Temperature dependence of density and viscosity of
vegetable oils
Bernat Esteban, Jordi-Roger Riba*, Grau Baquero, Antoni Rius, Rita Puig
Escola d’Enginyeria d’Igualada (EEI-Escola d’Adoberia), Universitat Polite
`
cnica de Catalunya, Plac¸a del Rei 15, 08700 Igualada,
Catalunya, Spain
article info
Article history:
Received 28 January 2011
Received in revised form
27 January 2012
Accepted 14 March 2012
Available online 5 April 2012
Keywords:
Viscosity
Density
Straight vegetable oil
Diesel engine
Combustion
abstract
The straight use of vegetable oils as fuel in diesel engines entails adjusting several physical
properties such as density and viscosity. By adequately heating the vegetable oil before
entering the injection system, its physical parameters can reach values very close to that of
diesel fuel. Consequently, by properly adjusting the temperature of vegetable oils used as
fuel, it is possible to improve their combustion performance, thus avoiding premature
engine aging due to incomplete burning. In this study the density and viscosity of several
vegetable oils are studied within a wide variety of temperatures. The optimal range of
temperatures at which each vegetable oil should operate in order to adjust its properties to
those of automotive diesel and biodiesel is then found. Additionally an empirical rela-
tionship between the dependence of viscosity with density is presented. Thus, by means of
the above-described relationship, through measuring the density of a given oil, its viscosity
can be directly deduced.
ª 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Diesel engines are being extensively utilized worldwide due to
their high economic advantage and durability [1,2]. They have
appealing features including robustness, high torque, and
lower fuel consumption under certain conditions. According
to Moron et al. [3] they are prevalent in sectors such as road
and train transport, agriculture, military, construction,
mining, maritime propulsion and stationary electricity
production. Diesel engines can use several fuel types,
including diesel fuel, straight vegetable oils (SVO), biodiesel e
transesterified vegetable oil e and short chain alcohols. Diesel
engines may also function with hybrid fuels, including SVO
mixtures in different proportions with diesel or diesel/
ethanol.
At the present time there is an increasing demand for
energy, concerns about global warming and a growing interest
in renewable energy sources; particularly in biofuels [4,5]. This
is due to diminishing reserves and price instability of the
world’s petroleum fuel. These challenges are in part due to the
diesel engines themselves. Consequently, it is an urgent
matter to reduce hazardous pollutants that diesel engines
emit such as NO
x
, CO, CO
2
and particulate matter (PM).
According to Lee at al. [1] this can be achieved by using new
combustion technology, by improving fuel characteristics and
or by applying after-treatment technology. It is well known
that utilizing biofuels with internal combustion engines may
contribute to reduce greenhouse gas emissions [6]. Small-
scale produced SVOs are considered attractive options for
renewable fuel because of environmental benefits [7]. Small-
scale use of vegetable oils is also considered an interesting
option because they can be obtained from agricultural or
industrial sources with very simple processing. This process-
ing includes cold pressing and refining stages that avoid
* Corresponding author. Tel.: þ34 938035300; fax: þ34 938031589.
E-mail address: jordi.riba@eei.upc.edu (J.-R. Riba).
Available online at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
biomass and bioenergy 42 (2012) 164e171
0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biombioe.2012.03.007
chemicals and complex processes involved in biodiesel
production. To summarize, the straight use of vegetable oils in
diesel engines avoids the transesterification stage required to
obtain biodiesel, lowering energy consumption and reducing
considerably environmental impacts due to lower polluting
emissions and less chemicals consumption [7].
There are some similarities between most available vege-
table oils and diesel fuel, making the vegetable oils appealing
to be used as fuel. For example, the lower heating values
(LHVs) of vegetable oils are very close to that of diesel fuel [8].
