Antiknock Agents. 0
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H. A. Beatty and W.G. Lovell E T a n COBPOBATION. DETROIT, MIM.
NE of the outstanding feaL a r g e amounts of gasoline are used to support our way of living and over 85% of it has turas of modem living is the. added to it about 1.5 ml. of tetraethyllead per gallon si that the combustion system of an enormous amount of fuel conaverage ear has passed through it, i n its life, over 10 pounds of lead compounds. This sumed. Especially popular as lead is added to reduce or prevent knock which can be destructive to the engine. T h e fuels are the combustible liquids prevention of fuel knock makes it possible to burn the fuel more efficiently and conserve called petroleum that come out of the gnalinc supply; and i t a use har remlted in an estimated annual saving of over holea in the earth. These are 50,000,Wll barrels of gasoline t year. T h e mechanism of this combustion control is by egsy to handle; produce a lot of mema of catalytic control of ee& of the chemical reactions that take plnee during comenergy for a relatively small bustion of hydrocarbons i n the engine. There arc many antiknock agents but the weight; and hum without leaving principal one in commercinl ure is tetraethyllead, which ir used with chlorine and bromine -lid residues or ashes. As a recomprmnds to remove the lead from the engine. Tetraethyllead has an effect on r e sult, about a third of the total moving knock dependent on the amount used, the hydrocarbon fuel on which it is used, energy used in this country wmes and the engine conditions under which it is used. Conaequcntly, its beat UM requires CD. from petroleum, almost as much operation between engine rmnuf.eturer, f u d producer, and antiknock compound producer. as from coal at present. One of the principal i~ of petroleum is for fueling internal combustion engines; &t half of the petroleum produced is chemical reactions that occur during burning may be about as converted into gasoline for use in these engines. The volatile important as the heat content, or the heat evolved in going from gasoline fractions of petroleum are relatively m y to mix with air the initialto the h a l materials. It is a matter of chemicsl kiand put intothecombustion chsmher of an engine, thereby making netics as much as thermodynamics. The antiknock agents, of thia part of the mechanical device easier to build. Once the fuelcourse, only work on the kinetic part. air mixture gets into the engine, i t is ignited and burned, and the Antiknock agents affect the efficiency with which gasalines can hot gascs push on the piston 80 that a fraction of the heat is conhe burned because they reduce or eliminate knock. It is the verted into useful work which does all sorta of things. Chiefly, knock that is the important thing in thia respect, because its presence acts as a barrier to the development of more efficient the mechanical work is used to propel automobiles, trucks, buses, and airplanes. A wide variety of engines could be used to propel engines. This harrier has been advanced considerably by the theze vehiclfflbut there are sound reasom for believing that, for adoption of antiknock agents whose use it is estimated is saving the pwenger cnr in particular and many nf the other vehicles, an over 5O,O00.000 barrels of gasoline a year. This is a result of the Ottc-cycle engine which coqsumes gasoline will r& the best inherent thermodynamics of our present internal wmbustion for a long time to come. A s ; b been observed, people, especially engines. in thia country, like to go places sitting down, and the consump Fundamentally, for automobile type engines, it is B matter of tion of motor fuel now rum about 2,000,M)O barrels a day. compression ratio. When the compreaeion ratio of an engine is More than 85% of the gasoline used as motor fuel has sdded to raised, or the ratio between the volume in the engine cylinder it, on the average, 1.5 ml. of tetraethyllead per gallon &s an antiwhen the piston is at the top 88 compared with a t the bottom, knock compound. Thus, in the life of an average automobile the efficiency goes up and more useful work ia obtained from approximately IOpounds of lead passes through its combustion burning the eame amount of fuel. Engine designers would like system. The antiknock additive increases the efficiency of the to use the high ratios: these result in a smaller displacement engasoline so that i t gives more useful work with as little heat laas as gine for the eame power output and leas fuel consumption or more miles per gallon. But when the ratio is increased fuels begin to n w . The amount of work obtained from a given amount of gasoline knock, reaulting in rapid rates of pressure rise. The compression depends not only on bow much heat is evolved when i t burns, but ratio at which knock occurs depends on the particular fuel and ala0 on how i t bums and how fast. In other words, the kind of on the engine operating conditions. Knock becomes worse as
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
