Spontaneous Ignition Temperature of Liquid Hydrocarbons at Atmospheric Pressure Effect of Concentration of Fuel Vapor and Oxygen: Fuel Ratio F. J. DYKSTRA -4SD G R ~ H ~E J DG I A R , Ethyl Gasoline Corporation, New York, ?J. Y.
A
RENEWED i n t e r e s t in the spontaneous ignition temperatures of motor fuels is evidenced in the recent l i t e r a t u r e ( 2 , 4, 8, 9 ) , as well as a t t e m p t s to relate such ignition temperatures to e n g i n e p e r f o r m a n c e . This s u b j e c t w a s s t u d i e d by this laboratory some years ago and was found to be more complex than much of t h e l i t e r a t u r e would indicate. This paper contains data selected to show the influence of certain v a r i a b l e s o n t h e vapor-phase i g n i t i o n temperature of h y d r o c a r b o n fuels.
Vapor-phase ignition temperatures of a sample of gasoline and of n-octane and 2,2,4-trimelhylpentane are measured in a glass apparatus under a Dariety of conditions. For some given fuel-oxygen-nitrogen mixtures, trvo types of ignition temperatures can be obtained f o r the same fuel: a low-temperature ignition, practically independent qf fuel concentration or oxygen-fuel ratio: and a high-temperaturc ignition, temperature varying markedly with uariations in these variables. These two ignition temperatures are separated by a zone of nonignit ion. Ignition of a hydrocarbon fuel is a complicated phenomenon, and care should be employed in interpreting experimental data.
zation. 0 xy gen-n i t rogen mixtures in varied proportions entered the outer tube at a fixed rate of 200 cc. per m i n u t e (at n o r m a l t e m p e r a t u r e and p r e s s u r e ) . There were, therefore, two parallel streams of gas, one of fuel vapor c o n t a i n i n g a s m a l l amount of nitrogen, and the other of oxygen and nitrogen. T h e s e were separately heated and then mixed at the orifice of the inner tube, 18 cm. from the exit. A thermocouple placed at 2 . 5 cm. from this orifice measured the temperature of the mixed gases.
The flow of oxygen, nitrogen, and fuel were adjusted to the desired values, and heat was then applied, the temperature rising slowly while the fuel flow remained constant. The tempera! L P P A R A T U S AND P R O C E D U R E ture of the mixed gases a t the After considerable experinientation with the Noore (5) time when explosion occurred was taken as the spontaneous type of ignition apparatus, it was found that this apparatus ignition temperature of that particular mixture. The temdid not permit control of oxygen-fuel ratio or concentration of perature readings were accurate to 2' C., the gas volumes to fuel, and it was therefore abandoned in favor of a n apparatus 2 per cent, and the fuel volume to 3 per cent. designed to measure ignition temperatures of completely The total volume of oxygen and nitrogen, for the experiments cited here, mas always 230 cc. per minute (N. T. P.). vaporized mixtures. For a given rate of fuel feed the oxygen: fuel ratio was varied The apparatus was of the concentric-tube or Dixon ( 3 ) type by changing the percentage of oxygen in the mixed gases. and consisted of an outer t,ube of Pyrex glass, 75 cm. long and I n other experiments the composition of the oxygen-nitrogen 3.4 cm. inside diameter, and an inner tube of Pyrex, 0.65 cm. inside diameter, drawn out to a 2-mm. orifice. The orifice of mixture was kept constant and the fuel feed varied to give the inner tube was 18 cm. from the exit of the outer tube. The varying percentages of fuel. outer tube was wound with resistance wire throughout its entire Certain experiments were carried out with a modified techlength and lagged with air cell asbestos over the heating element. nic, discussed below. Capillary tubing attached to a gravity feed delivered fuel Figure 1 illustrates the effect of oxygen: fuel ratio upon the into the inner tube, where it was vaporized by means of heat. A small amount of nitrogen (30 cc. per minute) aided the vapori- ignition temperature of a commercial gasoline, with conctant
0 . ? 4 6 8 / 0 PERC€NT COM8USTl8lC (VOL UML)
250 !
