LavoieHeywoodKeck CombustionSciTechnol 1 313 1970

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Combustion Science and Technology, 1970, Vol. 1, pp. 313-326 © GORDON AND BREACH, SCIENCE PuBLISHERS LTD.

Printed in Belfast , N. Ireland

Experimental and Theoretical Study of Nitric Oxide Formationin Internal Combustion Engines

GEORGE A. LAVOLE, JOHN B. HEYWOOD, AND JAMES C. KECK

Massachusetts Institute Technology ambridge Massachusetts

Abstract-The nonequilibrium formation of nitric oxide within the internal combustion engine cylinder is examinedA thermodynamic model which predic ts the propert ies of the burnt and unburnt gases dur ing the combust ionprocess isdeveloped. A set of reactions which govern the formation of nitric oxide isproposed, an d rate equationsfor nitric oxide concentr tions as a function of time in the post flame gases are derived The results of time resolvedmeasurements carried alit on a CF R engine are described, where emitted light intensities at wavelengths selectedto record radiation from the CO 0 and NO 0 continua were used to determine the nitric oxide concentrationThe comparison between theoretical and experimental results for fuel-lean mixtures confirms that the importantfeatures of the model presented are correct.

I INTRODUCTIONIt is now accepted that nitric oxide in the exhaust

gases of combustion devices is one of the pollutantswhich react in the atmosphere to form photochemical smog. The automobile is the largestsingle source of nit ric oxide. It contr ibutes about50 per cent overal l of the total emitted , though inthe Los Angeles bas in , where the smog problem ismost severe, its contribution is significantly higher.The processes which govern the production ofnitric oxide inside the engine cylinder are no t wellunderstood, however. Since legislation which willlimit the amount of nitric oxide which automobileengines can exhaust into the atmosphere is antic ipated in the near future, a more basic understandingof the production mechanism is essential.

Most of the previous work on nitric oxide production has involved correlations of nitric oxideconcentrations measured in the exhaust gases withthe engine operat ing parame te rs such as fuel/airratio, spark advance, engine speed, exhaust manifold pressure (e.g., Huls and Nickol (1967), Hazenand Holiday (1962), Alperstein and Bradow (1966),Wimmer and McReynolds (1961)). From suchstudies a qualitative understanding of the processhas been buil t up. Fo r example, modifications inengine operating conditions which increase the peakcycle temperature, or the oxygen concentration inthe burnt gases increase the nit ric oxide concentra tion in the exhaust. Th e measured nitric oxidelevels are found to be nearer the equil ibrium concentration corresponding to the peak cycle temperature and pressure, than to the equilibriumconcentration at exhaust conditions.

Two previous papers have proposed possible

kinetic schemes to account for this nonequilibriumconcentration. Newhall and Starkman (1967)using a kinetic model based on the Zeldovichmechanism, Zeldovich (1946), wi th 0 an d Nassumed in equilibrium with O 2 an d N 2 , respectively,predicted immediate freezing of the assumedequil ibrium composi tion at the peak tempera tureand pressure for homogeneous burnt gases. Theirtime-resolved spectroscopic measurements at 5.3 flo

gave ni tric oxide concentra tions considerably inexcess of equil ibrium concentra tions at the peakpressure and temperature; their measurements alsoshowed no decompositi on dur ing the expansionstroke. Later Newhal l (1968) with cal cula tionsusing a more detailed kinetic model again predictedthe freezing of nitric oxide at the assumed peakequilibrium concentr at ion . Eyzat and Guibet(1968) considered the forma tion process. Theyassumed nitric oxide was formed by the reactionN 2

O 2 2NO an d carried ou t kinetic calcu

l ations using a perfect gas model for the thermodynamic properties of the unburnt and burnt gases,wi th the burnt gases t aken to be homogeneous.They concluded that in fuel-lean mixtures theformation process was too slow for nitric oxide toreach equilibrium at the peak pressure and temperature. However, the reaction used to follow theformation of nitric oxide does no t proceed directlyand is not the rate determining reaction.