further increases in compression ratio are made. This constitutes a very real barrier t o increases in compression ratio, since this undesired type of combustion called knock not only disturbs the driver of the vehicle but in extreme cases actually destroys the engine. I n general, any considerable intensity of knock cannot be tolerated, and in some particular cases, the incidence of knock can result in complete destruction of a n engine in a few minutes or less. If a perfect fuel, which would not knock under any conditions, could be manufactured at a permissible cost, this problem would be solved. However, it now appears that it is impossible to make such a perfect fuel and even the best hydrocarbons known will fail under severe conditions. Fortunately, the addition of tetraethyllead to gasoline allows its use at higher compression ratios than would be possible without it. That is the useful thing that antiknock materials do, and that is the reason they are used so widely. The addition of antiknock agents to gasoline, in amounts usually of the order of less than 0.1%, increases the potential value of a fuel so that i t can be used under more economic conditions because of the removal of knock. The question of how large these gains may be is a complex one, and one of the principal problems in the use of antiknock agents is that of using them so as to derive the greatest advantage. Their utility or effectiveness depends on how much is used, the fuel in which they are used, the engine in which they are burned, and even the conditions under which the engine is operated. An idea of how effective antiknocks may be in producing more power, but not more efficiency, from an engine may be derived from the fact that the performance number of iso-octane with 3 ml. of tetraethyllead per gallon is 146.6. This means that the knock-limited power output of a somewhat arbitrary average of various engines, with varied amount of supercharge, when using iso-octane containing 3 ml. oflead, is 146.6% of what it is when the fuel contains no lead. The more important corresponding gains in thermodynamic efficiency which accompany increases in compression ratio made possible by the use of lead are about a quarter of the supercharged power increase, but these gains are very important. The high compression automobile described by Kettering (a) showed an increase in economy of about 35% in terms of miles per gallon (with constant performance) when fuel of about 100 research octane number was used instead of fuel of about 85 octane number. Under favorable conditions and in suitable stocks, the addition of 3 ml. of tetraethyllead can move the knock barrier about the effective distance between these two fuels. So this becomes a very real thing in terms of miles per gallon and efficiency. The mechanism by which these antiknock compounds do their work is, of course, by catalytic control of certain of the chemical reactions that take place during combustion of hydrocarbons in the engine. The many chemical reactions that occur in the conversion of, say, heptane into carbon dioxide and water, are far from completely charted. The reactions that occur take place in a short space of time. (An engine operating at 4000 r.p.m. allows only 0.015 second for an up and down motion of the piston during which the charge must be compressed, burned, and expanded, and the combustion time is only about a tenth of this.) During combustion, temperatures run u p to 2000" C. and the pressures to hundreds of pounds per square inch, so i t is not a good place for test-tube chemistry, or even isothermal or isopressure work. Such information as is obtained from engine combustion comes from records of pressure changes, tcmperature changes, emitted light, absorbed light, and optical effects of varied gas densities, and also by often misleading analogy with combustion that occurs outside of an engine. Information is obtained also from experiments of intuitive interpretation analogous to what the mathematician calls integration by ingenious devices. I n general, however, i t is believed that most flames and ex-
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plosions fall into the category of chain reactions in which the reaction is extended from one molecule to another by direct molecular interaction rather than by general diffusion of heat, thus giving a series or chain of consecutive reactions. The continuation of such a chain is conditioned by the efficiency with which the molecular interaction occurs. I n a n Otto-cycle engine the normal type of flame moves but moderately fast, since the end gas (the unburned portion of the charge ahead of the flame front) is relatively slow to react-that is, the efficiency of the chain-continuation process is low. This results from the fact that the different kinds of fuel molecules which may be present in significant amounts are all, relatively speaking, quite stable as regards inflammation. Hence the intrinsic velocity of normal flame combustion is not greatly affected by wide variations in fuel composition or moderate changes in pressure and temperature, or by the presence of antiknock agents. Before the arrival of the flame front, slow oxidation occurs in the end gas, probably instigated by catalytically active surfaces or hot spots and continued by a chain-reaction mechanism. Normally, the amount of such oxidation is slight and the products formed have no important effect on the subsequent inflammation. However, when the temperature of the gas and the