Evaluation of Adding an Olefin to Mixtures of Primary Reference Fuels

Aug 23, 2016 - School of Chemistry, University of Leeds, Leeds LS2 9JT, United Kingdom. •S Supporting Information. ABSTRACT: The impact of adding an...
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Evaluation of Adding an Olefin to Mixtures of Primary Reference Fuels and Toluene To Model the Oxidation of a Fully Blended Gasoline J. C. G. Andrae*,† and T. Kovács‡ †

J A Reaction Engineering, SE-183 32 Täby, Sweden School of Chemistry, University of Leeds, Leeds LS2 9JT, United Kingdom



S Supporting Information *

ABSTRACT: The impact of adding an olefin to ternary mixtures of toluene and primary reference fuels to mimic the oxidation of a fully blended gasoline was examined with kinetic modeling. Reactions for the oxidation of 2,4,4-trimethyl-1-pentene (DIB-1), which is the major constituent in diisobutylene (DIB), were added to a previously developed semidetailed mechanism for ternary mixtures. The merged kinetic mechanism was revised and successfully checked for validity against data for neat fuel components as well as fuel mixtures at conditions relevant to engine combustion. The validated kinetic model was then used to model a fully blended research gasoline. By using a nonlinear-by-volume blending model for octane numbers, a four-component surrogate fuel was formulated which consisted of 51% isooctane, 18% n-heptane, 26.4% toluene, and 4.6% DIB-1 by liquid volume. The surrogate fuel reflected molecular-structure class composition, research octane number, motor octane number, density, and H/C ratio of the target gasoline. Ignition delay times for gasoline measured in a shock tube, rapid compression machine, and an HCCI engine were then compared to simulated results using the quaternary mixture and ternary mixtures with similar octane numbers and H/C ratio as the target gasoline. Adding DIB-1 to a ternary mixture had a small but significant effect on the autoignition of gasoline surrogate fuels. The quaternary mixture showed better agreement when compared to measurements, especially at higher temperatures. The simulated ignition delays at shock tube and rapid compression machine conditions were also well-correlated with the combustion phasing in an HCCI engine defined as the temperature required at bottom dead center to achieve 50% heat release (CA50) at top dead center. Similar results were achieved when comparing with other published mechanisms. Simulations with neat and binary mixtures combined with a rate-of-production and sensitivity analysis with multicomponent mixtures show that the reason for the increased reactivity and shorter ignition delay when adding DIB-1 to the ternary mixture is that DIB-1 promotes toluene ignition more than isooctane at these conditions.

1. INTRODUCTION There is much demand for chemical kinetic models to represent practical fuels such as gasoline. The practical fuel is a complex mixture of hydrocarbons containing hundreds of components whose identity and amount are often unknown. However, the components can be divided into various molecular-structure classes such as straight-chain alkanes, branched alkanes, cyclic alkanes, alkenes, and aromatics. This facilitates the development of chemical kinetic models for “surrogate fuels” that can used to model the practical fuel of interest.1 One approach is to include a pure component from each class of compounds, and such models are very useful to understand for example autoignition of a wellcharacterized fuel.2−12 Autoignition quality of fuels has to be understood in terms of empirical measures such as research and motor octane numbers (RON and MON).13 A common gasoline surrogate fuel in kinetic modeling is the ternary mixture which includes toluene and primary reference fuels (PRF).14−18 Recently it was shown that such mixtures could be formulated to mimic both RON and MON of the target fuel.19 Nevertheless, Dryer suggests that at the very least H/C (and O/C ratio) should also be the same as the target fuel if appropriate kinetically related and scaling behaviors of low-, intermediate-, and high-temperature heat release rate are to be achieved.20 © XXXX American Chemical Society

Regardless of the blending model for RON and MON, it is still very difficult to accurately match RON, MON, and H/C ratio of the target fuel by utilizing a ternary mixture. Cai and Pitsch recently formulated a ternary mixture that could reasonably well reflect octane numbers and H/C ratio of the target fuel in the form of a fully blended gasoline.12 However, the relative percentage of the molecular-structure classes in the target fuel cannot be taken into account using only ternary mixtures. For example the fully blended gasoline RD387 consists of around 5 vol % olefins.8 Detailed description of olefin chemistry is part of a number of gasoline surrogate kinetic models,2,3,7,9,10 but a systematic evaluation of the specific impact on olefin addition to mixtures of toluene and primary reference fuels to model the oxidation of a fully blended gasoline has not been given much attention. Perez and Boehman showed by using statistical modeling that fuel chemistry has significant effects on engine performance under HCCI conditions.21 Having n-heptane and the olefin 1-hexene in the gasoline surrogate fuel was beneficial to achieve satisfactory combustion phasing (CA50) but detrimental to thermal Received: May 18, 2016 Revised: August 8, 2016