However, some physical and chemical differences exist. For
example, the cetane number is a variable that affects the
ignition quality and therefore determines the flammability of
the fuel [9]. Diesel fuel has higher cetane number than vege-
table oils, implying a shorter ignition delay [10] and a small
change in engine efficiency [11]. However, the difference in
ignition delay between diesel fuel and SVOs is not significant
[9] and may be compensated by adjusting the ignition delay
[12]. It is an accepted fact that viscosity has a significant effect
on spray characteristics. Higher viscosity leads to: inferior fuel
atomization, higher Sauter mean diameter ethe ratio of the
mean volume to the mean surface of the fuel dropletse and
lower spray speed than conventional diesel fuel [13e16]. The
atomization effectiveness depends on the geometry of the
injection system, various fuel properties such as viscosity,
surface tension and density [3,17]. SVO viscosity has
a profound effect on the flow through the fuel system, thus
influencing how the oil sprays from the injector.
It is also an accepted fact that the large molecular sizes of
the triglycerides contained in vegetable oils results in higher
viscosity, higher density and lower volatility compared to
diesel fuel. That, in turn, causes poor fuel atomization due in
part to the large size of droplets upon injection into the
cylinder and also due to high-spray jet penetration. Optimal
atomization improves mixing and complete combustion in
diesel engines, which has great impact on emissions and
efficiency [18]. Higher viscosity fuels cause the jet to become
a solid steam instead of a spray composed of small droplets
[19] resulting in poor combustion that produces black smoke
and provokes the development of deposits in the combustion
chamber. Furthermore, the introduction of unburnt fuel,
which flows down the cylinder wall into the crankcase, dilutes
the vegetable oil in the lubrication engine oil [20]. Higher
viscous fuels tend to form larger droplet size, which fosters
other competitive reactions, such as charring or coking and
polymerization. Contrarily, as pointed out by Nwafor et al.
[20], viscous fuels have a higher lubricating effectiveness. It is
recognized that indirect injection engines are less affected by
viscosity differences than direct injection engines [21,22].
Observations gathered using SVO as fuel in unmodified
diesel engines draw attention to the need to fit the most
relevant physical properties, which narrows this study to
density and viscosity [23]. When using SVO as fuel in an
unmodified diesel engine, its viscosity has to be lowered to
allow appropriate atomization. If not, incomplete SVO
combustion and carbonization will eventually damage the
engine. Consequently, when using SVO in these engines,
some precautions must be taken. Often, vegetable oils are
preheated to reach appropriate density and viscosity values
before reaching the injectors by using a heater. Once heated,
SVO becomes very similar to diesel fuel in terms of physical
properties, which will be discussed further in this study.
Several studies have been carried out that focus on
lowering the viscosity of vegetable oils by appropriate heating.
One of the earliest studies was carried out by Murayama et al.
[24] who suggested increasing the temperature of rapeseed oil
to 200
C to achieve efficient combustion for direct injection
diesel engines. More recently, Agarwal et al. [25] found that
heating the Jatropha oil between 90
C and 100
C before
combustion in a diesel engine was adequate to lower the
viscosity within a close range to diesel. Moreover this study
concluded that preheating the Jatropha oil does not lead to
change in optimum fuel injection pressure. Additionally,
some of the available commercial kits applied to use SVO in
automotive diesel engines utilize a heat exchanger to raise the
temperature of vegetable oil. These kits frequently use the
water of the cooling circuit to heat the SVO up to about
70e80
C. All of this information makes it clear that there is
discrepancy in the data concerning the optimal temperature
at which each particular SVO should be preheated in order to
obtain improved combustion.
Although density and even more so viscosity play an
important role in the evaluation of fuel performance, there is
very little published information about the optimal tempera-
ture level at which the SVOs have improved performance in
diesel engines. Hence, it would be extremely beneficial to
know the values of these properties within the vast range of
temperatures to find out the most appropriate heating
temperature for each one of the analyzed vegetable oils.
