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PreprinUCRL-JC-15185

US. epartmentof EnergyinLiver moreLaboratory

Fuel and AdditiveCharacterization for HCClCombustionS.M. Aceves, D. Flowers, J. Martinez-Frias,F. Espinosa-Lopez, W. J. Pitz, R. Dibble

This article was submitted to2003 JSAEEAE International Spring Fuels & Lubricants Meeting,Yokohama, Japan, May 19-22,2003

February 12,2003

Approved for public release; further dissemination unlimited

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DISCLAIMERThis document was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor the University of California nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately owned rights. Reference herein to any specificcommercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California. The views and opinions of authors expressed herein do notnecessarily state or reflect those of the United States Government or the University of California, andshall not be used for advertising or product endorsement purposes.This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may bemade before publication, this preprint is made available with the understanding that it will not be citedor reproduced without the permission of the author.This work was performed under the auspices of the United States Department of Energy by theUniversity of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.

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JSAE 20030130SAE 2003-01-1814Fuel and Additive Characterization for HCCl Combustion

Salvador M. Aceves, Dan iel Flowers, Joel Martinez-Frias,Francisco Esp inosa-Loza, William J. PitzLawrence Livermore National Laboratory

Robe rt DibbleUniversity of California Berkeley

Copyright (B2003 Society of Automotive Engineers of Japan, Inc.

ABSTRACTThis paper shows a numerical evaluation of fuelsand additives for HCCl combustion. First, a long listof candidate HCCl fuels is selected. For all the fuelsin the list, operating conditions (compression ratio,equivalence ratio and intake temperature) aredetermined that result in optimum performanceunder typical operation for a heavyduty engine.Fuels are also characterized by presenting Log(p)-Log(T) maps for multiple fuels under HCClconditions. Log(p)-Log(T) maps illustrate importantprocesses during HCCl engine operation, includingcompression, low temperature heat release andignition. Log(p)-Log(T) diagrams can be used forvisualizing these processes and can be used as atool for detailed analysis of HCCl combustion.The paper also includes a ranking of many potentialadditives. Experiments and analyses have indicatedthat small amounts (a few parts per million) ofsecondary fuels (additives) may considerably affectHCCl combustion and may play a significant role incontrolling HCCl combustion. Additives are rankedaccording to their capability to advance HCClignition. The best additives are listed and anexplanation of their effect on HCCl combustion isincluded.INTRODUCTIONHCCl is a combustion process that has someadvantages with respect to both spark-ignitedengines and diesel engines. HCCl combustion hasbeen identified as a global autoignition process[I].This is significantly different to the flamepropagation that occurs in a SI engine, and it is alsovery different to the stratified combustion of dieselengines. Considering the great differences betweenthese combustion processes, it is natural to expectthat the fuels that have been optimized for SIengines and diesel engines may not be optimum forHCCl engines.

One of the advantages of HCCl combustion is itsintrinsic fuel flexibility. HCCl combustion has littlesensitivity to fuel characteristics such as lubricityand laminar flame speed. Fuels with any octane orcetane number can be burned, although theoperating conditions must be adjusted toaccommodate different fuels, which can impactefficiency. An HCCl engine with variablecompression ratio or variable valve timing could, inprinciple, operate on any hydrocarbon oroxygenated liquid fuel, as long as the fuel isvaporized and mixed with air before ignition.The literature shows that HCCl has been achievedwith multiple fuels [2-4].he main fuels that havebeen used are gasoline, diesel fuel, propane, naturalgas, and single- and dual-component mixtures ofthe gasoline primary reference fuels, PRF(iso-octane and n-heptane). Other fuels can be usedand some have also been tested to a lesser extent(methanol, ethanol, acetone). Previous research hasshown that it is not optimal to use familiar fuelparameters such as octane or cetane ratings toguide in the definition of performance ratings offuels for HCCl combustion [5]. Therefore, a betterapproach and a new set of ratings are needed toidentify attractive fuels for HCCl engines.In addition to the opportunity of identifying optimumfuels for HCCl combustion, there is also theopportunity of identifying optimum additives forHCCl combustion. Experiments and analyses haveindicated that small amounts (a few parts permillion) of secondary fuels (additives) mayconsiderably affect HCCl combustion and may playa significant role in controlling HCCl combustion [6-91.While it may be impractical to carry two fuels onboard of a vehicle, it may be possible tomanufacture the additive in the vehicle [7,10].Otherwise, the additive may be stored in the vehicleand replenished at maintenance intervals. This ismore likely if the additive is used in small quantities.This paper presents an analysis of fuel and additivecharacterization for HCCl combustion. First,

