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The Oil Engine's Del Modern Oil Refinery

9th January 1942, Page 26
9th January 1942
Page 26
Page 27
Page 26, 9th January 1942 — The Oil Engine's Del Modern Oil Refinery
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Which of the following most accurately describes the problem?

, a S. Campbell

INTENSIVE study, development and competition over !the past 15 years have produced the modern transport oil engine. Those three factors in its evolution will continue to prevail; likewise with the petrol engine, which, even at this day, offers scope for still further research.

The fundamentals of the oil engine, although closely similar to the petrol unit, necessitated particular attention to internal stresses, in view of the materially higher pressures encountered. Early units were clumsy and designers erred in favour of a high factor of safety; development of better metals, however, has kept pace with design and the modern oil engine has, in consequence, a reasonable power-weight ratio.

Whilst in gen6ral it is a reasonably efficient unit, the od engine must, like all other mechanical appliances, depend for its continuous operation on a number of factors. Each of these will have a definite influence on the ultimate cost of operation. Bearings, valves, cylinder material, injection equipment, and lubricant filters all affect the service obtained from this prime mover.

Design characteristics and operating conditions govern the economic value of any mechanical appliance, and in particular our subject, the transport oil engine. The oil industry would not wish to reflect on the oil-engine manufacturer; the former is in rather a similar position to the latter, in that progress in oil refining practice in recent years. has enabled the production of improved lubricants.

Engine's influences on Lubricant and Vice Versa To enable one to grasp the significance of oil refining practice in its effect on our transport oil engines one must have, a clear idea of the factors in such units, and their influence, if any, on the lubricating oil. Those influences— each to a varying degree—will affect, to some extent, the result in lubrication.

The effects on operation produced by eight main engine features are as follow: Combustion—varying carbon production; fuel-sprayer design and condition—fuel dilution; piston and ring design —blowby; cylinder cooling—temperature of cylinder walls; sump capacity—oil cooling effect; oil-pump size—pressure at bearings; big-end bearings—oil throw to cylinders; ventilation of engine—temperature of sump.

With regard to combustion, normal oil-engine design determines that on a varying load, on a road vehicle, combustion is incomplete over the phase of speed variation, due to -the governor's inability to calibrate fuel accurately enough, in relation to the varying load and speed. This incoMpIete combustion produces unburnt carbon particles, . some of which will pass the pistchis, with consequent thickening of the oil.

Dilution of Oil Not a Serious Factor Fuel dilution, associated with fuel-sprayer design and condition, whilst measurable and depending on the condition of the sprayers in the main, is by no means a serious factor in to-day's engines. Whilst abnormal dilution will affect the character of the oil, regular attention to the sprayers will preclude this possibility. Fuel dilution, in short, generally is a negligible factor in oil-engine operation.

Due to inefficient pistons and rings, blowby, although not prevalent in new engines, is of particular significance in its effect on lubricant, Hot gases, embracing, in sonic cases, chemical impurities of the fuel, exert, an oxidizing influence on the lubricant. Increasing oxidation of the cpil is the main factor in determining sticking of piston rings and valves, and the formation of sludge in an engine. Oxidation of the lubricant is influenced also by the normal contact between the oil, when hot, and air, Cylinder cooling and the actual temperature of The cylinder walls, at the various points, will naturally determine the useful body of a lubricant at these points. For that reason, this feature will influence not only the consumption of lubricant, but also the actual lubrication of the pistons and cylinder walls. Sump capacity governing, as it does, the ratio between the quantity of oil in circulation and the quantity in the sump, will affect the temperature of oil under normal operating conditions. As oils oxidize more readily at higher temperatures, it is obviously desirable to maintain a reasonable temperature of the sump, by its design. Sump temperatures should be within 15 degrees F. either, way of 140 degrees F. Higher temperatures tend not only to oxidation, but also to further thinning.

Fewssure at the bearings, apart from the specific body of the oil, is governed by the size and speed of the oil pump. That feature should also take into consideration an efficient suction filter. This demands particular care in design. Whilst retaining solid particles, it should present a surface which is not readily plugged. For that reason it should have ample vertical height, so ensuring a clear surface even though the lower extremities are restricted.

