R. W. HURN and K. J. HUGHES Bureau of Mines Petroleum Experiment Station, Bartlesville, Okla.
Ignition Accelerators and Autoignition Environ T H E increased demand for Diesel fuel has resulted in the use of increasingly greater percentages of cracked distillates and other low-quality Diesel fuels. Since these fuels often fail to meet specifications of the cetane number, there is growing interest in the use of ignition-promoting additives to upgrade these fuels. At the same time there are serious questions regarding the true value of the additives. Improvement in the cetane number may be determined, but there is doubt that this improvement necessarily will reflect a real proportionate gain in engine performance under conditions of operation that require use of the ignition promoter. The studies reported herein were made to determine how the response of ignition-promoting materials varied with changes in the conditions under which autoignition was initiated. The data were intended to serve a dual purposeto answer the very practical question of whether ignition promoters are equally effective under different autoignition conditions; and, to provide fundamental information that might be valuable in understanding the mechanism by which additives promote ignition. A variety of compounds qualify as ignition accelerators (7). These additives, in general, have some chemical
characteristics in common and for the most part tend to be thermally unstable. The most popular are nitrate and peroxide compounds. The mechanism of an ignition accelerator has not been clearly established; however, it has been suggested that the ignition promoters enter into chain-branching reactions (2, 3 ) . I t is postulated that the course of the early chain reactions can be sufficiently altered so that the time required to bring about the final stages of combustion can be materially reduced. As the additives are generally thermally unstable, it would be expected that their decomposition products are the effective agents in altering the course of the early chain reactions. For instance, it has been established that an additive decomposition product as nitrogen dioxide will accelerate the reaction between oxygen and hydrogen and between carbon monoxide and oxygen. The ignition-delay data reported herein were obtained using a constantvolume combustion bomb. Extensive work has been done with the bomb, and its utility as a combustion research tool has been established (4). Different fuels may show different changes in ignition delay with changes in bomb temperatures and pressures. Therefore, since the results obtained in the bomb depend upon the test conditions, it may
be expected that these data will not necessarily correlate with a unique fuel characteristic, such as cetane number. To the contrary, the significance of these results lies in departure of the data, both in scope and possibly in direction, from those obtained in conventional tests. Apparatus a n d Experimental Procedures
The apparatus used in these studies has been previously described (4, 5 ) . The bomb is an electrically heated stainless steel pressure cylinder, approximately 5 inches long and 3 inches in diameter, fitted with openings for fuel injection and pressure-sensing apparatus; accessory instrumentation is provided to show an oscillogram of the pressuretime history of the bomb contents with a correlated diagram of the fuel-injectionvalve motion. The injector equipment allows single-shot injection of fuel, and the control equipment permits precise control of bomb pressures and temperatures over a wide operating range. Oscilloscope records are photographed on 35-mm. film; ignition delays are measured by an electronic time-interval meter. Figure 1 shows typical combustion records. These oscillograms show bomb pressure (vertical deflection) as a func-
*
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250 psi
5
ADD/T/V€ C
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10 15 20 0 5 IO T I M E A F T E R I N J E C T I O N OF F U E L , m S e c
15
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I
I G N I T I O N D E L A Y OF C L E A R F U E L , M I L L I S E C O N 3 S
20
Figure 1 . Typical bomb combustion records with additive of commercial amyl nitrate
1904
1
3 % ADD
.-1.39O0F.
