Aviation Engines Part 14

Second--The lubricant must not coagulate or gum; must not injure the parts to which it is applied, either by chemical action or by producing injurious deposits, and it should not evaporate readily.

Third--The character of the work will demand that the oil should not vaporize when heated or thicken to such a point that it will not flow readily when cold.

Fourth--The oil must be free from acid, alkalies, animal or vegetable fillers, or other injurious agencies.

Fifth--It must be carefully selected for the work required and should be a good conductor of heat.

DERIVATION OF LUBRICANTS

The first oils which were used for lubricating machinery were obtained from animal and vegetable sources, though at the present time most unguents are of mineral derivation. Lubricants may exist as fluids, semifluids, or solids. The viscosity will vary from light spindle or dynamo oils, which have but little more body than kerosene, to the heaviest greases and tallows. The most common solid employed as a lubricant is graphite, sometimes termed "plumbago" or "black lead." This substance is of mineral derivation.

The disadvantage of oils of organic origin, such as those obtained from animal fats or vegetable substances, is that they will absorb oxygen from the atmosphere, which causes them to thicken or become rancid.

Such oils have a very poor cold test, as they solidify at comparatively high temperatures, and their flas.h.i.+ng point is so low that they cannot be used at points where much heat exists. In most animal oils various acids are present in greater or less quant.i.ties, and for this reason they are not well adapted for lubricating metallic surfaces which may be raised high enough in temperature to cause decomposition of the oils.

Lubricants derived from the crude petroleum are called "Oleonaphthas"

and they are a product of the process of refining petroleum through which gasoline and kerosene are obtained. They are of lower cost than vegetable or animal oil, and as they are of non-organic origin, they do not become rancid or gummy by constant exposure to the air, and they will have no corrosive action on metals because they contain no deleterious substances in chemical composition. By the process of fractional distillation mineral oils of all grades can be obtained. They have a lower cold and higher flash test and there is not the liability of spontaneous combustion that exists with animal oils.

The organic oils are derived from fatty substances, which are present in the bodies of all animals and in some portions of plants. The general method of extracting oil from animal bodies is by a rendering process, which consists of applying sufficient heat to liquefy the oil and then separating it from the tissue with which it is combined by compression.

The only oil which is used to any extent in gas-engine lubrication that is not of mineral derivation is castor oil. This substance has been used on high-speed racing automobile engines and on airplane power plants. It is obtained from the seeds of the castor plant, which contain a large percentage of oil.

Among the solid substances which may be used for lubricating purposes may be mentioned tallow, which is obtained from the fat of animals, and graphite and soapstone, which are of mineral derivation. Tallow is never used at points where it will be exposed to much heat, though it is often employed as a filler for greases used in transmission gearing of autos. Graphite is sometimes mixed with oil and applied to cylinder lubrication, though it is most often used in connection with greases in the landing gear parts and for coating wires and cables of the airplane.

Graphite is not affected by heat, cold, acids, or alkalies, and has a strong attraction for metal surfaces. It mixes readily with oils and greases and increases their efficiency in many applications. It is sometimes used where it would not be possible to use other lubricants because of extremes of temperature.

The oils used for cylinder lubrication are obtained almost exclusively from crude petroleum derived from American wells. Special care must be taken in the selection of crude material, as every variety will not yield oil of the proper quality to be used as a cylinder lubricant. The crude petroleum is distilled as rapidly as possible with fire heat to vaporize off the naphthas and the burning oils. After these vapors have been given off superheated steam is provided to a.s.sist in distilling.

When enough of the light elements have been eliminated the residue is drawn off, pa.s.sed through a strainer to free it from grit and earthy matters, and is afterwards cooled to separate the wax from it. This is the dark cylinder oil and is the grade usually used for steam-engine cylinders.

PROPERTIES OF CYLINDER OILS

The oil that is to be used in the gasoline engine must be of high quality, and for that reason the best grades are distilled in a vacuum that the light distillates may be separated at much lower temperatures than ordinary conditions of distilling permit. If the degree of heat is not high the product is not so apt to decompose and deposit carbon. If it is desired to remove the color of the oil which is caused by free carbon and other impurities it can be accomplished by filtering the oil through charcoal. The greater the number of times the oil is filtered, the lighter it will become in color. The best cylinder oils have flash points usually in excess of 500 degrees F., and while they have a high degree of viscosity at 100 degrees F. they become more fluid as the temperature increases.

The lubricating oils obtained by refining crude petroleum may be divided into three cla.s.ses:

First--The natural oils of great body which are prepared for use by allowing the crude material to settle in tanks at high temperature and from which the impurities are removed by natural filtration. These oils are given the necessary body and are free from the volatile substances they contain by means of superheated steam which provides a source of heat.

