Assessing the Effect of Mass Transfer on the Formation of HC and CO Emissio 2

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Assessing the effect of mass transfer on the formation of HC and CO emissions

in HCCI engines, using a multi-zone model

N.P. Komninos *

School of Mechanical Engineering, National Technical University of Athens, 9 Heroon Polytechniou St., Zografou Campus, 15780 Athens, Greece

a r t i c l e i n f o

Article history:

Received 29 June 2008Accepted 27 January 2009

Available online 24 February 2009

Keywords:

HCCI

Multi-zone model

Mass transfer

Emissions formation

Hydrocarbons

CO

a b s t r a c t

The focus of the present study is to assess the effect of mass transfer on the formation of unburned HC

and CO emissions in HCCI engines. A multi-zone model was modified and used for this purpose. The

new feature of the multi-zone model is its ability to switch between two distinct simulation modes,

i.e. either including or excluding mass transfer between zones. The switch between modes occurs at a

user-defined point in the engine closed cycle. Apart from mass transfer, the two modes use identical

sub-models for the heat transfer between zones and to the cylinder wall and for combustion simulation,

which is modeled using a reduced set of chemical reactions coupled with a chemical kinetics solver.

Using the modified multi-zone model, four cases were simulated and compared: one including mass

transfer throughout the closed cycle, and three cases whereby mass transfer is neglected after the initi-

ation of the 1st or 2nd heat release or after the completion of main heat release. The simulation results

reveal that mass transfer affects the HC and CO accumulated at the colder regions during combustion and

governs the HC partial oxidation and CO production during expansion. For the operating conditions stud-

ied, neglecting mass transfer during combustion results to an underprediction of HC by as much as 50%

and of CO by 45% relative to the case where mass transfer is considered for. Omitting mass transfer only

during expansion, results to an overestimation of HC by 9% and to an underestimation of CO by 26%.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Homogeneous Charge Compression Ignition (HCCI) engines

have the potential of high thermal efficiency and low NOxand soot

emissions as compared to conventional CI or SI engines[1]. How-

ever, operational and environmental issues arise since ignition

and combustion cannot be directly controlled and moreover un-

burned HC and CO emissions remain at a high level[13]. It is be-

lieved that HC and CO are formed at relatively cold regions within

the combustion chamber, i.e. in the crevice regions and the thermal

boundary layer[48]. Modeling results also support this hypothe-

sis, i.e. that CO results from the partial oxidation of fuel originating

from the crevices and the thermal boundary layer[9,10].

Several models have been constructed for the description of

HCCI combustion. Easley et al. [9]used six zones to describe the

physical processes within the engine cylinder; three adiabatic con-

stant mass core zones, an outer core zone exchanging mass with

the fifth zone, representing the boundary layer and a crevice zone.

The thermal boundary layer was assigned constant thickness. Mass

and heat exchange was not considered between all zones. The

combustion process was modeled using chemical kinetics and a

set of chemical reactions. The temperature and mass distribution

inside the combustion chamber at IVC was predefined. It was

shown that both the crevice regions and the boundary layer have

an impact on unburned HC and CO emissions.

The same structure was used by Ogink and Golovitchev [11].

The authors used nine zones and estimated the temperaturevol-

ume fraction distribution at IVC and an estimation for the thermal

boundary layer thickness which is held constant throughout com-

pression and expansion. They also did not include both heat and

mass transfer between all zones, which affects the distribution

and magnitude of CO and unburned HC emissions as well as the

combustion mechanism. Xu et al.[12]also used the scheme intro-

duced by Easley et al., whilst neglecting heat and mass transfer be-

tween zones. The model used underpredicted CO emissions and

overpredicted HC emissions.

Aceves et al. [13,14]used the KIVA code to generate tempera-

turemass distributions during compression and up to ignition. A

10-zone model was then used to simulate the combustion pro-

cesses. No interaction between zones, neither heat nor mass ex-

change, was allowed. In [13] both unburned HC and CO

emissions were underpredicted, whilst in[14]unburned HC emis-

sions were overpredicted for most of the cases examined, whilst

CO emissions were underpredicted. The demonstration of the

importance of mixing on the HC and CO emissions formation was

realized in a later study by Flowers et al.[15]by coupling CFD code

and chemical kinetics.

0196-8904/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.enconman.2009.01.026

* Tel.: +30 210 7723651.

E-mail address: [email protected]

Energy Conversion and Management 50 (2009) 11921201

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The present study focuses on the effect of mass transfer on the

formation of HC and CO using a multi-zone model. The multi-zone

model used is a modification of a previously presented model[1618]. The major modification is the ability to switch between two

distinct modes of simulation; one mode considers for mass transfer

between zones and the other neglects mass transfer. The switch

between the two modes can be realized at a user-defined point

of the closed cycle in terms of crank angle. Thus the effect of mass

transfer on the performance and emissions formation can be deter-

mined in a straightforward manner.

