Combustion characteristics of a compression-ignition ...

Applied Thermal Engineering 26 (2006) 327–337 www.elsevier.com/locate/apthermeng

Combustion characteristics of a compression-ignition engine fuelled with diesel–dimethoxy methane blends under various fuel injection advance angles Yi Ren, Zuohua Huang *, Deming Jiang, Liangxin Liu, Ke Zeng, Bing Liu, Xibin Wang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China Received 11 January 2005; accepted 19 July 2005 Available online 2 September 2005

Abstract The combustion characteristics and emissions under various fuel injection advance angles in a compression-ignition engine fuelled with diesel–dimethoxymethane blends were investigated. The results showed that the ignition delay, the rapid burning duration and the total combustion duration increase with the advancing of fuel injection angle for both diesel fuel and diesel–dimethoxymethane blends. At a specific fuel injection advance angle, the ignition delay increases and the heat release rate in the premixed burning phase increases with the increase of dimethoxymethane fraction in the blends. The center of the heat release curve moves close to the top-dead-center, and the brake specific fuel consumption (bsfc) decreases and the thermal efficiency increases with advancing the fuel injection advance angle. Maximum cylinder gas pressure increases with advancing the fuel injection advance angle, and the maximum cylinder gas pressure of the blends gives a higher value compared to that of diesel fuel. The maximum rate of pressure rise and the maximum rate of heat release increase with advancing the fuel injection advance angle. The exhaust smoke concentration increased and the exhaust NOx concentration decreased with decreasing the fuel injection advance angle for both diesel fuel and the blended fuels, and the behaviors were more obvious with decreasing the fuel injection advance angle. 2005 Elsevier Ltd. All rights reserved. Keywords: Combustion; Emissions; Fuel blends; Fuel delivery advance angle; Compression-ignition engine

1. Introduction The advantage of diesel engine compared to gasoline engine is the fuel economy benefits, while the high NOx and smoke emissions still remain the main obstacles for its increasing application due to the increasing concern in environmental protection and implementation of more stringent exhaust gas regulations. Thus, further reduction in engine emissions becomes one of major tasks in engine development. However, it is difficult to * Corresponding author. Address: School of Energy and Power Engineering, XiÕan Jiaotong University, XiÕan 710049, PeopleÕs Republic of China. Tel.: +86 29 82660456. E-mail address: [email protected] (Z. Huang).

1359-4311/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2005.07.009

simultaneously reduce NO and smoke in traditional diesel engine due to the trade-off relationship between NO and smoke. One promising approach to solve this problem is to use the oxygenated fuels or to add the oxygenated fuels in diesel to provide more oxygen during the combustion. In the application of pure oxygenated fuels, Fleisch et al. [1], Kapus et al. [2] and Sorenson et al. [3] have studied the dimethyl ether (DME) in the modified diesel engine, and their results showed that the engine could achieve ultra-low emission prospects without fundamental change in combustion systems. Huang et al. [4] investigated the combustion and emission characteristics in a compression-ignition engine with DME and found that the DME engine has high thermal efficiency, short premixed combustion and fast diffusion combustion

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Y. Ren et al. / Applied Thermal Engineering 26 (2006) 327–337

Nomenclature A wall area (m2) ATDC after top-dead-center beq bsfc equivalent to diesel fuel (g/kW h) bmep brake mean effective pressure (MPa) BTDC before top-dead-center bsfc brake specific fuel consumption (g/kW h) Cp constant pressure specific heat (kJ/kg K) Cv constant volume specific heat (kJ/kg K) hfd fuel delivery advance angle C wt.% mass fraction of carbon in fuel blend dp/du pressure rise with crank angle (MPa/CA) (dp/du)max maximum rate of pressure rise with crank angle (MPa/CA) dQB/du heat release rate with crank angle (kJ/CA) (dQB/d/)max maximum rate of heat release with crank angle (kJ/CA) dQW/du heat transfer rate with crank angle (kJ/CA) hc heat transfer coefficient (J/m2 s K)

duration, and their work was to realize low noise, smoke-free combustion. Kajitani et al. [5] studied the DME engine by delaying the injection timing to realize both smoke and NOx emissions.

