Emission Spectra on Autoignited Heutane-Air Mixtures W. J. LEVEDAIII, A N‘D H. P. BROIDA National Bureau of Stando rds, IVashington, D. C. The explosive knook reaction i n internal cornbustion engines is known to be caused by multistage autoignition, although its mechanism has not been tisfaetorily explained. A n apparatus was deloped to obtain emission spectra on the -1-flame id hot-flame stages of autoignition in an internal mbustion engine and to remrd total emission innsity as a function of time during the combustion cycle. Spectra of the mol-flame stage of n-heptane autoignition were identical to those of formaldehyde fluorescence. Hot &me reactions showed CH and OH under all operating conditions, while CP bands appeared i n rich mixtures. Hydrooarbon flame bands were observed in lean mixtures. Cool-
M.
UGH study has been devoted to explosive knock reaction
m an internal combustion engine but BS s e t there is no complete and satisfactory explanation of its mechanism. The autoignition theory (16) has been used successfully to explain several of the phenomena associated with knock, and many research programs are being conducted within the framework of this theory. Such investigations may eventually lead to a better understanding of the complex physical and chemical processes involved. In a spark ignition engine the expansion of burned gases in and behind the flame front together with the motion of the piston compresses the unburned gases ahead of the flame to high temperatures and pressures. If sufficiently high temperatures and pressures persist for a long enough time, the “end gas,” or the last part of the unburned charge, may ignite spontaneously and burn a t an extraordinarily fast rate. This oauses a pressure wave to reflect hack and forth across the cylinder, resultinginaudible knock. It is experimentally very difficult to isolate and study the end gas reaction in the ordinary spark-ignition engine. Therefore, many attempts have been made to study the temperatures and pressures under conditions vhich approximate those producing knock. Severalinvestigators(1,8-10,1S,15,20) have subjectedtheenginefuel-air charge t o conditions comparable to those in the end gas by motoring the engine without spark rtt compression ratios sufficient t o cause reaet,ion to occur. This vork has shown t~hat most aliphatic fuels show two exothermic stages in their combustion. The first of these stages releases a small amount of heat and simultaneously emits low intensity radiation (“cool flames”), while the second or hot flame stage is highly exothermic and gives more intense radiation. Rapid eompression machines (b, 6-7, 18, 171, which compress the charge very rapidly to a predetermined pressure and then maintain constant volume throughout
flame intensit) rose rapidly to a peak, th en decreased sharply before the onset of the much nnore intense hot flame. It is indicated that the entiire -1-flame emission is caused by excited formaldehyde. The decrease in cool-flame intensity prior to the initiation of hot flames shows that the rate offormation of excited formaldehyde reaches a maximum and subsequently decreases before the onset of the hot flame. The technique of osoillographically record. . .. .rletwterl -. ... .. - -, hv aing the intensity of filtered radiation photomultiplier tube is extreme1y useful because of its simplicity, sensitivitj, and I&ability. A m e dium-size spectrograph was fuunId to he successful in identifying a number of w e l l - k mown emitters.
. .
the subvequent combustion reactions, nave also proaucea man? significant results. Flame speotroscopy in engines has received comparatively little attention as a method for analysis of combustion reactions. In the early 1930’s.Withrow and Rassweiler ($1,d Z ) studied the emission and absorption spectra of combustion in a sparkignition engine under both knocking and nonknocking conditions. I n emission ( d l ) , CZ,CH, and OH radicals and various metals were identified. Formaldehyde (2%) was detected by adsorption in the gases ahead of the flame front. Recently Hirschfeld and Miller (4)have confirmed this absorption work. However, no work has been published previously showing the results of spectroscopic studies of autoignited combustion.
Figure 1. Apparatus
1?76
VOLUME
1777
2 4 , NO. 11, N O V E M B E R 1 9 5 2
drop across a calibrated orifice and fuel was metered through a float-type flanmeter. No spark was used; reaction \\-as caused splely by compression and the resulting polytropic temperature rme. Two of the holes in the cylinder contained fused quartz aind o w 0.5 inch thick and a/8 inch in diameter which could bo easily removed for cleanine. Outside one of the \\-inclaws \US mounted
F i g u r e 2.
