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Performance and emissions of a turbocharged,high-pressure common rail diesel engineoperating on biodiesel/diesel blendsX-G Wang, B Zheng, Z-H Huang*, N Zhang, Y-J Zhang, and E-J Hu
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an
The manuscript was received on 31 March 2010 and was accepted after revision for publication on 21 July 2010.
DOI: 10.1243/09544070JAUTO1581
Abstract: The effect of biodiesel addition to diesel on engine performance, combustion, andemissions were studied in a turbocharged, high-pressure common rail diesel engine. Biodiesel/diesel blends with different biodiesel fractions were used and compared with neat biodieseland diesel at different engine loads and speeds. The results show that the brake thermalefficiency increases slightly as biodiesel is added to diesel. Exhaust gas temperature is notsignificantly affected at low engine speeds and decreases gradually at high engine speeds withan increase in biodiesel fraction. Fuel injection includes both pilot and main injections. Dieseland biodiesel give a similar start to the heat release. The first peak in the heat release rate forbiodiesel is lower than that of diesel, while the second peak is higher for biodiesel. The heatrelease rate curve for biodiesel indicates that the use of biodiesel increases thermal efficiencyand NOx emission compared to that of diesel especially at high engine loads. Hydrocarbon andCO emissions maintain very low values and little variation is seen for the different fuels. CO2
emission decreases with increasing biodiesel fraction in the blends. The level of NOx emissiondecreases slightly at low engine loads and increases at high engine loads with increasingbiodiesel fraction. Biodiesel reduces particulate matter (PM) emission significantly and PMreduction effectiveness is increased at high engine loads and/or speed. The oxygen in biodieselplays a key role in reducing PM emission. Biodiesel/diesel blends can improve performanceand decrease emissions for turbocharged, high-pressure common rail diesel engines.
Keywords: diesel engine, biodiesel, performance, emissions
1 INTRODUCTION
The high fuel efficiency of diesel engines has led to
their use in many areas including transportation and
agricultural machinery. However, the exhaust emis-
sions from diesel engines, especially NOx and parti-
culate matter (PM), have a significant impact on the
environment. Moreover, it is difficult to simulta-
neously reduce these emissions since there is a trade-
off between NOx and PM emission levels. Increases in
the price of oil coupled with continuously tightening
emission control regulations have led to significant
interest in renewable energy sources. One such en-
ergy source is biodiesel; the use of which can lead to
reductions in petroleum consumption and carbon
dioxide emissions [1]. One advantage of biodiesel is
that it contains almost 11 wt% oxygen and these
oxygen molecules enhance the combustion process
and inhibit soot formation in diesel engines [2]. There
is an extensive literature on experimental investiga-
tions on the combustion and emission characteristics
of biodiesel fuel in diesel engines [1–15]. From these
papers it is clear that the use of biodiesel blends and
neat biodiesel can decrease CO, hydrocarbon (HC),
and soot emission levels compared to normal diesel
fuels. Unfortunately, these decreases are accompa-
nied by an increase in NOx emission levels [7–15].
The majority of the reports in the literature con-
cern studies carried out on conventional pump-line-
*Corresponding author: State Key Laboratory of Multiphase Flow
in Power Engineering, Xi’an Jiaotong University, Xi’an, 710049,
People’s Republic of China.
email: [email protected]
127
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
injector injection engines with only limited interest
being focused on high-pressure common rail en-
gines [1, 16]. For engines equipped with conven-
tional pump-line-injector injection systems, the in-
crease in NOx emissions can be attributed to the
earlier start of injection for biodiesel because of its
higher bulk modulus of compressibility [16]. How-
ever, in a common rail injection system, such as
found in modern turbocharged high-pressure com-
mon rail diesel engines, the actual injection timing
between diesel and biodiesel can be ignored, which
can remove the influence of injection timing for
different fuels on the combustion process and NOx
emissions.
A high injection pressure is an effective method to
improve direct injection diesel engine performance
and decrease PM emission levels due to improved
spray atomization and fuel–air mixing [17–19]. Mul-
tiple injection strategies have been tested for emis-
sion reduction at different operating conditions and
it has been reported that appropriate configurations
could offer simultaneous soot and NOx reductions
while maintaining a reasonable fuel economy [20].
