Construction TipsFor Oil System

In aircraft engines, the lubri-cation system is designed to meetthe problems of high temperatures,high bearing stresses, and properfunctioning in all flight attitudesof the aircraft, except inverted.The high temperature of the var-ious engine parts tends to thinout the lubricant (lower its vis-cosity), which decreases its effec-tiveness in overcoming metallicfriction. Therefore, provisionsmust be made to cool the oil ex-ternally either by radiation fromthe sump or by means of a separ-ate radiator. The cooled oil, onreentering the lubrication systemof the engine, materially assistsin reducing the high temperaturesof the various parts, particularlythe bearings.

The oil pressure pump and itsdistributing lines and passages cir-culates the oil under pressure tothe various working parts of theengine so long as the oil is sup-plied to the inlet side of the pump.This then is the function of thelubrication system; to provide anadequate supply of cooled oil tothe engine.

OIL SYSTEM OPERATION.

In a wet-sump engine, the entirelubrication system is incorporatedin the engine. In a dry-sump en-gine, an oil sumip located in thelowest portion of the crank-casecollects all surplus oil drainedfrom the pressure system. A sca-venging pump removes the oilfrom the sump and forces it backinto the external lubrication sys-tem where it is cooled, collected,and recirculated to the oil pres-sure pump. It is primarily withregard to this external lubricationsystem that the following "GoodPractice" comments are concerned.

OIL SYSTEM ARRANGEMENT

The major units in an oil systeminclude the supply tank, the nec-essary piping and connections, theoil temperature-regulator assem-bly (oil cooler or radiator), the oiltemperature gauge, and the oilpressure gauge. Some modern lub-rication systems should be arrang-ed so as to furnish oil by gravityto the inlet side of the oil pressurepump during all normal flightattitudes. It should be readilypossible to remove all air, water,or cleansing compounds from thesystem after overhaul. Foaming is

promoted by any of these foreignmaterials.

OIL TANKS. GENERAL

Oil supply tanks usually areconstructed of aluminum alloy, orstainless steel and are of such de-sign as to permit installation inthe aircraft as close to the engineas possible. The design and streng-th practices outlined previouslyherein for fuel tanks should alsobe applied to oil tanks so as topreclude failures from vibration,inertia, or fluid loads. For oil tankshowever, the tank should be cap-able of withstanding an internalpressure of 5 psi instead of the3% psi recommended for tfueltanks.

CAPACITY AND E X P A N S I O NSPACE. The tank capacity shouldbe sufficient to assure a supplyof oil which is adequate for thetotal-fuel supply. The customaryratio for nontransport-type air-craft is approximately one gallonof oil for every 25 gallons of fuelcapacity, but not less than onegallon for each 7 maximum con-tinuous horsepower of the engineinvolved. This does not includethe oil in the piping, oil tempera-ture regulator, and engine. In ad-dition to providing for the ratedoil capacity of the tank, the tankvolume should be such as to in-clude an expansion space whichcannot be inadvertently filled withoil when the airplane is in thenormal ground attitude. Such ex-pansion should be at least 10%of the tank volume except thatit should be in no case less thanone-half gallon. This can be ac-complished by locating the fillerlip with respect to the groundangle so that it controls the max-imum level. , ' • i -

OUTLET

The tank outlet usually is lo-cated at its lowest section to per-mit complete drainage while theaircraft is in the ground positionor in normal flight attitude. Thetank-inlet line from the oil tem-perature regulator usually entersthe top of the tank and should beof the same size as the outlet line.

VENTS

The vents should be located atthe top of the tank and shouldnot be attached to the filler neckor incorporated in the filler cap.They should be so arranged thatthe oil tank will be properly vent-ed at all flight attitudes of the air-plane when the tank is filled toits rated capacity.

In most instances, the crankcaseis vented through suitable piping

to the top of the oil supply tank.Two vent lines are used on high-powered engines, one to vent thepower section and the other tovent the accessory section. Whereoil tanks are not vented to theengine crankcase, special pre-cautions should be taken to prev-ent over-How or a fire hazard.No traps or pockets should existin oil-tank vent lines.

QUANTITY INDICATOR

A suitable means should be pro-vided at the tank to determine theamount of oil in the tank. An oilgauge of the bayonet type is com-sidered suitable as a means ofdetermining the amount of oil,provided it is marked in gallonsand indicates the oil level downto the last 20 % of the oil ca-pacity. The gauge need only be as-cessible during filling. Eventhough a cockpit gauge may beprovided, there should be a gaugeon the tank.

OIL TANK INSTALLATION

The oil supply tank should pref-erably be located as high abovethe pump inlet as practicable. Ac-tually, the tank should be locatedso that its outlet is sufficientlyabove the engine oil pump inletwhen the aircraft is in its groundposition, to provide a positive headof oil at the sump. Suction liftsbetween the tank outlet and thepump inlet should be avoided.

As applicable, the same pro-visions and practices recommendedpreviously herein for installingfuel tanks should be applied tothe installation of the oil tank.These provisions and practicesconcern (a) the method of sup-porting the tank to assure properdistribution of loads, (b) protec-ting the tanks against chafing andcorrosion, (c) protection of flex-ible liners, if used, and (d) vent-ilation and drainage of tank com-partments.

