530
Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 536-540
Nomenclature A = TAN, mg of KOH g-’ B = TBN, mg of KOH g-’ C = carbon residue concentration, wt % Z = insolubles concentration, wt ‘70 I,, = concentration of coagulated n-heptane insolubles, wt % = concentration of coagulated benzene insolubles, wt % kl, = overall first-order rate constant for lubricant transport,
ZBB
h-l k,,,, = overall zero-order rate constant for increasing ideal tracer, wt % h-’ kadd = rate constant for fresh oil addition, h-’ kf = rate constant for the outward flow of lubricant from engine system, h-’ k, = rate constant for oil sampling (averaged value on assuming continuous sampling), h-’ k, = rate constant for the volatilization of lubricant, h-’ kA = rate constant for increasing TAN, mg of KOH.(g h)-’ kB = rate constant for decreasing TBN, mg of KOH4g h1-l kc = rate constant for increasing carbon residue, wt % h-’ kI = rate constant for increasing insolubles, wt % h-’ k,,, k, = rate constant for increasing ZBH and ZBB, respectively, wt % h-l
Q = quantity of engine oil charged, g Radd = mass rate for fresh oil addition, g h-l S = sulfated ash concentration, wt % T = ideal tracer concentration, wt % tind = induction period for the oxidation of lubricant in oil sump, h X = dummy variable for the parameter of engine oil, unit Subscripts m = equilibrium conditions 0 = initial conditions tind = conditions at tind Literature Cited A b b o t A. D.; Bowman. L. 0. J. Jpn. Pet. Inst. 1966, 9 , 184. Knight, C. R.; Welser, H. SA€ Pap. 1976. No. 760721. Kusama, K.; J. Marine Eng. Soc. Jpn. 1973, 8 , 868. Mahoney, L. R.; Otto, K., Korcek, S., Johnson, M. D. Ind. Ens. Chem. prod. Res.-L?ev. 1980, 19, 11. Marquardt, D. W.; J . SOC. I d . Appl. Math. 1963, 1 1 , 431. Parsons, J. C.; J . Inst. Pet. 1969, 55, 256. SmRh, I. B.; Chowings, A. R. SAEPap. 1976, No. 760723.
Received for review November 20, 1980 Revised Manuscript Received April 20, 1981 Accepted April 28, 1981
Kinetic Approach to Engine Oil. 2. Antioxidant Decay of Lubricant in Engine System Seijiro Yasutomi, Yoshlhlro Maeda, and Tsutomu Maeda Lubricants & Petroleum Products Laboratory, Nippon Mining Co.,Ltd., 17-35, Nh-minami 3-chome, Toda-shi, Saitama-ken 335, Japan
Antioxidant decay of lubricants within a lightduty diesel engine was studied by using specially formulated oils containing p ,p’-dioctyldiphenylamine. The decay curve of the antioxidant fotlows the same function consisting of first-order and zero-order terms as that for the changes in other typical properties of lubricants. However, its first-order rate constant is much greater than that derived from lubricant transport. This descrepancy may be attributed to the possibility that the antioxidant is completely consumed in the piston-cylinder area and that there is no reflux back to the oil sump. The analysis of the zero-order term suggests that the oxidation mode of engine oil in the oil sump is “initiated oxidation” caused by radical species produced in the piston-cylinder area.
