Experimental study of combustion and emission characteristics of gasoline compression ignition (GCI) engines fueled by gasoline-hydrogenated catalytic biodiesel blends

Energy 187 (2019) 115931

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Experimental study of combustion and emission characteristics of gasoline compression ignition (GCI) engines fueled by gasolinehydrogenated catalytic biodiesel blends Yanzhi Zhang a, Zilong Li b, Pachiannan Tamilselvan c, Chenxu Jiang b, Zhixia He a, *, Wenjun Zhong c, Yong Qian b, **, Qian Wang c, Xingcai Lu b a b c

Institute for Energy Research, Key Laboratory of Zhenjiang, Jiangsu University, Zhenjiang, 212013, PR China Key Lab. for Power Machinery and Engineering of M. O. E., Shanghai Jiao Tong University, Shanghai, 200240, PR China School of Energy and Power Engineering, Jiangsu University, Zhenjiang, 212013, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 February 2019 Received in revised form 5 August 2019 Accepted 10 August 2019 Available online 12 August 2019

Gasoline compression ignition (GCI) engines have received more and more attention owing to their high thermal efficiency and low harmful emissions. However, GCI engines fueled by pure gasoline with low reactivity are limited to poor combustion stability at low loads and high pressure rise rate at high loads. To this end, a kind of second-generation hydrogenated catalytic biodiesel (HCB) from waste cooking oil with high reactivity is blended into the China 95#gasoline with different volume ratios, and the effect of blended ratio on the combustion and emission characteristics of a heavy-duty diesel engine was explored in the present study. The results indicate that ignition performance is significantly improved as the increase in HCB proportion, maximum combustion pressure can be effectively suppressed, and the combustion stability under low load conditions is much enhanced. Furthermore, HCB blended ratio should match combustion phasing controlled by the start of injection (SOI) to obtain better engine performance and wider operation range. In terms of emissions, the gas emissions of nitrogen oxides, carbon monoxide, and unburned hydrocarbon can be significantly reduced with increasing HCB fraction, however, the particulate matter emissions are increased slightly as a penalty. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Gasoline compression ignition (GCI) Advanced combustion mode Hydrogenated catalytic biodiesel Internal combustion engine

1. Introduction Conventional diesel combustion (CDC) has high thermal efficiency, but it produces a large amount of nitrogen oxides (NOx) and soot emissions owing to high combustion temperature and limited fuel-air mixing [1]. In CDC mode, the high injection pressure is generally employed to promote fuel-air mixing and to reduce equivalence ratio and soot emissions. Meanwhile, a large amount of exhaust gas recirculation (EGR) is needed to control NOx emissions [2]. On the other hand, spark plug-ignited gasoline engines can effectively suppress soot and NOx emissions due to better fuel-air mixing (nearly premixed) and lower compression ratio (CR).

* Corresponding author. Institute for Energy Research Jiangsu University Zhenjiang, 212013, PR China. ** Corresponding author. Key Lab. for Power Machinery and Engineering of M. O. E., Shanghai Jiao Tong University, Shanghai, 200240, PR China. E-mail addresses: [email protected] (Z. He), [email protected] (Y. Qian). https://doi.org/10.1016/j.energy.2019.115931 0360-5442/© 2019 Elsevier Ltd. All rights reserved.

However, a lower CR results in lower thermal efficiency, and a too high CR is limited to engine knocking [3]. In order to combine the advantages of diesel and gasoline engines, researches on advanced combustion modes have become hot spots. Researchers have proposed several modes such as homogeneous charge compression ignition (HCCI) [4e8], premixed charge compression ignition (PCCI) [9e13], reactivity controlled compression ignition (RCCI) [14e18], gasoline compression ignition (GCI) [19,20] and other advanced combustion modes. Currently, HCCI and PCCI have great potentials under low-to-medium load conditions, but they are difficult in extending to high loads, which seriously affects the wide applications of these two combustion modes [21]. RCCI can achieve relatively clean combustion under medium-to-high load conditions; however, it employs two different injection systems, leading to additional complexity and cost compared to other combustion modes. At the same time, carbon monoxide (CO) and unburned hydrocarbon (HC) emissions are also too high at low loads owing to the low temperature combustion (LTC) nature of RCCI mode [22]. Alternatively, GCI fueled by

2

Y. Zhang et al. / Energy 187 (2019) 115931

Nomenclature AHRR CDC CD CR CO CS EGR EVO GCI HCCI HC HCB HSDI

apparent heat release rate conventional diesel combustion combustion duration compression ratio carbon monoxide concentration stratification exhaust gas recirculation exhaust value opening gasoline compression ignition homogeneous charge compression ignition unburned hydrocarbon hydrogenated catalytic biodiesel high-speed direct-injection

