PODE3–4 mixtures

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Improvement of combustion performance and emissions in diesel engines by fueling n-butanol/diesel/PODE3–4 mixtures ⁎

Haozhong Huang , Qingsheng Liu, Wenwen Teng, Mingzhang Pan, Chang Liu, Qingxin Wang College of Mechanical Engineering, Guangxi University, Nanning 530004, China

H I G H L I G H T S effects of loads in CI engine fueled with n-butanol/diesel/PODE were studied. • The effects of PODE on particulate emission were studied for the first time. • The can improve efficiency and reduce emissions significantly. • PODE addition of PODE in n-butanol/diesel blend reduces CO and THC. • The • PODE can improve the accumulated particulate matters emission. 3–4

3–4

3–4

3–4

3–4

A R T I C L E I N F O

A B S T R A C T

Keywords: n-Butanol/diesel/PODE3–4 blends Diesel engine BMEP Combustion efficiency Emission characteristics

Polyoxymethylene dimethyl ethers (PODEn) are an excellent biofuel with no CeC bond and substantial sootreduction potential. The effects of BMEPs on the characteristics of combustion performance and emissions in a four-cylinder direct injection diesel engine with n-butanol/diesel/PODE3–4 blends were investigated. Mechanism of the PODE blends on soot reduction is discussed. The experimental results indicate that upon adding PODE3–4 to the blend of n-butanol with diesel can improve the thermal efficiency and combustion efficiency with an increment in the brake specific fuel consumption (BSFC). As the BMEP increased, a decreasing trend was observed in the emissions of soot, CO, and THC, while increasing NOx formation. Under a BMEP of 1.2 MPa, the soot emissions from the combustion of BD20, BDP10, and BDP20 reduced by 61.5%, 80.7, and 91.1%, respectively, compared to that from pure diesel. Under equal BMEP value, adding PODE3–4 to n-butanol/diesel blend reduced the soot, CO and THC emissions, and the lowest soot and THC emissions were found for BDP20, followed by BDP10, BD20, and D100. The number concentration of the accumulated particulate matter as well as the mass concentration of total particulate matters can be decreased by adding PODE3–4. The chemical kinetics simulation results reveal that CeO bonds break and CH2O is first produced in the pyrolysis of PODEn; as the value of n increases, more CH2O is produced and further oxidized to form HCO, which is finally transformed into CO and CO2, avoiding the production of soot precursors.

1. Introduction At present, controlling the emission from diesel engines is a matter of primary importance for environmental reasons. An effective way to pursue this goal is to add oxygen-containing compounds to the diesel fuel [1,2]. With the aggravation of the global energy crisis, the world is experiencing a change in energy-related structures. Energy is no longer associated with petroleum resources only, but it tends to be diversified. By changing the physical and chemical characteristics of fuels, the combustion performance can be improved, promoting complete combustion and reducing the pollutant emissions of diesel engines without

⁎

great modifications of their structure [3]. The component to be added should not only have a high cetane number value, but also show good mutual solubility with diesel, easy degradation and low cost; in addition, the raw materials should be easily accessible. Recently, a number of alternative fuels have been studied, including natural gas [4–7], alcohol fuels [8–10], dimethyl ether [11–13] and biodiesel [14–16]. Biodiesel, as a diesel engine fuel, has many advantages [17]. First, it is a non-toxic biodegradable alternative fuel, which can be obtained from renewable sources. Second, it has been reported that diesel–oxygenate blends can yield lower exhaust emissions, especially in terms of particle emission. Third, the addition of

Corresponding author at: College of Mechanical Engineering, Guangxi University, Daxuedong Road 100, Xixiangtang District, Nanning 530004, China. E-mail address: [email protected] (H. Huang).

http://dx.doi.org/10.1016/j.apenergy.2017.09.088 Received 11 January 2017; Received in revised form 12 September 2017; Accepted 13 September 2017 0306-2619/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Huang, H., Applied Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.09.088

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oxygenates in the mixture can decrease the amount of aromatic hydrocarbons and sulfur in the fuel. Therefore, it is essential to investigate the effect of the content of oxygenates on the emission and combustion performance of the fuel in a diesel engine. Among oxygenated fuels, n-butanol has a higher calorific value, cetane number and flash point with respect to methanol and ethanol, but a lower evaporation pressure and combustion temperature. In addition, it is more easily vaporized and is compatible with a larger range of air-fuel ratio values. It can also overcome the shortcomings of lowcarbon-content alcohols applied to diesel engines [18]. In recent years, a great deal of research has been conducted on the effects of the addition of n-butanol to diesel, in terms of combustion performance and exhaust emissions [19–24]. These studies indicated that n-butanol is a promising additive for diesel. Zheng et al. [25] experimentally studied the effects of blending n-butanol into diesel fuels on the combustion performance and emissions characteristics in a modified single-cylinder diesel engine under two-stage injection and high EGR rate conditions. Their results showed that n-butanol/diesel blended fuels can reduce soot emissions compared to diesel, at the expense of increasing the NOx emissions and the maximum pressure rise rate. Gerardo et al. found that the addition of n-butanol to diesel fuels improved soot and NOx emissions because of decreasing cetane number and increasing oxygen content [26]. In addition, with increasing amount of n-butanol in the fuel, the soot and CO emissions decreased, while the HC emissions increased [27]. However, other studies have demonstrated that a large amount of n-butanol is detrimental to the combustion performance, leading to a higher specific fuel consumption and a lower thermal efficiency because of the low cetane number and heat value of n-butanol [28]. Polyoxymethylene dimethyl ethers (PODEn), CH3O(CH2O)nCH3, are a new type of additive for diesel fuels. PODEn molecules show a favorable mutual solubility with diesel. In particular, PODE3–4, with an oxygen content of 46.98%, has a higher cetane number than diesel [29], exhibits good ignitability and high volatility, which can improve the formation of the mixture between fuel and air in the cylinder and promote complete combustion. The high cetane number of PODE3–4 can help overcome the low ignitability of n-butanol caused by its low cetane number. Previous research results indicate that PODE3–4 is, in fact, a promising additive in diesel fuel [29]. In recent years, some researchers have paid close attention to the application of PODE3–4 as an additive for diesel [30–33]. Xiao et al. [34] investigated the compatibility of PODE3–4/diesel mixtures with different proportions, finding that the mixtures with a content of PODE3–4 over 40% exhibited density stratification. Liu et al. [29] studied the effect of the addition of 10–20% PODE3–4 (in volume) to diesel on combustion performance and exhaust composition. Their results show that blending PODE3–4 with diesel can improve the engine efficiency and significantly reduce the emissions of harmful substances, especially soot. In 2016, Liu et al. [35] also studied the emission characteristics and thermal efficiency of diesel engines fueled with different mixtures of gasoline, diesel, and PODE3–4. Their results demonstrate that soot emissions are lower in the case of gasoline/diesel blends, and the addition of PODE3–4 can further decrease soot emissions, also increasing the combustion efficiency and thermal efficiency. Tong et al. [36] experimentally investigated the combustion characteristics of a gasoline/PODE mixture in a Reactivity Controlled Compression Ignition (RCCI) diesel engine. Their results indicate that the addition of PODE can improve thermal efficiency, decrease soot emission, and improve engine stability. A few studies concerning the addition of PODE into n-butanol/diesel blends are available [29]. From the detailed discussion above-mentioned, it is evident that PODE3–4 demonstrates a significant potential as an additive component for the optimization of the properties of n-butanol/diesel mixtures. In this study, D100 (pure diesel fuel), BD20 (with 20% n-butanol in diesel), BDP10 (obtained by adding 10% of PODE3–4 to BD20) and BDP20 (20% of PODE3–4 added to BD20) were prepared, and their

