THE J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
594
Vol. 14, No. 7
Carburetor Adjustment by Gas Analysis'3z By A. C. Fieldnera and G. W. Jones4 GAS LABORATORY CHEMICAL DIVISION, PITTSBURGH EXPERIMENT STATION, U. S. BUREAUOR MINES, PITTSBURGH, PA.
The following paper records experimental work on the determination of carbon dioxide in exhaust gas, which has a direct bearing on carburetor adjustment. The per cent carbon dioxide in the exhaust gas bears a direct relation to the completeness of combustion and air-fuel ratio. Curves are gioen showing the carbon dioxide percentage relation to air-fuel ratio and also to completeness of combustion, both by laboratory and road tests.
Characteristic curoes are given of two types of carburetors the results of which were plotted f r o m road tests, showing how the air-fuel ratio changes with change of mixture rate through carburetor. Procedure f o r sampling exhaust gases while adjusting carburetors on the road are gioen and a portable carbon dioxide indicator is described. Examples of carburetor adjustment by gas analysis are gisen.
ESTS made by the Bureau of Mines on a large number of motor vehicles, to determine the amount and composition of the exhaust gases produced under varying operating conditions, have shown that the average automobile and truck as used in service waste from 20 to 30 per cent of gasoline by incomplete combustion.6 The average driver uses a rich mixture, which does not contain sufficient air to burn the gasoline completely. In these road tests the average ratio of pounds of air to gasoline was found to be approximately 12.5 to 1. Gasoline as used at the present time requires approximately 15 lbs. of air for complete combustion. Less than 15 lbs. of air causes the appearance of carbon monoxide, hydrogen, methane, and very small quantities of unsaturated hydrocarbons (0.1 per cent or less in most cases) in the exhaust gas. The heat losses due to combustibles in the exhaust gases of the motor vehicles tested average for all tests approximately 30 per cent of the heat value of the gasoline used. I n some instances this heat loss was over 50 per cent. At least 50 per cent of this could have been prevented by correct carburetor adjustment. Gas analysis is a positive method for determining the completeness of combustion and thus affords a ready means of adjusting the carburetor for maximum economy of gasoline. As shown by the curves and tests that follow, the carbon dioxide percentage of the exhaust gas gives directly the air-fuel ratio of the gas mixture entering the engine and percentage of completeness of combustion that has taken place.
is from 11.5 : 1 to 14 : 1 at the lowest loads shown. Berry6 and others have obtained curves of a similar nature which were based on actually measuring the air and gasoline supplies instead of on exhaust gaq analysis. The air-fuel ratios for the above curve were obtained by calculation from a complete analysis of the exhaust gas and the carbon content of the gasoline used. The following formula was used for computing the air-fuel ratio : Lbs. of air per Ib. of fuel = a 0.0764 0.0317 (c 4-d e ) X 0.791 where a = percentage of carbon in fuel. b = percentage of nitrogen by volume in exhaust gas, c = percentage of carbon dioxideby volume in exhaustgas. d = percentage of carbon monoxide by volume in exhaust gas. e = percentage of methane by volume in exhaust gas. 0.0764 = wt. of 1 cu. f t . of air a t 60" F. and 29.92 in. Hg. 0.0317 = wt. of carbon in 1 cu. ft. of COS,CO or CH(at60" P. and 29.92 in. Hg. 0.791 = percentage of nitrogen in normal air.
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DERIVATION OF AIR-FUELRATIO AND PERCENT COMPLETENESS OF COMBUSTION CURVES
6
The air-fuel ratio for maximum power at full load extends from about 11 : 1 on the rich side to 15 : 1 on the lean side. These results were obtained from block tests with a 19.6 h. p. Lycoming engine at a speed of 1000 r. p. m. The inlet air temperature varied from 89" to 101" F. and the cooling water was kept at 15OOF. It is seen from Fig. 1 that the maximum power range decreases as the load is decreased and
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1 Presented before the Division of Industrial and Engineering Chemistry, a t the 63rd Meeting of the American Chemical Society, Birmingham, Ala., April 3 to 7, 1922. 2 Published by permission of the Director, U. S. Bureau of Minps. a Supervising Chemist, Pittsburgh Experiment Station, U. S. Bureau of Mines. 4 Assistant Chemist, Pittsburgh Experiment Station, U. S. Bureau of Mines. 5 A. C. Fieldner, A. A. Straub and G. W. Jones, "Gasoline Losses Due to Incomplete Combustion in Motor Vehicles," THISJOURNAL, 13 (1921), 51; J . SOC.Aulomotive Eng., 8 (1921), 295; Bur. M i n e s , Refits. Inwestiga-
$ions,2425.
