934
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
in January 1948. Bv January 1949 test work on the two coache5 had reached approximately 70,000 miles. In view of the satisfactory performance of the fuel in this time, as judged by operation of the vehicles in service and by partial engine inspections, it was decided to continue the test run for several more month< Up to January 1949 approximately 50,000 gallons of fuc.1 containing 2,P-dinitropropane i$ ere consumed in thesf, field tesl operations. I n ordei to coiihrin and auppleinent the observatioris ni the Greyhound fleet regarding exhaust smoke, further tests M PI e made using a series of fuels which included Diesel fuel blmds with dinitiopropane. These smoke tests were made in urban busses poa ered by 4-cylinder, 2-cycle engines operated hv thc Trentor Transit Company of Trenton, AT. J. (Table IX). The data from this work permitted the simple conclusiori thai additive cetane number is as effective a- natural retane niimhrr in reducing exhaust smoke.
Vol. 41, No. 5
Literature Cited (1) Ayjohn and Nelson, Petroleum Refiner, 27, No. 7 , 346 (1948),
(2'1 Rroeae and Hinze, "Experiments with Doped Fuels for High
Speed Diesel Engines," paper greaented before S.A.E. meeting May 2 2 , 3939. ( 3 ) Bureau of Fxplosivw, Association of (41
American Railroads, private communication t o Commercial Solvents Corp. from Office of Chief Inspector, Dee. 13, 1946. Denton, Bishop, Nygaard. anti Toland, 1x0. E x i . CHGM.,40,
381 (1948). (51 Holaday, Albright. Apjohn, arid Miller, Oil Gus J . , 45, No. 51, 112 (1945). (6) Kettering Laboratory of -4pyIied Physiology, College of Medicine, University of Cincinnati, privata communication t,o Commercial Solvents Corp., Jan. 27, 1947. (7) Miller and Nelson, Oil Ga,s J . , 46, No. 7 , 8 2 (1947). (8) Smolak and Kelson, Petroleum Refiner, 27, No. 8 , 405 (1948), (9) War Dept ., Chief of Ordnance, private corniniinicat~ion t,o Conirnercial Solvents Corp.r K O Y 16. . 1945. RE'CF-IVED AUgllSt 1 1,
3.948,
Oxidation of Lubricating Oils at High Temperatures G. H. Denison and 0. L. Harle CALIFORNIA RESEARCH CORPORATION, RICHMOND, CALIF.
M o s t laboratory investigations of lubricating oil oxidation are performed at temperatures corresponding to those prevailing in bearings and crankcases of internal combustion engines. To assist in extending this type of information to temperatures prevalent in the piston zone, techniques were developed for determining the rates of oxygen absorption at temperatures as high as 340" C. The rates of oxygen absorption over the range 170' to 340 ' C. were shown to obej the Arrhenius law within the accuracy of the present experimental method. The data indicate that in general at piston area temperature of 290' C. and an engine sump temperature of 90 C., assuming other rate-controlling factors equal, the over-all oxidation rate in the piston area is 200,000-fold more rapid. All the oils tested show experimental activation energies for the initial step of oxidation centering about 33 kg.-cal. in agreement with values given by other experimenters for reactions performed at lower temperatures on hpdrocarbons.
