Formaldehyde Formation by Preflarne Reactions in an Engine Spectroscopic Study LLOYDWITHROWAND GERALDM. RASSWEILER, General Motors Research Division, Detroit, Mich. A number of experiments have been performed with a n engine to study the spectral absorption in that portion of the combustion space where knock occurs when present. The results are as follows: (1) Formaldehyde has always been found in the noninjlamed gases under knocking conditions. (2) W h e n either isopropyl nitrite or diethyl peroxide is added to the fuel to induce knock, the formaldehyde concentrat ion increases. (3) Two fuels which produce the same intensity of knock under the same sets of engine conditions m a y not f o r m the same maximum concentrations of formaldehyde in the noninflamed gases. (4) Formaldehyde is frequently detected in the
T
HIS investigation is a continuation of work previously
reported (4, 6) on the study of the chemical reactions in the noninflamed gases in an engine, just prior to knock. The fact that the occurrence of knock is preceded by chemical changes in the noninflamed gases has been established by studying the spectral absorption of initially transparent fuel-air mixtures when they were producing different degrees of knock in an engine. Just prior to its inflammation, the portion of the charge which subsequently produces knock shows region of continuous absorption between 2100 and 3400 A. upon a portion of which is superimposed some banded structure. Formaldehyde is responsible for the banded structure; other compounds effect the continuous absorption. Under the engine conditions maintained in these earlier experiments, there seemed to be a definite relationship between formaldehyde formation in the noninflamed gas and the occurrence of knock immediately thereafter, as evidenced by two sets of observations: First, the formaldehyde bands were detected in the knocking zone with a variety of fuels under knocking conditions and were not detected with a number of nonknocking fuels. Second, a n experiment in which the ratio of constituents in a blend of ethanol and diethyl ether was varied to decrease and finally to remove knock, showed that the knock and the formaldehyde bands disappeared almost simultaneously. Inasmuch as formaldehyde is an oxidation product which can easily be identified in the noninflamed gases by spectroscopic means, it was hoped that the concentration of this material might furnish a criterion of the extent of the reactions. The study of the relationship between formaldehyde formation and knock has therefore been extended to cover a wide range of engine conditions. The effects of such variables as spark advance, mixture ratio, heat content of the intake mixture, speed, and fuel have been investigated, together with the effect of knock inducers and suppressors. This paper summarizes the results of these experiments. APPARATUS The spectrograph, the pressure indicator, and the engine with its two windows, one on each side of a rectangular-
noninflamed gases under nonknocking conditions, but the absorption bands disappear if the octane number of the fuel is suficiently raised by some means other than adding tetraethyllead or if the other engine variables are adjusted to reduce suficiently the tendency of the engine to knock. (5) Addition of tetraethyllead to suppress knock does not affect the formaldehyde concentration appreciably even though the concentration of tetraethyllead in the fuel is much more than suficient to remoue all the evidence of knock. On the other hand, addition of enough aniline to the fuel io suppress knock completely, decreases the formaldehyde concentrat ion. ended combustion chamber, have already been described (6). Only one change in the apparatus merits mention. The slotted disk which acts as a stroboscopic shutter is now mounted directly on the crank shaft of the engine. An auxiliary disk rotating a t cam-shaft speed cuts off the light from the source during the intake stroke. This change in the apparatus eliminates the lash in the train of gears which previously linked the disk to the crank shaft, thereby assuring a constant exposure angle and allowing this angle to be adjusted statically. ,111 the spectra discussed herein were exposed in the knocking zone-in other words, a t the end of the combustion chamber farthest from the spark plug. The region investigated extended out 3 / ~inch (0.48 em.) from the end wall of the combustion space. The light sources were an incandescent-tungsten-band lamp in a quartz envelope and an underwater spark between beryllium electrodes, both of which have already been described (4, 6 ) . Owing to the fact that the gasoline was only partially vaporized in the intake manifold, measurement of temperature in the intake mixture by any ordinary means was questionable. But in order to estimate qualitatively the amount of heat added to the fuel-air mixture, a n exposed thermocouple was suspended near the intake port in the middle of the gas stream. The intake temperatures referred to herein are readings on this thermocouple. The nomenclature for the gasolines is the same as t h a t employed in earlier papers where they are described (4,B). DESCRIPTION OF DATA Inasmuch as a previous paper (4) outlines the difficulties involved in the photography and interpretation of spectra taken to study the formaldehyde concentration in t h e engine, a detailed discussion of these complications will b e omitted in describing the data in the present paper. Nevertheless, when interpreting the spectra the following pertinent factors have been given due consideration : (1) Formaldehyde is formed in the noninflamed charge but i t has only been detected very close to the flames; (2) the flames advance
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INDUSTRIAL AND ENGINEERING
along a curved front; (3) the portions of the charge which burn last are located in the corners of the combustion chamber and therefore light knocks occur in the corners. Thus, if spectra are exposed just before the occurrence of light knocks, the optical path is not homogeneous; that is, the gases located therein are partially inflamed and partially
FIGURE1. FORMlLDEHYDE BANDSDIS.iPPEAR ]?-HEY KNOCK Is REMOVED BY RICHENING THE MIXTURE 1. Source.
