INDUSTRIAL AND ENGINEERING CHEMISTRY
1512
TESTS AT LOWER CONCENTRATIOSS
I n Table X, the data on further tests of some of the more active compounds obtained during this investigation are reported. Soil burial tests were run on strips of cotton cloth which were impregnated with a 1% concentration of the candidate fungicide instead of a 2% concentration. These values, with the results of soil burial tests on cloth protected with 2% of the compound, are listed together for purposes of comparison. With tests on 2 weeks’ soil burial, the differences in the activity a t different concentrations are insignificant, and well within the limits of eiperimental error. There is differentiation in activity at a 1% concentration in the soil burial tests lasting for 4 weeks. These results indicate that the most effective compounds found during this investigation are 5-(2-thenylidene)rhodanine and 5 - ( p
chlorobenzy1idene)rhodanine. ACKSQWLEDGJIEST
The work described in this paper v a s support,ed by a contract with t’he Army Chemical Corps. The author. express t,heir appreciation to Elise S . Lawton and Marny Potter. who assisted in the testing and synthesis of compounds.
Vol. 46, No. 7
LITERATURE CITED
(1) dndreasch. R.. Monatsh.. 10. 73 118891. (2) Andreasch, R., and Zipser, A., Ibid., 24, 499 (1903).
(3) Ibid., 25, 159 (1904). (4) Ibia.,26, 1191 (1905). (5) Bradsher, C. K., Brown, F. C., and Grantham, It. J., J . iim. Chem. SOC.,73, 5377 (1951). ( 6 ) Ibid.,76,114 (1954). (7) Brown, F. C., Bradsher, C. K., and Bond, S. hI., IKD.E m . CHEM.,45, 1030 (1953). (5) Brown. F. C., Bradsher, C. K., Bond, S. SI., and Potter, hl., J . Am. Chen~.Soc., 73, 2357 (1951). (9) Brown, E’. C., Bradsher, C. K., and Lawton, E. S . , IND. ENO. CHEM.,45,1027 (1953). (10) Brown, F. C., Bradsher, C. K., SlcCallum, S.G., atid Potter, AI.. J . Oro. Chem.. 15. 174 (19503. (11) Crowe, B., and Nord, F. F., bid.. 15, 81 (1950). (12) Emerson, \Ti. S., and Patrick, T. AI., Ibid.,14, 790 (1949). (13) Geiger, W. B., and Conn, J. J., J . Am. Chem. Soc., 67,112 (1945). (14) Granicher, C., Gerd, M., Ofner. -4.. Klopfenstein, 9.. and Schlatter, E., H e h . Chinz. -4ck2, 6 , 458 (1923). (15) Libermann, D.. Himbert, J.. and Hengl, L., BUZZ. SOC. chim. France, 15, 1120 (1948). (10) Plucker, J., and S m s t u t z , E. D.. J . Am. Chem. Soc.. 62, 1512 (1940). (17) Zipser, A , , Monelah., 23, 958 (l902j. R X C E I Y Gfor D review December 1 2 , 1953
.$CCEPTED
1 I a r c h 2 2 , 19.54.
Ignition Be avior of the Hexanes J. ENOCH JOHNSON, JOHN W. CRELLIN, AND HOMER W. CARHART Naval Research Laboratory, Washington 25, D . C .
