IhTD USTRIAL A N D ENGINEERING CHEMI8TRY
476
Vol. 15, No. 5
MOTOR-FUEL SYMPOSIUM ~~
~
~
Papers presented before a joint meeting of the Divisions of Petroleum Chemistry and Gas and Fuel Chemistry a t the 65th Meeiing of the American Chemical Society, N e w Haven, Conn., April 2 to 7, 1923
Fuel Requirements of Internal-Combustion Engines’ By Stanwood W. Sparrow BURBAU OB
STANDARDS, W A S H I N G T O N ,
D. c.
VER twelve million automobiles are in operation in
0
this country. Evidently, the “fuel requirements of intercombustion engines” have been met whether they have been understood or not. It would be easy to state the characteristics of a satisfactory fuel. This discussion attempts something more difficult-namely, to suggest how various characteristics of a fuel affect its ability to fulfill the requirements of internal-combustion engines. In what follows fuel has been considered from the standpoint of (a) availability, (b) usability, and (c) power-producing ability. AVAILABILITY To burn a gallon of alcohol one thing is essential-namely, a gallon of alcohol. No discussion is necessary as to this phase of availability, nor is there any doubt that availability means available on a commercial scale a t a reasonable price. But availability means more It means that fuel available in New Haven to-day should be available in New York to-morrow and in Detroit next week. Why? Consider alcohol. To use alcohol as a fuel carbureter jets must be larger than when gasoline is used. Making the carbureter right for alcohol makes it wrong for gasoline. It may be relatively easy to use either alcohol or gasoline as a fuel. It would be extremely difficult to use first one fuel and then the other. There is still another angle to this question of availability. In citing the advantages of fuels such as alcohol and benzene, stress is placed on their antiknock value. It is pointed out-and rightly-that this quality permits the use of high compression ratios and the attainment of higher efficiencies than would otherwise be possible. These high compression ratios will not be employed, however, until there is assurance that it will never be necessary to use fuels of less antiknock value whose performance in a high-compression engine would be extremely unsatisfactory. France and Germany a t least have given serious thought to the military side of availability. They have learned that motor vehicles should operate in time of peace upon fuels such that no changes in the induction system of the engine would be necessary to obtain satisfactory operation with the fuels available in time of war. As a result both countries have drawn up specifications for what they term “national fuels.” The United States is not greatly dependent upon imports for its fuel supply. The question is frequently raised, however, as to how much the supply of niotor benzene for fuel would be affected by the demand for toluene in the manufacture of explosives. USABILITY Usability is a matter of comparison. The question is not whether i t is possible or impossible to use a certain fuel, but whether it is more or less difficult to use one fuel than another. For automotive work one must consider the diffi1
Published by permission of the Actinc Director, Bureau of Standards.
culty involved: (1) in carrying safely an adequate supply of the fuel, (2) in preparing the fuel for combustion, (3) in distributing the fuel to the various cylinders, and (4) in burning the fuel in the time available. The supply of fuel that must be carried to travel a given distance is dependent upon the calorific value of the fuel. This is discussed later. Some of the other factors which affect the fuel’s usability are-explosive range, distillation range, latent heat of evaporation, flash point, freezing point, separation point, viscosity, detonation characteristics, spontaneous ignition temperature, and corrosiveness. The order of importance of these factors depends upon the type of service in which the fuel is used. EXPLOSION RANm-This is defined usually as “the difference between the minimum and maximum volume that will explode when mixed with unit volume of air.” Narrowing the explosion range increases the difficulty of engine operation. It often results in poorer fuel economy by preventing operation with mixtures as lean as could otherwise be used. Laboratory measurements of explosion range are not ordinarily directly applicable to engine conditions. In the engine, conditions of temperature, pressure, the degree of mixing, and the amount of inert gas present are different from those under which laboratory determinations are made. Nevertheless, it is highly probable that a fuel shown by laboratory tests to possess a greater explosion range than another will have a greater explosion range than the other when used ip the engine.
