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Effects of low-temperature reforming products of PRF50 on combustion and emission characteristics in an HCCI engine
Applied Thermal Engineering 151 (2019) 451–458
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Effects of low-temperature reforming products of PRF50 on combustion and emission characteristics in an HCCI engine Yang Wang, Hu Wang, Mingfa Yao
T
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State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China
H I GH L IG H T S
external fuel reforming device strategy was verified by the fuel of PRF50. • The effects of reforming temperature on reformed products were investigated. • The addition of reformed products may alter the engine combustion phasing. • The • The addition of reformed products can improve the engine thermal efficiency.
A R T I C LE I N FO
A B S T R A C T
Keywords: PRF50 Temperature Reform Reaction pathway Engine
In this study, the application of external fuel reforming system in an HCCI engine has been investigated at the reforming temperature from 450 to 750 K. Experiments were conducted in a single-cylinder engine fueled with PRF50, and related simulation analysis was conducted with CHEMKIN package. The temperature of 600 K was the transition point for reformed products, and the composition of reformed products changed variously at this temperature. The results of which reveal the affection of reforming temperature of the external reformer to engine combustion phasing: When the temperature was lower than 600 K, the engine combustion phasing could be advanced by introducing reformed products, and vice versa. The combustion stability could be improved (CoVIMEP < 5%) with reformed products when the temperature was lower than 650 K. By reasonably controlling the operating condition of external reformer, CO and UHC emissions could be reduced, and the indicated thermal efficiency could also be improved by 4% (44%).
1. Introduction Global warming and fossil resources shortage have become worldwide topics. For the automobile industry, there are opportunities and challenges [1–4]. It is very important to solve this issue by developing advanced combustion technology in internal combustion engine. Therefore, lots of novel combustion technologies and concepts [5–14] were proposed by the worldwide researchers, such as homogeneous charge compression ignition (HCCI), reactivity controlled compression ignition (RCCI) and gasoline compression ignition (GCI), etc. Compared with the conventional combustion mode, all above-mentioned novel combustion technologies regulate the in-cylinder fuel concentration and the reactivity of the fuel-air mixtures to optimize the fuel chemical reaction pathway, and hence to improve the thermal efficiency and combustion performance. Actually, the engine fuel reforming technologies are intended to reduce the combustion irreversible loss and
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improve the engine combustion controllability. The engine thermal efficiency could be improved from 36.8% to 44.9%, and the total exergy loss could be reduced from 30.9% to 16.86% by optimizing the fuel reaction pathways [15,16]. As a promising engine technology, the fuel reforming strategy has been extensively investigated. The conventional means of reforming the fuel and then changing the fuel-air mixture reactivity was introducing the fuel during the negative valve overlap (NVO) period of an engine cycle in the low-temperature combustion process [17–26]. When introduces the fuel into the O2-deficient NVO environment, some of the fuel will be converted to products including H2 and CO, and will produce other short-chain hydrocarbons simultaneously by means of partial oxidation, thermal cracking, and water-gas shift reaction. There is a strong correlation between the concentration of these reformed products and injected fuel type and timing, but exists a weaker dependencies on initial temperature and NVO duration, indicating that
Corresponding author. E-mail address: [email protected] (M. Yao).
https://doi.org/10.1016/j.applthermaleng.2019.01.088 Received 30 August 2018; Received in revised form 12 January 2019; Accepted 26 January 2019 Available online 28 January 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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fuel mass flow was regulated with an electronic controller. The flow rates for port injection fuel was measured by fuel consumption meters (AVL 733S). The port fuel injection pressure was set to 0.3 MPa. The specifications for port injection system were shown in Table 3. As shown in Fig. 1, the working cycle that is the fuel being reformed was supplied by the fuel injection pump, and then mixed with the air from #2 air compressor in the evaporation mixer to be “homogeneous” (the vaporized fuel mixed well with the air in the evaporation mixer, which was considered “homogeneous” in this work); then the mixtures were introduced into the reformer, and finally the reformed products participated in combustion with fresh air from #1 air compressor and fresh fuel from intake port injector in engine. The temperature of evaporation mixer was fixed at 380 K, higher than the boiling point of n-heptane and iso-octane (371.4 K and 372.3 K), respectively. As shown in Fig. 1, there were four temperature monitoring points on the reformer, the thermocouples numbered #1, #2, and #3 were evenly equipped on the external reformer, and the thermocouple numbered #4 was located at the outlet of the external reformer, which was used to detect the realtime temperature of the reformed products. The thermocouple numbered #5 was used to monitor the temperature of evaporation mixer. The thermocouples numbered #6 and #7 were used to detect the temperature of fresh air form #1 air compressor and reformed mixtures from reformer, respectively. The measurement accuracy of these thermocouples is ± 3 K. The FTIR (Fourier Transform Infrared Spectroscopy), located at the outlet of the external reformer, could on-line analyze the composition of the reformed products. The external fuel reforming system specifications are shown in Table 4. In this work, considering the engine working stability, the selected operative conditions are the stable working condition of the engine, which obtained through repeated experimental tests. The engine speed was fixed at 1200 r/min, and the intake air temperature was 365 K. The intake air pressure from both air compressor numbered #1 and #2 was 103.2 KPa, and the injection pressure of fuel injection pump was 110 KPa. The detailed engine operation conditions are given in Table 5. The engine operated on the small load under the HCCI mode, and the total fuel mass was fixed at 20 mg/cycle. 25% of the total fuel mass (per cycle) was reformed in the external fuel reforming system, and the other fuel was injected in the intake port to form homogeneous charge. The fuel injection strategies of this work were shown in Table 6. The reforming temperature of the external reformer was varied from 450 K to 750 K, and the variable temperature interval was 50 K. The equivalence ratio of external reformer was fixed at 8, and the total equivalence ratio including external reformer and engine was 0.32. The air flow of #2 air compressor was 4.35 L/min, and the residence time of fuel being reformed in the external reformer was about 17.3 s. This study adopts the CHEMKIN package to simulate the reaction pathway analysis of key low-temperature reforming products [37], and the flow reactor model was selected as the simulation model. In the simulation process, the geometric size of the flow reactor model was consistent with the actual reformer, and the initial boundary conditions (temperature, pressure, equivalence ratio, and residence time) were also consistent with the actual conditions. The reaction pathway of key low-temperature reforming products was conducted with the Reaction Path Analyzer of CHEMKIN package. The detailed primary reference fuel reaction mechanism from Lawrence Livermore National Laboratory was adopted in this work [38]. The kinetic model in this work, including 1389 species and 5723 reactions, has been validated against experimental results extensively [39–45]. The results showed that the proposed kinetic model achieved good effects over wide ranges of temperature and pressure and can be used for engine combustion simulations.