pure gasoline or blended fuels (mixtures of gasoline and other fuels with high reactivity) attracts much more attention recently owing to that it is not necessary to modify current injection system and has higher thermal efficiency and lower harmful emissions compared to diesel engines [23]. In GCI engines, gasoline-like fuel with low reactivity (high research octane number (RON)) and high volatility shows physical and chemical properties that are favorable for the utilization in LTC regimes. The high RON of gasoline-like fuel extends ignition delay and promotes fuel-air mixing. Additionally, the high volatility will speed up the vaporization of liquid fuel, which in turn also results in better fuel-air mixing and lower equivalence ratio. Kalghatgi et al. [20] proposed the GCI mode in 2006 with using high RON gasoline fuel in a compression ignition diesel engine. The longer ignition delay can effectively promote fuel-air mixing, the injection and combustion can be completely separated without the aids of a large amount of EGR, early start of injection (SOI) and high CR, leading to low NOx and soot emissions and high thermal efficiency. After then, Kalghatgi et al. [24,25] conducted a series of studies on the GCI mode. In their researches, gasoline is directly injected into cylinder near the top dead center (TDC). With the near-TDC injection, combustion phasing can be effectively controlled, and on the other hand, sufficient premixed charge can be obtained owing to high volatility gasoline. Therefore, much lower NOx and soot emissions can be obtained simultaneously in a wide operating range. However, multiple injection strategies should be employed to lower the pressure rise rate (PRR) and combustion noise for GCI mode under high load conditions. GCI has been widely studied in the literature, which can be divided into two main classifications, namely concentration stratification (CS) and reactivity stratification (RS) [26e30]. The former method utilizes the CS to control the combustion phasing and to achieve the desired loads, in which part of the fuel is injected as the prior injection in the intake stroke and the remainder of the fuel is delivered during the compression stroke. Based on SOI level, the CS can be further divided into partial fuel stratification, moderate fuel stratification and heavy fuel stratification [31]. CS can effectively control the combustion phasing and lower maximum PRR, which is beneficial in extending to higher loads. However, CS still faces difficulties in the ignition, combustion stability, and excessive CO and HC emissions at low loads [19]. The RS usually adds other fuels with high reactivity into gasoline-like fuels, which can significantly improve the ignition characteristics and combustion stabilities, especially at low loads. Xu et al. [32] pointed out that the gasoline-diesel blend can effectively extend GCI engine operation ranges and improve combustion

ISFC IMEP IVC LTC NOx PCCI PRR PM RCCI RON RS RME SOI SME TDC

indicated specific fuel consumption indicated mean effective pressure intake valve closing low temperature combustion nitrogen oxides premixed charge compression ignition pressure rise rate particulate matter reactivity controlled compression ignition research octane number reactivity stratification rapeseed methyl ester start of injection soybean methyl ester top dead center

stability. Meanwhile, NOx and smoke emissions can be significantly reduced compared to diesel fuel. A research group from Tianjin University has systemically studied the GCI combustions fueled by blended fuels, and the fuels with high reactivity including diesel, PODEn, etc. [26,33]. They indicated that blended fuels can remarkably improve the LTC characteristics and lower the soot emissions. Biodiesel as a renewable substance is attracting more and more attention due to its abundant sources. Biodiesel generally consists of macromolecular ester substances such as soybean methyl ester (SME) and rapeseed methyl ester (RME) obtained by the transesterification of vegetable oil. This kind of biodiesel blending with gasoline can reduce CO, HC and soot emissions, but its high oxygen content could cause serious NOx emissions as a penalty. Putrasari et al. [34] studied the effect of biodiesel volume ratio (5e20%) on the combustion and emission characteristics of a GCI engine. The results showed that the thermal efficiency of the GCI engine fueled by the blended fuel is comparable to that of diesel engine, the HC emissions are significantly reduced while the NOx emissions are much increased as the biodiesel fuel fraction increases. Adams et al. [35] studied the effect of biodiesel proportion on the combustion stability of a GCI engine. It was found that the addition of biodiesel can effectively reduce the requirement of intake temperature and increase combustion stability at low loads. As well known, the biodiesel with high polyunsaturated fatty acids has poor oxidation stability, which is not suitable for long time storage. Alternatively, hydrogenated catalytic biodiesel (HCB) is a high quality fuel derived from waste cooking oils through the hydrodeoxygenation catalytic process, and the main species of HCB consists of n-pentadecane, n-heptadecane, and n-hexadecane [36]. The advantages of HCB fuel over other kinds of biodiesels include high cetane number (nearly 100) with good ignition characteristics; low sulfur content (corresponding to good emission performance), good oxidation stability; low acidity, low corrosiveness to injection system, fully compatible with fossil fuels (like gasoline and diesel fuel which is quite important for blending stability) and free from oxygen and aromatic hydrocarbons [37]. However, it should be noted that the pure HCB cannot be directly employed in engines owing to its high freezing point (about 14
C). The blending of high quality HCB with high reactivity and gasoline may present potential to remarkably improve the GCI performances under low or high load conditions. However, the combustion and emission characteristics of GCI engines fueled by gasoline-HCB blends have not been studied so far. Therefore, the HCB fuel is blended into the China 95# gasoline in different volume ratios, and the effects of blended ratio on the combustion, operation boundary and emission characteristics of a heavy-duty diesel

Y. Zhang et al. / Energy 187 (2019) 115931

engine were explored under different SOI and indicated mean effective pressure (IMEP) conditions in the present study.

