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    GIFT OF

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    ELEMENTSOF AVIATION ENGINES

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    ELEMENTS OFAVIATIONENGINESBy JOHN B. F. BACON, PH. B.

    Instructor, Engines DepartmentU. S. Schoolof Military Aeronautics

    Berkeley, California

    PAUL ELDER AND COMPANYSAN FRANCISCO M CM XVIII

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    COPYRIGHT, 1918, BY

    fi-W JOHN B. F. BACON

    BERKELEY, CAL.

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    CONTENTSPAGE

    INTRODUCTION / . . VIICHAPTER ITHE AVIATION ENGINE . . 3CHAPTER II

    APPLICATION OF THE BASIC PRINCIPLE . .' . . 7CHAPTER III

    ENGINE SPECIFICATIONS .16CHAPTER IV

    ENGINE PARTS .... . , . .V . 22CHAPTER VCARBURETION ;JY .... ... . . 47

    CHAPTER VIIGNITION. > ,; ,"; ' . .. ^ . , . . 57

    CHAPTER VIILUBRICATION . . . '^ . . ,. . . , 71

    CHAPTER VIIICOOLING . . . . . , . - ^- - 80CHAPTER IXROTARY ENGINES . . . . . . . . . 84

    CHAPTER XTHE LIBERTY MOTOR . .'. 96INDEX . 105

    III]

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    ILLUSTRATIONSFACING PAGE

    THRUST BEARINGS 36DIAGRAM TO ILLUSTRATE THE CURTISS Ox VALVEACTION . 42THE MILLER AVIATION CARBURETOR .... 50A HALF SECTION VIEW OF A ZENITH CARBURETOR . 52DIAGRAMS TO ILLUSTRATETHE LOCATION OF THE CORE

    IN A SHUTTLE TYPE MAGNETO 58WIRING DIAGRAM OF A MAGNETO SYSTEM . . .62DIAGRAM TO ILLUSTRATE THE PRINCIPLE OF REVOLV-

    ING POLES ON THE DIXIE MAGNETO .... 64DIAGRAM TO ILLUSTRATE POSITION OF ROTOR IN THE

    DIXIE MAGNETO WHEN THE CORE is MAGNETIZED 66DIAGRAM TO ILLUSTRATE POSITION OF ROTOR IN THE

    DIXIE MAGNETO WHEN THE CORE is DEMAGNE-TIZED . . . 66

    DIAGRAM OF A BATTERY SYSTEM OF IGNITION WITH ANON VIBRATING COIL . 68GEAR PUMP 76DIAGRAM TO ILLUSTRATE THE OPERATION OF A VANEPUMP .76CENTRIFUGAL PUMP 82DIAGRAM TO ILLUSTRATE THE PRINCIPLE OF AROTARY ENGINE 84

    IVJ

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    INTRODUCTIONHAVING been forcibly impressed with thefact that many of those who take up thestudy of aviation are not familiar with gasolineengines and have little mechanical inclination,it has been the endeavor of the writer to explainin a simple way some of the points that appearto cause beginners the greatest amount of trouble.While it may aid those who are conscientiouslyreviewing the subject, it is far from the purposeof this book to provide a short cut to passingmarks on examination papers.All of the information herein contained hasbeen before the engineering public at one time oranother. Realizing that certain new develop-ments must not appear in print during thiscritical period every precaution has been takento observe strict avoidance

    of revealing confiden-tial information.The writer wishes to express his gratitude tothe members of the Engines Department in theS. M. A. ofBerkeley for their assistance. Specialthanks is due Mr. James Irvine for his sugges-tions which have resulted in many improvements.JOHNB.F.BACON,

    818th Aero Depot Squadron, U. S. A.

    Berkeley, Cal., August, 1918.[VII

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    ELEMENTSOF

    AVIATION ENGINES

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    ELEMENTSOF AVIATION ENGINESCHAPTER ITHE AVIATION ENGINE

    IN TAKING up a new subject it is often bestto fix clearly in mind just what is meant bythe name of the subject, so in beginning a dis-cussion upon aviation engines it seems well tostart with a rough definition of the term avia-tion engine. A simple statement that an in-ternal combustion engine so designed that it iscapable of lifting from the ground and sustain-ing in flight a heavier than air flying machinewill suffice as a definition for our subject. Bythe term internal combustion, engine is com-monly meant simply a gasoline engine, becausein such an engine the power is derived from theforce of an explosion within a cylinder. Thiswill make clear what we mean by our subject.The question at once arises : Why must avia-tion engines be internal combustion engines in-stead of steam engines, and why not propelaeroplanes by aid of electricity? The answer issimply that maximum power and minimum

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    ; ; ; CLEMENTS OF AVIATION ENGINESweight can be best obtained with the internalcombustion engine. In the study of aeronau-tics weight is a tremendous factor, and it is in-teresting to note that not until the gasolineengine had reached its modern developmentwas human flight practical. On account of theunlimited use of gasoline as a motive powerand the increasing interest of technical men inthe problems of aviation, the gasoline enginehas been developed to such a point that it maydeliver 1 H.P. for every 1.8 pounds of itsweight. To a mechanical mind this seems oneof the greatest achievements of the twentiethcentury.

    Since gasoline engines have been used so ex-tensively and with such marked success inautomobiles, the aviation student will at onceinvoluntarily compare the aviation engine withthat in an automobile, and oftentimes he com-pares them wrongly by stating that the avia-tion engine develops a vastly greater speedthan the engine of an automobile is capable ofattaining. This is incorrect and is a poor wayof comparing the two. The main difference isthat of lightness. Aviation engines are of thelightest possible construction and are designedto run continuously at their highest speed.

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    THE AVIATION ENGINESeldom are the frail supporting members forthe engines in a horizontal plane, and often theengine is called upon to do its work while com-pletely inverted. These are conditions that theautomobile engine does not have to meet. Inorder to attain a construction that will fulfillthe requirements imposed upon aviation en-gines, it is natural to expect that some sacri-fice must be made. This accounts for their lowdegree of durability. When we examine theheavy construction of a 400 H.P. marine gaso-line engine and then regard the frail parts of a400 H.P. aviation engine there is not theslightest doubt which engine will continuelonger in its operations. However, since lightconstruction is an absolute necessity, it is use-less to expect much in the way of durability,and as a means of knowing what an aviationengine will stand it is interesting to note thatafter every seventy-five hours of operation theengine should be rebuilt.As a compact and light power plant the avia-tion engine is the highest attainment of me-chanical genius. It has been developed fromthe type that propels the automobiles, andjust as the old types of automobile engines donot resemble in appearance the types used to-

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    ELEMENTS OF AVIATION ENGINESday, so the first aviation engines have littleresemblance to those of the present time. Thedevelopment has been rapid, and it is difficultto predict what will be the effect upon aviationif the rapid strides taken during the past tenyears continue to add to the efficiency and re-liability of the aviation engine during the nextten years to come.

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    CHAPTER IIAPPLICATION OF THE BASIC

    PRINCIPLE

    THEWORKING principle of an aviation en-

    gine is identically the same as that of theordinary gasoline engine. In the middle of thenineteenth century it was satisfactorily proventhat the explosive force of gasoline could beused to actuate a piston, and this has givenrise to the adoption of a new form of motivepower. Since that time gasoline engines havebeen developed along two lines, one beingcalled the two-stroke cycle engine, and theother the four-stroke cycle engine, but since theformer has not been used extensively in aviationwork little attention will be given to it here.A two-stroke cycle engine is one in which anexplosion takes place in the cylinder everytime the crank shaft makes one revolution. Acharge of combustible gas is slightly com-pressed within the crank case by the pistontraveling downward. Near the bottom of thisdownward stroke the piston uncovers a port inthe cylinder wall allowing some of the com-pressed gas to enter the cylinder. Then the

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    ELEMENTS OF AVIATION ENGINESpiston moves upward, closing the port andcompressing the gas. The charge is ignitedwhen the piston is near the end of its upwardstroke, and the result is that the force of theexplosion violently drives the piston down-ward. An exhaust port on the opposite side ofthe cylinder from the intake port is uncoveredas the piston sweeps downward, and the forceof the explosion starts the burnt gas rushingout of the cylinder. The intake port havingalso been uncovered by this time will allow afresh charge to enter. By using a deflector onthe piston head the fresh charge is hinderedfrom rushing straight to the exhaust port andis diverted upward, serving admirably to expelthe remaining burnt gases. Now the piston isready to go upward again, and the same opera-tions are repeated. In this way the pistonmakes two strokes to complete a cycle, henceit is spoken of as the two-stroke cycle engine.Some confusion may be caused by notknowing the exact meaning of the word cycle,so it may be well to insert here a definition. Acomplete series of events occurring in regularsequence and ending so that the same opera-tion can be repeated in the same order is calleda cycle.