Figure 2. quality
'-1,100"
F ,
'-900°F
Reduction in delay as a function of fuel ignition
A D D I T I V E S IN FUELS tion of time (horizontal deflection). I n each record the start of fuel injection is coincident in time with the left origin of the pressure trace. A record of the injector-valve velocity is shown on one oscillogram-in this record an upward deflection of the injection curve denotes a n opening velocity, a downward deflection a closing velocity. These records show the slight decrease in pressure that accompanies fuel injection, followed by a marked pressure rise due to combustion after a suitable induction, or delay, period. This delay period varies with pressure and with fuel-ignition-improver concentration. It is impossible to fix a unique point a t which combustion may be said to have begun, since the combustionpressure curve is relatively smooth in most instances. For this reason, the ignition-delay period has been arbitrarily defined as that interval of time between the following two events: 1. The start of rapid fuel injection, as indicated by the first detected motion of the injection-valve pintle. 2. The point at which the bomb pressure exceeds the initial pressure by either 10 or 100 pounds per square inch. If no measurement datum is specified, the figure of 10 pounds per square inch applies. These respective levels are indicated by the small pips on the pressure traces of Figure 1. For 250 pounds per sqQare inch (Figure l), ignition delay measured to the lower pressure level does not adequately define the point of rapid combustion since there is a low (15 to 20 pounds per square inch) pressure step which precedes the main pressure rise. However, ignition delay measured to the 10-pound-per-inch level closely approximates that point a t which early heat release starts. I n spite of the arbitrary system of defining ignition delays, it has been found that, for most cases of normal combustion, the 10pound-per-square-inch-level measurement is adequate to describe the point of initial rapid heat release. Data shown in Figures 1 and 7 were taken with the fuel delivery adjusted to maintain a ratio of fuel to air of 0.03 by weight. All other data were taken with the injection equipment adjusted to deliver 0.12 gram of n-decane per injection, which gave ratios of fuel to air of 0.02 to 0.07, depending on bomb pressure and temperature. The latter procedure resulted in essentially constant volume delivery of the fuels handled. The fuels used in these tests are described in Table I. Most of the bombcombustion studies were made with the three commercial Diesel fuels. Some physical properties of the four additives used are shown in Table 11. The additives were assigned code designations
Y2% A D D I T I V E 40
W
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360
I
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5- 2 0
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LL
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z o 0
60
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3 % ADDITIVE
0 3
D
$ 40 w
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a a 20 W > a 0
1,000 1,200 BOMB T E M P E R A T U R E , O F.
600
Figure 3.
800
1,400
Influence of temperature on averaged response to all additives
Average d a t a for additives A, B, C, and D used individually in each of three fuels-Texas straight run, Texas Gulf Coast light cycle, and West coast straight run
that are used throughout the remainder of the report. Throughout the report, additive concentration is expressed as weight per cent of the untreated fuel.
ignition accelerator is the percentage by which the ignition delay of the clear fuel can be reduced by adding the ignitionaccelerating material. However, if this evaluation-method is used, it must be demonstrated that the percentage reduction values are a real measure-of the
Discussion One measure of the effectiveness of an
Table 1.
Gulf Coast
Fuel Properties
Texas Gulf Coast Texas GulfCoast West Coast U-6, Low Cetane Straight Run Light Cycle Oil Straight Run Reference Fuel Cetane Viscosity S.S.U., 100° F. Gravity, OAPI 60' F. Sp. gr., 60/60 Aniline point, O F. Flash point PMCC, 'F. Bromine No. Cetane No. R.I. n2$ R.I. n2$ ASTM distillation IBP 10%
50% 90% Max. temp.
35
37
40
33
38.2 0.8340 157.5
27.1 0.8925 118.7
26.9 0.8933 120.5
51.7 0.7726 188
175 2.17 55.9 1.46380 1.47628
220
200
120
369 440 512 576 618
460 509 542 584 610
10.82
36.91.51177 1.53291
5.42 . ._ 31
36 50.1 0.7792 203
22.9
100 1.4347
1.49015 1.50385 470 499 526 560 579
VOL. 48, NO. 10
348 373 391 496 608
526
OCTOBER 1956
1905
\ -
I %
ADDITIVE
I
IO 600
60
1,OOO 800 BOMB TEMPERATUQE
200
1,400
"F
Figure 6. Influence of bomb temperature on effectiveness of individual additives on average data
1 L TGC TLC WC 3 % ADDITIVE
40
wc
TLC
TGC Figure 4.