A

DOI: 10.1021/acs.energyfuels.6b01193 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Conceptual Model for DIB-1 Low-Temperature Chemistry

(

E

)

k f = AT n exp − RT . Units: A, mol·cm·s; E, cal/mol.

a

To investigate the effect of DIB-1 addition in the surrogate fuel, measurements of ignition delay in shock tube, rapid compression machine, and HCCI engine for the target gasoline are then compared to simulated results using the quaternary mixture and three ternary mixtures with similar octane numbers and H/C ratio as the target gasoline. A sensitivity and rate-ofproduction analysis is carried out to further analyze the results. Finally the effect of kinetic model on the results is investigated where predictions of other kinetic models for gasoline surrogate fuels in the literature are compared to the results with the model in this work.

efficiency and indicated mean effective pressure (IMEP). On the other hand, high contents of isooctane or toluene were not desirable for HCCI combustion because of low ignitability and high burning rates but must be present at optimal concentrations (for example around 30% for toluene) to keep high IMEP and thermal efficiency. Ignition of neat and binary mixtures involving the olefin diisobutylene (DIB) was first studied experimentally by Sturgis in a cooperative fuels research (CFR) engine22 and more recently with the isomer DIB-1 in a rapid compression machine and DIB in a motored engine.23,24 Important results from those studies are that DIB-1 is more resistant to autoignition than isooctane at low temperatures but less resistant at high temperatures. Moreover, the addition of small amounts of DIB-1 and isooctane to toluene results in a drastic reduction in ignition delay where the effect of DIB-1 is stronger that isooctane.23 Also, DIB addition to nheptane and isooctane inhibits ignition.22,24 Recently, Kukkadapu et al. compared the ignition delay (τi) in a rapid compression machine of a four-component gasoline surrogate fuel consisting of 57 vol % isooctane, 16 vol % nheptane, 23 vol % toluene, and 4 vol % 2-pentene and the Stanford A surrogate which is a ternary mixture consisting of 63 vol % isooctane, 17 vol % n-heptane, and 20 vol % toluene.25,26 They found that the quaternary mixture better reproduced the ignition behavior of the full blend RD387 gasoline. It should be noted that the Stanford A surrogate does not as well reflect the H/C ratio and RON of RD387 compared to the quaternary mixture which may affect the comparison. For further understanding of the minimum number of components required in a surrogate fuel to mimic the oxidation and ignition behavior of a target gasoline, the RON, MON, and H/C ratio for the surrogate fuels to be compared should be as close as possible. In this work, for the impact of adding an olefin to mixtures of toluene and primary reference fuels to mimic the oxidation, a full blend gasoline fuel has been examined with kinetic modeling where RON, MON, and H/C ratio are similar for the fuels studied. Reactions for the oxidation of 2,4,4-trimethyl-1-pentene (DIB-1), which is the major constituent in DIB, are added to a previously developed semidetailed mechanism for ternary mixtures. The validated kinetic model is used to model a fully blended research gasoline (RD387) which consists of around 68 vol % saturated hydrocarbons, 27 vol % aromatics, and 5 vol % olefins.8 By using a nonlinear-by-volume blending rule, a quaternary mixture is formulated which consists of 51 vol % isooctane, 18 vol % n-heptane, 26.4 vol % toluene, and 4.6 vol % DIB-1. This surrogate fuel reflects molecular-structure class composition, RON, MON, density, and H/C ratio of the target gasoline.