time available for its oxidation are sufficiently increased, the chaincontinuation efficiency increases up to a critical point. At this point the chains multiply rapidly in number, and the partial oxidation of the unburned fuel increases abruptly. The products of this oxidation are unstable compared with the original fuel, and burn a t an extremely rapid rate. Their sudden spontaneous ignition and subsequent extra-rapid combustion starts pressure waves in the burned gases; these are responsible for the sound of knock. Antiknock agents act as inhibitors for the slow oxidation reactions in the end gas. Their effect is to break short the reaction chains and so prevent the chain development from reaching that critical point a t which its multiplication begins. The advantages in the use of antiknock materials are thus very great. The problem is to decide what antiknock compound to use in what amount in what fuel in what kind of engine and under what conditions to get the best over-all technical and economic result. Obviously, there is no straightforward answer to such a question; the answer lies in cooperation between the people who make the gasoline, and those who make the antiknock compound, and those who make the engine. The customer should receive the maximum benefit by proper adjustment of all of the factors. The selection of the best antiknock material is not simple Just as there is no perfect fuel, there is likewise no expectation of finding a perfect antiknock, or one which will eliminate knock entirely under all conditions, and have no deleterious effects. There are described in the literature many compounds of metals such as tin, titanium, selenium, tellurium, iron, nickel, bismuth, and lead which have antilrnock action but vary widely in their effectiveness, depending on how they are used. Obviously, a satisfactory metallic antiknock compound should contain an element which has antiknock action, and i t should be soluble in gasoline and sufficiently volatile and also thermally unstable enough to be effective. The metallic antiknocks are oxidized during use and the end products should be in such a form as t o do no serious harm t o the engine mechanism. Finally, all these compounds are poisonous t o a greater or lesser degree because the human race is not yet accustomed to living in an atmosphere containing volatile metals, so that their use must be governed accordingly. Balancing all of these factors is not a simple matter. The principal metaIlic antiknock now in commercial use is one of the tetraalkyllead compounds, or tetraethyllead. Many studies have been made of other lead alkyls, especially the more volatile ones, in an effort to improve the distribution of the antiknock agent through the engine. B u t tetraethyllead still appears to be the best general selection of the lead compounds when all factors are considered. Its intrinsic antiknock action generally
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is highest; it is less variable in its action over the whole range of hydrocarbon types encountered in gasolines; its lower volatility, though less desirable from the standpoint of distribution, is better from the standpoint of safety in handling: and it is cheaper to manufacture. Tetraethyllead is always used together with a n artfully blended mixture of chlorine and bromine compounds which may be thought of as combining with the lead oxide formed in the combustion of tetraethyllead to form lead halides which are sufficiently volatile so that substantially all of the lead passes out of the engine. The selection of the best amount and type of halide for optimum results depends on a large number of conflicting factors related to various types of fuels and engines. The other metallic antiknock compound which has been used commercially, but only for short periods of time and especially in Europe, is iron carbonyl. The principal disadvantage to its use is that the combustion products are iron oxides which wear out engines a t a n extremely rapid rate; extensive research has failed to overcome this obstacle. Another class of antiknock compounds is the aromatic amines of which aniline and xylidine are well known examples, A great deal of time has been spent in investigations of the amines, especially during the war. They seem to have a different method of action than do the metallic compounds, and generally speaking, are much less effective. Tlicir use has not been extensive. The problem of how much antiknock material to use is also an important one. If engine performance is considered rather than octane numbers, in general, equal proportional successive additions of tetraethyllead, in contrast to some other antiknock materials, produce equal proportional successive improvements in the utility of the fuel, and the constant of proportionality depends primarily on the fuel. For most paraffinic hydrocarbon fuels, the proportionality constant is substantially the same, so that the better the fuel, the greater the gain when a fixed amount of lead is added. With other classes of fuels such as olefins and aromatics, the proportionality constant is profoundly affected by the specific hydiocarbon compounds so that even a t this date there is only a useful geneial pictuie of the type of phenomenon to be considered.