The following questions concerning nitric oxideproduct ion in the engine cylinder still remain unanswered. First ly, what is the s ta te of the combustion products? No account has yet been taken ofnonuniformities in the post-flame gases. I t isknown, Rassweiler and Withrow (1935), that a



314 GEORGE A. LA VOlE, JOHN B. H EYW OO D, A ND J AM ES C. KECK

THERMODYNAMICS OF INTERNALCOMBUSTION

At the t empera ture and d ens it ie s in an i nt er na lcombustion e ngi ne, it is a reasonable first approximation to assume that t he v ol um e of the reactionzone is negligible a nd t ha t the gas within a cyl inderconsists of a burned fraction at thermodynamicequilibrium plus an unburned fraction frozen at itsoriginal composition. If we also assume that thepressure is uniform throughout th e cylinder, thenthe condi tions in t he b ur ne d an d unburned gasesar e determined by the equation of state for theburned gas

pv. = R.T., (I )the equation of s ta te f or t he u nb ur ne d gas,

pv = R T , (2)conservation of mass,

s ub st an ti al t em pe ra tu re g ra di en t ( 1T 200 OK)exists du e to the diffe rent t ime his to ries of gaselements which burn early or late in the combustionprocess. Such temperature differences would bee xpe cte d to re su lt in substantial nonuniformity innitric oxide concentration. Secondly, what reactionsmust be inc luded in the nitr ic oxide kinet ic schemeto f ol lo w b ot h f or ma ti on an d decomposition? Arelated question is whether nitric oxide formationwithin th e flame front is important compared withth e rate-limited processes occurring in th e postflame g as es ? T he t hi rd q ue st io n c on ce rn s experimental verification of the theoret ical model . S incenitric oxide formation is a nonequilibrium process,th c details can only be confirmed with time-resolvedmeasurements of the nitric oxide concentrationwithin the engine cylinder.

In this study, w hi ch is d ir ec te d at these threep ro bl em a re as , o ur ap pr oa ch is based on th efollowing assumption. In genera l, the rates of th eencrgy-producing reactions ina flame ar e sufficientlyfast so that th e burned gases ar e c lose to thermodynamic equilibrium. Th is pe rm it s us, in ou rtheoretical model, to separate the problem of nitricoxide f or ma ti on i nt o t wo parts: Th e first involvesthe determination of th e temperature, density, andconccntration of major species in t he b ur ne d gasassuming that th e carbon-oxygen-hydrogen systemis in e qu il ib ri um ; t he second involves integration ofthe reaction rate equations for nitric oxide in thepreviously determined environment.

6

9= U:T dX )/

Mx=

where

( I I )

and

where cv. , C v,, ' hI ' an d h/ are cons tants obtained byfitting straight lines to the specific energy versustemperature curves.

Substi tuting equations I , 2 , 5 an d 6 into 3 an d 4 , we obtain

pV M = xR. f . I - x)R f (7)

R f . = R T (p V - MR f,,) Mx, (12)

where y = c. c is th e specific heat r at io , and thesubscript 0 refers to an y convenient reference state

an d

an d

(E - W - Q) M = x(c .f. h/.) I - x

x (c,. T + h/,,) (8)

(E - W - Q) M = febdx ' Le dx', (4)

where P is the pressure, v is th e specific volume, Ris t he g as constant, T is th e temperature, V is thecylinder volume, M is th e massof gas in th e cylinder,x is the mass fraction burned, E is the to ta l energy,Wis the work done on the piston, Q is the heat loss,e is th e specific internal energy, an d the subscr iptsband u refer to burned a nd u nb ur n ed gas, respectively.

In g en er al equations 3 an d 4 form a se t ofcoupled integral equations which cannot be solvedanalytically. However, over the temperature rangesof interest, it is a reasonable approximation toassume constant specific heats for both b ur n ed a ndunburned gases. Under these conditions

e. = cv.T. h/., 5

an d

7 = LT dX 1- x 10

a re the mean temperatures of t he b ur ne d an d unburned gases. Equations 7 an d 8 may now besolved to obtain

pV - PoVo (y - I)(W Q+ (y . - y )Mc,,,.(f,, - To)

and conservation of energy,

3IM = Cv.dx ' r v dx',



EXPERIMENT L N D T H EO RE TI L S T UD Y OF NITRI OXIDE FORM TION 3t5

prior to ,ignition for which x = Note that bydefinition

Tb(x , x)/0,(X ) = [p(x)/p(x )] l ,-1I / 1 (16)

where Tb(x , x) is the tempera ture of the elementwhich burned at the pre ssu re p x ), when thepressure is p x); and

Tb(x ) = [hl « hl b Cp T,,(x )]/c pb (17)

is the temperature resul ting f rom the isentha lp iccombustion of the gas at the pressure p x ).Equations (16) and (17) together with I) , I I) and(15) then give the state of the burned gas. This casecorresponds to zero heat conductivity in the burnedgas, an d no mixing. This unmixed case represents the opposite limit to the fully mixed case, andin view of well-known measurements of the temperature g radi en t existing in the burned gases, isprobably closer to reality.

