HEAVY-DCTY MOTOR OILS.

HEAVY-DUTY MOTOR OILS.

I3

By K. T. ARTER, B.Sc. (Eng.), M. Inst. P.*

(ASSOCIATE MEMBER).

December, 1943.

ONE of the notable features of the past twenty-five years has been the steady advancement in the performance of high-speed internal- combustion engines, as measured both by thermal efficiency and power output per unit displacement. This has been accompanied by improve- ments in mechanical reliability and a lengthening of the periods run between servicing and major overhauls. These developments could not have been wholly achieved without concomitant progress in the manufacture and blending of lubricating oils. Thus the characteristics of engine oils have been improved in many respects, such as redu,ced carbon formation, greater resistance to oxidation and sludge forma- tion, and the maintenance of fluidity at very low temperatures. The trend of these developments up to 1935 has been described by Brame.1'

Such improvements in quality were effected largely by advances in refining methods, but more recently it came to be appreciated that refining could be carried too far, with the result that valuable natural inhibitors were removed. This resulted in a loss of high-temperature stability which could not be entertained a t a time when engine operat- ing conditions were tending to become increasingly severe. Further progress was made by the introduction of a new technique, whereby chemical addition agents are introduced in small percentages into the oil. These are added for specific purposes, such as inhibition of oxida- tion and the protection of the newer alloy bearing metals from corro- sion by the products of high-temperature oxidation, which are liable to be formed in the untreated lubricant.

In the United States of America the period 1936 to 1940 was one of intense activity in the development of engine-oil additives, and it

* Esso European Laboratories, London. (I) A list of references is given at the end of the paper.

3

14 THE IXSTITYTION O F AUTOMOBILE ESGIXEERS.

marked the acceptance of an entirely new class of internal-combustion engine lubricants which have come to be known as “ Heavy-Duty ” or “ H.D.” oils.e These new oils originated from the rapid growth in the use of small high-speed compression-ignition engines in the United States, which became particularly marked after 1935. It has been stated3 that the estimated annual sales of such engines of less than zoo h.p. increased from 1.450 in 1934 to more than 21,000 in 1936, and their numbers have steadily increased in succeeding years. The price differential between petrol and gas oil has always been much less in America than in this country. Consequently there is less incen- tive to use the C.I. engine, which has therefore had to compete on a weight per horse-power basis with the petrol engine. Hence, American oil engine development has laid emphasis on power output rather than fuel economy, and some engine manufacturers ran into trouble in their efforts to equal petrol engine performance.

THE CAUSES OF SLUDGING AND RING-STICKING IN C.I. ENGINES.

It is well known that incomplete combustion of the fuel and dirty exhaust result from increasing the quantity of fuel injected beyond a certain point. It has not been so generally appreciated that such conditions have a deleterious effect on the lubricating oil, which rapidly becomes contaminated with the sooty products of incomplete com- bustion. These contaminants are responsible for the considerable quantities of black sludge which are commonly found in C.I. engines, and it is notable that such sludge shows a carbon content of 70 to go per cent in comparison with 30 to 35 per cent in the sludge from petrol engine^.^

The relation between combustion chamber conditions and the sludging of C.I. engine oils was shown very clearly by some experi- ments carried out by the author about five years ago. These were made on a 4-cyl. C.I. engine of IOO mm. bore and IZO mm. stroke, which will be referred to as engine M. It was operated under constant load, and samples of the oil in circulation were withdrawn at intervals. The percentage of sooty (naphtha-insoluble) material in the samples was determined, and, knowing the quantity of oil in the engine and the quantity consumed, it was possible to calculate the total weight of sooty contaminants formed a t various stages of the test. It was found that a t any given load this quantity was directly proportional to the period of operation, as shown by the straight lines in Fig. I . I t is also very evident that the rate of formation a t full load (93Ib. per sq. in. b.m.e.p.) was considerably greater than at a b.m.e.p. of of 68 lb. per sq. in. A series of such tests was therefore made under various loads, using the following two fuels :-

’

~~

................................................... B Fuel

Cetane number Specific gravity .................................... 0.846 0.864 I 46 .................................... I 56 I

The lubricating oil used was a straight mineral type of S.A.E. 30 viscosity classification. The rate of formation of soot in grams

HEAVY-DUTY MOTOR OILS. 15

per hour was determined for each load and is plotted in Fig. z against b.m.e.p. It is significant that the rate of contamination began to increase markedly between 90 and about 931b. per sq. in. b.m.e.p.

0 4 8 12 10 20 24 HOURS.

Fig. I.-Rate of formation of sooty contaminants in oil-Engine M.

The latter figure represents the approximate point at which the engine exhaust becomes visibly dirty. It will also be seen that fuel A, having the higher cetane number, contaminated the oil somewhat more rapidly

70 80 90 100 B.M.E.P. LBS./SQ. IN.

