May, 1926
IiVDLiSTRlAL A S D ESGI,YEERlSG CHEMISTRY
I n Figure 14 the difference in heating effect for the ironon-iron bearing caused by increasing the load is very pronounced, although fluid film lubrication prevailed a t both loads. Likewise there is only a slightly lower temperature rise for the iron-on-magnesium bearing than for the iron-oniron bearing, although there is a marked difference in the thickness of film developed, all other operating condition. having remained unchanged. Figure 14 shows how the film thickness and temperature rise may be varied for a given bearing by merely changing the oil to one of different viscoqity.
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The investigation has raised many questions, has brought out the fact that fluid film lubrication is materially influenced by surface action, that the friction developed in bearings may accordingly be expected to depend on these surface forces as well as upon viscosity, and that, therefore, a study of bearings according to the four general types discussed in the first section of this paper is highly desirable. I t is expected that different bearings will give different frictional results. A more complete mathematical analysis of the experimental data is reserved for a future paper.
Diesel Engine Lubrication By P. L. Scott ‘ 0 SCOTT ST., C l i r C A G O , I L L .
The demands of oil engines, especially those of high power output, make necessary a wide range of pressures, speeds, temperatures, and materials, and it is very important that the proper oil is used. O n elaborate installations several kinds of oil are often used. Lubricating difficulties in these engines are usually due to failure of the moving parts to get oil, failure of the oil film due to improper design or improper oil, or dirty oil. Explosions due to oil vapor have occurred not infrequently. The oil requirements for the different parts of the engine and the various mechanisms for feeding oil to these various parts in order to prevent the first two sources of trouble are described. The value of a clean oil is brought out by two illustrations, and several methods for accomplishing this are suggested.
PICTURE of the practical lubrication problems encountered by designers and operators of Diesel type oil engines may be of assistance to the cheinists and oil refiners on whose continuing cooperation we must rely for their solution. The demands of the oil engine for steady running at full load are much more severe than on the automobile, and yet it is demanded that it use much less oil. Few automobiles will run 1000 horsepower hours per gallon of oil, assuming proper drainage of crank-case oil. Even the poorest oil engine must do as well as this and most concerns will guarantee 2000 horsepower hours per gallon and some as high as 4000 and 5000 horsepower hours. The true Diesel antl all engines of high power output, with brake mean effective pressures from 60 to 90 pounds per q u a r e inch require the utmost efficiency in lubrication. Engines using brake mean effectives of 30 or 40 pounds per square inch can use simpler and cheaper systems. The large double-acting two-stroke cycle engine (Figure 1) presents the most serious problem as far as cylinder, piston, and stuffing box are concerned because of the relatively large heat flow. Four-stroke cycle engines add the complication of an elaborate valve gear, though even when double acting they do not have such seyere heat conditions.
A
General Lubricating Requirements in an Oil Engine
I n an oil engine the following four lubricating demands are to be met: (1) high rubbing velocity with moderate presures, (2) low rubbing velocity with high pressures, (3) low pressures with intense heat, and (4)miscellaneous relatirely small parts lightly loaded as a rule, but requiring a complexity of feeds.
