Insight into fuel reactivity effects on thermochemical fuel reforming (TFR)

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Insight into fuel reactivity effects on thermochemical fuel reforming (TFR) Qian Sun, Bolun Li, Ang Li, Yu Shao, Zhuoyao He, Xingcai Lu, Lei Zhu*, Zhen Huang Key laboratory of Power Machinery and Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China

highlights
Reasons for enriched fuels with different reactivities reaching enrichment limit.
Isooctane enrichment help accelerate combustion at large equivalence ratios.
Effect of enriched fuel reactivity on the NOx emission and fuel consumption.

article info

abstract

Article history:

This study uses a port-injection spark-ignition four-cylinder natural gas engine to achieve

Received 27 November 2019

TFR (Thermochemical fuel reforming) mode. To study the effects of fuel reactivity on

Received in revised form

combustion, reforming process, emissions and fuel economy, chemicals including n-

31 December 2019

heptane, PRF50 and isooctane are respectively used as enriched fuel. The results show that

Accepted 9 January 2020

the higher the reactivity of the enriched fuel, the better the combustion and cycle stability

Available online xxx

of the reforming cylinder. However, n-heptane enrichment with high reactivity has the problem of knocking at large equivalence ratio. The enrichment limit of PRF50 is the

Keywords:

highest, which combines the properties of n-heptane and isooctane. The H2 production

Natural gas engine

abilities of three enriched fuels are similar, but that of isooctane is slightly lower under

In-cylinder fuel reforming

large equivalence ratios. In terms of fuel economy, the three perform similarly at small

Fuel reactivity

equivalence ratios. Whereas it’s lower with isooctane enrichment at large equivalence

Hydrogen

ratios, which is at the expense of increased NOx emission. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The shortage of fossil resources and increasingly stringent emission regulations have brought many challenges to the automotive industry [1]. In this case, the search for new clean vehicle energy becomes a key issue. Natural gas is considered to be one of the most promising alternative fuels because of its abundance, cleanness during combustion and good explosion

resistance [2]. At the same time, in order to improve the working performance of natural gas engines, many combustion modes have been designed, such as gasoline/NG bi-fuel mode and diesel/NG dual fuel mode. Besides, HCNG mode also has been proved to increase the brake thermal efficiency, proving to be an effective method [3]. Studies have shown that H2 can significantly increase the concentration of O, H and OH radicals and thus several key reaction rates during combustion [4,5]. Therefore, natural gas

* Corresponding author. E-mail addresses: [email protected], [email protected] (L. Zhu). https://doi.org/10.1016/j.ijhydene.2020.01.045 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Sun Q et al., Insight into fuel reactivity effects on thermochemical fuel reforming (TFR), International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.045

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Abbreviations ATDC after top dead center BMEP brake mean effective pressure BSemission break specific emission BSFC brake specific fuel consumption CA10 crank angle at which 10% of total heat is released CA50 crank angle at which 50% of total heat is released CA90 crank angle at which 90% of total heat is released COVIMEP coefficient of variations for IMEP COVTFRIMEP COVIMEP for reforming cylinder D-EGR Dedicated exhaust gas recirculation FCE flexible cylinder engine FDD flame development duration GDI gasoline direct injection HCCI homogeneous charge compression ignition HCNG hydrogen-compressed natural gas blended fuel IMEP indicated mean effective pressure NVO negative valve overlap PRF50 primary reference fuel, n-heptane and isooctane in a 50:50 vol ratio PRF90 primary reference fuel, n-heptane and isooctane in a 10:90 vol ratio RCD rapid combustion duration RON research octane number TFR thermochemical fuel reforming

blended with H2 can significantly improve the flame propagation speed [6,7]. The method of adding H2 to the engine can effectively improve the combustion stability and thermal efficiency [8,9], and reduce HC emissions [10]. However, as H2 is flammable and explosive, it is inconvenient to store and transport. Mixing hydrogen directly with fuel does not meet the conditions for practical application at present. While the fuel reforming method has enabled on-line H2 production and thus been extensively studied. At present, there are mainly three methods to reform H2 production: steam reforming, partial oxidation reforming and dry reforming [11]. Among them, the steam reforming method can be realized under both aerobic and anaerobic conditions, and is thus widely used. Partial oxidation reforming method undergoes an incomplete oxidation reaction to form H2, CO and other incomplete oxidized products. The dry reforming process utilizes the endothermic reaction between the reformed fuel and CO2, so that CO2-rich engine exhaust can be used for reforming. Many researchers use the extra device to realize reforming to obtain H2 outside the engine structure, which is then passed into the engine cylinder to optimize combustion. Ji et al. [12,13] designed and installed an ethanol reforming reactor at the exhaust pipe of a spark-ignited gasoline engine. It uses exhaust heat and catalyst to promote steam reforming in ethanol to produce syngas, which contains mainly H2 and CO. This syngas is shown to be helpful in shortening the combustion duration and reducing the emission of HC and