Variation of additive response in different fuels
It'",
60L!
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1
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!!
'
Average data, bomb pressures, 6 5 0 and 250 Ib./sq. in., bomb temperatures, 1 390°, 1 1 OO', 900°, 700°,and 600" F . TGC, Texas Gulf Coast slraight run TLC, Texas Gulf Coast light cycle W C , West Coasl straight run
additive effect and not a systematic function of the ignition delay of the clear fuel. The data of Figure 2 are arranged to allow inspection for any such correlation. Here the percentage of reduction in delay obtained by adding an improver is plotted against the delay of the untreated fuel; delays Ivere determined under comparable test conditions. Under test conditions of 250 pounds per square inch the percentage of reduction in delay generally increases 140,
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E
n
60 0
1/2
I P E Q C E N T A D D I T I V E IN F U E L
Figure 5. Influence of additive concentration on comparative effectiveness A. B.
Average d a t a for fuels and bomb test conditions indicated for Figure 4 Average d a t a as above, except 600' F. test data omitted
1906
with the delay of the clear fuel, but this does not hold true for 650 pounds per square inch. In general, there is little true correlation between the percentage of reduction in delay and the delay of the clear fuel; therefore, percentage of reduction in delay is used as a measure of the effectiveness of an additive. regardless of the magnitude of the ignition delays considered. Data in Figure 3 show reduction in delay for three addithe concentrations a t several bomb test temperatures. These data indicate that bomb temperature has no significant influence on effectiveness of additive. At the higher pressure with 1 and 3y0 additive concentrations there may be some trend toward increased effectiveness a t the higher temperatures. but the data are inconclusive. Data are shoLvn in Figure 3 for each of the two test pressures. 250 and 650 pounds per square inch. Without exception, these data show that a n increase of bomb pressure from 250 to 650 pounds per square inch reduces the effectiveness of additives. Similar results have been obtained from tests using other fuels. The effect of changing pressure could br attributed to either an increase in oxygen partial pressure or to the effects of increased density and heat capacity. To determine the influence of oxygen concenrration, some tests were made in which the oxygen partial pressure was varied and the pressure of the inert gas also varied to maintain a fixed total gas pressure. Fixed total
INDUSTRIAL AND ENGINEERING CHEMISTRY
uI 1.400
Figure 7. Influence of bomb temperature on comparative effectiveness of additives on average data
pressures of 250 and 650 pounds per square inch were used. Summary data are given in Table 111, which is arranged to show the decrease in ignition delay caused by either-a change in oxygen pressure with total pressure constant, or a change in oxygen pressure with change in total pressure. In all instances increased oxygen pressure caused less delay reduction with the fuels containing ignition promoters. That is, the clear fuels were more responsive to increased availability of oxygen. Whereas both increased availability of oxygen and the presence of ignitionpromoting materials are effective in reducing autoignition delay, the two effects are not linearly cumulative. From the data, Table 111, the reduc-
A D D I T I V E S IN FUELS T E X A S GULF COAST S.R.
T E X A S LIGHT CYCLE O I L
average percentage of reduction obtained by use of the given additive, compared to the average percentage reduction shown by all additives. Additives B and C are shown to be particularly effective in the Texas Gulf Coast fuel; this and other differences of less magnitude clearly demonstrate that fuel sensitivity must be considered in evaluating ignition-promoting materials. The influence of additive concentration on relative effectiveness is shown in Figure 5. These curves are based upon an average of data obtained at the several temperatures and pressures; data from tests using bomb test temperature of GOO0 F. are omitted from Figure 5B to emphasize the material differences in response that occur at this low temperature. For a realistic evaluation, these low-temperature test data should not be considered. Based on Figure 5B the following observationi may be made :
W E S T COAST S.R .