Practically, adding some oxygenated compounds into diesel fuel to reduce engine emissions without modifying Table 3 Fuel properties of the diesel/DMM blended fuels Fuel blend #1

Table 1 Engine specifications Items

Value

Bore (mm) Stroke (mm) Displacement (cm3) Compression ratio Shape of combustion chamber

100 115 903 18 x shape in the bottom of bowl-in-piston 11 kW/2300 rpm 0.3 4

Rated power/speed Nozzle hole diameter (mm) Number of nozzle holes

lower heating value (MJ/kg) Hu H wt.% mass fraction of hydrogen in fuel blend m mass of cylinder gases (kg) O wt.% mass fraction of oxygen in fuel blend p cylinder gas pressure (MPa) pmax maximum cylinder gas pressure (MPa) R gas constant (J/kg K) T mean gas temperature (K) Tw wall temperature (K) TDC top-dead-center V cylinder volume (m3) uc crank angle of the center of heat release curve (CA degrees ATDC) ue crank angle of heat release ending (CA degrees ADTC) us crank angle of heat release beginning (CA degrees BTDC)

DMM in the blends (vol.%) DMM in the blends (wt.%) Lower heating value (MJ/kg) Heat of evaporation (kJ/kg) Cetane number C (wt.%) H (wt.%) O (wt.%)

Base fuel

Blended fuel

Type of fuels

Diesel

Chemical formula Mole weight (g) Density (g/cm3) Lower heating value (MJ/kg) Heat of evaporation (kJ/kg) Self-ignition temperature (C) Cetane number C (wt.%) H (wt.%) O (wt.%)

C10.8H18.7 148.3 0.86 42.5 260 200–220 45 86 14 0

Dimethoxy methane C3H10O2 32 0.865 22.4 318.6 430 30 47.4 10.5 42.1

Mass fraction of each fuel wt.%

Table 2 Fuel properties of diesel and dimethoxy methane

Fuel blend #2

Fuel blend #3

Fuel blend #4

5

10

15

20

5.14

10.27

15.38

20.47

41.47

40.44

39.41

38.38

303.7

347.3

44.23 84.02 13.82 2.16

390.7

43.46 82.04 13.64 4.32

434

42.69 80.06 13.46 6.48

41.93 78.1 13.28 8.62

100 80

DMM

60 diesel

40 20 0 fuel 1

fuel 2

fuel 3

fuel 4

Fig. 1. Mass fraction of the fuel blends.


the engine design seems to be a more attractive approach. Huang et al. tested the gasoline–oxygenate blends in a spark-ignited engine and got the satisfactory result on emission reduction [6], and they also investigated the combustion and emission characteristics of diesel/oxygenated additives blends in a compressionignition engine [7–9]. Murayama et al. [10] studied the emissions and combustion of diesel/dimethyl carbonate (DMC) blends with EGR. Ajav et al. [11] studied the diesel/ethanol blends for emission reduction and Huang et al. investigated the engine performance and emissions of diesel engine fuelled with diesel/methanol blends [12]. Miyamoto et al. [13] and Akasaka et al. [14] also conducted research on diesel combustion improvement and emission reduction by the use of various types of oxygenated fuel blends. In addition, Robert et al. [15] studied the exhaust emissions of

fuel 4 fuel 3

9

fuel 2 6

fuel 1

3

0 5

10

15

20

25

Volume fraction of DMM vol.% Fig. 2. Oxygen mass fraction in fuel blends.

200

160

200

(a) 1800 rpm bmep=0.14 MPa = -18 CA BTDC fd

diesel fuel 2 fuel 4

160

dQB/d J/CA

dQB/d J/CA

120

fuel 2

80

0

0

0

10

20

30

-40 -20

40


-10

(c) 1800 rpm bmep=0.14 MPa

160

fd

160

20

30

40

(d) 1800 rpm bmep=0.7 MPa fd= -22 CA BTDC

fuel 4

= -22 CA BTDC

120

dQB/d J/CA

120


fuel 4 80

40

0


80

40

0

-10

0

10

20

30

-40 -20

40

-10

Crank angle CA degree 200

fd

160

30

40

J/CA

bmep=0.7 MPa = -25 CA BTDC fd

fuel 4

120


80

dQB/d

80

20

(f) 1800 rpm


fuel 4

10

200

= -25 CA BTDC

120

0

Crank angle CA degree

(e) 1800 rpm bmep=0.14 MPa

160

40

0

-40 -20

10

200

200

dQB/d J/CA

0



-40 -20

(b) 1800 rpm bmep=0.7 MPa = -18 CA BTDC fd

80

40

-10

fuel 4

120

40

-40 -20

J/CA

0

dQB/d

Oxygen mass fraction wt.%

15

12

329

40

0

-10

0

10

20


30

40

-40 -20

-10

Fig. 3. Heat release rate of the blends.

