I
K. J. PIPENBERG and A. J. PAHNKE Petroleum Laborqtory, E. 1. du Pont d e Nemours & Co., Inc., Wilmington, Del.
Spectrometric Investigations of n-Heptane Preflame Reactions in a Motored Engine
I9
The chemical reactions responsible for the cool, blue, and intense blue flames observed in motored engines and preceding autoignition of end gases and knock in fired engines involve progressively more complete degradation and oxidation of the fuel. The tetraethyllead retards further reaction of carbonyl intermediates, possibly by deactivating free radicals acting as chain carriers. G E N E R A L L Y accepted theory (75) considers knock in spark ignition engines as an abnormally rapid combustion of the unburned fuel-air mixture ahead of the normal flame front. This is believed to be the final step in a series of preflame reactions occurring in the “end gas” region, The extremely rapid and complex nature of these preflame processes has been a major deterrent in fundamental understanding of the knock problem. In recent years, many experimental techniques have been developed to overcome the many difficulties encountered in studying knock in engines. Considerable progress has been made in expanding our knowledge of the
physical and chemical processes associated with the knock phenomenon. Further work is necessary, however, before knock is completely understood, particularly investigations defining the chemical reactions which lead to its occurrence. Much of the available information concerning the identities and concentrations of various reactants taking part in the preknock reactions has been obtained by using a variety of gas sampling techniques (4, 7, 76, 78). This approach, while providing very valuable information concerning the general nature of preflame reactions, is subject to severe limitations inherent in the sampling operation (20). Spectrometric analysis of the reacting mixture in the combustion chamber, although experimentally difficult, overcomes these limitations by permitting direct analysis of the gases during the reaction without disturbing the reaction. Preflame reactions of paraffin hydrocarbons are believed to involve two or more reaction processes, which can be defined on the basis of observed radiation effects. A pale blue luminescence, known as cool flame radiation, is associated with an early stage in these preflame reactions, and has been the object of considerable study (3, 6, 73, 77). Recently, a second radiation phenomenon described as a blue flame or bright blue flame has been observed during a
Engine spectrometer used t o study preAame reactions
..... ... .Optical _..-.-Mechanical _ - -Electrical
ENGINE CHAMBER MECHANICAL TAKE OFF FROM ENGINE
..-
later stage of the reaction (6, 78). The reactions accompanying the appearance of blue flame radiation have been cited by Sturgis (78) as those just preceding the final autoignition process. An objective of the present investigation was to obtain a clearer understanding of the various radiation phenomena and the reactions which accompany them. nHeptane, a hydrocarbon which has been investigated extensively in the past, was selected for these spectrometric studies.
Experimental A special spectrometer, designed and developed in cooperation with the Farrand Optical Co. for engine investigations of combustion phenomena, was used in conjunction with an ASTM Supercharge method engine to study the preflame reactions of n-heptane. This instrument automatically record? emission or absorption spectra of &e reactants in the combustion chamber of the engine, and with appropriate changes in optics can cover the wave length region from 0.3 to 15 microns. A unique mechanical shutter arrangement contributes to the versatility of the instrument in studying the very rapid processes occurring in the engine. One mode of shutter operation opens the shutter stroboscopically at a given point in the engine cycle and permits a complete spectrum to be obtained at this preselected point. I n a second type of operation the shutter opening position slowly precesses through the engine cycle, making it possible to follow the spectral intensity a t any preselected wave length as a function of engine crank angle. These two types of shutter operation provide the means for identifying chemical species present at any time in the engine cycle and to follow the concentration of selected species throughout the reaction. The spectrometer consists of a light source and a monochromator located on opposite sides of the engine. Two special fused quartz or sodium chloride windows mounted in opposite walls of the combustion chamber provide an optical path for the spectrometer. The source image is focused at the center of the combustion chamber and then refocused at the entrance slit of the monochromator. A standard Farrand U V I R double monochromator is used with interchangeVOL. 49, NO. 12
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DECEMBER 1957
2067
I A
COOL FLAME B3L U E F L1 A M E
COOL F L A M E
B
A
C
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Table I.
