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Ignition delay time measurements and modeling for gasoline at very high pressures
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Ignition delay time measurements and modeling for gasoline at very high pressures D.F. Davidson a,∗, J.K. Shao a, R. Choudhary a, M. Mehl b, N. Obrecht c, R.K. Hanson a a Mechanical
Engineering Department, Stanford University, Building 02-660, Room 104, Stanford CA, USA Science Division, Lawrence Livermore National Laboratory, Livermore, CA 94551 USA c Total Marketing Services, 92800 Puteaux, France
b Materials
Received 29 November 2017; accepted 18 August 2018 Available online xxx
Abstract Ignition delay times (IDT) for high-octane-number gasolines and gasoline surrogates were measured at very high pressures behind reflected shock waves. Fuels tested include gasoline, gasoline with oxygenates, and two surrogate fuels, one dominated by iso-octane and one by toluene. RON/MON for the fuels varied from 101/94 to 106.5/91.5. Measurements were conducted in synthetic air at pressures from 30 to 250 atm, for temperatures from 700 to 1100 K, and equivalence ratios near 0.85. Results were compared with a recent gasoline mechanism of Mehl et al. (2017). IDT measurements of the iso-octane-dominated surrogate were very well reproduced by the model over the entire pressure and temperature range. IDT measurements for the toluene-dominated surrogate were also reproduced by the model to a lesser extent. By contrast, IDT measurements for the neat gasoline and gasoline with oxygenates, show excellent agreement with the trends of the Mehl et al. model only below 900 K. Above 900 K, the model returned IDT values for the two gasolines that were approximately 1.6× the measured values. Finally, we observed that IDT measurements for the toluenedominated surrogate fuel and the two gasolines, near 70 atm and below 900 K, appeared to be shortened, possibly by non-homogeneous ignition or non-ideal gas processes. This dataset provides a critically needed set of IDT targets to test and refine boosted gasoline models at high pressures. © 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Gasoline; High pressure; Ignition delay time; Negative temperature coefficient regime
1. Introduction In recent years, fuel economy requirements and CO2 emission regulations have been added to the existing vehicle emissions standards, driving ∗
Corresponding author. E-mail address: [email protected] (D.F. Davidson).
the development of more efficient powertrains. Consequently, car manufacturers have developed new solutions to reduce overall fuel consumption and CO2 emissions, commonly including higher compression ratio, turbocharging and engine downsizing. With the increase of power density and higher pressure operation, abnormal combustion modes such as knock or superknock are becoming a major limitation of engine efficiency.
https://doi.org/10.1016/j.proci.2018.08.032 1540-7489 © 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Please cite this article as: D.F. Davidson et al., Ignition delay time measurements and modeling for gasoline at very high pressures, Proceedings of the Combustion Institute (2018), https://doi.org/10.1016/j.proci.2018.08.032
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Prolonged knock must be avoided as it can cause severe engine damage to the pistons, cylinder head or cylinder head gasket. As the power density of modern engines increases, thermodynamic conditions of the fuel and air mixture in the combustion chamber during the compression stroke increase and amplify knock sensitivity. One of the common strategies to eliminate engine knock when detected is by retarding the spark timing, but at the cost of reducing engine performance and increasing CO2 emissions. Therefore, formulating fuels with enhanced knock resistance can help to improve engine performance and reduce fuel consumption. Fuel knock resistance is usually defined by indices such as Research Octane Number (RON) and Motor Octane Number (MON) [1,2] and by Octane Index (OI) [3,4]. Previous studies have looked at relationships between octane rating and gas phase ignition delay times measured in shock tubes or rapid compression machines at various pressures and temperatures [5–8]. However, these studies have been performed at low to medium pressures ranging typically up to 60 atm. In order to formulate better high-performance fuels, it is paramount to understand fuel auto-ignition phenomena at engine-relevant conditions and explore high to very-high pressures ranging from 100 to 200+ atm that can typically be seen in highly downsized engines. In the present study we investigated fuels with RON/MON ratios from 101/94 to 106.5/91.5. Modern light- and heavy-duty diesel engines are designed for 200 atm peak pressure, and some OEMs are considering going to 250 atm. In response to this, single-cylinder compression-ignition research engines are being designed for 300 atm. Modeling of combustion processes in gasoline compression ignition engines, in which the chemical kinetics play a crucial role in the combustion process, would benefit from this data collected at these elevated pressures. Earlier shock-tube work, which has been limited mostly to below 60 atm barely reaches mid-load in a modern diesel engine. Data at very high pressure will also improve understanding and development of high efficiency spark ignition gasoline engines. When these engines are operated without spark retard (for example when running on fuel resistant to end-gas autoignition), peak pressures can easily exceed 100 atm at high load. Future SI engines operating with a dilute boost or lean boost strategy at high load to avoid knock, can similarly reach high peak pressures approaching those of diesel engines. To address to these needs, programs such as Co-Optima [9] were instituted with the aim of providing the fundamental background to assist screening of promising future fuel candidates and identify advanced engine combustion strategies that leverage fuel properties at their best. In the gasoline space, this has led to the identification of synergistic blending behavior among fuel compo-
