Prediction of Pre-ignition Re Activity and Ignition Delay for Hcci Using a Reduced Chemical Kineti

8/14/2019 Prediction of Pre-ignition Re Activity and Ignition Delay for Hcci Using a Reduced Chemical Kineti

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28CHAPTER 3. PREDICTION OF PRE-IGNITION REACTIVITY AND IGNITION

DELAY FOR HCCI USING A REDUCED CHEMICAL KINETICMODEL*

3.1 Overview

Homogeneous Charge Compression Ignition (HCCI) engines have the

possibility of low NOx and particulate emissions and high fuel efficiencies. In

HCCI the oxidation chemistry determines the auto-ignition timing, the heat

release rate, the reaction intermediates, and the ultimate products of combustion.

This chapter reports an initial effort to apply our reduced chemical kinetic model

to HCCI processes. The model was developed to study the pre-ignition

characteristics (pre-ignition heat release and start of ignition) of primary

reference fuels (PRF) and includes 29 reactions and 20 active species. The only

modifications to the model were to make the proscribed adjustments to the fuel

specific rate constants, and to enhance the H2O2 decomposition rate to agree

with published data. Simulations were compared with measured and calculated

data from our engine operating at the following conditions: speed - 750 RPM,

inlet temperature - 393 K to 453 K, fuels PRF 20, PRF 50 and PRF 20 with

alkenes and aromatics, and equivalence ratio - 0.4 and 0.5. The simulations are

in good agreement with the experimental data including temperature, pressure,

ignition delay, and preignition heat release. This demonstrates the model has

potential for use in predicting the behavior of HCCI engines. From both the

experiments and the reduced kinetic model, the results show that for PRF 20 the

firststage ignition begins at 707 K, the second-stage ignition temperature is 910-

924 K, and significant reaction occurs during the preignition process resulting in

* This chapter was the basis for SAE paper No. 2001-01-1025, SAE Trans.110 [Zheng et al., 2001]


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29pre-ignition heat release of about 7%-10% of the potential heat release. The

second-stage ignition time varies with inlet temperature, equivalence ratio,

engine load and octane number. When the engine is run with a volumetric

efficiency of 71% at 750 RPM, inlet temperature 423 K, equivalence ratio 0.4 and

PRF 20, the duration from the first to second-stage ignition is 25 CAD.

3.2 Introduction

Society mandates that future engines must be more efficient and pollute

less. Homogeneous Charge Compression Ignition (HCCI) is a possible

alternative operating mode for the reciprocating engine in which a homogeneous

charge of fuel and air, as in a spark ignition (SI) engine, is compressed to auto-

ignition, as in a compression ignition engine (CI). Due to high temperatures and

heterogeneous combustion of the atomized fuel, CI engines emit a large amount

of NOx and soot. In a spark ignition (SI) engine, combustion proceeds as a

flame front propagates through a premixed homogeneous charge of fuel and air.

As flame propagation occurs, local temperature at the front --- a thin zone of

intense chemical reaction --- is high, so abundant NOx formation occurs in the

post-flame hot combustion products. Stratified charge SI engines, while

attempting to avoid these undesirable operating conditions, still have problems

with high emissions [Aoyama et al., 1996].

In HCCI engine operation, a highly diluted mixture of fuel and air with

varying amounts of EGR [Ryan and Callahan, 1996] is used to control the rate of

the chemical reactions and produce reasonably slow combustion with a bulk gas

temperature much lower than in an SI engine. This low bulk gas temperature


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30leads to lower NOx emissions. Since HCCI engines operate with high

compression ratio and under lean-burn conditions, they have high efficiencies

similar to diesel engines [Suzuki et al., 1997, 1998; Ishibashi and Asai, 1996;

Dickey et al., 1998; Christensen et al., 1997]. Further, as the combustion

initiation occurs almost homogeneously in an HCCI engine, the cycle to cycle

variations of the combustion process are very small [Christensen et al., 1998].

However, HCCI is not perfect. It generates more unburned hydrocarbons

(HC) than typical SI engines and operates with lower Indicated Mean Effective

Pressure (IMEP). Also, currently there is an inability to control the initiation and

rate of combustion over the whole speed-load range [Ryan and Callahan, 1996]

in HCCI engines. These factors presently are limiting the commercialization of

the HCCI concept.