Fig. 2-Etiect of load on rate of soot formation.

over a given period than fuel B. This was in line with some observa- tions of exhaust colour which were made by means of a photo-electric

3 ( 2 )

16 THE INSTITUTION O F AUTOMOBILE ENGIXEERS.

apparatus, and which confirmed that, for a given load, fuel A gave a slightly dirtier exhaust. The engine in question, which was of the precombustion chamber type, had been designed to operate on fuels of about 45 cetane number. The fact that combustion appeared to be less clean with the fuel of higher cetane number was probably due to reduced combustion turbulence. The point it is desired to emphasize here is that the rate of contamination of the oil was closely related to the degree of cleanliness of combustion.

Determinations of fuel consumption were made in the course of the above tests, and it was therefore possible to express the weight of soot formed in the oil as a percentage of the weight of fuel supplied to the engine in the same time. The results in this form are given in Fig. 3. Some further investigations carried out on a similar type of engine operating in passenger-carrying service showed that the amount of soot formed was again directly proportional to the mileage run, and

d W P

0 70 80 90 100

B.M.E.P. LBS./SQ. IN.

Fig. 3.-Relation of rate of soot formation to fuel consumption.

represented 0 - 0 2 per cent of the fuel consumed ; thus the rate of con- tamination in actual service agreed closely with that observed in the bench tests. Another design of engine used in the same type of omni- bus, and which gave a somewhat dirty exhaust under most running conditions, contaminated the lubricant much more rapidly, corre- sponding to 0.06 per cent of the fuel used.

The solids thus formed in the lubricating oil of C.I. engines are too great in quantity, and a proportion of them too fine, to be completely removed by any filter of reasonable size for a road vehicle engine; consequently they are circulated through the engine and result in dirty pistons and accumulation of carbon behind the rings. The carbon ultimatcly causes sluggish ring action, and leads to increased blow-by of the combustion gases past the piston. The blow-by, in turn, raises the piston temperatures and accelerates oxidation of the oil reaching the pistons and cylinder walls. Thus the ill effects of incomplete com-

HEAVY-DUTY MOTOR OILS. I7

bustion have a cumulative effect on deterioration, both of the lubricat- ing oil and engine condition. The result of attempting to increase engine output beyond a certain limit is the occurrence of ring-sticking, scuffing of the piston rings, increased wear and the formation of lacquer or varnish on the pistons. The relation of ring-sticking to mechanical design has been more fully discussed by Rosen.5 6

DETERGENCY.

Ring-sticking became a serious problem on some types of Amezican engines and appeared likely, at one time, to set a limit to further improvement in performance. At this point, however, research work by some of the leading oil companies in that country led to the develop- ment of addition agents of an entirely new type, which had the property

A. Mineral oil.

B. H.D. 611.

Fig. 4.-Dirpersion of carbon black.

of largely preventing the deposition of undesirable products on the vital engine parts. Such compounds are commonly described as ‘‘ detergents,” although they appear to operate by a combination of detergency and peptizing action. The latter term in this case implies an ability to disperse the solid contaminants throughout the liquid phase (i .e. , the oil) and to prevent them from coagulating to any appre- ciable extent ; the mechanism is of a physico-chemical nature and is not to be confused with solvent action, wherein undesirable crankcase contaminants are dissolved in the oil itself. The major function of detergency in a crankcase oil is to prevent fouling of the pistons and consequent ring-sticking, although the reduction of sludge accumulations in other parts of the engine is also advantageous.

The ability of a detergent additive to retard the settling of solids in the oil has been demonstrated quite convincingly by a simple laboratory test.4 The two bottles shown in Fig. 4 contain equal quantities of engine

18 THE INSTITUTION OF AUTOMOBILE ENGINEERS.

Viscosity.

......................... Redwood No. I at 140" F Centistokes at 100' C. ...........................

, I 50" C. ...........................

oil of the same viscosity, but whereas the oil in A was of a straight mineral type, that in B embodied the addition agent. To each of the bottles was added the same weight of carbon black, which was thoroughly mixed with the oil ; the bottles were then allowed to stand for zz hours a t zooo F. It will be seen that a t the end of this period the carbon black had settled in the uncompounded oil to a marked degree, whilst it was still completely dispersed in the detergent oil.

Very similar conditions prevail in the engine where carbon particles normally tend to aggregate, particularly in the presence of traces of water such as are usually found in used oils. In the absence of a dis- persing agent, these aggregates settle out on engine parts and form the black pasty type of sludge which is particularly common in C.I. engines.

Naphthenic base oil.

148

64 8.0

RESISTANCE TO OXIDATION.

The early detergent oils contained soap-type addition agents, such as aluminium naphthenate, calcium naphthenate, and calcium dichloro- stearatc3 The disadvantages of these oil-soluble soaps was that they could only be used in a limited class of naphthenic base oils. Whilst oils of this type are entirely satisfactory in some applications, they have poor viscosity-temperature characteristics. A further disadvantage of these early detergent blends was that they were insufficiently resistant to high-temperature oxidation and were consequently corrosive to copper-lead and other alloy-type bearing metals which were then becoming necessary for reasons of mechanical strength a t the higher power outputs.