nents that could improve fuel performance at high pressures typical of high-pressure cycles obtained in down-sized boosted engines. Important questions remain open to modelers interested in fuel chemistry and blending behavior and now, more than ever, there is a growing need for fundamental IDT experiments in the 100 to 200+ atm range typical of highly downsized engines. 2. Previous studies Little distillate gasoline IDT data exist at the pressure conditions (>100 atm) typical of downsized engines. Early shock tube work by Gauthier et al. [6] at Stanford investigated one gasoline fuel and two surrogates at pressures ranging from 15 to 60 atm and temperatures above 900 K, effectively providing the first test data for modern formulations of high-pressure gasoline IDT. Kukkadapu et al. [10,11] used a rapid compression machine to study gasoline IDT at 20-40 atm. More recently, Sarathy et al. [8] investigated several FACE gasolines at pressures up to 40 atm at KAUST. Of particular importance and relevance to the current study is the observation that the shock tube derived IDT values below 950 K in the KAUST study also exhibit a near-abrupt change in activation energy and were significantly shorter than predictions with their gasoline model. Other studies of relevance include early shock tube ignition delay time measurements of gasoline surrogate components at pressures up to 45 atm as reported by Fieweger et al. [5], and IDT measurements of a gasoline/ethanol mixture (RON/MON 109/101) near 30 atm as reported by Cancino et al. [12] at IVG Duisburg. Most recently, Davidson et al. [13] investigated IDT measurements for a wide range of distillate fuels and found, at least for high temperature conditions above 1000 K, that IDT values were nearly identical for all studied distillate fuels, gasoline included. However, the absence of validation data for the high pressures relevant to boosted DGI (Direct Gasoline Injection) engines limits the ability of testing numerical models used by engine designers to simulate combustion chemistry at these conditions. This work provides the first instance where the ignition behavior of real fuels is systematically studied in a well-characterized environment at pressures above 100 atm, providing a fundamental stepping stone to understanding gasoline behavior in highly downsized engines. 3. Fuel composition and modeling The properties and composition of the four fuels studied are shown in Table 1. The simulations
Please cite this article as: D.F. Davidson et al., Ignition delay time measurements and modeling for gasoline at very high pressures, Proceedings of the Combustion Institute (2018), https://doi.org/10.1016/j.proci.2018.08.032
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D.F. Davidson et al. / Proceedings of the Combustion Institute 000 (2018) 1–8 Table 1 Composition of liquid fuels A, B, C and D in mass%. Fuel composition
A
B
C
D
Olefins Aromatics N-Paraffins Iso-Paraffins Napthenes Oxygenates Total MW RON MON
9 2 3 81 5
19 81
17 36 2 34 11
100 103.4 101 94
100 90.2 104 91
17 29 2 32 12 8 100 90.1 106.5 91.5
100 97 105 92
were performed using an extensive gasoline surrogate mechanism generated by the collaborative effort of LLNL, NUI Galway and KAUST [Mehl et al. (2017) to be submitted]. This detailed kinetic model includes a selection of n-alkanes, iso-alkanes, cycloalkanes, olefins and aromatics relevant to gasoline fuels (e.g., C4– C9 carbon number). While PRF mixtures (blends of the primary reference fuels, n-heptane and isooctane, used to define the octane scale) are often used to represent the behavior of gasoline fuels in traditional applications, their strong propensity to undergo low-temperature degenerate branching makes them inadequate to mimic real gasolines, particularly at high pressure, where the O2 addition reactions are thermodynamically favored. In order to capture the reactivity of high performance fuels, where the low temperature reactivity is suppressed by their unsaturated compounds content, more complex surrogates are needed, therefore the availability of components different from alkanes in the surrogate model is a necessity. Modeling surrogates for the fuels are given in Table 2. The composition of these multi-component surrogates was selected based on the results of a detailed hydrocarbon analysis of the real fuels and their octane numbers were estimated using a method developed by LLNL where the ignition behavior at 25 atm is correlated to their standardized knock metrics RON and MON [14]. The calculated RON are well in line with the ones of the target fuels, while the MON values are somewhat higher than expected. This discrepancy can be either result from limitations in the correlations used, or from shortcomings in the composition of choice or the kinetics of its components. 4. Experimental methods 4.1. High-pressure shock tube Ignition delay time experiments for all test mixtures were performed using the Stanford high-purity, high-pressure shock tube (HPST). The stainless steel driven section had an internal diameter of 5 cm and was heated to 90 C to pre-
3
Table 2 Composition of surrogates for fuels A, B, C and D in mass %. Molecule identifiers as in [15]. IC8D4 = 2,4,4trimethylpent-1-ene. Surogate component
A
IC8 IC5H12 IC8D4 C6H12-1 C4H10 c-C5H10 C6H5CH3 T135MBZ C6H5C2H5 P-XYL O-XYL C2H5OH Total MW RON MON
66 10 5 9 3 5 2
B
81
100 98.7 99.3 95.2
100 90.4 105.3 94.6
19
C
D
21 3 10 17 2 12 19 5 4 5 2
21 1 10 17 2 12 13 3 5 6 2 8 100 86.3 104.9 95.4
100 93.0 104.2 95
vent condensation of the test gas mixture. Burst diaphragms were made of aluminum and steel (with cross-scribing). Typical uniform test times for non-reactive synthetic air mixtures are 2 ms when helium is used as the driver gas. Uniform test times were extended to beyond 5 ms where needed using tailored He/N2 driver gas mixtures and driver inserts. Fuels were obtained from TOTAL Marketing & Services. High-purity synthetic air (21% O2 /N2 ) was supplied by Praxair as the bath gas. The liquid fuel was injected into a 12.8-liter stainless-steel mixing tank at 110 C. A test gas mixture of fuel/air was then prepared manometrically and was stirred using a magnetically-driven vane assembly for at least 15 min prior to the experiments. As the test pressures of the experiments in this study extend to 250 atm, non-ideal Equation of State (EOS) effects may begin to appear in both the determination of the reflected shock conditions and in the kinetics simulations [16]. To estimate the magnitude of these EOS effects on the test conditions of the IDT experiments, reflected shock conditions were calculated using both the ideal and the Peng-Robinson EOS. For incident shock wave speeds that produce reflected shock conditions in air of 800 K, 200 atm and 88.3 kg/m3 calculated using the Ideal EOS, the Peng-Robinson EOS predicts values of 793 K, 196 atm and 81.9 kg/m3 [17]. Based on this limited variation (1% in T5 , 2% in P5 and 7% in ρ 5 ), the ideal gas EOS was used for all temperature and pressure determinations in this study. The influence of non-ideal EOS on the kinetic simulations (e.g., fugacity effects on equilibrium constants) was not addressed in this study [18].
