INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1955
a s materials of construction around sources of high energy radiation. ACKNOWLEDGMENT
The assistance of Forest Stark and Louis Piasecky, who prepared and tested these samples, is gratefully acknowledged. LITERATURE CITED (1)
Boyer, R. F., Dow Chemical Co., unpublished work, February
1947. (2) Charlesby, A.,Nature, 173, 679 (1954).
(3) Dewey, D. R., High Voltage Engineering Corp., Boston, Mass.,
private communication.
(4) Flory, P. J., and Rehner, J., Jr., J. Chem. Phys., 11, 521 (1943).
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( 5 ) Johansson, 0. K., Dow Corning Corp., unpublished work, 1955. (6) Katz. L.. and Penfold. A. S.. Rev. M o d e r n Phus.. 24. 28 (1952). (7) Lawton, E. J., Balwit, J. S.,and Bueche, d;. M., IND.ENG. CHEX.,46, 1703 (1954). (8) IIonk, G. S., “Coloration of Optical Materials bv - High-Enerev -Radiations,” ANL-4536 (Juiy 1950). (9) Osthoff, R. C., Bueche, A. M., and Grubb, T. C., J. Am. Chem. Soc., 76, 4669 (1954). (10) Sun, K. H., Modern Plastics, 32, 141 (1954). (11) Warrick, E. L., and Lauterbur, P. C., IND. ENG.CHEM.,47, 486 (1955). ,
>
,
RECEIVED for review May 3, 1955. ACCEPTED July 22, 1955. Division of Polymer Chemistry, Symposium on Effect of Radiation on Polymers, 127th Meeting, ACS, Cincinnati, Ohio, March-April 1955. Contribution from the Multiple Fellowship on silicones sustained a t hlellon Institute by the Corning Glass Works and Daw Corning Corp.
Ignition Temperatures of Lead
Compound-Carbon Mixtures GEORGE J. NEBEL AND PAUL L. CRAMER Research Laboratories Division, General Motors Corp., Detroit, Mich.
I
SORGANIC compounds are known to influence the combus-
tion of carbon. This subject has been studied extensively, for the most part, by investigators concerned either with the purely scientific aspects of the problem or with the combustion of solid fuels. I n the past few years, however, automotive research workers have become interested in carbon combustion. Their interest arises from the belief that the tendency of carbonaceous material in engine deposits t o glow or ignite is one of the most important factors influencing preignition, a complicated phenomenon threatening to limit both the smooth operation and efficiency of modern high-compression automotive engines. The most common form of preignition occurs when the fuelair mixture in the combustion chamber of an automotive engine is ignited by glowing combustion chamber deposits rather than by the spark discharge. Engine deposits containing lead have been found more harmful than unleaded deposits in causing preignition (6, 8),probably because the lead compounds cause the carbon t o glow or ignite more readily and a t lower temperatures. Various investigators (1-5, 7 ) have shown that the chemical form of the lead in engine deposits influences the tendency of the deposit t o ignite or glow. These considerations have suggested that preignition may be prevented by converting the lead in deposits to some “harmless” or noncatalytic form. As a result, several preignition-controlling gasoline additives which are believed t o function in this manner are currently marketed.
