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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 44, No. 7
Variations in the stabiiitieq of the diesters could be related to molecular configurations. Tertiary C-H bonds contributed t o reactivity, but their proximity to such structures as carbonyl oxygen or a quaternary carbon made for more stable compounds. This stability was probably due to a screening effect.
Dornte, R. W., Ibid., 28, 26 (1936). Dornte, R. IT.. and Ferguson, C. V., Ibid., 28, 863 (1936). George, P., Rideal, E. K., and Robertson, A, Nature, 149,
ACKNOWLEDGMENT
Larsen, R. G., Thorpe, R. E., and Armfield, F. h., IND.ENG.
The authors wish to express their appreciation to their colleagues, J. G. O’Rear, for his synthesis of a number of the diester fluids and George Cohen, for his assistance in some of the experimental work.
Lewis, J. S., J . Chem. Soc., 1927, 1555. Miller, R. IT., Craig, P. N., and Wolfe, J. K., NRL Rept P-
LITERATURE CITED
O’Rear, J. G., X R L R e p t . 3891 (1951). Shell Development Co. Tech. Bept. No. 5 ; 13239 (Oct. 22, 1950). Steacie, E. IT7. R., and Szwarc, M., J . Chem. Phvs., 19, 1309
(1) Am. SOC.Testing Materials, “Standards on Petroleum Products and Lubricants,” Designation D 974-48T, Philadelphia, 1950.
(2) Atkins, D. C., Baker, H. R., Murphy, C. &I., and Zisman, W. A, IND. ENG.CHEM., 3 9 , 4 9 1 (1947). (3) Bried, E. M., Kidder, H. F., Murphy, C. M., and Zisman, W. A., Ibid., 3 9 , 4 8 4 (1947). (4) Denison, G. H., Jr., Ibid., 36,477 (1944). (5) Denison, G. H., Jr., and Harle, 0. L., Ibid., 41, 934 (1949).
601 (1942).
Glavis, F. J., and Stringer, H. R., “Symposium on Synthetic Lubricants,” A S T M Special Pub. 77, 16 (1947). Kreulen, D. J. W., and Kreulan Van Selms, F. G., J . I n s t . Petroleum, 34, 930 (1948).
CHEM.,34,183 (1942). 2573 (1945).
Murphy, C. M., and Ravner, H., Ibid., C-3380 (1948). Murphy, C. M., Ravner. H., and Smith, N. L., IND.E m . CHEM.,42,2479 (1950).
(1951).
Walsh, A. D., T r a m . Faraday SOC., 42,269 (1946). Ibid,, 43,297 (1947). RECEIVED for review December 8, 1951.
~ ~ C C E P T E February D 26, 1962. The opinions or assertations contained in this paper are t h e authors’ and are not to be construed as official or reflecting t h e views of the Navy Department.
Autoignition Properties of Certain Diesel Fuels J. ENOCH JOHNSON, JOHN W. CRELLIN, AND HOMER W. CARHART Naval Research Laboratory, Washington 25, D . C .
T THE present time the evaluation of Diesel fuels in terms of ignition quality is accomplished by means of the C F R engine method. This procedure requires relatively large quantities of fuel, which presents a severe handicap in any research on fuels, additives, pure hydrocarbons, and other compounds when only small amounts of material are available. Furthermore, the engine provides data under limited conditions, which are often difficult t o interpret, although in recent years new techniques, such as t h a t used by Levedahl and Howard (Q),have widened its scope. Various methods have been used t o simulate the engine and yet eliminate Pome of the difficulties associated with it by the design of special devices such as the rapid compression machines (8, 1 4 ) and the constant-volume bombs (4,11) which restrict themselves t o the development of a single combustion cycle or a fraction thereof. Studies of ignition and slow oxidation in combustion tubes under both static and flow conditions have provided data which have been valuable in the interpretations of ignition phenomena (1,16). To achieve a better understanding of the relationship between fuel composition and ignition properties and to study other factors influencing ignition, it was desired t o have a laboratory method capable of utilizing small quantities of material. A considerable amount of information has been published by many investigators on the temperatures of self-ignition of combustibles as measured in apparatus of various designs based on the Moore oildrop method. Much of this data has been obtained using air a t atmospheric pressure, although some of the work has been done with atmospheres other than air. Although ignition of fuel in Diesel engines occurs a t relatively high pressures, it was felt t h a t investigations a t atmospheric pressure, having the advantage of simplicity of equipment and operational procedure, would be of considerable value in studying ignition phenomena
APPARATUS AXD PROCEDURES
