1077 Effects of the addition of ethanol and cetane …gr.xjtu.edu.cn/upload/PUB.1643.4/Effects+of+addition+of...Effects of the addition of ethanol and cetane number improver on the

Effects of the addition of ethanol and cetane numberimprover on the combustion and emissioncharacteristics of a compression ignition engineY Ren*, Z-H Huang, D-M Jiang, W Li, B Liu, and X-B Wang

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, People’s Republic of

China

The manuscript was received on 4 January 2007 and was accepted after revision for publication on 27 February 2008.

DOI: 10.1243/09544070JAUTO516

Abstract: Combustion and emission characteristics of a direct-injection diesel engine fuelledwith diesel–ethanol blends were investigated. The results show that the ignition delay and thepremixed combustion duration increase, while the diffusive combustion duration and the totalcombustion duration decrease with increase in the oxygen mass fraction in the blends. Theaddition of 0.2 per cent volume fraction of cetane number improver (isoamyl nitrite) couldmean that the ignition delay and the premixed combustion duration of the fuel blends with10 vol % ethanol fraction recover to those of diesel fuel. Meanwhile, with the increase in theethanol fraction in the fuel blends, the centre of the heat release curve moves closer to the topdead centre. The brake specific fuel consumption increases, while the diesel equivalent brakespecific fuel consumption decreases with increase in the ethanol fraction. The exhaust smokeconcentration increases and exhaust nitrogen oxide (NOx) concentration decreases onprolonging the fuel delivery advance angle for both diesel fuel and the blended fuels. For aspecific fuel injection advance angle, the exhaust smoke concentration shows a large decreaseand the exhaust NOx concentration a small decrease on ethanol addition.

Keywords: combustion, emission, diesel, ethanol, oxygenated fuel blends

1 INTRODUCTION

The advantages of a diesel engine compared with a

gasoline engine are the fuel economy benefits and

high power output; however, the high nitrogen

oxides (NOx) and smoke emissions are still consid-

ered the main obstacles for its increasing application

with growing concern in environmental protection

and implementation of more stringent exhaust gas

regulations; therefore, further reduction in engine

emissions becomes one of the major tasks in engine

development. However, it is difficult to reduce NOx

and smoke simultaneously in the traditional diesel

engine owing to the trade-off relationship between

NOx and smoke. One promising approach to solve

this problem is to use the oxygenated fuels or to add

oxygenate additives in diesel 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 diesel engine,

and their results showed that the engine could

achieve ultra-low-emission prospects without fun-

damental 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 a high

thermal efficiency, short premixed combustion, and

fast diffusion combustion 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 reduce both smoke and NOx

emissions.

Practically, using some oxygenate compounds in

pure diesel fuel to reduce engine emissions without

*Corresponding author: School of Energy and Power Engineer-

ing, Institute of Internal Combustion Engines, Xi’an Jiaotong

University, Xi’an, 710049, People’s Republic of China. email:

[email protected]

1077

JAUTO516 F IMechE 2008 Proc. IMechE Vol. 222 Part D: J. Automobile Engineering

modifying the engine design seems to be a more

attractive approach. Huang et al. [6] tested gasoline–

oxygenate blends in a spark-ignited engine and

obtained a satisfactory result on emission reduction

[6]; these workers [7–9] also investigated the combus-

tion and emission characteristics of diesel–oxygenate

blends in a compression ignition engine. Murayama

et al. [10] investigated the emissions and combustion

of diesel–dimethyl carbonate blends with exhaust gas

recirculation (EGR). Ajav et al. [11] studied diesel–

ethanol blends for emission reduction and Huang et

al. [12] investigated the engine performance and

emissions of diesel engine fuelled with diesel–metha-

nol blends. Miyamoto et al. [13] and Akasaka and

Sakurai [14] also conducted research on diesel

combustion improvement and emission reduction

by the use of various types of oxygenated fuel blend.

