A Study of Carburetion of Gasoline, Including the Design and Testing of a Venturi-Type Carburetor Citation Beattie, Joseph Andrew (1917) A Study of Carburetion of Gasoline, Including the Design and Testing of a Venturi-Type Carburetor. Bachelor's thesis, California Institute of Technology. doi:10.7907/c09b-7w58. https://resolver.caltech.edu/CaltechTHESIS:07172024-172858726 Abstract No abstract. Item Type: Thesis (Bachelor's thesis) Subject Keywords: (Mechanical Engineering) Degree Grantor: California Institute of Technology Division: Engineering and Applied Science Major Option: Mechanical Engineering Thesis Availability: Public (worldwide access) Research Advisor(s): Unknown, Unknown Thesis Committee: Unknown, Unknown Defense Date: 1 January 1917 Record Number: CaltechTHESIS:07172024-172858726 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:07172024-172858726 DOI: 10.7907/c09b-7w58 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 16562 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 17 Jul 2024 22:29 Last Modified: 17 Jul 2024 22:30 Thesis Files PDF - Final Version See Usage Policy. 10MB Repository Staff Only: item control page

wA STUDY OF CARBURETION OF GASOLINE,INCLUDING THE DESIGN AND TESTING OF A VENTURI-TYPE CARBURETOR." By JOS. A. BEATTIE CLASS OF NINETEEN 'HUNDRED AND SEVENTEEN, DEPARTMENT OF MECHANICAL ENGINEERING . THROOP COLLEGE OF TECHNOLOGY PASADENA, CALIFORNIA PART ONE I - Introduction - II - Requirements of a Carburetor III - Desoription of Apparatus IV - Method of Making Tests V - Formulas used VI - Data. and Results of Tests VII~ Graphical Presentation of Results VIII .:. Conclusions IX - Photographs X - Original Data XI - Appendix PART TWO I - Design of Carburetor II - Appendix I N T R ODUC T I ON ----------~It has been the writer's belief for some time tba.t the automobile gasoline engine has been highly developed in all of its parts except the carburetor. This apparatus, so essential to the proper and efficient operation of the engine, has not received the attention whioh its importance demands. To reach its present state 1t has passed through various stages of experimental evolution, in fact it has been developed through experimentation, and not from a study of the fundamental principles underlying its operation. As a result of this there are dozens of carburetors on the market at the present time, each claiming superiority over the others. The construction, arrangement of details, and operation, a.re a.s varied as the different types of carburetors are numerous. Some have no adjustments what- ever, while others have as many as five o A carburetor which seems to work well on one make of engine, works very badly on another . Mr . Browne, in the introduction to his "Handbook of Ca.rburetion" expresses the above idea very cle~ly where he says in part: -1- nconoensus of public opinion, both technical and lay, would undoubtedly be singularly unanimous in weloom1ng the complete abolition of the carburetor. No pa.rt of a motor vehicle is less understood or more abused, in thought a.nd deed. No other pa.rt of the entire mechanism of the oar is subjected to the indignities that are heaped upon the carburetor. This condition will continue to exist until the genius which has already made such colossal strides in automobile engineering turns its serious attention to an understanding of the fundamental laws governing carburetion. ------ the lack of uniformity exhibited by the great and ever-increasing variety of carburetors on the market proves that, as yet, no oomprehensi~e principle of automatic regulation of the gas to air ratio has been generally recognized.• It is the purpose of this thesis to make a study of oarburetion, with the object in view of designing a carburetor which is simple and positive, and which will maintain a constant ratio of fuel and air over a wide range of operation. The study will be both theoretical and practical; the theoretical in order to know the fundamental laws on which to base the design; and the praotio-al, to know the actual service requirements of an automobile carburetor. The latter will be studied by testing several types of standard carburetors on an engine, noting especially the operating ohara.oter1st1cs, requirements, and fuel consumption. -3- REQUIREMENTS OF A