This paper investigates the potential of utilizing the exhaust waste heat using an integrated mechanical device with internalcombustion engine for the automobiles to increase the fuel economy, the useful power, and the environment safety. One of theways of utilizing waste heat is to use a Stirling engine. A Stirling engine requires only an external heat source as wasted heat for itsoperation. Because the exhaust gas temperature may reach 200 to 700∘C, Stirling engine will work effectively. The indication work,real shaft power and specific fuel consumption for Stirling engine, and the exhaust power losses for IC engine are calculated. Thestudy shows the availability and possibility of recovery of the waste heat from internal combustion engine using Stirling engine.
1. Introduction
Today, the energy researches take a wide place in the world;the automobile is so significant that it consumes more thanhalf of the total energy used by all types of transportationcombined. Numerically, the energy consumption of auto-mobiles accounts for 52% of all energy used by the entiretransportation; less than 35% of the energy in a gallon ofgasoline reaches the wheels of a typical car; the remainingheat is expelled to the environment through exhaust gasesand engine cooling systems [1]. Figure 1 illustrates the energylosses of internal combustion engine (ICE). The figure showsthe thermal losses take 60% approximately, and 33% of thepower is expelled with exhaust gases; in another mean two-thirds of our fuels’ money was spent in the environment.Since most of the energy consumed by an internal combus-tion engine is wasted, capturing much of that wasted energycan provide more power and efficiency. Many researchersexamine how to utilize that lost energy and many methodswere used such as thermoelectric generation, piezoelectricgeneration, thermionic generation, thermophotovoltaic, andmechanical turbo [2]. But these entire components wereconsidered as electric or electronic methods and they cannot
give largest power at high temperature. Another methodnow used to recover the heat from exhaust gas is calledorganic Rankin cycle (ORC). Figure 2 is based on the steamgeneration in a secondary circuit using the exhaust gasthermal energy to produce additional power by means ofa steam expander. The principle working of the organicRankin cycle is the same as that of Rankin cycle: the workingfluid is pumped to a boiler where it is evaporated, passedthrough an expansion device (turbine or other expander),and then passed through a condenser heat exchanger whichit is finally recondensed [3, 4]. Another method to recoverexhaust heat is by the use of the Stirling engine technique.This method has more activity and it is considered anexternal combustion engine to produce mechanical work.The recovery and utilization of waste heat not only conservefuel, usually fossil fuel, but also reduce the amount of wasteheat and greenhouse gases damped to environment [2]. Theuse of Stirling engine has many advantages which can besummarized as follows [5–7]: high potential efficiency up to45%, reversible operation, cleaner emissions, quiet operation,low vibrations, low maintenance, smooth torque delivery,and ability to run at different fuels; finally Stirling enginedoes not have valves, carburetor, ignition system, or boilers.
Hindawi Publishing CorporationJournal of EnergyVolume 2015, Article ID 495418, 8 pageshttp://dx.doi.org/10.1155/2015/495418
2 Journal of Energy
Fuel energy 100%
Exhaust 33%
Cooling 29%
Mechanical power 38%
Friction losses 33%
Air drag 5%
Engine 11.5%
Transmission 5%
Rolling resist 11.5%
Brakes 5%
Air drag 5%
Energy used to move the
car 21.5%
Thermodynamicslosses
Total energy losses
Figure 1: Energy losses of internal combustion engine.
Gas
Heat
Reduce heat
Gasifier
Pump Liquid
Liqu
id Gas
Cooler (radiator)
Electric generator
Expender Heat
exchanger(radiator) (turbine)
Sealed system
Power+$$
Figure 2: Organic Rankin cycle (ORC).
On the other side the main disadvantages and limitations canbe concluded as long start-up time at cold starting, typicallynot self-starting, and finally being quite large and heavy.
Themain objectives of this paper could be summarized intwo points: using Stirling engine to recover the waste powerthrough exhaust manifold to generate electrical power andalso showing the effect of raising the entire operating pressurefor Stirling engine to get more power in practical size for theautomobile and internal combustion engine.
