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Applied Thermal Engineering 65 (2014) 597e604

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Numerical study of heat transfer and combustion in IC engine with aporous media piston region

Lei Zhou a,*, Mao-Zhao Xie b, Kai Hong Luo a,c

aCenter for Combustion Energy, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University, Beijing 100084,Chinab School of Energy and Power Engineering, Dalian University of Technology, Dalian 116024, ChinacDepartment of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK

h i g h l i g h t s

* Corresponding author.E-mail address: zhouleiquan@l63.com (L. Zhou).

1359-4311/$ e see front matter � 2014 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2013.12.06

g r a p h i c a l a b s t r a c t

� Two-temperature treatment studiesthe working process of the PMengine.

� Self-balancing temperature of the PMdetermines the continued and stablework.

� Stronger heat exchange occurs be-tween gas and PM with smallerporosity.

� The PM engine can have lower levelsof NOx, unburnt HC and COemissions.

Porous medium

Cylinder

Axis

Piston

a r t i c l e i n f o

Article history:Received 29 August 2013Accepted 28 December 2013Available online 11 January 2014

Keywords:Porous medium engineCombustionHeat transferEmissions reductionNumerical simulation

a b s t r a c t

Based on superadiabatic combustion in porous medium (PM), the porous medium engine as a newcombustion concept is proposed to achieve high combustion efficiency and low emissions. In this paper,an axisymmetric model with detailed chemistry and two-temperature treatment is implemented into avariant of the KIVA-3V code to simulate the working process of the PM engine. Comparisons with thesame engine but without PM are conducted. Temperature evolution of the PM and its effects are dis-cussed in detail. Key factors affecting heat transfer, combustion and emissions of the PM engine, such asporosity, the initial PM temperature and equivalence ratio, are analyzed. The results show that thecharacteristics of heat transfer, emissions and combustion of the PM engine are superior to the enginewithout PM, providing valuable support for the PM engine concept. In particular, the PM engine is shownto sustain ultra lean combustion.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Due to the increasingly stringent regulation on fuel efficiencyand exhaust emissions, the internal combustion (I.C.) enginecommunity worldwide is resorting to new combustion concepts toachieve high efficiency and low emissions [1,2]. The reciprocating

All rights reserved.6

superadiabatic combustion in porous medium (PM) is an advancedcombustion concept with a potential to achieve these goals [3e5].This technology combines several outstanding features, such aslean-burn combustion, extended flammable limits, nearly constantand homogeneous combustion temperature, due to the excellentoverall heat transfer properties, high heat capacity and large innersurface area of the PM. Application of this technique to I.C. enginemight bring about an innovation in the engine industry [6,7].Essentially, the PM engine is a new concept for implementing ho-mogeneous combustion in I.C. engines. Based on previous studies,

Nomenclature

c specific heat capacitydt time stepdm average pore diameterDdj:j: thermal dispersion coefficient

Dim diffusion coefficient of species iDp equivalence particle diameter of PMDdjjm species dispersion coefficient

hv volume convective heat transfer coefficienthi molar enthalpy of species iL combustor lengthP pressurePe Peclet numberPPC pores per centimeterppm part per millionPr Prandtl numberVoln volume of cell nXi mole fraction of species iWi molecular weight of species ix spatial coordinateYi mass fraction of species i

Greek symbolsf equivalence ratiob temperature exponent of Arrhenius expressiona diffusion coefficient3 porositym viscositys StephaneBoltzmann constantl thermal conductivityr densityq crank angleqr radiant heat fluxRedp Reynolds numberR universal gas constantT temperatureu velocityumag magnitude of the velocityVi diffusion velocity of species i

Subscriptseff effectivei index for spatial direction or speciesk index for speciesg gass porous media

L. Zhou et al. / Applied Thermal Engineering 65 (2014) 597e604598

there are two main types of PM engines: in the first type, PM is thecombustion platform inwhich the combustion processes occur. Themost classical cases are the two configurations proposed by Durstand Weclas [6], where PM is located at the cylinder head or pistonbowl. In the second type, PM plays the role of thermal storage andheat exchanger, and combustion does not occur in PM. An exampleis the regenerative PM engine proposed by Ferrenberg [8], wherePM is attached to a rod and moves in cylinder following a certainmotion.