In both modes, the multi-zone model consists of zones, which

assume spatial locations and dimensions. One of the zones repre-

sents the crevice regions, since these regions apart from being a

source for unburned HC, decrease the effective compression ratio

and the available chemical energy during main combustion. At

the mode where mass transfer is considered for, the zones ex-

change mass to maintain the in-cylinder pressure uniform, takinginto account their spatial location. As regards heat transfer and

combustion, the two modes use identical sub-models, features of

which have been used by other researchers [1921]. Specifically,

in the work of Jia et al.[19]the influence of mass transfer has also

been pointed out using features of the sub-models presented

herein. The variation of mixture composition due to combustion

is determined using chemical kinetics and Chemkin v.4 libraries

[22].

Four cases where examined and compared; in the first case

mass transfer is included for the entire closed cycle, i.e. throughout

compression combustion and expansion. In the second and third

case mass transfer is considered for up to the initiation of the 1st

and 2nd heat release respectively. Beyond this point, the switch

between modes is realized and the zones retain constant mass.In the fourth case mass transfer is included up to the completion

of the 2nd heat release, beyond which the constant zone mass

mode is applied.

The comparison of the aforementioned cases revealed that masstransfer affects to some extent the temperature field and peak

combustion pressure, due to the significant enthalpy transfer

occurring between zones mainly during combustion. As regards

HC and CO emissions, mass transfer affects the HC and CO accumu-

lated at the colder regions during combustion and governs the HC

partial oxidation and CO production during expansion. For the

operating conditions studied, neglecting mass transfer during com-

bustion results to a severe HC and CO underprediction relative to

the case where mass transfer is considered for. Omitting mass

transfer only during expansion, results to a mild overestimation

of HC and to a significant underestimation of CO.

Although the results presented herein where obtained from a

multi-zone model, they are qualitative valid. Data obtained from

CFD models or their hybrids could be very useful for further inves-tigation and for the more accurate description of mass transfer in-

side the combustion chamber. CFD data can also be used to further

validate the multi-zone model presented herein. These issues are

the subject of future research.

2. Model description

The basic structure of the multi-zone model used in the present

study has been presented in the past[1618]. The major modifica-

tion made herein is the ability to switch between two modes; the

mode which includes mass transfer between zones (with mass

transfer WMT mode) and the mode which neglects mass transfer,

assuming constant zone mass (CZM mode). The switch between

the two modes is realized at a user-defined crank angle (CA) inthe closed cycle. Apart from mass transfer, the two modes of the

Nomenclature

ah, bh, ch heat transfer constantsB cylinder bore (m)K thermal conductivity (W/mK)MW average zone molecular weightn moles (kmol)

_q heat flux (W/m2)Ru universal gas constant 8.314 (kJ/kmol K)Rmf fuel burning rate (kg/m

3 s)r distance (m)S cylinder height (m)T temperature (K)t thickness (m), time (s)u velocity (m/s)V volume (m3)Y mass fraction ()W molecular weight of speciesZ number of zones

Greek symbolsa constantj Karman constantl dynamic viscosity (kg/m s2)q density (kg/m3)_x molar rate of production (moles/(cm3 s))

Subscriptscyl cylinderg gasl laminarmin minimum

n normalprod producedt turbulenttrans transferredtot total

w wall

Superscripts characteristic value+ dimensionless value

Dimensionless numbersPr Prandtl numberRe Reynolds number

AbbreviationsCA crank angleCFD computational fluid dynamicsCI compression ignitionCR compression ratioCZM constant zone massEVO exhaust valve openingHC hydrocarbonsHRR heat release rateIVC inlet valve closingSI spark ignitionTDC top dead centerWMT with mass transfer

N.P. Komninos / Energy Conversion and Management 50 (2009) 11921201 1193


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model use identical sub-models for the description of heat transfer

and combustion.

2.1. Zone configuration

The configuration of the zones in both modes is identical. As

shown inFig. 1the combustion chamber is divided into three dif-

ferent types of zones, i.e. the core zone, the outer zones and thecrevice zone. The core zone is a cylinder, each of the outer zones

is a cylindrical annulus and the crevice zone lies beneath the out-

most zone. The crevice zone represents all the crevice regions

which communicate directly with the combustion chamber, i.e.

the region above and behind the first compression ring, the head

gasket crevice, etc.

The difference between the two modes lies in the calculation of

zone volumes and thicknesses. For the WMT mode, the crevice vol-

ume is a fraction of the clearance volume, whilst the volume of the

remaining zones varies throughout the engine cycle and is calcu-

lated according to their thickness. The thickness of the core zone

equals half its height and therefore varies throughout the engine

cycle. The thickness of all outer zones is the same in the x and

y direction and is constant during the cycle. From the previous

it is concluded that at TDC the sum of zone thickness must equal

half the cylinder clearance height:

Xzi2

ti

TDC

Smin

2 1

For the CZM mode, in which mass transfer is neglected, the vol-

ume of each zone, including the crevices, is calculated from ther-

modynamic considerations. The thickness of the zones, excluding

crevices, is thereafter determined from the zone configuration

and the volume of each zone. The sum of all zone volumes must

equal the combustion chamber volume and therefore condition

(1) is superfluous in this mode. It is obvious that in CZM mode

the thickness of the zones varies during the calculations and gen-

erally differs from the zone thickness of the WMT mode. The com-putational procedure is explained in detail in Section2.6.