Oil filler openings should beplainly marked with the ratedcapacity and the word "oil". Theopening should be provided witha satisfactory cap which shouldfit tightly to avoid oil leakageand prevent loss of oil in flight.With systems which provide avent to the crankcase, the capshould be tight with no holes forventing contained in it. All re-cessed filler necks should be pro-vided with suitable drains.

OIL RADIATOR

The oil radiator or cooler shouldbe suitably mounted in the re-turn line (between the scavengingpump and supply tank) andequipped with a suitable drain ordrain plug. A relief valve should

be provided either built into theradiator in order to preclude ex-cessive pressures being built upin the core. Various size coolersare used depending upon theamount of oil circulated throughthe particular engine. The oiltemperature may be controlled bymeans of a thermostatically-oper-ated valve, to control the passageof oil through the radiator, and bya shutter assembly installed inthe air-vent side of the radiator.

The mounting of the oil ra-diator should be given particularattention so that excessive vi-bratory stresses will not developin the cooler elements and theirattachments. Where mounting lugsare provided, they should be ofrugged construction and theirattachment design should be suchas to avoid local concentration ofstresses. In general, the radiatorshould be mounted on nonabsorb-ent pads or other cushioning ma-terial.

OIL TEMEPERATURE GAUGE..

A suitable means should be pro-vided for measuring the oil tem-perature near the engine inlet. Asplice or "Y" connection is al-uminum tubing for the purposeof inserting the thermometer bulbis satisfactory providing the tubinghas a wall thickness of .049 inchor more. When the engine manu-facturer provides a standard con-nection to the engine crankcasefor an oil temperature bulb, sucha location for the bulb should beused. Calibration of an oil tem-perature gauge may be readily ac-complished by immersing the tem-perature bulb in boiling water.

OIL SYSTEM LINES, FITTINGSAND ACCESSORIES

Oil-system plumbing is gener-ally simples than fuel systemplumbing and the lines are largerin diameter. Nevertheless, the pre-cautionary details and plumbingpractices described previouslyherein for the fuel system shouldbe complied with, as applicable, inthe design and installation of theoil-system lines, fittings and ac-cessories.

OIL SYSTEM DRAINS

With regard to the oil systemdrains, one or more accessibledrains should be provided at thelowest point in the system to drainall the major parts of the systemwhen the airplane is in its normalposition on level ground. Whenthe system employs an oil tem-perature radiator, it is usuallynecessary to provide a drain bothin the radiator and in the system.These drains are not intended toremove oil from the engine crank-

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case which should be drained byremoving the engine-sump plug.It should be possible to drainpractically all the oil in the linesby means of the tank and radiatordrains. The drain valves shouldincorporate means for lockingthem in the closed position.

The following are recommendedas fire-protection measures:

(a) A fire resistant, oil inletline consisting of a fire resistanthose with assembled end fittingsrather than hose clamps.

(b) A means of shutting off theoil flow forward of the firewall.The shut-off should be immediate-ly operable from the cockpit inthe event of an emergency.

ENGINE BREATHER LINES

i Crankcase breathers are pro-vided on the engine to relieve in-ternal pressure resulting from hightemperatures and high speed pis-ton operations. The condition re-sulting from a clogged breatherline may be hazardous and there-fore precautions should be takenin the installation to preclude thepossibility of the breather linefreezing during cold weather. Thefollowing installation practice isrecommended:

(a) The breather line should notdischarge into the carburetor coldair intake. Such an arrangementintroduces water vapor into theengine induction system which inturn is conducive to carburetoricing. Consequently, the practiceis considered dangerous.

(b) It is preferable to run thebreather line down inside and tothe rear of the cowling, and sorouted that it is not exposed toa direct blast of cold air fromopenings in the cowl. The lineshould terminate just outside orflush with the cowling, or just

I inside the co.vling if possibleI where warm air from the engine

will flow over the breather and< prevent the freezing of moisture

condensed from the breather va-pors. In any event the dischargeshould be so disposed that (1) thepossible hazard of ice accretionsbuilding up and clogging the lineis minimized, (2) no fire hazardwill exist from the vapors orspray, and (3) the possibility ofoil spraying on the windshield, incase of engine malfunctioning, andthus impairing the pilot's vision,will be avoided.

(c) In no case should the brea-ther line run outside the cowlinglor an appreciable distance with-out adequate insulation.