2. Experimental setup 2.1. Experimental system The experiments were conducted on a four cylinder high-speed direct-injection (HSDI) heavy-duty diesel engine with a highpressure common-rail injection system. Only the forth cylinder is selected as the test cylinder, and brief diagram of the experimental setup is shown in Fig. 1. A pressure transducer (Kistler model 6125B) with a voltage amplifier (Kistler 5015A) is employed to measure in-cylinder gas pressure, and apparent heat release rate (AHRR) and the gas temperature are determined by a D2T combustion analyzer. The heated chemiluminescent analyzer (HCLD CAI 600), the heated flame ionization detector (HFID CAI 600) and the non-dispersive infra-red analyzer (NID CAI 602P) were employed for the measurements of NOx, HC and CO emissions, respectively. The particulate matter (PM) emissions were measured by a fast particulate analyzer (DMS500) with a frequency of 1 Hz for 5 min. Detailed information about the experimental system can be referred to Refs. [38,39]. In the present paper, the injection pressure was maintained at 60 MPa to protect the high pressure pump for blended fuels with high gasoline proportions. The engine speed was fixed at 1500 rpm, and no EGR was introduced for all the measurements. Initially, the engine was warmed up to reach the lubrication and coolant oil temperature to 85
C. Then all the experimental results were recorded under stable conditions and were averaged from 200 consecutive cycles for each condition. Detailed specifications of the engine can be found in Table 1.

3

Table 1 Engine specifications and operation conditions. Parameter

Value

Bore  Stroke (mm  mm) Displacement (L) Connecting rod (mm) Compression ratio Hole number Hole diameter (mm) Intake temperature (K)/pressure (bar) Intake valve closing (IVC) (
CA ATDC) Exhaust valve opening (EVO) (
CA ATDC) IMEP (bar)

114  130 1.352 216 18 7 0.176 293, 1.0 145 112 5, 8

2.2. The test fuels and properties The test fuels used in the present paper were the blended fuels of China 95# gasoline and the HCB with different volume ratios. The HCB fuel is produced from waste cooking oils through one-step catalyzed hydrogenation processes. The hydro-isomerization process with Pt catalyst was neglected to significantly reduce the production cost and at the same time to preserve the straight-chain structure and then high cetane number. The main species of HCB includes n-pentadecane, n-hexadecane, and n-heptadecane. Detailed physical and chemical properties of the gasoline and the HCB fuel used in the present study can be found in Table 2. The HCB concentrations used in the blended fuels were set as 20, 30 and 40% by volume, which is called as G80H20, G70H30, and G60H40, respectively, where the term “G” represents gasoline and “H” stands for HCB.

Exhaust Gas Analyzer

Air Filter

Inter Cooler

Smoke Analyzer

Turbo Charging

Combustion Analyzer

Exhaust Gas

Dynamo Meter

Intake Gas Common Rail Injection System Fig. 1. Diagram of the experimental setup.

Air Filter

Y. Zhang et al. / Energy 187 (2019) 115931

Table 2 Properties of gasoline and HCB.

Cetane number Density at 20
C (Kg/m3) Viscous at 20
C (mm2/s) Low heat value (MJ/Kg) Boiling Temp (
C) T10 T50 T90

Gasoline

HCB

15 700 0.7 43e45

100 780 4.7 44

59.7 107.3 160.2

e 303.0 315.0

3. Results and discussion 3.1. Combustion characteristics The in-cylinder pressure, AHRR, and combustion phasing (CA50) are important parameters which determine the engine performance. For pressure, the maximum pressure and maximum PRR will affect the engine durability and noise. For AHRR, too high AHRR leads to unacceptable combustion noise, while too low AHRR results in more combustion losses and then lower fuel economy. For CA50, too early CA50 leads to much high combustion noise and wall heat losses, while too retarded CA50 results in lower fuel economy. Therefore, the effect of HCB blended ratio on in-cylinder pressure, AHRR and CA50 will be given in this section. Effect of HCB blended ratio on the pressure and AHRR at different SOIs for IMEP ¼ 5 bar is shown in Fig. 2. It can be seen that the ignition timing is gradually advanced as the HCB proportion increases. Meanwhile, the AHRR peak decreases with increasing the HCB fraction. The above characteristics can be explained by the physical and chemical properties of the blended fuels. Firstly, the cetane number of the blended fuel increases as the HCB fraction increases, making the blended fuel more easy to compression ignition and then more advanced ignition timings. Secondly, due to the low volatility of the HCB fuel, the degree of fuel-air mixing is gradually reduced with the increase of HCB fraction, resulting in less premixed combustion. The above two aspects lead to more advanced ignition timing and lower AHRR peak as the HCB proportion increases. By retarding SOI to -7
CA ATDC, the comparison of pressure and AHRR for different blended fuels is illustrated in Fig. 2(b). Similar to the results as shown in Fig. 2(a), higher HCB blended ratio leads to more advanced ignition timing and lower AHRR. However, the difference between G80H20 and G70H30 becomes larger at retarded SOI compared to the results under advanced SOI conditions. It demonstrates that the blended fuel with higher gasoline

90

500

90

500

(b) SOI=-7°CA ATDC

G60H40 G70H30 G80H20

G60H40 G70H30 G80H20

300 200 30

Pressure (bar)

400

60

AHRR (J/°CA)