2.2. Heat transfer

In the present study a modification has been made for the cal-

culation of the heat transfer rate to the cylinder wall, in order to

reduce the user defined constants, which where used in the previ-

ous versions of the model [1618]. In those versions an Annand-

like correlation was used for the estimation of the heat flux to

the cylinder wall[23]:

_qw ah k Rebh

B Tg Tw ch T

4g T

4w 2

whereTg is the temperature of the outmost zone, Twthe wall tem-

perature and ah, ch and bh, were the user defined constants.

In the present study the no-slip condition has been applied in-

stead[24]. According to the no-slip boundary condition, the fluid in

contact with the combustion chamber wall assumes the velocity of

the boundary (wall) and is considered therefore stationary. Conse-

quently, heat is transferred through this thin fluid layer only via

conduction. Thus the wall heat flux is estimated by:

_qw kw@T

@rn

rn0

wall boundary condition 3

where kw is the conductivity of the charge at the cylinder wall

temperature, rn is the normal distance from the wall and @T@rn

rn0

the temperature gradient at the wall. The temperature gradient at

the wall is approximated numerically by the relations:

@T

@rn

rn0

ffi T2 Tw

t2=2

2T3 T2

t2 t3=2

1; when Tw< T2< T3

and Tw< T 4a

@T

@rn

rn0

ffiT2 Twt2=2

; in all other cases 4 b

whereTis the mean charge temperature. The use of Eq. (4a) for the

estimation of heat flux gives better results than the simpler (4b)

during the latter stage of compression, combustion and expansion,

since it takes into account the change of temperature gradient when

moving from zone 3 to zone 2 to the wall. It is shown subsequently

that these equations provide an adequate agreement between

experimental and calculated pressure traces, which suffices for

the present study. A further discussion of the wall heat transfer is

out of the scope of the investigation presented, since the main focus

is the effect of mass transfer on the formation of HCCI engine emis-

sions. The determination of the wall heat flux of an HCCI engine is

an active area of research and a heat transfer model commonly ac-

cepted has not yet been presented.

In the multi-zone model, heat is also transferred between zoneswith a mechanism similar to conduction, i.e. the heat flux between

neighbouring zones is based on their temperature difference and

mean distance (Eq.(5)):

_q ktot@T

@rn5

The amount of heat exchanged is calculated by multiplying the

heat flux by the surface area that separates them, and the time step

used.

For the determination of the total conductivity in Eq.(5), the ap-

proach of Yang and Martin[25]is followed:

ktot kl kt 6

The Yang and Martin approach has also been used by other

authors[26]for the CFD modelling of an HCCI engine, for the esti-

mation of wall heat transfer in premixed charge engine combus-

tion[27]as well as in HCCI multi-zone models[1921]. The ratio

of turbulent to laminar conductivity is calculated using the follow-

ing formula:

ktkl

PrlPrt

lt

ll7

The formula presented above, presupposes swirl dominated

flows and is used in the absence of other data. The viscosity ratio

of Eq.(5)is calculated from the formula:

lt

ll jr

n 1 exp 2ajr

n

8

B

S2 3 i z

cylinder head

x

y

liner

1

ti

ti

1

Fig. 1. Geometric configuration of the multi-zone model.

1194 N.P. Komninos/ Energy Conversion and Management 50 (2009) 11921201
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and

rn u

lw

Z rn0

qdrn 9

where j = 0.41 is the von Karman constant, a = 0.06 and rn is the

normal distance from the wall. The characteristic velocity is consid-

ered to be proportional to the engine speed. The proportionality

constant is taken equal to 0.10 for the purposes of the present studyand was chosen to match the cylinder pressure trace during com-

pression, combustion and expansion.

2.3. Mass transfer

In the WMT mode, in which mass transfer in considered for,

mass flows between zones to maintain the pressure uniform inside

the combustion chamber, which is the actual case in reciprocating

engines. The cylinder pressure is calculated at each CA step using

the ideal gas relation for all the zones:

PcylVi miMWi

RuTi; i 1;z 10

Solving(10)for the mass of each zone and summing for all the

zones, we end up to:

Xzi1

mi mcylXzi1

PcylViMWiRuTi

11

The mean cylinder pressure is thus estimated from:

Pcyl mcylRuPz

i1ViMWi

Ti

12

At each CA the mass of each zone is calculated using the equa-

tion of state for an ideal gas:

mi PcylViMWiRuTi

; i 1;z 13

The mass change of each zone during the CA step is therefore:

Dmi mi mprevious CAi ; i 1;z 14

The mass change of each zone equals the net mass flow from its

neighbouring zones. Taking into account the configuration of the

zones, mass flows only between neighbouring zones:

Dmi mflowi1!i

mflowi!i1 15

where Dmi is the total mass variation of zone i and mflowi1!i

is the

mass flow from zone i to zonej.