the NVO fuel reforming strategy is kinetically limited [27]. The dedicated exhaust gas recirculation (D-EGR) theory was proposed by researchers from Southwest Research Institute [28]. According to the D-EGR strategy, partial gasoline fuel was reformed catalytically to H2 and CO in a dedicated cylinder firstly, which are then routed with fresh gasoline into other cylinders for combustion. The results exhibited that the average thermal efficiency was increased by 12–15% with the D-EGR strategy. Zhu et al. [29] developed a new combustion mode of thermochemical fuel reforming (TFR), and verified it in a natural gas engine experimentally. In this study, a separate cylinder is adopted to produce CO and H2 by reforming nature gas therein, which were then routed with fresh nature gas into other cylinders. The results revealed that the emissions of brake specific carbon monoxide (BSCO), brake specific oxides of nitrogen (BSNOX), and brake specific hydrocarbon (BSHC) were decreased by 8%, 35% and 12%, respectively. According to Tsolakis et al. [30], reformed exhaust gas recirculation (REGR) is applied to a HCCI engine fueled with bioethanol as well as to a diesel engine fueled with biodiesel (a mixture of 50% rapeseed methyl ester and 50% ultra-low sulphur diesel) due to the higher hydrogen content of REGR. By adopting REGR instead of EGR, the bioethanol had higher tolerance to EGR ratio and the biodiesel fuel consumption was decreased by 3%. Yap et al. [31] experimentally investigated the affection of nature gas fuel reforming on the load and emissions with an external reformer in an HCCI engine, and the results showed that this strategy could expand the operation range and reduce emissions in the HCCI engine. Although above-mentioned D-EGR, TFR or external reformer strategy can improve the thermal efficiency and reduce emissions through reforming fuel to CO and H2, these strategies cannot control the fuel reaction pathway in engine flexibly due to the limited reformed products (only CO and H2). Furthermore, considering that the engines usually operating on a wide range of operation conditions which requiring the reformed products with varied reactivities, we proposed the low-temperature fuel reforming concept in our previous works [32–36]. Moreover, the previous studies revealed that the reforming temperature exhibited a significant effect on reformed products. Therefore, this work focuses on reforming temperature effect, the formation process of reformed products, the engine combustion phasing, combustion stability, emissions and efficiency. All of these arrangements aimed at the practical route of the external fuel reforming device-assisted fuel reforming strategy. Moreover, according to our previous results, given that the reactivity range of reformed products and the engine combustion stability in the experiment under different reforming temperature, the primary reference fuel of PRF50 (a mixture of iso-octane and n-heptane with the volume ratio of 1 to 1) was adopted as the target fuel in this work. 2. Methods As shown in Fig. 1, the external fuel reforming system is summarized briefly hereinafter. All the experiments were carried out on a sixcylinder diesel engine, and the sixth cylinder thereof is separated from the others to perform a single-cylinder research. The engine original intake and exhaust manifolds were replaced by prototype manifolds, and the sixth cylinder was isolated from those of the other five cylinders. Thus the single-cylinder experiments could be performed with no cross-contamination from other cylinders independently, and the detailed engine specifications are given in Table 1. The uncertainty of the measurement instruments is shown in Table 2. The in-cylinder pressure was measured for 100 cycles at 0.5 °CA resolution, which arranged to calculate apparent heat release rate, cumulative heat released, indicated mean effective pressure (IMEP) and the crank angle at 50% heat release (CA50). The emissions were measured with a Horiba MEXA 7100 DEGR analyzer. The fuel was injected at the intake port via a Delphi injector to be homogeneous charge, and an in-line fuel pump was used to provide upstream pressure for the port fuel delivery and the 452
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Fig. 1. The schematic diagram of engine combined with external fuel reforming system. Table 1 Engine specifications. Engine name Engine type Number of valves Bore/stroke Connecting rod length Bowl volume Compression ratio Electronic control unit Inlet valve close Exhaust valve open
Table.5 Engine operation conditions. YC6J220-30 In-line, four-stroke, water-cooled, single-cylinder 4 105 mm/125 mm 210 mm 6.5 L 16:1 EDC7 (BOSCH) −133 CA ATDC 125 CA ATDC
Engine speed Intake air pressure Intake air temperature Total fuel mass EGR rate Cooling water temperature
Table.6 Injection strategies for fixed total fuel mass.
Table 2 Uncertainties of the measurement instruments. Instrument
Uncertainties
Gaseous analyzer (HORIBA 7100DEGR) Gaseous analyzer (AVL SESAM i60 FTIR) In-cylinder pressure (KISTLER 5125C11) Intake air temperature (k-type thermocouple) Intake air pressure (pressure transmitter)
± 0.5% full scale ± 2.0% full scale < ± 1% ±1 K ± 1 kPa
1200 r/min 103.2 KPa 365 K 20 mg/cycle 0% 358 K
Case#
Intake port (%)
Reformer (%)
Reforming temperature (K)
1 2 3 4 5 6 7 8
100% 75% 75% 75% 75% 75% 75% 75%
/ 25% 25% 25% 25% 25% 25% 25%
/ 450 500 550 600 650 700 750
Table 3 Specifications for port injection system. Number of holes Included spray angle Injection pressure Steady flow rate
4 15° 0.3 MPa 700 mL/min
Table 4 External fuel reforming system specifications. Reformer
Evaporation mixer
Temperature controller #1 Gas flowmeter #2 Gas flowmeter #1 Air compressor #2 Air compressor Fuel injection pump
Length Diameter Wall thickness Length Diameter Wall thickness Ruipu Electric RP-100P Endress Hauser (85-260VAC) STEPS-S49 33/MT RONFEN (Y160L-2) Fusheng Z-0.10/8 (D-2(II)) Longer pump LSP01-2A
1000 mm 40 mm 5 mm 300 mm 30 mm 3 mm
Fig. 2. The temperature profile of reformer.
