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Fuel 87 (2008) 2691–2697

Combustion and emissions of a DI diesel enginefuelled with diesel-oxygenate blends

Yi Ren, Zuohua Huang *, Haiyan Miao, Yage Di, Deming Jiang, Ke Zeng,Bing Liu, Xibin Wang

State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University,

Xi’an 710049, People’s Republic of China

Received 15 June 2007; received in revised form 17 February 2008; accepted 20 February 2008Available online 14 March 2008

Abstract

Combustion and emissions of a DI diesel engine fuelled with diesel-oxygenate blends were investigated. The results show that thereexist the different behaviors in the combustion between the diesel-diglyme blends and the other five diesel-oxygenate blends as the dig-lyme has the higher cetane number than that of diesel fuel while the other five oxygenates have the lower cetane number than that ofdiesel fuel. The smoke concentration decreases regardless of the types of oxygenate additives, and the smoke decreases with the increaseof the oxygen mass fraction in the blends without increasing the NOx and engine thermal efficiency. The reduction of smoke is stronglyrelated to the oxygen-content of blends. CO and HC concentrations decrease with the increase of oxygen mass fraction in the blends.Unlike conventional diesel engines fueled with pure diesel fuel, engine operating on the diesel-oxygenate blends presents a flat NOx/Smoke tradeoff curve versus oxygen mass fraction.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Oxygenate; Combustion; Diesel engine

1. Introduction

With the more concerning for environmental protectionand human health, the increasingly stringent emissionsstandards are implemented all over the world. The seekingof new methods in reducing the harmful exhaust emissionsand improving the fuel economy of engines becomeurgently. The advantage of the diesel engine compared withthe gasoline engine is better fuel economy benefits and highpower output; however, the high NOx and smoke emissionsare still the main obstacle for the development of dieselengines. Thus, the reduction of engine emissions becomesone of major researches in the engine development.

In the conventional diesel engine, it is difficult to simul-taneously reduce NOx and smoke due to the tradeoff rela-tionship between NOx and smoke. For stringent emissions

0016-2361/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2008.02.017

* Corresponding author. Tel.: +86 29 82665075; fax: +86 29 82668789.E-mail address: zhhuang@mail.xjtu.edu.cn (Z. Huang).

regulation, the wide ranges of technologies including com-bustion improvement, fuel improvement and exhaust treat-ment should be taken, in which fuel improvement hasbecome one of the most promising approach in realizinglow emissions engine. Oxygenated fuels are beneficial tothe reduction of smoke or particulate matter emissions indiesel engines. Thus, using diesel-oxygenate blends candecrease the engine emissions without large modificationof the diesel engine and has a wide applicability in currentlyused vehicle.

In recent years, many experiments on oxygenated fuelsin the diesel engine were performed, and the researchesproved that smoke emissions from the engine could bereduced when oxygenated fuels were blended with dieselfuel [1–9]. Moreover, previous reports showed that com-paring with diesel fuel, the use of diesel-oxygenate blendsin the diesel engine did not bring the increase of NOx emis-sion [10–13]. Miyamoto et al. [9] found that the amount ofsmoke reduction depended on the total oxygen mass frac-

Notation

ATDC after top-dead-centerbmep brake mean effective pressure (MPa)BTDC before top-dead-centerbsfc brake specific fuel consumption (g/kW h)hfd fuel delivery advance angle (CA BTDC)C, wt.% mass fraction of carbon in fuel blenddQB/du heat release rate with crank angle (kJ/CA)

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 blendTDC top-dead-centeruc crank angle of the center of heat release curve

(CA degrees ATDC)

2692 Y. Ren et al. / Fuel 87 (2008) 2691–2697

tion in the blends, and species of each oxygenated fueladded into the diesel fuel had approximately the sameimpact in reducing the smoke emissions with the same totaloxygen-content of the blends. Adelbert [14] investigated theeffects of oxygenates blended with the diesel fuel on theparticulate matter (PM) emissions in a compression–igni-tion engine, and they found that PM reduction was onlycorrelated to the oxygen-content of the blends and PMwas reduced by about 3.5% for each 1% of fuel oxygenby mass. Westbrook et al. [15] used the detailed chemicalkinetic modeling to simulate soot reduction in diesel engineand their results showed that oxygenate species influencedthe smoke reduction level since the amount of soot precur-sor was varied for different oxygenate additives

In practice, comparing with gaseous oxygenates likeDME, adding some liquid oxygenate into diesel fuel inreducing engine emissions without modifying fuel systemseems to be a more attractive method. Up to now, reportson utilization of oxygenate additives in diesel engine giveindividual influence and lack the comprehensive study oncombustion and emissions of the diesel engine fuelled with

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 9.5 kW/2000 rpmNozzle hole diameter (mm) 0.3Number of nozzle holes 4

