Effect of Tetraethvllead on Preflame Reactions in J
an Engine
A
T PRESEST considerable i n t e r e s t centers
LLOYD WITHROW .4ND GERALD M. RASSWEILER General Motors Research Division,
Such an investigation is being made in this laboratory and some of the data obtained to date are recorded.
about the c h e m i c a l Detroit, Mich. r e a c t i o n s which are affected when knock is supwessed with tetraethyllead. Strong evidence exists for the conclusion Apparatus that lead in some way inhibits preflame reactions; but, as yet, The spectra were photographed through a single-cylinder, no experimental evidence obtained in an engine has been ell-head engine fitted with two quartz windows on opposite recorded to demonstrate the occurrence of such an effect, sides of the combustion chamber above the piston (10). For an experimental test of this possible action of lead to be Since the spark plug was located near the other end of the combeyond reproach, it must be so carried out that it does not bustion chamber, the portion of the charge which knocked disturb the environment in which the combustion reactions lay between the windows. All of the engine spectra were exoccur. In this respect it appears that a study of engine composed with the light beam passing through the knocking zone bustion by means of absorption spectra is most satisfactory. in a region about inch (0.48 cm.) wide bounded on one The present paper describes spectroscopic data which indicate side by the end wall of the combustion chamber. The time that lead does influence preflame reactions and which show that and duration of the exposure in each explosion was accurately atomic lead is present a t the time antiknock action is effected. controlled and held constant with a stroboscopic disk attached I n previously described studies of the spectral absorption to the crank shaft (11). by that portion of the charge which reacts chemically with the In order to record continuously the changes in the appearemission of light a t the instant knock occurs (5, 10,l1 ) ,it was ance of the spectra during the period that knock was being reestablished that just prior to such inflammation there appear moved with tetraethyllead, the plate holder on the spectroin the spectra: (1) formaldehyde absorption bands and ( 2 ) graph was fitted with a motor drive which moved it continua wid: continuous absorption band extending from about ously in the direction of the length of a spectrum line. An 3000 A. down through the ultraviolet. This latter effect is index on the plate holder and a scale on the spectrograph observed under knocking conditions when the experiments made it possible to follow this motion and to record the posiare made with a fuel that is initially transparent in this spection of the plate a t any time. The velocity of movement of tral region. On the other hand, the continuous absorption the plate was 0.22 mm. per second when the carbon arc was is almost entirely absent in spectra exposed when using transbeing used as a light source and 0.5 mm. when the underparent hydrocarbon fuels which do not knock. I n subsewater spark was being used. quent exDerimentsthe effect of tetraethyllead on the intensities The d e n s i t o m e t e r used for df the formaldehyde absorption studying the change in blackenbands was examined in the engine ing on t h e p l a t e s w a s kindly (11). No effect was observed Absorption spectra of the gaseous charge l o a n e d b y E. J. Martin and even though considerably more in an engine indicate that when tetraethylW. W. Perkins who designed and lead was added than was neceslead is added to the fuel to prevent knock, built the i n s t r u m e n t i n this sary to remove knock. This rethere is an effect on the preflame reactions laboratory. It consisted essensult is in agreement with data tially of a photronic cell behind o b t a i n e d i n previous unpubin that portion of the charge which, in the a slit onto which was projected lished experiments in this laboraabsence of tetraethyllead, suddenly inan enlarged image of the spect o r y w i t h a s a m p l i n g valve flames at the time of knock. Atomic lead trum. The length of the slit located in the knocking zone. has been identified in the noninflamed corresponded to about 1 mm. In c o n t i n u i n g this work, a charge in the knocking zone at the moment on the original plate. further study of the ultraviolet spectral absorption in the knockthat antiknock action is being effected, ing zone, with the charge conbut no accompanying absorption by lead Materials taining sufficient tetraethyllead monoxide has been observed. When lead The fuels c o n s i s t e d of nto suppress a knock, was needed is vaporized in a hot nichrome tube, the heptane, 2 , 2 , 4-trimethylpento determine: (1) whether tetratane, and gasoline C (a specially absorption spectra exhibit lead monoxide ethyllead exerts any effect on purified fraction of a standard the continuous absorption band bands at lower temperatures than atomic r e f e r e n c e fuel used in knock which was observed prior t o inlead lines. Comparison of experiments in r a t i n g ) . The n-heptane and flammation in knocking exploand out of the engine indicate that lead gasoline C were rendered optisions and (2) whether the lead is monoxide is being reduced in the nonincally clear b y r e f i n i n g with present as the metal, or the oxide, s u l f u r i c a c i d , w a s h i n g with flamed charge. or both, prior to inflammation water, drying over calcium chlowhen s u p p r e s s i n g a knock. 872
AUGUST, 1933
INDUSTRIAL -4ND ENGIXEERING CHEMISTRY
ride, and distilling over sodium. I n an effort to prepare leaded fuels which were optically clear in the spectral region being examined, tetraethyllead which had previously been washed with sulfuric acid, washed with dilute sodium carbonate solution, dried over sodium sulfate, and vacuumdistilled was immediately dissolved in optically clear 2,2,4trimethylpentane, and an absorption spectrum of the solution was photographed. As shown by the spectra in Figure 1, this fuei absorbed strongly throughout the ultraviolet below 2700 A. Similar results were obtained when spectra of tetraethyllead and ethylene dibromide, in sufficient quantities to suppress a moderate knock and subsequently to remove most of the lead from the engine, were dissolved in gasoline C. On the basis of these spectra more absorption would be expected by the fuel itself when operating the engine on a leaded fuel than when operating on straight gasoline. The extent of this uncertainty is rather difficult to estimate because, a t the time the absorption spectra are exposed in the engine, the tetraethyllead is probably completely decomposed. I n any case, as will be shown later, the extra absorption shown by the leaded fuel does not invalidate the effects observed.