Also the materials moving one upon the other differ widely. This wide range of pressures, speeds, temperatures, and materials would seem to require a dozen different oils, which is obviously out of the question. On the more elaborate installations three kinds of oil are used-cylinder oil, crank-case oil, and a light oil for some of the smaller parts. As the requirements decrease two kinds, and even one, suffice. Too much >tress cannot be laid on the selection of the proper oil. Common Lubricating Difficulties
Three things may happen as the result of an error in engine operation, design, or selection of oil-the moving parts may fail to get oil; clean oil may break its film too readily; the oil: may be dirty. The best correction for the first trouble is strict and adequate supervision. The second is due to selection of an iniproper oil and can be corrected by careful study of condition.. The film may fail because the oil will not stand high rubbingvelocity, a high pressure, or a high temperature. The designer must keep within known limits in stressing the parts. Fouling may be caused by dirt in the air, by carbon from the cylinder, by too rapid breaking down of the oil itself, by saponification or emulsion from admixture of water, or by leakage of cylinder oil into the crank case. Dirty oil means rapid wear, burnt bearings, scored cylinders, and often clogging of passage3 so that no oil reaches a given part. The remedy is merely a matter of finding why the oil gets dirty, stopping the cause if possible, and in any event cleaning the oil regularly. Often in overhauling an engine inspection of the oil passages i> neglected, with the result that a piece of waste, or sludge. clogs, and a big repair bill results. Another cause of serious trouble is explosion of oil vapor in the crank case of a two-stroke cycle crank-case scavenging engine and even in the scavenging belt of pump-scavenged engine. Such explosions have resulted fatally. The writer has had two engines run away on vapor-charged air from the scavenging belt where oil had been allowed to collect. Both cases require that lubrication be sufficient, not in excess. Figure 2 shows a typical crank-case scavenging engine, antl the method of careful lubrication from a mechanical oiler to a “banjo-ring” in quantities just sufficient to meet the need of the crank pin, and also from this oiler through the cylinder wall into a slot in the piston in quantities just sufficient for the wrist pin. Such engines and the scavenging belt of pumpscavenged engines should be so arranged that they can be drained regularly to remove accumulations. Figure 3 shows a modification of a piston-scavenging engine, where the problem is not so difficult as with crank-case
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n
Figure 1-Cross
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drilled into the inside passage in the shaft to the crank pin. This system usually includes the wrist pin in the circuit through a hollow connecting rod. The wrist pin has relatively low rubbing velocity but high pressure and is fed, as mentioned, from the hollow shaft system or by taking a little excess from the crank pin furnished by other methods, by scraping some oil from the cylinder wall plus splash (practically obsolete), or by having a special lead from a mechanical oiler feeding through the cylinder wall into a groove cut in the piston and from thence to the pin bearing. The piston and cylinder lubrication present some of the most trying problems. The pressures are light-ring and normal wall pressure-but a large portion of the cylinder wall over which the piston passes is swept by flame every stroke in twostroke cycle and every other stroke in four cycle and this flame starts out with a temperature of at least 2000’ F. Under such conditions the problem of keeping friction down and keeping rings free to seal properly is exceedingly difficult. Although small bores are not difficult and although the cylinder-wall area increases only as the bore so that the heat conditions might not a t first be considered serious for large bores, they do become much more serious as size increases, as is witnessed by both increased lubricating and cooling difficulties. The amount of heat liberated increases as the square of the diameter and the greatly increased heat effect is probably due t o radiant dissipation of heat, for this has been shown experimentally to be a very important factor in heat loss from the burning charge to the walls. Increased convection may also have something to do with it, for the intensity of the reaction is undoubtedly more severe in the charge when chance of loss of heat by conduction is cut in the ratios of the proportions 1 : ato 1 : 2 2 . The more severe the service the more the need for a separate oil to lubricate the cylinder and it is undesirable to let this oil mix with the crank-case oil. Low-duty engines generally use
Section through Cylinder of Double-Acting Two-Stroke Cycle Engine
scavenging, owing to the use of a cross-head, piston rod, and separate chamber under the piston for building up scavenging pressure. This division has a n additional advantage that cylinder oil does not foul crank-case oil, if two oils are used. Many large cross-head engines pass the piston rod through a stuffing box in order to prevent such contamination, the space under the piston being left open. Oil Requirements for Various Parts of Engine
The requirements of the different principal parts of the engine vary according to the class of lubrication required. The main bearings and crank pins are essentially the same except that the method of feeding oil to them must be different. They are in the class of moderate pressure and rather high rubbing velocity. Oil may be fed to the main bearings by pressure through a header and a hole to the bearing. It may be carried up on a ring loosely riding on the bearing and lying in a groove cut in the cap but large enough to dip in a reservoir below the bearing. It may be fed by a positive mechanical oiler. Oil may be fed to the crank pin through a “banjo-ring” from a mechanical oiler, the shaft being drilled through the cheek of the crank and the pin; by pressure through a shaft hollow the entire length, first to each main bearing and then through a circumferential groove in the bearing, and through a hole
Figure 2-Crank-Case
Scavenging Engine Showing Lubrication System
the same oil for both cylinders and bearings. If special oil is used for the cylinder, it is always fed by a mechanical oiler direct to one or more points near outer dead-center position. K i t h ports near the base of the cylinder the problem of preventing oil from being blown out by exhaust or carried away by incoming scavenging air is introduced. If the leads are below the ports oil is wiped off on the ports and lost. If the leads are above the ports and oil is fed during exhaust
May, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
