January 1951
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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
thin laminar flame wall which would not diverge too greatly from the maximum flame temperature. I n conclusion, the carbon black flame is a region of ovoidal cross section, bounded by walls of high temperature flaming gases, whose interior is a preheating and cracking zone for hydrocarbon gases, diluted by products of combustion and hydrogen. The flame, used in this research, burning 44 standard cubic feet per tip per 24 hours through a 0.044-inch slot a t a tip height distance of 2.75 inches ranges in thickness from 0.25 t o 0.48 inch and in width, from 0.85 inch a t a height of 0.25 inch above the tip, through 1.20 inches a t 1.25 inches to a width of 1.50 inches at a point of 0.05 inch below the channel. The temperature gradient data together with visual observation show that the flame may be divided into three more or less diffusely defined regions. T h e bottom part of the flame, near the tip and the interior of the flame generally, consists of a lower temperature region wherein little carbon exists and in which gas phase primary reactions of the hydrocarbons may be taking
181
place. The temperatures in this region range from near 500“ to near 1000’ C., the higher temperatures being nearer the channel. The second zone, that of incipient carbon formation, surrounds this inner portion, being rather thin toward the bottom outer edges of the flame. This secondary portion blends into the bright reddish yellow top portion of the flame whose temperatures range around 1450’ C., in which the soot density is quite high and in which, evidently, the majority of the carbon-forming reactions occur. LITERATURE CITED (1) Bone, W.A.,and Townend, D. T. A., “Flame and Combustion in Gases,” London, Longmans, Green & Co., Inc., 1927.
(2) Hottel. H., and Broughton, H. F., IND. ENQ.CHEM.,ANAL.ED.,4,
166 (1932). (3) Ihid., p. 167. (4) Kurlbaum, F., Physik. Z.,3, 187 (1902). (5) Lewis, B.,and Von Elbe, G., “Combustion, Flames and Explosions of Gases,” Cambridge, Cambridge Press, 1938. (6) Wiegand, W. B., IND.ENG.CHIM., 23, 178 (1931). RECEIVEDJuly 3, 1950.
Evaluation of Fuels Using the Jentzsch Ignition Tester
I
RUTH B. GILMER AND HARTWELL F. CALCOTE Experiment Incorporated, Richmond, Vu. I n the search for a small scale laboratory method for Otto cycle fuel evaluation, the German Jentzsch ignition tester was examined with respect to pure fuels, mixtures, and additives. The results of these tests were then correlated with the critical compression ratios found in the literature. In general, the variation in engine conditions has a greater effect on the correlation than the difference aooredited to the Jentzsch tester. The observations may be rationalized by comparing them with a pressure-temperature ignition diagram where in this instrument the pressure is simulated by oxygen flow. As a means of attaching a single number to a fuel to indicate its tendency to knock, the Jentzsch ignition tester is as adequate as any single engine test. I t therefore offers a simple and inexpensive means of evaluating the knocking tendency of fuels.
equation with “bubbles per minute plus one”-the “plus one” allows for the diffusion of air into the unit. After demonstrating the usefulness of wizardry, i t is possible, to a limited extent, to rationalize the principles involved with autoignition phenomena.