Second--Another grade of these natural oils which are filtered again at high temperatures and under pressure through beds of animal charcoal to improve their color.

Third--Pale, limpid oils, obtained by distillation and subsequent chemical treatment from the residuum produced in refining petroleum to obtain the fuel oils.

Authorities agree that any form of mixed oil in which animal and mineral lubricants are combined should never be used in the cylinder of a gas engine as the admixture of the lubricants does not prevent the decomposition of the organic oil into the glycerides and fatty acids peculiar to the fat used. In a gas-engine cylinder the flame tends to produce more or less charring. The deposits of carbon will be much greater with animal oils than with those derived from the petroleum base because the const.i.tuents of a fat or tallow are not of the same volatile character as those which comprise the hydro-carbon oils which will evaporate or volatilize before they char in most instances.

FACTORS INFLUENCING LUBRICATION SYSTEM SELECTION

The suitability of oil for the proper and efficient lubrication of all internal combustion engines is determined chiefly by the following factors:

1. Type of cooling system (operating temperatures).

2. Type of lubricating system (method of applying oil to the moving parts).

3. Rubbing speeds of contact surfaces.

Were the operating temperatures, bearing surface speeds and lubrication systems identical, a single oil could be used in all engines with equal satisfaction. The only change then necessary in viscosity would be that due to climatic conditions. As engines are now designed, only three grades of oil are necessary for the lubrication of all types with the exception of Knight, air-cooled and some engines which run continuously at full load. In the specification of engine lubricants the feature of load carried by the engine should be carefully considered.

_Full Load Engines._

1. Marine.

2. Racing automobile.

3. Aviation.

4. Farm tractor.

5. Some stationary.

_Variable Load Engines._

1. Pleasure automobile.

2. Commercial vehicle.

3. Motor cycle.

4. Some stationary.

Of the forms outlined, the only one we have any immediate concern about is the airplane power plant. The Platt & Washburn Refining Company, who have made a careful study of the lubrication problem as applied to all types of engines, have found a peculiar set of conditions to apply to oiling high-speed constant-duty or "full-load" engines. Modern airplane engines are designed to operate continuously at a fairly uniform high rotative speed and at full load over long periods of time. As a sequence to this heavy duty the operating temperatures are elevated. For the sake of extreme lightness in weight of all parts, very thin alloy steel aluminum or cast iron pistons are fitted and the temperature of the thin piston heads at the center reaches anywhere between 600 and 1,400 Fahr., as in automobile racing engines. Freely exposed to such intense heat hydro-carbon oils are partially "cracked" into light and heavy products or polymerized into solid hydro-carbons. From these facts it follows that only heavy mineral oils of low carbon residue and of the greatest chemical purity and stability should be used to secure good lubrication. In all cases the oil should be sufficiently heavy to a.s.sure the highest horse-power and fuel and oil economy compatible with perfect lubrication, avoiding, at the same time, carbonization and ignition failure. When aluminum pistons are used their superior heat-conducting properties aid materially in reducing the rate of oil destruction.

The extraordinary evolutions described by airplanes in flight make it a matter of vital necessity to operate engines inclined at all angles to the vertical as well as in an upside-down position. To meet this situation lubricating systems have been elaborated so as to deliver an abundance of oil where needed and to eliminate possible flooding of cylinders. This is done by applying a full force feed system, distributing oil under considerable pressure to all working parts.

Discharged through the bearings, the oil drains down to the suction side of a second pump located in the bottom of the base chamber. This pump being of greater capacity than the first prevents the acc.u.mulation of oil in the crank-case, and forces it to a separate oil reservoir-cooler, whence it flows back in rapid circulation to the pump feeding the bearings. With this arrangement positive lubrication is entirely independent of engine position. The lubricating system of the Thomas-Morse aviation engines, which is shown at Fig. 76, is typical of current practice.

[Ill.u.s.tration: Fig. 76.--Pressure Feed Oiling System of Thomas Aviation Engine Includes Oil Cooling Means.]

