Theoretical Investigation of the Combustion of PRF90 under the

Nov 22, 2017 - Fuel reforming strategy is a promising new technology to improve the combustion performance of the fuel in engines. The flexible cylind...
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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Theoretical Investigation of the Combustion of PRF90 under the Flexible Cylinder Engine Mode: The Effects of Cooling Strategies on the Mode Yang Wang,† Lixia Wei,‡ and Mingfa Yao*,† †

State Key Laboratory of Engines, Tianjin University, Tianjin 300072, People’s Republic of China College of Mechanical Engineering, Guangxi University, Nanning, Guangxi 530004, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Fuel reforming strategy is a promising new technology to improve the combustion performance of the fuel in engines. The flexible cylinder engine (FCE) mode is a new concept, by optimizing the property of the fuel via low-temperature reforming in a flexible cylinder. The FCE mode consists of three processes, i.e., reforming, cooling, and working processes. In this work, the primary reference fuel of PRF90 was adopted to demonstrate the FCE mode in a homogeneous charge compression ignition engine, mainly comparing the effects of three typical cooling strategies, i.e., linear, natural, and delayed cooling, on the reformed products and, hence, the combustion performance of the fuel in the normal cylinders theoretically. The simulations were performed using the CHEMKIN package. The ignition delay time was decreased dramatically under the FCE mode with the three cooling strategies. The potential in decreasing the ignition delay time is in the order of linear cooling > normal cooling > delayed cooling. The laminar flame speed was increased slightly under the FCE mode. The delayed cooling strategy performed best in reducing the harmful emissions of carbon monoxide and unburned hydrocarbon among the three cooling strategies. From the point view of energy efficiency and emissions, the delayed cooling strategy should be adopted in practical engines under FCE mode. emissions was decreased by about 56.8%. Fennell et al.21 experimentally explored the efficiency and emissions of a GDI engine with the addition of H2-rich exhausts from catalytically reformed gasoline by platinum−rhodium. It was found that this reformed exhaust gas recirculation strategy improved the indicated engine efficiency and reduced the particulate matter emissions relative to the normal EGR. These reforming strategies require an external fuel reforming device and, hence, the redesign of the engine system layout. Moreover, in these strategies, the fuel was injected into the negative valve overlap (NVO) period as a common way to reform the fuel and, hence, to control the combustion phasing in a HCCI engine.22−24 When the fuel is injected into the O2deficient NVO atmosphere, part of the fuel will be converted to a mixture containing significant levels of H2 and CO. Other short-chain hydrocarbons will be produced by means of watergas shift, thermal cracking, and partial oxidation reactions simultaneously. Wolk et al.25 examined the impact of gasoline pilot fuel injection during the NVO period on the main period combustion performance of four fuels, i.e., isooctane, n-heptane, ethanol, and 1-hexene, on a direct-injection, single-cylinder research engine. It was found that this strategy decreased the reactivities of n-heptane and 1-hexene only at very high reformate fractions (>80%). Ethanol exhibited sharp reactivity increases at low reformate fractions ( normal cooling > delayed cooling. The ignition delay time in the case of the linear cooling process was decreased by about 2° CA than that of the delayed cooling process. As shown in Figure 8, the ignition delay time in the case of the delayed cooling is closer to the TDC than the other two cooling strategies. Thus, the delay cooling process is more energy-efficient than the other two cooling processes.32,33 Because combustion is a free radical process, the radicals in the reformed products should have some effect on the ignition delay time. Besides, during the cooling process, the radicals should be eliminated by the wall reactions as a result of their high reactivities. To clarify the effects of the radicals on the ignition delay time, additional simulations were performed by removing the free radicals from the reformed products, as shown in Figure 3. It can be seen that the simulated results altered marginally with and without the radicals in the reformed

altered substantially. Reaction R6 contributed overwhelmingly to the production of CH3O2H in the case of linear cooling and natural cooling, while reaction R4 contributed dominantly to the formation of CH3O2H in the case of delayed cooling. In reaction R4, XC8H17 denotes any of the four isomers of C8H17 radicals, i.e., 1-(2,2,4-trimethyl)pentyl (AC8H17), 3-(2,2,4trimethyl)pentyl (BC 8 H 17 ), 2-(2,4,4-trimethyl)pentyl (CC8H17), and 1-(2,4,4-trimethyl)pentyl (DC8H17). In fact, a high temperature favors the reactions with higher activation energies. Thus, the contributions of reactions R4 and R5 increased substantially with the formation of CH3O2H. In reaction R5, C7H15-X denotes any of the four C7H15 isomers produced from n-heptane, i.e., 1-heptyl (C7H15-1), 2-heptyl (C7H15-2), 3-heptyl (C7H15-3), and 4-heptyl (C7H15-4). Reaction R3 had no effect on the formation of CH3O2H at 0.01 s in the delayed cooling process. CH3O2H was mainly consumed by decomposition to form methoxyl (CH3O) and hydroxyl radical in the delayed cooling strategy at 0.01 s. CH3O2 H = CH3O + OH