(a) SOI=-13°CA ATDC

Pressure (bar)

ratio is much more sensitive to the ambient temperature and pressure when injection occurs compared to the blended fuel with lower gasoline proportion. Furthermore, by increasing IMEP to 8 bar, the corresponding pressure and AHRR at different SOIs are given in Fig. 3. Besides the same general trends of pressure and AHRR with a variation of HCB blended ratio as shown in Fig. 2, it is interesting to find that the difference in the ignition timing for different blended fuels is reduced, while the difference in the AHRR peak is increased by comparing the results as shown in Figs. 2 and 3. With increasing the IMEP, more fuel is injected into the combustion chamber, more HCB leads to closer ignition timing, and more gasoline results in more premixed combustion and then higher AHRR. Detailed comparisons of ignition behaviors for different blended fuels are revealed in Fig. 4, in which CA50 and combustion duration (CD) are presented. As shown in Fig. 4, CA10, CA50 and CA90 are defined as the crank angles where the accumulative heat rates account for 10%, 50%, and 90% of the total accumulative heat rate, respectively. CD is defined as the duration between CA90 and CA10. From Fig. 4(a), it can be seen that the ignition timing is advanced and CD becomes longer as the HCB fraction increases. Longer CD can effectively reduce the AHRR peak, which is beneficial to extend to high loads. Considering the combustion stability, it can be seen that the difference in CA50 is relatively small at early SOIs compared to the results under late SOI conditions, as shown in Fig. 4(a). By increasing the IMEP to 8 bar, it can be observed from Fig. 4(b) that overall ignition timings at higher loads are little retarded compared to the results at lower loads. Furthermore, the variations of CA50 are much reduced with increasing the load, and also achieving more stable combustion at higher loads even under the later injection conditions. To show the effect of SOI and HCB blended ratio on CA50 clearly, Fig. 5 illustrates the relationship between CA50 and SOI for different blended fuels at different loads. It can be clearly observed from Fig. 5(a) that CA50 has a nearly linear relationship with the SOI. For different blended fuels, CA50 is generally advanced as the HCB fraction increases. Furthermore, the sensitivity of CA50 on SOI becomes weaker as the HCB fraction increases in the blended fuels. This indicates that higher HCB proportion can efficiently reduce the engine cycle-to-cycle variations. With increasing the IMEP to 8 bar, as displayed in Fig. 5(b), the differences in the CA50 for different blended fuels are reduced, especially for the G70H30 and G80H20. Moreover, it is interesting to find that CA50 is a little retarded as the HCB proportion increases, showing the opposite trend under low load conditions as shown in Fig. 5(a). Under higher load conditions, combustion efficiency for the blended fuel with lower HCB fraction is much improved, leading to more concentrated heat release and shorter

400

60 300 200 30

100 0

0 -10

0

10

20

30

Crank angle (°CA ATDC)

40

100 0

0 -5

0

5

10

15

20

25

30

Crank angle (°CA ATDC)

Fig. 2. Effect of HCB blended ratio on the pressure and AHRR at different SOIs for IMEP ¼ 5 bar.

35

40

AHRR (J/°CA)

4

Y. Zhang et al. / Energy 187 (2019) 115931

500

90

500

(b) SOI=-7°CA ATDC

G60H40 G70H30 G80H20

G60H40 G70H30 G80H20

60 300 200 30

Pressure (bar)

400

AHRR (J/°CA)

Pressure (bar)

(a) SOI=-13°CA ATDC

400

60 300 200 30

100 0

100

0 -10

0

10

20

30

AHRR (J/°CA)

90

5

0

0

40

0

Crank angle (°CA ATDC)

10

20

30

40

Crank angle (°CA ATDC)

Fig. 3. Effect of HCB blended ratio on the pressure and AHRR at different SOIs for IMEP ¼ 8 bar.

Fig. 4. Comparison of the ignition delay and CD for different blended fuels at different SOIs and loads.

CD. On the contrary, the heat release is more uniform for the blended fuel with higher HCB fraction (referred to the AHRR as shown in Fig. 3), thus the CA50 is slightly retarded as the HCB fraction increases. It also can be observed from Fig. 5(b) that the sensitivity of CA50 on SOI for G80H20 is much reduced, indicating that GCI can get excellent combustion stability under higher load conditions even employing the blended fuels with low HCB fractions.

3.2. Operation boundary characteristics As well known, the operation ranges of engines are mainly

limited to the maximum PRR and fuel economy, which has a close relationship with CA50. In order to explore the effect of HCB blended ratio on the operation ranges of GCI engines, the distributions of maximum PRR and indicated specific fuel consumption (ISFC) with CA50 for different blended fuels at different loads are shown in Figs. 6 and 7, respectively. The maximum PRR and ISFC boundaries as shown in Figs. 6 and 7 are predefined, which are referred from the similar studies of Li et al. [15,17,40]. It can be seen from Fig. 6 that the maximum PRR is gradually reduced when CA50 is retarded. By increasing the HCB fraction, the sensitivity of maximum PRR on CA50 is much reduced, which can effectively avoid a limitation of the PRR boundary, especially under higher load

15

20

(a) IMEP=5 bar

(b) IMEP=8 bar

CA50 (°CA ATDC)

CA50 (°CA ATDC)

18 12

9

6

G60H40 G70H30 G80H20

16 14 12 10 G60H40 G70H30 G80H20

8

3

6 -13

-11

-9

SOI (°CA ATDC)

-7

-5

-13

-11

-9

SOI (°CA ATDC)

Fig. 5. Relationship between CA50 and SOI for different blended fuels at different loads.

-7

-5

6

Y. Zhang et al. / Energy 187 (2019) 115931

25

25

(b) IMEP=8 bar

G60H40 G70H30 G80H20

20

Maximum PRR (bar/CA°)

Maximum PRR (bar/CA°)

(a) IMEP=5 bar

PRR Boundary

15 10

As S O I retard

5

ed

G60H40 G70H30 G80H20

20

PRR boundary

15 10 5

0

As SOI re

tarded

0 3

5

7

9

11

13

15

7

9

CA50 (°CA ATDC)

11

13

15

17

19

CA50 (°CA ATDC)

Fig. 6. Distribution of maximum PRR with CA50 for different blended fuels at different loads.