NOx. It is also found to improve engine thermal efficiency at 3%e4%. Liao et al. [14] designed a catalytic methanol reforming reactor, which can be heated by the exhaust heat. It is discovered that the reforming efficiency of methanol increases with the increase of the rate of heat exchange of the reformer. It is worth noting that when the S/C ratio was set to 1.2 and methanol supply is at 15.8 g/min, the H2 yield can stably approach 75%. Tartakovsky et al. [15,16] studied the effects of reforming with methanol and ethanol on engine performance. The results also reveal that H2-rich reformed gas can significantly improve in-cylinder combustion and optimize the performance of the whole engine. Perry Leung et al. [17] designed a small bioethanol reformer that was intended to be installed in the exhaust pipe with PteRh catalyst. It has been found that H2O and CO2 are consumed in the reformings, and the steam reforming is considered to be the main endothermic reaction. With more O2 in the exhaust, the threshold for heat recovery is predicted to be lower. In addition, some researchers use reformers to recover energy from exhaust gas. Pashchenko [18e20] proposed a model of thermochemical recuperation (TCR), which used steam methane reforming to recover exhaust energy. Through thermodynamic analysis, the effects of temperature, pressure and H2O/CH4 ratio on TCR efficiency can be determined. Since reforming outside engine needs additional reforming reactor with large dimensions, and the reactor working in a catalyst environment would encounter problems such as catalyst poisoning and aging, many scholars has paid attention to the method of H2 production in engine cylinder. Willand et al. [21] proposed the concept of in-cylinder fuel reforming by means of negative valve overlap (NVO). It leaves a part of the high-temperature exhaust gas in the cylinder through the early closing of the exhaust valve and the late opening of the intake valve. During this time, a small amount of fuel is injected in cylinder for reforming. Using NVO, Tomonori Urushihara et al. [22] achieved in-cylinder fuel reforming mode on an HCCI gasoline engine to optimize combustion. The research shows that NVO reforming method can not only broaden the operating range of the HCCI gasoline engine, but also further reduce NOx emissions. Besides, NVO reforming also has certain optimizing effects on the fuel consumption of the whole engine. Terry Alger et al. [23] proposed D-EGR operation mode on a 2.4 L four-cylinder naturally aspirated gasoline engine. The experiment showed that the H2 and CO generated by the dedicated cylinders can increase the burning velocity of the other 3 cylinders and broaden the EGR bearing rate, thus improving the thermal efficiency and reducing the emissions of the whole engine. Christopher Chadwell et al. [24] used the D-EGR concept and modified a GDI engine with methods such as increasing the compression ratio and optimizing the booster system. The study found that in-cylinder reforming products can restrain knocking and reduce the overall fuel consumption rate by at least 10%. Wang Yang et al. [25] proposed the FCE working mode on the HCCI engine. The simulation was calculated using CHEMKIN 2.0 software with n-heptane as the working fuel. The simulation results showed that the n-heptane reforming gas produced by the flexible cylinder can shorten the ignition delay time of other cylinders and increase the flame speed, and this mode can effectively reduce the harmful emissions.

Please cite this article as: Sun Q et al., Insight into fuel reactivity effects on thermochemical fuel reforming (TFR), International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.045

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Considering that natural gas is a better alternative fuel for engines, Zhu Lei et al. [26] proposed a working mode of TFR (thermochemical fuel reforming) on a premixed ignited natural gas engine. In this mode, by means of in-cylinder fuel reforming, the fourth cylinder gains reformed gas containing H2 and CO, which would be mixed with the intake air to promote the combustion in the whole engine. Studies have shown that TFR mode can significantly improve engine cycle stability, reduce overall fuel consumption, and reduce NOx emissions by nearly 65% compared to those of the original machine, so TFR has proven to be an efficient combustion mode. Fuel reactivity has an important effect on in-cylinder combustion performance of engine, such as anti-knock performance in SI engines, ignition performance in CI engines, etc. Moreover, some studies have found that fuel reactivity may have some effects on reforming [27,28]. Significantly, It has been found that the higher the reforming fuel RON is, the greater the chemical reactivity range of the reformate for incylinder fuel reforming on HCCI engine [29]. In that case, PRF90 was studied to be used as the reforming fuel for incylinder reforming. Results showed that the reforming reaction can be stably carried out at a low temperature, and the initial temperature required increases with the equivalence ratio. Furthermore, the highly reactive OH and HO2 radicals produced by the reforming reaction are effective in shortening the ignition delay. Hence, fuel reactivity is an important factor for in-cylinder thermochemical fuel reforming, but there is almost only rare research on it. Therefore, this study explores the effect of enriched fuel reactivity on in-cylinder fuel reforming in TFR mode detailedly. N-heptane, PRF50 and isooctane were selected as enriched fuels for reforming cylinders. In this study, the performance of fuel enrichment limit, combustion, reformed gas production, emissions and fuel economy were discussed. And then, the effects of enriched fuel reactivity can be comprehensively analyzed.