(R7)

However, the absolute reaction rate of reaction R7 is much lower than those of reactions R3−R6, the formation reactions of CH3O2H. Consumption of CH3O mainly led to the formation of CH2O, which is a stable and common species during the combustion of hydrocarbon fuels. CH3O + O2 = CH 2O + HO2

(R8)

CH2O may increase the ignition delay time of the fresh fuel or even lead to misfire, as shown in Figure 3. Besides reaction R8, CH 2O may also be produced from other oxygenated intermediates. NEOC5H11O = CH 2O + TC4H 9

(R9)

XC8H17O = Y/PC7H15 + CH 2O

(R10)

CH3COCH 2O = CH3CO + CH 2O

(R11)

CH 2O + OH = HCO + H 2O

(R12)

Figure 7 describes the reaction pathways of CH2O in the cooling process at 0.01 s. H-abstraction of CH3O by O2 contributed more than 35% for the formation of CH2O (reaction R8) in all three cooling processes. The decomposition of 2-(2,3-dimethyl)propanyloxyl [(CH3)3CCH2O*, NEOC5H11O] contributed 30.8% to the production of CH2O E

DOI: 10.1021/acs.energyfuels.7b02216 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 8. Effect of three cooling processes (linear cooling, natural cooling, and delayed cooling) on the temperature of the normal cylinder fueled with the mixture of PRF90 and all of the reformed products. The inset panel shows details of the bumps of the temperature profiles. Figure 9. Mole fraction profiles of (a) OH and (b) H2O2 in the normal cylinder with the addition of all of the reformed products under the three cooling processes.

products. Again, this can be attributed to the very low mole fractions of the radicals in the reformed products. In fact, it is common that the mole fractions of radicals are very low in the gas at ambient pressure and temperature. It can also be concluded that the reformed products without the radicals have a significant effect on the ignition delay time of the fresh PRF90. Thus, the contributions of the key species in the reformed products were further investigated separately by mixing them with the fresh fuel of PRF90 separately to investigate the effects of these species on the combustion, including several stable species with relatively higher mole fraction in the reformed products after cooling, such as H2, CH2O, CH3COCH3, and CO. Because CH3O2H, H2O2, and KETs are typical low-temperature combustion intermediates, the effects of these species were also simulated, as shown in Figure 3. It can be seen that CH3O2H, H2O2, and C3KET13 decreased the ignition delay time significantly, with the potential of CH3O2H > C3KET13 > H2O2 in the linear cooling and natural cooling processes and CH3O2H > H2O2 > C3KET13 in the delayed cooling process. As shown in Figure 3, CH3O2H was the most potent in decreasing the ignition delay time of the fresh PRF90 compared to other species in the reformed products. The potencies of C3KET13 and H2O2 were affected by their concentrations and were reversed in the three cases as the result of reversed concentrations. CO and H2 had little effect on the ignition delay time of PRF90 in all three cooling processes. The additions of CH3COCH3 and CH2O resulted in misfire or delayed ignition delay time in all three cases. It is known that the ignition delay time is related to the formation of OH. Thus, the mole fraction of OH was investigated, as shown in Figure 9a, under the FCE mode with the addition of all of the reformed products in the three cooling processes. There are three peaks in each mole fraction profile of OH in Figure 9a. The first peak begins at about −25° CA, and the peak timing is in the sequence of linear cooling < natural cooling < delayed cooling. Reaction pathway analysis of OH with the addition of all of the reformed products into PRF90 at −25° CA in the normal cylinder is shown in Figure 10. It indicated that the delayed cooling process decreased the contributions from the small hydroperoxides (reaction pathway 1) and increased the contributions from the typical low-temperature reactions

(reaction pathways 2−30) obviously. The results are in accordance with those in Figure 9a: enhanced contributions from the small hydroperoxides reflect the advance of a combustion process over another one in the low-temperature region. Contributions from the decomposition of H2O2 are all zero as a result of the lower temperature at −25° CA of about 750 K than the decomposition temperature of the same species in the gas phase of about 1000 K. Because the cooling process altered the reaction pathways in the normal cylinders, the absolute rate of production analysis of OH was compared, as shown in Figure 11. It can be seen that the four most important reactions for the formation of OH are basically the same for the three cases AC8H16OOH‐C = IC8ETERAC + OH