The Jcntzsch ignition apparatus ( 5 )is a convenient and versatile instrument for studying ignition properties of materials a t atmospheric pressure, as has been demonstrated at various laboratories ( 3 , 6 , 17, 18). Certain modifications of t,he apparatus Tvere made from time t,o time in order to make the instrument more suitable for specific investigations. Essentially, the apparatus consists of an electrically heated ignition chamber coupled with an accurate oxygen-mctcring system. In the original Jentzsch method, the flow rate of oxygen supplied to the ignition chamber is controlled by means of a fine adjustment needle valve and is measured in terms of bubbles per minute by passing t’he oxygen through a water bubbler. As the bubble rate is decreased, however, more and more air diffuses into the chamber, which lowers the effective oxygen concentration. Therefore, this method was modified by metering pure oxygen and nitrogen into the ignition chamber by means of independent bubblers. The bubble rates were so adjusted that their sum was 300 per minute or about 25 cc. per minute. The oxygen concentration of the dried gas stream \vas measured continuously by means of a Pauling oxygen meter (1.2). The stainless stecl ignition chamber of the Jentzsch apparatus contains four cylindrical cells open a t the top, three of which are interconnected and receive oxygen from a central inlet near the bottom. The fourth cell is used for the measurement of the block temperature and contains a thermocouple made of No. 24 B &- S gage, Fiberglas-insulated, duplex iron-constantan wire, shielded by a two-hole ceramic tube over the portions of the wire inside the ignition chamber. At the bottom of the ceramic tube, the wires arc spread and spot-welded t o a stainless steel crucible of the type used for ignition tests. During the ignition tests, one of the shallow ignition crucibles is placed in the bottom of each cell as a fuel receptacle. K i t h the
July 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
1613
block heated t o the desired temperature, a drop of fuel is added to the crucible in the front cell by means of a medicine dropper and the ignition phenomena are observed. After each test, the chamber is purged with air and, since it has been found that dirty crucibles give less reproducible results, the used crucible is replaced with a clean heated one from one of the other cells. With a view toward investigating the influence of chamber design on ignition, a single-cell chamber was constructed having the same total volume as the three-cell Jentzsch chamber. The opening at the top of the new chamber has the same diameter a s the opening in each of the three cells of the Jentzsch chamber. The new chamber is recessed at the bottom to hold the ignition crucible and has two ports near the bottom for admission of the oxygen mixture. The Diesel fuels and blends used, and their cetane numbers, were obtained from the U. S. Naval Engineering Experiment Station, Annapolis, Md.: Fuel Designation Fischer-Tropsch Nav Spec. 7-0-2e 2 9 - d t a n e , cat. cracked Blend 2,400/ Fiacher-Tropsch 60% 29-cetane Blend 6,50 29 cetane 5 0 2 7-6-2e" Blend 7, Fischer-Tropsch" Doped with proprietary ignition improver.
Cetane Number 83.7 52.0 29.Q
50.1 50.0
110 (est.)
The usual procedure was to set the gas flow a t a given oxygen concentration (or bubble rate) and determine the ignition behavior of the fuel from the upper ignition range (500" t o 600"C.) to the lowest ignition temperature. This procedure was repeated a t several different oxygen concentrations, so chosen to give a delineation of the different types of ignition behavior for a given fuel as a function of oxygen concentration and temperature a t atmospheric pressure. IGNITION CURVES
The ignition curves shown in Figure 1 are for a Fischer-Tropsch fuel and are more or less typical of the general shape of the curves found for the other fuels and blends studied. Curve A was obtained by premixing the oxygen and nitrogen and maintaining a constant flow rate of gas t o the chamber. Curve B was obtained by supplying pure oxygen to the chamber and changing the flow rate so that dilution resulted by back diffusion of air into the chamber. Figure 1 illustrates the close similarity in the curves obtained by the two methods-Le., the various inflection points occur a t the same temperatures. Figure 2 shows in more detail how the ignition data were plotted and the curves drawn. At the lower temperatures, the ignition boundaries could be determined to &2' C. However, as the temperature was increased the reproducibility became poorer and the boundaries could not be as clearly defined. Curve A (Figure 1) will be discussed here in detail for illustration of the principles involved in some of the ignition phenomena. The region of "positive ignition" (visible and/or audible combustion when observed under ordinary laboratory conditions) lies above and t o the right of the ignition curve represented by the solid line. The area labeled cool flames denotes the conditions under which a type of reaction or ignition was observed which did not fit the definition of positive ignition. Cool flame ignition was evidenced by a weak bluish flame invisible in ordinary light but readily observable in a carefully darkened room. I n order to attempt an explanation of the ignition curves, it is desirable t o consider the vapor-phase oxidation of hydrocarbons. There is much experimental evidence to indicate that in such oxidations there exist two reaction mechanisms which are usually referred to a8 the low temperature and high temperature oxidations. The low temperature reaction is the more predominant up to about 350" to 400" C., and is generally believed to depend
TEMPERATURE
Figure 1.
