Combustion and Emission Characteristics of a Compression

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Combustion and emission characteristics of a compressionignition engine fuelled with Dieseldimethoxy methane blends

Z.H. Huang a,*, Y. Ren a, D.M. Jiang b, L.X. Liu b, K. Zeng b, B. Liu b,X.B. Wang b

a State Key Laboratory of Multiphase Flow in Power Engineering, Xian Jiao Tong University, Xian, Peoples Republic of Chinab School of Energy and Power Engineering, XianNing West Road 28, Xian Jiao Tong University, Xian 710049, Peoples Republic of China

Received 24 August 2004; received in revised form 22 February 2005; accepted 25 August 2005Available online 6 October 2005

Abstract

The combustion and emission characteristics of a compression ignition engine fuelled with Dieseldimethoxy methane(DMM) blends were investigated. The results showed that the initial combustion duration experienced a slight variationwith the increase of DMM fraction in the fuel blends, while the rapid combustion duration and the total combustion dura-tion decreased with the increase of DMM fraction in the fuel blends, and the crank angle of the centre of heat release curvemoved closer to the top dead center. This would be due to the increase in the amount of combustible mixture for the initialpremixed combustion phase because of the fast evaporation of DMM and the promotion of the subsequent diffusive com-

bustion phase resulting from the oxygen enrichment. The maximum cylinder pressure and the maximum mean cylindertemperature showed a slight increase with the increase of DMM addition. The enrichment of oxygen by injecting the oxy-genate fuel is responsible for the combustion improvement. The maximum rate of pressure rise and the maximum rate ofheat release will increase with the increase of the DMM fraction in the fuel blend, and this would also be due to the increasein the amount of combustible mixture for the premixed combustion phase fuelled with the DieselDMM blends. A remark-able reduction in the exhaust CO and smoke can be achieved when operating on the DieselDMM blend. Flat NO xsmokecurves are presented when operating on the DieselDMM fuel blends, and a simultaneous reduction in both NOx andsmoke can be realised at large DMM additions. Thermal efficiency and NOx give the highest value at 2% oxygen massfraction (or 5% DMM volume fraction) for the combustion of DieselDMM blends. 2005 Elsevier Ltd. All rights reserved.

Keywords: Combustion; Emissions; Dieseldimethoxy methane blends; Compression ignition engine

1. Introduction

Reduction of engine emissions is a major research aspect in engine development with the increasing concernon environmental protection and the stringent exhaust gas regulations. It is difficult to reduce NOx and smoke

0196-8904/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.enconman.2005.08.020

* Corresponding author. Tel.: +86 292668726; fax: +86 292668789.E-mail address: [email protected] (Z.H. Huang).

Energy Conversion and Management 47 (2006) 14021415

www.elsevier.com/locate/enconman
mailto:[email protected]:[email protected]


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simultaneously in normal Diesel engines due to the trade off curve between NOx and smoke. One prospectivemethod to solve this problem is to use oxygenated alternative fuels or to add the oxygenated fuels in the Dieselfuel to provide more oxygen during combustion. In the application of pure oxygenated fuels, Fleisch et al. [1],Kapus and Ofner [2] and Sorenson and Mikkelsen [3] have studied dimethyl ether (DME) in a modified Dieselengine, and their results showed that the engine could achieve ultra-low emission prospects without a funda-mental change in the combustion systems. Huang et al. [4] investigated the combustion and emission charac-teristics in a compression ignition engine with DME and found that the DME engine has high thermalefficiency, short premixed combustion and fast diffusion combustion, and their work was to realize low noise,smoke free combustion. Kajitani et al. [5] studied the DME engine with delaying the injection time to reduceboth smoke and NOx.

