Technologies to recover exhaust heat from internal combustion engines

Full text

Technologies to recover exhaust heat from internal combustion engines
R. Saidur
a
, M. Rezaei
a
, W.K. Muzammil
a
, M.H. Hassan
a
, S. Paria
a
, M. Hasanuzzaman
b,
n
a
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
b
UM Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R& D, University of Malaya, 59990 Kuala Lumpur, Malaysia
article info
Article history:
Received 11 August 2011
Received in revised form
8 May 2012
Accepted 8 May 2012
Keywords:
Waste heat recovery
Waste heat recovery technologies
Internal combustion engine
abstract
The focus of this study is to review the latest developments and technologies on waste heat recovery of
exhaust gas from internal combustion engines (ICE). These include thermoelectric generators (TEG),
organic Rankine cycle (ORC), six-stroke cycle IC engine and new developments on turbocharger
technology. Furthermore, the study looked into the potential energy savings and performances of
those technologies. The current worldwide trend of increasing energy demand in transportation sector
are one of the many segments that is responsible for the growing share of fossil fuel usage and
indirectly contribute to the release of harmful greenhouse gas (GHG) emissions. It is hoped that with
the latest findings on exhaust heat recovery to increase the efficiency of ICEs, world energy demand on
the depleting fossil fuel reserves would be reduced and hence the impact of global warming due to the
GHG emissions would fade away.
&2012 Elsevier Ltd. All rights reserved.
Contents
1. Introduction .....................................................................................................5650
2. Thermoelectric energy conversion technology ..........................................................................5651
2.1. Background of thermoelectric generator.........................................................................5651
2.2. TEG in the automotive industry ...............................................................................5652
2.3. Challenges of TEG. ..........................................................................................5652
2.4. Recent development of TEG in automotive industry ...............................................................5652
3. Six-stroke internal combustion engine cycle ...........................................................................5653
4. Rankine bottoming cycle technique ..................................................................................5653
4.1. Background of the technique ..................................................................................5653
4.2. Working fluids in Rankine cycle ...............................................................................5654
4.3. Considerations of wor king fluids ...............................................................................5654
4.3.1. Types of working fluids . ............................................................................. 5654
4.3.2. Latent heat, density and specific heat of working fluids . . .................................................. 5654
4.4. Analysis of Rankine bottoming cycle in a vehicle . .................................................................5655
4.5. Rankine bottoming cycle in the automotive industry ..............................................................5655
5. Turbocharger ....................................................................................................5655
5.1. Introduction . . . ............................................................................................5655
5.2. Challenges of turbocharger ...................................................................................5656
5.2.1. Variable geometry turbine—Reducing turbo lag ........................................................... 5656
Contents lists available at SciVerse ScienceDirect
journal homepage: www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
1364-0321/$ - see front matter &2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.rser.2012.05.018
Abbreviations: A/R, Aspect ratio; B, Biodiesel; BASIC, Beginner’s all-purpose symbolic instruction code; BiTe, Bismuth telluride; BPV, Bypass valve; BTDC, Before top dead
center; CeFeSb, Skutterudite; CHRA, Center housing and rotating assembly; CI, Compression ignition; CO, Carbon monoxide; DF, Diesel fuel; DI, Direct injection; EGR,
Exhaust gas recirculation; GDP, Gross domestic product; GHG, Greenhouse gas; HCCI, Homogenous charge compression ignition; HCPC, Homogenous charge progressive
combustion; HEV, Hybrid electric vehicle; ICE, Internal combustion engine; ISFC, Indicated specific fuel consumption; K, Total thermal conductivity; MEP, Mean effective
pressure; MPPT, Maximum power point tracker; NA, Naturally aspirated; NO, Nitric oxide; NO
x
, Nitrogen oxide; ORC, Organic Rankine cycle; PV, Photovoltaic; PVG,
Photovoltaic generator;
r
, Electrical resistance; S, Thermo power; SI, Spark ignition; Sin, Sustainability index; SiGe, Silicon germanium; SnTe, Tin telluride; T, Absolute
temperature; TEG, Thermoelectric generator; TU, Turbocharged; VGT, Variable geometry turbocharger; VNT, Variable nozzle turbocharger; WEDACS, Waste energy driven
air conditioning system; WHR, Waste heat recovery; Z, Figure of merit; ZnBe, Zinc-beryllium;
c
, Exergy efficiency
n
Corresponding author. Tel.: þ603 22463246; fax: þ603 22463257.
E-mail addresses: hasan@um.edu.my, hasan.buet99@gmail.com (M. Hasanuzzaman).
Renewable and Sustainable Energy Reviews 16 (2012) 5649–5659