About this time the General Motors two-stroke oil engine reached the production stage and introduced some additional lubricating problems. This engine does not require a very high degree of detergency,

Paraffinic base oil.

-

164 11.3 60

I ,800 30

but the high piston temperatures resulting from the two-stroke cycle made new demands on the oil in regard to oxidation resistance and the prevention of carbon deposits on the underside of the piston crown.

Intensive development has overcome all these difficulties and a t the present time there are several additives which can be used in high- quality paraffinic base stocks of excellent viscosity-temperature characteristics. The exact composition of these newer types of additives is rarely disclosed, but the majority of them are complex metallo- organic compounds, They combine the property of inhibiting oxidation of the oil with that of detergency and are entirely satisfactory for use

I9 HEAVY-DUTY MOTOR OILS.

Oil,

-__ -- ........................... Naphthenic oil, uncompounded

H.D. oil, naphthenic base .................................... Paraffinic oil, uncompounded .............................. H.D. oil, paraffinic base.. .....................................

with any type of alloy bearing material now on the market. The improvement in viscosity-temperature characteristics will be apparent from the comparative figures in Table I.

Oils containing certain of these additives have been proved, by pro- longed and severe testing, to give exceptional results in all types of American C. I. engines in respect of detergency, resistance to oxidation, and avoidance of bearing corrosion. The description of such lubricants as " Heavy Duty " oils is fully justified by their performance.

Ring-sticking Demerit . Rating.

L -- 4.78 1.42

1 0 ' 0 1.26

IMPROVEMENTS EFFECTED BY H.D. OILS I N COMPRESSION-IGNITION ENGINES.

Owing to the unusual circumstances created by the war, there have been only limited opportunities of conducting properly observed tests of these oils on British engines in this country, and some of the most interesting results cannot a t present be disclosed in detail. The author therefore makes no apology for quoting mainlv from the published results of tests on American engines4 to illustrate the improvements derived from the use of these oils.

Ring-Sticking and Rang Zone Condition. The beneficial effect of H.D. oils in reducing ring-sticking was clearly

established by tests of a six-cylinder engine developing about go h.p., which will be referred to as engine H. This was particularly subject to ring-sticking and especially with paraffinic oils. The tests were of 84 hours duration a t 95 per cent load. The amount of ring-sticking was assessed by a numerical " demerit " rating, whereby the worst possible condition was represented by the figure 10 and perfect condition by 0. Intermediate conditions were designated by numbers ranging between these two extremes. The following comparative results were obtained :-

TABLE I1

Thus the addition of the detergent-inhibiting compound to either type of base oil reduced the degree of ring-sticking by some 70 per cent. Another design of engine, which also sticks rings with uncompounded oils, showed the following striking results4 :-

With a straight paraffinic oil, ring-sticking occurred after 126 hours. When a straight naphthenic oil of poor viscosity-temperature

characteristics was used i t was possible to run for joo hours before ring- sticking commenced.

20 THE INSTITUTION OF AUTOMOBILE ENGIKEERS.

When using paraffinic or naphthenic oils containing an additive of the detergent-inhibiting type, a 1,000-hour test was completed without any indication of ring-sticking, and the piston after removal appeared capable of satisfactory operation for many more hundreds of hours.

Remarkably similar results were obtained in field tests of 1,000 to 1,200 hours’ duration under more severe conditions than those used in the laboratory engine.

Fig. 5 shows the pistons of a high-speed C.I. engine after running for 265 hours on a straight mineral oil. Nine rings were stuck, the pistons

Fig. S.--Mineral oil. 265 hours.

were fairly well covered with black varnish and the oil ring slots were partially clogged. Fig. 6 shows the pistons from a similar engine which had operated for 1,000 hours on an H.D. oil. All rings were free and the piston skirts and ring grooves were practically clean.

In addition to freedom from ring-sticking, the pistons invariably exhibit an exceptional degree of cleanliness when a detergent additive

Fig. 6.-Hcavy-duty oil, 1,OOO hours.

is used. This improvement is illustrated in Fig. 7, which shows one of the pistons of engine M after running on (a) a straight mineral oil of good quality, (b) a detergent oil. In both cases the test consisted of a Ioo-hour run a t about go per cent of full load. This engine is not particularly subject t o ring-sticking troubles with uncompounded oils, but the piston invariably strips in a very dirty condition, and the upper ring

HE.\VY-DCTTY MOTOR OILS. 21

grooves are " packed " with hard carbon. After tests on the H.D. oil the pistons have never shown more than a thin film of black oil, which can be easily wiped off with a soft cloth, leaving the clean metal beneath. Similarly, deposits of hard carbon in the ring grooves are almost completely absent.

