Simulation of the Impact of N2-Doped Intake

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Simulation of the Impact of N2-doped Intake Air on both NOx Emissions and Combustion

Efficiency for n-Heptane Oxidation in Diesel Engine Conditions

F. Lagrze*1,2, G. Morac1, L. Unger1, H. Duval2, J.B. Guillot2

1 Renault s.a.s, 1 av du Golf,78288 Guyancourt France2 LGPM, Ecole Centrale Paris, 92295 Chtenay-Malabry France

AbstractThe purpose of this study is to assess the impact on both nitrogen oxides emissions and combustion efficiency, ofn-heptane combustion by nitrogen doped air. Experiments were carried out on a Renault test Diesel engine withconventional Diesel fuel and nitrogen doped intake air. Comparison between experiment and simulated results froma 246-species n-heptane oxidation mechanism has shown the same tendencies when slightly rising N2 fraction inintake air: (1) NOx emissions neatly decrease; (2) CO2 emissions remain almost unchanged.

* Corresponding author : [email protected] Proceedings of the European Combustion Meeting 2009

IntroductionNitrogen oxides (NOx), comprising NO and NO2, are

among the most worrying pollutants because of their

multiple sanitary and environmental impacts such asozone pollution, acid rains, eutrophication, green houseeffect and toxic compounds generation if combined withother pollutants [1]. In 2004, 50% of NOx emissionssued from transportation [2] and especially from Diesel

vehicles. In Europe, Diesel-powered passenger carsaccount for 30.3% of the fleet and 53.3% of the newcars bought in 2007 were Diesel-powered [3]. One cantherefore understand how much is at stake in controllingthese emissions. European official standards (Table 1)regulate levels for automotive pollutants namely, carbonmonoxide, nitrogen oxides, particulate matter andhydrocarbons.

Table 1. Regulated pollutants levels for personalvehicles ( < 2.5 t in g/km). Euro 4 and Euro 5

Take intoforce

NOx PM COHC +NOx

Euro4

Jan. 2005 0.25 0.025 0.50 0.30

Euro5

Sept. 2009 0.18 0.005 0.50 0.23

The challenging point consists in NOx andparticulate matter emissions reduction. Particulatematter is efficiently removed from exhaust gas thanks to

Diesel Particulate Filter (DPF). Regarding NOxtreatment, three-way catalytic systems, effective for gasengine, cannot be used for Diesel vehicle because oflarge oxygen contents (up to 19%) in the exhaust gas.However, NOx trap or a Selective Catalytic Reduction(SCR) system can be used [4]. SCR consists inconverting nitrogen oxides into N2 and water by addinga gaseous reductant (such as ammonia or urea). A NOx-trap is generally composed of a NOx adsorption systemassociated with a catalytic converter.

After-treatment is not the only way to reduce NOxemissions, indeed combustion improvement could be a

solution. According to Figure 1, NOx mainly form attemperature greater than 2200 K and for air/fuel ratiosgreater than 0.7. Beside, particulate matter is present for

air fuel/ratios lower than 0.6 and temperatures rangingfrom 1400 K to 2750 K.

Figure 1. PM and NOx formation with respect to thetemperature and air/fuel ratio of the mixture [5]

Combustion in Diesel engines being non-homogenous, the full range of air/fuel ratio can befound. Locally stoichiometric air/fuel ratios induce highflame temperature which generates large quantities of

NOx in presence of abundant N2 and O2. Exhaust gasrecirculation (EGR) is effective to reduce NOxemissions since it lowers the temperature of the burningmixture [6]. Exhaust gas mainly comprise N2, O2, CO2,

H2O and species traces of unburned and partiallyoxidized hydrocarbons. However, for practical reasons

N2 could be used as synthetic EGR [7].In this work, the impact of nitrogen-doped intake air

on fuel combustion and emissions was investigated. Thefirst step of our work consisted in a set of experimentscarried out on a test engine. In order to provide furtherinsight on the influence of the extra N2, simulationswere performed with chemical software.


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Experimental and Modelling StudiesExperiment

Experiments were performed on a Renault G9T600test Diesel engine with the operating conditions reportedin Table 2. Nitrogen concentration was monitored byway of tanked nitrogen injection upstream from the air

filter in order to guarantee the homogeneity of the N2-doped air.

Table 2. Test engine operating conditions

EngineSpeed

M.E.P. Hygrometry Fuel

1500 rpm 7 bar 50%Conventional

Diesel

Nitrogen levels in intake air varied from 79.3%vol, N2(standard air composition) up to 83.0%vol, N2 (Table 3).

