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A critical test for models of the nitric oxide formation process in spark-ignition engines
A CRITICAL TEST FOR M O D E L S OF THE NITRIC OXIDE F O R M A T I O N P R O C E S S IN S P A R K - I G N I T I O N E N G I N E S WILLIAM R. AIMAN
General Motors Research Laboratories, Warren, Michigan An entirely satisfactory model for the formation of nitric oxide (NO) in spark-ignition engines has not yet been developed. To further elucidate the NO-formation process, various quantities of pure NO were added to the intake air of a single-cylinder engine at five different air-fuel ratios. The experimental results contradict the predictions of the equilibrium-at-peak-cycle-temperature model and also the predictions of the formation-rate-controlling model. Models in which formation and decomposition processes are both important may explain the results of these experiments. The formation and decomposition processes seem to be of equal kinetic importance at the air-fuel ratio corresponding approximately to peak exhaust NO concentration (93% of stoichiometric fuel). The formation process becomes more important at leaner mixtures, and the decomposition process becomes more important at richer mixtures. In addition, a change in chemical mechanism is suggested at the stoiehiometric air-fuel ratio. equilibrium at peak cycle temperature, but does not decompose as the temperature decreases on the expansion stroke. The formation-rate-controlling (FRC) models assume that equilibrium is not attained, and that the NO concentration is controlled by the rate at which NO can form. Either the direct molecular reaction, 7
Introduction
Nitric oxide emission from spark-ignition engines is affected to a certain extent by almost every engine parameter, but air-fuel ratio, spark timing, and charge dilution are the parameters of primary importance2 -a (Charge dilution is the fraction of a given cylinder charge which is not fresh air-fuel mixture, i.e., residual exhaust, recirculated exhaust, humidity, or other diluents.) These three parameters probably affect the exhaust NO concentration by: (1) changing the oxygen concentration in the burned gases, (2) changing the temperature of the burned gases, and (3) changing the time available for reaction. Nitrogen concentration is unimportant, kinetically, because it is an excess reactant. 4 A number of contradictory models have been proposed to describe the NO-formation process. Among the simple models proposed are an equilibrium-limitation model, 5 and several formation-rate-controlling models.6-s The possibility that decomposition reactions during the expansion process may control the over-all process has also been considered? The equilibrium-limitation modeP ~-also called the equilibrium-at-peak-cycle-temperature (EPCT) model] assumes that NO attains
N 2 + 0 2 = 2 NO, or the Zel'dovitch mechanism, s N2-}-O= N-t-NO, N+02= NO+O, has been used as the rate-limiting step. (The reverse reactions were assumed to be negligible.) The possibility that NO emission is controlled by the rate of the NO decomposition during the expansion process has also been suggested? A decomposition-rate-controlling (DRC) model would assume NO equilibrium at peak cycle temperature, as in the E P C T model. However, during the expansion process, the concentration would decrease b y a rate-limited process. The NO decomposition would stop (or "freeze") when the
861
POLLUTANT FORMATION AND DESTRUCTION IN FLAMES
862
z 0
~ 5
I FRCMODEL
(.~ z 0 C, z
AIF: 13
DRCANDEPCT MODELS
INTAKE NO CONCENTRATION
FIG. 1. Predicted NO-addition lines. The predicted response of exhaust NO concentration to the addition of NO to the intake per the FRC, DRC, and EPCT formation models. Two all-fuel ratios are represented, which causes the different intercepts on the ordinate.
temperature falls sufficiently. This sequence has been described as part of more-complex models.l°-12 All of these simple NO-formation models predict more or less the same results for experiments where the normal engine parameters (air-fuel ratio, spark timing, etc.) are varied. Thus, the results of such experiments are not very useful in distinguishing between NO-formation models. The models do predict different results for an experiment in which various quantities of NO are added to the intake air, and the exhaust NO is measured. Because different results are predicted, this experiment can provide a critical test for NO-formation models. If the formation rate controls the over-all process (FRC model) then the amount of NO formed by the reaction would just add to the amount present in the fresh charge. A plot of exhaust NO concentration versus intake NO concentration would be a straight line with unity slope. (If the rate-controlling reaction were auto-catalytic or auto-inhibited, curved lines would result. In auto-catalytic and auto-inhibited reactions, the concentration of the product influences the rate of the reaction. The rate is increased in auto-catalytic and decreased in auto-inhibited reactions.) If the decomposition rate controls the over-all process (DRC model), then there must be a stage where the NO concentration is very near
equilibrium2 This equilibrium stage would erase ally influence of NO in the intake air, and a plot of exhaust NO versus intake NO would have zero slope. The EPCT model would also imply a zero-slope plot for identical reasons. Figure 1 shows the expected NO-addition lilies for two air-fuel ratios, as predicted by the FRC models or the EPCT and DRC models. The variations in the intercepts on the ordinate are due to the ordinary variation in exhaust NO with air-fuel ratio. In the experiment to be described, small quantities (up to 1.5%) of NO were added to the intake air, while exhaust NO was analyzed. At five air-fuel ratios (constant air flow, spark timing, and engine speed), exhaust NO concentration was measured as a function of intake NO concentration. The results of this experiment provide a critical test for evaluating NO formation models. The test is applied to the FRC, DRC, and EPCT models and its application to more complex models m-12 is discussed.