Practically, adding some oxygenated compounds to fuels to reduce engine emissions without engine mod-ification seems a more attractive proposition. Huang et al. tested the gasoline-oxygenate blends in a spark igni-tion engine and got satisfactory results on emission reduction [6] and also investigated the combustion andemission characteristics of Diesel/dimethyl carbonate (DMC) in a compression ignition engine [7]. Murayama

et al. [8] studied the emissions and combustion with EGR (exhaust gas recirculation) and dimethyl carbonate

Nomenclature

A wall area (m2)ATDC after top dead centrebmep brake mean effective pressure (MPa)BTDC before top dead centreCp constant pressure specific heat (kJ/kgK)Cv constant volume specific heat (kJ/kgK)C (wt.%) mass fraction of carbon in fuel blenddp/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/du)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/m

2 s K)Hu lower heating value (MJ/kg)

H (wt.%) mass fraction of hydrogen in fuel blendm mass of cylinder gases (kg)O (wt.%) mass fraction of oxygen in fuel blendp cylinder gas pressure (MPa)pmax maximum cylinder gas pressure (MPa)R gas constant (J/kgK)T mean gas temperature (K)Tmax maximum mean gas temperature (K)Tw wall temperature (K)TDC top dead centreV cylinder volume (m3)

uc crank angle of centre 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)hfd fuel delivery advance angle (CA degrees BTDC)Duic initial combustion duration (CA)Durb rapid combustion duration (CA)Dutc total combustion duration (CA)

Z.H. Huang et al. / Energy Conversion and Management 47 (2006) 14021415 1403


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(DMC). Ajav et al. [9] studied Diesel/ethanol blends for emission reduction, and Huang et al. investigated theengine performance and emissions of a Diesel engine fuelled with Diesel/methanol blends [10]. Miyamoto et al.[11] and Akasaka and Sakurai [12] also conducted research on Diesel combustion improvement and emissionreduction by the use of various types of oxygenated fuel blends.

Dimethoxy methane (DMM) has a high oxygen fraction and relatively high cetane number, and this makes

it a better oxygenate additive for a Diesel/oxygenate blended fuel. Some preliminary studies revealed that thereduction of particulate emissions [13] and toxic gas pollutants could be achieved by fuelling with the Diesel/DMM blends [14]. However, those previous works just reported the emission results for a specific DMM addi-tion and did not give the results on combustion characteristics and/or heat release analysis under variousDMM additions. However, the combustion behaviors are directly linked to the engine performances andthe mechanism of emission reduction operating on various fractions of DieselDMM blends. In order to ac-quire a comprehensive evaluation for DieselDMM blends, many aspects are still worth investigating, espe-cially in a quantitative level. These quantitative parameters are expected to supply more information onengine combustion operating on oxygenated fuels and provide more practical measures for the improvementof combustion and reduction of emissions.

Based on the authors previous study, the objectives of this study will investigate the combustion charac-teristics based on the heat release analysis for different fractions of DMM addition in Diesel fuel and expect

to increase the understanding of the combustion parameters versus the DMM fraction and/or the oxygen frac-tion in the fuel blends of a compression ignition engine operated on DieselDMM blends.

2. Test engine and fuel properties

The specifications of the test engine are listed in Table 1. In the study, Diesel fuel is the base fuel, whileDMM is used as the oxygenated additive. Four fractions of the DieselDMM blends were designated forthe study, and the volume fraction of DMM in the fuel blends are 5%, 10%, 15% and 20%, respectively. Fuelproperties and the fraction of the four blends are given in Table 2, Table 3, Fig. 1 and Fig. 2, and the massfraction of oxygen in the fuel blends ranges from 2.16% to 8.62% as shown in Fig. 2 and Table 3. The fuelproperties show that DMM has a high oxygen content while the heat value and cetane number are low com-

pared to those of pure Diesel fuel. In the experiment, the above four fuel blends with different DMM fractionswere tested on the engine. Meanwhile, the engine exhaust emissions were analyzed under the same brake meaneffective pressure (bmep). The resolution for NOx measurement is 1 ppm, the resolution for CO is 0.01% andthe resolution for smoke measurement is 0.1 Bosch unit. Furthermore, these parameters were made in com-parison with those of pure Diesel combustion in order to clarify the effect of oxygenate additives on combus-tion. A Horiba exhaust gas analyzer was used to measure the combustion products.