Table 3. Intake flow composition tested for 1.2 bar

intake pressure and 305 K intake temperatureN2 content in

intake air (%vol)Intake air flow

(kg/h)Fuel flow

(kg/h)

79.3 113 4.353

81.0 114 4.448

81.5 114 4.472

82.0 115 4.418

82.5 116 4.459

83.0 116 4.447

ModellingCalculations require a proper chemical mechanism

and a definition of a combustion system.The mechanism used in this study resulted from the

combination of a 246-species mechanism (2309reactions) describing the oxidation ofn-heptane [9] anda 29-species mechanism governing the reactions ofnitrogenous species [10]. A homogeneous reactor with

variable volume and pressure as a function of time(engine cycle) was defined. This tool is called HCCIin DARS (Digital Analysis of Reaction System) [11].The compression law is determined from engineparameters (Table 4) and simulates, to some extent, thepiston stroke in an engine. Crank angle varied from 180degrees BTDC (before top dead centre) to 180 degreesATDC (after top dead centre).

Table 4. Engine parameters used for simulation

Enginevolume

Compressionratio

BoreConnectingrod length

Stroke

2188cc

17.587

mm149.96

mm92

mm

Diesel fuel is mainly composed of paraffinic species[12]. Heptane and decane are commonly used in Dieselsurrogates from the literature [13] [14]. In this study, themodel fuel used to describe Diesel fuel combustion is n-heptane.

Initial pressure and temperature had to be properlyset in order to allow fuel ignition. The engine model andthe surrogate fuel used for the simulation did not take

into account facilitating factors such as additives oracoustic effects, etc. Therefore, fuel ignition was notpossible in the simulation with the same (T, p)conditions as in experiments thus, initial temperaturehad to be set to 800 K. Nitrogen concentrations testedwere the same as for the experiment. Air was assumed

to be a binary mixture of nitrogen and oxygen. Initial(180 degrees BTDC) mixtures composition are reportedin Table 5

Table 5. Initial mixtures composition for varyingnitrogen contents in intake air

Mixture composition(mass fraction)

N2 content inintake air (%vol)

Oxygen Nitrogen n-heptane

79.3 0.205 0.784 0.011

81.0 0.188 0.801 0.011

81.5 0.183 0.806 0.011

82.0 0.178 0.811 0.011

82.5 0.173 0.816 0.011

83.0 0.168 0.821 0.011

Results are presented in the following section.Chemical and physical impact of nitrogen-doping will

be discussed.

Results and DiscussionImpact of N2-doped air on fuel consumption, in-

cylinder pressure, temperature, and species emissionshas been observed.

In Cylinder Pressure and TemperatureAs shown in Figure 2, maximum in-cylinder

pressure is reached at 3.5 degrees BTDC and equals20.3 bar. The evolution of the pressure inside the reactoris not dependent upon the nitrogen concentration in theinitial mixture.

Press ure (Pa)

0.00E +00

5.00E +05

1.00E +06

1.50E +06

2.00E +06

-180 -120 -60 0 60 120 180

crank angle

79.3% N2

81.0% N2

81.5% N2

82.0% N2

82.5% N2

83.0% N2

Figure 2. Reactor pressure vs. crank angle for differentnitrogen contents in intake air

The same observation applies to temperature curves(Figure 3). Maximum temperature is reached at 12.3degrees BTDC and equals 2590 K.


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Temperature (K)

0

500

1000

1500

2000

2500

3000

-180 -120 -60 0 60 120 180

crank angle

79.3% N2

81.0% N2

81.5% N2

82.0% N2

82.5% N2

83.0% N2

Figure 3. Reactor temperature vs. crank angle fordifferent nitrogen contents in intake air

It can be stated that, in this ideal model, the engineefficiency remains constant for the whole range ofnitrogen levels tested. Neither the ignition timing northe maximum pressure and temperature depend on N2

concentration for the range of compositions simulated.However, extreme nitrogen-doping could have drasticeffects on combustion efficiency as discussed in thelimitations subsection.

Carbon dioxide and n-heptane

Carbon dioxide formation is plot in Figure 4. Thereis no impact of N2 variation on CO2 emissions. FinalCO2 mass fraction equals 0.115.