Sludge. Detergent additives have a pronounced ability to minimize the

accumulation of sludge on engine surfaces. For example, in engine H the use of such an additive effected a reduction of 60 to 75 per cent in the amount of sludge deposited on various parts of the engine, such as the crankcase, valve gear, and filter. It must be emphasized that the additive does not prevent the formation of sludge contaminants in the oil, but i t acts as a disperser in the manner already described. In these

(a) Xineral oil. ( b ) Heavy-duty oil.

Fig. 7-Appearance of pistons from engine M after 100-hour tests.

circumstances i t is not surprising to find that the H.D. oil itself becomes dirtier than an uncompounded oil. This fact is illustrated by some further data obtained on engine M. During a Ioo-hour test at 86 ib. per sq. in. b.m.e.p., with a fuel consumption of 0.40 lb. per b.h.p.h., the sooty contaminants in a straight mineral oil were found to represent 0.018 per cent of the fuel consumed. When an H.D. oil was used this proportion increased to 0 - 035 per cent, or approximately double the previous figure.

The solid contaminants dispersed in the oil ultimately leave the engine when the crankcase is drained and it will therefore be clear that a regular draining procedure is desirable in order to obtain the full advantage of such oils. If the solid matter in the drained oil is separated, analysis shows that the proportion of additive in the sludge-free oil is less than was present in the original oil before use. On the other hand, some of the elements of which the additive is composed can be identified in the separated sludge. Evidently the additive has an affinity for the sludge particles which largely prevents them from coalescing and adher- ing to the metal surfaces of the engine. Thus, in effect, the additive becomes used up in the course of time. Normally the additive con- sumed in this manner is replaced by fresh material introduced with make-up oil, and under any normal conditions of service it is extremely

22 THE INSTITUTION OF AUTOMOBILE ESGISEERS.

unlikely that the quantity of active additive remaining in the oil would become small enough to be ineffective. It will, at the same time, be appreciated that this is a further reason for following a regular drainage procedure, particularly i f the oil consumption is low. It is impossible to recommend a precise figure for drainage periods, since this will obviously vary with engine design and operating conditions. It is. however, improbable that oil changes will need to be made more fre- quently than with any straight oil. Oil life is, in general, improved in comparison with straight mineral oils and, moreover, the better piston condition should ensure that a lower average oil consumption is main- tained over a long period of operation.

Pitrging. When a highly detergent oil is supplied for the first time to an engine

which has previously run for a considerable period on straight mineral oil, a pronounced purging action results, whereby accumulations of sludge and carbonaceous material become detached from the engine surfaces and are taken into suspension in the oil.' This situation may arise in practice from a decision to change over a fleet of vehicles to the H.D. type of oil, or it may be brought about deliberately with the intention of cleaning the interior surfaces of a dirty engine. It is advisable when any engine is changed over to a detergent blend for it to be thoroughly flushed with the new oil and drained as completely as possible after running for a nominal period.

Some users adopt the practice of filling up occasionally with a detergent oil for the purpose of flushing out a dirty engine. Experience has shown that sludge can be removed in this manner, although the effect is restricted mainly to parts of the engine which are well washed by circulating oil. Sludge accumulations in the valve chest and similar places are not greatly affected. There have been a few instances in which the oil consumption of a dirty engine is increased to a marked degree after changing to a detergent oil. The explanation of this appears to be that carbon and varnish-like deposits are removed from the piston and piston-ring surfaces with the result that the clearances are increased and there is some loss of oil control. This occurrence is usually only of a temporary nature and it is commonly found that a normal rate of consumption is restored after running for a time with the H.D. oil, probably because a better piston ring seal is established.

Falters. In comparison with straight oils the filter usually has an appearance

of greater cleanliness when H.D. oil is used. This is an effect of the sludge-dispersing property of the additive ; the sludge particles are so finely divided that many of them pass through the filter. At the same time, the oil more rapidly assumes a dirty appearance. It is therefore inadvisable to rely on visual inspection of the filter or the oil on the dip- stick for guidance as to when either should be changed. An exception to this occurs when the change is first made from straight mineral oil to H.D. oil. The resultant purging action may cause the filter t o choke with old deposits washed down into the crankcase. In view of this the oil pressure and the state of the filter should be carefully watched until stable conditions are established.

HEAV\--DUTl- MOTOR OILS. 23

It has been found that a few types of filter tend to remove a pro- portion of the additive from the oil, but there is as yet no evidence that this is such as to impair the special properties of H.D. oils. It is possible that the filter becomes saturated after a time and subsequently has no more effect on the additive. It seems reasonable to regard the functions of the filter and the additive as complementary. Whereas the object of the filter is to remove as much as possible of the solid contaminants in the oil, the primary purpose of the additive is to preserve freedom of the piston rings and retard the formation of deleterious oxidation products in the oil.