QUARTZ TUBE FURNACE AIR
PORT
In the present investigation, ignition temperatures of various lead compound-carbon mixtures were determined under controlled laboratory conditions. Those lead compounds reducing the carbon ignition temperature appreciably in these experiments are probably the most objectionable in the engine from a preignition standpoint, and vice versa. Therefore, ignition temperature measurements are a useful guide in selecting various classes of compounds for test as preignition-controlling fuel additives. MATERIALS
The carbon used throughout these experiments was Excelsior grade channel black produced by the Binney and Smith Co. The chemical composition was reported as 94.8% fixed carbon and 5.2y0volatile matter. (The volatile matter consisted of complex carbon oxides.) The ash content was very low, less than 0.01yo. The mean particle size was 21 mp. All lead compounds were C.P. or best available grade, with one exception. Lead pyrophosphate could not be obtained from commercial sources and therefore was prepared in this laboratory. SAMPLE PREPARATION
Each lead compound was screened through a 200-mesh sieve. The undersized portion was collected and mixed with carbon black. The amounts of carbon and lead compound were such that the mixture contained 500 gram-moles of carbon for each gram-mole of lead (in the form of the particular compound being tested). T o ensure uniform dispersion of the lead compound throughout the carbon, the mixture was thoroughly ground with a mortar and pestle. Reproducibility was much poorer if the grinding was omitted. The mixture was pelleted in a tableting machine. Neither wetting agents nor binders were used, with the result that the pellets were rather fragile. The pellets were cylindrical in shape, measuring l / 4 inch in diameter and inch high. Because no attempt was made to charge the tableting machine with exactly the same amount of mixture each time, the pellet weights varied somewhat, from 65 to 85 mg. This small variation in weight had no noticeable effect on the ignition temperature. APPARATUS
Figure 1. Schematic diagram of apparatus
A schematic diagram of the apparatus is shown in Figure 1. The basic components were the air supply system, the reactor and the furnace.
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The reactor consisted of a quartz tube " 4 inch in inside diameter and a device for suspending the pellet in the tube and recording its temperature. The details are shown in Figure 2. The pellet rested upon a perforated porcelain disk, which was supported by a stainless steel carriage welded to the thermocouple rod. The sides of this cylindrical carriage were partially cut away, so that the pellet was constantly exposed to fresh air. The thermocouple junction was located about 1/32 inch above the top of the pellet. The Chrome1 and Alumel wires forming the thermocouple were inserted through a Monel rod inch in outaide diameter and were insulated from this rod by tightly packed magnesia. The rod was attached to a male borosilicate glass taper by means of a connector plug. The clearance between the Monel rod and the glass was sealed with de Khotinsky cement to prevent leake. The borosilicate glass taper fitted into the top of the uartz tube. The ?urnace containing the reactor was an electrically heated tubular type mounted in a vertical position. The wattage rating was 750 a t 115 volts and the maximum o erating temperature was 1950" F. The electrical leads from tEe furnace were connected to the secondary of a Variac autotransformer. The output
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formly until the pellet ignited. At that time the pellet temperature increased suddenly. The ignition temperature was taken as that where the curve deviated noticeably from its normal slope. When the pellet temperature reached 50" or so below the expected ignition temperature, gas samples were collected a t 4minute intervals until the pellet burned completely. The samples were collected by the displacement of acidified brine solution and were analyzed for carbon dioxide, oxygen, and carbon monoxide in conventional Orsat equipment. The rate of air flow, 300 cc. per minute, permitted the 250-cc. gas sample to be collected over a reasonably short time interval, about 1 minute. At the same time, the concentration of combustion products was still great enough to be a reliable indication of the extent of reaction. Total carbon oxide concentrations of 4% or more were usually observed shortly after ignition began. IGNITION OF PURE CARBON
The data obtained from a typical experiment with pure carbon are shown in Figure 3. The pellet temperature and furnace temperature (dashed curve) are plotted as a function of time along the left ordinate. The concentration of carbon dioxide and carbon monoxide is also shown. With pure carbon, ignition did not begin suddenly. The temperature rise was rather small and gradual and some carbon oxides were detected before the pellet thermocouple indicated any reaction, so it was rather difficult t o define an exact ignition temperature. The value of 1030' F. was chosen because, there, the pellet temperature curve deviated somewhat from its normal slope and the concentration of carbon oxides rose sharply. The principal combustion product was carbon monoxide.