Please cite this article as: D.F. Davidson et al., Ignition delay time measurements and modeling for gasoline at very high pressures, Proceedings of the Combustion Institute (2018), https://doi.org/10.1016/j.proci.2018.08.032
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4.2. Shock tube diagnostics Two diagnostics were employed during the experiments: laser absorption and sidewall pressure. Both diagnostics were located 1.1 cm away from the end wall. Initial fuel concentrations and relative hydrocarbon time-histories were measured directly in the shock tube by laser absorption with a 3.28 μm NanoplusTM DFB interband casade laser. Fuel concentrations were determined using Beer’s Law (I/I0 ) = exp (−σ N L ) = exp(−α)
(1)
where (I/I0 ) is the ratio of the transmitted and incident laser power at 3.28 μm, σ is the fuel crosssection at the same wavelength, N the fuel density, L the pathlength, and α is the absorbance. Fuel crosssections used in this determination of the fuel loading were measured in neat fuel vapor separately. In situ fuel loading measurements ensured accurate determination of equivalence ratio of the test gas mixtures. Ignition delay times were determined from both the normalized pressure time-histories monitored using a high-pressure KistlerTM model 607C1 piezoelectric pressure transducer (PZT) (that replaced a failing lower pressure sensor) and by the relative hydrocarbon time-histories measured by the 3.28 μm laser absorption signal. Though the absorbances measured behind the reflected shock waves were large (∼5–7) the stability of the laser system enabled useful information to be extracted from the small transmitted signals. In this study, the 1st and 2nd stage ignition delay times are defined as the time interval between the arrival of the reflected shock and the onset of 1st or 2nd stage ignition, determined by extrapolating the maximum slope of the 3.28 μm absorbance record to the baseline. Similar definitions were used for the pressure profiles and resulted in nearidentical IDT values. In many cases the 1st stage ignition was identifiable only in the absorbance record, as changes in the pressure record were very small. Uncertainties (2σ ) in IDT measurements are typically ± 15%. Uncertainties (2σ ) in initial reflected shock temperature T5 are ± 1%. Accurate measurements of the incident shock speed (with uncertainties typically ± 0.2%) translate into accurate determinations of reflected shock temperatures and pressures. The shock arrival times in the driven section of the shock tube were recorded by five axially-spaced PZTs (PCBTM 113A) placed over the last meter of the driven section. The velocity of the incident shock at the end wall was then determined by extrapolation, allowing calculation of the initial reflected shock temperature and pressure with ± 1% (2σ ) uncertainty, by using the ideal one-dimensional shock-jump relations and assuming vibrational equilibrium and frozen chemistry. The uncertainty in the determination of the individual ignition delay data points is typically ± 15% increasing to ± 20% (2σ ) for the highest
Fig. 1. IDT data for gasoline/air mixture near 40 atm and φ = 1. Solid line: Mehl et al. model (PRF84); dashed line: Davidson et al. [13] correlation. Current measurements (Fuel C and d) shown as stars with error bars.
pressure experiments. The primary contribution to the uncertainty is from the initial reflected-shock temperature. Other minor factors include determination of the ignition delay time interval based on the slope intercept criterion, fuel concentration uncertainty, and the influence of gradients (dP5 /dt) in reflected shock pressure with time.
5. Results and discussion 5.1. 30–50 atm studies A small selection of IDT measurements at 32 atm were performed to anchor the present study to earlier work, i.e., the 40 atm Sarathy et al. FACE fuels [8], 50 atm Gauthier et al. RD387 and Surrogate B [6], and 30 atm Cancino et al. IVG RON/MON 109/101 [12] fuel measurements. All data presented in Fig. 1 were normalized to 40 atm and an equivalence ratio of 1 using 1/P and 1/φ scalings. The similar IDT behavior of these fuels at these high temperatures is consistent with the distillate fuel correlation of Davidson et al. [13] and begin to diverge from this correlation at lower temperatures. The IDT values of all fuels above 950 K (and the RD387 IDT values down to 900 K) are in good agreement with a simulation using a PRF84 (n-heptane/iso-octane) surrogate. 5.2. Fuel A Fuel A has a high iso-paraffinic content dominated by iso-octane. Measurements were made at pressures from 54 to 248 atm and temperatures from 705 to 1060 K. Individual test conditions and IDT data for this and the other fuels are provided in the supplementary material. Representative laser absorbance and normalized pressure data for four
Please cite this article as: D.F. Davidson et al., Ignition delay time measurements and modeling for gasoline at very high pressures, Proceedings of the Combustion Institute (2018), https://doi.org/10.1016/j.proci.2018.08.032
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Fig. 2. Example ignition delay time and laser absorbance measurements for Fuel A/air. 3.28 μm laser absorption (upper traces) and relative pressure (lower traces). Reflected shock conditions: 871 K, 248 atm, φ = 0.75; 895 K, 223 atm, φ = 0.70; 761 K, 155 atm, φ = 0.97; 760 K, 152 atm, φ = 0.93. Ringing, evident in the early part of the pressure profiles, is a result of a failing PZT sensor.
Fig. 3. IDT measurements and model for Fuel A/air mixtures. Solid lines: 2nd stage IDT simulations.
experiments are shown in Fig. 2. Of importance is the increase in laser absorbance just before ignition in the lower temperature experiments. This is indicative of the formation of a series of strongly absorbing, but late-forming intermediate product species. A complete interpretation of the 3.28 μm absorption signal is not yet possible as the lateforming intermediate species have not been identified nor have their IR absorption cross-sections been measured at these test conditions. Differences in the temperature and pressure dependences of the intermediate and product species absorption crosssections and concentrations may cause the different absorbance behavior seen at higher and lower temperatures. A rise in pressure, simultaneous with the change in laser absorbance, was seen in all cases.
5
Fig. 4. Example ignition delay time measurements for Fuel B/air. 3.28 μm laser absorption (upper traces) and relative pressure (lower traces). Reflected shock conditions: 786 K, 176 atm, φ = 0.76; 797 K, 157 atm, φ = 0.73; 824 K, 202 atm, φ = 0.82; 789 K, 128 atm, φ = 0.90;.
Fig. 5. IDT measurements and model for Fuel B/air mixtures. Solid lines: 2nd stage IDT simulations. Black crosses: lower temperature 60 atm measurements.
A summary of the IDT measurements and simulations are shown in Fig. 3. Data in this figure and others has been scaled by 1/P and 1/φ to the stated nominal values. Excellent agreement is seen between the model and the 2nd stage IDT data at all pressures. The simulated 1st stage IDT values are in very good agreement with the measurements. We believe this is the highest pressure IDT data for fuel dominated by branched alkanes currently available. 5.3. Fuel B Fuel B has a high aromatic content dominated by toluene with a minor 1-hexene component. Representative data for four experiments are shown in Fig. 4. The first strong delay in absorbance is attributed to 1st stage ignition. Small rises in pressure occur with the observed change
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Fig. 6. Example ignition delay time measurements for Fuel C/air. 3.28 μm laser absorption (upper traces) and relative pressure (lower traces). Reflected shock conditions: 787 K, 185 atm, φ = 0.77; 796 K, 190 atm, φ = 0.77; 803 K, 204 atm, φ = 0.95; 831 K, 214 atm, φ = 0.77; 851 K, 259 atm, φ = 0.88.
Fig. 7. IDT measurements and model for Fuel C/air mixtures. Solid lines: 2nd stage IDT simulations.
in absorbance; pressure rises coincident with the absorbance changes are evident in the two lower traces attached to the 824 K and 797 K absorbance traces. The IDT values for these four experiments are sensitive to pressure and equivalence ratio, as well as temperature; the shortest IDT occurs for the 824 K, 202 atm, φ = 0.82 example; the longest for the 786 K, 176 atm, φ = 0.76 example. A summary of the IDT measurements and simulations is presented in Fig. 5. Simulations show an explicit pressure dependence of order P−1 . The higher temperature (above 900 K) 60 atm simulations are in fair agreement with associated data. The 140 and 190 atm 2nd stage IDT data, follow the trend of the simulations but have IDT values approximately 1/2 of the modeled values. The 1st stage ignition values based on initial fuel removal at 60, 140 and 190 atm are also shown.