Wear. In some circumstances the use of H.D. oils results in a reduction of

piston ring and cylinder wear. Some observations of ring wear were made in engine M during Ioo-hour tests a t 1,500 r.p.m. and 86 lb. per sq. in. b.m.e.p. These tests were run a t constant load for periods of eight hours followed by a shut-down for sixteen hours. With a com- pounded oil containing a detergent and inhibiting additive the loss of weight of the rings was 36 per cent less than with a straight mineral oil of good quality. In another type of engine running under continuous high load the reduction was 35 per cent. It is unlikely that this decreased rate of wear is directly attributable to any essential difference in film strength between the oils. A more rational explanation is that the H.D. oil conferred better lubricating conditions by preserving freedom of ring movement and thus preventing the consequential evils of excessive blow-by and overheating.

That wear reduction is not necessarily an inherent property of H.D. oils was shown by some comparative 75-hour tests in small single- cylinder petrol engines.4 When the jacket temperature was maintained a t 200' F. there was no difference in ring wear between the H.D. oil and a straight mineral oil. However, when the temperature was reduced to 100' F. the wear increased by 84 per cent with the mineral oil and only by 29 per cent with the H.D. oil. Thus, under the same low- temperature corrosive conditions the latter was effective in reducing ring wear by 30 per cent in comparison with the uncompounded oil.

Bearing Corrosion It is important that lubricants for modern high-speed oil engines

should be non-corrosive to hard alloy bearings, which are being used to an increasing extent. The degree of corrosion resistance required to give adequate protection of bearing surfaces may, however, vary considerably between different designs of engine, whilst it is also related to the particular operating conditions employed and to the micro- structure of the bearings themselves. Many of the earlier detergent- type additives greatly accelerated bearing corrosion. Wolf2 pointed out that such compounds may attack alloy bearings, particularly of the copper-lead type, by a mechanism entirely independent of the base oil. With additives of this type, it is therefore necessary to pro- vide inhibitors which not only prevent corrosion by products of oil oxidation, but also by the detergent additive itself. Some of the additives now available are themselves non-corrosive, and, in addition, retard the development of corrosive oil oxidation products. As an

24 THE IXSTITUTION OF AUTOMOBILE ENGINEERS.

illustration of the performance of such an additive may be quoted4 comparative 500-hour endurance tests in a 4-cyl. 106 b.h.p. engine operating with a crankcase temperature of 230' F. With an uncom- pounded mineral oil, the test had to be discontinued after 301 hours of operation owing to severe corrosion, which averaged 7-1 grams per bearing. By contrast, the additive-containing oil completed the full period of test with almost negligible bearing corrosion (0.048 grams per bearing).

H.D. OILS IN PETROL ENGINES

It will be evident that H.D. oils were, in the first place, developed exclusively to meet certain problems associated with oil engines operating under severe conditions of load and temperature. The petrol engine is not subject in quite the same manner to contamination of the oil by insoluble products of incomplete combustion, but other diffi- culties have arisen in some instances where very high crankcase tem- peratures are experienced. If petrol engines are operated for any length of time a t crankcase temperatures exceeding, say, 240 OF., the oxidation process is greatly accelerated, resulting in excessive sludging and the accumulation of varnish or lacquer on the pistons. The latter, again, is favourable to ring-sticking, and there have been instances of piston varnish virtually causing the engine to seize up. It cannot be denied that the first step to take in countering such troubles is improvement of the oil cooling so as to maintain more reasonable crankcase temperatures. Unfortunately, the trend of modern body design has rendered it increasingly difficult to do this, and in such circumstances the heavy-duty oils have proved highly advantageous in preserving a satisfactory engine condition for lengthy periods of high-temperature operation. In the case of petrol engines designed for very high outputs, oils containing detergent and inhibiting additives are likely to prove increasingly valuable in the future, and there is a wide scope for further development.

An interesting demonstration of the effects of a detergent and inhibiting additive was obtained on a highly boosted air-cooled petrol engine used for fuel research work. This engine has to operate under border-line detonating conditions a t high cylinder head temperatures, with the result that ring-sticking rapidly occurs when a straight mineral oil of high quality is used. For this reason the engine has to be given a top overhaul every 30 to 35 hours. After one such overhaul, the lubricating system was cleaned and filled up with a fresh charge of the mineral oil. After the normal run-in period, the b.m.e.p. a t the border- line detonation point on a particular fuel was 251 lb. per sq. in. This is termed the " peak " b.m.e.p. It is a measure of fuel periormance, and does not necessarily represent the maximum output of the engine. Operation was continued for 73 hours, after which time there was a noticeable increase in oil consumption, as shown by Table 111. This was accompanied by an increased tendency towards detonation. To maintain the initial intensity of detonation, it was necessary to reduce the boost pressure with the result that the " peak " b.m.e.p. fell to 247 lb. per sq. in. After eight hours the additive was introduced into the oil in the form of a concentrate whilst the engine was running. During the succeeding 24 hours the oil consumption improved to

HEAVY-DUTY MOTOR OILS. 25

............... Mineral oil after run-in

Oil doped with additive ...............