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There is a tendency t o believe that the question of how much tetraethyllead to use aIso should be weighed from the standpoint of deposit accumulation in the combustion chamber and its relation to increase in octane requirement of the engine with use. Extensive t'ests in passenger cars over the past 2 years offer convincing evidence that the amount of tetraethyllead used in the fuel has no significant effect on the octane requirement increase due to its use. The better the scavenging of the lead accomplished by the halides, the better the valve and spark plug life, provided there is no accompanying increase in corrosion rate. Although better scavenging does reduce the t,otal weight of engine deposits due to lead, this factor appears to bear little relation to increase in the octane requirement of the engine with use. Certain classes of unstable sulfur compounds, such as may occur in some gasolines, have a bad effect on the usefulness of lead ( I ) , Consequently, this is another problem that must be considered in the application of antiknock compounds. Finally, the usefulness of lead is influenced by the engine and engine condit'ions under which i t is used. Many unleaded fuels appear relatively superior to others when they are used in engines, or under conditions where the temperatures of t,he charge are relat,ively low. I n such fuels, especially, the effectiveness of lead is much better under those mild conditions. These, then, are some of t,he major problems of the use of antiknock compounds. It is apparent that the best solution to these problems is that which furnishes the best product for the customer who drives an automobile or uses an engine, and the best product for him is the maximum possible pleasant miles per dollar. Neither t,he fuel, nor the antiknock compound, nor the engine is any good by itself in producing transportation or power. It takes the patient cooperation of refiner, antiknock producer, and engine manufacturer to find the best solution tG their mutual problems, so that fuels may be used to the best possible advantage.
Literature Cited (1) IND.ESG.CHEM.,41, 885 (1949). (2) Kettering, C. F.. S.A.E. Quart. T T U I L 1, S . ,659 (10171
teracti H. K. Livingston E. I. DL' PONT DE NEMOURS & COAMPANY, INC., WILMINGTON, DEL,
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ASOLINES containing Pignificant amounts of sulfur give a smaller increase in octane number on the addition of tetraethyllead than sulfur-free or 101%sulfur garolines of similar hydrocarbon composition and base octane number. This antagonism of sulfur toward tetraethyllead was first reported by Rndo (S) in 1934. Much Jvork has been devoted since to the development of refining techniques for removing sulfur compounds, especially those that have the more deleterious effect on tetraethyllead, from gasoline. No complete explanation of the nature of the sulfurtetraethyllead interaction has ever been attempted, however. A thorough understanding of the sulfur-tetraethyllead interaction is of considerable importance in a t least two respects. Sulfur antagonism significantly limits the maximum beneficial results obtainable in the commercial utilization of tetraethyllead, and i t offers an unusually attractive indirect method for investigating the mechanism of the antiknock action of tctraethyllead. A comprehensive investigation of the antagonifitic effect of sulfur compounds on the antiknock action of tetraethyllead is
now in progress ( 6 ) . The present paper will not be conccrned with the detailed rcsults of this investigation, except in so Jar as they relate to the quantitative interpretation to be developed. All sulfur compounds that have the same type of sulfur linkage have the same effect on tetraethyllead when compared a t the same sulfur concentration (6, 8). The data for compounds with aliphatic sulfide structure (6) are typical of the agreement observed. The other important classes of sulfur compounds arc thiols, disulfides, and thiophenes. The early work of Schulzc and Ihiell (9) established the order of antagonism as disulfiriP > sulfide > thiophene. Thiols are about equivalent to disulfides (8, 10) in their effect on tetraethyllead.
Effect of Tetraethyllead Concentration For any given sulfur compound and concentration, thc proportional lead response is the same for each increment of tetracthyl-