0

10

20


30

40

330


Based on the authorsÕ previous study, the objectives of this study is to investigate the combustion and emission characteristics through heat release analysis of diesel/DMM blends under various fuel injection advance angles, and expect to increase understanding on combustion and emissions of diesel/DMM blends under various fuel injection advance angles.

heavy-duty diesel engine operated on several diesel/ oxygenated blends. Dimethoxy methane (DMM) has high oxygen fraction, relative high cetane number, better volatility and better solubility with diesel fuel, thus it is regarded as a better oxygenate additive for diesel/oxygenate blends. Some preliminary studies revealed that reduction of particulate emissions and toxic gas pollutants could be achieved when fuelled with the diesel/DMM blends [16,17]. However, those previous work just reported the exhaust emissions for a specific DMM addition but still lacked of information about the combustion characteristics which are very useful for interpretations of the behaviors of emission reduction fuelled with such blends. Meanwhile, the combustion and emissions of engine fuelled with diesel/DMM blends under various operating conditions such as fuel delivery advance angle are still unknown, where the combustion and emission characteristics under various operating conditions are very important for engine operation and evaluate the applicability of the blended fuel in diesel engine. In order to acquire a comprehensive evaluation for diesel/ DMM blends, many aspects still worth of investigating, especially in a quantitative level, where these quantitative parameters are expected to provide more information on engine combustion fuelled with the oxygenated fuels and provide more practical measures for combustion improvement and emission reduction.

2. Test engine and properties of fuel blends In this study, diesel fuel is the baseline fuel while dimethoxymethane (DMM) is used as the oxygenate additive. Four fractions of diesel–DMM blends were investigated in the study, and the volume fractions of DMM in the blended fuels are 5%, 10%, 15% and 20%, respectively. The specifications of the test engine are listed in Table 1. Fuel properties and fractions of four blends are given in Tables 2 and 3, as well as in Figs. 1 and 2, and the oxygen mass fraction in the fuel blends ranges from 2.16% to 8.62% as shown in Fig. 2 and Table 3. It can be found that DMM has high oxygen-content while the heat value and the value of cetane number are lower than those of pure diesel fuel. In the experiment, the above four fuel blends and pure diesel fuel were tested in a direct-injection diesel engine. Meanwhile the cylinder pressure and emissions under various fuel injection advance angles were measured 12

12

8

6

4 -28

8

6

4

-26

-24

-22

-20

-18

-16

-28

-14

12

-24

-22

-20

-18

-16

-14

12


diesel fuel 1 fuel 2 fuel 3 fuel 4


10

Ignition delay CA

10

Ignition delay CA

-26

Fuel delivery advance angle CA degree BTDC


8

6

4 -28

(b) 1800 rpm bmep=0.14 MPa


10

Ignition delay CA

10

Ignition delay CA

(a) 1200 rpm bmep=0.14 MPa


(d) 1800 rpm bmep=0.7 MPa

8

6

-26

-24

-22

-20

-18

-16


-14

4 -28

-26

-24

-22

-20

-18

-16


Fig. 4. Ignition delay of the blends versus fuel injection advance angles.

-14

Y. Ren et al. / Applied Thermal Engineering 26 (2006) 327–337 30

30

Rapid burn duration CA


27

24

21

18

-26

-24

-22

-20

-18

-16

21

18

15 -28

-14

-26

-24

-22

-20

-18

-16

-14

Fuel delivery advance angle CA degree BTDC 40

40


35

(c) 1200 rpm bmep=0.7 MPa Rapid burn duration CA



24


30

25

20 -28




27

15 -28

331

-26

-24

-22

-20

-18

-16

35

30

25

20 -28

-14



-26

-24

-22

-20

-18

-16

-14



Fig. 5. Rapid burn duration of the blends versus fuel injection advance angles.