Fig. 3 gives the fuel properties of the DieselDMM blends versus DMM volume in the blends. The figureshows that the cetane number decreases slightly with the increase of DMM addition due to the low value ofcetane number of dimethoxy methane, and this will probably lead to an increase in ignition delay of theDieselDMM fuel blends. The lower heating value of the blends decreases, and the heat value of evaporationincreases with the increase in DMM addition, the former factor requires more fuel to be supplied for obtainingthe same power output (bmep) and the latter would lead to a temperature drop of the cylinder gas due to theevaporation of the injected fuel.

Table 1Engine specifications

Bore (mm) 100Stroke (mm) 115Displacement (cm3) 903Compression ratio 18Shape of combustion chamber x shape in the bottom of bowl-in-pistonRated power (speed) 11 kW/2300 rpmNozzle hole diameter (mm) 0.3

Number of nozzle holes 4

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3. Instrumentation and method of calculation

The cylinder pressure was recorded by a piezoelectric transducer with the resolution of 10 Pa, and thedynamic top dead centre (TDC) was determined by motoring. The crank angle signal was obtained from

an angle generating device mounted on the main shaft. The signal of cylinder pressure was acquired for every

Table 2Fuel properties of Diesel and dimethoxy methane and blended fuel constitutions

Base fuel Blended fuel

Type of fuels Diesel Dimethoxy methaneChemical formula C10.8H18.7 C3H10O2Mole weight (g) 148.3 32

Density (g/cm3) 0.86 0.865Lower heating value (MJ/kg) 42.5 22.4Heat of evaporation (kJ/kg) 260 318.6Self-ignition temperature (C) 200220 430Cetane number 45 30C (wt.%) 86 47.4H (wt.%) 14 10.5O (wt.%) 0 42.1Fuel blend #1 (vol.%) 95 5Fuel blend #2 (vol.%) 90 10Fuel blend #3 (vol.%) 85 15Fuel blend #4 (vol.%) 80 20Fuel blend #1 (wt.%) 94.86 5.14

Fuel blend #2 (wt.%) 89.73 10.27Fuel blend #3 (wt.%) 84.62 15.38Fuel blend #4 (wt.%) 79.53 20.47

Table 3Fuel properties of the Diesel/DMM blended fuels

Fuel blend #1 Fuel blend #2 Fuel blend #3 Fuel blend #4

Lower heating value (MJ/kg) 41.47 40.44 39.41 38.38Heat of evaporation (kJ/kg) 303.7 347.3 390.7 434Cetane number 44.23 43.46 42.69 41.93C (wt.%) 84.02 82.04 80.06 78.1

H (wt.%) 13.82 13.64 13.46 13.28O (wt.%) 2.16 4.32 6.48 8.62

fuel 1 fuel 2 fuel 3 fuel 40

20

40

60

80

100

Mass

fractiono

feac

hfue

lw

t.%

diesel

DMM

Fig. 1. Mass fraction of the fuel blends.



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0.5 CA, and the acquisition process covered 254 completed cycles. The average value of these 254 cycles wasoutputted as the pressure data used for calculation of the combustion parameters. Bmep was calculated fromthe engine power, speed and configuration specifications.

A thermodynamic model is used to calculate the thermodynamic parameters in this paper. The model ne-glects the leakage through the piston rings [7], thus the energy conservation in the cylinder can be written asfollows:

dQB

du

dQW

du

dmu

du p

dV

du mCV

dT

du p

dV

du1


2

4

6

8

10

Oxygenmass

fractionwt.%

Fig. 2. Oxygen mass fraction in fuel blends.

0 5 10 15 20

DMM volume in fuel blends Vol.%

0

2

4

6

8

10

Oxygenmass

infue

lblen

ds

wt.%

(a)

0 5 10 15 20

DMM volume fraction in fuel blends Vol.%

35

40

45

50

Es

tima

tedce

tanenum

ber

(b)

0 5 10 15 20

DMM volume fraction in fuel blends Vol.%

20

30

40

50

60

Lower

hea

tva

lue

MJ/kg

(c)

0 5 10 15 20200

240

280

320

360

400

Hea

to

fevapora

tion

kJ/kg

(d) DMM volume fraction in fuel blends Vol.%

Fig. 3. Fuel properties of Diesel/DMM blends versus DMM volume in blends.