Durst and Weclas [6] firstly proposed the concept of the PMengine based on the PM combustion technique. They performed asystematic experimental study on a test engine, which was amodified diesel engine by inserting a silicon monocarbide (SiC) PMinto the cylinder head between the intake and exhaust valves. Fuelwas injected into the PM volume, and consequently, all combustionevents, i.e. fuel vaporization, fueleair mixture formation and ho-mogenization, internal heat recuperation, as well as combustionreactions occurred inside the PM. Their results demonstrated manyattractive characteristics of the PM engine in comparison with theconventional one, such as very low emissions, high cycle efficiencyand low combustion noise. The measured NOx and CO emissionswere found to be significantly reduced to a very low level comparedwith conventional engine. Meanwhile, there is a noticeablereduction in soot formation.

Based on a multi-zone combustion model, Macek and Polá�sek[9] modeled the working process of an IC engine equipped with aPM at the cylinder head fueled with methane and hydrogen,respectively, and discussed some important issues concerningpractical applications of the PM engine. The purpose of the PMmatrix use was to ensure reliable ignition of lean mixture and tolimit maximum temperature during combustion. The resultsdemonstrate that with PM can achieve a homogenize combustionprocess. In PM engine, with its high porosity, the pressure loss inthe PM area is not a significant influence.

Recently, Xie, Zhao and colleagues [10,11] simulated theworkingprocess of a PM engine, characterized by a permanent contact typeand fueled with methane. They investigated the compression

ignition and combustion characteristics of a PM engine using a two-dimensional numerical mode. The results show that at a givencompression ratio, the initial PM temperature and the PM structuresignificantly affect the heat transfer between the solid and gasphases, which also determining the compression ignition of the PMengine. The fuel injection timing is also a key factor which in-fluences the autoignition timing. A late fuel injection timing cannotlead to an autoignition process. Liu and Xie [12] analyzed thecombustion and working processes of a specific PM engine andevaluated its thermodynamic performance based on single-zoneand two-zone thermodynamic models considering the influencesof the mass distribution, heat transfer from the cylinder wall, massexchange between zones and heat transfer in PM. The two-zonesimulation results show the same results with three-dimensionsimulation as above mentioned that auto-ignition timing isdependent on the initial PM temperature, especially at PM tem-peratures exceeding 1000 K. But in their study due to the lack of theconvection and heat diffusion in two-zone model, the averagetemperature in the PM zone is higher than that in the cylinder,which probably causes a slightly higher NO emission.

The above mentioned studies are all based on the PM engineconcept proposed by Durst and Weclas. The thermal regenerationconcept for diesel engines was examined by Park and Kaviany [13]for the roles of the porous insert motion and the fuel injectionstrategies on the fuel evaporation and combustion, using a two-zone model and a single-step reaction mechanism. They claimedthat the regenerative engine using an in-cylinder reciprocatingporous regenerator can result in a more uniform fuel-vapor dis-tribution and a dominant premixed combustion regime due to theincrease in the adiabatic flame temperature. According theirstudies, the thermal efficiency also increased for a promotion of 10%for thermal efficiency.

To further understand the fundamental mechanism of the PMengine, Computational Fluid Dynamics (CFD) is a useful tool. Theaim of this work is to study a new engine with the PM located atpiston head using an axisymmetric model with detailed chemistryand two-temperature treatment based on a modified version of the

Cylinder

L. Zhou et al. / Applied Thermal Engineering 65 (2014) 597e604 599

engine CFD software KIVA3V [14]. Effects of relevant geometricaland operating parameters on the engine performance are analyzedand discussed.