In an HCCI engine, the autoignition process, controlled by low and

intermediate temperature chemistry [Ryan and Callahan, 1996], plays a critical

role. Heat release occurs as the result of this reactivity and leads to an increase

in cylinder pressure and temperature and eventually to ignition. In order to

understand these processes chemical models must be developed which are valid

for the conditions of HCCI engine operation, i.e., low to intermediate

temperatures and fuel lean mixtures.

The chemical kinetics of combustion is extremely complicated, and this is

especially so for autoignition processes. In the 1970s, based on degenerate-

branched-chain and class chemistry concepts, reduced kinetic models were

developed for prediction of autoignition delay time in an engine [Halstead et al.,


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311975] and this work formed the basis for later development of several reduced

chemical kinetic models [Hu and Keck, 1987; Li et al., 1992]. Since the 1980s,

detailed chemical kinetic models relevant to end gas autoignition of

stoichiometric combustion have emerged. However, there are hundreds of

species and chemical reaction equations needed to simulate the ignition and

combustion process even for simple hydrocarbons such as the butanes

[Cernansky et al., 1986; Green et al., 1987a, 1987b]. Needless to say, the

situation gets more complicated as one moves to more complex, higher

molecular weight fuels such as n-heptane [Curran et al., 1998]. Nonetheless,

several modeling studies using detailed chemical mechanisms have been

reported to simulate HCCI combustion. Some used a single-zone model [Kelly-

Zion and Dec, 2000; Aceves et al., 1999] and others used a multi-zone model

along with CFD [Kraft et al., 2000 and Aceves et al., 2000]. Recent modeling

work by Aceves et al. [2000] concluded that decomposition of H2O2 was the main

cause for HCCI ignition.

As noted, detailed models contain a large number of reactions and

species and, when they are coupled with multi-dimensional fluid dynamic models

for autoignition and combustion predictions in engines, they can be

computationally expensive and impractical. In these situations, reduced

chemical kinetic models are desirable. One of the first successful reduced

models was developed by Halstead et al. [1975] and their methodology (used in

References [Hu and Keck, 1987; Cowart et al., 1990]) can generally reproduce

the overall trend for ignition delay of specific hydrocarbons of interest. However,


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32significant physical features such as preignition fuel consumption, cumulative

heat release, and species concentrations are not modeled very well [Li et al.,

1996].

To resolve these issues a new reduced kinetic model was developed by Li

et al. [1996] for application to PRF 87 (87% iso-octane and 13% n-heptane) and

PRF 63 at selected engine conditions. This model predicted the ignition delay

and the pre-ignition heat release for these fuels to within 15% [Li et al., 1996].

The model was then applied to n-butane and iso-butane blends, where the model

predictions agreed well with experiments with only minor adjustments in three

pre-exponential rate parameters [Wang et al., 1996a].

Encouraged by the success of this reduced kinetic model, we have

applied it to HCCI conditions in hopes of gaining insights into factors that may

allow control of the HCCI combustion process. This is one component of a larger

research program on HCCI combustion.

3.3 Overview of the reduced kinetic model and thermodynamic model

As mentioned, the baseline reduced model for this study was reported by

Li et al. [1996] and is shown in Table 3-1. It was developed to model the

autoignition of the SI primary reference fuels (n-heptane and iso-octane) and

their mixtures by expanding the work of Hu and Keck [1987]. Eleven reactions

and seven active species were added to their model in order to produce higher

specific heat release without completely consuming the fuel and to include the

chemical pathways for CO production. The new reactions included pathways to


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33Table 3-1. Reduced Chemical Kinetics Model of Li et al. [1996]

A. 20 Active Species

1. RH 2. O2 3. R 4. RO2 5. QOOH

6. OOQOOH 7. OQO 8. OH 9. HO2 10. HOOH

11. OQOOH 12. RCHO 13. C=C 14. RCO 15. Rs

16. RsO2 17. RsOOH 18. RsO 19. RO 20. ROOH

B. 29 Reactions (units: mole, s, kcal)Arrhenius parameters of rate constants k=Ae

-E/RT

Equilibriumk

+k

-

Reaction H300 Log A E log A+

E+

log A-

E-

1. RH+O2 R+HO2 46.4 1.5 46.0 13.5 46.0 12.0 0.0

2. R+O2 RO2 30.1 -1.4 -27.4 12.0 0.0 13.4 27.4

3. RO2 QOOH

n-heptane 7.50 0.9 8.0 11.9 19.0 11.0 11.0iso-octane 7.50 0.0 11.24 11.0 22.4 11.0 11.0