The body structure of this paper starts by highlightingobjectives, advantages, limitations, and related research inSection one. The main factors that have an effect on theperformance of Stirling engine were displayed in Section two.Thermodynamic model analysis in terms of Schmidt cycle,waste exhaust recovery, Stirling engine power, and exhausttemperature profile were covered in Section three. Our pro-posed exhaust’s heat recovery system in Section four follows.Section five presented the experimental methodology andsetup. The calculation and results were depicted in Sectionsix. Finally, Section seven summarizes the entire paper andshows the main conclusion.
2. Stirling Engine Effective Factors
Usually the design point of a Stirling engine will be some-where between the two limits of (1)maximumefficiency pointand (2) maximum power point. There are many factors thatmay affect the out power and mechanical efficiency for theStirling efficiency, which can be concise as the following.(i) Swept volume: the area under the P-V diagram indicatesto the network that if the volume expands the power willbe increased; (ii) regenerator efficiency: the regenerator hasmesh wires to store the heat while the working gas transfersbetween the hot side and cold side; theoretically if the enginedoes not have a full regenerative, the major trouble will bein the stream flow losses through the regenerator; (iii) meanpressure: it is the average pressure inside the engine at themaximum and lower temperatures; the problem appearedwhen the inside pressure is more than the atmosphericpressure; unbalance on the pistonwill occur; (iv)working gas:the type of gas in the Stirling engine takes a major factor; toget more power, the filled gas must have high specific heatcapacity so that the gas will gain and lose the heat rapidly;
Journal of Energy 3
then the piston is moving rapidly to produce a positive speed[5]. The hydrogen has the lowest molecular weight so ithas great efficiency but low safety. Helium (He), N
2, and
air were considered a working gas for Stirling engine; andfinally (v) temperature difference: as any heat engine, themechanical efficiency stands upon the hot temperature andcold temperature so more difference gives more efficiency.
3. Thermodynamic Analysis
3.1. Schmidt Cycle Analysis. The Schmidt cycle is defined as aStirling cycle in which the displacer and the power piston orthe two power pistons move sinusoidally and dead volumesare found.The assumptions upon which the Schmidt analysiswas based are as follows [8]: (i) sinusoidal motion of parts;(ii) gas temperatures known and constant in all parts of theengine; (iii) absence of gas leakage; (iv) working fluid follow-ing perfect gas law; and finally (v) at each instant in the cyclethe gas pressure being the same throughout the working gas.
In this paper the Schmidt cycle will be evaluated numer-ically. The performance of the engine can be calculated usingP-V diagram.The volume in the engine is calculated using theinternal geometry. When the volume, mass of the workinggas, and the temperature are decided, then the pressure iscalculated using an ideal gas method in PV = mRT equation.