Apparatus An A S T M - C F R , single-cylinder engine, with a disk-shaped combustion chamber and a shrouded intake valve, was used. Engine air came from a critical-flow system as shown in Fig. 2. The humidity of this air was about 23 gr/ lb.13 A timed fuel-injection system delivered fuel into the intake manifold. The exhaust pipe was 8-in. long, and ended in an expansion tank. The exhaust pressure in the expansion tank was I in. Hg. NDIR instruments measured llitric oxide and carbon monoxide concentrations, and a polarographic analyzer measured oxygen concentration. This instrumentation was used to determine the exhaust concentrations and the NO
TABLE I Engine conditions Engine speed Air flow Spark timing Fuel Compression ratio Measured charge dilution Intake air temp. Coolant and oil temp. Exhaust pressure
1500 rpm 0.400 lb/min 15° BTC Indolene 30 (C~H2~, MW-95) 8:1 9.8 vol. % J26°F 188°F 1 in. Hg
NITRIC OXIDE FORMATION PROCESS
863
DRIERS
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NITR I COX I DE METERING VALVE
_.41=--
INTAKE SAMPLE LINE
OXIDE
SUPPLY
X EXPANSION TANK MIXING TANK ICE BATH
\ SURGE TANK
EXHAUST 7 " SAMPLING LINE EXHAUST
REGULATED -~HIGH-PRESSURE AIR SUPPLY
CRITICAL FLOW ORIFICE
- EXHAUST BACK-PRESSURE CONTROL VALVE
FIG. 2. Experimental apparatus. TABLE II Results of adding nitric oxide to engine intake Air-fuel ratio
Fuel equiv. ratio
Intake NO
Exhaust NO (wet basis)
13.1
1.13
0 2,050 6,100 10,000 13,100
1,100 1,650 2,400 3,100 3,750
14.0
1.06
0 2,200 5,800 9,200
1,950 2 550 3 650 4,600
14.8
1.00
0 1,750 5,650 8,450 11,600 14,800
2 3 4 5 6 8
600 200 650 700 850 350
15.8
0.934
0 2,100 5,600 8,250 10,500
2,750 4,200 6,250 7,500 8,100
17.5
0.843
0 2,250 6,550 9,400
9OO 2,80O 6,450 8,650
concentration in the intake air (see Fig. 2). The measured intake NO concentrations were corrected for oxidation to N 0 : (see Appendix I). Air-fuel ratio was determined from the exhaust analysis}4 All instruments were zeroed oi1 nitrogen and were calibrated periodically with gases of known composition. The engine conditions used are listed in Table I. At each air-fuel ratio, various quantities of "pure" NO (Matheson Technical Grade, 98.5% NO minimum) were added to the intake system above the surge tank. Results and Discussion
NO concentrations in the exhaust are shown in Fig. 3 as a function of intake NO concentration for each air-fuel ratio. (All results are listed in Table I I . ) The data resulted ia a family of straight lines, with varying intercepts on the ordinate, and with slopes which increased with air-fuel ratio. The linear variation of exhaust NO with intake NO indicates that the ratelimiting reactions are not significantly autocatalytic or auto-inhibited. The slopes of the lines in Fig. 3 varied from 0.20 to 0.84. Thus, equilibrium considerations do not control the over-all N0-formation process, nor is the process controlled only by tile formation rate or by the decomposition rate. Perhaps both formation and decomposition processes are kinetically important. The nearness of the line's slope to 1.0 or 0.0
864
POLLUTANT FORMATION AND DESTRUCTION IN FLAMES
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INTAKENOCONCENTRATION,PPM" 1000 FIG. 3. NO-addition lines. Exhaust NO as a function of intake NO and Mr-fuel ratio, in an experiment with 0.400 lb/min air-flow, 15° BTC spark, and 1500 rpm. may indicate the relative importance of the formation and decomposition processes, respectively. These slopes are plotted in Fig. 4 as a function of air-fuel ratio. This plot shows the air-fuel ratio effect on the relative importance of the formation and decomposition processes. The increase in the slopes at higher air-fuel ratios indicates that the formation process is becoming more important relative to the decomposition process. Extrapolation of the curve in Fig. 4 indicates that, at about 18:1 and 11:1 air-fuel ratios, the slopes approach 1.0 and 0.0, respectively. This extrapolation implies that the process may become purely formation controlled at very high air-fuel ratios, and purely decomposition controlled at very low air-fuel ratios. The air-fuel ratio effect o,1 the relative importance of formation and decomposition appears to change at stoiehiometric. (See the break in the curve in Fig. 4.) This change in the effect of air-fuel ratio may be due to a change in the chemical mechanism of NO formation. On the rick side of stoichiometric, the detailed chemical mechanism may be different from that on the lean side. This change in mechanism (as the air-fuel ratio becomes richer than stoichiometric) is probably due to the low concentration of molecular oxygen on the rich side of stoichiometric. In
this region, another elementary reaction m a y become more important than
N+O~= N0+O. Figure 5 shows the ordinary variation of exhaust NO with air-fuel ratio (fixed spark). 1.0 / .8
o=
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13
14
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15
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AIR-FUEL RATIO, LB/LB
FIG. 4. Slopes of NO-addition lines, dNOex/dNOin vs. air-fuel ratio with 0.400 lb/min air-flow, 15° BTC spark, and 1500 rpm. Error bands are shown.
NITRIC OXIDE FORMATION PROCESS This figure is a plot of the data obtained with no NO addition to the intake. The maximum of Fig. 5 occurs at an air-fuel ratio of about 15.5, which is interesting because 15.5:1 corresponds to dNOex/dNOin=0.5 (see Fig. 4). If dNO~:/dNOi~ does reflect the relative importance of the formation and decomposition processes, then maximum exhaust NO occurred at the air-fuel ratio where the formation and decomposition processes were of equal iraportance kinetically.