Porous medium

Axis

Piston

Fig. 1. An axisymmetric model of PM engine with PM insert on piston head.

2. Numerical model

2.1. Physical model

In this study, a single cylinder and four-stroke diesel engine wasused as the reference prototype model. The geometric and oper-ating parameters are shown in Table 1. To save computation cost,only an axisymmetric model engine is considered. In the PM en-gine, a PM reactor is mounted on the piston head as shown in Fig. 1which shows the schematic diagram of the computational domain.The height and radius of the cylinder PM are equal to those of thebowl. The motion of PM follows the movement of the pistonmotion.

2.2. Governing equations

To simplify the computation, the following assumptions areintroduced in this work:

1) The geometry of the cylinder-PM heat exchanger assembly isaxisymmetric, with the fuel injector located at the center of thecylinder head.

2) The PM used is non-catalytic, homogeneous and optically thick.The thermophysical properties of the PM are isotropic and takento be constant [15].

3) Solid radiation is taken into account by using the Rosselandapproximation.

4) Instead of liquid fuel, methane is used as fuel for the combustionsimulation of the engine.

Based on the above assumptions, the governing equations forgas phase energy, solid phase energy, gas species and momentumin the PM domain can be written as follows. Other relevant con-servation equations can be found in Ref. [14].

Gas� phase continuity equation :v�

3rg

�vt

þv�

3rgui�

vxi¼ 0

(1)

Gas� phase momentum equation :v

vtðruiÞ þ

v

vxj

�rujui

�

¼ �vpvxi

þ v

vxj

"m

vuivxj

þ vujvxi

!#þ Si

(2)

The last term on the right-hand side of Equation (2) representsthe resistance caused by the PM and is computed by Ergun model[16]:

Table 1The basic parameters of the PM engine.

fuel Bore Stroke Compression

CH4 0.135 m 0.15 m 17.5Start of

computationEnd ofcomputation

Injection velocity Start ofinjection

155.0BTDC

120.0ATDC

500.0 m/s 15.0BTDC

Si ¼ ��m

C1ui þ C2

12rumagui

�;

where C1 is the permeability of the PM and C2 is the inertialresistance factor: C1 ¼ ðD2

p=150Þð 33=ð1� 3Þ2Þ, C2 ¼ ð3:5=DpÞðð1� 3Þ= 33Þ, Dp ¼ 1:5ð1� 3= 3Þdm, and umag is the magnitude of thevelocity, dm is the average pore diameter.

The gas� phase energy equation : vvt

�3rgcgTg

�þV �

�rgcguiTg

�þ 3

Xi

u:

ihiWi ¼ V ��l0gVTg

�þ hv

�Ts � Tg

�(3)

where the first term on the right-hand side is the thermal con-duction; the second term represents the convection heat transferbetween the gas and the solid. l0g is the corrected thermal con-ductivity of the gas inside the PM [17],l0g ¼ ð 3lg þ 3rgcgD

dj:j:Þ, in

which thermal dispersion ðDdj:j:Þ effect of the mixture in the PM is

taken into account by Ddj:j:=ag ¼ 0:5 Pe, where Pe is Peclet number:

Pe ¼ rCpudm=lgThe volumetric heat transfer coefficient hv between the solid

and gas phases was here estimated from the experimental corre-lation suggested by Fu and Viskanta [18], which can be expressedfor a foam sample of length L as: hvd2m=lg ¼ ½0:0426þ1:236=ðL=dmÞ�Redp where dm ¼ ffiffiffiffiffiffiffiffiffiffiffi

4 3=pp

=PPCðcmÞ, Redp ¼rudm=m

Solid phase energy equation :v

vt½ð1� 3ÞrscsTs�

¼ V$�leffVTs

�� hv

�Ts � Tg

�(4)

where, the first term on the right-hand side represents the solideffective conduction. The radiant heat flux in the PM is modeled bythe simplified Rosseland model [19] and then combined with thesolid conduction. Thus the effective conductivity of the solid isobtained as leff¼le þ lr, with le ¼ ls(1� 3) and lr ¼ 16sT3

s dm=

9ð1� 3Þ.

