March, 1927
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
389
A Laboratory Method of Determining the Starting Properties of Motor Fuels’ By Wheeler G. Love11 and John D. Coleman, with T. A. Boyd RESEARCH LABORATORIES, GENERALMOTORSCORP., DETROIT,MICH.
One of the most important properties of a motor fuel is the ease with which engines may be started on it. The starting characteristics of any given fuel are a complex function of its physical and chemical properties, and of the temperature of its mixture with air. Hence, the starting characteristic is a property which cannot be determined accurately from conventional distillation curves, or from dew points and similar measurements. Ease of starting may be determined directly in a n engine, of course, when proper means are provided for controlling conditions. But, since a large amount of equip-
ment is needed for such a test, an attempt has been made to devise a laboratory method of determining the readiness with which an explosion may be obtained with any given fuel at the temperatures desired. This method consists in measuring directly the air-fuel ratio necessary to produce an explosive mixture at any given temperature. It has been applied to the testing of a considerable number of fuels of widely different properties, over a broad temperature range, and has been found to yield results that are comparable with those obtained in starting tests on actual engines,
I K M B E R of methods have been used for estimating the relative volatilities of motor fuels. Most of these do not give directly a measure of the starting performance of the fuels, however, and the estimation of starting characteristics from the data obtained is only an approxhation a t best.
I n order to measure the starting characteristics of a gasoline, or its effective volatility as applied to starting conditions, it is necessary to determine how readily the gasoline, or a portion of it, evaporates a t temperatures that prevail where the engine must be started. This involves a determination of how readily enough fuel evaporates under the given conditions to make an explosive mixture in the engine.
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Starting tests can be made directly on engines,’,* of course; b u t such a method can hardly constitute a convenient one for use in the laboratory, because it requires a complicated and expensive apparatus. An estimate of the effective starting volatility of a gasoline may be made from the conventional distillation curve. It is evident t h a t a fuel having a low boiling range throughout will evaporate quite readily. Except in the case of fuels t h a t are markedly different, however, this method does not give much information on starting properties, and i t usually breaks down entirely in the case of distillation curves t h a t are close together, or of those t h a t cross each other. It is recognized generally t h a t the estimation of the starting volatility of a gasoline from the initial point of its distillation is not a dependable method. The dew point, or the temperature a t which a liquid first begins to separate from a mixture of completely vaporized gasoline and air, has been employed for estimating the Volatility of motor fuels. This is a property that may be measured either by static or by dynamic and with good accuracy It may be estimated also, although possibly with less accuracy, by means of a n equilibrium distillation of the gasoline4 where a liquid in equilibrium with its vapor is prepared. As an extension of this latter, rules have been laid down for estimating the dew point in a simple manner by calculation from the temperature a t which 85 per cent of the fuel has boiled away in the usual distillation test. But methods involving the dew point relate to conditions of complete evaporation of the fuel, and are therefore not applicable directly to starting conditions. where the temperature is often far below the dew point and even below the initial distillation point, and where only a small portion of the fuel may be vaporized. Closely allied t o dew point methods and subject t o their disadvantages is the use of the end point suggested by Stevenson and Stark, or the temperature of complete vaporization of a fuel under equilibrium conditions a t atmospheric pressure.5 Another means of determining the volatility of a gasoline is by estimation from its vapor pressure. However, this method has an obvious inaccuracy in t h a t gasoline is a complex mixture, the vapor pressure of which decreases as evaporation proceeds. Still a fourth methods consists in determining the percentage of the fuel that evaporates when fuel and air are passed through a tubular helix under definite conditions of temperature and relation of amount of fuel to air. From the standpoint of measurements of starting characteristics, this method has the disadvantage that it requires the establishment of a n assumed value to the fuel-air ratio necessary to get an engine to start. Presented before the Division of Petroleum Chemistry a t the 72nd Meeting of th8 American Chemical Society, Philadelphia, Pa., September 6 to 11, 1926. * Numbers in text refer t o bibliography.at end of article.