~"
3
The FRC, DRC, and EPCT models are incorrect, according to the data shown in Fig. 3. The slopes of the addition lines (Fig. 4) range from 0.20 for the richest mixture to 0.84 for the leanest mixture, whereas equilibrium-step models (EPCT and DRC) imply 0.0 slope, and the FRC model implies 1.0 slope. For rich mixtures, the slopes are nearer 0.0, and the equilibrium-step models (EPCT or DRC) should be more nearly correct. For lean mixtures, the slopes are nearer 1.0, and the FRC model should be more nearly correct. Newhall and Starkman's EPCT model 5 only used data for rich air-fuel ratios, and Eyzat and Guibet's FRC model ~ only used data for lean air-fuel ratios. Thus, the disagreements between these models may have been due, in part, to the different air-fuel ratio ranges chosen by the experimenters. A simple competing-reaction model might explain the NO-addition data. If both formation and decomposition processes were important, and if the relative importance of the two processes varied with air-fuel ratio, then the data of Fig. 3 might be matched by the model's predictions. In this competing-reaction model, the predominately important process would be the formation process when the stoichiometry was very lean, and would be the decomposition process when very rich. The relative importances would be equal at peak-NO air-fuel ratio, and the nature of the chemical mechanisms themselves would change at stoichiometric. Such a model might predict all of the general features of the NO-addition data, and also the features of the "normal engine experiment" data (such as peak NO at about 15.5:1 air-fuel ratio, higher NO concentration with advanced spark, etc. ). Simple models have a utility in developing an intuitive understanding of a process, and also allow simple explanations of experimental re-
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Implications for Modeling
865
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AIR-FUELRATI0, LB/LB FIG. 5. Intercepts of N O - a d d i t i o n lines. E x h a u s t
NO vs. air-fuel ratio, with zero intake NO, 0.400 Ib/min air-flow, 15° BTC spark, and 1500 rpm.
sults. However, the details of the NO-formation process are too complex to be well described by such a simple model. An example of these complexities is the substantial temperature gradient (>200°K) across the burned gas}5 A much more sophisticated model is available, 1°-m which accounts for many of these complications. The burned gas is treated as a series of elements. Each element experiences a different temperature-time history, because of different inflammation times. These different inflammation times cause the gradient in temperature and also cause a gradient in the final NO concentration. The basic assumptions of the gradient model are: no mixing until late in the process, adiabatic combustion for each element of the cylinder gas, and a Zel'dovich NO-reaction mechanism. Both formation and decomposition processes are important in this model, and the relative importance of the two processes does change with air-fuel ratio. 16 If a change of mechanism were also included, the gradient model would seem capable of fitting the NO-addition data. This modified gradient model seems to be the most promising model for future development.
Conclusions This experiment suggests four conclusions about the NO-formation process in sparkignition engines: (1) both formation and decomposition processes are important kinetically, (2) the chemical mechanism seems to change at stoichiometric air-fuel ratio, (3) the formation and decomposition processes may be equal in importance, at the air-fuel ratio corresponding
866
POLLUTANT
FORMATION
AND
to peak exhaust NO concentration, and (4) the rate-controlling reactions seem to be neither auto-catalytic nor auto-inhibited. The NO-addition data provide a critical test for models of NO formation in spark-ignition engines. The results contradict the predictions of the EPCT model, the FRC model, and the DRC model. To match the results from the NO-addition experiment, a model probably must involve both formation and decomposition processes. The proposed competing-reaction model and the gradient model contain this feature, and they probably can match the NO-addition data; thus, these models seem to be the most promising for future work.
DESTRUCTION
IN FLAMES
The latter equation [-Eq. (5A)] was used to correct the analyzer readings and yield intake NO concentrations. Oxidation in the intake system caused N02 to be present in the intake charge inducted by the engine. The concentration of N02 in the intake was calculated by considering the surge tank to be a well-mixed flow reactor. dNO2/dt = - - d N O / d t = k p 3 (02) (NO)2,
(NO2)in= k]- (30-- Vin )/30~ 3(02)in (NO)in 2 At, (7A) At =
volume of surge tank/volume flow rate of air,
APPENDIX
(8A)
Correction for Oxidation of Nitric Oxide Oxidation of NO to NO2 in engine exhaust gases is usually ignored. This oxidation can be neglected because the oxygen (02) concentration is low (0.1%-4%), and the NO concentration is very low (0.1%-0.5%). However, during this experiment the concentration of NO reached 1.48% in the intake system, where the O2 concentration was 21%. The combination of higher O~ and NO concentrations meant that oxidation of NO could not be ignored. NO reacts with oxygen at room temperature, as follows'7: -
-
d N O / d t = kp ~(02) (N0)2,
k= (1.57:t:0.09)X109 ppm-2 mia-~ atm-3.
(1A) (2A)
There were two separate effects of this reaction: (i) oxidation in the sample line between the intake pipe and the analyzer (see Fig. 2) caused errors in the concentrations recorded, and (2) oxidation in the intake system caused NO2 to be present in the mixture inducted by the engine in addition to the NO. Oxidation in the sample line caused errors in the measurements of intake NO concentrations, because the analyzer does not respond to NO2. This error was corrected by using the reactionrate equation [Eq. (IA)~ integrated for a plugflow reactor at atmospheric pressure. 1/(NO)in=l/(NO)anal-kp3(O2)At,
At=0.11 min (measured),
(6A)
(3A) (4A)
1/(NO )i~= 1/(NO )~.a~--3.65)410 -~ ppm-I.
