) 30% EGR.
emissions were recorded by means of the intensified CMOS APX PHOTRON camera (15 000 frames per second, 256 × 256 pixels2), placed in front of the mirror (Figure 1). This camera associated (15) Morimoto, S. S.; Kawabata, Y.; Sakurai, T.; Amano, T. Operating characteristics of a natural gas-fired homogeneous charge compression ignition engine (performance improvement using EGR). SAE Tech. Pap. 2001-01-1034, 2001. (16) Dubreuil, A.; Foucher, F.; Mounaı¨m-Rousselle, C. Effect of EGR chemical components and intake temperature on HCCI combustion development. SAE Tech. Pap. 2006-32-0044, 2006.
with a GEN II intensifier allows for the recording of the light emission from 300 to 700 nm. Because of the different emission levels between the cool and main flame, the gate width of the intensifier needs to adapt to adjust as well as possibly the signal-to-noise ratio. For the cool flame, the gate width was set to 35 µs, with the highest intensifier gain value. Both cycles, 20 images of luminescence, were recorded from 27 crank angle degrees (CAD) before top dead center (BTDC). For main flame visualization, the 4 µs intensifier gate width was used and 60 images were recorded for both cycles, from 20 CAD BTDC. For both of these studies, the images were obtained using an 85 mm f/1.4 NIKON visible-lens objective without a spectral filter. Because the A f X emission of OH is centered at 306.4 nm,17 to detect the spatial distribution of OH radical, a narrow-band filter with a central wavelength of 307.1 nm ((10 nm) was placed between a 105 mm NIKON UV-lens objective and the intensified CMOS camera. A sequence of 50 images was recorded from 20 CAD BTDC at both cycles. (17) Gaydon, A. G. The Spectroscopy of Flames; Chapman and Hall: London, U.K., 1974.
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EGR Effect on the Cool and Main Flame Intensity. Figure 2 shows average luminescence images recording during the first
step of the HCCI combustion, i.e., the cool flame, for 0 and 30% EGR. These images are normalized by the arbitrary maximum intensity of the 0% EGR case. Despite a high acquisition frequency, the cool flame is distinguishable during five degrees. As a function of the EGR rate, the average intensity decreases and is slightly delayed in the cycle. By considering the spatial distribution, the intensity is not spatially similar, showing that the cool flame initiation does not occur homogeneously despite the mixture homogeneity. Indeed, at -23.4 CAD, one can note that the intensity is more important especially in the zone close to the wall, on the right of the image. At -22.8 CAD, the global intensity and the spatial distribution increases to become more homogeneous; a maximum level is reached at -22.2 CAD. With the increase of the EGR rate, the light emission is globally lower because of the mixture dilution but the spatial distribution remains unchanged. The air-fuel-EGR mixture homogeneity had been checked previously;16 therefore, the spatial distribution of the flame must be linked to the temperature heterogeneity. In Figure 3, the evolution of the mean intensity is plotted in comparison to the heat release rate for 0 and 30% EGR. One can observe the good concordance between the heat release rate and the light emission maxima for both EGR rates. In Figure 4, the global effect of the EGR rate on the main flame can be observed. The average luminescence intensity images during the main flame phase are presented from -10.8 to -8.4 CAD and from -7.8 to -5.4 CAD, respectively, for 0 and 30% EGR. The combustion process seems to occur throughout the cylinder. However, a slight parietal effect is observed for the 0% EGR case at -9.6 CAD. A kind of ring is formed closed to the wall with a lower intensity than at the center of the chamber. This confirms the impact of the colder wall temperature and the cylinder temperature heterogeneity. Nevertheless, one can observe a relatively important degree of homogeneity on a macroscopic scale. The EGR effect affects the main flame luminescence without degrading the average homogeneity of the combustion process. The normalized average main flame luminescence signal is plotted with the normalized heat release rate, during the main combustion phase, in Figure 5 for both EGR cases (0 and 30%). For both EGR cases, the intensity signal is not phased with the heat release rate, contrary to the cool flame case. The combustion visualization was recorded on the large visible wavelength range: therefore, the signal can be due to different species and also the CO-O* radical emission, as in premixed flames.21 Indeed, this radical species has a large wavelength emission range, between 300 and 600 nm, covering a large part of the visible spectrum. Kim et al.,22 in their study of the HCCI combustion, showed experimentally that CO-O* emission dominates the visible spectrum and that its signal is clearly superimposed on the heat release rate. This result is not in good agreement with this present study. However, Kumano et al.,23in their study of the DME HCCI combustion performed in a rapid compression machine, showed that the main flame luminescence
(18) Lutz, A. E.; Kee, R. J.; Miller, J. A. Senkin: A Fortran program for predicting homogeneous gas phase chemical kinetics with sensitivity analysis. Report SAND87-8248, Sandia National Laboratories, Albuquerque, NM, 1988. (19) Chang, J.; Gu¨ralp, O.; Filipi, Z.; Assanis, D.; Kuo, T.-W.; Najt, P.; Rask, R. New heat transfer correlation for an HCCI engine derived from measurements of instantaneous surface heat flux. SAE Tech. Pap. 200401-2996, 2004. (20) Dubreuil, A.; Foucher, F.; Mounaı¨m-Rousselle, C.; Dayma, G.; Dagaut, P. HCCI combustion: Effect of NO in EGR. Proc. Combust. Inst. 2007, 31 (2), 2879–2886.