Table 2Fuel properties of diesel and oxygenic fuels

Diesel Dimethoxymethane Diglyme

Chemical formula C10.8H18.7 C3H8O2 C6H14O3

Molecular weight 148.3 76 134Theory air/fuel ratio kg/kg 14.36 7.24 11.24Density (g/cm�3) 0.83 0.85 0.94Cetane number 45 30 126Lower heating value (MJ/kg) 44 22.4 24.5Heat of evaporation (kJ/kg) 260 385 322Boiling point (�C) 180–330 43 161.3C (wt.%) 86 47.4 53.7H (wt.%) 14 10.5 10.5O (wt.%) 0 42.1 35.8

diesel-oxygenate blends. The general behaviors of oxygen-ate additives in engine combustion and emissions have notclarified yet, making it difficult to provide the guidance forengine operation and selection of oxygenates.

Based on the authors’ previous study [16–19], the objec-tives of this study are to investigate the general behavior incombustion and emissions of a compression–ignition engineoperating on various types of diesel-oxygenate blends. Thestudy provide the common insights on combustion andemissions operating on the diesel-oxygenate blends.

2. Experimental setup and procedures

In this study, diesel fuel is the base fuel while dim-ethoxymethane (DMM), diglyme (DGM), dimethyl car-bonate (DMC), diethyl carbonate (DEC), diethyl adipate(DEA) and ethanol are used as the oxygenic additive.The six selected oxygenated fuels can reflect ethers, estersand alcohols, respectively. For the structure of molecule,esters include the C–O2 moiety with one C@O structure,ethers have the C2–O moiety, and alcohols have the C–OH moiety. These oxygenates were individually blendedwith pure diesel fuel to make the diesel-oxygenated blends,and the volume fractions of oxygenates in the blends werefrom 0% to 20%. These blended fuels and pure diesel fuelwere tested in a direct-injection diesel engine. For the testengine, the original fuel delivery advance angle is 25 CABTDC, and engine speed at maximum torque is reachedat 1400 rpm. The detailed specifications of the test engineare listed in Table 1, and fuel properties are listed in Table2. The tested diesel fuel was provided by China Petroleum

Diethyl adipate Dimethyl carbonate Diethyl carbonate Ethanol

C10H18O4 C3H6O3 C5H10O3 C2H6O202 90 118 4613.25 4.59 8.79 12.151.00 1.06 0.97 0.79– 35 – 825.5 13.5 21.1 26.78295.1 369 360.4 854�904127 90.9 126.8 78.459.4 53.3 50.8 52.28.9 6.7 8.5 1331.7 40 40.7 34.8

Fig. 1. Heat release rate of the blends.

Fig. 2. Ignition delay of the blends.

Y. Ren et al. / Fuel 87 (2008) 2691–2697 2693

and Chemical Corporation and the solidifying point of die-sel fuel is 273.15 K. It should be noted that the oxygenatefuels have the lower energy density compared with thatof pure diesel fuel as shown in Table 2.

In the experiment, the beginning timing of nozzle valvelifting was measured by the needle lift detecting apparatus;AVL Di-smoke was used to measure the exhaust smoke,and the exhaust NOx was measured by AVL Di-Gas4000 light. The cylinder pressure was recorded with theKistler type cylinder pressure sensor, and the data wererecorded for every 0.1 crank angle. The crank angle wasmeasured with the Kistler type crank angle sensor. Mean-while, the cylinder pressure and emissions were measuredand analyzed under the same brake mean effective pressure(bmep) and engine speed, and combustion analysis wasmade based on the cylinder pressure information.

3. Results and discussion

3.1. Combustion characteristics

The ignition delay is defined as the time interval fromthe beginning timing of nozzle valve lifting (i.e. the startof fuel injection) to the separation timing between rate ofcylinder pressure rise and rate of motoring pressure rise(it is regarded as the start of combustion). The total com-bustion duration is the duration from the start of combus-tion to the end of combustion.

Thermodynamic model is used to calculate the thermo-dynamic parameters in this paper, the model neglects theleakage through the piston rings, and thus the energy con-servation in cylinder is written as follows:

dQB

du� dQW

du¼ dðmuÞ

duþ p

dVdu

¼ mCVdTduþ mT

dCV

duþ p

dVdu

ð1Þ

Gas state equation is

pV ¼ mRT ð2ÞThe variation of gas state equation with crank angle is

given by:

pdVduþ V

dpdu¼ mR

dTdu

ð3Þ

Heat release rate dQB

du can be derived from formula (1) and(3) as follows:

dQB

du¼ p � Cp

RdVduþ CV V

Rdpduþ mT

dCV

duþ dQW

duð4Þ

where heat transfer rate is given by

dQW

du¼ hc � A � ðT � T WÞ ð5Þ

Heat transfer coefficient hc uses the Woschni’s correla-tion formula, and Cp and CV are temperature-dependentparameters, their formulae are given in literature [20].