230

230

(a) IMEP=5 bar 220

G60H40 G70H30 G80H20

220

ISFC Boundary

ISFC (g/kWh)

ISFC (g/kWh)

(b) IMEP=8 bar

G60H40 G70H30 G80H20

210 200 190

ISFC Boundary 210

200

190

180

As SOI retarded 170

I retard As SO

ed

13

15

180 3

5

7

9

11

13

15

7

9

CA50 (°CA ATDC)

11

17

19

CA50 (°CA ATDC)

Fig. 7. Distribution of the ISFC with CA50 for different blended fuels at different loads.

higher HCB ratio results in worse fuel economy, which is easily limited to the ISFC boundary. However, on the other hand, lower HCB fraction leads to higher maximum PRR and is easily limited to the PRR boundary. Combing the results as shown in Figs. 5e8, it can be concluded that the HCB blended ratio should match CA50 controlled by SOI to obtain better engine performance and wider operation range. Overall, the characteristic of GCI combustion is remarkably affected by fuel properties. According to the above information, the impacts of fuel properties on the combustion characteristics of GCI engines fueled by gasoline-HCB blends are summarized in Fig. 9. Compared to the HCB fuel, gasoline has higher RON, which results in retarded CA50 and more unstable combustion. Meanwhile, owing to the faster combustion rate of gasoline than that of the HCB fuel, heavier knock and better fuel economy can be obtained for gasoline. On the other hand, HCB has much lower RON, leading to

conditions. Considering the fuel economy, it can be observed from Fig. 7 that fuel economy is gradually improved when CA50 is advanced. Under low load conditions, the fuel economy generally becomes better with the HCB fraction decreasing, except for G80H20. High gasoline proportion in G80H20 leads to low combustion efficiency at low loads. However, with increasing load, combustion efficiency for G80H20 is much improved, resulting in comparative performance to G70H30, as shown in Fig. 7(b). To illustrate the relationship between maximum PRR and fuel economy for different blended fuels clearly as shown in Figs. 6 and 7, Fig. 8 displays the distribution of maximum PRR and ISFC for different blended fuels at different loads. Fig. 8 clearly shows that the maximum PRR and fuel economy has obvious “trade-off” relationship, especially under high load conditions, which is consistent to the numerical studies of Li et al. [40,41]. On one hand,

(a) IMEP=5 bar

G60H40 G80H20 G70H30

200

190

G60H40 G70H30 G80H20

ISFC Boundary

210

200 PRR Boundary

PRR Boundary

ISFC (g/kWh)

ISFC Boundary

210

(b) IMEP=8 bar

220

ISFC (g/kWh)

220

190

180

180 3

6

9

12

15

Maximum PRR (bar/°CA)

18

3

6

9

12

15

18

Maximum PRR (bar/°CA) {Li, 2018 #854;Li, 2016 #570;Li, 2013 #878}

Fig. 8. Distribution of maximum PRR and ISFC for different blended fuels at different loads.

Y. Zhang et al. / Energy 187 (2019) 115931

3.3. Emission characteristics

Faster Combustion Rate

Higher RON

Retarded CA50

Larger PRR

Shorter CD

More Unstable Combustion

Heavier Knock

Better Fuel Economy

For Same CA50 More Stable Combustion

Lighter Knock

Worse Fuel Economy

Advanced CA50

Lower PRR

Longer CD

Lower RON

HCB

Slower Combustion Rate

Fig. 9. Impacts of fuel properties on the characteristics of GCI combustion.

advanced CA50 and more stable combustion. Moreover, lighter knock and worse fuel economy can result from lower PRR and longer CD under same CA50 condition for the HCB fuel owing to its lower combustion rate. Therefore, gasoline-HCB blends can effectively overcome the issues encountered by GCI engines fueled by pure gasoline. However, the blended ratio should be coupled with CA50 carefully, as discussed above.

1200

(a) IMEP=5 bar

G60H40 G70H30 G80H20

NOx (ppm)

1000

(b) IMEP=8 bar

G60H40 G70H30 G80H20

1000

NOx (ppm)

1200

Fig. 10 gives a detailed comparison of the gas emissions with the variation of SOIs at different loads. Under low load condition as shown in Fig. 10(a), it can be seen that NOx emissions have clear “trade-off” relationship with the CO and HC emissions at different SOIs. NOx emissions are significantly reduced when SOI is retarded, while the CO and HC emissions show the opposite trend. This is because with SOI retarding, the CA50 is gradually retarded, the movement of the piston in the expansion stroke will effectively reduce the in-cylinder temperature, resulting in lower NOx emissions while higher CO and HC emissions. Considering different blended fuels, it can be observed that all the gas emissions can be significantly suppressed as the HCB fraction increases. More HCB proportion can effectively extend CD, smooth the AHRR with improved combustion efficiency, and finally result in lower gas emissions. By increasing IMEP to 8 bar, it can be observed from Fig. 10(b) that the NOx emissions exhibit the similar trend to the results at low loads as shown in Fig. 10(a). However, the CO and HC emissions are reduced as SOI retards, showing opposite trends. CD at higher load is much increased compared to that at lower load, and the CA50 is retarded as the SOI retards. The above factors can effectively oxidize the CO and HC emissions during the combustion period. Moreover, it also can be seen from Fig. 10(b) that the CO and HC emissions for the blended fuels with lower HCB (e.g. G80H20) are much improved owing to the high combustion efficiency under higher load conditions. Concerning the PM emissions, Fig. 11 gives the comparison of PM emissions with the variation of SOIs at different loads. As can be clearly seen, the blended fuel with lower HCB fraction illustrates huge advantages in controlling the PM emissions. As the HCB fraction increases, the total number of PM emissions is much increased, especially at high load conditions. Higher HCB ratio will result in shorter ignition delay (see Fig. 4), worse fuel-air mixing, higher equivalence ratio, and then higher PM emissions. However,

