INDUSTRIAL A N D ENGINEERING CHEMISTRY
486
Vol. 18, No. 5
Principles Underlying the Use of Equilibrium Oils for Automotive Engines By Robert E. Wilson and Robert E. Wilkin STANDARD OIL Co. (INDIANA), WHITING,I N D .
The dilution of crank-case oils with the heavy ends of gasoline constitutes the outstanding present-day problem in the lubrication of automotive engines. This paper presents first a theoretical analysis of dilution a n d the development of the fundamental laws, assuming t h a t dilution approaches a n equilibrium condition in the crank case. Experimental data from road tests and dynamometer trials were obtained to determine the constants and t o test the validity of these laws. These data show that, under average conditions, the crank-case oil loses about two-thirds of its viscosity in the first 150 miles of winter service and thenceforth remains practically unchanged if operating conditions are constant. In order to make a n oil which will give satisfactory starting and cold lubrication and a t the same time have sufficient viscosity a t all times in the average car, it is suggested t h a t a heavy oil of from 425 to 525 seconds viscosity a t 100” F. (37.8’ C.) be blended with 10 to 12 per cent of a distillate having a boiling range substantially identical with t h a t found in the average crank case a t equilibrium. By this means it is possible to produce a n oil with a n initial viscosity of about 200 seconds, which gives easy starting and adequate cold lubrication and yet is so near the average equilibrium dilution t h a t in general i t maintains a n optimum viscosity throughout its entire service. The results of extensive tests on this “equilibrium” or of “nondiluting” type of oil are presented.
HE outstanding problem in the lubrication of automo-
T
bile engines is the situation produced by the fact that the heavy ends of the motor fuel tend to accumulate in the crank-case oil and greatly reduce its viscosity, especially in winter operation. I n fact, the average car in winter dilutes to about 15 per cent in 150 miles service and its viscosity drops to about one-third of its initial value. This means that the motorist has the choice of using either a light oil which will operate satisfactorily a t the start but thins out to a dangerously low viscosity in a hundred miles or so, or of starting with a very heavy oil which is satisfactory after it has become diluted, but gives excessively hard starting and little or no lubrication while the engine is warming up. Evidence is accumulating that the second alternative is responsible for much of the scoring of cylinder walls and bearings formerly attributed to the excessive thinning out of the oil. Dilution-viscosity curves for various oils made by this company are shown in Figure 1. Although much progress has been made in determining the effect of different factors on the dilution of the crank-case oil, and some interesting appliances have been brought forward which should reduce dilution if installed on new cars, nothing has been thus far suggested which offers reasonable promise of reducing i t markedly on existing cars, or of totally eliminating it even on specially equipped new cars. This situation seems to justify the presentation of this paper, which summarizes the theoretical basis for, and some additional data bearing on, a new method of attack on the dilution problem, first proposed by the writers a t a recent meeting of the Society of Automotive Engineers.l 1
J . SOC.Aulomolive Eng
, 18,
163 (1926).
It appears to be well recognized213 that the two factors of most importance in producing dilution are cold starting with a choke, which introduces considerable liquid gasoline into the cylinders and thence to the crank case, and excessively cold cooling water, which causes the heavy ends of gasoline to condense in the cold oil film on the cylinder walls and wash down into the crank case. Air temperature, the use of hot spot manifold, and the “blow-by” of vapors past loose rings seem to be much less important than has frequently been assumed. Acting in the opposite direction are certain factors tending to eliminate this diluent from the oil. Again the cooling water temperature is important, for whenever it heats up to 160” F.-the diluent is a proper operating temperature-say eliminated fairly rapidly from the film on the cylinder walls, which is constantly being replaced by fresh oil from the crank case. The crank-case oil temperature is also important, especially if there is much “breathing” of air into ahd out of the crank case, as in the case of four-cylinder cars. Any “blow-by” of hot exhaust gases would undoubtedly help to eliminate dilution by this route, especially if the crank-case oil were warm. Theoretically, the rate of elimination by all of these methods must be in approxiFIGURE-/ mately direct propor- .I EFFFCT OF DILUTION tion to the amount of diluent present in the 5 A Y B O L T ON VISCOSITV oil, as the greater the POLARINE O/LS quantity present the higher its vapor pressure. As the net, or observed, rate of dilution is the difference between the true rate 700 at which diluent enters the oil and the rate a t which it is eliminated, it would loo be expected on theoretical grounds that the observed dilution w o u l d increase rapidly a t first and then /O 20 more slowly as the amount of diluent and hence the rate of elimination increased. Eventually a point of equilibrium should be approached a t which the rate of eliminating diluent would just balance the rate a t which it comes in. Even then the amount of dilution would have temporary increase just after starting and decrease just after running hot for some time, but the average amount of diluent present should remain substantially constant under fairly uniform operating and weather conditions. Expressing this mathematically, if x represents the per cent dilution and m the miles traveled, dx/dm represents the net rate of dilution, and should equal K-K’x,where K is the 2