Fig. 8. Example ignition delay time measurements for Fuel D/air. 3.28 μm laser absorption (upper traces) and relative pressure (lower traces). Reflected shock conditions: 838 K, 136 atm, φ = 0.80; 782 K, 190 atm, φ = 0.83; 811 K, 150 atm, φ = 0.91; 817 K, 215 atm, φ = 0.89; 954 K, 163 atm, φ = 0.89.
Shown as black crosses are Fuel B/air IDT measurements attempted at 60 atm and below 950 K. Measurements made at these conditions were expected, based on current modeling, to have IDT values that were significantly longer than 2 ms. However, in the NTC temperature regime, these measurements reached an IDT plateau near 1–2 ms. Interestingly, a similar plateau was also seen in the Sarathy et al. 40 atm FACE gasoline data [8]. The lower temperature 60 atm IDT data (900–750 K) appear to be coincident with the higher pressure (140 and 190 atm) IDT values. Non-local ignition, flame propagation, or 1st stage-2nd stage IDT interactions may be responsible for the early onset of IDT in this pressure and temperature range. To resolve this issue in future work, we propose to use high-speed endwall imaging to monitor the combustion process and identify the occurrence of homogeneous or inhomogeneous ignition events [19]. 5.4. Fuel C Fuel C is a high-performance gasoline with significant aromatic and iso-paraffinic components. Representative IDT data for five experiments are shown in Fig. 6. For this fuel, early changes in laser absorbance associated with fuel removal and 1st stage ignition, is coincident with only small changes in pressure. A distinct beginning to the change in absorbance signaling 1st stage fuel removal is evident in the 787 K, 796 K and the 803 K examples. In the higher temperature examples at 831 K and 851 K, fuel removal is evident, but there is not a clear transition to a linear change in absorbance that is seen in the lower temperature examples. IDT simulations and measurements are shown in Fig. 7. At temperatures above 900 K, where
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IDT values for Fuel C and show little apparent effect of oxygenate addition on fuel reactivity. The Mehl et al. model accurately captures the 140 and 220 atm data, while again over-estimating the IDT values for 70 atm. 5.6. Modeling observations
Fig. 9. IDT measurements and model for Fuel D/air mixtures. Solid lines: 2nd stage IDT simulations.
little influence of NTC chemistry is expected, the modeled IDT values are approximately 1.6× the measured IDT values. At higher pressures and NTC temperatures, as is particularly evident in the 220 atm data, the model accurately captures the IDT trends in pressure and temperature variation. 5.5. Fuel D Fuel D includes approximately 8% by mass of oxygenates. Representative IDT data for five experiments are shown in Fig. 8. Laser absorbance and normalized pressure traces are very similar to those seen in the Fuel C data. 1st stage ignition is easily identifiable in the two cases where a linear change in absorbance is seen, i.e., for 782 K and 817 K examples. The absorbance profile at 838 K has a structure that is similar to the absorbance profiles seen in the toluene/air experiments, though the IDT value is consistent with surrounding data and simulation. IDT simulations and measurements are shown in Fig. 9. Measured IDT values for these two oxygenated fuels are very similar to the measured
Modeling results show general agreement with the high pressure data but appear to overestimate the ignition delay times of fuels B to D at high temperature. These results are consistent with the high MON values predicted using the correlations developed at LLNL. A sensitivity analysis on the reaction rates used in the model was performed to understand which fuel components and reactions are responsible for the deviations observed at high temperature between the model and the experiments (see Fig. 10). At these temperatures and pressures the ignition process is controlled by HO2 chemistry (HO2 recombination and H2 O2 decomposition) and its interactions with the fuel molecules (mainly via abstraction reactions). As expected, the next reaction in order of importance for fuel A is the HO2 abstraction on iso-octane, the prevalent component in the blend. For fuel C and D, and to a lesser extent for fuel A too, cyclopentane chemistry appears to be important. Finally, fuels C and D chemistry appears to be affected by diisobutylene and ethanol (fuel D only). A flux analysis (not presented in detail here) shows how, in the initial stages of the oxidation process (2.5% conversion of the fuel), the fate of HO2 is primarily to recombine or abstract an H from cyclopentane and ethanol to form H2 O2 (which later decomposes to 2 OH radicals). The radicals formed by the cycloalkane and the alcohol react with oxygen in a 1 or 2 step process to form HO2 again, feeding an autocatalytic cycle. These reactions end up promoting the reactivity of the blend in the model but it is not clear to which extent this mechanism accurately reflects the correct branching behavior
Fig. 10. A factor rate sensitivity analysis for the three fuels surrogates at 70 atm, φ = 0.85, 1000 K. Molecule identifiers as in [15]. IC8D4 = 2,4,4-trimethylpent-1-ene.
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occurring in the real fuel. This determination is made harder by the scarcity of fundamental studies on the blending behavior of components such as cyclopentane. Targeted studies at high pressures are needed to improve the understanding of the ignition behavior of fuel blends relevant to gasolines.
Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10. 1016/j.proci.2018.08.032. References
6. Conclusions For the first time IDT values for highperformance gasolines were measured at high pressures up to 250 atm. In general, excellent agreement was seen between model and data at very high pressures and NTC temperatures for gasolines. Simulations of this data using the current Mehl et al. model capture the trends in pressure and temperature, but overestimate the magnitude of the IDT values at higher temperatures. Work is currently proceeding on identifying the chemistry and surrogate species components that are affecting the reactivity of this model, possibly taking advantage of more targeted studies on the blending behavior of fuel components at these conditions. Significant differences between the model and experiment at large IDT values also point to the need to confirm the assumption of homogeneous ignition in these experiments or to investigate the role of real gas EOS. Studies of the combustion process using high-speed endwall imaging would provide one path to resolving this issue. Studies using real-gas CHEMKIN-like codes would provide another. Acknowledgments This work performed at Stanford University was supported by TOTAL Marketing & Services, Paris France and the authors would like to thank Roland Dauphin for his support on the fuels characteristics. The modeling work performed at LLNL was performed under the auspices of the U.S. Department of Energy (DOE), Contract DE-AC52-07NA27344 and was sponsored by (DOE) Bioenergy Technologies Office (BETO) and Vehicle Technologies Office (VTO) under the DOE Co-Optimization of Fuels and Engines Initiative. We would also like to thank an anonymous reviewer who provided further justification, included in the introduction, for this study.