Thus, the question must be asked: could the high-
pressure common rail injection system in a diesel
engine be used to create a high injection pressure
and realize multiple injection strategies, consequent-
ly reducing both NOx and PM emissions?
The objectives of this study are to investigate the
performance, combustion, and emission character-
istics of biodiesel and biodiesel/diesel blends in a
modern turbocharged, high-pressure common rail
diesel engine, and to examine the possibility of
improving engine performance and decreasing emis-
sions simultaneously with biodiesel/diesel blends.
The effects of biodiesel fraction on engine perfor-
mance, combustion, and emissions are discussed and
compared with those of diesel and biodiesel.
2 EXPERIMENTAL SET-UP AND PROCEDURE
2.1 Test engine and apparatus
A commercial light-duty direct injection diesel engine
GW2.8TC was used in this study. It is a four-stroke
four-cylinder turbocharged high-pressure common
rail diesel engine. The engine specifications are listed
in Table 1. The diesel engine was coupled to an eddy-
current dynamometer, as shown in Fig. 1. The real-
time engine speed, torque, and power, as well as
exhaust gas and coolant temperatures and lubricating
oil pressure were monitored by a Powerlink Engine
Control System (Type FC2000). The glow plug of one
cylinder was replaced by a Kistler piezoelectric
transducer (Type 6055Csp), which has the same size
as the glow plug and was used to record cylinder
pressure with a resolution of ¡10 Pa. The dynamic
top-dead-centre (TDC) was determined by motor
operation. The crank angle signal was obtained from a
Kistler crank angle encoder (Type 2614A) mounted on
the main shaft. The temporal curves of the cylinder
gas pressure and crank angle were recorded by a
Yokogawa DL750 data acquisition system. The signal
of the cylinder gas pressure was acquired for every
0.1u increment in crank angle over 100 completed
cycles. The averaged value of the cylinder gas
pressure was used to calculate the heat release rate
Table 1 GW2.8TC engine specifications
TypeIn-line four-cylinder common railinjection, turbocharged diesel engine
Combustion chamber v typeBore6stroke 93 mm6102 mmDisplacement 2.771 lCompression ratio 17.2:1Rated power/speed 70 kW ¡ 3/3600 r/minPump Bosch CP1HCommon rail Bosch LWRInjector Bosch CR1P2, 660.137 mm
Fig. 1 Schematic diagram of experimental system
128 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
using the method in Heywood [21]. A high-precision
electronic balance with an accuracy of ¡0.1 g was
used to determine fuel consumption by weighing the
fuel mass at the beginning and end of each test
condition. For each fuel and test condition, fuel
consumption was recorded over 5 min periods. Based
on the power output for each test condition, the brake
specific fuel consumption (BSFC) and brake thermal
efficiency were calculated.
A Horiba MEXA-700l analyser was used to measure
the excess air ratio with an accuracy of ¡0.1. A Horiba
MEXA-554JA analyser was used to measure unburned
HC, CO, and CO2 concentrations in the exhaust. The
accuracies for HC, CO, and CO2 are ¡12 ppm, ¡0.06
per cent, and ¡0.5 per cent respectively. A Horiba
MEXA-720 NOx analyser was used to measure NOx
concentration in the exhaust, with an accuracy of
¡30 ppm. An ELPI4.0 analyser was used to measure
PM emission over a 150 s period when the engine was
operating in steady state. Detailed information about
ELPI can be found in Tsolakis [22].
2.2 Test fuels
The pure fuels used in this study were an ultra-low
sulphur diesel fuel (, 50 ppm) and soybean-derived
biodiesel with diesel fuel being used as basis for
comparisons. The properties of the diesel and bio-
diesel fuels are given in Table 2. It can be seen that
the biodiesel fuel has a lower low heating value and
higher oxygen content than the diesel fuel. Five
diesel/biodiesel blends, D90B10, D80B20, D60B40,
D40B60, and D20B80, were used to study engine
performance and improvement in emission levels
with blended fuels, where D(X)B(100-X) denotes that
the fuel blends are composed of X% diesel and (100-
X)% biodiesel by volume. For consistency and con-
venience, the diesel and biodiesel fuels are denoted
as D100B0 and D0B100 respectively in this paper.