Fig. 1 shows the heat release rate of the blended fuels,respectively. Due to the variation of fuel properties of theblended fuels with the addition of oxygenates, the heatrelease rate of the diesel-oxygenated blends gives differentbehaviors compared to those of pure diesel fuel. The figuresreveal that the combustion initial phase is postponed andthe maximum rate of heat release is increased for the mostdiesel-oxygenate blends except the diesel-DGM blend. Thehigh cetane number of DGM leads to the advancement ofthe combustion initial phase and the decrease of the maxi-mum rate of heat release. Thus, the combustion phase andthe heat release rate are strongly related to the cetane num-ber of the oxygenate fuel.

Fig. 2 illustrates the ignition delay versus oxygen massfraction in the blends. The ignition delay strongly dependson the cetane number of fuels. As shown in the figure, theignition delay of the diesel-DGM blend decreases and theignition delay of other diesel-oxygenate blends increaseswith the increase of the oxygen mass fraction in the blends.This is also due to the influence from the cetane number ofthe oxygenate fuel. The cetane number of DGM is higher

Fig. 3. Total combustion duration of the blends. Fig. 4. Crank angle of the center of heat release curve (uc) of the blends.

Fig. 5. Brake specific fuel consumption (bsfc) of the blends.

Fig. 6. Effective thermal efficiency (get) of the blends.

2694 Y. Ren et al. / Fuel 87 (2008) 2691–2697

than that of diesel fuel and the cetane numbers of otheroxygenates are lower than that of diesel fuel. It is wellknown that the fuel with the high cetane number givesthe short ignition delay while the fuel with the low cetanenumber gives the long ignition delay.

Fig. 3 shows the total combustion duration versus oxy-gen mass fraction in the blends. The total combustionduration decreases with the increase of oxygenate fractionin the blends. The decrease of the heating value of theblends by adding oxygenates requires more fuels to beinjected into the cylinder to obtain the same power output.This is favorable to the extension of the total combustionduration of the diesel-oxygenate blends. However, addingoxygenate additive in the diesel creates more oxygen avail-able for promoting combustion, especially the diffusivecombustion. The comprehensive influence makes thedecrease of the total combustion duration for the diesel-oxygenate blends.

Fig. 4 gives the crank angle of the center of the heatrelease curve (uc) versus oxygen mass fraction in theblends. For all test-blends, the center of the heat releasecurve moves close to the top-dead-center (TDC) with theincrease of the oxygenate fraction in the blends. This isdue to the improvement combustion process anddecrease in combustion duration. A compact heat releaseprocess is presented in the case of the diesel-oxygenateblends.

Fig. 5 plots the brake specific fuel consumption (bsfc)versus oxygen mass fraction in the blends. The results showthat bsfc increases linearly with the increase of the oxygenmass fraction in the blends. Two factors are considered toexplain the behavior. One is the decrease in the heatingvalue of the blends by adding oxygenates, and this requiresmore fuel to be injected into the cylinder to get the samepower output, leading to the increase in the bsfc. Anotheris the improvement of the combustion by oxygen enrich-ment, and this will decrease the bsfc. The comprehensiveinfluence makes the increase of bsfc in the case of diesel-oxygenate blends.

The effective thermal efficiency get versus oxygen massfraction in the blends is illustrated in Fig. 6. The effectivethermal efficiency shows a slight increase with the increaseof the oxygen mass fraction in the blends. This indicates

Fig. 9. Extinction coefficient (Kvalue) of the blends.

Y. Ren et al. / Fuel 87 (2008) 2691–2697 2695

that the addition of the oxygenated fuels can increase theengine thermal efficiency. Short and compact heat releaseprocess results in an increase of engine thermal efficiency.

Figs. 7 and 8 give the cylinder maximum mean gas tem-perature (Tmax) and the duration of the temperature over1800 K versus oxygen mass fraction in the blends. In thisstudy, the temperature was calculated with the thermody-namic model based on the cylinder pressure. Little varia-tion in Tmax is presented at the engine speed of 2000 rpmand a slight increase in Tmax is presented at engine speedof 1400 rpm. The duration of temperature over 1800 K willdecrease remarkably with increasing the oxygenate fractionin the blends. The combustion improvement by oxygenenrichment leads to the increase of gas temperature, butthe decease of the combustion duration leads to thedecrease of the duration of gas at high temperature.

Figs. 9 and 10 show the exhaust smoke concentrationand its reduction rate versus oxygen mass fraction in theblends, respectively. In this experiment, a smoke meter ofAVL Di-smoke 4000 was used to measure the exhaust

Fig. 7. Maximum temperature (Tmax) of the blends.

Fig. 8. Duration of gas temperature over 1800 K of the blends.