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Fig. 3. The components of reformed products under different reforming temperatures. Fig. 4. Comparisons between experimental and predicted mole fraction profiles of major low-temperature reformed species.
3. Results and discussion 3.1. The effect of temperature on low-temperature reforming process
analyzed. The process of “the first oxygen addition → the first isomerization → the second oxygen addition → the second isomerization” is the key section to form the major reformed products during the lowtemperature reaction process. As shown in Fig. 5, C2H2, C2H4, C4H6, and CH3CHO in the reformed products were mainly produced by nheptane, and the formation reaction pathways of C2H2, C2H4, and C4H6 were different at the temperature of 600 K and 750 K. These major reformed products were produced through the typical low-temperature branching reaction process at the temperature of 600 K, but the pyrolysis of PRF50 fuel started to dominate the reforming process at the temperature of 750 K. The formation pathway of CH3CHO was consistent at the temperature of 600 K and 750 K, which is NC7H16 → C7H15-4 → C7H15O2-4 → C7H14OOH4-2 → C7H14OOH4-2O2 → NC7KET42 → CH3CHO. When the reforming temperature was set at the temperature of 600 K, C3H6 was mainly generated by the reaction pathway of IC8H18 → BC8H17 → BC8H17O2 → BC8H16OOH-A → BC8H16OOHAO2 → IC8KETBA → IC3H7COC3H6-T → IC3H7 → IC3H7O2 → C3H6. There were two major formation pathways for C3H6 at the temperature of 750 K, which are IC8H18 → CC8H17 → IC4H8 → C3H6 and IC8H18 → AC8H17 → IC4H9 → C3H6. The formation pathway of CH2O varied with different temperature. When the temperature was at 600 K, CH2O was mainly generated by R1, R2, R3, R4, R5, and R6; as the temperature increased to 750 K, R7, R8, R9, and R10 constituted the critical formation pathway of CH2O. Meanwhile, CO was produced through R11 and R12 at this temperature.
Fig. 2 shows the temperature measurement results of the four temperature monitoring points on the external fuel reforming system. It can be found that the temperature profile of the four thermocouples increased slightly compared with the setting temperature in case 2-case 4. There were stronger temperature lift in case 5-case 8 due to the higher reforming temperature (650–750 K), wherein the fuel released heat to increase the system temperature through low-temperature reaction. However, considering the relatively low reforming fuel mass (25% of the total fuel mass per cycle), the overall temperature fluctuation in this external fuel reforming system was still thought to be acceptable. The reforming temperature exhibited significant effects on the formation of key low-temperature reformed products, and the reformed products measured by FTIR were given in Fig. 3. When the reforming temperature was lower than 600 K, the main components of reformed products were PRF50 fuel, ethane (C2H6), acetaldehyde (CH3CHO), and a small amount of short-chain alkane, alkene, and alkyne, such as methane (CH4), acetylene (C2H2), ethylene (C2H4), propylene (C3H6), propane (C3H8), and butyne (C4H6). When the reforming temperature was higher than 600 K, significant amount of CO was generated in the external fuel reforming system, which concentration could reach about 1.24 × 105 ppm in case 8. The concentration of CO and CO2 increased with the temperature increased, and PRF50 fuel was almost entirely consumed in case 8. When the reforming temperature at 600 K (case 5), most of PRF50 fuel (76%) was reformed, and the concentration of other small molecule species significantly increased at this reforming temperature. The components of reformed products underwent a drastic change at case 5, thus the temperature of 600 K was regarded as a transition point in the reforming process. The comparison between experimental and computed mole fraction profiles of major low-temperature reformed products is shown in Fig. 4. The results indicate that the simulation with detailed PRF mechanism predicted the mole fraction profiles of major reformed products well, which means that the main combustion chemistry of PRF50 can be well represented with this detailed mechanism. As shown in Fig. 5, the key low-temperature reforming products in the reaction pathway at the temperature of 600 K and 750 K are 454
IC8H18 + O2 < = > BC8H17 + HO2
(R1)
BC8H17 + O2 < = > BC8H17O2
(R2)
BC8H17O2 < = > BC8H16OOH-A
(R3)
BC8H16OOH-A + O2 < = > BC8H16OOH-AO2
(R4)
BC8H16OOH-AO2 < = > IC8KETBA + OH
(R5)
IC8KETBA < = > CH2O + IC3H7COC3H6-T + OH
(R6)
IC8H18 + O2 < = > CC8H17 + HO2
(R7)
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Fig. 5. The reaction pathway analysis of key low-temperature reformed species at 600 K and 750 K.
CC8H17 < = > IC4H8 + TC4H9
(R8)
CH3O2 + TC4H9 < = > CH3O + TC4H9O
(R9)
CH3O + O2 < = > CH2O + HO2
(R10)
CH2O + HO2 < = > HCO + H2O2
(R11)
HCO + O2 < = > CO + HO2
(R12)
combustion phasing could be advanced by introducing the reformed products, with higher in-cylinder pressure and heat release rate. In other words, the low-temperature reformed products could accelerate the HCCI combustion reaction rate. According to our previous results [32,33], by introducing low-temperature reformed products into the engine, the concentration of hydroxide radical (OH) would be improved in the combustion process. The higher OH concentration accelerated the removal of first hydrogen atom from the initial fuel molecules (nheptane and iso-octane), which had a faster combustion effects than fueled with pure PRF50 in case 2-case 5. In case 6-case 8, when the reforming temperature was higher than 600 K, CO and CO2 in the reformed products were increased accordingly as shown in Fig. 3, which resulted in postponing the combustion phasing after adding the reformed products. Fig. 7a shows the comparison of reformed products on CA50 under different reforming temperatures compared with case 1, with the change sequence of combustion phasing of case 5 > case 4 > case 3 > case 2, and the maximum advanced crank angle of CA50 was about 7.5 CA at the reforming temperature of 600 K (case 5). When the reforming temperature was higher than 600 K, the sequence of retarding ability of reformed products on CA50 was that of case 8 > case 7 > case 6, and the maximum retarded crank angle of CA50 was about 3.5 CA at reforming temperature of 750 K (case 8). A reactivity coefficient (RC) has been proposed to assess the reactivity of reformed products in our previous work [36], and the definition formula is:
3.2. The effect of reformed products on combustion Fig. 6 shows the experimental results of the mixtures after the reformed products from external reformer back into the engine at different reforming temperatures. As shown in Fig. 6, when the reforming temperature was lower than 600 K (case2-case5), the engine
RC =
CA50(fresh fuel + reformed products ) − CA50(fuel ) mole fraction of (fresh fuel + reformed products )
wherein the RC is the reactivity coefficient; CA50 (fresh fuel + reformed products), the crank angle degree of 50 percent of the mixture of the fresh fuel and reformed products combusted in a normal cylinder, CA50 (fuel), crank angle degree of 50 percent pure fuel combusted in normal cylinder, mole fraction of (fresh fuel + reformed products), the total mole fraction of the mixture of the fresh fuel and reformed products in the
Fig. 6. Comparisons of in-cylinder pressure and heat release rate at different reforming temperatures. 455
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Fig. 8. The effect of reformed products on combustion efficiency at different reforming temperatures.