T
HIS study of the ignition behavior of the isomeric hexanes was undertaken to further the knowledge of the relationship between the molecular structure of hydrocarbons and their ignition properties. The over-all objective is the development of information which will correlate t,he composition of fuels with performance in internal coiiibustion engines. The controlled-oxygen ignition meter developed in these studies presents a simple and convenient system for evaluating fuels and pure compounds with small amounts of material. Previous work (8, 9) with this apparatus has supplied data which correlate very well with those obtained by other invest’igators using substantially different techniques and apparatus; the results gained tend to supplement and extend their work. The ignition patterns developed in the present study give a clear picture of the differences in ignition character of compounds as affected by changes in oxygen partial pressure and temperature. Particularly under conditions in xhich ignition is controlled by the mechanism of low temperature oxidation, the results reveal the effects of molecular structure on ignition and illustrate the complexity of the processes involved. The isomeric hexanes were chosen for study because they exemplify the transition from straight-chain hydrocarbons to highly branched compounds having the same molecular weight and because they could be obtained highly purified. I n addition, other investigators have utilized the hexanes for related studies, so that correlation with their work is made easier. APPIRATUS I Y D PROCEDURE
The controlled-oxygen ignition meter has been used previously a t this laboratory to study the ignition behavior of fuels and hydrocarbons and has been described in detail (8). Essentially it consists of an electrically heated ignition chamber of 21-ml. capacity containing thermocouples in both the block and free space inside the chamber. Pure oxygen or mixtures of osygen and nitiogen are preheated and supplied to the chamber
a t the rate of 25 nil. per minute. A single drop of the hydrocarbon is introduced int,o the chamber and the resultant phenomena are observed. The ignition delay is measured by noting the time that, elapses between the addition of the fuel drop and any evldence of ignition. After each such procedure the chamber is purged with air and the ignition crucible, which is used in the bottom of the chamber, is replaced with a clean one. Studies are usually made a t a fixed osygen concentration, the block temperature being lowered gradually unt,il the minimum ignition temperature is reached. This process is repeated at several different oxygen concentrations to provide data from which the ignition diagrams are derived. ilt the lower temperat,ures the ignition boundaries could be determined to k 2 O C., but a t the higher temperatures they could not be delineated so clearly. All hexanes used in this study were obtained froin the €’hillips Petroleum Co. Each mas specified t o have a purity of not less than 99 mole % and vias used as rweived. IGNITION DIAGRAMS FOR T H E HEXASES
Ignition diagrams were prepared from the data for each of the five hexanes, as illustrat’ed for 2-niethylpentane in Figure 1, in which t’he curve is drawn t o represent the boundary between the various ignition zones. The ignition pattern for iso-octane (2,2,4-trimethylpentme) has been included for comparative purposes. I n the diagrams presented in Figure 2, the temperature of the ignit,ion chamber was plotted against the per cent oxygen in the gas mixture supplied to the chamber. The ignition patterns for n-hexane, 2-methylpentane, and 3rnethylpentane shown in Figure 2 are typical of those found previously for several pure hydrocarbons and Diesel fuels (8, 9). The regions of positive or “hot” ignition lie above and to the right of the curves. The regions of cool flame lie below the curve. The boundary between the positive ignition and nonignition zones drops almost vertically from 100% oxygen to less than 40% before any significant deviation occurs. The temperature requirement for the vertical boundary increases on going from n-hexane to 2-methylpentane to 3-methylpentane,
July 1954
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY x 0
1513
POSITIVE IGNITION COOLFLAMES
0
100
80
z W
> a
g
10
60
I-
z
$
40
W D:
a 20
Figure 1. Ignition Diagram for 2-Methylpentane 250