CARBURETOR It 1s generally supposed that mixtures of various ratios of fuel and air a.re required over the different conditions of operation imposed on gasoline engines. This supposition is without a doubt erroneous~ and has a.risen from the fact that on starting an engine, and possibly at low engine speeds, an excessive amount of fuel must be delivered by the carburetor in order that the low velocity air column may carry into the cylinders sufficient vaporized fuel to form a combustible mixture. It has 1:een shown by many experiments that a mixture with a certain definite ratio of gasoline vapor and air reaches a maximum pressure in the shortest space of time when ignited; whereas any deviation from this ratio, either towards richness or leanness, produces a lower maximum pressure in a longer time with a consequent decrease in power. a.Te Diagram No.l clearly shows this fact. These the results of many experiments conducted at the Massachusetts Institute of Technology. The correct mixture need not neoessa.rily be the theoretically perfect one, in fact most investigations have shown that a slight excess of fuel gives the best results. This 1s directly opposite to Steam and Gas Engineering practice; as in gas engine design it ie customary to allow 15% excess air; while in boiler work as high as 50% excess air is allowed for the proper combustion of coal, and a somewhat smaller exoess for oil. This difference can be attributed to the fa.ct that in the gaseline engine a certain proportion of the fuel is not properly vaporized, consequently an excess is necessary in order that the vaporized portion may form a oorreot mixture with the air present. Some very interesting tests have been carried on at Purdue University in conneotion with carburetor performance. Diagram No.2 is here reproduced from these experiments. The curve shows that the engine will run with a mixture of less than .055 lb of gasoline per pound of air, but will not pull well with so lean a mixture. As more fuel is added, the power increases rapidly until nearly full power is reached, when the curve becomes almost horizontal, increasing slowly to a maximum, then decreasing slowly for a time, but finally reaching a point where it falls off rapidly. The richest mixture with which the engine could be run was .155 lb. of gasoline per pound of air; nearly three times as rich as the leanest mixture. In other words, a carburetor may be adjusted to as lean a -5- mixture as oan be used to carry full load, after which the amount of gasoline can be nearly doubled without seriously affecting the power capacity of the engine. This explains in part why oars of the same make show such different mileages per gallon of gasoline. The function of a carburetor does not end with the oorreot proportioning of fuel and aire Its further duty is to atomize the fuel and to thoroughly mix the minute particles with the air, in order that they ma.y be quiokly vaporized and form a homogeneous explosive gas with it. Though vaporization be- gins in the carburetor, the process continues through the manifold and valve chambers, and even in the cylinder. The sooner the vaporization is completed the bet- ter, as most of the fuel entering the cy.linder in liquid form 1s cracked by the heat of combustion, and forms carbon, which 1s deposited in the cylinder and on the spark plug. Also a part of the liquid fuel finds 1 ts way into the ora.nkoa.se where it mixes with the oil,lessening the latter's usefulness as a lubricant. The majority of automobile engines now provide some means of heating the air supply, and on certain machines the fuel also is heated. Vaporization is greatly aided by using heated air and fuel, but the advantages are to some extent offset by -6- the loss in volumetric efficiency, resulting from too , much heat. This loss is nearly 2% for every 10 ° F. increase in the temperature of the charge. The latest development in the application of heat 1s in the •Hot-spot manifold" . In this the hot exhaust gases are utilized to heat a plate on the intake manifold. The hot plate is located at a bend in the intake manifold., so that the heavier fuel pa.rtioles in the mixture strike against it on making the turn, and are quiokly vaporized. In this way heat is applied where most needed, and does away with the