2. Theoretical air, 60 per cent; not knocking. air, 80 per cent; knocking.
3. Theoretical
noninflamed. The same condition exists in comparable exposures made under nonknocking conditions. It becomes apparent then that the length of the absorling column of noninflamed gas is not the same in all experiments. Under certain conditions, the intensity of the formaldehyde absorption is influenced by this changing length as Tell as by the concentration of the-absorbing material. Another variable which influences the extent of the formaldehyde absorption is the total pressure in the combustion chamber. As the burning gases expand, they compress the noninflained portion of the charge and the absorption exhibited by a given concentration of formaldehyde is increased. In comparing spectra exposed under knocking and nonknocking conditions, the effects of pressure and of length of absorbing column can usually be canceled because of the fortunate circumstance that, prior to knock, the rates of pressure rise as well as the rates of flame travel are almost the same in knocking and nonknocking explosions ( 5 ) . The comparisons of formaldehyde concentrations are only semi-quantitative. The complete disappearance of the formaldehyde bands from the spectra indicate.; that the formaldehyde concentration has dropped below some limiting value which can no longer be detected by the method used. The magnitude of this concentration has not heen determined spectroscopically, but quantitative determinations of the formaldehyde concentrations in samples taken from the same engine fitted with a sampling valve on a different cylinder head give an indication of the concentrations dealt with spectroscopically. These data will be described in a later paper. In general, in studying the effect of an engine variable on the formaldehyde concentration, a series of spectra wa5 photographed under each of two sets of engine conditions. The various members of each series were exposed at different crank-shaft angles-that is, a t different times with respect to the arrival of flames in the optical path. The duration per explosion of each exposure was about 3 crank-ahaft degrees. The individual spectra in each series were photographed a t intervals of 3" or 4' of crank-shaft revolution
CHEMISTRY
12.57
throughout the part of the engine cycle in which formaldehyde bands could be detected. Since, on the spectra so obtained, the formaldehyde bands are very diffuse and therefore difficult to reproduce, microphotometer records are presented in Figures 1 to 10 in place of the spectra. Photometer records from all the spectra exposed while studying the effect of a single engine variable are not presented. Instead, photometer records mere made from the one or two spectra in a group Tvhich showed the maximum intensity of the formaldehyde bands, or, if no bands appeared in any member of the group, from the spectra exposed a t those angles where the bands would be most likely to appear. In some figures there is also included a trace made from the spectrum of the source without absorbing material in the light path. Before discussing the microphotometer records separately, their common features will be pointed out. Except for Figure 1, the-records cover the spectral range between 3200 and 3330 A . which includes two formaldehyde ban4s having their short wave-length edges a t 3250 and 3285 A. These two edges are indicated in each figure. The dips in the curves, or the changes in slope, t o the right of the marks represent formaldehyde absorption bands; the small variations in each trace should be disregarded. Inasmuch as the backgrounds in comparable spectra were of very nearly the same intensity, it was necessary t o shift the zero of the galvanometer in order to avoid superimposing the traces. While exposing each spectrum, a composite pressure record of sev4 era1 explosions was g 5 photographed. Perti2 nent pressure c u r v e s ? 6 are shown in F i g u r e 2 11. T h e y a r e n u m bered according to the 20 j corresDonding uhotomI 1 e t e r t r a c e . On some 4 4 of the records there is, N in addition to the zero !k pressure line, a second FIGURE 2. IKDUCING KNOCKBY h o r i z o n t a l l i n e on ADVANCING THE SPARK INTEXSIFIES which a p_ p_ e a r s a deFORMALDEHYDE BANDS flection used for tim- 4. Source 5 . Engine not knocking; spark, ing purposes. On each 13'. 6. Engine knocking llghtly; spark, 28'. p r e s s u r e card a vertical white line indicates the angle at which the spectrum was exposed. The width of this line represents approximately the duration of the exposure.