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Ignition delay time measurements and modeling for gasoline at very high pressures D.F. Davidson a,∗, J.K. Shao a, R. Choudhary a, M. Mehl b, N. Obrecht c, R.K. Hanson a a Mechanical
Engineering Department, Stanford University, Building 02-660, Room 104, Stanford CA, USA Science Division, Lawrence Livermore National Laboratory, Livermore, CA 94551 USA c Total Marketing Services, 92800 Puteaux, France
b Materials
Received 29 November 2017; accepted 18 August 2018 Available online xxx
Abstract Ignition delay times (IDT) for high-octane-number gasolines and gasoline surrogates were measured at very high pressures behind reflected shock waves. Fuels tested include gasoline, gasoline with oxygenates, and two surrogate fuels, one dominated by iso-octane and one by toluene. RON/MON for the fuels varied from 101/94 to 106.5/91.5. Measurements were conducted in synthetic air at pressures from 30 to 250 atm, for temperatures from 700 to 1100 K, and equivalence ratios near 0.85. Results were compared with a recent gasoline mechanism of Mehl et al. (2017). IDT measurements of the iso-octane-dominated surrogate were very well reproduced by the model over the entire pressure and temperature range. IDT measurements for the toluene-dominated surrogate were also reproduced by the model to a lesser extent. By contrast, IDT measurements for the neat gasoline and gasoline with oxygenates, show excellent agreement with the trends of the Mehl et al. model only below 900 K. Above 900 K, the model returned IDT values for the two gasolines that were approximately 1.6× the measured values. Finally, we observed that IDT measurements for the toluenedominated surrogate fuel and the two gasolines, near 70 atm and below 900 K, appeared to be shortened, possibly by non-homogeneous ignition or non-ideal gas processes. This dataset provides a critically needed set of IDT targets to test and refine boosted gasoline models at high pressures. © 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Gasoline; High pressure; Ignition delay time; Negative temperature coefficient regime
1. Introduction In recent years, fuel economy requirements and CO2 emission regulations have been added to the existing vehicle emissions standards, driving ∗
Corresponding author. E-mail address: [email protected] (D.F. Davidson).
the development of more efficient powertrains. Consequently, car manufacturers have developed new solutions to reduce overall fuel consumption and CO2 emissions, commonly including higher compression ratio, turbocharging and engine downsizing. With the increase of power density and higher pressure operation, abnormal combustion modes such as knock or superknock are becoming a major limitation of engine efficiency.
https://doi.org/10.1016/j.proci.2018.08.032 1540-7489 © 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Please cite this article as: D.F. Davidson et al., Ignition delay time measurements and modeling for gasoline at very high pressures, Proceedings of the Combustion Institute (2018), https://doi.org/10.1016/j.proci.2018.08.032
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Prolonged knock must be avoided as it can cause severe engine damage to the pistons, cylinder head or cylinder head gasket. As the power density of modern engines increases, thermodynamic conditions of the fuel and air mixture in the combustion chamber during the compression stroke increase and amplify knock sensitivity. One of the common strategies to eliminate engine knock when detected is by retarding the spark timing, but at the cost of reducing engine performance and increasing CO2 emissions. Therefore, formulating fuels with enhanced knock resistance can help to improve engine performance and reduce fuel consumption. Fuel knock resistance is usually defined by indices such as Research Octane Number (RON) and Motor Octane Number (MON) [1,2] and by Octane Index (OI) [3,4]. Previous studies have looked at relationships between octane rating and gas phase ignition delay times measured in shock tubes or rapid compression machines at various pressures and temperatures [5–8]. However, these studies have been performed at low to medium pressures ranging typically up to 60 atm. In order to formulate better high-performance fuels, it is paramount to understand fuel auto-ignition phenomena at engine-relevant conditions and explore high to very-high pressures ranging from 100 to 200+ atm that can typically be seen in highly downsized engines. In the present study we investigated fuels with RON/MON ratios from 101/94 to 106.5/91.5. Modern light- and heavy-duty diesel engines are designed for 200 atm peak pressure, and some OEMs are considering going to 250 atm. In response to this, single-cylinder compression-ignition research engines are being designed for 300 atm. Modeling of combustion processes in gasoline compression ignition engines, in which the chemical kinetics play a crucial role in the combustion process, would benefit from this data collected at these elevated pressures. Earlier shock-tube work, which has been limited mostly to below 60 atm barely reaches mid-load in a modern diesel engine. Data at very high pressure will also improve understanding and development of high efficiency spark ignition gasoline engines. When these engines are operated without spark retard (for example when running on fuel resistant to end-gas autoignition), peak pressures can easily exceed 100 atm at high load. Future SI engines operating with a dilute boost or lean boost strategy at high load to avoid knock, can similarly reach high peak pressures approaching those of diesel engines. To address to these needs, programs such as Co-Optima [9] were instituted with the aim of providing the fundamental background to assist screening of promising future fuel candidates and identify advanced engine combustion strategies that leverage fuel properties at their best. In the gasoline space, this has led to the identification of synergistic blending behavior among fuel compo-