Introduction Oxidation stability is one of the most important requirements of engine oil in order to maintain proper functions of a lubricant for a long period. Oxidation of engine oil has been studied by means of laboratory methods as well as authorized engine tests. Most of the laboratory methods employ heterogeneous metal catalysts in order to accelerate the degenerated oxidation at high temperatures. However, dominant mechanisms of the oxidation within an engine system have not yet been clarified quantitatively. The kinetic analysis described in part 1has emphasized that the characteristics of an engine as a flow reactor are of great importance concerning the degradation of lubricant. Bardy and Asseff (1970) also indicated that the oxidation of engine oil is greatly influenced by the amount of charged oil and the rate of fresh oil addition. On the other hand, Mahoney et al. (1978) and Korcek et al. (1978) have reported a series of interesting results based upon their original methodology using an initiator to yield the 0196-4321/81/ 1220-0536$0i.25/0
information on “total antioxidant capacity”. This new parameter, which indicates the radical scavenging capacity of engine oil, will provide more useful information in conjunction with the evidence of “initiated oxidation” caused by radical species within an engine system. This study deals with a kinetic analysis of the antioxidant decay in a light-duty diesel engine in order to investigate the initiation mechanism for the oxidation in an oil sump. It is well known that Zn-dialkyldithiophosphate (ZDDP), which has been widely used for typical engine oils, acts not only as a peroxide decomposer but as a radical scavenger (Burn, 1968; Howard et al., 1973). The inhibition behavior of ZDDP has many reaction stages which make the analysis much more difficult. On the contrary, diphenylamine has been studied by many workers (Boozer and Hammond, 1955; Thomas and Tolman, 1962; Buchochenko et al., 1961), and the material is not sublimed dissimilar from some phenolic antioxidants. Therefore, simply formulated oils containing an antioxidant analogous to diphenylamine were utilized. 0 1981 American Chemical Society
Ind. Eng.
Table I. Formulation and Properties of Sample Oils oil name Oil-1 Oil-2 base oil
Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981 537
Oil-1
2.0
hydrofinished mineral oil
additives overbased Casulfonate antioxidant a antiforming agent
wt %
4.0
4.0
wt % ppm
1.0 25
2.0 25
0
Oil-2
properties sp. gr. ( 1 5 / 4 "C) flash point (COC) pour point viscosity (37.8 "C) (98.9 "C) viscosity index carbon residue sulfated ash TAN
TBN (D-2896) (D-664)
-
"C "C cSt cSt wt % wt % mg of KOH gm g o f KOH g- ' m g o f KOH
0.8894 0.8879 2 64 270 -7.5 -10.0 114.5 111.6 11.7 11.5 98 98 1.26 1.28 1.46 1.46 0.81 0.90 12.0 10.6
12.7
10
10.5
30
40
50
Figure 1. Decay curve of antioxidant in engine system.
ga
20
Test Duration
p , p '-Dioctyldiphenylamine.
Experimental Section Sample Oil. p,p'-Dioctyldiphenylamine (Vanlube 81) obtained from R. T. Vanderbilt Co. was used as an antioxidant. A calcium sulfonate was added as a detergentdispersant additive. Prior to use, it had been confirmed through an initiated oxidation test that the sulfonate indicated little effect as an antioxidant. Formulations and typical properties of the sample oils are summarized in Table I. The code number for the oils is the same as in part 1. Bench Engine Test. Details of bench engine test were previously described in part 1. Lubricant Analysis. The antioxidant concentration was determined by high-pressure liquid chromatography. Carbonyl and nitrate groups produced during the engine test were followed by the measurements of the absorbance a t 1705 and 1630 cm-', respectively. Fresh oil was used as a reference. Prior to the IR analysis, soot contaminant was removed by means of a high-speed centrifuge. Data Analysis. The values of kinetic parameters were determined by the method of Marquardt (1963). However, in order to visualize the overall first-order rate constant, klBt,for the antioxidant decay, a graphical method proposed by Guggenheim (1926) was applied. According to eq 1, (Xt-At is represented by eq 2.
c
v
6
4
b
V
+. 3
h
'i
cn
$
2
-
m
E
2 4
1
c 0 Test Duration
(hr)
Figure 2. Increase in TAN, TC, and
TN
for Oil-1.
x,)
X = X, - (X, - Xo) exp(-kl,,t)
(1)
W - A-~Xt) = (X,- Xo)[exp(kl,tAt) - 11 e x p ( - k d )
c
(2)
When time interval, At, is fixed, the linear relation between t and In (Xt-A,- X,) can be obtained In (Xt-At- X,) = -kl,,t
+ constant
(3)
Results and Discussion Decay curves of the antioxidant during the engine test are illustrated in Figure 1. The most remarkable feature is the drastic decreases in its concentration in the early stage of the test. In contrast, during thermally initiated oxidations, antioxidants are scarcely consumed in their induction period, and subsequently rapid decay is observed, as indicated by Harie and Thomas (1957) and Denisov (1963).