T
HE possibility of evaluating Otto cycle engine fuels without
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actual engine tests has intrigued scientists for many yeare ( 3 ) . A simple test which could be carried out with small quantities of material would be especially valuable in research to appraise the usefulness of scarce new compounds and in routine control or testing of fuel quality. Such a test was invented in Germany by Jentzsch as early as 1926 (2) and used extensively by the Germans during the last war (7). This is attested by the large number of directives on the subject to the German navy and by the fact that the unit was evidently manufactured on a commercial scale. Nevertheless, fuel evaluation in this country is still carried out almost exclusively in standard test engines ( 5 ) . Although i t must be admitted a t the outset that any final conclusion as to a fuel’s utility must come from performance tests in the engine in which i t is t o be used, the advantages of the Jentzsch ignition tester as a practical tool cannot be ignored. Probably the greatest impedance to applying the Jentzsch ignition tester in this country is the apparent witchcraft involved in a method which plots bubbles per minute against temperature and then uses an
Figure 1. Jentzsch Ignition Tester
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. P
TABLE I. GEXERALRnQvIREmxrs Designation T U
c
I or 1v
Tu b,
c W
Diesel Fuels The lower, the sooner ignition takes place T h e higher, the lower the amount of air required and the greater the tendency t o ignite The shorter the quicker the fuel jgnites (high speed Diesels require short ignition lags which however, lead some: times 'to preignition; so called chemical whips)
FOR -4 GOOD FUEL
Spark Ignition Fuels The higher, the better t h e antiknock quality The lower the better the antiknock qdality, but may lead to soot if too low The longer, the better t h e antiknock quality
lower ignition temperature or the lowest temperature a t which spontaneous ignition takes place with a n excess of oxygen = lowest oxygen bubble number in the preignition region = - Tu = lower ignition value bu 1 = - To = - Tu characteristic ignition value bu 1 = ignition lag. The time between introduction of a fuel drop into the ignition chamber and ignition =
+ +
T h e general directional trends of these quantities for good Diesel fuels or good spark ignition fuels-Le., Otto engine fuels-are summarized in Table I. Only spark ignition fuels will be considered further in this paper. Figure 2.
Effect of Octane Number on Ignition Characteristics
I n this paper a brief description of the Jentzsch ignition tester will be given and some experimental results presented. An empirical relationship will be shown t o exist between the results obtained with the ignition tester and knock performance in a n engine. The correlation is in agreement with the assumed importance of autoignition on knock (8). DESCRIPTION OF INSTRUlMENT AND PROCEDURE
T h e Jentzsch ignition tester is essentially a hot plate-type ignition temperature measuring device del- oped by the Germans cion engines. The sigto evaluate fuels for Diesel and spark i oxygen to simulate presnificant feature of the unit is the USF sure so t h a t ignition curves (oxy ( ' , I flow versus temperature), roughly equivalent to pressurc tcbmperature ignition diagrams, may be obtained. The unit (Figure 1) consists of four chambers of equal size (diameter, 0.594 inch; depth 1.59 inches) arranged symmetrically in a stainless steel block which is heated by a n electric furnace. One of the chambers holds a thermometer or thermocouple, while the remaining three are used as ignition chambers. The fuel is added dropwise into the chamber with a small medicine dropper. Each of the three ignition chambers is supplied with oxygen through a passage down the center of the steel block connecting a t the bottom with each of the three chambers. T h e ignition chambers mag be looked upon as engine cylinders in which the temperature is provided by heating, the pressure by the admission of oxygen, and scavenging of the chambers by means of a n easily removable stainless steel dish. The rate of oxygen feed to the chamber is determined by a bubbletype flowmeter in which 60 bubbles equal 5 cc. of oxygen. When the rate of oxygen feed (in bubbles per minute) is plotted against the temperature, a curve is obtained which bears an empirical relationship to the usual pressure-temperature ignition diagram (8,IO). Several characteristic parameters which are peculiar t o a given fuel may be taken from such curves. These are defined below and are indicated in Figure 2 for n-heptane and iso-octane. T o = upper ignition temperature or the lowest temperature a t which spontaneous ignition takes place without extra oxygen
An experiment is performed in the following manner: The oxygen bubble number is set with a fine control needle valve and is determined either by counting the bubbles over a measured time interval or by closing the bubble counter with a pet cock and observing the rate of water rise in t h e calibrated measuring tube (Figure 1). The temperature is controlled by regulating the current t o the electric furnace with a rheostat and is determined with a thermometer or thermocouple. A small ignition dish, which just fits into the ignition chambers, is placed in the bottom of each chamber. Only the front chamber is actually used in making ignition tests; the other two are used merely to preheat the dishes, a new dish being used for each test. A small drop of fuel is carefully added and the ignition lag is determined. Ignition is indicated by a flash of flame, sometimes accompanied by a sharp report. If no ignition occurs within 30 seconds, the temperature is allowed either t o increase or decrease slowly and the test repeated until an ignition region is found. The dishes must be carefully cleaned between each test. I n the authors' experiments this was done by heating over a Bunsen flame and scouring with a steel brush. The ignitions and nonignitions are plotted to give the characteristic ignition curve, oxygen flow versus temperaturr.