C arbon Dioxide

0.00

0.04

0.08

0.12

-180 -120 -60 0 60 120 180

crank angle

massfraction

79.3% N2

81.0% N2

81.5% N2

82.0% N2

82.5% N2

83.0% N2

Figure 4. Mass fraction of carbon dioxide vs. crankangle for different nitrogen contents in intake air

n-Heptane (Figure 5) consumption does not dependon nitrogen doping. n-Heptane is totally consumedbetween 35 and 13 degrees BTDC.

As for temperature and pressure results, nitrogen-doping does not have any effect on n-heptaneconsumption and CO2 emissions. The start ofcombustion is not affected by such variations.Therefore, there is no direct chemical impact on fuelconsumption.

n -heptane consumption

0.000

0.020

0.040

-180 -120 -60 0 60 120 180

crank angle

massfraction

79.3% N2

81.0% N2

81.5% N2

82.0% N2

82.5% N2

83.0% N2

Figure 5. Mass fraction ofn-heptane vs. crank angle fordifferent nitrogen contents in intake air

Nitrogen Oxides

NOx (NO+NO2) emissions vs. crank angle arepresented in Figure 6. For each case, NOx start to format 4 degrees ATDC. The increase of nitrogen doping

from 79.3% to 83.0% leads to a 23%decrease in NOxemissions.

Nitrogen Oxides

0.000

0.004

0.008

0.012

0 60 120 180

crank angle

massfraction

79.3% N2

81.0% N2

81.5% N2

82.0% N2

82.5% N2

83.0% N2

Figure 6. Mass fraction of nitrogen oxides vs. crankangle for different nitrogen contents in intake air

Decrease in NOx by N2-doping may result from adilution effect and/or from a direct chemical action onthe kinetic mechanism. If N2 behaved as a diluent, adecrease in temperature would be observed, but as seenabove (Figure 3), temperature does not depend upon N2-doping. Hence, according to this work, there is likely nosignificant dilution effect. However, there is an impact

of nitrogen-doping on NOx emissions (Figure 6). It isknown that NOx generation in hydrocarbons combustionmainly sues from a temperature dependant mechanism[8]. But in this study, it has been shown that variationsin NOx emissions appear without changes in mixturetemperature (Figure 3). Therefore, one explanation tonitrogen oxides reduction could be a variation of NOxformation/oxidation reactions. In this work, the mainspecies (> 1 ppm) are NO, NO2 and HONO (Figure 7).Reaction flow and sensitivity analyses were performed

but are not presented herein. Nevertheless, analysesshowed that only NOx sub-mechanism is affected by N2

variation. Reaction channels are the same for all the

cases but the concentrations of nitrogenous species aremodified (Figure 8).


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Figure 7. Distribution of nitrogenous species at 180 degrees ATDC for standard intake air -The green line stands for the detection limit with conventional techniques.

Figure 8. Relative variations of nitrogenous species for intake air composition of 81.0%vol, N2and 83.0%vol, N2 with respect to standard air composition.


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One may observe that the decrease in NO and NO2is compensated by an increase in minority species suchas HNO, HNCO or H2NO. Though some of the speciesare dramatically risen (e.g. H2NO, CN, N2H2), theirconcentrations remain lower than 1 ppm.

LimitationsSo far, the observations above may let think that

intake N2 content can be lowered as much as wanted toreduce NOx emissions without any impact oncombustion efficiency. This is of course false since theintake air has to provide the mixture with enoughoxygen to ensure the fuel combustion. The calculationwas repeated with an intake air composition of 90 %vol,

N2. Maximum reactor pressure and temperaturerespectively decrease from 20.3 bar down to 18.8 barand from 2590 K down to 2330 K. Furthermore, lowercarbon dioxide and higher carbon monoxide levels

indicate that the combustion process has not gone tocompletion. Hence a maximum nitrogen content (or aminimum oxygen content) limit exists. This limitationrange will be further investigated in upcoming work.

Comparison with experimentsExperimental conditions are summarized in Table 2

and Table 3. Experimental CO2 and NOx emissions arereported in Table 6.