In addition, McCormick et al. [15] studied the exhaust

emissions of a heavy-duty diesel engine operated

using several diesel–oxygenated blends.

Ethanol is a promising biomass fuel, which can be

produced from crops. Ethanol has a high oxygen

content and an abundant source; thus it is regarded as

a better oxygenate additive or a good alternative fuel

in engines. Previous investigations revealed that the

reduction in particulate emissions and toxic gas

pollutants could be achieved when using diesel–

ethanol blends [16–18]; however, these previous

studies mainly focused on the experimental results

under different engine conditions (engine speed and

engine load) and used a specific proportion of ethanol

in blends. As the information is very important for the

clarification of combustion phenomenon and appli-

cation of such blends, further investigation needs to

be conducted, especially on a quantitative scale.

These quantitative results are expected to supply

more information on engine combustion fuelled with

oxygenated fuels versus oxygen mass fraction in the

blends and to provide more practical measures for the

improvement in combustion and reduction in emis-

sions of engine fuelled with diesel–oxygenate blends.

Based on the present authors’ previous analysis, the

objective of this study is to investigate engine

combustion and emission characteristics of diesel–

ethanol blends with cetane number (CN) improver,

extending understanding of the combustion and

emission characteristics of diesel–ethanol blends and

providing practical guidance for engine optimization.

2 FUEL PREPARATION AND APPROACH

In this study, diesel fuel is the base fuel while ethanol

is used as the oxygenate additive. A CN improver was

used to recover the CN of the blends as ethanol has a

low CN. A small fraction of surfactant, which is

composed of carbon, hydrogen, and oxygen, was

used to make the blends uniform and stable. Four

blends without CN improver, designated E5, E10,

E15, and E20, were prepared in which the volume

fractions of ethanol in the diesel–ethanol blends are

5 per cent, 10 per cent, 15 per cent, and 20 per cent

respectively, and those with 0.2 per cent volume

fraction of CN improver (isoamyl nitrite) were

designated E5A, E10A, E15A, and E20A respectively.

The base fuel is diesel fuel (E0). The fuel properties

are given in Table 1 and Table 2, as well as in Fig. 1.

It can be seen from Fig. 1 that adding 0.2 per cent

CN improver made little difference to the blended

fuels.

In the experiment, the above eight fuel blends and

pure diesel fuel were tested in a direct-injection (DI)

diesel engine. The original fuel delivery advance

angle of the engine is 25u crank angle (CA) before top

dead centre (BTDC), and the specifications of the

test engine are listed in Table 3. The initial time of

the nozzle valve lifting was measured with a needle-

lift-detecting apparatus. An FQD-201B smoke meter

was used to measure the exhaust smoke, and the

exhaust gases (NOx, carbon monoxide, and hydro-

carbons) were measured with a AVL DiGas 4000 light

emission tester.

The cylinder pressure and emissions were re-

corded under various engine conditions, and com-

bustion analysis was performed on the basis of the

cylinder pressure information. Furthermore, com-

parisons in combustion and emissions were con-

ducted among these blends to clarify the behaviours

of engine fuelled with diesel–ethanol blends.

3 RESULTS AND DISCUSSION

3.1 Combustion characteristics

The heat release rate dQb/dQ is calculated using the

formula

Table 1 Fuel properties of diesel and ethanol

Base fuel Oxygenates fuel

Types of fuel Diesel EthanolDensity (g/cm3) 0.86 0.79Lower heating value (MJ/kg) 42.5 26.78Heat of evaporation (kJ/kg) 260 854–904Self-ignition temperature (uC) 200–220 636CN 45 8Carbon (wt %) 87 52.2Hydrogen (wt %) 12.6 13Oxygen (wt %) 0.4 34.8

1078 Y Ren, Z-H Huang, D-M Jiang, W Li, B Liu, and X-B Wang

Proc. IMechE Vol. 222 Part D: J. Automobile Engineering JAUTO516 F IMechE 2008

dQb

dQ~p

Cp

R

dV

dQz

CV V

R

dp

dQz

dQW

dQð1Þ

where the heat transfer rate is given as

dQW

dQ~hcA T{TWð Þ ð2Þ

where the heat transfer coefficient hc uses the

Woschni heat transfer coefficient [18].

