LEAN FLAMMABILITY LIMIT AND MINIMUM SPARK IGNITION ENERGY JOHN 8. FENN Experiment Inc., Richmond, Va. The left hand side of This study was stimulated by the availability of a large HIS laboratory has Equation 1 is the classical amount of data which had been accumulated in a long been engaged for some Arrhenius expression for a range study of cornbustionin ramjet engines. The usefulyears in the study of airsecond-order reaction rate. ness of lean limit flame temperatures in correlating some fuel combustion in ducted Pressure has not been incharacteristics of ramjet burners hinted that this property burners of the ramjet type. cluded as a variable since of a fuel might afford an approach to some of the more Theramjet burner is characonly observations a t atmosfundamental problems of the combustion process. terized by high velocity pheric pressure will be conIt was found, by assuming that the over-all combustion flow and attendant sensitivity to actual reaction reaction was second-order with respect to initial fuel and sidered immediately. The oxygen concentrations, that a correlation between miniconsiderations of Seinenov rate since the sojourn time mum spark ignition energiesand lean limit flame temperaof the air-fuel mixture in and others are taken as justitures could be obtained. By evaluation of the empirical fication for assuming that the reaction zone is quite constants, prediction of one from the other was possible the reactions in the flame short. Consequently, the with reasonable accuracy. Values of the over-all activafront occur a t or near the kinetic aspects of the conition energies for the explosive reaction of fuel with oxygen flame temperature (6, 9). bustion process have been were obtained. This is indicated by the presof particular interest to the A fairly successful description of ignition by sparks was ence of Tf in the exponent, workers here. Some moderdeveloped without recourse to any but thermal consideraThe particular reaction for ate success has been entions. The association of lean limit flame temperatures which E is the activation countered in correlating with activationenergies should prove useful in the elucidaenergy is not indicated. It burner performance over a tion of other combustion phenomena. is implicit that it relates to wide variety of fuel types with familiar kinetic comthe ratedetermining process, bustion quantities, such as burning velocities,&ammability limits, which is also assumed to be second-order with respect to the ignitionenergies, and thelike. Much of this work remains classiconcentrations of primary fuel and oxygen. Assumption of any fied and has not been published. However, out of buch correlations reaction system that i s firsborder with respect to fuel concentrathere have been precipitated relationships between some of the tion would appear equally as valid. classical quantities themselves which have not been previously The right hand side of the equation is the simplest possible discussed. This paper will describe such a relationship between expression for a rate of heat loss from the flame front. The conlean flammability limits and minimum spark ignition energies. stant, k , obviously includes the thermal conductivity of the The first portion of the paper attempts to relate flame temperagas and is specific for the particular apparatus in which the lean ture a t the lean flammability limit to the activation energy of the limit is determined. In addition, it presumes a similar temperarate-determining reaction in a flame-front. The remainder of the ture gradient for all cases. All these simplifying assumptions discussion develops'a theory of minimum spark ignition energy are probably not too brutal in the case of lean limit mixtures where based on this concept of activation energy. the properties of the air-fuel mixture are beginning to approach those of pure air and where flame temperatures are low enough to avoid complications because of dissociation and nonlinear specific ANALYSIS LEANLIVITFLAME TEMPERATURE AND ACTIVATION ENERQY. heat capacities. Equation 1 can be rearranged as follows Consider a flame front in an air-fuel mixture containing just
T
enough fuel to maintain propagation. Using a purely thermal interpretation, it can be assumed that the rate of heat generation in the flame front is just equal to the rate a t which heat is lost from the flame front, or per unit area
- -E
-