The aim of this paper is to obtain insightful knowledge
about the temperature dependencies of the critical physical
parameters, such as density and viscosity of commonly used
vegetable oils, including rapeseed, sunflower, soybean, palm,
corn and grapeseed. This includes the most significant vege-
table oils produced worldwide during 2009, i.e. palm, soybean,
rapeseed and sunflower [26]. The results presented in this
study will allow adjusting the density and viscosity values of
the vegetable oils with that of other fuels currently being used
in diesel engines. Mathematical expressions of temperature
dependences of these physical parameters are given to char-
acterize the vegetable oils studied. Additionally, the densities
and viscosities of the vegetable oils analyzed are compared
with those of commercial pure biodiesel and automotive
diesel fuel in order to compare their physical parameters.
From this comparison and from the limit values proposed in
Section 2.2, the heating temperature to match the density and
viscosity values of automotive diesel and biodiesel is obtained.
In order to characterize the vegetable oils and pure bio-
diesel samples used in this study, their composition has been
analyzed by means of gas chromatography. Table 1 gives
a listing of the samples analyzed in this paper. Their fatty acid
composition is given in mass percent.
2. Methodology
2.1. Temperature dependence of density and viscosity
As discussed in the introduction, density and viscosity play an
important role in the atomization process, which in turn
biomass and bioenergy 42 (2012) 164e171 165
determines if complete combustion is carried out in a diesel
engine which consequently influences engine emissions and
efficiency. Therefore, in order to achieve the correct fuel atom-
ization, proper values of density and viscosity are required.
Moreover, both, density and viscosity are highly temperature
sensitive. In this section these dependences are analyzed and
mathematical equations to set them are explained.
2.1.1. Density
Density is an important physical characteristic of any
substance, and is a measure of the mass per unit of volume of
that substance. It is an accepted fact that vegetable oil density
decreases linearly with increasing temperature. This rela-
tionship can be expressed mathematically as [27],
r ¼ a þ b,T (1)
where r is the density expressed in g cm
3
, T is the tempera-
ture expressed in
C, a is the intercept and b is a negative
slope.
2.1.2. Viscosity
Viscosity is a measure of the resistance offered by a fluid to
flow. According to Krisnangkura et al. [28] viscosity may be
considered the integral of the interaction forces of molecules.
When heat is applied to fluids, molecules can then slide over
each other more easily making the liquid to become less
viscous. The effect of temperature on the kinematic viscosity
of liquid is described by means of the Arrenhius equation as,
h ¼ A
1
,exp
E
a
RT
(2)
h being the kinematic viscosity, E
a
the activation energy for
flow, R the universal gas constant and T the absolute
temperature. Additionally, A
1
¼ N
A
h/V, where N
A
, V and h are
the Avogadro’s number, the molecular volume and the Plank’s
constants, respectively. The SI physical unit of kinematic
viscosity is m
2
s
1
.
In the case of vegetable oils, Equation (2) can be rewritten
as in Equation (3) which is known as the Andrade equation
[27,28]
h ¼ A
1
,exp
B=T
(3)
where T is the absolute temperature and A
1
and B are specific
constants to be adjusted for each specific oil. By applying
logarithms to both sides of Equation (3) it leads to,
ln
ð
h
Þ
¼ A þ B=T (4)
Equation (4) allows us to linearize the Equation (3) by
applying the least-squares method and making 1/T the inde-
pendent variable. Additionally, Azian et al. [29] suggested
modifying the Equation (4), which is especially useful when
dealing with wide temperature ranges,
lnðhÞ¼A þ B=T þ C=T
2
(5)
Sometimes, the dynamic or absolute viscosity m is applied,
which can be calculated from the kinematic viscosity h and
the density r as,
m ¼ h,r (6)
The SI physical unit of dynamic viscosity is the Pa s, which
is identical to kg m
1
s
1
.
2.1.3. Interdependence between viscosity and density
As both, density and viscosity are highly temperature sensi-
tive, it is possible to find out the dependence between them.