(a
C.)
Sample Ignition Curves
Fischer-Tropsch fuel
on chain-branching processes which involve the formation and decomposition of alkyl hydroperoxides. The high temperature mechanism which predominates above 350' to 400 O C. is believed t o depend a t least partially on pyrolysis prior t o and during oxidation. These ideas may be used t o explain the larger features of the ignition curves. Point N (Curve A , Figure 1) represents the temperature of greatest reactivity of this fuel due t o the low temperature mechanism. As the temperature is increased beyond N , there is a decrease in this reactivity due t o instability of certain heat-sensitive intermediates so that more oxygen is required to furnish the necessary supply of these intermediates for the reaction t o attain a rate sufficient for positive ignition. At higher
60
POSITIVE IGHiTlOH oCOOL F L A M E S
50
= 40 Y
0
*
Q x
30 c r
= Y
0
::20 IO
0 230
240
I
I
250
260
270
2.80
TEMPERATURE
Figure 2.
290
300
I
I
310
320
I
330
(%)
Experimental Points for Part of Ignition Curve
Fischer-Tropsch fuel Numbers e t top of curves indicate time lags in seoonds
this effect, it is believed that the ignition curve would dmp gradually and smoothly through this temperature range.
i27 ,
I
I
POS 1 TiVE IGNITION
I I
I
I
I
_ - ----I 340 380 420 460 T E M P ER ATU RE ( 'C.1 Effect of Air Pressure and Temperature on Ignition of n-Octane
3
Figure 3.
Replotted from Maccormac and Townend (10)
temperatures, OP, the high temperature mechanism becomes effective and the reactivity increases again, permitting ignition at lower oxygen concentrations. These curves have been obtained a t atmospheric pressure and the partial pressure of oxygen has been changed by regulating the mole fraction of oxygen supplied either by premixing (Curve A ) or by back diffusion (Curve B ) . It, is significant that curves containing the same features were obtained by Maccormac and Townend ( I O ) using a different experimental method. An example of this is shown in Figure 3 in which their ignition curve for n-octane and air in a closed vessel is replotted. The partial pressure of oxygen was regulated by changing the total pressure of air without changing the mole fraction. The similarity of t,he curves in Figures 1 and 3 extends even to the inflection point designated as M ,Figure 1, which has been found for the other undoped fuels in the present work and for other hydrocarbons by Townend (15). It appears, therefore, that ignition behavior is affected more by the partial pressure than by the concentration of oxygen. The bulge in the curve a t 0, Figure 1, is not considered significant. It is believed to be due to the physical phenomenon of nonwetting of the crucible by the fuel drop ( 7 ) . Were it not for
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Vol. 44, No. T
INDUSTRIAL AND ENGINEERING CHEMISTRY
1614
EFFECT O F CERTAIN VARIABLE FACTORS ON AUTOIGNITIOS TEMPERATURE
The spontaneous ignition of a fuel-oxygen mixture is dependent on a number of variables, such as temperature, pressure, concentration of react,antp, composition of the fuel, catalysis, design of the reaction chamber, etc., and it is known that several of these factors are interdependent. The main emphasis in the studies to be described here has been centered around t h e effects of oxygen concentration and type of fuel on the temperature of autoignition. It is evident from the ignition curves that in most cases the oxygen concentration could be reduced to 50% or less before the self-ignition temperature of the fuel was changed significantly. I n this connection it is interesting to note that Cullis and Hinshclwood ( I ) , who studied the slow oxidation of hydrocarbons a t low temperatures in a closed system, found t'hat the rate of reaction is nearly independent of the partial pressure of oxygen once a certain minimum pressure is reached. They observed further that the addition of nitrogen to a given hydrocarbon-oxygen mixture had relatively little effect on the maximum rate of reaction. In Figure 4 are presented the ignition curves for t'hree fuels of widely different cetane number: the 29-cetane catalyt~ically cracked fuel, 52-cetane 7-0-2e fuel, and €%-cetane Fischer-Tropsch fuel. The essential features of the curves are strikingly siniilar for all three fuels. The main difference is that in the low temperature region the curves are displaced toward lower temperatures and oxygen concentrations as the cetane number increases. This is in keeping with the concept that cetane number is in a way a measure of the ease of ignition of a fudl in an engine. The additive nat,ure of ignit,ion behavior is illustrated in Figure 5 which gives the ignit.ion curves for 29-cetane fuel, a FischerTropsch fuel, and a blend which contains 60% 29-cetane fuel and 40% Fischer-Tropsch fuel. I t may be seen that the blended fuel gives an ignition curve which is intermediate between t,hose of the unblended fuels. However, the blend will ignite under conditions under which the higher cetane Fischer-Trapsch fuel does not ignite (area &, Figure 5). The same type of phenomenon i r ~ also y be seen in Figure 4. The effect of adding a cetane improver is seen in Figure 6 in which Fischer-Tropsch fuel is compared to a Fischer-Tropsch fuel doped with a proprietary material and a blend doped with t h e same material. The addition of the dope to the Fischer-Tropsch