4. QOOH+O2 QOOHOO -30.1 -1.9 -27.4 11.5 0.0 13.4 27.4

5. OOQOOH=> OQOOH+ OH 26.6 11.3 17.0

6. OH+RH => H2O+R -23.5 13.3 3.0

7. OQOOH => OQO+OH 43.6 15.6 40.08. HO2+HO2 => HOOH+O2 -38.5 12.3 0.0

9. HOOH+M => 2OH+M 51.4 16.88 46.0

10. OQO => 2RCHO+RCO

n-heptane 17.5 14.0 15.0

OQO => 2RCHO+Rs

iso-octane 18.5 14.0 15.0

11. QOOH => C=C+RCHO+OH -3.0 14.4 31.0

12. RO2+RCHO => ROOH+RCO -0.6 11.45 8.6

13. HO2+RCHO => HOOH+RCO -0.6 11.7 8.64

14. C=C+HO2 =>Epox+OH 0.23 10.95 10.0

15. HO2+RH R+HOOH 8.0 0.9 8.0 11.7 16.0 10.8 8.0

16. RO2+RH ROOH+R 8.0 1.1 8.0 11.2 16.0 10.1 8.0

17. RCHO+OH => RCO+H2O

n-heptane -31.5 13.22 0.0iso-octane -31.5 13.57 0.0

18. RCO+M => Rs+CO+M 10.7 16.78 15.0

19. Rs+O2 RsO2 -31.0 -1.4 -27.4 12.0 0.0 13.4 27.4

20. RsO2 => C=C+OH 17.5 11.75 28.9

21. RCHO+RsO2 =>RsOOH+RCO -0.6 11.53 8.6

22. RH+RsO2 RsOOH+R 8.0 1.18 8.0 11.28 16.0 10.1 8.0

23. RsOOH => RsO+OH 43.6 15.6 43.0

24. RsO+O2 => RsO+HO2 -26.5 10.6 2.14

25. C=C+OH => 2OXY+OH 75.5 12.72 -1.04

26. ROOH = RO+OH 43.6 15.6 43.0

27. RO => Rs+RCHO -10.0 13.3 15.0

28. RO2 => C=C+HO2 4.0 9.85 23.0

29. RO2=> ether+OH

n-heptane -25.0 9.48 18.0

iso-octane -25.0 8.78 18.0


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34oxidize aldehydes (RCHO), olefins (C=C), carbonyl radicals (O=R) and smaller

allyl radicals (Rs). To predict other classes of species (for example heterocyclic

ethers which are a major oxidation product for alkane fuels), several reaction

paths were considered based on measurements of species concentrations from

Li et al. [1992 and 1996]. Furthermore, based on our current understanding of

autoignition chemistry, the isomers ROOH and OQOOH were treated individually

in contrast to the identical treatment in previous models.

The thermodynamic model for our engine was based on the work of

Ferguson et al. [1987] in which the cylinder was modeled as an open system with

mass loss via blowby. This model was adapted to calculate heat release from

measured pressure data up to the point of autoignition [Li, 1995].

The reference point for all of the measurements and calculations is inlet

valve closing (IVC) where the in-cylinder composition and conditions can be

determined. Combining the measured intake manifold flows and conditions with

the known residual fraction allows IVC conditions to be calculated. The residual

fraction was experimentally determined [Li, 1995; Henig et al., 1989] and the

residual gas is assumed to consist of ten species in chemical equilibrium

whereas the unburned gas is treated as a frozen ideal mixture of fuel and air.

For our experimental conditions, residual fraction is 0.1. Beyond the IVC point,

mean gas temperature (T) was calculated from the engine geometry and

measured pressure (P), and then P and T were used to calculate internal energy.