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    APPLICATION OF THE BASIC PRINCIPLEThe four-stroke cycle engine has proven the

    more satisfactory of the two types, and sinceit is the one used in connection with aviation,it is very desirable to fully understand it. Thistype differs from the two-stroke cycle in thatit has two distinct mechanically-operatedvalves in the cylinder which, of course, necessi-tate a few more working parts. Instead of thegas being stored and compressed within thecrank case, this engine draws its explosivecharge directly from the carburetor by openingthe inlet valve as the piston goes downwardand making use of the suction thus exerted.The charge is compressed by the reversal of thepiston's motion and the closing of the inletvalve. Near the end of this compression strokethe charge is ignited, resulting in an explosiveforce being exerted on the piston when it isready to go downward again. Near the end ofthis succeeding downward stroke the exhaustvalve is opened permitting the force of the ex-plosion to give the burnt gases their initialoutward impulse. The valve remains openduring the entire upward stroke of the pistonto insure all of the burnt gases being expelled.The clearing out of the cylinder is often re-ferred to as scavaging the cylinder. Generally

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    ELEMENTS OF AVIATION ENGINESthe exhaust valve closes after the piston hasreached its uppermost position. This brings usto the opening of the inlet valve and with thatthe sequence of events is repeated.By the stroke of the piston is meant themovement of the piston in one direction. Itfollows from this that the length of the strokeis the linear distance the piston travels fromits uppermost position to its lowest position orvice versa. The term stroke has come to meansimply the number of inches between top cen-ter and bottom center, thus designating thetwo extreme positions of the piston. To makeclear the four strokes of the piston in a four-stroke cycle engine, the first one in which thepiston goes down and draws in a charge iscalled the intake stroke. The next upwardmotion is the compression stroke. Then comesthe explosion which drives the piston down-ward. This is the power stroke. Finally theexpulsion of the burnt gases is the exhauststroke, and this completes the cycle.

    In aviation engines it is customary to ignitethe charge near the end of the compressionstroke instead of at the beginning of the powerstroke. The speed of the engine justifies this.If ignition were to take place when the piston[10]

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    APPLICATION OF THE BASIC PRINCIPLEwas at top center or a little afterward, theforce of the explosion would be exerted uponthe piston head at such a late time that thepiston could not deliver its maximum impulseto the crank shaft. When the piston is nearingbottom center its effectiveness for transmittingforce is negligible. Consequently by openingthe exhaust valve at the end of the powerstroke instead of at the beginning of the ex-haust stroke, the force of the explosion serves tostart the burnt gases rushing outward withoutlosing power. The exhaust valve is generallyheld open until the beginning of the intakestroke. This aids in scavaging the cylinder asit permits more time for the operation, and thedanger of retaining some of the burnt gasesis avoided since the out-going exhaust willpossess a certain amount of inertia. Differentmakes of engines have different times for open-ing the intake valve. On some there is asmall interval between the closing of theexhaust valve and the opening of the inletvalve, as is the case with the Curtiss OX andthe Hall-Scott. This permits the downwardmotion of the piston to establish somewhat ofa rarefication within the cylinder, so that whenthe inlet valve is opened there will be a ten-

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    ELEMENTS OF AVIATION ENGINESdency for the gas to enter more promptly.The closing of the inlet valve occurs at the be-ginning of the compression stroke. The gaspassing through the manifold will have someinertia which will maintain a flow into thecylinder during the first part of ensuing up-ward stroke. By thus keeping the valve openpast bottom center a larger amount of gas isplaced in the cylinder.The question often arises: Why are not two-stroke cycle engines used for aviation work, onaccount of the decrease in weight due to theless number of working parts, the more fre-quent power impulses, and the need of anengine that will do its best work when runningat top speed? The two-stroke cycle engine ful-fills all the requirements demanded of anaviation engine except for the fact it will notordinarily run satisfactorily at low enoughspeeds to allow the propeller to idle. Since asuccessful aviation engine must be able to runslow enough without stopping to allow theplane to glide, it can be easily seen that thepresent form of two-stroke cycle engine ispoorly suited for aviation work.So far in explaining the different operationsinvolved in a cycle, only one cylinder has been

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    APPLICATION OF THE BASIC PRINCIPLEconsidered. It is advisable to have frequentpower impulses and to avoid vibration asmuch as possible. This is accomplished byusing a number of cylinders which decreasesthe weight of the reciprocating parts.

    Vibration is due to the shifting of the cen-ters of gravity of pistons and connecting rods.In a single cylinder engine of required powerturning at a speed suitable to drive a propeller,the amount of vibration would be prohibitive.The greatest bearing pressure in an engine athigh speeds comes not so much from the ex-plosion, but from the effort of starting andstopping the weight of the piston and connect-ing rod. To decrease this reciprocating weightit is necessary to resort to the basic law ofvolumes and areas. If we make a body half thedimensions of another, it will have but onequarter of the area and only one-eighth of theweight. This can be applied to pistons. Thus apiston can be replaced by four smaller ones halfas large, and the area of the four will equal thatof the larger one. However, these four pistonswill weigh practically one-half as much as theoriginal single piston. This illustrates the wayreciprocating weight is lessened andshows plain-ly the demand for a larger number of cylinders.

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    ELEMENTS OF AVIATION ENGINESThe way the cylinders are arranged serves as

    a means of classifying aviation engines. If thecylinders stay in a fixed position in respect tothe crank shaft, it is spoken of as a fixed cylin-der engine, but if the cylinders revolve aboutthe crank shaft it is called a rotary engine.Various difficulties in construction are encoun-tered when the number of cylinders is increased,so fixed-cylinder engines are not confined tothe vertical style but are often built in a Vform to permit a shorter crank shaft. A pecu-liar style of fixed-cylinder engine is that withan additional row of cylinders between thetwo rows that go to make up the V. This is thedesign of the Sunbeam Engine. Another styleof fixed-cylinder engine is one in which thecylinders radiate from the crank case allowingthe force of all explosions to be exerted uponthe same crank pin. The Anzani engine is ofthis design. The rotary engines have not somany variations. As a means of increasing thenumber of cylinders a second bank of cylindersis often added, which of course necessitatestwo throws on the crank shaft. Rotary enginesare limited to those having one and two banks.In both the fixed-cylinder and the rotary typesthe growing demand for an increased number

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    APPLICATION OF THE BASIC PRINCIPLEof cylinders has resulted in the adoption of en-gines of the designs just referred to for aviationwork.

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    CHAPTER IIIENGINE SPECIFICATIONSA A BASIS of comparing aviation enginescertain specifications are used. It must beremembered that all engines are not called up-on to do the same work, and furthermore thatthey are not all designed by one man or evenby a group of men holding the same views onvarious mechanical problems. This will ac-count for the wide range in specifications. Inorder to become familiar with the points whereengines differ, a few items will be taken up here.The first point to consider is whether theengine has fixed cylinders or is a rotary. If itis a fixed-cylinder engine, the arrangement ofthe cylinders should be noted. Generallyspeaking, rotary engines are used for very fastbut brief flights, while fixed-cylinder enginesserve better for long flights where speed is notso important.The horse-power of an engine is probably thematter of greatest interest. All planes are notof the same size and weight, so there is needfor engines of different power. One horse-power is the power required to lift 33,000

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    ENGINE SPECIFICATIONpounds a distance of one foot in one minute.The horse-power necessary to operate a planeis calculated by multiplying the total air re-sistance of the plane, expressed in pounds, bythe speed in feet per second, then by 60 sec-onds in a minute, and dividing the product by33,000. The actual horse-power that an enginedevelops is spoken of as brake horse-power.It may be found by measuring the torqueexerted by the engine running with a propellerattached. By torque is meant the moment oftangential effort, or to put it more roughly, aforce tending to produce rotation. The torqueis allowed to be exerted upon an arm whichdelivers the force to a platform balance. Bymultiplying the force in pounds by the dis-tance in feet through which it acts in one revo-lution by the R.P.M. and dividing the productby 33,000, the actual horse-power is obtained.The distance through which the force acts isthe circumference of a circle having the powerarm as a radius. This distance will be 6.2832times the arm's length, so if we make the armexactly 5J4 feet long, the distance throughwhich the force acts will be 33 feet. This per-mits us to reduce our fraction to the lowestterms, making the denominator 1,000 instead

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    ELEMENTS OF AVIATION ENGINESof 33,000. The horse-power can then be ob-tained by multiplying the torque expressed inpounds by the R.P.M. and then dividing by1,000, which simply amounts to moving thedecimal point three places to the left.The weight of an engine is of great import-ance, for it determines the engine's fitness. Ashas been said before, aviation work requiresmaximum power for minimum weight. Light-ness is the keynote of the whole engine, so theaviation engine is devoid of all unnecessaryequipment. Self-starters are seldom used onaccount of their weight and mufflers never, onaccount of their weight and resistance also.Aviation engines avoid the use of a fly-wheel,on account of the large number of cylindersand also on account of the steadying effect ofthe propeller. In speaking of the weight of anengine, the weights of tanks and radiators arenot included, nor does oil or water enter intothe engine's weight. By dividing the weightby the horse-power the weight per horse-poweris obtained. This is a very significant figureand is widely used in comparing engines. Themost modern types of aviation engines rangefrom two to three pounds in weight for everyhorse-power developed.[18]