Oxygen contents and low heating values of various
fuel blends as well as those of the pure diesel and
biodiesel fuels are shown in Fig. 2. With the increase
of biodiesel fraction in fuel blends, oxygen content is
increased and low heating value is decreased. The
combination of different fuel compositions and
properties for the biodiesel and diesel fuels may
create the conditions to improve engine perfor-
mance, reduce emissions, and lower the amount of
consumed diesel.
2.3 Experimental procedure
An extended warm-up period was used to ensure that
the coolant reached approximately 80 uC. Then the
engine was loaded to test the engine speed and
torque. In the experiment, engine speed and torque
variations were controlled within ¡10 r/min and
¡0.1Nm. Exhaust gas analyses were conducted
during steady operating conditions. During this
steady process, the cylinder gas pressure and crank
angle were recorded simultaneously. In this study, the
effect of fuel blends on engine performance and
emissions were evaluated for each fuel at engine
speeds of 1600 and 2600 r/min. Five engine torques of
34, 68, 101, 135, and 169 Nm, corresponding to brake
mean effective pressure (BMEP) levels of 0.154, 0.308,
0.458, 0.612, and 0.766 MPa were selected. The test
matrix covers the main conditions that a diesel engine
can achieve. During the completion of the engine test
matrix, no adjustments were made to the engine
operating parameters.
3 RESULTS AND DISCUSSION
3.1 Engine performance
Figure 3 shows a comparison of BSFC levels for the
investigated fuels as a function of engine load. At
both 1600 and 2600 r/min, the BSFC decreases
Table 2 Physical and chemical properties of the dieseland biodiesel fuels
Diesel Biodiesel
Low heating value (MJ/kg) 43.1 37.4Density (15 uC) (kg/m3) 840 881.5Viscosity (30 uC) (mm2/s) 3.4 4.27Cetane number 53 51.5Carbon content (wt%) 86.1 77.1Hydrogen content (wt%) 13.8 11.8Oxygen content (wt%) 0 10.9(A/F)st 14.69 12.69
Fig. 2 Oxygen content and low heating value of fuelblends
Performance and emissions of a turbocharged common rail diesel engine 129
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
monotonically with increasing BMEP. When the
engine speed is increased from 1600 to 2600 r/min
losses due to friction increase and this leads to the
higher BSFC values observed at this engine speed.
Compared to those of neat diesel, the BSFC levels of
D90B10 and D80B20 are slightly decreased. As
shown in Fig. 2, the low heating values of D90B10
and D80B20 are 98.6 and 97.2 per cent of that of
diesel, respectively. Thus, improved combustion
appears to compensate for the slightly decreased
low heating values for D90B10 and D80B20. With
further increasing biodiesel volume fraction in the
blends, BSFC increases monotonically. The BSFC
was compared under the same engine speed and
BMEP. To maintain the same power output, more
fuel needs to be consumed in the case of a decreased
low heating value. The gradual decrease in low
heating value, as shown in Fig. 2, is responsible for
the monotonically increasing BSFC level as the
biodiesel percentage exceeds 20 per cent by volume.
The oxygen content in biodiesel can promote the
combustion process in the combustion chamber of
the diesel engine, and thus can decrease fuel
consumption at low diesel concentrations. However,
a large biodiesel fraction still results in high fuel
consumption due to the lower heating value even
though combustion is improved.
Brake thermal efficiency can be used as a sur-
rogate measure that reflects fuel economy when the
engine is operated with different fuels. Figures 4(a)
and (b) show the brake thermal efficiency versus
engine load for different fuels. With the addition of
biodiesel to diesel, the thermal efficiency is in-
creased slightly. As previously discussed, the oxygen
content in biodiesel promotes burning rate, and
improves combustion efficiency and thermal effi-
ciency. Figures 4(c) and (d) plot the thermal effi-
ciency versus biodiesel fraction in the blends. The
results clearly show that biodiesel and blends of
biodiesel/diesel give slightly higher thermal efficien-
cies than those of pure diesel. The results on BSFC
and thermal efficiency suggest that using diesel/
biodiesel blends does not lead to a decrease in the
engine’s thermal efficiency, and this provides the
possibility to use the oxygen molecules in the
biodiesel to create low emissions levels.