T
HE lack of agreement betM een the stabilitj of 811 ull a b ~iirasw e d by laboratory tests and its actual service stabilit) in arj internal combustion engine has been the cause of much EpPculation and study. A moderate knomledge of oxidation processeq under mild laboratory conditions has been developed ( I S ) and ha5 been useful in treating service problems involving mild oxidation, bur no valid 01 generally useful extension of such knowlrdge to the conditions prevailing in internal combustion engines -foi example, on piston walls-has been made. M o s t laboratory investigations of oil oxidation are performed a t temperature3 of approximately 170" C. This report describes preliniinarj data obtained 011 0x1 gen absorption tests carried out a t temperatures correspoiiding to those possible in the piston ring zone and shorn 3 that no unpredictablp discontinuity arises in correlating oxygen absorption dai a over the range 170" to 340" C. Rosen (IO) reports tenipeiatures in B Diesel engine a t full load of 256" C. a t the top of the top piston-r+ing groove. IJnderwood and Chtlin ( I I ) ieport top ~irrg-groovctwn-
perat,ures up to 282" C. in Diesels, and Anderson ( 1 ) reports temperatures up t o 280" C. at, the top of the top ring groove in aircraft pistons. Whether higher 1 emperatures than these exist on the piston walls or underneath the crown and play a part in oil oxidation is undetermined. To furnish a margin of safety and to make firmer the data in the range of piston wall temperatures reported above, t,est,vwere extended to 340 a C. In addit>ionto helping clarify thinking in regard to oil oxidatiurl at piston temperatures, the correlation of oxidation rate with temperature also furnishes suggestions regarding the initial st8cpin oil oxidat,ion. Calculat,ion of the experimental activation energy of the initial reaction from the rate data nbt'ained over the broad temperature range inv&igated furnishes values not Ridely dive'rgent for different, oils. The value6 obtained tend to center on 33 kg.-cIal. per mole, which is in agreement with the value 28 * 2.5 Icg.-c:al. found by George, Rideal, and RohertPon, for several pure hydrocarbons tit lower temperatlures (51, The present work is part of a program started before the war and designed to aid the corrt?lation of laboratory and engine test, data. The present status of this work permits some extension of insight into the kinetics of oil oxidation, but clear-cut relations l,et,ween this work and the customary engine performance test,s h v e iiot as yet, been ascertained. Although automatic oxygen re#-ordingapparatus for use a t high temperat'ures is now in use and one set of data, obtained with this equipment is presented, it is believed that the older data obtained by more crude tools are nevertheless valid. The older data const>iiuiPJ l e main subst>anrro f i h e present paper.
Experimental (ixygen absorption iri the region 170 ' t o ZOO C. was ineasiired rn apparatus similar to that used in previous work (3).
The apparatus consisted of a closed all-glass system (except for a Sylphon hellows pump) which circulated air a t the rate of 2 liters per minute. T o reduce the explosion hazard in the oxidation tests a t high temperature, air rather than oxygen was used throughout. The oil sample was cont'ained in a 800-ml. cell with a sealed-in sintered-gla,ss disk through which t,he gas passed to ensure ade-
quate contact with the oil. Constant temperature was maintained by means of a thermostated oil bath. Fifty-gram oil samples were used. A run was started with t h e air pressure at 1 atmosphere and allewed t o proceed until a pressure drop of 2 or 3 cm. of mercury resulted, at which time the pump was stopped, the pressure drop recorded, and fresh oxygen admitted t o return the pressure to 1 atmosphere. This process was repeated until the desired degree of oxidation had been attained. Experimentally determined factors were used to convert pressure drop to cubic centimeters of oxygen absorbed.
Table I. Oxidizer Test on Oil B at 171" C.
T o minimize the explosion hazard further and also t o simplify operation, t h e customary 500-ml. oxidizer cell was replaced with a 25-ml. cell and oil samples of 2 to 10 grams were used instead of 50gram samples. Convenient temperature control was obtained by using vapor baths of 1-methylnaphthalene (240 "C.), I-bromonaphthalene (280" C.), and mixtures of triamylbenzene with diamylnaphthalene (320" C. plus). T h e test temperatures were determined by a glass-encased thermocouple immersed within the test oil. The first results at the higher temperatures made i t evident that a correction for vaporization of oil was necessary. No cor-
8 190% 204'C.
600
500
400 K2'0.23
300
=I O
4 0
200
190'C.
0
a
100
V'
(50-gram sample) 4P 4v Cm. H g CC. 02/lb0 G. 16 0.5 28 0.9 28 0.9 94 3.0 82 2.6 110 3.5 119 3.8 2.6 82
Time, Hours 0.5 1.0 1.5 2.78 3.5 4.5 5.5 6.0
Typical treatment of the results of a test of this type are shown in Table I. The differential volumes expressed for 100 grams of oil are given in column three and the total volumes per 100 grams of oil in column four. For investigations between 204" and 340' C., deviations from the typical procedure as described above were necessary.
700 OIL
935
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1949
v.