nents that could improve fuel performance at high pressures typical of high-pressure cycles obtained in down-sized boosted engines. Important questions remain open to modelers interested in fuel chemistry and blending behavior and now, more than ever, there is a growing need for fundamental IDT experiments in the 100 to 200+ atm range typical of highly downsized engines. 2. Previous studies Little distillate gasoline IDT data exist at the pressure conditions (>100 atm) typical of downsized engines. Early shock tube work by Gauthier et al. [6] at Stanford investigated one gasoline fuel and two surrogates at pressures ranging from 15 to 60 atm and temperatures above 900 K, effectively providing the first test data for modern formulations of high-pressure gasoline IDT. Kukkadapu et al. [10,11] used a rapid compression machine to study gasoline IDT at 20-40 atm. More recently, Sarathy et al. [8] investigated several FACE gasolines at pressures up to 40 atm at KAUST. Of particular importance and relevance to the current study is the observation that the shock tube derived IDT values below 950 K in the KAUST study also exhibit a near-abrupt change in activation energy and were significantly shorter than predictions with their gasoline model. Other studies of relevance include early shock tube ignition delay time measurements of gasoline surrogate components at pressures up to 45 atm as reported by Fieweger et al. [5], and IDT measurements of a gasoline/ethanol mixture (RON/MON 109/101) near 30 atm as reported by Cancino et al. [12] at IVG Duisburg. Most recently, Davidson et al. [13] investigated IDT measurements for a wide range of distillate fuels and found, at least for high temperature conditions above 1000 K, that IDT values were nearly identical for all studied distillate fuels, gasoline included. However, the absence of validation data for the high pressures relevant to boosted DGI (Direct Gasoline Injection) engines limits the ability of testing numerical models used by engine designers to simulate combustion chemistry at these conditions. This work provides the first instance where the ignition behavior of real fuels is systematically studied in a well-characterized environment at pressures above 100 atm, providing a fundamental stepping stone to understanding gasoline behavior in highly downsized engines. 3. Fuel composition and modeling The properties and composition of the four fuels studied are shown in Table 1. The simulations
Please cite this article as: D.F. Davidson et al., Ignition delay time measurements and modeling for gasoline at very high pressures, Proceedings of the Combustion Institute (2018), https://doi.org/10.1016/j.proci.2018.08.032
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D.F. Davidson et al. / Proceedings of the Combustion Institute 000 (2018) 1–8 Table 1 Composition of liquid fuels A, B, C and D in mass%. Fuel composition
A
B
C
D
Olefins Aromatics N-Paraffins Iso-Paraffins Napthenes Oxygenates Total MW RON MON
9 2 3 81 5
19 81
17 36 2 34 11
100 103.4 101 94
100 90.2 104 91
17 29 2 32 12 8 100 90.1 106.5 91.5
100 97 105 92
were performed using an extensive gasoline surrogate mechanism generated by the collaborative effort of LLNL, NUI Galway and KAUST [Mehl et al. (2017) to be submitted]. This detailed kinetic model includes a selection of n-alkanes, iso-alkanes, cycloalkanes, olefins and aromatics relevant to gasoline fuels (e.g., C4– C9 carbon number). While PRF mixtures (blends of the primary reference fuels, n-heptane and isooctane, used to define the octane scale) are often used to represent the behavior of gasoline fuels in traditional applications, their strong propensity to undergo low-temperature degenerate branching makes them inadequate to mimic real gasolines, particularly at high pressure, where the O2 addition reactions are thermodynamically favored. In order to capture the reactivity of high performance fuels, where the low temperature reactivity is suppressed by their unsaturated compounds content, more complex surrogates are needed, therefore the availability of components different from alkanes in the surrogate model is a necessity. Modeling surrogates for the fuels are given in Table 2. The composition of these multi-component surrogates was selected based on the results of a detailed hydrocarbon analysis of the real fuels and their octane numbers were estimated using a method developed by LLNL where the ignition behavior at 25 atm is correlated to their standardized knock metrics RON and MON [14]. The calculated RON are well in line with the ones of the target fuels, while the MON values are somewhat higher than expected. This discrepancy can be either result from limitations in the correlations used, or from shortcomings in the composition of choice or the kinetics of its components. 4. Experimental methods 4.1. High-pressure shock tube Ignition delay time experiments for all test mixtures were performed using the Stanford high-purity, high-pressure shock tube (HPST). The stainless steel driven section had an internal diameter of 5 cm and was heated to 90 C to pre-
3
Table 2 Composition of surrogates for fuels A, B, C and D in mass %. Molecule identifiers as in [15]. IC8D4 = 2,4,4trimethylpent-1-ene. Surogate component
A
IC8 IC5H12 IC8D4 C6H12-1 C4H10 c-C5H10 C6H5CH3 T135MBZ C6H5C2H5 P-XYL O-XYL C2H5OH Total MW RON MON
66 10 5 9 3 5 2
B
81
100 98.7 99.3 95.2
100 90.4 105.3 94.6
19
C
D
21 3 10 17 2 12 19 5 4 5 2
21 1 10 17 2 12 13 3 5 6 2 8 100 86.3 104.9 95.4
100 93.0 104.2 95
vent condensation of the test gas mixture. Burst diaphragms were made of aluminum and steel (with cross-scribing). Typical uniform test times for non-reactive synthetic air mixtures are 2 ms when helium is used as the driver gas. Uniform test times were extended to beyond 5 ms where needed using tailored He/N2 driver gas mixtures and driver inserts. Fuels were obtained from TOTAL Marketing & Services. High-purity synthetic air (21% O2 /N2 ) was supplied by Praxair as the bath gas. The liquid fuel was injected into a 12.8-liter stainless-steel mixing tank at 110 C. A test gas mixture of fuel/air was then prepared manometrically and was stirred using a magnetically-driven vane assembly for at least 15 min prior to the experiments. As the test pressures of the experiments in this study extend to 250 atm, non-ideal Equation of State (EOS) effects may begin to appear in both the determination of the reflected shock conditions and in the kinetics simulations [16]. To estimate the magnitude of these EOS effects on the test conditions of the IDT experiments, reflected shock conditions were calculated using both the ideal and the Peng-Robinson EOS. For incident shock wave speeds that produce reflected shock conditions in air of 800 K, 200 atm and 88.3 kg/m3 calculated using the Ideal EOS, the Peng-Robinson EOS predicts values of 793 K, 196 atm and 81.9 kg/m3 [17]. Based on this limited variation (1% in T5 , 2% in P5 and 7% in ρ 5 ), the ideal gas EOS was used for all temperature and pressure determinations in this study. The influence of non-ideal EOS on the kinetic simulations (e.g., fugacity effects on equilibrium constants) was not addressed in this study [18].