Figure 5 plots the variation in the excess air ratio
as a function of engine load for different fuels at
engine speeds of 1600 and 2600 r/min. The results
show that the excess air ratio is insensitive to engine
speed and fuel type. This suggests that the oxygen
levels taken from the air are the same under same
engine speed and load for all fuels, thus the in-
fluence of this oxygen should be the same for all
fuels. Thus, any differences in oxygen contribution
to combustion and emissions for different diesel/
biodiesel blends are a result of the difference in
oxygen content in the fuel blends. The excess air
ratio is decreased as engine load is increased, and
this will influence combustion and emissions under
different loads.
A comparison of measured exhaust gas tempera-
tures for different fuels is shown in Fig. 6. When the
engine speed increases from 1600 to 2600 r/min the
exhaust gas temperature increases at a specific en-
gine load. This is due to a decrease in heat loss to the
coolant and postponed heat release. The exhaust gas
temperature increases monotonically with the in-
crease in engine load. More fuel is injected and more
heat is released at high engine load, resulting in an
increase in cylinder gas temperature and exhaust gas
temperature. At the engine speed of 1600 r/min, little
variation in exhaust gas temperature is observed
Fig. 3 Brake specific fuel consumption
130 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
between the different fuels. However, at the engine
speed of 2600 r/min, with the increase of biodiesel
fraction, the exhaust gas temperature decreases
gradually. Moreover, the difference in exhaust gas
temperature among different fuels is obvious at high
engine loads. For example, at the lowest engine load
of 2600 r/min, the exhaust gas temperature is
decreased from 227 to 220 uC when the fuel is
changed from diesel to biodiesel, while the exhaust
temperature changes from 473 to 432 uC at the
highest engine load. The calculated adiabatic flame
temperature and measured flame temperature of the
biodiesel fuel are lower than those of the diesel fuel
[4, 23, 24]. The lower temperature of the burning gas
is responsible for the decreased exhaust gas tem-
perature for biodiesel and biodiesel blends. More-
over, differences in the heat release rate will also
influence the exhaust gas temperature. The de-
creased exhaust gas temperature caused by using
biodiesel has also been reported in Ozsezen et al.
[25].
3.2 Combustion analysis
The cylinder gas pressure and heat release rate of the
diesel and biodiesel fuels are illustrated in Fig. 7. The
cylinder pressure and heat release rate curves of the
diesel/biodiesel blends are between those of pure
diesel and biodiesel. Thus, Fig. 7 only plots the
cylinder pressure and heat release rate for the pure
diesel and biodiesel fuels. The heat release rate curve
demonstrates two-stage heat release, which is dif-
ferent to the behaviour observed for premixed and
diffusion combustion in a plunger-pump-injection
diesel engine. The individual heat release rates re-
flect the pilot and main injections.
From Fig. 7 it can be concluded that that the diesel
and biodiesel fuels have similar initial heat release
behaviours. In a common rail injection system, the
difference between the actual injection timings of
the diesel and biodiesel fuels can be ignored. This is
different to the case of a conventional pump-line-
injector system, where the actual injection timing of
biodiesel is earlier than that of diesel because of its
Fig. 4 Brake thermal efficiency
Performance and emissions of a turbocharged common rail diesel engine 131
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
higher bulk modulus [26]. Therefore, in the common
rail diesel engine, the similar initial behaviour of the
combustion for the diesel and biodiesel fuels (diesel/
biodiesel blends included as well) is the result of the
injection timings being almost equivalent. (It should
be noted that the diesel and biodiesel fuels used in
this study give almost the same Cetane number, as
shown in Table 2). Even though the initial combus-
tion behaviour is similar, there is an obvious dif-
ference between the heat release rate curves for the
biodiesel and diesel under each engine condition:
the first peak in the heat release rate for biodiesel is
lower than that of diesel, while the second peak is
higher for biodiesel. This tendency becomes more
obvious with increasing engine load and/or speed.