Subject to the boundary conditions:

Dm1 mflow1!2 16

since mass does not flow through the wall and

Dmz mflowz1!z 17

The total mass exchange is obtained as follows:

DmcylXzi1

Dmi mflow1!2

Xz1i2

mflowi1!i mflowi!i1

mflowz1!z 0

18

Since blowby is not described in the model, the previous condi-

tion must be satisfied.

The transfer of species and enthalpy is based on the assumption

that the mass flowing from a zone to its neighbouring one has the

thermodynamic properties (i.e. temperature and chemical speciescomposition) of the zone from which it originates.

2.4. Combustion

Combustion is described using the set of chemical reactions cre-

ated at Chalmers University for isooctane[28]consisting of 84 spe-

cies and 412 reactions. These reactions describe the oxidation of

isooctane and the formation of combustion products. Soot emis-

sions are not included due to the premixed nature of combustion.

The rate of production (or destruction) of each species is calculatedand the set of differential equations obtained is solved using the

Chemkin[22]libraries to determine the variation of mixture com-

position for each zone:

dYjdt

i

_xjWjq

i

; j 1; 84 and i 1;z 19

where Yjis the mass fraction of species j in zone i, xjis the mo-

lar rate of production of species j (moles/(cm3 s)), Wjis the molec-

ular weight of species j, q is the density of zone i (g/cm3) andzis

the number of zones.

At each time step, first the change of composition of each zone

is determined assuming no mass exchange between them. Then

mass is transferred between zones to keep the pressure uniform

throughout the cylinder. Therefore, the change of composition of

each zone is the result of combustion and mass transfer from its

neighbouring zones in the mode where mass transfer is considered

for.

2.5. Pollutant formation

The formation rate of CO and HC in each zone is estimated using

the chemical reactions involved in the Chalmers reduced mecha-

nism for isooctane oxidation[28]. Since it is believed that CO and

HC emissions are largely affected by the existence of the crevice

volumes and the temperature distribution inside the combustion

chamber, the interaction of the crevices with the outer- and there-

fore colder-zones is considered. To implement this interaction, ex-

change of species is considered between the zones including the

transfer of combustion products in and out of the crevices.The formation of nitrogen oxides (NOx), i.e. NO and NO2, is ac-

counted for in the reduced set of chemical reactions created at

Chalmers University[28]. This set includes the chemical reactions

listed in Table 1. The NOx formation mechanism includes the ex-

tended Zeldovich mechanism (reactions 13) for NO formation,

the reactions leading to NO2 (reactions 47) and reactions includ-

ing N2O, which have been found to play an important role in NOxformation[29].

2.6. Constant zone mass mode description

The two modes of the multi-zone model used are identical as

regards combustion and heat transfer between zones and to the

Table 1

NOx formation reactions.

1 N + NO = N2+ O

2 N + O2= NO + O

3 N + OH = NO + H

4 NO + HO2= NO2+ OH

5 NO2+ O = NO + O26 NO2+ H = NO + OH

7 NO + O + M = NO2+ M

8 N + CO2= NO + CO

9 N2O + O = NO + NO

10 N2O + O = N2+ O211 N2O + H = N2+ OH

12 N2O + M = N2+ O + M

13 N2O + OH = N2+ HO2



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cylinder wall. In the WMT mode, mass transfer is determined as

described in Section 2.3. In the CZM mode, which neglects mass

transfer, the mass of each zone is considered constant after a

user-defined CA has been reached, i.e. species and enthalpy trans-

fer are omitted.

From a computational point of view, in the WMT mode, the vol-

ume of each zone is determined at the beginning of each CA step

based on the configuration and the (constant) thickness of thezones. In this mode, mass transfer results from thermodynamic

considerations, to maintain uniform pressure within the combus-

tion chamber (Section2.3).

In the CZM mode, the mass of each zone remains constant and is

therefore known at the beginning of each CA step. The volume of

each zone is subsequently determined using thermodynamic con-

siderations to maintain uniform pressure. The thickness is thereaf-

ter calculated based on the volumes and the configuration of the

zones. In the CZM mode the configuration of the zones is identical

as in the WMT mode, but the thickness of each zone varies.

3. Engine specifications operating and modeling conditions

Although the main focus of the paper is to investigate the effect

of mass transfer on emissions formation using a multi-zone model,

a comparison of the predictions of the model to experimental re-

sults is provided for a rudimental validation of the model.