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The gas state equation is

pV mRT 2

The variation of the gas state equation with crank angle is given by

pdV

du V

dp

du mR

dT

du3

The heat release rate dQBdu

can be derived from Eqs. (1) and (3) as follows:

dQBdu

pCp

R

dV

duCVV

R

dp

du mT

dCVdu

dQWdu

4

where the heat transfer rate is given by

dQWdu

hc A T TW 5

The heat transfer coefficient hc uses the correlation formula given by Woschni [15]. Cp and CV are temper-ature dependent parameters whose formulae are given in Ref. [15].

The primary data is the cylinder pressure-crank angle data. Using these primary data and the above for-mulae, the maximum cylinder gas pressure pmax, the mean gas temperature T, the maximum mean gas tem-perature Tmax, the rate of pressure rise (dp/du), the rate of heat release (dQB/du), the maximum rate ofpressure rise (dp/du)max and the maximum rate of heat release (dQB/du)max can be calculated.

The initial combustion duration Duic is defined as the crank angle interval from the ignition start to thatwhere 10% of the heat release is reached; the rapid combustion duration Durb is defined as the crank angleinterval from the crank angle where 10% of the heat release has occurred to the crank angle where 90% ofthe heat release is reached; and the total combustion duration Dutc is the crank angle interval from the begin-ning of heat release (crank angle of heat release curve starts rising after fuel injection) to the ending of heatrelease (crank angle of heat release curve falls into zero). The premixed combustion phase refers to the com-bustion of the combustible mixture prepared within the ignition delay period, while the diffusive combustionphase refers to the subsequent mixing controlled combustion period. The crank angle of the centre of the heatrelease curve is determined by the following formula:

uc

Zueus

dQBdu

u du

Zueus

dQBdu

du

6

in which us is the crank angle at the beginning of heat release, and ue is the crank angle at the end of heatrelease.

4. Results and discussion

Fig. 4 gives the combustion duration of the DieselDMM blends versus the DMM volume fraction in thefuel blends under various engine loads (bmep). In the case of the initial combustion duration Duic, a slightvariation ofDuic versus the fraction of DMM addition is observed for the specific bmep at both high enginespeed (1600 rpm) and low engine speed (1200 rpm), see Fig. 4a and b. However, a different behavior ofDuicversus the engine load is shown at high and low engine speeds, that is, a slight difference is presented at lowengine speed (1200 rpm), while the initial combustion duration shows an increase with the increase of engineload at high engine speed (1600 rpm). The following interpretation can explain this behavior. With the increaseof engine speed, the amount of fuel for 10% of the heat release will increase, and this will subsequently increasethe duration to burn this fraction of fuel. On the other hand, a high load will increase the cylinder gas tem-perature, and this will supply the environment for increasing the combustion speed. The combined effects ofthese two factors would make the initial combustion duration increase with the increase of engine load. For a

given duration of time (ms), the duration in crank angle will increase with the increase of engine speed, and a



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relative large difference in the initial combustion duration under different engine loads is presented at the highengine speed. The results indicated that under the same bmep condition, the addition of DMM in the Dieselfuel had little influence on the initial combustion duration.

In the case of the rapid combustion duration Durc, a slight decrease ofDurc is observed with the increase ofDMM in the fuel blends at both high (1800 rpm) and low (1200 rpm) engine speeds, see Fig. 4c and d, whileDurc shows an increase with the increase of engine load. Increasing the DMM fraction in the fuel blend willincrease the oxygen fraction in the fuel blend, and this will contribute to improvement of the combustion,especially for improvement of the diffusive combustion phase where the enrichment of oxygen will promotethe combustion process, indicating a better advantage by oxygenate fuels in Diesel fuel. The long durationofDurc at high engine load would be due to the increase in fuel amount and the increase of the duration

for reaching 90% of the heat release.