Fig. 7. Comparison of CA50 and the reactivity of reformed products under different reforming temperatures.
normal cylinder. When adding reformed products into engine advances the CA50 timing compared with fueled with pure fuel, the RC value should be less than 0, and the reformed products are defined as high reactivity, and vice versa. Furthermore, when the RC value equals to 0, the reactivity values of reformed products and fresh fuel are identical. A larger RC absolute value indicates the stronger positive/negative reactivity of the reformed products, indicating their relative capability to advance/delay the combustion phasing in HCCI combustion. When the reforming temperature were at the temperature of 450 K (case2), 500 K (case3), 550 K (case4), and 600 K (case5), the addition of reformed products can advance the CA50 timing compared with fueled with pure PRF50 in case1. According to the above-mentioned definition and shown in Fig. 7b, the values of RC were less than 0 in case 2-case 5, which indicated higher reactivity of these reformed products at the temperature of 450–600 K, and the sequence of | RC (case 5)| > | RC (case 4)| > | RC (case 3)| > | RC (case 2)| demonstrated that increasing the reforming temperature shall enhance the reactivity of reformed products. In case 6-case 8, the RC values were positive when the reforming temperature were at 650 K (case 6), 700 K (case 7), and 750 K (case 8), indicating their inhibiting effects on combustion phasing, as shown in Fig. 7a. The effects of adding reformed products on combustion efficiency are shown in Fig. 8, it is seen that when the reforming temperature was lower than 600 K (case 2-case 5), the combustion efficiency can be improved compared with case 1 (baseline), with the highest combustion efficiency of 96.95% in case 5, a 2.51% improvement. In contrast, lower the combustion efficiency is observed as the reforming temperature was continued to increase, as shown in case 6-case 8. The effects of the reforming temperature on the coefficient of variation in indicated mean effective pressure (CoVIMEP) are shown in Fig. 9. It is seen that lower reforming temperature was of benefit in improving the combustion stability, but the combustion stability were decreased (the engine became unstable) in case 7 and case 8. The key factors affecting the fuel combustion stability are H radicals due to the important chain branching reaction H + O2 < = > OH + O [46]. After introducing the reformed products into the engine, the reaction H + O2 < = > OH + O were promoted, resulting in lower CoVIMEP in case 2-case 6. When the reforming temperature was higher than 650 K (case 7 and case 8), the CoVIMEP exceeded 5%, indicating an unstable condition, especially at reforming temperature 700 K (case 7).
Fig. 9. CoVIMEP as a function of reforming temperature.
Fig. 10. The engine emissions of CO and UHC under different reforming temperatures.
3.3. The effect of reformed products on emissions and thermal efficiency Fig. 10 illustrates the effects of reforming temperature on emissions, showing a decrease of concentration of CO and unburned hydrocarbon (UHC) as the reforming temperature increased from 450 K (case 2) to
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Acknowledgements This work is funded by the Major Research Plan of the National Natural Science Foundation of China 91541205 and 51506145. References [1] M. Masiol, C. Agostinelli, G. Formenton, et al., Thirteen years of air pollution hourly monitoring in a large city: potential sources, trends, cycles and effects of car-free days, Sci. Total Environ. 494 (2014) 84–96. [2] M. Chertok, A. Voukelatos, V. Sheppeard, et al., Comparison of air pollution exposure for five commuting modes in Sydney – car, train, bus, bicycle and walking, Health Promotion J. Australia 15 (1) (2004) 63. [3] Z. Zhang, W. Li, C. Zhang, et al., Climate control loads prediction of electric vehicles, Appl. Therm. Eng. 110 (2017) 1183–1188. [4] M. Cepeda, J. Schoufour, R. Freak-Poli, et al., Levels of ambient air pollution according to mode of transport: a systematic review, Lancet Public Health 2 (1) (2017) e23. [5] M.F. Yao, Z.Q. Zheng, H.F. 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Fig. 11. The engine energy under different reforming temperatures.
600 K (case 5). One possible reason to explain the emissions of CO and UHC decreased by 17.02% and 37.98% is that the addition of reformed products enhanced the formation of OH in the engine, which in turns advanced the engine combustion phasing, and improved the fuel combustion efficiency. In the external fuel reforming mode, the overall energy distribution is shown in Fig. 11. The energy required for the external reformer to reform the fuel came from electrical heating, which increased as the reforming temperature increases, and when the reforming temperature was 750 K (case 8), the proportion of this energy in the total energy increased to 3% of the total energy contained in the fuel. The heat transfer losses accounted for about 30% of the total energy at case 1case 8. Compared with case 1, the addition of reformed products could reduce the exhaust losses. Meanwhile, the indicated thermal efficiency could be improved to 44%, a 4% improvement than the baseline case. 4. Conclusions In this work, the application of external fuel reforming device-assisted fuel reforming in an HCCI engine has been conducted at different reforming temperature through both experiments and simulations. Compared to our previous results, the addition of PRF50 reformed products in engine can also reduce emissions and improve indicated thermal efficiency, while providing better combustion stability. The major conclusions of the current study can be summarized as follows. (1) When the reforming temperature was no more than 600 K, the major reformed products were PRF50 fuel, ethane (C2H6), acetaldehyde (CH3CHO), and a small amount of short-chain molecules; when the reforming temperature was higher than 600 K, significant amount of CO was generated in the external fuel reforming system. (2) Reforming temperature of 600 K was the transition point for the engine combustion phasing. When the reforming temperature was lower than 600 K, the engine combustion phasing could be advanced by introducing the reformed products, and vice versa. (3) The engine can work steadily (CoVIMEP not exceeding 5%) after the addition of reformed products, when the reforming temperature was lower than 650 K. The addition of reformed products of PRF50 could reduce the emissions of CO and unburned hydrocarbon (UHC), and the indicated thermal efficiency could be improved to 44%.