ratio Engine speed Intake pressure Intake temperature

1500 r/min 0.1 MP 400 KDuration ofinjection

Gaseous fueltemperature

10.0 CA 500.0 K

L. Zhou et al. / Applied Thermal Engineering 65 (2014) 597e604600

Gas� phase species equation :v

vt

�3rgYi

�þ V$

�rguiYi

�þ V$

�rg 3YiVi

�� 3u

:

iWi ¼ 0(5)

where the diffusion velocities are given by the mixture-averagedformulation modified properly to include the dispersion effect:Vi ¼ �ðDim þ Dd

jjmÞð1=XiÞðvXi=vxÞ. For species diffusion [20]:

Ddjjm=Dim ¼ 0:5 Pem.

The last right term is the chemical reaction source term,which iscalculated from CHEMKIN3.0 code [21] linked with KIVA-3V.

2.3. Gas injection model

In this study, methane was used as fuel for the engine simula-tion. The temperature of the fuel was assumed to be 500 K and theinjection mass varied according to the operation conditions. Thegaseous fuel is injected through the injector located at the center ofthe cylinder head into the cylinder, forming a conical spray with atop angle of 120�. The injection velocity was specified to be 500m/sand the injection duration was 10� crank angle (CA). Details aboutthe gas injection model can be found in Ref. [22].

2.4. Initial conditions and solution

In this paper, an axisymmetric model with detailed chemistryand two-temperature treatment were implemented into KIVA-3Vcode to simulate the working process of the PM engine. In orderto compute the combustion and, especially, the emission charac-teristics, the chemical kinetics package Chemkin3.0 was alsoemployed and incorporated into the KIVA-3V code. The detailedreaction mechanisms GRI-3.0 including 53 species and 325elementary reactions were used for describing methaneoxidation. Fig. 2 shows the computational mesh. The computationcovers the period from the intake valve close (IVC) to the exhaustvalve open (EVO). For boundary conditions, constant temperatureswere specified for the main cylinder boundaries; the side boundaryof the PM was assumed to be adiabatic. Thus, the initial tempera-ture of the PMwas set at a constant value (of 1250 K), which shouldbe approximately equal to the average temperature of the PMduring a continuous operation. The initial temperature for the bulk

z(cm)

2

4

6

8

10

12

14

16

cylinder

Porous media areain Piston

Fig. 2. The computation grids of the PM engine.

gas phase in the cylinder volume and PM region was specified as400 K.

Due to the absence of experimental data in the PM engine, theabove numerical model for combustion processes in PM has beenvalidated against experimental data for a PM burner with unidi-rectional and reciprocating flows obtained by Zhdanok et al. [23],whomeasured the propagation of thermal and combustionwave inthe PM. Based on the same models as used in this paper, we per-formed an analysis of filtration combustion of gaseous mixture in aporous packed bed in a previous work [10], and the results were ingood agreement with the measurements by Zhdanok et al. [23].

3. Results and discussion

3.1. Overall combustion characteristics of the PM engine

Fig. 3 illustrates the simulated average pressure and gas andsolid temperatures over the entire cylinder against crank anglewith porosity of 0.87, heat capacity of 420.6 kJm�3 K�1 and equiv-alence ratio of 0.3, respectively, which reveals some basic workingcharacteristics of the PM engine. In this paper, the gas temperaturerefers to the average gas temperature over the entire cylinderincluding the PM region, unless otherwise stated. Although theinitial gas temperature in the PM region is set as 400 K, the tem-perature of gas phase goes up rapidly and approaches the tem-perature of the PM very soon, due to the high thermal capacity anda sufficient heat transfer rate between the PM and gas.