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4.2. Shock tube diagnostics Two diagnostics were employed during the experiments: laser absorption and sidewall pressure. Both diagnostics were located 1.1 cm away from the end wall. Initial fuel concentrations and relative hydrocarbon time-histories were measured directly in the shock tube by laser absorption with a 3.28 μm NanoplusTM DFB interband casade laser. Fuel concentrations were determined using Beer’s Law (I/I0 ) = exp (−σ N L ) = exp(−α)
(1)
where (I/I0 ) is the ratio of the transmitted and incident laser power at 3.28 μm, σ is the fuel crosssection at the same wavelength, N the fuel density, L the pathlength, and α is the absorbance. Fuel crosssections used in this determination of the fuel loading were measured in neat fuel vapor separately. In situ fuel loading measurements ensured accurate determination of equivalence ratio of the test gas mixtures. Ignition delay times were determined from both the normalized pressure time-histories monitored using a high-pressure KistlerTM model 607C1 piezoelectric pressure transducer (PZT) (that replaced a failing lower pressure sensor) and by the relative hydrocarbon time-histories measured by the 3.28 μm laser absorption signal. Though the absorbances measured behind the reflected shock waves were large (∼5–7) the stability of the laser system enabled useful information to be extracted from the small transmitted signals. In this study, the 1st and 2nd stage ignition delay times are defined as the time interval between the arrival of the reflected shock and the onset of 1st or 2nd stage ignition, determined by extrapolating the maximum slope of the 3.28 μm absorbance record to the baseline. Similar definitions were used for the pressure profiles and resulted in nearidentical IDT values. In many cases the 1st stage ignition was identifiable only in the absorbance record, as changes in the pressure record were very small. Uncertainties (2σ ) in IDT measurements are typically ± 15%. Uncertainties (2σ ) in initial reflected shock temperature T5 are ± 1%. Accurate measurements of the incident shock speed (with uncertainties typically ± 0.2%) translate into accurate determinations of reflected shock temperatures and pressures. The shock arrival times in the driven section of the shock tube were recorded by five axially-spaced PZTs (PCBTM 113A) placed over the last meter of the driven section. The velocity of the incident shock at the end wall was then determined by extrapolation, allowing calculation of the initial reflected shock temperature and pressure with ± 1% (2σ ) uncertainty, by using the ideal one-dimensional shock-jump relations and assuming vibrational equilibrium and frozen chemistry. The uncertainty in the determination of the individual ignition delay data points is typically ± 15% increasing to ± 20% (2σ ) for the highest
Fig. 1. IDT data for gasoline/air mixture near 40 atm and φ = 1. Solid line: Mehl et al. model (PRF84); dashed line: Davidson et al. [13] correlation. Current measurements (Fuel C and d) shown as stars with error bars.
pressure experiments. The primary contribution to the uncertainty is from the initial reflected-shock temperature. Other minor factors include determination of the ignition delay time interval based on the slope intercept criterion, fuel concentration uncertainty, and the influence of gradients (dP5 /dt) in reflected shock pressure with time.
5. Results and discussion 5.1. 30–50 atm studies A small selection of IDT measurements at 32 atm were performed to anchor the present study to earlier work, i.e., the 40 atm Sarathy et al. FACE fuels [8], 50 atm Gauthier et al. RD387 and Surrogate B [6], and 30 atm Cancino et al. IVG RON/MON 109/101 [12] fuel measurements. All data presented in Fig. 1 were normalized to 40 atm and an equivalence ratio of 1 using 1/P and 1/φ scalings. The similar IDT behavior of these fuels at these high temperatures is consistent with the distillate fuel correlation of Davidson et al. [13] and begin to diverge from this correlation at lower temperatures. The IDT values of all fuels above 950 K (and the RD387 IDT values down to 900 K) are in good agreement with a simulation using a PRF84 (n-heptane/iso-octane) surrogate. 5.2. Fuel A Fuel A has a high iso-paraffinic content dominated by iso-octane. Measurements were made at pressures from 54 to 248 atm and temperatures from 705 to 1060 K. Individual test conditions and IDT data for this and the other fuels are provided in the supplementary material. Representative laser absorbance and normalized pressure data for four
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Fig. 2. Example ignition delay time and laser absorbance measurements for Fuel A/air. 3.28 μm laser absorption (upper traces) and relative pressure (lower traces). Reflected shock conditions: 871 K, 248 atm, φ = 0.75; 895 K, 223 atm, φ = 0.70; 761 K, 155 atm, φ = 0.97; 760 K, 152 atm, φ = 0.93. Ringing, evident in the early part of the pressure profiles, is a result of a failing PZT sensor.
Fig. 3. IDT measurements and model for Fuel A/air mixtures. Solid lines: 2nd stage IDT simulations.
experiments are shown in Fig. 2. Of importance is the increase in laser absorbance just before ignition in the lower temperature experiments. This is indicative of the formation of a series of strongly absorbing, but late-forming intermediate product species. A complete interpretation of the 3.28 μm absorption signal is not yet possible as the lateforming intermediate species have not been identified nor have their IR absorption cross-sections been measured at these test conditions. Differences in the temperature and pressure dependences of the intermediate and product species absorption crosssections and concentrations may cause the different absorbance behavior seen at higher and lower temperatures. A rise in pressure, simultaneous with the change in laser absorbance, was seen in all cases.
5
Fig. 4. Example ignition delay time measurements for Fuel B/air. 3.28 μm laser absorption (upper traces) and relative pressure (lower traces). Reflected shock conditions: 786 K, 176 atm, φ = 0.76; 797 K, 157 atm, φ = 0.73; 824 K, 202 atm, φ = 0.82; 789 K, 128 atm, φ = 0.90;.
Fig. 5. IDT measurements and model for Fuel B/air mixtures. Solid lines: 2nd stage IDT simulations. Black crosses: lower temperature 60 atm measurements.
A summary of the IDT measurements and simulations are shown in Fig. 3. Data in this figure and others has been scaled by 1/P and 1/φ to the stated nominal values. Excellent agreement is seen between the model and the 2nd stage IDT data at all pressures. The simulated 1st stage IDT values are in very good agreement with the measurements. We believe this is the highest pressure IDT data for fuel dominated by branched alkanes currently available. 5.3. Fuel B Fuel B has a high aromatic content dominated by toluene with a minor 1-hexene component. Representative data for four experiments are shown in Fig. 4. The first strong delay in absorbance is attributed to 1st stage ignition. Small rises in pressure occur with the observed change
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Fig. 6. Example ignition delay time measurements for Fuel C/air. 3.28 μm laser absorption (upper traces) and relative pressure (lower traces). Reflected shock conditions: 787 K, 185 atm, φ = 0.77; 796 K, 190 atm, φ = 0.77; 803 K, 204 atm, φ = 0.95; 831 K, 214 atm, φ = 0.77; 851 K, 259 atm, φ = 0.88.
Fig. 7. IDT measurements and model for Fuel C/air mixtures. Solid lines: 2nd stage IDT simulations.
in absorbance; pressure rises coincident with the absorbance changes are evident in the two lower traces attached to the 824 K and 797 K absorbance traces. The IDT values for these four experiments are sensitive to pressure and equivalence ratio, as well as temperature; the shortest IDT occurs for the 824 K, 202 atm, φ = 0.82 example; the longest for the 786 K, 176 atm, φ = 0.76 example. A summary of the IDT measurements and simulations is presented in Fig. 5. Simulations show an explicit pressure dependence of order P−1 . The higher temperature (above 900 K) 60 atm simulations are in fair agreement with associated data. The 140 and 190 atm 2nd stage IDT data, follow the trend of the simulations but have IDT values approximately 1/2 of the modeled values. The 1st stage ignition values based on initial fuel removal at 60, 140 and 190 atm are also shown.