QN.rNoAe R T J = k (Tf- T o ) where Q = beat of combustion in calories per mole of fuel N t = mole fraction of fuel N o = mole fraction of oxygen A = Arrhenius constant E = activation energy in calories per mole R = gas constant (1.98 calories per mole degree) T j = flame temperature, O K. T o = teTperature of air-fuel mixture ahead of flame front, K. k = unknown constant having the dimensions calories per degree second
Since (TI T o )is proportional to QNf and since all other terms in the middle expression are nearly constant for lean limit mixtures, it is apparent that to a good approximation, B is a constant. If Equation 2 is now written in log form
E
[-RlogB] TI= aT/
(3)
where OL is a convenient constant. In other words the activation energy of the rate-determining reaction in a lean limit flame is proportional to the lean limit flame temperature. Thus, if the lean limits are determined in the same equipment for a large number of fuels, the relative activation energies can be obtained directly by calculation of the flame temperature. This turns out to be a useful concept. It will be applied in this paper to the
2865
2866
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
development of a theory for minimum spark ignition energy. Khen the experimental values for minimum spark ignition energy are employed in the theory, evaluation of the constant, C Y , in Equation 3 is achieved so that absolute values of activation energy are obtained in so far as the theory is true. According to Equation 3, in a given apparatus the lean limit flame temperature is largely determined by the activat'ion energy which is usually interpreted as a function of the chemical species only. Therefore, it follows that the lean limit flame temperature should be independent of initial temperature. This predicted independence is entirely consistent with experiment. For example, White found for several fuels t,hat the lean limit flame temperatures were nearly constant in the face of a variation in initial temperature from room temperature to 400" C. (7').
Vol. 43, No. 1 2
ductivity and is specific to a particular apparatus, in this caw the electrodes in the spark gap. The use of r in the denominator of the right side of the equation to determine the temperature gradient is a somewhat presumptive approximation. The thickness of the reaction zone interface, which really determines the gradient, should be inversely proportional to the square root of the reaction rate, if it can be considered as a flame front ( 3 ) . Equation 4 as rearranged in Equation 5 indicates that r is also inversely proportional to t,he square root of the reaction rate. Therefore, the thickness of the reaction zone was assumed to be proportional to the radius of the critical sphere and written as CT. Equation 4 can nolv be solved for r to give E
THEORY OF MINIMUM SPARK IGNITION ENERGY
Lewis and von Elbe have recently I'educed the process of ignition of gaseous mixtures by electrical sparks to a well defined system of theory and experiment (6). Proceeding from the assumption that pract'ically the entire energy of a spark discharge becomes thermal in nature within a very short time, they have succeeded in developing a theory which relates the minimum spark energy requirement for ignition to normal burning velocity and the quenching distance. Essentially their theory contemplates a critically small volume of combustible mixture which just satisfies thc necessary and sufficient condition for propagation that the rate of heat loss equals the rate of heat generation. They then define an excess enthalpy content of a plane combustion wave, which in t'he limiting case of the critically sniall volume element is identified with the ignition energy. This energy is determined by integration using quenching distance data to determine the limits and burning velocity data to define the required temperature and energy distributions. The present treatment derives froin the same basic considerations exckpt that some simpler and perhaps more sanguine assumptions are made, particularly in connection with the rate of heat generation. The result is a relationship betxeen lean limit flame temperature and minimum spark ignition energy involving only two unknown constants. These constants might be calculated but are evaluated here empirically from data over a wide variety of fuels. For convenience, the basic assumptions used are listed: 1. The energy of the spark is considered to be purely thermal; thus the ternis heat and energy are used interchangeably. 2. The combust,ion reaction is assumed t o be second-orderi.e., first-order with respect t o fuel and oxygen. 3. Diffusion processes are ignored. 4. The necessary and sufficient condition for explosive combustion to occur is assumed to be only t h a t the rate of heat generation shall be a t least equal to the rate of heat loss. Consider a homogeneous volume element of such size that the rate of heat generation therein is exactly balanced by the rate of heat loss. This necessary and sufficient condition for pi'opagation-Le., ignit'ion-can be expressed