1
50
50
40
60
Figure 8. Cetane number-ignition delay correlation of average data for additives A, B, C, and D
tion of ignition delay by use of an additive varies with fuel. Figure 4 has been arranged to show this sensitivity of the additives to fuel. The data of Figure 4
are based upon the average data from tests of the fuels at five bomb temperatures and two bomb pressures. Each plotted point represents the relative
Table II. Additive A
BO/C
Viscosity S S U , 100' F .
CsHiiNOa
133
0.998
81 Centistokes
B
(CHa)3COOC(CHs)3
146
0,7940
Structure
The influence of bomb temperature upon effectiveness of the individual
Additive Properties
Gr.
MoE. W t .
sp.
1. Additive C is relatively high in effectiveness at all concentrations. 2. B is relatively less effective in low concentration; A is relatively more effective in the 0.5% concentration. 3. Additive'D becomes relatively less effective as concentration is increased'
n2g 1.413 1.3890
Flash P t . , O F. 108 (PMCC) 65 (tag tester)
B.P., 'F.
M.P., O F.
305-314 -40
176 at 284 mm. Hg
Below - 78O C.
193 at 7 mm. Hg Decomposes at 266
127.4
334
0
11
C
CsH7-N--CO-CgHs
176
D
N0z CH3--C(NO&-CHa
134
I
1.113 Den. 24.5' C.
30.9
1.4382
164 (PMCC)
A.
Amyl nitrate (commercial blend) B. Di-tert-butyl peroxide C. Ethyl-N-isopropyl-N-nitro carbamate D. 2,2-Di-nitropropane
Table 111.
Sensitivity of Ignition Improved Fuels to Change in Bomb and Oxygen Pressures Ignition Delay Change, Milliseconds Change in Pressure, Lb./Sg. I?. Bomb Oxygen Texas Gulf Coast S.R. W e s t Coast S.R. From To From To Clearfuel 0.6%A 0.6%B 0.6%C 0.6%D Clearfuel 0.6%A 0.6%B 0.6%C
0.6%D
Bomb Temperature, 1 1 OOo F.
250 250 650
250 650 650
55.7 92.8 139.7
92.8 139.7 232.8
1.22 0.53 0.77
0.85 0.36 0.63
250 250 650
250 650 650
55.7 92.8 139.7
92.8 139.7 232.8
2.36 1.93 2.11
1.09 1.45 0.52
0.89 0.31 0.63
0.88 0.27 0.59
Bomb Temperature, 900'
1.28 1.20 1.15
1.02 1.41 1.28
1.08 0.32 0.72
1.37 1.66 1.38
0.70 1.10 1.13
0.58 1.18 1.02
0.70 1.08 1.01
0.74 1.08 1.06
4.95 4.48 1.83
2.08
1.52 2.48 1.38
2.17 1.68 2.0
2.48 2.38 1.95
F. 2.17 0.91 1.54
2.18
1.37
VOL. 48, NO. 10
OCTOBER 1956
1907
additives is shown in Figures 6 and 7. The curves of Figure 6 show the actual percentage reductions in delay by use of the additives in all fuels tested-curves of Figure 7 show comparative effectiveness of the additives. From these curves additive D becomes relatively more effective a t the higher temperatureadditive C less effective. Otherwise, the trends are not significantly consistent with temperature; again, the data obtained a t 600' F. are believed to be too erratic for use in any generalized interpretation. Differences between the effectiveness of the different additives become markedly greater at the lower test temperatures; however, this may result from greater dispersion of data at the lower test temperatures. These investigations were not concerned with a correlation of cetane number and reduction in bomb ignition delay; however, the question of such correlation is pertinent. The curves of Figure 8 and the data in Table IV show the relationship between fuel cetane number and the average ignition delays measured in tests with all of the additives. The ignition-delay data are therefore nonspecific, and use of the curves should be restricted to generalized application. The discussion of the data is beyond the scope of this article and is given only for reader's reference. For a more fundamental study of the effect of ignition promoters it is useful to know the influence of these materials on the minimum autoignition temperature of fuels to which they are added. Pertinent data are summarized in Table V. Although the fuels differ in their minimum ignition temperature, it does not appear that any of the additives tend to affect the minimum ignition temperatures to a significant degree. The effect of ignition promoters on heat release of fuels during the early
Table IV. Fuel
Texas Gulf Coast straight run TexasGulf Coast light cycle oil West Coast straight run
Clear 55.9 36.6 31.