I .S pints per hour, and the " peak " b.m.e.p. was restored to its initial value. Fuel testing was continued with the compounded oil until the engine had completed 50 h.ours. At this point the " peak " b.m.e.p. with the same fuel was actually 3 lb. per sq. in. higher than a t the commencement, and the oil consumption was very satisfactory. The

251 249'5

247

251 254

TABLE 111.

Hours run.

5 ' 0 6.5 7'5 8 . 0 10.5 50

Remarks. " Peak " b.m.e.p.,

lb. per sq. in.

oil con- sumption,

pints per hr.

engine was then dismantled for inspection and the condition of the rings and grooves was found to be excellent, with no sign of ring- sticking. The engine has since run a total of 95 hours on the com- pounded oil and testing was only discontinued a t this point because of a mechanical failure in another part of the equipment.

It has been established that under such hignly boa5ted conditions lubricating oil reaching the combustion chamber has a pronounced pro-knock effcct. The fall in the b.m.e.p. corresponding to border-line detonation is a clear indication that the increased oil consumption after 74 hours was due to more oil passing the piston. These results provide strong circumstantial evidence that the detergent additive served to check incipient ring-sticking. This conclusion is further supported by the excellent mechanical condition of the engine after 50 hours.

Similar indications were obtained on another type of liquid-cooled research engine, which had a " peak " b.m.e.p. of 163 Ib. per sq. in. after 67 hours operation on straight mineral oil. The circulating oil was doped with additive after 72 hours but on further operation of the engine it was found that the power continued to fall until a total of 76 hours had been completed. At this point there was a sudden marked improvement and the " peak" rose to 161 lb. per sq. in. After a further 4 hours this was restored to the same value as a t the 67-hour point, viz., 163 lb. per sq. in. It would appear that, after a period of operation with the additive in the oil, the rings were freed, and this was the reason for the improvement in engine performance.

PRESENT AND FUTURE USE OF H.D. OILS.

But for the outbreak of war, H.D. oils would undoubtedly have appeared on the British market in 1940. Limited supplies of detergent oils have, in fact, been available since that time, but their use is

26 THE INSTITL'TION OF AUTOMOBILE ENGINEERS.

restricted by the Petroleum Department to certain American tractors, which impose severe requirements on the lubricant. Heavy-duty oils of the latest detergent-inhibiting type are not obtainable by com- mercial and private operators. The reason for this is that all available supplies of approved H.D. oils are required for military purposes. I t has been officially stated8 that a decision was taken in 1941 to use only oils of this type in the U.S. Army Motorized Ground Forces. The U.S. Army Specification No. 2-104~ (covering " All-Purpose Engine Oil ") lays down engine test requirements which cannot be met by any known oil not containing a detergent and inhibiting additive. The main features of these severe engine tests are given in the Appendix. The US. Navy has also made considerable progress in the establish- ment of engine test procedures for H.D. oils, and some results obtained in medium and high-speed naval diesel engines have been described by Klemgard.9

Whilst H.D. oils are obviously not essential to the operation of all classes of military equipment, the desirability of making them univeis- ally availabl: is evident. Apart from the simpiification of supply problems, these oils will confer an additional margin of dependability a t times when it is impossible to comply with scheduled maintenance procedures. Under the Allied pooling arrangements, the British Army will also receive supplies of H.D. oils, and arrangements to this effect are alreadv in force.

When these oils become generally available for civilian use in this country they should prove very advantageous to operators of com- pression-ignition engines. British oil engines are highly developed, and have proved extremcly reliable. Nevertheless, their reliability should be enhanced by use of the new compounded oils and the mileage between overhauls increased. For the same reasons H.D. oils should find ready acceptance among some classes of commercial vehicle operators using petrol eagines, more particularly where heavily loaded vehicles are used over long distances. The cost of such compounded oils is likely to be a little higher than that of straight mineral products, owing to the additional manufacturing operations involved, but this should easily be offset by economies in maintenance costs and oil consumption.

Economic considerations of this nature are not usually so important to the private car owner. However, it seems probable that post-war developments will be in the direction of smaller and more highly rated engines, which will inevitably make more severe demands on the lubricating oil. The extent of the contribution which H.D. oils can make to such developments will have to be determined in due course by comprehensive testing. There is a great deal of truth in the saying that the lubricant is one aspect of engine design, and it is the author's belief that a much closer degree of co-operation between the automobile and petroleum industries would be advantageous to both.

ACKNOWLEDGMENTS

The author wishes to acknowledge permission to quote the results of tests carried out in Esso European Laboratories, and his indebtedness to numerous colleagues for helpful suggestions and criticisms.

REFERENCES.