Th e above equations have been programmed forsolution on a digital computer. In addition,computer programs have been developed to calculate the required th ermod yn amic properties for

FIG. J. Measured pressure, p, and calculatedmass fraction burned as functions of crankangle Conditions are equivalence ratio 0 9engine speed 1200rpm, residual fraction 0.05.

x

both equilibrium and frozen mixtures of fuel, air,and co mb ustion produ cts c on tain ing up to fiftyspecies.

Some examples of results for the CFR engineused in the exper iments d iscussed in Section V aresho wn in F igu re s 1 and 2. Figure 1 shows a measured pressure curve and the cor re sp onding massfraction burn ed as fun ction s of crank angle

Engine conditions were equivalence ratio, = 0.9,engine speed N = 1200 rpm an d residual fractione = 0.05. Measurements of oxygen concentrationin the exhaust confirm that the charge was no tcomple te ly burned. As described in Section V(i) ,the specia l cyl inder head was des igned for ease ofoptical measurements, an d as a result high com-bustion efficiency has been sacrificed. The decay inx a fte r the maximum results from the constant

assumption, with the value of chosen to matchequilibrium data over a range of temperatures.

Figure 2 illus trates the difference between the mixed and unmixed models. shows, as afunct ion of x the mass fraction burned: i the

(14)

(13)

Iv

w = pc/V .1 0

If we now assume the unburned gas is ini tial lyuniform and undergoes isentropic compression then

T,,/To = (p/Poy ,,-11/1 . (15)

This equation together with equat ions II ) and(12) enables one to determine both x and T, fromthe thermodynamic properties of the burned an dunburned gases an d measured values of p, V, Mand We will take Q to be zero in the subsequentanalysis.

To proceed further, we must make some assumptions about condi tions in the burned gas, andwe s hall consider two limiting cases. In the firstcase, we assume the temperature of the burned gasis uniform so that T, = Tb • The state of the burnedgas is then given by equat ions I) , II) , (12) an d(15). This case corresponds to infinite heatconductivity in the burned gas. We will alsoassume the b urned gas co mpo sition is uniform,which corr esponds to a fully mixed model andrepresents one limiting condi tion. In the sec ondcase, we assume that no mixing of the burnedgases occurs, and each gas element is isentropicallycompressed after combustion so that

and



316 GEORGE A. LAVOIE, JOHN B. H EY WO OD , A ND J AM ES C. KECK

(2) N + O 2 NO + 0 + 31.8;k

2 = 2 X 1O - l I e - 7. /II1

I d[N]V V dt

= -fJ aR, + R 2 + Ra) + R, + a R 2 + Ra ) 19

I d[ N ]V

V dt

= y R . + R 5 + R G+ R. + R 5 ry,2RG• 20)

R, is the one-way equil ibrium rate of the ithreaction (e.g., R, = kdNJ.[NO]. with subscript edenoting equilibrium concentration); and a =[NO]/[NO]., = [N]/[N]., and y = [N ] f ]N ] ..i.e., the concentration of NO , N a n d N dividedby their equilibrium concentration. Vis the volume

• his assumption may not be satisfactory towards theend of the expansion s troke. However, we shall show tha tnitric oxide levels freeze early in the expansion stroke so theerror will not be significant.

(3) N + OH ~ NO + H + 39.4;

k 3 = 7X

10 -11

(4) H + N 2 ~ N 2+ OH + 62.4;k. = 5 X 10- 11 e- IO·8 / 1/7

1 d[NO]V

- = -a fJR, + R 2 + R3 + 2aR G V dt+ R, + f R 2 + R3 + 2y R G 18)

(5) 0 + N 2 ~ N 2+ O 2+ 79.2;k

5= 6 X 1O-lIe-24.o/wr

(6) 0 + N 2 ~ NO + NO + 36.4;kG = 8 X 10- 11 e- 24.0/ 111

where the exotherrnicities of the reactions are givenin kcal, the exothermic rate constants k , are givenin ern sec I , and the activation energies are given inkcal. Th e rate constants are taken from Schofield(1967) for reactions I, 2, 4, 5 and 6, and fromCampbell (1968) for reaction 3. Note that we haveno t included the reaction 2N O ~ N 2 + O 2 which isvery slow and does no t proceed directly.

n the present investigation, we are consideringonly the processes occurring in the burned gasbehind the reac tion zone. The rates of the energyproducing reactions in a flame are sufficiently fastso that the burned gases are close to thermodynamicequilibrium. We maythereforeassumeequilibriumconcentrations for 0 , O 2 , H, OH and N 2 • Therate equat ions for the remaining species NO , N,and N can then be written in the form:

1.0 3 ~ ~ ;:o ;; ----;;:- ;;;,....-_--;-;::

o

2900

KINETICS OF NITRIC OXIDE FORMATION

FIG. 2. Calculated temperature distributions in theburned gas for mixed and unmixed models: (i)Th.emean t empe ra tu re of the b urned gas, 1 , x); (u)the tempera ture of the element burned at x, T. x);(iii) the tempera ture of the first e lement burned,T, O, x) ; (iv) the temperature distribution for theunmixed case when half the charge is burned,T, x, 0.5).

mean temperature of the bu rned gas, f. x ; (ii)the temperature of the element burned at x, T. x);(iii) the temperature of the first element burnedT O x ; and (iv) the temperature distribution forthe unmixed case at a time when half the gas isburned, T,, x,0.5 . Note that the unmixed modelpredicts that the temperature of the first elementburned is substantially higher than the meantemperature and tha t when the charge isfully burnt,a temperature difference of about 400 oK existsacross the post-flame gases.

The process of nitric oxide formation in variousgases has been extensively studied in shock tubes,flow systems, and flames, and rate constants for theimportant reac tions have been determined. Thiswork indicates that, under conditions prevailing ininternal combust ion engines, the process will begoverned primarily by the following reactions:

I) N + NO ~ N2+ 0 + 75.0;k, = 2 X 10- 11

2700::>

«a:w0-:: :w



EXPERfMENTAL AN D THEORETICAL S TU DY O F NITRIC OXIDE FORMATION 317

TABLE II

ratio of R to R 1 • Values of R /R 1 are given inTable I can be seen that, except for leanmixtures at low tempera tures, most of the nit ricoxide formation will occur via the extended Zeldovich mechanism. During the removal process,however, when 0( becomes very large, the N. Omechanism may become dominant. On the timescale of interest, this has relatively little effect sinceboth mechanisms are slow and effectively frozen.The N. O mechanism does become important innitric oxide formation in furnaces where lowertemperatures and excess ai r are the norm. React ion(3) affects the value of K, ; in rich mixtures thevalue of K 1 can be significantly reduced.

IV. CALCULATION OF NITRIC OXIDE CONCENTRATIONS IN TH E BURNED GAS

Equations (18)-(20) and equation (21) have beenprogrammed for integration on a digital computer.The required equilibrium reaction rates, equilibriumcompositions, and thermodynamic propert ies areobtained from a thermodynamic equilibriumcomputer program. The results obtained for

internal combustion engine conditions confirm thatthe steady-state approximation is valid, an d henceonly a so lut ion to equa tion (21) is required. Theintegration of equation (21) is linked with thethermodynamic model of the combustion processin the following way. We will consider only theunmixed case.

The path followed by each element burnt isshown in Figure 3. Consider the element that burnsat Pi For P < Pi it is compressed along the isentrope Su to state Pi hi). After isenthalpic combustion its entropy is SOl and since no mixingoccurs, the element is further compressed along theisentrope Sb . Element} which burns at a later time

is compressed along the isent rope SOl after combustion. Du e to the difference in specific heats ofburnt and un burnt gases, it has a lower temperaturethan element i. Thus given Pi and i and p as afunction of time, So, is computed, and the statetime history of the element obtained. Equation (21)can now be integrated along SOl

of the burned gas: For the unmixed case V is thevolume of a small gas element of fixed mass ; forthe fully mixed case V is the total volume of burnedgas in the cylinder, and its mass increases with time.

Rough estimates of the magni tude of the termsin equat ions (18), (19) an d 20) indicate that therel axation times for equa tions (19) and 20) areseveral orders of magnitude shorter than that ofequation (18). I t is therefore an excellent approximation to assume steady-state concentrations for Nand N 2 and set the right-hand sides of equations(19) and (20) equal to zero. We can then eliminate and y from equat ion (18), and the equation fornitric oxide formation becomes

d[NO V = 2(1 - 0(2) 1 R, -----. ....-)V dt O K 1 K 2

(21)where K 1 = R / R 2 R 3 ) and K, = R / R R.).Note that we have no t assumed N is in equil ibriumwith N 2 through the dissociat ion react ion N 2

M N N M. This reaction is too slow tobring N into equilibrium at the pressures andtemperatures of interest.

The first term in the large brackets on the rightside of equat ion (21) is the resul t of react ions (1)(3). corresponds to the Zeldovich mechanismextended by adding thereaction between Nand OH .Th e second term is the result of reactions (4)-(6)which involve N 2 as an intermediary. is thisterm which under some conditions is kineticallyequivalent to the 2N O N 2 O 2 reaction.