Fig. 10. Smoke reduction rate of the blends.

smoke, and the exhaust smoke concentration is scaled bythe extinction coefficient (Kvalue). Smoke reduction ratiois defined by the formula of ½KvalueðdieselÞ � Kvalue

ðblendsÞ�=KvalueðdieselÞ. The figures reveal that the smokeconcentration decreases with the increase of the oxygenmass fraction in the blends, and this behavior is more obvi-ous at high engine loads. As smoke mainly produces duringthe diffusive combustion phase, adding the oxygenates todiesel fuel can reduce the engine smoke duo to the improve-ment of the diffusive combustion and the promotion ofpost-flame oxidation of smoke in the late expansion andexhaust processes. Meanwhile, the decrease of the amountof the fuel burned in the diffusive combustion duration isalso beneficial to the reduction of the exhaust smoke. Theresults reveal that the smoke reduction is strongly relatedto the oxygen mass fraction in the blends and is less relatedto the type of oxygenate. Thus, smoke reduction of diesel-oxygenate blends can be controlled by adjusting the oxygenfraction in the blends. The smoke reduction rate increases,but its increment shows a decrease trend with the increaseof the oxygen mass fraction in the blends Ten percent of

2696 Y. Ren et al. / Fuel 87 (2008) 2691–2697

the oxygen mass fraction in the blends can decrease theengine smoke by 30–40%.

Fig. 11 illustrates engine NOx emission versus oxygenmass fraction in the blends. Under the same engine speed,engine load and fuel delivery advance angle, the engineNOx concentration shows a slight decrease with the

Fig. 11. NOx concentration of the blends.

Fig. 12. CO concentration and HC concentration of the blends.

increase of the oxygen mass fraction in the blends. Twofactors influence the NOx for diesel-oxygenate blends,one is the gas temperature and another is the duration ofgas at high temperature. In one aspect, using diesel-oxygen-ate blends increases the cylinder maximum mean gas tem-perature and this tends to increase the NOx

concentration. In other aspect, the usage of diesel-oxygen-ate blends decreases the duration of gas at high tempera-ture and this tends to decrease the NOx concentration.The comprehensive influence makes the NOx behaviors ofthe diesel-oxygenate blends.

Fig. 12 gives the exhaust CO concentration and theexhaust HC concentration versus oxygen mass fraction inthe blends, respectively. Similar to the behavior of thesmoke, the engine CO concentration of the engine withthe diesel-oxygenate blends decreases with the increase ofthe oxygen mass fraction in the blends. This is also dueto the combustion promotion from the oxygen enrichment.HC concentration decreases with oxygenate flues addition,and this suggests that adding oxygenate fuels can decreaseHC from the locally overrich mixture. Furthermore, oxy-

Fig. 13. Relationship between get/NOx and smoke (Kvalue).

Y. Ren et al. / Fuel 87 (2008) 2691–2697 2697

gen enrichment is also favorable to the oxidation of HC inthe expansion and exhaust processes.

The relationship between thermal efficiency and smoke,between NOx and smoke are plotted in Fig. 13. Adding theoxygenate fuels in diesel fuel can decrease the exhaustsmoke concentration without decreasing the effective ther-mal efficiency. In conventional diesel engines fueled withpure diesel fuel, there exists the tradeoff curve betweenthe smoke and the NOx. However, in the case of diesel-oxy-genate blends, a flat tradeoff cure between NOx and smokeis presented. This suggests that smoke can be decreased byadding the oxygenate in diesel fuel without increasing theNOx concentration. Thus, exhaust gas recirculation(EGR) can be used to eliminate the NOx emissions. Oxy-genate fuels have high EGR tolerance due to oxygenenrichment.

4. Conclusions

Combustion and emissions of a compression–ignitionengine fuelled with diesel-oxygenate blends were investi-gated, and the main results are summarized as follows:

(1) Ignition delay decreases with the increase of diglymefraction in the blends due to the high cetane numberof diglyme. Ignition delays of the other five diesel-oxygenate blends increase with the increase of oxy-genate fuels fraction in the blend the low cetane num-bers of the oxygenate fuels. Combustion durationdecreases with the increase of oxygenate fraction inthe blends and a compact heat release process is pres-ent in the case of diesel-oxygenate blends.

(2) The centre of heat release curve moves close to theTDC with the increase of the oxygen mass fractionin blends.

(3) Smoke concentration decreases with the increase ofthe oxygen mass fraction in the blends withoutincreasing NOx CO and HC decrease with theincrease of the oxygen mass fraction in the blends.A flat NOx/smoke tradeoff curve is presented whenoperating on the diesel-oxygenate blends.

Acknowledgments

This study was supported by National Natural ScienceFund of China (50576070, 50521604), the Doctoral Foun-dation of Xi’an Jiaotong University (DFXJTU2005-04).

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