GNOME TYPE ENGINES USE CASTOR OIL

The construction and operation of rotative radial cylinder engines introduce additional difficulties of lubrication to those already referred to and merit especial attention. Owing to the peculiar alimentation systems of Gnome type engines, atomized gasoline mixed with air is drawn through the hollow stationary crank-shaft directly into the crank-case which it fills on the way to the cylinders. Therein lies the trouble. Hydrocarbon oils are soon dissolved by the gasoline and washed off, leaving the bearing surfaces without adequate protection and exposed to instant wear and destruction. So castor oil is resorted to as an indispensable but unfortunate compromise. Of vegetable origin, it leaves a much more bulky carbon deposit in the explosion chambers than does mineral oil and its great affinity for oxygen causes the formation of voluminous gummy deposit in the crank-case. Engines employing it need to be dismounted and thoroughly sc.r.a.ped out at frequent intervals. It is advisable to use only unblended chemically pure castor oil in rotative engines, first by virtue of its insolubility in gasoline and second because its extra heavy body can resist the high temperature of air-cooled cylinders.

HALL-SCOTT LUBRICATION SYSTEM

[Ill.u.s.tration: Fig. 77.--Diagram of Oiling System, Hall-Scott Type A 125 Horse-Power Engine.]

The oiling system of the Hall-Scott type A-5 125 horse-power engine is clearly shown at Fig. 77. It is completely described in the instruction book issued by the company from which the following extracts are reproduced by permission. Crank-shaft, connecting rods and all other parts within the crank-case and cylinders are lubricated directly or indirectly by a force-feed oiling system. The cylinder walls and wrist pins are lubricated by oil spray thrown from the lower end of connecting rod bearings. This system is used only upon A-5 engines. Upon A-7a and A-5a engines a small tube supplies oil from connecting rod bearing directly upon the wrist pin. The oil is drawn from the strainer located at the lowest portion of the lower crank-case, forced around the main intake manifold oil jacket. From here it is circulated to the main distributing pipe located along the lower left hand side of upper crank-case. The oil is then forced directly to the lower side of crank-shaft, through holes drilled in each main bearing cup. Leakage from these main bearings is caught in scuppers placed upon the cheeks of the crank-shafts furnis.h.i.+ng oil under pressure to the connecting rod bearings. A-7a and A-5a engines have small tubes leading from these bearings which convey the oil under pressure to the wrist pins.

A bi-pa.s.s located at the front end of the distributing oil pipe can be regulated to lessen or raise the pressure. By s.c.r.e.w.i.n.g the valve in, the pressure will raise and more oil will be forced to the bearings. By uns.c.r.e.w.i.n.g, pressure is reduced and less oil is fed. A-7a and A-5a engines have oil relief valves located just off of the main oil pump in the lower crank-case. This regulates the pressure at all times so that in cold weather there will be no danger of bursting oil pipes due to excessive pressure. If it is found the oil pressure is not maintained at a high enough level, inspect this valve. A stronger spring will not allow the oil to bi-pa.s.s so freely, and consequently the pressure will be raised; a weaker spring will bi-pa.s.s more oil and reduce the oil pressure materially. Independent of the above-mentioned system, a small, directly driven rotary oiler feeds oil to the base of each individual cylinder. The supply of oil is furnished by the main oil pump located in the lower crank-case. A small sight-feed regulator is furnished to control the supply of oil from this oiler. This instrument should be placed higher than the auxiliary oil distributor itself to enable the oil to drain by gravity feed to the oiler. If there is no available place with the necessary height in the front seat of plane, connect it directly to the intake L fitting on the oiler in an upright position. It should be regulated with full open throttle to maintain an oil level in the gla.s.s, approximately half way.

An oil pressure gauge is provided. This should be run to the pilot's instrument board. The gauge registers the oil pressure upon the bearings, also determining its circulation. Strict watch should be maintained of this instrument by pilot, and if for any reason its hand should drop to 0 the motor should be immediately stopped and the trouble found before restarting engine. Care should be taken that the oil does not work up into the gauge, as it will prevent the correct gauge registering of oil pressure. The oil pressure will vary according to weather conditions and viscosity of oil used. In normal weather, with the engine properly warmed up, the pressure will register on the oil gauge from 5 to 10 pounds when the engine is turning from 1,275 to 1,300 r. p. m. This does not apply to all aviation engines, however, as the proper pressure advised for the Curtiss OX-2 motor is from 40 to 55 pounds at the gauge.

The oil sump plug is located at the lowest point of the lower crank-case. This is a combination dirt, water and sediment trap. It is easily removed by uns.c.r.e.w.i.n.g. Oil is furnished mechanically to the cam-shaft housing under pressure through a small tube leading from the main distributing pipe at the propeller end of engine directly into the end of cam-shaft housing. The opposite end of this housing is amply relieved to allow the oil to rapidly flow down upon cam-shaft, magneto, pinion-shaft, and crank-shaft gears, after which it returns to lower crank-case. An outside overflow pipe is also provided to carry away the surplus oil.

DRAINING OIL FROM CRANK-CASE

The oil strainer is placed at the lowest point of the lower crank-case.