I ‘ Lubricants : Some Recent Developments,” J. S. S. Brame, Proc, I.A.E., Vol. XXX, page 100.

a “ Crankcase Oils for Heavy-Duty Service,” H. R. Wolf, S.A.E. Journal, April, 1941, Vol. 48 (4), page 128.

3 “ Improvements in Diesel-Engine Lubricating Oils,” Bray, Moore, and lIerrill, S.A4.E. Journal, Jan., 1939, Vol. 44 ( I ) , page 35.

“ Lubrication of Severe-Duty Engines,” McNab, Winning, Baldwin, and Miller, S.;4.E. Journal, Aug., 1941. Vol. 49 (2), page 309.

6 “ Engine Temperatures as Affecting Lubrication and Ring-Sticking,” C. G. A. Rosen, S.A.E. Journal, April, 1937, Vol. 40 (4). page 165.

6 “ Cylinder Lubrication of Small-Bore Diesel Engines,” C. G. A. Rosen, I. Mech. E., General Discussion on Lubrication, Vol. I , page 559.

7 “ Recent Developments in Diesel Lubricating Oils,” G. L. Neely, S.A.E. Journal, Nov., 1939, Vol. 45 (5), page 485.

8 ‘‘ Fuels and Lubricants for U.S. Army Motorized Ground Forces,” G. -4. Round, S.A.E. Journal, July, 1942, Vol. 50 (7), page 269.

9 “ Heavy-Duty Lubricating Oils for Naval Diesel Engines,” E. N. Klem- gard, S.A.E. Journal, July, 1942, Vol. 50 (7), page 284.

APPEND1 X.

LABORATORY ENGINE TEST PROCEDURES USED FOR THE EVllLUATlON OF HEAVY-DUTY OILS IN THE U.S.A.

~.-CATERPILLAR ENGINE TESTS

The following outline is abstracted from the “ Diesel Lubricant Test Manual ” (February, 1943). issued by the Caterpillar Tractor Co., Peoria, Ill.

Test No. I-A : 480-Hour Endurance Test. The engine is a special one-cylinder unit, which can be dismantled in twenty

minutes for inspection of the liner, piston, and rings. The piston is oil-cooled. Bore 54 in., stroke 8 in. The test is conducted to determine the effect of the lubricant on ring-sticking, wear, and accumulation of deposits.

Test Conditions.-After a preliminary six-hour run-in, the engine is operated continuously for 480 hours a t 19.8 b.h.p. (75 lb. per sq. in. b.m.e.p.) at 1,000 r.p.m.

Cooling water temperature ..................... 1 7 5 ~ - 1 8 0 ~ F. Oil to bearings .................................... 145~-150” F.

Oil changed after six-hour run-in and thereafter a t end of each I Z O hours’ operation. One U.S. quart of make-up oil added whenever oil level drops I quart below I ‘ full ” level (6 quarts).

Final Inspection.-Condition of liner, piston, and rings recorded and photo- graphed, together with record of quantity and type of sludge on oil filter element. Liner wear is measured and must not exceed 0.001 in. in transverse direction at a position I& in. down from top of cylinder liner.

28 THE ISSTITLTTIOK OF AUTOMOBILE ENGINEERS.

B.h.p. -

Tesl N o . 2 : Accelerated Run-In and High-Load Tesl.

The engine used in test No. I-A is modified by fitting a special piston and cylinder head assembly. The piston is not oil-cooled ; the pre-combustion chamber is designed t o cause severe conditions of heat flow. The test is con- ducted to check the ability of the lubricant to prevent scuffing under con- ditions of accelerated run-in and high load.

Test Conditions.-The test consists of a t least two runs of 3 hours 20 min. each, preceded by a 6-hour run for calibration of the fuel pump rack setting. New liner and piston assembly fitted before first tes t ; if first run passes without scratching of piston rings and liner, second run can be made using same piston, but new rings and liner. Test load is adjusted on basis of fuel supply rate equivalent to a specified heat input, as follows :-

S.A.E. viscosity grade ............ 10 ... 20 ... 30

* Based on higher calorific value of fuel.

Heat input, B.Th.U. per min .*... 2,330 ... 2,460 ... 2,460

An input of 2,460 B.Th.U. per min. corresponds approximately to 15 b.h.p. (63.5 lb. per sq. in. b.m.e.p.). The test schedule is as follows :-

R.p.m. Time, min.

Idle 2.0

I 0 f I I0 j- I

I80 f 2

* Governed by fuel rate.

Cooling water temperature ...... S.A.E. 30 or 20 ... 175' F. & 5 S.A.E. 10 ... 140'F. f 5

Inlet air temperature ... goo F. & z ............ Oil temperature .................. ... 140°F. j - 5

Final Inspection.-The liner, piston, and rings are inspected after each run and are required to be free from scratching.

Test No. 3-A : 120-Hour High Tem9erature Oil Stability and Bearing Corrosion Test.