where most of the symbols have the same significance as before and Tr is again taken as the adiabatic flame temperature of the mixture. I n addition, r is the radius of the sphere, p is the gas density, and C is a proportionality constant. The density term appears because the concentration of each reactant is the product of the mole fraction and the density. The constants 9'and k' are not quite the same as in Equation 1, the difference being indicated by the primes. In Equation 1, A was of such dimensions that the rate of heat generation mas related to a unit area of plane flame front. I n Equation 4, A' relates to a unit volume of homogeneously reacting gas. Again, k' is an effective thermal con-
where it has been assumed that 7'1 - T o ,the temperature rise by combustion, is proportional to r , Q , the amount of heat, released upon combustion per mole o i mixture. This assumption is not strictly true a t high temperatures where theqecific heats are no longer linear with temperatuw. Holvever, since all the flame temperatures treated here are in the same range, and sincci the properties of the combustion products are similar in all cases because of the dominHiny influence of nitrogen, the assuniption is a reasonable approximation. The required constants arc all summarized in 8. It is noteworth?, that the relationship bctweon P and T i as finally expressed in Equation 6 is true only for the case where the temperature of the gas mixture in the sphere is equal to the flame temperature which would be reached by coinbustion. of the mixture. Consequently, T j and, therefore, T , become fixed when the composition and initial temperature of the niixture are specified. The heat' necessary to raise the temperature of the critical sphere of radius, T , to Y ' j , the flame temperature, must come from some external source. This folloivs from the basic assumption of reaction rate necc.ssary to maintain the heat balance in Ilkpation 4. If some of the heat came from combustion, t,he concentrations of reactants would be decreased and, therefore, the reaction rate also. The heat thus required from an external source can be considered an ignition energy. The question of whether this energy represents the miniinuiii required for ignition is more complicated. It depends on the variation of the tot'al enthalpy of t8hecrit,ical,sphere, as T and 1') vary in accordance nith Equation 5. It is not immediatoly evident,, for example, if the temperature ver'e higher than t,he flame temperature, as n-oulti be quite possible with a spark, t'hat the enthalpj- content of the resulting smaller sphere ~ o u l t l not be smaller than the enthalpy content of the larger sphere a t a lower temperature. Detailed knowledge of specific heats a t high temperature would be required to determine this. Belox flame temperature, of course; where the specific heats do not vary abruptly Kith temperature, the total enthalpy of a critical sphere increases with decreasing temperature and increasing radius. I n any event it would seem reasonable to presume that the minimum energy requirement for ignition should be proportional to the energy requirement defined as the heat necessarj- to raise the sphere of critical volume to the flame temperature or
H =K.
53 d C , , p
(2'f - T o )
(7)
where H is the minimum spark ignition energy, C, is the mean constant pressure heat capacity on a weight basis, and K is a proportionality constant having a probable value equal to or less than one. If the value of r from Equation 6 is now substituted, there is obtained
t
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
2867
APPLICATION OF THEORY TO EXPERIMENTAL DATA Table
1.
Ignition Energy and Lean Limit Data for Various Fuels 4 = Equivalence Ratio H = Ignition . . ?eGpF12mKe. Tf(1esn) Lean Ignition Lean Ignition Energy