The diesel-equivalent b.s.f.c. beq and effective

thermal efficiency get are calculated respectively

from the formula

beq~beHuð Þblends

Huð Þdiesel

ð3Þ

get~3:6|106

Huð Þdiesel|beqð4Þ

The CA Qc of the centre of heat release curve is

determined from the formula

Qc~

ÐQe

Qs

ðdQb=dQÞQ dQ

ÐQe

Qs

ðdQb=dQÞdQ

ð5Þ

The ignition delay is defined as the time interval

from the initial time of the nozzle valve lifting (i.e.

the start of fuel injection) to the initial time of the

rapid pressure rise (it is regarded as the start of

combustion); the premixed combustion duration is

the time interval from the start of combustion to the

time of the first trough on the heat release rate curve;

the diffusive combustion duration is the time

interval from the time of the first trough on the heat

release rate curve to the end of combustion; the total

combustion duration is the duration from the start

of combustion to the end of combustion.

Figure 2 gives the heat release rate of the diesel–

ethanol blends. The results show that for the same

engine load (b.m.e.p.), engine speed, and fuel delivery

advance angle, the initial combustion phase gives

changes, owing to the addition of ethanol. Moreover,

the maximum rate of heat release increases with

increase in the ethanol mass fraction in the blends,

and this value gives a low value in the case of CN

improver addition compared with the value without

the CN improver. This indicated that the CN has a

large influence on the maximum rate of heat release.

A long ignition delay and better evaporation of

ethanol increase the fraction of combustible mixture

prepared during the period of ignition delay, con-

tributing to the increase in the maximum rate of heat

release. In addition, a similar curve is revealed in the

early stage of combustion between fuel E0 and fuel

E10A, and similar behaviours are also presented for

fuel E5 and fuel E15A, and for fuel E10 and fuel E20A.

This indicated that the addition of 0.2 vol % of CN

Table 2 Fuel properties of the diesel–ethanol blended fuels

FuelEthanol in theblends (vol %)

Lower heatingvalue (MJ/kg)

Heat of evaporation(kJ/kg) Carbon (wt %) Hydrogen (wt %) Oxygen (wt %)

E0 0 42.5 260 87 12.6 0.4E5 5 41.6 296 85.1 12.6 2.25E5AE10 10 40.7 331 83.3 12.6 4.07E10AE15 15 39.8 3667 81.5 12.7 5.85E15AE20 20 39.0 400 79.8 12.7 7.26E20A

Fig. 1 Mass fraction of the fuel blends

Table 3 Engine specifications

Bore 100 mmStroke 115 mmDisplacement 903 cm3

Compression ratio 18Shape of combustion

chamberv shape in the bottom of the bowl

in pistonRated power; speed 10.5 kW; 2000 r/minNozzle hole diameter 0.3 mmNozzle opening pressure 19 MPaNumber of nozzle holes 4

Effects of addition of ethanol and CN improver on a compression ignition engine 1079


improver (isoamyl nitrite) in the blended fuels could

mean that the ignition delay recovers to those with

10 vol % ethanol addition. Compared with fuel E0, the

heat release curves of fuel E10A finished early,

indicating the decrease in diffusive combustion

duration, and this would be the oxygen enrichment

by ethanol addition.

Figure 3 illustrates the ignition delay of the blends

versus the oxygen mass fraction in the blended fuels.

For a specific fuel delivery advance angle, the

ignition delay shows an increase with increase in

the oxygen mass fraction in the blends, and adding a

CN improver into the blends can mean that the

ignition delay of the fuel blends recovers to those

with 10 vol % less ethanol addition. The ignition

delay increases on delaying the fuel delivery advance

angle for both diesel fuel and diesel–ethanol blends.