Table 1 Technical specification of test engine. Model

Specification

Number of cylinders Cylinder diameter (mm) Number of valves Stroke (mm) Displacement (L) Maximum torque (Nm) Compression ratio Rated power (kW)/Speed (r/min)

4 85 16 88.1 1.99 286 16.5 100/4000

combustion performance was compared in a four-cylinder direction injection diesel engine. This study aims to explore the potential advantages of obtaining high combustion efficiency and emission reduction by adding PODE3–4 to n-butanol/diesel blends, especially the effects of PODE3–4 addition on the particle characteristics (including PM size distribution, PM number concentration, and PM mass concentration), and the pyrolysis mechanisms of PODEn for soot reduction are also discussed in this study. 2. Experimental apparatus and procedures 2.1. Test engine and apparatus The test was conducted on a four-cylinder diesel engine. Table 1 lists the major parameters of the engine, and Fig. 1 illustrates the whole test system. The engine speed was maintained at 1600 rpm (corresponding to the maximum brake torque conditions) in this test. A pressure sensor (Kistler 6052CU20) was used to measure the cylinder pressure. The pressure was recorded for every increment of the crank angle, and 200 consecutive pressure cycles were measured and stored at each operating point. The INCA software was applied to control the fuel injection system. The engine intake pressure was 0.15 MPa, and the intake temperature was (30 ± 2) °C. The Exhaust Gas Recirculation (EGR) rate was controlled by the EGR valve; the EGR rate and exhaust gas emission were measured using a Horiba MEXA 7500DEGR; soot emission was measured using an AVL 415S system; and the particle emission was measured using a Cambustion DMS500. Table 2 lists the experimental uncertainties of the instruments. 2.2. Test fuels Diesel, purchased from the market in Nanning, China, n-butanol, and PODE3–4 were used as the base fuels. The detailed properties of the three base fuels are listed in Table 3. Four fuels were tested in this work. Pure diesel (denoted as D100) was used as a baseline comparison, and the other three fuels were obtained by blending diesel, n-butanol and PODE3–4 at different volume ratios. Specifically, for BD20, 20% of nbutanol was added to pure diesel; for BDP10, 10% of PODE3–4 was added to BD20; for BDP20, 20% of PODE3–4 was added to BD20. The fuel compatibility test results of BDP20 reveal that 20% PODE3–4 is successfully soluble in the BD blend fuel at 20 °C, without stratification occurring. The composition and cetane number value of these mixed fuels are listed in Table 4. D100 has the highest cetane number of 54, whereas BD20 has the lowest cetane number of 45.4. The addition of PODE3–4 significantly improves the cetane number, the values for BDP10 and BDP20 being 48.7 and 52, respectively, both of which are higher than that of BD20 and slightly lower than that of D100. 2.3. Operating conditions and test procedure Table 5 lists the engine operating conditions. Four loads were tested at an engine speed of 1600 rpm. The engine was operated in the mode 2

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Fig. 1. Schematic diagram of the experimental system.

1: Diesel fuel tank. 3: Fuel consumption monitor. 5: ECU. 7: ECU controller. 9: Eddy-current dynamometer. 11: Crank angle sensor. 13: Pressure sensor. 15: Heat exchanger. 17: Heat exchanger. 19: Cambustion DMS500. 21: Smoke meter.

2: Fuel filter. 4: High-pressure fuel pump. 6: Data acquisition. 8: Dyamometer controller. 10: Diesel engine. 12: Common-rail. 14: Direct injector. 16: EGR valve. 18: Air filter. 20: HORIBA MEXA7100DEGR. 22: Back pressure valve.

