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of 14
typinjection and atomiz
bus
. Vana
ing and
nt, Belg
Antonio
ce Engi
um
Pump-line-nozzle system
s. The results reported
t volume combustion
nder non-evaporating
medium speed diesel
type and temperature.
oils and animal fats.
igh speed camera was
enetration and spray
parameter to control
because it significantly affects the fuel properties. Both the injection timing and injection
roperties on the spray
ly deteriorated atomi-
td. All rights reserved.
These days, it is clear that the energy availability from non-
renewable sources is limited [1]. Furthermore, fossil fuels
have now been identified as one of the main culprit of climate
change and environmental pollution [2,3]. Therefore,
duction are required.
The automotive industry and the transport sector in general
have to meet future stringent emission limits and zero net
carbon dioxide emissions in the long-termview [4]. This aim is
also driven by the limit of the worldwide oil reserves and the
rising consumption and price of fossil fuels. As a first, short
* Corresponding author. Tel.: 32 9 264 34 53.
Available online at www.sciencedirect.com
.co
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8E-mail address: [email protected] (J. Galle).Sprays
Medium speed diesel engines
duration are affected by the fuel properties. The influences of these p
development were less pronounced. At low temperatures, a strong
zation of oils and fats was observed.
2013 Elsevier L
1. Introduction solutions for carbon dioxide emission re22 June 2013
Accepted 5 July 2013
Available online 3 August 2013
Keywords:
Biofuels
Constant volume combustion
chamber
Physical fuel properties
affecting atomization, spray development and combustion processe
in this paper have been obtained by experimentation with a constan
chamber. The influences of physical fuel properties on injections u
conditions are studied, using a pump-line-nozzle system from a
engine with injection pressures up to 1200 bar, by changing the fuel
Experiments were conducted for diesel, biodiesel, straight vegetable
Injection pressure and needle lift measurements were analyzed. A h
used to visualize the spray, which enabled us to study the spray p
angle. Our results show that the fuel temperature is an importantReceived in revised form However, these fuels have different physical, chemical and thermodynamic propertiesin an optical com
J. Galle a,*, S. Defruyt a, CA. Verliefde c, S. VerhelstaGhent University, Faculty of Engineer
Sint-Pietersnieuwstraat 41, B-9000 GebTechnical University of Havana JosecGhent University, Faculty of Bioscien
Coupure links 653, B-9000 Gent, Belgi
a r t i c l e i n f o
Article history:
Received 30 August 20120961-9534/$ e see front matter 2013 Elsevhttp://dx.doi.org/10.1016/j.biombioe.2013.07.ation of liquid biofuelstion chamber
de Maele a, R. Piloto Rodriguez b, Q. Denon c,
Architecture, Department of Flow, Heat and Combustion Mechanics,
ium
Echeverria, Centro de Estudios de Energa Renovable, Havana, Cuba
neering, Department of Applied Analytical and Physical Chemistry,
a b s t r a c t
Due to the scarcity of fossil fuels and the future stringent emission limits, there is an
increasing interest for the use of renewable biofuels in compression ignition engines.influence of fuel e and properties on the
Experimental investigation concerning thehttp: / /www.elsevierier Ltd. All rights reserved004m/locate/biombioe.
Tf Fuel temperature
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8216term solution, considerable effort has been made to reduce
harmful emissions from diesel engines, to meet these stricter
emission standards [5]. However, as fossil fuels are limited, a
second, long term solution is essential. The use of renewable
fuels can be a solution for the above two problems simulta-
neously [6]. Biofuels can offer a significant contribution in
reducing harmful emissions. Besides, it is a biologically
derived product and thus one of the most efficient ways of
Abbreviations& Symbols
AF Animal Fats
ASOI After Start Of Injection
B Bulk modulus of the fuel
CI Compression Ignition
CVCC Constant Volume Combustion Chamber
D Diesel
DI Direct Injection
FLP Full Lift Period
fps Frames per Second
GUCCI Ghent University Combustion Chamber I
RSO Rapeseed Oil
LL Liquid Length
NL Needle Lift
NOP Needle Opening Pressure
PLN Pump-Line-Nozzle
PO Palm Oil
RME Rapeseed Methyl Ester
RSO Rapeseed Oil
SMD Sauter Mean Diameter
SVO Straight Vegetable Oilsreducing the carbon dioxide emissions [7].
It is generally accepted [8e11] that the fuel atomization
process is extremely important for the reduction of fuel con-
sumption and exhaust emissions of diesel engines. A good
atomization leads to an improvement of the airefuel mixing,
causing a more complete and more efficient combustion
process that results in a higher performance and less
pollutant emissions. So, in order to fulfill future emission
regulations, the atomization process has to be investigated
thoroughly. Although there are many literature reports
related to the review on performance and exhaust emissions
of compression ignition (CI) engines fueled with biofuels,
these studies mostly treat the engine as a black box, without
studying in depth how biofuels affect the injection process or
what are the repercussions of its use on the airefuel mixing
[6,12].
Biofuels have different properties, affecting atomization
and thus combustion. In addition, fuel properties are affected
by operation temperature and pressure. As an example, fuel
viscosity in the cold start phase can be doubled compared to
normal operating conditions. However, there is little knowl-
edge about the influences of the different fuel properties on
the atomization process [7,13]. This knowledge is essential in
order to adapt engines for the use of alternative fuels and to
ensure reliable calibration of the injection parameters at all
operating conditions and for all fuels [7]. Furthermore, this
knowledge is important facing the arrival of new alternativefuels and could be used to design fuels displaying better
characteristics [14].
We used a traditional pump-line-nozzle injection, as
implemented in current medium speed diesel engines system
to conduct single-shot injection experiments in an optically
accessible constant volume combustion chamber (CVCC)
under an inert, non-evaporating atmosphere. The experi-
mental conditions were varied and different fuels were used.