Experimental setup and research method Experimental setup The schematic view of experimental setup is shown in Fig. 1. The engine used in this study is a port-injection, spark-ignited four-cylinder natural gas engine. The main parameters of it are shown in Table 1. The fourth cylinder was separately analyzed as the reforming cylinder, and the other three cylinders were set as normal cylinders, from which the second cylinder was selected as a representative for analysis. In order to meet the design requirements, the fourth cylinder was modified to achieve in-cylinder fuel reforming. An extra liquid fuel injector was installed at the fourth cylinder intake manifold to achieve in-cylinder rich combustion. The injector was connected to a self-designed liquid fuel tank through a pipe, which was pressurized with high-pressure nitrogen gas. In this test, the pressure was set to 11 bar. Exhaust from the fourth cylinder was separated from that from other three cylinders, so that it can be mixed with the total intake air in the gas mixer after going through the supercharger. Then, the

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mixed gas was cooled in an intercooler to reduce the effect of temperature increase on the reduction of the intake air amount. At the same time, the evenness of the intake of the four cylinders was maintained. The exhaust of the other three normal cylinders pushed the exhaust turbine to drive the supercharger to increase the pressure of fresh intake. The extra injector on the fourth cylinder was controlled by the single-chip computer program, which used the fourth cylinder exhaust oxygen sensor signal to achieve closed-loop feedback control of the in-cylinder equivalence ratio. The engine used compressed natural gas (CNG) as the primary fuel, and the gas injector was mounted on the intake manifold behind the intercooler, controlled by Woodward's electronic control unit. Table 2 lists the main components of the compressed natural gas used in this study. In order to explore the effect of enriched fuel reactivity, n-heptane, isooctane and PRF50 (Primary Reference Fuel, n-heptane and isooctane in a 50:50 vol ratio) were used as liquid enriched fuels in the fourth cylinder. Table 3 shows the main characteristic parameters of the liquid enriched fuel. N-heptane, with its high cetane number and good ignitability, is regarded as a representative of highly active fuels. Moreover, there is the “cold flame” phenomenon [30] at low temperatures. On the contrary, isooctane has a higher octane number, higher antiknock property and chemical stability, making it a typical low chemically active fuel. To increase the reactivity gradient, the PRF50 fuel in this experiment was prepared with a 50:50 vol ratio of two fuels. Such a ratio is representative of the reactivity inbetween n-heptane and isooctane. The main equipment and parameters of this study are shown in Table 4. Kistler cylinder pressure sensors were installed in the second and fourth cylinders to collect pressure data, which was analyzed by the combustion analyzer. To analyze the exhaust from the reforming cylinder, gas collecting bags were used to collect the exhaust gas before exhause in the fourth cylinder cools down. Then, the mole percentages of H2, CO, CH4 and CO2 in the reformed gas were measured off-line using a gas chromatograph. For the purpose to measuring the whole engine emissions, the emission analyzers (CAI) were placed after the normal cylinder exhaust turbine.

Research method In the study, some analysis and definitions of combustion need to be explained in advance. The stability of in-cylinder combustion is characterized by the point distribution of CA50 (Crank angle degree corresponding to 50% accumulated heat release rate) and IMEP in 200 acquisition cycles, and higher concentration of point distribution signifies better incylinder combustion stability. Furthermore, COVIMEP is used as an indicator parameter for the stability of the in-cylinder combustion cycle, with higher cyclic volatility signifying worse in-cylinder combustion stability. (1) COVIMEP ¼ （sIMEP = x）  100% 2 00 P In this function, x ¼ xi =200 is the average value of 200 i¼1 acquisition cycles IMEP, xi is the measurement value (bar) of

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Fig. 1 e Experimental system.

Table 1 e The basic parameters of experimental engine. Model

SC5DT

Engine type Bore  Stroke Compression ratio Displacement Maximum power Maximum torque Ignition timing

Lined, 4 cylinders, 2 valves, water-cooled, turbocharged 114 mm  130 mm 13:1 5.3 L 135 kw/2300 rpm 645 N m/1500 rpm 20 CAD BTDC

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 00 P IMEP in a single cycle and sIMEP ¼ ðxi  xÞ2 =ð200  1Þ is the i¼1 standard deviation of 200 acquisition cycles IMEP. To analyze the pre and medium term combustion conditions, the crank angle from the ignition time to CA10 time is regarded as the flame development duration (FDD) of the incylinder combustion, and the crank angle from CA10 to CA90 time is taken as the rapid combustion duration (RCD). In order to evaluate the fuel economy of the whole engine, the amount of enriched liquid fuel is converted to the amount

of natural gas with the same calorific value, which is added to the amount of natural gas injection at the intake manifold to calculate the total fuel consumption of the whole engine. Meanwhile, the fuel consumption and emissions amount for per unit of power (BSFC and BSemission) are analyzed to evaluate the fuel economy and emissions of the whole engine under different loads. BSFC ¼ ðhCNG  mCNG þ hE  mE Þ=ðhCNG  PÞ

(2)

Table 2 e Components of natural gas used in experiments. CNG Components

CH4

C2H6

C3H8

C4H10

CO2

N2

Others

Mole fraction/%

94.88

4.17

0.32

0.12

0.14

0.35

0.02

Table 3 e The physical and chemical properties of the enriched fuels [31]. Properties Mole weight/(g/mol) H/C atomic ratio Density (298 K)/(kg $ m3) Cetane number () Octane number () Heat of vaporization/(kJ $ mol1) Heat of combustion/(kJ $ mol1) Boiling point/(K)

n-Heptane

Isooctane

100.20 2.28 679.5 56 0 31.77 4849.2 372.0

114.23 2.25 692.0 13 100 30.79 5463.0 372.4

In this function, hCNG , hE are the calorific values of natural gas and enriched fuel (kJ/g), respectively; mCNG , mE are the mass flow rates of natural gas and enriched fuel (g/h), respectively and P is the engine power (kW). BSemission ¼ memission =P

(3)