Firstly, the volumes of the expansion and compressioncylinder at a given crank angle will be determined. The in-stantaneous expansion volume 𝑉
𝐸is
𝑉𝐸=𝑉𝑆𝐸
2(1 − cos 𝜃) + 𝑉
𝐷𝐸, (1)
where𝑉𝑆𝐸
is a swept volume of the expansion piston and𝑉𝐷𝐸
is an expansion dead volume under the condition.The instantaneous compression volume𝑉
𝐶is determined
by
𝑉𝐶=𝑉𝑆𝐸
2[1 − cos 𝜃) +
𝑉𝑆𝐶
2[1 − cos (𝜃 − 𝜑)] + 𝑉
𝐷𝐶− 𝑉𝐵,
(2)
where 𝑉𝑆𝐶
is a swept volume of the compression piston, 𝑉𝐷𝐶
is a compression dead volume, and 𝜑 is phase angle.The total instantaneous volume is calculated in
𝑉 = 𝑉𝐸+ 𝑉𝑅+ 𝑉𝐶. (3)
In the Beta type Stirling engine, the displacer piston and thepower piston are located in the same cylinder. When bothpistons overlap, an effective working space is created. Theoverlap volume 𝑉
𝐵is
𝑉𝐵=𝑉𝑆𝐸+ 𝑉𝑆𝐶
2− √
𝑉𝑆𝐸
2
+ 𝑉𝑆𝐶
2
4−𝑉𝑆𝐸𝑉𝑆𝐶
2cos𝜑. (4)
The engine pressure 𝑃 based on the mean pressure 𝑃mean, theminimum pressure 𝑃min, and the maximum pressure 𝑃max isdescribed in [8]:
𝑃 =𝑃mean√1 − 𝑐
2
1 − 𝑐 ⋅ cos (𝜃 − 𝐴)=
𝑃max (1 − 𝑐)
1 − 𝑐 ⋅ cos (𝜃 − 𝐴)
=𝑃min (1 + 𝑐)
1 − 𝑐 ⋅ cos (𝜃 − 𝐴),
(5)
where
𝐴 = tan−1V sin𝜑
𝑡 + cos𝜑 + 1,
𝑆 = 𝑡 + 2𝑡𝑥𝐷𝐸+4𝑡𝑥𝑅
1 + 𝑡+ V + 2𝑥
𝐷𝐶+ 1 − 2𝑥
𝐵,
𝑥𝐵=𝑉𝐵
𝑉𝑆𝐸
,
𝐵 = √𝑡2 + 2 (𝑡 − 1) V cos𝜑 + V2 − 2𝑡 + 1,
𝑐 =𝐵
𝑆, 𝑡 =
𝑇𝐶
𝑇𝐸
, V =𝑉𝑆𝐶
𝑉𝑆𝐸
, 𝑥𝐷𝐸
=𝑉𝐷𝐸
𝑉𝑆𝐸
,
𝑥𝐷𝐶
=𝑉𝐷𝐶
𝑉𝑆𝐸
, 𝑥𝑅=𝑉𝑅
𝑉𝑆𝐸
,
(6)
where 𝑡 is temperature ratio, V is a swept volume ratio, and 𝑥is dead volume ratio.
The net indicated work per cycle [8] is described by
𝑊net =𝑃mean𝑉𝑆𝐸𝜋𝑐 (1 − 𝑡) sin𝐴
1 + √1 − 𝑐2
=𝑃min𝑉𝑆𝐸𝜋𝑐 (1 − 𝑡) sin𝐴
1 + √1 − 𝑐2⋅√1 + 𝑐
√1 − 𝑐
=𝑃max𝑉𝑆𝐸𝜋𝑐 (1 − 𝑡) sin𝐴
1 + √1 − 𝑐2⋅√1 − 𝑐
√1 + 𝑐.
(7)
3.2. Waste Heat Energy Calculation. The quantity of wasteheat contained in an exhaust gas is a function of both thetemperature and the mass flow rate of the exhaust gas:
�� = �� ∗ 𝑐𝑝∗ Δ𝑇, (8)
where �� is the heat loss (kJ/s); �� is the exhaust gas mass flowrate (kg/s); 𝑐
𝑝is the specific heat of exhaust gas (kJ/kg⋅K); and
Δ𝑇 is temperature gradient in K.The mass flow rate of exhaust gas ��
𝐸
��𝐸= ��𝑓+ ��𝑎. (9)
Mass flow rate of air (��𝑎) can be evaluated according to
��𝑎= 𝜇𝛾∗ 𝜌𝑎∗ V𝑠∗ 𝑁 ∗ 2. (10)
Mass flow rate of fuel (��𝑓)
��𝑓=
��𝑎
(𝐴/𝐹)ratio. (11)
The volumetric efficiency (𝜇𝛾) has a range 0.8 to 0.9.