Tc Injector cooling temperatureV Volume of fuel between the plunger of the pump
and he injector needle
Greek symbols
a Cam angle position
Roman symbols
DP Pressure drop across the nozzle orifice
Cd Discharge coefficient
Fp Force of the fuel acting on the injector needle
Fs Force of the spring acting on the injector needle
Fv Viscous force on the injector needle
NLstart The cam position when the needle starts to lift
[CA]Pa Ambient pressure, pressure inside the
combustion chamber
rf Fuel density
ra Ambient density, gas density inside the
combustion chamber
Ta Ambient temperature, temperature inside the
combustion chamberWe studied the influences of engine parameters, physical fuel
properties and in-cylinder conditions on the injectionpressure
andneedle lift andon the spray development andatomization.
2. Experimental method
2.1. Experimental setup and data acquisition
The experiments were conducted in the Ghent University
Combustion Chamber I (GUCCI). This constant volume com-
bustion chamber has an internal volume of 1603 mm3 and
optical access in 2 directions through quartz windows of
150 mm in diameter. The injector is placed along the space
diagonal (Fig. 1). The aim of this setup is to carry out and
visualize single shot injections with different types of fuels.
The injection system is an exact copy of the Pump-Line-
Nozzle (PLN) system, used by the engine manufacturer
(256 mm bore engines). The injection pump in the setup is
operated by a camshaft which is driven by an electric motor
with a power of 2.2 kW. Considering that the instantaneous
power from the electric motor is not enough to operate the
pump with a low coefficient of irregularity, the required en-
ergy is stored in a flywheel mounted on the camshaft.
A single shot injection is obtained by the pneumatic
operation of the control rod of the pump. At rest a pneumatic
cylinder retracts the control rod into zero load position.
co
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8 217A Bryce Woodward heavy-duty sac-hole nozzle with eight
440 mm orifices was used. During the experiments, the atmo-
sphere in the combustion chamber consisted of an inert gas, N2,
at a maximum pressure Pa of 80 bar and a maximum tempera-
ture Ta of 150 C. As a consequence, only the atomization andspraydevelopmentofnon-evaporatingsprayswas investigated.
All optical measurements are obtained with shadowgraph
imaging [15]. Continuous light from an Xe-arc lamp is focused
onto a pinhole of 1 mm to simulate a point source. The light is
collimated by a 1200 mm focus biconvex lens (f/8) to a parallel
light beam. Planar mirrors steer the beam through the com-
bustion chamber. A schematic overview of the setup is given
in Fig. 2.
The authors refer to [1] for a more detailed description of
Fig. 1 e Left: Mounting of the injector on the constant volume
the x-axis.the injection equipment, the optical measurement equip-
ment, the sensors and the data acquisition of the GUCCI setup.
2.2. Tested fuels
The fuels we tested are diesel (D), a biodiesel derived from
rapeseed oil (rapeseedmethyl esters, RME), animal fats (AF) and
twostraight vegetableoils (SVO): rapeseedoil (RSO) andpalmoil
(PO). Inthiswork,2differentbatchesof rapeseedoil, indicatedas
RSO1 and RSO2, are used of which the origin is unknown. Inter-
esting tomention is that theodor andcolorof the2batcheswere
different. When investigating some physical properties, rele-
vant for atomization, small but significant differences were
noticed (Figs. 3e5). A GCeMS analysis (cfr. Fig. 6) reveals a dif-
ference in chemical composition which will probably influence
the ignition and combustion process more than the atomiza-
tion. The combustion behavior is not studied in this work. No
significant differencewas between RSO1 and RSO2 in the optical
spray images. Nevertheless, for all results shown along this
study, theRSObatch is specified. Inorder to study the influences
of the physical fuel properties on the injection process, knowl-
edge of the exact values of these properties at different fuel
temperatures is needed. The dynamic viscosity was measured
using a Brookfield DV II viscometer, measurements of thesurface tension were based on theWilhelmy plate method and
for the density measurements an Anton Paar DMA5000 density
meter was used. All measurements were conducted at atmo-
sphericpressure.Theresults forviscosityanddensityareshown
inFigs. 3 and4.Thebulkmoduliof fuelswerenotmeasured.The
bulk modulus is a measure for the compressibility of a fluidum
and is defined as the ratio of the pressure increase to the
resulting relative decrease of the volume. Since the bulk
modulus isamainparameteraffecting the injectionpressure for
PLN-systems, data on the bulk moduli of fuels were gathered
from literature. These data are represented in Fig. 5.
2.3. Fuel temperature control
mbustion chamber. Right: Injector cooling and definition ofFuel properties strongly depend on the fuel temperature.
Therefore it is very important to know and control the fuel
temperature Tf, defined as the temperature of the injected fuel
at the moment of injection, for every experiment.
Between two injections, a certain amount of fuel resides
inside the injector. Since the internal volume of the injector
exceeds the injected volume (2000mm3} per injection), all fuel
that will be injected has been inside the injector since the end
of the previous experiment. The time between two single shot
experiments is sufficiently long to assume that the fuel inside
the injector adopts the equilibrium temperature of the
injector. Based on this assumption, we conducted tempera-
ture measurements inside the injector in order to determine
the regime temperature of the fuel.
The injector is both in contact with the chamber walls and
injector cooling (Fig. 1), resulting in a temperature gradient
inside the injector. The chamber walls are electrically heated
in order to control the gas temperature Ta inside the chamber.
The temperature Tc of the injector cooling oil is controlled as
well. Our goal was to set up a correlation, which expresses the
fuel temperature Tf as a function of the PID-controlled tem-
peratures Tc and Ta.