Table 2 Uncertainties and experimental measurement techniques/ instruments. Measurement

% Uncertainty

Measurement technique

EGR

± 0.5

Soot Load Speed Temperature Air flow rate Diesel fuel measurement N-butanol measurement Gas fuel measurement Pressure pickup Crank angle encoder

± 0.1 ± 0.2 ± 0.1 ± 0.15 ±1 ±1

Gaseous analyzer (HORIBA MEXA7100DEGR) Light transmittance method Strain gauge type load cell Magnetic pickup principle Thermocouple Orifice meter Volumetric measurement

±1 ±1 ± 0.1 ± 0.2

Volumetric measurement Volumetric measurement Magnetic pickup principle Magnetic pickup principle

Table 3 Properties of fuels. Diesela

n-Butanolb

PODE3–4c

Molecular formula

C12eC25

C4H10O

Cetane number Research octane number Oxygen content (%) Density (g mL−1) Low heat value (MJ kg−1) Boiling point (°C) Kinematic viscosity (m2 s−1@20 °C)

54 – – 0.82 42.8 180–360 4.8

– 96 21.62 0.81 33.2 117 3.64

CH3O(CH2O) nCH3 78.4 – 46.98 1.019 19.05 156.202 1.05

a b c

Properties of diesel are from ASTM D975. Properties of n-butanol are from Ref. [37]. Properties of PODE3–4 are from Ref. [31].

Table 4 Composition of blend fuels.

without EGR. The A/F (air/fuel) ratio decreased as the load increased. A combination of pilot injection and main injection was adopted as an injection strategy; the injection pressure varied with the application of different loads. This injection strategy was based on the strategy map of the original engine. The main injection duration was adjusted to achieve the same engine load. In the following discussion, the notation “A/F ratio” indicates the mass ratio of intake air to the injected fuel in one cycle. The engine underwent a warm-up process for 10 min before carrying out the tests. To avoid the effect of the difference of atmospheric humidity and temperature on measurements, all the experiments were performed on the same day. Each test was repeated three times for reducing experimental uncertainties and increasing the reliability of the test results.

Component volume percentage

D100 BD20 BDP10 BDP20 a

3

Cetane number

Diesel

N-butanol

PODE3–4

100 80 72 64

0 20 18 16

0 0 10 20

Estimated by Ref. [38].

54 45.4a 48.7a 52a

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injection stage and a lower in-cylinder temperature, which is not favorable for the ignition in the main-injection stage, eventually delaying the main-injection heat release. Under a relatively small BMEP of 0.4 MPa, as shown in Fig. 2(a), the pilot-injection fuel occupied a large portion of the total fuel quantity, and the heat release in the pilot-injection stage had significant effects on the heat release in the maininjection stage and was only slightly affected by the cetane number. Thus, after the addition of n-butanol with its low heat value, heat release in the main-injection stage is delayed. In addition, the heat value of PODE3–4 is the lowest among the investigated fuels (diesel, n-butanol, and PODE3–4); the addition of PODE3–4 induces further delay in the main-injection heat release of BDP10 and BDP20, allowing a more significant mixing between the air and fuel and increasing the premixed combustion ratio, thereby increasing the mean in-cylinder pressure and the peak value of the main-injection heat release rate. Under a BMEP of 1.2 MPa, the pilot-injection fuel quantity was relatively large, and the pilot-injection time was at least, as listed in Table 4; as a result, the pilot-injection fuel had more time to mix with the air in the cylinder. Besides, since the mixed fuel is more volatile, the air–fuel mixing favored, leading to a more uniform gas mixture and a higher premixed combustion ratio, thus increasing the in-cylinder pressure and the peak value of the pilot-injection heat release rate and the advance of the main-injection heat release time. As shown in Fig. 2(c), the combustion of these four fuels only involved the main-injection heat release stage. The initial heat release point during the combustion of D100 was the least, followed by BD20, BDP10, and BDP20. Similarly, the peak value of the heat release rate during the combustion of D100 was the largest, followed by those of BD20 and BDP10, with BDP20 exhibiting the lowest value. This is because of the fact that, under a large BMEP of 1.6 MPa, only one injection occurred (Table 4), and the in-cylinder combustion temperature was high enough to reach the ignition limit. However, because of the

Table 5 Operating conditions. D100/BD20/BDP10/BDP20 Pilot plus main injection

BrakeMean Effective Pressure (BMEP) (MPa) Intake pressure (MPa) A/F ratio Injection pressure (MPa) Pilot injection timing (BTDC) Pilot injection mass (mg/cycle) Main injection timing (BTDC)

0.4 0.14 42.3 62 14.7 1.52 −3.4

0.8 0.14 28.5 81.4 16 1.04 -2.7

1.2 0.14 23.5 107 17 1.52 1

1.6 0.14 21.0 115 – 0 2.4

3. Results and discussion 3.1. Effects of different BMEP on the combustion characteristics of blended fuels Fig. 2 displays the variations in in-cylinder pressure, heat release rate, and maximum pressure rise rate as a function of the crank angle during the combustion of the four tested fuels under different BMEPs. As shown in Fig. 2(a) and (b), the fuel injection included two stagespilot injection and main injection, leading to the presence of two different heat release stages in these two injection stages. Compared to the case of pure diesel, the times of pilot-injection heat release during the combustion of BD20, BDP10 and BDP20 delayed, and the main-injection heat release times were different under different loads. Under a BMEP of 0.4 MPa, the main-injection heat release during the combustion of D100 occurred at the earliest time, followed by the combustion of BD20 and BDP10, and finally by the combustion of BDP20. It is wellknown that the increase in the cetane number can lead to an anticipation of the ignition time; however, at a fixed pilot-injection fuel quantity, a lower heat value leads to a lower heat release in the pilot-

175 150

5

125

4

D100 BD20 BDP10 BDP20

3 2

100 75 50 25

1

0

10

Heat release rate(KJ/deg) Cylinder pressure(MPa)

Cylinder pressure(MPa)

BMEP:0.4MPa

6

0 -20

200

200

7

0

10

20

30

40

150 125 100

6

75 4 2

-10

0

4 2 -10

0

10

30

-25 40

20

30

3.25 D100 BD20 BDP10 BDP20

3.00

MPRR(bar/deg)

6

350 325 300 275 250 225 200 175 150 125 100 75 50 25 0 -25 40

Heat release rate(KJ/deg)

Cylinder pressure(MPa)

BMEP:1.6MPa

8

0 -20

20

0

(b)

14

10

10

25

Crank Angle(deg)

(a)

12

50

D100 BD20 BDP10 BDP20

Crank Angle(deg)

D100 BD20 BDP10 BDP20

175

8

0 -20

-25 -10

BMEP:1.2MPa

2.75 2.50 2.25 2.00 1.75 1.50 1.25 0.4

Crank Angle(deg)

0.8

1.2

BMEP(MPa)

(c)

(d)

Fig. 2. Cylinder pressure, heat release rate, and maximum pressure rise rate for four fuels at various loads.