neoess1ty of overheating the whole charge. The air velocity through carburetor and intake manifold deserves careful consid- eration. High velocity produces thorough atomization and prevents deposition of fuel particles in the manifold. passages. But the advantages of high velooi ty must be balanced against the volumetric loss by velocity, which increases rapidly with increase in air velocity. The carburetor and manifold passages should therefore be as large as possible consistent with proper perfo~manoe at the slowest engine speeds. In summary, the requirements of a oa.rburetor a.re as follows: l = Constant ratio of fuel and air 2 = Thoroughly atomized, homogeneous mixture 3 - Least possible volumetric loss,- viz: low velocity, small friction loss, a.nd small loss due to heating the charge. -s- DESCRIPTION OF APPARATUS E NGI NE . ------A stook Ford engine was used in all of the tests. Specifications as follows: Four cylinders, four cycle. Cylinders ca.st en bloo, with w~ter jaokets and upper half of ora.nk oase integral. Cylinder bore three and three-quarters inches; piston stroke four inches. full twenty horsepower. The power developed was measured by means of a Froude type water brake, having a. Rotor about six inches in diameter by two inches wide. CAR B. U RE TORS ----------l - Holly Standard 1917. Model for Ford Engines 2 - Stromberg Type B-#3, Carburetor 3 - Ensign Type G Carburetor size l inch for Ford engines. 4 - Modified Venturi-type carburetor designated as B-19 OTHER APPARATUS • Fairbanks-Morse Scales for weighing gasoline and cooling water, and for measuring the power developed. Schuchardt and Solmtte Tachometer for obtaining engine speed. Thermometers for obtaining temperature ~ POWER MEASUREMENTS The power developed by the engine was measured in all cases by a Froude Water Brake. The Power equation for this type of brake 1s the ea.me as that for a Prony type bra.lee, i.e.: 2 ft R N W H•• P = 33 .ooo Where R = distance in feet from centre of shaft to knife edge on end of lever ~m. N = Revolutions per minute W = Pounds foroe exerted by end of lever arm. The lever a.rm R. in this brake is 6.30 inches. Therefore the horsepower deYeloped when W is one pound and N is one revolution per minute, 1s 2,a X 6.30 X l X l . H.Pe: 12 x 33.000 or H.P.: • 0001 .0001 X N X W : CALORIFIC~VALUE OF FUEii;,, Several samples of the gasoline used in the different runs were tested for heating value in a Parr Calorimeter. As these determinations require considerable time, the following approximate formula taken from Hirshfeld and Barnard's Heat-Power Engineering, was used in most oases: nThe higher calorific value of u. s. petroleum and its distillates, ranging from crude oil to gasoline, varies quite regularly with the specific gravity of the material, and is expressed approximately by the following formula, which may be as sumed correct within 2 per cent: lS,650 plus 40 (B - 10) B.T.U. per pound: in which B = degrees on the Baume hydrometer -- "· To correct the specific gravity for temperature variation, the following formula was used: Where Ss s 1 -.0007 ct-60> s- specific gravity at 60° F. s : t - It tt tt to F. temperature in F. 0 This formula was taken from Browne's Handbook of CarbUI"etion. To reduoe specific gravity to the Baume scale, the following formula,taken from the same book, was used. Specific gravity a 140 + (130 plus Degrees B) Curve I shows the results of these calculations. The curve 1s practically a straight line, exoept near the lower limits where the head Hs influences the Velocity Vf very much. At a certain veloeity of air the fuel velocity becomes zero. This velocity may be found as follows: :,·.00165 Va,2 - 1.34 : 0 = 1.34 . 00165 - Va2 812 Va= 2S.5 ft.per seo . If Hs were equal to zero, then Vf would va:ry directly as Va: 2 Ha _- -Va / 2 gha 2 g Wf = Hf w;.- Wt arid Wa :: Wt. of equal volumes a.nd fuel. • • • Vr 2 2 :~ and Vr = Va = Va and for a constant ratio of !