Values of K 1 and K, are given in Table 1 for arange of temperatures and equivalence ratios. Ingeneral K 1 an d K 2 are of order unity or less, andthus for small 0( the relative importance of the twomechanisms just described is determined by the

TABLE I

(a) Values of K,1>= 0.8 1.0 1.2

3000 OK 0.3 0.3 0.22500 0.4 0.3 0.12000 0.4 0.4 0.06

1500 0.1 0.8 0.02(b) Values of K,

1>= 0.8 1.0 1.2T = 3000 oK 0.3 0.2 5 X 10-

2500 0.4 8 x 10- 5 X 10 - 3

2000 0.5 3 x 10- 7 x 10-1500 0.8 5 x 10- 4 X 10-

Values of R,/R,

3000 K250010001500

1>= 0.84 X 10-9 X 10-5 X 10- 1

6

1.0I X 10-3 X 10-5 X 10-I X 10-

1.2I X 10-4 X 10-9 X 10-5 X 10-



318 GEORGE A L VOIE JOHN B HEYWOOD ND J MES C. KECK

T

Pi

~_

I II I: II II II I

~ -

UNBURNEDGAS

Typical results for nitric oxide mass fractions inthe burnt gas for two elements which burn atdifferent times are shown in Figure 4. The equivalence rat io is 0.9, the engine speed 1200 rpm theresidual fraction is 0.05. 0 = 3 is the first partof the charge to burn; at 0 = 100

, about onethird of the charge is burnt. Equilibrium nitricoxide concent ra tions correspond ing to the localtemperature and pressure are also shown. Note thatfreezing occurs in the rate-controlled solutions earlyin the expansion stroke. Whetherthe rate-controlledsolution reaches nitric oxide concentrations equalto the equili brium value at the peak temperature e.g., the 0 = _30 solut ion) will depend on thetemperatures attained and the engine speed.

The average exhaust nitric oxide concentration isobtained from the integrali Pj P

Sbi

b j

~ ~ ~ R ~ N EI I S

I II II I Su = const

I II II

hi

FIG. 3. I llus trat ion of calcula tion procedure fornitric oxide concentration in unmixed case on

pressure-temperature-enthalpy plot. 22)

FIG 4. Nitric oxide mass fractions as a function oftime in the burnt gas for two elements which burnat different times. 0 = _30 0 is the fi rst par t of thecha rg e to bur n; at 0 = 100

, one third of the chargeis burnt . Equivalence ratio is 0.9, engine speed is1200 rpm, residual fraction is 0.05.

o

zWNo0:1L

FIG Frozen nitric oxide mass fraction as afunction of value of at which e lement burned

lntegrated value gives exhaust concentration.

1 . 0 r . . . .

o

where {NO}, denotes the frozen nit ric oxide massfraction of the element which burned at x Figure 5shows {NO} as a function of x for r = 0.9. Theexhaust concentration is predicted to be about4000 ppm. N ot e t ha t the first part burned has sixtimes the frozen nitric oxide concentra tion of thelast part burned.

ms

0.4

zo

u«0:L



EXPERIMENT L N D THEORETI L S T U D Y O F N I TR I O X ID E F OR M T I ON 319

V. TIME RESOLVED MEASUREMENTS OFN IT RI C O XI DE C ON CE NT RATI ON

I Method and Experimental Equipment

To investiga te n it ric oxide concent ra tions in anoperating engine cylinder, we have employed timeresolved spectroscopic techniques. Briefly, themethod involves the simultaneous measurement oflight emission intensities at two wavelengths characteristic of the two recombination continua: CO + > CO 2 + an d NO + 0 > NO. + hv Theintensities are

leo = Kt( T)[CO][O] and I ~ n= K2 T [ N O ] [ 0 ]

where the K s are known functions of temperaturean d wavelength; Hartunian et al (1966), Myersand Bartle (1967), Fontijn et al (1964). Using theassumption of equilibrium concentrations of speciesin the C O H system, described in Section II , themeasurement of le o and the pressure p allows thedetermination of temperature T carbon monoxideconcentration [CO], and oxygen atom concent rat ion [0]. The measurement of I ~ othen gives thenitric oxide concentration [NO].

The experiments were conducted on a singlecyl inder, L-head, C.F.R. engine. A speciallydesigned head allowed light emission measurements

to be made through a number of small quartzwindows at different positions along the flame path.The engine head and combustion chamber areshown in Figure 6. Spark plug, window locat ions ,an d p or ts f or the Li Strain Gauge pressure transducer are indicated . The engine compression rat iowas 4.9, and the e xhaust was pumped down to apressure of 6 psia to im prove the operating characteristics. Unleaded fuel was used.