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different fuels are tested for predicting the maximumpower output and efficiency that can be obtainedwithin the constraints of peak cylinder pressure andcombustion timing. The purpose is to comparedifferent fuels for any given set of enginerequirements (efficiency and power output).The paper also shows Log(p)-Log(T) diagramsgenerated for four different fuels. Log(p)-Log(T)diagrams show chemical heat release rates as afunction of temperature and pressure. Log(p)-Log(T)diagrams can be used to explain the behavior offuels under different HCCl operating conditions. Thefour fuels used for the analysis include a fuel withlittle low temperature heat release (iso-octane), afuel with substantial heat release at low temperature(n-heptane), and two fuels with equal researchoctane number (RON) and different chemicalstructure (n-pentane and a primary reference fuel,PRF, both with a 61.7 RON).Finally, the paper shows how additives can beranked according to their capability to advanceHCCl ignition. It has been discovered that a very lowconcentration (10 ppm) of the best additives canadvance HCCl combustion by almost 11 crankangle degrees. This is a big effect, equivalent toheating the charge by over 30 K, and it offerspotential for successful control of HCCl engines.

StrokeBore

FUEL CHARACTERIZATION

10cm10cm

Fuel characterization for HCCl combustion isconducted by performing multiple runs with adetailed chemical kinetics code running in singlezone mode. Single zone runs assume that the airand fuel mixture is perfectly homogeneous incomposition as well as temperature, and areexpected to predict ignition timing with goodaccuracy [ I l l . On the other hand, single zonemodels are known to overpredict peak cylinderpressure [Ill.HCT [I21 is used for all the runspresented here. Two detailed mechanisms areused. A small mechanism with 179 species and1125 reactions is used for all the short-chain fuelsand a big mechanism with 913 species and 3620reactions is used for iso-octane. The mechanismsare available on our website [13].Parameters defining engine geometry and speedare listed in Table 1.These parameters are used forall the calculations in this paper. Table 2 showsengine operating conditions specific to thecalculations described in this section. Theseconditions are representative of heavy-duty enginesrunning at maximum power. The compression ratiois kept as a free parameter, while the intaketemperature and equivalence ratio are adjusted tomeet two constraints: the maximum heat releaseoccurs at S'ATDC, and the peak cylinder pressure isless than 250 bar. The inlet temperature can bevaried in the range between300and 400 K, and theequivalence ratio has a maximum value of 0.5 tolimit NO, emissions. Compression ratio is variedbetween 12 and 20. Intake pressure and enginespeed are kept constant (3 bar absolute and 1800rpm). A constant fraction of residual gases is

assumed (2 %). Composition of the residual gasesis calculated assuming full combustion of the fuelinto C02and H2O. The charge temperature at intakevalve closing is determined by a publishedprocedure(141.This procedure considers the mixingof residuals with fresh gases as well as theexpansion during blowdown, but it does not take intoaccount heat transfer to the wall. A long list of likelyHCCl fuels is analyzed, including propane, methane,natural gas, methanol, hydrogen, ethanol and iso-octane.Table 1. Engine geometry and operating conditionsfor all the single zone chemical kinetics runsconducted in this paper.I Parameter I Description 1

Connecting rod length I15cmSpeed I1800rpm

Table 2. Engine operating parameters used in thefuel characterization section of the paper (Figures 1-4).