I
1
I
TEMPERATURE,
Figure 3.
I
OC.
Effect of Antiknock Compounds on Igni tion Characteristics
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3anuary 1951
TEMPERATURE, ‘C.
Figure 4.
Effect of Structure on Ignition Characteristics
RESULTS AND DISCUSSION
The effect of changing the octane number by varying the composition on an n-heptane-iso-octane mixture (definition of octane
number) is demonstrated in Figure 2. With the exception of isooctane, these curves are comparable t o the usual pressuretemperature diagram if the ordinate is looked upon as equivalent to pressure. This point of view is rationalized by arguing that a pressure increase is effective only as far as i t increases the concentration of available osygen. Some justification of this has been found in actual engine tests ( 1 ) . The addition of oxygen t o the intake manifold in an Otto engine showed a n effect on the tendency to knock which was equivalent to a pressure increase. However, the failure of iso-octane to show an ignition peninsula with the Jentzsch tester indicates that the simile between pressure and oxygen flow should not be carried too far; iso-octane normally gives an ignition peninsula on a pressure-temperature diagram (6). All of the compounds tested having a critical compression ratio below iso-octane showed an ignition peninsula, while those having a critical compression ratio above iso-octane did not. The ignition lag, which also may be important in determining knock (I), is indicated in seconds along the curves. Note particularly the long ignition delays for the 75 octane mixture. Ignition failures were sometimes observed in the lower regions of the ignition peninsula. Figure 3 illustrates the effect of additives on the characteristic ignition curve. Here again, decreasing the tendency t o knock decreases the size of the ignition peninsula as well as increasing the lower ignition temperature. It also noticeably increases the peak in the curves. I n Figure 4 the effects of structure are as expected when compared with the extensive studies of the behavior of pure compounds in engines ( 5 ) . No further comment on this figure is necessary other than t o point out that although the upper ignition temperature-Le., with no oxygen added-is essentially the same for all fuels, the addition of oxygen brings out large differ-
7n
I
DICYCLOPENTADIENL 0 0 CYCLOPENTANE
BENZENE
TOLUENE
.2 4
2
6
8
IO
12
\
14
2
(6OOr.p. m.-350°.F.)
Figure 5.
a.
Correlation of Critical Compression Ratio with Characteristic Ignition Value Authors’ values 0. Literature values (9, 11)
4
6
CRITICAL COMPRESSION
CRITICAL COMFRESStON RAT10
e
IO
12
RAT10
le000 r.p.m. -L?50°F.)
Figure 6.
e.
Correlation of Critical Compression Ratio with Characteristic Ignition Value Authors’ values 0. Literature values (9, 11)
INDUSTRIAL AND ENGINEERING CHEMISTRY
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where the engine speed is changed from 600 to 2000 r.p.m. and the jacket temperature is either 350” or 212’ F. Some of the fuels having a critical compression ratio below the normal curve under one set of conditions have a critical compression r&o above the normal curve for another set of conditions. Some of these variations are summarized in Table 11. Variation in engine conditions ha3 a greater effect on the correlation than the difference between the predicted critical compression ratio and the critical compression ratio determined in a n engine test. Therefore, it might be concluded that as a means of attaching a single number t o a fueb to indicate its tendency to knock, the Jentzsch ignition tester is a8 adequate as any single engine test. Of course, other factors may also be important in determining knock. Ignition lag, the peak in the curve, and the position of the curve on the temperature scale have already been mentioned. The significant point is t h a t the characteristic ignition value correlates sufficiently to give an indication of fuel quality.
\
!
~
0 3-METHYL HEXANE