This dependence is often used in the oil industry [30]. The
measurement of density is very simple and quick because it
requires a low-cost hydrometer that is quite easy to use. On
the other hand, the measurement of the viscosity is more
complex and time consuming, requiring a more complex and
expensive assessment instrument employed by qualified
laboratory technicians. If the relationship between density
and viscosity is known, one only needs to measure the density
of a given oil to deduce its viscosity. According to Rodenbush
et al. [27], when dealing with vegetable oils the dependence
between density and viscosity can be expressed as illustrated
in Equation (7).
r ¼ D þ E=h
1=2
(7)
Equation (7) is particularly useful because it can be adjusted
by means of a least-squares linear regressionbychoosing1/h
1/2
as an independent variable.
Table 1 e Fatty acid composition of the analyzed vegetable oils.
Carbon number Systematic name (common) Rapeseed Sunflower Soybean Palm Corn Grapeseed Biodiesel
C12:0 Dodecanoic (lauric) eee0.1% ee e
C14:0 Tetradecanoic (myristic <0.1% <0.1% <0.1% 0.9% <0.1% <0.1% 0.3%
C16:0 Hexadecanoic (palmitic) 4.7% 6.7% 11.2% 45.6% 11.4% 7.2% 18.6%
C16:1 Hexadecenoic (palmitoleic) 0.3% 0.3% 0.2% 0.4% 0.3% 0.1% 0.8%
C18:0 Octadecanoic (estearic) 1.3% 2.9% 2.9% 3.8% 1.7% 3.9% 4.5%
C18:1 Octadecenoic (oleic) 65.3% 38.7% 25.2% 38.5% 32.8% 20.2% 32.0%
C18:2 Octadecadienoic (linoleic) 19.2% 51.3% 55.4% 10.5% 53.3% 68.4% 39.7%
C18:3 Octadecatrienoic (linolenic) 8.3% <0.1% 5.0% 0.1% 0.5% 0.2% 4.0%
C20:1 Eicosenoic (gadoleic) 0.7% <0.1% 0.1% e <0.1% <0.1% e
C22:1 Docosenoic (erucic) <0.3% eeeee e
Table 2 e European standards related to automotive
diesel fuel, biodiesel and rapeseed oil.
Standard Fuel Density (g cm
3
) Viscosity (mm
2
s
1
)
15
C40
C
EN 590 Diesel fuel 0.820e0.845 2.00e4.50
EN 14214 Biodiesel 0.860e0.900 3.50e5.00
DIN 51605 Rapeseed oil 0.910e0.925 Max. 36.00
biomass and bioenergy 42 (2012) 164e171166
2.2. Density and viscosity ceiling values
In this section different European standards regarding the
density and viscosity requirements of various automotive
fuels sold in the European Union are examined. The European
standards analyzed are EN 590:2004 [31] which pertains to
automotive diesel fuel and EN 14214:2008 [32] that is associ-
ated with biodiesel. The German norm DIN 51605 [33] con-
cerning the straight use of rapeseed oil in diesel engines is also
included in this comparison. Table 2 summarizes the limit
values of density and viscosity for diesel fuel, pure biodiesel
and rapeseed oil agreed upon by these European standards.
Concerning the performance of diesel engines, it is
acknowledged that the effects of using vegetable oils greatly
depend on the engine sensitivity to fuel injection and on the
combustion characteristics of the vegetable oil [34,35]. Current
diesel engines have fuel injection systems that are susceptible
to viscosity changes [36]. According to Totten et al. [19], the
recommended viscosity values for diesel engine fuels range
from 1.8 to 5.8 mm
2
s
1
at 38
C. The lower viscosity limit is
imposed to provide adequate lubrication for injection system
components and to prevent leakage. More recently, Mohamed
Y.E. Selim [18] recommended a range lying between 1.6 and
7mm
2
s
1
when operating at 40
C. Thus, viscosity limit values
imposed by EN 590:2004 and EN 14214:2009 [31,32] are
compatible with viscosity values close to 5.8 mm
2
s
1
. This
value is recommended in [19], and the range 1.6e7mm
2
s
1
is
suggested in [18]. Consequently, as deduced from technical
literature, viscosity of vegetable oils should be lowered at least
to values close to 6 mm
2
s
1
when used as fuel in diesel
engines to match the values of the European standards, thus
avoiding the problems related to higher viscosity fuels. Addi-
tionally, from the abovementioned standards a conservative
density value for SVO of 0.860 g cm
3
is proposed as a ceiling
value because it meets the well accepted EN 14214:2008 [32]
standard for biodiesel.