800 600

800

600

400 200

800

400

800

600 600

CO (ppm)

400

400

200

150

200

150

0

HC (ppm)

HC (ppm)

125

100

100

50

75

50 -13

-11

-9

SOI (°CA ATDC)

-7

-5

0 -13

-11

-9

-7

SOI (°CA ATDC)

Fig. 10. Comparison of the gas emissions with variation of SOIs at different loads.

-5

CO (ppm)

Gasoline

7

8

Y. Zhang et al. / Energy 187 (2019) 115931 9E+07

4E+08

(b) IMEP=8 bar

G60H40 G70H30 G80H20

PM total number (N/cc)

PM total number (N/cc)

(a) IMEP=5 bar

6E+07

3E+07

0 -13

-11

-9

-7

3E+08

2E+08

1E+08 -13

-5

G60H40 G70H30 G80H20

-11

SOI (°CA ATDC)

-9

-7

-5

SOI (°CA ATDC)

Fig. 11. Comparison of the PM emissions with variation of SOIs under different load conditions.

(a) SOI=-13°CA ATDC

(b) SOI=-7°CA ATDC

8E+07

1.5E+08

IMEP=5 bar

6E+07

dN/dlogDp (N/cc)

G60H40 G70H30 G80H20

4E+07

2E+07

5E+07

8E+08

0

6E+08

4E+08

2E+08

1

10

2

10

3

6E+08

4E+08

2E+08

0 10

PM diameter (nm)

1

10

2

10

PM diameter (nm)

Fig. 12. Distribution of the PM diameter for different blended fuels under different conditions.

(a) NOx and PM

(b) CO and HC 110 G60H40 G70H30 G80H20

100

HC (ppm)

6E+07

4E+07

2E+07

90 80

IMEP=5 bar

0

3.5E+08

2.5E+08

2E+08

IMEP=8 bar 400

500

600

700

NOx (ppm)

800

900

1.5E+08 1000

110

60

PM total number (N/cc)

3E+08

300

G60H40 G70H30 G80H20

70

IMEP=5 bar

100 90 80 70 60

HC (ppm)

8E+07

50 40

IMEP=8 bar 50

150

250

350

450

550

30 650

CO (ppm)

Fig. 13. Distributions of NOx and PM emissions, and HC and CO emissions for different blended fuels at different loads.

3

0

dN/dlogDp (N/cc)

IMEP=8 bar

dN/dlogDp (N/cc)

IMEP=8 bar

10

G60H40 G70H30 G80H20

1E+08

8E+08

0

PM total number (N/cc)

dN/dlogDp (N/cc)

IMEP=5 bar

Y. Zhang et al. / Energy 187 (2019) 115931

it also can be seen from Fig. 11 that the differences in the PM emissions for different blended fuels are much reduced as SOI advanced, especially under higher load conditions. As SOI advanced, the injected fuels have more timing to mix with the air before auto-ignition owing to the relatively lower ambient temperature and pressure when injection occurs, leading to better fuelair mixing, and subsequently allowing the employment of blended fuel with higher HCB ratio without the limitation of PM emissions. Therefore, the SOI should be adjusted carefully to lower the harmful emissions simultaneously. Considering the detailed information about size distribution of the PM emissions, Fig. 12 shows the size distribution of PM emissions for different blended fuels under different conditions. As can be seen, the PM diameter exhibits a weak “double-peaks” characteristic under low load conditions. However, with the load increasing, the PM diameter shifts to a larger value and shows “single-peak” characteristic. Furthermore, it also can be observed from Fig. 12 that the peak diameter of PM emissions is much reduced for the blended fuel with lower HCB fraction, which can effectively prevent the formation of soot emissions. As a summary, distributions of NOx and PM emissions, and HC and CO emissions for different blended fuels at different loads are revealed in Fig. 13. For the NOx and PM emissions, it can be observed from Fig. 13(a) that the blended fuel with higher HCB ratio faces serious problem of higher PM emissions, while the blended fuel with lower HCB fraction is mainly limited to higher NOx emissions. Furthermore, it is interesting to find that the famous “trade-off” relationship between the NOx and PM emissions can be gradually broken up with decreasing the HCB fraction, especially under higher load conditions. Concerning the CO and HC emissions, it can be seen from Fig. 13(b) that it faces serious HC emissions problem at lower load, while CO emissions are more serious under higher load conditions. Furthermore, the blended fuel with high HCB fraction exhibits huge advantages in controlling the CO and HC emissions under different load conditions. 4. Conclusion In the present study, the HCB fuel with high reactivity prepared from waste cooking oil is blended into China 95# gasoline in different volume ratio, and the effects of different blended ratios on the combustion and emission characteristics of a GCI engine were explored. The following conclusions can be obtained: 1) As the HCB blended ratio increases, the ignition performance of blended fuel is significantly improved, the maximum combustion pressure is effectively suppressed, and the operation stability under low load conditions is much enhanced. 2) The sensitivity of combustion phasing on SOI decreases as the HCB proportion increases for the blended fuels. 3) The operation range of the blended fuels with higher HCB fraction is mainly limited to the fuel economy boundary, and the operation range of the blended fuels with lower HCB ratio is mainly controlled by the maximum PRR boundary. Furthermore, HCB blended ratio should match the combustion phasing controlled by SOI to obtain better engine performance and wider operation range. 4) PM diameter exhibits a weak “double-peaks” characteristic under low load conditions. However, with the load increasing, the PM diameter shifts to a larger value and shows “single-peak” characteristic. Furthermore, the peak diameter of PM emissions is much reduced for the blended fuel with lower HCB fraction. 5) The gas emissions of NOx, CO and HC can be significantly suppressed with the HCB fraction increasing. However, the PM emissions are increased slightly as a penalty.