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Fig. 2 gives the brake thermal efficiencies obtained under the same conditions as given for the tests shown in Fig. 1. The curve shows that a t wide open throttle positions maximum efficiency is obtained at an air-fuel ratio of 16.5 to 17.5 : 1, while at lighter loads the value decreases to around 8 0. C. Berry, "Mixture Requirements of Automobile Engines," J . SOC.Automotrve E n g . , S (1919). 364.
July, 1922
THE JOURNAL OF' INDUSTRIAL AhlD ENGIhTEERIMG CHEMISTRY
595
Fig. 4 shows a curve obtained by plotting the carbon dioxide percentages and air-fuel ratios obtained in road tests under different conditions of operation, using different types of motor vehicles and many different grades of gasoline. It may therefore be taken as an average curve for the presentday grade of gasoline and may serve as a basis for carburetor adjustment by gas analysis. Most of the points occur a t air-fuel ratios for rich adjustments. This curve follows closely the curve obtained from the laboratory block tests to the point for complete combustion. By determining carbon dioxide in the exhaust gas the airfuel ratio can be read from the curves and the carburetor may be adjusted accordingly.
RELATION BETWEEN PERCENT CONPLETENESS OF COMBUSTION AND P E R CEXT C l R B O N DIOXIDE I N THE EXHSUST GAS
FIG. 2
13.5 : 1. Thermal efficiency is a measure of the relative mileage which may be obtained from a given quantity of fuel. To obtain high mileage the carburetor adjustment should be at a higher air-fuel ratio than that for maximum power. Power must be sacrificed to some degree in securing the greatest mileage. However, an air-fuel ratio may be chosen which will give reasonably good economy of gasoline and a t the same time not sacrifice power and flexibility appreciably. Several cars tested by the Bureau showed that an air-fuel ratio of 13.7 to 14.7 : 1 can be obtained with flexible and easy operation. Each particular type of car has a maximum air-fuel ratio at which it can be operated satisfactorily. The temperature of the intake air, type of carburetor, design of the manifold, velocity of the air-fuel mixture, and turbulence in the engine govern the leanest mixture which may be used. The relation of the air-fuel ratio to the percentage of carbon dioxide in the exhaust gas is shown in Fig. 3. This curve was obtained from laboratory block tests with only one type of gasoline. The region for maximum power, perfect combustion, and limit of maximum thermal efficiency are given as found by Figs. 1 and 2. The perfect combustion line is the theoretical ratios of air to gasoline for complete combustion found by analysis of the gasoline.
The exhaust gas from different gasolines should contain from 14 to 14.5 per cent carbon dioxide a t complete combustion of a theoretically correct mixture with air (without excess of either gasoline or air). As the combustion becomes less complete or the air is supplied in excess of that required, the percentage of carbon dioxide decreases proportionately. It is therefore possible to construct curves showing the relation between the percentage carbon dioxide and completeness
RIR N E L RflTtO
FIG. 4-sHOWING RELATION BETWEEN Cor PER CENT IN EXHAUST GAS AND AIR-FUEL RATIO POINTSO N CURVETAKEN FROM TESTSUNDER ACTUALROAD CONDITIONS MADEON 101 MOTORVEHICLES CSINGDIFFERENT BRANDS O F GASOLINE
of combustion. These curves may then be used in estimating the completeness of combustion from a carbon dioxide determination in the same manner as the power plant engineer checks up the efficiency of combustion in a boiler. Fig. 5 shows such a curve, the points of which were taken from tests made with 53 trucks of 1.5- to &ton capacity and tested at 10 mi. per hr. up a 3 per cent grade under summer conditions. The curve shows at a glance the wide range of combustion efficiency found with different trucks operated under the same conditions. In some cases about 45 per cent of the heat value of the gasoline passes out in the exhaust as a result of rich carburetor adjustment. The percentage completeness of combustion of these tests was calculated by the folIowing formula, making use of the exhaust gas analysis and ultimate analysis of the fuel: CALCULATIONS FOR DETERMINING COMPLETENESS OF COMBUSTION
FIG. 3
From the ultimate analysis of the gasoline by weight: Per cent hydrogen X 0.5 -ratio of water vapor to carbon dioxide by volume on complete combustion = A . Per cent carbon X 0.0833
-
THE JOURXAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
596
FER CENT C O M P I L - N E ~O~F COM8UST9N
FIG. 5 Taking the heat of combustion of carbon as 14,540 B. t. u. and that of hydrogen as 62,000 B. t. u., 14,540 X per cent carbon per cent heat units due t o carbon = B. (14,540 X per cent carbon) 4- 62,000 X hydrogen per cent heat units 62,000 X per cent hydrogen due to hydrogen 14,540 X per cent carbon f 62,000 X per cent hydrogen
-
=
c.