The behaviours can be explained by the decrease in

Fig. 2 Heat release rate of the fuel blends



CN and the increase in the heat of evaporation of the

blended fuels with ethanol addition. The premixed

combustion duration and the amount Qpremixed of

heat release in the premixed combustion duration

versus the oxygen mass fraction in the blended fuels

are shown in Fig. 4; the results showed that the

premixed combustion duration and the amount of

heat release in the premixed combustion duration

increase with the advancement of fuel delivery

advance angle for both diesel fuel and the diesel–

ethanol blends, and this is due to the increase in

ignition delay with the advancement of fuel delivery

advance angle. For a specific fuel delivery advance

angle, the premixed combustion duration and the

amount of heat release in the premixed combustion

duration increase with increase in the oxygen mass

fraction in the blended fuels. Two factors are

considered to cause this behaviour: one is the

increase in the amount of combustible mixture

prepared during the ignition delay since the addition

of ethanol increases the ignition delay, and the other

is that the addition of ethanol would promote the

formation of a combustible mixture due to the

oxygen enrichment and better volatility of ethanol.

The addition of a CN improver can decrease the

premixed combustion duration of diesel–ethanol

blends; the results show that the premixed combus-

tion duration can recover to those with 10 vol % less

ethanol addition on the addition of 0.2 vol % of CN

improver. This behaviour is similar to that of ignition

delay for the blended fuels. This suggests that the

change in the CN of the blended fuels strongly

influences the ignition delay and the premixed

combustion duration for diesel–ethanol blends. The

diffusive combustion duration and the amount

Qdiffusive of heat release in the diffusive combustion

duration versus the oxygen mass fraction in the fuel

blends are given in Fig. 5. The results reveal that the

diffusive combustion duration and the amount of

heat release in the diffusive combustion duration

decease with increase in the oxygen mass fraction in

the blended fuels, and this is regarded as diffusive

combustion improvement due to oxygen enrich-

ment by adding oxygenates. The improvement in

diffusive combustion is favourable to a reduction in

Fig. 3 Ignition delay versus oxygen mass fraction

Fig. 4 Premixed combustion duration and amount ofheat release in premixed combustion durationversus oxygen mass fraction



exhaust smoke. The total combustion duration

versus fuel delivery advance angle is illustrated in

Fig. 6. The blended fuels presented a slight decrease

with increase in the oxygen mass fraction for fuels

with and without the CN improver. At the same fuel

delivery advance angle, more fuel should be injected

for diesel–ethanol blends compared with pure diesel

fuel to obtain the same engine load (b.m.e.p.) since

the heat value of diesel–ethanol blends is less than

that of pure diesel fuel, and the value will decrease

with the increase in the ethanol fraction in the

blends. However, the total combustion duration

shows a slight decrease with increase in the oxygen

mass fraction (or ethanol mass fraction) in the

blends. As explained above, the enrichment of

oxygen owing to the ethanol addition is helpful to

the improvement in diffusive combustion, decreas-

ing the diffusive combustion duration, and finally

contributing to the reduction in the total combus-

tion duration.

Figure 7 exhibits the CA Qc of the centre of the heat

release curve versus the oxygen mass fraction in

blended fuels. The figure shows the decrease in Qc

with increase in the oxygen mass fraction in blended

fuels. This can be explained as follows; the improve-

ment in the diffusive combustion phase and the

decrease in the total combustion duration contribute

to making the heat release process closer to the top

dead centre (TDC).