Internal-CombustionEngine Principles

S. J. Dzik

SERIES - IICompressing the charge -In a conventional internal -

combustion engine the mixture offuel and air must be highly com-pressed in order to obtain effic-ient combustion and a reasonableamount of work. If the charge isignited at atmospheric pressure,the combustion is slow and muchheat is radiated and lost. The re-sulting pressure or power, dueto the combustion of the charge,is relatively low. Thus, an en-gine compressing the charge to100 pounds per square inch willonly develop approximately 1horsepower for every 4 cubic in-ches displaced by the piston, whilecompressing the charge to 140pounds per square inch will in-crease the output to a point where1.8 cubic inches of piston dis-placement will produce 1 horse-power. Some aircraft engines inuse at the present time have com-pression ratio as high as 7.25 to 1.In general, it may be said thatthe power and efficiency of an in-ternal - combustion engine in-crease with the degree of com-pression. Therefore an aircraft en-gine with a compression of 150pounds per squar3 inch has amuch higher output for a givensize cylinder than an engine witha compression of only 110 poundsper square inch. Unfortunately,compression pressures are limitedby the fuel used and the servicefor which the engine is intended.Hence a compression pressurethat would be highly desireablefrom an efficiency point of viewwould DC impossible because ofcertain conditions. Extremely highcompression pressures result indetonation and set up heavystresses in the structural partsof the engine. -

The effects of compression,up to certain critical limits, inincreasing the actual efficiencyand output of an engine are two-fold.

(1) The compression producesheat, which aids in the vaporiza-tion of the fuel.

(2) At the point of highestcompression, the volume is at aminimum and combustion is ac-complished more rapidly becauseof the smaller space through whichthe flame spreads.

The exact compression obtainedin any cylinder depends upon anumber of factors. The followingare the most important:

(1) The ratio of the total vol-ume of the cylinder to the com-pression space. The compressionpressure of the cylinder variesas the ratio of the volume of theclearance space, plus the volumeslivept by the piston to the clear-ance space. This ratio is known asthe compression ratio, and inaircraft engines varies from 5 to 7.For example, in a cylinder havinga piston displacement of 100 cu-bic inches and a cylinder chamberspace of 20 cubic inches, the com-pression ratio is:

100 + 20 120 6

20 20 ~~ 1or, as it is commonly written, 6:1.

(2) The pressure of the chargein the cylinder when the com-pression process begins. Thecompression pressure in the cyl-inder varies as the initial pressureof the charge at the beginning ofthe compression stroke. The in-itial pressure is determined by thevolume and density of the chargeadmitted to the cylinder, or vol-umetric efficiency.

In simple terms, volumetric ef-ficiency is the volume of thecharge admitted into the enginecylinder of 100-cubic-inch dis-placement has a volumetric .ef-ficiency of 95 percent, ' i " v

95 •****•——=0.95 = 95100

percent. With the advent of sup-erchargers incorporated or instal-led on present day aircraft enginesthe volumetric efficiency may beincreased above 100 percent, be-cause the charge is forced intothe cylinder at pressures aboveatmospheric. Thus, the higher thecompression pressure of the chargeresulting in higher power output.

Horsepower calculations. - . Aconvenient and frequently usedmethod of comparing the output ofdifferent engines is to designatethe number of cubic inches ofpiston displacement required perbrake horsepower. This is obtain-ed by dividing the total cubic-inch piston displacement by itsrated brake horsepower. In manyautomotive racing engines theaircraft engines this figure variesfrom 1.5 to 4 cubic inches perhorsepower, and in some casesis as low as 1 cubic inch, becauseof the high compression pressuresand piston speeds. Much dependson the volumetric efficiency. Withthe high temperatures and restric-tions of the inductior system ex-

perienced with some cylinders,the volumetric efficiency is ser-iously affected. The use of two in- "let and two exhaust valves percylinder and supercharging greatlyincreases the volumetric effici-ency, with a corresponding in-crease in the power per unit ofdisplacement volume.

In the inward movement of thepiston in an engine cylinder, aspecific volume is displaced fromits upper extreme to its lower ex- \vtreme position on the admission =*or intake stroke. This space or ,volume displaced by the pistonis called "piston displacement,"and consists of the products ofthe area of the cylinder bore and •the length of the stroke of thepistons. In case of multiple cyl- :inder engines, this product is ;multiplied by the number of cyl-inders comprising the engine andmay be expressed by either ofthe following formulas:

Displacement^ 0.7854 D2LN •(all dimensions in inches).Displacement's.1416 R2LN(all dimensions in inches).

Where D=Cylinder diameter,L=Plston stroke,N=Number of cylinders,R=Cylinder radius.

Employing the first formula, -the piston displacement of a 12-cylinder engine having a 5-inchbore and 7-inch stroke would becomputed as follows:0.7854X52X7"X 12=: 1,659.34 cu. in.

Low weight per horsepower isan essential requirement for air-craft engines; therefore, it is quitenatural that weight per horse-power should be a basis used forcomparing engines. The term"weight per horsepower" may beapplied to an engine with fourdifferent meanings.

(1) Dry weight of complete en-gine with carburetors, air stack,magnetos, and pumps.

(2) Weight with attached ac-cessories such as magnetos, car-buretors, and pumps, and withcoolant in the cylinder jackets, •connecting pipes, and coolantpump.

(3) Weight with attached ac-cessories such as radiators, tanks,batteries, and instruments, or inother words, the entire powerplant.

(4) Weight including all ac-cessories and supplies. This basisis the most important and is de-pendent upon the length of re-quired flight and fuel and oilconsumption of the engine.

In comparing engines it may befound that an engine light inweight may prove to be low in

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