In the above function, memission is the mass flow rate (g/h) of a certain emission; P is the engine power (kW). In the experiment, test conditions were set as 1500 rpm of maximum torque and the BMEP (Brake Mean Effective Pressure) at 0.305 MPa and 0.765 MPa respectively (hereinafter

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Table 4 e The experimental equipment and basic parameters. Equipment

Manufacture

Dynamometer ECU Combustion analyzer Cylinder Pressure Transducer Natural gas flow meter Gas Chromatography Emission analyzer

CAMA (China) Woodward (USA) D2T Osiris (France) Kistler (Switzerland) Emerson (USA) Shimadzu (Japan) CAI (USA)

referred to as small load, large load) to investigate the performance of the engine under two loads. The normal cylinder maintained combustion at equivalence ratio, while the reforming cylinder was enriched with three different liquid fuels with the equivalence ratios increasing from 1.0 by a gradient of 0.05 until the engine output was unstable or the cylinder combustion was abnormal. Table 5 shows the specific test conditions.

Results and discussions Effect of enriched fuel reactivity on reforming cylinder Effect of enriched fuel reactivity on enrichment limit of reforming cylinder The enrichment limit is the maximum equivalence ratio of the reforming cylinder to maintain stable working. According to previous studies [32], higher enrichment limits often correspond to better reforming performance. Fig. 2(a) shows the curves of the changing cylinder pressure and heat release rate in reforming cylinder at different equivalence ratios. As the equivalence ratio increases, the peak of the heat release rate first increases and then decreases. Accordingly, the peak of cylinder pressure also shows the same changes. The “first increase” is because that with moderately rich fuel in cylinder, the flame speed would increase [33]. Thus the combustion condition and the power capacity of the engine are improved, which means a “power mixture” is formed. On the other hand, moderately rich fuel increases incomplete combustion in the cylinder. The CO generated would increase the molecular variation coefficient m of the in-cylinder combustion, meaning that the total number of molecules after incomplete combustion is increased than that of complete combustion, that brings about higher efficiency of the in-cylinder working

Model CW260B-18007500 PCM128-HD D2T Evolution3 6118BFD35Q03A41 CMF025 GC-2014 (TCD) NOx HC CO

Accuracy

HCLD (600) HFID (600) NDIR (602P)

Ttq 0.1% e e 10 Pa 0.35% 0.01% 1% 1% 1%

substance. As the equivalence ratio further increases, the oxygen content in the cylinder would decrease, causing more fuel to be incompletely combusted. As a result, the in-cylinder combustion condition deteriorates, which is manifested in a decrease in the peak of heat release rate and a delay in the exothermic phase. Specifically, when BMEP at 3.05 bar, the combustion heat release rate curve of n-heptane enrichment shows “bumps” at 1.4 and 1.45 equivalence ratios. To explore the “bump” phenomenon, Fig. 2(b) shows the cylinder pressure and heat release rate curves of three characteristic cycles and mean cycles in 200 acquisition cycles at 1.45 equivalence ratio. It shows that regardless of the heat release rate peak value of a single acquisition cycle, the heat release rate curve always has a “bump” near the top dead center. This means that the occurrence of this phenomenon has nothing to do with the ignition and combustion conditions in the cylinder. Possible cause should be the higher gas pressure in the cylinder near the top dead center, and more n-heptane releasing heat in low temperature due to its high reactivity. Apart from these, obvious “knocking sounds” occurred during the test at 1.5 equivalence ratio for the small load and 1.3 equivalence ratio for the large load. It can be verified that the knockings occur inside the cylinder, so test data under these equivalence ratios are not collected for the purpose of protecting the engine equipment. The above analyses show that when the concentration of n-heptane in the reforming cylinder is relatively large, knocking combustions would occur, putting a limit to further increase of the enrichment ratio. Fig. 3(a) shows the variations of the cylinder pressure and heat release rate in response to the changes of equivalence ratio in the reforming cylinder with isooctane enrichment. It can be seen that at the small load, the peak value of the heat release rate increases slightly with the equivalence ratio, and then decreases, similar to the changes occurred during n-

Table 5 e Test conditions with different enriched fuels. Engine speed (rpm) 1500

BMEP (Bar)

Enriched fuel type

Equivalence ratio of reforming cylinder FTFR ()

Equivalence ratio of normal cylinder FN ()

Ignition timing (CAD ATDC)

3.05 Small load

n-Heptane PRF50 Isooctane n-Heptane PRF50 Isooctane

1e1.45 (With a gradient of 0.05, the same below) 1e1.75 1e1.60 1e1.25 1e1.50 1e1.40

1

20

7.65 Large load

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Fig. 2 e Combustion performance for n-Heptane enrichment.

