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ABSTRACT

Many exhaust gas analyzers only calculate the Air Fuel Ratio

(AF) based on equations derived for the special case of

combustion without exhaust gas recirculation (EGR), and

therefore do not represent the combustion AF during lean

operation with EGR. Equations that are not subject to this

limitation will be presented. Using these equations to provide

measurements of the combustion AF and total inert EGR is

encouraged as they relate more directly to what governs the

combustion process.

INTRODUCTION

Many test facilities utilize exhaust gas analysis equations thatare deduced from the combustion reaction equation for a

general hydrocarbon fuel that may contain oxygen and does

not include EGR:

(1)

An example of literature using this equation is [1]. Other

literature investigating variations on this equation are [2,3,4].

This paper presents the derivations for Air Fuel Ratio (AF)

and EGR concentration calculations based on the general

combustion equation including EGR:

(2)

The implications of the revised equations are relevant for lean

operation and especially for simultaneous lean and EGR

operation and thought advantageous as they relate more

closely to what governs the combustion process. This type ofoperation is relevant for diesel engines and more recently the

advent of direct injection gasoline engines and Homogenous

Charge Compression Ignition (HCCI) engines. Furthermore

equations for inert EGR and exhaust inert concentration are

given. Finally, the calculation of carbon balanced exhaust

oxygen concentration is given as a good complement to the

actual exhaust oxygen measurement.

It is suggested that ideally these new measurements be made

readily available by commercial exhaust analyzers.

It should be noted that the reaction equation (2) used as a

basis in this paper is still somewhat idealized. For example, idoes not consider the effects of humidity, assumes no water

in the fuel, ignores water drop out in cooled EGR systems

and ignores the effect of measurement disturbance from the

finite samples of the analyzers. Some discussion of these

effects are included in [2,3,4].

THEORETICAL ANALYSIS &

RESULTS

The data illustrated in the following was obtained on a Direc

Injection Gasoline engine running both stratified and

homogeneous lean combustion as well as homogeneous

stoichiometric combustion. The measurements of intake andexhaust constituents were done with industry standard

emissions analyzers typical in vehicle emission laboratories

(VEL) of ULEV2 accuracy.

General Air Fuel Ratio and EGR Definitions and

their Calculation from Emissions

2010-01-1285

Published

04/12/2010

Martin MllerDelphi Corp.

Copyright 2010 SAE Internationa

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AIR/FUEL RATIO (AF) EQUATIONS &

DEFINITIONSThe oxygen and carbon balanced AF for the general

combustion reaction equations of (2) becomes:

(3)

(4)

where

(5)

As opposed to the special case equations for (1), the general

equations for (2) contain an EGR dependency. They becomeidentical for EGR=0. The AF that is calculated depends on

which EGR is used in the equation as shown in Table 2. This

means that the AF that is commonly reported by emission

benches and Wide Range Air Fuel Ratio (WRAF) sensors is

the fresh airflow AF, also referred to as the throttle AF, since

it does not account for any EGR. The ratio between the

throttle AF and combustion AF is shown vs. throttle AF and

total EGR in Figure 2, and is seen to differ up to 30% during

lean operation with significant EGR. It can be argued that the

combustion process relates more closely to the combustion

AF and inert EGR, which is why providing those

measurements is advantageous.

The practical implications of this are the required additional

measurements of EGR when measuring combustion, or intake

port, AF as compared to the common practice when

measuring throttle (fresh air) AF. External EGR is measured

by CO2 in the intake and exhaust and is commonly available.

Until recently measuring internal EGR was impractical, but a

new more practical technique was recently described in [5,6].

Figure 1. Lean operation results in several AF

definitions. They all co-incide for stoichiometric

operation. The WRAF sensor AF and common emission

bench calculated AF are both measurements of throttle

(= fresh) AF which only accounts for the fresh airflow

which has not been recycled.

Table 2. AF definitions and their EGR dependencies.

Table 3. Relation between AF definitions.

Table 1. Editorial notes

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Figure 2. AF-X / AF-Throttle = f(AF-Throttle, EGR-Y)

where X = Intake Port for Y = external and X =Combustion for Y = Total. The surface is a curve fit of

the measured data points obtained by post-processing

measured exhaust concentrations usingequations (3)

and(4).