The experimental data used in the present work were provided

by Lund Institute of Technology from a relevant experimental

investigation. The details of this investigation have been published

in[5,6]. The engine considered is a Volvo TD100 Diesel engine, the

specifications of which are given inTable 2.

The operating conditions used for the purposes of the present

study, i.e. the study of mass transfer and its effect on emissions,

the results for a naturally aspirated case of isooctane fuel are used

as specified inTable 3. The air was heated to 105C before entering

the cylinder.

Sixteen (16) zones were used for the multi-zone model simula-

tion, including the crevice zone (zone 1). For the WMT case the cre-

vice volume is considered constant throughout the engine cycle

and estimated to be 1.5% of the clearance volume[30]. The thick-

nesses of the zones at IVC are given in Table 4.

More zones and a finer zone configuration near the wall, com-

pared to the previous studies (10 zones of equal thickness at TDC

in [1618]), was necessary for the more accurate description of

the thermal boundary layer near the combustion chamber wall,

which affects the emissions formed in this region.

The temperature and composition at IVC are uniform through-

out the cylinder as already mentioned and the wall temperature

was estimated around 430 K. Details for the computational proce-dure can be found in previous papers of the author [1618]. The

computational time for the case of 16 zones is approximately

40 min on an Intel Core Duo 2 GHz Processor, which is relatively

acceptable for such calculations, i.e. including chemical kinetics.

4. Computational procedure

Table 5presents the four cases, which were simulated and com-

pared. In the first case (with mass transfer WMT), which is the

base of comparison, mass transfer is allowed between zones

throughout the closed cycle, i.e. during compression combustion

and expansion. In all CZM cases the multi-zone model runs in

WMT mode only up to a certain point in the closed cycle, after

which the mode switches to CZM. In CZM mode mass and speciestransfer is prohibited and zone mass is maintained constant for the

remaining cycle. In particular, case CZM 1st HR includes mass

transfer up to the initiation of the 1st heat release, case CZM 2nd

HR includes mass transfer up to the initiation of the 2nd heat re-

lease (10 deg aTDC) and in case CZM Expans mass transfer is

neglected after the completion of main combustion (10 deg

aTDC).

It is evident form the aforementioned, that all CZM cases pro-

duce identical results with the WMT case, up to the point where

mass transfer is no longer considered for.

5. Results and discussion

5.1. Comparison to experimental results

InFig. 2the experimental pressure trace and heat release rate

are compared to the ones obtained from the multi-zone model

with mass transfer throughout the closed engine cycle (case

WMT). The comparison between the pressure traces and heat re-

lease rates is adequate. The heat release rate and the peak pressure

are slightly overestimated. The overestimation of heat release rate

could be attributed partly to the unavoidable smoothing of the

experimental heat release rate and also to the different heat trans-

fer models used during combustion.

Table 6presents the corresponding exhaust measured and cal-

culated emissions at EVO. All emissions are underpredicted by

about 6% for CO and 11% for HC emissions. NOx emissions are not

a concern in HCCI combustion since they remain at a very low levelfor the case studied.

Table 2

Engine data.

Compr. ratio 17

Bore (mm) 120.65Stroke (mm) 140

Connect. rod (mm) 260

EVO (deg BBDC) 39

EVC (deg BTDC) 10

IVO (deg ATDC) 5

IVC (deg ABDC) 13

Table 3

Operating conditions.

Fuel Isooctane

Engine speed (rpm) 1000

Lambda 2.95

imepnet (bar) 3.8

Inlet conditions Naturally aspirated

Residual fraction (est.) (%) 5

Table 4

Zone thickness.

Zone Thickness (mm)

26 0.047882

79 0.239409

1015 0.488194

16 (at TDC) 0.488194

Table 5

Description of simulated cases.

Case Mass transfer inclusion

WMT (base) Entire closed cycle

CZM 1st HR Up to 1st heat release initiation (20 deg aTDC)

CZM 2nd HR Up to 2nd heat release initiation (10 deg aTDC)

CZM Expans Up to 2nd heat release completion (+10 deg aTDC)



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5.2. Mass transfer description

The multi-zone model results for the in-cylinder mass flow have

been presented in previous studies by the author [1618], there-fore only a brief description is provided herein for completeness.

Mass flow is affected by three processes, i.e. compression, expan-

sion and combustion. The upward piston motion during compres-

sion induces mass flow from the core zone to the outer zones.

Expansion reverses the flow and induces mass flow from the outer

zones to the inner zones. As soon as combustion commences in a

zone, mass is transferred from this zone to its neighbouring ones.

The combined effect of these phenomena, determines the direction

of the flow between neighbouring zones. The effect of compression,

combustion and expansion on mass transfer between zones is

shown schematically inFig. 3.