Fuel Acetaldehyde Acetone Acetylene Acetylene Acetylene Acrolein Allyl chloride Bensene 1 3-Butadiene ;-Butane n-Butyl chloride Carbon disulfide Carbon disulfide Cyclohexane Cyclopentane Cyclopropane Cyclopropane Diethyl ether Dihydropyran Diisobutylene 2,2-Dimethylbutane 2,2-Dimethylpropane Ethane Ethyl acetate Ethylene Ethylene oxide Ethylene oxide Ethylene oxide Furan n-Heptane Hydrogen Hydrogen Hydrogen Isobutane Iso-octane Isonentane Isobropyl mercaptan Isopropyl chloride Isopropyl ether Isopropylamine Methane Methylacetylene Methylacetylene Methyl ethyl ketone Methyl eth 1 ketone Methyl sulzde n-Pentane n-Pentane n-Pentane n-Pentene-2 Propane Propane Propane Propane Propionaldehyde n-Propyl chloride Propylene Propylene oxide Propylene oxide Tetrahydrofuran Tetrahydropyran Triethylamine Triptane
limit 0.567 0.593 0.307 0,307 0.307 0.494 0.646 0.571 0.499 0.629 0.614 0.305 0.305 0.578 0.579 0.527 0.527 0 535 0.605 0.649 0.599 0.618 0.551 0.607 0.453 0.434 0.434 0.434 0.474 0.744 0,229 0.229 0.229 0.586 0.693 0.573 0.597 0.695 0.606 0.816 0.597 0.469 0.469 0.569 0.569 0.552 0.565 0.565 0.565 0.555 0.574 0.574 0.574 0.574 0.518 0.679 0.521 0.481 0.481 0.616 0.693 0.744 0.594
mixt. 1.00 1.00 0.65 0.75 1.00 1.00 1.oo 1 .oo 1.00 1.00 1.00 0.80 1.00 1.00 1.00 0.84 1.oo 1.00 1.oo 1.00 1.00 1.00 1.00 1 .oo 1.00 0.70 0.80 1.00 1.00 1.00 0.59 0.78 1 .oo 1.oo 1.oo 1.oo 1.oo 1.00 1.00
limit 1675 1700 1275 1275 1275 1540
mixt. 2300 2210 2040 2250 2580 2340
i%i
1590 1765 1740 1030 1030 1670 1675 1650 1650 1610 1740 1825 1705 1740 1600 1705 1475 1515 1515 1515 1540 1980 985 985 985 1680 1825 1660
2340 2365 2280 2225 1950 2250 2225 2235 2080 2350 2305 2230 2290 2220 2220 2195 2125 2340 1920 2060 2425 2390 2275 1820 2180 2345 2260 2210 2250
1%
2250
1.oo 0.90 1 .oo 0.85 1.00 1.00 0.75 0.80 1.00 1.oo 0.71 0.80 0.90 1 .oo 1 .oo 1.oo 1.00 0.80 1.oo 1.oo 1.oo 1.00 1 .oo
1650 1600 1600 1650 1650
2200 2240 2450 2095 2210
1635 1635 1635 1650 1640 1640 1640 1640 1565
2095 2170 2275 2320 2260 1860 1950 2060 2310
1555 1555 1900 1900
iiio
2320 2040 2360 2430 2270
1690
2250
1.00
.. ..
..
..
..
..
T
t o 0.729 0.771 0.626 0.566 0.495 0,656 0.839 0,735 0.671 0.771 0.779 0.530 0.457 0,750 0.745 0.782 0.701 0.700 0.780 0.797 0.769 0.784 0,730 0.800 0.630 0.790 0.736 0.610 0.645 0.865 0.540 0.45 0.421 0.744 0.859 0.735 0.78 0.840 0.771 0.916 0.760 0,713 0.653 0.784 0.748 0,720 0.784 0.756 0.716 0.713 0,726 0,885 0.842 0.796 0.678 0.840 0.694 0.763 0.658 0.781 0.835 0.866 0,750
Under a contract with the Navy Bureau of Aeronautics ovei the past two years, this laboratory has accumulated a considerable amount of spark ignition data for a large number of fuels. The actual experimental procedures used and a large portion of the available data have already been reported ( I , 2 ) . In addition, the lean flammability limit composition for downward propagation in a 25-mm. tube has been determined for the same fuels under sponsorship of Project Bumblebee for the Navy Bureau of Ordnance. These data will be reported in the near future. For present purposes the pertinent data from both sources are summarized here. Table I shows the minimum spark ignition energies, the flame temperatures, and the ratio of lean limit flame temperature to flame temperature of the mixture ignited by the spark for various fuels. The lean limit equivalence ratio, $L, is also given. Equivalence ratio is defined as the ratio of actual quantity of fuel to the stoichiometric quantity. Most of these data refer to stoichiometric mixtures, but some results for leaner mixtures are included. The flame temperatures are from several sources and were calculated by a variety of methods. Enough check calculations were made to ensure reasonable consistency. Actually, in the case of the lean limits it was found that dissociation could be ignored so that values could be obtained by interpolation using limit composition data and heats of combustion. For some of the fuels, particularly those containing elements other than carbon, hydrogen, and oxygen, flame temperature data were not available. In those cases, resort was made to a straight line plot of $L us. T/(lean,/Tf for all fuels for which flame temperatures were known, and values were interpolated for T,(lean)/ Tf. T o determine T, - To, the value for the nearest hydrocarbon was multiplied by the ratio of the heats of combustion. Thus, the data used for the chlorides, mercaptans, and amines are only approximate. All the data in Table I are for an initial temperature of 298" K. and an initial pressure of 1 atmosphere. In Table I1 are the data for variation of spark ignition energy with initial pressure for a number of representative fuels. Table I11 shows the effect of initial temperature
Joules X I b - 4 3.76 11.5 0.44 0.37 0.2 1.37 7.75 5.5 1.75 3.8 12.4 0.47 0.15 13.8 2.38
....