The engiue is a specially equipped four-cylinder, 4) in. by 5f in. unit without oil cooler. The test is conducted to check the stability and bearing corrosion characteristics of the lubricant. At least two new liners and piston assemblies are used for each test and two big ends are fitted with Copper-lead precision type bearing shells.

Test Conditions. --After a preliminary six-hour run-in the engine is loaded to 37 b.h.p. a t 1,400 r.p.m. (67 lb. per sq. in. b.m.e.p.) for 120 hours.

Water outlet temperature ........................ Oil to bearings .......................................

The crankcase oil is topped up once every twenty-four hours. No oil change

zooo F. f z

z I zo F. f z Intake air temperature ........................... 140' F . & 5

is made during the test period.

Fixall nspe.cclion.-'€he liners, pistons, and rings are inspected for scratching, The loss of weight due to corrosion ring-sticking, and lacquer formation.

must not exceed 100 mgm. per whole bearing.

II.-GENER.4L 310TORS 5OO-HOUR TEST.

This method of test is described in a pamphlet issued by the Detroit Diesel Engine Division of General Motors Corp., dated April 3rd, 1940. A production Series 7 1 (two-cycle) engine. with three, four or six cylinders, is used ; fan and generator 2re not fitted. The test is conducted to determine the suitability of oils for service in this particular type of engine under extreme temperature conditions. New pistons, rings, liners, and alloy bearings are used for each test.

Test Conditions.-The preliminary run-in and checking schedule totals 21 hours. The test is run continuously for 500 hours, except for stops a t each 48 hours for inspection of air box, air inlet ports, piston rings, and skirts. This can be done through the air box cover plates.

Speed .................................... ?,ooo r.p.m. Load .................................... 80 5 3- b.1i.p. for 3-cgl.

106 t 3 b.h.p. for 4-cyl. 160 i 4 b.h.p. for 6-cyl.

\.\rater outlet temperature ......... 180" F. & 3 Oil sump temperature ............... 230" F. f I Intake air temprratu re ............ 105" F. f 5

The first 144 hours are run without an oil filter. The oil is not changed during a test. The procedure for sampling and topping-up is closely controlled.

Final Inspertior.-The comprehensive inspection requires details a.nd photographs of piston deposits, piston scuffing, stuck rings, clogging of a.ir intake ports, coking of piston undersides, bea.ring corrosion, filter deposits, etc.

III.-36-HOUR CWEVROLET TEST.

Full details of this have been published as a " Proposed Method of Test for Oxidation Characteristics of Heavy-Duty Crankcase Oils," in the October, 1942, edition of A.S.T.M. Standards on Petroleum Products and Lubricants (published by the American Society for Testing Materials). In this test the lubricant is evaluated in respect of resistance to oxidation, bearing corrosion, and the deposition of intrinsic decomposition products ; it is not a test of detergency.

The enginr is a 6-cyl. Chevrolet of 3$ in. bore and 34 in. stroke. Test Conditions.-After a preliminary %hour run-in the engine is operated

for a total of 36 hours in periods of not less than 8 or more than 16 hours. The engine is stopped for a t least 8 hours between periods.

R.p.m. .......................................... 3.150 5 25 ............................................. J3.h.p. 30 * f

W'ater outlet temperature .................. 200' F. z Oil temperature : S.A.E. 30 ............... 280' F. = 2

S.A.E. I O W or 10 ... 26.5 t 2 .4ir!fuc.l ratio .................................... 14'5 ;I 0.5 to I

4

I.’iaton skirtr .................................... Varn is13 Varnish Sludge

OiI screen ....................................... Sludge Rocker-a.rm cover.. ............. Varnish and sludge Pu3h-rod cover plate ........................ Varnish and slrmdge Crankcase o i l pan.. ............................ Varnish and sludge

’Ihr copper-lead bearing ha1r.w are weighed to determine I- d u c t t r corrosion. Oil samglt-5 are examined to am%% detrrhration through osida tion.

1,aboratory and enginr test3 for H.D. oils asst’ d i a r u m l in the foltowin:

“ Crankcave Oils for Heavy-Duty Scxvice.” H. R. \I’olf. S.A . E . jorr~wcc!.

“ The Testing of Heavy-Duty &lotor Oils,” t-l. C. Jlougey and J . A . Mollcr,

“ The Correlation of Laboratory Oil Bench Tests with Full-Scale Engine

“ Comparison of Laboratory Diesel Engine Tests wlth Service Perform-

l’apers :--

April. 1941, Vol. 48 (4), page 128.

.Y..q.E. J O W W Z d , OCt., 1942. VOl. 50 ( I O ) , page 417-

Tests,” C. W. Georgi, S.il.E. .Jozrrnal, Feb., 1943, Vol. j ,I (2 ) : pa.ge 52.

ance,” R. S. \l’etmiIlcr and R. Hegernan, .$,A .E. Pveprhl . June, 1943.