5.3. Effect of mass transfer on pressure traces and temperature profiles

The inclusion of mass transfer affects the predicted pressure

trace, as shown inFig. 4, whereby the four cases simulated are pre-

sented. Comparing cases WMT and CZM 1st and 2nd HR, it is con-

cluded that earlier ignition and a higher peak combustion pressure

results when neglecting mass transfer after the initiation of com-

bustion. The temperature field is also affected by the inclusion of

mass transfer. Fig. 5 presents the temperature profile prior to

and during combustion for the two extreme cases, i.e. with the

inclusion of mass transfer (WMT) and neglecting mass transfer

after 1st heat release initiation (CZM 1st HR). The different zone

thicknesses for the two cases, which could induce alterations to

the heat transfer estimations, does not seem to be the source of

this discrepancy since the temperature profiles remain essentially

identical up to TDC. Therefore this difference must be attributed to

mass transfer during combustion.To investigate this hypothesis, the heat release, net heat trans-

fer and net enthalpy transfer rates for each zone, are depicted in

Fig. 6for the WMT case. The net heat transfer rate of each zone

is positive when more heat is gained than lost from the zone. Sim-

ilarly, the net enthalpy transfer rate is positive, when the enthalpy

-40 -30 -20 -10 0 10 20 30 40

CA deg aTDC

0

20

40

60

80

100

Pressure(bar)

Experimental

Calculated

0

400

800

1200

1600

2000

HRR

(J/deg)

Fig. 2. Experimental and calculated (case WMT) pressure traces and heat release

rates.

Table 6

Measured and calculated emissions.

Species Measured (mg) Calculated (mg)

CO 0.9682 0.9082

HC 2.536 2.244

NOx 3.521 103 2.764 103

1

23

Compression Expansion

z

i

i+1

i-1

Mass Flow

Crevice Flow

Piston Motion

Combustion

Combustion

Fig. 3. Schematic of mass flow during compression, combustion and expansion.

-10 -5 0 5 10 15

CA deg aTDC

30

40

50

60

70

80

90

Pressure

(b

ar)

WMT

CZM 1st HRR

CZM 2nd HRR

CZM Expans

Fig. 4. Comparison of pressure traces for the cases examined.

0 1 2 3 4 5

Distance from Wall (mm)

400

600

800

1000

1200

1400

1600

1800

2000

Temperature(K)

CZM 1stHR

WMT

-15oaTDC

-5oTDC

2o

3o

4o

5o

CombustionChamberWall

Fig. 5. Temperature profiles comparison for the WMT and CZM 1st HR cases.



7/10

inflow is higher than the outflow. The negative peaks in the net en-

thalpy transfer diagram occurring between 3 and 5 deg aTDC cor-

respond to the inner zones. This net enthalpy transfer rate

observed during main combustion is comparable and for some

zones even greater than net heat transfer rate. As soon as combus-

tion occurs in a zone, mass flows out of this zone to the neighbour-

ing zones, resulting to a net outflow (loss) of enthalpy, whichwould have remained in the zone if mass transfer was neglected.

This explains the milder temperature profile and the delayed tem-

perature increase of WMT case as compared to the CZM 1st HR

case.

5.4. Effect of mass transfer on emissions formation

In Table 7 the emissions results for all simulated cases are

shown, both in absolute values (g) and relative (%) to the baseline

case (WMT). A significant deviation is observed between case WMT

and CZM cases. Neglecting mass transfer after the 1st or 2nd heat

release initiation, results to an underestimation of CO and HC by at

least 30% and at most 53%.

Comparing cases CZM 1st HR and CZM Expans, it is observed

that if mass transfer is neglected prior to 1st heat release, HC emis-

sions are significantly underestimated, whilst if the constant zone

mass assumption is applied at the beginning of expansion, HC

emissions are overestimated. Therefore, the timing at which the

constant zone mass assumption is applied to the multi-zone model

alters both quantitatively and qualitatively the emissions

prediction.

In order to explain the aforementioned trends, the in-cylinder

HC and CO history is discussed in the subsequent sections.

5.4.1. Effect of mass transfer on HC emissions formation

To clarify the effect of mass transfer on HC emissions, a compar-

ison of the in-cylinder HC history for the four cases examined ispresented inFig. 7. It is observed that when mass transfer is ne-

glected after the 1st or 2nd heat release initiation, the amount of

in-cylinder HC present during expansion is low and essentially

constant throughout the expansion stroke, i.e. HC oxidation does

not seem to take place during expansion.

Neglecting mass transfer after the completion of combustion

(CZM Expans), results to an overestimation of HC relative to the

WMT case, due to the apparent lower HC oxidation rate during

expansion.

The source of these deviations from the baseline case can be lo-

cated by shifting the focus to the zone HC distribution during the

latter stages of combustion and the early stages of expansion,

which is provided inFig. 8.