From gas analysis by volume: 2CO per cent C H I =volume carbon containing gases = D. Per cent COI Per cent Cor (0.3 X per cent C O ) - completeness of combustion of carbon = E Per cent CO, per cent CO f per cent CHI D X A = equivalent volume of water vapor formed on complete coabustion per cent H2 per cent C H 4 completeness of combustion of hydrogen = F. 1D X A Hence, completeness of combustion of gasoline (B X E) C ( C X F).
+ + + +
+
-
-
the automobile should be tested a t other speeds and a record kept of the carbon dioxide percentages and carburetor settings. Then, by plotting the carbon dioxide percentages found at the different speeds, the carburetor setting giving the highest average carbon dioxide percentage at the different speeds and also flexible operation should be chosen as the best adjustment. If a characteristic curve of the carburetor being tested is known, only one test is necessary to adjust for most conditions under which the automobile may be used. Fig. 6 shows such a curve of the metering pin type of carburetor and gives the air-fuel ratio which the carburetor meters into the engine a t the different mixture rates passing through the carburetor. The curve was obtained by averaging the tests from three automobiles using this type of carburetor and shows that as the velocity of gas mixture increases the air-fuel ratio likewise increases to a certain point, above which it remains fairly constant. This particular carburetor gives the needed rich mixture at low velocities for flexibility in driving at low speeds and then increases the air ratio as the speed increases, which permits economy of gasoline. The tests made on these three automobiles gave low percentages of carbon monoxide, good mileage, and high values for completeness of combustion. The carbon dioxide percentage in the exhaust gas is over 12 per cent at mixture rates over 9 cu. ft. per min. In adjusting a carburetor of this type it is necessary, having the characteristic curve as shown in Fig. 6, only to test the car a t one speed, say at 20 mi. per hr. on level grade, and to adjust the carburetor to give a carbon dioxide percentage of 12.5, as shown in Fig. 6. At low speeds and idling positions the mixture is rich enough to give good operation.
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The factors A, B, and C are practically constant for any given grade of gasoline. This greatly simplifies the calculations when a series of tests are made.
SAMPLING EXHAUST GASES The exhaust gas should be sampled from the exhaust pipe at a point between the engine and the muffler. The location should be as near the muffler as possible in order that the gases from the different cylinders may have the maximum mixing before sampling. If samples are taken from the muffler they may become contaminated with air due to leakage, and in some types of motor vehicles the pressure may not be sufficient to cause the exhaust gas to flow through the sampling arrangement.