Fig. 3 presents the influence of the PM physical features on thegas temperature. During the early compression stroke, there is notany remarkable variation of PM temperature but the gas temper-ature in the cylinder region and in the PM region increases. It wasnoted that with upward motion of piston, the heat exchange pro-cess between PM and compressed gas is promoted and the pore gastemperature in the PM region and PM temperature reach a relativeequilibrium period due to the enhancement of the convection of airin the cylinder and more heat transfer from the PM region to allspace. At the top dead center (TDC) the air is compressed most andin close contact with the PM volume and near the TDC ofcompression stroke fuel is injected into the PM volume. Aftercombustion has started, the gas phase temperature increasesrapidly, and simultaneously the PM temperature goes up due tostrong heat transfer from high temperature gas to PM, while thehigher gas velocity enhances the heat exchange process.We can seefrom Fig. 3 that the average gas temperature and PM temperature

Crank Angle (dgree)

Temperature(K)

Pressure(MPa)

225 270 315 360 405 450400

600

800

1000

1200

1400

1600

1800

2000

0

1

2

3

4

5

6

7

8pressureporous temperatureporous gas temperaturegas temperature

Fig. 3. Variation of temperature and pressure with crank angle.

L. Zhou et al. / Applied Thermal Engineering 65 (2014) 597e604 601

reach the highest value at this time. During the post-combustionperiod, the heat exchange between the hot PM and gas in the PMvolume causes the former’s temperature to decrease, while a littleof enthalpy is transferred from the PM back to the gas phase and thegreat majority of energy was trapped in the PM for the next cycle.So the compression ignition of PM engine can continue to work.

During the period of combustion the energy is deposited at theporous medium and at the beginning of the next cycle the enthalpyis transferred from the PM back to the gas phase to heat up the air incylinder. So the porous medium has the heat storage and exchangefunction. In the working process of the PM engine, there are someimportant parameters, such as initial temperature, the porosity ofPM and the equivalence ratio, which affect the evolution of thesolid and gas temperatures independently and interactively. Themain focus of this study is the influence of these parameters asdiscussed in the following.

3.2. Self-balancing temperature of PM

In this study, the self-balancing temperature of PM is theconcept that the engine can sustain the constant temperature or a

a) gas temperature

b)PM temperature

Crank Angle (degree)

Temperature(K)

320 340 360 380 400 420600

800

1000

1200

1400

1600

1800

2000

1150 K1250 K1350 K

Crank Angle(degre)

Temperature(K)

Temperature(K)

225 270 315 360 405 450

1100

1200

1300

1400

1500

1100

1200

1300

1400

1500

1150 K1250 K1350 K

Fig. 4. Variation of average gas temperature and PM temperature with PM initialtemperature.

small range of variation in temperature at the start and finish of theworking process, which is one of the crucial factors in determiningthe realization of compression ignition and controlled peak tem-perature and pressure in the combustion period. In this paper, theself-balancing temperature of PM is set as the initial temperature ofPM.

The curves in Fig. 4a) reflect the ignition timing is different fordifferent initial temperatures of PM. At the initial temperature of1150 K, the ambient gas in the combustion chamber cannot obtainenough heat from the hotter PM by the heat transfer way, and themethane is a fuel that is not easy to be compression ignited. Hence,with the initial temperature of PM at 1150 K, compression ignitioncannot be realized around the TDC. For this temperature, a longertime is necessary for the ambient gas in the chamber obtainingsufficient heat from the PM and its temperature reaching a certainextent. Thus, the ignition occurs significantly later and the tem-perature evolution shows difference trend than other PM initialtemperatures. when the initial PM temperature is equal to or above1250 K, the mixture in the engine is ignited very soon after fuel isinjected into the PM and it does not significantly affect the averagegas temperature, owing to the fact that a part of combustion energyis stored in the PM. Fig. 4b) presents the variation of PM temper-ature with different initial temperatures of PM. When the initialtemperature of PM is higher than 1250 K, the PM temperature atthe end of the computation is lower than that at the start of thecomputation. So in this paper the initial temperature of PM is set as1250 K to realize the self-balancing temperature of PM during allthe working processes. At the same time, a high level of the initialtemperature of PM will result in an increase in the combustiontemperature and less enthalpy storage during combustion stage,which leads to an increase in NOx emissions.