2.4 4.9 3.65 9.6 16.4 15.7 2.85 14.2 0.96 2.6 1 5 0.87 2.25
7.0
0.26 0.21 0.20 5.2 13.5 7.0 5.3 15.5 11.4 20.0 4.9 1.72 1.52 14.75 5.3 4.8 28.0 20.5 4.9 7.0 5.88 10.5 5.5 3.1 3.25 10.8 2.8 3.7 1.9 5.4 12.1 11.5 10.0
m
where u combines all of the undefined constants. It is implicit that the density term, p , relates to the flame temperature. A useful approximation is that a change in initial temperature'results in a proportional change in flame temperature of about half the amount. Assuming that pressure is everywhere the same, it can, therefore, be assumed to a good approximation for comparable 'experiments that p is proportional to p/To. Accordingly, Equation 8 can be written in common log form and rearranged to give
HN3 =L log 2 Tf - To
+ 210g To - 2logp, + 0.65 aT'f(Iea3 R T,
(9)
where E = a T f ( l e a n )has been substituted from Equation 3 and all the undetermined constants have been combined in L. Equation 9 is readily amenable to test by experiment.
8
:
+$0
I
-2
$$.
4
+.f
"1/
71'
NORMALIZED TO THIS POlM
PO
QN-PENTANE A N-HEPTANE *PROPANE ISOOCTANE A PROPYLENEOXIDE W CARBON BISULFIDE
38
23
Figure 1.
24
Effect
of
.
25
26
27
LOG T. Initial Temperature on Minimum Spark Ignition Energy -40" to POO' C.
2868
INDUSTRIAL AND ENGINEERING CHEMISTRY Table
II.
In Figure 2 a similar plot was made with log p a as the ordinate. Once again the theoretically required slope of 2 represented by the straight' line is affirmed. Here again the data mere adjusted to coincide a t 1 atmosphere. Figure 3 s h o w data for all fuels with the ratio of the lean limit flame temperature to the observed flame temperature as the variable. There is a fair ainount of scatter from the predicted linear relationship. JF-hen the wide range of the data is consiclercd, however, the agreement is rcmarkable. The slope of the line in Figure 3 can be used with Equation 9 to determine 01 in Equation 3, from which the activation energies can be calculated directly using the lean limit flame temperatures in Table I. It turns out that c( has the value 16; this gives the following energies for representative fuels,
Variation of Ignition Energy with Pressure Pressure, Atmospheres 1. o 0.5 0.25
Fuel Acetylene
0.10
0.04 1.0
Carbon disulfide
0.0 0.36 0.23
1. o 0.75 0.60 0.28 0.17 0.10 3.0 2.0 1 5 1.0
Hydrogen
Pentane
0.63
0,30
0.38 0.25 0.13 0.13 2.0 1.3 1.2;
Propane
Fuel Pentane Propane Ethylene Carbon disulfide Hydrogen Acetylene
400.