The heat transfer coefficient accounting for heat loss was set to match the

temperature profile during the early stage of compression where no apparent


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35heat release occurred. The blowby was measured experimentally. Based on

these assumptions and engine calibration data, heat release is calculated

according to the first law of thermodynamics. Using the experimentally derived

temperature and heat release profiles calculated using the thermodynamic model,

the ability of the reduced kinetic model to reproduce the ignition delay and the

pre-ignition heat release was investigated.

3.4 Experimental apparatus and procedures

The data used in this study are from a single cylinder, four stroke, air

cooled research engine which was originally modified at Sandia National

Laboratory, with subsequent modifications at Drexel University. In its present

configuration, the engine has a 76.2 mm bore, a 82.6 mm stroke and a 8.2

compression ratio. The engine has been described along with the general

experimental facility in previous publications [e.g., Henig et al., 1989; Yang et al.,

2000]. A key feature of the facility is the ability to preheat the intake charge over

a range of temperature, 300-500 K, and to independently control the manifold

pressure.

The engine was operated at a constant engine speed of 750 rpm. PRF 20

and PRF 50 mixtures of the primary reference fuels and mixtures of PRF 20 with

alkenes and aromatics were examined. The test fuel was injected into the air

stream of the heated inlet manifold well upstream of the intake valve to assure

complete vaporization and mixing. The fuel delivery system was capable of

maintaining the equivalence ratio within 5% of the desired set point. HCCI

operation was obtained at inlet air temperatures of 393-453 K, inlet manifold


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36pressure of 760 Torr, equivalence ratios between 0.4-0.5 and inlet air flow rates

of 65-80 l/min (volumetric efficiencies of 71% and 89%). At each experimental

condition, the cylinder pressure history was measured using a water-cooled

piezoelectric pressure transducer. The experimental pressure data were

averaged for 11 cycles prior to being input to the thermodynamic model for

calculation of temperature and heat release profiles.

3.5 Results and discussion

Using measured pressure data, the average and core temperature and

heat release were calculated using a locally modified version of a standard heat

release model (HRM) [Ferguson et al., 1987; Li, 1995]. The reduced kinetics

model (RKM) was modified to simulate the core temperature using the same

methodology as the heat release model [Ferguson et al., 1987] and then was

tested using the rate parameters suggested by Li et al. [1995] for PRFs, with the

fuel specific rate parameters adjusted in order to adapt the model to the specific

test fuels. For example, the rate parameters for the alkylperoxy radical

isomerization reaction (reaction 3) were calculated from the proportions of n-

heptane and iso-octane as suggested in reference [Li et al., 1996], yielding

A3+=8.45*1011 and E3

+=20.08 for PRF 20. Similar adjustments were made for

reactions 10, 17, and 29. Using these rate parameters, the temperature and heat

release were calculated and compared with experimental data for different inlet

temperatures, equivalence ratios and engine loads. Further, the characteristics

of preignition were calculated for different inlet temperatures using the reduced


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37kinetic model. We also explored the application of the reduced kinetic model for

fuel mixtures (PRF 20+10% propene, pentene or toluene).

3.5.1 Comparison of the baseline model to experimental data for PRF 20

and PRF 50

For PRF 20, Figure 3-1 presents the experimental data and calculations

from the HRM and RKM at T=423 K. Experimentally, ignition occurred at 348

CAD and cumulative heat release from the HRM was 17.4 J (7%) up to the

ignition time. The baseline RKM did not predict those characteristics properly.

The predicted ignition time was 354 CAD and with a cumulative preignition heat

release of 21.4 J (10%). We investigated logical steps to improve the agreement.

Hu and Keck [1987] and Li et al. [1996] suggested that the simplest and

most rational way to include the effects of variation in fuel for reduced models is

to adjust the equilibrium constant of the RO2isomerization reaction, RO2

QOOH. This reaction also determines the extent of reaction. Because the

mechanism of PRF reaction is a degenerate branching reaction, one QOOH can

generate two OH so that more QOOH leads to the more active radicals. The

RO2 radical does not branch, so adjusting A3+ or E3+ is an efficient way to

account for the different fuels. Setting E3+=19.0 (kcal/mol) for 20 PFR and

E3+=22.4 (kcal/mol) for PRF 50 [Li et al., 1992], the value of the forward pre-

exponential constant A3+

was adjusted to improve agreement with the heat

release. We finally selected A3+=3.3*1011 for PRF 20 and A=3.5*1011 for PRF 50.