Whilst the most highly stressed part of the engine, bigend bearings must also fulfil a function in the lubrication of the cylinder walls. Side clearance must be sufficient, unless special holes are provided in the shoulder of the rod, to ensure an adequate " bleed " of oil to the cylinder walls, even at the "starting body" of the lubricant.

Ventilation is important in its effec‘ on sump temperatures, and in ensuring a reasonable scavenging of gases from the base, gases which, under poor fuel conditions, may contain sulphur.

Carbon and, Temperature] ' Vitally important For the main consideration, then, those influences can now be resolved into two main factors, carbon and temperature. Fuel dilution and purely mechanical matters can be disposed of, the former as of little importance in the oil engine and the latter as beyond the scope of this article. We have, therefore, to accept those two influences carbon and temperature as being vitally important, and to understand their particular relation, both to an engine and to oil refining practice.

Carbon must be considered in two distinct senses.

Flea, carbon from combustion, or correctly, incomplete combustion of the fuel, is produced in a finely divided state, in the form of soot, some of which passei the pistons into the base and lubricating system. That factor is entirely removed from the carbon-producing propensities of the lubricant itself, and they bear no relation one to the other.

Combustion carbon is a result of the prevailing engine combustion characteristics, and its presence in the lubricating oil in volumetric proportion should be used as a guide in determining crankcase drainage. Better lubricants, which resist oxidation, usually retain this finely divided carbon in suspension, and on draining, a clean engine will result. Poor lubricants, however, which oxidize readily are found to produce sludge deposits, formed of the oxidized oil and carbon.

In the case of an oil which has become oxidized in service, it is invariably found, on analysing it, that a lower carbon figure is obtained than with a high-quality oil in service, the reason being that the balance of the combustion carbon, in the case of the poor oil, is left in the engine in the form of sludge deposits. Combustion carbon, therefore, in relation to oil refining, whilst primarily a result of engine combustion characteristics, is governed in its effect by oil quality.

Secondly, the carbon-producing propensities of the lubricant itself bear a definite relation to the degree of refining reached, and to the stability of the lubricant in service. Stability is a measure of the proportion of stable hydro-carbons present in the lubricant. Stable hydrocarbons are formed of a higher proportion of hydrogen than carbon and in consequence are less likely to deposit carbon. The degree of refining reached, in eliminating unstable hydro-carbons, will, therefore, determine not only the general stability of that lubricant, but also its carbonforming tendency.

With regard to temperature, the influence of this factor on a lubricant is two-fold; there are its effect on the body, or viscosity, and its tendency to accelerate oxidation of the lubricant. Influence on viscosity is purely physical, pending the oil being oxidized. Mineral lubricating oils do not permanently change their body in service, except by the addition of foreign matter (fuel) or continued subjection to temperatures above normal. For that reason they will regain their initial standard viscosity, on. cooling to standard temperature, so long as they are not oxidized.

However, the viscosity temperature characteristic of a particular lubricant will determine its behaviour over a range of operating temperatures. It is obviously desirable that the body or viscosity shall vary as little as possible over a range of temperatures. A " flatter curve," as a better viscosity temperature characteristic is known, can be a result of improved refining practice.

The second influence, but the more important, which temperature exerts on a lubricant, is that of oxidation, whereby, depending on the stability of the particular lubricant, it will absorb oxygen. Poor lubricants absorb oxygen or become oxidized more readily than superior oils. Increasing oxidation of the lubricant in an engine is mainly responsible for ring sticking, "varnished pistons," and in most cases sludge deposits and permanent thickening.

Removal of Unstable

Hydro-carbons Desirable

The "resistance to-oxidation " which las lubricant may possess is in direct proportion to the stable and unstable hydra-carbons present. Unstable material, which can be removed by a modern refining process, will more readily absorb oxygen. It is, therefore, desirable that a refining process be utilized which will remove the maximum proportion of unstable hydro-carbons from a lubricant.