The two heat release processes correspond to the
two independent injections. The difference in the
first peak in the heat release rate is due to differences
in spray atomization. It is generally recognized that
biodiesel has poor spray atomization characteristics
due to its high surface tension and viscosity lev-
els. Moreover, the first heat release is from the pilot
injection, which has a short time scale. During this
short pilot injection duration, the lift and fall of the
needle in the injector occupy a high percentage of
the injection time. Thus, the spray atomization is
more prone to be affected by fuel properties. The
poor spray atomization properties of biodiesel are
responsible for the lower heat release rate. For the
main injection duration, biodiesel gives a higher
release rate which compensates for the lower heat
release rate of the first heat release. With increasing
engine load and/or speed, the differences in heat
release rate between biodiesel and diesel become
more obvious as the injected fuel amount is in-
creased.
Another tendency in heat release rate between
biodiesel and diesel is that the second heat release
curve of biodiesel moves closer to TDC than does the
diesel and this tendency becomes more obvious at
Fig. 5 Excess air ratio Fig. 6 Exhaust gas temperature
132 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
high engine loads. The incompletely burned biodie-
sel from the pilot injection might advance the main
combustion. This behaviour of the main heat release
indicates that the overall heat release of biodiesel is
more compact and thus a higher thermal efficiency
will be created. This is consistent with the results in
Fig. 4, that biodiesel has a higher thermal efficiency
than diesel. Moreover, the heat release rate of bio-
diesel moves closer to TDC and thus the combustion
process is finished earlier and a lower exhaust gas
temperature is created in this case.
Due to the advanced main heat release for bio-
diesel at high engine loads, the cylinder gas tempera-
ture of biodiesel is higher compared to that of diesel
at high engine loads, as shown in Figs 7(c) and (f).
This is reflected in a higher cylinder pressure for
Fig. 7 Comparison of cylinder gas pressure and heat release rate for diesel and biodiesel
Performance and emissions of a turbocharged common rail diesel engine 133
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
biodiesel. This higher cylinder temperature may lead
to the increased NOx formation in the cylinder.
3.3 Engine emissions
Figure 8 shows the influence of biodiesel fraction on
unburned HC emission from the exhaust. The results
show that HC emissions for all fuels and loads have
low values in this turbocharged common rail diesel
engine. No observable difference in HC emission
levels can be seen for any diesel/biodiesel blend or
biodiesel. Generally, HC emissions from diesel en-
gines are a result of poor fuel/air mixing and they
consist of fuel droplets that are either completely
unburned or only partially burned [21]. Due to the
improved fuel/air mixing and combustion processes
in the turbocharged, high-pressure common rail
diesel engine, the effect of biodiesel addition on HC
emissions is limited. This is different to HC emis-
sions in a plunger-pump-injection diesel engine
where biodiesel addition to diesel has been shown
to significantly decrease HC emissions [7, 8]. This
study indicates that HC emission levels are low and
are not significantly influenced by fuel type in the
high-pressure common rail injection diesel engine.
Figure 9 shows CO emission versus engine load.
CO emission levels are low for all fuels. The studied
engine has low CO emission levels in diesel opera-
tion, thus the effect of biodiesel addition on CO
levels is difficult to demonstrate. CO is generated by
incomplete combustion processes [21], and these
are strongly inhibited by the high excess air ratio and
good combustion process properties in the turbo-
charged, high-pressure common rail diesel engine.
CO2 emission is regarded as a main factor in global
warming. Figure 10 shows the exhaust CO2 emission
versus engine load for various fuels. With an increase
in biodiesel fraction in the blends, CO2 emission
decreases monotonically. It is believed that CO2
concentration has a strong relationship with the
carbon–hydrogen ratio in the fuel [25, 27]. Actually,
biodiesel has a low carbon content and thus na-
turally produces less carbon dioxide in the exhaust
gas.
Figure 11 shows exhaust specific NOx emission
versus engine load for different fuels. NOx concen-
tration increases monotonically with increase in
engine load except for the lowest engine load. The
increase in NOx is attributed to the increased
temperature of the burned gas. More fuel is injected
and burnt at high engine loads, leading to an in-
creased cylinder gas temperature. The results show
little variation in NOx level as a function of engine
speed. With increasing engine speed, more fuel is
injected and a higher temperature of the burning gas
is generated. This is beneficial for NOx generation.