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38With the heat release in hand, we turned our attention to addressing the

discrepancy in ignition time. The intermediate temperature region is dominated

by the reactions of HO2 radicals and the characteristic stable products are

Figure 3-1. Pressure, temperature and cumulative heatrelease profiles using HRM and RKM for PRF 20 at 71%

volumetric efficiency, Tin=423 K, and =0.4 with baselineRKM parameters


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39alkenes and methane. If the reactions reach the high temperature region, the

reactions will be dominated by OH. For the transition from the intermediate

temperature region to ignition, the concentration of OH is also key, and the

reaction, HOOH+M=>2OH+M, should be considered. If E9+ = 46 (kcal/mol) and

A9+ = 7.59*1016 are used [Table 3-1], ignition does not occur until 451 CAD. To

address this, we decided to select E9+=45.5 (kcal/mol) and A9+ = 1.202*1017, the

same values as in reference [Westbrook and Dryer, 1984]. Figure 3-2 shows the


volumetric efficiency, Tin=423 K, and =0.4 withadjusted RKM parameters


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40effect of making these adjustments to the RKM. It appears that through simple

and logical parameter adjustment, the reduced kinetic model can predict ignition

time and heat release of the primary reference fuel mixtures used in this study.

3.5.2 Comparison and prediction of ignition time and heat release fordifferent inlet temperature, equivalence ratio, engine load and octanenumber

Without any additional changes to the rate parameters, the behavior of

PRF 20 at Tin = 453 K was simulated using the RKM. The model predictions and

experimental data are in excellent agreement, as illustrated in Figure 3- 3.

Equally good agreement occurred for Tin = 393 K.

Keeping the same kinetic parameters as above for PRF 20 and PRF 50,

we calculated the CAD or time to ignition at the conditions of speed - 750 rpm,

fuel - PRF 20 and PRF 50, volumetric efficiencies - 71% and 89%, inlet

temperature 393, 423, and 453K, and equivalence ratio - 0.4 and 0.5. Results

are shown in Tables 3-2, 3-3 and 3--4, which illustrate that the calculated ignition

times are consistent with the experimental data.

There is another observation that can be made based upon the results

shown in Figures 3-2 and 3-3. The time between the first-stage and the second-

stage ignition was about 25 CAD. During this period, the preignition reactions

continue and the chemistry moves from the low temperature region through the

intermediate temperature region to ignition. Negative Temperature Coefficient

(NTC) behavior of the fuels is important in controlling this transition. Therefore, it

is the chemical reactions in the NTC region that determines ignition time for PRF.


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41This verifies the importance of low and intermediate temperature chemistry in

autoignition processes and in HCCI engine operation.


volumetric efficiency, Tin=453 K, and =0.4 withadjusted RKM parameters


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42Having confirmed the applicability of our model over a range of conditions,

we investigated the effects of inlet temperature on the first and second-stage

ignition temperature, the cumulative heat release up to the second-stage ignition,

Table 3-2. Ignition time variation for selected inlettemperatures at 71% volumetric efficiency, 20 PFR,

=0.4

Table 3-3. Ignition time variation for selectedequivalence ratios at 71% volumetric efficiency, PRF 20and Tin=423 K

Table 3-4. Ignition time variation for selected volumetric

efficiencies at =0.5, 50 PFR and Tin=453 K

InletTemperature

(K)

Ignition Time(CAD)

EXP. RKM

393 357 356423 348 348

453 345 344

EquivalenceRatio Ignition Time(CAD)

EXP. RKM

0.4 348 348

0.5 343 344

VolumetricEfficiency (%)

Ignition Time(CAD)

EXP. RKM

71 364 364

89 351 352


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43and the first and second-stage ignition time. The results are presented in Figure

3-4. We observed that:

Figure 3-4. Temperature, CAD and cumulative heatrelease at the 1

stand 2

nd stage ignition time as a

function of inlet temperature for PRF 20


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441. The temperature of the first-stage ignition is about 707 K, independent of inlet

temperature.

2. The temperature at which the second-stage ignition occurred is approximately

920 K for all conditions. The temperature increases about 14

o

C withincreasing inlet temperature from 393 K to 453 K.