Test bratton
(hr)
Figure 3. Increase in TAN, TC, and
TN
for Oil-2.
The corresponding changes in TAN (totalacid number), A , the absorbance of carbonyl group, TC, and that of nitrate group, T N , for Oil-1 and Oil-2 are shown in Figures 2 and 3, respectively. For Oil-1, the oxidation in oil sump occurs after about 20 h, while, for Oil-2, such oxidation does not occur. However, even during the induction period for the oxidation in the oil sump, both oils indicate considerable increases in TAN, T ~and , T N due to the oxidation in the piston-cylinder area. Rate constants for some physical factors in the "flow reactor" model described in part 1 are summarized in
538
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981
Table 11. Kinetic Parameters for Physical Factors in Engine System .- __ __. ____I__._ . - - __ -rate constant unit Oil-1 Oil-2 t k,)
sSo,
10-’;1-’ 10e2wt% h wt % wt %
‘?add
1 0 Zh-’
(/if
kadd’so
’
10-Zh-l 10 W 1 10 211 I
kf ks k\
-
~~
1.78 3.70 1.45 2.09
2.15 4.30
2.56 0.86
2.95 1.26
0.92
0.89
0.78
0.80
3.5-
1.46
30.
2.00 u!
? 2.5-..
E
I
..
02.0-
-.. I C
~
h
0
3
0
31
0
c
-y 1 . 5 -
TbZ, --r
12 TbY
4 3 2
1 .o
~
0.5-
h
5 4 -
cn
oat, 1
-
00
02
0 4
06
08
50
40
(hr)
tionship between t and ([InH],-,, - [InH],). This provides the validity of the equation d[InH]/dt = -kl,,[InHl + k,,,, (9)
10
exp(-(kf*k5)t)
Figure 4. Linear Increase in TAN, T~ as a function of exp(-(kf + k,)t).
Table 11. These values were determined from the increase in sulfated ash. Prior to the analysis of the antioxidant decay, the increases in TAN and 7c will be analyzed. Equations 4 and 5 may hold for TAN and 7 c , respectively. dA/dt = -(kf + k,)A + (kaddAO + k ~ ) (4) dTc/dt = -(kf + k , ) T c
+ k,,
(5)
where kf is the rate constant for the outward flow of lubricant from the engine system, k, is the rate constant for oil sampling, kadd is the rate constant for the addition of fresh oil, k A is the rate constant for increasing TAN, and k, is the rate constant for increasing TC. Integration of eq 4 and 5 gives the following equations represented by using a dummy variable X. During the induction period X = X, - (X,- X o ) exp[-(kf + kJt1 (6) After the induction period
x = x, - (x,- xt,n,,) exp[-(kf
k,)(t - tlnd)] ( 7 )
where x
20 30 Test Duration
Figure 5. Guggenheim plot for antioxidant decay: -, during induction period; - - -, after induction period.
a 6
io
m
=
(kaddxo
+ kX)/(kf + ks)
(8)
As Figure 4 shows, the linear relations of TAN and TC against exp(-(kf 123) support the fact that eq 6 and 7 hold for the increases in both parameters. Moreover, the instantaneous increase of TAN and 7 C in steady state after the induction period suggests that oxidation in the oil sump is initiated a t a constant rate by radical species supplied from the piston-cylinder area. Therefore, the antioxidant decay is analyzed on the premise that the oxidation mode in the oil sump is the “initiated oxidation”. A Guggenheim plot (Guggenheim, 1926) for the antioxidant decay in Figure 5 demonstrates the linear rela-
+
where klst is the overall first-order rate constant and k,,,, is the overall zero-order rate constant. It should be noticed that the two lines are approximately parallel but their slope of klst is about four times as large as (k,+ k,) derived from the lubricant transport. There is a possibility that the antioxidant decay involves first-order reaction mechanisms. The following reactions can be considered as the dominant process of termination. 2R00. ROO.