It can be concluded that for a specific PM engine it should have aspecific self-balancing temperature. The self-balancing tempera-ture can not only induce the accomplishment of compressionignition and guarantee the normal operation for the next cycle, butalso avoid the rise of NOx emissions. Of course, the physical prop-erties of the PM, the equivalence ratio, the heat transfer betweenthe gas and solid phase of PM and the volume of PM all have in-fluences on the PM self-balancing temperature. Hence, these fac-tors must be comprehensively considered to understand theworking character of the PM in I.C. engines.

3.3. Effect of PM porosity

One of the key parameters of porous structure is porosity,because it affects substantially the heat capacity and heat transferbetween the gas and the PM. To study the influence of PM structureon the realization of compression ignition of the PM engine, threedifferent porosities were examined with the initial PM temperatureof 1250 K and fuel/air equivalence ratio of 0.3.

Fig. 5 shows the variation of the average gas temperature andPM temperature with different porosity. It is seen that the burningtime is advanced and the peak average gas temperature in cylinderdescends with decreasing porosity of the PM at equivalence ratio of0.3, which can lead to reduction in NOx emissions as shown inFig. 5b). This is mainly because stronger heat exchange occurs be-tween gas and PM with smaller porosity ( 3¼ 0.7), which reducesthe peak gas temperature in the cylinder during the combustionprocess (Fig. 5a). Fig. 5a) also gives the simulated PM temperaturechanges with the porosity of the PM. It is obvious that less changein the PM temperature is obtained for smaller porosity ( 3¼ 0.7),while larger change corresponds to larger porosity ( 3¼ 0.87). This isdue to the fact that with decreasing porosity of the PM, the heatstorage capacity in the PM is enhanced, which reduces the tem-perature variation in the PM.

b)

Crank angle (degree)

GasTemperature(K)

PMTemperature(K)

320 340 360 380 400 420800

1000

1200

1400

1600

1800

1100

1150

1200

1250

1300

1350

=0.7

=0.8

=0.87

Gas

PM

Crank angle (degree)

NOx(ppm)

300 350 400 4500

200

400

600

800

1000

=0.7=0.8=0.87

a)

Fig. 5. Variation of average gas temperature and PM temperature with porosity.

a) average gas temperature

b) NOx emissions

Crank angle (degree)

GasTemperature(K)

330 360 390 420 450 480600

800

1000

1200

1400

1600

1800

2000=0.2=0.3=0.35=0.4=0.2 (no-PM)=0.3 (no-PM)=0.4 (no-PM)

Crank Angle (degree)

NOx

270 300 330 360 390 420 450 4800

200

400

600

800

1000

1200=0.2=0.3=0.35=0.4=0.2 (no-PM)=0.3 (no-PM)=0.4 (no-PM)

Fig. 6. Variation of average gas temperature and NOx emissions at equivalence ratioswith and without PM.

L. Zhou et al. / Applied Thermal Engineering 65 (2014) 597e604602

3.4. Effect of equivalence ratio

3.4.1. Variations of temperature and emissions compared with andwithout PM

To deeply understand the effect of PM on the engine perfor-mance, comparisons of simulated combustion and emission char-acteristics in IC engines with andwithout PM are carried out, whichhave the same geometries and specifications parameters. To ensurea similar burning processes in both engines and eliminate theignition influence, certain energy was added to the engine withoutPM at the same burning zone as the engine with PM, The sentencemeans that we set a high combustion area in without PM enginewhich is similar to the combustion areawith PM.Wemake them sothat both engines (with and without PM) have the same ignitionlocation and combustion time duration. The results are shown inFigs. 6e8. The results are calculated under the conditions of theinitial PM temperature of 1250 K, porosity of 0.87 and fuel/airequivalence ratios of 0.2, 0.3, 0.35 and 0.4, respectively. Fig. 6a)indicates that the average gas temperature increases withincreasing equivalence ratio. And the average gas temperature inthe engine without PM is higher than in the engine with PM at thesame equivalent ratio. It is mainly due to the fact that a part ofreaction heat is transferred from the high temperature gas to thePM during combustion period through both convection and radi-ation. Fig. 6b) shows that the NOx concentration sharply increasesafter ignition and reaches a peak shortly after the TDC. Its level thendecreases and finally remains nearly constant during the subse-quent combustion process. Regardless of whether or not PM is usedin the engine, the gas temperature in cylinder increases with