0 2 4 6 8 10

Oxygen mass fraction in fuel blends wt.%

0

4

8

12

16

20

Initialcom

bus

tion

dura

tion

CA

1200 rpm

fd = 25 CA BTDC

fuel 1 fuel 2 fuel 3fuel 4

diesel

0 2 4 6 8 10


0

4

8

12

16

20

Initialcom

bus

tion

dura

tion

CA

1600 rpm

fd = 25 CA BTDC


diesel

0 2 4 6 8 10


0

5

10

15

20

25

30

Rap

idburn

ing

dura

tion

CA

(b)

(c)

(d)

(e)

(f)

1200 rpm

fd = 25 CA BTDC

bmep = 0.70 MPabmep = 0.56 MPabmep = 0.42 MPabmep = 0.28 MPabmep = 0.14 MPa

bmep = 0.70 MPabmep = 0.56 MPa

bmep = 0.42 MPabmep = 0.28 MPabmep = 0.14 MPa




diesel

0 2 4 6 8 10


0

5

10

15

20

25

30

Rap

idburnin

gdura

tion

CA

1600 rpm

fd = 25 CA BTDC


diesel

0 2 4 6 8 10


20

28

36

44

52

60

To

talcom

bus

tion

du

ration

CA

1200 rpm

fd = 25 CA BTDC

fuel 1 fuel 2 fuel 3fuel 4

diesel

0 2 4 6 8 10


20

28

36

44

52

60

To

talcom

bus

tion

dura

tion

CA

1600 rpm

= 25 CA BTDC


diesel

fd



(a)

Fig. 4. Combustion durations of the Diesel/DMM fuel blends.



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The total combustion duration Dutc versus DMM fraction in the fuel blend, Fig. 4e and f, exhibits a similarbehavior to that of the rapid combustion duration, the shortening of the rapid combustion duration is con-sidered to contribute to the decrease of the total combustion duration. Thus, the results indicate that addingthe oxygenate fuel in the Diesel fuel can shorten the combustion duration due to the oxygen enrichment, and itis beneficial to the increase of engine thermal efficiency.

Fig. 5 illustrates the crank angle of the centre of heat release curve uc for the DieselDMM fuel blends. Thefigure also shows the decrease ofuc with the increase of DMM addition. As explained above, the decrease incombustion duration and the improvement of the diffusive combustion phase will bring the heat release pro-cess closer to top dead centre.

The maximum cylinder pressure (pmax) and the maximum mean cylinder temperature (Tmax) versus DMMfraction in the fuel blend is plotted in Fig. 6. For a given engine load (bmep), the maximum cylinder pressureand the maximum mean gas temperature show a slight increase with an increase of the DMM addition. Theamount of blend will increase with the increase of the DMM fraction in the fuel blend in order to get the same-bmep, thus more combustible mixture will be available for the premixed combustion phase due to the low boil-ing point of the DMM. Moreover, the enrichment of oxygen by injecting the oxygenate fuel will also promotethe combustion. The results suggest that DieselDMM blends would be helpful for the reduction of engineexhaust CO and smoke, since it could increase the in cylinder gas temperature and oxygen enrichment.

Fig. 7 gives the maximum rate of pressure rise (dp/du)max and the maximum rate of heat release (dQB/du)maxversus the DMM fraction in the fuel blend. In the case of an engine speed of 1200 rpm, (dp/du)max and (dQB/du)max show an increase with the increase of DMM fraction in the fuel blend at all engine loads except for thelowest engine load (bmep = 0.14 MPa). The low temperature and cylinder swirl under the small engine loadis considered to be responsible for this behavior. In the case of an engine speed of 1600 rpm, (dp/du)max and(dQB/du)max show an increase with the increase of DMM fraction in the fuel blend, and little difference isobserved for the engine loads between bmep = 0.70 MPa and bmep = 0.28 MPa while a large difference is

0 2 4 6 8 100

4

8

12

16

20

c

CA

ATDC

1200 rpm

fd = 25 CA BTDC

fuel 1 fuel 2 fuel 3 fuel 4diesel

0 2 4 6 8 10



0

4

8

12

16

20

c

CA

ATDC

1600 rpm

fd

= 25 CA BTDC


diesel

(a)

(b)bmep = 0.70 MPabmep = 0.56 MPabmep = 0.42 MPabmep = 0.28 MPabmep = 0.14 MPa



Fig. 5. Crank angle of the centre of heat release curve for the Diesel/DMM fuel blends.