t
U
A
+
LC
EFFECTox FORMALDEHYDE BAI~DS OF SUPPRESSING KNOCK BY CHASGIKG hIIXTURE RATIO,SPARK -%DVANCE, I S T A K E hfIXTURE TEhfPERATURE, AND EKGINE SPEED Figure 1 is a n example of the results obtained when suppressing knock by richening the mixture. Trace 3, which was made from a spectrum exposed chiefly through noninflamed gases with the engine knocking on gasoline, reveals distinctly six formaldehyde bands in the spectral region covered. (The photometer records in Figure 1 were made up in sections. The abrupt peaks a t the ends of the sections are reference marks and should be disregarded.) Trace 2 was taken from a spectrum that was exposed about the same distance ahead of the flames as trace 3, but under nonknocking conditions. A comparison of trace 2 with trace 1 made from a spectrum of the source without absorbing material in the light path, shows clearly the absence of deflections attributable to formaldehyde. Pressure records 2 and 3 in Figure 11 denote the magnitude of knock removed and show that the pressure a t the time of exposure in the
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nonknocking run was only slightly lower than in the knocking run. These data thus indicate that suppressing knock completely by richening the mixture removes all traces of formaldehyde bands from the absorption spectra. Figure 2 shows the effect on the formaldehyde bands of inducing a knock by advancing the spark. Record 5 shows little or no trace of formaldehyde bands, but they are distinct
Vol. 26, No. 12
the results of which are also illustrated in Figure 4, was performed under slightly different conditions. As shown by traces 14 to 16, inclusive, the data are similar to those obtained in the first experiments. In the experiments just described, the evidence of formaldehyde formation in the noninflamed gas either disappeared or became very weak when knock was completely removed by changing the mixture ratio, spark advance, mixture temperature, or engine speed.
EFFECT02; FORMALDEHYDE BANDSOF SUPPRESSING KNOCK BY CHANGIXG COP~STITUEXTRATIOSIN FUEL BLENDSUKDER SEVERAL SETSOF ENGIKE CovDITIONS At this point several expressions should be explained. Despite the fact that the term “knock” is not uniquely defined and its units of measurement are not universally agreed upon, there is little ambiguity when one states that “the intensity of knock was reduced.” Throughout the present series of papers the intensity of knock has been estimated by the amount of the sudden pressure rise; and, although the authors hold no brief for this method, it has been found convenient and satisfactory for qualitative work. If one gradually reduces the knock by retarding the spark, for example, the knock finally d i s a p p e a r s ; the condition lo where it is barely detectable on one pressure card in about fifty, is designated herein “inc i p i e n t k n o c k . ” There is greater difficulty in describing the situation with regard t o knock when the spark is still further retarded. Such sets of engine conditions will be designated as “outside t h e knocking range” or, for variety, as “subknocking conditions” l4 or as “below incipient knock.” To date, no convenient way has been found for expressing quantitatively how far below incipient knock a given set of engine conditions is, and for lack of a better method the spark advance or the octane number of the fuel under the given set of conditions is sometimes compared with the spark advance or octane number of the fuel required to produce incipient knock, other conditions remaining unchanged. 4 .a The detailed study of t h e relation of knock to formaldehyde formation was continued FIGURE 4. FORMALDEHYDE BANDSDISAPPEARwHEN b y e x p o s i n g sets of spectra KNOCK 1s R E ~ ~ ~ o ~ E D while operating the engine on INCREASING ENGINESPEED a s e r i e s of blends the compositions of which were varied INTAKE THERMOt o c h a n g e progressively the COUPLE TRACESPEED READING KXOCK knocking tendency of the enR.p.m. OC. gine, Figure 5 presents 10% G A S O L I N E A -!s o m eexperiment of the datap e from 1500 90% Q A S O14 L I N E CNone such r f o r mone ed 10 None with blends of gasolines A and 11 1500 18 Moderate 12 600 13 600 18 Moderate c,with an engine speed of 1500 r. p. m., and with the fuel-air GASOLINE C 14 1500 m i x t u r e s t r o n g l y heated. 16 15 ‘ io”:120 None These engine conditions difA
”1
1
FIGURE 3. FORMALDEHYDE BANDS IIVTEN~IFIED BY HEATINGINTAKEMIXTURETO INCREASE KNOCK 7. 8. 9.
