ON THE POWER AND EFFICIENCY OF INTERNAL COMBUSTION ENGINES
;: General Motors Rerearch Laborstorirr have a/way3 been engaged in research on internal combur-
tion engine fuels from the standpoint o f enginc builders. The objective has been to find out what can be done about the chemistry of combustion of the fuel in the engine to improve the power and rHicicncy o f the combination, fuel PIUS engine. This research early rhowed that limitetions are imposed on the luel-plur-engine combination by the destructive combustion disturbance called "knock". Although both fuel and engine arc involved, the power md efficiency of my one derign of engine arc dependent upon the extent to which the fuel urcd is free from knock. The Iotter, in turn, i s influenced by the addition of materiels such as tetrecthyllcad to the fuel and, s t i l l mote importantly, by even slight changer in the molecule< structure of the fuel itself, or the w a y in which even the same numberofstomrare linked togetherin space to form molecules. Suchrtudierofmolecolarrtructurehave shown that long thin m ~ l ~ ~ u knock l e s badly end short compact ones of theidme weight,oolya little, and that the diHerenccr between them may amount to severdl fold changes in the potential power output o f en engine using them. In this rerearch compound calicd "tript,nc"(e,e,3-trimethylbutdne) was found to be one of the best fuels. Recently W % pore triptsnc hsr been msdc in tank cdr qwntiticr. Sufficieotqudntiticrhsve been obtained in 8 newpilotplsnttapermitcxtenrive tqting, even in dirplencf. The gains possible with triptene, especially when tctracthyllrad i p added, depend upon the particular rngine and conditions of opera. tion, and h a w amounted to as much A S four timer the power and a quarter ICII fuel than i s obtained with prcscnt 100-octane g e d i n c . The objective of this rcrcarch i s to Rod thc bcrt combination of engine and fuel to give the greatest output of useful work or power per total dollar, ifrespective of what form the engine-fuel combination may tekc. The results show that there i s still a w r y largc field forimprovemcnt, both in the fuels thcmrolvrr and in the derigning o f enginer to use such fuclr to b r r t advantage.
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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
1080
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WO great industries in this country, petroleum refining and internal combustion engine manufacturing, produce the coordinating factors of our automobile, aircraft, and Diesel horsepower. These two industries are roughly the same size, and they have a common technical meeting ground in chemistry, since a chemical reaction produces the power. We call this reaction rrcombustion’’because it nicely conceals our lack of knowledge on jus$ what takes place in the engine cylinder. Upon this poorly understood reaction depends the service that these two industries render the public. I am speaking as a representative of our own research laboratories and of a number of other organizations which have been of assistance to us in working out further
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Vol. 36, No. 12
advances in the field which this paper covers. When conditions permit, public acknowledgment of the great, contributions of these collaborating organizations will be made. It is essential for both the engine maker and the fuel refiner to recognize that each has a n individual responsibility in supplying to his industry the best that science affords. It is also necessary to emphasize the importance of this technical meeting ground to the future of these two industries. To understand the real importance of this, we should know that the installed horsepower of internal combustion engines in our automobiles just before the war was thirty times that of our central power-generating stations. This central station installed power was 50,000,000 horsepower. During the war the manufacturing horsepower of airplane engines for a single month is ten times as much as that developed by Boulder Dam. I n 1912, just about the time battery ignition and the selfstarter were introduced, people became conscious of a combustion disturbance we call “knock”, and the public recognized knock as a barrier to high power and efficiency. I t was natural for motor car manufacturers and users to blame engine knocking on the new type of ignition. This circumstance caused us (then the Delco Company) to undertake a study of knock; ever since that time we, as General Motors Research, have been experimenting with the relation between fuels, ignition, and engines. We have done this as builders of engines and equipment because we fully realized that i t was the combination of engines and fuels, and not either of them alone, that develops useful power. I n the early phases of this research when we were trying to develop the problem, we knew that both fuel and engines were involved, and so we tried first to find out what part belonged to each. We soon discovered that, for any given design of engine, its production of power and its efficiency were dependent on the extent to which the fuel used was free from knock. At that time the common method of grading all fuels was by specific gravity, and i t was regarded as a n axiom that the higher the gravity, the more the fuel would knock. Certain of the early experiments q a d e in our laboratory, then located in Dayton, indicated that this was not necessarily so. It was in this early
Figure I. Aniline Equivalents of Paraffin Hydrocarbons
I C.CCcc .C
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5 Figure 2.