3
J. SOC.Aufomotive E n g . , 16, 69 (1924) Clayden, I b t d . , 17, 58 (1926). Elsinger,
INDUSTRIAL A N D ENGINEERTNG CHEMISTRY
May, 1926
constant for the rate of diluent addition and K‘ that for the rate of elimination. These terms are true constants for any given car operating under fixed conditions. The fundamental equation is therefore -a.x =
K-K’x
dm
(1)
which is most readily integrated when written as m =
$-K -dxK’x
observed. Figure 2 shows the agreement between the observed dilution and the theoretical curve for Buick No. 12-B, which is fairly typical. Table I shows the values calfrom a few members of a series of culated for K , K’, and cars run under fairly constant conditions during the winter of 1924-5. As indicated later, about 85 per cent of all cars dilute between 10 and 25 per cent in winter operation, but this table includes several extreme cases in order to throw some light on the cause of excessive dilution.
x.
Table I-Calculated 0
5-c 12-B 14-B 16-C 18-B 13-A Average
dx
2;
=
K-K’X,
= 0
K = K’X, or X , = K / K ’
whence
(2) (24
where X, = the concentration of diluent a t equilibrium. substitution of this value for K , Equation l a becomes
On
which integrates directly m =
-
+
log, (X.-x)
+c
(4)
The constant of integration, C, may be readily evaluated, as when m = 0, z also must be zero. Therefore
c
=
1 log, K
--j
x,
(5)
which gives the dilution a t any desired mileage for the particular conditions represented by K and K‘. K may be readily calculated from (2a) once K’ and X , are known. This gives an integrated equation with two unknowns, connecting dilution and mileage. By drawing a smooth curve through the observed points on the dilution-mileage curve for any car it is possible to determine the two constants and then plot the theoretical curve to compare with the actual one. By selecting the points of reference a t 60 and 180 miles, the solution of the equation is fairly simple and the resulting calculated curves in general correspond well with those
X,
0.168 0.194 0.143
Per cent 6.8 12.0 14.0
0.263 0.317 0.314
0.0116 0.0126 0.0066 0.0102 0,0099 0.0086
15.4 21.3 25.9 32.4 36.6
0,195
0.0103
0.108 0.060 0.178
10-B 14-D
Values for K a n d K’
K’ 0.0159 0.0050 0.0127
K
Car 11-B
When equilibrium is reached the net rate of dilution becomes zero because the rate of elimination is then exactly equal to the rate of diluent addition. This fact may be represented by
487
14.6
I n studying this table it will be noted that the rate a t which diluent enters the crank case, K , increases fairly steadily as the equilibrium dilution increases, showing a maximum variation between 0.060 and 0.317. On the other hand, the rate of elimination, K’, does not vary so widely or so consistently, most of the values being in the neighborhood of 0.01. Excessive rates of diluent addition, rather than low rates of elimination, are evidently the main cause for high equilibrium dilutions. By assuming the average figure of 0.01 for K’, it becomes possible to calculate an approximate dilution-mileage or viscosity-mileage curve for a given oil in a given car if we know only the equilibrium dilution characteristic of the car under given operating conditions. This possibility is utilized later. “Nondiluting” or Equilibrium Oils
I n view of the foregoing discussion i t is apparent that if one should start with an oil containing that amount of diluent which represents equilibrium for a given car, the viscosity should remain substantially constant in service, and if the base oil used in making up the mixture is heavy enough, it should be possible to have substantially the ideal operating viscosity throughout the entire period of service of the oil, F/GUR€ 3
I S D U S T R I A L A N D ENGINEERISG CHEMISTRY
488
instead of starting with an oil which is too heavy and ending with one which is too light, as is general practice today. Whether or not such a solution of the problem would be entirely practicable for general use obviously depends upon the answer to the following questions: 1-Does dilution actually approach an equilibrium in a given car under given operating conditions? 2-If so, about how soon is such an equilibrium substantially reached? 3-What is the magnitude of this equilibrium dilution for different cars under different operating conditions and how much does it vary? 4-What is the composition of this diluent, and how much does it vary? 5 - 1 s i t possible to get a single blended oil of this “nondiluting” type which will give satisfactory results in practically all cars?