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Declarations of interest None. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 51776088, 51706088 and 51876083), Natural Science Foundation of Jiangsu Province, China (Grant No. SBK2019040799), China Postdoctoral Science Foundation (Grant No. 2019M651733), Jiangsu Planned Projects for Postdoctoral Research Fund (Grant No. 2018K106C) and High-tech Research Key Laboratory of Zhenjiang (Grant No. SS2018002). References [1] Heywood JB. Internal combustion engine fundamentals. New York: Mcgrawhill; 1988. [2] Dec JE. Advanced compression-ignition enginesdunderstanding the incylinder processes. Proc Combust Inst 2009;32:2727e42. [3] Lu X, Han D, Huang Z. Fuel design and management for the control of advanced compression-ignition combustion modes. Prog Energy Combust 2011;37:741e83. _ [4] Calam A, Solmaz H, Yılmaz E, Içingür Y. Investigation of effect of compression ratio on combustion and exhaust emissions in A HCCI engine. Energy 2019;168:1208e16. rez FE, Sim J, Chang J, Im HG, [5] An Y, Jaasim M, Raman V, Hern andez Pe Johansson B. Homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) in compression ignition engine with low octane gasoline. Energy 2018;158:181e91. [6] Lu X, Qian Y, Yang Z, Han D, Ji J, Zhou X, Huang Z. Experimental study on compound HCCI (homogenous charge compression ignition) combustion fueled with gasoline and diesel blends. Energy 2014;64:707e18. [7] Yao M, Zheng Z, Liu H. Progress and recent trends in homogeneous charge compression ignition (HCCI) engines. Prog Energ Combust 2009;35(5): 398e437. [8] Onishi S, Jo S, Shoda K, Jo P, Kato S. Active thermo-atmosphere combustion (ATAC)-a new combustion process for internal combustion engines. SAE Technical Paper 1979:790501. [9] Xu G, Jia M, Li Y, Xie M, Su W. Multi-objective optimization of the combustion of a heavy-duty diesel engine with low temperature combustion under a wide load range: (I) Computational method and optimization results. Energy 2017;126:707e19. [10] Xu G, Jia M, Li Y, Xie M, Su W. Multi-objective optimization of the combustion of a heavy-duty diesel engine with low temperature combustion (LTC) under a wide load range: (II) Detailed parametric, energy, and exergy analysis. Energy 2017;139:247e61. [11] Jia M, Li Y, Xie M, Wang T. Numerical evaluation of the potential of late intake valve closing strategy for diesel PCCI (premixed charge compression ignition) engine in a wide speed and load range. Energy 2013;51:203e15. [12] Jia M, Xie M, Wang T, Peng Z. The effect of injection timing and intake valve close timing on performance and emissions of diesel PCCI engine with a full engine cycle CFD simulation. Appl Energy 2011;88(9):2967e75. [13] Lee SS. Investigation of two low emissions strategies for diesel engines: premixed charge compression ignition (PCCI) and stoichiometric combustion. PhD Thesis. University of Wisconsin-Madison; 2007. [14] Zheng Z, Xia M, Liu H, Wang X, Yao M. Experimental study on combustion and emissions of dual fuel RCCI mode fueled with biodiesel/n-butanol, biodiesel/ 2,5-dimethylfuran and biodiesel/ethanol. Energy 2018;148:824e38. [15] Li Y, Jia M, Chang Y, Xie M, Reitz RD. Towards a comprehensive understanding of the influence of fuel properties on the combustion characteristics of a RCCI (reactivity controlled compression ignition) engine. Energy 2016;99:69e82. [16] Qian Y, Wang X, Zhu L, Lu X. Experimental studies on combustion and emissions of RCCI (reactivity controlled compression ignition) with gasoline/ n-heptane and ethanol/n-heptane as fuels. Energy 2015;88:584e94. [17] Li Y, Jia M, Chang Y, Liu Y, Xie M, Wang T, Zhou L. Parametric study and optimization of a RCCI (reactivity controlled compression ignition) engine fueled with methanol and diesel. Energy 2014;65:319e32. [18] Kokjohn S, Hanson R, Splitter D, Reitz R. Fuel reactivity controlled compression ignition (RCCI): a pathway to controlled high-efficiency clean combustion. Int J Engine Res 2011;12(3):209e26. [19] Kavuri C, Paz J, Kokjohn SL. A comparison of reactivity controlled compression ignition (RCCI) and gasoline compression ignition (GCI) strategies at high load, low speed conditions. Energy Convers Manag 2016;127:324e41. €m H. Advantages of fuels with high resistance [20] Kalghatgi G, Risberg P, Ångstro to auto-ignition in late-injection, low temperature, compression ignition combustion. In: SAE Technical Paper; 2006. 2006-01-3385. [21] Zhao F, Asmus T, Assanis DN, Dec JE, Eng JA, Najt PM. Homogeneous charge compression ignition (HCCI) engines: key research and development issues. Warrendate, PA: Society of Automotive Engineers; 2003.