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observed between bmep = 0.14 MPa and the other loads. High cylinder swirl at high engine speed will increase

the amount of combustible mixture for the premixed combustion phase and result in the increase of (dp/du)maxand (dQB/du)max with the increase of the DMM addition.

The NOx concentration of Diesel/DMM blends under various brake mean effective pressures (bmep) isshown in Fig. 8. The behavior of NOx versus the oxygen mass fraction gives a similar trend under all engineloads and engine speeds, while NOx reaches its highest value at 2% oxygen mass fraction (or 5% DMM volumefraction), indicating the improvement of combustion at this fraction of DMM addition. NOx slightly decreaseswith the increase of DMM in the Diesel fuel, and the decrease in heating value and the increase in fuel injec-tion duration of the blends is contributed to the lowering of the NOx concentration. Generally speaking, for agiven bmep, NOx shows little variation with the addition of DMM in the Diesel fuel, and this is consistentwith the results obtained by Yeh et al. [16] whose work showed that little or no change in NOx emissionswas observed with the oxygenated fuels.

Fig. 9 shows the exhaust CO concentration of the Diesel/DMM blends and its reduction rate versus theoxygen mass fraction at various brake mean effective pressures (bmep). At all engine loads (bmep) and enginespeeds, the CO concentration shows a decrease with the increase of the oxygen mass fraction (DMM massfraction) of the Diesel/DMM fuel blends. A remarkable decrease in CO with an increase in oxygen mass frac-tion is presented at the high engine load, while a slight decrease is observed at the middle and low loads. Thissuggests that oxygen containing fuel blends could effectively decrease the locally rich spray regions where COis mainly formed, especially at the high engine load where a larger fraction of the locally rich spray region willexist. Moreover, the oxygen enrichment can also improve the post-flame oxidation of CO in the late engineexpansion and exhaust processes, and high load results in high cylinder temperature and, consequently, highdegree of CO oxidation. The results also revealed that 40% reduction of CO on average could be achievedwhen 10% of oxygen was available in the DieselDMM blends for both high engine speed (1600 rpm) andlow engine speed (1200 rpm), and the average CO reduction rate gives a linear reduction with the increase

of the oxygen mass fraction in the blends.

0 2 4 6 8 10


0

2

4

6

8

10

Pmax

MPa

1200 rpm

fd = 25 CA BTDC


diesel

0 2 4 6 8 10


0

2

4

6

8

10

Pmax

MPa

1600 rpm

fd = 25 CA BTDC

fd = 25 CA BTDC


diesel

0 2 4 6 8 10


0

500

1000

1500

2000

2500

3000

T

max

K

1200 rpm

fd = 25 CA BTDC


diesel

(a)

(b)

(c)

(d)

0 2 4 6 8 10


0

500

1000

1500

2000

2500

3000

Tmax

K

1600 rpm


dieselbmep = 0.70 MPabmep = 0.56 MPabmep = 0.42 MPabmep = 0.28 MPabmep = 0.14 MPa




Fig. 6. pmax and Tmax of the Diesel/DMM fuel blends.



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0 2 4 6 8 10


0

400

800

1200

1600

2000

NO

xppm

1200 rpmbmep = 0.70 MPabmep = 0.56 MPa


diesel

fuel 1 fuel 2 fuel 3

fd = -25 CA BTDC

fuel 4

0 2 4 6 8 10


0

400

800

1200

1600

2000

NO

xppm

1600 rpmbmep = 0.70 MPabmep = 0.56 MPabmep = 0.42 MPabmep = 0.28 MPabmep = 0.14 MPa

diesel

fuel 1 fuel 2 fuel 3

fd = -25 CA BTDC

fuel 4

(a)

(b)

Fig. 8. Exhaust NOx concentration of the fuel blends.