CARBURETOR ADJUSTMENT To adjust a carburetor by gas analysis, tests should be made on the road under the different conditions under which the automobile or truck is driven by the operator. The automobile should be run until it is thoroughly warmed up and operating satisfactorily and then tested at a given speed, say 20 mi. per hr. on the level, by taking exhaust gas samples and analyzing for carbon dioxide content. By referring to Fig. 4 the air-fuel ratio can be obtained. Should the percentage of carbon dioxide be low, the carburetor is given a slightly leaner adjustment and again tested, and a record is kept of the carbon dioxide percentage and carburetor adjustment for each test. If the carburetor has two or more independent adjustments they should be investigated separately to determine the effect of each adjustment at the given speed. When the highest carbon dioxide percentage consistent with flexible operation at this speed has been obtained,
Vol. 14, No. 7
FIG. 6
July, 1922
T H E JOURKAL OF INDefSTRiAL AND ENGINEERING CHEMISTRY
Fig. 7 gives a similar curve showing the characteristics of a surface evaporation type of carburetor. As the air velocity increases, the mixture becomes richer, while for economy the air-fuel ratio should go the other way. At low velocity the mixture is leaner; it should be richer for good idling qualities and light loads. Under the usual driving condi-
Cff F T W - f l R M/XRJRE PA5S/M T H R W CARaURETok PER MNffTE
Fro. 7
597
in Fig. 8. In the upper end of this jet are several very fine holes evenly spaced about the top at an angle of 45' with the horizontal. From these holes dilute caustic soda solutiorf is forced out of the jet for absorbing the carbon dioxide. From the bulb of the buret extends a slender tube of the proper size to cover the carbon dioxide percentages which may be found in the usual type of gases. For exhaust gases the scale extends from 5 to 14.5 per cent. For gas which may have a higher carbon dioxide content a tube slightly larger in diameter must be used. C and H are wide-mouth bottles of 500-cc. capacity; J is a 250-cc. wide-mouth bottle. The buret and bottles with capillary connections and rubber tubing are mounted on a suitable stand, as shown in Fig. 9. Bottle C ie filled to the point indicated in the figure with 5 per cent sulfuric acid solution. The height may vary somewhat without appreciably affecting the accuracy of analysis, but sufficient solution must be present to form a seal for the lower end of the buret when the buret is filled with liquid and ready for taking a sample. The dilute acid solution is introduced into C by removing the small glass plug B and slipping a piece of hollow glass tubing into the rubber connection at B the other end of which dips into the reserve supply of solution. By suction on tube -4, the acid is drawn into bottle C until filled to the proper height. The glass tubing is then removed and the glass plug reinserted. Bottle J is filled with a 10 per cent caustic soda solution by removing glass plug M and inserting a small funnel in its place. Enough water is poured into bottle H to form a water seal with the rubber tube extending into the bottle from G. After the caustic soda solution is blown to the tip of jet K by blowing on rubber tube L and opening pinch clamp T the indicator is ready for use.
tions the carbon dioxide percentage cannot be increased so high as that shown in Fig. 6, for the mixture would be too lean a t low speeds to give satisfactory operation. Until curves similar to the above are obtained by actual road tests for each type or class of carburetor definite methods or values for percentage of carbon dioxide, which the exhaust gas should contain, cannot be given. The two above curves are given as examples to show the types of curve, one of which gives very good mileage, completeness of combustion, operation, and carbon dioxide analysis, and the other in which the carburetor functions in the wrong direction and makes it very difficult to get a good. combustion efficiency.
CARBON DIOXIDE ILVDICATOR A carbon dioxide indicator for adjusting carburetors on the road has been devised, by means of which the exhaust gas sample is taken directly into the buret while the car is in motion and the carbon dioxide percentage is determined while stopping until the analysis is made, or on smooth roads while the car is in motion. The indicator was designed to be as nearly leak-proof as possible and easily and speedily operated by the layman, yet accurate within the limits of sampling the gases. Two minutes are required to sample and analyze the gas, a marked saving in time over the usual method. The time is so short that temperature changes do not affect the result appreciably. Tar and soot in the exhaust gas do not clog the indicator because its only stopcock has a large (6-mm. bore) opening. The only special part of the indicator is the buret, which is provided with a rugged stopcock having large openings in the plug and connecting branches to give an easy passage for the gas. Below the stopcock is the body of the buret, which has a glass jet extending nearly to the top of the bulb as shown