Meanwhile, the actual time that the burning gas is at
the high temperature is reduced with increasing en-
gine speed. The combination of these two competing
effects results in the observed insensitivity of NOx
emission with engine speed.
It is also noted that, at the lowest engine load, NOx
concentration decreases slightly when biodiesel is
added to diesel; while at the highest engine load,
NOx level clearly increases when biodiesel is added
to diesel. Song et al. [28] thought that the combina-
tion of decreased low heating value and leaner
overall mixtures with using oxygenated fuels was
responsible for the slightly decreased NOx emissionFig. 8 Brake specific HC emission
Fig. 9 Brake specific CO emission
134 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
levels. At low engine loads, the lower temperature of
the biodiesel spray flame [4, 23, 24] results in slightly
decreased level of NOx emission.
As indicated in Fig. 7, biodiesel gives an obviously
advanced and compact heat release rate curve
compared to diesel at high engine loads, leading to
an increased cylinder gas temperature of biodiesel
operation and higher NOx emission levels. Moreover,
at high loads, a very rich core is generated as more
fuel is injected. NOx emission levels are likely to be
highly influenced by the existence of this high-
temperature fuel-rich core since the oxygen atom in
biodiesel can be used to thermally generate NOx.
These two factors contribute to the increased NOx
concentration for biodiesel at high loads.
Figure 12(a) shows the PM concentration versus
engine load for different fuels. When the engine
speed is increased from 1600 to 2600 r/min, PM
emission levels decrease significantly. At high engine
speeds, more fuel is injected and burned, thus the
temperature of the burnt gas increases. Meanwhile,
the excess air ratio remains almost constant as the
engine speed is increased from 1600 to 2600 r/min.
The high temperature of the burning gas in the
cylinder might be beneficial to oxidize the already
formed PM. The variation of PM emission versus
engine load shows different characteristics to those
observed in a naturally aspirated diesel engine,
where PM emission levels generally increase with
an increase in engine load [8]. The use of a
turbocharger increases the intake air mass at high
engine loads, which provides an opportunity to
reduce PM emission at high engine loads.
Biodiesel shows significant reduction in PM emis-
sion levels regardless of engine speed and load. The
addition of biodiesel to diesel provides more oxygen
to the combustion reaction and promotes complete
combustion especially for those areas at the core of
the fuel spray [7]. Moreover, the oxygen in biodiesel
inhibits cyclic-carbon-molecule formation. There-
fore, the addition of biodiesel decreases PM emission
levels. The clear effect of oxygen content in a fuel on
PM reduction has been previously reported in [7, 8].
Figure 12(b) is a plot of PM reduction rate versus
biodiesel fraction for the blends. PM emission levels
decrease with increasing biodiesel fraction. The
influence of biodiesel fraction on PM reduction varies
with engine speed and load. At a specific biodiesel
fraction and engine load, PM reduction rate increases
when the engine speed is increased from 1600 to
2600 r/min especially at low biodiesel fraction levels
in the blends. Moreover, PM reduction rate is also
increased at high engine loads. At low engine loads,
where the overall mixtures are much leaner, the
oxygen in biodiesel has a limited influence on PM
emissions. While at high engine loads, more fuel is
injected and burned, thus a relative rich core exists. In
this high-temperature fuel-rich core, the oxygen
atoms from biodiesel can consume the soot precur-
sors through forming a OH radical [29]. This effect
results in a significant reduction effect on soot
formation. A similar effect can be used to explain
the increased PM reduction rate when the engine
speed is increased from 1600 to 2600 r/min. This
behaviour of oxygenated fuel on PM reduction at high
engine loads has previously been reported by Song
et al. [28]
3.4 Discussion
As previously described, at a specific engine speed
and load, the excess air ratio is almost unchanged for
different diesel/biodiesel blends, however, the PM
Fig. 10 Brake specific CO2 emission
Performance and emissions of a turbocharged common rail diesel engine 135
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
concentration reduces significantly with an increase
in biodiesel fraction in blends. This characteristic
reflects the role of oxygen in biodiesel on PM
reduction. The PM emission level versus excess air
ratio is plotted in Fig. 13. The oxygen content of
biodiesel is about 11 wt%, while the oxygen content
of the charge is estimated to be six to fifteen times
that of the injected biodiesel. Thus, from a quanti-
tative viewpoint, the oxygen contribution from
biodiesel compared to that from the intake air is
small and could be ignored. This is the reason why a
near constant excess air ratio was used for both
diesel and biodiesel experiments. However, the small
quantity of oxygen in biodiesel results in a signifi-
cant reduction in the level of PM. This reveals that
the oxygen in biodiesel plays an important role on
PM reduction. Wang et al. [24] investigated soot
formation in a biodiesel spray flame in a constant-
volume combustion chamber, and their results in-
dicate that the oxygen in biodiesel plays a significant
role in the reduction of soot levels. This phenom-
enon can be used to garner valuable insight into
PM emission reduction using biodiesel in a diesel
engine. It might be argued that sulphur content in
diesel fuel may also result in high PM emission.