'
7 - 0 - 2 e FUEL, FISCHER
-
5 2 C N.
T R G P S C H F U E L , 84 C N
f IO
T E M P E R A-URE
Figure 4. Effect of Cetane Number on Ignition
I
-B L E ' I O P P
! -I
,,
I
I I
I
I
I I
l l I
I
0
,
1 I
( 4 0 % F l , 60% 2 9 - C ) 50 C.N FISCHER-'ROPSCH F U E L 8 4 C ti I
I
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
July 1952
-FISCHER-TROPSCH F U E L -_ -FISCHER-TROPSCH FUEL WITH IMPROVER( R L E N W 7 )
I
I 60.
01
220
----B L E N D F B
I I
I
’
I’
260
’
is15 F I SCHER-TROPSCH a 4 c N.
90
---- 2 9 - C E T A N E
W I T H IMPROVER
I
I
I
I
I
I
I
I
300
340
380
420
460
500
540
580
f UEL,
FUEL
TEMPERATURE (‘C.)
Figure 6.
Effect of Addition of Cetane Number Improver on Ignition
fuel displaces its ignition curve toward lower temperatures and oxygen concentrations and in this case causes the disappearance of the first inflection point. However, the ignition curve for blend 6, which contains the same additive in the same coneentration, still retains this inflection point. In Figure 7 are shown the ignition curves obtained with the new single-cell chamber and Figure 8 shows, for comparison, the ignition curve of Fischer-Tropsch fuel obtained using the threecell Jentzsch ignition chamber with that from the single-cell chamber. The principal features of the curves are similar but three main differences are apparent which have been observed also with the other fuels compared in this manner. In the first place, ignition seems t o occur at lower oxygen concentrations in the single-cell chamber as indicated by the per cent oxygen supplied t o the chamber. In the second place, it is evident t h a t the temperature of self-ignition in the single-cell chamber is signifiGantly lower in the region associated with low temperature oxidation. The effect is as if the entire left-hand portion of the ignition curve has been shifted t o lower temperatures. I n the t h i r d place, the inflection points in the low temperature region a r e much more pronounced in the curves obtained using the aingle-cell chamber. I n the low temperature oxidation region four sets of observations have been presented which may be grouped together for discussion. These are
Figure 7.
TEMPERATURE ( O C . ) Ignition Curves Obtained with Single-Cell Chamber
The chain reactions may be quenched by active molecules striking inert molecules on t h e walls of the container or in t h e gaseous mixture with a transfer of energy which results in inactive molecules incapable of propagating the chain. I n addition the active molecules may be inactivated by fragmentation. As an example of the wall-quenching effect, Spence and Townend (IS) have found t h a t with small diameter tubes (larger wall areavolume ratio) more stringent conditions were required for ignition than with larger diameter tubeb. One further factor which must be considered is the nature and stability of the intermediate chaininitiating and propagating products formed under a given set of conditions. Thus, it is t o be expected t h a t different fuels having different hydrocarbon compositions will give rise t o intermediate products of differing degrees of stability and reactivity. lo
--T H R E E - C E L L -SINGLE-CELL
1. The shift t o conditions of higher temperature and oxygen concentration with decreasing cetane number 2. The ignition of a lower cetane fuel under conditions which .do not allow ignition of a higher cetane fuel 3. The effect of a dope on the ignition behavior of t h e base fuel 4. The shift of the ignition curve t o conditions of lower temperature and oxygen concentration when using the single-cell “rhamberin place of t h e three-cell chamber
In order to explain these observations, consideration must be given t o t h e theoretical aspects of the phenomena occurring immediately preceding and during ignition and t o the experimental conditions prevailing. It has been pointed out t h a t oxidation may proceed by two mechanisms. I n the low temperature mechanism it has been postulated that peroxides and hydroperoxides are among the first oxidation products formed which decompose or react t o initiate branched-chain reactions. Neither eo01 flames nor positive ignition can occur if the rate of quenching *ofthese chains is greater than the rate of initiation. Thus there must exist a critical concentration of these or similar intermediate products under a given set of conditions before either type of ignition can occur.