Residual gas fraction I 0.02Heat transfer to walls I adiabaticThe results are presented in Figures 1-4. Figures 1and 2 respectively show the intake temperature andthe equivalence ratio that are necessary to obtain250 bar of maximum pressure when maximum heatrelease occurs at 5"ATDC. Figure 1 showstemperature as a function of compression ratio. Thefigure shows that temperature is a decreasingfunction of compression ratio. Increasing thecompression ratio reduces the amount of heatingrequired for satisfactory combustion timing. Figure 1also shows that the temperature for satisfactorycombustion increases monotonically with octanenumber. The fuel with the lowest octane numberamong the fuels considered (iso-octane, RON=100)requires the coolest operating temperature, and thefuel with the highest octane number (methane)requires the highest operating temperature.Figure 2shows the equivalence ratio necessary toobtain the required ignition timing and peak cylinderpressure, as a function of compression ratio. Thefigure shows that equivalence ratio is a decreasingfunction of compression ratio. This trend is similar tothe trend obtained for temperature in Figure 1.However, Figure 2shows that the equivalence ratiolines for all fuels behave very similarly, with all of

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them falling in a narrow band along the diagonal ofthe figure.Figures 1 and 2 show the likely range ofcompression ratios that can be used for the differentfuels over typical HCCl combustion. Iso-octane islimited to relatively low compression ratios,especially for the high intake pressure consideredhere. Use of iso-octane at high compression ratiosmay require cooling of the intake charge. On theother hand, methane and natural gas are limited tohigh compression ratio engines. Use of methane atlow compression ratios may require considerableheating of the intake (over 400 K intaketemperature), and this may prove impractical undermany conditions.

320!- '.,

Figure 1. Intake temperature necessary to obtain 250 bar ofmaximum pressure when maximum heat release occurs at5OATDC as a function of compressionratio, for all the fuels beingconsidered.

Figures 3 and 4 respectively show engine indicatedefficiency and engine indicated mean effectivepressure (IMEP) as a function of compression ratio,for all the fuels being considered. Figure 3 showsthat efficiency is an increasing function ofcompression ratio, and Figure 4shows that IMEP isa decreasing function of compression ratio. IMEP isa decreasing function of compression ratio becausethe peak cylinder pressure has to be kept under250bar, Meeting this restriction requires reducing theequivalence ratio as the compression ratio isincreased.Figures 3 and 4 also show that the lines for all fuelsfall within a relatively narrow band of efficiency andIMEP. However, within the narrow band significantdifferences are observed. In Figure 3, natural gasand methane have about 4% lower efficiency thanother fuels (ethanol, methanol). Methane andnatural gas have a higher equivalence ratio than theother fuels for a given compression ratio (Figure 2).As a result, the specific heat ratio (y), is lowest formethane and natural gas, resulting in lower engineefficiency. Figure 4 shows that hydrogen fallsnoticeably below the IMEP-compression ratio linefollowed by all the other fuels. This is because burn

duration is extremely short for hydrogen, resulting ina high peak cylinder pressure. Considering that theengine has a restriction on peak cylinder pressure(250 bar), an HCCl engine running on hydrogen islimited to operate at a low equivalence ratio (Figure2), resulting in a low IMEP.

P.-3 -m 0.3 -

02 -tt-+--c-4--c-+

12 13 14 15 18 17 18 18 x )0.1' ' " ' ' ' "comprassion ralioFigure2. Equivalence ratio necessary to obtain 250 bar ofmaximum pressure when maximumheat release occurs at5OATDC as a function of compression ratio, for all the fuels beingconsidered.

70 1

29 2 13 14 IO 16 17 18 19 20compression ratio

Figure 3. Engine indicated efficiency as a function of comp-ression ratio, for a ll the fuels being considered.