Emission I n t e n s i t y - T i m e T r a c e 3 cycle*, cool flames
The experimenbs discussed in this paper ncre dcvised to determine whether it is feasible to makc spectroscopic analyses of emission from excited particles in the combustion chamber of an autoignition engine, and to decide what types of instrumentation are most likely to produce fruitful results. These preliminary investigations have confirmed the earlier work of Withrow and Rassweiler, and identified the emission spectrum of cool flames in an engine with that of formaldehyde fluorescence. They have also shown the potentialities of such st,udies and indicate the type of apparatus to be used in future nark. F i g u r e 3. APPARATUS
(8, 9 ) . -Figure 1 shows the equipmeit with the spectrograph in place. T h e cylinder head contained four holes into which instruments or windows could he inserted, and the compression ratio was continuously variable from about 5 t o 1 to ahout 22 t o 1. Fuel and air were thoroughly mired in a vaporizing tank before entering the engine. Air flow was measured from the pressure
...
4000 WAVELENGTH, ..
Figure "4
~~~~~
~
~~~~
~
m d the phototube. Current h m the tuhe was shown hy the height of a photographically recorded oscilloscope trac! whose horizontal sweep was synchronized with crankshaft rotatlan and time coordinated.
4243
i
Emission I n t e n s i t y - T i m e T r a c e
3 oycles, cool flames followed hy hot flames
4360
4447 4573 4707
.
4400
BNESTROM~
4800
1778
ANALYTICAL CHEMISTRY OPERATIhG CONDITIORS
The engine was turned by a dynamometer a t 900 or 1200 r.p.m. and the cylinder jacket temperature was maintained a t 212" F. Homogeneous n-heptane-air mivtures of 0.75, 1.0, 1.25 and 1.50 stoichiometric fuel concentration were inducted a t 28-inch mercury pressure and 220"F. Compression ratios were varied with each mixture to give three different operating conditions: (1) low compression ratio which allowed only cool flame reaction to occur; (2) nonknocking or lightly knocking two-stage reaction with a medium compression ratio: and (3) violently knocking two-stage reaction at high compression ratio. Several exposure times were used under each set of operating conditions. Most of the two-stage reactions were carried out using a 0.05-mm. slit and exposure times of 5 seconds to 15 minutes. When the spectra of the weak cool flame only were being photographed, it was found necessary to use a wide (1-mm.) slit and exposure times up to 5 hours. RESULTS
Total Light Intensity. The first experiments were made with a '931 A. photomultiplier tube, utilizing its total spectral response region of approximately 3000 to 6500 A. Figure 2 shows a typical radiation-time diagram made a t a compression ratio low enough so that the cool flame reaction did not begin until the piston was nearly a t top center. Subsequent rapid downward motion of the piston with the resultant expansion and cooling of the gases precluded the hot flame reaction. The photograph shows radiation from three successive cycles, in each of which radiation begins shortly before top center and rises abruptly to a peak. The intensity then decreasej, possibly because of the formation of reaction-inhibiting combustion products. Figure 3 is a radiation-time diagram made under the same operating conditions which prevailed in Figure 2, except that the compression ratio has been raised. Reaction began earlier in the cycle and high pressures and temperatures were maintained long enough to cause the rapid hot flame autoignition to take place. The traces in Figures 2 and 3 have identical shapes up to the point where emission from the cool flame dropped to about one third its peak value. The intensity from the hot flame reaction then increases rapidly to a peak several hundred times higher than that of the cool flame. The hot flame begins about 2 milliseconds after initiation of the cool flame. Cyclic reproducibility of the btart of the cool flnmr reaction is better t h m one crank angle