However, the diesel fuel used in this study has a low
sulphur content. Therefore, PM reduction must be
mainly attributed to the oxygen in biodiesel. The
strong PM reduction effect by oxygen in biodiesel
can be attributed to the fact that the oxygen in
biodiesel finds it easy to participate in the combus-
tion reaction. For the oxygen taken from the air, the
fuel needs to be atomized and mixed with this air if it
is to take part in combustion reaction.
PM emission is reduced using biodiesel, while NOx
emission is increased slightly at high engine loads.
PM versus NOx emission using all the data in Figs 11
and 12 is plotted in Fig. 14. At the lowest engine load
(a BMEP of 0.154 MPa), with increasing biodiesel
fraction in blends, PM emission reduces significantly
Fig. 11 Brake specific NOx emission
136 X-G Wang, B Zheng, Z-H Huang, N Zhang, Y-J Zhang, and E-J Hu
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
whereas NOx emission reduces only slightly. In other
words, PM and NOx emissions reduce simultaneously
using biodiesel/diesel blends and pure biodiesel. At
a middle engine load (a BMEP of 0.308 MPa), PM
emission clearly reduces whereas the NOx emission
level remains effectively constant with increasing
biodiesel fraction. At high engine loads, PM emission
decreases significantly whereas the NOx emission
increases slightly as biodiesel fraction is increased.
The significant reduction in PM emission indicates
that the biodiesel/diesel blends (and biodiesel) con-
tain oxygen and thus have a high exhaust gas recircu-
lation (EGR) tolerance. This suggests that the combi-
nation of biodiesel/diesel blends and EGR could allow
the simultaneous reduction of PM and NOx emis-
sions.
4 CONCLUSIONS
Performance, combustion, and emissions of a high-
pressure common rail, turbocharged diesel engine
fuelled with biodiesel/diesel blends as well as neat
diesel and biodiesel have been investigated. The
main conclusions that can be drawn from this work
can be summarized as follows.
1. The brake thermal efficiency increases slightly as
biodiesel is added to diesel. The exhaust gas tem-
perature varies only to a small extent among the
different fuels at low engine speeds, but decreases
with an increase in biodiesel fraction in the blends
at high engine speeds.
2. Biodiesel gives a low heat release rate at pilot
injection and high heat release rate at main
injection.
Fig. 12 PM emission and PM reduction rate
Fig. 13 PM emission versus excess air ratio
Fig. 14 PM versus NOx emission
Performance and emissions of a turbocharged common rail diesel engine 137
Proc. IMechE Vol. 225 Part D: J. Automobile Engineering
3. HC and CO emissions vary only to a small extent
among the different fuels. The level of NOx
emission decreases slightly at low engine loads
and increases at high engine speeds for biodiesel
and biodiesel/diesel blends. Biodiesel and bio-
diesel/diesel blends significantly decrease PM
emission.
4. The oxygen in biodiesel plays a key role in
reducing PM emission. The combination of
biodiesel/diesel blends and EGR could allow the
simultaneous reduction of PM and NOx emis-
sions.
ACKNOWLEDGEMENTS
This work was supported by the National NaturalFoundation of China (50821064). Technical supportfrom Great Wall Motor Company Ltd is gratefullyacknowledged.
F Authors 2011
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