One of the greatest challenges of HCCl engines istheir low power output. Power output is typicallylimited by the peak cylinder pressure that the enginecan withstand. In HCCl engines, combustion is verysudden, producing a high peak cylinder pressurethat limits the power output that can be obtainedfrom the engine. It would be desirable to find fuelswith a longer burn duration that may result in higherpower output. However, Figure 4 shows that nosuch fuel was identified in this analysis. The effect ofthe different fuels on maximum IMEP is relativelysmall.

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20I \?

Figure 4. Engine indicated mean effective pressure (IMEP) as afunction of compression ratio, for all the fuels being considered.

HEAT RELEASEAND IGNITION IN HCCIENGINESEngine processes are well visualized in Log(p)-Log(T) diagrams. The benefits of using thesediagrams have been extensively discussed in arecent publication [15], where the procedure wastitled "visual thermodynamics." As described in 1151,many diesel engine processes can be easilyillustrated in Log(p)-Log(T) diagrams, includingcompression, combustion, expansion, ignition, whitesmoke generation, etc. Log(p)-Log(T) diagrams alsohave applicability to illustrate HCCl engineprocesses leading to autoignition.This section shows Log(p)-Log(T) diagrams for fourdifferent HCCl fuels. These include a fuel with highoctane number (iso-octane), a fuel with low octanenumber (n-heptane), and two fuels with the sameresearch octane number (RON) but differentcomposition (n-pentane and a primary referencefuel, PRF, both with a 61.7 RON). All thecalculations presented in this section are performedwith a detailed chemical kinetic code in single zonemode (121. A detailed mechanism is used thatcontains iso-octane as well as n-heptane. Thismechanism includes 1036 chemical species and4238 chemical reactions. The analysis considers theengine dimensions listed in Table 1 and the engineoperating conditions listed inTable 3.The Log(p)-Log(T) diagrams for the four fuels areshown in Figures 5-8. These diagrams include threetypes of lines, described next.Compression lines. These lines show thetemperature and pressure history during thecompression stroke. Twenty-one runs wereconducted for each fuel by varying the intaketemperature between 300 K and 500 K at intervalsof 10 K. Figures 5-8 generally include threecompression lines out of the 21 compression linesthat were calculated. One for the lowest intaketemperature considered in the analysis (300 K), one

for the lowest intake temperature for which ignitionoccurs, and one for the highest intake temperature(500 K). For n-pentane and n-heptane, the fuelignites even when the intake temperature is 300 Kso only two compression lines are presented.Compression lines appear as straight lines inLog(p)-Log(T) diagrams, up to the point wheresignificant chemical heat release occurs.Table 3. Engine operating parameters used in theheat release and ignition section of the paper(Figures 5-8).Parameter 1 DescriptionIntake pressure I 1bar absolute 1Intake temperature rangeCompression ratio I 1 6 1300-500 KI Equivalence ratio I 0.3Residual gas fraction I 0.02Heat transfer to walls I adiabatic

Contour lines for chemical heat release rate. Heatrelease is calculated as a function of pressure andtemperature during the compression stroke. Contourlines are then generated by interpolation betweenthe heat release values calculated on thecompression lines. Heat release is calculated inWatts per gram of fuel and then plotted in theLog(p)-Log(T) diagrams over the whole range ofoperation between the minimum and the maximumtemperature.Ignition line. This is the line at which hightemperature (fast) combustion starts during HCClcompression. The ignition line is determined byfinding the points at which the concentration of H202reaches a maximum. According to a previouspublication [16], HCCl ignition occurs when the H202molecule decomposes into two OH radicals.Figure 5 shows the Log(p)-Log(T) diagram for n-heptane. This is typical of low octane number fuels,in which there is a significant amount of chemicalheat release at a relatively low temperature (800-900 K). Heat release reaches a maximum at anintermediate temperature (the cool flame region)and then drops as the temperature increases. Hightemperature ignition then occurs as the temperatureis further increased. This is typically called two-stageignition [17]. Low temperature heat release is sointense for n-heptane that even the coldest case(Ti,=300 K) eaches ignition. Low temperature heatrelease is most intense at high pressure, andtherefore has a bigger effect on cases with a lowintake temperature. Compression lines deviate fromlinearity very early in the process. The ignition line isalmost a horizontal line, indicating that pressure hasa very small effect on ignition for n-heptane. This isin agreement with the ignition lines plotted in [15],which were obtained from experiments done on aconstant volume combustion vessel with cetanefuel.