showing an increasing resistance to spontaneous ignition, At temperatures slightly below this boundary exothermic preignition reactions take place. This is evidenced by the temperature of the gases in the chamber, which rises above the block temperature but not sufficiently to cause ignition. At lower oxygen levels the two peninsulas characteristic of the more easily ignitable hydrocarbons become apparent. The inflection point of the first peninsula for these three hexanes lies between 30 and 40% oxygen. The fact that the oxygen requirement a t this point is essentially the same indicates that this peninsula may be ascribed to a similar oxidation process in each case. I t may be significant that the minimum oxygen requirement a t this inflection point for the Diesel fuels studied by this method also lies between 30 and 40% oxygen. The minimum oxygen requirement for the second peninsula in the ignition curve for n-hexane is considerably lower than that for the methylpentanes. This further illustrates the greater ease of ignition of n-hexane, already shown by its lower minimum ignition temperature. This peninsula lies a t about the same oxygen level for the two methylpentanes, although there is a temperature shift corresponding to the difference in minimum ignition temperatures. The portion of the ignition curves lying in the region of high temperature ignition is very much the same for these three hexanes, indicating a close similarity in the oxidation processes in this temperature region. The ignition diagrams for 2,2-dimethglbutane, 2,3-dimethylbutane, and iso-octane are given in the upper half of Figure 2. These three hydrocarbons show a marked displacement in the oxygen concentration required for positive ignition in the low temperature region compared to n-hexane and the methylpentanes. For the most part, ignition a t low temperatures for these highly branched paraffins is confined to cool flames. Only 2,2dimethylbutane shows a vestige of the phenomena exhibited by the more easily ignitable hydrocarbons. The single peninsula found for 2,2-dimethylbutane is believed to correspond to the second or larger peninsula in the ignition curve of n-hexane. Although 2,3-dimethylbutane and iso-octane show no positive ignition below 395" and 435' C., respectively, even a t 100% oxygen, cool flames were observed with these two hydrocarbons at much lower temperatures as denoted by the cool flame boundary curves. Below 50% oxygen the cool flames for 2,3-dimethylbutane and iso-octane became so indistinct that the boundaries could not be clearly delineated. A t higher temperatures the positive ignition limits for the di-
I
I
1
I
I
I
I
I
290
330
370
410
450
490
530
570
TEMPERATURE ( " C )
Figure 2.
Ignition Diagrams for the Hexanes and Iso-octane
methylbutanes and. iso-octane tend to come together and, because of their much steeper slopes, converge toward the ignition curves of the other three hexanes. DISCUSSION OF IGNITION DIAGRAMS
The relative ease of ignition of the isomeric hexanes is reflected in their ignition diagrams. The effect of branching is observed in the minimum ignition temperatures for positive ignition and cool flames, the oxygen requirement a t a given temperature, the areas of positive ignition, and the contours of the positive ignition curves. However, in order to develop a reasonable understanding of these phenomena, it is advisable to examine the current theories of oxidation of hydrocarbons. 20 18 16
-8
14
z
0
8
12
I",
2
10
-1
z F
-
8
z ' 6 4
2
260 *
280
300 320
340 360
300 400
0
TEMPERATURE ('GJ
Figure 3. Ignition Lag Data for Cool Flames and Positive Ignition for the Hexanes
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
1514
" 1560 1600 1640 1680 I720 I760 1800 1840 1880 I920 I / T X IO6
Figure 4. Arrhenius Plots of Ignition Data for the Hexanes
It, is generally agreed that the oxidation reactions may be separated into a low temperature region and a high temperature region, with the transition from one to the other occurring in the neighborhood of 400" C. a t ordinary pressures. Oxidation in the low temperature region appears to be a composite of several reactions which give rise to positive ignition limit curves, showing two or even three distinct lobes or peninsulas as pointed out by hfalherbe and Walsh (28). Inspection of the ignition diagrams in Figure 2 bears out the admitted complexit'y of the ignition process. The peninsulas in the positive ignition curves indicate definite zones of increasing and diminishing reactivity. The positive ignition limit curves for 2,3-dimethylbutane and iso-octane would seem to indicate that a simpler mechanism might apply. However, there are other considerations which niake this simplicity unlikely. The production of cool flames M-ith these compounds a t temperatures much lower than positive ignition, the primitive peninsula shown by 2,2-dimethylbutane, and the high pressure data of Maccormac and Townend ( 1 6 ) for iso-octane all point to the likelihood that even with 2,3-dimethylbutane a t least one low temperature peninsula would be found a t higher pressures of oxygen. It is very likely that the low t'emperature phenomenon observed with all the hexanes is contingent on the formation and decomposition or further react,ion of peroxides. Ralsh ( 2 0 ) has proposed the initial formation of an alkyl hydroperoxide which takes place a t a tertiary carbon atom in preference to a secondary and finally t o a primary. The several peninsulas in the ignition diagrams of many hydrocarbons indicate changes in the rate of oxidation as the temperature changes. These changes are attributed by Malherbe and Walsh (18) to the different ways in which an oxygen molecule may attack the alkyl hydroperoxide molecule. These reactions are illustrated simply as: R
+
+ 02--ROO
+
ROO RH ROOH R ROOH 0% -.t radicals --f
+
It is pioposed by Malherbe and Walsh t,hat if the hydroperoxide has two or more types of C-H bonds, the oxygen molecule will attack them in two or more different t,emperature ranges. Fallah, Long, and Garner ( 4 ) have shown that in the oxidation of n-heptane there are a t least two distinct maxima in the amount of peroxide formed as a function of temperat.ure. It may well be that' the t\vo peninsulas obtained in the present work with n-hexane and t'he methylpentanes correspond to these
Vol. 46, No. 1
maxima in peroxide formation. The fact that the first peninsula for these three hexanes shows nearly the same minimum oxygen requirement would indicate that the peroxides formed under these conditions are of a similar type. This seems contradictory to Walsh's theory (20) that the first attack on the molecule should be a t a secondary carbon for n-hexane and a t the tertiary carbon atom for the methylpentanes. Furthermore, attack a t the tertiary carbon atom would leave only secondary C-H for the two methylpentane peroxides, although the peroxide from n-hexane would contain both secondary and tertiary C-H. This Fould not account for the two peninsulas rvhich were found in the ignition curves for the methylpentanes, if we assume that these peninsulas are dependent on the presence of secondary and tertiary C-H in the hydroperoxide molecule, as suggested by Malherbe and Walsh. This apparent disagreement may be resolved, however, if we assume that in the methylpentanes the point of initial oxygen attack is not solely at the tert'iary C-H. If we utilize instead the met'hyl retardation rule of Hinshelnood (6) as a method of estimating the point of oxygen at'tack as illustrated in Table I the attack would occur a t a secondary C-H in all three of these hexanes. This view is strengthened further by the study of the slow oxidation of 2-methylpentane b y Cullis ( Z ) , in which his data led him to the conclusion that attack a t the secondary CEIe group was much more important t'han attack a t the tertiary CH. By assuming attack a t CH2 for both of the methylpentanes, the resulting hydroperoxides would t'lien satisfy the conditions suggested by Malherbe and Walsh as necessary for the existence of two prominent peninsulas for these hydrocarbons.
' ik0
250
1
1
1
270 2!30 290 300 MINIzMU'M IGNITION POINT ( ' C )
260
1 310 2 2 0
Figure 5. Comparison of Minimum Ignition Point with Relative Oxidation Rate for the Hexanes
Only one peninsula was found for 2,Z-dimethylbutane, as may be seen in Figure 2, although the diagram as shown may not be definitive, since the data were obtained close to the limiting experimental conditions. However, the appearance of only one peninsula agrees with the hydroperoxide oxidation theory, as the hydroperoxide most probably formed from 2,2-dimethylbutane n-ould leave only a tertiary C-H and no secondary C-H. The peninsula observed lies in the temperatwe range indicated by Malberbe and Valsh to be typical of attack on the remaining secondary C-H of an alkyl hydroperoxide, yet no such C-Hbond is available in this instance. The ignition limit diagram for iso-octane derived by Maccormac and Townend (16) shows only one peninsula, which would be the case whether first attack occurred at the tertiary carbon according to Walsh or a t the secondary carbon as indicated by the methyl retardation theory. However, the similarity in temperature ranges of the peninsula found by Maccormac and Townend for iso-octane and that found in the present work for 2,Z-dimethylbutane suggests that the peninsula foI iso-octane also may be
July 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY 3
I
TABLEI. SELECTION OF POINTOF INITIAL OXYGENATTACK Walsh (80)
Compound
n-Hexane
c c-e-c-e-c-c
Z-Methylpentane
c-c-c-c-c