2. CHEMICAL KINETIC MODEL DEVELOPMENT AND VALIDATION 2.1. Mechanism Development. The starting mechanism for model development was a semidetailed chemical kinetic mechanism for ternary mixtures with toluene and PRF involving 137 species and 635 reactions.17 It has a revised toluene submodel compared to the original model by Andrae et al.15 for better prediction of species data in flow and perfectly stirred reactors. The mechanism consists of a detailed reaction mechanism for toluene, and skeleton mechanisms for the combustion of isooctane (2,2,4-trimethylpentane) and n-heptane. The mechanism contains no specific reactions for DIB-1 (JC8H16) oxidation other than a formation and decomposition reaction:

AC8H17•+ O2 ⇆ JC8 H16 + HO2• JC8 H16 → IC4H8 +

C3H5•+ CH3•

(1) (2)

Although eqs 1 and 2 might provide enough information for qualitative studies involving fuel mixtures,4 a more comprehensive description is needed. In this work the DIB-1 oxidation mechanism (2,4,4-trimethyl-1pentene subset) by Metcalfe et al.27 is added to the model for ternary mixtures, an approach previously utilized to incorporate DIB-1 chemistry to existing surrogate models.3,9,10 However, the DIB model by Metcalfe et al. was only validated at high temperatures and low pressures. Therefore, some low-temperature reactions analogue to the skeletal isooctane submechanism were added to the DIB-1 submechanism (see Table 1). Rate constants were estimated to reflect that DIB-1 is more resistant to autoignition than isooctane at lower temperatures and shows no negative-temperature-coefficient (NTC) behavior.23 Some modifications of the submodels in the model for ternary mixtures were necessary to incorporate the DIB-1 chemistry, and also updates on specific rate constants were conducted as part of the model development based on the work in refs 9, 12, 16, and 28−36. Table S1.1 and S1.2 of the Supporting Information describe these changes in detail and rate constants for the DIB-1 submechanism, respectively. The overall kinetic model consists of 159 species and 734 reactions. As part of this study a reduced version of the mechanism was also developed by utilizing the program KINALC37 which runs together with CHEMKIN and SENKIN codes.38,39 By using the “Connect” option in KINALC, a number of species (and their associated reactions) contained in the toluene and benzene submechanisms were systematiB

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Energy & Fuels cally removed without losing accuracy at conditions relevant for model application. The reduced version consists of 123 species and 583 reactions. Both the detailed and reduced mechanism are given in electronic format as Supporting Information. 2.2. Reactor Model Assumptions. Simulations were performed with the software CANTERA.40 For ignition delay calculations a constant volume assumption was used where total ignition delay (τi) was defined as the time when the temperature had increased 400 K from the starting temperature. For diluted mixtures at low pressure, definition of ignition delay as suggested in the original reference (such as position of CH peak) was also compared to when possible and no significant difference could be seen. For flow reactor simulations a constant pressure assumption was used. Laminar flame speed simulations were carried out using the model for a freely propagating flame using mixtureaverage transport properties. To obtain grid independent solutions, values of slope and curve controlling the addition of points in regions of high gradients and high curvature were set to 0.05−0.075 and 0.15, respectively. For ignition timing in HCCI where reactor volume changes with time, a single-zone modeling approach was employed which treats the in-cylinder charge as a single lumped mass with uniform composition and thermodynamic properties. The in-cylinder mass is compressed and expanded adiabatically using the standard slider−crank relationship.41 The variation of specific volume as a function of time could be, with negligible error ( 900 K and the fuel with the highest content toluene and lowest H/C ratio is least reactive for T < 770 K (see Figure 10a,b). This is even more pronounced at lower fuel−air ratios (see Figure 10c−f). At T < 740 the ternary mixture with the least amount of toluene and highest H/C ratio becomes less resistant to autoignition than the fuel containing DIB-1. The observed deviation of the shape for the simulated results E

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Figure 10. (a−f) Comparison of measured ignition delays of gasoline RD3878,50 and simulated (lines) total ignition delays using a constant volume assumption. Surrogate fuel compositions as isooctane/n-heptane/toluene/DIB-1 by vol %.