heptane enrichment. With the large load, the peak of heat release rate does not increase with equivalence ratio, indicating that the addition of isooctane is less effective in promoting in-cylinder combustion. With either load, the isooctane enrichment limit occurs because of the deterioration of the stability of the whole engine and the large fluctuations of power output. In particular, it is found that under the large load, with equivalence ratio being 1.35, the exothermic phase is advanced and the peak of the heat release rate is higher. Regarding this abnormal combustion, the following 200 acquisition cycles under this condition are investigated. In this study, an acquisition cycle with the peak value of heat release rate 1.5 times higher than that of the mean cycle is regarded as a “rough combustion” cycle. Similarly, an acquisition cycle with a peak value 2 times higher than that of the mean cycle is regarded a “surge cycle”. As shown in the right-side diagram of Fig. 3(b), there is a “rough combustion” cycle at the large load of 1.4 equivalence ratio, and the peak value of the heat release rate and cylinder pressure are much higher than the mean cycle. In the left graph of Fig. 3(b), the

statistical results of two abnormal cycles with the equivalence ratio of 1.3e1.4 are shown. It can be seen that as the equivalence ratio increases, the number of “rough combustion” cycles in the 200 acquisition cycles increases greatly, while the “surge cycle” begins from the equivalence ratio of 1.35 and the number increases exponentially. This shows that the greater the equivalence ratio of isooctane enrichment is, the more cycles of “rough combustion” would be in the cylinder, which results in the increase of mean heat release rate peak. Fig. 4(a) shows the curves representing the changes of cylinder pressure and heat release rate in the reforming cylinder corresponding to the different equivalence ratios with PRF50 as enriched fuel. It shows that as the equivalence ratio increases, the variations of heat release rate peak show similar patterns as those enriched by the above two fuels respectively. At 1.10 equivalence ratio under small load and 1.05 equivalence ratio under large load, the peaks of heat release rate are both higher than those of equivalent combustion. Therefore, the combustion performances are better than those of low reactivity isooctane. Under small load, when

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Fig. 3 e Combustion performance for Isooctane enrichment.

the PRF50 enrichment reached limit due to deterioration of the working stability of the whole engine, no knockings appear in the in-cylinder combustion nor is there any “bump” on the heat release rate curve. This means that PRF50 combines the properties of the above two fuels, achieving relatively higher reactivity and explosion resistance, thereby broadening the enrichment limit in the reforming cylinder. At large load and 1.50 equivalence ratio, “bump” appears near the top dead center in the heat release rate curve and the heat release rate peak value increases significantly. In order to analyze the phenomenon, Fig. 4(b) compares three characteristic acquisition cycles with the mean cycle of the reforming cylinder under this condition. As shown in Cycle 1, sudden increases in the peak value of heat release rate and cylinder pressure verify this cycle as knocking cycle. Therefore, more cycles with extremely high peak values of heat release rate would cause the increase of that in the mean cycle. Moreover, the heat release rate curves of the three cycles under this condition all have “bumps” near the top dead center. These should be mainly due to the high reactivity component of nheptane in the PRF50 fuel.

Effect of enriched fuel reactivity on combustion of reforming cylinder Above analyses indicate that certain differences in the combustion conditions of the reforming cylinder exist when three fuels with different reactivity are used as enrichments. This issue is further explored below. This study chooses to make the comparison at higher equivalence ratio in order to fully demonstrate the differences in combustion of different fuels. Fig. 5 compares the curves of cylinder pressure and heat release rate in the reforming cylinder at 1.45 equivalence ratio under small load and at 1.25 equivalence ratio under large load. It shows that the peak pressure of the cylinder is the highest with n-heptane enrichment, and the combustion is the best. The peak value of heat release rate and cylinder pressure with PRF50 enrichment is higher than that with isooctane enrichment, which is consistent with the order of fuel reactivity. This means that a higher reactivity would promote the combustion in cylinder to a larger extent. In particular, under small load, a distinct “bump” appears on the heat release rate curve of n-heptane enrichment. However, there was no such “bump” with PRF50 enrichment, indicating

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Fig. 4 e Combustion performance for PRF50 enrichment.

Fig. 5 e Cylinder pressure and heat release rate of the reforming cylinder for three enriched fuels.

that low reactivity isooctane inhibited the occurrence of low temperature heat release. Taking the 1.25 equivalence ratio as an example, Fig. 6 displays the distribution of IMEP-CA50 for three enriched fuels under large load to analyze the stability of combustion in reforming cylinder. By a comparison of the three kinds of enriched fuels, it can be found that the distribution of IMEPCA50 for n-heptane enrichment is the most concentrated,

and the number of misfire cycles is the least. That means nheptane can help the natural gas to form a more stable combustion. The results of PRF50 and isooctane enrichment show that the numbers of misfire cycles increase, with a larger number in the latter. In addition, the distributions of IMEPCA50 for them are more dispersed than that for n-heptane enrichment, and the average values of IMEP indicated by red dots are lower. It shows that with the decrease of enriched

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Fig. 6 e The distribution of IMEP-CA50 for reforming cylinder with different fuels.

fuel reactivity, the cycle stability and working ability of the reforming cylinder are reduced. Comparing the two loads, it can be seen that as the load increases, the number of combustion misfire cycles decreases, and the IMEP-CA50 distribution becomes more concentrated, which means the combustion stability becomes better. In particular, when the equivalence ratio of n-heptane enrichment is 1.25 under large load, there is almost no misfire cycle. The above analysis shows that the increase in load at the same equivalence ratio enhances the combustion stability, so a larger load is beneficial to the rich combustion in the reforming cylinder. In order to compare the speed of combustion, Fig. 7 shows the flame development duration (FDD) and the rapid combustion duration (RCD) for the three enriched fuels under large load. The result shows that higher fuel reactivity brings about shorter combustion duration, except for cases at large equivalence ratios. At the initial increasing phase of the rich equivalence ratio, the enrichments of n-heptane and PRF50 show a slight shortening of FCD and RCD. It means that the slightly rich mixture fuel can increase the burning speed and