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The Explosive Limit Mixture
When liquid gasoline is exposed in the presence of a large quantity of air, evaporation begins, and the vapor pressure of the liquid is then a t a maximum. If there is too small an amount of air, even the very low boiling portion of the fuel will not evaporate completely, but some will remain unvaporized in the liquid, and maintain there a vapor pressure equal to that exerted by the vapor of the part already evaporated. I n the case in which there is a n infinite amount of air, the partial pressure of the fuel vapor, which is a function of its vapor concentration, will be close to zero; so that ultimately all of the fuel will evaporate. Such a condition as this is below the practical limit, so far as the automobile is concerned, because, in order for the engine to start, there must be enough fuel vaporized into the air in its cylinders to form an explosive mixture. This explosive mixture may be obtained either by placing in contact with the air present a large amount of fuel, only a small portion of which evaporates, or by using a fuel that will maintain a high vapor pressure after a considerable portion of i t has evaporated, thereby forming a combustible mixture from a small total amount of fuel. On this basis, then, a fuel that will give easy starting of a gasoline engine is one which, when mixed with air, evaporates so completely that only a little of it is required to yield an explosive mixture. From a practical point of view a volatile gasoline is one that requires little “choking” in order to get a vapor mixture that is rich enough to ignite in a cold engine. Consequently, the effective volatility of a gasoline from the standpoint of starting an engine is related t o the explosive limit of a mixture of a portion of the gasoline with air. Since gasolines differ in chemical properties, and since all hydrocarbons do not have the same explosive limits, the effective volatility of a gasoline is a combination of chemical and physical properties. A direct method of determining the effective starting volatility of a gasoline would be to take a definite amount of gasoline with respect t o a given amount of air, bring them to
ISDlYA9TRI-41, ASD E-YGIiVEERINC: CHE.WIXTRY
LIarch, 1927
A calibration of the entire apparatus was made with a sample of pure pentane. Burrell and Boyd' determined the explosive limit of a light gasoline vapor, which had an average composition expressed by pentane, C5HI2. As a mean of the various conditions, the explosive limit was found to be 1.5 per cent of pentane by volume, which corresponds to an airfuel ratio of 26.2: 1. 1 careful determination made in the present apparatus gave a value of 23.2: 1. From this close agreement, it is reasonable to conclude that the results ob90
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That is to say, in practice, the carburetor must be "choked" to various degrees for the different fuels in order to start engines on them. Hence, the data obtained by this method might be considered as an '(index of choking." The curves for the fuels shown in Figure 3 are regular, but they are by no means parallel. It is apparent from this chart, and also from other data presented later, that the effective starting volatility of a fuel does not change with the temperature similarly for all gasolines. This indicates the I
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Figure 4-Air-Fuel Ratio Required t o Start Engine in Twelve Seconds a t Various Temperatures Calculated from Bureau of Standards Data on Five Fuels
importance of making determinations of the effective starting volatility a t somewhere near the temperatures that will be encountered in the actual starting of engines. The case of the benzol blend, No. 7 may be cited in illustration of this point. At the higher temperatures, the ignition curve of this blended fuel parallels that of a straight gasoline, No. 2. But a t about - 5 " C. (23" F.) it breaks sharply to the left, possibly because a t that temperature the benzol in the mixture begins to have such a low vapor pressure that large amounts of the fuel must be present in order for enough of the aliphatic portion of the mixture to be evaporated to form an explosive mixture. This forms a good illustration of a fuel that is quite volatile a t usual temperatures, but whose volatility decreases very rapidly a t lower temperatures. Because of interest in this connection, data are presented for a single hydrocarbon, as distinguished from a mixture of a large number, such as gasoline. Pentane was chosen for this, its ignition curve being the vertical dotted line a t the right of Figure 3. Because pentane is a pure substance, its vapor p r e s s u r e d o e s not change with the ld amount e v a p o r a t e d . I I Consequently, as long ___ ~as the temperature is such that the vapor pressure of the liquid is high enough to give an explosive mixture, all of Q ____.__ it will evaporate (provided, of course, that I an e x c e s s of l i q u i d 8 12 Yd .zV above saturation is not *= r"=& nT?a @Ta??LmKm@Z present), and the Figure &Comparison of Results of mixture ratio to give Engine Starting Tests and Laboratory Determinations for Five Fuels a t Various an explosion remains Temperatures unchanged by the temperature. For temperatures such that the vapor pressure of the pure liquid is not sufficient t o form a n explosive mixture, the presence of a larger amount of liquid would not change the condition a t all.
