viable combined cycle design for automotive applications

International Journal of Automotive Technology, Vol. 13, No. 3, pp. 401−407 (2012)

DOI 10.1007/s12239−012−0037−0

Copyright © 2012 KSAE/ 064−06

pISSN 1229−9138/ eISSN 1976-3832

401

VIABLE COMBINED CYCLE DESIGN FOR AUTOMOTIVE

APPLICATIONS

K.-B. KIM1), K.-W. CHOI2) and K.-H. LEE2)*

1)Department of Mechanical Engineering, Chungbuk National University, Chungbuk 361-763, Korea2)Department of Mechanical Engineering, Hanyang University, Seoul 133-070, Korea

(Received 24 August 2011; Revised 7 October 2011; Accepted 25 October 2011)

ABSTRACT−A relatively new approach for improving fuel economy and automotive engine performance involves the use

of automotive combined cycle generation technologies. The combined cycle generation, a process widely used in existing

power plants, has become a viable option for automotive applications due to advances in materials science, nanotechnology,

and MEMS (Mico-Electro Mechanical Systems) devices. The waste heat generated from automotive engine exhaust and

coolant is a feasible heat source for a combined cycle generation system, which is basically a Rankine cycle in the context of

this study. However, there are still numerous technical issues that need to be solved before the technology can be implemented

in automobiles. A simulation was performed to examine the amount of waste energy that could be recovered through the use

of a combined cycle system. A simulation model of the Rankine cycle was developed using Cycle-Tempo software. The

simulation model was ultimately used to evaluate the rate of waste heat recovery and the consequential increase in the overall

thermal efficiency of the engine with the combined cycle generation system under typical engine operating conditions. The

most effective automotive combined cycle system recovered 68% of the waste heat from the exhaust and coolant, resulting in

a 6% improvement in engine efficiency. The results are expected to be beneficial for evaluating the feasibility of combined

cycle generation systems in automotive applications.

KEY WORDS : Combined cycle, Rankine cycle, Automotive engine, Heat recovery

1. INTRODUCTION

Skyrocketing fuel prices and stringent emission regulations

have spurred auto manufacturers to develop automobiles

with better fuel efficiencies and lower harmful engine-out

emission levels. While remarkable progress has been

achieved, such research has been somewhat biased toward

an optimization of the combustion process or the injection

system. It is generally known that the maximum thermal

efficiency of an automotive engine is rarely above 40%. It

has been assumed that further improvements in the thermal

efficiency through optimization of the combustion process

have reached a technical limit.

In recent research aimed at improving automotive

energy efficiency, specific trends have emerged. One trend

involves the development of electric hybrid cars. Research

on hybrid automobiles is actively being cond-ucted by

many automotive companies (Kim., 2000; Walters et al.,

2001; Hirose et al., 2002; Liu et al., 2011; Shin et al., 2011;

Suh et al., 2011; Wang and Luo, 2011; Kim et al., 2010).

Another trend is the downsizing of the internal combustion

engine, which is also considered to have great potential in

improving the thermal efficiencies of engines (Hauet and

Maroger, 2002; Zaccardi et al., 2009; Fraser et al., 2009;

Lee et al., 2010; Park et al., 2010; Liu et al., 2010). A

relatively new approach for improving the overall energy

efficiency of vehicles is the implementation of the

cogeneration concept (Ringler et al., 2009; Freymann et

al., 2005; Endo et al., 2007; Kadota and Yamamoto, 2008).

It has been noted that energy loss in modern engines may

reach approximately 60%. Therefore, recovering heat waste

through an exhaust and cooling circuit seems to be a very

effective way to improve the overall thermal efficiency.

While cogeneration systems have been successfully adopted

in many power plants, there is a concern that such systems

may be too large for use in automobiles. However, advances

in materials science, nanotechnology, and MEMS devices

have made cogeneration applicable in automobiles.