Figure 12. BDC temperatures required to maintain combustion phasing (CA50) at TDC for fully premixed fueling. Experiments by Hwang et al.63 compared to simulations. Conditions: ϕ = 0.28, pBDC = 1 bar, and 1200 rpm. Simulations performed with compression ratio of 15.8. Figure 11. Mass-averaged temperature trace for toluene measured in a HCCI engine at 1200 rpm by Hwang et al.63 compared to simulations (lines) using a single-zone model. Conditions at BDC: ϕ = 0.28, pBDC = 1 bar, and TBDC = 162 °C (simulation).

mixtures of fuels that are not present for neat fuels, as shown in an experimental work with a multicomponent mixture.65 4.2.2. Gasoline. Figure 13 shows how simulated results for surrogate fuels as defined in Table 4 compare against the measured value for gasoline. The temperatures required at bottom dead (TBDC) center to achieve 50% heat release (CA50) at top dead center in an HCCI engine are well-correlated with the simulated ignition delays shown in Figure 10. The quaternary mixture shows the highest reactivity with increased temperature and also best agreement against shock tube ignition delay data for gasoline (see Figure 10). The quaternary mixture also needs a TBDC in HCCI that is slightly lower and closer to the measured one for gasoline compared to the ternary mixtures (see Figure 13). Perez and Boehman could also well-correlate their measured ignition delays in an ignition quality tester (IQT) with the combustion phasing (CA50) observed in an HCCI engine.66 The results according to the simulations above show that adding DIB-1 to ternary mixtures with PRF and toluene slightly

using the detailed LLNL mechanism.35,64 Observed lowtemperature heat release rate for PRF80 could not be reproduced by the kinetic model, and they had to use a much higher content of n-heptane (PRF40) in their simulations. More work is clearly needed on the low-temperature chemistry and pressure dependence in kinetic models for PRF to improve the results for such fuels. It is preferred that a model is well-balanced and is able to predict data for both neat fuels and fuel mixtures as well as possible. However, it should be noted that although a kinetic model could fail to predict neat fuels at all tested conditions, it may still succeed to predict multicomponent fuel mixtures. This is because of the effect of shared reactive intermediates in F

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Figure 15. Modeled ignition delay times for binary mixtures of toluene/ isooctane and toluene/DIB-1. Constant volume simulation. Conditions: ϕ = 1.0 (in air) and p = 40 bar.

Figure 13. BDC temperature required to maintain combustion phasing (CA50) of gasoline at TDC for fully premixed fueling. Experiment by Hwang et al.63 compared to simulations with surrogate fuels with composition as isooctane/n-heptane/toluene/DIB-1 by vol %. Conditions: ϕ = 0.28, pBDC= 1 bar, and 1200 rpm. Simulations performed with a compression ratio of 15.8.

alters the reactivity of surrogate fuel, especially as the temperature increases over 900 K. A four-component surrogate fuel containing DIB-1 also shows slightly better agreement compared to ternary fuel mixtures against measured combustion timing of gasoline in a HCCI engine. These results give support to the fact that octane numbers do not indicate autoignition uniquely.66−68 Matching simulated ignition delays of the surrogate fuel with experimental data of the fully blended fuel provides a better representation of autoignition quality. Figure 16. HCCI engine simulation with fuel composition and TBDC as in Figure 13. Conditions: ϕ = 0.28, pBDC = 1 bar, engine speed = 1200 rpm, and compression ratio = 15.8.

5. DISCUSSION Experimental results by Mittal and Sung show that, at low temperatures, neat DIB-1 exhibits longer ignition delays than neat isooctane, whereas, at higher temperatures, ignition delays of neat DIB-1 are shorter than those of isooctane. They also showed that the addition of a small amount of DIB-1 or isooctane to toluene resulted in a drastic reduction in ignition delay, with the effect of DIB-1 addition being much stronger.23 Figure 14 shows modeled ignition delay of neat DIB-1 and isooctane while Figure 15 shows ignition delay for binary

DIB-1. Overall, the modeled results in Figures 14−16 confirm previous experimental findings for binary fuel mixtures and help to explain why quaternary mixtures containing the olefin DIB-1 become less resistant to autoignition than ternary mixtures at higher temperatures. 5.1. Rate-of-Production and Sensitivity Analysis. The results were further analyzed by conducting a net rate-ofproduction and brute force sensitivity analysis. For this purpose, first SENKIN38 was used to perform constant volume simulations with sensitivity analysis. Then KINALC37 was utilized to extract rate-of-production data and sensitivity coefficients from the SENKIN simulations. Finally, the most sensitive reactions were used to perform a brute force sensitivity analysis where the pre-exponential (A) for each reaction was doubled and the sensitivity percentage expressed as S(τ ) =

Figure 14. Modeled ignition delay times for neat isooctane and neat DIB-1. Constant volume simulation.