Hinshelwood ( 6 )
*
c-c-c-c-c-c c c-c-c-c-c
Y
b
C I Y
c-c-c-c-c
3-Methylpentane
k C 2,2,4-Trimethylpentane
C-b-C-C-C
I
C
C
2
2
2
0 I-
dr
z
C
b
b i :
1
25
c c-c-e-c-c I t: l e c-c-c-c-c
Y
eau
15
W
=
I
-I
I
e COOL
c
IGNITION DELAY STUDIES
The data on ignition delay for the hexanes taken a t 100% oxygen are plotted against temperature in Figure 3. The shape of the curves is typical of those previously found for fuels and hydrocarbons by this method. Ignition delay curves for hydrocarbons obtained by such methods as the constant-volume bomb ( 7 ) and the rapid-compression machine ( 1 9 ) also have similar shapes, The ignition delay for cool flames follows the pattern found for positive ignitions, as is shown by comparing the curve for 2,3dimethylbutane with those of the other hexanes. Also, in general, the ignition delays for the isomers increase in the same order as the difficulty of ignition a t any given temperature. The data on ignition delay obtained with the hexanes are given in Figure 4 in the form of Arrhenius plots. In all cases the resulting plots are essentially straight lines with some deviation a t the extremes of temperature. This was also observed by McEwan and Tipper ( 1 7 ) for the slow oxidation of cyclopropane. The slopes of these straight lines were used to derive values for the apparent activation energies required for ignition and are given in Table 11. These values indicate that the energy of activation becomes higher as the hexanes become more highly branched and the ease of ignition decreases, and a t best are b e lieved to indicate relative differences only and not absolute values,
FLAME
05
0
due to a hydroperoxide of the type resulting from initial attack a t the CH2 group. It may be concluded from the foregoing discussion that the current theories concerning ignition of hydrocarbons are compatible with the present study of the ignition of the hexanes. The Hinshehood theory of methyl retardation is helpful in explaining the ignition phenomena observed with these hydrocarbons and is referred to again during the discussion of the significance of the h i m u m ignition temperatures. The views of Malherbe and Walsh concerning the further attack of the hydroperoxide initially formed have also been utilized in the explanation of the contours of the positive ignition curves. I t was anticipated that the ignition behavior of the methylpentanes and n-hexane n-ould be similar, judging from the iindings of Cullis ( a ) regarding 2-methylpentane and n-hexane. I t was also expected that these three hexanes should differ markedly in their ease of ignition from 2,%-dimethylbutane and 2,3-dimethylbutane when one considers the oxidation studies of Kahler, Bearse, and Stoner (12) with the hexanes.
1515
Figure 6.
250
260 270 280 290 300 MINIMUM IGNITION POINT ("C)
310
320
Relationship of Minimum Ignition Point to Methyl Retardation Factor
However, these derived values are consistent with those calculated by others for the oxidation of hydrocarbons, which range from 25 to 40 kcal. per mole (11, 14). The value of 29.5 kcal. for 2,2dimethylbutane is in good agreement with the value of 27.3 kcal. obtained by Fenn (6) for the activation energy required for the explosive reaction of the same compound with oxygen as calculated from spark ignition energy measurements. Additional investigation will be necessary to establish the validity and application of the present method for the determination of these apparent activation energies. SIGNIFICANCE OF THE MINIMUM IGNITION POINT
The lowest temperature a t which positive or hot ignition occurs in the ignition meter a t 100% oxygen for a given material has been defined as the self-ignition point (SIP) (IO). However, it was observed in this study that, in the case of 2,3-dimethylbutane and iso-octane, cool flames were found a t temperatures well below the self-ignition point. It has been postulated (8) that positive ignition a t the lower temperatures is always preceded by a cool flame in a two-stage reaction which is often indistinguishable experimentally. Therefore, it is reasonable to say that the self-ignition point is also the minimum temperature for the occurrence of cool flames for materials that show positive ignition in the low temperature region. However, in order to avoid confusion it was advisable to define the minimum temperature for the occurrence of either positive ignition or cool flames a t 100% oxygen as the minimum ignition point ( M I P ) to dif-
I20
TABLE 11. APPARENTEXERQY OF ACTIVATION Compound %Hexane 2-Methylpentane 3-Methylpentane 2,P-Dimethylbutane 2,3-Dimethylbutane
E, Koal./Mole 25.0 26.7 28.3 29.5 33.9
540
Figure 7.