(hA)
At--- [- (3.5 ft3X 0.075 Ib/ft3)/rhai, E (30-- Vin)/30~,
(9A)
(NO2)ia-- 5.10X 10 -16
X {~ (30-- Vin)4(02)in (NO) i2"]/fhMr}, (10A) (NO2)i~--2.67X10-n(30--Vin)4(NO)i~2. (llA) This final equation [-Eq. (llA)~ was used to calculate the concentration of NO2 which was inducted into the engine. Since the intake vacuum varied only slightly, these concentrations depended primarily on NO concentrations. The median concentration of N02 in the intake was 115 ppm. Since the NO2 is proportional to the square of the NO, then if NO2 did affect the results of this experiment, the lines in Fig. 3 should not be straight. The straightness of these lines implies that the effect of the NO2 was small.
Nomenclature k
reaction-rate constant, ppm -2 rain-1 atm -~ rhair mass-flow rate of air into intake, 0.400 Ib rain-I (NO)anal volume fraction of NO in analyzer, ppm (NO)i~ volume fraction of NO in intake, ppm (NO2)in volume fraction of NO2 in intake, ppm (02)i~ volume fraction of 02 in intake, ppm p pressure, arm At time available for reaction, rain Via intake vacuum, in. Hg
NITRIC OXIDE FORMATION PROCESS
Acknowledgment
867
8. NEWttALL, H. I(. AND SnAHED, S. M.: Thir-
A. E. Fincham was of great help in constructing the apparatus and instrumentation and in the conduct of the experiment. REFERENCES 1. NEnEL, G. J. AND JACKSON, M. W.: J. Air Pollution Control Assoc. 8, 213 (1958). 2. BnNSON, J. D. AND STEBAR, R. F.: Paper 710008, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1971. 3. A~MAN, W. R.: Paper 720256, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1972. 4. QUADER, A. A.: Paper 710009, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1971. 5. NEWHALL, H. K. AND STARKMAN,E. S.: Paper 670122, SAE Trans. 76, 743 (1968). 6. SPINDT, R. S., WOLFE, C. L., AND STEVENS, D. R.: SAE Trans. 64, 797 (1956). 7. EYZAT, P. AND GUIBET, J. C.: Paper 680124, SAE Trans. 77, 93 (1969).
teenth Symposium (International) on Combustion, p. 381, The Combustion Institute, 1971. 9. SMITH, R. D.: J. Appl. Chem. 10, 1 (1960). 10. IJAYOIE, G. A., HEYWOOD, J. B., AND KECK, J. C.: Combust. Sei. Technol. 1, 313 (1970). 11. HAYWOOD,J. B., MATHEWS, S. M., AND OWEN, B.: Paper 710011, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1971. 12. MvzIo, L. J., STaRKMAN, E. S., AND CARETTO, L. S.: Paper 710158, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1971. 13. CORNELIUS,W. AND CAPLAN,J. D. : SAE Qtrly Trans. 6, 666 (1952). 14. D'ALLEVA, B. A. AND LOVELL, W. G.: SAE Trans. 31, 90 (1936). 15. RASSWEILER, G. M. AND WITHROW, L.: SAE Trans. 30, 125, April, 1935. 16. BLUMBERG, P. AND KUMMER, J. T.: Combust. Sci. Technol. 4, 73 (1971). 17. GLASSON,W. A. AND TUESDAY, C. S.: J. Amer. Chem. Soc. 85, 2901 (1963).
COMMENTS John B. Heywood, Massachusetts Institute of Technology, Cambridge, Mass. The results reported in this paper can be qualitatively explained b y models which follow both the NO formation and decomposition processes, and take account of the gradient in temperature across the burned charge in a spark-ignition engine. For fuel-lean mixtures, NO formation is rate controlled in the post-flame gases, but little NO decomposition occurs after peak pressures are reached, and freezing occurs early in the expansion stroke. There is some decomposition of NO in the early-burned parts of the charge, because these reach higher temperatures, but no decomposition of NO occurs in late-burning elements. Thus, almost all the added NO will appear in the exhaust, since decomposition of the added NO will only occur in the first part of the charge to burn. For rich mixtures, NO concentrations in the burned gases are close to equilibrium concentrations at peak cylinder pressures and temperatures, and substantial decomposition of NO occurs during the expansion stroke before concentrations freeze. Therefore, almost all the added NO in elements burning early will be decomposed; a smaller fraction of the added NO
will decompose in elements burning later in the combustion process, since these are at a lower temperature for a shorter time. Thus, only a small fraction of the added NO appears in the exhaust. The same kinetic mechanism, the Zeldovich mechanism extended to include the reaction of N with OH:
N2+O~NO+N, N+O~--NO+O, N+OH~NO+H, describes the NO formation and decomposition process in both rich and lean mixtures. In rich mixtures with higher H concentrations, the third reaction allows a more rapid NO decomposition than in lean mixtures. Both our own research on this topic, and that of Blumberg and Kummer (Ref. 16 of the paper), support this explanation.
Author's Reply. In general, I agree with Prof. Heywood's comments. The gradient model does seem capable of explaining the main features of the N0-addition data.
868
POLLUTANT FORMATION AND DESTRUCTION IN FLAMES
One must keep in mind that not even the complex gradient model matches the actual process in all details. Ignored are mixing and heat-transfer processes, and numerous elementary reactions which proceed to at least a limited extent. Hopefully, the ignored features of the process do not have substantial effects.
Because we cannot match the actual process in all details, we should not expect to be perfect in our attempts to match empirical results. The main values of a model are to give a general description of the trends and, most importantly, to say why the trends exist.