4

1.6

Heat release rate(KJ/m3/deg)

Fuel Injection strategy

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32

22

CA50(deg)

21 20

Combustion duration(deg)

D100 BD20 BDP10 BDP20

19 18 17 16

D100 BD20 BDP10 BDP20

30

28

26

24

22 0.4

0.8

1.2

1.6

0.4

0.8

BMEP(MPa)

(a) CA50 of the four fuels under different loads

1.6

(b) Combustion duration of the four fuels under different loads

340

100.00

300

Combustion efficiency (%)

D100 BD20 BDP10 BDP20

320

BSFC(g/kW·h)

1.2

BMEP(MPa)

280 260 240 220 200

D100 BD20 BDP10 BDP20

99.98 99.96 99.94 99.92 99.90

0.4

0.8

1.2

1.6

0.4

0.8

BMEP(MPa)

1.2

1.6

BMEP(MPa)

(c) BSFCs of the four fuels under different loads

(d) Combustion efficiency of the four fuels under different loads

Excess air coefficient

3.5

D100 BD20 BDP10 BDP20

3.0

2.5

2.0

1.5

1.0 0.4

0.8

1.2

1.6

BMEP(MPa)

(e) Excess air coefficients of the four fuels under different loads Fig. 3. Combustion characteristics of the four fuels under different loads.

mixed fuel, the lower the value of CA50 and combustion duration. This may be attributed to the higher volatility and combustibility of PODE3–4 with respect to diesel and n-butanol. A higher volatility can in fact accelerate the mixing between fuel and air, and a higher combustibility can enhance the fuel combustion rate. Therefore, increasing the amount of PODE3–4 in the fuel can reduce the value of CA50 and the combustion duration. With increasing BMEP, the value of CA50 and combustion duration first decreased and then increased, after reaching a minimum at a BMEP of 0.8 MPa. Fig. 3(c) shows the variations in the brake specific fuel consumption (BSFC) as a function of BMEP during the combustion of the four fuels. With increasing BMEP, the values of BSFC for all the fuels initially

higher volatility of the mixed fuel, evaporation lowered the in-cylinder temperature and delayed the initial heat release point; at that time, the combustion occurred after the top dead center (TDC), and the piston moved downward increasing the in-cylinder volume. As a result, the incylinder pressure decreased, along with the peak value of the heat release rate and the maximum pressure rise rate. Fig. 3(a) and (b) show the variations in CA50 and combustion duration as a function of BMEP for the four investigated fuels. Both CA50 and the combustion duration decreased for the mixed fuels than that of pure diesel; after the addition of PODE3–4, the values of CA50 and the combustion duration of the mixed fuels further decreased. Moreover, the higher the ratio of CA50 and combustion duration in the 5

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decreased and then increased slightly. At a fixed BMEP value, the BSFC was higher for the blends containing PODE3–4. This variation trend can be explained by considering two aspects. The BSFC is inversely proportional to the brake thermal efficiency and is given by the product of the indicated thermal and mechanical efficiencies. The mechanical efficiency is given by the ratio of the average effective pressure to the average indicated pressure. Under a low BMEP, both the average mechanical loss pressure and mechanical efficiency are small, increasing the BSFC; as the BMEP increases, the mechanical efficiency increases and the BSFC decreases rapidly. Under a BMEP of 1.2 MPa, the BSFC reached to its minimum value. As the BMEP was further increased, since the excess air coefficient in the cylinder was smaller than 1.5 (Fig. 3(e)), the combustion deteriorated, leading to an increase in the BSFC. In contrast, the heat value of PODE3–4 is lower than those of diesel and n-butanol; thus, under the same conditions, more PODE3–4 should be consumed in order to generate the same power as in the combustion of diesel and n-butanol. Fig. 3(d) shows the trends in the combustion efficiency as a function of BMEP during the combustion of the four fuels. The trends are similar to one another, displaying a quick increase followed by a gradual flattening as the BMEP increases, and can be explained as follows. Under a small BMEP, the fuel injection quantity per cycle is low, the excess air coefficient is too large, the gas mixture is too thin, and the combustion temperature is low, all of which are unfavorable for the oxidation combustion of the fuel, thus decreasing the combustion efficiency; under a large BMEP, the excess air coefficient is too low and the locally too-thick regions expand, which is also detrimental to the combustion efficiency. As far as comparing the four fuels is concerned, the combustion efficiency was highest for BDP20, followed by BDP10 and BD20, all of which are higher than that of pure diesel. In contrast, 46.98% of oxygen in PODE3–4 can promote the fuel oxidation combustion; in contrast, as stated above, PODE3–4 shows high volatility and combustibility and can be better mixed with air, thus resulting in a short combustion duration (Fig. 3(b)) and a higher combustion efficiency.