5.3. Recent developments of turbocharger...........................................................................5656
5.3.1. Two-stage turbocharger.............................................................................. 5656
5.3.2. Turbocharging for a new type of engine ................................................................. 5657
6. Economical view and environmental impact ...........................................................................5657
7. Conclusion ......................................................................................................5658
Acknowledgment . . ...............................................................................................5658
References ......................................................................................................5658
1. Introduction
In recent years the scientific and public awareness on environ-
mental and energy issues has brought in major interests to the
research of advanced technologies particularly in highly efficient
internal combustion engines. The number of vehicles (passenger and
commercial vehicles) produced from 2005 to 2010 shows an overall
increasing trend from year to year despite major global economic
downturn in the 2008–2010 periods (Fig. 1). Note that China’s
energy consumption in transportation sector is the lowest (13.5%)
[1] although the country produced the highest number of vehicles in
2009 to 2010 as compared to the other countries (Table 1).
Viewing from the socio-economic perspective, as the level of
energy consumption is directly proportional to the economic
development and total number of population in a country, the
growing rate of population in the world today indicates that the
energy demand is likely to increase. It is also expected that the
average increase in population growth between 2010 and 2020 is
projected to be 10.74% [3–5]. For instance, the current population
of Malaysia is expected to rise from 28 million in 2010 [6] to 33
million by the year 2020 [7]. In consequence, Malaysia Gross
Domestic Product (GDP) saw a stable increase from RM 87,280
million in 1980 to RM 675,825 million in 2009 [8]. From 1980 to
2009, the per capita income also recorded an increase from RM
6341 to RM 24,604 (US$1¼RM 3.50) [8]. The energy demand of
Malaysia is thus presented in Fig. 2 [9] showing a stable
percentage values for transportation sector in 2002 period. As
discussed before, the growing number of population puts trans-
portation sector in a very crucial role due to its dependability
towards the continuous and rapid development of a nation urban
areas and the standard of living for its people. For instance, in
0
2,000,000
4,000,000
6,000,000
8,000,000
10,000,000
12,000,000
14,000,000
16,000,000
18,000,000
20,000,000
2005 2006 2007 2008 2009 2010*
No. of Vehicles
Year
China Germany Japan
Malaysia UK USA South Korea
Fig. 1. Production of vehicles from 2005 to 2010 for selected countries [1,2].
Table 1
Terminal energy consumption structure by region and sector (unit: mtoe) [1].
Regions Total
consumption
Energy consumption by sector
Industry Transportation Agricultural/
commerce/civil
Non-energy use
China 597 327 (54.8)
a
80.5 (13.5) 165 (27.6) 24.5 (4.1)
USA 1597 394 (25.3) 623 (40.0) 475 (30.5) 65.4 (4.2)
EU (15) 1057 320 (30.3) 321 (30.4) 386 (36.5) 30.2 (2.8)
Japan 359 135 (37.6) 94.4 (26.3) 119 (33.2) 10.5 (2.9)
OECD 3692 1106 (30.0) 1242 (33.6) 1120 (33.0) 125 (3.4)
Total in the world 6212 2144 (34.5) 1831 (29.5) 2035 (32.8) 201 (3.2)
a
Percentage value inside the parenthesis.
Fig. 2. Final energy usage by the main sector of Malaysia in 2002 [9].
R. Saidur et al. / Renewable and Sustainable Energy Reviews 16 (2012) 5649–56595650

2002, the transportation sector of Malaysia used about 40% of the
total energy consumed as shown in Fig. 2.
A number of irreversible processes in the engine limit its
capability to achieve a highly balanced efficiency. The rapid expan-
sion of gases inside the cylinder produces high temperature differ-
ences, turbulent fluid motions and large heat transfers from the fluid
to the piston crown and cylinder walls. These rapid successions of
events happening in the cylinder create expanding exhaust gases
with pressures that exceed the atmospheric level, and they must be
released while the gases are still expanding to prepare the cylinder
for the following processes. By doing so, the heated gases produced
from the combustion process can be easily channeled through the
exhaust valve and manifold. The large amount of energy from the
stream of exhausted gases could potentially be used for waste heat
energy recovery to increase the work output of the engine [10].
Consequently, higher efficiency, lower fuel consumption by
improving fuel economy, producing fewer emissions from the
exhaust, and reducing noise pollutions have been imposed as
standards in some countries [11,12]. Hatazawa et al. [13], Stabler
[14], Taylor [15], Yu and Chau [16] and Yang [17] stated that the
waste heat produced from thermal combustion process generated
by gasoline engine could get as high as 30–40% which is lost to
the environment through an exhaust pipe. In addition, 12–25% of
the available energy in a fuel will be used to drive the wheels and
other accessories which technical descriptions of those literatures
are heavily discussed in Refs. [14,15,18–20].
In internal combustion engines a huge amount of energy is lost
in the form of heat through the exhaust gas. Conklin and Szybist
[21] investigated that the percentage of fuel energy converted to
useful work only 10.4% and also found the thermal energy lost
through exhaust gas about 27.7%. The second law (i.e., exergy)
analysis of fuel has been shown that fuel energy is converted to
the brake power about 9.7% and the exhaust about 8.4% as shown
in Fig. 3. In another research [22] the value of exhaust gases
mentioned to be 18.6% of total combustion energy. It is also found
that by installing heat exchanger to recover exhaust energy of the
engine could be saved up to 34% of fuel saving [23].
2. Thermoelectric energy conversion technology
2.1. Background of thermoelectric generator
Being one of the promising new devices for an automotive
waste heat recovery, thermoelectric generators (TEG) will become
one of the most important and outstanding devices in the future.
Within the recent years, the revival of interests into clean energy
production has brought TEG technology into the attention of
many scientists and engineers. Mori et al. [24] studied the
potentials of thermoelectric technology in regards to fuel econ-
omy of vehicles by implementing thermoelectric (TE) materials
available in the market and certain industrial techniques on a 2.0 l
gasoline powered vehicle. Hussain et al. [25] studied the effects of
thermoelectric waste heat recovery for hybrid vehicles. Stobart
and Milner [26] explored the possibility of thermoelectric regen-
eration in vehicles in which they found out that the 1.3 kW
output of the TE device could potentially replace the alternator of
a small passenger vehicle. Stobart et al. [27] reviewed the
potentials in fuel saving of thermoelectric devices for vehicles.
They concluded that up to 4.7% of fuel economy efficiency could
be achieved. From these articles, the understanding of TEG
technology has been comprehensively discussed as a promising
new technology to recover waste heat from internal combustion
engines. Studies on thermoelectric devices are still an ongoing
matter.
TE devices may potentially produce twice the efficiency as
compared to other technologies in the current market [28].TEG is
used to convert thermal energy from different temperature
gradients existing between hot and cold ends of a semiconductor
into electric energy as shown in Fig. 4. This phenomenon was
discovered by Thomas Johann Seebeck in 1821 and called the
‘‘Seebeckeffect’’. The device offers the conversion of thermal
energy into electric current in a simple and reliable way. Advan-
tages of TEG include free maintenance, silent operation, high
reliability and involving no moving and complex mechanical
parts as compared to Rankine cycle system [29] which will be
discussed in the next section of this study. In regards with the
applicability of TEG in modern engines, the ability of ICEsto
convert fuel into useful power can be increased through the
utilization of the mentioned device. By converting the waste heat
into electricity, engine performance, efficiency, reliability, and
design flexibility could be improved significantly. The fuel effi-
ciency of gasoline powered, diesel and hybrid electric vehicles
(HEVs) that utilize the power generation of IC engine is as low as
25% and conversely as much as 40% of fuel energy can be lost in
the form of waste heat through an exhaust pipe [30]. An increase
of 20% of fuel efficiency can be easily achieved by converting
about 10% of the waste heat into electricity [17,31]. Furthermore,
secondary loads from the engine drive trains can be eliminated
with the help of TEG system, and as a result torque and
Brake
Work
10.4%
Exhaust
27.7%
Friction
Coolant,
and Other
61.9%
1st Law Fuel Energy Distribution
Brake
Work
9.7% Exhaust
Exergy
8.4%
Irreversib
iilities,
Friction,
Coolant,
and Other
81.9%
2nd Law Fuel exergy Distribution
Fig. 3. 1st law and 2nd law energy and exergy distribution in an internal combustion engine [21].
R. Saidur et al. / Renewable and Sustainable Energy Reviews 16 (2012) 5649–5659 5651