General Motors Research Laboratories, Warren, Michigan An entirely satisfactory model for the formation of nitric oxide (NO) in spark-ignition engines has not yet been developed. To further elucidate the NO-formation process, various quantities of pure NO were added to the intake air of a single-cylinder engine at five different air-fuel ratios. The experimental results contradict the predictions of the equilibrium-at-peak-cycle-temperature model and also the predictions of the formation-rate-controlling model. Models in which formation and decomposition processes are both important may explain the results of these experiments. The formation and decomposition processes seem to be of equal kinetic importance at the air-fuel ratio corresponding approximately to peak exhaust NO concentration (93% of stoichiometric fuel). The formation process becomes more important at leaner mixtures, and the decomposition process becomes more important at richer mixtures. In addition, a change in chemical mechanism is suggested at the stoiehiometric air-fuel ratio. equilibrium at peak cycle temperature, but does not decompose as the temperature decreases on the expansion stroke. The formation-rate-controlling (FRC) models assume that equilibrium is not attained, and that the NO concentration is controlled by the rate at which NO can form. Either the direct molecular reaction, 7
Introduction
Nitric oxide emission from spark-ignition engines is affected to a certain extent by almost every engine parameter, but air-fuel ratio, spark timing, and charge dilution are the parameters of primary importance2 -a (Charge dilution is the fraction of a given cylinder charge which is not fresh air-fuel mixture, i.e., residual exhaust, recirculated exhaust, humidity, or other diluents.) These three parameters probably affect the exhaust NO concentration by: (1) changing the oxygen concentration in the burned gases, (2) changing the temperature of the burned gases, and (3) changing the time available for reaction. Nitrogen concentration is unimportant, kinetically, because it is an excess reactant. 4 A number of contradictory models have been proposed to describe the NO-formation process. Among the simple models proposed are an equilibrium-limitation model, 5 and several formation-rate-controlling models.6-s The possibility that decomposition reactions during the expansion process may control the over-all process has also been considered? The equilibrium-limitation modeP ~-also called the equilibrium-at-peak-cycle-temperature (EPCT) model] assumes that NO attains
N 2 + 0 2 = 2 NO, or the Zel'dovitch mechanism, s N2-}-O= N-t-NO, N+02= NO+O, has been used as the rate-limiting step. (The reverse reactions were assumed to be negligible.) The possibility that NO emission is controlled by the rate of the NO decomposition during the expansion process has also been suggested? A decomposition-rate-controlling (DRC) model would assume NO equilibrium at peak cycle temperature, as in the E P C T model. However, during the expansion process, the concentration would decrease b y a rate-limited process. The NO decomposition would stop (or "freeze") when the
861
POLLUTANT FORMATION AND DESTRUCTION IN FLAMES
862
z 0
~ 5
I FRCMODEL
(.~ z 0 C, z
AIF: 13
DRCANDEPCT MODELS
INTAKE NO CONCENTRATION
FIG. 1. Predicted NO-addition lines. The predicted response of exhaust NO concentration to the addition of NO to the intake per the FRC, DRC, and EPCT formation models. Two all-fuel ratios are represented, which causes the different intercepts on the ordinate.
temperature falls sufficiently. This sequence has been described as part of more-complex models.l°-12 All of these simple NO-formation models predict more or less the same results for experiments where the normal engine parameters (air-fuel ratio, spark timing, etc.) are varied. Thus, the results of such experiments are not very useful in distinguishing between NO-formation models. The models do predict different results for an experiment in which various quantities of NO are added to the intake air, and the exhaust NO is measured. Because different results are predicted, this experiment can provide a critical test for NO-formation models. If the formation rate controls the over-all process (FRC model) then the amount of NO formed by the reaction would just add to the amount present in the fresh charge. A plot of exhaust NO concentration versus intake NO concentration would be a straight line with unity slope. (If the rate-controlling reaction were auto-catalytic or auto-inhibited, curved lines would result. In auto-catalytic and auto-inhibited reactions, the concentration of the product influences the rate of the reaction. The rate is increased in auto-catalytic and decreased in auto-inhibited reactions.) If the decomposition rate controls the over-all process (DRC model), then there must be a stage where the NO concentration is very near
equilibrium2 This equilibrium stage would erase ally influence of NO in the intake air, and a plot of exhaust NO versus intake NO would have zero slope. The EPCT model would also imply a zero-slope plot for identical reasons. Figure 1 shows the expected NO-addition lilies for two air-fuel ratios, as predicted by the FRC models or the EPCT and DRC models. The variations in the intercepts on the ordinate are due to the ordinary variation in exhaust NO with air-fuel ratio. In the experiment to be described, small quantities (up to 1.5%) of NO were added to the intake air, while exhaust NO was analyzed. At five air-fuel ratios (constant air flow, spark timing, and engine speed), exhaust NO concentration was measured as a function of intake NO concentration. The results of this experiment provide a critical test for evaluating NO formation models. The test is applied to the FRC, DRC, and EPCT models and its application to more complex models m-12 is discussed.