The b.s.f.c. and the diesel equivalent b.s.f.c. beq

versus the oxygen mass fraction in the blended fuels

are plotted in Fig. 8. The results show that the b.s.f.c.

of blended fuels increases with increase in the

oxygen mass fraction in fuel blends. However, the

diesel-equivalent b.s.f.c. decreases with increase in

the oxygen mass fraction in the fuel blends. With

respect to the behaviour of the b.s.f.c. versus the

oxygen mass fraction, two aspects should be taken

into account. One aspect is that the addition of the

ethanol in the blended fuels would result in the

increase in the amount of fuel burned in the

premixed burn phase, and the centre of heat release

curve moves close to the TDC, leading to the

decrease of b.s.f.c. Another aspect is the decrease

in the heating value of the blended fuels with

Fig. 5 Diffusive combustion duration and amount ofheat release in diffusive combustion durationversus oxygen mass fraction

Fig. 6 Total combustion duration versus oxygen massfraction



increase in the ethanol fraction, in order to obtain

the same b.m.e.p. and engine speed; more fuel

should be injected and this increases the b.s.f.c. The

comprehensive results showed the increase in the

b.s.f.c. with increase in the ethanol fraction. How-

ever, the diesel-equivalent b.s.f.c. beq would decrease

with increase in the ethanol fraction owing to the

improvement in combustion. The effective thermal

efficiency get is in an inverse ratio to the diesel-

equivalent b.s.f.c. as indicated in equation (4), and so

they must reflect the same phenomenon. Therefore,

the effective thermal efficiency get versus the oxygen

mass fraction in the blended fuels is given in Fig. 9,

and the result shows that get increases with increase

in the ethanol fraction in the blended fuels.

The NOx concentration of diesel–ethanol blends

versus the oxygen mass fraction of the blended fuels

is illustrated in Fig. 10. The results show that the

NOx concentration of the fuel blends increases with

the advancement of fuel delivery advance angle,

while NOx gives a slight decrease with increase in the

oxygen mass fraction (ethanol mass fraction) in the

blended fuels. In the early case, the increase in

premixed combustion causes the increase in the NOx

concentration while, in the late case, the tempera-

ture drop due to the high value of ethanol evapora-

tion leads to the decrease in the NOx concentration.

Figure 11 gives the smoke concentration and its

reduction rate versus the oxygen mass fraction in the

blended fuels. The smoke reduction rate is defined

by the formula [Kvalue(diesel) 2 Kvalue(blends)]/Kvalue

(diesel). The purpose of adding the oxygenate to

diesel fuel is expected to decrease the engine smoke

by providing more oxygen and making it burn

completely. The results clearly show that the exhaust

smoke could be decreased markedly on the addition

of ethanol to diesel fuel with and without the CN

improver. This suggests that the oxygen-containing

fuel blends can reduce the rich spray region and

promote the post-flame oxidation of the formed

soot. The results also reveal that the smoke con-

centration of the blended fuels without CN improver

gives a lower value than those with a CN improver.

The addition of CN improver decreases the ignition

delay and the amount of fuel burned during the

premixed combustion phase and increases the

Fig. 7 CA Qc centre of the heat release curve versusoxygen mass fraction Fig. 8 B.s.f.c. and diesel-equivalent b.s.f.c. beq



amount of fuel burned during the diffusive combus-

tion phase. This causes the increase in the engine

smoke on CN improver addition. The smoke reduc-

tion rate increases with increase in the oxygen mass

fraction (ethanol fraction). For the same engine

speed and engine load (b.m.e.p.), engine smoke

gives a high reduction rate with increase in the

oxygen mass fraction in the blended fuels in the case

of a small fuel delivery advance angle, and this

reveals that the ethanol addition has a large

influence on smoke reduction in the case of a small

fuel delivery advance angle.

The relationships between NOx and smoke of

diesel–ethanol fuel blends at various b.m.e.p. and

fuel delivery advance angles are plotted in Fig. 12.

Unlike the engine operating on pure diesel fuel,

which has a trade-off behaviour between NOx and

smoke, a flat NOx–smoke trade-off curve is pre-

sented when operating on the diesel–ethanol fuel

blends. Simultaneous reduction in NOx and smoke

could be observed with ethanol addition at high

engine loads. The results also reveal that NOx

concentration shows a decrease, and smoke con-

centration an increase, on delaying the fuel delivery

advance angle.