INTENSE BLUE F L A M E
Engine speed, r.p.m. Jacket temp., C. Inlet air temp., O C.
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Ratio
Manifold Air Pressure, Inches Hg -4bs.
0.066
25
0.033 0.016
30
Fuel-4ir
300 CRANKANGLE -DEGREES
Figure 1.
130
TOG
30
60
C R A N K A N G L E - DEGREES
CRANKANGLE-DEGREES
Total preflame radiation oscillograms IP28
Photomultiplier detector unit
A 0.033
Fuel-air ratio Manifold air pressure, inches Manifold air temperature, C. Compression ratio
30
93 6.7:l
able fused quartz, lithium fluoride, and sodium chloride prisms. Three radiation detectors span the various regions of the spectrum: 1P21 photomultiplier tube for 0.3 to 0.8 micron, Eastman Ektron lead sulfide cell for 0.7 to 3.3 microns, and a Farrand-Hornig thermocouple for 1.5 to 15 microns. The detector signal developed by the pulsating light signal is amplified by Farrand alternating current amplifiers tuned to light chopping frequencies of 5 to 20 cycles per second which correspond to engine speeds of 600 to 2400 r.p.m. The amplified signal is rectified and filtered before going to the Leeds &
Engine Experimental Conditions 1800 100 93
Compression Ratio Cool Cool flame flame and (1 max- blue flame imum) (2 maxima) 7.2 6.7 7.2
30
>7.2" 8.0
9.9
a Autoignition occurred before a stable blue flame appeared.
C
6 0.033 30 93
0.016 30 122
8.0:l
12.9:l
wave length and a plot of spectral intensity u s . crank angle position is obtained. Total radiation diagrams were obtained simultaneously with the recording of the spectral data by mounting a unit containing a 1P28 photomultiplier detector directly on the engine, utilizing an additional opening in the combustion chamber wall. The amplified signal wa5 observed on a dual-beam oscillograph whose horizontal sweep was synchronized with the engine and was used to set the shutter opening position of the spectrometer to correspond to cool flame and blue flame regions. The ASTM Supercharge method engine used in these investigations was operated at the motoring conditions shown in Table I. The stroboscopic shutter was set for an opening time of 20 crank angle degrees. 'iVith this shutter opening, cool flame radiation and blue flame radiation could be studied independently.
Northrup 10-mv. recorder having a response speetL of 1 second for full-scale deflection. The monochromator and recorder are driven through a variable ratio reduction gear by a constant speed motor. The stroboscopic shutter driven through an auxiliary drive shaft on the engine can be set to open at any desired crank angle position and with this setting, a complete spectrum can be obtained. The use of the auxiliary phase advance mechanism causes the shutter opening position to precess slowly through the complete engine cycle. With the latter operation the monochromator is set at a specified
Figure 2. Ultravioletvisible emission spectrum of cool flame
Prefiame Radiation
Three distinct radiation phenomena-cool flame, blue flame, and intense blue flame-have been experimentally observed to accompany the preflame reactions of n-heptane in a motored engine. Total radiation oscillograms showing the
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MAP
= 25
M4T=93'C, CR=7.2:1
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Figure 3.
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Near-infrared emission spectrum of cool flame
INDUSTRIAL AND ENGINEERING CHEMISTRY
4 0
5 0
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WAVE LENGTH-MICRONS
Figure 4.