Intake thermocouple reading, 15’’ C.; incipient knock Intake thermocouple reading, 16’ C ; incipient knock Intake thermocouple reading, 224‘ C.; light knock
on record 6 which was taken from a spectrum exposed after advancing the spark to induce knock. The interpretation of this and similar spark advance experiments is rather difficult because, on advancing the spark, the pressure in the combustion chamber, just before the arrival of the flames, is increased (compare records 5 and 6, Figure 11) and the sensitivity of the spectroscopic method, as it is now used, is likewise increased somewhat. Just how much effect this changed sensitivity has on the formaldehyde bands has not yet been determined. The data in Figures 3 illustrate the effect of heat on the formaldehyde absorption by the noninflamed gases when the engine was running on a mixture of heptane and isooctane (2,2,4-trimethylpentane). With no heat added to the intake mixture, knock was so light i t could scarcely be detected. Trace 7 shows no formaldehyde bands and trace 8, representing conditions a t a slightly later angle, shows only a slight indication of the formaldehyde band a t 3250 A. As is evidenced by trace 9, the addition of heat to the in-going mixture plainly increases the intensity of the formaldehyde bands. Pressure records 8 and 9 (Figure 11) shorn the degree of knock introduced. Since the pressure at the time of exposure was less in the knocking than in the nonknocking run, the greater intensity of the formaldehyde bands in the former case cannot be attributed to difference in total gas pressure. Therefore, it follows from Figure 3 that increasing the degree of knock by the addition of heat to the fuel-air mixture before i t enters the engine, definitely increases the formaldehyde concentration. Figure 4 illustrates two different experiments in which knock was reduced by increasing the engine speed from 600 to 1500 r. p. m. Traces 10 to 13, inclusive, represent the results of the first experiment that was made with a blend of gasolines A and C. No heat was added to the intake mixture during these runs. As shown by pressure records 11 and 12 (Figure ll), the engine knocked moderately a t 600 r. p. m. and no evidence of knock was observed a t 1500 r. p. m. The two spectra represented by traces 12 and 13 were both exposed just prior to knock, though a t slightly different crank-shaft angles. There is strong evidence of formaldehyde bands in these two traces. Traces 10 and 11, which are comparable with 12 and 13, respectively, so far as crank-shaft angle and duration of exposure of the spectra are concerned, show only very slight indications of the presence of formaldehyde bands. The second experiment,
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INDUSTRIAL AND ENGINEERIKG CHEMISTRY
fered from those maintained in a previously reported experiment (4) that was performed a t 600 r. p. m. with ethanol-diethyl ether blends and with no heat added to the in-going mixture. Kevertheless, the results in the tR-o experiments were alike, for in both cases, when the constituents in the blends were gradually varied to reduce knock and finally to r e m o v e it, t h e e v i d e n c e of formaldehyde formation ahead 17 of the flames disappeared a t nearly the same time as the 18 knock. The t h e e traces in Figure 5 illustrate this 19 phenomenon for the gasoline blends. The degree of knock represented by trace 19 may b e j u d g e d f r o m t h e corresponding pressure r e c o r d in Figure 11. F o r m a l d e h y d e bands appear distinctly on this trace. Trace 18 r e p r e s e n t s conditions of incipient knock. Here the bands are very weak. A further change of 10 per cent in the ratio of the constituents removed the knock completely FIGURE 5. FORMALDEHYDE( r e c o r d 17, Figure ll), and BANDS DISAPPEARWITH only suggestions of the formb O C K AT 1500 Fi. P. M. aldehyde bands are present on WITH INTAKE 4IIXTURE HE.4TED trace 17, Figure 5. The above result is not always obtained. This fact is illustrated by the next experiment performed under the same conditions as the last but INTAKE with no heat added