The AF calculations derived for (1) are identical to (3) and

(4) with EGR = 0. The AF dependency on EGR is purely a

multiplicative effect on the exhaust oxygen term and is most

clearly apparent in the oxygen balanced form, equation (3).

As a special case, this means that its absence in the non-

general equations is not of consequence for stoichiometric

and rich operation.

The Throttle AF is calculated when ignoring recycled

exhaust. The Combustion and Throttle AF ratio relate as:

(6)

Stoichiometric combustion utilizes all oxygen, leaving the

exhaust gas practically oxygen free, . For this

special case , and a distinction between throttle

and combustion AF is irrelevant. This special case is the

common current practice. Not including EGR dependence is

essentially not properly accounting for the air (oxygen and

nitrogen) that is present in the exhaust due to recycling. For

lean operation the distinction becomes necessary, e.g. for

diesel engines or direct injection gasoline engines running

stratified combustion.

The data points in all the figures were obtained on a Direct

Injection Gasoline engine running both stratified and

homogeneous lean combustion as well as homogeneous

stoichiometric combustion under a wide range of speed, load

AF and EGR conditions. All the following figures with meas

points as independent axis have AF and Intake EGR vs

measurement point number ordered as shown in Figure 3.

Figure 3. Combustion AF and Intake EGR vs.

measurement points illustrated in the following figures.

ENGINE MANAGEMENT SYSTEMS

(EMS) IMPLICATIONSIn order to take full advantage of providing the new AF and

EGR measurements, EMS control algorithms should in

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concert convert to these units. This may be done relatively

easily by implementing an EGR estimation model in the EMS

and using this together with Throttle AF measured by a

WRAF sensor to look up for example the combustion AF in a

tabulation of the surface shown in Figure 2.

EGR EQUATIONS AND DEFINITIONSThe following EGR definitions are used:

(7)

(8)

(9)

(10)

(11)

This is a total of 8 EGR definitions when applying (11) to(7,8,9,10), and the equations for their calculation from

exhaust and intake constituent measurements are listed

below. The measurement most commonly provided by

emission bench measurements is an approximation of

. However, it can be argued that the Total Inert

EGR concentration is what most directly affects

combustion. [7] includes an example of such a discussion.

Total EGR

The oxygen and carbon balanced EGR equations for (2)

becomes:

(12)

where

(13)

The coefficients A,B and C are specific of the balancing

method.

Oxygen balanced

(14)

(15)

(16)

(17)

(18)

Carbon balanced

(19)

(20)

(21)

(22)

(18)

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The exhaust measurement is corrected for the water not

reaching the exhaust analyzer:

(23)

It is assumed that the water in the intake sample is negligible,

giving:

(24)

Inert EGR and Exhaust Inert concentration

During lean operation only part of the recycled gas is inert.

Therefore does not represent the inert dilution of the

combustion fresh charge as is the case for stoichiometric andrich operation. Inert EGR affects combustion most directly,

which is why it should be readily available as a measurement.

It is the product of total EGR and inert percentage of the

exhaust. The exhaust inert percentage measurement should

also be made readily available.

(25)

where

(26)

(27)

and the coefficients A,B, E and F are specific of

the balancing method.

Oxygen balancing

: measured is corrected to a wet basis

using (37).

, are given by (14) and (15).

(28)

(29)

Carbon balancing

: carbon balanced is corrected to a

wet basis using (37).

, are given by (19) and (20).

(30)

(31)

is given by (35). It is the carbon balanced exhaus

oxygen concentration.

The implemented exhaust inert percentage calculationsshould be truncated to 100%.

It is interesting to note that only depends on

AF and not on , and it is not very different to that of air

Therefore, the ratio of the molar weights of air and exhaust

gas varies slightly with air fuel ratio. Since the calculations

describing this variation are rather complex, and the variation

from unity is small, it is natural to investigate how wel

approximates unity. Figure 4 shows

approximation errors of 5% for lean operation, which would

be considered significant enough for its inclusion. Howeverapparent measurement errors cause the measured exhaus

molar weights to be too low, so the real approximation errors

are less than 5% and the approximation of = 1

is reasonable.

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Plots a,b,c are all ordered with respect to lambda illustrated in

plot c (right y-axis). Plot c shows how well

approximates unity. The approximation errors are small, and

decrease with lean operation as expected.

Plots b,d show that these errors are reduced when calculating

and instead increase with lean operation

due to the multiplicative effect of .

