Exergy efficiency; Nanolubricants; Engine friction;
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During the past two decades, exergy analysis has been attractive to many scientists,
researchers and engineers in many different areas. Exergy analysis is determination the maximum
theoretical amount of operation capability of system in many applications [1, 2]. One of the most
important applications of exergy analysis is, it can be implemented to reduce losses and
destructions and therefore the efficiency of system can be evaluated [3, 4]. For example, exergy
analysis is able to be used for reducing losses and minimizing irreversibilities in engines. One of
the best methods of reducing irreversibilities; especially irreversibilities related to friction; is to
improve the quality of lubrication in engines. A simple way to improve quality of lubricants is to
add some additives to standard engines oil, and in this case the nanoparticles are useful additives
. Results of researches conducted in recent years show that adding nanoparticles provides
special properties for materials, including friction reduction through mending effect, rolling
effect, third body material transfer, ball bearing effect, colloidal effect, protective film [5, 6].
∗Corresponding author : Tel:+60-010-298-2705; Fax:+60-010-298-3541.
Email address: email@example.com (M. Mohammadnejad).
Energy Education Science and Technology Part A: Energy Science and Research
2011 Volume (Issue) 27(2): 447-454
Estimating the exergy efficiency
of engine using nanolubricants
M. Mohammadnejad*, M. Ghazvini, F. S. Javadi, R. Saidur
University of Malaya, Department of Mechanical Engineering, 50603 Kuala Lumpur, Malayasia
Received 12 October 2010; accepted 19 December 2010
Friction is an effective factor in the reduction of the exergy efficiency in engines. In this study, the
effects of using nanolubricants on friction reduction, thereby increasing the engine exergy efficiency, have
been investigated. First, studies done on exergy, engine lubrication and friction reduction with
nanolubricants have been reviewed. Based on these studies the mathematical formula for estimating the
exergy efficiency of an engine that uses nanolubrication was derived. Finally, it has been found that
adding CuO nanoparticles to SF oil has the greatest effect on the exergy efficiency improvement and the
maximum amount of this improvement is 4.6%.
448 M. Mohammadnejad et al. / EEST Part A Energy Science and Research 27 (2011) 447-454
In this study, it has been focused on the possible exergy efficiency improvement in engines by
using nanolubricants in reducing rubbing friction. Related equations of friction losses and exergy
for engine have been performed to estimate exergy efficiency using nanolubricant. Firstly, the
thermodynamic equation of friction losses in engine and also the relationship between exergy and
thermodynamic properties of system have been provided, and then the required information about
friction coefficient of nanolubricants has been mentioned. Finally with combining both of them
the exergy efficiency of engines using nanolubricant has been estimated in different temperature
in comparison with engines using standard lubricant.
For energy saving, exergy and energy analysis have been applied to some utility sectors such
as chillers, refrigerators and room air conditioners in some recent research [3, 7-9]. Also using
renewable energies especially solar energy, wind energy and biofuel energy has received more
attention than before [10-16]. On the other hand there are many researches in different areas of
nanotechnology [17, 18], but there is no work on estimating exergy efficiency improvement of an
engine with the lubricant which contains nanoparticles. The objective of this work is to estimate
the exergy efficiency of an engine using nanolubricants.
2. Studies done on exergy and engines lubrication
2. 1. Exergy equation related to system
The concept of exergy is stated according to both first and second laws of thermodynamic.
Exergy is known as the maximum amount of generated work by a system or flow, up to when the
system or flow reaches to equilibrium with reference environment. When a system or flow is not
completely in stable equilibrium with environment, it changes. Exergy is a measure for
evaluating this change. In actual process a part of exergy destroys due to irreversibilities .
In exergy analysis it is necessary to be specified the characteristics of reference environment
including temperature , pressure and chemical composition . In general exergy efficiency is a
measure of approach to the ideal case. Therefore, the result of exergy analysis is useful for
determining that how much the efficiency of a system can be improved .
In fact the aim of exergy analysis is to determine the causes and to calculate the magnitude of
exergy losses. The exergy of a thermodynamic system (regardless of exergy associated with
kinetic and potential energy) is determined by Eq. 1:
where T0 is the temperature of environment and S0 , h0 are the initial entropy and enthalpy of
system respectively. Q is the heat rejected to the cool medium (the waste heat) .
2. 2. Theory of engine lubrication
In estimating the magnitude of friction of different engines it is necessary to identify the
friction coefficient of lubricants. The analysis of friction behavior in lubricants can be done with
Stribeck curve, which has been shown in Fig. 1.
Fig. 1 shows the variations of friction coefficient as a function of the combined variables of
) 1 ()()(
M. Mohammadnejad et al. / EEST Part A Energy Science and Research 27 (2011) 447-454 449
Fig. 1. Stribeck curve and the regime of lubricants
rotation speed, the viscosity of lubricant and normal force (ην/p). Totally this figure illustrates the
relationship between friction coefficient and lubricant condition . When rotation speed is low
and normal force is high (ην/p is low), the friction coefficient is completely related to the
properties of contacting surface, and this regime is called Boundary lubrication. But the
Hydrodynamic lubrication regime occurs when the rotation speed is high and normal force is low
(ην/p is high ), the friction coefficient is related to the properties of lubricant film [5, 23]. This
type of lubrication regime occurs in engine bearing, between piston skirt and cylinder liner and
between piston rings and liners for high sliding velocity. Between these two regimes there is a
regime which the friction coefficient of lubricant is related to both of contact surfaces properties
and lubricant film, which is called mixed lubricati
3. Studies done on friction reduction by nanolubricant
In research that has been already done , three types of nanoparticles (Cuo, TiO2, Nano-
diamond ) were added into two different types of standard lubricating oils ( SF and base oil ) for
analyzing the variations of friction coefficients (the difference between SF and base oil is, SF oil
has friction reduction and anti wear additives in similar viscosity grade ). The weight percent of
each nanoparticles additives (Cuo , TiO2, Nano-diamond) is 0.1 in both standard lubricating oils
. The friction coefficient of SF oil with and without nanoparticles additives have been shown
in Fig. 2. The X-coordinate shows temperature of lubricant, and displays the passed time from the
start of the test. The friction coefficients of the SF oil with nanoparticles, specially the CuO and
TiO2 additives, are lower than the SF oil without nano additives.