TEMPERATURE
Figure 8.
CHAMBER
CHAMBER
C)
Effect of Chamber Design on Ignition Fischer-Tropmh fuel
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
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Vol. 44, No. 7
OCCL-RRENCE OF COOL FLAMES
7 - 0 - 2 e A T 65 B U B B L E S OF O X Y G E N
50
T E M P E R A T U R E ( 'C,) Figure 9. Successive Ignition from Single Drops of Fuel
In observation 1, it is apparent that the higher cctane fuel produces chain-initiating and/or propagating intermediates more readily than a low cetane fuel, hence the shift in the ignition curve. I n general, the more paraffinic a fuel, the higher its cetane number. It has been demonstrated (1) t h a t in the vapor phase the rate of oxidation of straight-chain paraffins is greater than that of the branched-chain or aromatic hydrocarbons. This shows t h a t the normal paraffins give rise t o the necessary intermediates under less stringent conditions. I t follows that a higher cetane fuel, being richer in paraffinic constituents, should ignite more readily. In observation ( 2 ) it appears that, once formed, the intermediates resulting from t h e lower cetane fuel are more stable than those formed from a higher cetane fuel. I n general, the more highly branched hydrocarbons have a lower cetane number than those with a straighter chain. On oxidation of the former, it would be expected that a higher proportion of tertiary peroxides t o secondary peroxides would be formed than from the straighter chain hydrocarbons. The tertiary peroxides and hydroperoxides are known t o be much more stable thermally than the secondary ones. Therefore, if the stability of these compounds is a controlling factor in the chain-propagating mechanism, the lower cetane fuels, being more highly branched, would be expected to ignite under conditions of higher temperature and lower oxygen concentration than the higher cetane fuel (area Q in Figures 4 and 5). In observation 3 the addition of a dope furnishes the necessary intermediates (probably chain initiators) under less stringent conditions than can be created from the fuel. These intermediates add the necessary impetus required t o ignite the fuel. I n observation 4 the lowering of the ignition temperature and oxygen concentration required for the single-cell chamber may be attributed in large part t o a reduced wall-quenching effect since the ratio of wall area t o volume is considerably smaller. However, the shift must be tempered by the knowledge that a greater amount of back diffusion of air occurs in the three-cell chamber. Therefore, at higher oxygen percentages the effective oxygen concentrations for the single-cell chamber is greater. Howeyer, it is believed t h a t the back diffusion of air does not account for the whole difference because the effect still persists down t o concentration of oxygen slightly below atmospheric. Also the lower temperature of self-ignition in the single-cell chamber cannot be attributed entirely to higher effective oxygen concentrations since the temperature shift still exists in the presence of excess oxygen.