degree (0.2 millisecond a t 900 r.p.m.), as can be seen from the superimposed traces of the three firings. The decay lines from the hot flame are irregular, however, and the three traces are separated. There is little doubt that this cool flame reaction is the same as that found under fixed pressure and temperat'ure conditions (17 ) . The decrease in emission from the first-stage reaction while temperature is increasing is compatible with the observed negative temperature coefficient of reaction rate observed in tube experiments in the neighborhood of 350" to 400" C. ( 11, 14, 18). Several engine operating variables were changed independently to determine their effect on the maximum cool flame intensity. When the fuel concentration was varied from 0.25 to 1.5 times iitoichiometric, the maximum intensity was the greatest when the mixture ratio was about 0.8 stoichiometric. Variations in engine speed, and consequently rate of compression, had only a small effect on the shape and intensity of the emission traces. An increase in compression ratio increases peak pressure, peak temperature, and rate of compression. As the compression ratio was continuously raised above that required to cause the first trace of reaction, the peak intensity of the cool flame increased rapidll- a t first and then leveled off to a maximum value which would not increase further even when subsequent violent second-stage autoignition occurred. Three sets of interference and glass filters were used to isolate bands of Cz a t 5165 A. and bands of CH a t 4315 A. and 3890.4. All these filters had transmittance half-widths of 150 A. and peaks near the heads of the respective bands. The filters were inserted hetiyeen the window in the engine cylinder and the photomultiplier tube. It had been int,cndcd to use these filters to observe the change of C2 and CH in the hot flame reaction under various gperating conditions. Unfortunately, these bands are superimposed on a strong continuum and it has not been possible t'o relate the intensities to the concentrations of the two molecules. Spectra. Emission spectra of only the cool flame reaction were obtained by holding engine conditions similar to those used to obtain Figure 2 . The output. from the photomultiplier tube was used as a monitor during the entire exposure, ensuring that no hot flame reaction occurred. With a 0.75 stoichiometric mixture, a 5-hour exposure time (about 180,000 cycles) was required to
3250
3650 WAVELENGTH, ANGSTROMS
Figure 5.
Densitometer Tracing of Hot Flame Autoignition Spectrum n-Heptane, 1.50 stoichiometric mixture, heavy knock
4050
V O L U M E 2 4 , NO. 11, N O V E M B E R 1 9 5 2
1779
HYDROCARBON FLAME BANDS
I
I'
3250
3050 Figure 6.
3650 WAVELENGTH, %NGSTROMS
43 00
Densitometer Tracing of Hot Flame Autoignition Spectruim n-Heptane, 0.75 stoichiometric mixture, light knock
obtain enough blackening of a 103-0 plate to obtain an identifiable spectrum. I n view of the correspondence betrreen the numerical values of wave length observed by Emeleus in the case of formaldehyde fluorescence ( 3 ) and shown in Figure 4 Kith th6 cool-flame spectrum obtained in this study, there is little doubt t h a t the tivo are identical. The emitter is probably excited formaldeh\.de. A large number of spectra were taken with both stages of the reaction occurring. Since the intensity of light emission in the hot flame is several orders of magnitude greater than that from the cool flame, exposure times of a few seconds t o several minutes were sufficient t o obtain the spectra of this more intense reaction. The lines and bands which were identified in these spectra are listed as X's in Table I, along with the conditions under n-hich they n w e obtained. Hydroxyl radicals appeared strongly in all spectra of the eecondstage reaction (Figure 5 ) . The CH bands \>-erefairly prominent under lightly knocking conditions, but with heavy knock the i n t e n d > - relative to that from OH was greatly reduced. EmisTable I.
Identified Lines and Bands in the SecondStage Reaction
Emitter
OH CH c2 CaO
Na Hydrocarbon flame bands Continuum a
WaveLength 3064 4312
3820 4365 4680 5550 6260 5293 2940-4100
Fraction of Stoichiometric Fuel 0.75 1.00 1 . 2 5 X" x x
x
x x x x
x x x x x
x
x
x
x
x
x
x
X
s
X (1ig ti t knock)
1.50
X
2400 t o lone n a v e X lengths Mixture strength a t which lines and bands were observed.