FIO.I-DISTILLATIOND A T A ON F U E L 3 FOR ROADT E S T S .CObPERATIVB FUELRESEARCH
DISTILLATION RANGE-It is customary to show the distillation characteristics of a fuel by plotting temperature against per cent distilled. Typical curves for four fuels are shown in Fig. 1. Such curves are very informative as to the starting characteristics of the fuel and as to the probable ease of preparing it for combustion and distributing it t o the
May, 1923
477
INDUSTRIAL AND ENGINEERING CHEMISTRY.
c. cylinders. The fuel in the charge must be mixed with the Gasoline ..... .. . .. . . . -25 to -30 air if it, is to be burned. The more finely divided the fuel the Motor benzene.. . . . . . - 11 16 easier will become the task of thorough mixing. Vaporizing 95 per cent alcohol.. .. fuel is one of the simplest methods of obtaining fuel in this The flash point in the engine would be somewhat different finely divided state. Moreover, fuel in the vapor form occupies many (in the case of gasoline about two hundred) times because of the influence of the pressure of the mixture at the the volume it occupies when a liquid. For this reason vapor- time of ignition, the amount of inert gas present, the thoroughization is a material aid in the problem of mixing and distribu- ness of the mixing, etc. Nevertheless, the fact that, as tion. The function of distillation curves is to indicate the rela- measured, the flash point of alcohol was more than 40 degrees tive amount of vaporization to be expected from different fuels higher than that of gasoline would indicate that under some at any given temperature. Starting characteristics of a fuel circumstances starting an engine would be more difficult with are dependent upon the temperatures at which the first 5 or 10 alcohol as a fuel than with gasoline. Such difficulty is reper cent is distilled. If this temperature is too high starting ported from countries using alcohol fuel blends in automobiles. FREEZING POINT-TO define a desirable freezing point is will be well-nigh impossible. The fuel-air ratio that must be furnished the engine for starting depends upon the amount easy: it is any temperature lower than the lowest ever of fuel that is vaporized under starting conditions. If only reached in service. Most fuels are entirely satisfactory in 5 per cent is vaporized the fuel content of the charge must this respect. Benzene is an exception, freezing at temperabe twice as great as when 10 per cent is vaporized. The tures often encountered in winter in a considerable portion objection to supplying this excess of fuel is not the cost of the of this country as well as in flight. Fuel sold under the name “motor benzol” ordinarily contains sufficient toluene to fuel but the probable dilution of the oil by this excess fuel. Perhaps a word as to the limitations of the distillation lower the freezing point materially. SEPARATION PoIwr-Many fuels are, and many probable curve may not be amiss. It would be a mistake to determine from distillation data the temperature necessary for com- fuels of the future will be, blends. There is usually some plete vaporization, and then, having determined that the temperature, termed the “separation point,’’ below which one charge entered the engine at a higher temperature, to con- constituent of the blend separates from the remainder. The clude that the fuel must have been completely vaporized. usability of a blended fuel ceases when that fuel ceases to The time required for vaporization must be considered and be a blend. Hence, one requirement of a blended fuel is the size of the liquid drops as well as the temperature. With that its separation point be a t a lower temperature than is an engine operating a t 3000 r. p. m. a complete cycle is com- reached by the fuel in service. VIscosITY-This is included to call attention to the effect pleted in 0.04 sec. The Bureau of Standards, in cooperation with the American Petroleum Institute and the Society of changes in viscosity rather than to suggest any range of of Automotive Engineers, is now making tests to determine viscosities as being particularly desirable. The flow of fuel engine performance with the four fuels whose distillation through the ordinary carburetor jet is considerably influenced characteristics are shown in Fig. 1. Interest in these tests by the viscosity of the fuel. In changing fuels, as for example centers about the fact that more than 30 per cent more D than from gasoline to alcohol with a viscosity twice as high, the proper jet size would be governed by the change in viscosity A fuel can be produced per barrel of crude oil. Under certain conditions a fuel may give trouble from being as well as by the change in fuel density and in the air-fuel too volatile as well as from not being volatile enough. With ratio. Viscosity plays an important part in the change in a certain fuel altitude chamber tests of an engine were made mixture ratio resulting from a change in temperature. If impossible by the fact that the fuel vaporized in the fuel an engine is operated on a cold day with the same carbureter lines. This not only wrought havoc with the metering adjustment as on a warm day there are two influences which characteristics of the carbureter, but eventually stopped the tend to make the mixture leaner on the cold day. The weight of air received by the engine in unit time increaseswith decrease flow of fuel entirely. Extremely high volatility is, of course, undesirable because in temperature. Under these conditions the viscosity of the of the waste that takes place when the fuel is stored and be- fuel increases and its flow decreases. Both influences are in the direction of leaner mixtures with colder air temperatures. cause of the fire hazard. L.4TENT HEAT OF EVAPORATION-vaporization is neces- The greater the change in viscosity with a given change