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    ENGINE SPECIFICATIONThe speed of most aviation engines is gener-

    ally about 1,400 R.P.M. being a compromisebetween the most efficient propeller speed andthe most efficient engine speed. An ordinarypropeller will do its best work when turningfrom 900 to 1,000 R.P.M. If it is driven con-siderably faster than that, it will cause whatis known as cavitation, which means that theblades are working in an unfavorable mediumso far as their usefulness is concerned. Thiswill show the undesirability of having pro-pellers turn at speeds which a high-grade auto-mobile motor can easily attain. Consequentlysince the speed of an engine is normallygreater than 900 or 1,000 R.P.M. it is advis-able to compromise by driving the propeller alittle faster than it ought to turn and runningthe engine at a reduced speed. The efficiencyof an engine, which roughly speaking is theproportion between the energy received aswork and the energy supplied as fuel, can beincreased if the engine is permitted to runfaster than 1,400 R.P.M. Since the propellerspeed has limitations, engines running athigher speeds must have a gear reduction re-garding the propeller. This is ordinarily ac-complished by driving a jack shaft carrying

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    ELEMENTS OF AVIATION ENGINESthe propeller by spur gears one above theother. Sometimes internal gears are used andthen the propeller will turn in the same direc-tion that the engine turns.The disadvantages of a geared propeller arethat more weight is added and a slight amountof power is consumed by the gears.The direction of rotation of an engine shouldbe considered. When standing directly in front

    of the propeller and noting that it turns coun-ter-clockwise, the engine is spoken of as havinga normal rotation. Should the propeller turnclockwise the engine has an anti-normal rota-tion. One reason for building both normal andanti-normal engines is that in case a plane hastwo engines as is sometimes the case withbombing planes, then normal and anti-normalengines are used to equalize the torque effect.The number of cylinders and their bore,meaning the internal diameter, is an important

    item. The stroke of the piston which has beenmentioned before is often spoken of in connec-tion with the bore. Various engines use differ-ent strokes with different bores, but for thesake of illustration, the stroke averages aboutone and one-quarter times the bore. If boththe bore and the stroke are large, there will be[20]

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    ENGINE SPECIFICATIONa tendency to develop heat on the compressionstroke providing the compression chamber issmall. The total piston displacement is calcu-lated by squaring half the bore, multiplying by3.1416, then multiplying by the stroke, andfinally by the number of cylinders. The resultwill be in cubic inches. The horse-power percubic inch of piston displacement, which isobtained by dividing the horse-power by thedisplacement, is a figure of much interest.Efficient motors will give from .17 to .27 H.P.for each cubic inch of displacement.

    Ignition, carburetion, and cooling enter intothe specifications of an engine, but since separ-ate chapters are devoted to them later, theyneed not be dealt with here.

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    CHAPTER IVENGINE PARTSr I lo TAKE up all the parts of an engine andJL describe them fully would be a big under-taking, and might not prove interesting tothose beginning this subject. Consequentlyonly the principal parts will be included anddealt with in a very brief manner.The cylinders of a gasoline engine are vari-ously constructed. They may be made as in-dividual units, or several may be cast in block.The advantage of the former method of con-struction is that more complete jacketing canbe accomplished, while rigidity is the advan-tage of the latter type. In case an engine hadfour cylinders cast in block and one becamedamaged, then the three good ones would haveto be discarded in order to replace the onecylinder that caused the trouble. This wasteis not encountered when each cylinder is aseparate replaceable unit. However, from thestandpoint of compactness the block construc-tion is by far the more preferable. Individualcylinders are made of cast iron, semi-steel, andsteel. When cast in block their material is[22]

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    ENGINE PARTSusually aluminum alloy. A peculiar form ofconstruction is that used in the Curtiss cylin-ders, where each cylinder is of cast iron with aband of some non-corrosive metal such asmonel metal to act as a water jacket. Thecylinders of the Hispano-Suiza are unusual indesign, being steel thimbles that screw into analuminum alloy water jacket designed to holdfour cylinders. The Sturtevant cylinders areinteresting in that they are of aluminum alloycast in pairs with a steel liner shrunk in to actas a cylinder wall.The location of the valves determines the

    shape of the cylinder head. If the valves oper-ate in extensions on opposite sides of the com-bustion chamber the cylinder is said to have aT head, since its shape is that of a T. This con-struction necessitates two independent camshafts besides being rather bulky, so is of littleimportance from the standpoint of aviationwork. If a cylinder has only one extension inwhich a valve or valves work, its shape willresemble that of a Greek letter gamma or sim-ply an inverted L. It is therefore called an Lhead. When a cylinder has no extensions oneither side but has two valves located in itshead, it is called an I head cylinder. This type

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    ELEMENTS OF AVIATION ENGINESof cylinder is the most popular for aviationengines, because it does away with an irregu-larly-shaped combustion chamber. In the caseof a T or L head cylinder the space above thevalves may be regarded as a pocket, and veryoften it is difficult to scavage these pockets.The placing of both valves in the head permitsthe combustion chamber to be made slightlyspherical in order to reduce the surface areaand lessen the amount of heat carried away atthe time when an explosion takes place.Some cylinders are made so that the headmay be removed without disturbing its base.This is known as a detachable head and hasthe advantage of providing an easy means ofremoving carbon and working upon the valves.However, a little more material is required inthis construction, and it brings into accountcompression leaks and also water leaks sincethe cylinder heads must be jacketed.The crank case is generally divided into twoparts; the top section serving as a base for thecylinders and the bottom section carrying asupply of oil. The sump is that part whichholds the oil. As a rule crank cases are alumi-num castings, and in case the motor is a V typegreat care is taken to strengthen the upper sec-[24]

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    ENGINE PARTStion by means of partitions or webs to preventthe strain exerted by explosions on oppositebanks from cracking the upper section. Thecrank shaft bearings are generally held in theupper section. Sometimes the lower halves ofthe bearings are held in partitions in the lowersection of the crank case, as in the Hispano-Suiza. The difficulty in this construction isthat the lower section can not be removedwithout disturbing the crank shaft. As ameans of retaining the oil in the sump whenthe engine is momentarily inverted, splashpans are placed in the lower section. They donot retain all of the oil, but aid in reducing theamount that would otherwise rush into thecavities of the pistons. The vents on crankcases are called breathers. These maintainatmospheric pressure in the crank case eventhough compression leaks are present.That part of the engine which is drivendownward within the cylinder by the force ofan explosion is the piston. Pistons have re-ceived as much if not more attention by de-signers than any other part of the engine, andthe result has been to secure satisfactory oper-ation at high speeds and at high temperatures.The material used in piston construction is

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    ELEMENTS OF AVIATION ENGINESgenerally aluminum alloy, although cast ironis sometimes used. The use of aluminum aspiston material serves to lessen vibration andincrease the speed, lessening the weight of re-ciprocating parts. Another reason for its useis the rapidity with which it conducts heat.The piston head may be either convex, flat, orconcave, and all of these shapes are in use atpresent. The convex or domehead brings intoaccount the ability of an arch to withstandstrain. Greater strength for a given amount ofmaterial is obtained by using a convex head.The flat head is the common type. By having aflat surface less area of the piston is exposed toabsorb heat. This results in a slightly cooler pis-ton, which is a big advantage, as it is impossibleto cool the piston in the same way that the cyl-inder is cooled. The concave head has been ex-tensively used on rotary engines because itpermits a shorter cylinder and thus lessens thecentrifugal force. This shape of piston head al-lows the combustion chamber to assume a spher-ical form. By the bosses are meant the twoprojections within the piston that hold the wristpin, and it follows that the upper end of the con-necting rod must fit between the two bosses.The lower portion of a piston is termed the skirt.[26]

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    ENGINE PARTSDue to more material at the head and also

    on account of the top surface coming in directcontact with the heat of each explosion, it willbe seen that the upper part of the piston willexpand more than the skirt. This necessitatesallowing more clearance between the cylinderwall and the piston at its head than at itsskirt. Some idea of this difference can be hadby pointing out that a five-inch piston may becleared .020 inch at the skirt and as much as.027 at the head.To prevent compression and the force of an

    explosion from passing down between the pis-ton and the cylinder wall, piston rings areused. These fit in grooves in the piston andbear upon the cylinder wall. Besides prevent-ing leaks these rings prevent much oil fromgetting upon the piston head where it wouldresult in the formation of carbon. The ringsare made of cast iron, and each piston gener-ally requires two or three of them. When thetwo ends of a ring come together squarelythe ring is said to have a butt joint. When theends meet each other diagonally it is called adiagonal joint. Likewise if the ends are madeso that they meet each other in the form of astep, it is called a step or lap joint. Obviously