combustion of D100 were the highest. The addition of n-butanol can provide a certain amount of oxygen for combustion, promoting the mixing between the fuel and air and reducing the content of aromatic hydrocarbons; the higher volatility and lower content of aromatic compounds of n-butanol can also reduce the soot formation [39,40]. Thus, soot emission from the combustion of BD20 was lower than that from the combustion of D100. A high oxygen content in the fuel can also reduce the local equivalent ratio and promote the oxidation of soot, contributing to the reduction of soot emission [41,42]. The addition of PODE3–4 further increases the fuel’s oxygen content; thus, soot emissions from the combustion of BDP10 and BDP20 further decreased compared to that from the combustion of BD20. As shown in Fig. 4(b), a higher amount of PODE3–4 in the blend significantly reduces soot emissions, especially under a large BMEP. Overall, under a BMEP of 0.4 MPa, the soot emissions from the combustion of BD20, BDP10, and BDP20 were reduced by 47.9%, 65.8%, and 81.9%, respectively, compared to those from the combustion of D100; under a BMEP of 1.2 MPa, the soot emissions from the combustion of BD20, BDP10, and BDP20 reduced by 61.5%, 80.7%, and 91.1%, respectively, with respect to pure diesel. These results indicate that the C–O bond in the PODE3–4 molecules is quite effective in inhibiting the soot formation during the combustion process. Fig. 5(a) shows the NOx emissions from the combustion as a function of BMEP for the four tested fuel blends. High temperature and oxygen enrichment are the main factors, leading to the formation of NOx. The NOx emissions increased with increasing BMEP for all the fuels, because of the fact that the increase in BMEP increases the injection fuel quantity per cycle and rises the in-cylinder combustion temperature, thus providing the high-temperature environment required for the formation of NOx. With regard to the mixed fuels, the oxygen in the molecules and the high volatility of the blend can expand the local oxygen enrichment regions in the cylinder. Under a low BMEP, the addition of PODE3–4 to the mixed fuel of n-butanol and diesel did not impose significant effects on NOx emissions. In those conditions, the excess air coefficient was too large (Fig. 3(e)), the environment was oxygen-rich, and the combustion temperature constituted the dominant factor. The effects of PODE3–4 addition on NOx emissions were also negligible at medium and high BMEP values, at high combustion temperature and additional oxygen contributed by PODE3–4, expanding the oxygen enrichment regions and promoting the formation of NOx. At a fixed BMEP value, NOx emissions from the combustion of BDP20 and D100 were the highest and the lowest, respectively. Under a BMEP of 1.2 MPa, the NOx emissions from the combustion of BDP20 increased by 40.4% and 32.3% compared to those from the combustion of D100. Fig. 5(b) and (c) show the CO and THC emissions from the combustion of the four fuels under different BMEPs. The CO emissions from

3.2. Effects of BMEP on the emission characteristics of blended fuels Fig. 4(a) shows the trend in the soot emission as a function of BMEP for the four tested fuels. It is worth to point out that soot formation can be compensated by reoxidation, and the measured soot emission is affected by these competing effects. The soot emissions from the combustion of all four fuels decreased with increasing BMEP. Under a small BMEP, the in-cylinder combustion temperature was low, which is unfavorable for the oxidation of soot, and thus, soot emissions were high. In addition, at a fixed BMEP value, the soot emissions from the 0.020

1.2MPa BMEP

80

Soot ration/%

Soot(g/kW·h)

0.015

0.4MPa BMEP

100

D100 BD20 BDP10 BDP20

0.010

47.9%

60

61.5%

65.8%

81.9% 80.7% 91.1%

40

0.005 20

0.000

0

0.4

0.8 1.2 BMEP(MPa)

BD20

1.6

(a) Soot emissions of the four fuels under different loads

BDP20

(b) The soot ratio of blended fuels to D100

Fig. 4. The soot emissions characteristics of the four fuels.

6

BDP10 Fuels

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3.00

D100 BD20 BDP10 BDP20

2.75

NOx(g/kW·h)

2.50 2.25 2.00 1.75 1.50 1.25 1.00 0.4

0.8

1.2

1.6

BMEP(MPa)

(a) NOx emissions 1.0

0.18

CO(g/kW·h)

0.8

0.6

0.4

D100 BD20 BDP10 BDP20

0.15

THC(g/kW·h)

D100 BD20 BDP10 BDP20

0.2

0.12 0.09 0.06 0.03 0.00

0.0

0.4

0.8

1.2

0.4

1.6

0.8

1.2

1.6

BMEP(MPa)

BMEP(MPa)

(c) THC emissions

(b) CO emissions

Fig. 5. Emissions from the four fuels under different loads.

PODE3–4, the Guassian software was employed to simulate and predict the pyrolysis mechanisms of PODEn. PODEn compounds are the synthetic product of methylal and methyl alcohol [31], and their physicochemical properties vary as a function of n. According to literature [45], the combustion-induced pyrolysis process of dimethyl ether, (CH3OCH3) includes four sub-processes, namely a dehydrogenation reaction, the decomposition of the produced CH3OCH2 into CH2O, the production of HCO from CH2O, and finally the production of CO and CO2. PODE is a kind of ether, presenting a molecular structure, which is similar to dimethyl ether. The Guassian software was adopted for investigating the pyrolysis mechanisms of the free radicals produced during the dehydrogenation of PODE3 molecules. The transition stages in several main decomposition reactions and the related reaction rates were explored for developing the detailed mechanisms in future studies. In order to explore whether a transition state exists during the reaction CH3O(CH2O)3CH2 → CH3O(CH2O)2CH2 + CH2O, we first used the ‘B3LYP/3-21g∗’ basis set in the Gaussian software to optimize the structure of the free radical CH3O(CH2O)3CH2 and then scanned the potential energy surfaces of the bonds to be broken in this radical for the prediction of transition states. The ‘UB3lYP/3-21g∗’ basis set was used in the scanning; then, the transition states produced in the pyrolysis reaction were calculated using the ‘UB3lYP/3-21g∗ opt = (ts,noeigentest,CalcFC) freq’ method; the activation energy, pre-exponential factor, and temperature coefficient were determined based on the optimized radical structure (i.e., the product), and the transition state structure was calculated, and finally, the reaction rate k was calculated. Using the above-described method, the main reactions R1, R2, R3, and R4 during the pyrolysis reaction of PODE3 were simulated (see Table 6), while the parameters of reactions R5–R7 were same as reported in the literature [45]. PODEn molecules do not have CeC bonds.