I
F/GURE4
V/srosirv
YS
JUMMPR
M/LTAGP
TEsr
E1
S A L E S DEPARTNENT ‘ ~ N o N o ~ ~ u r I N011 G
,I
NONOILOTING”.WL - AYERAGE
1 . 1
98%OlLUNON
FIGUP?€
BLOCK
5
TESTS
O F 9 5 C A P C h 7ROCh‘S
MEDIUM POlARfNf-AV€RAGZ Of 91 CAPS 8 TROcflS
B 100
‘
400
MEDlUH WL1.Q1*Ix
P$
I n this figure the abscissas represent the original viscosity of the oil before adding any diluent and the ordinates represent the percentage dilution. The curved lines represent the \-iscosity of the blends thus produced. For instance, a n oil of 200 viscosity (third curve from the top) can evidently be made either from an oil of 200 viscosity with 0 per cent diluent, an oil of 400 original viscosity with 8.3 per cent diluent or an oil of 500 viscosity with 11.0 per cent diluent, with many intermediate possibilities. I n selecting a satisfactory oil the first requirement to insure easy starting in wi&er and good cold lubrication is that the initial viscosity should not be greater thanabout 220 seconds a t 100” F. (37.8’ C.),or not above that of light motor oil. The lower limit for the initial viscosity was set at 180 seconds, largely in order to avoid excessive sales resistance from motorists who would not believe a lighter oil could possibly lubricate properly. The selection of these limits gives the curved cross-hatched band between 180 and 220 viscosity, within which limits any blend would satisfy the above conditions.
‘,
8 8 1 D,‘ur/m
Y
Vol. 18, No. 5
‘WONOILUTING’~IL,460 v/s OIL +io% D
fllLEAG,E 100
200
3w
U U ~ N ~
!
4w
so0
IO0
,DURATION 2
4
OF
6
RUN
- Houns 8
/O
Test engine, Waukesha 4 cylinder 3% X 4 X inches. Runs 1 and 2 at 1250 r. p. m. and 16 hp: temperature. water outlet 165’ F., water inlet 135’ F.. oil sump 130’ F a’nd air inlet l i o n F. Runs 3 and 4 at 1000 r. p. m. and, 11 hp.: temperailre: water outlet 110’ F., water inlet 75’F., oil sump 110 F., and air inlet 120’ F.
The other limits which determine the selection of the best oil are not based upon its condition as marketed, but after it has been in use in different kinds of service, since a single “nondiluting” oil should give satisfactory service in cars which dilute anywhere within the range of 5 to 20 per cent. The vertical lines a t the left indicate the viscosity of the original oil which must be selected in order to give the specified viscosities a t 20 per cent dilution. It was considered that the oil used should have a minimum viscosity of 100 seconds when diluted to this extent, and this establishes the left-hand limit as indicated. The vertical lines a t the right indicate the viscosities of the original oil required to give the specified viscosities a t 5 per cent dilution, and here the maximum permissible viscosity has been tentatively set a t 340 seconds. Even this would be too high for easy starting from a cold garage, but very few cars kept in cold garages in winter would ever drive out diluent until only 5 per cent is left. This established the fourth limit, and left the area between the two vertical lines, 100 viscosity a t 20 per cent dilution and 340 viscosity a t 5 per cent dilution, as that which satisfied the last two conditions. The small, doubly cross-hatched area where the two bands cross represents oils which will fulfil all the prescribed conditions as discussed, and the star shows the composition of the oil now recommended. This recommended oil, which has given excellent service in several hundred cars and trucks operated both winter and summer, consists of 480 viscosity oil to which about 10.5 per cent of artificial diluent has been added. This artificial diluent must correspond very closely in composition to that
I N D U S T R I A L A N D ENGINEERING CHEiMISTRY
May, 1926
found in crank case-thus a lighter diluent would evaporate too rapidly and a heavier one would be ineffective because i t would not evaporate fast enough to make up for the diluent coming in. The recommended oil has the following constants: Viscosity a t 100' F Flash Pour test (depending o n type of oil used) Viscosity when diluted 15 per cent
200 seconds 170' t o l S O F F. 0' to 20' F. 150 seconds
Although 10.5 per cent dilution generally lowers the pour test of an oil about 10" F., i t is more difficult to secure a low pour test for such a mixture of a heavy oil plus diluent than for a
lk\
I
I
