Energy Fuels 2010, 24, 5404–5409 Published on Web 10/04/2010
: DOI:10.1021/ef100938u
Spectroscopic Measurements of Low-Temperature Heat Release for Homogeneous Combustion Compression Ignition (HCCI) n-Heptane/Alcohol Mixture Combustion Peerawat Saisirirat,†,‡ Fabrice Foucher,‡ Somchai Chanchaona,† and Christine Mounaı¨ m-Rousselle*,‡ †
Combustion and Engines Research Laboratory (CERL), King Mongkut’s University of Technology Thonburi (KMUTT), 126 Pracha-uthit Road, Bangmod, Tungkru, Bangkok 10140, Thailand, and ‡Institut PRISME, Universit e d’Orl eans, 8 rue L eonard de Vinci, 45072 Orl eans Cedex, France Received July 23, 2010. Revised Manuscript Received September 17, 2010
The effects of the alcohol fuel fraction on the diesel homogeneous combustion compression ignition (HCCI) combustion are explained by analyzing simultaneously chemiluminescence data and cylinder pressure measurements. In parallel, experimental results are compared to the simulation results from a zerodimensional (0D) model. n-Heptane was selected to represent diesel fuel, while ethanol and 1-butanol were the considered alcohol fuels. The results showed that the main combustion was delayed when the alcohol fuels were blended, but species radiation results showed that the impacts on chemical processes were not distinguishable during the main combustion. This delay was linked to the impact of the blended fraction of alcohols on the lowtemperature heat release (LTHR). This influence can be separated in two ways. First, the heat release rate results showed that the LTHR quantity decreased, inducing a decrease of the air-fuel mixture temperature after LTHR. Thus, the mixture required more time for the temperature to rise sufficiently to initiate the main combustion processes. Second, the chemiluminescence intensity of CH2O* decreased with an increase in the fuel fraction of alcohols. Therefore, the reaction processes during the main combustion were slower because of less supplied active intermediate species. Moreover, the simulation results from the model showed a similar trend. The results can be used to increase the understanding of the chemical aspects of diesel HCCI combustion.
LTHR, is characteristic of chemical oxidation at low temperature of the hydrocarbon fuel.7-10 The LTHR of diesel fuel with the diesel engine compression ratio leads to the following main combustion occurring earlier than the optimized theoretical period, thus ensuring a higher thermal efficiency. Moreover, the earlier onset of the main combustion creates a stronger pressure gradient because of the shorter combustion duration. Recent studies have shown that, to decrease the effect of LTHR and, thus, to slow the main combustion, the diesel fuel should be blended with a single-stage ignition fuel, such as high-octane fuel.5,6,11-13 The present study continues our previous investigations14-16 into the effects of alcohol fuel on diesel HCCI combustion. A further reason for investigating the impact of alcohol fuel is that, because it can be produced from the agricultural waste, it can partly replace fossil fuel. In our previous works,14-16 the effects of fuels blended with two alcohols on diesel HCCI combustion were investigated,
Introduction In automotive applications, the potential of the homogeneous combustion compression ignition (HCCI) combustion process, discovered several decades ago,1-3 resides in its thermal efficiency and low NOx and soot emissions.4 However, HCCI combustion cannot cover the entire automotive operating range and, therefore, has to be used as one mode of the dual-mode engine to optimize the HCCI performance during the low to medium operating load range. For diesel HCCI, diesel fuel with a high ignitability presents a two-stage heat release: a low-temperature heat release (LTHR), followed by a main heat release (MHR). This is a characteristic feature of many diesel fuels, such as straight carbon-chain paraffin fuels,5 DF-2 diesel fuels, Fischer-Tropsch naphtha,4 and dimethyl ether (DME).6 The earlier short heat release, *To whom correspondence should be addressed. E-mail: christine.