    [271

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    ELEMENTS OF AVIATION ENGINESa ring having a step joint will offer more resis-tance to the passage of gas than those havingbutt or diagonal joints. A precaution to takein placing a piston in a cylinder is to make surethe joints in the rings are at equal intervalsaround the circumference of the piston.The wrist pin is made of steel, usually hol-low and case hardened, and is used to form amovable joint between the piston and the con-necting rod. Its length depends upon thediameter of the piston. There are three gen-eral ways of retaining the pin in its right posi-tion. It may be held rigidly in the connectingrod by means of a clamp or a set screw whichresults in the pin turning in the piston bossesas the connecting rod moves back and forth.This method is used in the Curtiss OX. An-other way is to pin the wrist pin in the bossesso that it is securely held in a fixed position.The connecting rod will then turn on the wristpin which means the bearing will be in theconnecting rod. Such a construction necessi-tates a bearing at both ends of the connectingrod. The Hall-Scott ASA has its wrist pinsheld rigidly in the piston bosses. The floatingmethod such as used in the Sturtevant allowsthe pin to move either in the bosses or in the[28]

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    ENGINE PARTSconnecting rod. Brass ends on the pin or capsover the ends of the bosses protect the cylinderwalls.The connecting rod is a steel arm used to

    convert the reciprocating motion of the pistoninto the revolving motion of the crank shaft.The majority of connecting rods have a crosssection resembling an I, although H and tubu-lar rods are not uncommon. In cases wherethe wrist pin is held in the bosses the upper endof the connecting rod is supplied with a bronzebushing that acts as a bearing surface. Thelower bearing, in which works the crank pin,is given more attention. Babbitt is employedas a bearing metal and is generally backed bybronze to take its place should enough heat bedeveloped to fuse the babbitt. Lower connect-ing rod bearings are made in two pieces topermit the crank pin being put in position.Between the two halves of thebearingareplacedstrips of metal called shims. These are .001,.002 and .005 inch thick and as the bearingwears away these can be removed insuring abetter fit. In a V motor, when the vertical axisof opposite cylinders are in the same verticalplane, the connecting rods of opposite cylin-ders will meet the crank pin at the same point.

    [29]

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    ELEMENTS OF AVIATION ENGINESThis will necessitate the forked or straddledconstruction in which one rod works betweenthe fork of another. It makes rather a com-plicated and costly bearing, but it is a favoritedesign and is being used extensively. TheHispano-Suiza has this type of lower connect-ing rod bearings. Another and simpler way isto have the cylinders "staggered" by placingthe cylinders on one bank a little ahead or be-hind those on the opposite bank, therebyallowing two lower connecting rod bearings towork side by side on one crank pin. A wiseprecaution to take in assembling a motor is tomake sure the lower connecting rod bearing issuch that it allows the wrist pin to be abso-lutely parallel with the crank pin. If it isotherwise the piston will not work freely withinthe cylinder.The crank shaft is the driving shaft of theengine to which the power impulses are trans-mitted by the pistons and connecting rods.It is needless to say that this is the most im-portant moving part of an engine, and for thisreason it is made with great precision fromselected pieces of high-grade steel by dropforging and subsequent turning. The principalparts of a crank shaft are the main bearings,[30]

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    ENGINE PARTSthe cheeks, and the pins on which the connect-ing rods work. Two cheeks and a pin arespoken of as a throw. Thus the number ofcylinders govern the number of throws, andalso upon the number of cylinders depends thenumber of degrees between the throws. In avertical motor, if the cylinders are cast separ-ately, there is generally a main bearing be-tween every two throws. Where the cylindersare cast in block there is not so much spacebetween the pistons which often means a de-crease in the number of main bearings. Thecrank shaft used in a V motor is identically thesame as one used in a vertical motor havinghalf the number of cylinders. Two connectingrods are fitted to each throw, and if the cylin-ders are cast separately a main bearing isplaced between every two throws.For the main bearings of a crank shaft thelining is babbitt usually backed by bronze verysimilar to the lower connecting rod bearings.Babbitt, which essentially consists of lead andantimony, is used as bearing metal because of itsanti-friction properties, its sufficient hardness,and the ease with which it can be replaced.Lead alone possesses considerable anti-frictionproperties, but is impracticable on account

    [31]

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    ELEMENTS OF AVIATION ENGINESof its softness. The addition of some anti-mony will materially harden the lead with-out lessening its anti-friction properties. Theuse of babbitt also permits the liner to bescraped to secure an exact bearing surface.By coating the journal with Prussian blue, thehigh spots can be detected on the liner, andthese can be successively removed by scraping.To have evenly placed power impulses thethrows on a crank shaft must be placed at cer-tain angles with one another. In any four-stroke cycle motor all cylinders will fire oncein two revolutions of the crank shaft or oncein 720 degrees. In a four-cylinder motor therewould be four explosions in 720 degrees, and toget equal spacing the power impulses wouldhave to come one-fourth of 720 or 180 degreesapart. This will explain why the angle be-tween two throws that receive impulses, onedirectly after the other, is 180 degrees for afour-cylinder crank shaft. The throws in a six-cylinder crank shaft are 120 degrees apart,since there will be six power impulses in 720degrees.

    In determining the order in which the cylin-ders will deliver their power impulses to thecrank shaft, it is the custom to fire them so[32]

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    ENGINE PARTSthat the vibrations set up by one explosion willserve to counteract the vibrations caused by aprevious explosion. To accomplish this an ex-plosion at one end of the shaft is followed byan explosion near the other end.Here we come to what is known as the firing

    order, which simply means the order in whichthe cylinders do their work. In order to discussthe firing order it is first necessary to explainhow the cylinders are numbered. In Americanpractice cylinder No. 1 is always that one atthe pilot's end of the engine, and the number-ing is in regular order toward the propeller.In V engines No. 1 is the first cylinder on theleft bank viewed from the pilot's cock pit.Some engines have the left bank numberedLI, L2, L3, L4, and the right bank Rl, R2,R3, R4. Others number the left bank 1, 2, 3, 4in the regular way and then start with thecylinder nearest the propeller on the rightbank calling it 1' followed by 2', 3' and 4'going toward the pilot's end. The Curtiss OXhas the peculiar way of starting with No. 1 onthe left bank nearest the cock pit and desig-nating as No. 2 the opposite cylinder on theright bank. No. 3 is the next one on the leftbank, and in this way the odd numbers are on

    [33]

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    ELEMENTS OF AVIATION ENGINESthe left bank and the even numbers on theright bank.To return to firing orders, it is best to startwith a four-cylinder engine. The cylinders insuch an engine can be fired in a 1, 2, 4, 3 orderor in a 1, 3, 4, 2 order. From this it can be seenthat throws 1 and 2 are 180 degrees apart and3 and 4 are also that distance apart. Likewiseit is evident that with a four-cylinder crankshaft, pistons 1 and 4 travel together and also2 and 3 are coming up or going down together.The two usual ways for a six-cylinder engine tofire are 1, 5, 3, 6, 2, 4, and 1, 4, 2, 6, 3, 5. Herethe throws are 120 degrees apart, and the pis-tons that travel together are 1 and 6, 2 and 5,and 3 and 4. V engines use the basic four-cylinder and six-cylinder firing orders to firethe two banks. The explosions will alternatebetween the two banks starting with the cylin-der at the pilot's end on the left block and fol-lowed by the forward cylinder on the rightblock. Explosions will occur on the left bankaccording to either one of the two firing orders,and those on the right bank in like mannerexcept that on the right bank we will be work-ing from the propeller end toward the pilot'send. Where an engine is numbered LI, L2,[34]

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    ENGINE PARTSetc., and Rl, R2, etc., its firing order may be:

    LI, R6, L5, R2, L3, R4, L6, Rl, L2, R5,L4, R3.Where the left bank is numbered 1, 2, 3,etc., and the right bank 1', 2', etc., in the oppo-site direction, the firing order may be:

    11' 55' 3 V 66' 22' 44'1,1 , 3,0 , O,O , U,U , Z,,Z, , 't,'* .The Curtiss OX with its peculiar cylindernumbering already referred to has the follow-ing distinctive firing order for normal rota-tion:

    1, 2, 3, 4, 7, 8, 5, 6.For an anti-normal engine it would be:2,1,4,3,8,7,6,5.

    or to start the cycle with an explosion in cylin-der No. 1 it would be:

    1,4,3,8,7,6,5,2.In order that the thrust exerted by thepropeller upon the crank shaft may be trans-

    mitted to the crank case and then to thefuselage, a thrust bearing is placed upon thecrank shaft very near the propeller hub.Thrust bearings are generally ball bearings hav-ing either one or tworows of balls and very oftenthey are designed to take a load directed atright angles towards the center of the shaft aswell as taking care of the thrust. In an engine