Fig. 7 e The variation of combustion duration for reforming cylinder at large load.

promote combustion, and higher fuel reactivity brings better promotion effect. However, such promotion effect was not identified with isooctane enrichment. With PRF50 and isooctane enrichment, the combustion periods are both shortened at a larger equivalence ratio. In addition, the RCD of isooctane enrichment is shorter than that of PRF50 enrichment. According to the analysis for combustion performance of isooctane enrichment above (Section Effect of Enriched Fuel Reactivity on Enrichment Limit of Reforming Cylinder), the shortening of the mean cycle combustion duration may have a relationship with the occurrence of the “rough combustion” cycles. This issue is further discussed below. Taking the main heat release stage-RCD as an example, data related to RCD are analyzed to investigate the reason for the shortening of combustion duration, with results shown in Fig. 8(a). The study compares the values of three equivalence ratios when RCD starts to experience shortening with PRF50 and isooctane enrichments. In the figure, the histogram shows the number of cycles of each type in 200 acquisition cycles and the dotted line stands for the average length of RCD in the corresponding cycles. As can be seen from Fig. 8(a), the RCD of the “rough combustion” at the same equivalence ratio is significantly shorter than “other cycles”. As the enriched equivalence ratio increases, the number of “rough combustion” increases and the RCD of mean cycle continues to decrease. As a result, the number of “other cycles” is continuously decreasing. The shortenings of RCD in “other cycles” is much fewer than those in “rough combustion”. It can be judged that when isooctane and PRF50 are used as enriched fuels, the shortening of mean cycle RCD is mainly due to the increased number of “rough combustion”. The “rough combustion” burns violently, and the burning velocity of it is higher. It occurs mainly because the combustible mixture in the cylinder is too rich and unevenly distributed, as analyzed above. But what are the chemical kinetics reasons for the faster combustion in cylinder with isooctane enrichment? Studies have shown that HO2 and OH are important reactive radicals in the combustion of n-heptane and isooctane,

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Fig. 8 e The analysis for the shortening of RCD with PRF50 and isooctane enrichment.

which participate in and greatly help H-abstraction [34,35]. Abundant HO2 and OH radicals can effectively shorten the ignition delay time and promote more intense combustion. It can be seen that with equivalence ratio at over 1.3 under large load, the RCD of the isooctane enrichment becomes shorter than that of the PRF50 enrichment. Therefore, the closed homogenous batch reactor model in ChemkinPro software was used to analyze the chemical reaction process at the conditions of 1.4 equivalence ratio. In order to express the incylinder combustion of RCD, this study selected the violent combustion process of 10e20 CAD for analysis. Therefore, the initial calculation conditions are set according to the corresponding in-cylinder temperature and pressure at 10 CAD. The average temperature in the cylinder at 20 CAD are set as the condition of observation point. Fig. 8(b) compares the absolute rate of major elementary reactions that generate HO2 and OH radicals under this condition. It can be seen that the reaction rate of the six elementary reactions is faster when isooctane is used as enriched fuel. It is indicated that at the equivalence ratio of 1.4, when isooctane is used as enrichment during combustion, more HO2 and OH reactive radicals are formed in the cylinder. They can effectively help methane to remove H (CH4þOH<¼>CH3þH2O) and further generate OH (CH3þHO2>CH3O þ OH), etc. All these can promote in-

cylinder combustion so that it is more intense than that with PRF50 enrichment.

Effect of enriched fuel reactivity on reformate The main products of the reforming cylinder are analyzed and compared, including H2, CO, CO2 and CH4. Fig. 9(a) shows the changes of mole percentages of H2 and CO in response to the changing equivalence ratios under both loads. It can be seen that the productions of H2 and CO with different enriched fuels follow similar trends in changes with the increase of equivalence ratio. Except for instability when the equivalence ratio is less than 1.05, the H2/CO ratio in each enriched fuel have always been at 1.3e1.5. This indicates that there is a fixed quantitative relation between the amount of H2 and that of CO produced by the in-cylinder reforming reactions. Under the same load, isooctane enrichment has a slightly weaker ability to produce H2 and CO at large equivalence ratios. Otherwise, the productions of H2 and CO for three fuels are similar to each other. Under the small load with increasing equivalent ratio, the production of H2 and CO shows a first rising and then falling trend, reaching a peak at 1.45 equivalence ratio. It can be seen from the above analysis that PRF50 fuel can broaden the enrichment limit, but does not show any advantage in the production of H2 at small load. At large load,

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Fig. 9 e Amount of main products produced by reforming cylinder for three enriched fuels.

the productions of H2 and CO in the reforming cylinder show an increasing trend with the increase of equivalence ratio. Since the PRF50 fuel broadens the enrichment limit, the maximum yield of H2 and CO is higher than those of n-heptane and isooctane enrichment. Through a comparison of the two loads, it can be found that the maximum productions of H2 and CO in the reforming cylinder are similar, and the

corresponding equivalence ratios are similar, too. This indicates that the load barely affects the production of H2 and CO in the reforming reaction, while the equivalence ratio is a more important factor. Fig. 9(b) shows the changing mole percentages of CH4 and CO2 in response to changing equivalence ratios under both loads. Under the same load, CO2 content decreases with the