1. o 0 73 0.50 0 30
23.0 68 0
0.23
Propylene oxide
II 0 . 18.5 0.8 3.6 26 93. 0.13 0.60 1.21 11.5 0.20 0.44 1.25 6.0 15.5 98.0 1.33 2.2 3.2 4.1) 18 0 23. 33 114, 330. 1.1 2.1 2.7 3 .5 6.0 10.1
2 0 1,3
1.0
.
1.35
4 4 20 5 :,7 0 08 0
0.25 0.13 0.10
Activation Energy, Kg-Cai. per Mole 26.1 26.1 23.6
16.0 15.8 20.4
These values are at the low end of the range 25 to 50 kg.-cal. usually used by others ( ( i , 6 , 8 ) .
0.31) 0.31
0.5
Vol. 43, No. 12
Table 111.
Variation of Ignition Energy with Initial Temperature
Fuel Carbon disulfide
To, ' C.
rf
23 100
0 76 0 5 14.5 6.7 3.2 27.0 11.0 4.8 45.0
n-Heptane
25
100 171 25
Iso-octane
100
variation. The spark energy data in Table I11 were taken using flanged electrodes. Results with these two electrode systems are not identical in absolute magnitude but are directly related as has been shovvn by Calcote ( 4 ) . Because the pui pose here is only to determine a slope in one term of the equation, the differences in absolute magnitude can be ignored. Equation 9 has essentially three variables: initial tempei aTf(~eun)
ture, To;pressure, po; and fuel;----.
Ti
I
10
(CONST)
Figure 2.
10
100 171 176 - 40 - 30 - 20 25 57 82 100 204
Propane
Propylene oside
Effect of Pressure on Minimum Spark Ignition Energy
7.8 4.2 2.3
2.5
11.7
9,7
8.4
5,s
4.2 3.6
3.5 1.4
25 100 182
2.4 1.3 O.Q
The question arises as to whether the dispersion of the data in the plots used to test the relations developed here is due to experimental inaccuracies or to real phenoinena for which the
1 I
100
HNY e X1 0
14.5
125
A convenient test oi the
equation with the available data is to fix two of the variables and to plot the remaining Itariation. Figure 1 shorvs such a plot using log Toas the abscissa for six different fuels. Since the intercept would be different for each fuel, the data ivere normalized to coincide at 298" K. That is, the constants mere adjusted so that ordinate for each fuel was the same a t 298" IC A straight line with a slope of two as required by Equation 9 was drawn through this point. There is some scatter of the data, but in general the predicted slope of two is verified.
01
171
- 80 - 20
n-Pentane
I 10
HNVXIO'
I 10
I
..
t . T 1
Figure 3.
Minimum Spark Ignition Energy and Lean Limit Flame Temperature
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
theory does not account. Of course, both of these factors may be involved. However, the data, particularly in the lean limit work, leaves much to be desired. It is fairly certain that the values for the high molecular weight compounds are not entirely consistent. The use of a relatively small diameter tube accentuates the effects of heat loss to the wall. Convection effects may be exceedingly variable. The use of a spark to initiate combustion near the limiting composition raises many questions as to energy requirements for initiating a sustained flame front. I n short, it is believed that the determination of true lean limits offers a fruitful area for future study both in the development of improved experimental techniques and in the understanding of other combustion phenomena. ACKNOWLEDGMENT
The author appreciates the cooperative assistance of many of the staff of Experiment Inc. In particular, he is grateful to H. F. Calcote, Ruth Gilmer, and C. E. Setliff.