To set up this correlation, a set of experiments was con-
ducted. An injector was provided with a drilling along its
tan
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8218central axis, making it possible to use a K-type thermocouple
tomeasure the temperature inside the injector. This was done
along the central axis, using a stepsize of 1 cm and starting at
the sac volume (x 0 cm) up to the place where the fuel entersthe injector (x 18 cm). The location x along the axis is shownin Fig. 1. This was done for different combinations of Tc and Ta(Fig. 7). Note that the conditions inwhich Tc exceeds Ta are not
considered, since this is not relevant for engine research. To
make the correlationmore robust, artificial pointswere added;
these points correspond with the situation in which the tem-
perature of the injector cooling and chamber cooling are equal.
In these cases, the fuel temperature should be equal to the
cooling and chamber temperature as well. Afterward, the in-
ternal fuel volume of the injector per unit of length along the x-
axis was used to calculate the fuel temperature as the mass
Fig. 2 e Schematic overview of the consweighted average of the temperatures measured along the x-
axis. This resulted in a Tf-value for every measurement point
of Fig. 7. Finally, these datawere used to fit the coefficients of a
correlation by the use of the LevenbergeMarquardt algorithm
Fig. 3 e Viscosity as a function of temperature for the fuels
used in our experiments.[16], which expresses the fuel temperature Tf as a function of
Tc and Ta. The fitting of the coefficients resulted in differences
between the predicted and experimental fuel temperatures
which are below 1 C.
2.4. Measurements and standard deviation
2.4.1. Injection pressure profile and needle displacementFor PLN injection systems, the injection pressure, which is the
pressure of the fuel inside the injector and the injection pipe, is
not constant during an injection. Furthermore, this pressure
actsuponthe injectorneedle,affectingtheneedledisplacement.
We conducted an analysis on the influences of engine parame-
ters, fuel properties and in-cylinder conditions on the injection
pressure and the needle displacement. The injection pressure
t volume combustion chamber GUCCI.was measured using a Kistler 4067A2000 pressure sensor
mounted half-way the injection pipe. The standard deviations
on these data are below 2% during the injection event. To
Fig. 4 e Density as a function of temperature for the fuels
used in our experiments.
monitored by PID controllers.
article.)
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8 219measure the needle displacement, a small magnet was fixed at
the topof the injectorneedle. In theupper regionof the injectora
hall-effect sensorwasmounted, deliveringa signal proportional
to the distance of the magnet. Both the injection pressure and
the needle position weremeasured every 1/10 cam angle (ca).
2.4.2. Spray imagesA high speed CCD camera (Photron FastcamAPX RS) was used
to obtain two dimensional spray images with a pixel resolu-
tion of 512 512 at a frame rate of 10 kHz. Image processingwas able to extract the spray penetration length, spray edge
and spray angle. In literature, many different definitions of
these characteristics can be found. This might lead to
different results among institutes or evenmisinterpretation of
the users of the data. Our definitions are based on those re-
Fig. 5 e Bulk modulus as a function of temperature. Data
obtained from literature.ported and used by Siebers [17]. Details of the used image
processing are described by Galle et al. [1].
Similar standard deviations were obtained for all mea-
surement conditions. The standard deviation percentage is
not significantly reducedwith the amount of experiments, but
becomes more stable over the time after start of injection
(ASOI). This was shown previously by Galle et al. [1]. Galle et al.
obtained a stable value for the spray penetration of about 4%.
Fig. 6 e Fatty acid composition by GCeMSIn this work, smaller standard deviations were found with a
stable value being around 2%. Higher standard deviations for
the spray angle were noticed. However, once the spray is fully
developed the deviation on the spray angle varies between 1
and 3 for most experimental conditions.The reason for the lower standard deviations in thiswork is
meanly the changes to the setup for the capability of more
accurate setting of the boundary conditions: deviations on the
injection profile were reduced by the change of the fuel cir-
culation pump, the chamber pressure was controlled by a
more accurate pressure sensor, and temperatures wereFig. 7 e Visualization of the experimental measurement
points (red diamonds). The blue line represents Ta [ Tc.
The open circles represent artificial measurement points.
(For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this3. Results and discussion
3.1. Injection pressure profile and needle displacement
3.1.1. General view on the injection pressure profileBefore discussing the detailed results of the analysis of the
different phases of the fuel injection, we start with a general
for the investigated fuels in mass%.
view on the injection pressure profile. The main parameter
affecting these processes is the engine speed (dotted lines in
Fig. 8). The higher the engine speed, the higher the injection
pressure and the longer the injection duration expressed inca. On a time based scale however, the injection period de-creases with increasing engine speed. The period between the
start of pressure buildup and the end of injection for diesel is
approximately 18, 12 and 9ms for injections at 400, 700 and1000rpm respectively.
In this work, the main focus is the study of the influences
of fuel properties on the injection process. Fig. 9 and the solid
lines on Fig. 8 illustrate the significant effect of fuel properties
on the injection pressure profile. The height of the injection
pressure is mainly determined by the bulk modulus of the
fuel, while the injection duration strongly depends on the fuel
density. This will be discussed later on.
da V da
where a is the cam angle position. B is pressure and temper-
ature dependent and dV/da is determined by the geometry of
the cam only. The secondary parameters affecting the pres-
sure are the expansion of the fuel pipe when pressure rises
and the fuel leakages along the plunger. As a conclusion, the
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8220The pressure profile is quite wavy due to pressure waves
traveling along the injection pipe and the injector. However,
the exact same wave pattern is observed for different mea-
surements at the same conditions. Even under different con-
ditions, the wave pattern is very similar, as can be seen in
Fig. 9. The frequency of these waves is quite constant during
the injection. At 1000rpm for example, the distance betweentwo adjacent peaks is around 0.7ca. Small variations wereobserved between different fuels due to changes in the speed
of sound of the fuel. The amplitude of the waves is attenuated
due to the viscosity of the fuel [18]. For the more viscous SVO
and AF the curves are rather smooth at the top of the injection
pressure profile. This is not the case for the less viscous fuels
RME and diesel (Fig. 9). This effect is also illustrated by the
solid lines of Fig. 8: the lower the fuel temperature and thus
the higher the viscosity, the more smooth the injection pres-
sure profile. At the end of injection, after the closure of the
needle, the pressure signal is strongly oscillating. This is
caused by pressure waves due to the sudden closure of the
needle. In this region the waves are not completely repro-
ducible resulting in high standard deviations in this region.