3. The cumulative heat release at ignition is large (about 23 J or 11%) at the

lowest inlet temperature and decreases to a plateau of 17 J (7%) as inlet

temperature increases.

The increase in the bulk gas temperature in the cylinder is the result of

piston compression and heat release by chemical reaction. When only the heat

release is considered, 1 J in additional energy results in a bulk gas temperature

increase of about 5 K at equivalence ratio = 0.4 and volumetric efficiency =

71%. If 7.3% (about 17 J) of the potential heat release occurs at a = 0.4

condition, the temperature will increase by about 100 K. The amount of heat

release depends on the reaction rate and the time available for chemical reaction.

The cumulative heat release up to the second-stage ignition point consists of two

parts. The first and major part occurs before the cylinder gas mixture reaches its

NTC region. Subsequently in the NTC region, heat release is slower although

chemical reactions and heat release continue. When the inlet temperature is low

the inlet charge actually has more reaction time before it moves into the NTC

region. Therefore the cumulative heat release is larger at low temperatures than

at higher temperatures.

The above results indicate that the current reduced model can be used as

a framework for predicting ignition time and heat release of pre-ignition in HCCI.


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45The reduced kinetic model is helpful for us to further study and optimize HCCI

engine operating parameters.

3.5.3 Extension of the current reduced model to mixtures of PRF 20 + 10%

propene, pentene or toluene

The various neat fuels and blends have different chemical behaviors in the

low and intermediate temperature regions. To investigate the interaction among

different fuels on HCCI engine operation, propene, pentene and toluene were

added into PRF 20 and tested.

Following the procedures suggested in previous work about prediction of

preignition reactivity for n-butane and iso-butane blends [Wang et al., 1996a], the

values of the pre-exponential parameters of reaction equations R3, R17 and R29

were adjusted in order to make the reduced model fuel specific. Because the

fuel consists mainly of PRF 20, the enthalpy changes were not adjusted. For

PFR 20+10% pentene the parameters were the same as the PFR 20. For PFR

20+10% propene and for PFR 20+10% toluene it was necessary to modify the

fuel specific parameters as follows to achieve good agreement.

A3+ = 2.1*1011 and E3+ = 19.0

A17+

= 2.44*1013

A29+ = 2.1*109

The experimental and calculated results also are shown in Figure 3-5 and

Table 3-5. Keeping the same rate parameters at different volumetric efficiency

(71%-89%), the calculated ignition times agreed well with experimental data.


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46

Figure 3-5. Pressure, temperature and cumulative heatrelease profiles using HRM and RKM for PRF 20 at 71%volumetric efficiency


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47Table 3-5. Ignition time variation for different fuel

mixtures at =0.4, 71% volumetric efficiency, andTin=423 K

Ignition Time(CAD)

Fuel

PRF 20 + EXP. RKM--- 348 348

10% Toluene 356 356

10% Propene 356 356

10% Pentene 347 348

These results indicate that the current reduced kinetic model appears to be

applicable for different fuels and their mixtures.

3.6 Conclusion

An existing reduced kinetic model for predicting pre-ignition reaction

behavior has been compared with the temperature and heat release profiles

measured in a stable HCCI engine operating with several fuels. With proscribed

adjustment of the fuel specific rate constants (R3, R17, and R29) and with an

enhanced rate for R9 per published data, the model properly simulates the

experimental data. The following observations can be made:

1. The reduced kinetics model can be used to predict the HCCI pre-ignition

behavior including temperature, pressure,ignition delay and heat release.

2. The temperature of the second-stage ignition is about 910-924 K.

3. The cumulative heat release at the time of ignition is about 7%-10% of the

potential heat release. The pre-ignition reaction and heat release are very

important for HCCI operation.


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484. For PRF 20, the temperature of the first-stage ignition is about 707 K, with the

second-stage ignition occurring approximately 25 CAD later.

5. The results also show that the presence of an NTC region affects the pre-

ignition behavior. The time spent in the pre-NTC and NTC regions affects theignition timing.

An obvious next step would be to enhance the model to include

calculations through the ignition event in order to evaluate burn duration. This

model enhancement is reported in Chapter 4.

Prediction of Pre-ignition Re Activity and Ignition Delay for Hcci Using a Reduced Chemical Kineti

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