Two different methods were employed t o establish the occurrence of cool flames-visual observation in the dark and the potentiometric response from a thermocouple placed in one of the ignition cells. The conditions under which cool flames were observed are shown on the graphs of the ignition curves. Direct observations of cool flames were impractical above 510" t o 520" C. because a t these temperatures the ignition chamber began t o glow sufficiently bright red so as t o interfere with their visual identification. Cool flames were observed even when pure nitrogen was supplied to both the three-cell and single-cell chambers a t 25 ml. per minute. Even a t this flow rate of gas, there is sufficient back diffusion of air t o supply the low amounts ol oxygen required for this type of ignition. Under certain conditions below 300" C., successive cool flames were observed with a single drop of fuel as illustrated in Figure 9. T h a t is, after a cool flame appeared, it glowed briefly, then expired, was followed by another which again disappeared, etc. This cycle has been repeated as many as five or six times. In some instances the cycIe was terminated by positive ignition. The time interval between successive cool flames decreases with increase in temperature. At higher temperatures, the tendency is for the cool flame to "burn" continuously for many second,. before expiration. It has been observed by other investigators (9, I S , 16) t h a t under some conditions cool flames or some related phenomena precede positive ignition. This was observed in the present work also. In such cases the cool flames rapidly increased in intensity, culminating in a bright yellow flame or sharp report or both. However, the time interval between cool flames and positive ignition was usually very short, which would explain why the cool flames were not generally observed in the present work. The cool flames were often accompanied by puffs of smoke emitted from the chamber. Where multiple cool flames appeared, each was followed by a puff of smoke. Under sonic conditions the smoke puffs were difficult t o distinguish and therefore this method was not used for determining the occurrence of cool flames. In his experiments, Townend (15) found an upper limit of temperature for cool flames. For example, he encountered n o cool flames with n-octane above 400" C. as shown in Figure 3. This is typical of his results with other hydrocarbons. In t h e present work, cool flames mith a number of fuels as well as noctane were observed up to block temperatures of 500" C or more. There is good reason t o believe that this phenomenon is due to the cooling effect of evaporation of the fuel drop and that the cool flames observed a t higher block temperatures are actually occurring in a zone immediately above the drop which is a t a considerably lower temperature. To simplify the explanation of the various phenomena associated with cool flames, it is expedient t o make use of the following assumptions:
1. A critical concentration of intermediates, XI must be attained before t h e rapid reaction evidenced by cool flames can occur. 2. Certain reactants, Y , part of which are dependent on t h e cool flame reaction, are necessary intermediates for the reactions that result in positive ignition in the low temperature region. 3. Y must attain a critical concentration before positive ignition can occur. Concepts similar to these have been proposed by other investigators ( 2 , 16). The fact t h a t a certain minimum temperature is required for production of cool flames is explained in terms of assumption 1. Above this minimum temperature the aecumulation of Xgradually reaches the critical concentration necessary t o produce a cool flame. Below this temperature the preliminary oxidation also occurs, but proceeds too slowly to overcome the concurrent loss
July 1952
INDUSTRIAL AND EN
= 60
Y
0
) .
0 x
c =
0 w
50
Y 0.
40
50 30
I "
220
260
300
340
380 420 T E M P E R A T U R E ('C.)
460
500
540
Figure 10. Comparison of Temperature Kick with Ease of Ignition 29-cetane fuel
'
of X by chemical reaction, degradation, and physical removal. The rate of formation of X is very dependent on temperature but relatively independent of the partial pressure of oxygen. (This is evidenced further by the ignition delay studies described later.) Hence, the sharp, clearly defined, and nearly perpendicular boundary between the zones of nonignition and ignition. The observation t h a t cool flames immediately precede positive ignition is explained by assumption 2. At the higher partial pressures of oxygen, when the cool flame reaction is reached, the increase in concentration of Y is so rapid t h a t positive ignition results and the over-all ignition occurs as a two-stage process. Though t h e time lag for the appearance of the cool flame may be relatively long, the interval between cool flame and positive ignition is very short, in many instances being so short as t o be indistinguishable. Furthermore, the much greater violence of the positive ignition masks the observation of the cool flame. Therefore, the two-stage process is not often observed even though i t occurs. As may be seen from the various ignition curves, only cool flames are exhibited at the lower partial pressures of oxygen, whereas a t the same temperatures but higher oxygen levels, positive ignitions occur. This may be explained by assumption 