6
7
8
Aniline Equivalents of Olefin Hydrocarbons
6
Figure 3.
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Aniline Equivalents of Aromatic Hydrocarbons
December, 1944
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
work that tke knock characteristics of fuel were found to depend t o a large extent upon molecular structure. I n 1919 and 1920 the late President of the AMERICAN CHEMICAL SOCIETY and the writer published articles on this subject is (4, 11). One of the things said (4) was: “The main point that the fuel problem, so far as it concerns future engine design, is practically a question of chemical construction and make-up.” We have always considered the fuel to be as much a part of the engine as the pistons or the valvea, and one cannot work either with fuel or engines without taking the other into account. The researches which discovered the effects of antiknock compounds, such as aniline and tetraethyllead, increase the output of internal combustion engines without changing the molecular structure of the fuel used (6,l a ) . From this work it became evident that the problem had two important factors. The first is that the specific molecular structure of the fuel greatly influences the performance, and the second is that materials can be added to fuel of any structure and affect the knock either upward or downward.
...
1081
to measurements on the pure compounds; but most of the compounds have since been measured in the pure state. Figures 1 to 7 show representative data on these structural relations for paraffins, aliphatic olefins, aromatics, six- and five-carbon ring naphthenes, alcohols, and various oxygen-containing compounds. Marks on each chart show the approximate relations between the aniline scale and the octane scale. A t the time
Additional research on fuels emphasized still more the importance of its molecular structure, or the specific way in which a number of atoms were grouped t o form a molecule. As a matter of historical interest, some of the results of investigations made in the 1920’s (6, 7, 8, 9) may be presented first. This work was done before the octane-number scale existed, and so these early but extensive measurements were made in terms of aniline as a n effective antiknock agent or in terms of “aniline equivalents”. The measurements were made in a one-gram-mole-per-liter solution in a reference gasoline. The aniline equivalent is strictly described as the number of centimoles of aniline added t o the reference gasoline (or to the solution in the case of negative values) to match the solution with regard t o knock in a n engine. Measurements in solution do not necessarily correspond directly Figure 5.
Aniline Equivalents of Five-Carbon Ring Naphthene Hydrocarbons
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Aniline Equivalentt of Six-Carbon Ring Naphthene Hydrocarbons
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Aniline Equivalents of Saturated Aliphatic Alcohols
1082
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 36, No. 12
Experimental Triptane Plant
these measuremetits were xnde, h e were mw way in which molecular ~ I w c t , ~ rerponded re thsn we were tu t.lir quantitative relation tbmuyh the usable conrentrat,iom. [Jndoubtcdly, if thofie rhsrts were replotted i n the light, of later knowledge. there would be some slteratianh in relative position. AH B more specific example of the Prost effect that molecular atructure cnn heve, the date in Figure 8 are preqented. The behsvinrh of isomeric ht;ptanes are shown and also one wt,sne (isooctaneor2,2,4-trimethylpeetane)forcurn~~~~i~~on. Theseven carbon ntomb and sixteen hydrogen stomx of the heptanes m y be arranged in nine diflwent, way8 to produce nine oompounds, some of which hsvo very similar physioal prupcrtiea. Tliese we shorn by conventional structural formulas along the bottom of Figure 8,and above esch is represented its critical eompression ratio, or the rompres4on ratio which m n be wed withaut knock when the engine uses that hydrocarbon gs s fuel under B set of standardized cinditions. These individual cootprmion ratios cannot he exceeded withont the incidence of knock, and if they are grestly exceeded. t,he d&ruction of the engine resulta.