FIG.8
To make a determination cock E is turned to the position as shown in section A--4 (Fig. S), and by blowing in rubber tube A, liquid is forced into the buret; when the liquid reaches cock E, it is turned 180°, thus filling the buret completely with the liquid. Fig. 10 shows the arrangement for sampling the exhaust gas from a motor vehicle while in motion. The gas coming through the exhaust tube causes a stream of gas
THE JO UR,VAI, OF IiVD CSTRIAL A N D BSGT.\'BERING
598
Vol. 14, No. 7
CHEMISTRY
At the m d of '$0 ICC cork E
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turned 90" to the left. and
Weather-clear; temperahire--68' I?.: humidity-65. Ail tests mid* over il I-mi. course and UP an averege 3 per rei,( ~ m d e . Course was macadamized and in good condition. TItsr No. 1
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There i s now a dcfinitc measiired volume of gas in the buret at the tcmpnrature of the indicator. Caustic S O ~ Ris forced into the buret, from the jet K liy blowing on L and a t the same time opening pinch clamp T. Six separat,c small eliarges or niorc of the caustic soda solution are forced into tlrc buret in succession, allowing sufficient time for the caustic solut,ion to drain down the buret before the next. clinrge is introduced. An interval of 10 see. has bcen foiind sufficient. The highest, point reached by tlic solution is blic eorrcct re'ding. The indicator is made reedy for aiiotlier analysis by :+ii blowing into tube A and filling the buret as previously dirccted. The dililtc siiiiurie acid solution in (>wlren forced into the huri:t. neutralizes the cniisiic soda solution each time so tliere is no a!xorption of carbon dioxide whan the sample is being taken arid hmught to the surrounding ternperaturo. A siiinll :imoont of solut,ioii is blown caelr time ovcr into bottle W when the h r e t is filled so that t,he liquid Iieiglit iii C is kept approximately a t the same level in C. A few drops of phenolphthalein solution arc ai11ir.d to the caustic soda solution which t.urns the solut,ioii in U pink when it has become eshausted. The bottle J holds sufficient caustic solution to imke from 25 to 35 determinations, depending on how economically the solution is used. It is very necrssary when t,lle first, charge of caustic soda solution is blown into the burct that only enough pressure is applied barely to cause the catistic solution to ruii down the sides of the jet, otherwise a small bubble of gas will be blown out of the bottom of tlic buret and this in turn will give too large a percentage indication of carbon dioxide. Laboratory tests show the indicator to be accurate to 0.2 Der cent vith carbon dioxide contents of 11 Der cent and more accurate at lower percentages. On actual road tests with automobiles where conditions are very unfavorable, as a result of temperature changes and the jolting action of the machine in motion, the error may be 8s high as 0.5 per cent. This. however. is wit,hin the accuracy of laking sanmles under these conditions. The following -sanmles . show the gain in mileage which can be obtained by proper carburetor a2justment. Table I shows tests made on a Ford automobile. The driver had used the adjustment shown in test for some time. By adjusting t,hc carburetor to give a higher carbon dioxide percentage, as shown in Test 2, the mileage was increased 23.5 per cent. Talde I1 shows results obtained with a six-cylinder automno1)ilc. B y adjusting the carburetor l o gi& ZI carbon dioxide percentage as shown in Test 3 a marked increme in mileage was accomplished.
hli. "ei rncreare Gal. Per cent 13.6 17.0 23.5 16.0 ,.,. 13.0 23.1
....
to {>ass hhrough cock 1: and out through the water seal IT. Khcn the automobile is operating a t the desired spcrd a sample is taken into the indicator by turuin:: cook E to the Icft 900, t,lws allolving the liquid in the buret to fall and pull in a mmple of gas. The exhaust pressure is sufficient to force tlie gas out through lire hot,toan of the h r e t int.o bottle C. Cock I?is then turned !IOo to tire rig!it and tlie gas in the buret allowed to stand 30 wc. to pcniiit the gas t,o come to the surrounding temperature and allow the buret to drain. Allowing the gas to stand and cool draws some liquid froin C into the bottom of the buret on account of contraction of the gas.
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turn to right A 1.3 iiirnr to left B same 1 s Test 10 A 1.5 turns t o left n same as T r J t 10 A 1.7 tmnE to left I3 same as Test 10 h same as T e \ l 3 B 2 turns to kit A 1.9 turns to lei, B same as Test 4 A 2.1 turns to i e l i B same as Test 6
July, 1922
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
599
FIG.10
Table I11 shows results obtained with a n eight-cylinder automobile. The adjustment which has been used previous t o the tests is given by Test 10. By readjusting the carburetor the mileage was increased as shown. The adjustment
shown by Test 6 was found to be too lean when tested a t low speed on hills and the carburetor was changed back to the setting given by Test 4 and operated a t that adjustment satisfactorily.