sion from C2 was evident only in very rich mixtures with light knock. The Vaidya hydrocarbon flame bands (19) were found only with lean mixtures and nonknocking second-stage reaction (Figure 6 ) . A continuum was always present, and its intensity increased with increasing wave length, fuel concentration, and intensity of knock. The sodium D line a t 5892 A. was always prominent. Several other weaker lines were found, but their identity is not yet. clear. It is probable that some of them are metal lines and others may possibly be the Schumann-Runge bands of oxygen. SUM.MARY
The technique of oscillographically recording the intensity of filtered radiation detected by a photomultiplier tube is extremely useful because of its simplicity, sensitivity, and reliability. I n an autoignited engine the intensity of the cool flame increases t o a peak within a fraction of a millisecond, after which a considerable decrease is apparent during the induction period prior t o the start of much more intense hot flames of the second-stage reaction. Spectra obtained from cool flames show this radiation to be identical with that of formaldehyde fluorescence. The hydrocarbon flame bands, found a t lean mixtures, have not previously been reported in engine studies. Except for such ident'ifications, the applicabilit'y of the present apparatus is limited because the spect'ra are integrat'ed over the entire cycle. Only when spectra are observed as function of time can the results be used for quantitative interpretation. LITERATURE CITED
(1) Broida, H. P., Levedahl, W. J., and Howard, F. L., J . Chem. Phys., 19, 797 (1951). (2) Draper, C. S., and Li, Y . T., J . Inst. Aero. Sci., 16, 10 (1949). (3) Emeleus, H. J., J . Chem. Soc., 1926, 2948. (4) Hirschfeld, M. A,, and Miller, C. D., Natl. Advisory Comm. Aeronaut., T N 1408 (1947).
ANALYTICAL CHEMISTRY
1780 (5) Jost, IT., “Explosion and Combustion Processes i n Gases,” New York, IlcGraw-Hill Book Co., 1946. (6) Leary, R.A . , Taylor, E. S., Taylor, C. F., and Jovellanog, J . E., Natl. Advisory Comm. -4eronaut. TN 1332 (1948). (7) Ibid., 1470 (1948) (8) Levedahl, W. J., and Howard, F. L.. Ind. Eng. C h o n . . 43, 2805 (1951). (9) Levedahl, I T J., and Howard, F. L., J . Research .\-ut/. Bur. Standalds. 46, 301 (1951). (IO) Levedahl, IT-. J., and Sargent, G. W.,Jr., thesis, l3.P.. XI:., \ Massachusetts Institute of Technology, 1948. (11) Lewis, B.. and \-on Elbe, G., “Combustion, Flanirs niid Explosions of Gases,” S e n York, Academic Press. 1951. ( 1 2 ) Livengood, J. C . , aiid Lenly, IT.A, I n d . Eng. C ’ / I V ~ . 43, “97 (1951).
Pastell, D. L., SAE Quart., 4, 438 (1950). Pease, R. N.,J . A m . Chem. Soc., 51, 1539 (1929). Retailliau, Ricards, and Jones, S A E Q ~ a r t . 4, , 438 (1950). Taylor, C. F., and Taylor, E. S., “The Internal Combustion Engine,” New I-ork, International Publishers, 1938. (17) Taylor, C. F.,Taylor, F.. S.,Livengood, J . C.. Russell, W ,A , , and Leary, I T . -1.. S B E Quart.. 4, 232 (1950). (18) Townend, D. T. A , , Chem. Reus.. 21, 259 (1937). (19) Vaidya. K.AI., Proc. Rou. Soc., 147, 513 (1934). ( 2 0 ) Walcutt, C.. and Rifkin. E. B., I n d . Ena. Chem., 43, 2844 (19521. 121) Kithrow. L., and Rassueilel, G. M.. &., 25, 1358 (1933). (13) (14) (15) (16)
( 2 2 ) Ihid.. 27, 872 (19%).
Spectrophotometric Determination of Uranium by Thiocyanate Met hod in Acetone Medium CARL E. CROUTHAMEL AND CARL E. JOHNSON, Argonne National Laboratory, Chicago, 111. The spectrophotometric methods for uranium current11 in use are all limited seriously in usefulness by either relatively low sensitivity or large numbers of cationic and anionic interferences. The peroxide methods which have been most widely used are relatively insensitive. The aqueous thiocyanate procedure has also been w-idely applied, but is subject to a large number of anionic interferences. The use of the thiocyanate acetone media has eliminated the majority of the anionic interferences of aqueous thiocyanate method, increased the sensitivity, increased the stability of the color, and made the correct color development independent of pH in the acid region. Standard deviations in precision and accuracy of &0.5% were obtained. .4nalyses of a large variety of aqueous and organic uranjl solutions are now possible with no separation of uranium from sulfate, citrate, phosphate, fluorsilicate, fluoride, copper, zirconium, iron, tin, mercury, nickeI, and manganese.