40

Total combustion duration CA


40

35

30

25


20

15

10 -28

-26

-24


-22

-20

-18

-16

35

30

25

15

10 -28

-14




40

35

20 -28

-24

-22

-20

-18

-16

-14

55

45

25

-26



50

30


20


-26


-24

-22

-20

-18

-16


-14

50

45

40

35

30 -28


-26

(d) 1800 rpm bmep=0.7 MPa -24

-22

-20

-18

-16


Fig. 6. Total combustion duration of the blends versus fuel injection advance angles.

-14

12

10



8

6

4

2

0 -28

-26

-24

-22

-20

-18

-16

-14

Crank angle for centre of dQB /dϕ CA ATDC

Y. Ren et al. / Applied Thermal Engineering 26 (2006) 327–337 Crank angle for centre of dQB/dϕ CA ATDC

332

12

10

8

6

2

0 -28

8

6


0 -28

-26

-24

-22

-20

-18

-16

-14


Crank angle for centre of dQB /dϕ CA ATDC

Crank angle for centre of dQB/ dϕ CA ATDC


2

-26

-24

-22

-20

-18

-16

-14


12

4


4


10


12

10


8

6


4

2

0 -28

-26

-24

-22

-20

-18

-16

-14


Fig. 7. Crank angle for center of the heat release curve of the blends versus fuel injection advance angles.

and analyzed under the same brake mean effective pressure (bmep) and engine speed, and combustion analysis was taken based on the cylinder pressure information. Furthermore, comparisons in combustion and emissions were conducted among these blends to clarify the behaviors of engine fuelled with diesel–oxygenates blends under various fuel injection advance angles.

3. Results and analysis B Heat release rate dQ is calculated by using the followdu ing formula:

dQB C p dV C v V dp dQW þ þ ¼p ; R du du R du du

ð1Þ

where the heat transfer rate is given by dQW ¼ hc A ðT T W Þ. du

ð2Þ

Here, the heat transfer coefficient hc uses the WoschniÕs heat transfer coefficient [18]. The ignition delay is the time interval from the beginning timing of nozzle valve lifting to the beginning timing of rapid pressure rising; the rapid burning duration is the time interval from the beginning timing of rapid pressure rising to the timing of 90% accumulated heat release, the total combustion duration is the period from

the beginning timing of rapid pressure rising (it is regarded as the beginning time of the heat release) to the ending timing of heat release. The crank angle of the center of heat release curve is determined by the following formula: R ue dQB u du us du . uc ¼ R ue dQB du us du

ð3Þ

Fig. 3 gives the heat release rate of the diesel/DMM blends under three fuel injection advance angles. The results showed that for the same engine load (bmep), engine speed and fuel injection advance angle, the fraction of premixed burning phase increases with the increase of DMM mass fraction in fuel blends. It is suggested that the formation of fuel–oxygen mixture is promoted when using the oxygenated blends, the enrichment of oxygen and the increase in the fraction of combustible mixture prepared during the period of ignition delay as DMM will evaporate rapidly in high temperature environment. Meanwhile, the figure shows large difference in the early stage of combustion and relatively small difference in the late stage of combustion. High volatility and high oxygen-content of DMM will increase the amount of combustible mixture within the ignition delay period, subsequently increase the combus-


8




7

Pmax MPa

Pmax MPa

7


6

6

5 -28

333

-26

-24

-22

-20

-18

-16

5 -28

-14

-26

-24

-22

-20

-18

-16

-14



10

9

Pmax MPa

Pmax MPa

10 8

7

6 -28



-26

-24

-22

-20

-18

-16


9


-14


8 -28

-26

-24

-22

-20

-18

-16

-14


Fig. 8. pmax of the blends versus fuel injection advance angles.

tion rate of the premixed combustion phase and result in the increase in the maximum heat release rate with the increase of DMM mass fraction in the blends. Fig. 4 shows the ignition delay of the blends versus the fuel injection advance angles. It can be seen that the ignition delay will increase with advancing the fuel injection advance angle for both diesel fuel and diesel/ DMM blends. For a specific fuel injection advance angle, the ignition delay remains little change or shows a slight variation with the increase of DMM mass fraction in the blends. Two factors will influence the behavior of the ignition delay versus DMM addition, one is fuel evaporation phenomenon as high fraction of DMM blends will bring large temperature drop due to fuel evaporation, and this is favorable to the increase of ignition delay. Another is oxygen enrichment for the combustible mixture prepared during the ignition delay, as high DMM fraction in blends will provide more oxygen for the combustible mixture, and this is favorable to the decrease of ignition delay. The comprehensive result gives a slight variation of ignition delay under various fractions of DMM addition. In respect to the rapid burning duration as shown in Fig. 5, it was found that the rapid burning duration would increase with advancing the fuel injection advance angle for both diesel fuel and the diesel/DMM blends. Under the same fuel injection advance angle, the rapid burning duration gave a