in sary. Vaporization in the time available requires heat. temperature the more sensitive will be the carbureter to such The higher the latent heat Qf evaporation the more heat must changes. DETONATION CHARACTERISTICS-Detonation has received be supplied to secure a given degree of vaporization. Supplying heat is difficult. The greater the amount of heat to a great deal of attention during the past few years. Mr. be supplied the greater is the difficulty. Hence, a low latent Midgley has presented the results of some of his work before heat of evaporation is desirable. this SOCIETY.To the motor-car driver detonation often is Before leaving this subject it may be of interest to mention known as spark knock. It causes the metallic ringing sound a condition occasionally found in aviation engine work which results when a hill is climbed with the spark too far Some tests were being made at the Bureau when the engine advanced or with the engine badly carbonized. This sound performance suddenly became very erratic. The trouble is accompanied by extremely high pressures which are often was traced to the formation of a miniature snowstorm inside destructive to spark plugs and piston heads. Severe detothe intake system. Air containing considerable moisture nation usually results in a higher loss of heat to the jacket entered the carburetor, its temperature was lowered because water and in a reduction of engine power. of the heat expended in vaporizing the fuel, and as a result Ricardo, in England, and Midgley, in the United States, water condensed and formed snow. This condition was have devoted a great deal of attention to the measurement of I emedied by supplying an additional amount of heat sufficient detonation, or the comparing of fuels on the basis of their t o vaporize the fuel. detonation characteristics. Ricardo3 has rated various fuels FLASH PomT-In engine work this term is applied to the by means of what he terms [‘toluenevalues.” He defines this minimum mixture temperature at which ignition at one point value as “the tendency of a fuel to detonate in terms of its will be propagated through the entire mixture. Ormandy2 equivalent toluene content, taking standard aromatic gasoline. gives the following values of flash point: as having zero toluene value and toluene as having a value O
2
Automobile Enqineer, March, 1922.
8
Automotive Induslrzes, April 21, 1921.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
478
of 100.” In making determinations Ricardo used both a variable compression engine and a supercharging engine. In one instance he obtained a toluene value for one fuel of 28 by one method and of 18 by the other method. For another fuel the value determined by one method was 35 and by the other method 24. It would appear then that the toluene values as thus determined are not highly accurate, although useful as a basis of comparison. Midgley4 has measured detonation by what he calls the “bouncing-pin apparatus” and has published the results of tests of some blended motor fuels. He has rated the blends on an “equivalent xylidine in kerosene by volume per cent” basis. A table of the detonation characteristics of various blends of alcohol and kerosene follows: ALCOHOL-KEROSENE BLEND Kerosene Determined Equivalent Xylidine in Kerosene Per cent by Volume, Per cent 2.30 85 4.60 75 7.20 65 12.60 50
Alcohol
by Volume
by Volume
Per cent 15 25 35 50
This table is given to call attention to one phase of the blended-fuel problem that a t first glance seems inconsistent. If 2.30 parts of xylidine are equivalent to 15 parts of alcohol when blended with kerosene, it would not be unnatural to 2 30 assume that - (50) parts of xylidine would be necessary 1 5_ -
to give the equivalent antiknock characteristics of 50 parts
of alcohol.
2 Ao (50) 15
=
7.7, whereas the table shows 12.6
parts of xylidine to be necessary. A partial explanation lies in the fact that with the alcohol-kerosene blend when the knock-suppressing agent has increased from 15 to 50 parts the 50 constituent whose knock is to be suppressed is only -as 85 great, namely, 59 per cent. With the xylidine-kerosene mix87.4 as ture the constituent whose knock is to be suppressed i,p97.7 great, or 90 per cent. The problem is further complicated by the fact that the alcohol and xylidine differ in detonation characteristics as well as in their ability to suppress detonation. As far as this discussion is concerned it is sufficient merely to call attention to the fact that the relative amounts of two antiknock agents that are necessary to suppress detonation are not independent of the amount of either that is required. High antiknock value is desirable even if the fuel is to be used in an engine which could be operated satisfactorily with a fuel of much lower antiknock value. Detonation increases with the accumulation of carbon. When the detonation reaches a certain degree of severity, the carbon must be removed from the engine. The higher the antiknock value of the fuel the greater can be the accumulation of carbon before the detonation will become so severe as to necessitate carbon removal. Hence, an increase in antiknock value makes engine overhaul less frequent. SPONTANEOUS-IGNITION TEMPERATURE-The spontaneousignition temperature of a mixture of fuel and air is the temperature to which it must be raised in order to spontaneously ignite throughout its mass. Determinations of spontaneousignition temperatures are made in several ways, the results of which are not in as close agreement as might be desired. One method is to compress the air and fuel until they ignite from the heat of compression. Another method is to heat air and fuel separately and determine the temperature at which they will ignite upon being brought together. A third method is to let a single drop in the liquid state fall into a 4
J.