However, the frequency seems to be quite constant for all
measurements. This oscillatory region after the end of injec-
tion will not be studied in detail in this text.
In the next paragraphs we will discuss some parts of the
injection process in more detail. We will focus on the mostFig. 8 e Injection pressure profiles of injections with rapeseed o
setpoints.important influences of fuel properties on the injection
process.
3.1.2. Pressure build-upFor a PLN injection, the movement of the injection pump
plunger is camshaft driven. Pressure is built up inside the
pump, causing the delivery valve of the pump to open. As the
camshaft is rotating, the plunger moves upward and the vol-
ume V of fuel between the plunger of the pump and the
injector needle is reduced. The increase of pressure related to
this decrease in volume is mainly determined by the bulk
modulus B of the compressed fuel:
dp BP;TdV (1)
Fig. 9 e Injection pressure profiles of injections at 1000 rpm
engine speed, using different fuels. Tc[ 50 C, Ta[ 140 C,Tf [ 56.5c and Pa [ 80 bar.il for different fuel temperatures and different engine speed
nt temperatures (Tf[ 26 and 45 C), 1000 rpm and Pa[ 80 bar.
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8 221bulk modulus is the most important parameter affecting the
pressure build-up.
A higher bulk modulus results in higher injection pres-
sures. This is illustrated by Fig. 9, which shows that the fuels
with the highest B have the highest injection pressure and the
solid lines of Fig. 8, which shows that the decrease in B for
higher fuel temperatures, results in a decrease of the injection
pressure.
3.1.3. Needle liftThe volume V is compressed until the injector needle lifts.
This moment is determined by the forces acting on the needle
and is defined as the point where the needle lift exceeds 4% of
the full lift. Same definition holds for the closing of the
injector. On the one hand there is the force of the spring (Fs),
who keeps the injector needle on its seat, on the other hand
there is the pressure of the fuel acting on the needle resulting
in the force Fp. Once the needle opening pressure (NOP) is
reached, F overcomes F and the needle starts to lift. The
Fig. 10 e Spray images of diesel and RSO1 injections at differe
Penetration lengths are around 180 mm.p s
dynamics of the needle displacement can be expressed as
follows:
mat Fpt Fst Fvt (2)where a(t) is the upward acceleration of the needle. Fs is pro-
portional to the needle displacement. The viscous force Fv is
proportional to theneedlesurfaceareaAandthe fuelviscositym:
Fv mAdvtdy (3)
dv(t)/dy is the velocity gradient in the fuel surrounding the
needle and depends on the needle velocity.
These equations show that the inertia of the needle with
mass m and the viscous forces of the fuel, slow down the
needle displacement. This causes a significant delay between
the moment when the NOP is reached and the moment when
the needle starts to lift. During this period pressure buildup
continues. A higher bulk modulus of the fuel causes a faster
increase in Fp, resulting in an earlier start of needle lift.
Fig. 11 shows NLstart, which is the cam position at the
moment when the needle starts to lift, as a function of fueltemperature for different fuels. The most important fuel
properties affecting the moment of needle lift are the bulk
modulus and the viscosity. A higher bulk modulus causes a
higher injection pressure, a larger Fp and thus an earlier start
of needle lift. A higher viscosity increases the viscous forces
and delays the start of needle lift. The effect of viscosity
retarding the needle displacement was already observed for
common rail (CR) injections [19,20]. For CR injection systems,
the injection pressure is constant, so there is no effect of the
bulk modulus counteracting the effect of viscosity.
For diesel and RME the changes in viscosity are rather
small when temperature is varied (Fig. 3). The needle lift is
delayed when Tf increases, due to the decrease in B. For the
SVO and AF, the changes in viscous forces are more impor-
tant, especially in the low temperature range (Fig. 3). The ef-
fects of B and viscosity counteract each other resulting in an
NLstart which is less dependent on the fuel temperature. In the
high temperature range (70e90 C), where the viscosity israther constant, the effect of the bulk modulus becomesmore dominant: the needle lift is retarded with increasing
temperature.
Fig. 11 e NLstart [ca] as a function of fuel temperature fordifferent fuels. Average values of 9 experiments at engine
speed set point of 1000 rpm. Standard deviation
One can conclude that at elevated temperatures (above
60 C) the higher bulk moduli of SVO and AF lead to an earlierstart of needle lift compared to diesel injections.
The duration of the needle lift period did not seem to vary a
lot with temperature. The needle lift period is defined as the
time between opening and closing of the nozzle. The observed
increase of 0.1ca between 40 and 90 C is possibly the result ofa decreasing bulk modulus. The differences between different
fuels, however, are more clear. The needle lift period of diesel
injections (1e1.1ca) is longer than the one of RME injections(0.9ca) at 1000rpm engine speed. For injections at 700rpmno significant difference was observed (1.3e1.5ca) betweenthese two fuels. The needle lift period of SVO andAF injections
is shorter (0.8ca at 1000 rpm and 1e1.1ca at 700rpm). Thiscan be related to the higher bulk modulus of SVO and AF,
increasing Fp acting on the needle during needle lift. The
viscous forces do not seem to have a significant effect on the
needle lift period. Combinedwith the earlier start of needle lift
(at high temperatures), the higher bulk moduli of SVO and AF
result in 0.5ca earlier injections for these fuels compared todiesel (Fig. 12).
The earlier injection timing for SVO and AF will impact
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8222engine emissions and performance. Experiments of Bari et al.