In a cogeneration system, waste energy may be

recovered in three different ways: turbo compounding,

thermo-electric conversion, or thermodynamic processing

(e.g., the Rankine cycle) (Saqr et al., 2008). The Rankine

steam cycle has exhibited better efficiency than both

thermo-electric devices and turbo compounding, which

utilizes the kinetic energy of the exhaust (Ringler et al.,

2009). A cogeneration system may also be categorized in

terms of whether the recovered energy is utilized as

auxiliary power or as electricity. BMW proposed a system

in which dual turbines in Rankine cycles are directly linked*Corresponding author. e-mail: [email protected]

402 K.-B. KIM , K.-W. CHOI and K.-H. LEE

to the engine crank shaft. The results of a rig test showed the

potential to boost the thermal efficiency to approximately

5% within the operating range of the combustion engine. In

research by Honda, an operating electric generator was

attached to turbines to charge the battery (Ringler et al.,

2009; Freymann et al., 2005; Endo et al., 2007; Kadota and

Yamamoto, 2008). A vehicle test was then performed with

a Honda Stream equipped with a 2.0 L engine and a

cogeneration system. An increase in the thermal efficiency,

from 28.9% to 32.7%, was attained at a constant vehicle

speed of 100 km/h.

In this study, a simulation was performed to validate the

above experimental results and to examine the amount of

waste energy that could be recovered through the use of a

combined cycle system. A simulation model of the Rankine

cycle was developed using Cycle-Tempo software. The

model was used to evaluate the rate of waste heat recovery

and the consequential increase in the overall thermal

efficiency of the engine with the combined cycle system by

varying the system layout and the working fluid under

typical engine operating conditions. System engineering

considerations such as the capital fee of the system are

beyond the scope of this study. Instead, the focus is on the

technical features of the system.

2. METHODOLOGY

2.1. Working Fluid Investigation

The Rankine cycle is an ideal vapor power cycle in which a

working fluid is repeatedly vaporized and condensed. The

cycle efficiency is directly associated with a change of state

in the working fluid. Therefore, it is important to determine

the optimum working fluid for a specific thermal system.

Steam is the most common working fluid that has been

utilized in vapor power cycles because it has many desirable

characteristics such as low cost, high availability, and high

enthalpy of vaporization. However, other types of working

fluid, including organic fluids and various refrigerants, are

often employed for many industrial applications in

accordance with their special purpose. The advantages and

drawbacks of working fluids can be found in the literature

(Taki et al., 1992; Teng et al., 2007a, 2007b; Teng and

Renger, 2009; Bombarda et al., 2010). Thermodynamic

properties of working fluids that are commonly used in a

thermal system are summarized in Table 1.

To select the most suitable working fluid for heat recovery

from automobile engine exhaust and coolant, consideration

should be given to the waste heat recovering rate, the system

efficiency, system size, manufacturing costs, safety issues,

and environmental factors. Unfortunately, no fluid that fully

meets the aforementioned requirements has been found.

However, water possesses optimal characteristics when a

high quality heat source is available.

As shown in Table 1, the latent specific heat (i.e., the

evaporation enthalpy) of water is higher than those of any

of the other fluids; thus, the mass flow rate can be lower.

This is advantageous because the size of the thermal

system, including the heat exchanger, can be reduced. As

water itself is not expensive, the reduced size of the system

could lead to lower manufacturing costs. A small size and a

low weight are crucial factors in transportation systems.

Furthermore, the use of water is ideal with respect to safety,

cost, and environmental impact.

Unfortunately, water is not an effective working fluid

when the temperature of the heat source is low because the

high evaporation temperature decreases the recovery rate

of the waste heat. In addition, if moisture content of the

steam exceeds 10 percent, it could degrade the turbine

efficiency and erode the turbine blade. This issue could

easily be solved with a volumetric-type expander, such as

an axial piston-type expander. The freezing temperature of

water (0oC) is another concern that manufactures must

consider when using water in a Rankine cycle for

automotive applications. Special treatment is required to

protect the water from freezing during the winter season.

Despite the drawbacks of water, it is still the preferred

choice for recovering waste heat from engine exhaust,

which is approximately 700oC in the most common driving

mode. The other waste heat source available in a vehicle is

the engine coolant. Normally, the temperature of the engine

coolant is approximately 100oC but could reach 130oC with

a low flow rate. As mentioned above, water is not a suitable

working fluid for maximizing the use of a low-grade heat

source; an alternative fluid should be considered for such

applications. Refrigerants have a low enthalpy of

evaporation, which results in low system efficiency and a

high global warming potential (GWP), an estimate of how

much a given mass of greenhouse gas could contribute to

global warming. Such substances are subject to restrictions

in the Kyoto protocol and are not considered when

selecting a working fluid for this study. The low enthalpy

of evaporation of a working fluid is beneficial from the

viewpoint of heat recovery, but a higher mass flow and

larger heat exchanger surfaces are required to improve the

Table 1. Thermodynamic properties of commonly used

working fluids in thermal systems.