The optical set up is shown in Figure 7. Radiation is emitted from a small volume of gas encompassed by a cone of l ight rays passing througha 2 X 2 mm source area in the center of the combustion chamber. The light cone is split up by afour-part mirror M 2 , into four quadrants each ofwhich is brought to a s ep ar at e focus by M 3 and M•.Three of the channels were monitored with photo-multiplier-interference filter pairs ; the channels at0.38 I and 0.61 had a band pass of 100 A whilethe channel at 0.75 I had a band pass of 300 A.The fourth channel was monitored with a Jar re llAsh model 82-000 monochromator and photomultiplier. The band pass could be varied from1.6 A to 22 A depending on the slit width. Theent ire opt ical sys tem was mounted in a solid f ramewhich could be moved on rails to any window position. The entire optical system including the actualwindow used was cal ibra ted before and after each

ENGINE HE DAN D

COM USTION

CH M ERTOP

VIEw

: E X HA U S TI ,I ,

/ G J ~ - W -

- - ; ® I = l / P , I V / tyj I

A I \ / \: J \ AL I \1 1 0 8 8 - 8 8 : _Jf- - 1 / - I \,2 3 4

\ C \INL T \ V \ I J C \

--_\_ r ;;.../ @ ~~ - -- _ :

S SPARK PLUGW WINDOWP PRESSURE GAUGEv VALVE

SECTIONA A I

, , I

- -- I I I

INLET j- - 1 1

I

PISTON

TOP OF STROKE

FIG. 6. Engine head and combustion chamber design for L.head C.F.R. Engine.



O P T I C A L T R A I NSCHEMATIC



322 GEORGE A L VOIE JOHN B HEYWOOD N D J M ES C KECK

10 10 . . . . . .

®

10 9 «I L t

d d n d

®®• r s

s cm 3

10 8@

@10 7

¢ 1.2 ¢ • 0 .9

FIG. II . Spectral intensity observed at three consecutive times during the cycle, before, at an d a ft er peak pressureas shown the corresponding circled numbers on the pressure time schematic

as can be seen from the photographic spectraFigure 8, the radia tion is free from discre te contr ibutions an d should representtheCO Ocontinuum.If this is so the radiation can be used to give anins tantaneous tempera ture as out lined in SectionV I ). The temperatures determined in this waywere compared with those calculated using the unmixed model of Section and the known pressurecurves. The two temperatures agreed absolutely towithin the accuracy of the radia tion constant andthe experimental residual gas fraction. Themeasured temperatures. showed the expected nearlyisentropic compress ion and expansion as well as atemperature difference of , ,50 K between the twowindows.

In the second experiment, nitric oxide was addedto the airflow in known amounts and the change inrad iat ion was measured. The extra rad ia tion was

proportional to the added nitric oxide an d had thesame shape as that observed when no nit ric oxide isadded. The effect of adding nitric oxide to theairflow is shown in Figure 12 where data recordsare displayed for two cycles, similar in every respectexcept for the amount of nitric oxide added. Thetraces of radiation intensity at 0.38 are effectivelyalike, but the traces at 0.61 /l- and 0.68 u for nitricoxide added show a much larger jump in radiationat the f ront than for no nitric oxide added.

The radiation intensities occurring just after thereaction zone for a number of data records areplotted against per cent nitric oxide added inFigure 13. Fo r clari ty, only the data points for 54 are shown. The straight lines are leastsquares fits to data with similar scatter. The figureshows that for all practical purposes the radiation at 38 u and hence the temperature after the reaction



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f i \ df,75 ku8acd am al~e ptrI3li4hcd r;iich ~i.~~jil h ~eq~xilikri8un-i mhia of P ) 10 NO, for thi5 teanllac~atairc, 3 3 1 ~ 6 : fSJ'= II.4 fit% he i.iitt>i ,E~ae~~ertnd i

rmog irm scriotas dii9ia~f-c.eramci1t c,g-ahidcring the t2r.n-

cert;llncEes irmt f~e thapc.tand $he ~-;ldi;atioi~ *ca~-ast2xfif\re1atrs.e to tme ; i~~ oil i(e r, he tnDuc o f f ' = 0-4 axi i \