0
5 IO /5 20 OXYGEN/f UEL RA TI0 O OLUME)
FIGURE1. EFFECTOF OXYGES:FUEL RATIOON SPONTAKEOUS IGNITION TmfPERATURE OF GASOLINE
0 2 HRCEN
r
4
6
8
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0
cOMBusr/mE @ L U M ~
FIGURE 2. EFFECT OF CONCENTRATION OF FUELVAPOR ON SPONTA-
FIGURE 3. EFFECTOF COVCENTRATIOV OF FUELVAPORON SPONTA-
NEOUS IGNITION T E M P E R h T U R E OF G.ASOLINE AT COSST.4ST OXYGEN : FUELRATIO
YEOUS
509
IGVITIOV
TEMPERATURE O F
G ~ S O L I VAT E C O N S T 4 \ T NITROGEN ' OXYGENR ~ T I O
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INDUSTRIAL AND ENGINEERIKG CHEMISTRY
percentages of fuel vapor in the mixture. Figure 2 illustrates the effect of the percentage of fuel vapor a t constant oxygen: fuel ratios. Figure 3 illustrates the conjunctive effect of varying the percentage of fuel vapor, keeping the oxygen: nitrogen ratio constant. These curves are calculated from the dits in Figures 1 and 2. T h e r e s u l t s are rather striking. For a given percentage of fuel v a p o r i n the mixture the ignition temperatures are lowered smoothly as the o x y g e n : fuel ratio is i n c r e a s e d , until suddenly an a b r u p t drop of about 200" C. o c c u r s with only a small change in the oxygen : fuel r a t i o 250 ( c u r v e for 3.1 per 2 4 6 8 / 0 / 2 OXYGEN/FUEL RATIO poiurn0 cent f u e l v a p o r in FIGURE 4. ZONES OF IGNITIONAND Figure 1). NONIGNITION FOR GASOLINE (4.6 PER Similar a b r u p t CENT COMBUSTIBLE) changes in the ignition temDerature are shown in Figure 2, corresponding to small changes in the percentage of fuel vapor a t constant oxygen: fuel ratio. From Figure 3, the effect of an increase in percentage of fuel vapor with constant oxygen: nitrogen ratio (decreasing oxygen: fuel ratio) is a drop in ignition temperature followed by a range of approximately constant ignition temperature and a final rise in ignition temperature at a high percentage of fuel vapor. This effect is pronounced only for nitrogen: oxygen ratios of 70:30 or lower. With a ratio of 79:21 (air) only a slow decrease in ignition temperature remains noticeable as the fuel percentage is raised. The data contained in Figures 1, 2, and 3 were difficult to interpret a t the time, and it was not until a change in technic was decided upon that a satisfactory explanation of the results was obtained. A few experiments were carried out as follows: using a given concentration of fuel vapor (say 4.6 per cent) and a very low oxygen: fuel ratio, the temperature was kept constant a t a point intermediate between the low and high values of Figure 1, and the oxygen : fuel ratio was slowly increased by increasing the percentage of oxygen in the oxygen-nitrogen mixture. Surprisingly enough, ignition did not occur until an oxygen]:fuel ratio was reached far greater than that indicated by the line connecting the high and low points in the corresponding curves of Figure 1. A number of experimental points were obtained by this technic. It was then found that these points could be duplicated, using the original technic, by continuing to raise the temperature (within certain ranges of oxygen: fuel ratio) above that of the lower ignition temperature. For example, with gasoline as fuel (Figure 4) 4.6 per cent fuel vapor, and 8.2:l oxygen: fuel ratio, ignition was obtained in a rising temperature experiment a t 280" C. The temperature was allowed to rise farther, with ignition occurring a t short intervals until the temperature reached 314O, when ignition ceased. When the temperature reached 444" C., ignition commenced again, the explosions being somewhat less violent than those in the range of 280" to 314" C. By lowering the temperature slowly, the same zones of ignition and nonignition were passed through. The data in Figures 4 and 5 were thus obtained. Figure 5 illustrates the results obtained with two pure isomeric hydrocarbons, n-octane and 2,2,4trimethylpentane. n-Octane shows the zones of ignition and nonignition similar to the corresponding data for gasoline.
Vol. 26. N o . 5
The 3.8 per cent fuel vapor mixture of 2,2,4trimethylpentane showed no ignition temperature below 523' C. and a relatively slight effect of oxygen:fuel ratio on the ignition temperature.
DISCUSSION OF RESULTS A study of the experimental results shows clearly that for certain fuels a t least two separate and distinct ignition temperatures can be obtained, under proper concentrations of fuel vapor and oxygen:fuel ratio. Under other conditions only one ignition temperature can be obtained, and for certain fuels only one ignition temperature was obtained under the conditions reported here. (0xygen:fuel ratios given here refer, of course, to those measured at the furnace inlet. During the process of mixing, the composition of the gas a t any one point may vary substantially from those figures.) The low ignition temperature (where both types are obtained) is almost unchanged by wide variations in the composition of the combustible mixture, while the high ignition temperature is sensitive to even small variations in the mixture. I a n i t ion temperatures in air, --+ which a p p e a r t o correspond usually to the high type, should be carefully controlled if reproducible results are to be obtained. Ignition temperatures in oxygen would appear to be usually of low type and are much less sensitive to conditions. "oi f/ = Presumably, zones of nonigI nition, similar to 2 4 6 8 lo It I4 those of Figures - . 4 MYG.CN/FML RA n o CYOLUUE.) and 5 , could have been obtained for FIGURE5 . IGNITION CHARACTERISTICS
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1 *-I- I ~
OF WOCTANE AND 2,2,4-TRIMETHYLPEN-
the mixtures TANE studied in Figures 1 and 2, the sudden drop in ignition temperature being merely a change from the high to the Iow ignition temperature. Abrupt changes of this type are characteristic of chain reactions, to which class these oxidations undoubtedly belong ( I , 6, '7). It is clear from these data that the ignition of hydrocarbon vapor is not a simple phenomenon, and that determinations of ignition temperatures under conditions not carefully controlled form a rather uncertain basis for comparing different hydrocarbons. A single determination, even under a given set of controlled conditions, may likewise be distinctly misleading. Caution in the interpretation of ignition temperature data is therefore decidedly advisable. ~~
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LITERATURE CITED (1) Beatty-and:Edgar, J . Am. Chem. Soc., 56, 102-14 (1934). (2)ICummings, Trans. Am. Soc. Mech. Engrs., Aeronaut. Eng.,5.
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65-73 _ _ . - (19.13). _-,.
(3)'Dixon, J. Chem. Soc., 95,514(1909). (4)-Grebel, BUZZ.mem. soc. i n g . civils. France, 85, 67-128 (1932). (5) Moore, Chem. Age (London), 10 (1924). (6) Pope, Dykstra, and Edgar, J. Am. Chem. Soc., 51, 1875, 2203, 2213 11929). (7) Semeno;, J . Phys. Chem. (U.S.9. E.),4,4-17 (1933). (8) Taylor, Aircraft Eng..,5, 136-7 (1933). (9) Townend and Mandlekar, Proc. Roy. SOC. (London), 141A 484-93 (1933). R ~ C E I V EDecember D 9, 1933.