4 Journal of Energy
0.008
0.002
363
Large. Well-designedhigh-efficiency
engines with goodcooling
Limit forconventionalstainless steelheater parts
A B
A B
heater partsCeramic
High-alloyheaterparts
Smaller. Moderate efficiencyengines designed for
economy. Long life. Orlimited cooling availability
0.02
0.015
0.01
0.005
0600 800 1000 1200
Heater temperature (K)
Beal
e num
ber (𝒫
0/(PfVE))
Figure 3: Graph of Beale number vs. heater temperature for a rangeof Stirling engines [5].
3.3. Engine Power Output. Power output can be estimatedusing a variety of methods which takes into considerationmany things like temperature difference, operating speed andpressure, expansion and compression space volumes, andregenerator effectiveness. The Beale and West numbers wereused to determine the Stirling engine power.
3.3.1. Beale Number. It is an empirical number that char-acterizes the performance of Stirling engine. It is used toestimate the power output of Stirling through relating anindicated power,P
𝑜(W), tomean pressure𝑃 (bar), operating
frequency𝑓 (Hz), and expansion space volume𝑉𝐸(cm3) with
the Beale number 𝐵𝑛:
P𝑜= 𝐵𝑛𝑃𝑓𝑉𝐸. (12)
Beale number can be estimated from Figure 3, showing agraph plotted by measuring data frommany Stirling engines.The solid line in the middle is typical of most Stirling engineswhile the upper and lower lines denote unusually high or lowperforming engines [5].
3.3.2. West Number. It is similar to the Beale number exceptit takes direct account of the temperature difference. Theformula is expressed as
P𝑜= 𝑊𝑛𝑃𝑓𝑉𝐸(𝑇ℎ− 𝑇𝑐
𝑇ℎ+ 𝑇𝑐
) , (13)
where𝑊𝑛is the West number, which has an average value of
0.25–0.35, and a higher number represents a more efficientengine.
3.4. Temperature Profile in Automotive Exhaust Systems. Theflow rate and temperature profile of the burnt gases passing
4003503002502001501005000 20 40 60 80 100
Length along exhaust system (inch)
0 20 40 60 80 100
Length along exhaust system (inch)
Exha
ust t
empe
ratu
re (∘
C)Ex
haus
t tem
pera
ture
(∘C)
Idle, 700 rpm
Accelerated, 2000 rpm
PredictedActual
700
600
500
400
300
200
100
0
Figure 4: Illustration of the exhaust gas temperature versus tailpipelength [9].
through the exhaust system are required to determine themost effective location for Stirling engine placement. Figure 4shows the comparison of the estimated and measured tem-peratures for almost identical runs along the exhaust lineof the test vehicle for both idling and accelerated enginespeeds [9]. The surface temperature measurements methodwas used in nearly stable conditions after initial warm-up. Exhaust temperatures for idling conditions were muchlower compared to accelerated conditions. When movingaway from the exhaust valve, the gases temperatures willbe decreased. Exhaust temperature varies with engine load;more loads or speed means more exhaust temperature due todecreasing in expansion cooling.
4. Proposed Exhaust’s Heat Recovery System
Figure 5 presents the unit of heat recovery for exhaust sys-tem (3D/section) views. Stirling engine components are (1)exhaust gases outlet; (2) exhaust gases inlet; (3) hot heatexchanger; (4) displacer piston cylinder; (5) coolant jacket;(6) crank case container (to keep high pressure behind powerpiston); (7) displacer; (8) power piston; and finally (9) crankshaft and flywheel.
Afterwards, the exhaust gases leave the combustionchamber, they will enter through hot exchanger’s pipe 1, andthen they leave from heat exchanger pipe 2 and then to cat-alytic converter, muffler, and tailpipe.
Hot heat exchanger must be near to exhaust valve orisolate the inlet pipe in Rockwool to prevent the exhaustgases’ heat escaping before it entered the heat exchanger. Thecoolant jacket is used to get difference in engine temperaturealso, to improve good contraction for working fluid forStirling engine in cold side of Stirling engine; coolant jacket
Journal of Energy 5
1
2
34
5
6
7
8
9
Figure 5: Exhaust’s heat recovery unit.