The first observation is that zones 1, 2 and 3, i.e. the outmost

zones, are the ones mainly responsible for the HC contained in

the cylinder immediately after combustion (10 deg aTDC) in all

cases. The accumulation of mass in zones 1 and 2 during the latter

stages of compression and during combustion observed when

mass transfer is included during these periods (WMT case), is not

captured by the CZM 1st HR and CZM 2nd HR cases. This is the

main reason for the low HC amount contained in zones 1 and 2

in cases CZM 1st HR and CZM 2nd HR. The difference in the HC

amount between these two cases is attributed to the timing of

the constant zone mass assumption; in case CZM 2nd HR more

mass has been accumulated in zones 1 and 2 relative to case

CZM 1st HR.

Another observation is that neglecting mass transfer apparently

affects the HC oxidation rate during expansion. As soon as mass

transfer is prohibited, the HC amount of zones 1 and 2 remainsunaltered, due to the low zone temperature, whilst in zone 3 al-

most all HC are oxidized. This prohibits further HC oxidation and

produces zero-slope HC history for all CZM cases during expansion

(Fig. 7). It also justifies the lower HC amount found in case WMT

relative to case CZM Expans, since the HC transfer from the colder

zones to the hotter ones, and their subsequent partial oxidation is

only captured in the WMT case.

-2 0 2 4 6 8

CA (deg aTDC)

0

50

100

150

200

250

HRR

(J/deg)

-80

-40

0

40

Htra

nsfernet(J/deg)

-80

-40

0

40

Qlossnet(J/deg)

Fig. 6. Zone heat release, net heat transfer and net enthalpy transfer rate, WMT

case.

Table 7

HC and CO emissions for the cases studied.

Case HC (g) HC reduction

rel. to base (%)

CO (g) CO reduction

rel. to base (%)

WMT (b ase) 2 .2 4E03 9.08E04

CZM 1st HR 1.05E03 53 5.04E04 45

CZM 2 nd HR 1 .5 5E03 31 4.88E04 46

CZM Exp ans 2 .4 4E03 +9 6.70E04 26

5 10 15 20 25 30 35 40 45 50

CA (deg aTDC)

0.0x100

2.0x10-3

4.0x10-3

6.0x10-3

8.0x10-3

HC(g)

Mass Transfer

CZM 1st

HRCZM 2ndHR

CZM Expans

Fig. 7. Total in-cylinder HC for the cases examined.



8/10

Cases WMT and CZM Expans are identical up to 10 deg aTDC.

For the CZM Expans case inFig. 8, it is observed that HC in zone3 increase just prior to 10 deg aTDC and then decrease as soon as

mass transfer is neglected. This implies that in zone 3 the temper-

ature at this instant is high enough to oxidize the HC present. On

the contrary, in case WMT the HC contained in zone 3 continue

to increase after 10 deg aTDC, due to mass transfer from zone 2.

This leads to the conclusion that during this period, HC flow from

zone 2 to zone 3 prevails and determines the HC amount in zone 3.

Moreover, since the temperature in zone 3 is high enough, partial

HC oxidation must take place for the WMT case. This can also be

demonstrated by studying the CO emissions formation, as is shown

subsequently.

5.4.2. Effect of mass transfer on CO emissions formation

Fig. 9depicts the total in-cylinder CO history at the latter stagesof combustion and during expansion. Since the CO values near

5 deg aTDC were high, the y-axis scale was modified to focus on

the differences between cases during expansion.

At about 8 deg aTDC the total in-cylinder CO is similar for the

four cases. However, in cases CZM 1st and 2nd HR the total CO oxi-

dation rate is higher, as implied by the steeper CO reduction.

Accordingly, the final CO amount is much lower in these cases rel-

ative to the base line case (WMT) by about 45%. The CO reduction

in case CZM Expans, although significant, is much less (26%),

which implies that including mass transfer between zones during

combustion is essential for the prediction of CO emissions.

In all CZM cases a near-zero CO production rate is obtained dur-

ing expansion, as implied by the zero-slope CO history. This is the

main source for the differences in the final in-cylinder CO betweencases WMT and CZM Expans.

Comparing Figs. 7 and 9for cases CZM 1st and 2nd HR it is seenthat although the HC amount during expansion is quite different

between the two cases, a corresponding difference is not observed

for CO.

To investigate further the differences between cases, the CO his-

tory in each zone is provided inFig. 10. In the cases where mass

transfer is neglected after the 1st or 2nd heat release initiation,

the contribution of zone 1 to the total CO present is non-existent

5 10 15 20 25 30 35 40 45 50

CA (deg aTDC)

0

0.001

0.002

0

0.001

0.002

0

0.001

0.002

0

0.001

0.002WMT

CZM 1stHR

CZM 2stHR

CZM Expans

HC

(g)

1

2

3

12

3

12

3

1

2

3

1

23

4...

Fig. 8. Zone HC distribution vs. CA for the cases examined.

5 10 15 20 25 30 35 40 45 50

CA (deg aTDC)

0.0x100

2.0x10-4

4.0x10-4

6.0x10-4

8.0x10-4

1.0x10-3

CO

(g)

WMT

CZM 1stHR

CZM 2nd HR

CZM Expans

Fig. 9. Total in-cylinder CO for the cases examined.