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Figure 5. Log(p)-L ogm diagram for n-heptane under HCClcompression conditions. The figure shows two compressionlines, one for Th=300 K, ph=lbar and another for Th=500 K,pin=lbar. Compression lines show the temperature and pressurehistory during the compression stroke. The figure also includesmultiple heat release contour lines. These lines show chem icalheat release as a function of pressure and temperature, in Wattsper gram of fuel, in powers of I O . Finally, the figure includes anignition line, which is the lin e at which high temperature (fast)combustion occurs during HCCl com pression.

Figure 6 shows a Log(p)-Log(T) diagram for iso-octane. Iso-octane has very little early heat release,and no cool flame region. Considerable heat releaseoccurs only as the compression temperatureapproaches the ignition line. The initial temperaturehas to be raised to 390 K before ignition occurs.Pressure has a considerable effect on ignition of iso-octane, as shown by the non-horizontal ignition line.Figures 7 and 8 show the Log(p)-Log(T) diagramsfor two fuels with the same research octane number(RON) and different composition. One is n-pentane(Figure 7) and the other is a primary reference fuel(PRF) with a 61.7 RON (Figure 8). Figures 7 and 8are qualitatively similar but quantitatively different.Both show cool flame regions, but these areconsiderably smaller than for n-heptane (Figure 5).Cool flame regions in Figures 7 and 8 exist only athigh pressure (over 20 bar). They therefore affectonly cases in which the intake temperature isrelatively low.Figures 7 and 8 show that n-pentane hasconsiderably more low temperature heat releasethan the 61.7 octane PRF, even though both havethe same RON. For n-pentane, ignition is achievedeven at the lowest intake temperature (300 K). Forthe PRF, the minimum intake temperature forignition is 330 K. On the other hand, thecompression line for Tin=500K is virtually identicalfor the two fuels. These lines are also similar to theTin=500K compression line for iso-octane (Figure6), indicating that there is very little low temperatureheat release when the intake temperature is high.

Figure 6. Log(p)-Logm diagram for iseoctane under HCClcompression conditions. The figure shows three compressionlines, one for Th=300 K, pn=l bar. other for Th=390 K, @"=Iar(the lowest temperature to achieve ignition) and another forTh=500 K,~ = lar. Compression lines show the temperatureand pressure history during the compression stroke. The figurealso indude s mu ltiple heat release contour lines. These linesshow chemical heat release as a function of pressure andtemperature, inWatts per gram of fuel, inpowers of IO . Finally,the figure indude s an ignition ine, which is the line at which hightemperature (fast) com bustion occurs during HCC l compression.

m U PPressue,Bar.-s I 7 I * t o

Figure 7. Log(p)-Log(T) diagram for n-pe ntane under HCClcompression conditions. T he figure showstwo compressionlines, one for Th=300 K. p h=l bar and another for T A O O K,b=lar. Compression lines show the temperature and pressurehistory during the compression stroke. The figure also includesmultiple heat release contour lines. These lines show chemicalheat release as a function of pressure and temperature, inWattsper gram of fuel, in powers of I O . Finally, the figure includes anignition line, which is the line at which high temperature (fas t)combustion occurs during HC Cl compression.