(R13)

AC8H16OOH‐BO2 = IC8KETAB + OH

(R14)

IC8KETAB = IC3H 7CHO + TC3H6CHO + OH

(R15)

DC8H16OOH‐BO2 = IC8KETDB + OH

(R16)

It can also be seen that the absolute rate of production of OH is in the order of linear cooling > natural cooling > delayed cooling. This difference led to the different mole fractions of OH under the three cooling strategies, in accordance with the comparison of the OH mole fractions in Figure 9a. The second peak in each mole fraction profile of OH in Figure 9a is the result of H2O2 decomposition. H 2O2 = OH + OH

(R17)

Figure 9b shows the mole fraction profiles of H2O2 in the normal cylinder under the three cooling processes. It can be seen that the mole fraction of H2O2 increases gradually from about −22° CA and sharply decreases at about −6° CA. The decrease of H2O2 coincides with the increase of the second peak of OH in the normal cylinder, indicating the relations between these two species. In the low-temperature process, reaction R17 had no contribution to the formation of OH. The low-temperature reactions of PRF90 accumulated much more H2O2 than the typical low-temperature reaction products of F

DOI: 10.1021/acs.energyfuels.7b02216 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 10. Reaction pathway analysis of the formation of OH with the addition of all of the reformed products into PRF90 at −25° CA in the normal cylinder under the three different cooling processes. Normal text, linear cooling; italic text, natural cooling; and bold text, delayed cooling.

Figure 11. Absolute rate of production analysis of OH with the addition of all reformed products into PRF90 under the three cooling processes (linear cooling, natural cooling, and delayed cooling) at −25° CA in the normal cylinder •

OOQOOH and KETs; thus, the decomposition of H2O2 should result in a much higher peak value in the mole fraction profile of OH. This trend was reflected in the sharp peaks in the mole fraction profiles of OH, as shown in Figure 9a. 3.2.3. Effects of Cooling Processes on the Flame Speed. The reformed products under the different cooling processes may affect the laminar flame speed of the fresh fuel. The effects were investigated theoretically, as shown in Figure 12. The simulated laminar flame speed of fresh PRF90 was provided as a benchmark. It can be seen that the reformed products under the delayed cooling strategy increased the laminar flame speed of the fresh PRF90 slightly. The reformed products under the linear and natural cooling strategies basically had no effect on the flame speed of PRF90. Because the laminar flame speed is related to the formation of OH in the high-temperature region H + O2 = OH + O

Figure 12. Flame speed of the mixture of fresh fuel and all of the reformed products under the three cooling processes (linear cooling, natural cooling, and delayed cooling) at p = 1 and 50 atm.

flame speed of the mixture of PRF90 and all of the reformed products. However, the mole fractions of H and OH at 1 and 50 atm under the linear cooling strategy and natural cooling strategy are almost identical to those of the fresh PRF90 flames. The enhanced formation of H and OH in the case of the delayed cooling strategy led to the increased laminar flame speed, in turn, as shown in Figure 12. 3.2.4. Effects of Cooling Processes on the Emissions. Table 3 shows the comparisons of the emissions of C2H2, C2H4, CH2O, CO, and UHC under the three cooling processes fueled with the mixture of all of the reformed products and PRF90 in the normal cylinder at 125° CA, together with the results of PRF90 as a benchmark. It can be seen that the mole fractions of C2H2, C2H4, and CH2O decreased dramatically under the FCE mode with all three cooling strategies. The mole fraction of CO decreased by about 8.4% under the delayed cooling strategy relative to the benchmark, and those for the other two cooling

(R18)

The mole fraction profiles of H and OH at 1 and 50 atm are compared among the three cases, together with that of the fresh PRF90 flames, as shown in Figure 13. It can be seen that the mole fractions of H and OH under the delayed cooling strategy were increased slightly. This, in turn, increased the laminar G

DOI: 10.1021/acs.energyfuels.7b02216 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 14. Reaction pathway analysis of the formation of CH2O with the addition of all of the reformed products into PRF90 at 125° CA in the normal cylinder under the three different cooling processes. Normal text, PRF90; underline text, PRF90 + all reformed products (linear cooling); italic text, PRF90 + all reformed products (natural cooling); and bold text, PRF90 + all reformed products (delayed cooling).

Figure 13. Comparisons of the mole fraction profiles of H and OH in the flames of fresh PRF90 with and without the addition of the reformed products under the three cooling strategies (linear cooling, natural cooling, and delayed cooling) at p = 1 and 50 atm and ϕ = 1.1.