~ Wa the velocity of fuel will if equal a constant times the velocity of air. of this type A carburetor (H 8 : 0) should keep a constant ratio of air to gas. provided the sp. gr. of the fuel remained constant. This type has the practical difficulties of preventing a loss of fuel when the engine is not running, also to pre- vent erratic operation of the carburetor due to vibration of the machine. From Curve I the 1mposs1b111ty of setting the needle valve for constant mixture at all speeds oan be seen, especially when attempting to do this at the low speeds where it is usually done. This lower range of velocities is used in order to reduce volumetric loss which becomes excessive at the' upper range. For example 7 if it is assumed that the proper mixture is obtained with Vf = 2 and Va= 60 keeping this ratio, for Va• 600 Vf should be 20; but from the curve Vf = 24.3, or an in- crease of 21.5%. So far three solutions seem possible: l - To adjust the simple type of carburetor (H8 :K =0) for the high range; 2 - To devise a means of keeping the mixture constant when adjusted at the low range; 3 - To make use of the type whe,:e H8 = 0. The first method would mean using the upper range of velocities with a considerable volumetric loss (about g,a.% at 400 feet per second - Chart I in Browne,Handbook of Ca.rburetion). Due to the high volumetric loss this method will not be considered. The second method necessitates a close adjustment at a slow speed of engine 1 and a means of keeping this ratio constant. By using the low velocity range, volumetric loss will be out down to the lowest possible value. The third method will not be considered, due to its practical difficulties already mentioned1 but the advantages offered by the second warrant its further development. V = / 2 gh - VF - lI 2 ghf Va - 2 gha Vf = Va - vf - Va/ ~ghf 7 2gha, ( Hf)f = Val hf 7~ - Ira Water weight - 62.355 lb. per ou.ft. at 62° F. Air n .0761 • n n n 62° F. Assume sp .gr. of gasoline - •74- at 60° F •. ~.p.. Hf = Vlf • -Px Wa • Wa • .0761 Ha, P Wf -PWf .74 X ~2°355 Wa .0406 Va If the carburetor is now set so that with an air velocity of 6a ft per seo~nd, a fuel velocity of 2 feet per second, results: in order to keep this mixture constant an air velocity of S40 feet per second would require a fuel velocity of 2g feet per second. From the ourve it is seen that the actual fuel velocity is 34.l feet per second or 6.1 feet per second too great. 6•1 6.1 -- a Va of 701i-06 = 150.2 A The fuel velocity may be reduced the required amount by direoting the fuel nozzle into the air current with the proper angle, so that the component of the air velocity along the fuel nozzle axis shall equal the excess fuel head of 6.1 feet per second. Assume that¼ of the fuel velocity is along line of air velocity. Va = 84-0 plus ¼x 28 : Vf(exoess) = 6.1 g47 •.c!o!: l50.2(ft/seo.of air) Vax COS 0: cos e = 150.2 1 5°• 2 = 847 .1773 e~ 79° 47• To find what effect this will have at other points along the our·v e, for example when 480 480 plus 483.5 X .1773: 85,,S Vf = .0406 x 85.s = 3.4s and 19.5 - 3.48 = 16.02 instead of 16 even Jia = 60 60 plus .17 x 2 = 60.4 60.4 X .1773 = l0.7 Vf: .0406 X 10.7 • 2.14 - Va: .4350 .43 = 1.71 instead of 2 120 120 t .177 X 4: 120.71 120.71 X .1773: Vf: .0406 X 21.4 • 21.4 .87 4.73 - .S7: 3.86 instead of 4 At the lower range the mixture will be a little weakened but the decrease is within 3 or 4%. Q will be made= S0° To aid in the evaporation of the fuel after it leaves the nozzle, a piece of brass, shown in the sectional drawing, is placed in the tube beyond the nozzle. This piece is given an approximate stream line form to reduce friction and also to insure co.n t·a;o:t of the air over the entire surface. The area of the tube should expand gradually from the throat to the maximum diameter therefore it is necessary to shape that part of the tube which contains the evaporating piece so that the annular area will increase evenly. The proper shape is deter- mined as shown in Drawing #1. To insure ease of starting a valve 1s placed at the entrance to the tube. By closing this the suction on the fuel jet is increased, causing a rapid flow of fuel. A butterfly type of throttle valve is placed at the other end of the tube to control the speed of the engine. THROOP COLLEGE Of TECHNOLOGY ~ ,__ ~ ~ + +-+ "' ~ '\l "'\ ~ "') ~ '\J I() I\J ~ '\l ~ '\I ~ '\J V) ~ ~ ~ '\J ~ ~ -~ ....... ~ ~ ~ ~ C ~ -~ ' ~~ ~ ~ ~ ~ 'O c.40 f Joo "'180 -~~-------~~-~ Velil~i ty Qf_Met/- lh a /2.<? _s_e._ . C.S.& 5 . 