Figure 6: SI Robin engine single cylinder, air cooled andwith direct-injection.
must be connected directly with a separated radiator. Thedisplacer owns a vertical hole with mesh material because ofbrief in engine size; the container also has compensated valveto modify entire pressure of the engine.
5. Experimental Setup
Anumber of experiments were carried out in Tafila TechnicalUniversity in the automotive laboratories. A Robin enginesingle cylinder, air cooled and with direct-injection engine,was used in this work as shown in Figure 6. The enginespecifications are listed in Table 1. To measure the gasolineengine torque, the engine was coupled to dynamometer. Thereading of engine parameters was recorded after 135 sec ofengine operation and depicted in Table 2. The Stirling enginewas coupled to ICE. The specification of Stirling engine waslisted in Table 3.
6. Results and Calculations
To evaluate the wasted exhaust power, the air and fuel flowthrough the combustion process should be estimated. Fromthe recorded information during experiment in Table 2, thefuel consumption is 21.6 g in interval 135 seconds. So the
Table 1: Specifications and parameters of the internal combustionengine.
ICE specificationsBrand Robin engines single cylinderModel EX13D-4 strokeDisplacement 126CCMax. output 3.2 KW at 4000 rpmMax. torque 8N.m at 2500 rpmFuel Unleaded gasolineSpark plug NGK B4 (recommended)Cooling Air coolingLubricant API/SE or SAE 10W-30
Table 2: Readings of ICE powered by Jordanian gasoline/octane rate95 after 135 seconds at beginning of the engine operation.
ICE readingExhaust gas temperature 200∘CAmbient temperature 24.5∘COil temperature 85.8 CFuel temperature 23.8 CEngine speed 2585 rpmEngine power 2.3 KWAir flow speed 1.5m/s
Table 3: Parameters and specifications of Stirling engine.
Stirling engine parametersEngine configuration Stirling engine-Beta typeHot swept volume 200CCCold swept volume 150CCHot dead volume 25CCCold dead volume 20CCRegenerator volume 30CCMean pressure 20 barsHot side temperature 200∘CCold side temperature 35∘CPhase angle 90∘
Engine speed 1500 rpm (25Hz)Working fluid Air
fuel and air flow are 0.16 g/s and 4.32 g/s, respectively. Thebrake power and recovery exhaust power are 0.901 kW andthe percentage difference for exhaust and brake power is 61%.The result of availability of heat recovery by Robin engine is
6 Journal of Energy
500
1000
1500
2000
2500
3000
200 250 300 350 400 450
Pres
sure
(kPa
)
Volume (cc)
Figure 7: Relation between entire pressures and changing in volumefor Stirling engine.
0
50
100
150
200
250
300
350
0 50 100 150 200 250 300 350 400
�e�c
Crank angle (𝜃∘)
Volu
me (
cm3)
Figure 8: Relation for volume changing of expansion and contrac-tion space versus crank angle.
similar to 04 Prius 1NZ-FEX and BMW M135i which give(58% and 56%, resp.) percentage difference.
Figure 7 represents the relation between entire pressuresand changing in volume for Stirling engine, the curve beingformed as cam lobe due to dead volume affected and non-sinusoidal motion for the drive mechanism; Schmidt cyclegives ideal assumption, else dead volume and nonsinusoidalmotion, too. The area enclosed in P-V curve was indicatedwork per one cycle. Figure 8 illustrates the relation for volumechanging of expansion and contraction space with crankangle. From this diagram the location of the volumes ofexpansion can be determined and contraction space must beequal relatively to crank angle. Also it can be provided as anindication for critical point when the volume was convertedor changed from increasing to decreasing or inverting versuscrank angle. Figure 9 shows the relation between totalvolumes for Stirling engine versus crank angle. From thisdiagram the total volume changing relatively to crank anglecan be determined; also the flow rate of working gas insideStirling engine can be estimated at any instant. After thatthe flow losses through the engine can be evaluated. The
050
100150200250300350400450
0 50 100 150 200 250 300 350 400
Tota
l vol
ume (
cc)
Crank angle (𝜃∘)
Figure 9: Changing of the total volume versus the crank angle forStirling engine.