4 CONCLUSIONS

A stabilized diesel–ethanol blend was used to study

the combustion characteristics and emissions of the

oxygenated blends in a compression ignition engine,

and the main results were summarized as follows.

1. Ignition delay increases with increase in the

ethanol fraction owing to the decrease in CN of

the blends. Premixed combustion duration and the

amount of heat release in the premixed combus-

tion duration increase, while the diffusive combus-

tion duration and the amount of heat release in the

diffusive combustion duration decrease with in-

crease in the ethanol fraction in the fuel blends.

2. The addition of 0.2 vol % CN improver (isoamyl

nitrite) can mean that the ignition delay and

premixed combustion duration of fuel blends

with 10 vol % ethanol fraction recover to those of

Fig. 9 Effective thermal efficiency versus oxygen massfraction

Fig. 10 Exhaust NOx concentration versus oxygenmass fraction



diesel fuel. The CN of the blended fuels is a key

influencing factor on the ignition delay and the

premixed combustion duration for diesel–ethanol

blends.

3. The centre of the heat release curve moves close to

the TDC with increase in the oxygen mass fraction

in blended fuels. The diesel equivalent b.s.f.c.

decreases with increase in the ethanol fraction.

4. A flat NOx–smoke trade-off curve exists when

operating on the diesel–ethanol fuel blends.

Utilization of diesel–ethanol blends combined

with delaying the fuel delivery advance angle

can simultaneously decrease both the smoke and

the NOx emissions.

ACKNOWLEDGEMENTS

This study was supported by the National NaturalScience Fund of China (50576070 and 50521604) andthe Doctoral Foundation of Xi’an Jiaotong Univer-sity. The authors acknowledge the teachers andstudents of Xi’an Jiaotong University for their helpwith the experiment. The authors also express theirthanks to their colleagues at Xi’an Jiaotong Uni-

Fig. 12 Relationship between NOx and smoke of theblended fuels

Fig. 11 Exhaust smoke concentration and its reduc-tion rate versus oxygen mass fraction



versity for their helpful comments and advice duringthe manuscript preparation.

REFERENCES

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2 Kapus, P. and Ofner, H. Development of fuelinjection equipment and combustion system for DIdiesels operated on dimethyl ether. SAE Trans.,1995, 104(4), 54–69.

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7 Huang, Z. H., Lu, H. B., Jiang, D. M., Zeng, K., Liu,B., Zhang, J. Q., and Wan, X. B. Combustionbehaviors of a compression-ignition engine fuelledwith diesel/methanol blends under various fueldelivery advance angles. Bioresource Technol.,2004, 95(3), 331–341.

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APPENDIX

Notation

ATDC after top dead centre

b.m.e.p. brake mean effective pressure (MPa)

b.s.f.c. brake specific fuel consumption

be brake specific fuel consumption (g/

kW h)

beq diesel equivalent brake specific fuel

consumption (g/kW h)

BTDC before top dead centre

dQb/dQ heat release rate with crank angle

(kJ/degree crank angle)

(Hu)blends lower heating value of diesel–oxyge-

nates blends (MJ/kg)

(Hu)diesel lower heating value of pure diesel

fuel (MJ/kg)

Qpremixed amount of heat release during pre-

mixed combustion duration

Qdiffusive amount of heat release during diffu-

sive combustion duration

TDC top dead centre

get effective thermal efficiency



hfd fuel delivery advance angle

(degrees crank angle before top dead

centre)

Qc crank angle of the centre of the heat

release curve (degrees crank angle

after top dead centre)



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1077 Effects of the addition of ethanol and cetane …gr.xjtu.edu.cn/upload/PUB.1643.4/Effects+of+addition+of...Effects of the addition of ethanol and cetane number improver on the