[21] revealed that oils (waste cooking oil) and diesel respond
identically to injection timing changes. So, based on the
knowledge on diesel combustion, we can try to predict the
influences of the injection timing on the performances and
emissions of engines running on SVO or AF. The earlier start of
injection implies that the temperature and pressure inside the
cylinder will be lower at the start of injection. As a result, the
ignition delay will increase for the SVO and AF. The poorer
atomization and lower volatility of the bio-fuels can even
reinforce the increase of the ignition delay compared to diesel
fuel. The later start of ignition results in an increased amount
of heat release in the premixed combustion phase, increasing
the probability of diesel knock and causing a higher
Fig. 12 e Comparison of needle lift as a function of ca fordiesel, RME, animal fat and rapeseed oil injections.Tc [ 90 C, Ta [ 140 C, Tf [ 91.5 C, engine speed setpoint [ 1000 rpm, Pa [ 60 bar.temperature during the subsequent diffusive combustion
phase [22]. The period during which the mean temperature
inside the cylinder is above 1500 K, is a very important factor
affecting the formation of thermal NOx. As a result, the
increased ignition delay due to the earlier injection, leads to
higher NOx emissions [22]. However, the earlier injection
timing also has some advantages. First of all, more fuel is
burned in the premixed combustion phase. During a premixed
combustion less local over-rich spots are present compared to
the diffusive combustion phase. This results in a more com-
plete combustion and lower CO, HC and smoke emissions [22].
3.1.4. Pressure drop caused by needle liftWhen the needle rises, fuel starts to leave the nozzle through
the orifices and enters the chamber. This sudden decrease in
pressure at the nozzle tip results in a pressure wave traveling
upstream.When it arrives at the pressure sensor, a drop in the
pressure profile is observed. In Fig. 9 this drop can be seen
between 345 and 346ca. The distance between the sensor andthe needle tip is about 40 cm, the delay between the start of
the needle lift and the start of the drop is 0.8ca at 1000rpm(970rpm effectively) and 0.6ca at 700rpm engine speed. Bothcorrespond to 280 ms, which is the time needed to travel those
40 cm at approximately 1400 m/s, which is the velocity of
sound in the fuel. Although the differences are smaller than
the standard deviation on the delay, it was possible to detect
an increasing trend of the delay with increasing fuel temper-
ature, due to the decrease in speed of sound of the fuel.
3.1.5. Injection periodTo express the volumetric flow rate through the nozzle ori-
fices, the theoretical volume flow rate of the fuel is multiplied
with a discharge coefficient Cd:
_V Cd _Vth CdA2DPrf
s(4)
with rf the fuel density, A the orifice cross section and DP the
pressure drop across the orifice.
Different authors [7,14] stated that the discharge coeffi-
cient is independent of the fuel density for every DP. Although
CR injection systems were used for their experiments, we
expect their conclusions to be valid for PLN injections as well.
So _Vis directly proportional to the square root of the ratio of DP
to rf. For PLN systems, _V is not constant during an injection
because DP (and thus Cd) is not constant. These changes in DP
cause only very small changes in rf, so we can assume rf to be
approximately constant.
If leakages are neglected, the total injected volume of fuel
is constant for PLN systems at constant pump position
(2000 mm3 for our experiments). Using the simplification that
DP is constant, equation (4) expresses that the injection
duration is directly proportional torf
p: This simplified ana-
lyses shows that the fuel density is amain parameter affecting
the injection duration. This was confirmed by our experi-
mental results. We observed a linear relationship between the
full lift period (FLP) andrf
p(Fig. 13). The FLP was defined as
the duration of the period of full needle opening. We canconclude that the higher density of SVO and AF results in
longer injections at lower volumetric flow rates, but higher
the height of the partial lift is between 25 and 40% of full lift,
compared to 8e20% for diesel and RME (Fig. 14). This results in
larger amounts of fuel injected during the partial needle lift for
SVO and AF. There is a slight effect of temperature on the
height of the partial lift (Fig. 15). The height decreases at
higher temperatures because of the temperature dependent
behavior of B. A more important parameter however, is the
chamber pressure. The chamber pressure acts up on the
needle tip, resulting in an extra upward force. At high engine
speeds, these forces were negligible compared to the fast and
high pressure buildup. At 400rpm, on the contrary, this af-fects the height of the partial lift significantly. For SVO and AF
an increase of 10% of full lift was observed when the chamber
pressure was increased from 40 to 80 bar (Fig. 15).
3.2. Spray development
All experiments were conducted in an (N2) atmosphere at
chamber pressures ranging from 40 bar to 80 bar, engine
speeds from 400rpm to 1000rpm. The chamber gas temper-
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8 223mass flow rates. This effect can be slightly reinforced by the
reduced leakages for SVO and AF, caused by their higher vis-
cosity and resulting in a slightly higher injected fuel volume
per injection [23].
Based on the longer injection duration of SVO and AF
compared to diesel, we would expect a longer combustion
duration as well. This would result in a decrease in thermal
efficiency. However, the earlier start of injection increases the
ignition delay, increasing the amount of fuel that is burned
abruptly in the premixed combustion phase. This effect
shortens the total combustion period because less fuel has to
be combusted during the diffusive combustion phase. It can
Fig. 13 e Full lift period [ca] as a function of square root ofrf for injections at 1000 rpm using different fuels. The solid
data have standard deviations below or equal to 0.13ca.The others have standard deviations above 0.22ca.partially counteract the effect of the longer injection period.
3.1.6. Partial needle lift at low engine speedsAt low engine speeds, the needle lift was observed to consist
of two phases. This can be explained as follows: on a degree
cam angle based scale, the pressure buildup before needle lift
is independent of the engine speed. Once the fuel pressure
exceeds the NOP, the dynamics of the needle can be expressed
using equation (2). If the needle starts to lift, a sudden pres-
sure drop is induced. At 400rpm, this pressure drop, causesthe pressure to fall below the NOP again. Thus, during a short
period of time, the resulting pressure on the needle is directed
downwards again. The needle drops again, but does not fully
fall back on its seat. During this partial needle lift, a small
amount of fuel is injected at low speed and is probably poorly
atomized. The influences of this partial needle lift on the spray
penetration are discussed in Section 3.2.1.