3, in that there is insufficient oxygen t o allow the concentration of Y t o reach t h e critical value. It is apparent, therefore, t h a t the rate of formation of Y is much more dependent on the partial pressure of oxygen than the rate of formation of X . The explanation of the occurrence of successive cool flames under certain conditions makes use of all three assumptions. It is postulated that after the first cool flame appears the concen' tration of Y generated is below the critical level and therefore positive ignition is not reached. This cool flame is soon quenched, since during the burning of the cool flame the consumption of X is considerably greater than the production of X . If sufficient original reactants survive t o produce the critical concentration of X again, a second cool flame appears. This process may be repeated several times under the proper conditions. The continuous burning of the cool flames a t higher temperatures may be explained by assuming t h a t the rate of generation of X is equal to or greater than the rate of decomposition. TEMPERATURE CHANGES CAUSED BY FUEL ADDITlON
I n order to observe gas-phase temperature changes due t o fuel addition t o the three-cell chamber, a bare thermocouple (No. 30 B & S gage) was suspended in the center cell about 5 mm. above the
T E M P E R A T U R E ('C.) Figure 11. Continuity of Ignition Lags for Cool Flames and Positive Ignition
crucible. The temperature changes were followed by means of a Brown Electronik indicator. The addition of a drop of fuel suddenly lowered t h e temperature in the center cell (about 4' t o 6" C. at 300" C. block temperature) because of the cooling effect of evaporation. The temperature drop in t h e side cells was negligible. Following t h e initial drop in temperature there was a slight gradual rise during the time lag preceding either cool flames or positive ignition. This temperature rise was more than a return t o the normal temperature of the block. The fixed thermocouple in the rear cell measured only the block temperature. It did not respond t o either the temperature lowering caused by evaporation of the fuel drop, or the temperature changes resulting from oxidation reactions, including positive ignition. Similar results were observed with the one-cell chamber. When a positive ignition or cool flame occurred the suspended thermocouple indicated a sudden temperature rise or "kick" which was quite apparent. An example of this is shown in Figure 10, in which the temperature changes indicated by the suspended thermocouple are plotted against the block temperature a t the time of each ignition test. These results are typical of those obtained with other fuels and also at other oxygen concentrations. At a fixed block temperature and for a single fuel, as the percentage of oxygen in the gas mixture is reduced, the temperature kick due t o ignition or cool flames becomes smaller. The response of the indicator is almost instantaneous and can be used not only t o identify cool flames without darkening the room, but also t o determine the time lags of cool flames. This was particularly applicable in the low temperature range, i.e., below 350" C. It is apparent that oxidation reactions preceding cool flames or positive ignition liberate some but comparatively little heat as evidenced by t h e slight temperature rise during the induction period. Furthermore, a t temperatures below those a t which either type of ignition will occur, there is also the evolution of heat in the gas phase as shown by a slight gradual rise in temperature above the original block temperature. On the other hand, the principal heat-evolving reactions occur within a short time, since the temperature kick begins at the same instant as does t h e cool flame or positive ignition. This agrees well with the hypothesis previously discussed t h a t certain intermediates are formed which must reach a critical concentration before a cool flame or positive ignition can occur. Examination of Figure 10 shows t h a t the temperature kicks observed for cool flames are approximately as large as those for
1618
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
positive ignitions a t adjacent temperatures. This was found t o be true for any of several thermocouple positions in the lower part of the ignition cell. With the thermocouple in the upper part of the cell, positive ignitions gave much larger kicks t’hancool flames. Because of the experimental procedures used, it is quite probable that a t the time of ignition there is a deficiency of oxygen in the IoTver part of the cell because of the relatively large volume of fuel vapor generated by its rapid evaporation. It, is reasonable, then, t h a t under these conditions only very incomplete combustion, such as cool flames, will occur in this part of the cell and t.he temperature kick observed n-ould be indcpendent of the type of ignition occurring in the rest of the chamber. This would explain the continuity of t8hetemperature kick curve in Figure 10 in passing from one tSpe of ignition zone to another. However, with the t.hermoeouple in the lower part of the cell, an apparent correlation exists between the magnitude of the kick and ease of ignition. Thus, the t.eniperature a t which the great’est,kick is observed corresponds to the temperature at which the lowest percentage of oxygen is required. This is shown by the reciprocal agreement of the curve in Figure 10 when compared t,o the corresponding ignition curve. IGYITIOV D E L A Y STLDIES