6 0
70
8 0
9 0
LENGTH-MICRONS
Infrared emission spectrum of cool flame
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AUTOIGNITION
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Figure 5. Cool flame and blue name emission spectra in ultraviolet-visible region
existence of these radiation effects have been obtained using the 1P28 photomultiplier detector unit (Figure 1). At a one-half stoichiometric fuel-air ratio and a compression ratio of 6.7 to 1, a single cool flame maximum was obtained with peak intensity occurring slightly after top dead center (Figure 1,A). When the engine conditions were made more severe by increasing compression ratio, the cool flame peak intensity increased and shifted forward. A second maximum described as a blue flame developed on the descending slope of the cool flame trace ( B ) . With further increase in engine severity, the blue flame maximum increased in intensity, became very unstable, and appeared to shift to later in the cycle. Autoignition, as evidenced by a sharp rise in the radiation intensity, occurred later in the cycle than the initial blue flame emission. This tends to support previous findings (78) that the blue flame reactions precede the autoignition process. T o stabilize the radiation effects occurring just prior to the point of autoignition, the fuel-air ratio was reduced to one fourth stoichiometric conditions. Under this type of operation, a rather unstable emission described as an intense blue flame was obtained having an intensity as much as 50 to 100 times greater than cool flame emission. An oscillogram of this emission pattern is reproduced in C. The relationship of the intense blue flame to the blue flame has not been definitely established, although observations made at one half stoichiometric fuel-air ratio conditions suggested that the intense blue flame may be nothing more than the blue flame with greater intensity; the possibility exists, however, that the intense blue flame is an entity in itself, distinctly different from the blue flame, since a weak maximum possibly attributable to the blue flame can be seen in C. It was definitely estab-
Figure 6. Cool flame and blue flame emission spectra in nearinfrared region
lished that the intense blue flame was not autoignition, as increasing compression ratio resulted in autoignition. Previous investigators (5, 70, 74) have suggested pregame processes of n-heptane to be a multistage sequence of reactions and have charxterized individual steps on the basis of observed radiation and ignition behavior. The reactions accompanying blue flame and intense blue flame emissions are very important, in that they appear to precede final autoignition. Sturgis (78) has presented gas sampling analyses of the reaction mixture demonstrating that the course of the preflame reactions changes markedly when blue flame reactions occur. He concluded that the blue flame reactions were the start of the autoignition reactions leading to the formation of the "hot" flame.
Cool Flame Spectra Emission spectra obtained in the cool flame region with the engine spectrom-
eter are reproduced in Figures 2 to 4. These spectra, covering the wave length region from 0.3 to 12 microns, were obtained under stoichiometric fuel-air ratio conditions which produced a total radiation oscillogram similar to the one of Figure 1,A. Emission spectra for the ultravioletvisible region '(Figure 2) are in excellent agreement with results of Emeleus (8), Fowler and Pearse (see Ubbelohde, 79), Kondratjew (9), and Levedahl and Broida ( 7 7). Excited formaldehyde is the predominant cool flame emitter present in this region. There was some indication of emission bands in the wave length interval of 0.50 to 0.65; however, the intensity of these bands was too low for positive identification. They may be similar to those reported by Downs, Street, and Wheeler ( 5 ) . Near-infrared (1 to 3.5 microns) emission spectra are presented in Figure 4 for both a stoichiometric n-heptaneair mixture and a n-heptane-nitrogen
FIA 0 . 0 3 3 MAP= 3 0 IN. MATS 93-C C R = 8.0:I
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Cool flame and blue flame emission spectra in infrared region VOL. 49, NO. 12
DECEMBER 1957
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6 0 70 L E N G T H -MICRONS
mixture having a n-heptane molar concentration equal to that of the n-heptaneair mixture. The following bands were isolated in the n-heptane-air spectrum:
Wave Length, Microns
Emitter
1.86 1.93 2.33 2.45 2.56 2.68 3.30
Hz0 Hz0 C-H C-H Hz0, COz C02, H20 C-H
.