    (351

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    ELEMENTS OF AVIATION ENGINESlike the Curtiss OX, where the crank shaft ex-tends several inches between the last maincrank-shaft bearing and the propeller hub, thethrust bearing will be the last point where theshaft may be supported. Now if a shaft isallowed to revolve without a radial bearing atits end vibration will result and this must beavoided. Consequently on the Curtiss OX andall other engines having a nose, the thrustbearing must be capable of taking both radialand thrust loads. Some thrust bearings havinga single row of balls will only take thrust in onedirection. This makes it necessary to reversethe bearings if an engine is transferred from atractor plane to a pusher plane or vice versa.The cam shaft is that part of the engine hav-ing irregularities upon its surface that openand close the valves at the proper time. Theirregularities are called cams and are usuallyaccurately shaped projections upon the shaftfor imparting the necessary motion to a valve.Cam shafts are always made of high-gradesteel and the cams are forged integral with theshaft. When gasoline engines were first beingdeveloped it was the practice to have as a camshaft a plain piece of shafting with the camkeyed or pinned to it in the right position.[36]

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    \

    CURTI&S OX

    r

    THRUST BEARINGS

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    ENGINE PARTSThis resulted in an endless amount of cam-shaft trouble as the cam would often comeloose causing a valve to operate at the wrongtime or possibly not operate at all. Now thatthe cams and shaft are made in one piece, thisdifficulty is no longer encountered.The location of the cam shaft has been amatter of much discussion. The old practicewas to have it located at the base of the cylin-

    ders as this was the most convenient positionwhere T and L head cylinders were used.Since I head cylinders are more favorablylooked upon, the overhead position of the camshaft is being used more and more, as it doesaway with the numerous push rods used tooperate the overhead valves. However, a camshaft so placed necessitates a pillar shaft andbevel gears to drive it. V engines that use thebase position of the cam shaft usually have thecylinders placed in a staggered position. Thismakes it much easier for one cam shaft locatedat the bottom of the V to operate the valveson both banks of cylinders. When a V engineuses the overhead position two cam shafts arenecessary.

    In all four-stroke cycle engines the cam shaftalways travels at half the crank-shaft speed.

    [37]

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    ELEMENTS OF AVIATION ENGINESThe reason is that it takes two revolutions ofthe crank shaft to complete a cycle and thatthe individual valves must open but once dur-ing a cycle. For instance, one cylinder will fireonce during two revolutions of the crank shaft.In order that it may function, an inlet valvemust open once to let a new charge in. Thenthe intake valve will open once during tworevolutions of the crank shaft which meansthat the cam operating that valve must re-volve once to two revolutions of the crankshaft.Upon the valves depend to a great extent

    the success of the engine, for aviation enginesseem particularly susceptible to valve trouble.The two general types of valves for gasolinemotors are the poppet or mushroom type andthe sliding sleeve type. The former is univer-sally used for aviation work largely because thelatter type brings into account a little moreweight. A poppet valve consists primarily ofa disk with a bevelled edge and a stem joiningthe disk at its center. The bevelled edge isusually at an angle of 45 degrees with the planeof the disk, although other angles are not un-common. By having the valves open inwardlythe force of an explosion or the force of a com-[38]

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    ENGINE PARTSpression stroke will tend to push the valvefirmly against the bevelled portion of the cy-linder referred to as the seat, and in this waythe greater the force within the cylinder themore tightly will the valve be held in its closedposition. It is safe to say that valves in avia-tion motors should be as large as possible. Theuse of I head cylinders restricts the size of thevalves, so it is often impossible to put in avalve of a satisfactory diameter. T and Lhead cylinders permit the use of larger valveson account of the extension to the combustionchamber. The object of using large valves issimply to charge and scavage a cylinder morerapidly.When we consider under what conditionsthe valves must do their work, it will be seenwhy a great deal of attention has been paid tothe materials of which they are made. Theexhaust valve opens on the power strokeallowing the highly-heated gases to escapearound it. Particles of carbon will invariablybe carried outward and some will at times becaught between the valve and its seat at theinstant it closes. The valve having been highlyheated on account of its direct contact withthe explosion, will be somewhat soft, and when

    [39]

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    ELEMENTS OF AVIATION ENGINESit snaps against the particle of carbon a smallindentation will be caused. This is called pit-ting, and to lessen it to a great extent it hasnow become the custom to make the exhaustvalve of tungsten steel. Of the two valves theinlet is less subject to pitting, since the incom-ing gas tends to cool it, and furthermore lesscarbon collects on its seat. Nickel steel is thematerial sometimes used for inlet valves.

    In order to make a valve seat more firm afteran engine has run considerably, and to preventleaking, it is necessary to grind a new surfaceboth on the valve and its seat in the cylinder.The abrasive is called grinding compound. Itis applied as a very thin paste to either thevalve or its seat, whereupon the valve is in-serted in its usual position and vigorouslyturned back and forth. If care is taken to fre-quently unseat the valve the compound willbe kept evenly distributed over the grindingsurface, and there will be little danger of cut-ting rings in either the valve or its seat. Prus-sian blue can be used to determine the fit.Frequently a valve may become warped or ashoulder may develop on the seat. A reamercan then be used to good advantage, but itmust be followed by grinding. Sometimes the[40]

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    ENGINE PARTSguide in which the valve stem works will be-come worn, making it useless to grind a valveuntil a bushing or new guide has been supplied.The springs used to close the valves deserveattention. On some engines double-coil springsare used, and then in case a spring breaks therewill still be one to close the valve. Occasion-ally the exhaust valve springs will be a littleheavier than those on the inlet valves. This isto allow for any decrease in strength caused bythe heat from the exhaust valve and also toprevent any possibility of the exhaust valvebeing pulled down on the intake stroke.The ways in which the force of a revolvingcam is brought to bear upon a valve stem arenumerous and interesting. With an L headcylinder where the cam shaft runs directlyunder the valve, it is a simple matter to have afollower riding the cam and a tappet rod be-tween the follower and the valve stem. Whenthe cam comes up the valve will be pushed up.As the cam goes on the spring will bring thevalve back to its seat. This is simplicity itself,but the use of I head cylinders makes neces-sary other means of transmitting the camthrust.The usual way of operating valves in the

    [41]

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    ELEMENTS OF AVIATION ENGINEScylinder head by a cam shaft located at thebase of the cylinders is to use push rods con-nected to rocker arms working on fulcrumsattached to the tops of the cylinders. As thepush rod is forced up, one end of the rockerarm goes up and the other end goes down,pushing inwardly on the end of the valve stem.This is the way the valves are operated on theCurtiss VX and the Sturtevant. The peculiar-ity in the Sturtevant is that the side thrustimparted to tappet is avoided by having apivoted arm ride the cam and on this arm reststhe tappet. Worn guides are reduced to aminimum in this way.The inlet valve operation on the Curtiss OXis interesting inasmuch as it brings into ac-count a new form of cam, and also because thevalve is pulled open instead of receiving adirect thrust. Upon the cam shaft for eachinlet valve are two cams that would be com-pletely circular but for a flat space on each.The space between these two cams is takenup by the exhaust valve cam, which is theordinary type of cam. Upon the two roundcams rests a tappet to which is attached a rod,or strictly speaking a tube, having a coil springheld about it at the top by a strap and at the[42]

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    DIAGRAM TO ILLUSTRATE THE CURTISS OX VALVE ACTION

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    ELEMENTS OF AVIATION ENGINESgage so that the clearance is that given by themanufacturer. Valve clearance varies between.010 and .030 inch, and its purpose is to allowfor expansion of the stem and also to obtain avery accurate adjustment of a particular valve.It is the custom to time on the opening of anintake valve or on the closing of an exhaustvalve. After the clearance has been adjustedfor one valve and after the cam shaft gearshave been unmeshed, the engine is turned inthe direction it is intended to rotate until thepiston in the selected cylinder is exactly in theright position for the inlet valve to open or forthe exhaust valve to close. Then the cam shaftis revolved by hand in the direction it is in-tended to turn until the inlet valve is juststarting to open or the exhaust valve has justclosed as the case may be. The next step is tomesh the cam shaft gears. Sometimes theteeth come directly together and when that isthe case it is necessary to "split a tooth."Different engines have ways of doing this, butit generally amounts to providing some meansof revolving the gear wheel upon the camshaft the distance of half a tooth, which isenough to allow the teeth to be meshed with-out disturbing the cam shaft.[44]

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    ENGINE PARTSAll of the other valves on the engine are

    timed by adjusting the clearance for each one.A piston is placed in the right position for avalve to open or close and if it does not func-tion correctly the clearance is changed until itopens or closes on time. From this it can beseen that if the cam shaft is out of time allvalves will be affected, while if the clearance isset wrong it will affect only the valve havingthe wrong clearance. In other words, the camshaft affects every valve, while clearanceaffects the individual valve. In cases wheretoo much clearance is given above a valve stemthe valve will open late and close early. Whenthere is too little clearance, the valve will openearly and close late.When spark plugs are placed in the cylinderhead it is possible to determine the position ofa piston at any time by removing a plug andinserting a steel scale. If valves are timed bydetermining a piston's position in this manner,it is spoken of as the linear method of timing.Inaccuracies may result from the use of thismethod where the pistons have convex headsand where particles of carbon are deposited onthe piston heads. A more accurate method isto make use of a timing disk attached to the

    [45]

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    ELEMENTS OF AVIATION ENGINEScrank shaft near the propeller hub. The cir-cumference of this disk is divided into degreeswith the points for opening and closing of eachvalve plainly marked upon it. If the disk isplaced accurately upon the crank shaft it fur-nishes an excellent means of timing the valves,because no linear measurements need be taken.Since the angle of a crank throw must be usedwhen working with a timing disk, this is calledthe angular method of timing valves.