CC. 0z/100 G 16 44 72 166
248 35s 477 559
Table 11. Oxidizer Test on Oil B at 314" C. Time, Min. 0.6 1.0 1.5 2.0 2.5
Cm. 4P, Hg
Estimated in Cell, OilGrams
2.0 2.4 2.3 2.3 1.15
5.9 5.6 5.3 5.1 4.8
AV V, Co. Oz/lbO G . CC. Oz/lOO G. 870 870 1100 1970 1120 3090 1160 4250 620 4870
rection is necessary a t temperatures below 204 O C. Referring t o Table I1 which contains the data for a test at 314" C., 6.0 grams of oil B were charged t o the cell. At the end of the test 4.4 grams of oil remained. Correction for this vaporization has been made in a linear manner, as indicated in column three of Table 11. Thus it, is assumed t h a t 5.9 grams of oil were present between 0 and 0.5 minute, 5.6 grams between 0.5 and 1.0 minute, etc. Because the tendency t o vaporize is a function of the boiling range of the oil and because the oxidation tendency of various fractions of the oil will vary somewhat, a more rigorous treatment of the vaporization is not justified. Hence the high temperature data cannot be analyzed too critically, although, as a n analysis of the d a t a obtained will indicate, the error incurred by this crude treatment is not excessive. Calculation of the remaining data in Table I1 is similar to t h a t described in the discussion of Table I. In most cases runs were continued t o a total absorption of 1000 t o
U
z 7000
z
P 7oo.
OIL D 196'6 204'C
1
OIL D 239: 2!2"&326'C.
600 500.
A
T I M E IN MINUTES
0
2
4
6
0
IO
20
30
8 - " 40
TIME I N M I N U l E S
Figure 1. Oxygen Absorption of Oils B and D in Temperature Range 190' to 340 O C.
C
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
936
TabIe 111. Inspections of Oils Tested Gravity,
A.P.I. A 29.5
O
Viscosity at loOD F.,
S.U.S.
210O F., 8.C.S.
480
66.7
B
Oil
C 21.3 26.7 25.7 27.5 27.4
J 27:s
A
+ 1%
Oil B 122 121 126 142 119
1727 1980 1950 2007 1618 1724
Viscosity Index 113 Detergent 16
57.4
636
D
E F G H I
Viscosity at
Sulfur
%
Carbon Residii~
0.13
0.05
0.64
0.03
4- 17’ Detergent 97 85 92 104 lo4 93
118
0.41
0.32 0.24 0.04 0.18 0.22
1.16
matic recording gasometer an initial slope can be obtained bl IIXspection with considerable precision. However, in treating d a h taken in the older oxidizer, wherein oxygen uptake is determined by points derived from the periodic recording of pressure decrcments, the initial slope is so sensitive to error in thc initial data point that it is best to use a semiempirical derived function to nhtain the initial slope. In deriving the function t o be used it wat. assumed that the rate of oxygen uptake could be expressed R S a differential equation in the form
0.36 0 08
0.73 0 68 0.28
2000 cc. of oxygen per 100 grains of oil. Table 111contains typical inspections of the oils tested. In Figure 1, data for oils B and D a t several temperatures are presented in graphic form. At the lower temperatures, a \yell-defined autocatalytic rate increase was observable during the course of a n experiment; but for data taken a t 282 C. and higher temperatures, essentially linear rate curves were obtained in experiments wherein an extrapolation of the treatment uscd for the low temperature data would have required strongly autocatalytic increases of rate. One series of data was obtained using a newer apparatus similar to the high temperature oxidizer described, but employing a continuous delivery of oxygen from a continuously recording automatic gasometer t o maintain constant pressure. I n using this apparatus, only the initial oxidation rate was measured. Correction for volatilization was unnecessary in these cases, as only the initial slope of the rate curve vias deemed of interest. .4n example of such an automatic recording is given in Figure 2.