increasing equivalence ratio, which leads to higher NOx concen-tration. However, it should be noted from Fig. 6 that when usingPM, the NOx concentration is lower than that without using PM atthe same equivalent ratio. In general, with increasing equivalentratio (from 0.1 to 1.2), the average inecylinder gas temperaturerises and, consequently, the NOx emission increases. Thus, in thecase without PM in the engine, the average gas temperature andNOx for equivalent ratio of 0.4 should be higher than those forequivalent ratio of 0.3. It is even more remarkable that for theequivalence ratio (f¼ 0.4), the peak temperature of the in-cylinderaverage gas temperature in the PM engine is higher than that of theengine without PM for the equivalent ratio f ¼ 0.3, but the NOx

level of the former is still lower than that of the latter due to theeffect of heat transfer with PM inserted at the local high temper-ature area. The above phenomenon also can be explained fromFig. 7 which compares the gas local temperature distribution withand without PM in engine at TDC under the conditions of fuel/airequivalence ratios of 0.3. It can be seen that the gas local temper-ature especially high temperature field in PM engine are substan-tially lower and relatively homogeneous than those without PM inengine. So the presence of PM can absorb the reaction heat in thelocal high temperature region to reduce the local peak temperatureso that less NOx is produced.

Fig. 7. The gas local temperature with and without PM in engine at equivalent ratio0.3.

a)

re(K) 1300

1350

1400

=0.07

L. Zhou et al. / Applied Thermal Engineering 65 (2014) 597e604 603

The concentrations of hydrocarbon compounds (HC), CO andNOx versus the equivalence ratio at the end of the computation ofboth engines are presented in Fig. 8. Obviously, with increasingequivalence ratio HC concentrations are reduced, while the NOx andCO emissions are increased. This is mainly due to the raised com-bustion temperature with more fuel burned. However, at allequivalence ratios, lower levels of HC, NOx and CO are generated inthe PM engine compared with the equivalent engine without PM.This indicates that the PM engine can attain more homogenouscombustion, which makes the combustion process more completeand reduces emissions.

Crank angle (degree)

Temperatu

350 355 360 365 370 3751100

1150

1200

1250

=0.7

=0.8

=0.87

3.4.2. Extra lean burn in the PM engineOne of the most interesting phenomena is that extra lean burn

can be sustained in the PM engine. This can be explained as follows.Fig. 9 shows the average temperature of the gas in the PM engine atequivalence ratios of 0.4 and 0.07, respectively, in relationship tovarying porosity. And the average temperature of the gas atequivalence ratio of 0.3 was shown in Fig. 5. It can be seen inFig. 9aa) that combustion occurs (i.e. average gas temperature risesabove the PM temperature) at a very low equivalence ratio of 0.07for all three values of porosity, indicating that extra lean burn

Equivalence ratios

CO,HC(ppm)

NOx(ppm)

0.15 0.2 0.25 0.3 0.35 0.4 0.450

100

200

300

400

500

600

0

200

400

600

800

1000

1200

1400COHCNOX

Solid lines: with PMDashed lines: without PM

Fig. 8. Variation of CO, HC and NOx with equivalence ratios.