255
260 270 280 290 300 MINIMUM IGNITION POINT ("GI
310
320
Relationship of Minimum Ignition Point to Research Octane Number
I N D IJ S TR I A L A N D E N G IN E E R I N G C H E M I S T R Y
1516
ferentiate it from the self-ignition point. The minimuin ignition point values for the five hexanes, iso-octane, n-octane, and n-heptane are recorded in Table 111.
TABLE 111. lIIsn\rrrai IGXITIOK POINT Relatir e RIethyl Oxidation Retardation Research Compound MIP, C.a Rateb FactorC Octane Yo.d 262 0.44 24 8 n-Hexane 1580 2-XIethylpentane 279 560 1.00 73 4 1.44 74 5 293 3-Methylpentane 60 2.2-Dimethylbutane 303 12 2 00 91.8 318e 1 2.67 101.7 2,3-DimethyIbutane Below 0 249 42,000 0 15 n-Octane n-HeDtane 262 ... 0 22 0.0 2,2,4~Trimethylpentane 30Ee ... 1.67 100 a Minimum temverature a t which either cool flame or uositive ignition occursit 100% oxygen. b Data from Cullis and Hinshelwood ( 5 ) . c Calculated according to method of Hinshelwood ( 6 ) . d nst9 f.r o.~~~ m i l i .. j.,
Cool flame meajurement.
Based on studies of the slow oxidation of paraffin hydrocarbons by Cullis and Hinshelwood ( S ) , Hinshelwood derived the theory of methyl ret'ardation ( 6 ) to explain the effects of molecular structure on oxidation. He shoved a good correlation between the derived methyl retardation factor and log of relative oxidation rate for a number of hydrocarbons. As the minimum ignition point is also a measure of oxidation rate, it was of interest to t,he present study t,o discover how well minimum ignition point values xould correlate with the relative oxidation rates found by Cullis and Ilinshelwood which \yere obtained a t lower temperatures rvit,hout the occurrence of cool flames or ignition. These values are given in Table 111. A very good correlation XTas found, as shown in Figure 5 , in which the minimum ignition point values are plotted against the log of the relative oxidation rates. The greatest deviation from the straight-line relationship is shomm by n-hexane. It seems remarkable that such a close relationship should exist bet,ween data obtained under such markedly different condit,ions and in entirely different types of apparatus. Because of Ihe excellent agreement with slow oxidation rate data, it seemed evident that the same struct,ural effects were operative in bot,h cases. Consequently, the mininium ignition point values were plotted against Hinsbel\Tood's methyl retardation factor in Figure 6. Again: excellent agreement resulted, shelving a regular decrease in minimum ignition point as the methyl retardation factor decreases. Only iso-octane departs significantly from the relationship curve. As iso-octane vias not included in the elow oxidat,ion study, it is not ltnoxii Fvhether there is better agreement of methyl retardation factor rrith osidation rate. n-Hexane, Tvhich did not fit the plot of minimum ignition point and log rate in Figure 5 as n.ell as might be desired, shows a much better conformity to the minimum ignit'ion point-methyl ret,ardation factor curve. The plot in Figure 6 tends to curve more a t the loner values. The same characteristic may be observed in the relationship between methyl ret,ardation fact,or aiid log oxidation rate ( 6 ) . It may be concluded t#hatHinshelwood has derived a very useful relationship between the Etructure of paraffin hydrocarbons and their oxida,tioii and ignition behavior. Furtliermore, it appears that the minimum ignition point is also a measure of the relationship of molecular structure to ease of ignition or slols oxidation. I n his study dealing with the correlation of the knock resistance of purc hydrocarbons with chemical structure, 1,ivingston ( 1 5 ) extended the methyl retardation concept and proposed a factor called the structural retardation factor (SRF). The best available octane number data xere utilized for the derivation of a series of rules for calculating the structural retardation concept. By so doing Livingston arrived a t a reasonable correlation of octane number with molecular structure.