r-1
500
w ir
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100
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1,000" F
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A
2
4 6 8 IO 12 0 2 4 6 8 10 TIME A F T E R F U E L INJECTION, MILLISECONDS Figure 9. Fuel heat release from ignition-improved fuels with additive of commercial amyl nitrate
'0
ignition period is also of interest in fundamental studies. Results of some limited work along these lines are presented in Figure 9. From these data the additive-bearing cetane starts releasing heat after a shorter interval than the clear cetane, but the rates of heat release are almost identical for both clear and treated fuels. The rates of heat release shown for U-6, the secondary Diesel reference fuel, are comparable for both the clear and treated fuels, with the significant exception that, in ignition of the additivebearing fuel, heat release is accelerated during the intermediate stage of the heatrelease period. This effect may be seen also in Figure 1, where it is demonstrated that the intermediate stage of the heatrelease period is shortened as additive concentration is increased. These data are the results of preliminary studies
relating fuels, additives, and heat release. It is expected that further investigation along these lines will contribute to better understanding of the manner in which ignition accelerators influence ignition and subsequent combustion of fuels. Literature Cited (1) Rogen, J . S., Wilson, G. C., U.O.P. Bull. 260, Universal Oil Products Co., Chicago, Ill., 1944. ( 2 ) Rroeze, J. J., Hinze, J. O., J . Z m t . Petroleum 27, 348-68 (1941 ). (3) Garner, F. H., others, Zbid., 38, 301 -43 (May 1952). ( 4 ) Hurn, R. W., Hughes, K. J., S.A.E. Quart. Trans. 6, No. 1, 24-35 (January 1952). (5) Hurn, R. W., Smith, H. M., TND. ENG.CHEW43, 2788 -93, (1931).
RECEIVED for review October 17, 1955 ACCEPTED JULY30, 1956
Cetane Numbers for Clear and Ignition Improved Fuels % A
0.6
61.8 43.9 38.2
1
3
71.1 48.5 44.
85 59.9 55.8
-
0.5
61.9 43.8 38.7
% A
0.5
%B-_-1
3
71.1 48.1 45.6
86.1 59. 56.
Minimum Ignition Temperatures,
_I______
Cleai
/
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Table V.
FueE
71
650 psi I
I
LII
m
250 psi
1
3
0.5
-%-C-.-
% D
__
0.5
1
3
0.6
1
61.4 41.8 38.1
70.7 46.2 43.1
85.2 58.5 54.
66.3 43.8 39.
73.5 50.1 45.4
d 88.2 64.5 61.7
F.
Additiaes and Concentrations -___ gzc % B _________ 1 3 0.5 1
~~
3
____ " / O D 0.5
1
3
250 Lb./Square Inch
Texas Gulf Coast straight run Texas Gulf Coast light cycle oil West Coast straight run
472 509 492
477 505 493
471 503 492
480
Texas Gulf Coast straight run Texas Gulf Coast light cycle oil West Coast straight run
483 529 503
482 520 497
483 519 501
482 523 495
473 503
472 504 489
471 504 487
468 502 484
472 504 487
472 505 487
472 506 489
472 504 489
463 507 485
469 502 485
483 521 501
490 526 500
483 527 501
485 521 498
484 526 497
486 522 500
486 518 495
6 5 0 Lb./Square Inch
1908
INDUSTRIAL AND ENGINEERING CHEMISTRY
487 528 501
484 525 498