Procedure The method of photographing the engine spectra on a moving plate and thereby obtaining a continuous record of the change in t.he spectra as lead removed the knock was suggested by the follawing three considerat'ions: (1) When operating the engine on gasoline containing enough lead to remove a moderate knock, the windows were rapidly coated with a deposit which, even in very thin layers, absorbed strongly in the ultraviolet; the effect of this deposit was appreciable 40 seconds after changing to the leaded fuel. (2) About 25 of these 40 seconds were required for the leaded fuel to replace the straight gasoline in the line to the intake manifold and to reach sufficient concentration in the engine to remove knock. (3) The remaining 15 seconds were barely sufficient to obtain a satisfactory exposure even with a wide spectrograph slit. The procedure followed in photographing the spectra in Figure 2 is as follows: The windows were cleaned and the engine was then operated on gasoline, adhering closely to the conditions listed wit'h Figure 2. With the engine knocking moderately and with the plate moving upward past the slit image at the rate of 0.22 mm. per second, the exposure was started. A pressure record was photographed at once, and, when the plate had moved 10 mm., the fuel was changed to gasoline containing 0.053 per cent of tet,raethyllead and an equal concentration of ethylene dibromide. The time at which knock completely disappeared was subsequently determined by watching t'he pressure-time records on the oscillograph screen, and at this instant the position of the plate was read. A second pressure record was photographed immediately after the disappearance of knock. The fuel was then changed back to straight gasoline and the position of the plate when knock reappeared was determined by observjng the pressure-time records as before. A third pressure record was then photographed. At the end of the exposure, with the engine still knocking, a sample of exhaust gas was removed and analyzed to obtain information about the mixture ratio. The percentage of explosions during which flames were visible through the stroboscopic shutter was also checked. After changing back to the leaded fuel and again removing the knock, another sample of exhaust gas was analyzed, and the percentage of flames visible through the disk was estimated. This latter estimate is inexact, but it gives: (1) a general idea of the time at which exposure was made with respect to the arrival of the flames in the optical path and (2) a rough measure of the number of explosions during which a portion of the gas in the optical path v a s inflamed when the exposure was made. The spectrum in Figure 4 was exposed essentially in the same way as Figure 2; the only difference was that the underwater spark \vas used for a source and that the fuel was changed only once during the course of the exposure.
873
Data Using the technic outlined, the ten spectra in Figure 2 mere exposed, each a t different crank-shaft angles varying progressively from 25" after ignition (6" to 4" before top dead center) to 51" after ignition (20' to 22" after T.D. C.). Thus, when the various exposures were made, the flame fronts were a t different positions with respect to the light beam which was passing along the end wall of the combustion chamber where the last part of the charge burns. Spectrum 1 was taken well ahead of the flames. Spectrum 10 was photographed after the gas in the optical path was completely inflamed. On the right- and left-hand edges of each spectrum are four short vertical lines. The lengths of these lines represent the height of the image of the spectrograph slit-that is, the length of the spectrum lines with the plate a t rest. These index lines have been placed adjacent to the several spectra in Figure 2 to show the respective positions of the slit image when each of the four following events took place, reading from top t o bottom: (1) fuel changed from straight gasoline to leaded gasoline, ( 2 ) evidence of knock completely disappeared from pressure cards, (3) fuel changed back to straight gasoline, and (4) knock reappeared on pressure cards. Thus, in Figure 2 , for example, the portion of spectrum 1 which is bounded by the black rectangle was being exposed a t the instant the valve, below the float bowl, was changed over to admit the leaded gasoline. It should be borne in mind that, as the slit image moved from mark 2 to mark 4 , the engine was not knocking. If adding tetraethyllead to the fuel to remove knock has any effect on the preflame reactions which produce the material responsible for the continuous absorption band in the ultraviolet, a change in absorption should appear between
SOURCL:
EMPTY C E L L MY. L A Y & R
4
2 YY. I MY.
FIGURE1. ULTRAVIOLET ABSORPTION
BY 0.2 PER SOLUTION OF TETRAETHYLLEaD IN O C T l V E
LAYER
LAYER
- .
..