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Sl'on. / / f PART TWO CARBURETOR DESIGN Fundamental principle of all suction type carlru.retors: V• V2 gh Let Va= Vel. of a.1r, ft. per sec. Vf: " Ha= - n " fuel" Head in ft. of air n n n " fuel n Hs • float chamber. n n n Hf n n nozzle above fuel level in Va2 2 g Head of fuel corresponding to air head= = H x de.nsity a.ir =. ~- x Wa a ·o nsity fuel -a Wf Wa Wt l ou.ft.a.ir Wf = a n " •fuel n n n "water Ww = --= Ha xWa spgt x Ww • • • Neglecting fuel head Hs ~ w. nozzle-:: Vf : 7-• 2 gua spgr Ww and fuel friction through Bu:t fuel head must be raised a dietanoe H8 against gravity: • • Vf -- / 2 ,.,.,.. _ l:)46a Wa Ww spgr ~ 2 gh8 whioh nagleots only the friction losses. The density of air varies inversely as the absolute temperature. l cu.ft. air at 62° Fo weighs • •. at Temp. t° F. weight l ou.ft. .0761 lbs . .0761 x 460 f 62 460 f! t 0 3 The density of gasoline has been found to va:ry as follows: Spgr • Vf 0.72 (l-.0007 (t0 -60) ) = li WE2 f ~~ X • 0761 . 2 - 62.355 X S(l-.0007(t0 -60))X Va ~ 2 gbs 62.355 •Wt.of l cu.ft. water at 62° F. S ... Spgr at 600 F . Assume air temp. to be 1000 F. (intake to carburetor) •460. 460 .. 62 = 55ga X • O761- : 100 Vf varies as the eguare root of the air d~nsity or /~'0161 = .276 dif. .010 Assume gasoline temp. 100° F. and Sp.gr . • 74 at 600 62.355 x .74: 46.l /46.l • 6.79 62.355 X .74(1-.0007(100-60)): 62.355 X •74 X •972 =44.g /44.S ~ 6.69 6.79 - 6.69: .10 .10 x 100 = 10 : 1.5% plus 6.79 6.79 The increase in the temperature of the air diminishes its density and consequently the fuel velocity is decreased; while the increase in the fuel temperature diminishes its density and its velocity of flow will be increased. Therefore the differences in velocity due to temperature changes tend to balanoe ea.ch other. • • -3.6 plus 1.5: about 2% dif. in Vf The difference is so small that the change in densities due to temperature changes will be neglected. Therefore the formula reduces to the following when gasoline of sp.gr. of .74 at 600 F. is used; and air temp. 62° F. .1.0161 f '-62 . 355 x. 74 V Va2 - 2 ghs = / .00165 Va2 - 2gh8 or for fuel of any other density this beoomes: The distance H8 va.ries from about l-116" to¼" in car- buretor praotioe. In the design of this oarburetor ¼ will be used (this faotor makes considerable difference in the calculations). o .. 02os rt. =l .00165 Va2 1.34 The velocity of the air is determined principally by starting conc11tions. When an engine is cranked at starting the velocity of air ;J through the carburetor must be sufficient to carry into the cylinders a combustible chargea This velocity is estimated at 7,000 to 9a000 feet per minute, or about 115 to 150 feet per second. (Page, The Modern Gasoline Automobile, page 2SS). Browne Handbook of Carburetion, page 18, states that 60 to 90 feet per second is suf- ficient with a well designed carburetore This carburetor is to be designed for a Ford enginee Assume speed range of engine 150 to 1500 R.P.M. Cylinders 3¾" x 4 (Engine 4 cyl. 4 cycle.) 3•7~2 !- x 4 x 2 x RPM x suction factor• piston disp. per min. in cu.in . • 88.5 x RPM x F - where F • suction factor. At slow speed F will approach unity and at the high speed it may decrease to .6 or .7 but to allow for speeds above 1500 no correction for F will be made in calculating air displacements a.nd velocities. 150 RPM Se 88.5 X 150 • 13,280 cu.in.per min. 1500 RPM S: 132,aoo " n n n n To out down volumetric loss (see Browne,Handbook of Carburetion, page lS) an initial velocity through the venturitube of 60 ft per second will be used. a = 6013,2SO X 60 X 12 a = 811 e d! A! -- 4 X .307 4 . = .307 d ~ u; tr s 491 •✓ /.391 = .625 or 5/gn diameter of tube at throat. Using a simple venturitube carburetor, the results would be as follows: For Va :a: Vf 60 = /.00165 X 602 - l.j4 • For Va _= 600 2.144 Vr = /.00165 x 6002 - 1.34. 24 . 36 Thus increasing air supply 10 times has increased fuel ~~i~ = 11.35 times supply and the mixture will be richer. Evidently this form of carburetor would give a varying ratio of mixture of air to gas. To further study these relations,: curves .- will be plotted of air velocity against fuel velooi ty. Va:: 60 vf - Va= 120 2 Vf : f.00165 X 120 - Va= 240 2.144 1.34: 4 . 73 - 1.3IJ. = 9.67 - Vf :}.00165 X 2402 -- Va:: 360 vf =/.00165 x 3602 - 1.34: 14.59 Va: 600 Vf :/.001ei5 x b002 - i.3}i: • 24<36 Va= 900 Vf •/ . 00165 X 900 2 - 1.34 = 36.50