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INDUSTRIAL A N D ENGINEERING CHEiMISTRY
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Correlation of Results with Engine Starting Tests
Using an actual engine, Eisinger’ has determined the starting properties of the five fuels of Figure 3. These engine data give the amounts of fuel that were used and the times required for starting the engine under carefully controlled conditions, and a t various temperatures. If i t is assumed that the volumetric efficiency of the engine is 100 per cent, it is possible to compute from Eisinger’s data the air-fuel ratio necessary to give a start in 12 seconds for various tempera-
Vol. 19, No. 3
presented in this paper, is plotted against the mixture ratio necessary to start the engine in 12 seconds a t various temperatures, as computed from the data of Eisinger, a straight line should result, provided that the two sets are comparable. Such a plot is shown in Figure 5. The points, representing five different fuels and temperatures ranging from 30” C. to 0” C. (88” F. to 32” F.),some twenty points in all, fall fairly closely along a straight line. Considering the wide difference in the methods used and in the range of the variables present, this would appear to be quite a significant agreement. That the slope of the line is not unity indicates that the mixture ratios calculated from the engine experiments may possibly be too low. This is due to the fact that the volumetric efficiency of the engine is not 100 per cent, and that the vaporization was not perfect. It may be due, also, in part to the fact that time allowed for ignition in the bomb was longer than it is in the engine. T h i s correlation of the results i n d i c a t e s that the l a b o r a t o r y \ method of measuring @ the mixture ratio re- \ 9 quired to give an explosive mixture, as described above, yields \/@ results which in general a r e p r o p o r t i o n a l to those obtained by enIn.#/,/,,, gine starting tests. I n o t h e r w o r d s , these , laboratory results have I
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Figure 6-Starting Characteristics a t Various Temperatures of Gasolines Used in CoGperative Fuel Research
tures. Although the volumetric efficiency was not 100 per cent, it was probably constant; so that while any one of the values obtained does not represent the actual mixture ratio, it should be proportional to it. The time of 12 seconds was chosen for use in making the computations, because data upon all five fuels were available only for this particular time period. These data upon mixture ratio as computed from the Bureau of Standards results are shown in Figure 4, where the mixture ratio upon which the engine starts in 12 seconds is plotted against the temperature. As may be seen by comparing these curves with those of Figure 3, they are similar to those obtained by the laboratory method, and the fuels lie in the same order. The two sets of data are not identical, however, probably on account of the unavoidable uncertainties in the computation, and of the fact that while equilibrium conditions may prevail in the bomb, they do not in the engine experiments. If the mixture ratio necessary to give an explosive mixture with each of the different fuels, determined by the method
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and not of the engine, Figure 8-Relationship of Starting and Distillation Data for and the method gives Characteristic Ten Straight R u n Gasolines Tralues which are indicative of the effective starting volatility of the fuel in a “perfect” engine, with a “perfect” carburetor. Relationship of Results to the Dew Point
Through the courtesy of W. A. Gruse of the Mellon Institute of Industrial Research, samples of gasolines A , C, and D,used in the Cooperative Fuel Research,8 were secured. The air-fuel ratios necessary to give an explosive mixture for these fuels were determined, by the method presented in this paper, over a wide temperature range. The results are shown in Figure 6. Dew point measurements of these three fuels are available, determinations having been made by the dynamic method of Gruse,2 by the direct method of KennedyJ3and also by computation from the 85 per cent point, and from data upon equilibrium solutions developed by Wilson and B a r r ~ a r d . ~ These data are given here in tabular form. METHOD
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Figure 7-Relationship of Dew Point and Starting Characteristic of CoEperative Fuel Research Gasolines A , C, a n d D (1) Dew points (Wilson and Barnard) at 15 1 ratio. (2) Dew points (Kennedy) at 12 1 ratio
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As a means of comparing the volatility determined by the method presented herein with the dew point measurements of these gasolines, a plot has been made, as shown in Figure 7, of the dew point a t a 12 : 1 ratio against the air-fuel ratio to give an explosive mixture. Because these curves are not straight lines, there seems to be no direct relationship between