Sac. Automotive
E n s . , June, 1922.
Vol. 16, No. 5
crucible heated to the required temperature and full of oxygen at that temperature. A low spontaneous-ignition temperature is undesirable if the fuel is to be used in an engine operating on the Otto cycle, since it increases the liability of pre-ignition. Pre-ignition causes overheating and loss of power. If the fuel is to be p e d in an engine of the Diesel type, a low spontaneous-ignition temperature is desirable. I n such engines the scheme of operation is to compress air to such a temperature that the fuel will ignite when injected into it. CORROsIvENEss-This property is among the undesirables. It may cause the failure of vital parts. It increases the wear of moving parts and consequently makes engine operation more noisy. How much corrosiveness should be permitted is dependenbupon the class of service in which the fuel is to be used. Failure of a fuel line in an automobile means delay, in an airplane it may mean death. The presence of water or sulfur appears to be particularly undesirable from the standpoint of corrosion.
POWER-PRODUCIXG ABILITY Fuel should have a high calorific value. Such a statement rolls easily from the tongue. If asked which is preferable, a high calorific value per unit weight of fuel, per unit volume of fuel, or per unit volume of combustible mixture, the answer may come more slowly. The calorific value of hydrogen per pound is nearly three times as great as that of gasoline. ilt 60” F. and atmospheric pressure hydrogen is a gas and gasoline a liquid. Under such conditions a cubic foot of gasoline has more than three thousand times the calorific value of hydrogen. A unit volume of combustible mixture has from 10 to 20 per cent higher calorific value when the fuel is gasoline than when it is hydrogen. Hydrogen is not suggested seriously as a fuel likely to be used extensively in internalcombustion engines. It serves to illustrate the difference between calorific value per unit weight, per unit volume, and per unit volume of combustible mixture. According to Ricardo, the total energy obtainable per unit volume of combustible mixture is the same within 3 per cent for hexane, heptane, benzene, toluene, xylene, and ethyl alcohol. The calorific power of these same fuels varies more than 60 per cent, considered on a basis of unit weight or unit volume. It is probable that the caIorific power per unit volume of combustible mixture is of most importance, since this determines the possible output of the engine per cycle. In all automotive work high calorific power per unit weight is desirable since a portion of the energy of the fuel must be expended in transporting the fuel, 9nd high calorific power per unit volume is desirable because i t permits the use of a small fuel container with a consequent low weight and wind resistance. These factors are most important in aviation work where the weight of fuel and tank is a much greater percentage of the total weight than in other branches of automotive work. Calorific determinations tell how much energy is in a pound of fuel. It is more important to know how much energy can be obtained from a pound of fuel. Perhaps one may yet rate a fuel upon some such fantastic sounding basis as “obtainable foot-pounds per pound of fuel per proper engine.’, It is certain that the engine should be considered in conjunction with the fuel. At the Bureau of Standards aviation gasoline and motor benzene were operated in an engine having a compression ratio of 5.4. Both gave the same power. With aviation gasoline as a fuel satisfactory operation was not possible a t a higher compression, but with the motor benzene the engine was operated at a compression ratio of 14: 1 and an increase in power of 33 per cent was obtained with a corresponding decrease in specific fuel consumption.
May, 1923
INDUSTRIAL AND ENGINEERING CHEMISTRY
A discussion of this sort is apt to prove disappointing in that it fails to specify definite requirements. It states what is better and what is worse rather than what is right and what is wrong. It is hoped, however, that the discussion may have thrown some light on the various requirements of internal-combustion engines and their relative importance.
Economic Aspects of Motor-Fuel Supply fiom Petroleum’ By F. W. Lane and A. D. Bauer BUREAUO F MINES,WASHINGTON, D. C!