horsepower losses from the engine can be reduced. This would
help to reduce engine weight and direct the most of the increased
power to the drive shaft, which would in turn help to improve the
performance and fuel economy. Additionally, the possibility of
minimizing the battery needs and exhaustion of vehicle battery
life while permitting operation of specific accessories during
engine off can be achieved by utilizing TEG [17].
2.2. TEG in the automotive industry
For an automobile engine, there are two main exhaust heat gas
sources which are readily available. The radiator and exhaust gas
systems are the main heat output of an IC engine [34]. The
radiator system is used to pump the coolant through the cham-
bers in the heat engine block to avoid overheating and seizure
[30]. Conversely, the exhaust gas system of an IC engine is used to
discharge the expanded exhaust gas through the exhaust mani-
fold. Zhang and Chau [30] reported that presently TEG is mostly
installed in the exhaust gas system (exhaust manifold) due to its
simplicity and low influence on the operation of the engine.
Furthermore, TEG system including the heat exchanger is com-
monly installed in the exhaust manifold suitable for its high
temperature region [17]. Basically, a practical automotive waste
heat energy recovery system consists of an exhaust gas system, a
heat exchanger, a TEG system, a power conditioning system, and a
battery pack as shown in Fig. 5; with the operation of the TEG
waste heat recovery system described as follows [16]:
i) During the normal operation of an internal combustion
engine, the produced waste heat released through the exhaust
manifold is captured by the heat exchanger mounted on the
catalytic converter of the exhaust gas system.
ii) Electricity is then generated from the thermal energy cap-
tured by the heat exchanger after it is transferred to the TEG
system.
iii) Power conditioning is performed by the power converter to
achieve maximum power transfer.
2.3. Challenges of TEG
The primary challenge of using TEG is its low thermal effi-
ciency (typically
Z
tho4%) [35]. Thermoelectric materials effi-
ciency depends on the thermoelectric figure of merit, Z; a material
constant proportional to the efficiency of a thermoelectric couple
made with the material. Karri et al. [36] stated that future
thermoelectric materials show the promise of reaching signifi-
cantly higher values of the thermoelectric figure of merit, Z, and
thus higher efficiencies and power densities can be obtained.
Materials such as BiTe (bismuth telluride), CeFeSb (skutterudite),
ZnBe (zinc–beryllium), SiGe (silicon–germanium), SnTe (tin tell-
uride) and new nano-crystalline or nano-wire thermoelectric
materials are currently in development stage to improve the
conversion efficiency of TEGs[37]. BiTe-based bulk thermoelectric
material is mostly used in waste heat recovery power generation
due to its availability in the market and high applicability in low
and high exhaust gas temperature range [37]. The performance of
a thermoelectric material can be expressed as ZT¼S
2
T/k
r
, where S
is the thermo power, Tthe absolute temperature,
k
the total
thermal conductivity, and
r
the electrical resistance [33].
Another challenge which is considerable is bigger size of the
radiator and extended piping to the exhaust manifold. This
problem can be mitigated by using a nanofluid in a radiator
system. By using nanofluid, the size and weight of an automotive
car radiator could be reduced without affecting its heat transfer
performance [38–40].
2.4. Recent development of TEG in automotive industry
TEG could be coupled with various other devices to maximize
its potential. Yu and Chau [16] has proposed and implemented an
automotive thermoelectric waste heat recovery system by adopt-
ing a Cuk converter and a maximum power point tracker (MPPT)
controller into its proposed system as tools for power condition-
ing and transfer. The other exciting development of TEG is the
combination of thermoelectric and photovoltaic (PV) systems
which can be called as a hybrid system. Zhang and Chau [30]
proposed the TE-PV system coupled with MPPT controller to
achieve maximum power output. They reported that the power
improvement is recorded from 7.5% to 9.4% when the hot-side
temperature of the TEG is heated from 100 1C to 250 1C and
the irradiance of PV generator (PVG) is fixed at 1000 W/m
2
.
Fig. 5. A typical waste heat energy recovery system [16].
Fig. 4. Schematic of a typical thermoelectric device [32,33,67].
R. Saidur et al. / Renewable and Sustainable Energy Reviews 16 (2012) 5649–56595652