Themost important parameter affecting this partial needle
lift at low engine speeds is the bulk modulus. First of all, B
affects the pressure rise before and during the needle lift and
determines the pressure at start of needle lift. Furthermore,
the pressure rise together with the viscous forces will control
the moment, the velocity and the height of the first partial lift.
For SVO and AF, an earlier start of needle lift is observed.
Furthermore, the pressure at start of needle lift is higher andFig. 14 e Needle lift at 400 rpm engine speed as a functionature (Ta) was varied between 20 C and 150 C. Fuel temper-ature was changed by varying the injector cooling
temperature and the chamber gas temperature according to
the obtained correlation.
3.2.1. Spray length3.2.1.1. Influences of the engine parameters. The influences ofengine parameters and in-cylinder conditions on the spray
penetration can be investigated by equation (5). This equation
expresses the spray penetration as a function of time S(t) in
the fully developed zone for a CR injection [24]:
St
2DPrf
d0t
svuut (5)withDP the pressure drop across the nozzle, ra the chamber gas
density and d0 the orifice diameter. The influences of chamberof ca for different fuels. Tc [ 70 C, Ta [ 140 C,Tf [ 73.6 C and Pa [ 60 bar.
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8224gas density on spray penetration were investigated by varying
chamber pressure (Fig. 16). A higher chamber gas density racauses a higher aerodynamic drag [14,25]. More momentum is
transferred from the fuel to the ambient gas, resulting in lower
penetration rates [26]. Furthermore, the influence of chamber
gas density is non-linear [17,27]. This was also observed in our
experiments: the differences in spray penetration between 20
and 40 bar are larger than the differences between 40 and
60 bar, which are again larger than the differences between 60
and 80 bar. Further increase of the pressure and density will
have less and less impact on the liquid length but can increase
the efficiency and power output of the engine.
The influences of the engine speed on the spray penetration
Fig. 15 e Needle lift at 400 rpm engine speed as a function
of ca for rapeseed oil at different fuel temperatures andchamber pressures.can also be derived from equation (5). The higher the engine
speed, the higher the pressure difference across the nozzle and
thus the higher the velocity of the fuel at the nozzle exit,
resulting in fasterspraypenetrations.Thiswasalsoobserved for
our experiments aswas already reported inRef. [1]. Injectionsat
400rpmfordiesel, RMEandRSO1are shown inFig. 17. Thespray
Fig. 16 e Influence of ambient pressure on spray
penetration. Injections with Diesel at Tf [ 60 C. Pa [ 20,40, 60 and 80 bar at engine speed set point 1000 rpm.penetration is quite different at SOI for 400rpm injectionscompared to the injections at 700 and 1000rpm. In the earlystages, the spray tip penetration is very slow, while after some
time, the spray tip velocity increases. This can be explained by
the needle lift, which was found to consist of two phases at
400rpm (see the inset of Fig. 17). During the first phase, theneedle lifts partially and a small amount of fuel is injected. Af-
terward, the needle drops again and less liquid is injected. This
results in a low spray penetration velocity. When later on the
main needle lift starts, a sudden increase in spray penetration
velocity is observed. This is clearly reflected in the spray pene-
tration progress: for RME and RSO1, a significant decrease in
penetration speed is observed, which corresponds to the small
drop of the needle after the first phase of the needle lift. For
Fig. 17 e Spray penetration for diesel, RME and KKZO as a
function of time ASOI. The inset shows the corresponding
needle lift for RME and KKZO. Engine speed [ 400 rpm,
Pa [ 60 bar and Tc [ 70 C.diesel, the partial needle lift is rather small and probably no fuel
is injected before themain needle lift.
3.2.1.2. Influences of the fuel properties. The injector temper-ature Tcwas varied between50and 90 Cand experimentsweredoneat Pa 40, 60and80bar. Results of PO injections are shownin Fig. 18. The curves corresponding to the same Pa show a very
similar penetration behavior. So, the influence of the chamber
pressure on the spray penetration is more important than the
one of the fuel temperature and thus the fuel properties. This
wasobserved for all fuels thatwere tested. The sameconclusion
was drawn when studying the effect of the engine speed: vary-
ing the engine speed affects the spray penetration more signif-
icantly than varying the fuel temperature.
The influences of the fuel properties are further investigated
in Fig. 19: the penetration length for injectionswithDiesel, RME,
RSO1 and PO at 80 bar, 1000rpm and Tf 65 C are compared.During the early stages of spray formation, the penetration is
quite similar for all fuels. This observation differs from most
results in literature for CR systems [14,19,28], which report a
decreased penetration for bio-fuels at the early stages of the
injection. However, this can be attributed to the PLN system.
Injection pressures at the needle lift are higher for SVO due to
their higher bulk modulus, so that the outlet velocity and thus
coefficient of the droplet. First of all, this equation shows that
the higher density of SVO causes a decrease in droplet decel-
eration, causing the longer spray penetrations. Secondly, the
droplet sizes of SVO are accepted to be larger than those of
diesel and biodiesel [6]. Larger droplets experience less
deceleration and result in longer spray penetrations. The
larger droplet sizes are mostly attributed to their higher vis-
cosity [14], surface tension [13] or both [6]. These droplet size
studies were all conducted for CR systems. For PLN systems,
the higher injection pressures of the SVO will also decrease
droplet sizes [29]. This effect only partly counteracts the in-
fluences of viscosity and surface tension on droplet sizes.