slight variation with the DMM addition. The total combustion duration versus fuel injection advance angle is illustrated in Fig. 6. Both diesel fuel and diesel–DMM blends present a decrease trend with advancing the fuel injection advance angle. Under the same fuel injection advance angle, more fuel should be supplied for blended fuels in order to achieve the same engine load (bmep) due to the decrease of lower heating value of blended fuels with DMM addition. However, the total combustion duration did not give increasing trend but shows slight decreasing trend with the increase of DMM mass fraction in the blends, the enrichment of oxygen due to the oxygenated fuel addition will be beneficial to the improvement of diffusive combustion phase, subsequently decreasing the duration of diffusive combustion phase as well as decreasing the total combustion duration with the increase of DMM fraction in diesel fuel. Fig. 7 illustrates the crank angle for the center of the heat release curve uc versus fuel injection advance angles. The figure shows that uc moves close to the topdead-center with advancing the fuel injection advance angle, and this behavior is reasonable since advancing the fuel injection advance angle will increase the fraction of premixed combustion phase due to the long ignition delay and decrease the fraction of the subsequent diffusive combustion phase. Generally speaking, for a specific fuel injection advance angle, uc will decrease with

Y. Ren et al. / Applied Thermal Engineering 26 (2006) 327–337 1.4

1.4

1.2

1.2

1.0

1.0

(dp/dϕ)max MPa/CA

(dp/dϕ)max MPa/CA

334

0.8 0.6


0.4 0.2 0.0 -28


-26

-24

-22

-20

-18

-16

0.6 0.4 0.2 0.0 -28

-14

1.2

1.2

1.0

1.0

(dp/dϕ)max MPa/CA

(dp/dϕ)max MPa/CA

1.4

0.8

0.2


0.0 -28


-26

-24

-22

-20

-18

-16

-26

-24

-22

-20

-18

-16

-14


1.4

0.4


0.8


0.6


0.8 0.6

0.2

-14



0.4

0.0 -28

(d) 1800 rpm bmep=0.7 MPa -26

-24

-22

-20

-18

-16

-14


Fig. 9. (dp/du)max of the blends versus fuel injection advance angles.

the increase of DMM mass fraction. More oxygen and more evaporated DMM fuel will prepare more combustible mixture, and this contributes to the increase in the fraction of premixed combustion phase and decrease the fraction of the subsequent diffusive combustion phase. Meanwhile, high rate of heat release contributes to the increase in the fraction of the premixed combustion phase while short diffusive combustion duration contributes to the decrease in the fraction of diffusive combustion phase. The maximum cylinder gas pressure (pmax) versus fuel injection advance angles is plotted in Fig. 8. It can be seen that pmax increases with advancing the fuel injection advance angle, and this can also be explained by the high premixed burning rate due to long ignition delay and more combustible mixture prepared within the ignition delay period. Meanwhile, early injection timing will bring the premixed combustion phase more closing to the TDC, resulting in a high value of cylinder pressure. The results show that the value of pmax gave lowest value for diesel fuel under the same fuel injection advance angle, while pmax shows an increase with the increase of DMM mass fraction in the blends since the fraction of the premixed combustion phase increases with the increase of DMM mass fraction in the blends due to better volatility of DMM additive. Figs. 9 and 10 give the maximum rate of pressure rise (dp/du)max and the maximum rate of heat release (dQB/