Crude oil or crude petroleum is found in many parts of the world and the oils in the various places differ widely in character and in their suitability for specific uses. One must understand that in every sample of crude oil there is a large number of different hydro-carbon compounds, varying in proportion according to the " crude."

Whilst one crude may disclose a predominance of good petrol hydro-carbons, the lubricants which can be obtained from it may be of poor character, also it is possible that asphalt products obtained may show desirable qualifies. Certain crudes produce little if any asphalt, and again, others produce little if any paraffin wax. Until recent years the oil refining industry accepted those facts and utilized the then known processes of refining. After distillation of the various products from a particular crude, petrol, paraffin and light, medium and heavy lubricating oils, each received a washing treatment to remove suspended impurities carried over in distillation. After washing, the resulting products were ready for the market or blending.

There was no refining process known, over a period of many years, which could effect a change in the chemical character of a particular lubricant as it came from the crude. In other words, a, lubricAt from a particular source had cgitain characteristics goverad by its inherent chemical composition, which could not be changed.

Although a lubricant, as produced, contained 15 per cent. of the best lubricating oil, it was not possible to separate that portion from the 85 per cent. of relatively inferior material, and the mass remained 100 per cent, relatively poor lubricant from a particular crude.

The object, 'therefore, is to eliminate from a lubricant, which may contain a relatively small percentage of desirable material, the undesirable portion, and this until recent years was not possible. We have established that for modern engine lubrication an oil must maintain its body to the best possible degree over a range of temperatures, and that at the same time it must be as stable as possible in service, resisting oxidizing influences of heat and air. We have further established those features as a measure of the highest possible proportion of stable (or saturated) hydrocarbons present. Modern oil refining practice is designed to remove, by chemical means, the undesirable, material from a lubricant, so producing a finished product which is infinitely more stable in service.

The stable hydro-carbona which go to make up the major portion of a good-quality lubricating oil are called stable because each contains a full complement of hydrogen, and is, in consequence, known as a saturated hydro-carbon. It has been established in recent years that certain chemical agents have an absorptive affinity for the saturated hydrocarbons, and that those various agents will, under favourable conditions of time and temperature, absorb the saturated material present in a lubricating oil. Certain other agents will at the same time absorb the unsaturated material and on settling, a clear separation takes place.

011 Separation by Solvent Relining Process It is then possible to remove the ails from their respective agents, so producing two lubricating oils, the one of high quality and the other relatively poor. , Such processes are generally krfown as solvent refining, and the process can be made extremely effective.

As indicated above, the factors of time and temperature influence the result so much so, indeed, that the economic feature may not allow of carrying the process, whichever it may be, to the ideal standard.

Lubricants produced by an efficient solvent process resist oxidizing influences better than ordinary-process oils; they minimize sludge deposits and lessen piston-ring and valve sticking. Varnish-like deposits, too, are less likely.

Clearly, the most important quality which an oil-engine lubricant should possess is that of resistance to oxidation, and in view of the importance of that feature in operation the oil industry, through vision, has taken the matter even farther. The finest lubricating oil made by the latest process is not good enough. Still more to retard oxidation, a chemical additive, or inhibitor, may possibly be added.

Beneficial Results from Oxidation Inhibitors Developments during the past two years have proved that certain chemicals, when added to lubricating oils in minute quantity, have the faculty of inhibiting oxidation. As a result oil engines show no sludge deposits, even after very extended mileages. Thus there is a higher factor of safety, as there is a smaller chance of oilways being choked.

There have been many complications during the development of satisfactory inhibitor-treated oils; one or more inhibitors, Whilst retarding oxidation, bad a corrosive effect on certain bearing alloys. Another excellent inhibitor could be added only to a relatively poor lubricant as it was unstable in better oils. As research proceeded, however, anti-oxidants were developed which are most efficient in function, non-corrosive with any bearing metals, and capable of being stabilized in the best types of oil. This latest development in oil-engine lubrication, whilst of considerable value to the operator, must not be regarded lightly. Inhibitor-treated lubricants must be stable in service and certain inhibitors are not stable in certain oils.

To-day, the transport oil engine, with lubricants " tailored" for the job, can look forward to a future of even greater successes than it enjoys at the present.

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