Plot b shows less than 100% for

stoichiometric operation due to some measured exhaust

oxygen content (see Figure 6).

External EGR

The definition gives:

(32)

Intake EGR

For a non-zero residual fraction, , the

external EGR is not equal to the intake EGR. Intake EGR

relates the externally recycled mass (flow) to the total mass

that the cylinder inducts from the intake manifold. External

EGR relates the externally recycled mass (flow) to the total

cylinder mass, which includes residuals.

(33)

Internal EGR (residuals)

Until recently, measuring internal EGR was very difficult, but

a new practical technique was recently described in [5-6]

[5-6] presented the Residual Estimation Tool (RET) which

provides on-line residual estimation. It does so by adjusting

key parameters of a crank-angle resolved combustion mode

until its behavior matches quantities that can more easily be

measured. Required measurements include crank angle

resolved cylinder pressure and combustion boundary

conditions such as average intake and exhaust pressures and

temperatures as well as air flow. The residual estimate of the

converged model is the provided virtual measurement. The

key advantage of this approach is that it does not rely on in-

cylinder gas sampling nor crank-angle resolved boundary

pressures, which keeps the equipment requirement relatively

simple and the convergence time low enough to run on-line.

Residual measurement was not performed during collection

of the data shown in this paper as the technique of [5-6] was

not available at the time. Since the engine used for data

collection did not have cam-phasing, the very coarse

approximation of the theoretical relationship between

geometrical compression ratio and internal EGR was

assumed, which was 8%. Clearly this is a coarse

approximation since any valve overlap at low load will have

significantly higher residuals. However, it is sufficient to

Figure 4. Illustration of the impact of approximating .

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illustrate the influence of residuals on the various definitions

presented here.

Comparison with commonly used calculation

Commonly implemented calculations do not distinguish

between intake and external EGR as the effect of residuals is

not included. The commonly used approximation of external/

intake EGR is:

(34)

The error of the approximation is illustrated below, and is

found to be relatively small.

Figure 5. Illustration of the error when calculating

intake and external EGR as the ratio of intake to exhaust

dry concentrations.

MISCELLANIOUS CALCULATIONS

Carbon balanced exhaust oxygen concentration

calculation

A sanity check of the measured exhaust concentration can

be made with that calculated through the measurements ofother exhaust species. Carbon balancing gives:

(35)

is on the same wet, dry or partially wet basis as

the other concentrations.

Note that it is not EGR dependent.

Figure 6. Comparison of measured exhaust oxygen

concentration with that calculated by carbon

balancing shows good correspondence.

Several regions show apparent erroneous

measurements. This illustrates the advantage of

as a check on measurements. Note the effect

of measurement errors on for example AF as seenin Figure 3.

(H2O) calculation

is calculated by assuming a fixed value for the water/

gas equilibrium constant, K, which relates the concentrations

of , , and water vapor under equilibrium

conditions:

(36)

where is on the same wet, dry or partially dry basis as

the other concentrations. According to [1] K=3.8 is

recommended.

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Converting exhaust emission concentrations to a

wet basis

Usually, the exhaust sample passes through an ice bath that

removes any water present so that the analyzers measure a

dry sample, in which case . When the wet

concentration is needed the following correction should be

applied, exemplified for the constituent XX:

(37)

SUMMARY/CONCLUSIONS

Emission analyzers today typically only provide a

measurement of Throttle (=Fresh) Air Fuel Ratio (AF) and an

approximation of External EGR. However, the combustion

process relates more directly to the Combustion AF and TotalInert EGR concentration, and is especially relevant for diesel

engines and direct-injection gasoline engines running

stratified combustion and most recently Homogeneous

Charge Compression Ignition (HCCI) engines. The two AF

definitions may for example differ by 30% during typical

stratified operation.

In response, this paper presents the several AF and EGR

definitions that exist for the general combustion reaction

which includes EGR. Their definitions and calculation from

exhaust and intake constituent measurements have been

presented. It is encouraged that exhaust analysis equipment

should provide this expanded list of measurements that relate

more closely to what governs the combustion process. It is

acknowledged that adding AF and EGR definitions increases

complexity and that the additional AF measurements are

more involved in practice as they require simultaneous

measurements of EGR. This is particularly challenging when

measuring internal EGR (residuals) although recent advances

reported in the literature are making this substantially easier.