du)max versus fuel injection advance angle. The figures reveal that (dp/du)max and (dQB/du)max will increase with advancing the fuel injection advance angle for both diesel fuel and diesel–DMM blends. Meanwhile, the results also show an increase in (dp/du)max and (dQB/du)max for the diesel–DMM blends. As being explained in the above sections, advancing the fuel injection advance angle will increase the fraction of the premixed combustion phase and the rate of heat release of the premixed combustion phase, making the heat release process close to the topdead-center, and bringing high cylinder pressure rise and high rate of heat release. For a specific fuel injection advance angle, (dp/du)max and (dQB/du)max will increase with the increase of the DMM mass fraction in the blends, and this would also be due to the increase in the fraction of the premixed combustion phase as more combustible mixture is available in the case of high DMM addition blends, and more mixture will join in the combustion process during the premixed combustion phase. The brake specific fuel consumption (bsfc) of fuel blends versus fuel injection advance angles is showed in Fig. 11. It can be seen that bsfc will decrease with advancing the fuel injection advance angle for both diesel fuel and the blended fuels due to the increase in the premixed combustion phase, the decrease in the total combustion duration, and closing of the center of heat release curve to the top-dead-centre as shown in Figs. 6 and 7. Under the same fuel injection advance angle,


300

120

(dQB/dϕ)max J/CA

(dQB/dϕ)max J/CA

250

200

150


100

50

0 -28


-26

-24

-22

-20

-18

-16

90

60

30

250

250

(dQB/dϕ)max J/CA

(dQB/dϕ)max J/CA

300

200

150


0 -28


-26

-24

-22

-20

-18

-16


-26

-24

-22

-20

-18

-16

-14


300

100



0 -28

-14


50

335

200

150

50

-14


100


0 -28

-26

-24

-22

-20

-18

-16

-14


Fig. 10. (dQB/du)max of the blends versus fuel injection advance angles.

300

bsfc g/kw.h

280

260


240

220

200 -26

1800 rpm bmep=0.7 MPa

-24

-22

-20

-18

-16


280

beq g/kw.h

bsfc shows an increase with the increase in DMM mass fraction in the blends. Due to the decrease of heating value of the blended fuel, more fuel should be injected into cylinder in order to get the same engine load (bmep). Fig. 12 gives the diesel-equivalent bsfc (beq), calculated by bsfc · (Hu)blended fuels/(Hu)diesel, and brake thermal efficiency of blended fuels versus fuel injection advance angles. As thermal efficiency is inversely proportional to the diesel-equivalent bsfc, as indicated in the relation 6 ge ¼ ðH u3:610 , they will reflect the same phenomenon in Þdiesel beq a different way. It can be see from the figure that the brake thermal efficiency will increase with advancing the fuel injection advance angle and this trend is obvious in the range of fuel injection advance angle from 22 CA degree BTDC to 18 CA degree BTDC. The increase in thermal efficiency with advancing the fuel injection advance angle is also related to the increase in the fraction of premixed combustion phase and decrease in the subsequent diffusive combustion phase. For a special fuel delivery advance angle, the thermal efficiency shows little variation between diesel fuel and the blended fuels. Fig. 13 shows the exhaust smoke concentration and exhaust NOx concentration of fuel blends versus fuel injection advance angles. The exhaust smoke concentration is scaled by the extinction coefficient K in the unit of m1. The results show that exhaust smoke concentration will increase with decreasing the fuel injection advance



260

240

220

200 -26

-24

-22

-20

-18

-16


Fig. 11. Brake specific fuel consumption of fuel blends and dieselequivalent bsfc of the fuel blends versus fuel injection advance angles.

336

Y. Ren et al. / Applied Thermal Engineering 26 (2006) 327–337 0.40

Thermal efficiency

1800 rpm bmep=0.7 MPa 0.35


0.30

0.25

0.20 -26

-24

-22

-20

-18

-16


Fig. 12. Diesel-equivalent bsfc and thermal efficiency of fuel blends versus fuel injection advance angles.

angle, and the decreasing trend is more obvious in the range of fuel delivery advance angle from 22 CA degree BTDC to 18 CA degree BTDC for both diesel fuel and the oxygenated fuels, and this revealed that decreasing the fuel injection advance angle will increase the fraction of diffusive combustion phase. Since the amount of smoke formation is strongly related to the fraction of diffusive combustion phase, the increase in the fraction of diffusive combustion phase will result in an increase in smoke formation. Addition of oxygenate in diesel fuel can provide more oxygen for combustion and improve combustion of the diffusive combustion phase. For a 1.0