Apparatus An A S T M - C F R , single-cylinder engine, with a disk-shaped combustion chamber and a shrouded intake valve, was used. Engine air came from a critical-flow system as shown in Fig. 2. The humidity of this air was about 23 gr/ lb.13 A timed fuel-injection system delivered fuel into the intake manifold. The exhaust pipe was 8-in. long, and ended in an expansion tank. The exhaust pressure in the expansion tank was I in. Hg. NDIR instruments measured llitric oxide and carbon monoxide concentrations, and a polarographic analyzer measured oxygen concentration. This instrumentation was used to determine the exhaust concentrations and the NO
TABLE I Engine conditions Engine speed Air flow Spark timing Fuel Compression ratio Measured charge dilution Intake air temp. Coolant and oil temp. Exhaust pressure
1500 rpm 0.400 lb/min 15° BTC Indolene 30 (C~H2~, MW-95) 8:1 9.8 vol. % J26°F 188°F 1 in. Hg
NITRIC OXIDE FORMATION PROCESS
863
DRIERS
iiiiiiiiiii,iiiiiiiliiiiiiiiiiiiiiii
®
, ROTAMETERS
PUMP
'*Hiiiiiiiii ''
iiiiiiiiiiii%iiiiiiiiiiiiiiim~m~iiiigiiiiiii REGULATED NITRIC
NITR I COX I DE METERING VALVE
_.41=--
INTAKE SAMPLE LINE
OXIDE
SUPPLY
X EXPANSION TANK MIXING TANK ICE BATH
\ SURGE TANK
EXHAUST 7 " SAMPLING LINE EXHAUST
REGULATED -~HIGH-PRESSURE AIR SUPPLY
CRITICAL FLOW ORIFICE
- EXHAUST BACK-PRESSURE CONTROL VALVE
FIG. 2. Experimental apparatus. TABLE II Results of adding nitric oxide to engine intake Air-fuel ratio
Fuel equiv. ratio
Intake NO
Exhaust NO (wet basis)
13.1
1.13
0 2,050 6,100 10,000 13,100
1,100 1,650 2,400 3,100 3,750
14.0
1.06
0 2,200 5,800 9,200
1,950 2 550 3 650 4,600
14.8
1.00
0 1,750 5,650 8,450 11,600 14,800
2 3 4 5 6 8
600 200 650 700 850 350
15.8
0.934
0 2,100 5,600 8,250 10,500
2,750 4,200 6,250 7,500 8,100
17.5
0.843
0 2,250 6,550 9,400
9OO 2,80O 6,450 8,650
concentration in the intake air (see Fig. 2). The measured intake NO concentrations were corrected for oxidation to N 0 : (see Appendix I). Air-fuel ratio was determined from the exhaust analysis}4 All instruments were zeroed oi1 nitrogen and were calibrated periodically with gases of known composition. The engine conditions used are listed in Table I. At each air-fuel ratio, various quantities of "pure" NO (Matheson Technical Grade, 98.5% NO minimum) were added to the intake system above the surge tank. Results and Discussion
NO concentrations in the exhaust are shown in Fig. 3 as a function of intake NO concentration for each air-fuel ratio. (All results are listed in Table I I . ) The data resulted ia a family of straight lines, with varying intercepts on the ordinate, and with slopes which increased with air-fuel ratio. The linear variation of exhaust NO with intake NO indicates that the ratelimiting reactions are not significantly autocatalytic or auto-inhibited. The slopes of the lines in Fig. 3 varied from 0.20 to 0.84. Thus, equilibrium considerations do not control the over-all N0-formation process, nor is the process controlled only by tile formation rate or by the decomposition rate. Perhaps both formation and decomposition processes are kinetically important. The nearness of the line's slope to 1.0 or 0.0
864
POLLUTANT FORMATION AND DESTRUCTION IN FLAMES
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INTAKENOCONCENTRATION,PPM" 1000 FIG. 3. NO-addition lines. Exhaust NO as a function of intake NO and Mr-fuel ratio, in an experiment with 0.400 lb/min air-flow, 15° BTC spark, and 1500 rpm. may indicate the relative importance of the formation and decomposition processes, respectively. These slopes are plotted in Fig. 4 as a function of air-fuel ratio. This plot shows the air-fuel ratio effect on the relative importance of the formation and decomposition processes. The increase in the slopes at higher air-fuel ratios indicates that the formation process is becoming more important relative to the decomposition process. Extrapolation of the curve in Fig. 4 indicates that, at about 18:1 and 11:1 air-fuel ratios, the slopes approach 1.0 and 0.0, respectively. This extrapolation implies that the process may become purely formation controlled at very high air-fuel ratios, and purely decomposition controlled at very low air-fuel ratios. The air-fuel ratio effect o,1 the relative importance of formation and decomposition appears to change at stoiehiometric. (See the break in the curve in Fig. 4.) This change in the effect of air-fuel ratio may be due to a change in the chemical mechanism of NO formation. On the rick side of stoichiometric, the detailed chemical mechanism may be different from that on the lean side. This change in mechanism (as the air-fuel ratio becomes richer than stoichiometric) is probably due to the low concentration of molecular oxygen on the rich side of stoichiometric. In
this region, another elementary reaction m a y become more important than
N+O~= N0+O. Figure 5 shows the ordinary variation of exhaust NO with air-fuel ratio (fixed spark). 1.0 / .8
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AIR-FUEL RATIO, LB/LB
FIG. 4. Slopes of NO-addition lines, dNOex/dNOin vs. air-fuel ratio with 0.400 lb/min air-flow, 15° BTC spark, and 1500 rpm. Error bands are shown.
NITRIC OXIDE FORMATION PROCESS This figure is a plot of the data obtained with no NO addition to the intake. The maximum of Fig. 5 occurs at an air-fuel ratio of about 15.5, which is interesting because 15.5:1 corresponds to dNOex/dNOin=0.5 (see Fig. 4). If dNO~:/dNOi~ does reflect the relative importance of the formation and decomposition processes, then maximum exhaust NO occurred at the air-fuel ratio where the formation and decomposition processes were of equal iraportance kinetically.