A
RIETHOD which is relatively free from both anionic and
cationic interferences has been developed for the deterinination of uranyl ion. About 0.01 mg. of uranium per milliliter \ d l develop an absorbance of about 0.20 in 1-em. cells. Thc molar absorbance index is approximately 3850. Relatively large amounts of copper, zirconium, tin, mercury, manganese, sulfate, fluoride, acetate, chloride, and nitrate do not interfere. More than unit molar ratios of foreign ion-uranyl ion may be present if the foreign ion is fluosilicate, phosphate, citrate, nickel, chromium, or iron. Lead, cobalt, niobium, and molybdenum interfere a t low concentrations. The thiocyanate method as developed by Currah and Beamish ( 4 ) and other (6, 12) has relatively high sensitivity and freedom from cation interferences. However, anion interferences have severely limited the applications of this method. Other spectrophotometric methods for uranium ( 5 , 14, 15) have been examined in this laboratory and found t o require excellent separations of iron, copper, zirconium, and many other interfering cations. -4 thorough investigation of the thiocyanate method indicated that very good results in both precision and accuracy ( & 0 . 5 9 ) could be obtained at 375 m p using fresh aqueous thiocyanate solutions and using several drops of 10% sodium thiosulfate to reduce interfering ferric ion. Stannous chloride, in the presence of uranium and ammonium thiocyanate in aqueous solution, as recommended by Currah and Beamish, was found to generate an interference
peak around 375 mk, which \vas not too serious when the thiocyanate was fresh but became large and rapid in forming as the thiocyanate solution aged. I n aqueous solution reducing agentP stronger than stannous chloride generally react more rapidly x i t h the thiocyanate and cannot be used to remove the iron interference. Fortunately, in acetone media the attack of thiocyanate by the stannous chloride was greatly inhibited and its use is reconimended. Thiosulfate will reduce iron in the aqueous solution ; however, the addition of the acetone-thiocyanate solution reversed the reaction and the red ferric thiocyanate complex slowly returned. The use of amber storage bottles did not decrease the aging effect or the sensitivity of the ammonium thiocyanate to reducing agents. The major difficulties with the aqueous thiocyanate method are the rather poor color stability and the large number of anion interferences. Thus, for the purpose of obtaining a medium which would eliminate the anion interferences and >-etretain the original sensitivity and freedom from cation interference, the authors turned to a system of lower dielectric constant in which the ionization of the large majority of weakly acidic anion3 ie greatly suppressed. The acetone-water system used in this procedure will protect the analyst from the color bleaching effects of such moderately strong acids as the sulfate and fluosilicate ions by suppressing their ionization (see Figure 5 ) . In the latter case the same effect x a s not accomplished by using a medium of 4 31 perchloric acid. The use of strong acid solutions to suppress the ionization of interfering anions was tried and judged not as effective as t,he acetone. I n strong perchloric acid the color stability was poor and precipitat’es of many cations int,erfered. The ammonium or sodium salt of thiocyanic acid is recommended over the potassium salt, as these salts caused no insoluble salt formation a t the high acetone-water ratios used in the procedure. Full advantage of the fact that thiocyanic acid is a very strong acid (?, 1.3) is not reilized in removing anion interferences unless the high acetone-water ratio is employed. The use of acetone in a somewhat similar manner has been reported for cobalt (11, 1 6 ) and iron ( 1 ) . REAGENTS
.Immonium thiocyanate, reagent grade, Baker and Adamson. Acetone, reagent grade, Baker and Adanison. saturated with ammonium thiocyanate a t room temperature (3.25 t o 3.50 Ai‘). Stannous chloride, reagent grade, Baker and Adamson. Ten grams dissolved in 10 ml. of concentrated hydrochloric acid, diluted t o 100 ml. and fikered through Whatman 42 paper. Standard solutions were prepared from Sational Bureau of Standards U,Os, LIS-ST, dissolved in perchloric acid. Concentrated sulfuric acid, reagent grade, Baker and Adamson.