IhTDUSTRIALANDIENGINEERING CHEMISTRY
March, 1927
the dew point of a fuel and its starting characteristics as determined by the method presented in this paper. This is to be expected, in a way, because the dew point is based upon complete vaporization of the fuel, whereas starting an engine I
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It is of special interest in this connection to see how two gasolines compare which have practically identical distillation curves, as determined in the usual manner. A group of four gasolines of different chemical composition was chosen, and from a series of naphthas from midcontinent crude oil another set of gasolines was made up so that for each of the original gasolines there was a "synthetic" gasoline having a distillation curve identical to it, within the experimental error. These distillation curves are shown in Figures 11B, C, D, and E. The original gasolines probably had rather widely different chemical constitutions, as they consisted of two vapor-phase cracked gasolines, a California gasoline, and a gasoline made by cracking Smackover crude in the liquid phase. The latter sample was obtained through the courtesy of Gustav Egloff of the Universal Oil Products Company, and those of the vapor-phase products were supplied by C. K. Reiman of A. D. Little, Inc. The samples of the straight-run naphthas were obtained by the courtesy of J. Bennett Hill of the Atlantic Refining Company. Determinations of the air-fuel ratio to give an explosive mixture for these eight gasolines were made over a considerable temperature range. Comparative curves for these fuels are shown in Figure 9. It is apparent from this chart that there may be a considerable difference in volatility between two gasolines having al-
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chemically. The data of Figure 3 show that the effective volatility of gasolines changes with the temperature a t different rates. Hence, because the volatility of different gasolines changes unequally as the temperature varies, no measure of the effective starting volatility of a fuel can be given, unless a definite temperature is specified to n-hich it corresponds.
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Distillatfon Curves of Fuels
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INDUSTRIAL AND ENGINEERING CHEMISTRY
394
most identical distillation curves. At first thought, it might be considered that this difference was due to a difference in chemical constitution. This point was not established in this investigation, however. I n Figure 11F are distillation curves of two gasolines prepared by mixing naphthas in slightly different proportions. The distillation curves are remarkably close. Their volatilities, however, as determined over a temperature range, differ, as shown in Figure 10. This slight difference in the distillation curve, with gasolines of similar chemical constitution, which makes a relatively large difference in the volatility as determined, would indicate that the differences in volatility previously observed for the pairs of fuels of different chemical constitution may be due in part a t least to differences in distillation curves, or of boiling points, which do not appear upon the distillation curves. I n the light of the present information, therefore, i t cannot be stated definitely whether a change in chemical constitution of a fuel, unaccompanied by a change in boiling point, does or does not alter the starting volatility as determined by the present method. On the other hand, it is evident from these data that dis-
VOl. 19, x o . 3
tillation curves of the Engler type cannot be of sufficient accuracy to make dependable estimations of relative volatility, especially a t low temperatures. Probably this is due in part to the fact that a t these low temperatures only the very volatile portion of the fuel is evaporated. This is the portion which either distils over in the Engler distillation as an uncondensed gas, or boils off so rapidly as not to appear clearly in the initial distillation point as recorded in usual practice. Bibliography 1-Eisinger, J . Soc. ;lutomotiue Eng., 18, 147 (1926); 19, 3 (1926). 2-Gruse, I n d . Eng. Chem., 16, 796 (1923). 3--Kennedy, Bur. Standards, Sci. Paper 600 (1925). I--Wilson and Barnard, J . Soc. Automotive Eng., 12, 287 (1923); I i l J . Eng. Chem., 17, 428 (1925). 5-Stevenson and Stark, Ibid., 17, 679 (1925). 6-Sligh, J . Soc. Automotive Eng., 18, 393 (1926); 19, 151 (1926). 7-Burrell a n d Boyd, BUY.Mines,Tech. Paper 116 (1915). 8-Carlson, J . Soc. Automotive Eng., 12, 139 (1923); Lee, I b i d . , 18, 3 (192:1) , Birdsell, Ibid., 14, 267 (1924); Eisinger, Ibid., 16, 69, 333 (1924); Spnrrow and Eisinger, I b i d . , 16, 237 (1925); Eisinger, I b i d . , 17, 52 (192.5).