Nevertheless, providing these new measurements may prove

worthwhile especially for sensitive combustion processes

such as HCCI.

It should be noted that Engine Management System (EMS)control algorithms may relatively easily be converted to such

new AF and EGR units. This includes the conversion of

measured Throttle AF by the WRAF sensor by using an EGR

estimation model and a tabulated relationship.

Finally, it is found prudent to sanity check the measured

exhaust oxygen with that calculated from Carbon-balancing.

REFERENCES1. General Motors Automotive Engine Test Code, For

Spark Ignition Engines, Seventh Edition, The Engine Test

Code Committee, 1994.

2. Chan, S. H., Zhu, J., Exhaust Emissions Based Air-Fuel

Ratio Model (I): Literature Reviews and Modelling, SAE

Technical Paper 961020, 1996.3. Silvis, W. M., An Algorithm for Calculating the Air/Fuel

Ratio from Exhaust Emissions, SAE Technical Paper

970514, 1997.

4. Jones, J. C. P., Muske, K. R., A Generalized Chemical

Balance Analysis Tool for Combustion and Catalytic

Reactions, SAE Technical Paper 2005-01-0680, 2005.

5. Sinnamon, J. F., Sellnau, M. C., A New Technique for

Residual Gas Estimation and Modelling in Engines, SAE

Technical Paper 2008-01-0093, 2008.

6. Sellnau, M., Sinnamon, J., Oberdier, L., Dase, C., Viele,

M., Quillen, K., Silvestri, J., Papadimitriou, I., Development

of a Practical Tool for Residual Gas Estimation in ICEngines, SAE Technical Paper 2009-01-0695, 2009.

7. Schppe, D., Geurts, D., Balland, J., Schreurs, B.,

Integrated Strategies for Boost and EGR Systems for Diesel

Engines to achieve most stringent Emission Legislation,

10th Supercharging Conference 2005, Technische

Universitaet Dresden.

DEFINITIONS/ABBREVIATIONS

f, fuel

f signifies fuel per carbon atom. Note f(uel) refers to f.

:concentration of XX, concentration type described by

sub- and super-scripts. The partially wet super-script is

implied if nothing else included. Unit in [%].

:special case o

This is equivalent to the exhaust concentrationmeasurements. Note that this could be on a wet, dry or

partially wet basis why this super-script is n

included.

:Nitrogen / Oxygen ratio of air, 3.774 used in order to

account for trace amounts of argon, , , etc in

atmospheric air, according to [1].

http://www.sae.org/technical/papers/2009-01-0695http://www.sae.org/technical/papers/2008-01-0093http://www.sae.org/technical/papers/2005-01-0680http://www.sae.org/technical/papers/970514http://www.sae.org/technical/papers/961020

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K

:water/gas equilibrium constant. Use 3.8 according to

[1].

:percentage of theoretical exhaust water content that

reaches exhaust analyzers. Usually 0. Unit in [%].

y

:Hydrogen / Carbon ratio of fuel

z

:Oxygen / Carbon ratio of fuel

n

:moles (in general) or molar coefficient for air in

combustion reaction equation

:effective number of carbon atoms per fuel molecule

:molar weight

:cylinder content before combustion

:oxygen balanced

:carbon balanced

stoich

:stoichiometric

tot

:total

int

:intake manifold

exh

:exhaust

(xxx)

:equation xxx

[xxx]

:reference xxx

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APPENDIX

INTRODUCTIONThe chemical combustion equation including EGR for a

general hydrocarbon fuel that may contain oxygen is:

(A1)

This is to be compared to the combustion equation used in [1]

which does not include EGR:

(A2)

Note that both combustion reaction equations have been

normalized by the effective number of carbon atoms in a fuel

molecule, .

PREPARATORY DERIVATIONS

(a). Normalized combustion reaction equation

The combustion reaction equation of (A1) origins from:

(A3)

Normalizing with gives:

(A4)

where

(A5)

and

(A6)

(b). Relationship of molar coefficients to measured

concentrations

is used as an example:

(A7)

differs from by the moles of tha

are removed from the exhaust sample before reaching the

exhaust analyzers. The sample may be wet, dry or partially

wet.