Treatment of Data The objective of analysis of the data was t o determine whether over-all oxidation rates at temperatures existing in the lubricated zones of a piston were those predictable by use of the Arrhenius equation, and whether the experiniental activation energies derivable from such data would shed light on the kinetics of oil ouidation. In treating the data it was decided not t o use the empirical equation frequently applied, Ti” = ki where 5’ is volume of oxygen taken up, 2 is an arbitrary constant, k is a rate constant, and t is time, because this equation allows no theoretically rational method of obtaining activation energy from the temperature coefficient of the rate constant. Instead i t was felt t h a t the simple use of the initial slope of the plot of oxygen uptake against temperature might reasonably be expected t o yield this information, From the data obtained with the newer auto-
Vol. 41, No. 5
+ k:l’ + k3V2 + .
dt = kl
wherein 121 represents an initial slope independent of reaction pi oduct and the higher order terms represent the lumped reactioiib whose rates depend upon various powers of the concentration of oxidation products. It was found sufficient to ignore all but the first two terms in the expression, yielding an integrated equation
The procedure was to obtain the best possible fit to all poiuts 01 the extended oxygen uptake curve by the use of the two constants. and to use kl so determined as the initial slope. This fit, in alt cases in which it was attempted, was almost exact. This treatment assumes that kl, the product-independenl ratt, may represent a chain reaction of indeterminate length wherein the rate-determining step, the temperature coefficient of which i i subsequently to be determined, is the step of chain initiatioir Following this assumption it may be valid further to assume that kz represents lumped chain-branching and chain-breaking steps dependent upon the concentration of the total reaction product of oxidation. From the fragmentary data available, a temperature coefficient has also been determined for rate constant k2. I n Figure 3 the values of log kl arc plotted against the rcciprocal of the absolute temperature for oils A, B, C, and D. Values oi kz where calculated-that is, for oils B and D-are shown in thr lower part of the chart. D a t a for Figure 3 were obtained by the pressure decrement method. The fit of the experimental points leaves little doubt but t h a t within the limits of test accuracy the rates of oxygen absorption a t temperatures even somewhat abovc those anticipated on piston malls obey the Arrhenius equation foi temperature dependence,
k = Ae-Eod/RT
l
1
l
1
l
1.8
,
1.9
~
2
3
t (MINUTES) Figure 2. Automatic Recording of Oil at 314’ C.
I
l
l
340
320
300
Figure 3.
280
I 2.0
[I/T.JX I
(3
nhere h: is the rate constant, A a eonstant, Eactthe experjrucriia, activation energy, R the gas constant, and T the test ternperaturt in degrees Kelvin. I n Figure 4 a similar plot is given for all oils
I .7
7
0
a
2 . : .I I 2.1
/
I
,
2.2
IO3
I
I
I
I
260
240
220
200
I
I60
T (‘GI Temperature Dependence of Selected Oila
1
May 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
C
-PRESSURE "'AUTOMAT
I. 7
1.8
1.9
I
I
320
B
2 .O
2. I
io3
( I/T.,)x 340
A
DECREMENT METHOD IC RECO ROE R
2.2
I
I
I
I
1
I
300
280
260
240
220
ZOO
I
I
180
T ('c) Figure 4. Temperature Dependence of
R1
tested, except t h a t in this case data points have been omitted, because they would be so close together as t o overlap. From the data shown in Figure 4 i t is evident t h a t most of the oils tested show similar dependence of oxidation rate on temperature. Values of the experimental activation energy, E,,t, for both the primary chain and chain-branching reactions as obtained from Figure 4 are presented in Table IV.
Discussion I n discussions of the mechanism of hydrocarbon oxidation, which is generally treated as a chain reaction, various hypothetical chain-initiating steps have been considered. An initiating step involving the production of a n alkyl radical (which subsequently reacts with oxygen) would presumably require a n activation energy of about 100 kg.-cal., required for carbon-hydrogen bond rupture, or about 70 kg.-cal. (8), required for carbon-carbon bond rupture. Otherwise, a dependency of chain-initiation rate upon oxygen pressure would be required for some chain-initiating step involving oxygen in the production of the required radical (6). An initiating step involving merely the production of an energy-rich hydrocarbon molecule, as suggested by Medvedev (9) and subsequently by George, Rideal, and Robertson (6),would, however, be independent of oxygen pressure, and might conceivably require any activation energy less than 70 kg.-cal. The experimental activation energy determined from kl, 33 3 kg.-cal., as well as the 28.2 =t2.5 k g s a l . of George, Rideal, and Robertson and the 26 kg.-cal. of Medvedev, all fall far short of the bond-rupture energies (70 t o 100 kg.-cal.). I t has been felt that the activation energy derived from k~ may be treated as a chain initiation activation energy, even though it is derived for an overall reaction, because the temperatures are so high in these experiments that the chain length will probably be short enough to render the variation of chain length with temperature insignificant. Upon this basis, the experimental activation energy here determined is comparable with the values taken from Medvedev and
Table IV. Oil
Experimental Activation Energies Eoot for kl, Kg.-Cal.
