GOOD ADVICE ON ENGINE LUBRICANTS
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rtrol technicians discuss operators' problems with particular reference to oil litives and review some lubrication problems in relation to engine design P. A. C. Brockington, AMI Medi E
ANY technically minded fleet managers are in a quandary regarding the amount of knowledge of engine lubricatechnology they should acquire and whether increasing knowledge of the subject could assist them to operate vehicles more efficiently or more economically. In a typical the manager bases his policy on strict adherence to the nmendations of the engine maker by employing one of the I grades, by changing engine oil at the stipulated mileage t more frequent intervals and by paying scrupulous attention Iter-element renewal and to cleanliness generally. He thus ins a highly satisfactory engine life in terms of bore and ing wear.
is manager may consider that a little knowledge would . dangerous thing in that it would encourage him to make • experiments, and in this attitude he has the wholehearted ort of vehicle manufacturers and of the leading oil comm. For example, Castrol emphasizes that any deviation this policy is likely to reduce margins of mechanical safety to be uneconomic on a long-term basis.
astrol qualifies this general advice, however, with the obitions that special recommendations may be applicable to et operated in "controlled conditions" and that beneficial Its may be obtainable by using a type of oil, not listed by Tlanufacturer, if running conditions are continuously severe. company also agrees that it is admissible to employ one e of oil for the vehicles of a large fleet (if approved by the :ompany) to simplify stocking and sump replenishing, and because variables are so diverse, periodic laboratory checks of ail condition may, in some cases, enable the interval between ;hanges to be increased without detriment to engine life.
an explanation of these observations, it is appropriate to don the main additives used in varying proportions in l-engine lubricants, and of related interest a brief account is of engine design factors cited by Castro( that influence the cc of additives or create special problems. Castrol agrees some knowledge of oil technology is beneficial if it en'ages the operator to seek the best advice in the event of ication troubles or dissuades him from accepting bad advice. ecause the level of detergency is the most important proy of diesel-engine oils, it is normally indicated by the code c of the oil. A Series 2 oil has a higher detergency than a plement 1 and a Series 3 a higher level than a Series 2. For e time, however, only the Series 3 has been produced to a Caterpillar-approved specification, the other two codes being used to indicate a level of quality, based on the original specifications. This allows considerable latitude and has been the cause of some confusion.
According to Castrol, detergency should be renamed hightemperature dispersancy, its function being to prevent the formation of carbon on rings and in ring grooves and on the underside of the piston by holding the carbon in suspension. In general, the proportion of detergent required increases with specific output and piston temperature, a Series 3 oil being normally considered obligatory for turbocharged engines.
In common with other additives, a detergent is relatively expensive and "wears out" with continued use because it is subject to depletion, the rate depending, to some extent, on operating conditions and the condition of the engine components. Continuous operation with faulty injectors can, for example, hasten depletion of the detergent and other additives and cause premature onset of ring sticking and scuffing.
Here then, is one valid argument against chancing engine failure by prolonging the interval between oil changes or by using an oil of doubtful additive quality. But, as mentioned, it may be possible to vary the normal recommendations, after consultation with the oil company, if the conditions of fleet operation are "controlled" to the extent that the routes are known in detail, that drivers are disciplined, that there is little variation in speed and loading and that the vehicles are examined by qualified mechanics at regular intervals. Given that the additives level is adequate, sustained high loading should not produce hot sludge.
A Series 3 oil contains about 12 per cent of detergent, whilst the level of Supplement 1 is around 3 per cent.
Law-temperature dispersant
In the opinion of Castrol, a dispersant additive should be known as a low-temperature dispersant, because it is designed to prevent the formation of cold sludge (which is a mixture of carbon and water) in the valve chest, sump and so on. Cold sludge in quantity causes blockage of the oil passages and filters, and if it is not dispersed, the amount produced by engine overcooling over a relatively short period can starve all the working parts of the engine.
Cases are known in which engine life has been reduced to a fifth of the normal life despite (or because of) low average loading, a concomitant factor in these cases being the nonevaporation of acidic products of combustion.
A dispersant property can, therefore, be an all-important characteristic of an oil, but is of little value if, in the main, the engine operates at higher temperatures. A cooling system that promotes rapid warming up and provides efficient control of temperatures minimizes the need for a low-temperature dispersant, but this would not apply to a vehicle making very frequent stops (such as a bakery van) because insufficient heat is produced by the engine to provide an adequate working temperature even if a wax-type thermostat is allied to radiator shutters and a thermostatically controlled fan.
In practice, both types of dispersant are combined in one additive, but if low-temperature dispersancy were not required, a detergent-only additive could be employed in its place. This would appreciably reduce the cost of the oil.