After 10 minutes from the start of the test, when the temperature increases above 50 0C,
friction coefficient of SF oil containing CuO becomes lower than the other two nanoparticles.
The maximum friction coefficient reduction, in this case, is about 18.4% in comparison with the
SF oil without nanoparticles . Fig. 3 shows the friction coefficients of base oil with and
As can be seen in this figure the friction coefficient of the base oil containing CuO is reduced,
whereas by adding nano-diamond this improvement does not exist. In addition the average of
reducing friction coefficient in case of adding CuO to base oil is 5.8% . According to
mentioned research, CuO additive has the greatest effect on reducing friction coefficient of
450 M. Mohammadnejad et al. / EEST Part A Energy Science and Research 27 (2011) 447-454
Fig. 2 . Friction coefficient of SF oil with and without nanoparticles .
Fig. 3 . Friction coefficients of base oil with and without nanoparticles .
4. Mathematical formulation to estimat exergy efficiency of an engine using nanolubricants
Exergy of system without nanolubricantion:
Noting that the variation of temperature of system due to changing lubricant is negligible and
h = h(T). Exergy of system with nanolubrication :
) 1 (][)(
) 1 (][)(
) 1 (M
M. Mohammadnejad et al. / EEST Part A Energy Science and Research 27 (2011) 447-454 451
where the notation ( .′) shows the properties of system with nanolubrication. From Eqs. 2 and 3:
Noting to this point internal energy of system due to friction changes negligibly .
where is friction coefficient of standard oil without nanoparticles and is friction coefficient
From Eqs. 6 and 7:
From Eqs. 4 and 8, it can be resulted:
That in this Equation the term is related to engine and the term depends on
Based on research done , the amount of the exergy losses due to rubbing friction has been
estimated 20% of the exergy of system. Therefore from Eq. 5:
Then Eqs. 9 and 10 are combined:
Also the exergy efficiency is :
M 2 .0 2 .1
to due friction
) 1 (
2 . 0
452 M. Mohammadnejad et al. / EEST Part A Energy Science and Research 27 (2011) 447-454
Exergy supplied is constant when the lubricant changes, thus :
From Eqs. 11 and 13 Eq. 14 is derived:
η′ : Exergy efficiency of engine using nanolubricant
η : Exergy efficiency of engine without nanolubricant
In Eq. 14, M can be calculated from Figs. 2 and 3 in different temperatures. Therefore it is
possible to evaluate the percentage of exergy efficiency improvement, in different temperature
for each type of nanolubricants. The achieved results have been displayed in Figs. 4 and 5.
Fig. 4. Exergy efficiency improvement using of engine nanolubricants (SF oil with CuO, TiO2,
As can be seen in Fig. 4 adding CuO nanoparticles to SF oil has the greatest effect on the
improvement of exergy efficiency. The maximum amount of this improvement which occurs
around 80 0C is 4.6% . But on the other hand, adding nano-diamond to SF oil has the least effect.
The improvement of exergy for this nanolubricant is less than 1%.
Fig. 5 shows adding CuO nanoparticles to base oil has positive effect on the exergy efficiency
improvement. The maximum amount of this improvement is 2.3%; but on the other hand adding
nano-diamond to base oil has adverse effect on the exergy efficiency. In the worst condition the
exergy efficiency decreases to 3.8%.
The other result from Figs. 4 and 5 is that adding nanoparticles to SF oil provides better
condition for exergy efficiency in comparison with base oil.
2 . 0
2 . 1
M. Mohammadnejad et al. / EEST Part A Energy Science and Research 27 (2011) 447-454 453
Fig. 5. Exergy efficiency improvement of engine using nanolubricants (Base oil with CuO,
As the result of this study, the exergy efficiency improvement of an engine with
nanolubricants compared to one without nanolubricants can be calculated by using Eq. 14. With
this equation it has been found that adding CuO nanoparticles to SF oil has the greatest effect on
the improvement of exergy efficiency and the maximum amount of this improvement which
occurs around 80 0C is 4.6%. But on the other hand, adding nano-diamond to SF oil has the least
effect. The improvement of exergy for this nanolubricant is less than 1%.
It was also found that adding CuO nanoparticles to base oil has a positive effect on exergy
efficiency improvement, the maximum amount of improvement is 2.3%. On the other hand
adding nano-diamond to base oil has an adverse effect on the exergy efficiency. In the worst
conditions, the exergy efficiency decreases to 3.8%. Totally, it can be said that adding
nanoparticles to SF oil provides better conditions for exergy efficiency as compared to base oil.
Future studies should investigate other effects of nanotechnology on the performance of
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