    [46]

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    CHAPTER VCARBURETIONIN ORDER that gasoline may be mixed withthe right amount of air to form an explosivemixture within the cylinders, it is necessary tomake use of a device known as a carburetor.A great deal of attention has been devoted tothe designing of carburetors, for it can bereadily seen that the fuel consumption of anengine will be governed largely by the perform-ance of the carburetor. Also of late muchattention has been given to the carburetion oflower grade fuels, so the subject of carburetorsis becoming a broad field.A suitable mixture for an aviation engine isone pound of gasoline to fifteen pounds of air.A richer mixture would be one having moregasoline, while one having more air would bea leaner mixture. It has been found that themost practical way to obtain this mixture is tospray the gasoline into the air, and this is bestaccomplished by making use of a jet attachedto a reservoir and lessening the atmosphericpressure about the jet. If the level of gasolinein the reservoir is slightly lower than the tip

    [47]

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    ELEMENTS OF AVIATION ENGINESof the jet and the jet is located in an air-sup-plied chamber having a connection with theinlet valves, the downward motion of the pis-tons will result in less pressure being exertedupon the gasoline in the jet than that in thereservoir, where atmospheric pressure is ex-erted. Gasoline in this way will be madeto flow from the jet, and since considerableair is being drawn past the jet it will tendto form a spray of the gasoline that is beingdelivered.To restrict the amount of gasoline that issupplied to the float chamber which in turnhas a great deal to do with the gasoline deliv-ered by the jet, the float with which the floatchamber is supplied, actuates a pin that opensand closes the main supply valve. Upon thetop of the float rest the ends of two pivotedarms having the other ends in contact withthe needle valve stem. As gasoline enters thefloat chamber the float will rise causing one endof the arms to rise and the other end to exerta downward pressure upon the needle valve.The result will be to seat needle valve allowingno more gasoline to enter until some has beendrawn off by the delivery from the jet. Fromthis it can readily be seen that the float cham-

    F481

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    CARBURETIONher is essentially a reservoir supplied with anautomatic valve.The space around the jet is called the mixingchamber. To admit the necessary air an open-ing is located somewhere below the level of thejet which insures all of the air passing the jet.As a means of diverting the air nearer the tipof the jet and thus securing more of a drawingeffect, the space around the jet through whichthe air passes is lessened by the insertion of achoke tube or a venturi as it is often called.Its purpose is to increase the velocity of airas it passes by the jet and thus increase thesuction at the tip of the jet. To regulate thespeed of the engine a butterfly valve is locatedjust a little distance above the choke tube.This valve, which is nothing more than a diskof metal, is often referred to as the throttle.When it is opened the speed of the engine isincreased on account of a greater volume ofgas being taken by the engine. As it is broughttoward a closed position, less gas will be sup-plied, and the result is to decrease the speed ofthe engine. Stop screws are provided to pre-vent the throttle from closing completely, forthat would cause the engine to cease runningaltogether.

    [49]

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    ELEMENTS OF AVIATION ENGINESSo far the most elementary type of carbu-

    retor has been discussed. It is one that consistsprimarily of a float chamber and one jet in aregularly shaped mixing chamber. This iscalled a simple jet carburetor, and its chiefweakness lies in the fact that at high speeds itwill deliver a richer mixture than when theengine is running slowly; the reason for thisbeing that as the speed is increased the suctionis greatly increased, which means more gaso-line in proportion to the air at high speeds thanat low speeds. A simple jet carburetor ad-justed for low speeds will use too much gaso-line at high speeds, while one that is adjustedfor high speeds will not supply enough gasolineat low speeds. Consequently simple jet car-buretors are not satisfactory for aviation en-gines.In order to secure the right mixture at bothlow and high speeds, several modifications ofthe simple jet carburetor have been used withmore or less success. One way is to have themixing chamber supplied with an auxiliary airvalve that is held in place by a weak spring.At low speeds the spring holds the valve closed,but as the speed is increased the valve isdrawn open due to the increase in suction.[50]

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    THE MILLER AVIATION CARBURETOR

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    ELEMENTS OF AVIATION ENGINESonly a limited amount. At high speeds the in-creased amount of gasoline from one will bejust enough to take care of the additionalamount needed. At low speeds both jets workharmoniously. Such departures from the sim-ple jet carburetor are spoken of as speed com-pensations.The Zenith carburetor has been widely usedin connection with aviation engines, and forthat reason it will be well to become familiarwith its operation. The principle used is thatof two small jets with one having only a lim-ited amount of gasoline to supply. In appear-ance it closely resembles a simple jet carbu-retor except for a narrow cylindrically-shapedwell between the float chamber and the mixingchamber. Gasoline is supplied from the floatchamber to this well through a small hole in aplug that forms the bottom of the well. Theplug is called the compensator. In the upperpart of the well is a hole which allows atmos-pheric pressure to be exerted upon the gasolinewithin. One jet is placed within the other, andthe inside jet is that one connected directly tothe float chamber. Obviously this jet, which isknown as the main jet, will act the same as onein a simple jet carburetor causing a richer mix-[52]

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    SLOW spfco scacw

    A HALF SECTION VIEW OF A ZENITH CARBURETOR

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    . .

    ' "

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    CARBURETIONture at high speeds than at low speeds. Theoutside jet or cap jet, as it is called, receives itssupply of gasoline from the well, and since theamount of gasoline furnished to the well islimited by the hole in the compensator, it canbe seen that the amount of gasoline deliveredby the cap jet is restricted to that amount thatwill flow by gravity through the hole in thecompensator. At low speeds both jets worknormally, but as the speed is increased themain jet will furnish more and more gasolinewhile there will be a tendency to draw moregasoline from the cap jet than can be suppliedby the hole in the compensator. The resultwill be to exhaust the supply in the well anduse instantly that which is fed to it. Sincethere is an air hole near the top of the well un-due suction upon the compensator will be pre-vented. It should be noted that air will enterthe well and be drawn out the cap jet at veryhigh speeds, but it is absolutely wrong to re-gard the air hole in the upper part of well as anauxiliary air valve. The compensation effectcomes from the fact that the increased amountof gasoline supplied by the main jet is enoughto make up for that which is not supplied bythe cap jet.

    [53]

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    ELEMENTS OF AVIATION ENGINESAt idling speed very little air is drawn in,

    and this is not sufficient to fully overcome thesurface tension of the gasoline in the jets. Bysurface tension is meant the force that tendsto resist breaking the surface of the column ofgasoline in the jet and drawing it outward. Toinsure a good mixture at idling speed theZenith is equipped with an entirely separatecarburetor that supplies its gas at a pointwhere the air passes by the nearly closedthrottle, and, on account of the small space,considerable suction is developed at this point.This carburetor gets its supply of gasolinethrough a tube leading down near the bottomof the well. The tube is held in what is calledthe priming plug, which acts as a cover for thewell. The size of the hole in the priming pluggoverns the amount of gasoline fed to theidling carburetor. The amount of air that isallowed to enter the mixing chamber of theidling carburetor is controlled by a thumbscrew known as the slow-speed screw.To facilitate starting, a strangler valve isplaced in the air inlet. If it is brought toward aclosed position a greatly increased amount ofgasoline will be drawn from the jets, and fromthis increased amount the more readily vola-[54]

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    ELEMENTS OF AVIATION ENGINESism passes through the core one instant andnot the next, then more electricity will be gen-erated. The core offers an easy path for themagnetism. Soft iron is used for the core be-cause it can very quickly become magnetized,and what is just as important it will quicklygive up its magnetism. By revolving such acore between the poles of a horseshoe magnet,it will amount to successively plunging a mag-net in a coil and rapidly drawing it out again.The magnetic lines of force from the core,when it is magnetized, will of course be cut bythe coil which accounts for the current.As the core is revolved between the twomagnetic poles, which are distinguished bycalling one the North pole and the other theSouth pole, the core is magnetized when in ahorizontal position almost connecting the twopoles and demagnetized when it has turned90 degrees to a vertical position. Consequentlyin one complete revolution of the core it willbe magnetized twice and demagnetized twice.It so happens that a little more current is gen-erated in the coil when the core loses its mag-netism than when it receives its magnetism,which means that maximum current is ob-tained when the core is approximately in a[58]

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    NL

    tPOSITION or GORE POSITIOH or CORE.