Also, when the irradiance of the PVG is controlled from 200 W/m
2
to 1000 W/m
2
and the hot-side temperature of the TEG is fixed at
250 1C, the power improvement as much as 4.8% to 17.9% can be
achieved. As a result the potential use of the system opens up
many possibilities for engine efficiency.
3. Six-stroke internal combustion engine cycle
The concept of a six-stroke internal combustion engine cycle is
fundamentally based on the basic four-stroke engine cycle but
with two added cycles to produce higher efficiency and reduce
emissions. There are many patents having been awarded for
designs on six-stroke cycle engine which are discussed in Refs.
[41–48]. However, very limited articles on the subject of six-
stroke combustion engine cycle performances have been pub-
lished. Hayasaki et al. [49] proposed a six-stroke direct injection
(DI) dual fuel diesel engine that has second compression and
combustion processes as opposed to the typical four-stroke diesel
engine. They used diesel–methanol fuel which reduces nitric
oxide (NO) and soot emissions to almost zero especially for soot
emission. Furthermore, a slightly lower indicated specific fuel
consumption (ISFC) of the six-stroke diesel engine than that of the
four-stroke engine was achieved. Hence, based on these papers, it
can be concluded that a six-stroke cycle engine has better thermal
performance and low fuel consumption potentials.
A typical four-stroke cycle involves (1) intake stroke, (2) com-
pression stroke, (3) combustion stroke and (4) exhaust stroke.
However, in a six-stroke internal combustion engine proposed by
Conklin and Szybist [21], the expanded exhaust gas from the
fourth stroke is trapped and recompressed by two additional
strokes. Theoretically, with the addition of a couple of power
strokes, more output work can be produced without any extra
fuel injected into the cylinder, thus improving fuel economy of
the engine. Consequently, water is injected and the steam/
exhaust mixture is expanded. By closing the exhaust valve earlier
than usual, the residual gas inside the cylinder will be trapped.
The injected liquid water will receive energy from the recom-
pressed gases which causes it to expand and hence increasing the
pressure inside the cylinder. Hence, more work is produced
through the expansion process. However, all the multi-stroke
engine cycles explained in Refs. [42–46,48] employ a complete
exhaust stroke during crank angle 540–7201which produces
impingement on the combustion chamber surfaces when water
is injected into the cylinder. Conklin and Szybist [21] believe that
an engine cycle that utilizes water injection to absorb the heat
directly from the exhaust gas is more practical than using the
combustion chamber surfaces as the primary heat source.
By employing an ideal thermodynamics model of the additional
two strokes i.e., exhaust gas recompression, water injection and
expansion, Conklin and Szybist [21] found out that the net mean
effective pressure of the steam expansion stroke (MEP
stream
) can
be maximized by modifying the exhaust valve closing timing
during the fourth stroke. As a result, the range of calculated
MEP
stream
was found out to be 0.75–2.5 bars which show a
potential increase in engine efficiency and fuel consumption as
the combustion mean effective pressures (MEP
combustion
) of gaso-
line powered IC engine are typically up to 10 bars [21].
4. Rankine bottoming cycle technique
The low-grade temperature heat from the exhaust cannot be
efficiently converted to electrical power by using conventional
methods as seen in industrial waste heat recovery systems. In this
section, a study on converting these low-grade temperature heat
sources using Rankine cycle is discussed. There are many other
thermodynamic cycles proposed to generate electricity from
exhaust heat. These are Kalina, supercritical Rankine, organic
Rankine, trilateral flash and Goswami cycles. Interestingly, Kalina
and organic Rankine cycles have been compared in many studies
in the past few years. DiPippo [50] reported that even though
there have been claims of up to 50% of more power output for the
same input for Kalina cycles as opposed to organic Rankine cycles,
data from actual operations only show a difference of about 3% in
favor of Kalina cycle as compared to organic Rankine cycle under
similar conditions. Vaja and Gambarotta [51] mentioned that a
12% increase in the overall efficiency with respect to the engine
with no bottoming. They added Organic Rankine Cycle (ORC) can
recover only a small fraction of the released heat by the engine
trough the cooling water.
4.1. Background of the technique
Rankine bottoming cycle is a derivative of the Rankine cycle.
Because of the low-grade heat sources, the efficiency of the cycle
depends on the selected working fluids and operating conditions
of the system. Chen et al. [52] reviewed 35 different types of
working fluid under different operating conditions. It may be
noted that the best working fluids with the highest efficiency
cycles may not be the same for other operating conditions and
different working fluids. Fig. 6 shows a configuration of a Rankine
cycle system and its processes plotted in a T-sdiagram.
AWHR Rankine bottoming cycle system consists of a wet, dry
or isentropic fluid as the working fluid, a pump to circulate the
working fluid (increase in pressure), an evaporator/boiler to
Fig. 6. Rankine cycle system [53].
R. Saidur et al. / Renewable and Sustainable Energy Reviews 16 (2012) 5649–5659 5653