:mot uh8aericd it) tar:, to tx itkin the experinaenr;i$era-or, th n,~ lgltu~a t he rringe of casx~ itic%a~> sx-c~~xa~ntered tiah: cupcrgsa-nent [rich a i d 1caa-a- ia=rricoxis%e dtied ;; 9~i floe ;tdsled)arld hcea~ ;thaera at.

he c0~14f;azat ii%r lat:a redirctir' ln,

i n prin-ocipiie, ~irn~iltralaee.an~, ecorsli ns csf E%fc%\h$fC:uad light i~~ne~-a.iitic..aPI a giws I P C ; ~ ~ ~ L , E B'r0~31% 5 0

~ ~ a % c i e r ~ g t k ~cjarc"ac:~$X~mgiite O and F; T

ctslatgn-aua, yiie1J1 ihs: tcga-nper;attlre, C91 C), am4 S(1cotmccs-mtra1 tam ;lip O ~ E ~ ~ ~ X P C L ~t % Sect tfa 'Q 1 . Is2

pt-actice, ~ne,.asbsre~lsensh 'TQP~P;I gi~ela iz~sfe ~c fe



324 GEORGE A L VOIE JOHN B HEYWOOD ND J MES C. KECK

d I n e x

o

FIG. 13. Radia tion intensi ty observed jus t afterthe reac tion zone at var ious wavelengths plo ttedagainst percent NO added. For clarity only pointsfor 0.54 It are shown.

show a great deal of scatter du e to the fact tha t theoptical paths for the four channels are not coincident on a scale of mm s, and on this scale there aresignificant turbulent fluctuations in temperature.As result it was necessary to average data from anumber of selected cycles.

Because of the cycle to cycle variation in pressure,burning velocity, and the resulting exponentialcycle to cycle light intens ity var ia tion , a s traightforward random selection of cycles was unsuitable.Instead cycles were chosen on the basis of peakpressure, selecting only those cycles with peakpressure within ± 10 psi of the chosen pressure.The resulting flame arrival times were within 5crank-angle degrees of one ano ther, and the lightintensities at a given wavelength for the rest of thecycle excluding the flame reaction zone were withina ±30 per cent band on either side of a smooth linedrawn through the data. Average intensity at 0.38IJ is used to represent the C O O continuum, andthe average intensity at 0.61 l after correction forthe underlying C O O continuum and mul tipl icat ion by f = 0.4 for the presence of NO z, is usedto represent the N O O continuum. Then as outlined in Section V I) values of temperature, CO

an d NO can be calculated as a function of timeduring the cycle.

4 Results

The results of such calculations are given inFigures 15 and 16 where experimental nitric oxideconcentra tions are shown versus time and crankangle for lean conditions = 0.9. The error barsare ±30 per cen t corr espond ing to the scat ter inaveraging and F indicates flame arrival time. Displayed also are the rate-limited computer solutionsfor parts of the charge that burned early and late inthe cycle.

Figure 15 shows a compari son between nitricoxide concen tra tions observed at two differentwindows, W z being closer to the spark than W 3 andshows higher nitric oxide due to higher temperatures. Both curves exhibit the expected nonequilibrium behavior and fall within the predictedlimits. No significant amount of nitric oxide isproduced at the flame front under these conditions.

In Figure 16 measured nit ric oxide is plot ted forbot h 0 per ce nt and 1.0 per cent added nitric oxideas seen at W a The temperature and reaction rates

s m

FIG. 14. Incremental intensities per mole fractionof added NO plotted against wavelength comparedwith theore tica l shapes. The solid curves are theshapes or the chemiluminescence radiation f 1.0,and thermal radiation f = 0 where f is the fractionof total radia tion at 0.61 It due to the chemiluminescent component



E X PE R IM E NTA L A N D THEORETICAL S T U D Y O N IT Ri C O XI DE F OR MATI ON 325

- 3 0 2 0 1 01.5

R N K N G L E

o 10 20 30 4 0 50 6 0

L T E

10

RATE CONTROLLEDSOLUTION

. : . EXPERIMENT

WINDOW W 2

WINDOW W 3

/

,/ ~ E R L Y/ .... < ~ U R N

/

/1 . F F3 •• '---

1 0

0 5

NO

T I M E MS

FIG 15. Exper imenta l NO concent ra tions observed at two windows. W is closer to the sp ar k than W Dolledcurves are computer solutions for the kinetic NO behavior of early and late burned gas, 4> = 9

R N K N G L E

o 10 2 0 3 0 4 0 50 6 030 - 2 0 1 0I. 5 ~ . . . _ _ _ _ _ _ _ _ _ _ r _ _ _ _

15

F······5

RATE CONTROLLED / - , E R LY

SOLUTION / B U R N E D- - - - - 1.0 NO ADDED /

/ ~ 0 0 NO ADDED ~

F

//

//

//

//;

o

1.0

0 5

N O

TIME MS

FIG 16. Experimental NO concentrations at W with and without NO added. Kinet ic solut ions are shown, ' = 0.9.