combustion can be sustained in the PM engine. The simulated re-sults are consistent with the flammable limit ( 3¼ 0.026) in PMwitha reciprocating flow system, which was reported by Hoffmann et al.[1]. Another interesting observation from the three figures is thatthe peak average temperature of gas is higher with larger porosityat equivalence ratio of 0.4 and 0.3, while the trend is the opposite atequivalence ratio of 0.07. This happens regardless of the fact thatbefore combustion starts and in the early part of combustion, theaverage gas temperature is higher with lower porosity for the threeequivalence ratios, due to stronger heat transfer from the PM to thegas. However, because of the very low equivalence ratio of 0.07,combustion heat release is relatively small and the heat transferfrom the burnt gas to the PM is relatively weak, compared with thecases at equivalence ratio 0.3 and 0.4. Therefore, the final peakaverage gas temperature is influenced more by the combustionprocess rather than the heat transfer from the burnt gas to the PM.In Fig. 9a) (equivalence ratio 0.07), the gas temperature rises fasterand higher before combustion and in the early part of combustionat the lower porosity 3¼ 0.7 than at higher porosity, because thePM can heat up the gas more easily (with less amount of gas inpores). The resulting higher gas temperature at porosity 3¼ 0.7facilitates combustion. This leads to more complete combustionand higher average gas temperature. On the contrary, in PM withgreater porosity, it is more difficult to build a high temperaturezone inside the PM, which leads to incomplete combustion and a

b)

Crank angle (degree)

Temperature(K)

340 360 380 400 420800

1000

1200

1400

1600

1800

2000

=0.7

=0.8

=0.87

=0.4

Fig. 9. Variation of average gas temperature with porosity at equivalent ratio 0.07and0.4 respectively.

L. Zhou et al. / Applied Thermal Engineering 65 (2014) 597e604604

lower average temperature in the cylinder. In summary, the trendsof average temperature variation with different porosity andequivalence ratio indicate that the presence of PM in engines canrealize the superadiabatic combustion and lean combustiontechnology.

4. Conclusion

In this paper, the characteristics of heat transfer, combustionand emissions of the PM engine with the PM located at piston headwere computed and analyzed using an axisymmetric model withdetailed chemistry and a two-temperature model. The resultsdemonstrate that the PM engine offers the realization of homoge-neous combustion with a controlled temperature in the PM com-bustion chamber. The temperature control is directly driven by theheat recuperation in the porous media and the equilibrium tem-perature of the PM should be specified properly according to therelevant influencing factors. In this study the concept of self-balancing temperature of the PM is presented, which determinesthe continued and stable work of PM engines. The results indicatethat the characters of heat transfer and combustion vary differentlywith the porosity from 0.7 to 0.87 under different equivalence ra-tios. For all values of porosity tested, the PM helps to maintain ahigh enough local temperature inside the PM to allow for com-bustion at extremely low equivalence ratio of 0.07 in the PM engine.An important result is that compared with the engine without PM,the PM engine can have lower levels of NOx, unburnt HC and COemissions, due to more complete combustion and less temperaturevariation.

Based on the experimental work of Hoffmann et al. [1] on aporous combustor, the present study has extended the applicationsof PM combustion technique to a reciprocating engine configura-tion, providing valuable support for the PM engine concept. Sig-nificant benefits in terms of combustion efficiency and emissionsreduction may be obtained if the reciprocating PM engine becomesa technical reality. More detailed experimental and numericalstudies are required to gain deeper physical understanding of thecomplex interactions among the transfer phenomena, combustionand emissions in the PM engine. In particular, it will be useful toinvestigate how both macroscopic and microscopic features of thePM affect the heat (including conductive, convective and radiativeheat transfer), mass and momentum transport in the PM engine,and the consequences on engine performance and emissions.

Acknowledgements

The authors wish to acknowledge the support for this work bythe National Science Foundation of China (NSFC Grant No.51176021), the UK Royal Academy Engineering Research Exchange

with China Scheme, China Postdoctoral Science Foundation(2012M510437) and the Center for Combustion Energy of TsinghuaUniversity.

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