2kt
+ InH
inactive products
kd
inactive products
(10) (11)
A t sufficiently low concentrations of an antioxidant, self-termination reaction 10 may be dominant and the steady concentration of peroxy radical at a constant initiation rate, Ri, may be represented by the equation [ROO.] = (Ri/2kJ1/2
(12)
Hence, [InH] may decrease according to the first-order dependence as follows d[InH]/dt = ki,~[InHl~[ROO~l = kinh(Ri/2kt)1’2[InH]= constant [InH] (13) Equation 13 has been varified experimentally by Denisov and Aleksandrov (1964) in the range of [InH] from lo* to M. However, the initial values of [InHIo for Oil-1 and Oil-2 are 2.26 X and 4.52 X M, respectively. At the higher concentrations, chain breaking reaction 11 may be predominant and [InH] may decrease according to the zero-order behavior. Therefore, another possibility of observing an apparent term included in klst is present. Steady temperatures of the top-ring zone in the test engine are about 230 to 250 “C and more severe conditions may exist at local positions in the piston-cylinder area. As Figure 6 shows, it seems likely that the apparent term is observed in proportion to the flow rate of lubricant from the oil sump toward the piston-cylinder area and that the antioxidant are com-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 3, 1981
-
Table 111. Kinetic Parameters for Antioxidant Decay
kadd
Fresh O i l
7-7r7 l t
kv
rate constant 121,
(kf t k , ) = kr
k zero kadd[InH]c
Ri [InHI., [InHI, r igure o. x
IIOW
539
reactor moaei ror antioxiaanc aecay.
pletely consumed in the reflux fraction. Mahoney et al. (1980) has recently found that the rate of the antioxidant consumption in a synthetic oil used in various gasoline engines is normalized by plotting mole equivalents of the antioxidant remaining in the oil against the product of engine firing events and the surface area of the oil film per cylinder. From the fact, they have suggested that the antioxidant consumption occurs primarily in the pistoncylinder area of an engine. This supports the possibility of the apparent term involved in klut. Hence kist may be described by the equation (14) = k, + kf + k, where k, is the rate constant for the reflux of lubricant from piston-cylinder area into the oil sump. By the comparison of the rate constant for the total oil consumption (kf+ k,) with that for the total upward flow (12, + kf+ k,), it is estimated that one-fourth of the quantity is dissipated from the engine system through its piston-cylinder area. On the premise of the “initiated oxidation”, k,,,, may be described by the equation
kist
(15) = kadd[InHlO - Ri where Ri is the radical input rate from piston-cylinder area into the oil sump. Accordingly, the antioxidant decay may follow the equation d[InH]/dt = -(k, + kf + k,)[InH] + (kadd[InHl0- Ri) (16) Integration of eq 16 gives the following equations [InH] = [InH], + ([InHlo - [InHl,) exp[-(k, + kf + k,NI (17) kzem
where [InHI, (kadd[InHlO - Ri)/(kr + kf + ks) (18) Kinetic parameters for the antioxidant decay are summarized in Table 111,where nearly equal values of k, and Ri are obtained for both oils. Solid curves in Figure 1 represent the antioxidant decay calculated from eq 17. The fitness with the experimental data suggests that the oxidation mode in the oil sump of engines is the “initiated oxidation” as assumed previously. Miliotis et al. (1969a,b) reported that some phenate type additives containing sulfur play important roles in the oxidation of engine oils not only as a peroxide decomposer but as a chain-breaking agent. The inhibition effect of such compounds other than antioxidants should be taken into account in treating commercial oils containing various additives and natural inhibitors from base-stocks. In the present study, simply formulated engine oils were used and the direct analysis of specific antioxidant concentrations
unit
Oil-1 --
10-lh-l lO-’h-’ 10-Zh-’ 10 Iwt % h-l 10-’Wt % h10 -‘wt % h- * wt % wt %
Oil-2
7.53 1.78 5.75 -0.93
7.81 2.15 5.66 1.40
2.56
5.90
3.49
4.50
0.99 -0.12
1.99 0.18
a Determined from the Increase in Sulfated Ash listed in Table 11.