Eoct for kzl
Kg.-Cal.
937
George, Rideal, and Robertiion. Data taken in these experiments, which have been considered too inconclusive to present formally, seem t o indicate that the dependency of oxidation rate upon partial pressure of oxygen is of a n order between 0 and 1, which might be in accord either with the observations of George, Rideal, and Robertson who found a case in which the oxidation rate was oxygen-independent, or with the earlier findings of Dornte (4) who in various cases found rates of oxidation dependent upon powers of oxygen partial pressure between 0.25 and 1. This observation regarding a relative lack, in many cases, of dependency of oxidation rate upon oxygen pressure is recurrent in the literature (2, 6). It is not considered possible at this timp to suggest a scheme of chain initiation that is dependent in a simple way upon oxygen. Although the data from which the experimental activation energy of chain-branching, kp, is determined are so sparse as only t o suggest the value 38 kg.-cal., it may be pointed out that the initiation of the degenerate chain-branching - step - is frequently formulated as the cleavage of a n alkyl peroxide at the oxygenoxygen bond ( I d ) . The latter has been given bond energies from 32 kg.-cal. (activation energy of diethyl peroxide decomposition, 7 ) to 66 kg.-cal. (heat of oxidation of ethyl linoleate and linolenate, d). Because the former value is determined for a reaction comparable t o that postulated for the initial chain-branching step, it is somewhat encouraging that the experimental activation energy here derived from kz tends t o confirm this lower value, The autocatalytic (chain-branching) rate increase which was observed in experiments at lower temperatures, and which was treated by the use of the rate constant k ~ ,was not observed a t temperatures of 282 O C. or higher. The extrapolation of k2 values to these higher temperatures would require that the autocatalytic effect be pronounced. Although hypotheses may be offered t o explain the disappearance of the autocatalysis, firm experimental support for such suggestions, other than the data presented here, has not been developed. The problem is, therefore, left open, with the statement t h a t it is not considered likely that the effect is due to apparatus or experimental procedure.
Conclusions Techniques have been developed for determining the rates of oxygen absorption by lubricating oils at temperalures as high as 340 C. The rates of oxygen absorption over the range 170' t o 340' C. obey the Arrhenius law within the accuracy of the present experimental method. From the existing data i t may be estimated t h a t a t a piston area temperature of 290 O C. and a n engine sump temperature of 90 O C., other rate-controlling factors assumed equal, the over-all oxidation rate at piston area is 200,000-fold more rapid. All the oils tested show activation energies for the initial step in the oxidation centering about 33 kg.-cal., in agreement with values given by other experimenters for reactions performed at lower temperatures.
Literature Cited (1) Anderson, R. G., Product Eng.,14, 786 (1943). (2) Bolland, J. L., and Gee, G., Trans. Faraday SOC.,42, 244 (1946) (3) Denison, G. H., IND. ENG.CHEM.,36, 477 (1944). (4) Dornte, R. W., Ibid., 28, 26, 863, 1342 (1936). (5) George, P., Rideal, E. K., and Robertson, A., Proc. Roy. Soc. (London),A185, 288 (1946). (6) George, P., and Robertson, A , , I b i d . , A185, 309 (1946). (7) Harris, E. J., and Egerton, A.C., I b i d . , A168, 1 (1938). (8) Jost, W., "Explosion and Combustion Processes in Gases," p 395, New York, McGraw-Hill Book Co., 1946. (9) Medvedev, S. S., A d a Physicochim. U.I?.S S ,9, 395 (1938). (10) Rosen, C. G . A . , S.A.E. JozirnaZ,40, 165 (1937). (11) Underwood, A. F., and Catlin, A. A., Ibid., 48, 20 (1941). (12) Walsh, A. D., Trans. Faraday Soc., 42, 269 (1946). (13) Zuidema, H. H., Chem. Reus., 38, 197 (1946) ; bibhography.
RECEIVED September 2 4 , 1948.