to the inTHERMOOCTANE COUPLE going m i x t u r e . Here the No. READING KNOCK c. formaldehyde bands were ob66 126 None served in spectra exposed well 63 124 Very light 60 124 Moderate below t h e k n o c k i n g range. The three traces in Figure 6 were all taken from spectra photographed with the &$ne operating under subknocking conditions. M'hen spectrum 22 was exposed, conditions were so far below incipient knock that the spark had to be advanced 15" to bring in knock. Despite this fact, the formaldehyde bands are distinct. The octane number of the fuel was raised three units for trace 21, decreasing still farther the tendency of the engine to knock, but the formaldehyde bands persist. They are obviously much weaker in spectrum 20 and eventually disappear entirely when the octane number of the fuel is sufficiently high. It is noteworthy that formaldehyde can be detected ahead of the flames under conditions so far removed from knock, and it is no less significant that the evidence of formaldehyde did finally disappear when the tendency of the engine to knock was sufficiently reduced. When the speed was reduced to 600 r. p. in., again using blends of gasolines A and C and adding no heat to the ingoing mixture, the relation betveen knock and the formation of formaldehyde seemed more exact; that is, there was less spread between the octane number of the fuel which produced incipient knock and the maximum octane number for formaldehyde detection. When the engine was operated on blends of benzene and gasoline, the formaldehyde bands were still more persistent than when running on blends of gasolines A and C. With the former blends the formaldehyde bands continued to appear below the threshold of knock a t 600 as well as a t 1500 r. p. in. But on adding sufficient benzene to the mixture, the bands finally faded out.
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The preceding data thus show that the relationship between the appearance of knock and of formaldehyde in the noninflamed gases is not exact and that the formaldehyde concentration in the gases prior to their spontaneous inflammation is dependent on the engine conditions and the character of the fuel used. Having studied the formaldehyde concentration in the noninflamed gases when operating with fuels of different octane numbers it was of interest to examine the formaldehyde concentration with two different fuels of the same octane number. Figure 7 shows some of the results obtained when comparing gasoline C (octane number 75) and a blend of 75 per cent isooctane plus 25 per cent n-heptane. While taking these spectra, the engine was operated a t 600 r. p. m. with no heat added to the intake mixture and with 85 per cent of the theoretical air in fuel-air mixture. Under these conditions the spark advance for maximum power was 11" and the spark advance at which incipient knock appeared was 12". When the spark was advanced farther, knock came in a t about the same rate with both fuels; that is, as nearly as could be detected by ear or by viewing the pressure cards, the knocking characteristics of the two fuels were the same. But there appeared to be a slight difference in the rates of flame travel. In the heDtane-octane blend the flames traveled somewhat slower than in the gasoline. Consequently, a t any given angle there was a slight difference in the times of exposure with respect to the arrival of flames in the optical path. This latter information was obtained by visual observations of the flames through the stroboscopic shutter. The estimated p r o p o r t i o n s of explosions d u r i n g w h i c h flames were visible a t each setting of the stroboscopic shutter are given in Figure 7. Traces 24 to 27, inclusive, in Figure 7 represent spectra photographed with the spark advance set for maximum power and with the engine FIGURE6. FORMALDEHYDE conditions just outside the BANDS OBSERVED AT 1500 knocking range. As can be R. P. M . U~YDER NONKNOCKISG CONDITIOXS WITH INTAKE seen f r o m F i g u r e 11, the MIXTURE NOTHEATED pressure cards were nearly FUEL identical when the engine was TRACE Gasoline C Gasoline