Fluid Water Ammonia Ethanol R245fa R134a R1234yf

Tcritical (oC) 373.95 132.25 240.75 154.05 101.06 94.8

Pcritical (bar) 220.64 113.33 61.4 36.4 40.59 32.65

Tboiling@atm (oC)

100 -33.33 78.4 14.9 -26.07 -29.2

Tmelting@atm (oC)

0 -77.73 -114.3 -67.2 -96.6 -

hfg@atm (kJ/kg)

2256.5 1369.48 820 196.69 216.97 178.21

Type Wet Wet Wet DryIsen-tropic

Isen-tropic

GWP 0 0 0 950 1,300 4

VIABLE COMBINED CYCLE DESIGN FOR AUTOMOTIVE APPLICATIONS 403

heat transfer rate to be comparable to that of water.

Consequently, a significant increase in the size of the heat

exchanger is inevitable.

Researchers from BMW selected ethanol as a working

fluid for the low temperature loop in its Rankine cycle

system (Ringler et al., 2009; Freymann et al., 2005). The

enthalpy of evaporation of ethanol is approximately four

times greater than those of refrigerants. With regard to the

enthalpy of evaporation, ammonia may be a better choice

than ethanol if the issue of toxicity is not considered. The

fact that ammonia has been frequently used in after-

treatment systems, such as the Urea-SCR in on-road

vehicles, indicates that toxicity may not be a problem. The

technical feasibility of using a binary flow (i.e., a mixture of

water and ammonia) in a Rankine cycle for cogeneration in

an automobile warrants further attention. However,

mixtures of water and ammonia were not included in this

study and remain a topic for future study.

2.2. Modeling and Simulation Method

A thermodynamic analysis of the Rankine cycles for each

working fluid was performed to investigate the effects of

the working fluid and system layout on system thermal

efficiency. Simulation models were constructed with the

computational tool Cycle-tempo, which was developed in

the thermal power engineering department of the Delft

University of Technology with the aim of computing mass

flows, gas compositions, and thermodynamics properties in

a variety of energy systems. Figure 1 is the interface of the

computer program used for the system configuration

chosen in this study.

As mentioned above, there are two waste heat sources

available in an automobile: engine exhaust (a somewhat high-

grade energy source) and coolant (a low-grade source). The

exhaust gas temperature at the exhaust manifold of a gasoline

engine was 689oC at a driving condition of 120 km/h (Park,

2009). The coolant temperature was assumed to be 120oC,

which is higher than that of a typical coolant. However, this

temperature could be achieved with a low mass flow without

an adverse effect on the engine. The waste heat energy from

the exhaust was approximately 25.19 kW (Park, 2009); it is

possible that the engine may release a similar amount of heat

energy to the coolant. The temperatures of the exhaust and

coolant of the gasoline engine were measured; however, the

engine specifications are confidential. The system design

target was selected on the basis of the source temperature. For

a more effective heat recovery process, a binary loop system

with a high temperature loop (HTL) and a low temperature

loop (LTL) was employed. Water and ethanol would be

preferable for use in the HTL and LTL, respectively. For the

simulation, the maximum evaporation temperature of the

cycle was selected to be 300°C. The design of the heat

exchanger was not considered, but the heat exchanging

efficiency was chosen to be 90%. In addition, pumps and

turbines were assumed to be isentropic.

3. RESULTS AND DISCUSSION

3.1. Optimum System Conditions

To design an effective automotive combined cycle system,

two factors need to be considered: the system efficiency

and the recovery rate of the waste heat. The designer

should select the design point that optimizes both the

system efficiency and the amount of heat recovered. A

simulation was performed to determine the optimum

conditions for the heat recovery system. The heat recovery

system was divided into two Rankine cycles: HTL and

LTL. Because the HTL has a high-grade heat exhaust

source, water is preferable for use as the working fluid. The

maximum evaporation temperature from a super-heater in

Figure 1. Interface of cycle tempo, the computer program

used for the system configuration.

