June, 1930
649
INDUSTRIAL, A X D E,VGINEERING CHEMISTRY
(2) Andre, Comfit. r e n d . , 175, 107 (1922). (3) Elsdon, “Chemistry and Examination of Edible Oils and Fats,” 1926. (4) Gardner, Coleman, Paint Mfrs. Assocn., U. S., Tech, Circ. 120 (1921). ( 5 ) Gates, J . Phys. Chem., 15, 101 (1911). (6) Jacobson and Holmes, J . Biol. Chem., 26, 55 (1916). (7) Klimont, J . grukt .Chem., 109, 266 (1925). (8) Koling, J . A m . Chem. SOL.,36, 951 (1916). (9) Lee, .4nn. R e p t . .4gr. Mort. Res. .?tu., Unri.. Bristol, 1917, 39 (1918).
(10) Lewkowitsch, “Chemical Technology and Analysis of Oils, Fats, and Waxes,” Vol. I (1912). (11) Seave, Anolrst, 37, 399 (1921). (12) Oudemans, J . p r a k t . Chem., [ l ] , 81, 356 (1860); 99 (1866). (13) Rather and Reed, Arkansas Agr. Expt. S a . , Bull. 156, 32 (1918). (14) Veimarn, von. J . Russ. Phys. Chem. .Tot., Proc. 1916, 48, 703. (15) Wright and Mitchell, “Oils, Fats, Waxes, and Their Manufactured Products,” 1921
Motor-Fuel Volatility’ 11--S tarting Volatili ty*,3 George Granger Brown, Charles L. Nickols, and Paul Bigby UNIVERSITY OF MICHIGAN, ANN ARBOR,b f I C H .
HE m o s t i m p o r t a n t characteristic of a fuel is its ability to start the motor in which it is to be u s e d . M o t o r - c a r drivers readily recognize difft,’rences in the effect which various fuels have on the ease of starting and frequently judge the value of a fuel almost entirely on this basis. It is extremely important for both c o n s u m e r and producer to u n d e r s t a n d what qualities make a fuel effective in starting the motor easily and quickly.
T
Previous Investigations
In the first paper of this series, published previously (2, Part I), equilibrium volatility was defined and a satisfactory method was described for its determination. In the papers comprising the present group, motor-fuel volatility has been interpreted in terms of engine performance. The second paper of the series, which is the first of the present group, relates equilibrium volatility to ease of starting as determined by actual engine tests. The third paper defines effective volatility under driving conditions and indicates how it may be determined from the equilibrium volatility or A. S. T. M. distillation data. The fourth paper relates effective volatility to engine performance and suggests the A. S. T. M . distillation characteristics required for satisfactory performance under different conditions of atmospheric temperature. The fifth paper discusses the relation of vapor pressure to vapor lock and suggests the fuel characteristics requisite to insure freedom from this trouble.
During the past six years an extensive investigation of engine starting has been conducted at the Bureau of Standards as part, of the cooperative fuel research program. Cragoe and Eisinger (5)identify the vaporization of fuel in bhe carburetor and manifold of a motor during the cranking period as “engine distillation.” This engine distillation is not a simple equilibrium distillation, such as was described in Part I of this series 07). It does not take place at at’mospheric pressure nor necessarily at atmospheric temperature. On account of the choke and throttle, the absolute pressure in the manifold during t he cranking period may be considerably less than atmospheric. This would tend to cause more of the fuel to vaporize than would be the case in an equilibrium air distillation. The latent heat of vaporization of the fuel must come eit’her from the surrounding metal part of the carburetor and manifold, from t’he air, or from the liquid fuel itself. The absorption of bhis latent heat lowers the temperature of the mixture and tends t o decrease the amount of fuel vaporized. Furthermore, the short time interval for the completion of the engine distillation clearly indicates that equilibrium conditions would not be obtained during the cranking period. 1 The investigations reported in this series of papers have been carried out during the past four years in the Chemical Engineering Laboratory of the University of Michigan. Owing to the interdependence of various phases of the work, publication of the results has been delayed. More complete information than can be given in this series of papers will he found in Bulletin 14 of the Department of Engineering Research of the University of Michigan, available in the near future. * Received April 26, 1930. * Part of a thesis submitted by Charles I,. Nickols in partial fulfilment of the requirements for the degree of doctor of philosophy at the University of Michigan.