0

4E-009

8E-009

1.2E-008

1.6E-008

CO

(kmo

l)

8 10 12 14 16 18 20 22 24 26 28 30

CA (deg aTDC)

0

4E-009

8E-009

1.2E-008

1.6E-008

CO

(kmo

l)

0

4E-009

8E-009

1.2E-008

1.6E-008

CO

(km

ol)

0

4E-009

8E-009

1.2E-008

1.6E-008

CO

(kmo

l)

WMT

CZM 1stHR

CZM 2stHR

CZM Expans

3

2

41

6 57

3

2

4

6 57

3

2

4

6

57

32

41

6 57

1

23

4...

Fig. 10. Zone CO distribution vs. CA for the cases examined.



9/10

and the contribution of zone 2 very low. However, these zones con-tain significant amounts of HC (Fig. 8), which is not oxidized as al-

ready mentioned. Thus, the large difference in HC observed

between the two cases during expansion is not followed by a cor-

responding difference in CO.

In cases WMT and CZM Expans, in which mass transfer is taken

into account during combustion, both zones 1 and 2 contain a sig-

nificant amount of CO immediately after main combustion (10 deg

aTDC). The CO present in zone 1 in these cases is attributed to CO

transfer (inflow) during combustion, since the temperature in zone

1 is low for CO production via HC partial oxidation.

Since CO is both transferred between zones and formed by par-

tial HC oxidation, the net CO production rate in each zone is pro-

vided in Fig. 11. A positive net CO production rate implies that

more CO is produced than consumed. It is obvious fromFig. 11that

more CO is produced in zone 2 when mass transfer is included dur-

ing combustion. This can be attributed in part, to the higher HC

present in zone 2 in cases WMT and CZM Expans relative to cases

CZM 1st and 2nd HR.

More important, however, is the positive net CO production rate

during expansion in zone 3 and to a lesser extent in the other

zones , which is absent in all CZM cases. Partial oxidation of HC

transferred to the hotter, inner zones from zone 1 and 2, is the

main source of this CO production.

6. Summary and conclusions

In the present study an investigation on the effects of mass

transfer on the formation of the CO and HC emissions of an HCCI

engine was conducted. A multi-zone model running in two modes

was used for this purpose. In the first mode the model includes

mass transfer between zones, whilst in the second mode mass

transfer is neglected. The switch between modes occurs at a

user-defined point (CA) in the engine closed cycle.

Four cases were examined, differentiated by the point at which

the switch between modes occurs: (i) including mass transfer

throughout the cycle, (ii) including mass transfer up to the initia-

tion of 1st heat release, (iii) including mass transfer up to the ini-tiation of 2nd heat release and (iv) including mass transfer up to

the completion of 2nd heat release.

The comparison of the different cases revealed that neglecting

mass transfer during combustion influences the temperature field

due to the significant enthalpy transfer occurring during this peri-

od. As a result, a steeper pressure trace during combustion and a

higher peak combustion pressure are obtained when neglecting

mass transfer for the case examined.

As regards unburned hydrocarbon emissions, neglecting mass

transfer during combustion results to an under-prediction by at

least 30% and at most 53% relative to the case which includes mass

transfer, for the operating conditions studied. The main source of

this deviation is the failure to capture the mass accumulation to-

wardsthe relatively cold regions of thecombustion chamber during

combustion. Neglecting mass transfer after the completion of main

combustion results to an overestimation by about 9%, since the HC

transfer from the colder regions to the hotter ones and their subse-

quent oxidation during expansion, is not captured in this case.

CO emissions are underpredicted by about 45% when mass

transfer is neglected after the initiation of the 1st or 2nd heat re-

lease. This is attributed to the lower CO amount accumulated in

the colder regions during combustion and to the HC transfer from

the cold to the hot regions and its subsequent partial oxidation,

which is not captured when mass transfer is not considered for.

Neglecting mass transfer after the completion of combustion re-

sults to an underestimation of the final CO amount by about 25%

relative to the case which includes mass transfer, since a significant

amount of CO is produced during expansion via the aforemen-

tioned HC transfer and partial oxidation mechanism.

Acknowledgements

I would like to thank Professor Bengt Johansson, Magnus Chris-

tensen and Andreas Vressner from the Lund Institute of Technology

for providing the experimental data used in this paper, for their

willingness and for their useful comments. I would also like to

thank Professor D.T. Hountalas of the National Technical Univ. of

Athens for his help in developing the multi-zone model and Dr.

E.G. Pariotis of the National Technical Univ. of Athensfor his helpful

technical comments on the present study. To my sister, Dr. G.P.

Komninou, for her support during the preparation of this study.

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0

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-3E-009

-2E-009

-1E-009

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Assessing the Effect of Mass Transfer on the Formation of HC and CO Emissio 2