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increase of equivalence ratio, indicating that the degree of complete combustion in the cylinder is continuously reduced, and that C is converted more into CO than CO2 due to incomplete oxidation. In combination with the curve of CO, it can be seen that the CO content starts to decrease with equivalence ratio larger than 1.45 at small load, mainly due to the decrease of relative oxygen content and average combustion temperature in the cylinder. When n-heptane is used as enrichment in reforming cylinder with equivalence ratio lower than 1.2, the CH4 content is as low as the equivalent combustion, indicating that the in-cylinder combustion with n-heptane enrichment is better than that with other two enriched fuels. In contrast, the content of CH4 is the highest with isooctane enrichment, followed by PRF50 enrichment. The increase of CH4 is due to the fact that the increase of equivalence ratio results in an increase in the number of incylinder misfire cycles and “rough combustion” cycles. Among such cycles, more natural gas mixture did not get ignited in the misfire cycle, and thus the average temperature in the cylinder is lower, worsening the quenching effect. In the “rough combustion” cycle, the in-cylinder pressure is extremely high, which also increases the narrowing effect, therefore causing more unburned CH4 would be discharged from reforming cylinder.

Effect of enriched fuel reactivity on whole engine Effect of enriched fuel reactivity on normal cylinder Fig. 10(a) shows the effect of the equivalence ratio of the reforming cylinder on the IMEP and COVIMEP of normal cylinders (represented by the #2 cylinder). At different loads, the IMEP of normal cylinder increases as the equivalence ratio increases. It can be seen from the above that at small load with equivalence ratio below 1.45 or at large load, the contents of H2, CO and CH4 in the reforming cylinder exhaust gas increase with the increase of the rich equivalence ratio, while the CO2 content decreases. Such changes of conditions increase the energy density in the reforming cylinder exhaust thus increase the in-cylinder combustion energy into the normal cylinders. At the same time, more H2 can help speed up and optimize in-cylinder combustion. All these factors make the combustion in the cylinder more intense, thus enhancing the working ability of the normal cylinder. In addition, at small load, although the amount of H2 produced decreases when the equivalence ratio is greater than 1.45, the IMEP in normal cylinder still increases. This indicates that the increase of exhaust energy density under this condition is the main reason. Apart from that, the amount of H2 under this condition can also promote the combustion in normal cylinder. As the enrichment ratio of the reforming cylinder increases, the combustion in the reforming cylinder deteriorates and the working ability decreases. Therefore, the increase of IMEP for the normal cylinder compensates for the shortage in the reforming cylinder, thereby satisfying the output demand of the whole engine. Through a comparison of the three kinds of enriched fuels, it can be found that the IMEP of normal cylinders is the highest when isooctane is used as enrichment. This is mainly because the reforming cylinder is in poor combustion condition with isooctane enrichment, and the exhaust gas contains more CO and unburned HC such as

Fig. 10 e The working performance for normal cylinder.

CH4. These gases enter the normal cylinders to increase the energy, and enhance the intensity of combustion, resulting in a high IMEP value. When either of the three kinds of fuels are used for enrichment, the COVIMEP of normal cylinder varies with the equivalence ratio. Fig. 10(b) shows the results at different loads. It can be found that all test conditions meet the stable working requirements of less than 5% for the normal cylinder COVIMEP. At small equivalence ratios, COVIMEP decreases as the equivalence ratio increases. This is because the increase in equivalence ratio increased the amount of H2 and reduced that of CO2 produced in the reforming cylinder, thus enhancing the promotion effect on the combustion in the normal cylinder while lowering the combustion misfire rate, which finally enhances the cycle stability. When equivalence ratio increases to a higher level, COVIMEP increases with isooctane and PRF50 enrichments, but not with n-heptane enrichment. The rise of COVIMEP in normal cylinders is mainly due to the deterioration of the combustion in the reforming cylinders and the greatly increasing emission of unburned CH4. In addition, with high octane number and low reactivity,

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more CH4 increases the difficulty of stable ignition in normal cylinders. Moreover, there is also a huge difference in the exhaust gas compositions for different cycles in the reforming cylinder, causing the intake air of each cycle in the normal cylinder to be different, and thereby affecting the combustion stability there. Comparing the two loads, we can find that although the instability and CH4 displacement in the reforming cylinder are higher at small load, it does not cause a considerable increase of COVIMEP. This is mainly because the engine torque is small under small loads and the requirements for output work are low. Therefore, the unstable operation of the reforming cylinder has a relatively small influence on the mechanical connection of the whole engine. In addition, the amount of combustible gas mixture in a normal cylinder at small load is less than that under large load. Therefore, under the condition of constant ignition energy, the combustible mixture is easier to be ignited, which maintains stable combustion in the cylinder and reduces the misfire cycles. As a result, the smaller the load, the greater the tolerance of the whole engine to the rich equivalence ratio in the reforming cylinder, meaning greater enrichment limit. Comparing the three enriched fuels, we find that the highly active enriched fuel can help the normal cylinder to maintain a more stable combustion.