0
500
1000
1500
2000
2500
3000
0 50 100 150 200 250 300 350 400
Pres
sure
(kpa
)
Crank angle (𝜃∘)
Figure 10: Changing of the pressure versus the crank angle forStirling engine.
total pressure versus crank angle is depicted in Figure 10.The relation between pressure and total volume is inversehere. The expansion and contraction intervals cannot bedetermined precisely from P-V diagram. But on the otherside, the expansion and contraction can be easily evaluatedfrom V-𝜃 or P-𝜃 diagram.
According to the Schmidt cycle, the ideal expansionwork,pumping work, total network, and ideal net power are 88.5 J,−56.6 J, 31.9 J, and 0.772 kW at 1500 rpm, respectively. Forestimation the real output (shaft power) for Stirling engineis 0.42 kW based on the Beale method. From Figure 3, theBeale number is nearly 0.0042 at heater temperature 200∘C.So, mechanical efficiency for Stirling engine is 51.8%.
To determine the specific fuel consumption using Stirlingengine, the following equation was used:
s.f.c.with Stirling =s.f.c.without Stirling
1 +h, (14)
where h is lowering percentage for brake power; in this caseit is equal to 18%, so the s.f.c. with Stirling engine will lowerto 15% of s.f.c. without Stirling engine.h can be calculated bydividing real brake power for Stirling engine on brake powerfor IC engine:
h =Pbr.stiPbr.IC
. (15)
Journal of Energy 7
0
50
100
150
200
250
300
350
400
450
0 20 40 60 80 100 120Engine load (%)
With Stirling engineWithout Stirling engine
bs.f.
c. (g
m/k
W·h
)
Figure 11: Showing brake specific fuel consumption without Stirlingengine and with Stirling engine.
0
500
1000
1500
2000
2500
3000
0 100 200 300 400 500
Pres
sure
(kPa
)
1 bar5 bars
10 bars15 bars
Volume (cm3)
Figure 12: P-V diagram at different operating pressures.
The brake specific fuel consumption without Stirling engineand with Stirling engine is displayed in Figure 11. At begin-ning of engine loading, bs.f.c. for internal combustion enginewith and without Stirling engine are identical due to loweringin exhaust temperature (low load); subsequently when theengine load was increased, exhaust temperature will alsoincrease, so Stirling engine will work effectively to reducespecific fuel consumption. So at those moments bs.f.c. curvesare not identical.
Figure 12 displays the P-V diagram at different operatingpressures. The indicated work is represented by the area ofenclosed curve for each operating pressure. As shown in thefigure as the operating pressure increases, the area enclosedwill increase and it will increase the indicated work.
7. Conclusion
The utilization of the exhaust waste heat for ICE by the useof Stirling engine was investigated. The study results can besummarized into the following points.
(i) Waste heat of internal combustion engine is consid-ered great problem; two-thirds of that energy whichentered through the engine was lost to the environ-ment.
(ii) Waste heat recovery takes great benefits as raising fuelmileage and reducing greenhouse gases and fuel con-sumption, so that the IC efficiency will be increased.
(iii) Around 15% can be improved in vehicle fuel econ-omy through installing Stirling engine cross exhaustmanifold to recoverwaste heat in internal combustionengines.
(iv) The recovered power through Stirling engine can beconverted to charge vehicle’s batteries or to operatethe mechanical auxiliary such as oil pump, waterpump, A/C compressor, and power steering pump.
(v) Applications’ range for this project is not trapped onthe automobile only, but it can be applicable on elec-tricity generation planets, mining application cementplanets, and factories.
(vi) Three obstacles to using Stirling engine are as follows:(1) adding some weights to the automobile which isgoing to decrease its fuel efficiency, so in order to beviable it must be light; (2) backpressure through theexhaust system; (3) additional pumping power losses.
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper.
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8 Journal of Energy
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