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[31] [32]
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Effects of low-temperature reforming products of PRF50 on combustion and emission characteristics in an HCCI engine Yang Wang, Hu Wang, Mingfa Yao
T
⁎
State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China
H I GH L IG H T S
external fuel reforming device strategy was verified by the fuel of PRF50. • The effects of reforming temperature on reformed products were investigated. • The addition of reformed products may alter the engine combustion phasing. • The • The addition of reformed products can improve the engine thermal efficiency.
A R T I C LE I N FO
A B S T R A C T
Keywords: PRF50 Temperature Reform Reaction pathway Engine
In this study, the application of external fuel reforming system in an HCCI engine has been investigated at the reforming temperature from 450 to 750 K. Experiments were conducted in a single-cylinder engine fueled with PRF50, and related simulation analysis was conducted with CHEMKIN package. The temperature of 600 K was the transition point for reformed products, and the composition of reformed products changed variously at this temperature. The results of which reveal the affection of reforming temperature of the external reformer to engine combustion phasing: When the temperature was lower than 600 K, the engine combustion phasing could be advanced by introducing reformed products, and vice versa. The combustion stability could be improved (CoVIMEP < 5%) with reformed products when the temperature was lower than 650 K. By reasonably controlling the operating condition of external reformer, CO and UHC emissions could be reduced, and the indicated thermal efficiency could also be improved by 4% (44%).
1. Introduction Global warming and fossil resources shortage have become worldwide topics. For the automobile industry, there are opportunities and challenges [1–4]. It is very important to solve this issue by developing advanced combustion technology in internal combustion engine. Therefore, lots of novel combustion technologies and concepts [5–14] were proposed by the worldwide researchers, such as homogeneous charge compression ignition (HCCI), reactivity controlled compression ignition (RCCI) and gasoline compression ignition (GCI), etc. Compared with the conventional combustion mode, all above-mentioned novel combustion technologies regulate the in-cylinder fuel concentration and the reactivity of the fuel-air mixtures to optimize the fuel chemical reaction pathway, and hence to improve the thermal efficiency and combustion performance. Actually, the engine fuel reforming technologies are intended to reduce the combustion irreversible loss and
⁎
improve the engine combustion controllability. The engine thermal efficiency could be improved from 36.8% to 44.9%, and the total exergy loss could be reduced from 30.9% to 16.86% by optimizing the fuel reaction pathways [15,16]. As a promising engine technology, the fuel reforming strategy has been extensively investigated. The conventional means of reforming the fuel and then changing the fuel-air mixture reactivity was introducing the fuel during the negative valve overlap (NVO) period of an engine cycle in the low-temperature combustion process [17–26]. When introduces the fuel into the O2-deficient NVO environment, some of the fuel will be converted to products including H2 and CO, and will produce other short-chain hydrocarbons simultaneously by means of partial oxidation, thermal cracking, and water-gas shift reaction. There is a strong correlation between the concentration of these reformed products and injected fuel type and timing, but exists a weaker dependencies on initial temperature and NVO duration, indicating that
Corresponding author. E-mail address: [email protected] (M. Yao).
https://doi.org/10.1016/j.applthermaleng.2019.01.088 Received 30 August 2018; Received in revised form 12 January 2019; Accepted 26 January 2019 Available online 28 January 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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fuel mass flow was regulated with an electronic controller. The flow rates for port injection fuel was measured by fuel consumption meters (AVL 733S). The port fuel injection pressure was set to 0.3 MPa. The specifications for port injection system were shown in Table 3. As shown in Fig. 1, the working cycle that is the fuel being reformed was supplied by the fuel injection pump, and then mixed with the air from #2 air compressor in the evaporation mixer to be “homogeneous” (the vaporized fuel mixed well with the air in the evaporation mixer, which was considered “homogeneous” in this work); then the mixtures were introduced into the reformer, and finally the reformed products participated in combustion with fresh air from #1 air compressor and fresh fuel from intake port injector in engine. The temperature of evaporation mixer was fixed at 380 K, higher than the boiling point of n-heptane and iso-octane (371.4 K and 372.3 K), respectively. As shown in Fig. 1, there were four temperature monitoring points on the reformer, the thermocouples numbered #1, #2, and #3 were evenly equipped on the external reformer, and the thermocouple numbered #4 was located at the outlet of the external reformer, which was used to detect the realtime temperature of the reformed products. The thermocouple numbered #5 was used to monitor the temperature of evaporation mixer. The thermocouples numbered #6 and #7 were used to detect the temperature of fresh air form #1 air compressor and reformed mixtures from reformer, respectively. The measurement accuracy of these thermocouples is ± 3 K. The FTIR (Fourier Transform Infrared Spectroscopy), located at the outlet of the external reformer, could on-line analyze the composition of the reformed products. The external fuel reforming system specifications are shown in Table 4. In this work, considering the engine working stability, the selected operative conditions are the stable working condition of the engine, which obtained through repeated experimental tests. The engine speed was fixed at 1200 r/min, and the intake air temperature was 365 K. The intake air pressure from both air compressor numbered #1 and #2 was 103.2 KPa, and the injection pressure of fuel injection pump was 110 KPa. The detailed engine operation conditions are given in Table 5. The engine operated on the small load under the HCCI mode, and the total fuel mass was fixed at 20 mg/cycle. 25% of the total fuel mass (per cycle) was reformed in the external fuel reforming system, and the other fuel was injected in the intake port to form homogeneous charge. The fuel injection strategies of this work were shown in Table 6. The reforming temperature of the external reformer was varied from 450 K to 750 K, and the variable temperature interval was 50 K. The equivalence ratio of external reformer was fixed at 8, and the total equivalence ratio including external reformer and engine was 0.32. The air flow of #2 air compressor was 4.35 L/min, and the residence time of fuel being reformed in the external reformer was about 17.3 s. This study adopts the CHEMKIN package to simulate the reaction pathway analysis of key low-temperature reforming products [37], and the flow reactor model was selected as the simulation model. In the simulation process, the geometric size of the flow reactor model was consistent with the actual reformer, and the initial boundary conditions (temperature, pressure, equivalence ratio, and residence time) were also consistent with the actual conditions. The reaction pathway of key low-temperature reforming products was conducted with the Reaction Path Analyzer of CHEMKIN package. The detailed primary reference fuel reaction mechanism from Lawrence Livermore National Laboratory was adopted in this work [38]. The kinetic model in this work, including 1389 species and 5723 reactions, has been validated against experimental results extensively [39–45]. The results showed that the proposed kinetic model achieved good effects over wide ranges of temperature and pressure and can be used for engine combustion simulations.