0.8

Km

-1

0.6

0.4


0.2

0.0 -26

-25

-24

-23

-22

-21

-20

-19

-18

-17



2000

NOx ppm

1600

1200


800

400

0 -26

-25

-24

-23

-22

-21

-20

-19

-18

-17

special fuel injection advance angle, the exhaust smoke concentration will decrease with increase of DMM mass fraction in the blends. Exhaust NOx concentration shows decrease with decreasing the fuel injection advance angle, and this trend is more obvious at the late fuel injection advance angle due to the decrease in the fraction of premixed combustion phase. Under a same fuel injection advance angle, exhaust NOx concentration shows slight variation versus DMM addition. This behavior indicates that diesel–DMM blends can decrease engine exhaust smoke without increasing exhaust NOx emission. 4. Conclusions The combustion characteristics and emissions under various fuel injection advance angles fuelled with diesel–dimethoxymethane blends were investigated in a compression-ignition engine, and the main results are summarized as follows: 1. The fraction of premixed combustion phase increases and the fraction of the diffusive burning phase decreases with the increase of DMM mass fraction in the blends. 2. Advancing fuel injection advance angle results in the increase of the ignition delay, while the total combustion duration decreases due to the improvement of diffusive burning phase. The crank angle for the center of heat release curve moves close to the top-deadcenter, the diesel equivalent bsfc decreases and the brake thermal efficiency increases with advancing the fuel injection advance angle due to the increase in the fraction as well as the combustion rate of the premixed combustion phase and the decrease in the subsequent diffusive combustion phase with increase in the fraction of DMM addition. 3. The maximum cylinder pressure, the maximum rate of pressure rise and the maximum rate of heat release increase with advancing the fuel injection advance angle for both diesel fuel and diesel/DMM blends. 4. Exhaust smoke concentration increases and exhaust NOx concentration decreases with decreasing the fuel injection advance angle for both diesel fuel and the blended fuels, and these behaviors are more obvious at the late fuel injection advance angle. Diesel– DMM blends can realize the decrease in engine exhaust smoke without increasing exhaust NOx emission.

Acknowledgements


Fig. 13. Exhaust smoke concentration and exhaust NOx concentration of fuel blends versus fuel injection advance angles.

This study was supported by the National Basic Research Program (No. 2001CB209206 and No.


2001CB209208), the Ford-China Research and Development Fund (No. 50122166), the State Key Laboratory Awarding Fund (No. 50323001) and the Key Project of the National Natural Science Fund (No. 50136040). The authors acknowledge the teachers and students of XiÕan Jiaotong University for their help with the experiment. The authors also express their thanks to the colleagues of XiÕan Jiaotong University for their helpful comments and advice during the manuscript preparation. References [1] T. Fleisch, C. McCarthy, A. Basu, A new clean diesel technology: demonstration of ULEV emissions on a Navistar diesel engine fueled with dimethyl ether, SAE Transactions 104 (4) (1995) 42–53. [2] P. Kapus, H. Ofner, Development of fuel injection equipment and combustion system for DI diesels operated on dimethyl ether, SAE Transactions 104 (4) (1995) 54–69. [3] S.C. Sorenson, S.E. Mikkelsen, Performance and emissions of a 0.273 l direct injection diesel engine fueled with neat dimethyl ether, SAE Transactions 104 (4) (1995) 80–90. [4] Z.H. Huang, H.W. Wang, H.Y. Chen, Study on combustion characteristics of a compression ignition engine fueled with dimethyl ether, Proceedings of the Institution of Mechanical Engineers, Part D, Journal of Automobile Engineering 213 (D6) (1999) 647–652. [5] Z. Kajitani, L. Chen, M. Konno, Engine performance and exhaust characteristics of direct-injection diesel engine operated with DME, SAE Transactions 106 (4) (1997) 1568–1577. [6] Z. Huang, H. Miao, L. Zhou, D. Jiang, Combustion characteristics and hydrocarbon emissions of a spark ignition engine fuelled with gasoline–oxygenate blends, Proceedings of the Institution of Mechanical Engineers, Part D, Journal of Automobile Engineering 214 (D3) (2000) 341–346. [7] Z. Huang, H. Lu, D. Jiang, K. Zeng, B. Liu, J. Zhang, X. Wang, Combustion behaviors of a compression-ignition engine fuelled with diesel/methanol blends under various fuel delivery advance angles, Bioresource Technology 95 (2004) 331–341.

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Combustion characteristics of a compression-ignition ...

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