A operating on these two differ% % 20 90 10 ent fuels. On s p e c t r a ex21 80 20 posed while using the blend, 42 io 30 the formaldehyde bands were clearly e v i d e n t , b u t they could scarcely be detected on spectra photographed while 72 14 None o p e r a t i n g o n gasoline C. 69 12 None 66 12 None Thus i t a p p e a r s t h a t the concentration of formaldehyde formed in the engine was greater with t,he blend than with gasoline C. Comparison of photometer records of spectra exposed a t the same angle (24 os. 25, or 26 us. 27) and a comparison of photometer records of spectra taken a t similar angles with respect to the arrival of the flames (25 os. 26) bear out this conclusion. Spectra 29 to 32, inclusive, were exposed after advancing the spark from 11" t o 20" to bring in a light knock. The corresponding pressure records in Figure 11 show this degree
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Vol. 26, KO. 12
of knock and also indicate that knock occurs a t a later angle assist in removing the lead from the engine, a gray subwhen operating on the heptane-isooctane blend. This latter stance, consisting chiefly of lead bromide, deposited rapidly phenomenon appears to be on the windows. This material was so opaque to ultraviolet related to differences in light and was deposited so rapidly that, when the fuel was the rates of flame travel changed to the leaded variety (by means of a valve at the when operating on these float bowl), the effect of the coating on the windows sometwo fuels. Formaldehyde times became noticeable in the spectra even before the lead b a n d s a r e d e f i n i t e l y in the engine was exerting its maximum effect on the knock. present in spectra taken It was therefore necessary to make the following changes in with both fuels; but again, procedure : (1) The underwater they are stronger in runs spark was substituted for the made while operating with incandescent f i l a m e n t as the the heptane-octane blend. source of continuous radiation. Spectra, for which no This cut the required exposure corresponding records are from 10 minutes to 15 seconds. reproduced herein, were (2) After changing fuels, spectra then exposed to compare were e x p o s e d as soon as the the blend with g a s o l i n e leaded fuel had completely reC when sufficient h e a t placed the base fuel in the l i n e was added to the in-going that is, as soon as the effect of mixture to raise the intake lead on the knock had reached FIGURE7. GASOLINE C PROa maximum. (3) The spectra DUCES A LOWER FORMALDEHYDE t h e r m o c o u p l e reading taken with the leaded fuel were from 27" to approximately CONCENTRATION IN THE ENGINE THAN THE HEPTANE-OCTANE 200" C. With regard to given h e a v i e r exposures than BLEND OF THE SAMEOCTAKE formaldehyde formation those with the base fuel in order NUXBER the results were consistent to obtain backgrounds of comEXPOSCRE with those presented in parable densities. Extra expo.$ PAST rnr. 3 Figure 7. N o differences sure did not, however, correct N DGyn FLAMES in rates of flame t r a v e l entirely for the absorption by N ro CDNTER VISIBLE TRACE FUEL Degrees % w i t h t h e t w o different the coating o n t h e w i n d o w s E N G I N E XOT K N O C K I N G ; S P A R X , 11" FIGURE9. ADDITION OF fuels were observed when because t h i s a b s o r p t i o n in23 Source .. ANILINE TO FUEL TO 24 Heptane-octane 22-24 25 the heat was added to the c r e a s e d r a p i d l y as the wave REMOVE K N O C K 25 Gaeoline C 22-24 50 intake m i x t u r e but the length decreased. 26 Heptane-octane 24-26 50 DECRE.4SES FORMALDEIn the first experiments with 27 Gasoline C 24-26 75 h e p t a n e-o c t a n e blend HYDE CONCENTRATION C N Q I N E KNOCKING: SPARK 20' knocked harder than gaso- tetraethyllead, base fuels were 36. Squrce. 37. Gasoline C 28 Source .. + aniline. not knocking. line C under these condi- chosen which knocked severely, 29 Heptane-octane 141i6 25 38. Gasol(ne C + anjline' resulting in strong a l d e h y d e 30 Gasoline C 14-16 50 tions. not knocking. 39. Gasoline C! 31 Heptane-octane 16-18 50 knocking. 