The water and carbon dioxide bands were readily identified from literature values, while the carbon-hydrogen bands were identified from the thermal emission spectrum obtained in the nitrogen atmosphere, These near-infrared data are in essential agreement with findings of Agnew, Agnew, and Wark (2) and confirm Agnew's suggestion ( 7 ) that 2.3micron emission bands are due to the carbon-hydrogen bond. A comparison with the emission spectrum of compressed air shows the intensities of the carbon dioxide and water bands to be reduced significantly when n-heptane is present. presumably because of a temperature decrease caused by fuel vaporization. KO evidence was obtained for the strong hydrogen peroxide band at 2.92 microns, which arises from peroxide 0-H vibrations. O n the basis of earlier chemical studies (76): it is known that hydrogen peroxide and organic hydroperoxides are formed during the n-heptane preflame reactions. A reference standard containing 50 mmoles of tert-butyl hydroperoxide per mole of benzene was inducted into the engine under conditions giving a strong benzene emission spectrum. KO evidence was obtained for the peroxide 0-H vibrations, which indicates the instrument was too insensitive OF the hydroperoxide was thermally decomposed by the heat of compression. Infrared emission spectra from 1.5 to 10 microns are reproduced in Figure 4
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Figure 8. Cool flame and blue flame absorption spectra in infrared region
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for both the n-heptane-air and nheptane-nitrogen mixtures. A compressed air spectrum obtained under the same engine conditions is superimposed on the n-heptane spectra. Characteristic bands due to water, carbon dioxide, carbon monoxide, hydrocarbon, and carbonyl groups were obtained (Figure 4). A comparison of the cool flame and thermal emission spectra reveals a shift of the C--H band at 3.43 to 3.56 microns when cool flames are present. This shift in addition to the strong carbonyl band at 5.6 microns suggests that aldehydic carbonyl compounds are being formed in the cool flame region. The spectral changes in the bands at ? microns reflect degradation of n-heptane to other hydrocarbon species. Possible structural changes accounting for this effect include formation of olefins through dehydrogenation or decomposition of alkyl peroxy radicals, lormation oi" carbonyl compounds through oxidation, and changes in the ratio of CH2 to CH3 through cracking or branching reactions. Although emission bands were observed in the region beyond 10 microns, they were too broad to permit valid interpretation. The lack of detail in this region as well as in the remainder of the infrared region is due to optical losses through the combustion chamber window,
the thin film of lubricating oil covering the inside surface of the window, and the 20 crank angle degree shutter. Blue Flame Spectra
'The transition from the cool flame to the blue flame region on the basis of the present spectrometric investigations apparently involves only changes in the concentrations of various reactants, as no evidence pointed to formation of new chemical species. This is shown by comparison of the emission and absorption spectra for the blue flame region with similar spectra for the cool flame region (Figures 5 to 8). Both the blue flame and the cool flame spectra were obtained under the same engine conditions by varying the shutter opening position to correspond to the individual radiation regions as defined in Figure 1,B. In the ultraviolet-visible region (Figure 5), the intensity of the cool flame radiation is approximately three times that of the blue flame, indicating a considerable reduction in the number of excited formaldehyde molecules in the blue flame region. 'The poorer resolution of the spectra in Figure 5 compared to the spectrum given in Figure 2 is due to increased slit widths and greater flame instability. Near-infrared spectra of the cool flame and blue flame maxima (Figure 6) are similar in nature except for intensity. Contrary to the observations in the ultraviolet-visible region, the blue flame spectrum appears to be three to five times more intense than the cool flame spectrum. Increased concentration of water and carbon dioxide coupled with a higher gas temperature in the blue flame region is a possible explanation for this increased intensity. Comparisons of cool flame and blue flame infrared emission spectra (Figure ?), show the two preflame regions to be spectrally similar except for an over-all greater intensity of the blue flame emission. The increase in the intensit) of the unresolved carbon monoxide-
Figure 9. Cool flame, intense blue flame, and hot flame emission spectra in ultraviolet visible region
INDUSTRIAL AND ENGINEERING CHEMISTRY
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- MICRONS