3. Experimental
In this study samples of different types of refined vegetable
oils were analyzed including rapeseed, sunflower, soybean,
palm, corn and grapeseed. Pure commercial biodiesel (BD100)
and automotive diesel fuel were also included in the sampling.
The vegetable oils samples were purchased from different
local suppliers, whereas the biodiesel and automotive diesel
samples were acquired from local petrol stations.
Fig. 1 e Experimental setup used to measure the density
and viscosity of vegetable oils in the temperature range
10
Ce140
C.
Table 3 e Dependence of density with temperature for automotive diesel fuel, pure biodiesel and the analyzed vegetable
oils. SVO density values below 0.860 g cm
L3
are in bold.
Temp. (
C) Density (g cm
3
)
Diesel BD100 Rapeseed Sunflower Soybean Palm Corn Grapeseed
10 0.8376 0.8859 0.9210 0.9251 0.9254 e 0.9237 0.9259
20 0.8308 0.8798 0.9145 0.9169 0.9185 e 0.9167 0.9188
30 0.8242 0.8717 0.9080 0.9114 0.9127 e 0.9113 0.9126
40 0.8181 0.8641 0.9027 0.9043 0.9061 0.8996 0.9046 0.9060
50 0.8114 0.8583 0.8963 0.8994 0.8998 0.8922 0.8979 0.8998
60 0.8043 0.8513 0.8911 0.8926 0.8941 0.8845 0.8920 0.8941
70 0.7970 0.8433 0.8848 0.8877 0.8879 0.8789 0.8864 0.8874
80 0.7890 0.8372 0.8777 0.8798 0.8817 0.8721 0.8801 0.8813
90 0.7825 0.8287 0.8724 0.8743 0.8750 0.8664 0.8740 0.8754
100 0.7759 0.8229 0.8658 0.8670 0.8689 0.8595 0.8678 0.8695
110 0.7708 0.8150 0.8593 0.8602 0.8626 0.8536 0.8610 0.8626
120 0.7636 0.8075 0.8537 0.8536 0.8566 0.8457 0.8555 0.8570
130 0.7577 0.8002 0.8469 0.8472 0.8498 0.8407 0.8489 0.8505
140 0.7516 0.7912 0.8395 0.8408 0.8430 0.8325 0.8424 0.8440
biomass and bioenergy 42 (2012) 164e171 167
A set of calibrated hydrometers with spans ranging from
0.7 to 0.95 g cm
3
were used to measure the density. The
thermal expansion of glass has a slight influence on the
hydrometer reading. To compensate this effect the tempera-
ture correction according to the standard ISO 649-2:1981 [37]
has been applied.
Additionally, three Cannon-Fenske-type gravity flow
viscometers series 50, 100 and 150 were utilized to measure
the viscosity. They allowed measuring viscosity of all of the
analyzed oils and fuels with flow times within the recom-
mended range set by the standard ISO 3105:1994 [38]. The
viscometers were calibrated by the manufacturer and the
calibration constant C
0
(T ) is supplied with the instrument.
The viscosity is obtained from the following equation,
h
app
¼ t,C
0
ðTÞ (8)
h being the kinematic viscosity in centistokes, t the flow
time expressed in seconds and C
0
(T ) the characteristic cali-
bration constant of each viscometer measured at the working
temperature.
Additionally, to ensure precise and stable temperature
control during measurements, a resistance temperature
detector (RTD) was used to measure the temperature. The RTD
was connected to a digital temperature PID controller. The
controller allowed regulating the temperature of a heated oil
bath containing the hydrometer and the viscometer by means
of an electric heater. The temperature of the oil bath was
adjusted from 10
C to 140
C. Both, density and viscosity were
measured within this temperature interval. Furthermore, the
interdependence between density and viscosity was studied.