[email protected]. (1) Onishi, S.; Jo, S. H.; Shoda, K.; Jo, P. D.; Kato, S. SAE Tech. Pap. 790501; Society of Automotive Engineers (SAE): Warrendale, PA, 1979. (2) Noguchi, M.; Tanaka, Y.; Tanaka, T.; Takeuchi, Y. SAE Tech. Pap. 790840; Society of Automotive Engineers (SAE): Warrendale, PA, 1979. (3) Najt, P. M.; Foster, D. E. SAE Tech. Pap. 830264; Society of Automotive Engineers (SAE): Warrendale, PA, 1983. (4) Yoshinaka, T.; Nakagome, K.; Niimura, K. SAE Tech. Pap. 961163; Society of Automotive Engineers (SAE): Warrendale, PA, 1996. (5) Shibata, G.; Oyama, K.; Urushihara, T.; Nakano, T. SAE Tech. Pap. 2005-01-0138; Society of Automotive Engineers (SAE): Warrendale, PA, 2005. (6) Zheng, Z.; Yao, M.; Chen, Z.; Zhang, B. SAE Tech. Pap. 2004-012993; Society of Automotive Engineers (SAE): Warrendale, PA, 2004 (7) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Sci. Technol. 1994, 95, 233–260. (8) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Sci. Technol. 1994, 103, 315–336. r 2010 American Chemical Society
(9) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 1998, 114, 149–177. (10) Curran, H. J.; Pitz, W. J.; Westbrook, C. K.; Callahan, C. V.; Dryer, F. L. Proc. Combust. Inst. 1998, 27, 379–387. (11) Lu, X. C.; Hou, Y. C.; Zu, L. L.; Huang, Z. Fuel 2006, 85, 2622– 2631. (12) Lu, X. C.; Ji, L. B.; Zu, L. L.; Hou, Y. C.; Huang, C.; Huang, Z. Combust. Flame 2007, 149, 261–270. (13) Yates, A.; Bell, A; Swarts, A. Fuel 2010, 89, 83–93. (14) Saisirirat, P.; Foucher, F.; Chanchaona, S.; Mounaı¨ m-Rousselle, C. Proceeding of the European Combustion Meeting (ECM2009); Vienna, Austria, 2009. (15) Saisirirat, P.; Foucher, F.; Chanchaona, S.; Mounaı¨ m-Rousselle, C. SAE Tech. Pap. 2009-24-0094, 2009. (16) Saisirirat, P.; Foucher, F.; Mounaı¨ m-Rousselle, C.; Chanchaona, S. International Conference on Green and Sustainable Innovation (ICGSI2009); Chiang Rai, Thailand, 2009.
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Energy Fuels 2010, 24, 5404–5409
: DOI:10.1021/ef100938u
Saisirirat et al.
Table 1. Properties of Neat Fuel
chemical formula nomenclature density (kg/m3) research octane number (RON) sensitivity cetane number lower heating value (kJ/kg) autoignition temperature (°C) boiling point (°C)
n-heptane
ethanol
1-butanol
n-C7H16 n-hept 685 0 0 53 42510 285 98.0
C2H5OH EtOH 790 129 27 11 28865 422 79.0
n-C4H9OH BuOH 810 96 18 17 33075 343 117.7
namely, ethanol and 1-butanol. n-Heptane was selected as the surrogate diesel fuel. The fuel properties of n-heptane, ethanol, and 1-butanol are shown in Table 1. All test fuels are absolute-grade purity (ca. >99.5%). The addition of highoctane fuels is preferable to the use of a high exhaust gas recirculation (EGR) rate, because the energy output is not affected. The objective of this work is to combine spectroscopic measurements of the chemical species, heat release rate (HRR) estimates, and zero-dimensional (0D) modeling with a kinetic mechanism to analyze previous results.