    /S /?/?0AfEN ,DIAGRAMS TO ILLUSTRATE THE LOCATION OF THE CORE IN A

    SHUTTLE TYPE MAGNETO

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    IGNITIONvertical position. Since it is in this positiontwice during one complete revolution, it fol-lows that an ordinary magneto furnishes twosparks per revolution.So far the core has been considered as therevolving part. Identically the same result isobtained when the core is held stationary andthe magnet or magnets are revolved. To turnthe magnets is inconvenient on account of theirhorseshoe shape, so rotating poles are oftenused to accomplish the same result. This isreferred to as a revolving field. In the firstcase where the core is rotated, an armaturemade up of the core and the shaft that carriesit is used. In appearance the armature hassomewhat of a resemblance to a shuttle, onaccount of the windings about the core. Forthis reason the type of magneto using the re-volving core and coil is called the shuttle type,while the one in which the magnetic field re-volves and the coil remains stationary is knownas the inductor type. More attention will bedevoted to the inductor type after the shuttletype has been further explained.The current required to jump the gap be-tween the points of a spark plug under highcompression is much greater than the amount

    [59]

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    ELEMENTS OF AVIATION ENGINESsupplied by a single coil wound about a core.In order to have a self-contained unit it isnecessary to make use of another coil woundabout the first consisting of much finer wireand having several hundred times as manyturns as in the first coil. The coil wound near-est the core is called the primary coil, while theoutside one is the secondary coil. Now if wehave some automatic device to break the pathof the current from the primary coil at thesame time that the core loses its magneticcharge a high tension current will be inducedinto the secondary coil and will be suitable toconduct to the spark plugs. The principle isthat of a transformer.On the shuttle type magneto a breaker me-chanism through which the primary currentpasses is held on one end of the armature,causing it to be revolved at exactly the samespeed as the armature is turning. Cams on thebreaker housing force the breaker points toseparate for an instant, at the same time thatthe core loses its magnetism. All that is neces-sary to do in order to stop the magneto fromdelivering current and in turn stop the engineis to close a switch on a line that connects thetwo breaker points. This will short circuit[60]

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    IGNITIONthe primary, destroying the effectiveness of thebreaker points and prevent the primary frominducing any current into the secondary coil.To advance or retard the spark, the positionof the breaker cams is changed. This affectsthe time that the primary circuit is broken.Moving the cams with the direction of rota-tion retards the spark, while to advance thespark the cams are moved against the direc-tion of rotation. This brings us to one diffi-culty with the shuttle type magneto. In orderto get the maximum current the primary circuitshould be broken as the core loses its magneticcharge. If the spark is retarded, however, theprimary is broken a little later than the mag-netic lines of force are broken, which results in aweaker spark. The effect is frequently to hind-er starting as it is necessary to retard the sparkto prevent injuring the one who is cranking.

    In the primary circuit a condenser is placedin multiple with the breaker points. It con-sists of alternate sheets of a conductor and anon-conductor such as tinfoil and mica. Halfof the sheets of the conductor are attached toone terminal, and the other half are attachedto the second terminal. This provides a placefor the current to go momentarily after the

    [611

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    ELEMENTS OF AVIATION ENGINESbreaker points have separated. If a condenserwere not used there would be a tendency forthe current to continue flowing for an instantthrough the air between the separating points,which would result in arcing and pitting thepoints. Right here it should be noted that thebreaker points are of platinum and should notseparate more than .020 inch. Since the con-denser prevents arcing it also serves to makethe break in the primary circuit occur morequickly, which means that more voltage willbe induced into the secondary coil.As a means of conducting the secondary cur-

    rent to the right spark plug at the right time adistributor is used. It consists of as many seg-ments as the number of spark plugs that themagneto supplies. A distributor arm with acarbon brush directly connected with the sec-ondary coil turns about upon the distributorplate conducting secondary current to eachsegment in turn. With the spark fully ad-vanced, the distributor arm should just beentering a segment every time the breakerpoints separate. For convenience the primaryand secondary circuits are both grounded.Should the secondary circuit be left open, aswould be the case if a wire were not attached162]

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    IGNITIONto a spark plug, the result might be that thehigh pressure of the secondary would cause ashort circuit between the two coils. To avoidsuch a happening a safety gap is provided inthe secondary circuit. Its points are generallythree-eighths of an inch apart insuring no in-terference with sparking at the plugs.The electrical pressure is expressed in volts.The flow is expressed in amperes. One volttimes one ampere is equivalent to one watt,which is nothing more than a unit of work,being 1 /764 part of a horse-power. The wattageof an ordinary magneto is about twenty. Thevoltage in the primary circuit is from six to tenvolts, while that in the secondary is about tenthousand. The amperage of the primary islimited to only a few amperes, yet that of thesecondary is infinitely less, being only theslightest fraction of an ampere, for it should beremembered that when a current of highervoltage is obtained by induction the gain inthe number of volts will be accompanied by aloss in the number of amperes. Upon the speedof the armature and the number of windingsdepends the voltage of a magneto. The num-ber of amperes is dependent upon the strengthof the magnets.

    [631

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    ELEMENTS OF AVIATION ENGINESAn ordinary magneto can deliver but two

    sparks per revolution, so the speed of the arma-ture is governed by the number of explosionsin the engine during one complete cycle. Afour-cylinder engine will fire four times in tworevolutions or two times in one revolution.Since two sparks will then be necessary forevery revolution of the crank shaft, it followsthat the armature should turn at engine speed.In an eight-cylinder engine there will be fourexplosions per revolution, so the armature willhave to turn at twice engine speed to give thefour sparks at the right time. The speed of anarmature on a magneto supplying twelvecylinders would be three times engine speed.A convenient means of determining this rela-tive speed is to divide the number of cylindersby four. The distributor arm turns at cam-shaft speed owing to the fact that each cylin-der requires one spark in two revolutions of theengine.The Dixie magneto, which is a good exampleof the inductor type, has been widely used anddeserves consideration. In it the magnets areturned at right angles to the position that theyoccupy in the Bosch and Berling, which arerepresentatives of the shuttle type. A shaft164]

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    DIAGRAM TO ILLUSTRATE THE PRINCIPLE OF REVOLVINGPOLES ON THE DIXIE MAGNETO

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    IGNITIONcarrying two shoes or pole extensions separatedby a bronze block is placed in line with the twopoles of the magnets. This shaft having nowindings upon it is not called an armature,but is known as a rotor. As the rotor is re-volved the shoes, each being in contact withone pole and being separated by the non-mag-netic bronze, will always have their respectivemagnetic charges, and the effect will be muchthe same as though the magnets themselveswere revolved. Were it not for the bronze be-tween the two shoes there would be a directflow of magnetism through the rotor betweenthe two poles, and the shoes would then beuseless.At right angles to the rotor is placed the core

    carrying the primary and secondary coils. Itis located in the space between the rotor andthe top of the magnets. Extending downwardfrom both ends of the core are two bars of softiron known as field pieces, and it is betweenthese two field pieces that the shoes revolve.When the shoes are in a horizontal position,magnetism will pass from one shoe into thenearest field piece, then through the core, intothe other field piece, and thence into the oppo-site shoe. When the shoes move to a vertical

    [65]

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    ELEMENTS OF AVIATION ENGINESposition the core will receive no magnetism,but in moving another 90 degrees the shoeswill come again to a horizontal position, andmagnetism will pass through the core in a re-

    ' versed direction. Thus the core will be mag-netically charged one instant and not the next,resulting in the generating of electricity.The breaker assembly does not revolve onthe end of the rotor, but is worked by cams onthe end of the rotor shaft. To advance or re-tard the spark it is thus possible to move thewhole breaker assembly instead of changingthe position of fixed cams, as is done on theshuttle type. Since the coils and core do notrevolve, it is also possible to change the posi-tion of the core and field pieces with thechanging of the position of the breakers. Theresult is to break the magnetism in the corewith the breaking of the primary circuit eventhough the spark is fully retarded. This in-sures the same intensity of spark when crank-ing the engine as is obtained at top speed withthe spark fully advanced.A special type of Dixie magneto is one hav-

    ing four shoes instead of two. There are twoopposite North shoes and two opposite Southshoes. The two field pieces leading to the core[661

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    nnnnnnnnn n nnnn n n fin../l(/l/l/f/Ul/UI/UVUVUUUV

    DIAGRAM TO ILLUSTRATE POSITION OF ROTOR IN THE DIXIEMAGNETO WHEN THE CORE IS MAGNETIZED

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    / If I II /f // II II /I/I I

    'III.iI.unVUUUUUUl/ V u

    DIAGRAM TO ILLUSTRATE POSITION OF ROTOR IN THE DIXIEMAGNETO WHEN THE CORE IS DEMAGNETIZED

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    IGNITIONare shortened so that their ends will be wellabove the center of the rotor to allow unlikeshoes to connect the two ends. Since the oppo-site shoes have the same polarity, it would notdo to have the ends of the field pieces in linewith opposite shoes. The advantage of thistype of Dixie is that four sparks can be securedduring one revolution of the rotor, which per-mits a much slower running magneto on anengine having a large number of cylinders.The magnets themselves deserve attention.They are made of hard steel in order to retaintheir magnetism as long as possible. To re-charge a magnet it is simply necessary to windit with insulated wire and pass direct currentthrough the wire. When dismounted from themagneto it is necessary to provide a path forthe magnetism between the two poles. A stripof steel will answer for a keeper, or the unlikepoles of two magnets may be placed together,insuring a perfect magnetic circuit.

