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:
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
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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
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JAUTO516 F IMechE 2008 Proc. IMechE Vol. 222 Part D: J. Automobile Engineering
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
1080 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
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
Effects of addition of ethanol and CN improver on a compression ignition engine 1081
JAUTO516 F IMechE 2008 Proc. IMechE Vol. 222 Part D: J. Automobile Engineering
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
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Proc. IMechE Vol. 222 Part D: J. Automobile Engineering JAUTO516 F IMechE 2008
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
Effects of addition of ethanol and CN improver on a compression ignition engine 1083
JAUTO516 F IMechE 2008 Proc. IMechE Vol. 222 Part D: J. Automobile Engineering
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
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Proc. IMechE Vol. 222 Part D: J. Automobile Engineering JAUTO516 F IMechE 2008
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
Effects of addition of ethanol and CN improver on a compression ignition engine 1085
JAUTO516 F IMechE 2008 Proc. IMechE Vol. 222 Part D: J. Automobile Engineering
versity for their helpful comments and advice duringthe manuscript preparation.
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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
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Proc. IMechE Vol. 222 Part D: J. Automobile Engineering JAUTO516 F IMechE 2008
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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JAUTO516 F IMechE 2008 Proc. IMechE Vol. 222 Part D: J. Automobile Engineering