I
T IS EVIDENT to all who have to review chemical literature that. there are periodic waves of interest in particular subjects, or in various aspects of these subjects. At one time there is a large number of articles on some one topic, whereas a few months later the center of interest has changed entirely. In no field is this more true than in the petroleum industry. If we look back over the past fifteen years we find that there has been a steadily growing output of books and articles dealing with the economic aspects of this industry. At first these articles were largely descriptive, telling how the various products were made and used; then there began to appear a note of alarm-it was feared that the supply of c p d e oil was insufficient and that the demands made upon it would become too great. Of course, the war with its tremendous consumption of petroleum products intensified this alarmist literature and it continued to increase in size and importance up to the end of 1920. It seemed to be general throughout the petroleum industry, some of the ablest executives joining in the chorus. All of a sudden something happened. The price of crude oil dropped with a suddenness which shook the industry from top to bottom. As soon as the excitement caused by this sudden change subsided the industry tried to find out what really had occurred, and in the past two years a feeling has apparently grown up that our supplies of crude oil are inexhaustible, that all this alarmist literature was a mistake, that we are going to have sufficient oil for all our needs for ai1 indefinite period. Within the past thres months, however, the price of crude oil, at least east of the Rocky Pllountains, has begun to rise again, and the increases have been so frequent that the industry is again trying to take stock of what has happened. It is not necessary to go into details of the actual production by fields, but the statistics of the past few months indicate the following things. In the first place, California is producing a large excess of relatively high-grade oil, with the result that a considerable amount of California crude is being transported to the east coast by way of the Panama Canal for refining. In a way this seems to be providential, as this California crude is taking the place of the Mexican oil previously used, the past year having seen the beginning of a great decline of production in Mexico. Aside from California, however, the situation is not as hopeful as we might wish. There have been tremendous developments in Arkansas, but practically all the oil SO far produced a t Eldorado and Smackover is of the socalled “fuel-oil” grade. It is fairly high in sulfur and very little of it is used for the production of gasoline. On the other hand, the production in the Midcontinent district of the so-called “light crude”-that is, crude with a high gasoline content-seems at the present time to be very slowly declining, Published by permission of the Director, Department of the Interior, Bureau of Mines.
479
Prophecy is impossible, however, at least in details. We may at any time find new fieIds of light crude which will more than make up for the probable decline of the present producing fields. If such fields are not developed, however, it looks as if economy in the use of gasolioe and the development of possible substitutes will become more and more necessary.
HISTORY OF GASOLINE INDUSTRY It was in the nineties of the last century that gasoline was first used as fuel for road vehicles. A few steam carriages were built in earlier years, but they were not very successful, and until the internal-combustion motor was developed and placed on rubber-tired wheels the self-propelled road vehicle was only an inventor’s dream. The first successful automobiles used what was called “benzine” in those days, which was a by-product of the kerosene industry. This “benzine” or “gasoline,” as it soon came to be called, was a good deal like the “fighting gasoline” furnished our aviators at the front, which was designed to give extra speed and power in airplane motors specially tuned up to its use. For several years the quality of gasoline remained about the same. Then it began to be apparent that the supply must be increased. More cars were being put on the road every day, and as the demand increased the refiners were forced t o begin cutting deeper into the crude oil and putting more of it into the gasoline fraction. How far this process has gone is well known to every driver who remembers the gasoline of ten or twelve years ago. How much further it will go is a question on which some of our best technical men are working. It is doubtless true that we can use gasoline even less volatile than the present grades, but the steadily increasing demand for kerosene may make it necessary to stop raising the end-point of gasoline. In other words, there is an economic volume between the supply of straight run gasoline and kerosene. The last ten years have seen other changes in the character of gasoline. The refiners found that they would have t o make more motor fuel than could be recovered by ordinary distillation practice, and proceeded to do so by means of the so-called “cracking” processes. Gas oil, which lies between kerosene and lubricating oil in volatility, when heated under pressure will break down or “crack” to form gasoline. The early experimenters encountered difficulties such as too great a formation of fixed gases and carbon, which cut down the efficiency of the processes used and added to the expense of operation. Continued research, however, reduced these losses, and cracking is now being done at a profit. It is estimated that at least 20 per cent of our motor fuel is produced to-day in cracking stills. About the same time that the refiners began to add to the size of the gasoline fraction, thus raising the end-point of the resulting motor fuel, the manufacture of natural-gas gasoline was started. This material is very volatile and is carried as vapor by natural gas, but it can be removed in the liquid state if the gas is highly compressed and then cooled. It has proved to be a valuable addition to our gasoline, as it furnishes a much needed supply of low-boiling hydrocarbons which are lacking in many grades of crude oil. While natural-gas gasoline amounts to onlyabout 10 per cent of our total gasoline supply, it makes available as motor fuel a large amount of material that would otherwise be useful only as naphtha for solvent purposes. The older processes for making natural-gas gasoline depended on compression and refrigeration, but more recent ones have been developed in which the gas is passed through heavy oil or charcoal, which absorbs the gasoline. This is then recovered by distillation.