the NVO fuel reforming strategy is kinetically limited [27]. The dedicated exhaust gas recirculation (D-EGR) theory was proposed by researchers from Southwest Research Institute [28]. According to the D-EGR strategy, partial gasoline fuel was reformed catalytically to H2 and CO in a dedicated cylinder firstly, which are then routed with fresh gasoline into other cylinders for combustion. The results exhibited that the average thermal efficiency was increased by 12–15% with the D-EGR strategy. Zhu et al. [29] developed a new combustion mode of thermochemical fuel reforming (TFR), and verified it in a natural gas engine experimentally. In this study, a separate cylinder is adopted to produce CO and H2 by reforming nature gas therein, which were then routed with fresh nature gas into other cylinders. The results revealed that the emissions of brake specific carbon monoxide (BSCO), brake specific oxides of nitrogen (BSNOX), and brake specific hydrocarbon (BSHC) were decreased by 8%, 35% and 12%, respectively. According to Tsolakis et al. [30], reformed exhaust gas recirculation (REGR) is applied to a HCCI engine fueled with bioethanol as well as to a diesel engine fueled with biodiesel (a mixture of 50% rapeseed methyl ester and 50% ultra-low sulphur diesel) due to the higher hydrogen content of REGR. By adopting REGR instead of EGR, the bioethanol had higher tolerance to EGR ratio and the biodiesel fuel consumption was decreased by 3%. Yap et al. [31] experimentally investigated the affection of nature gas fuel reforming on the load and emissions with an external reformer in an HCCI engine, and the results showed that this strategy could expand the operation range and reduce emissions in the HCCI engine. Although above-mentioned D-EGR, TFR or external reformer strategy can improve the thermal efficiency and reduce emissions through reforming fuel to CO and H2, these strategies cannot control the fuel reaction pathway in engine flexibly due to the limited reformed products (only CO and H2). Furthermore, considering that the engines usually operating on a wide range of operation conditions which requiring the reformed products with varied reactivities, we proposed the low-temperature fuel reforming concept in our previous works [32–36]. Moreover, the previous studies revealed that the reforming temperature exhibited a significant effect on reformed products. Therefore, this work focuses on reforming temperature effect, the formation process of reformed products, the engine combustion phasing, combustion stability, emissions and efficiency. All of these arrangements aimed at the practical route of the external fuel reforming device-assisted fuel reforming strategy. Moreover, according to our previous results, given that the reactivity range of reformed products and the engine combustion stability in the experiment under different reforming temperature, the primary reference fuel of PRF50 (a mixture of iso-octane and n-heptane with the volume ratio of 1 to 1) was adopted as the target fuel in this work. 2. Methods As shown in Fig. 1, the external fuel reforming system is summarized briefly hereinafter. All the experiments were carried out on a sixcylinder diesel engine, and the sixth cylinder thereof is separated from the others to perform a single-cylinder research. The engine original intake and exhaust manifolds were replaced by prototype manifolds, and the sixth cylinder was isolated from those of the other five cylinders. Thus the single-cylinder experiments could be performed with no cross-contamination from other cylinders independently, and the detailed engine specifications are given in Table 1. The uncertainty of the measurement instruments is shown in Table 2. The in-cylinder pressure was measured for 100 cycles at 0.5 °CA resolution, which arranged to calculate apparent heat release rate, cumulative heat released, indicated mean effective pressure (IMEP) and the crank angle at 50% heat release (CA50). The emissions were measured with a Horiba MEXA 7100 DEGR analyzer. The fuel was injected at the intake port via a Delphi injector to be homogeneous charge, and an in-line fuel pump was used to provide upstream pressure for the port fuel delivery and the 452
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Fig. 1. The schematic diagram of engine combined with external fuel reforming system. Table 1 Engine specifications. Engine name Engine type Number of valves Bore/stroke Connecting rod length Bowl volume Compression ratio Electronic control unit Inlet valve close Exhaust valve open
Table.5 Engine operation conditions. YC6J220-30 In-line, four-stroke, water-cooled, single-cylinder 4 105 mm/125 mm 210 mm 6.5 L 16:1 EDC7 (BOSCH) −133 CA ATDC 125 CA ATDC
Engine speed Intake air pressure Intake air temperature Total fuel mass EGR rate Cooling water temperature
Table.6 Injection strategies for fixed total fuel mass.
Table 2 Uncertainties of the measurement instruments. Instrument
Uncertainties
Gaseous analyzer (HORIBA 7100DEGR) Gaseous analyzer (AVL SESAM i60 FTIR) In-cylinder pressure (KISTLER 5125C11) Intake air temperature (k-type thermocouple) Intake air pressure (pressure transmitter)
± 0.5% full scale ± 2.0% full scale < ± 1% ±1 K ± 1 kPa
1200 r/min 103.2 KPa 365 K 20 mg/cycle 0% 358 K
Case#
Intake port (%)
Reformer (%)
Reforming temperature (K)
1 2 3 4 5 6 7 8
100% 75% 75% 75% 75% 75% 75% 75%
/ 25% 25% 25% 25% 25% 25% 25%
/ 450 500 550 600 650 700 750
Table 3 Specifications for port injection system. Number of holes Included spray angle Injection pressure Steady flow rate
4 15° 0.3 MPa 700 mL/min
Table 4 External fuel reforming system specifications. Reformer
Evaporation mixer
Temperature controller #1 Gas flowmeter #2 Gas flowmeter #1 Air compressor #2 Air compressor Fuel injection pump
Length Diameter Wall thickness Length Diameter Wall thickness Ruipu Electric RP-100P Endress Hauser (85-260VAC) STEPS-S49 33/MT RONFEN (Y160L-2) Fusheng Z-0.10/8 (D-2(II)) Longer pump LSP01-2A
1000 mm 40 mm 5 mm 300 mm 30 mm 3 mm
Fig. 2. The temperature profile of reformer.