2869
LiTERATURE CITED
(1) Calcote, H. F., Gregory, C. A., Jr., Barnett, C. M., and Gilmer,
Ruth B.. “Spark Ignition I. Effect of Molecular Structure,” paper presented at the 119th Meeting of the AMERICAN CHEMICAL SOCIETY, Cleveland, Ohio. Calcote, H. F., Gregory, C. A,, Jr., and King, I. R., “Spark Ignition 11. Effect of Initial Temperature and Pressure,” paper presented a t the 119th Meeting of the AMERICANCHEMICAL SOCIETY, Cleveland, Ohio. Gaydon, A. G., and Wolfhard, H. G., Proc. Roy. Soc. (London), 196, 105 (1949). Jost, W. (Croft), “Explosions and Combustion Processes in Gases,” New York, McGraw-Hill Book Co., 1946. Lewis, B., and von Elbe, G., J. Chem. Phys., 15,803 (1947). Semenov, N. N., Uspekhi Fis. Nauk, 24 (1940), translated by NACA TM No. 1026 (September 1942). White, A. G., J.Chsm. Soc., 1925,672. Zeldovich, Ya. B., and Frank-Kamenetsky, D. A., Acta Phusiwchim. U.R.S.S., 9, 341 (1938). Zeldovich, Ya. B., and Frank-Kamenetsky, D. A., J. Phys. Chem. (U.S.S.R.), 12, 100 (1938). RECEIVED July 27, 1961. Presented before the Division of Gas and Fuel Chemistry at the 119th LIeeting of the . ~ X E R I C A NCHEMICAL SOCIETY, Cleveland, Ohio.
SPONTANEOUS IGNITION TEMPER ATU RES Commercial Fluids and Pure Hydrocarbons JOSEPH
L. JACKSON
Lewis Flight Propulsion La boratory, National Advisory Committee for Aeronautics, Cleveland, O h i o
Aitempts to understand the effect of molecular structure of hydrocarbons on spontaneous ignition temperature, and subsequent correlations of these temperatures with other combustion phenomena, require self-consistent data for a large number of pure hydrocarbons. Various compilations of spontaneous ignition temperature have been published, but as they were comprised of data obtained by many investigators, many inconsistencies exist. The data presented here are self-consistent because they were obtained in the same apparatus, by the same techniques, and by the same investigator. Tabulated values of
spontaneous ignition, and ignition delay, as determined in a crucible-type apparatus, are presented for 94 pure hydrocarbons and 15 commercial fluids. Although absolute values of ignition temperature depend on the apparatus and test procedure, it is believed that these self-consistent data will be useful to investigators who are concerned with thermal ignition processes, such as those encountered in internal-combustion engines. The data should also be helpful in establishing the relative fire hazard of the fluids investigated in the presence of a thermal ignition source under quiescent conditions.
N CONJUNCTION with the aircraft fire problem and as a
have been made (4-6). In general, the value obtained for the spontaneous ignition temperature of any substance will be dependent on the pressure, the nature of the igniting surface, the combustible to air or oxygen ratio, the movement of the mixture relative to the surface, and the time allowed for ignition to occur. Both individual and relative values of ignition temperature of different substances vary widely in the reported .data from different investigators. For example, the values quoted in the literature (3) for the spontaneous ignition temperature of benzene in air vary from370’ to 1062’ C. (690’ to 1944’ F.),
I
corollary t o the combustion research a t the Lewis Flight Propulsion Laboratory of the NACA, it was desirable to have self-consistent data on spontaneous ignition temperatures of hydrocarbons and aircraft fluids. Compilations of spontaneous ignition temperatures have been presented ( I , 4 ) but they are composed of data determined by many investigators using a variety of methods. Since different methods were used, there is no basis for comparing the ignition temperatures of the combustibles listed and it appeared desirable to redetermine the spontaneous ignition temperatures of a variety of pure hydrocarbons, fuels, and other liquids using a single procedure. I n this paper are reported the spontaneous ignition temperatures of 94 pure hydrocarbons and 15 commercial fluids as determined by one investigator using one method. 9 survey of the methods used for the determination of ignition temperatures is given in reference (3) and more rece,nt studies
APPARATUS AND PROCEDURE
The apparatus used in this work was the Scott, Jones, and Scott ( 4 ) modification of the ASTM crucible-type method ( 2 ) . Spontaneous ignition temperatures were determined by raising the temperature of the’ block and periodically dropping a few