Figure 2. Schematic diagram of the Rankine system for the

high temperature loop. The engine model provided by the

software represents an automotive engine although it

resembles a gas turbine engine. “G” represents a generator, “s”

denotes source, and “H” means a heat exchanger.


the HTL was selected after taking into account the system

efficiency and durability. Instead of using a large super-

heater to recover the waste heat at once, an additional heat

exchanger was installed downstream of the exhaust gas

line. This resulted in a reduction in the super-heater size. A

schematic diagram of the cycle is shown in Figure 2.

A typical pressure ratio of 10 was chosen for the system.

The thermodynamic efficiency and the amount of the heat

recovered were evaluated with the simulation model while

the evaporation and condensing pressures were changed.

As shown in Figure 3(a), the efficiency rapidly increased as

the system pressure was increased. The efficiency started to

converge to approximately 18.5% when the maximum to

minimum system pressure was 30 to 3. Increasing the

system pressure further has only a negligible impact on the

system efficiency; it does, however, increase the

manufacturing cost of the system. As shown in Figure 3(b),

the amount of heat recovered decreases as the system

pressure increases because the high condensing pressure

causes a high condensing temperature. There is a trade-off

between the cycle efficiency and the amount of heat

recovered. The trade-off is not an issue with regard to

additional power gained from a Rankine system. Note that

a multiplied value of the thermal efficiency and the amount

of heat recovered becomes the additional power recovered

by the system as shown in Figure 4. It was approximately

3.1 kW for maximum and minimum system pressures.

Figure 5 shows the working potential of the Rankine

system for maximum and minimum system pressures. It is

high when the minimum system pressure is low because

the condensing temperature is low, and it decreases with

increasing system pressure. To evaluate how much energy

could be effectively recycled through the system, the

second law efficiencies were investigated. The second law

efficiency is defined as the ratio of the actual thermal

efficiency to the maximum possible (reversible) thermal

efficiency under the same conditions. It increased as the

maximum and minimum system pressures were increased

as shown in Figure 6.

Although a similar amount of power recovered through

the Rankine system could be gained no matter what the

system pressure was, the high working potential but low

second law efficiency of the system with low pressure

Figure 3. Efficiency and the amount of the recovered heat

from the high temperature loop in the Rankine cycle as a

function of the maximum and minimum system pressure.

The superheating temperature was 300oC.

Figure 4. Additional power recovered from the high

temperature loop in the Rankine cycle as a function of the

maximum and minimum system pressures.

Figure 5. Work potential of the high temperature loop in the

Rankine cycle as a function of the maximum and minimum

system pressures.


means that the energy recycle rate is not effective. Based

on these results and a consideration of the system

durability, it was concluded that a maximum to minimum

system pressure of 30 to 3 is the optimum system pressure.

Low-grade heat sources are also available from the

coolant, which has a temperature very different from that of

the exhaust. As such, it is better to recover the heat using a

separate cycle, the LT Rankine cycle, in which ethanol is

the preferred working fluid. Because it may be hard to cool

the ethanol below a temperature of 70oC, the condensing

temperature of the LT loop was designed to be

approximately 70oC. The low pressure of the system was

fixed at 0.7 bar based on this temperature. In the same

manner, the system efficiency was evaluated as a function

of the pressure ratio. The maximum efficiency that could

be obtained with a pressure ratio of 6 is shown in Figure 7.

3.2. Optimum System Layout

It is important to investigate the efficiency with respect to

the system layout. The optimum results of this study are as

follows: the heat recovered is approximately 16.87 kW

from the HTL and 16.7 kW from the LTL in the case where

only the coolant was used as a heat source. In addition, the

efficiencies are 18.5% and 5.6% for the HTL and LTL,

respectively. The complete system that has combined HTL

and LTL is shown in Figure 8.

This system has two heat exchangers in the HTL and the

LTL that recover the waste heat from three heat

exchangers: the HTL condenser, the coolant, and the heat

exhaust. The heat exhaust from the engine therefore passes

through three heat exchangers. The temperature of the

exhaust at the inlet of the super heater is 689oC and is

reduced to 523oC after passing through the superheater. A

catalyst is mounted in the exhaust pipeline after the super-

heater to decrease the temperature of the exhaust to 250oC,

whereby it enters heat exchanger A. The exhaust

temperature is then 150oC at the inlet of heat exchanger B,

whereby it is cooled to 115oC. The purpose of heat

exchanger C is to recover heat from the engine coolant, and

heat exchanger D serves as a condenser for the HTL and as

a heater for the LTL. The heat from the steam is still a

valuable energy source because the temperature of the

condensed steam at heat exchanger D is higher than that of

the sub-cooled ethanol. The ethanol is first heated at

exchanger D, undergoes a phase change through heat

exchanger C, and becomes superheated vapor at heat

exchanger B. The superheated ethanol vapor is then used to

power turbine 1. In the HTL, the sub-cooled water passes

through heat exchanger A and is heated. The steam from

the heat exchanger is superheated at the super heater. After

being used to power turbine 2, a small amount of steam

condenses, transfers heat to the ethanol, and then

condenses further at the condenser.