absorb exhausted heat energy, an expansion machine (expander)
to release power by bringing the fluid to a lower pressure level
(organic vapor expands in the turbine to produce mechanical
energy), a condenser to release the heat from the fluid and
liquidize the fluid before starting the whole cycle again [54].
Evaporator/boiler for a Rankine cycle system is usually a heat
exchanger that absorbs heat from exhaust gas that operates at a
constant level of evaporating pressure [51,55].
4.2. Working fluids in Rankine cycle
Power generation by the Rankine cycle is a well adopted
technology. In most applications, wet working fluid or water is
used as the working fluid in the closed circuit of the cycle. Due to
the thermal stability of steam (water) it can be applied in
applications where the heat source temperatures are very high
without the fear of thermal decomposition. However, in applica-
tions that captures heat from low-grade sources and provides
output capacity smaller than 1 MW as in the case of automotive
engines, organic working fluid turbines generally have higher
efficiency than steam turbines due to design considerations with
smaller molecular weight of working fluids and better economics
[56,57]. The T-s diagram of working fluids can have positive slope
of saturation curve, negative slope or vertical slope. Accordingly,
these fluids are called wet, dry and isentropic fluids (Fig. 7).
Anorganic Rankine cycle (ORC) utilizes organic fluid (i.e., dry or
isentropic) instead of water as the working fluid. It can be said
that the efficiency of the cycle is greatly dependent on the
selection of the working fluid. An organic Rankine cycle generally
uses isentropic organic fluids due to their low heat of vaporization
and they do not need to be superheated to increase their recovery
efficiencies as needed for wet working fluid (water) [58]. Various
articles have been published discussing the ideal properties and
operating conditions of working fluids for a Rankine cycle. Badr
et al. [59] studied properties of different organic fluids as
candidates for regenerative Rankine-cycle units by using compu-
ter programs (BASIC) to predict optimal working fluid, design and
operating conditions of a proposed system. Gu et al. [60] found
out that the cycle efficiency of several working fluids is very
sensitive to evaporating pressure but insensitive to expander inlet
temperature. Hung [61] demonstrated that systems with lower
irreversibility would produce a better power output. However,
this would vary according to different types of working fluid and
heat source. Dai et al. [62] examined the performance of an ORC
system that utilized different working fluids. They found out that
the cycles with organic working fluids produce much higher
exergy efficiency than the cycle with water. In another research,
He et al. [63] mentioned that one of the important characteristics
of the working fluid used in ORC is the slope of the saturation
vapor curve. They noted that efficiency is extremely affected by
the evaporation temperature of the working fluid. Boretti [64,65]
stated that in a given temperature gradient for optimizing the
work output, the working fluid’s evaporation enthalpy should be
as high as possible. Furthermore, Larjola [66] stated that by using
an appropriate organic fluid as a substitute for water, superlative
efficiency and maximum power output can be obtained when
wasted heat energy with moderate temperature is placed as the
heat source at the inlet. This is due to the lower irreversibility
between the working fluid and the heat source.
4.3. Considerations of working fluids
As mentioned before, working fluid has the most important
role in determining the efficiency of the cycle. The selected fluid
may affect various aspects of the whole system including the
overall system efficiency, operating conditions, economic viability
and also environmental impact due to the chemical nature of the
working fluids. The selection criteria and properties of working
fluids for Rankine cycle are presented in Table 2 [52]:
4.3.1. Types of working fluids
As discussed in Section 4.2, there are three types of working
fluids which are dry, wet and isentropic fluids that are very much
dependent on the slope of the saturation vapor curve on a T-s
diagram (dT/ds). The type of working fluid can be determined
through a simplified Equation [67]:
E¼C
p
T
H
ðnT
rH
=1T
rH
Þþ1
T
2
H
D
H
H
ð1Þ
where E(ds/dT) refers to the inverse of the slope of saturated
vapor curve on T-s diagram, nis suggested to be 0.375 or 0.38 by
[68],T
rH
(¼T
H
/T
C
) refers to reduced evaporation temperature and
D
H
H
is the enthalpy of vaporization. Note that, values of Eare,
E40; a dry fluid, E¼0; an isentropic fluid and Eo0 for a wet
fluid. Also, Chen [52] recommended that the entropy and tem-
perature data are to be used directly if they are available to avoid
large deviations due to the simplification of Eq. (1). For an organic
Rankine cycle, it is strongly suggested to use isentropic or dry
fluids to avoid liquid droplet impingement in the turbine blades
during the expansion process.
4.3.2. Latent heat, density and specific heat of working fluids
There are different opinions on the influence of latent heats,
densities and specific heats of working fluids on the Rankine
cycle. One literature suggests that high latent heat and density
with low specific heat liquid are preferable for its advantage of
absorbing more energy from the source in the evaporator and as a
result reducing pump consumption, size and the required flow
rate [69]. Another literature suggested that a low latent heat fluid
would provide the best operating condition due to the saturated
vapor at the turbine inlet [70]. For a more complete conclusion, an
analytical investigation was conducted for enthalpy change
Fig. 7. Dry (e.g., isopentane), wet (e.g., water) and isentropic (e.g., R11) working
fluids [52].
Table 2
Specifications of the hybrid engine [53].
Engine type Naturally aspirated
Maximum torque 95 Nm
Maximum power 54.7 kW
Minimum BSFC 235 g/(kW h)
R. Saidur et al. / Renewable and Sustainable Energy Reviews 16 (2012) 5649–56595654