It will be noted that the "nondiluting" oil lost a little diluent and finished up with slightly less diluent and much higher viscosity than the medium Polarine. These same tests are being continued this winter, but all the results are not yet available. However, there can be no doubt as to the superior performance of the "nondiluting" type of oil. As a further check on the general theory of equilibrium oils, several block tests have been run under carefully controlled conditions. Figure 5 shows the results of two pairs of comparative t e d s between the two kinds of oil, on a Waukwha Type Z,4 cylinder 3' ': by 4' ':inch engine, one with very
-
CALCULATED M = - % ~&x$ ~ L ~ ~ K, =o.o/
I\I
--
489
I
"NOND/LUT/NGX=525 YfS O/L
\H€AVY uED/UM
tL I G H T
MOTOR O/L
+ N X D/LU€NT
I
I
I
MOTOR OIL
I M O T O R OIL
50
straight light oil of similar viscosity. Experiments in this laboratory have indicated, however, that the viscosity a t 0" F. measured by the pressure required to force oil a t a given rate through a small capillary, is considerably less for an oil of this type than for an ordinary light oil (215 seconds yiscosity a t 100" F.) with zero pour test, apparently because the temperature coefficient of viscosity for these oils is smaller than for any of the ordinary types. This is further borne out by the fact that the reduction in viscosity in going from 100" to 210' F. is abnormally small for these oils. The flash test is very low, judged by any ordinary motor oil specification, but it is slightly higher than the majority of oils found in crank cases after 200 miles of winter operation, and would not involve any source of danger in handling. The cost of producing the oil will be slightly higher than for the present medium oils, the extra cost of the heavier oils being only partly counterbalanced by the use of the 10 per cent diluent. Service Performance of Recommended Oil
Oil of this type and substantially this composition was given its first service test in about twenty-five cars during the winter of 1924-5. During the summer the test was extended to cover all the cars and trucks in this company's Chicago and Detroit garages. Figure 4 shows the average results on these cars and trucks operated alternately on medium Polarine and "nondiluting" oil during the summer.
I
MILEAGE
I
I
low jacket water and oil temperature, and the other under more normal conditions, as indicated in the figure. Small samples of oil were withdrawn and tested every hour. I n both cases the two oils reached substantially the same equilibrium dilution. The time required to reach equilibrium was greater than in ordinary road tests, probably partly because there was no cold starting, and partly because of the construction of the crank case, which prevents very little splashing of the crank-case oil. It was interesting to note that variations of only 5 to 10 degrees in jacket outlet temperatures cause marked irregularities in curves obtained in this manner. As to performance, the recommended oil has given entire satisfaction under both winter and summer conditions in about thirty different makes of cars and trucks. The only difficulty of any kind was a little hard starting when using an oil of about 220 seconds initial viscosity made from a 525-second base oil, but dropping these viscosities to 200 and 480, respectively, eliminated this trouble. Thus far time has not sufficed to run road tests on a variety of different oils under identical operating conditions, but the use of the theoretical equations derived in the first part of the paper, and abundantly verified by actual experimental results, permits Figures 6 and 7 to be drawn to show substantially what would happen in a car under typical summer and typical winter conditions using light, medium, and heavy motor oils compared with a nondiluting type of oil slightly heavier than the one finally recommended. On inspection of
INDUSTR141, A N D ENGINEEIUNQ CHEMISTRY
4'30
btresc figureti there a n c e r t i d y be no cluestion as to which ively type of oil is best. Of course, ill ears which dilutc ex IS oil cent,-evon the noniiil u t ing ty ctory, biit it is Iielter than any ordinary nsed, and sucb ex< ive dilutions are
Vol. 18, No. 5
gradually being climiilnt(d by better design and inore int & w t operrttimi. If the automobile industry can keep dilution 11elow say 20 per ocnt the recommended oil nhoitld certaitily give almost, ideal lubrication automatically thinning down a lirtle i n \viittm m i l tlrirkrning up in summer.