CENT
marks 1 and 2 in those spectra taken ahead of, but close to, the flame fronts. I n order to study the changes in absorption, densitometer readings were taken along ths spectrum line (characteristic of the source) at about 2875 A. These readings are plotted in Figure 2 as abscissa against position along the spectrum line. The resulting curves represent plots of absorption by the gases in the engine (along an arbitrary scale) against time, or more exactly against position of the image of the spectrograph slit on the spectrum plate. The gaseous pressures in the engine, when the spectra were exposed, may be estimated from the pressure-time curves in Figure 3. The serial number a t the left of each set of three pictures correqponds to the spectrum of the same number in Figure 2 . In each set of pictures, the first pressure-time curve was obtained before removing the knock, the second after the knock disappeared, and the third after knock reappeared. The white vertical line on each picture marks the time a t which the spectrum mas exposed, the width of the line being approximately equal to the duration of the exposure. Thus, in addition to gii-ing information concerning the gaseous pressures, these records show the relative times of exposure of each spectrum with respect to the time of knock. Each of the spectra in Figure 2 , together with its corresponding densitometer curve, n-ill be discussed separately.
874
INDUSTRIAL AND ENGINEERING CHEXIISTRY
DENSITOMETER READIRQS ON
1 84 0 28'75 LINE.
SPECTRA
VOL. 27, NO. 8
DENSITOMETER READINGS ON 2875 LINE.
6
28,5
1,
4
.rl
izo e
.rl
i240 10 a0 50 Absorption
Exposed from -6' t o -4O
2 g o 4
I I
4
4
$ 20 a
I
Y .r(
4 03
40
I
I
10 90 50 Absorpt%on
-- 10
80 50 hbsorpt ion
10 30 50 Absorption
...
Y
.d
Exposed from -2O
t o T.D.C.
_-
Exposed from T.D.C. t o + 2 O
_-
Exposed from +20 t o +40
..
I'
10 30 M) hbs orp t i on
Exposed from +e0 t o +loo
10 30 50 Absorption
Exposed from + l o t o +12'
10 30 50 Absorption
Exposed from +14' t o +16O
a
103
0
Cr 4
uj
--
10 30 50 Absorption
Exposed from +4O t o +e0
$40
IO 30 50 -Exposebfrom Absorption +a0t o +220
FIGURE 2 . DECREASE IN CONTIZrUOUS ABSORPTION CHARACTERISTIC OF NOXINFL4MED CHARGE P R I O R BY ADDITION OF TETRAETHYLLEAD TO FUEL Fuel Speed Spark Intake the-reading
Gasoline C with and without 2 cc. tetraethyllead per gal.
600 r. p. m. 30'
Small random variations of intensity were caused by variations in the light source, and therefore only general trends will be considered. It should be stated that theospectral region covere9 by these spectra extends from 2700 A. on the left to 3400 A. on the right. As is indicated by the first set of pressure records in Figure 3, spectrum 1 of Figure 2 was exposed about 14 crank-shaft degrees before the occurrence of knock. Visual observations through the stroboscopic shutter showed that during this
Mixture ratio Throttle Indicated horsepower: Knocking Not knocking
TO
KNOCK
S6yO theoretical air
Full 1.7 1.9
exposure none of the flames was reaching the optical path. When the engine was knocking, the noninflamed charge showed very little absorption; but when the leaded fuel entered the engine, the amount of absorption increased rapidly. According to the densitometer record for spectrum 1 in Figure 2, the absorption continued to increase until knock reappeared and then remained almost constant until the end of the exposure. This latter observation indicates that the increase in absorption in spectrum 1 was caused
I w w . S e w tlic iicjttoiii OS Figure 5, just h i v e the le, :iii iiinissinrr speetrutii i d lead t ~ t i dlead rmrroxide c is inoliideil for comp:rrisoa. A s tire teixqierature of tiic tiihe w w raised, tlie firat sixns of ahaorption bnnds of lead trionoride aplieared at 'JOO" C. At ttrii temperature nnly three l,nnd lieads, indii:a iIot.s, c~rriklhe oliserveil; but ;is tlic tempcr:&tiiro lva8 t,he numl,er and intensity of the nlwrbed Iranrl lrearl illy increiiwil. At 1150' '. the atomic le:id absorption lines at 3139, 31383, :itid -1058 A. appeared. The increased tibsorptiiin at liiglicr tartiperatiires grnlialdy rowlted in part from an iucrcnsed c~iiicentrat,iini of alrwiriiing Inaterial as well a b from t,he elevated teiiiperat,nro, Imause there was x u exi:ess of liquid lead in the tnbc at or pressure of lend :it '300" C. is 0.3 miti. .t Il,i0" C. it is 9.1 r i i i n . T l r o last spectrum 011 Vigure 5 which was ixposed nt OiNlo C. in the nic l m i i r i r tube arid wliicli ex$en(ls out into the ultravirilet spectr$ region begond 2800 8.iloes not show t,he lead line at 2833 A, which is tire first of tlic sevenll lead lines i n this spectr;il region t.o appear as the teiiipcratiire is raised. This 2833 A. line wts first diserved in the nichrmre tube when t,lic tenigerature was 025' C . ~-that ~ . is, slightly above the terriperat,irrcat which the lead monoxide linnds m r e first observed. Cniler the conditions