Table 6. NOx and CO2 emissions measured for varyingintake air concentration in N2

N2 content in

intake air (%vol)

CO2

(%)

NOx

(ppm)

79.3 8.09 680

81.0 8.25 364

81.5 8.17 287

82.0 8.17 234

82.5 8.18 185

83.0 8.15 164

Measurement uncertainty is 1% of the reading

The same tendencies as in simulation cases areobserved i.e., a neat decrease in NOx emissions andconstant carbon dioxide emissions when increasing

nitrogen-doping. Comparison of simulated andexperimental NOx is presented in Figure 9. The relativecut-off in NOx emissions is larger for experimental data.This difference may be explained by:the composition of real fuel which is not pure n-

heptane but Diesel fuel.the use of an ideal model such as homogenous

reactor which is far from real engine.higher admission temperature in the simulation than

in the experiment. As a matter of fact, initialtemperature was set to 800 K in the simulation toguarantee combustion. Hence, the simulationtemperature along the cycle is likely higher than the

engine temperature. Therefore NOx formation,which is a temperature dependent mechanism, ispromoted [8].

NOx (%N2)/NOx(standard)

0.0

0.2

0.4

0.6

0.8

1.0

78% 79 % 8 0% 81% 82% 83% 84 %

% N2

experiment

simulation

Figure 9. Comparison of the decrease in NOx forexperimental and modelling data - NOx levels arenormalized with respect to NOx levels for standard air

Only one set of engine parameters has been tested inthis study; however, the variation of engine speed,

compression ratio, etc could be interesting toinvestigate.

Conclusions

In this work, experimental and modelling impact ofnitrogen-doping of intake air on fuel combustion andemissions has been investigated.Simulations in reactor with variable volume andpressure and experiments show that increasing N2-doping cuts down NOx emissions but have nosignificant impact on fuel consumption and CO2emissions. However, decrease in NOx emissions islarger in experimental cases than in simulations.

Nitrogen-doping variations do not have any effect onsimulated in-cylinder pressure and temperature. Henceno diluting effect is observed. However NOx emissionssignificantly decrease with increasing N2-doping.Chemical effect has been identified. Nevertheless, thereis a maximum nitrogen concentration that cannot beexceeded to guarantee fuel combustion. This pointneeds to be further investigated. Additionalinvestigation could be also done on:

the use of various surrogate fuels (n-decane/-methylnaphthalene,)

the use of a more realistic engine model (1-D, 3-D codes, )

the variation of engine parameters (enginespeed, compression ratio)


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References

[1] U.S. environmental protection agency, Healthand Environmental Impacts of NOx, Availablefrom:http://www.epa.gov/air/urbanair/nox/hlth.html [last accessed 1.28.09].

[2] CCFA, Air quality and road transportscontribution, Available fromhttp://www.ccfa.fr/IMG/pdf/Qualite_air_GB.pdf[last accessed 1.28.09].

[3] CCFA, The French Automotive Industry,Analysis and Statistics, Available fromhttp://www.ccfa.fr/IMG/pdf/Analyse_Statistiques_2008_GB.pdf[last accessed 1.28.09].

[4] G. Plassat, Les technologies des moteurs devhicules lourds et leurs carburants, Lesvaluations de l'ADEME, 2005, chap. 6, ISBN 2-86817-817-0.

[5] F. Pischinger, H. Schulte, J. Hansen, VDI-Berichte 714, Die Zukunft des Dieselmotors,Dsseldorf , 1988, p61-93.

[6] M. Zheng, G.T. Reader, J.G. Hawley, EnergyConvers Manage 45 (6) (2004) 883-900.

[7] M. Sjberg, J.E. Dec, W. Hwang, SAE paper2007-01-0207 (2007).

[8] H.B. Palmer, D.J. Seery, Annu. Rev. Phys.Chem. 24 (1973) 235-262.

[9] S.S. Ahmed, F. Mauss, G. Morac, T. Zeuch,Chem. Phys. (9) (2007) 11071126.

[10] G. Morac, P. Dagaut, J.F. Roesler, M.Cathonnet, Combust. Flame 145 (2006) 512-520.

[11] DARS, http://www.diganars.com/, [last accessed1.28.09].

[12] P. Dagaut, Formation et rduction des polluantsen combustion, Journe des Doctorants enCombustion, Nov. 30 2004 Available from :http://www.gfcombustion.asso.fr/documents/ec2004-dagaut-2.pdf[last accessed 1.28.09].

[13] J.T. Farrell, N.P. Cernansky, F.L. Dryer, D.G.Friend, C.A. Hergart, C.K. Law, R.M. McDavid,C.J. Mueller, A.K. Patel, and H. Pitsch, SAE

paper 2007-01-2001 (2007).[14] W. Hentschel, K.P. Schindler, O. Haahtela, SAE

paper 941954(1994).

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Simulation of the Impact of N2-Doped Intake