I t is known that ignition delay is indicative of the ignition quality of Diesel fuels. In general, the ignition delay increases with decreasing cetane number for a given set of temperature and pressure conditions in a n engine. For this reason, it vim felt that attention should be given to the effect of various factors on the ignition delays obseived with Diesel fuels in the present work. It is appreciated that this delay may not be related directly t o the delay in an engine. However, the information obtained should be of value toward a better understanding of ignition phenomena. The ignition delay referred t o in this paper is the time in seconds which elapses between the addition of the fuel drop and occurrence of cool flames or positive ignition. Ignition lags of greater than 30 seconds were rarely encountered. Starting from the lower ignition limits, it has been observed that at a k e d oxygen concentration the ignition lag decreases rapidly as the temperature is increased as may be seen in Figure 11. This curve is typical of those obtained with other fuels and also at different concentrations of oxygen. The effect of oxygen concentration on ignition delay may be seen by referring t o Figure 2. It is evident t h a t ignition delay is scarcely affected by changes in oxygen concentration. It can be seen from Figure 11 t h a t transition from one type of ignition zone t o another does not affect the unifoim change in time lag. The sudden change in the ignition lag a t about 450” C., which is typical of the other fuels, is not considered significant, but is believed t o be due t o the nonwetting phenomenon discussed previously. The fact that the time lag is independent of the type of ignition may be explained by use of the assumptions made earlier in this paper, particularly the concept that cool flames precede positive ignition, The determining factor on the ignition lag would then be the time required t o reach the critical concentration of the intermediates necessary for cool flames. The reactions t h a t follow, regardless of their nature, are very rapid and would not be measured by the technique used, The decrease in time lags
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as the temperature is increased is due to the acceleration of t h e pre-ignition reactions so that the critical concentration of intermediates is attained more quickly. The relative independence of ignition lags from the partial pressure of oxygen is further evidence ( 1 ) t h a t above a certain small amount, the partial pressure of oxygen is not a controlling factor in t h r pre-ignition reactions. CONCLUSION
The apparatus and methods used in the present ~i-orkare rclatively simple, yet many aspccts of the results obtained can lie ~orrelat~ed very well n-ith studies made by other investigators using entirely different approaches and techniques. Accordingly, considerable weight can be given to interpretations and conclusions t h a t may be drawn from this type of experimentation. In view of the above and because of its simplicity and speed, t h k method offers considerable promise in the investigations of ignition phenomena. 4CKUOW LEDGME\T
The authois are indebted t o James K. bIusick, Cora .1 lIcLean, and Harry P. Richey foi their assistance during the course of the investigation, and to 1Irs. Jcwi Kilcox in the preparation of the manuscript, LITERATURE CITED
Cullis, C. F., Hiiishelwood, C. N., Mulcahy, M. F. It.,and Partington, R. G., Discussions Furndag Soc., No. 2, 111 (1947). ( 2 ) Gervart. Y.G.. and Frank-Kamenetskil. Bull. acnd. sci. C.R.S.S., CEasse SCL. chanz., 1942, 210-220; UOP Translation 41 0 (Dee. 24,1943). (3) Gilmer, R. B., and Calcote, H. F., IKD. EKG.@ H E Y . , 43, 181 (1)
(1951). (4) Hum, R. W., and Smith, H. M., Ibid., 43, 2788 (1951). (5) Jentzsch, H., Oel u. KohZe, 31, 255 (1950). (6) Johnson, J. E., Blizzard, R. H., and Carhart, H. W., Saval Research Lab., Rept. 3602 (February 1950). (7) Johnson, J. E., Ciellin, J. W., and Carhart, H. W., Ibid., 3859 (December 1951). (8) Jost, IT.,“Thiid Symposium on Combustion, Flame and Explosion Phenomena,” PP. 424-32, Baltimore, hId.. The Williams & Wilkins Co., 1949. (9) Levedahl, 77’. J., and Howard, F. L., IKD. ENG.CHEW,43, 2505 (1951). (10) Maccormac, M., and Townend, D. T. A., J . Chem. Sor., 1938, 238. (11) Michailova, 11. S . , and Seumann, h l . B., Natl. Advisoiy Comm. Aeronaut., Tech. Mem., 813 ((1936). (12) Pauling, L., Wood, R. E., and Sturdivant, J. H., J . Am. C ‘ h m . Soc., 68,796 (1946). (13) Spence, K., and Townend, D. T. A,, “Third Symposium on Combustion, Flame and Explosion Phenomena,” pp. 404-14, Baltimore, Md., The Xilliams & Wilkins Co., 1949. (14) Taylor, Taylor, Livengood, Russell, and Leary, S.A.E. @[orL. Tyans.,4, 232 (1950). (15) Townend, D. T. A., Chevz. Rem., 21, 259 (1937). (16) Walsh, A. D., T m n s . FaradagSoc., 43,305 (1947). (17) Zerbe, C., and Eckert, F , Angew. Chem., 45,593 (1932). (18) Zerbe, C., Eckert, F., and Jentzsch. H., Ibid., 46, 659 (1933). RECEIVED for review January 29, 1952. ACCEPTED kIarch 5 , 1952. T h e opinions a n d assertations cont,ained in this article are the p r i r a t e ones of the authors a n d are not t o be construed as reflecting the views of the N a v y Department or t h e naval establishment at large.