~"
3
The FRC, DRC, and EPCT models are incorrect, according to the data shown in Fig. 3. The slopes of the addition lines (Fig. 4) range from 0.20 for the richest mixture to 0.84 for the leanest mixture, whereas equilibrium-step models (EPCT and DRC) imply 0.0 slope, and the FRC model implies 1.0 slope. For rich mixtures, the slopes are nearer 0.0, and the equilibrium-step models (EPCT or DRC) should be more nearly correct. For lean mixtures, the slopes are nearer 1.0, and the FRC model should be more nearly correct. Newhall and Starkman's EPCT model 5 only used data for rich air-fuel ratios, and Eyzat and Guibet's FRC model ~ only used data for lean air-fuel ratios. Thus, the disagreements between these models may have been due, in part, to the different air-fuel ratio ranges chosen by the experimenters. A simple competing-reaction model might explain the NO-addition data. If both formation and decomposition processes were important, and if the relative importance of the two processes varied with air-fuel ratio, then the data of Fig. 3 might be matched by the model's predictions. In this competing-reaction model, the predominately important process would be the formation process when the stoichiometry was very lean, and would be the decomposition process when very rich. The relative importances would be equal at peak-NO air-fuel ratio, and the nature of the chemical mechanisms themselves would change at stoichiometric. Such a model might predict all of the general features of the NO-addition data, and also the features of the "normal engine experiment" data (such as peak NO at about 15.5:1 air-fuel ratio, higher NO concentration with advanced spark, etc. ). Simple models have a utility in developing an intuitive understanding of a process, and also allow simple explanations of experimental re-
--
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I 0
Implications for Modeling
865
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14
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18
AIR-FUELRATI0, LB/LB FIG. 5. Intercepts of N O - a d d i t i o n lines. E x h a u s t
NO vs. air-fuel ratio, with zero intake NO, 0.400 Ib/min air-flow, 15° BTC spark, and 1500 rpm.
sults. However, the details of the NO-formation process are too complex to be well described by such a simple model. An example of these complexities is the substantial temperature gradient (>200°K) across the burned gas}5 A much more sophisticated model is available, 1°-m which accounts for many of these complications. The burned gas is treated as a series of elements. Each element experiences a different temperature-time history, because of different inflammation times. These different inflammation times cause the gradient in temperature and also cause a gradient in the final NO concentration. The basic assumptions of the gradient model are: no mixing until late in the process, adiabatic combustion for each element of the cylinder gas, and a Zel'dovich NO-reaction mechanism. Both formation and decomposition processes are important in this model, and the relative importance of the two processes does change with air-fuel ratio. 16 If a change of mechanism were also included, the gradient model would seem capable of fitting the NO-addition data. This modified gradient model seems to be the most promising model for future development.
Conclusions This experiment suggests four conclusions about the NO-formation process in sparkignition engines: (1) both formation and decomposition processes are important kinetically, (2) the chemical mechanism seems to change at stoichiometric air-fuel ratio, (3) the formation and decomposition processes may be equal in importance, at the air-fuel ratio corresponding
866
POLLUTANT
FORMATION
AND
to peak exhaust NO concentration, and (4) the rate-controlling reactions seem to be neither auto-catalytic nor auto-inhibited. The NO-addition data provide a critical test for models of NO formation in spark-ignition engines. The results contradict the predictions of the EPCT model, the FRC model, and the DRC model. To match the results from the NO-addition experiment, a model probably must involve both formation and decomposition processes. The proposed competing-reaction model and the gradient model contain this feature, and they probably can match the NO-addition data; thus, these models seem to be the most promising for future work.
DESTRUCTION
IN FLAMES
The latter equation [-Eq. (5A)] was used to correct the analyzer readings and yield intake NO concentrations. Oxidation in the intake system caused N02 to be present in the intake charge inducted by the engine. The concentration of N02 in the intake was calculated by considering the surge tank to be a well-mixed flow reactor. dNO2/dt = - - d N O / d t = k p 3 (02) (NO)2,
(NO2)in= k]- (30-- Vin )/30~ 3(02)in (NO)in 2 At, (7A) At =
volume of surge tank/volume flow rate of air,
APPENDIX
(8A)
Correction for Oxidation of Nitric Oxide Oxidation of NO to NO2 in engine exhaust gases is usually ignored. This oxidation can be neglected because the oxygen (02) concentration is low (0.1%-4%), and the NO concentration is very low (0.1%-0.5%). However, during this experiment the concentration of NO reached 1.48% in the intake system, where the O2 concentration was 21%. The combination of higher O~ and NO concentrations meant that oxidation of NO could not be ignored. NO reacts with oxygen at room temperature, as follows'7: -
-
d N O / d t = kp ~(02) (N0)2,
k= (1.57:t:0.09)X109 ppm-2 mia-~ atm-3.
(1A) (2A)
There were two separate effects of this reaction: (i) oxidation in the sample line between the intake pipe and the analyzer (see Fig. 2) caused errors in the concentrations recorded, and (2) oxidation in the intake system caused NO2 to be present in the mixture inducted by the engine in addition to the NO. Oxidation in the sample line caused errors in the measurements of intake NO concentrations, because the analyzer does not respond to NO2. This error was corrected by using the reactionrate equation [Eq. (IA)~ integrated for a plugflow reactor at atmospheric pressure. 1/(NO)in=l/(NO)anal-kp3(O2)At,
At=0.11 min (measured),
(6A)
(3A) (4A)
1/(NO )i~= 1/(NO )~.a~--3.65)410 -~ ppm-I.