Figure 1. Schematic diagram of the combustion image setup. Table 2. Test Conditions speed (rpm) compression ratio equivalence ratio intake temperature (°C) based fuel blended fuel blended fraction (molar) (%) EGR type EGR fraction (%) averaged energy content (J/cycle)
Experimental Section Experimental Setup. A PSA DW10 diesel engine was modified to operate only with the fourth cylinder. An extended engine linear was set up, and the original piston was replaced by a transparent-bowl extended piston to study the combustion behavior from a 45° mirror, placed under the piston crown. Figure 1 shows a schematic diagram of the test engine and the combustion image pathway. Combustion images were recorded with a high-speed CMOS Fastcam Ultima APXi2 camera (Photron). The emitted light from 300 to 700 nm was recorded with a GEN II intensifier unit. In this work, the camera unit was set up to capture three combustion events: the combustion light emission during the main combustion stage and during LTHR and formaldehyde emission during LTHR. The camera was set to acquire the series of images at a resolution of 256 256 pixel2 and 15 000 flames/s (which is equivalent to a 0.6 crank angle degree between each image for a 1500 rpm engine test speed) with a Nikon lens (50 mm f1:1.8). To record CH2O emission (between 300 and 400 nm), UG5 and WG295 filters were selected. The UG5 filter allows for the light emission between 230 and 400 nm to pass through, while the WG295 filters allows for the wavelengths shorter than 300 nm to pass through. For each condition, a 50 cycle series of images was recorded. A system-controlled PC was also used to synchronize the cylinder pressure acquisition system, to acquire 100 cycles of cylinder pressure at 10 points/crank angle. A detailed experimental setup to acquire the cylinder pressure data, engine control unit, experimental heat release analysis, and simulated EGR can be found in our previous works.14-16 Simulated EGR, which is composed of N2, CO2, and additional air, was used instead of real EGR to avoid the time needed to stabilize the engine with the real EGR and to control the species introduced in the combustion chamber. The test conditions, selected from our previous works, are presented in Table 2. To prepare the fuel blends and prevent fuel separation, the alcohols were mixed before filling in the fuel tank, 5 min before the beginning of each experiment. Simulation Modeling. To advance the understanding of the effects of alcohol blends, the additional engine subroutine was included in the SENKIN program, part of the CHEMKIN II modeling package.17 The engine subroutine was modeled as a close homogeneous time-varied volume. Fuel oxidation was described by the detailed kinetics reaction mechanism from
1500 16:1 0.3 80 n-heptane ethanol and 1-butanol 0 and 37 simulated 0 40 392.7 330.9
ethanol and 1-butanol oxidation sub-schemes18,19 and a PRF oxidation scheme from the literature.20 This scheme was also used and validated in previous work.21
Results and Discussion Figure 2 shows the average images of natural chemical emission during the main combustion phase, when the engine was fueled with pure n-heptane (Figure 2a), a n-heptane/ethanol mixture (Figure 2b), and a n-heptane/1-butanol mixture (Figure 2c) at 63:37 molar percentage. The chemiluminescence (CL) images were captured at the 8-bit grayscale intensity level but are presented in false colors (see color scale in Figure 2d) to accentuate the CL intensity differences. The corresponding crank angle positions at which they were taken are indicated to the right. It can be seen that the emitted radiation seems to be homogeneously distributed within the combustion chamber, which is characteristics of HCCI combustion, except for near the walls, where a temperature gradient exists. The images show that the duration of light radiation is similar in all three cases; its high CL signal is shorter than two crank angle degrees. However, this period is linked to the alcohol fuel fraction. The evolution of average intensities and HRRs, deduced from cylinder pressure analysis, is plotted in Figures 3 and 4. These results are given for pure n-heptane and one blended alcohol fuel (37%) and two EGR rates: 0 and 40%. First, as the images clearly show, the effect of alcohol addition is to delay the main combustion stage to that of dilution by EGR. The discussion about the effect of EGR and alcohol addition was already explained in our recent work.14-16 The CL signal for both alcohols is not phased with the HRR; it always (19) Dagaut, P.; Togbe, C. Energy Fuels 2009, 23, 3527–3535. (20) Curran, H. J.; Pitz, W. J.; Westbrook, C. K. http://www-pls.llnl. gov/?url=science_and_technology-chemistry-combustion-prf (accessed on Dec 3, 2004). (21) Saisirirat, P.; Togbe, C.; Chanchaona, S.; Foucher, F.; Dagaut, P.; Mounaı¨ m-Rousselle, C. Proc. Combust. Inst. 2010, DOI: 10.1016/j. proci.2010.07.016.
(17) Lutz, A. E.; Kee, R. J.; Miller, J. A. Report SAND86-8209; Sandia National Laboratories: Livermore, CA, 1986. (18) Dagaut, P.; Togbe, C. Fuel 2010, 89, 280–286.