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Fig. 3. The components of reformed products under different reforming temperatures. Fig. 4. Comparisons between experimental and predicted mole fraction profiles of major low-temperature reformed species.
3. Results and discussion 3.1. The effect of temperature on low-temperature reforming process
analyzed. The process of “the first oxygen addition → the first isomerization → the second oxygen addition → the second isomerization” is the key section to form the major reformed products during the lowtemperature reaction process. As shown in Fig. 5, C2H2, C2H4, C4H6, and CH3CHO in the reformed products were mainly produced by nheptane, and the formation reaction pathways of C2H2, C2H4, and C4H6 were different at the temperature of 600 K and 750 K. These major reformed products were produced through the typical low-temperature branching reaction process at the temperature of 600 K, but the pyrolysis of PRF50 fuel started to dominate the reforming process at the temperature of 750 K. The formation pathway of CH3CHO was consistent at the temperature of 600 K and 750 K, which is NC7H16 → C7H15-4 → C7H15O2-4 → C7H14OOH4-2 → C7H14OOH4-2O2 → NC7KET42 → CH3CHO. When the reforming temperature was set at the temperature of 600 K, C3H6 was mainly generated by the reaction pathway of IC8H18 → BC8H17 → BC8H17O2 → BC8H16OOH-A → BC8H16OOHAO2 → IC8KETBA → IC3H7COC3H6-T → IC3H7 → IC3H7O2 → C3H6. There were two major formation pathways for C3H6 at the temperature of 750 K, which are IC8H18 → CC8H17 → IC4H8 → C3H6 and IC8H18 → AC8H17 → IC4H9 → C3H6. The formation pathway of CH2O varied with different temperature. When the temperature was at 600 K, CH2O was mainly generated by R1, R2, R3, R4, R5, and R6; as the temperature increased to 750 K, R7, R8, R9, and R10 constituted the critical formation pathway of CH2O. Meanwhile, CO was produced through R11 and R12 at this temperature.
Fig. 2 shows the temperature measurement results of the four temperature monitoring points on the external fuel reforming system. It can be found that the temperature profile of the four thermocouples increased slightly compared with the setting temperature in case 2-case 4. There were stronger temperature lift in case 5-case 8 due to the higher reforming temperature (650–750 K), wherein the fuel released heat to increase the system temperature through low-temperature reaction. However, considering the relatively low reforming fuel mass (25% of the total fuel mass per cycle), the overall temperature fluctuation in this external fuel reforming system was still thought to be acceptable. The reforming temperature exhibited significant effects on the formation of key low-temperature reformed products, and the reformed products measured by FTIR were given in Fig. 3. When the reforming temperature was lower than 600 K, the main components of reformed products were PRF50 fuel, ethane (C2H6), acetaldehyde (CH3CHO), and a small amount of short-chain alkane, alkene, and alkyne, such as methane (CH4), acetylene (C2H2), ethylene (C2H4), propylene (C3H6), propane (C3H8), and butyne (C4H6). When the reforming temperature was higher than 600 K, significant amount of CO was generated in the external fuel reforming system, which concentration could reach about 1.24 × 105 ppm in case 8. The concentration of CO and CO2 increased with the temperature increased, and PRF50 fuel was almost entirely consumed in case 8. When the reforming temperature at 600 K (case 5), most of PRF50 fuel (76%) was reformed, and the concentration of other small molecule species significantly increased at this reforming temperature. The components of reformed products underwent a drastic change at case 5, thus the temperature of 600 K was regarded as a transition point in the reforming process. The comparison between experimental and computed mole fraction profiles of major low-temperature reformed products is shown in Fig. 4. The results indicate that the simulation with detailed PRF mechanism predicted the mole fraction profiles of major reformed products well, which means that the main combustion chemistry of PRF50 can be well represented with this detailed mechanism. As shown in Fig. 5, the key low-temperature reforming products in the reaction pathway at the temperature of 600 K and 750 K are 454
IC8H18 + O2 < = > BC8H17 + HO2
(R1)
BC8H17 + O2 < = > BC8H17O2
(R2)
BC8H17O2 < = > BC8H16OOH-A
(R3)
BC8H16OOH-A + O2 < = > BC8H16OOH-AO2
(R4)
BC8H16OOH-AO2 < = > IC8KETBA + OH
(R5)
IC8KETBA < = > CH2O + IC3H7COC3H6-T + OH
(R6)
IC8H18 + O2 < = > CC8H17 + HO2
(R7)
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Fig. 5. The reaction pathway analysis of key low-temperature reformed species at 600 K and 750 K.