Fig. 8. Example ignition delay time measurements for Fuel D/air. 3.28 μm laser absorption (upper traces) and relative pressure (lower traces). Reflected shock conditions: 838 K, 136 atm, φ = 0.80; 782 K, 190 atm, φ = 0.83; 811 K, 150 atm, φ = 0.91; 817 K, 215 atm, φ = 0.89; 954 K, 163 atm, φ = 0.89.
Shown as black crosses are Fuel B/air IDT measurements attempted at 60 atm and below 950 K. Measurements made at these conditions were expected, based on current modeling, to have IDT values that were significantly longer than 2 ms. However, in the NTC temperature regime, these measurements reached an IDT plateau near 1–2 ms. Interestingly, a similar plateau was also seen in the Sarathy et al. 40 atm FACE gasoline data [8]. The lower temperature 60 atm IDT data (900–750 K) appear to be coincident with the higher pressure (140 and 190 atm) IDT values. Non-local ignition, flame propagation, or 1st stage-2nd stage IDT interactions may be responsible for the early onset of IDT in this pressure and temperature range. To resolve this issue in future work, we propose to use high-speed endwall imaging to monitor the combustion process and identify the occurrence of homogeneous or inhomogeneous ignition events [19]. 5.4. Fuel C Fuel C is a high-performance gasoline with significant aromatic and iso-paraffinic components. Representative IDT data for five experiments are shown in Fig. 6. For this fuel, early changes in laser absorbance associated with fuel removal and 1st stage ignition, is coincident with only small changes in pressure. A distinct beginning to the change in absorbance signaling 1st stage fuel removal is evident in the 787 K, 796 K and the 803 K examples. In the higher temperature examples at 831 K and 851 K, fuel removal is evident, but there is not a clear transition to a linear change in absorbance that is seen in the lower temperature examples. IDT simulations and measurements are shown in Fig. 7. At temperatures above 900 K, where
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IDT values for Fuel C and show little apparent effect of oxygenate addition on fuel reactivity. The Mehl et al. model accurately captures the 140 and 220 atm data, while again over-estimating the IDT values for 70 atm. 5.6. Modeling observations
Fig. 9. IDT measurements and model for Fuel D/air mixtures. Solid lines: 2nd stage IDT simulations.
little influence of NTC chemistry is expected, the modeled IDT values are approximately 1.6× the measured IDT values. At higher pressures and NTC temperatures, as is particularly evident in the 220 atm data, the model accurately captures the IDT trends in pressure and temperature variation. 5.5. Fuel D Fuel D includes approximately 8% by mass of oxygenates. Representative IDT data for five experiments are shown in Fig. 8. Laser absorbance and normalized pressure traces are very similar to those seen in the Fuel C data. 1st stage ignition is easily identifiable in the two cases where a linear change in absorbance is seen, i.e., for 782 K and 817 K examples. The absorbance profile at 838 K has a structure that is similar to the absorbance profiles seen in the toluene/air experiments, though the IDT value is consistent with surrounding data and simulation. IDT simulations and measurements are shown in Fig. 9. Measured IDT values for these two oxygenated fuels are very similar to the measured
Modeling results show general agreement with the high pressure data but appear to overestimate the ignition delay times of fuels B to D at high temperature. These results are consistent with the high MON values predicted using the correlations developed at LLNL. A sensitivity analysis on the reaction rates used in the model was performed to understand which fuel components and reactions are responsible for the deviations observed at high temperature between the model and the experiments (see Fig. 10). At these temperatures and pressures the ignition process is controlled by HO2 chemistry (HO2 recombination and H2 O2 decomposition) and its interactions with the fuel molecules (mainly via abstraction reactions). As expected, the next reaction in order of importance for fuel A is the HO2 abstraction on iso-octane, the prevalent component in the blend. For fuel C and D, and to a lesser extent for fuel A too, cyclopentane chemistry appears to be important. Finally, fuels C and D chemistry appears to be affected by diisobutylene and ethanol (fuel D only). A flux analysis (not presented in detail here) shows how, in the initial stages of the oxidation process (2.5% conversion of the fuel), the fate of HO2 is primarily to recombine or abstract an H from cyclopentane and ethanol to form H2 O2 (which later decomposes to 2 OH radicals). The radicals formed by the cycloalkane and the alcohol react with oxygen in a 1 or 2 step process to form HO2 again, feeding an autocatalytic cycle. These reactions end up promoting the reactivity of the blend in the model but it is not clear to which extent this mechanism accurately reflects the correct branching behavior
Fig. 10. A factor rate sensitivity analysis for the three fuels surrogates at 70 atm, φ = 0.85, 1000 K. Molecule identifiers as in [15]. IC8D4 = 2,4,4-trimethylpent-1-ene.
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occurring in the real fuel. This determination is made harder by the scarcity of fundamental studies on the blending behavior of components such as cyclopentane. Targeted studies at high pressures are needed to improve the understanding of the ignition behavior of fuel blends relevant to gasolines.
Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10. 1016/j.proci.2018.08.032. References
6. Conclusions For the first time IDT values for highperformance gasolines were measured at high pressures up to 250 atm. In general, excellent agreement was seen between model and data at very high pressures and NTC temperatures for gasolines. Simulations of this data using the current Mehl et al. model capture the trends in pressure and temperature, but overestimate the magnitude of the IDT values at higher temperatures. Work is currently proceeding on identifying the chemistry and surrogate species components that are affecting the reactivity of this model, possibly taking advantage of more targeted studies on the blending behavior of fuel components at these conditions. Significant differences between the model and experiment at large IDT values also point to the need to confirm the assumption of homogeneous ignition in these experiments or to investigate the role of real gas EOS. Studies of the combustion process using high-speed endwall imaging would provide one path to resolving this issue. Studies using real-gas CHEMKIN-like codes would provide another. Acknowledgments This work performed at Stanford University was supported by TOTAL Marketing & Services, Paris France and the authors would like to thank Roland Dauphin for his support on the fuels characteristics. The modeling work performed at LLNL was performed under the auspices of the U.S. Department of Energy (DOE), Contract DE-AC52-07NA27344 and was sponsored by (DOE) Bioenergy Technologies Office (BETO) and Vehicle Technologies Office (VTO) under the DOE Co-Optimization of Fuels and Engines Initiative. We would also like to thank an anonymous reviewer who provided further justification, included in the introduction, for this study.
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Please cite this article as: D.F. Davidson et al., Ignition delay time measurements and modeling for gasoline at very high pressures, Proceedings of the Combustion Institute (2018), https://doi.org/10.1016/j.proci.2018.08.032