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Figure 2. Averaged images of chemical emission during the main combustion: (a) pure n-heptane, (b) n-heptane/ethanol mixture at 63:37, (c) n-heptane/1-butanol mixture at 63:37, and (d) color intensity scale.
Figure 3. HRR and average CL intensity evolution during combustion for pure n-heptane and a n-heptane/ethanol mixture during the main combustion: (A) 0% EGR and (AA) 40% EGR.
appears after the main heat release, a phenomenon discussed in the literature.22-25 In fact, the total emissions, directly captured without any optical filter during the main combustion, are due to emission of CO-O*,24 an electronically excited state of CO2. The signal is emitted by CO-O* during its energy decrease from the excited state to the ground state. As this process occurs during the final stage of hydrocarbon oxidation, CL is emitted after the maximum of the HRR. This has been investigated in recent works22-24 to evaluate the effect of EGR. Similar results can be observed here in the AA and BB cases; the reaction rates of chemical processes and CO-O* reactions
are slowed when EGR is introduced. Thus, with EGR, a second stage in the HRR appears after the main combustion because of the conversion of CO to CO2. However, even if alcohol addition delays the main combustion to the dilution by EGR, the impact on the oxidation process differs, in that a second stage of heat release does not occur. To investigate this observation more closely, the first stage of HRR (LTHR) before the main combustion was studied in greater detail. Figures 5 and 6 show the comparison of HRR and average CL intensity (Figure 5 for total CL and Figure 6 for CH2O CL) during the first stage of heat release before the main combustion stage for ethanol and 1-butanol addition. For comparison, the impact of EGR addition is plotted in Figure 7. The EGR fraction considered was 40% because the onset of the main combustion was delayed by the same order as with the 37% alcohol blend. For all cases, pure n-heptane was considered as the surrogate for diesel HCCI combustion. First, the results of average total CL intensity are consistent with the HRR results in both their magnitude and phase.
(22) Dubreuil, A.; Foucher, F.; Mounaim-Rousselle, C. Energy Fuels 2009, 23, 1406–1411. (23) Kim, B.; Kaneko, M.; Ikeda, Y.; Nakajima, T. Proc. Combust. Inst. 2002, 29, 671–677. (24) Kumano, K.; Iida, N. SAE Tech. Pap. 2004-01-1902; Society of Automotive Engineers (SAE): Warrendale, PA, 2004. (25) Samaniego, J. M.; Egolfopoulos, F. N.; Bowman, C. T. Combust. Sci. Technol. 1995, 109, 183–203.
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Figure 4. HRR and averaged CL intensity evolution during combustion for pure n-heptane and a n-heptane/1-butanol mixture during the main combustion: (B) 0% EGR and (BB) 40% EGR.
Figure 5. HRR and average total CL intensity evolution during LTHR: case A, ethanol fraction addition; case B, 1-butanol fraction addition.
Figure 6. HRR and average CH2O CL intensity evolution during LTHR: case A, ethanol fraction addition; case B, 1-butanol fraction addition.
intermediate species, which can identify the chemical reactions during LTHR.26-28 The results of filtered radiation intensities
The emitted light signal is the combination of many intermediate species, such as CH2O* and CO-O* emission. Two filters were therefore selected to capture only CH2O* radiation emissions between 300 and 400 nm. This CH2O is the
(27) Ohta, Y.; Furutani, M. J. Thermodyn. Combust. Comm. Pol. Acad. Sci. 1991, 11, 43–52. (28) Hwang, W.; Dec, J.; Sj€ oberg, M. Combust. Flame 2008, 154, 387–409.
(26) Calvert, J. G.; Steacie, E. W. R. J. Chem. Phys. 1951, 19, 176–182.
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Figure 7. HRR and average CL intensity evolution during LTHR in the case of EGR addition.