CC8H17 < = > IC4H8 + TC4H9
(R8)
CH3O2 + TC4H9 < = > CH3O + TC4H9O
(R9)
CH3O + O2 < = > CH2O + HO2
(R10)
CH2O + HO2 < = > HCO + H2O2
(R11)
HCO + O2 < = > CO + HO2
(R12)
combustion phasing could be advanced by introducing the reformed products, with higher in-cylinder pressure and heat release rate. In other words, the low-temperature reformed products could accelerate the HCCI combustion reaction rate. According to our previous results [32,33], by introducing low-temperature reformed products into the engine, the concentration of hydroxide radical (OH) would be improved in the combustion process. The higher OH concentration accelerated the removal of first hydrogen atom from the initial fuel molecules (nheptane and iso-octane), which had a faster combustion effects than fueled with pure PRF50 in case 2-case 5. In case 6-case 8, when the reforming temperature was higher than 600 K, CO and CO2 in the reformed products were increased accordingly as shown in Fig. 3, which resulted in postponing the combustion phasing after adding the reformed products. Fig. 7a shows the comparison of reformed products on CA50 under different reforming temperatures compared with case 1, with the change sequence of combustion phasing of case 5 > case 4 > case 3 > case 2, and the maximum advanced crank angle of CA50 was about 7.5 CA at the reforming temperature of 600 K (case 5). When the reforming temperature was higher than 600 K, the sequence of retarding ability of reformed products on CA50 was that of case 8 > case 7 > case 6, and the maximum retarded crank angle of CA50 was about 3.5 CA at reforming temperature of 750 K (case 8). A reactivity coefficient (RC) has been proposed to assess the reactivity of reformed products in our previous work [36], and the definition formula is:
3.2. The effect of reformed products on combustion Fig. 6 shows the experimental results of the mixtures after the reformed products from external reformer back into the engine at different reforming temperatures. As shown in Fig. 6, when the reforming temperature was lower than 600 K (case2-case5), the engine
RC =
CA50(fresh fuel + reformed products ) − CA50(fuel ) mole fraction of (fresh fuel + reformed products )
wherein the RC is the reactivity coefficient; CA50 (fresh fuel + reformed products), the crank angle degree of 50 percent of the mixture of the fresh fuel and reformed products combusted in a normal cylinder, CA50 (fuel), crank angle degree of 50 percent pure fuel combusted in normal cylinder, mole fraction of (fresh fuel + reformed products), the total mole fraction of the mixture of the fresh fuel and reformed products in the
Fig. 6. Comparisons of in-cylinder pressure and heat release rate at different reforming temperatures. 455
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Fig. 8. The effect of reformed products on combustion efficiency at different reforming temperatures.
Fig. 7. Comparison of CA50 and the reactivity of reformed products under different reforming temperatures.
normal cylinder. When adding reformed products into engine advances the CA50 timing compared with fueled with pure fuel, the RC value should be less than 0, and the reformed products are defined as high reactivity, and vice versa. Furthermore, when the RC value equals to 0, the reactivity values of reformed products and fresh fuel are identical. A larger RC absolute value indicates the stronger positive/negative reactivity of the reformed products, indicating their relative capability to advance/delay the combustion phasing in HCCI combustion. When the reforming temperature were at the temperature of 450 K (case2), 500 K (case3), 550 K (case4), and 600 K (case5), the addition of reformed products can advance the CA50 timing compared with fueled with pure PRF50 in case1. According to the above-mentioned definition and shown in Fig. 7b, the values of RC were less than 0 in case 2-case 5, which indicated higher reactivity of these reformed products at the temperature of 450–600 K, and the sequence of | RC (case 5)| > | RC (case 4)| > | RC (case 3)| > | RC (case 2)| demonstrated that increasing the reforming temperature shall enhance the reactivity of reformed products. In case 6-case 8, the RC values were positive when the reforming temperature were at 650 K (case 6), 700 K (case 7), and 750 K (case 8), indicating their inhibiting effects on combustion phasing, as shown in Fig. 7a. The effects of adding reformed products on combustion efficiency are shown in Fig. 8, it is seen that when the reforming temperature was lower than 600 K (case 2-case 5), the combustion efficiency can be improved compared with case 1 (baseline), with the highest combustion efficiency of 96.95% in case 5, a 2.51% improvement. In contrast, lower the combustion efficiency is observed as the reforming temperature was continued to increase, as shown in case 6-case 8. The effects of the reforming temperature on the coefficient of variation in indicated mean effective pressure (CoVIMEP) are shown in Fig. 9. It is seen that lower reforming temperature was of benefit in improving the combustion stability, but the combustion stability were decreased (the engine became unstable) in case 7 and case 8. The key factors affecting the fuel combustion stability are H radicals due to the important chain branching reaction H + O2 < = > OH + O [46]. After introducing the reformed products into the engine, the reaction H + O2 < = > OH + O were promoted, resulting in lower CoVIMEP in case 2-case 6. When the reforming temperature was higher than 650 K (case 7 and case 8), the CoVIMEP exceeded 5%, indicating an unstable condition, especially at reforming temperature 700 K (case 7).
Fig. 9. CoVIMEP as a function of reforming temperature.
Fig. 10. The engine emissions of CO and UHC under different reforming temperatures.
3.3. The effect of reformed products on emissions and thermal efficiency Fig. 10 illustrates the effects of reforming temperature on emissions, showing a decrease of concentration of CO and unburned hydrocarbon (UHC) as the reforming temperature increased from 450 K (case 2) to
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Fig. 11. The engine energy under different reforming temperatures.
600 K (case 5). One possible reason to explain the emissions of CO and UHC decreased by 17.02% and 37.98% is that the addition of reformed products enhanced the formation of OH in the engine, which in turns advanced the engine combustion phasing, and improved the fuel combustion efficiency. In the external fuel reforming mode, the overall energy distribution is shown in Fig. 11. The energy required for the external reformer to reform the fuel came from electrical heating, which increased as the reforming temperature increases, and when the reforming temperature was 750 K (case 8), the proportion of this energy in the total energy increased to 3% of the total energy contained in the fuel. The heat transfer losses accounted for about 30% of the total energy at case 1case 8. Compared with case 1, the addition of reformed products could reduce the exhaust losses. Meanwhile, the indicated thermal efficiency could be improved to 44%, a 4% improvement than the baseline case. 4. Conclusions In this work, the application of external fuel reforming device-assisted fuel reforming in an HCCI engine has been conducted at different reforming temperature through both experiments and simulations. Compared to our previous results, the addition of PRF50 reformed products in engine can also reduce emissions and improve indicated thermal efficiency, while providing better combustion stability. The major conclusions of the current study can be summarized as follows. (1) When the reforming temperature was no more than 600 K, the major reformed products were PRF50 fuel, ethane (C2H6), acetaldehyde (CH3CHO), and a small amount of short-chain molecules; when the reforming temperature was higher than 600 K, significant amount of CO was generated in the external fuel reforming system. (2) Reforming temperature of 600 K was the transition point for the engine combustion phasing. When the reforming temperature was lower than 600 K, the engine combustion phasing could be advanced by introducing the reformed products, and vice versa. (3) The engine can work steadily (CoVIMEP not exceeding 5%) after the addition of reformed products, when the reforming temperature was lower than 650 K. The addition of reformed products of PRF50 could reduce the emissions of CO and unburned hydrocarbon (UHC), and the indicated thermal efficiency could be improved to 44%.
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