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Fig. 11 shows the variation of NOx emissions with the changing equivalence ratios in the reforming cylinder under different loads. It can be seen that NOx emissions increase as the equivalence ratio in the reforming cylinder increases. The NOx emissions are similar at small equivalence ratios for three kinds of enriched fuels, mainly due to similar combustion conditions in cylinder. As the equivalence ratio in the reforming cylinder continues to increase, the combustions in normal cylinders become more intense, thus causing high temperature which causes an increase in NOx emissions. Moreover, NOx emissions increase more under large load when isooctane is used as enriched fuel. This is mainly because the IMEP value of the normal cylinder is higher under

this condition, meaning a severe combustion condition in the cylinder, which is beneficial for NOx generation. Moreover, the proportion of “rough combustion” cycles in the reforming cylinders at large equivalence ratios will also increase. In this case, the combustion is extremely intense, and such a high temperature environment causes rapid generation of NOx. The specific power of fuel consumption is used to indicate the fuel economy of the engine. As can be seen in Fig. 12, the fuel consumption decreases as the equivalence ratio increases. This is mainly because the increase of equivalence ratio promotes the production of H2 in the reforming cylinder, and more H2 can improve the combustion condition in the normal cylinders, making the in-cylinder combustion more complete. When the equivalence ratio is relatively higher at small load, the H2 production in the reforming cylinder is lowered, so that the fuel consumption of the whole engine is increased. Compared with the large load, the fluctuations of the fuel consumption of the whole engine at small load are larger, and there is a significant increase at 1.05 equivalence ratio. Under this condition, the reforming cylinder produces very little H2 and CO, and its CO2 and CH4 contents are similar to those of equivalent combustion. This shows that the fuel in reforming cylinder is completely burned and the reforming effect is weak. Since the production of H2 is very low, the reforming cylinder exhaust has little effect on the combustion of three normal cylinders. On the contrary, the EGR effect on the normal cylinder is enhanced because the O2 content is lowered. Therefore, the liquid fuel injected under this working condition does not achieve the purpose of optimizing the combustion of the whole engine. As a result, the fuel consumption rate increases, but such inefficiency disappears with the increasing amount of H2 produced in the reforming cylinder. It can be found that under a larger equivalence ratio, the fuel consumption rate is the lowest when isooctane is used as enrichment. Under that condition, large equivalence ratio of isooctane enrichment makes the reforming cylinder burn relatively poorly compared to the other two fuels. This results in a greater amount of CH4 and other unburned HC in the reforming cylinder exhaust, which allows more energy enter

Fig. 11 e The variation of NOx emission for three enriched fuels.

Fig. 12 e The variation of engine fuel economy for three enriched fuels.

Effect of enriched fuel on emissions and fuel economy

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the normal cylinder through EGR. In this case, since the normal cylinder is maintained at the equivalent combustion by control, an increase in the exhaust energy in the reforming cylinder would cause a decrease of natural gas required from the intake manifold, thereby reducing the overall fuel consumption. The key to achieving lower fuel consumption is that although the reforming cylinder burns poorly with isooctane enrichment, H2 production remains at a relatively high level. Thus, it helps optimizing the combustion in the normal cylinder and achieve the target load. On the other hand, when the fuel consumption rate is reduced, the “rough combustion” cycles also cause a great increase in NOx emission.

Conclusion Based on a port-injection, spark-ignited four-cylinder natural gas engine, TFR (thermochemical fuel reforming) mode is achieved by injecting liquid fuel in the fourth cylinder (reforming cylinder) to make it rich combustion. This study systematically explored the effects of the reactivity of enriched fuel chemicals on the enrichment limits, combustion, reformed gas production, emissions and fuel economy. The conclusions are summarized as follows: (1) At different loads, the reason for reaching the enrichment limit with n-heptane enrichment is that the combustion in the reforming cylinder suffers from knocking. For isooctane enrichment, it is due to the unstable power output of the whole engine, that it is hard to reach the enrichment limit. In contrast, PRF50 (Primary Reference Fuel, n-heptane and isooctane in a 50:50 vol ratio) has the highest enrichment limit. It appears that the whole engine is unstable at the enrichment limit of small load, and knockings occur at large load. Therefore, the fuel with moderate reactivity can reach higher enrichment limit. (2) When the equivalence ratio in the reforming cylinder isn't particularly large, higher reactivity of the enriched fuels means better combustion condition in the reforming cylinder, namely: nheptane > PRF50 > isooctane. However, when the equivalence ratio is greater than 1.3 at large load, the RCD for isooctane enrichment becomes shorter than that of PRF50 enrichment. At this condition, a large number of HO2 and OH active radicals in the cylinder can promote in-cylinder combustion. However, the cyclic volatility is still high. (3) Under the small load, all three fuels reached the peak of H2 and CO production at the enrichment limit of nheptane with 1.45 equivalence ratio. However, with large equivalence ratio, the contents of H2 and CO in the reformed gas decrease with isooctane enrichment, while CH4 increases significantly. Under large load, the H2 and CO contents in the reformed gas continue to increase until enrichment limit. It is concluded that equivalence ratio is the main influencing factor for the reforming reactions, and fuel reactivity has little effect except for large equivalence ratios.

(4) NOx emission increases as the equivalence ratio of reforming cylinder increases, and it is higher with isooctane enrichment under large load. In terms of fuel economy, three types of enriched fuels are at a similar level at small equivalence ratios. However, at large equivalence ratios, the fuel consumption for isooctane with low reactivity is the lowest, but meanwhile the NOx emissions is higher.

Acknowledgements This work was supported by the National Science Foundation of China (Project No. 51436005; 51676125; 91741120). The authors also would like to thank the Shanghai Rising-Star Program.

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