has supplied available information on the oxidation mechanism of engine oils. However, for the typical commercial oils, an initiated oxidation test using an initiator at moderate temperatures may be much more suitable than the conventional analysis of individual components, as insisted by Mahoney et al. (1978). Finally, it has been recognized that nitrogen oxides contained in the atmosphere within an engine system can accelerate the oxidation of lubricant. Kreutz (1969) suggested that the accelerating effect of nitrogen oxides is primarily due to their ability to act as radical initiators for the chain oxidation mechanism. He proposed a reaction scheme in simplified form as
--
RH
NO,
RONO
RON02
R.
+ HNO,
+ NO RO. + NO2 RO.
(A)
(C)
(D) (E)
RO. inactive products In this scheme, both nitrites and nitrates decompose into nitrogen oxide and alkoxy radical which can initiate the chain reaction of oxidation. On the assumptions that nitrates are produced a t a constant rate and that they completely decompose in the piston-cylinder area, eq 19 may hold for the increase in nitrates. d[RONOz]/dt = -(k, + kf + k,)[RONOz] + k, (19)
where k, is the rate constant for increasing TN. Integration of eq 19 gives the equation [RON02] = [RON02],[1 - exp(-(k, + kf + k,)t)] (20) From eq 17 and 20, a linear relationship between ([InHIo - [InH]) and [RON02] is expected as follows
Figure 7 indicates the linear relationship between ([InH],,
- [InH]) and T N which may be proportional to [RON02]. Conclusion In the present study, the antioxidant decay in simply formulated lubricants within a light-duty diesel engine has been analyzed in order to elucidate the dominant factors in the oxidation of lubricant. The assumption of the oxidation mode in an oil sump as “initiated oxidation” a t a constant rate by radical species produced in an pistoncylinder area can fairly explain the observed curves of the antioxidant decay quantitatively. This result supports the validity and the availability of “total antioxidant capacity” proposed by Mahoney et al. (1978). In the near future,
Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 540-544
540 ._
k , = rate constant for the volatilization of lubricant, h-l = rate constant for increasing TAN, mg of KOH (g h)-l k x = rate constant for changing the parameter X,unit h-' k,, = rate constant for increasing TC, (mm h)-' k,, = rate constant for increasing T N , (mm h)-' Zk, = rate constant for bimolecular termination of peroxy radical, M-' s-' kinh = rate constant for inhibition with peroxy radical, M-' kA
-, E
125-
100-
E
075-
S-1 050.
0 25.
0 00
00
05
10
15
([lr~H]~-[lnH])
2.0
25
(wt%)
Figure 7. Linear relationship between ((InH],,- [InH])and TN: -, during induction period; - - -, after induction period.
it will be necessary to study how the values of k, and Ri are influenced by lubricant formulations or operating variables of engines.