during the turbine expansion by using the following Equa-
tion [52]:
D
h
isentropic
¼C
p
T
0
in
½1e
lðð1=T
1
Þð1=T
2
Þ=C
P
ð2Þ
where
D
h
isentropic
refers to the unit isentropic enthalpy drop (i.e.,
work output) through a turbine, T
0
in
is the turbine inlet tempera-
ture, Lis the latent heat of the working fluid, T
1
and T
2
are the
saturation temperatures of two points on the T-s diagram with
T
1
4T
2
[52]. Eq. (2) proves that the higher latent heat of a fluid
could produce higher unit work output when the other para-
meters are known. Also, fluids with higher density would produce
the same power output with smaller sized equipment. Study done
in [52] reaches the same conclusion as in [69] that working fluids
that have high latent heat and density with low liquid specific
heat would give rise to high turbine work output.
4.4. Analysis of Rankine bottoming cycle in a vehicle
Duparchy et al. [53] have studied a Rankine bottoming cycle
system implemented in a hybrid vehicle. The following discussion
attempts to summarize their findings. Table 2 provides the
specifications of engine they were testing with.
The operating points that were chosen based on for design and
simulation at constant speeds of 70 km/h and 120 km/h that
corresponds to rolling resistance of 5 kW and 20 kW, respectively
are listed in Table 3.Fig. 8 shows the layout of the Rankine
bottoming cycle system.
Main sources of waste energies can be found in the exhaust
gases and the cooling system of a vehicle. The following discus-
sion attempts to analyze the energy and exergy balances of these
two sources.
4.5. Rankine bottoming cycle in the automotive industry
In recent years, interests in a Rankine bottoming cycle have
prompted various automotive manufacturers to investigate its
potentials. Many researchers [71–74] reported that Honda and
BMW (Turbosteamer), respectively achieved a decrease in fuel
consumption up to or more than 10% for their passenger cars. For
commercial trucks, Nelson [75] reported that Cummins improves
10% of fuel consumption for their trucks by utilizing ORC. Many
other research studies on Rankine bottoming cycles have been
carried out which are discussed in Refs. [26,51,76–83].One nota-
bly exciting new research is the one proposed by Miller et al. [84]
in which they explored the use of organic Rankine bottoming
cycle integrated with TEG. A selective summary of various WHR
literature using Rankine bottoming cycles is presented in Table 4.
5. Turbocharger
5.1. Introduction
A naturally aspirated (NA) internal combustion engine pro-
duces large amount of waste heat. The combustion process of fuel
within the cylinder releases heat energy and exhausted through
the exhaust manifold and finally to the environment. This wasted
exhaust energy can be recovered using a turbocharger. Funda-
mentally, a turbocharger in its simplest definition is a type of
supercharger that is driven by exhaust energy. The other type of
automotive supercharger is the belt-driven supercharger. How-
ever, turbocharger will be the main focus of this subsection. A
turbocharger is a type of gas turbine where heat and pressure in
the expanding exhaust gas is used to increase engine power by
Fig. 8. Layout of the waste heat recovery Rankine bottoming cycle [53].
Table 4
Selective summary of WHR literatures using Rankine bottoming cycles.
Description of technique
used in the study
Accomplishment Reference
A specific thermodynamic
analysis is examined to
study the performance of a
stationary internal
combustion engine with an
ORC system.
Achieved 12% increase in
efficiency using Rankine cycles
from exhaust gas and engine
coolant. The latter only recorded
a small fraction of overall
improvement.
[51]
An ORC technique was used
with high-efficiency, low
emissions dual fuel low
temperature combustion
engine to examine the
potential exhaust WHR.
7% of improvement of fuel
economy was achieved. Average
emissions of NO
x
and CO
2
were
also reduced by 18%.
[76]
Examined system concepts
and control methods for
exhaust WHR in hybrid
vehicles through computer
simulation.
Potential fuel economy
efficiencies between 6–31% can
be achieved. Dynamic system
controls need to be investigated
and developed.
[78]
Potential exergy from WHR of
exhaust and coolant for 2.0 l
Honda Stream SI engine that
utilize ORC technique was
studied. Changes were made
on the engine to produce
maximum waste heat
energy.
Successfully showed an increase
in thermal efficiency from 28.9%
to 32.7% at a constant speed.
[77]
A study on WHR from dual-
cycle system for power
generation was presented.
The system uses TEG and
ORC technique to maximize
WHR.
Shows an overall improvement
mainly due to ORC that produces
most of the energy improvement.
Only small fraction of energy
generated through TEG but may
be useful for parasitic heat loss
i.e., fans and power steering
pumps.
[84]
Table 3
Operating points of parameter 1 and 2 [53].
Operating parameter 1 Operating parameter 2
Speed Constant at 70 km/h Constant at 120 km/h
Engine speed 1250 rpm 2500 rpm
BMEP 5 bar 10 bar
BSFC 275 g/(kW h) 237 g/(kW h)
P
exhaust
1 bar 1 bar
T
exhaust
550 1C 790 1C
Exhaust gas flow 6.06E03 kg/s 2.12E02 kg/s
R. Saidur et al. / Renewable and Sustainable Energy Reviews 16 (2012) 5649–5659 5655