and scavenging, much is carried away by gas or the air. So the timed mechanical oiler has come into use, which deposits oil only when the piston is in the proper position. Even when the same oil is used for both cylinder and bearings the mechanical oil is almost universal. There is one wellknown engine, however, which still depends on splash and fog in the crank case and operates well. Such gearings as are loaded do not present a serious problem, for there is always room for proper design and lubrication can usually be taken care of from the crank case. The upper gears of a cam-shaft drive, however, require a special feed. Chains are sometimes used for driving cam shaffs and they either run in a bath of oil a t the bottom and carry sufficient to the top or else special flood feed is furnished. The miscellaneous lightly loaded partscam s h a f t s , r o c k e r arms, auxiliary drives, etc.--often f u r n i s h an especially trying problem, not because of their demands on the lubricant but to get any kind of lubricant to them without m a k i n g t h e engine too complicated and a t considerable exp e n s e . Often cam shafts and as many auxiliaries as possible are placed inside the crank case and lubricated by the s p l a s h and fog. This makes inspection and repair difficult. When outside, oil baths placed under cams and such Figure 3-Transverse Section of PistonScavenging Engine Showing Arrange- other parts as possible m e n t of Scavengihg Air and Pistons are common. AuxilDirectly Oil Cooled iaries are often fed by overflows from the crank case, though positioned outside. Sometimes leads from the mechanical oiler, wick feeds, cups, and hand lubrication are used. One auxiliary, however, which was a source of considerable worry to the builder and operator and still is a costly piece of mechanism, though it gives the best efficiencies and reliability in the largest engines and dominates that field exclusively, is the air compressor, t o provide air a t from 800 to 1200 pounds for injection of fuel. Higher pressures have been achieved in other fields but they were never reliable enough for the requirements of an ocean-going power plant. The Diesel itself will burn almost any oil as a fuel and with compressions half as high in the compressor; even with three or four stage and with intercoolers the lubrication problem for reliable service is probably one of the most difficult. The writer has seen a two-stage compressor without an intercooler "get away" with 800 pounds final pressure but he never remained around that corner of the engine very long. Work has been done on soapy lubricants and emulsions but no use seems to have been made of such compounds. The oil for this service must be very carefully chosen and fed in just the right amounts if an explosion is to be avoided and the sticking of valves in the higher stages and carbon deposit in the piping eliminated. If the compressor does not function so reliably as the engine, especially on a ship, a separately driven spare must be carried and such compressors are not cheap.
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Means of Supplying Oil
There are numerous ways of properly supplying oil, among them being the earlier schemes of bearing lubrication by means of a banjo-ring or by a loose ring in the main bearing, and the splash system. These methods are still used in some very successful low-powered engines, as illustrated in Figure 2, but none of them will meet the exacting demands of engines having high unit power outputs. For heavier demands pressure feed is coming more and more into the fore, though direct feed from a mechanical oiler shares almost equal honors. I n pressure feed a gear pump or a big plunger pump supplies an excess of oil to a header, which feeds all the main bearings and thence through grooves and a drilled crank shaft, to Lhe crank pins and often the wrist pins. Sometimes this system is even arranged to supply cooling oil for the pistons. Additional advantages of excess feed are keeping down bearing temperatures and flushing dirt out a t a rapid rate. Figure 3 shows a good example of pressure feed, even though on a rather low-powered engine. With pressure feed the mechanical oiler is generally used for cylinder lubrication.
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Figure 4-Mechanical
Oiler with Timed Introduction of Oil
Gravity feed, with a tank placed some distance above the engine, drawing from the sump and furnishing to t h e tank, permits better settling of impurities but can be used only where very low pressures are intended. Some concerns use both systems on slightly different models of engines. There is another form of gravity feed in which the tank delivers to sight-feed glasses. There is some criticism of this type because of low pressure on small pipes and still smaller orifices, but one of our most important manufacturers has used this system for many years without serious trouble. The mechanical oiler is very widely used and the type in which introduction of oil can be timed has been mentioned. These oilers are a series of small plungers delivering a small, measured quantity of oil regularly to the desired points. They are positive in action on delivery though not on suction,
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I S D U S T R I A L A G D ESGI,VEERISG CHEVISTRY
and are very widely used for supplying oil to practically every point in a Diesel engine. At some important points they are used almost exclusively. Figure 4 shows a type of oiler which delivers oil to the piston a t the proper time. To aid in keeping an air-free lubricating pipe to the pistons and preventing gas blow back, several types of check valves have been developed for the cylinder wall inlet. It must be remembered that feed from a mechanical oiler must not fail even for a short time. These forms are the more important ones used to supply main bearings, crank and wrist pins, and cylinders. For lighter drives the bath is common, in which some receptacle. to be filled from time to time by hand or by an overflow system from the crank case, keeps a t least a part of the mechanism running in oil, ample oil being splashed and carried to the rest. Cup and wick oilers are common and often advantageously employed and hand oiling is resorted to in some cases. Oil Grooves
The subject of oil grooves has an important bearing on the success of a lubricating system and is a designer’s problem which can only be mentioned here. Diagonal grooves must be used with care, even though they distribute oil quickly. The position of holes in the crank pin and grooves in the bearing must be placed with regard to load conditions. I n the two-stroke cycle pressure is always in the same direction, while in the four-stroke cycle bearing pressures usually reverse on the exhaust stroke. Oil Cleaning
The question of cleaning lubricating oil is probably one of the most important features of good lubrication and long life of the engine. The causes of fouling of the oil have already been mentioned. Some installations must throw away their oil every 3 or 4 months when unpurified. With capacities for average power plant installations of from 100 to 500 or 600 gallons this is very serious. A number of cleaning methods are used-filtering, settling, chemical sludging, and centrifugal separation.