Each sample was tested three times, and the average
density and viscosity were calculated. Fig. 1 shows the
experimental setup used to determine the temperature
dependence of density and viscosity of the samples analyzed.
3.1. Density
As stated, it is an accepted fact that fuel density has a great
influence on the atomization process. Thus the knowledge of
the temperature dependence of this parameter is a highly
important component of the process of the study in order to
ensure optimal combustion of the fuel [18].
The density of the different vegetable oils, pure biodiesel
(BD100) and automotive diesel fuel samples were measured
according to the method explained in Section 2.1.1. The set of
measures carried out with in the range between 10
C and
140
C is shown in Table 3.
Results presented in Table 3 clearly show that the density
of diesel fuel is much lower than the density of the vegetable
oils studied. However, heating any of the vegetable oils at
a temperature of 120
C or higher is sufficient to ensure that
density is below 0.860 g cm
3
, corresponding to the inferior
density limit given by the EN 14214:2008 European standard
[32] for biodiesel fuel in diesel engines. Despite these obser-
vations, this temperature requirement can be lower depend-
ing on engine types and configurations. For example, in
engines used in applications such as combined heat and
power (CHP), vegetable oil may be injected at lower tempera-
tures [39].
Table 4 shows a summary of the most relevant parameters
related to the densityetemperature fit obtained by applying
Equation (1) when analyzing the whole set of samples. Once
Table 4 e Main parameters of the densityetemperature
fit.
Sample ab(10
4
) r
2
F
calc
(10
4
) F
tab
Diesel fuel 0.8442 6.7009 0.99911 1.3466 4.7472
Biodiesel 0.8938 7.2073 0.99932 1.7554 4.7472
Rapeseed 0.9273 6.1837 0.99945 2.1884 4.7472
Sunflower 0.9310 6.4145 0.99920 1.4907 4.7472
Soybean 0.9315 6.2790 0 .99985 8.0160 4.7472
Palm
a
0.9250 6.5612 0.99900 9.0182 5.1174
Corn 0.9295 6.2053 0.99983 6.9725 4.7472
Grapeseed 0.9314 6.2343 0.99982 6.7749 4.7472
a Analysis done for T 40
C, as palm oil is solid below 40
C.
Table 5 e Dependence of kinematic viscosity with temperature for automotive diesel fuel, pure biodiesel and the analyzed
vegetable oils. SVO viscosity values below 6 mm
2
s
L1
are in bold.
Temp. (
C) Kinematic viscosity (mm
2
s
1
)
Diesel BD100 Rapeseed Sunflower Soybean Palm Corn Grapeseed
10 5.39 9.00 119.48 118.72 107.62 e 113.39 100.63
20 4.15 6.78 74.19 73.45 67.12 e 70.29 64.32
30 3.30 5.30 48.88 48.46 44.69 e 46.54 42.94
40 2.70 4.26 34.06 33.78 31.42 45.34 32.53 30.19
50 2.26 3.51 24.68 24.48 23.00 28.19 23.74 22.29
60 1.92 2.94 18.62 18.52 17.47 20.84 17.96 17.04
70 1.64 2.51 14.48 14.44 13.67 15.60 14.01 13.34
80 1.43 2.16 11.58 11.53 11.17 12.35 11.39 10.88
90 1.27 1.90 9.45 9.44 9.13 9.94 9.34 8.96
100 1.14 1.69 7.89 7.78 7.71 8.21 7.83 7.53
110 1.03 1.51 6.70 6.50 6.58 6.88 6.65 6.42
120 0.93 1.36 5.86 5.62 5.68 5.85 5.70 5.49
130 0.85 1.23 5.09 4.91 4.99 5.02 4.93 4.77
140 0.78 1.13 4.47 4.37 4.45 4.38 4.34 4.21
biomass and bioenergy 42 (2012) 164e171168