I

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81.7 od.M PRF

Pnseure, 8.1

Figure 8. Log(p )-Logo diagram for a 61.7 PRF under HCClcornpression conditions. The figure shows three compressionlines, one for Th=300 K, pn=l bar; other for Th=330 K,~ = lar(the lowest temperature to achieve ignition) and another forTh=500 K, pm=lbar. Compression lines show the temperatureand pressure history during the compression stroke. The figurealso indudes multiple heat release contour lines. These linesshow chemical heat release as a function of pressure andtemperature. in Watts per gram of fuel, in powers of I O . Finally,the figure includes an ignition line, which is the line a t which hightemperature (fast) cornbustionoccurs during HCCl compression.

ADDITIVE CHARACTERIZATIONAdditive characterization for HCCl combustion isconducted by performing multiple runs with adetailed chemical kinetic code running in single zonemode. HCT [I21 is used for all the runs with adetailed mechanism for iso-octane that includes 913species and 3620 reactions. The mechanism isavailable on our website 1131. An initial run wasmade for a baseline condition typical of HCClcombustion with iso-octane fuel. Table 1 lists thegeometric characteristics of the engine and Table 4lists the operating conditions for the baseline run.The potential applicability of different species foradvancing HCCl combustion is then evaluated bymaking 913 additional HCT runs. In each of theseruns, the concentration of each of the 913 species isincreased by 10 ppm with respect to theconcentrations used for the baseline run. The timingfor maximum heat release is then evaluated foreach run and compared to the timing of the baselinerun. This calculation allows us to rank theapplicability of all 913 species for advancing theignition timing and therefore evaluate their potentialuse as additive for controlling HCCl combustion.The detailed chemical kinetic model has beenvalidated by comparing predicted speciesconcentrations to measured species concentrationsin flow reactors and jet-stirred reactors at elevatedpressures [la]. However, it is not possible tovalidate the model with respect to many of the 913species in the mechanism because theirconcentration is too small or their lifetimes are tooshort to be experimentally measured. In particular,we have not been able to directly validate the

mechanism for alkylhydroperoxide peroxy radicals,ketohydro-peroxides, and alkylhydroperoxidesadditives, which are discussed below. The bestavailable estimates for rate of production andconsumption of these species have been put intothe detailed chemical kinetic mechanism.Table 4. Engine geometry and operating conditionsfor the additive evaluation section of the paper.[Parameter I Description I

I

Many of the 913 species in the mechanism areclassified as unstable. Species are defined asunstable if their concentration drops by more than1% during the first 10 crank angle degrees of thecompression stroke. It is considered that speciesthat decompose rapidly at near ambient conditionscannot be successfully stored or generated in thevehicle, and therefore have little practicalapplicability for HCCl engine control. Many radicalsthat are known to have an effect on HCCI ignition[I51 are unstable (H, OH, 0). According to thisdefinition, 343 species out of the 913 species in themechanism are found to be unstable. It should benoted that the stability definition used here is notvery strict, and some unstable species may meetthe requirement imposed. This is further discussedin the results.The remaining 570 species are ranked according tohow effectively they advance combustion withrespect to the baseline run. Table 5 shows theresults. The table shows the five species that havethe greatest effect on advancing the time formaximum heat release, as well as a number ofselected species that may be of interest becausethey are simpler species that could possibly begenerated or stored in the vehicle. The table alsoincludes the combustion timing obtained byincreasing the intake temperature by 20 K, 30 K and40 K with respect to the base case, for comparingthe relative effect of additives with the effect oftemperature.Alkylhydroperoxide peroxy radicals are mosteffective at advancing combustion. They occupymost of the top 100 places in the ranking of fueladditives for advancing combustion. Someperoxides advance combustion by more than 11crank angle degrees. This is a big advancement incombustion for a very small concentration (10 ppm).To obtain the same advancement in combustion,the intake temperature would have to be increasedby over 30 K. Ozone is also very effective inadvancing combustion, and can be generated onboard a vehicle by a simple electrochemical reaction