The concentration measured by the emissions bench in

volume percent is:

(A8)

The rightmost term in (A7) is found by relating the number of

moles of fuel to the number of carbon-containing species

through a carbon balance:

(A9)

Substituting using (A8) and its equivalent for and

into (A9) results in:

(A10)

(A7), (A8) and (A10) now gives:

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(A11)

and similarly for the other molar coefficients:

(A12)

(A13)

(A14)

(A15)

(A16)

(c). , and exhaust concentrations

expressed through other measured species

As the concentrations of and are not measured, their

molar coefficients are expressed by measured molar

coefficients of other species.

Hydrogen balancing gives:

(A17)

Nitrogen balancing gives:

(A18)

There are several reasons for expressing exhaust

concentration through the measurement of other exhaus

species.

measurement not available

As a check of measured concentration

As a replacement of erroneous measurements

As will be seen in the following sections, both the AF and

derivations can be solved either using or not using

measured . Derivations using measured are based on

oxygen balancing while derivations not using measured

are based on carbon balancing. As an example, it is found

that AF=f( ). Therefore, four types of AF calculations are

possible depending on the combination of oxygen and carbon

based derivations. For consistency the following convention

is assumed unless explicitly stated otherwise:

O-bal = oxygen balance based derivation = measured

used in all calculations

C-bal = carbon balance based derivation = measured no

used in any calculations

Expressing the molar coefficient of by measured species

is found through oxygen balancing:

(A19)

where is used (derived below).

(d). (H2O) calculation

is calculated by assuming a fixed value for the water/

gas equilibrium constant, K, which relates the concentrations

of , , and water vapor under equilibrium

conditions:

(A20)

Substituting e with (A17) and applying (A11,A12,A13) and

(A16) gives:

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(A21)

Finally, inserting N of (A10) results in:

(A22)

where is on the same wet, dry or partially dry basis as

the independent terms. According to [1] K=3.8 is

recommended.

(e). Converting exhaust emission concentrations to

a wet basis

The molar coefficients of the combustion reaction equation

are wet, meaning they relate to the total mol amount which

includes water vapor. Usually, the exhaust sample passes

through an ice bath that removes any water present so that the

analyzers measure a dry sample, in which case .

The correction is exemplified for :

(A23)

where is the theoretical number of moles of water in theexhaust calculated by conserving species in the combustion

equation.

Combining (A10) and (A16) gives:

(A24)

which inserted into (A23) applying

and (A12) results in:

(A25)

This conversion applies to any species.

AIR/FUEL RATIO DERIVATIONSAF is defined as:

(A26)

The basis for intake air is

Air is defined as having the molar relation :

Cylinder air amount is therefore :

Which gives:

(A27)

can be expressed through either oxygen or carbon

balancing.

Oxygen balancing

Oxygen balancing gives:

(A28)

Substituting a,b,d,f,g and N gives:

(A29)

Inserting (A10), (A15) and (A29) in (A27) gives:

(A30)

and finally using (A26) results in:

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(A31)

It is noted that . However, the influence ofis only present for exhaust containing oxygen, why the

dependency effectively disappears for stoichiometric

and rich operations. The difference between

derived for the two combustion reaction equations is that the

term has the multiplier for (A1) but not

for (A2). Not including EGR dependence is essentially not

properly accounting for the air (oxygen and nitrogen) that is

present in the exhaust due to recycling.

Carbon balancing

was obtained through carbon balancing (see (A10)).

An alternative expression for is:

(A32)

where is the percentage of water reaching the exhaust

analyzers.

Substituting , and results in:

(A33)

(A34)

Substituting N, b,c and g gives:

(A35)

Combining (A19) and (A27) gives:

(A36)

Inserting gives after some

rearranging:

(A37)

where

(A38)

and finally using (A26) results in:

(A39)

Note that for the special case of =0 (A39) becomes:

(A40)

which is identical to stated in [1].

Stoichiometric Air/Fuel ratio

Stoichiometric air/fuel ratio is calculated for completecombustion yielding only , and . The

combustion equation becomes:

(A41)

The AF definition is restated:

(A42)

Intake air amount is :

Which gives:

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(A43)

Oxygen balancing gives:

(A44)

Carbon balancing gives :

(A45)

Hydrogen balancing gives :

(A46)

Combining (A42,A43,A44,A45,A46) results in:

(A47)

EGR DERIVATIONS

Total EGR

is defined as the percentage of exhaust mass recycled.