0 2 4 6 8 10


0

0.2

0.4

0.6

0.8

1

(dp

/d)max

MPa

/CA

1200 rpm

= 25 CA BTDC

fuel 1 fuel 2fuel 3

fuel 4

diesel

0 2 4 6 8 10


0

0.2

0.4

0.6

0.8

1

(dp

/d)

max

MPa

/CA

1600 rpm

= 25 CA BTDC

fuel 1 fuel 2fuel 3 fuel 4

diesel

0 2 4 6 8 10


0

40

80

120

160

200

(dQB/d)max

J/CA

1200 rpm

= 25 CA BTDC

fuel 1fuel 2

fuel 3 fuel 4

diesel

0 2 4 6 8 10


0

40

80

120

160

200

(dQB/

d)max

J/CA

1600 rpm

= 25 CA BTDC

fuel 1 fuel 2fuel 3 fuel 4

diesel

(a)

(b)

(c)

(d)

fd

fd

fd

fd





Fig. 7. (dp/du)max and (dQB/du)max of the Diesel/DMM fuel blends.



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The smoke and its reduction rate for the DieselDMM blends at various brake mean effective pressures(bmep) are illustrated in Fig. 10. The purpose of using the oxygen containing fuel blend is that it is expectedto decrease the engine smoke by supplying more oxygen to make it burn more completely. The resultsclearly showed that the engine smoke could be decreased remarkably with the addition of dimethoxy meth-ane in the Diesel fuel at all engine speeds and engine loads, and this is reasonable since the oxygen contain-ing fuel blends can reduce the rich spray regions and increase the post-flame oxidation of the formed soot.The results also showed 5060% reduction in smoke could be achieved when the oxygen mass fraction in theblends reached 10% by adding DMM in the Diesel fuel, and this is consistent with the results obtained byUllman et al. [17] whose study showed that smoke could be reduced by about 5% for each 1% increase inoxygenate addition. The reduction rate of smoke is more obvious at low engine speed. The combustionunder low swirl intensity depends largely on the airfuel ratio in the cylinder during combustion, and theoxygen enrichment will be beneficial to decrease the rich mixture region and decrease smoke formationin the cylinder. Furthermore, the long residence time during the engine expansion and exhaust processesat low engine speed will promote the post-flame oxidation of the combustion formed smoke and decreasethe exhaust smoke level.

Fig. 11 gives the exhaust smoke versus engine speed under high and middle loads. The results show that thebehavior of smoke versus engine speed for the Diesel/DMM blends showed a similar trend to that of Dieselcombustion, that is, exhaust smoke increases with the increase in engine speed. The short time for fuel evap-oration and mixing at high engine speed causes more rich mixture zones in the combustion chamber and in-creases the smoke formation during combustion, meanwhile, the short time of the expansion and exhaustprocesses at high engine speed restricts the oxidation of smoke in the post-flame process and gives a high ex-haust smoke level.

The relationship between NOx and smoke of the Diesel/DMM fuel blends at various brake mean effective

pressures (bmep) is plotted in Fig. 12. Unlike the engine operating on pure Diesel fuel, which has a trade off

0 2 4 6 8 10


0

0.2

0.4

0.6

0.8

CO

%

1200 rpm

diesel

fuel 1

fuel 2

fuel 3

fd

= -25 CA BTDC

fuel 4

0 2 4 6 8 10


0

0.2

0.4

0.6

0.8

CO%

1600 rpm

diesel

fuel 1

fuel 2 fuel 3

fd

= -25 CA BTDC

fuel 4

0 2 4 6 8 10 12 14


0

20

40

60

80

100

COre

duc

tionra

te%

1200 rpm

fd

= -25 CA BTDC

0 2 4 6 8 10 12 14


0

20

40

60

80

100

COre

duc

tion

rate%

1600 rpm

fd= -25 CA BTDC

(a)

(b)

(c)

(d) bmep = 0.70 MPabmep = 0.56 MPabmep = 0.42 MPabmep = 0.28 MPabmep = 0.14 MPa





bmep = 0.14 MPa


bmep = 0.42 MPa


Fig. 9. Exhaust CO concentration and its reduction rate of the fuel blends.