10

Y. Zhang et al. / Energy 187 (2019) 115931

[22] Cung K, Ciatti S. A study of injection strategy to achieve high load points for gasoline compression ignition (GCI) operation. In: Proceedings of the ASME; 2017 [Washington, USA]. [23] Adams C. Characterization of injection pressure effects on gasoline compression ignition combustion. PhD Thesis. University of Wisconsin-Madison; 2017. [24] Kalghatgi GT, Hildingsson L, Harrison AJ, Johansson B. Autoignition quality of gasoline fuels in partially premixed combustion in diesel engines. Proc Combust Inst 2011;33:3015e21. €m H. Partially pre-mixed auto-ignition of [25] Kalghatgi G, Risberg P, Ångstro gasoline to attain low smoke and low NOx at high load in a compression ignition engine and comparison with a diesel fuel. In: SAE Technical Paper; 2007. 2007-01-0006. [26] Liu J, Wang H, Zheng Z, Li L, Mao B, Xia M, Yao M. Improvement of high load performance in gasoline compression ignition engine with PODE and multiple-injection strategy. Fuel 2018;234:1459e68. [27] Huang H, Liu Q, Teng W, Pan M, Liu C, Wang Q. Improvement of combustion performance and emissions in diesel engines by fueling n-butanol/diesel/ PODE3e4 mixtures. Appl Energy 2018;227:38e48. [28] Liu J, Sun P, Huang H, Meng J, Yao X. Experimental investigation on performance, combustion and emission characteristics of a common-rail diesel engine fueled with polyoxymethylene dimethyl ethers-diesel blends. Appl Energy 2017;202:527e36. [29] Huang H, Zhou C, Liu Q, Wang Q, Wang X. An experimental study on the combustion and emission characteristics of a diesel engine under low temperature combustion of diesel/gasoline/n-butanol blends. Appl Energy 2016;170:219e31. [30] Mohamed Ismail H, Ng HK, Gan S, Lucchini T. Computational study of biodieselediesel fuel blends on emission characteristics for a light-duty diesel engine using OpenFOAM. Appl Energy 2013;111:827e41. [31] Dempsey AB, Curran SJ, Wagner RM. A perspective on the range of gasoline compression ignition combustion strategies for high engine efficiency and low NOx and soot emissions: effects of in-cylinder fuel stratification. Int J Engine

Res 2016;17(8):897e917. [32] Zhang F, Xu H, Rezaei SZ, Kalghatgi G, Shuai SJ. Combustion and emission characteristics of a PPCI engine fuelled with dieseline. In: SAE Technical Paper; 2012. 2012-01-1138. [33] Zheng Z, Yue L, Liu H, Zhu Y, Zhong X, Yao M. Effect of two-stage injection on combustion and emissions under high EGR rate on a diesel engine by fueling blends of diesel/gasoline, diesel/n-butanol, diesel/gasoline/n-butanol and pure diesel. Energy Convers Manag 2015;90:1e11. [34] Putrasari Y, Lim O. A study on combustion and emission of GCI engines fueled with gasoline-biodiesel blends. Fuel 2017;189:141e54. [35] Adams CA, Loeper P, Krieger R, Andrie MJ, Foster DE. Effects of biodieselegasoline blends on gasoline direct-injection compression ignition (GCI) combustion. Fuel 2013;111:784e90. [36] Xuan T, Cao J, He Z, Wang Q, Zhong W, Leng X, Li D, Shang W. A study of soot quantification in diesel flame with hydrogenated catalytic biodiesel in a constant volume combustion chamber. Energy 2018;145:691e9. [37] Zhong W, Xuan T, He Z, Wang Q, Li D, Zhang X, Huang YY. Experimental study of combustion and emission characteristics of diesel engine with diesel/ second-generation biodiesel blending fuels. Energy Convers Manag 2016;121:241e50. [38] Qian Y, Yu L, Li Z, Zhang Y, Xu L, Zhou Q, Han D, Lu X. A new methodology for diesel surrogate fuel formulation: bridging fuel fundamental properties and real engine combustion characteristics. Energy 2018;148:424e47. [39] Qian Y, Qiu Y, Zhang Y, Lu X. Effects of different aromatics blended with diesel on combustion and emission characteristics with a common rail diesel engine. Appl Therm Eng 2017;125:1530e8. [40] Li Y, Jia M, Chang Y, Xu Z, Xu G, Liu H, Wang T. Principle of determining the optimal operating parameters based on fuel properties and initial conditions for RCCI engines. Fuel 2018;216:284e95. [41] Li Y, Jia M, Chang Y, Fan W, Xie M, Wang T. Evaluation of the necessity of exhaust gas recirculation employment for a methanol/diesel reactivity controlled compression ignition engine operated at medium loads. Energy Convers Manag 2015;101:40e51.