Recycling happens both internally (residuals) and externally:

(A48)

(A49)

(A50)

Assuming atmospheric air is free, all CylPreComb

is from recycling:

(A51)

(A52)

Solving for EGR using (A52) would involve measuring the

concentration in the cylinder during the crank angle

interval where the intake valves are shut and combustion has

not yet begun. Such a measurement is in general no

available. Instead, (A52) is written as:

(A53)

In the following, the residual fraction, , is

assumed known.

The rightmost term of (A53) is rewritten as:

(A54)

The rightmost term of (A54) is:

(A55)

where

(A56)

and

(A57)

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Applying (A6) and (A11,A12,A13,A14,A15,A16) results in:

(A58)

where

(A59)

Inserting (A56),(A58) and (A48) into (A55) gives:

(A60)

The numerator of the first term is named:

(A61)

and are given by (A29) respectively (A35).

The remaining term to be solved is .

Oxygen balancing

Substituting and in (A56) gives:

(A62)

Substituting a,c,d,f,g gives:

(A63)

where

(A64)

(A65)

(A66)

Combining (A53),(A54), (A60),(A63) and (A66) results in:

(A67)

(A68)

where

(A69)

and

are measured.

The exhaust measurement is corrected for the water no

reaching the exhaust analyzer, using (A25):

(A70)

It is assumed that the water in the intake sample is negligible

giving:

(A71)

Carbon balancing

Subtituting e,h and f in (A56) gives:

(A72)

(A73)

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where

(A74)

(A75)

(A76)

Equivalent to oxygen balancing, one gets:

(A77)where

(A78)

INERT EGR DERIVATIONDuring lean operation only part of the recycled gas is inert.

Therefore does not represent the inert dilution of the

combustion fresh charge as is the case for stoichiometric and

rich operation. Inert EGR affects combustion most directlywhy it should be readily available as a measurement.

The two EGR definitions are:

Total EGR :

Inert EGR :

Introducing the exhaust inert mass percent:

(A79)

and

(A80)

and applying continuity:

(A81)

results in:

(A82)

Likewise for and .

It is defined that the exhaust can be purely partitioned into air

and inert, which gives:

(A83)

where

(A84)

and

(A85)

which is measured.

The term remains to be solved.

(A86)

Inserting (A11,A12,A13,A14,A15,A16) gives

(A87)

Oxygen balancing

From (A63):

(A88)

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The numerator of (A87) is solved by substituting e,h given by

(A17),(A18) and then substituting a,b,c,d,g given by

(A11,A12,A13,A14) and (A16), resulting in:

(A89)

where

(A90)

(A91)

Combining the numerator (A89) with the denominator (A88)

cancels the EGR dependency:

(A92)

It is interesting to note that only depends on

AF and not on . Since the molar weight of air and inert

are very similar and is in between, it is natural to

investigate the possible simplification of =1.

See Figure 4 in main report for evaluation of

approximation.

Carbon balancing

Equivalent to oxygen balancing, one gets:

(A93)

where the only difference between and is

that the exhaust oxygen measurement of is replaced

with that calculated by carbon balancing, .

is derived in section Carbon balanced exhaust

oxygen concentration derivation.

Therefore:

(A94)

(A95)

INTAKE EGR

For a non-zero residual fraction, , the

external EGR is not equal to the intake EGR.

Intake EGR relates the externally recycled mass (flow) to the

total mass that the cylinder inducts from the intake manifold.

External EGR relates the externally recycled mass (flow) to

the total cylinder mass, which includes residuals.

(A96)

(A97)

They are seen to relate as:

(A98)

The rightmost term can be rewritten as (applying

) :

(A99)

Finally, it is noted that and relate

equivalently.

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CARBON BALANCED EXHAUST

OXYGEN CONCENTRATION

DERIVATIONAs discussed in section Preparatory derivations there are

several reasons for expressing exhaust concentration

through the measurement of other exhaust species.

Equaling the two expressions for , (A15) and (A19), gives:

(A100)

Substituting and , (A10) and (A35) respectively,

results in:

(A101)

is on the same wet, dry or partially dry basis as

the other concentrations.

The Engineering Meetings Board has approved this paper for publication. It has

successfully completed SAE's peer review process under the supervision of the session

organizer. This process requires a minimum of three (3) reviews by industry experts.

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ISSN 0148-7191

doi:10.4271/2010-01-1285

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http://dx.doi.org/10.4271/2010-01-1285