Figure 8. Predicted results during the LTHR period for (a) pure n-heptane, (b) increased ethanol fraction, (c) increased 1-butanol fraction, and (d) EGR of 40%.
are shown in Figure 6. The light emission nearly disappears in the case of alcohol-blended fuels, but the effect of EGR addition is less marked; the CH2O chemiluminescence intensity decreases by only about 50% compared to pure n-heptane combustion without EGR. Kinetic simulations are therefore needed to explain the effects of the alcohol fraction on CH2O CL. Figure 8 shows the results during the LTHR period predicted by the 0D simulation, for HRR, formaldehyde (CH2O), hydroxyl radical (OH), and hydroperoxyl radical
(HO2) concentrations, and the reverse reaction rate for the reaction: H2O2 (þM) f 2OH (þM). As mentioned above, formaldehyde is the intermediated species during the LTHR period, while the hydroxyl radical can be considered as an indicator of the hydrocarbon oxidation reaction. The hydroxyl radical is produced during LTHR via the reaction of the alkylhydroperoxy radicals (QOOH), peroxy alkylhydroperoxy radicals (O2QOOH), and keto-hydroperoxides.7-10 It also influences many chemical processes, including these following 5408
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therefore appears to have two impacts: one is the thermal impact, i.e., the mixture temperature after LTHR decreases because of the reduction of LTHR, as observed in previous work,13-15 and the other is the chemical impact, explained above. Moreover, the 0D engine simulation presents more differences between two alcohol fuels than experimental results. Differences can be found at a higher blended fraction as in our previous work15,21 for 57 alcohol molar percentages. The same tendency is found; the impacts of ethanol addition on the deceleration of LTHR slowing and the reduction of intermediate species are higher than those of 1-butanol addition, as seen in Figure 8. Because the reaction rate of reverse reaction 4 does not appear positive for both alcohol additions, the main hydroxyl radical is produced by n-heptane lowtemperature chemical reactions.
reactions: CH2 O þ OH ¼ HCO þ H2 O
ð1Þ
HCO þ O2 ¼ CO þ HO2
ð2Þ
HO2 þ HO2 ¼ H2 O2 þ O2
ð3Þ
H2 O2 þ M ¼ 2OH þ M
ð4Þ
It can be seen that the simulation results of HRR exhibit the same tendency as the experimental results. CH2O* emission appears in the same HRR-relative period as the increase in the CH2O concentration in the model. This is reasonable because CH2O is formed as the high electronically excited first state and, after LTHR, CH2O concentration becomes constant and returns to the ground state. The impact of the alcohol fraction on the LTHR can be observed here in the reaction rate of the reverse reaction 4, which is significant during the developing period of LTHR, whether or not EGR is introduced. The opposite holds, however, in the cases of ethanol and 1-butanol blends. It can be concluded that the production of OH radicals is greater during LTHR for pure n-heptane than for alcohol blends; therefore, the reverse reaction 4 can be observed. Hence, the H2O2 concentration for pure n-heptane is greater via this reverse reaction 4. The HO2 concentration is also greater because it cannot reform via another reaction 3, as shown in Figure 8, again regardless of whether or not EGR is introduced. These results can be explained by the fact that the addition of the alcohol fraction affects HCCI combustion differently than EGR addition. While EGR addition does not affect OH radical production, it affects the energy content, thus delaying the initiation of the main combustion stage but to a lesser extent than the blended alcohols. The alcohol blends affect OH radical production and also slow the low-temperature chemical reactions, decreasing the concentrations of reactive intermediate species (CH2O, OH, HO2, etc.). The alcohol-blended fraction
Conclusion In this work, the influence of the alcohol fuel fraction on diesel HCCI combustion has been investigated by means of optical analysis. The following was found: (1) The alcoholblended fraction does not affect the chemical reactions during the main combustion. It impacts only on LTHR, which occurs before the main combustion and indirectly affects the initiation of the main combustion process. (2) The first impact on LTHR is the decrease in LTHR, which affects the charge temperature. The mixture therefore requires a longer temperature rise time for the charge to initiate the main combustion process. (3) Another impact is to reduce activated intermediate species after LTHR. Reaction processes are slower because of a lower supply of reactant, delaying the main combustion phase. (4) The results predicted by the chemical simulation model show satisfactory agreement with experimental results. The model can therefore be considered appropriate to explain the observed impact of the alcohol fuel fraction and EGR addition.
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