127 Performance and emissions of a turbocharged, high ...gr.xjtu.edu.cn/upload/PUB.1643.4/Performance+and+emissions+of+a... · The manuscript was received on 31 March 2010 and was

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


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


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



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



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



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



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



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



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



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



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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