the combustion of all the four fuels decreased gradually with increasing BMEP. Under a small BMEP, the combustion temperature was generally low, and the CO emissions mainly came from the combustion in the cylinder low-temperature regions [43]; as the BMEP increased, the incylinder temperature increased and the CO emissions decreased. It can also be observed that CO emissions increased after the addition of nbutanol to pure diesel. This happens because n-butanol has a low cetane number and shows poor combustibility, and a larger portion of overthin gas mixture was formed before the combustion, thus the combustion temperature was low. All of these factors are unfavorable for the oxidation of CO. Furthermore, after the addition of PODE3–4 to the mixed fuel, CO emissions further reduced owing to the higher cetane number and better combustibility of PODE3–4. At fixed BMEP values, the CO emissions from the combustion of BD20 were the highest, followed by BDP10 and BDP20, all of which were higher than pure diesel. THC emissions show similar variation trends. Under a large BMEP, the in-cylinder temperatures were relatively high, promoting fuel atomization and evaporation, accelerating the mixing between fuels and air, thus contributing to sufficient combustion and the reduction of THC emissions. Compared to the results from the combustion of D100, the THC emissions from the combustion of mixed fuels were lower, especially after the addition of PODE3–4. This is due to the fact that, after the addition of PODE3–4 with its higher volatility, the nonuniform regions in the gas mixture decreased; the higher combustibility of PODE3–4 also promoted the combustion of the gas mixture [44]. 3.3. Exploration of the pyrolysis mechanisms of PODEn In order to gain a thorough understanding of the reasons for the significant decrease in the soot emissions deriving from the addition of 7

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induced by soot emissions and accounted for larger ratios in the energy losses of exhaust gases; the energy losses induced by heat transfer and friction in the engine were significant, and F.Work was relatively small. Under larger BMEPs (1.2–1.6 MPa), the energy losses induced by CO and THC emissions, as well as the heat transfer and friction in the engine decreased gradually with increasing BMEP, whereas F.Work increased. This is due to the fact that, under medium and small loads, the injection fuel quantity per cycle was small, and the average temperature in the cylinder was low and CO and THC emissions were significant (Fig. 5(b) and (c)), leading to significant energy losses. Although less heat was exchanged among the in-cylinder working medium, the internal wall of the cylinder liner, the top surface of the piston and the bottom of cylinder cover, the friction-induced energy losses increased, decreasing the F.Work. Fig. 7 shows that with increasing BMEP, the values of F.Soot, F.THC, and F.CO during the combustion of the four fuels decreased gradually. At a fixed BMEP value, the values of F.Soot and F.THC during the combustion of D100 were the maximum, followed by those of BD20, BDP10, and BDP20; the value of F.CO during the combustion of BD20 was the highest, followed by BDP10, BDP20, and pure diesel. The reasons that account for the variation in soot, THC and CO emissions can also explain the variation trends of F.Soot, F.THC, and F.CO. In addition, at a fixed BMEP, the value of F.Work during the combustion of BDP20 was the maximum, followed by BDP10, D100, and BD20; the value of F.Qthe during the combustion of BDP20 was the highest, followed by the values during the combustion of D100, BDP10, and BDP20. After the addition of PODE3–4, the viscosity of the mixed fuel decreased, the volatility was enhanced, and the air/fuel mixing was more effective, thus leading to a better combustibility and a shorter combustion duration (Fig. 3(b)); less heat was transferred from the working medium to the wall, and thus the performance ability was strengthened, giving rise to a higher brake thermal efficiency. Conclusively, the addition of PODE3–4 to the mixed fuel of n-butanol and diesel improved the in-cylinder combustion, further reduced the energy losses induced by emissions, friction and internal energy transformation among working media, and globally enhanced the brake thermal efficiency.

Table 6 The main reactions for PODE3 pyrolysis process. No

Reaction

A

n

Ea (kJ/ mol)

R1

CH3O(CH2O)3CH2 → CH3O(CH2O) 2CH2 + CH2O CH3O(CH2O)2CH2 → CH3OCH2OCH2+CH2O CH3OCH2OCH2 → CH3OCH2+CH2O CH3OCH2 → CH3+CH2O

5.892E10

0.85

78.15

3.161E11 1.545E11 2.037E10

0.94 0.93 1.33

87.82 79.28 85.27

R2 R3 R4

As shown in Fig. 6, during the decomposition process of PODEn, CH2O is produced by breaking the CeO bond; the amount of produced CH2O increases with n. CH2O is further oxidized to from HCO and finally transformed into CO and CO2. Thus, we may conclude that no precursors of soot are produced in the pyrolysis of PODEn, which can account for the observed decrease in the soot emissions. 3.4. Measured energy distribution for four fuels under different loads According to the energy balance law, in each cycle part of the fuel mass energy is dissipated for the emission of soot, THC, and CO, while the remaining part is released in the form of heat release during the fuel combustion. This includes the energy cycle brake work, heat loss due to the heat transfer, variations in the internal energy of the cycle charge, and friction loss. For a combustion process, the energy balance can be expressed by the following equation [46].

mf ·Hu = mfSoot ·HuC + mfTHC ·HuTHC + mfCO ·HuCO + Wb + Qthe

(1)

where mf, mf Soot, mf THC, and mf CO denote the cycle mass of the fuels, soot, THC and CO (in milligrams), respectively; Hu, Hu THC, and Hu CO denote the low heat value of the fuels, THC and CO (in joules per milligram); Hu Soot denotes the low heat value of soot (in joules per milligram). The low heating values of HC and the fuel are assumed to be equal. HuC denotes the heating value of the carbon atom, which is assumed as the heating value of soot [35]; Wb denotes the engine cycle brake work (in joules); Qthe includes the cycle heat transfer loss, the change of internal energy of the cycle cylinder charge (in joules), and the friction loss. In this study, the energy loss fraction of soot, THC and CO emissions, and the fraction of brake work are denoted as F.Soot, F.THC, F.CO, and F.Work, respectively; the fraction of Qthe is denoted as F.Qthe. The fraction of brake work, F.Work, is taken as the representative of brake thermal efficiency. The combustion efficiency was calculated by using the following equation [31]:

ηc = 1−(mfCO ∗HuCO + mfTHC ∗HuTHC )/(mf ∗Hu)

3.5. Effects of different BMEP on particulate matter (PM) emissions during the combustion of blended fuels According to their morphology, particulate matter can be classified into nuclear particle matters and accumulated particle matter. Specifically, the former term refers to particles with a diameter < 50 nm and mainly includes volatile components (such as hydrocarbons and sulfuric acid) and some fundamental carbon particles; the latter refers to the particles with a diameter ranging from 50 to 500 nm and mainly includes soot and soluble organic components and sulfates adsorbed therein.