On the assumption that t h e decrease in pressure in the manifold tends to compensate for the decreased temperature due to the latent heat of vaporization and also for the short time interval allowed for engine distillation, the r e s u l t a n t mixture attained in the equilibrium air-distillation may be regarded as the ideal attainable only in infinite time in the engine d i s t i l l a t i o n . Thus the efficiency of the engine distillation may be defined as the ratio of the air-vapor ratio attained in the equilibr i u m d i s t i l l a t i o n to that attained in the engine distillation. This efficiencv depends only slightly upon such factors as temperature; fuel, mixture-ratio supplied, and cranking speed, but does depend almost entirely upon the number of engine revolutions. Cragoe and Eisinger concluded that about ten revolutions of the engine were required to obtain an explosive mixture in the cylinders when a mixture was supplied a t the lower end of the manifold, which would give a resultant air-vapor mixture of 12 to 1 under equilibrium conditions. They derived the following equation which fitted their experimental data with an accuracy of about *25 per cent: log R E ~ = D 1.301 -
where -READ N
2
y7 *V
resultant air-vapor mixture attained in equilibrium-air distillation = number of engine revolutions =
This equation corresponds to a 20-to-1 air-vapor mixture required in the cylinders for starting. Cold-Room Tests
Since these conclusions were based entirely upon tests made over a limited temperature range and with the special set-up described by Eisinger, it seemed desirable to make a thorough investigation of the ease of starting a cold motor with widely different fuels when the motor was driven by the electric starter supplied from the same battery as was used for ignition and with the standard carburetor mounted on the manifold instead of the special fuel system used in the tests a t the Bureau of Standards. E ilPPARATuS-The tests reported in this investigation were made on a small, water-cooled, four-cylinder, valve-in-head
I-VDUSTRIAL A N D ENGINEERISG CHE-MIXTRY
650 Table I-Relation
between Engine and A. S. T. M. Temperatures for Starting with a 1 : l Air-Fuel Ratio
I
wM
RESULTANT ENGINE VAPORIZED AIR-VAPOR REVOLUTIONS TOSTART T~~~~
1
P e r cent a
6 5 7 5 10 25
20 15 4 13 3
LO
4
.4verage of last four lines
m
20 13 7 3
Vol. 22, No. 6
1 2774 1 27 1 284
ENGINE TEMPERATURE 30' C. 86'F.
10' C. 50'F.
0°C.
32°F.
-1O'C. 14'F.
-2O'C. 4'F.
-
-3OoC. -22'F.
-4OOC. -40'F.
CORRESPONDING A. S. T . M. DISTILI~ATION TEMPERATURE
F.
O F .
232
I
20' C. 68'F.
237
228 216 214 209 216 213
passenger car motor of 311/1~-inch bore and 4-inch stroke. (Figure 1) The motor was standard equipped in every way except that the fan blades were removed, and in most tests a battery of larger capacity was used than is ordinarily supplied with this motor. Extensive tests showed that the use of the larger battery had no effect except to give slightly more consistent results and it required less frequent charging. T h e starting switch is interconnected with a solenoid, which automatically starts the stop watch as soon as the starter button is depressed and stops it upon release of the starter button. The revolution counter is actuated directly by the motor. I n order to determine the air-fuel ratio supplied by the carburetor, the rate of air-flow is measured by the orifice meter placed before two 5-gallon surge tanks in the air line to the carburetor, and the fuel used recorded as the differeiice between the readings on the fuel burets before and after the test, Since the revolution counter is direct connected to the tnotor, the total revolutions of the motor are recorded even if the motor continues to run for a few moiiients after the starter button and ignition are turned off.
REVOLUTION
Figure 1-Apparatus Used in Making Starting Tests
I'ROCEDUHE-I~starting a series of tests, the carburetor
>vas carefully wiped dry and the motor cranked for about 100 revolutions with no gasoline in the carburetor to insure that the tests would not be influenced by the fuel previously used. The burets were then filled with the fuel to be tested, and the bowl of the carburetor was lightly tapped to insure proper seating of the float needle valve. All tests were made a t a fixed throttle of about onequarter open. The stop watch was set to zero and the reading' on the revolution counter recorded. Room temperature and barometer readings were also recorded a t this time, and in most cases the specific gravity of the fuel was also recorded. The starter button was then depressed, autoiliatically closing the ignition switch and starting the stop
F. 205 193 192 187 193 191
a
F. 181 171 169 164 171 168
F. 158 147 146 141 147 145
O
F
135 125 122 119 125 122
F.