__
Automotive Engine Lubrication' By A. W. Pope, Jr. lV.i,
Kllll.i
Il,iTi,R
co.,W a o K i i s i l n , \Vi*
This paper aims to set forth t h e pertinent facts pertaining t o modern practice of automotive lubrication. The splash system is most widely used today, b u t because of certain shortcomings when used for heavy-duty high-speed work t h e pressure system is coming into increased favor. The pressure system also has its disadvantages, but a combination of t h e pressure and splash systems has been worked o u t t o give satisfaction for general automotive use. The only part of a n automotive engine giving any real dificulty in lubricating is t h e piston and piston rings. This difficulty has been overcome by certain changes i n construction arid t h e development of t h e fresh-oil or sidewall lubrication system. Oil purification by elimination of solids from t h e oil a n d control of dilution is discussed a n d t h e need for better cold-weather performance is expressed.
that a plain bearing rotnting in its journal has nil inlierent pumping action, t,ending t o take oil in through tlie slack side and delivering it from t,he load side of tlie bearing. It might at first appear feasihlc to develop this inlierent pumping action to a point where it would siipply suflicient quantities of oil to it.s hearing. However, the rnasimom theoretical pressure available to form oil into the hearing by this method would only be atmosplterir. Present racing-car prastice includes oil pressure in excess of 200 pounds per square inch Sor lubricating and cooliirg connecting-rod bearings. So t,he impossibilit,y of ever liaridling the situation with no more than atmosplrcric pressure is apparent.
Splash System
OD4Y the sjilasli system of lobrication is used oil Sar the ar est number of units. lt has somo fontores that are T 1 very g favorable to t.hesat.isf:ict.ory operation of an engine. It fills the arcas within the eugine with a uniform, concentrated, heavy oil mist, which pervades all spaces open to the crank case. This assures a supply of luhricatit to all moving parbs within tliesc spaces. Corrosion difficulty appears to be tninimizcd with the use of tho splash system, probably owing to the following influences: (1) A n iinmediate supply of lubricant is available after starting under all conditions of temperature. This reduces dilution and protects hearings which are suddenly put into operation with corroded surfaces. i%i The heavv of oil mist in the crank case ...... ~.. rmiccntration .~ may be suficient'to produce an oil film on the bearing surfaces which will resist the oxidizing influence of the attacking add collected in the oil. (3) The fine mist farmed by the splash system is more suited for the elimination of the rust-producing products from the oil: that is, contact of this fine oil mist with the cylinder and hot niston heads tends to keeo down the nercentare of harmful k c n t s in the oil. \_,
~~
~
~
Piston and cylinder lubrication with a splash system is very satisfactoiy. There is a iiiiiforrn supply US oil to all pistons during the life OS tho engine, which is not. influeiiccd by t.he viscosit,y of the oil. In addition to this uniformity of supply, the great density of the mist produces a desirable cooling effect on the piston. These same points apply to all other parts wllich are dependent upon the splash for their supply of lubricant. The weakness in a splash system is its inabi1it.y to create tl. hydraulic oil pressure within a rotating beitring which is great enough to prevetit metal-to-metal contact of the surfaces under heavy loads or with large clearaiices. It is true I Prceented by T. C. Coiemliii andX. W. Pope, Jr.. ueder tile title "A Study of Cylinder l\~dlLuhrieafion.''
Fresh Oil Pump and Distributor Assembly Showink AdJustin$ Screw and Delivery Pipe Conrlectiuns
When using a splash system it is necessnry to muintain close bearing clearances with consei~ucntclose manufacturing limits to maintain quieti1 This also means that only small amounts of wear are permissible before the machine will become very noisy. Pressure System
It is the general recogrtit,ion US these sliortoomings of the splash system whicli has brought the pressure system into almost univcrsal use for heavy-duty high-speed work. The trend is toward the pressure system. h e a t i n g hearings are operated nrosi sur:i:wsSully where lubricant is supplied to them under pressure. The necessary lubricant pressure is a direct function of the bearing load and speed. I n connecting-rod and crankshaft-bearing work the lubricant serves the triple pitrpose of lubricant, cushion, and coolant. If sufficient oil is supplied under suitahle pressure, cornparat.ivcly large bearing clearances can he used.