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The m.lue of clean oil is best brought out by the following analysis of sludge taken from an engine in a plant of one of the well-known manufacturers of Diesel engines, which was foriiied a t the rate of one pint per 20 hours for one 400 horsepon-er engine : Bearing temperature Amorphous carbon Paraffin Heavy oil binder Water and volatile oils Silicates Loss
l i 0 ’ F. Per cent 92.2 0.7 5.15 1.21 0.35 0.38
The air mas clean and there was no water seepage. On the ferry “Poughkeepsie” without cleaning it was found necessary to take up the connecting rods at the rate of 0.012 inch every 2 weeks. With clean oil the amount taken up was 0.001 inch on seven rods and 0.003 inch on seventeen in 5 months. Taking up rods and keeping bearings properly close i+ expensive, more so than the extra cost of correct oil and keeping it clean. One oil pumping station has used the same oil for three years after installing proper cleaning facilities. Conclusion
The successful lubrication of any Diesel engine depends upon four factors: Efficient supplying of the proper lubri-
cant, moderation in bearing loads, adequate oil purifying syatein, and thorough supervision during operation. The first three of these factors constitute problems for the engine designer and the manufacturer of lubricants. Obviously the last rests entirely in the hands of the user of the engine. The Diesel engine is still developing and many problems connected with the selection, application, and care of its lubricants yet remain to be solved. It is hoped that the oil companies will take an active part in this development. Diesel engines are more expensive than steam engines and require greater care, but their much higher efficiency is the reason for their rapid development. It is only through cooperation that their development can advance with the greatest posbible rapidity.
A Problem in Diesel Engine Lubrication By Fred Norton and Ralph R. Matthews ROXANAPETROLECM
In the early days of the development of Diesel engines the matter of efficient lubrication was not given the attention it should have had. I t was soon discovered, however, that this neglect was causing considerable trouble to the users, as for this reason the engine often could not be operated under full load successfully for any length of time. This paper considers such a condition as found in one plant and the steps taken to correct it.
HE plant under consideration is a central power plant using five Diesel engines, each unit rated 523 horsepower direct connected to alternating generators having a total connected peak load of 1875 horsepower with a standby steam plant for emergency use, the steam plant being kept ready for instant use at all times. The steam plant was necessary because this particular plant had encountered difficulty in keeping its units in continuous operation. The Diesel engines used in this plant were of the 2-stroke cycle, land type, and were operated a t 300 revolutions per minute. This type of engine is very difficult to lubricate as all moving parts are a t all times under pressure.
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At the end of the first 6 months of operating these five units the time lost due to all mechanical troubles had exceeded by far a figure which was considered necessary in good operation. The average running time was 91.71 per cent of the available running time, and it was later found that 87.53 per cent of all lost time was due to faulty lubrication. Analyzing the conditions in the plant a t this time, the following interesting facts were noted: cooling system properly designed and in excellent working condition; mechanical condition of the equipment very good; engines not overloaded; engine room well ventilated; proper grade of fuel being used; engines running a t rated speed. I n fact, all things pertaining to good operating conditions seemed to be in the very best of shape. The actual working conditions of each unit were then investigated. The room temperature was 73” F.; cooling water temperature leaving the jackets, 179” F. A water softener was in use and examination of jackets proved that there was no trouble from mud or scale, indicating rather conclusively that the troubles were not caused by low heat