(21) Samaniego, J.-M.; Egolfopoulos, F. N.; Bowman, C. T. CO2 Chemiluminescence in Premixed Flames. Combust. Sci. Technol. 1995, 109, 183–203. (22) Kim, B.; Kaneko, M.; Ikeda, Y.; Nakajima, T. Detailed spectral analysis of the process of HCCI combustion. Proc. Combust. Inst. 2002, 29 (1), 671–677. (23) Kumano, K.; Iida, N. Analysis of the effect of charge inhomogeneity on HCCI combustion by chemiluminescence measurement. SAE Tech. Pap. 2004-01-1902, 2004. (24) Dec, J. E. A computational study of the effects of low fuel loading and EGR on heat release rates and combustion limits in HCCI engine. SAE Tech. Pap. 2002-01-1309, 2002.
Figure 7. Modeling of the cool combustion phase: heat release rate and chemical species involved in CO oxidation for (a) 0% and (b) 30% EGR.
Zero D Modeling of HCCI Combustion. To better understand the processes involved during HCCI combustion, a zero D model was developed: a variable volume based on the engine geometrical data was considered, with a zero-dimensional singlezone reactor using the SENKIN code from the CHEMKIN II library18 associated with a modified Woschni heat loss model,19 to compare respective net heat releases. From the experimental data, we calculated Tbottom dead center (with the ideal gas law), which we introduced into the model as the initial condition at the beginning of the compression stroke. A detailed kinetic scheme of n-heptane/toluene (533 species and 2893 reactions) was used to simulate the n-heptane oxidation in engine HCCI mode. More details about the modeling and kinetic scheme, established from experimental data obtained in a jet stirred reactor, are presented in a previous paper.20 Operating Conditions Investigated. The experimental tests were performed with n-heptane as fuel (>99% pure). Two EGR rates (0 and 30%) were used at an equivalence ratio (φ) maintained at 0.3. The engine speed, the intake pressure (Pin), and the intake temperature (Tin) were kept constant, respectively, to 1500 rpm, 0.1 MPa, and 75 °C. The operating conditions are summarized in Table 2.
Results and Discussion
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Figure 8. Spatio-temporal averaged image intensity of the OH* chemiluminescence for 0 and 30% EGR.
emission appears just after the main flame and that the luminescence of the flame comes from CO-O* emission. The chemical reactions of the continuum CO-O* can be given by21 CO + O + M ) CO-O* + M CO-O* ) CO2 + hν
(R1) (R2)
CO-O* + M ) CO2 + M
(R3)
In the first step, reaction R1 involves CO and O in their electronic ground state, leading to the formation of electronically excited CO-O*. CO-O* returns to the ground state with reactions R2 and R3, with reaction R2 being responsible for the emitted light. It can be observed on the main heat release (Figure 5) that two peaks appear with the increase of the EGR rate. This phenomenon is the consequence of the difficulty to convert CO to CO-O*,23 by the following reaction:25,26 CO + OH ) CO2 + H
(R4)
When the EGR rate increases, the in-cylinder temperature is not sufficiently important for this reaction, which slows down. From the experimental results, it can be seen that the doublepeak phenomenon becomes more and more important with the increase of the EGR rate. The luminescence emission during the main combustion, dependent upon the CO oxidation, agrees well with this double peak. For the highest EGR rate, the luminescence can be collected during more crank angles, indicating the difficulty converting CO to CO2. Figure 6 presents the heat release, issued from the modeling of n-heptane HCCI combustion for 0 and 30% EGR, with the normalized mole fraction species taking part in CO oxidation (CO, O, OH, H, and CO2) and the reaction rates of R1 and R4. CO oxidation reactions (R1 and R4) take place rather late in the main heat release development: the reaction that is responsible for the double peak (R4) is well-phased with them. Reaction R1 is phased with reaction R4 at a reaction rate 10 times lower. When the EGR rate increases, reaction R4 becomes less effective and the amplitude of the second peak of the heat release increases slightly, as observed from experimental results. The emission signal seems to be in good accordance with reaction R1, with a decrease of its reactivity between 0 and 30% EGR. Thus, the data from the modeling completely explain the luminescence signal decrease, linked to the EGR rate. The (25) Bhave, A.; Kraft, M.; Montorsi, L.; Mauss, F. Sources of CO emissions in an HCCI engine: A numerical analysis. Combust. Flame 2006, 144, 634–637. (26) Ogink, R.; Golovitchev, V. Gasoline HCCI modeling: An engine cycle simulation code with a multi-zone combustion model. SAE Tech. Pap. 1999-01-0619, 2002.