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[7]. Ketohydroperoxides occupy most of the ranksbetween 100 and 200, advancing combustionbetween 6 and 8 crank angle degrees. Nitrogencompounds can also play a role as HCCl additives[6].hese include nitric oxide, which has previouslybeen proposed as an HCCl additive [9]. Hydrogenperoxide (H202) is not as effective as otheradditives, but H202 may be easier to store anddistribute [8] .The intake fuel concentration at the baselineconditions listed in Table 4 is 0.5% by volume or5000ppm. Therefore, the molar rate of consumptionof additive at 10 ppm would be 500 times less thanthe molar rate of fuel consumption. It may thereforebe reasonably easy to store the additive on avehicle, provided that it is stable andenvironmentally benign. Ultimately, the benefit ofusing additives to control combustion has to becontrasted to the possibility of adjusting the intaketemperature to obtain satisfactory combustion [19,201.Among the 913 potential additives that were used inthis analysis, many were found that couldconsiderably advance combustion, but none wasfound that could significantly delay com bustion. Themaximum ignition delay is less than 0.1 degrees,which is insignificant for practical purposes.Additives that delay combustion are mainly inertspecies. For inert species, combustion is delayedmainly through thermal effects (by decreasing thevalue of the specific heat ratio, y, reducingcompression heating). The chemical effect ofadditives considered in this work for inhibiting thereactions leading to HCCl ignition is negligible.However, other substances not considered in thisanalysis (fire retardants, i.e. bromine) may have aconsiderably greater effect in delaying HCClcombustion. Their stability and toxicity have to beevaluated before they can be applied for HCClcombustion control.THE CHEMICAL KINETICS OF HCClADDITIVESThe most effective additives in Table 5 foradvancing combustion are the alkylhydroperoxideperoxy radicals. These species have the generalmolecular structure of HOOROO. where R is ahydrocarbon molecular structure. Additives labeled1 hrough 5 in Figure 9 are in this family of additives.The reason why these species are particularlyeffective is because of the reactive sequence:HOOROO. => HOORC=O + OH (1 )HOORC=O => .ORC=O+ OH

radical. These species are among the few additivesconsidered that produce two OH radicals. If theseradical species were being considered forgeneration on board a vehicle, their lifetime on timescales and temperatures appropriate for additivegeneration and storage would need to be estimated.Additional reactions, beyond what is in the currentreaction mechanism for HCCl combustion, wouldneed to be considered.

Additive 3

Additive 103

Additive 122

Figure 9: Molecular structure of selective additives. Eachadditive is identifiedby its ranking number (Table 5).

Ozone is a very effective additive due to thereaction:

Ozones decomposition is slow enough so that itstable during the early stages of the compressionstroke but its decomposition increases rapidly withincreasing temperature, providing 0 atoms to initiatethe low temperature chemistry of iso-octane (71.The HNO radical is effective in advancingcombustion due to its decomposition to formreactive H atoms:HN O => H + NO (3)

where R is R minus one H atom. Note theproduction of two reactive OH radicals for theconsumption of one alkylhydroperoxide peroxy

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Table 5. Ranking of the different species according to their effect in advancing combustion in HCCl combustion,at the conditions detailed in Table 4 for the baseline run. Calculation assumes that 10 ppm of each additive areadded to the composition of Table 4. For the baseline condition, maximum heat release occurs at 6.4"ATDC.The timing obtained by heating the intake 20 K, 0 K and40 K with respect to the base case is also included forcomparison.Ranking Name Chemical formula Angle for maximum heat Advancement in

release rate, degrees combustion timing,

204

267302342369370422449514

Since it is a radical, its stability during additivegeneration and storage would have to be evaluatedfor it to be considered as a practical additive.The next series of additives which promote HCClcombustion contain the same molecular structureROOH where R is a hydrocarbon or carbonylstructure. These include peroxy formic acid (additive103, Table 5), the ketohydroperoxide group (e.g.additive 122), and the alkylhydroperoxide group (e.9.additive 204) and their example structures can beseen in Figure 9. The mechanism of their promotingeffect is simply:ROOH => RO. + OH (4)One reactive OH radical is produced for each additivemolecule consumed. The RO. produced is also aradical and may lead to further promotion. Forthe

additive peroxy formic acid, the 'RO." is HCOz and isparticularly reactive:HC03H => HC02 + OH (5)HCO2 => H + CO2An OH radical and an H atom are produced for eachadditive molecule consumed. The second reactionshows why the HCOZ radical is computed to be aneffective additive itself (Table5).Since the additives with the structure ROOH arestable compounds and not radical species theirlifetimes after generation and storage on board thevehicle are expected to be longer than the previouslydiscussed radical species.