τ(2A) − τ(A) × 100 τ (A )

(5)

In eq 5 τ is the ignition delay in milliseconds. Figures 17 and 18 show the rate of net rate of consumption of toluene in two multicomponent fuels. As expected the net rate of consumption of toluene is higher in the fuel that contains DIB-1 (see Figures 17 and 18). It can also be seen that a cross-reaction between toluene and IC4H7 (559) suggested by Sakai et al.16 contributes to toluene consumption at these conditions (see Figure 17). Figure 19 shows the results of a brute force sensitivity analysis for the same fuels. Only those reactions are shown which are

mixtures with toluene and isooctane and DIB-1 respectively at the same operating condition as in Figure 10b with the multicomponent mixtures. The simulated results confirm the findings by Mittal and Sung above. Figure 16 shows toluene conversion at HCCI conditions according to Figure 13. In the temperature range 800−1100 K toluene conversion is somewhat higher in the fuel that contains G

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fuel mixture results in increased sensitivity toward the decomposition reaction 479 (see Figure 19a): JC8 H16 ⇆ IC4 H 7 + TC4 H 9

The IC4H7 radicals further react with toluene, leading to an increased rate of consumption of that fuel (see Figure 17). In summary, the results presented in this section illustrate how the addition of an olefin (DIB-1) to a ternary mixture can lead to decreased ignition delay of a gasoline surrogate fuel containing toluene. 5.2. Effect of Kinetic Model. The model used in this work has been extensively validated with good results (see Figures 1−9 and Supporting Information). However, the impact of the kinetic model used may still be significant. Figure 20 shows results for a ternary mixture for the same initial conditions but with different kinetic models in the literature.

Figure 17. Rate of consumption of toluene. Constant volume simulation. Fuel composition as in Table 4. Conditions: ϕ = 1.0, T = 980 K, and p = 40 bar.

Figure 18. Rate of consumption of toluene. Constant volume simulation. Fuel composition as in Table 4. Conditions: ϕ = 1.0, T = 980 K, and p = 40 bar.

Figure 20. BDC temperature required to maintain combustion phasing (CA50) of gasoline at TDC for fully premixed fueling. Experiment by Hwang et al.63 compared to single-zone HCCI simulations with different kinetic models7,12,17,18 and surrogate fuel compositions as isooctane/n-heptane/toluene/DIB-1 by vol %. Conditions: ϕ = 0.28, pBDC = 1 bar, and 1200 rpm. Simulations performed with a compression ratio of 15.8.

Figure 20 shows that simulations with the model in this work are in relatively close agreement with the other models and that the ternary mixture is more resistant to autoignition than the quaternary mixture. Therefore, changing the kinetic model to simulate a ternary mixture does not change the fact that adding DIB-1 to the ternary mixture makes the surrogate fuel slightly less resistant to autoignition for the reasons explained above. The model by Andrae is more reactive than the other models but also shows with its two-step reaction model for DIB-1 (see Section 2.1) the increased reactivity when adding DIB-1 to the ternary mixture. Figure 21 shows results for neat fuels and indicates how well the different models are tuned for neat fuels at HCCI conditions. An improvement of the model in this work compared to the Andrae model is the prediction for toluene, DIB-1, and PRF80 (see Figure 21a,c,d).

6. CONCLUSIONS By using a nonlinear-by-volume blending model for octane numbers, a four-component gasoline surrogate fuel has been formulated which consists of 51% isooctane, 18% n-heptane, 26.4% toluene, and 4.6% DIB-1 by liquid volume. The surrogate fuel reflects molecular-structure class composition, RON, MON, density, and H/C ratio of the target gasoline.

Figure 19. (a, b) Brute force sensitivity coefficients. Conditions: p = 40 bar and ϕ = 1.0. Fuels as defined in Table 4.

above the limit of |S(τ)| ≥ 5%. Positive values mean that increasing the rate constant results in a longer ignition delay. Many reactions are similar for the two fuels; however, a significant difference is that the addition of DIB-1 to the ternary H

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mechanisms in the literature for which ternary mixtures could be simulated. The reason for the increased reactivity at higher temperatures when adding the olefin to the ternary mixture can be explained by that DIB-1 promotes toluene ignition more than isooctane at these conditions. The results presented give support to the fact that octane numbers does not indicate autoignition uniquely. Matching simulated ignition delays of the surrogate fuel with experimental data of the fully blended fuel provides a better representation of autoignition quality.