evolution of the R1 reaction rate during the cool flame is represented in Figure 7 for both EGR cases. The same conclusions as the main combustion can be made here concerning the effects of EGR on the R1 reactivity, explaining the decrease of the luminescence for the cool flame between 0 and 30% EGR. EGR Effect on the OH* Emission. This section discusses the EGR effect on the OH* chemiluminescence, recorded during the main flame. Figure 8 presents average images of OH* emissions from -12.2 to -11 CAD for 0% EGR and from -9.8 to -8.6 CAD for 30% EGR. It can be observed that the chemiluminescence signal is not homogeneous over the image surface: the center of the combustion chamber presents the highest intensity value. Because the combustion duration is very short, it was difficult to obtain sufficient images to completely follow the process during the cycle. However, it can be noticed that the spatial average intensity of the signal decreases as a function of the EGR rate, and its maximum is delayed between 0 and 30% EGR, as seen in Figure 8. By superimposing the heat release rate on the average intensity of OH* chemiluminescence (Figure 9), it can be observed that the peaks of average signal intensities appear just after the peak of the heat release. This phasing tends to increase with the increase of the EGR rate. According to Yamada et al.,27 the main chemical reactions inducing the main combustion appearance are HCHO + OH ) CHO + H2O
(R5)
CHO + O2 ) HO2 + CO
(R6)
HO2 + HO2 ) H2O2 + O2
(R7)
H2O2 + M ) 2OH + M
(R8)
To provide some explanations about the presence of the OH* signal linked to the main combustion phase, the kinetic modeling was used. Figure 10 shows the simulated heat release rate, the normalized OH mole fraction, and the R5-R8 reaction rates during a simulated HCCI combustion of n-heptane for 0 and 30% EGR at a fixed equivalence ratio of 0.3. The HO2 species is produced, in part, by reactions R5 and R6 and is recombined with itself (reaction R7) to produce hydrogen peroxide H2O2. The latter, which is accumulated throughout the negative temperature coefficient (NTC) process, begins to break up from reaction R8. This reaction is much more reactive than R7, with H2O2 strongly decreasing to form the (27) Yamada, H.; Goto, Y.; Tezaki, A. Analysis of reaction mechanisms controlling cool and thermal flame with DME fueled HCCI engines. SAE Tech. Pap. 2006-01-3299, 2006.
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Figure 10. Simulated main flame heat release rate associated with chemical species and R5-R8 reaction rates for (a) 0% and (b) 30% EGR. Table 3. Operating Conditions fuel
n-heptane
φ EGR rate (VEGR/Vtotal Pin (MPa) Tin (°C) engine speed (rpm)
Figure 9. Main flame heat release rate and the average image OH* intensity for (a) 0% and (b) 30% EGR.
OH species. It can be observed that the main heat release rate reaches its maximum when reaction R8 begins to produce OH. The reactivity increases and then decreases just after that, probably because of the kinetic competition process with reaction R4 (main flame double-peak phenomenon). Then, the reactivity increases again to form the rest of OH, which reaches its maximum after the main flame peak. That is in perfect agreement with the OH* chemiluminescence results. The use of EGR delays the combustion with a decrease of the main flame, induced by the global reactivity reduction of the present reactions. Conclusions In this paper, the HCCI combustion of n-heptane was experimentally studied inside an optical HCCI engine. Investigations about the EGR effect on the cool flame, main flame, and OH* emission were conducted using a two-dimensional chemiluminescence method. A zero-dimensional model was used to provide chemical details about the HCCI process. It was shown that the effect of the EGR delays both oxidation phases, the cool one and the main one, during the cycle. Using the rapid recording of the luminescence signals, it can be observed that the combustion is delayed when the EGR rate increases. Because the equivalence ratio was kept constant, this
intake)
(%)
0.3 0 and 30 0.1 75 1500
is the consequence of the dilution by CO2 and N2. The average light emission intensity of the flame, mainly because of the emission of the CO2* continuum, decreases with the EGR. The cool flame is initiated with a certain inhomogeneity, which is lower than that raised during the main flame development. Because the mixture homogeneity was checked before autoignition, the nonhomogeneity of the combustion emission must be due to the in-cylinder temperature heterogeneity. The increase of the EGR rate decreases the OH* chemiluminescence: this is linked to the reduction of the global combustion reactivity, and this assumption was proven using the kinetic simulation data. Acknowledgment. This work was sponsored in part by the French Agency for Environment and Energy Management (ADEME), the “Re´gion Centre”, and the PREDIT program under contract 081.1537.00.
Nomenclature BTDC ) before top dead center CAD ) crank angle degrees EGR ) exhaust gases recirculation HCCI ) homogeneous charge compression ignition HRR ) heat release rate NTC ) negative temperature coefficient PM ) particle matter TDC ) top dead center EF800533C