    In timing a magneto to an engine it is bestto start by selecting a cylinder and placing thepiston in that cylinder in the right position onthe compression stroke for the spark to occur.Then turn the distributor arm so that it isabout to enter the segment which has connec-

    [67]

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    ELEMENTS OF AVIATION ENGINEStion with the spark plug in the selected cylin-der. Next, after making sure the spark leveris fully advanced, turn the magneto in its rightdirection until the breaker points have justseparated. The distributor arm should now beupon the foremost part of the segment to in-sure that it will still be in contact with the seg-ment when the spark lever is retarded. Theremaining step is to connect the driving shaftwith the armature or rotor as the case may be.Upon those engines having double ignition toprocure a greater factor of safety and to reducethe time to fully explode the charge, the twomagnetos must furnish their sparks at the sametime or be synchronized as it is technicallycalled. To accomplish this the first magneto istimed to the engine and the second magneto istimed to the first. In this way the breakerpoints on each one can be made to separate atthe same instant.With a battery system either a vibrating or

    a non-vibrating coil may be used. Vibratingcoils will give a rapid succession of sparks atthe spark plugs. The primary circuit is madeto pass through the vibrator and the magnet-ism in core is allowed to separate the vibratorpoints which breaks the primary circuit and[68]

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    I WWW

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    IGNITIONdemagnetizes the core. The contact is thenmade again by the vibrator springing back andthe operation is repeated. When a non-vibrat-ing coil is used there must be mechanically-operated breakers to break the primary circuit,very similar to those used on magnetos.The wiring diagram for a battery systemusing mechanically-operated breakers is simi-

    lar to a magneto wiring diagram, except thatthe switch is not placed in the same position.In a battery system the switch is placed inseries in the primary circuit, and by openingthe switch the engine is stopped. Sometimestwo breakers are used instead of a single one.The two are then wired in multiple and aremade to break at the same time, thereby in-suring uninterrupted flight in case one refusesto close. A small resistance coil of iron wireis often placed in the primary circuit with aview to saving the battery during slow run-ning, or in case the switch is left closed whenthe engine is not running. Ordinarily whenthe engine stops the breaker points are to-gether, which, with a closed switch, affords adirect path for the current to pass from onepole of the battery to the other. The iron coilwill then be heated with the result that less

    [69]

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    ELEMENTS OF AVIATION ENGINEScurrent can pass through the heated iron wirethan when it was cold. In this way a batterywill not be exhausted so readily. A coil thatserves this purpose is called a ballast coil.

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    CHAPTER VIILUBRICATION

    THE PURPOSE of lubrication is to reducefriction. Even though two pieces of metalthat move one upon the other may have theirsurfaces highly polished and appear perfectlysmooth, it will be noticed upon examinationwith a microscope that the surfaces are veryirregular. In other words all sliding surfaces,no matter how carefully they may be finished,are known to consist of minute projections anddepressions. Consequently the projections andhollows on the contact faces tend to interlockand resist a sliding motion. From this it canreadily be seen that friction is nothing morethan the force which resists the relative motionof one body in contact with another body.Excessive friction results in the developmentof heat.As a means of minimizing friction, oil is in-

    troduced between the contact surfaces. Theoil will first fill the depressions and finallyform a film between the two surfaces, separat-ing them sufficiently to prevent the projectionson one surface from interlocking with the de-

    [71]

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    ELEMENTS OF AVIATION ENGINESpressions on the other. This is referred to asthe theory of lubrication. Perfect lubricationis greatly to be desired for it eliminates wear,and by reducing the power required to turn theengine it adds to the efficiency of an engine.

    After realizing the necessity for oil the nextstep is to ascertain what properties an oil musthave in order that it may be suitable for avia-tion engines. In testing an oil it is customary todetermine the gravity, viscosity, flash point, firepoint, and whether or not it has acid properties.The gravity of an oil has in reality no effectupon its lubricating merits, as there is con-siderable variation in the gravity of high gradeoils. However, it is usually determined andused principally in checking current deliveriesof a certain brand. The specific gravity is theratio between its weight and the weight of anequal volume of water. In the oil trade, though,it is customary to use the Baume gravity scalein which the gravity of water is 10 at 60 F.The lighter the oil is in body, the higher willbe the Baume reading. Hydrometers gradu-ated for either specific gravity or Baume areused to measure the gravity of an oil. The fol-lowing formula will serve to convert one scalein another:[72]

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    LUBRICATIONSpecific gravity =;130+ Baume reading*Viscosity is the technical name for what is

    popularly called "body." To express it morespecifically it is the fluidity of an oil. To ob-tain the viscosity the oil is put into a cup sur-rounded by water at about 212 F. When theoil has reached this temperature, a plug ofspecific size in the bottom of the cup is re-moved allowing 60 c.c. of the hot oil to run outinto a marked flask. The number of secondsrequired to draw the 60 c.c. is reported as theviscosity of the oil. Good cylinder oil will havea viscosity of about 75 seconds.The flash point is the lowest temperature atwhich the oil will ignite but not continue toburn. If the flash point is too low, the oil willnot remain on the cylinder walls and bearingswhen the normal heat is developed, leaving thefriction surfaces without lubrication. It is wellto use oil having a flash point above 325 F.The fire point is the temperature at whichthe ignited vapor from the oil will continue toburn. This temperature, which ranges be-tween 45 and 75 F above the flash point, isnot of much consequence from our standpointas it is always beyond the point where the oilwill cease to be useful.

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    ELEMENTS OF AVIATION ENGINESCertain mineral oils are treated with sul-

    phuric acid during the process of refinement.To protect the highly polished bearing sur-faces it may be necessary to ascertain if anyacid has remained in the oil. A simple way oftesting is to wash a sample of the oil withwarm water and test the water with litmuspaper. The presence of any acid will result inthe paper being turned pink.

    Lubricating oil we are accustomed to thinkof as being only mineral oil. With the develop-ment of aviation engines, castor oil, which is avegetable oil, has received considerable atten-tion. This oil, which has a gravity of 96Baume and a flash point a little higher thanmost mineral oils, will thicken to a marked de-gree upon standing. When heated it willreadily oxidize and exhibit acid properties,rendering it of little use in engines where theoil is used over and over again. Its universaluse in rotary engines is due to the fact that itwill not unite with gasoline. In these enginesthe crank case is filled with gasoline vapor whichtends to wash off any mineral oil that is suppliedto the bearings. Hence castor oil is resortedto, and as long as the oil is used but once inrotary engines, it serves very well as a lubricant.[74]

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    LUBRICATIONVery few engines have the same system of

    lubrication. The oil supply is generally carriedin the lower half of the crank case which iscalled the sump. Frequently auxiliary tankshaving connection with the sump are used.The splash system of oiling is not suitable foraviation engines, so it is possible to make useof what is called a dry sump. If no provisionis made to retain the oil in the sump when anengine is momentarily inverted there is greatdanger of the oil rushing into the cylinder andpiston cavities. However, if the lower half ofthe crank case has a false bottom the oil maybe carried in the compartment thus formedwith no danger of it rushing out. Another wayto obtain a dry sump is to collect the returningoil from the bearings in a trap at the bottom ofthe crank case and pump it away to a tankwhere the main supply is located.A gear pump is generally used to force theoil to the bearings. Its construction is remark-ably simple as it consists of two rotating gearsin a closely-fitted housing. Oil is caught in thespaces between the successive teeth of eachgear and carried around to the discharge of thepump. Plunger pumps and vane pumps arealso used. The Hispano-Suiza engine makes

    [751

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    ELEMENTS OF AVIATION ENGINESuse of the vane pump to develop a high oilpressure.A pressure relief valve is usually placed onthe oil line very near the pump. Such a valveconsists essentially of a poppet valve whichopens outwardly and which is held in pla