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AUTOIGNITION carbon dioxide band is proportionally greater than increases in either the C-H stretching band or the carbonyl band. In the infrared absorption spectra (Figure 8), an increase in the carbon monoxide absorbance and a decrease in the carbonyl absorbance are observed in passing from the cool flame to the blue flame regions. In Figure 8, the spectra have been displaced along the transmittance ordinate for ease of comparison. These observations indicate that reaction and conversion of the carbonyl compounds formed in the cool flame region occur in the blue flame region and lead to carbon monoxide, carbon dioxide, and water. The utility of infrared absorption data is considerably reduced by experimental difficulties in obtaining such "data in engines. As shown by Figure 8, the air supply contributes markedly to absorption at the water and carbon dioxide bands. Oil on the surface of the windows in the combustion chamber results in absorption at the C-H frequencies and decreases the sensitivity of the instrument in recording changes in absorption at these wave lengths due to changes in the gas phase. The near-infrared and infrared emission spectra are complicated by a self-absorption effect, particularly in the water and carbon dioxide regidns. which results in absorption bands being superimposed on emission bands, causing decreased intensities and shifted wave length positions. Intense Blue Flame
The intense blue flame appears to be a transitional stage in the course of the final autoignition reactions as reflected by the emission spectra for these phenomena (Figures 9 and 10). The intense blue flame spectra were obtained under engine conditions which produced an oscillogram similar to that of Figure 1,C. To stabilize the engine long enough to obtain the emission spectra, however, it was necessary to operate the engine at the following conditions: fuel-air ratio ( F / A ) 0.015; manifold air pressure (MAP), 60 inches of mercury absolute; manifold air temperature (MAT), 204' C.; compression ratio (C.R.), 7.5:l. Cool flame emission spectra and hot flame spectra from a firing engine are compared with intense blue flame spectra in Figures 9 and 10. I n the ultraviolet-visible region the emission spectrum for the intense blue flame (Figure 9), contains emission bands for carbon dioxide, fluorescent formaldehyde, and HCO radical, all superimposed on a carbon monoxide continuum. The emission bands at 0.330, 0.338, 0.342, 0.350, and 0.359 micron identified as HCO bands are the first direct evidence obtained in these spectrometric studies of preflame reactions which indicates free radical formation. No evidence was
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2_ I N T E N S E BLUE FLAME COOL FLAME
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Figure 10.
Carbonyl-Time Relationships
Carbonyl compounds are known to be important intermediates in the preflame oxidation of most hydrocarbons in engines. Pahnke, Cohen, and Sturgis (76) formulated a general mechanism in which
I
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1
3.5
Cool flame, intense blue flame, and hot flame in near-infrared region
found for the OH band at 0.31 micron, which is a strong indication that the intense blue flame is not hot. The ultraviolet-visible emission spectrum of the hot flame reproduced in Figure 9 is composed of OH, HCO, CH, and C Z emission bands superimposed on the carbon monoxide continuum. The near-infrared emission spectrum of the intense blue flame region (Figure IO) is very similar to the hot flame emission spectrum of the fired engine operating under lean conditions. No C-H emission bands at 2.3 to 2.4 microns were observed, indicating that a major portion of the n-heptane has been oxidized to compounds of low molecular weight. This indication of extensive n-heptane reaction, together with the finding that the intense blue flame is not a hot flame, suggests that the intense blue flame is a transitional state in the sequence of final autoignition reactions.
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3.0
L E N G T H - MICRONS
carbonyl compounds were suggested to be intermediates formed in the further reaction of alkyl peroxy radicals. Degradation of the carbonyl compounds to water, carbon monoxide, and carbon dioxide in a series of reactions involving free radicals formed from peroxide decomposition was postulated to be an important energy-releasing process occurring at the time of autoignition. In a later study, Sturgis (78) showed the final reactions leading to autoignition of nheptane to involve further reactions of higher carbonyl compounds formed in the early stages of the n-heptane preflame oxidation. To learn more about the role of carbonyl compounds, their formation and decomposition were followed throughout the course of the preflame reactions, using the engine spectrometer. This was accomplished by setting the spectrometer at the 5.8-micron carbonyl absorption band and using the shutter precessing mechanism, which permitted the carbonyl absorbance to be determined as a function of engine crank angle. Relative absorbance was converted to relative carbonyl concentration by using absorption coefficients experimentally determined in a motored engine with acetone as the reference standard.
n
RADIATION ~NTENS~TY
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j i COOL FLAME AND
A
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RADIATION INTENSITY71
COOL FLAME
BLUE FLAME
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RADIATION INTENSITY
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330
TDC
30
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CRANKANGLE -DEGREES
Variation in carbonyl concentration throughout engine cycle VOL. 49, NO. 12
DECEMBER 1957
2071
Figure 12. Relation of blue flame radiation to carbonyldegradation process
BLUE F L A M E RADIATION /\,/mIFFERENcE BETWEEN RADIATION I N T E N S I T I E S OF I FIGURES 1 2 A B 12 B 1 I \