326 GEORGE A LA VOlE JOHN B HEYWOOD ND J MES KECK

are low enough so that not much nitric oxide isproduced in the a per cent added case or decomposed in the 1.0 per cent added case.

At present only the data for lean conditions has

been analysed, but there do not appea r to be anyqualitative differences for rich conditions, and workis progressing on their analysis.

VI. CONCLUSIONS

The time-resolved experimental measurementsand the data reduction described in this paper confirm that these features should be included in modelsused to predict the nitric oxide concentrations foundin spark ignition engines.

I. Both the formation and decomposition ofnitric oxide in the post-flame gasesare rate limited atthe condi tions under which I.C. engines normally

operate.2. At least in the lean case, the nitric oxideformed in the flame front is negligible.

3. The state of the burned gas is not uniform; asubstantial temperature gradient, and hence nitricoxide concentr ation gradient exists behind thename zone.

This work l s supported in part by the M I TUrban Systems Laboratory from the Ford UrbanGrant by the Sloan Basic Research Fund and bythe Ford Foundation Fund

REFERENCES

Alperstein, M. and Bradow, R. L., 1966, Exhaust EmissionsRelated to Engine Combustion Reactions, pa pe rpresented at Fuels and Lubricants Meeting, SAEpaper 660781.

Campbell, I M. and Th ru sh, B. A., 1968, Reactivity ofHydrogen to Atomic Ni tro gen and Atomic Oxygen,Trails Faraday Sac 64, Part 5, 1265-1274.

Eyzat , P. and Guibet, J. C.; 1968, A New Look at Nit rogenOxides Formation in Internal Combust ion Engines

SAE paper 680124, presented at Automotive Engineering Congress.

Fontijn, A., Meyer, C. F. and Schiff, H. I 1964, AbsoluteQuantum Yield Measurements of the N O - O Reactionand Its Use as a Standard for Chemiluminescent

Reactions, Chem Phys 40, 64.Hartunian, R. A., Thompson, W. P. and Hewitt, E. W.,1966, Glow-Discharge Shock Tube for StudyingChemiluminescent, Surface-Catalytic, and Gas-PhaseReaction Rates; Temperature Dependence of N O - Oand C O - O Chemiluminescence, Chem Phys 441765.

Hazen, D. F. and Holiday, G. W., 1962, The Effects ofEngine Ope ra ti ng and Design Vari ables on Exhaus tEmissions, SAE paper 486C presented at SAE NationalAutomobile Week.

Huls, T. A. and Nickol , H. A., 1967, Influence of EngineVariable s on Exhaus t Oxides of Nitrogen Concent ra ti ons from a Mul ti -cyl inde r Engine , SAE paper670482 presented Mid-year Meeting.

Levitt, B. P. , 1965, Tempera ture and Wavelength Dependence of the Thermal Emission and O - N O Re

combination Spectra of NO Chem Phys 421038.

Newhall, H. K., 1968, Kinetics of Engine-GeneratedNit rogen Oxides and Carbon Monoxide, 12th Symposium Inte rna tiona l) on C om bu st io n, The C ombustion Institute.

Newha ll , H. K. and Starkman E. S. , 1967, Direc t Spect roscopic Determination of Nitric Oxide in ReciprocatingEngine Cycles, Paper 670122 presented at SAE Automotive Engineering Congress.

Myers, B. F. and Bartle , E. R., 1967, Shock-Tube Study ofthe Radiative Processes in Systems Containing AtomicOxygen and Carbon Monoxide at High Tempera ture , Chem Phys 47, 1783.

Rassweiler, G. M. and Withrow, L., 1935, Flame Temperatures Vary with Knock and Combustion ChamberPosition, SAE Trans.. 125-133.

Schofield, K., 1967, An Evaluation of Kinetic Rate Data forReactions Neutrals tmospheric Interest lanet

Space Sci 15, 643-670.Wimmer, D. B. and McReynolds, L. A., 1961, Nit rogen

Oxides and Engine Combustion, Paper 380E presentedat SAE Summer Meeting.

Zeldovich, Y. B.; 1946, The Oxidation of Nitrogen inCombustion Explosions, Acla Physicochimica U S S R21. 577-628.

LavoieHeywoodKeck CombustionSciTechnol 1 313 1970

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