With this layout, the thermal efficiencies of the HTL and

LTL were measured as 18.5% and 11.8%, respectively. In

addition, the amount of the heat recovered from the HTL

was approximately 16.87 kW, while that attained from the

Figure 6. Second law efficiency of the high temperature

loop in the Rankine cycle as a function of the maximum

and minimum system pressures.

Figure 7. Cycle efficiency of the low temperature loop in

the Rankine system as a function of the pressure ratio.

Figure 8. Automotive combined cycle system optimized

for heat recovery and system efficiency. The system

consists of two Rankine cycles.


LTL was approximately 18.94 kW. The thermal efficiency

decreased to 5.6%, and the amount of heat recovered from

the LTL was reduced to 18.37 kW when heat exchanger D

was decoupled with the HTL. With only heat exchangers B

and D and no coolant, the thermal efficiency of the LTL

was 3.99%, and the amount of heat recovered from the LTL

was 2.22 kW. When exhaust heat was the only source for

the LTL, the thermal efficiency decreased to 2% and 1.65

kW of heat was recovered.

3.3. Optimum System Layout

The efficiency of an internal combustion engine is typically

expressed as

(1)

where Wengine is the mechanical energy converted from the

fuel energy, Efuel. The fuel energy is defined as

(2)

Qexhaust and Qcoolant denote the heat losses through the

exhaust and coolant, respectively, and L represents other

losses due to auxiliary engine devices. It is generally

known that Wengine, Qexhaust, and Qcoolant each represent

approximately 30% of the fuel energy in an internal

combustion engine. Because the heat from the engine

exhaust and mechanical work were measured as 25.19 kW

and 26.7 kW in this study, the fuel energy may be

approximately 84 kW when excluding a conversion loss

due to incomplete combustion. From an automotive

combined cycle system (which is basically two Rankine

cycles), additional work (net work) could be gained as

follows:

(3)

(4)

Therefore, the thermal efficiency of an engine with a

combined cycle system is expressed as

(5)

An additional 5.36 kW of work was produced with the

combined cycle system used in this study. Consequently,

the thermal efficiency of the combustion engine improved

from 31.8% to 38.2%, corresponding to an approximately

16.7% relative improvement in the degree of efficiency.

However, when taking all irreversible phenomena into

account, such as flow friction loss (10%), heat loss (20%),

and the isentropic efficiency of the turbine (70%), the

relative improvement in efficiency and the additional work

would be approximately 8.4% and 2.7 kW, respectively.

(6)

where represents an overall isentropic

efficiency through the system, and ηturbine is an isentropic

efficiency of the turbine itself.

4. CONCLUSIONS AND FUTURE WORK

This study was designed to explore the technical feasibility

of a combined cycle system in an automobile. A simulation

was performed to evaluate the thermal efficiency and the

amount of the waste heat recovered depending on the

system layout and properties. The principle conclusions of

this study can be summarized as follows:

(1) Binary Rankine cycles can maximize both the waste

heat recovery and the overall system efficiency. In each

Rankine cycle, water is the preferred working fluid for

the high temperature loop, while ethanol is more

suitable for the low temperature loop.

(2) To recycle as much waste heat as possible from the low

temperature loop, the heat source could be the heat

released from the high temperature loop and the heat

exhaust as well as the coolant.

(3) With the combined cycle system, an additional 5.36 kW

of work was attained. This resulted in an approximately

6.4% improvement in the thermal efficiency of a

combustion engine, which corresponds to an approximate

16.7% relative improvement in the degree of efficiency.

ACKNOWLEDGEMENT−The authors acknowledge the

financial support for this research project provided by Korean

Ministry of Knowledge Economy. This work was done as a part

of Industry sources development project.

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