4. CONCLUSION

strategies of linear cooling and natural cooling were 3.8 and 6.1%, respectively. The simulated flow rate analysis of CH2O is shown in Figure 14, together with the simulated results of C2H2 and C2H4 in Figures S3 and S4 of the Supporting Information. It can be seen that the formation of CH2O was mainly due to the following reactions: C3H5‐T + O2 = CH 2O + CH3CO

(R19)

CH 2OH + O2 = CH 2O + HO2

(R20)

CH3 + O2 = CH 2O + OH

(R21)

IC3H6CHCHO + OH = TC3H6CHO + CH 2O

(R22)

In this work, the effects of the three cooling strategies (linear cooling, natural cooling, and delayed cooling) on the reformed products and ultimately on the FCE mode were investigated theoretically with the primary reference fuel (PRF90) as the substitute for gasoline, covering the ignition delay time, the laminar flame speed, and the emissions. The reformate cooling strategy may reduce the ignition delay time dramatically. The emissions were also reduced significantly under the delayed cooling strategy. (1) The cooling strategy may influence the combustion of the fresh fuel significantly, including the ignition delay time and the emissions, by altering the composition of the reformed products. The key species in the reformed products, such as hydroperoxyl methane, hydroperoxide, and KETs, may decrease the ignition delay time of the fresh PRF90 remarkably, while formaldehyde and acetone may increase the ignition delay time of PRF90 or even lead to misfire. (2) The delayed cooling strategy may reduce the ignition delay time of PRF90 to the TDC, from 27° to −5° CA, more closely than the other two cooling strategies, implying that it is more energy efficient. (3) The delayed cooling strategy may also reduce the emissions obviously than the natural cooling and linear cooling strategies, especially the emission of UHC, from 55.3 to 37.6 ppm. (4) The delayed cooling strategy is more preferable in the cooling process when adopting the FCE mode, from the point of view of engine efficiency and emissions.

This formation pathway (reaction R22) was significantly altered under the three cooling strategies, and the CH2O formation was increased from 45.1 to 59.5, 67.1, and 70.3%, respectively. The emission of UHC is an important source of pollution, and the UHC consists of unburned fuel, the pyrolysis products of fuel, the partial oxidation products of fuel, etc. The concentration of UHC is about 55.3 ppm when fueled with the benchmark of PRF90 but decreased to 37.6 ppm with the delayed cooling strategy, nearly decreased by 32.0%. The potential of reducing the emission of UHC with the three cooling processses is in the order of delayed cooling > natural cooling > linear cooling. In all, the delayed cooling strategy shows the best promise of decreasing the emissions from the normal cylinder.

Table 3. Comparison of Emissions of C2H2, C2H4, CH2O, CO, and UHC under the Three Cooling Processes in the Normal Cylinder at 125° CAa PRF90 C2H2 C2H4 CH2O CO UHC a

3.76 3.77 2.27 1.31 55.3

× × × ×

10−11 10−13 10−11 10−7

PRF90 + all reformed products (linear cooling) 5.03 2.84 1.15 1.26 51.8

× × × ×

PRF90 + all reformed products (natural cooling)

10−13 10−14 10−13 10−7

5.34 3.04 1.23 1.23 42.7

× × × ×

PRF90 + all reformed products (delayed cooling)

10−13 10−14 10−13 10−7

5.87 3.36 1.37 1.20 37.6

× × × ×

10−13 10−14 10−13 10−7

The unit of C2H2, C2H4, CH2O, and CO is mole fraction, and the unit of UHC is parts per million (ppm). H

DOI: 10.1021/acs.energyfuels.7b02216 Energy Fuels XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02216. Pressure profile of the cooling process (Figure S1), pressure profiles of the normal cylinder after introducing the reformed products under the three cooling strategies (Figure S2), reaction pathway analysis of the formation of C2H2 with the addition of all of the reformed products into PRF90 at 125° CA in the normal cylinder under the three different cooling processes (Figure S3), reaction pathway analysis of the formation of C2H4 with the addition of all of the reformed products into PRF90 at 125° CA in the normal cylinder under the three different cooling processes (Figure S4), and mole fractions of the reformed components in the reformed gas mixture before and after cooling (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-139-2080-5843. Fax: +86-22-2738-3362. Email: [email protected]. ORCID

Yang Wang: 0000-0003-0586-1187 Mingfa Yao: 0000-0002-7293-8714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by the Major Research Plan of the National Natural Science Foundation of China (91541205). REFERENCES

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DOI: 10.1021/acs.energyfuels.7b02216 Energy Fuels XXXX, XXX, XXX−XXX