Spontaneous Heating of Oils' Methods of Testing By Norman J. Thompson INSPSCTION D E P T . , ASSOCIaTED
FACTORY & ~ U T U A LFIREI N S G R A X C E
Chemical methods such as absorption of iodine or of oxygen are not suited to critical examination. The iodine number often has no significance, and the results obtained by absorption of oxygen are subject to uncontrollable errors. The original Mackey method is not suitable for indication except in the case of the more dangerous oils. By increasing the size of the sample to 30 grams, using clean cotton waste carrying an equal weight of oil, more significant results can be obtained with no more attention, even though the test time is longer. The method which in general gives the most information involves the maintenance of a small temperature differ-
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Chemical Methods
There has been a tendency in the past to class various oils on the basis of their iodine numbers; the hazards were assumed to be directly proportional to the iodine values. However, the iodine number does not indicate either the extent or the rate of oxidation except in a general way. For example, the lower the iodine number of linseed oil the more rapid will be its oxidation a t normal temperatures. This is true in nearly every case and can be explained by the fact that oxidation, while decreasing the iodine number, greatly assists further reaction due to the catalytic effect of the October 25, 1926.
ence between the sample and its surroundings. This method clearly shows that certain oils formerly thought to be relatively safe may, under favorable conditions, be more hazardous than other oils which have been classed as dangerous. If the temperature difference is taken care of automatically, the apparatus might be complicated and somewhat expensive. If the regulation is secured manually, the operator's constant attention is required. Regardless of the method employed, results can be interpreted only by comparison with tests on oils of known performance. Furthermore, the conditions under which the oil is to be used must be known in order to indicate correctly the probable hazard from spontaneous heating.
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ONG experience with fires caused by oils such as lard oil, red oil, and wood oil led to grave doubts regarding the value of available methods of testing for tendency to heat spontaneously. Accordingly, a study was made in order to obtain more definite information about existing methods, and, if necessary, to develop a new method which would more closely simulate the very favorable conditions often existing in practice.
1 Received
COMPANIES, BOSTON, M A S S .
oxidation products themselves. Furthermore, substaiices are frequently added to commercial oils which increase the iodine numbers but a t the same time decrease the oxidation rates. Another method of indicating the spontaneous heating or autoxidation of oils is to measure directly the absorbed oxygen. This method, however, is unsatisfactory for critical examination. When samples such as cottonseed oil or oils of even greater heating tendencies are tested by such a method, i t is found that the oxygen absorbed in a definite length of time is closely proportional to the heating rates of the oils as determined in other ways. However, when testing oils such as castor oil or mixtures of cottonseed and mineral oil, it is often found that instead of a gas absorption there is an increase in pressure. This pressure increase is due to the vapor pressures of the oil and of its oxidation products, and is great enough a t temperatures around 100' C. to make unreliable any figures obtained by the absorption method. Since we are most interested in obtaining significant data on oils of lesser heating tendencies than cottonseed oil, this method proves utterly lacking.