We can conclude that during the early stages of the injec-
tion, the reduction in spray outlet velocity due to the high
viscosity and density of the SVO and AF is counteracted by the
increase in injection pressure at SOI due to the higher bulk
moduli of SVO and AF. In the fully developed zone, the higher
Fig. 18 e Influence of the fuel temperature and ambient
pressure on the spray penetration. Injections with PPALM
at Tc [ 50 C (red curves), 70 C (blue curves) and 90 C(black curves). Pa [ 40 (longest penetration length), 60
(curves in the middle) and 80 bar (shortest penetration
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8 225penetration increases. This counteracts the increased friction
due to the higher viscosity and the lower outlet velocity due to
the higher fuel density (Bernoullis law). Together this results in
similar spray penetrations at SOI for all fuels tested. In a later
stage, the spray tip penetrations of PO, RSO1 and less clearly AF,
are faster than those of Diesel and RME. A first explanation is
the higher injectionpressures for SVOandAFwhen using a PLN
system, due to their higher bulk modulus. However, the longer
spray penetrations in the fully developed zone for SVO
compared to diesel were also observed for CR systems
[14,19,28]. So there should be other explanations as well. These
can be derived from the following equation expressing the
aerodynamic drag on a liquid droplet:
length). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version
of this article.)a 83rairrfuel
Cw1rv2 (6)
where r is the radius of the droplet, v the speed of the droplet
relative to the surrounding gas, a the acceleration of the
droplet, rf the density of the fuel droplet and Cw the drag
Fig. 19 e Influences of fuel type on the spray penetration.
Injections with Diesel, RME, KKZO, PPALM and AF are
compared. Pa [ 60 bar, Tc [ 60 C.injection pressure and the larger SVO and AF droplets result in
a longer penetration length.
3.2.2. Spray angle3.2.2.1. Influence of the engine parameters. The spray angle isdetermined by the entrainment rate of ambient gas into the
spray. Because of the velocity difference between the fuel and
the gas, gas will entrain into the fuel spray while the fuel
penetrates along the spray axis. Due to the ambient gas, the
droplets will experience drag forces. This drag corresponds to
a momentum transfer from the droplets to the ambient gas,
decelerating the droplets [14]. Thus, a higher chamber gas
density causes an increased deceleration of the droplets and
as a result, faster droplets will push the slower ones outside
more rapidly. This stronger dispersion of the droplets results
in wider spray angles. This is expressed by different authors
[24,30] using the relationship tan(q)wrax, with x between 0.17
and 0.20 in most publications. In Fig. 20, measurements with
diesel, RME and RSO1 at Tc 60 C are shown for Pa 40 barand Pa 80 bar. In these figures, an increase of approximately5 was observed between 40 and 80 bar. We found x to bearound 0.19, which is in close agreement with the results of
Desantes et al. [30] (x 0.17). No significant difference wasfound among the different fuels.
Fig. 20 e Influence of ambient pressure on the spray angle.
Injections with diesel, RME and KKZO. Tc [ 60 C, Pa [ 40
and 80 bar, engine speed set point 1000 rpm. Moving
average over 500 ms, equal to 5 spray images.
probably results in a significant reduction in droplet sizes and
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8226Furthermore, when comparing the spray structures, dif-
ferences between the sprays at 40 and 80 bar were observed
(Fig. 10). The outer ranges of the sprays at low chamber
pressure appear to be less dense. On the two dimensional
spray images, the spray is surrounded by a more clear region.
This was observed most clearly for injections with RSO1 and
will be discussed at the end of the next section.
The influences of the engine speed on the spray angle were
investigated by comparing spray angles at engine speed set-
points of 400, 700 and 1000rpm. When varying the injectionspeed between 700 and 1000rpm, we did not observe anydifferences in the steady state spray angle. Similar steady
state spray angles for different injection pressures are also
reported in literature. Takahashi et al. [31] showed that once
the injection pressures is sufficiently high, the spray angle no
longer depends on the injection pressure. Next, Desantes et al.
[30] stated that at high injection pressures, a further increase
in injection pressure can affect the spray angle through cavi-
tation only. Although the steady state spray angle is similar,
slightly higher spray angles were sometimes observed at SOI
for higher engine speeds. Takahashi et al. [31] and Galle et al.
[1] mentioned that for relatively low injection pressures, the
spray angle increases with increasing injection pressures. So,
because the injection pressure shortly ASOI is higher for in-
jections at 1000rpm, this can be a first explanation. However,differences in cavitation regime can also influence the spray
angle ASOI.
For 400rpm injections, mostly a steady state spray anglewas reached similar to the 700 and 1000rpm injections.However, at high chamber pressures, sometimes the spray
angle increased as a function of time during the whole injec-
tion event. In these cases, the spray diverged strongly from
the theoretical conical spray shape.
3.2.2.2. Influences of the fuel properties. The standard devia-tion of the spray angle (1e3) is mostly larger than differencesbetween experiments with different fuels or between experi-
ments at different fuel temperatures. So, the results that are
discussed here are trends that were discernible, but they have
to be verified by conducting more identical experiments for
every set of conditions. In Fig. 20, spray angles of different
fuels are very comparable. Varying the fuel temperature did
not alter the spray angle for diesel and RME. However, for PO
and AF, increasing the fuel temperature often resulted in an
increase in spray angle. The higher spray angles at higher fuel
temperatures observed for the SVO could be related to the
decrease in viscosity, density and surface tension. Larger (due
to high viscosity and/or high surface tension) and heavier (due
to high fuel density) droplets, have more momentum and
experience less deceleration (equation (6)). This reduces the
chance to get pushed outside, decreasing the spray angle.
However, this does not explain why spray angles between
diesel and SVO are similar. In general, the spray angle seems
to be altered only slightly by fuel properties for the PLN in-
jection system.
The spray angle is calculated by image processing soft-
ware. However, similar spray angles do not mean that there
are no structural differences betweendifferent fuel sprays. Forfuel temperatures above 45 C, no clearly visible structuraldifferences were observed between sprays of different fuels.explains why no clear region was observed around the spray
anymore. If the clear region around the spray was observed at
higher fuel temperatures, it was at 40 bar. Based on the pre-
vious explanation about the droplet sizes, this could be related
to a higher SMD at lower chamber pressures. However, the
influences of chamber pressure on SMD are not generally
accepted in literature [29], so this is only a hypothesis. Finally,
we observed that the clear region around the spray was more
explicit at lower engine speeds. Again, this can be related to
the higher SMD at lower engine speeds due to the lower in-
jection pressures [29]. Experimental investigations of SMD are
necessary to provide more insights into these phenomena.