Acknowledgment The authors are indebted to Dr. K. Yamazaki, Professor of Emeritus, University of Tokyo, for his valuable discussion and continual stimulation. The authors also express their appreciation to Mr. A. Matsunaga for his advice in the antioxidant analysis. Nomenclature A = TAN, mg of KOH g-' [InH] = concentration of antioxidant, M or w t % klst = overall first-order rate constant for antioxidant decay, h-' k,, = overall zero-order rate constant for antioxidant decay, M h-' or wt % h-' ka&J = rate constant for fresh oil addition, h-' k f = rate constant for the outward flow of lubricant from engine system, h-' k , = rate constant for the reflux of lubricant from pistoncylinder area into oil-sump, h-' k , = rate constant for oil sampling (averaged value on assuming continuous sampling), h-'
Ri = initiation rate or radical input rate from piston-cylinder area into oil sump, M s-' or wt % h-' tind = induction period for the oxidation of lubricant in oil sump, h [RON021 = concentration of nitrate, M or wt % [ROO.] = concentration of peroxy radical, M or wt % X = dummy variable for TAN or TC, unit TC = absorbance of carbonyl at 1705 cm-l, mm-' T N = absorbance of nitrate at 1630 cm-', mm-' Subscripts m = equilibrium conditions 0 = initial conditions tind = conditions at tind
Literature Cited Bardy, D. C.; Asseff, P. A. SA€ Pap. 1970, No. 700508. Boozer, C . E.; Hammond, G.S. J . Am. Chem. SOC. 1955, 77, 3233. Buchochenko,A. L.; Kaganskaya, K. Y., Neiman, M. B. Klnet. Katal. 1963, 2, 161. Burn, A. J. Adv. Chem. Ser. 1968, 75, 323. Denisov, E. T. Kinet. Katal. 1963, 4 , 508. Denisov, E. T.; Aleksandrov, A. L. Zh. Flz. Khlm. 1964, 38, 491. Guggenheim, E. A. Phil. Mag. 1926, 2, 538. Harie, 0. L.;Thomas, J. R. J . Am. Chem. SOC.1957, 79, 2973. Howard, J. A.; Ohkatsu, Y., Chenier, J. H. B., Ingold, K. U. Can. J . Chem. 1973, 51, 1543. Korcek, S.;Mahoney, L. R.,Johnson, M. D., Hoffman, S. SA€ Pap. 1978, No. 780955. Kreutz, K. L. Lubrlcation 1969, 55 (6), 53. Mahoney, L. R., Kwcek, S.,Hoffman, S.,Willermet, P. A. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 250. Mahoney, L. R.; Otto, K. Korcek, S., Johnson, M. D. Ind. fng. Chem. Prod. Res. Dsv. 1980, 19, 11. Marquardt, D. W. J. SOC. Ind. Appl. Math. 1963, 1 1 , 431. Milllotis, G.; Boudoncie, B., Parc, G. Bull. SOC. Chlm. Fr. 1960a, 647. Milllotis, G.; Boudoncle, B., Parc, G. Bull. Soc.Chim. Fr. 1969b. 4462. Thomas, J. R.; Tolman, C. A. J . Am. Chem. SOC. 1962, 84, 2930.
Received for reuiew November 20, 1980 Revised Manuscript Received April 20, 1981 Accepted April 28, 1981
Kinetic Approach to Engine Oil. 3. Increase in Viscosity of Diesel Engine Oil Caused by Soot Contamination Seljiro Yasutoml,* Yoshihiro Maeda, and Tsutomu Maeda Lubricants 61 Petroleum Products Laboratory, Nippon Mining Co.,Ltd., 17-35, Niizo-minami 3-chome. Toda-shi, Saitama-ken 335, Japan
Used diesel engine oil is regarded as a suspension composed of Newtonian oil phase and the dispersed phase of soot particles. Most used oils can be treated as a Newtonian suspension in the range of the shear rate of usual measurements with a capillary viscometer. The increase in viscosity as a function of soot concentration can be described by a "modified Brinkman's equation" including an empirical parameter a which corrects the difference in the abilii to increase the viscosity between soot and inert spherical particles. A reference value of a is obtained from the data with wide varieties of lubricants used in fiiM service, while some peculiar condfions in a bench engine test induce a little discrepancy from the reference value.
Introduction Soot contamination is one of the most important causes of the increase in viscosity of diesel engine oils. The increase in viscosity is very dependent on the design of diesel 0196-4321/81/1220-0540$01.25/0
engine utilized. Parsons (1969), Smith and Chowings (19761, and Knight and Weiser (1976) have demonstrated that indirect-injection type induces a much greater rate for the soot contamination than the direct-injection type. @ 1981 American Chemical Society