OF
111 102 99 95 102 99
79 77 74 79 77
88
watch. The time taken to obtain the first explosion was automatically recorded by the stop watch, and the revolutions of the motor when the first explosion was obtained were read from the revolution counter, a t which instant the starter button was also released. The average r. p. m. was obtained by dividing the revolutions required for starting by the fractional part of a minute required to obtain the first explosion. At the conclusion of such a test the carburetor bowl and burets were drained of fuel as a check on the readings of the burets. The carburetor, aftcr being allowed to drain for 5 or 10 minutes, was carefully wiped dry. The motor was then cranked for about 100 revolutions previously to making the next test. Each series of tests Tvas started with the choke open to see if a start could be obtained without choking. The following tests were then run a t the various fixed choke positions until a perfect start in three or four revolutions was obtained. -4t least three checks were obtained a t each choke setting. I n order to obtain temperatures below that of the laboratory, the motor was placed in a room cooled by direct expansion of ammonia. At the lower temperatures it was found necessary to dilute the oil with a t least 25 to 30 per cent of gasoline in order to make cranking possible. Many undiluted special low cold-test oils were tried, but without success. The capacity of the storage battery also approaches zero at about -40" F. and inany of the tests were made possible only by bringing a warm battery into the cold room. RESULTS-AS has been frequently reported, starting was found to be far more difficult at low cranking speeds such as 20 r. p. m., than at the higher speeds, such as 140 r. p. m. However, when the air-fuel ratio supplied by the carburetor was computed for these different conditions, it was found that the same number of engine revolutions were required for a given air-fuel ratio a t the carburetor regardless of cranking speed. By dividing the total fuel used by the total engine revolutions, the fuel used per engine revolution is obtained. By dividing the air flow in cubic feet per minute by the average revolutions per minute, the air supplied per revolution is siniilarly obtained. These data are plotted separately in Figures 2 and 3. It is seen that the points fall on smooth curves which may be used in determining the air-fuel ratio supplied by the carburetor a t any of the fixed choke settings and motor r. p. m., provided only the specific gravity of the fuel a t the temperature in the buret is known. All the data obtained regarding the number of revolutions required to start could then be plotted as a function of the air-fuel ratio supplied a t the carburetor for a given room temperature. By use of the relationship given by Brown and Skinner ( 2 ) and the continuous vaporization curves of the fuels used in these starting tests, as given in Figure 4, the resultant airvapor mixtures formed under equilibrium conditions from the air-fuel ratios supplied at the carburetor were computed.
I.VDCSTRIAL A N D ESGIA-EERING CHEiVISTRY
June, 1930
0
20
40
bo
80
100
I20
140
160
I80
651
ZW
SPLEO R P M
Figure 2-Volume of Fuel C o n s u m e d per Revolution of Motor Plotted a s a Function of t h e Cranking Speed for Different Choke Settings
In Figure 5 the log of the resultant air-vapor mixture is plotted as a function of the reciprocal of the number of revolutions required to start. For purposes of comparison, the straight line deduced by Cragoe and Eisinger is also shown in Figure 5 . The results of this investigation using standard commercial equipment agree within the experimental error with those obtained a t the Bureau of Standards using a motor specially equipped and cranked by a dynamometer.
SPEED
R.P.M
Figure 3-Volume of Air Consumed per Revolution of t h e Motor Plotted a s a Function of t h e Cranking Speed for Different Choke Settings 400
I
I 1
,
Effect of A . S. T. M . Distillation on Ease of Starting
-Although the equilibrium volatility of a fuel cannot be accurately predicted by any simple relationehip from the lower end of the -1.S. T. hl. distillation curve. the actual starting tests are consistent only within *20 per cent, so that any relationship accurate within these limits should prove satisfactory for all practical purposes. The original relationship given by Cragoe and Eisinger was apparently based on inadequate information, and the results appear optimistic. For example, it was suggested that any fuel having a 5 per cent point on the X. S. T. 31. distillation at or below 119" F. should d a r t a car with a 1:1 air-fuel ratio a t a temperature of -40" F. Fuels 5115 and 1041 used in the present investigation just about meet this requirement, but it was impossible to start the motor a t -26" F. with fuel 5115 in this air-fuel ratio. In fact, it required 30 revolutions to start with this fuel at this temperature with an airfuel ratio of 0.3:l. Similarly, it was found impossible to start the motor with fuel 1041 at -30" F. even with an airfuel ratio of 0.3. Just possible starting with fuel 1041 in an air-fuel ratio of 1: 1 was found to occur at about - 15" or - 17" F. -4gaiii Cregoe and Eisinger indicated p:rssible starting at 40" F. with a 1 : l air-fuel ratio if the f u d has 5 per cent distilled on the A. S. T. M. distillation at 230" F., but experiment show< that fuel 29 with 5 per rent distilled below 200" F. gives possible. starting with a 1 1 air-fuel ratio only at temperatures well above 40" F. Bridgemaii ( 1 ) has revised these relationships upon the basii of his first approximation of the equilibrium-air distillation from the .A. F. T. M. distillation. The results appear well 11itliin the experiniental error in determining ease of starting