(hA)
At--- [- (3.5 ft3X 0.075 Ib/ft3)/rhai, E (30-- Vin)/30~,
(9A)
(NO2)ia-- 5.10X 10 -16
X {~ (30-- Vin)4(02)in (NO) i2"]/fhMr}, (10A) (NO2)i~--2.67X10-n(30--Vin)4(NO)i~2. (llA) This final equation [-Eq. (llA)~ was used to calculate the concentration of NO2 which was inducted into the engine. Since the intake vacuum varied only slightly, these concentrations depended primarily on NO concentrations. The median concentration of N02 in the intake was 115 ppm. Since the NO2 is proportional to the square of the NO, then if NO2 did affect the results of this experiment, the lines in Fig. 3 should not be straight. The straightness of these lines implies that the effect of the NO2 was small.
Nomenclature k
reaction-rate constant, ppm -2 rain-1 atm -~ rhair mass-flow rate of air into intake, 0.400 Ib rain-I (NO)anal volume fraction of NO in analyzer, ppm (NO)i~ volume fraction of NO in intake, ppm (NO2)in volume fraction of NO2 in intake, ppm (02)i~ volume fraction of 02 in intake, ppm p pressure, arm At time available for reaction, rain Via intake vacuum, in. Hg
NITRIC OXIDE FORMATION PROCESS
Acknowledgment
867
8. NEWttALL, H. I(. AND SnAHED, S. M.: Thir-
A. E. Fincham was of great help in constructing the apparatus and instrumentation and in the conduct of the experiment. REFERENCES 1. NEnEL, G. J. AND JACKSON, M. W.: J. Air Pollution Control Assoc. 8, 213 (1958). 2. BnNSON, J. D. AND STEBAR, R. F.: Paper 710008, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1971. 3. A~MAN, W. R.: Paper 720256, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1972. 4. QUADER, A. A.: Paper 710009, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1971. 5. NEWHALL, H. K. AND STARKMAN,E. S.: Paper 670122, SAE Trans. 76, 743 (1968). 6. SPINDT, R. S., WOLFE, C. L., AND STEVENS, D. R.: SAE Trans. 64, 797 (1956). 7. EYZAT, P. AND GUIBET, J. C.: Paper 680124, SAE Trans. 77, 93 (1969).
teenth Symposium (International) on Combustion, p. 381, The Combustion Institute, 1971. 9. SMITH, R. D.: J. Appl. Chem. 10, 1 (1960). 10. IJAYOIE, G. A., HEYWOOD, J. B., AND KECK, J. C.: Combust. Sei. Technol. 1, 313 (1970). 11. HAYWOOD,J. B., MATHEWS, S. M., AND OWEN, B.: Paper 710011, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1971. 12. MvzIo, L. J., STaRKMAN, E. S., AND CARETTO, L. S.: Paper 710158, SAE International Automotive Engineering Congress, Detroit, Mich., Jan. 1971. 13. CORNELIUS,W. AND CAPLAN,J. D. : SAE Qtrly Trans. 6, 666 (1952). 14. D'ALLEVA, B. A. AND LOVELL, W. G.: SAE Trans. 31, 90 (1936). 15. RASSWEILER, G. M. AND WITHROW, L.: SAE Trans. 30, 125, April, 1935. 16. BLUMBERG, P. AND KUMMER, J. T.: Combust. Sci. Technol. 4, 73 (1971). 17. GLASSON,W. A. AND TUESDAY, C. S.: J. Amer. Chem. Soc. 85, 2901 (1963).
COMMENTS John B. Heywood, Massachusetts Institute of Technology, Cambridge, Mass. The results reported in this paper can be qualitatively explained b y models which follow both the NO formation and decomposition processes, and take account of the gradient in temperature across the burned charge in a spark-ignition engine. For fuel-lean mixtures, NO formation is rate controlled in the post-flame gases, but little NO decomposition occurs after peak pressures are reached, and freezing occurs early in the expansion stroke. There is some decomposition of NO in the early-burned parts of the charge, because these reach higher temperatures, but no decomposition of NO occurs in late-burning elements. Thus, almost all the added NO will appear in the exhaust, since decomposition of the added NO will only occur in the first part of the charge to burn. For rich mixtures, NO concentrations in the burned gases are close to equilibrium concentrations at peak cylinder pressures and temperatures, and substantial decomposition of NO occurs during the expansion stroke before concentrations freeze. Therefore, almost all the added NO in elements burning early will be decomposed; a smaller fraction of the added NO
will decompose in elements burning later in the combustion process, since these are at a lower temperature for a shorter time. Thus, only a small fraction of the added NO appears in the exhaust. The same kinetic mechanism, the Zeldovich mechanism extended to include the reaction of N with OH:
N2+O~NO+N, N+O~--NO+O, N+OH~NO+H, describes the NO formation and decomposition process in both rich and lean mixtures. In rich mixtures with higher H concentrations, the third reaction allows a more rapid NO decomposition than in lean mixtures. Both our own research on this topic, and that of Blumberg and Kummer (Ref. 16 of the paper), support this explanation.
Author's Reply. In general, I agree with Prof. Heywood's comments. The gradient model does seem capable of explaining the main features of the N0-addition data.
868
POLLUTANT FORMATION AND DESTRUCTION IN FLAMES
One must keep in mind that not even the complex gradient model matches the actual process in all details. Ignored are mixing and heat-transfer processes, and numerous elementary reactions which proceed to at least a limited extent. Hopefully, the ignored features of the process do not have substantial effects.
Because we cannot match the actual process in all details, we should not expect to be perfect in our attempts to match empirical results. The main values of a model are to give a general description of the trends and, most importantly, to say why the trends exist.