Vol. 46, No, I
I n view of the successful correlation of minimum ignition point with molecular structure based on methyl retardation factor, and Livingston's correlation of octane number with et'ructure, a plot which shows a relationship betxveen mininium ignition paint and research octane number for the hesanes is given in Figure 7 . The data for iso-oct,ane and n-heptane are also included. The plot, in Figure 7 shows a fairly good correlation betx-een minimum ignition point and octane number. It seems clear that the reactions which contribut'e t o engine knock are definitely related to those contributing to low temperat,ure autoignition and to slow oxidation of paraffin hydrocarbons. This support's the view that knock in spark-ignition engines is dependent on the spontaneous ignition of the "end gas." The findings of Levedahl and Howard (13) and others show that, in the two-stage spontaneous ignition of hydrocarbons, cool flames play an important role. This is also demonstrated in the present work by the close relationship between the minimum ignition point', which is essentially a ineasure of the minimum temperature for the formation of cool flames, and octane number, Previous work (10) on the correlation of self-ignition point, with cetane number shows that, in general, the self-ignition point. of Diesel fuels is inversely related to cetane number-Le., the higher the cetane number the lower the self-ignition point. In the ease of these Diesel fuels it must be borne in mind that the self-ignition point is coincident with the minimum ignition point. Although no cetane number data for the hexanes appear to be available, the known relationship of octane number t o cetane number further indicates the possible utility of the minimum ignit,ion point a6 a tool for estimating the cetane number of pure hydrocarbons. Work on the significance of the minimum ignition point and its value in the study of the ignition of fuels and pure hydrocarbons is being extended to include other types of hydrocarbons. LITERATURE CITED
American Petroleum Instimte, API Project 45, 14th Annual Report. Cullis, C. F.,Tmns. Faradag Soc., 4 5 , i o 9 (1949). Cullis, C. F., and Hinshelwood, C . S . .Discussions Faladay Soc., N o . 2, 117 (1947). Fallah, A., Long, R., aiid Garner, F. H., Fuel, 31, 4 (1952). Fenn, J. B., ISD. ENG.C m x , 43, 2 8 6 (1951). Hinshelwood, C. K , , J . Chem. Soc., 1948, 331. H u n , R. W., and Smith. IT. 11.. IXD.ENG.CHEM.,43, 2788 (1961). Johnson, J. E., Crellin, J. IT,, aiid Carhart, €1. JY., Ibid., 44, 1612 (1952).
I W . , 45, 1749 (1953). Johnson, J. E., Crellin, J. W,, arid Carhart, H. W,, Saval Re-
search Laboratory. Rept. 3839 (Dee. 21. 1951). Jost, IT., "Explosions aiid Coinbustion Processes in Gases," Kew York, XIcGraw-Hill Book Co., 1946. Kahler, E. J., Bearse. -1.E . , and Stoner, G . G., IND.ENG. CHEkf., 4 3 , 2777 (1951). Levedahl, 117. J., aad Howard. F.L., Ihid.,43, 2805 (1951). Lewis, B.. and T o n Elbe, G., "Combustion, Flames, and Explosions of Gases," Xew York, Academic Press. 1931. Livingston, H. K., IND.ENG.CHEM.,43, 2834 (1951). ilIaccormac, 11.,and Townend, D. T. A , J . Ci~em.Soc.. 1938, 23s. AIcEwan, -1.C..and Tipper. C. F.H., Suture, 170, 482 (1952). LIallierbe, F. E., and Walsh, A. D.. TiSam. Faiaday Soc., 46,
835 (1950). Taylor, C. F.,Taylor, E. S.,Livengood, .I. C., liussell, W. A., arid Leary, TV. d., 8.A. E. CZuart. Trans., 4,232 (1960). Walsh, A. D., Trans. Faruday Sac., 42, 269 (1946). R E C I X ~ Efor D rcview December 3, 1953. ACCEPTHD X a r c h lG, 1954. Presented before the Division of Petroleuin Chemistry a t the 1'24th Xceting of the B~sa1c.4XCREXICALSOCIETY, Chicago. 111. The opinions and aasertions contained in this article are the private ones of the authors and are not to be construed as reflecting the vie1v.s of the S a v y Department or the naval establishment a t large.