compressing the air that goes into the engine’s combustion
chambers. The turbine blades of the pump will be spun by the
hot exhaust gases leaving the cylinders. A turbocharger compo-
nent usually made up of (1) turbine, (2) shaft, (3) compressor,
(4) waste gate valve, (5) actuator and (6) center housing and
rotating assembly (CHRA). Fig. 9 shows a typical turbocharger.
Turbocharger technology make possible for engine downsizing
through reducing pump work in SI engines. Also, its ability to
increase power density greatly influences the revival of diesel
engines into the industry in which most of diesel engines today
are equipped with turbochargers. Turbocharging increases the air
mass flow rate into the engine which significantly reduces
particulates for diesel engines that are released into the atmo-
sphere. Furthermore, it has been reported that turbocharged
diesel engine can improve the fuel economy of passenger vehicles
up to 30–50% and downsized turbocharged gasoline engine by
5–20% [86]. Turbocharging was largely adopted in diesel engines
and recent motivation for more fuel efficient, economic and high-
performance engines. Turbo charging has also slowly been estab-
lished with gasoline engines although the demands are dissimilar
to that of diesel engines.
5.2. Challenges of turbocharger
The earliest development of exhaust-driven turbocharger was
recorded by Dr. Alfred J. B ¨
uchi in Switzerland between 1909 and
1912. In 1915, he proposed the first turbocharged diesel engine
but gain no interests from the community [87]. In its early
infancy, turbochargers were utilized mostly in heavy-duty appli-
cations. Traditionally, turbochargers had two important issues
which are the main reasons for its low acceptance in the
automotive industry. Turbochargers suffer turbo lag (i.e., hesita-
tion or transient response) during low speed acceleration and
there are major concerns with heated bearings. Turbo lag can
poorly affect the drivability and performance of the engine.
Park et al. [88] studied the mechanism of a turbocharger
response delay and stated that the interruption of boost pressure
response is due to a combination of issues. The primary reason is
due to the physical properties of the turbocharger systems i.e.,
weights of the turbine and compressor. The secondary reason
(which is a resultant problem of primary factor) is mainly due to
the reduced useful turbine energy occurred from disturbances in
the operating mechanism. To improve the transient response,
weights of turbine and compressor wheel can be reduced by using
new materials. Additionally, reducing intake and exhaust system
volume may also improve transient response of the turbocharger.
5.2.1. Variable geometry turbine—Reducing turbo lag
Variable Geometry Turbocharger (VGT) technology (also known
as Variable Nozzle Turbocharger, VNT) is a type of turbine where
the turbo controls the exhaust flow through the turbine blades by
using variable vanes. At low engine speeds, the effective aspect
ratio (A/R) is too large and the turbo will not be able to produce
boost. Conversely, at high engine speeds, the A/R ratio is too small
for the turbo which will choke the engine. As a result, increase in
exhaust manifold pressures, high losses in pumps and eventually
lower power output. A turbocharged engine equipped with VGT
has small movable vanes to direct the incoming exhaust flow
through the turbine blades. At different ranges of speed, the angle
of the vanes would vary to optimize the flow of the exhaust gas.
There are various articles discussing VGTs. Shimizu et al. [89]
examined the torque control of a small gasoline engine that was
equipped with a VNT turbocharger. Authors reported torque
improvement of about 27% at lower speed. Wang et al. [90]
designed an electronic control for a VNT turbocharger which they
demonstrated to have a good performance of pressure boost
control under steady and transient conditions. Andersen et al.
[91] investigated and benchmarked six closely matched turbo-
charged SI engines that are equipped with VGTs. They tested
various engine operations and conditions including engine per-
formance during low and high engine speeds, transient response
and flow capacity. They concluded that more than 10% improve-
ment were achieved during low and high engine speed torque for
VGT turbochargers compared to fixed geometry turbochargers.
Eichhorn et al. [92] explored the use of a turbocharger equipped
with VNT to power auxiliary load i.e., the air conditioning system
by using WEDACS (Waste Energy Driven Air Conditioning System).
This system uses the turbine to produce mechanical energy and
cold air. The mechanical energy is then converted into electrical
energy by using an alternator while the cold air is used to cool the
air conditioning fluid. The results show that from a 2 l engine,
50 W to 1.3 kW of power could potentially be recovered.
5.3. Recent developments of turbocharger
5.3.1. Two-stage turbocharger
An exciting development of turbocharger technology is the
introduction of a revolutionary -stage turbocharger. Basically, a
two-stage turbocharger has two different sized turbochargers
assembled in serial configuration. The smaller sized turbocharger
responds at lower speed by producing a higher torque that will
reduce fuel consumption on the road [93]. Furthermore, the larger
unit provides boost at higher engine speeds [93]. Based on the
cluster of engines, different characteristic ranges of different
Fig. 9. Typical turbocharger with compressor wheel and turbine [85].
R. Saidur et al. / Renewable and Sustainable Energy Reviews 16 (2012) 5649–56595656

engines can be deduced. In the lower left-hand side of the figure,
cost-effective engines or Base Engines utilized conventional
technologies for example port fuel injection, one or none inlet
cam phaser and simple mono-scroll turbines. In the lower right-
hand side of the figure, Peak Power Engines group that have
higher specific engine power range sacrifice low-end torque for
high performance goals. Then, Peak Torque Engines group have
achieved to increase their low-end torque at low specific power
by using direct injection technology and one cam phaser. These
VTG equipped Diesel engines can provide almost the same
performance as the Peak Torque Engines thus revealing the
potentials of gasoline engines for higher specific engine power.
From the conventional engines to one-stage turbocharging the
specific low-end torque and specific engine power is limited to
values of about 180 Nm/l at 85 kW/l up to 160 Nm/l at 100 kW/l.
This limited power and torque ranges can be solved using the
two-stage regulated turbocharging and up to 200 Nm/l of low-
end torque can be delivered with power densities of more than
110 kW/l [94]. Due to the restrictions of single-stage turbochar-
ging, the introduction of two-stage regulated turbocharging can
significantly boost for higher charging pressure over the entire
engine speed map. Furthermore, the respective stage loading of
low-pressure stage and high-pressure stage can be reduced and
efficiently distributed. In the two-stage turbocharger, there are
two main components (or stages) that are designed to function
very differently. The high-pressure turbocharger component is
responsible for charging pressure at low engine speeds to ensure
an increase of pressure during lower flow rate. It is situated
upstream from the compressor side or downstream from the
turbine side with respect to the main charging stage.
With the arrangement of the turbocharger, the low speed startup
torque can be represented by the high-pressure stage to enhance
the low-end torque of the engine. At higher power, low-pressure
turbocharger with higher maximum flow rate can be utilized. The
strategy in controlling the charging group is by using the turbine by
pass; waste gate and compressor bypass control elements. The share
of the required total turbine power for the turbocharger is deter-
mined by the turbine bypass which is generated by the high-
pressure turbine. At low speed and full load, the turbine bypass is
closed to allow for exhaust gases to pass both of the high-pressure
and low-pressure turbochargers. Simultaneously, the air is com-
pressed in two-stages at the compressor side. As the engine speed
increases, the total pressure ratio also increases until finally the
high-pressure turbocharger is fully deactivated by the opening of
the turbine bypass on the exhaust side. During this point, the high
pressure compressor is bypassed to reduce throttling losses. From
medium speeds to high speeds, the high-pressure turbine is not
requiredandonlythelow-pressureturbocharger is used to provide
boost. To further reduce losses, below the naturally aspirated full
load line; the waste gate is opened so that boost pressure is
generated above the line only [94].
The BMEP of R2S system shows significant increase as com-
pared to the conventional 1-stage turbo charging. Maximum
BMEP level of 25 bar was achieved from the speed of 2000 RPM
as compared to only about 21 bar for 1-stage turbocharger. The
limit of 25 bar of BMEP was not due to the limitation of the
turbocharger, but due to the engine. Concurrently, the specific
power showed an increase from 85 kW/l to 100 kW/l starting
from engine speed of 5000 RPM. There is also a reasonable
increase in BSFC due to the increase of BMEP. The absolute intake
manifold pressure also shows the advantageous of two-stage
turbocharging system. The pressure increases from about
1600 mbar to the peak value of 2750 mbar at speeds from
1000 RPM to 1400 RPM. These data show that the two-stage
turbocharger system can provide high low-end torque better than
a single stage turbocharger.
Thus, the study can conclude that the advantages of a 2-stage
turbocharger over a conventional 1-stage turbocharger are as
follows:

Total pressure ratios are higher than that of a 1-stage turbo-
charger. Higher power outputs are possible.

Better efficiencies at low pressure (LP) stage.

Produces better low-end torque.

Dynamic performances are better with smaller High Pressure
(HP) stage (low inertia).

Minimizes the turbo lag.
Whereas the disadvantages of the 2-stage turbocharger
includes more weight, bigger size, more required actuators and
a boost pressure control more complex than 1-stage turbocharger.
5.3.2. Turbocharging for a new type of engine
Turbocharger technology has also been simulated in a new
type of engine. Musu et al. [95] proposed a novel combustion
concept called Homogenous Charge Progressive Combustion
(HCPC) that permits reduction in soot and NO
x
emissions in all
operating conditions (during high and low engine loads). A
formation of pre-compressed homogenous charge is progressively
transferred into the cylinder to control the transfer flow rate and
increasing pressures without relying on exhaust gas recirculation
(EGR). This method is closely based on standard Homogenous
Charge Compression Ignition (HCCI). The authors believe that this
turbocharged concept permits engine speed to increase up to
6000 rpm, with indicated thermal efficiency of 45%, power den-
sity of 64 kW and 300 kPa of intake pressure.
6. Economical view and environmental impact
In the design and analysis of systems which are contributing
with energy, economy is combined with technical improvements
to achieve the highest outcome. Many researches [96] show
methods with details to calculate economic factors in presence
of efficiency improvement for industrial products and this paper
is not going into details for it. However some researchers [97,98]
have recommended that for considering the whole aspect of a
technology improvement, the exergy analysis of the system
should come into consideration too. The relation between sus-
tainability of a process, exergy efficiency and environmental
impact can be seen in Fig. 10. Sustainability and environmental
impact have reverse relation which shows that when sustain-
ability increases, environmental index will decrease.
For addressing sustainability issue and global environmental
aspects the concept of exergy should come into consideration and
sustainability index is a symbol to show the sustainability by
0 20406080100
Exergy Efficiency
Sustainability
Environmetal Impact
Fig. 10. Illustration of the relation between sustainability, environmental impact
and exergy efficiency in a process [98].
R. Saidur et al. / Renewable and Sustainable Energy Reviews 16 (2012) 5649–5659 5657

numbers. It can be calculated from below Equation [98]:
Sin ¼1=ð1
j
Þð3Þ
This Equation clearly shows if the exergy efficiency increases
from 0.8 to 0.9 is highly affect on the sustainability index
compared to exergy efficiency increasing from 0.1 to 0.2 and
finally can be seen that for generating a fix amount of power less
pollutions of SO
2
and NO
x
will produced which leads to less
environmental impact clearly.
7. Conclusion
From the study, it has been identified that there are large
potentials of energy savings through the use of waste heat
recovery technologies. Waste heat recovery entails capturing and
reusing the waste heat from internal combustion engine and using
it for heating or generating mechanical or electrical work. It would
also help to recognize the improvement in performance and
emissions of the engine if these technologies were adopted by
the automotive manufacturers. The study also identified the
potentials of the technologies when incorporated with other
devices to maximize potential energy efficiency of the vehicles. It
should be noted that TEG technology can be incorporated with
other technologies such as PV,turbochargerorevenRankine
bottoming cycle technique to maximize energy efficiency, reduce
fuel consumption and GHG emissions. Recovering engine waste
heat can be achieved via numerous methods. The heat can either
be ’’reused’’ within the same process or transferred to another
thermal, electrical, or mechanical process. The common technolo-
gies used for waste heat recovery from engine include thermo-
electrical devices, organic Rankine cycle or turbocharger system.
By maximizing the potential energy of exhaust gases, engine
efficiency and net power may be improved. Exergy efficiency is a
concept which helps to obviously show the environmental impact
by numbers. By increasing the exergy efficiency, sustainability
index will increase and leads to less production of pollutants like
NO
x
and SO
2
during creating the same amount of power.
Acknowledgment
The authors would like to acknowledge the financial support
from the High Impact Research Grant (HIRG) scheme (UM-MoHE)
project (Project No: UM.C/HIR/MoHE/ENG/40) to carry out this
research.
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