(2)

As shown in Fig. 7, under small and medium BMEPs (0.4–0.8 MPa), the energy losses induced by CO and THC emissions far exceeded those

R1 CH3O(CH2O)3CH2

R2 CH3O(CH2O)2CH2

R3 CH3OCH2OCH2

Fig. 6. The prediction map of main pyrolysis process for PODE3.

CH3OCH2 R4

CH2O R5 HCO R6 CO R7 CO2 8

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Fig. 7. Energy distribution of cycle fuel mass for four fuels under different loads.

0.002 0.000

0.060 0.045 0.030 0.015 0.000

0.045

F.THC(%)

D100 BD20 BDP10 BDP20

0.004

0.030 0.015 45 40 35 30 25

0.000

F.Q the(%)

75

F.Work(%)

F.CO(%)

F.Soot(%)

0.006

70 65 60 0.4

0.8

1.2

1.6

6x107

4x107 D100 BD20 BDP10 BDP20

5x107 4x107

Accumulation mode number concentration(n/cc)

Nucleation mode number concentration(n/cc)

BMEP(MPa)

3x107 2x107 1x107

3x10

D100 BD20 BDP10 BDP20

7

2x107

1x107

0 0.4

0.8 1.2 BMEP(MPa)

0.4

1.6

0.8 1.2 BMEP(MPa)

1.6

Fig. 8. Nucleation mode number concentration of the four fuels under different loads.

Fig. 9. Accumulation mode number concentration of the four fuels under different loads.

Fig. 8 shows the number concentration of nuclear particulate matter emitted from the combustion of the four fuels under different BMEPs. With increasing BMEP, the number concentration of nuclear particulate matter from the combustion of D100 and BD20 first decreased and then increased; the number concentration of nuclear particulate matter from the combustion of BDP10 and BDP20 first decreased, then increased and finally decreased slightly. As the BMEP increased, the temperature rose, improving the atomization of fuel and the mixing performance, thus decreasing the emissions of THC and fine particles produced by nonuniform mixing, as well as the production of the precursors of nuclear particulate matter. At higher loads, the air-fuel ratio decreased, more carbon black was produced, and the fundamental carbon particles were not fully oxidized, thus increasing the number concentration of nuclear particulate matter. In addition, after the addition of PODE3–4, the mixed fuels became more volatile and more oxygen-rich, which contributed to the improvement of oxygen deficiency in the local overdense regions and sufficient combustion. Thus, the number concentration of nuclear particulate matter from the combustion of BDP10 and BDP20 decreased. It can also be observed that after the addition of PODE3–4 to BD20, the number concentration of nuclear particulate matter increased, suggesting that the addition of PODE3–4 can contribute to the oxidation of large particles at the end of combustion, thus reducing the emission of large particles and increasing the emission of small particles. Fig. 9 shows the number concentration of accumulated particulate matter from the combustion of the four fuels under different BMEPs.

With increasing BMEP, the number concentration of accumulated particulate matter decreased for all the tested fuels. As the BMEP was increased, the in-cylinder combustion temperature rose, which was favorable for the oxidation of large particulate matter. Since n-butanol has more favorable volatility than pure diesel, the content of multi-ring hydrocarbons in the mixed fuels after the addition of n-butanol decreased and less soot was produced (Fig. 4). Thus, the number concentration of accumulated particulate matter from the combustion of BD20 was smaller than that from the combustion of D100. Furthermore, the number concentration of the accumulated particulate matter from the combustion of mixed fuels decreased upon adding PODE3–4 to the blend, as a result of the higher oxygen content and better combustibility. Fig. 10 shows the size distribution of the number concentrations of accumulated particulate matter from the combustion of the four fuels under different BMEPs. The highest peak value of the number concentration of accumulated particulate matter was found for the combustion of D100, followed by BD20, BDP10, and BDP20. Under a small BMEP, the sizes of the peak number concentration of accumulated show significant differences among the four different fuels. After the addition of PODE3–4, the size of the peak number concentration decreased, as the number concentration of large particles was lower and the number concentration of small particles was higher. Figs. 11 and 12 show the number and mass concentrations of the total particulate matter from the combustion of the four fuels under different BMEPs. With increasing BMEP, the number concentration of 9

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Accumulation mode number concentration (dN/dlogDp/cc)

Accumulation mode number concentration (dN/dlogDp/cc)

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8x107 D100 BD20 BDP10 BDP20

7x107 6x107 5x107

BMEP: 0.4MPa

4x107 3x107 2x107 1x107 0 100

1000

2x107 1x107 0 10

100

(b)

BMEP: 1.2MPa

1.25x107 1.00x107 7.50x10

3x107

(a) D100 BD20 BDP10 BDP20

1.50x107

4x107

Dp/nm

2.00x107 1.75x107

BMEP: 0.8MPa D100 BD20 BDP10 BDP20

5x107

Dp/nm

6

5.00x106 2.50x106 0.00 10

100

1000

Accumulation mode number concentration (dN/dlogDp/cc)

Accumulation mode number concentration (dN/dlogDp/cc)

10

6x107

1000

1.50x107 D100 BD20 BDP10 BDP20

1.25x107 1.00x107

BMEP: 1.6MPa

7.50x106 5.00x106 2.50x106 0.00 10

100

Dp/nm

Dp/nm

(c)

(d)

1000

1x10

8

9x10

7

8x10

7

7x10

7

6x10

7

5x10

7

4x10

7

3x10

7

2x10

7

1x10

7

Total mass concentration(ug/cc)

Total number concentration(n/cc)

Fig. 10. Size distribution of the accumulation mode number concentrations of different fuels at different loads.