An anti-oxidant zinc-based additive, known as ZDDP, prevents deterioration of the oil in high-temperature conditions by controlling acidity, which promotes lacquering and is particularly detrimental to copper-lead bearings. The additive also acts with the detergent to assist in the prevention of piston-ring sticking and is an essential element of most diesel oils. Engine makers are warned, however, that the zinc attacks certain types of phosphor bronze employed for little-end bushes. Correct selection of the bush material eliminates the possibility of attack.
An anti-wear additive is also regarded as an essential ingredient of all oils, because it reduces scuffing of steel on steel and thus increases the life of gears, cams, valve stems, rocker pads and so on. H ZDDP also provides this property, possibly in combination with other anti-wear additives.
The development of multi-grade oils was made possible by the availability of viscosity-index-improving additives, the use of which gives a greater viscosity improvement at high temperatures than at low temperatures, and enables an oil to be produced which provides a low viscosity for starting combined with an adequate viscosity at high temperatures. Whilst the main advantage of' using a multi-grade -lubricant is that it affords easier starting than an otherwise suitable mono-grade oil, its favourable stability in changing temperature conditions may be regarded as a bonus advantage.
In multi-grade oils, the v.i. improving additive usually provides sufficient pour-point depressant properties but, in straight grade oils, a separate pour-point depressant additive may be needed1 This controls the size of the crystals in the oil structure and reduces the temperature at which the oil remains fluid, a property that is also of particular value in low-temperature operations.
Whilst aeration conditions that cause very severe foam are not likely to be produced in typical diesel engines, foaming is promoted by a highly-detergent additive and an anti-foam additive is essential in some applications. In practice, the need for such an additive is often decided by the characteristics of the base oil, some base oils being more prone to foaming than others.
Last on the list, an anti-corrosion additive can be combined with a detergent or dispersant additive, the importance of which depends upon the extent that conditions tend to promote corrosion. In total the additive content may amount to about 20 per cent of the bulk of the oil.
Even this simplified appraisal of additive characteristics may confuse the thinking of some fleet managers who are considering a means of reducing their engine-oil bill by using a cheaper lubricant that promises the same advantages as a branded product or is cheaper because some additives can be eliminated from the specification by virtue of their non-applicability to the particular operating conditions of the fleet. At least they confirm -the plea that any operator who is considering the cost-saving offered by an oil that is not listed by the vehicle manufacturer should hasten very slowly unless he is in close liaison with oil company technicians.
If the size of the fleet and the type of operations warrant analysis of engine-condition factors in relation to oil properties, and the undertaking is backed by laboratory facilities, then and then only is an exercise in additive discrimination justified.
Damage by glycol
Employing a single grade of oil for a mixed fleet is permissible given that its quality is adequate for the engine type having the highest specific output, but the, warning is given that a highly detergent oil is not necessarily suitable for stop-start vehicles the engines of which should be operated on a highly dispersant oil. Used in engines for which a low-detergency oil is suitable, a highly detergent lubricant may enable the oil-change period to be extended.
The most pressing problem facing the laboratory staff relates to the damage that can be caused by glycol contamination of the lubricant in the case of wet-liner engines. Although some types of liner seal eliminate such contamination for an indefinite period, it is a persistent problem of some magnitude. And pending the day that an efficient seal is a feature of all wet-liner diesels, it will remain a problem unless an additive can be developed that combats its effects or an acceptable anti-freeze solution is produced that has no ill effects. Alcohol is acceptable as the basis for an anti-freeze but has other disadvantages. Glycol contamination can create severe sludging by promo a fall-out of soot and carbon, and it can also cause oxidatio the form of lacquer and consequent ring sticking. Current rels research work at the Castrol laboratories gives promise tit solution will be found to the sludging problem, but the lacque problem may be insurmountable.
Adequate sump capacity is a design feature of first imp ance in the Castrol view, and it is pointed out that doubling capacity more than doubles the amount of contaminant that be controlled by the additives and therefore the allowable inte between oil changes. Of potential interest to many operat Castrol states that the life of an oil can be extended by suits additive treatment, and this could be particularly advantage if the extra cost were more than offset by operational gain Whilst applying an oil cooler to a standard type of eni designed to operate without a cooler is considered permissi its advantages would, it is claimed, be at best marginal. The vs of a water-oil heat-exchanger in terms of more rapid warming is also discounted on the score that the rate of temperature ris to a large extent determined by heat input from the pistons.
hi the company's opinion, other design features of an eni that should receive particular attention include inlet-vs assemblies to reduce or eliminate leakage of oil past the st( and good ventilation is also cited as a feature of considers importance, notably when the engine operates at reduced ti peratures for long periods. Piping the valve chest or sumr the intake, which is common practice in the case of cart petrol engines, is strongly recommended, and the use of a drau tube is advocated.
In conclusion, Castrol emphasizes that additive treatment c not be employed to transform a poor-quality base oil into acceptable lubricant. It can only be used to correct (in so cases) a specific deficiency.