RING
IGNITION OF CARBON-LEAD COMPOUND MIXTURES
1
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TUBE
/V/-OUARTZ
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b W
To show the effect of a typical lead compound, data for carbon containing 0.2 mole % lead monoxide are shown in Figure 4. Comparing Figures 3 and 4, it may be seen that lead monoxide caused the following changes :
(lLSTIINLESS
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E
TI1 i
L
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E
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FURNACE
Ignition began a t 695" rather than 1030' F. The rate of combustion was increased. The temperature rise accompanying ignition was much more pronounced, permitting a more precise determination of the ignition temperature. Carbon dioxide rather than carbon monoxide was the principal combustion product. The combustion time was reduced from 20 t o 15 minutes. The ignition temperatures of other lead compound-carbon mixtures are shown in Table I. In each experiment the amount of lead compound added t o the carbon was such that the molar ratio of lead to carbon was 0.0020. Thus, the differences in ignition temperatures are due t o the effect of chemical structure, ~
Table I. Ignition Temperature of Lead Compound-Carbon Mixtures (Lead compound concentration 0.2 mole % based on Pb) Lead Compound Ignition Added to Concentration, Temp., 0 F. Carbon Wt. % AIR
INLET1
Figure 2. Details of reactor
voltage from the Variac was continually increased a t the rate of 1 volt every 3 minutes by means of a motor-reducer unit geared to the Variac. Increasing the heater voltage at this rate from an initial setting of 27 volts produced a uniform temperature rise of 6' to 7" F. per minute. The same heating rate was used in all experiments. PROCEDURE
With the pellet in position and the equipment assembled, the air flow through the reactor was adjusted to 300 cc. per minute. The initial heater voltage was set to 27 volts and then the furnace heater and Variac drive motor were turned on. Both the furnace temperature and pellet temperature were recorded automatically a t SO-second intervals. Both rose uni-
(None) Acetate Iodi,de Basic chloride Basic bromide Peroxide Bromide Monoxide Chloride Fluoride Carbonate Basic carbonate Basic sulfate Borate Formate Nitrate Arsenate
Sulfide Chromate Sulfate Pyrophosphate Orthophosphate
.. 5.95 7.14 4.15 4.82 3.82 5.76 3.58 4.42 3.96 4.26 4.18 4.19 4.93 4.73 5.23 5.46 3.83 5.10 4.80 4.81 4.31
1030 510 570 605 650 660 685 695 715 725 730 740 765 795 800 820 895 935 960 960 1030 1030
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November 1955
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4.0
1!
s I
In 3.0
0 X
eo 6 m K
0
IO
I20
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TIME
Figure 3.
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170
180
70
190
80
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90
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Ignition of pure carbon
Figure 4. 775
not concentration. I n any one experiment the ignition temperature could be determined to the nearest 5' F. Four to six tests were made with each lead compound. I n general, duplicate tests agreed to f 1 5 ' of the reported values. With the exception of the lead phosphates, each lead compound reduced the carbon ignition temperature. Data similar in general form to those shown in Figure 4 were observed with all such compounds, except that ignition began at different temperatures. The data obtained during the lead phosphate experiments were very similar t o those shown in Figure 3 for pure carbon.