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01193. Detailed description of model and model validation (PDF) Detailed model (TXT) Reduced model (TXT) Transport data (TXT)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.



REFERENCES

(1) Pitz, W. J.; Cernansky, N. P.; Dryer, F. L.; Egolfopoulos, F. N.; Farrell, J. T.; Friend, D. G.; Pitsch, H. SAE Tech. Pap. Ser. 2007, DOI: 10.4271/2007-01-0175. (2) Yahyaoui, M.; Djebaïli-Chaumeix, N.; Dagaut, P.; Paillard, C. E.; Gail, S. Proc. Combust. Inst. 2007, 31, 385−391. (3) Andrae, J. C. G. Fuel 2008, 87, 2013−2022. (4) Andrae, J. C. G.; Head, R. A. Combust. Flame 2009, 156, 842−51. (5) Ra, Y.; Reitz, R. D. Combust. Flame 2011, 158, 69−90. (6) Mehl, M.; Chen, J. Y.; Pitz, W. J.; Sarathy, S. M.; Westbrook, C. K. Energy Fuels 2011, 25, 5215−5223. (7) Mehl, M.; Pitz, W. J.; Westbrook, C. K.; Curran, H. J. Proc. Combust. Inst. 2011, 33, 193−200. (8) Kukkadapu, G.; Kumar, K.; Sung, C. J.; Mehl, M.; Pitz, W. J. Combust. Flame 2012, 159, 3066−78. (9) Wang, Y.; Yao, M.; Zheng, Z. Fuel 2013, 113, 347−356. (10) Zhong, B.-J.; Zheng, D. Fuel 2014, 128, 458−466. (11) Sarathy, S. M.; Kukkadapu, G.; Mehl, M.; Wang, W.; Javed, T.; Park, S.; Oehlschlaeger, M. A.; Farooq, A.; Pitz, W. J.; Sung, C.-J. Proc. Combust. Inst. 2015, 35, 249−257. (12) Cai, L.; Pitsch, H. Combust. Flame 2015, 162, 1623−1637. (13) Kalghatgi, G. T. Proc. Combust. Inst. 2015, 35, 101−115. (14) Chaos, M.; Zhao, Z.; Kazakov, A.; Gokularkrishnan, P.; Angioletti, M.; Dryer, F. L. A PRF+toluene surrogate fuel model for simulating gasoline. Proceedings of the 5th U. S. Combustion Meeting, Kinetics Paper E26; Western States Section, Combustion Institute: Pittsburgh, PA, USA, 2007. (15) Andrae, J. C. G.; Brinck, T.; Kalghatgi, G. T. Combust. Flame 2008, 155, 696−712. (16) Sakai, Y.; Miyoshi, A.; Koshi, M.; Pitz, W. J. Proc. Combust. Inst. 2009, 32, 411−418. (17) Andrae, J. C. G. Fuel 2013, 107, 740−748. (18) Liu, Y.-D.; Jia, M.; Xie, M.-Z.; Pang, B. Energy Fuels 2013, 27, 4899−4909. (19) Kalghatgi, G.; Babiker, H.; Badra, J. SAE Technol. Pap. Ser. 2015, 8, 505.

Figure 21. BDC temperatures required to maintain combustion phasing (CA50) at TDC for fully premixed fueling. Experiments by Hwang et al.63 compared to simulations using kinetic models from the literature.7,12,17,18 Conditions: ϕ = 0.28, pBDC = 1 bar, and 1200 rpm. Simulations performed with compression ratio of 15.8.

Results from simulations with a validated chemical kinetic mechanism in this work showed that adding the olefin DIB-1 to a ternary mixture of toluene and PRF slightly alters the reactivity of the surrogate fuel, especially as the temperature increases over 900 K. Against HCCI engine data, simulations using the quaternary mixture showed better agreement compared to ternary mixtures with similar octane numbers and H/C ratio. This result was not altered when comparing with other published I

DOI: 10.1021/acs.energyfuels.6b01193 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b01193 Energy Fuels XXXX, XXX, XXX−XXX