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0 2 4 6 8 10


0

1

2

3

4

5

1200 rpmbmep = 0.70 MPabmep = 0.56 MPabmep = 0.42 MPabmep = 0.28 MPabmep = 0.14 MPa

diesel

fuel 1

fuel 2

fuel 3

fd = -25 CA BTDC

fuel 4

0 2 4 6 8 10


0

2

4

6

8

Smo

ke

BSU

Smo

ke

BSU

1600 rpm


diesel

fuel 1

fuel 2fuel 3

fd

= -25 CA BTDC

fuel 4

0 2 4 6 8 10 12 14


0

20

40

60

80

100

Smo

kered

uc

tionra

te%


1200 rpm

fd = -25 CA BTDC

0 2 4 6 8 10 12 14


0

20

40

60

80

100

Smo

kere

duc

tionra

te%


1600 rpm

fd = -25 CA BTDC

(a)

(b)

(c)

(d)

Fig. 10. Engine exhaust smoke and its reduction rate of the fuel blends.

1000 1200 1400 1600 1800 2000

Engine speed rpm

0

2

4

6

8

SmokeBSU

(a)

bmep = 0.70 MPadiesel10% DMM in volume

fd = -25 CA BTDC

bmep = 0.42 MPa

1000 1200 1400 1600 1800 2000

Engine speed rpm

0

2

4

6

8

Smo

ke

BSU

(b)

bmep = 0.70 MPa

10% DMM in volume20% DMM in volume

fd

= -25 CA BTDC

bmep = 0.42 MPa

Fig. 11. Exhaust smoke versus engine speed for the fuel blends.



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behavior between NOx and smoke, a flat NOx/smoke trade off curve is presented when operating on theDiesel/DMM fuel blends. Even a reduction in NOx and smoke together is observed with a large additionof DMM in the Diesel fuel, and this means a simultaneous reduction in both smoke and NOx can be realisedwhen operating on the Diesel/DMM blends. Since the blends are oxygen containing fuels and have a hightolerance for exhaust gas recirculation, the combination of Diesel/DMM blends with exhaust gas recirculationcan make a further decrease of NOx without increasing the smoke emission.

5. Conclusions

The combustion and emission characteristics of a compression ignition engine fuelled with Dieseldimethoxy methane (DMM) blends were investigated, and the main results are summarized as follows:

1. The initial combustion duration shows a slight variation with the increase of the DMM fraction in the fuelblend, while the rapid combustion duration, the total combustion duration and the crank angle of the cen-tre of heat release curve decrease with the increase of the DMM fraction in the fuel blend due to theimprovement of the diffusive combustion phase.

2. The maximum cylinder pressure and the maximum mean cylinder temperature give a slight increase withthe increase of the DMM addition. The enrichment of oxygen by injecting the oxygenate fuel is responsiblefor the combustion improvement.

3. The maximum rate of pressure rise and the maximum rate of heat release will increase with the increase ofthe DMM fraction in the fuel blends. This would be due to the increase in the amount of combustible mix-

ture for the premixed combustion phase.

0 1 2 3 4 5

Smoke BSU

0

200

400

600

800

1000

NO

xppm

= - 25CABTDC

1200 rpm


DMM increase

0 1 2 3 4 5 6 7 8

Smoke BSU

0

200

400

600

800

1000

NO

xppm

= - 25CABTDC

1600 rpm


DMM increase

(a)

(b)

fd

fd

Fig. 12. Relationship between NOx and smoke of the fuel blends.



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4. A remarkable reduction in the exhaust CO and smoke is realised when operating on the Diesel/DMMblends. Flat NOx/smoke and thermal efficiency/smoke curves are presented when operating on theDiesel/DMM fuel blends and even a simultaneous reduction in both NOx and smoke can be realised atlarge DMM additions.

Acknowledgements

This study was supported by the National Basic Research Program (No. 2001CB209207 and No.2001CB209208), the Ford-China Research and Development Fund (No. 50122166), the State Key LaboratoryAwarding Fund (No. 50323001) and the Key Project of the National Natural Science Fund (No. 50136040).The authors acknowledge the students of Xian Jiao Tong University for their help with the experiment. Theauthors also express their thanks to the colleagues of Xi an Jiao Tong University for their helpful commentsand advice during the manuscript preparation.

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