5 I
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120
140
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Ignition of carbon containing lead oxide
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a
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z
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E
EFFECT O F LEAD COMPOUUD CONCEYTRATION
Because the reproducibility of the lead chloride experiments was very good, additional tests were made with this compound t o determine the effect of concentration upon ignition temperature. The concentrations of lead chloride, 0.0002 and 0.0200 mole 70, were one tenth and ten times that used in the other experiments. These test results, plotted in Figure 5 , show that the ignition temperature decreased with increasing lead chloride concentration. This was expected. By plotting the lead chloride concentration on a logarithmetric scale, a straight line could be drawn through the data points. S o significance has been attached t o this relationship. A very small amount of lead chloride, less than 0.5 weight %, reduced the carbon ignition temperature almmt 300" F
675
---
fiWY
0 02
MOL. %
0.20
2 DO
PbCI,
(LOG SCALE)
Figure 5. Effect of lead chloride concentration on ignition temperature
varied considerably because of the difference in lead compound concentration and experimental technique. At very high lead compound concentrations, Burke, Test, and Jackson actually found that lead sulfate and lead phosphate incrensed the carbon ignition temperature. PR E1GYITION
COMPARISON WITH WORK O F OTHER Ih3'ESTICATORS
I n these laboratory experiments, lead compounds commonly found in engine deposits, such as basic lead bromide, basic lead chloride, and basic lead sulfate, reduced the carbon ignition temperature several hundred degrees. In addition, the carbon once ignited burned more intensely in the presence of these lead compounds than without them. These observati3ns suggest
Ignition temperatures of lead compound-carbon mixtures determined by several other groups of investigators are shown in Table I1 along with some of the results of the present work. When ignition temperatures determined by different investigators are compared, it should be realized that the ahsoliitc values depend greatly upon the experimental methods and materials used. -4s investigaTable 11. Comparison of Results of Various Investigators tors employed widely different experimental techniques, Ignition Temperature, ' F., as Reported by Blayden, Burk, Test Jeffrey, Griffith, quantitative agreement is all Lead Compound Riley, and Withrow and Jackso; Dunning, and that can be expected. Added to Carbon Shaw ( I ) (2) Baldwin (4) ( 7) I n Table 11, the lead com(Kone) 915 1200 735 1200 pounds are listed in order of Acetate 509 .. Basic chloride .. 727 .. 740 decreasing catalytic effect as 727 Basic bromide .. .. .. Bromide . . 720 determined in the present exMonoxide 617 58'5 770 periments. With a few minor Chloride 727 .. 820 Carbonate 644 exceptions, each investigator 9iO Basic sulfate 650 880 Nitrate .~ 690 ranked the lead compounds in Sulfate .. 939 lo20 840 the same relative order, inOrthophosphate .. , . 1100 850 dicating g o o d q u a l i t a t i v e Concentration, wt. yo 10 5-7 85 Not reported agreement. As expected, the a Exact values given in Table I. actual ignition temperatures . I
Present work 1030 510 605 650 685 695 715 730 765
820 960 1030 3.5-7a
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that these same lead compounds are active catalysts in the engine and lend support to the generally accepted belief that leaded engine deposits glow or ignite more easily than unleaded deposits and, thus, are more likely to cause preignition. Lead compounds may also contribute to preignition by causing the deposit particles to burn more intensely and attain higher temperatures. Under these conditions, there is agreater possibility that the hotter particle will ignite the fuel-air mixture in the few milliseconds available for it t o do SO. The harmful effects of lead compounds in engine deposits may I be nullified in two ways:
additives. Of the 21 compounds investigated, only the two lead phosphates were found to be noncatalytic. Thus, on the basis of present information, if the preignition problem is to be solved or partially solved by the use of fuel additives which form deposits that do not ignite easily, compounds containing phosphorus would probably be most effective for this purpose,
1. Prevent the lead compounds, or carbon, or preferably both, from depositing in the combustion chamber. 2 . Convert the lead to some “harmless” or noncatalytic form by means of special fuel additives.
Campbell, J. M . (to General Motors Corp.), U. S.Patent 2,405,560 (1946). Jeffrey, R. E.,Griffith, L. W., Dunning, E., and Baldwin, B. S., Petroleum Refiner, 33, 92 (August 1954). Sabina, J. R., Mikita, J. J., and Campbell, 14. H., Proc. A m .
’
The fist solution is preferred, but the second is perhaps more easily achieved. Both are being or have been investigated by leading automotive and petroleum products companies. There is some evidence that phosphorus-containing fuel additives suppress preignition, presumably because they convert the lead to a harmless form ( 3 , 4,6, 7). One reason for doing this work was to find out whether any lead compounds were less catalytic than lead phosphate. If so, it was planned to test related organic compounds as preignition-controlling fuel
LITERATURE CITED
Blsyden, H E., Riley, H. L., and Shaw, F., Fuel, 22, 32 (1943). Burk, F. C., Test, L. J., and Jackson, H. R., Proc. Am. PctroZeum I n s t . , 34 (1111,270 (1964).
Petroleum Inst., 33
(III), 137 (1953).
\T7illiarns,R. I