TABLEIV-EFFECT OF CARBURETOR ADJUSTMENT ON GASOLINE CONSUMPTION AND EXHAUST GASANALYSISON LOADED 3.5-TON TEUCK Truck equipped with a 4-cylinder 33/a-in. bore X 53/4-in. stroke L head motor, having a White carburetor containing low and high speed nozzles of fixed size. The carburetor was not heated, the i h e t air t o carburetor was partly heated, and portion of the intake manifold is cast in cylinder block. The total weight of loaded truck was 8.55 tons. In was geared 13.3 t o 1in high gear and governed for maximum speed of 15 mi. per hr. Tests were conducted a t 15 and 10 mi. per hr up 3 per cent grade and on level grade of dry pavement. Choke on carburetor air was left wide open on all tests which were conducted with gasoline F.;50 per cent-262' F.; dry-433O F.; average 275' F. 57.4O B&, distillation 10 per cent-198 Gasoline Carb. Setting Consumption Lb. Air Per cent Diam. Nozzles Gal. Mi. per Lb. CompleteLow High R. P. M. per per --EXHAUST GAS ANALYStS, P e R CENT B Y VOLUME-Gasoness of In. In. Motor Mi. Gal. co2 0 2 eo CH4 Hz hTn line Combustion Results o f tests at 15 mi. P e r hr. U -P 3 -Per cent w a d e 0.032 0.032 1530 0.2571 3.48 10.5 0.4 6.1 0.8 1.5 81.4 12.6 78 0.032 0.035 1560 0.2741 3.65 1.7 11.6 0.3 4.5 0.4 81.5 12.7 82 1577 0.2805 3.57 0.033 0.038 10.4 1.3 4.6 0.9 1.5 51.3 13.2 77 0.033 0.040 1547 0.3373 2.96 0.5 7.9 0.6 3.7 78.3 11.5 69 9.0 0.037 0.046 1734 0.4679 7.8 2.14 5.5 1.0 13.4 1.4 70.6 8.8 48 Results of tests at 10 mi. Per hr. u0. 3 .aer ceitl W a d e 0.032 0.032 1096 0.3007 81.8 13.0 79 3.33 10.9 0.7 1.5 0.5 4.6 0.032 0.035 1067 0.2780 3.60 11.9 0.7 3.1 0.6 1.0 82.7 13.7 84 0.033 0.038 1090 0.2871 3.48 10.8 1.0 4.5 0.7 1.8 81.2 13.1 79 llt7 0.033 0.040 0.3060 3.27 3.5 79.1 12.0 75 10.0 0.4 6.4 0.6 1.99 5.9 1.3 12.5 1.8 6.2 72.3 9.2 51 0.037 0.046 0,3770 Results of tests at 15 mi. per hr. Eevel grade 80.8 12.4 5.61 11.0 0.5 5.3 0.5 1.9 79 1655 0.1780 8.032 0,032 82.2 13.3 1.4 82 3.8 0.6 0.1756 5.69 11.6 0.4 1642 0.035 0.032 52.0 13.6 1.4 82 4.1 0.5 0.1770 5.65 10.9 1.1 1662 0.035 0.033 77.7 11.3 3.7 68 7.7 0.8 0.1907 5.24 9.3 0.8 1669 0.040 0.033 9.5 0.2928 73.0 5.7 5.9 1.5 12.7 1.8 51 3.42 1642 0.046 0.037 Results of tesls at 10 m i . per hr. level grade 1.6 81.4 12.5 79 0.6 4.7 0.7 5.75 11.0 0.1727 1156 0.032 0.032 83.0 13.6 0.7 55 0.7 0.6 2.7 5.75 12.3 0.1727 1143 0.035 0.032 13.0 1.0 81.3 1.6 3.7 0.9 SI 5.73 11.5 0.1743 1143 0.038 0.033 69 3.0 11.7 75.8 1.0 0.8 6.9 5.54 9.5 0,1805 1130 0.040 0.033 49 4.2 10.2 73.6 2.6 3.5 10.7 3.33 5.4 0.3001 1133 0.046 0.037 1 Changed gears. REMARXS The mixture was not lean enough with any adjustment t o decrease power sufficiently so t h a t the truck would not meet conditions of tests. T h e motor did not backfire on any test after becoming thoroughly warmed, with air choke wide open. On tests with 0.032-in.-0.032-in. nozzles in carburetor (smallest used), the truck did not possess the flexibility in operation i t did with other adjustment, not being as sensitive t o throttle changes. With0.037-in.-0.046-in. nozzles in carburetor the mixture was too rich and the motor did not develop its normal power. With this adjustment the truck this test was run in third gear. would not ascend 3 per cent grade a t 10 mi. per hr. in fourth gear; the last part of. Nozzles 0.033 in.-0.040 in. are supplied by manufacturers as standard for thls truck for the Pittsburgh district.