D100 BD20 BDP10 BDP20

0.020

D100 BD20 BDP10 BDP20

0.016

0.012

0.008

0.004

0.000 0.4

0.8

1.2

0.4

1.6

0.8

1.2

1.6

BMEP(MPa)

BMEP(MPa) Fig. 11. Total number concentration of the four fuels under different loads.

Fig. 12. Total mass concentration of the four fuels under different loads.

the total particulate matters from the combustion of D100 and BD20 first decreased and then increased, while from the combustion of BDP10 and BDP20 showed an oscillating behavior. The experimental curves are consistent with the trends found for nuclear particulate matter (Fig. 9). The mass concentration of the total particulate matter from the combustion of the four fuels decreased with increasing BMEP, because of the fact that the mass concentration of the total particulate matter depends on the number and size of particulate matter. With increasing BMEP, both the in-cylinder pressure and the combustion temperature increased, thus promoting the oxidation of soot (Fig. 4) and decreasing the number of large emitted particles. Accordingly, the mass concentration of the total particulate matter decreased with increasing BMEP, following almost the trend as the number concentration of the accumulated particulate matter. The mass concentration of particulate matter mainly depends on the number of accumulated particulate

matter; although their number concentration was lower, the accumulated particles occupied larger volumes and thus showed large mass ratios.

4. Conclusions PODE3–4 with a high cetane number and oxygen content is a promising additive for n-butanol/diesel blends. Detailed combustion and emission characteristics under various BMEP conditions were studied to achieve high efficiency and optimized emissions by using n-butanol/ diesel/PODE3–4 blends. The development of reaction mechanisms of the PODE blends on the soot emission reduction is discussed, providing valuable data for controllable practical applications. The following conclusions can be summarized based on the obtained results: 10

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1. Compared to D100, the pilot-injection heat release times during the combustion of BD20, BDP10, and BDP20 gradually delayed, while the main-injection heat release times varied with the applied BMEP. Under equal BMEP value, adding PODE3–4 to the n-butanol/diesel blend improved the brake thermal efficiency and the combustion efficiency. 2. With increasing BMEP, a decreasing trend was observed in the emissions of soot, CO, and THC, while increasing NOx formation. Under a BMEP of 1.2 MPa, the soot emissions from the combustion of BD20, BDP10, and BDP20 reduced by 61.5%, 80.7%, and 91.1%, respectively, compared to that of pure diesel. Under equal BMEP value, adding PODE3–4 to n-butanol/diesel blend reduced the soot, and CO and THC emissions, and the lowest soot and THC emissions were found for BDP20, followed by BDP10, BD20, and D100. 3. During the pyrolysis process of PODEn, CeO bonds are broken down to form CH2O. With increasing n, more CH2O is produced. CH2O is further oxidized to form HCO, which is finally transformed into CO and CO2, i.e., the production of soot precursors can be effectively avoided. 4. With increasing BMEP, the number concentration of accumulated particulate matter and the mass concentrations of total particulate matter decreased. Under an equal BMEP value, the addition of PODE3–4 to BD20 increased the number concentration of nuclear particulate matter, while decreasing the number concentration of the accumulated particulate matter as well as the mass concentration of total particulate matter. Acknowledgement The research is sponsored by projects of Natural Science Foundation of Guangxi (project Outstanding Young Scholarship Award, Grant No. 2014GXNSFGA118005), Natural Science Foundation of China (Grant No. 51076033), and Guangxi Science and Technology Development Plan (1598007-44 and 1598007-45). This research is financially supported by the project of outstanding young teachers' training in higher education institutions of Guangxi. References [1] Yupei, Zhao, Zheng, et al. Mechanism of chain propagation for the synthesis of polyoxymethylene dimethyl ethers. J Energy Chem 2013;22(6):833–6. [2] Park W, Park S, Reitz RD, et al. The effect of oxygenated fuel properties on diesel spray combustion and soot formation. Combust Flame 2016;180:276–83. [3] Patil AR, Taji SG. Effect of oxygenated fuel additive on diesel engine performance and emission: a review. IOSR J Mech Civil Eng 2013:30–5. [4] Amirante R, Distaso E, Iorio SD, et al. Effects of natural gas composition on performance and regulated, greenhouse gas and particulate emissions in spark-ignition engines. Energy Convers Manage 2017. [5] Yousefi A, Birouk M, Guo H, et al. An experimental and numerical study of the effect of diesel injection timing on natural gas/diesel dual-fuel combustion at low load. Fuel 2017;203. [6] Wang Z, Zhao Z, Wang D, et al. Impact of pilot diesel ignition mode on combustion and emissions characteristics of a diesel/natural gas dual fuel heavy-duty engine. Fuel 2016;167:248–56. [7] Zhang Q, Li M, Shao S. Combustion process and emissions of a heavy-duty engine fueled with directly injected natural gas and pilot diesel. Appl Energy 2015;157:217–28. [8] Atmanli A. Effects of a cetane improver on fuel properties and engine characteristics of a diesel engine fueled with the blends of diesel, hazelnut oil and higher carbon alcohol. Fuel 2016;172:209–17. [9] Jamrozik A. The effect of the alcohol content in the fuel mixture on the performance and emissions of a direct injection diesel engine fueled with diesel-methanol and dieselethanol blends. Energy Convers Manage 2017;148:461–76. [10] Kumar BR, Saravanan S. Use of higher alcohol biofuels in diesel engines: a review. Renew Sust Energy Rev 2016;60:84–115. [11] Wang Y, Liu H, Huang Z, et al. Study on combustion and emission of a dimethyl etherdiesel dual-fuel premixed charge compression ignition combustion engine with LPG (liquefied petroleum gas) as ignition inhibitor. Energy 2016;96:278–85. [12] Ji C, Shi L, Wang S, et al. Investigation on performance of a spark-ignition engine fueled with dimethyl ether and gasoline mixtures under idle and stoichiometric conditions. Energy 2017;126:335–42. [13] Wei Y, Wang K, Wang W, et al. Comparison study on the emission characteristics of diesel- and dimethyl ether-originated particulate matters. Appl Energy 2014;130(5):357–69.

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