4. Conclusions
In this paper, spray measurements were conducted in a con-
stant volume combustion chamber in inert non-evaporative
atmospheres at elevated pressures (up to 80 bar). The
research focuses on medium speed diesel engines and a
pump-line-nozzle injection system is used. Fuels investigated
were diesel, rapeseed biodiesel (RME), rapeseed oil (RSO), palm
oil (PO) and animal fats (AF). The influences of fuel properties,
engine speed and chamber pressure on the injection pressure
profile, needle lift, spray length and spray angle were inves-
tigated. The main conclusions of this paper are as follows:
Fuel properties are strongly temperature dependent. Even
small changes in fuel temperature result in a significant in-
fluence on the injection pressure of a pump-line-nozzle sys-
tem. Therefore, fuel temperature at the moment of injection
was determined and controlled for all experiments.
- The bulk modulus, a measure for the resistance to
compression, is the main parameter affecting the injection
pressure profile.Galle et al. [1] reported a strong structural difference for RSO in
comparison with diesel and RME. RSO droplets are larger than
those of diesel and thus for a same volume of fuel, they take
less space on the two dimensional spray images than more
and smaller droplets. Due to the large droplets, some light can
pass through the outer regions of the spray cone and there-
fore, on the two dimensional spray images, RSO is surrounded
by a more clear area. For diesel and RME sprays, the large
amount of small droplets do not allow light to pass through
the spray, resulting in very homogeneous two dimensional
spray images. It is important to note that the experiments of
Galle et al. [1] were conducted at Tc 20 C. We also observedthis more clear area for our RSO1 experiments at 27 C. How-ever, when fuel temperaturewas raised to 45 C, the clear areasurrounding the spray was much less observed for RSO1(Fig. 10) (except for some experiments at low chamber pres-
sure, 40 bar). Based on the reasoning of Galle et al. [1], this
means that increasing the fuel temperature from 25 to 45 Cresults in a significant improvement of the atomization of
rapeseed oil. This can be attributed to the decrease in vis-
cosity: in the range from 25 to 45 C, viscosity of rapeseed oildecreases from 60 to less than 30 mPas (cfr. Fig. 10). This- The higher bulk modulus of straight vegetable oil and ani-
mal fat results in an earlier and faster needle lift for
- The higher density of biofuels results in longer injection
b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8 227durations for pump-line-nozzle injection systems.
- The spray length is strongly influenced by the engine speed
(or thus injection pressure) and the chamber gas density.
These parameters affect the spray penetration more
significantly than the fuel temperature or the fuel type.
- At start of injection, spray penetration is similar for all fuels
because of counteracting influences of bulk modulus, vis-
cosity and density. In the later stage of the injection, how-
ever, straight vegetable oil and animal fat show slightly
faster spray penetrations due to the higher injection pres-
sures and larger droplets of these fuels compared to diesel
and rapeseed biodiesel.
- The spray angle is not significantly influenced by engine
speed. Chamber gas density is the dominant parameter
affecting the spray angle. Similar spray angles were
observed for different fuels. However, an influence of fuel
temperature was often observed for the more viscous fuels.
- At low temperatures, despite similar spray angles, the spray
atomization for straight vegetable oil is clearly worse than
diesel and rapeseed biodiesel. Increasing fuel temperature
from 25 to 45 C already enhanced the spray atomizationsignificantly. This is most probably due to the strong
decrease in viscosity. Research on droplet sizes and droplet
distribution is necessary to quantify these influences of fuel
properties on atomization. Probably, the sauter main
diameter is the spray parameter that is most significantly
affected by the fuel properties.
Acknowledgments
The authors of this paper would like to acknowledge the sug-
gestions and technical assistance of Koen Chielens and Patrick
De Pue. The authors would like to thank Prof. Dr. ir. P. Van der
Meeren for the use of the lab facilities of the Department of
Applied Analytical and Physical Chemistry at Ghent University,
used in determining the fuel properties. J. Galle thanks the
Institute for the Promotion of Innovation through Science and
Technology in Flanders (IWT-Vlaanderen) for his Ph.D. grant
(SB-81139). The experimental equipment is financially sup-
ported by Anglo Belgian Corporation (ABC), the Ghent Univer-
sity Special Research Fund and the Institute for the Promotion
of Innovation through Science and Technology in Flanders
(IWT-Vlaanderen) within the R&D project Research and
development of a medium speed highly efficient internal
combustion engine with ultralow emissions, for diesel, dual
fuel, heavy fuel oil and alternative fuels (IWT110579).
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b i om a s s a n d b i o e n e r g y 5 7 ( 2 0 1 3 ) 2 1 5e2 2 8228
Experimental investigation concerning the influence of fuel type and properties on the injection and atomization of liquid ...1 Introduction2 Experimental method2.1 Experimental setup and data acquisition2.2 Tested fuels2.3 Fuel temperature control2.4 Measurements and standard deviation2.4.1 Injection pressure profile and needle displacement2.4.2 Spray images
3 Results and discussion3.1 Injection pressure profile and needle displacement3.1.1 General view on the injection pressure profile3.1.2 Pressure build-up3.1.3 Needle lift3.1.4 Pressure drop caused by needle lift3.1.5 Injection period3.1.6 Partial needle lift at low engine speeds
3.2 Spray development3.2.1 Spray length3.2.1.1 Influences of the engine parameters3.2.1.2 Influences of the fuel properties
3.2.2 Spray angle3.2.2.1 Influence of the engine parameters3.2.2.2 Influences of the fuel properties
4 ConclusionsAcknowledgmentsReferences