Figure 4-Equilibrium Vaporization Curves of Fuels Used in Starting Teats a n d in Tests for Effective Volatility
n.itli a 1:l air-fuel ratio. Computations based on these relationships are given in Table I. If the resultant air-vapor mixture of 20, requiring approximately an infinite number of revolutions for starting, be disregarded as impractical and only those mixtures which enable the motor to be started in 20 rei-olutions or less be considered, the -1. S. T. M. temperature for 6 to 25 per cent vaporized is practically constant for each engine temperature. If these temperatures are al-eragcd, a. has been done in the table, the average *I. P.T. h1. temperature may be used with an error of less than + A o E'. to indicate all practical degrees of ease of starting a t the corresponding engine temperature. Similar considerations for other air-fuel ratios indicate a
INDUSTRIAL AND ENGIXEERING CHEMISTRY
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T'ol. 22, No. 6
distillation temperature of 140" F. At the given air-fuel ratio, a fuel which is 10 per cent vaporized a t 140" F. in the A. S. T. M. distillation will enable the motor to be started in seven revolutions; 6.5 per cent vaporized will enable the motor to be started in 20 revolutions; while 5 per cent vaporized indicates a possible start in 100 or more revolutions. Since the A. S. T. M. distillation characteristics of these fuels are known and actual starting tests were made, these data serve as a check of the accuracy of the relationship between the A. S. T. M. and equilibrium-air distillations as well as that of the relationship derived by Cragoe and Eisinger between the resultant air-vapor mixture and the number of revolutions required to start. The agreement between the experimental data and the curves representing this equation seems to be within the experimental error in all cases except two.
50302010
12
moa
7
REV
e
5
TO START
4
3
Figure 5-Resultant Air-Vapor R a t i o Obtained under E u i l i b r i u m Conditions Plotted as a Function of &e Revolutions Required to Start Computed from the data of Figure 4, ,by the method outlined in the first paper in this series. The solid straight line is that deduced by Cragoe and Eisinger.
similar constancy of temperature on the A. S. T. M. distillation corresponding to particular engine temperatures. These temperatures are plotted for convenience in Figure 6. For purposes of comparison, actual experimental data obtained with the fuels whose A. S. T. M. distillation curves are shown in Figure 7 are plotted in Figure 6. The lines represent the relation between engine temperature and the corresponding A. S. T. M. temperature for the indicated air-fuel ratios. The percentages tabulated along each line indicate the percentage of fuel that must be vaporized in the A. S. T. M. distillation a t the corresponding temperature to insure a start in the indicated number of engine revolutions with the air-fuel ratio specified on the line. For example, a t an air-fuel ratio of 1, an engine temperature of 10" F. corresponds to an A. S. T. M.
0
0
I n past discussions starting has been regarded as possible if the motor could be made to fire in any number of revolutions up to an infinite number. It is very questionable if it is actually possible to start a motor in cold weather if more than 20 or 30 revolutions are required. By that time the capacity of the battery may have been completely exhausted and what may be regarded as theoretical possible starting becomes an actual impossibility. At low engine temperatures, oil or battery conditions limit the maximum number of revolutions in which a start must be obtained. With a low-viscosity oil recommended for coldweather operation the motor could not be cranked a t all at a temperature of -10" F. When this oil was diluted 50 per cent, only 19 revolutions could be obtained from a fully charged battery at an engine temperature of -20" F. For these reasons it is clear that possible starting a t low temperatures demands a fuel so volatile as to start within 10 or 20 revolutions. At higher temperatures 10 or 20 revolutions may be considered an easy start. Furthermore, the lower cranking speed at the low temperatures causes a more dilute mixture to be supplied by the carburetor, thereby making starting still more difficult. Proper allowance for this increased difficulty of cranking and starting a motor in cold weather should be made by using fuels of relatively greater volatility a t the lower temperatures. Literature Cited
A . S . T.M. TEMPERATUREOF,
Figure 6-Relationshi b e t w e e n E n g i n e Temperature a n d A. S. T. M. Distillation%emperature for S t a r t i n g a Cold Motor
(1) Bridgeman, J . SOC.Automolive Eng., 22, 447 (1928). (2) Brown and Skinner, IND. END. CHBM.,22, 278 (1930). (3) Cragoe and Eisinger, Trans. SOC.Automotive Eng., 22, Pt. I, 1 (1927).