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The additive NO is effective because it convertsrelatively unreactive H02 adicals produced during thecompression stroke into reactive OH radicals,promoting ignition:NO + HO2 => NO2 + OH (6)Hydrogen peroxide (H202)would be expected to be avery effective additive because of its decomposition toform two reactive OH radicals:

(7)However, the bond dissociation energy of H202 s high(52 kcal/mole, 217 kJlmole, corresponding to anactivation temperature of 2615 K). This means thatH202does not decompose in the compression strokeuntil the temperature reaches 900-IO00 K, which istoo late for hydrogen peroxide to be a highly effectiveadditive (Table 5). The additives with the ROOHstructure have bond dissociation energies of 3746kcal/mole. They decompose at temperatures below900 K and very effectively promote the lowtemperature chemistry.CONCLUSIONThis paper shows an evaluation of fuels and additivesfor HCCI combustion. The paper has three mainparts. First, a list of candidate HCCl fuels is selected.For all the fuels in the list, operating conditions aredetermined that result in optimum performance undertypical operation for a heavy-duty engine. Fuels arealso characterized by presenting Log(p)-Log(T) mapsfor multiple fuels under HCCI conditions. The Log(p)-Log(T) maps illustrate important processes duringHCCI engine operation, including compression, earlychemical heat release and ignition. The paper alsoincludes a ranking of many potential additives.Additives are ranked according to their capability toadvance HCCl ignition. The main conclusionsobtained from the paper are summarized next.Operating conditions for satisfactory HCClcombustion (compression ratio, equivalence ratio andtemperature) are limited by the characteristics of thefuel. Fuels with a relatively low octane number cannotoperate at a high compression ratio unless the intakemixture is cooled below ambient temperature. Fuelswith a high octane number may require considerablepreheating to operate at a low compression ratio.The efficiency and IMEP that can be obtained from anengine at a given compression ratio falls within arelatively narrow range for all the fuels considered inthe analysis. However, within the narrow rangesignificant differences are observed. Natural gas andmethane have about 4% lower efficiency than otherfuels because they have a higher equivalence ratiothan the other fuels for a given compression ratio. Asa result, the specific heat ratio (y), is lowest formethane and natural gas, resulting in lower engineefficiency. Hydrogen falls noticeably below the IMEP-compression ratio line followed by all the other fuels,mainly because burn duration is extremely short forhydrogen. Increasing the burn duration couldpotentially increase the power output that can be

obtained from an engine. However, no fuel was foundthat would result in a significantly extended HCClcorn bustion.Log(p)-Log(T) maps were generated for four differentfuels. These diagrams help visualize the effect ofoctane number and early heat release on HCCIoperation. The maps show that fuels with equalresearch octane number (RON) may haveconsiderably different heat release characteristics.Several additives were identified that advancecombustion by almost 11 crank angle degrees whenadded to the intake mixture at a concentration of 10ppm. This is a big effect, equivalent to increasing theintake temperature by more than 30 K. The bestadditives are alkylhydroperoxide peroxy radicals, butthe long-term stability of these compounds has to bedemonstrated before they can be applied to enginecontrol. Ozone is an excellent additive, and it is alsostable and easy to manufacture on board a vehicle.ACKNOWLEDGMENTSThis work was performed under the auspices of theU. S. Department of Energy by the University ofCalifornia, Lawrence Livermore National Laboratoryunder Contract No. W-7405-Eng-48.REFERENCES1.

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