32 Gasoline C 16-18 75 The effects on the form- bands in the absorption spectra, aldehyde formation of in- When amounts of tetraethvllead sufficient to remove such a knock completely were ducing knock with isopropyl nitrite and of suppressing i t with added to the fuel, no effect on the formaldehyde bands could either aniline or tetraethvllead be detected. Under such conditions this result was not have also been investigated. surprising in view of the experiments already described in 33 The data in Figure 8 show that which formaldehyde was detected under conditions below the action of isopropyl nitrite but close to incipient knock. is most marked; the formaldeIt seemed desirable, therefore, to perform experiments 34 hyde bands become very proin which the tetraethyllead would still further reduce the n o u n c e d when knock is in35 tendency of the engine to knock. The concentration of d u c e d w i t h 2 p e r cent by lead in the fuel could not well be increased for reasons alv o l u m e of isopropyl nitrite. ready discussed. Therefore a base fuel was chosen which The c o n v e r s e effect with 4 knocked incipiently. A blend of 50 per cent gasoline C and per cent aniline by volume, 50 per cent isooctane satisfied this requirement and the shown on Figure 9, is not so addition of 0.5 cc. of tetraethyllead per liter completely striking but it is quite definite. suppressed the knock. The results of this experiment are Trace 37 shows no formaldeillustrated by traces 40 and 41 in Figure 10. (The backhyde; trace 38 taken a t a ground produced by the underwater spark is not very unislightly different angle shows form, the hump on the right and the break a t B being charlittle or none; and the bands acteristic of the source.) Owing to the light degree of the are definitely present when knock, the formaldehyde bands are very weak on trace 40. . t h e engine w a s k n o c k i n g , But they are still visible on trace 41, and apparently their trace 39. intensity is not appreciably changed by the added tetraIn s t u d y i n g the effect of FIGURE8. ADDIXGIso- ethyllead. These data confirm the visual inspection of the tetraethyllead on the intensity PROPYL NITRITETO THE original spectra which included not only 40 and 41 but others FUEL TO INDUCEKNOCK taken a t different crank-shaft angles. The traces in Figure of the formaldehyde bands, a INCREASES FORMALDEHYDE rather serious difficulty was en10 show, then, that 0.5 cc. of tetraethyllead per liter of the CONCENTRATION countered. If the engine was fuel does not suppress all indications of formaldehyde forma33. Source. 34. Gasoline C 50 o p e r a t e d o n gasoline contion in the noninflamed gases even though knock is entirely per cent + isooctane 50 per cenk not knocking. 35. Same taining t e t r a e t h y l l e a d , toremoved. fuel as 34 + 2 per cent isopropyl gether with ethylene bromide to This result seemed inconsistent with that obtained when nitrite: knocking. 0
Y
2
aniline was used as :L knock suppressor, or when either isopropyl nitrite or diethyl peroxide was used as knock inducers. Therefore, anotlier set 41 of spectra was photograplied using 1.0 instead of 0.5 cc. of tetraethyllead per litcr of the blend, a n d t h e r e s i i l t s a r e represented by traces 42 and 43 in I'igure 10. Tire dips caused 48 by the forrnaideiiyde b a n d s are about equally prominent in the presence and absence of tetraethyllead. (TI1e s t e e p slope toward the right end of a3 trace 43 was caused hy the selective absorption of the deposit on tlie engine windows.) Tliese data t h i s indicate that theaddition of more than twice a.s much tetraetliyllead as is required to rernove knock does not affect appreciably tlie maximuiri formaldeliyde concentration in the noninflarried gases. 111 connection aitli the preceding result it sliould Ix added that tlie possibility of the fornialdehyde c o n c e n t r a t i o n being increased by the ethyl groups contained in the tetraethyllead niolecules has been investigated. S p e c t r a were photographed with the engine running on isoiictane which does not produce enough formaldehyde in the present engine to be detected by the I