The efficiency of nn engine improves greatly with Irisher c.omprwsion ratiou-consequently, the great importanre of having fuels that permit of high compre9sionr. Tho higher efficiency which we normally get from Ijieeel engine8 ia due to the feet that in those engines compremion Patios w e very high (sround 16: l ) , and consequent,ly the expsnsian ratios of the burned gares are best for economy. Our beat IXesel economies we about 40% in therms1 efficiency. Relatively recent experimental work indicates that, with fuels of high order of antiknock value, specific fuel economies can be obtained in spark-plug engines equal to that of Diexelr. Three pointn arc to be .wen from Figure 8. First, there is a fairly regulsr inarewe in 1he individual critical compression rstios as thestructure of the rnolecuk of heptane becomes more closely cent.ralized or highly branched. The fundament4 r e w n for this difference in the wey hydrocarbons of different struotureb burn i8 still B subject for argument, wen though it hss been, and etill i8. the basis of 8. large amount of research. A seeand important fact is shown by t.he dotted line of Figure 8 which givw the increase in oriticel compression ratio obtained by the addition of tetraethyllead. The freer from knock the hydrooarbon
i i i general, but not ~tlwbyr,the more eKe&vc lead is in mskine it, still bett,er. The t.hiril and most impor1,snt fact to be learned from Figure 8 ia the wide range of compreaion ratios covered. Iso-octane itself (2,2,4trimethylpentsne), which is rated at 100-oetane number and is the bmis of premnt 100uet.sne aviation fuels, ili only about half way u p the ~ e s l e .Arnonr( the isomeric heptanes, then, there are compounds greatly superior to I G c t s n e , as well tw n-heptane whirh i- the zero of the oetlme-number iCSlP.
ili itlielf, of
7 + The great effect of struct.ure'eJis shown in the hehsvior of 2,2,3trimethylbutane (triptsne). 11, w ~ shown s by engine t-18 run many years ago (6, IO) thst trimethylbutane is one of the hest fuelsknownasfaras knorkisconrerned. Itiumgwd thst,shen leaded, it is beyond the espheity of 80me of the older methods of measurement, as shown by its relation to iso-ortane. It is possible, however, to look at this heptane of special molecular structure from mother standpoint than high cornpression ratio, which mpmxenta gains in efficiency. This other standpoint is gains hy supercharging, which m e primarily inereages in power. Figure 9 shows the knock-limited power obtainable from B particular but probably fairly reprebentat,ive supercharged engine with vsried amount of supercharge pressure, for several fuels. Here the concern is with the upper range of fuels, and imoctane of 1Woctsne number ia near the bottom of the chsrt. The power obtained with leaded triptane is nearly three times as great as with isouetane. Hecause of them fwtors, it WBS decided to make enough trimethylhutane to find out more of ita practioal use. Once we have B knack-free fuel, engine design o m move ahesd on sn independent basis. A large number of problems which always arise relatink to engine design and use for special applications can only be intelligently evatwted by experiment. To do this large scale experimenting, a knock-free fuel must be avsilsble in qusntity. Himonr OF TRIFTANE. Trimethylbutane (triptane) was ftrst made 88 an individual ohemiesl oompound by Chavanne, B Belgian, in 1922 (e). It was again made by the Ethyl 0 -
December, 1944
line Corporation (5) and fiwt tested in gasoline solution in an engine in 1926 (6) incidental to the molecular structure investigations in General Motors Research Laboratories. This was when triptane was first found to be a n outstanding fuel component. It has been made by the laboratory methods of the Grignard reaction at various times for engine test. But triptane suffered from the limitation that it could not then be made cheaply in quantity. I n 1938,2 gallons of triptane were prepared by the Ethyl Corporation by the Grignard reaction and tested in a supercharged engine; as a result, Wright Field became interested in obtaining enough for test in a full-scale airplane engine. The Dow Chemical Company then prepared about 300 gallons for the Ethyl Corporation by a similar method at a cost of about $35 a gallon. At an earlier date the coined name “triptane” was suggested by Calingaert (I). The first full-scale engine tests were carried out a t Wright Field in 1941, and we understand that it was found to be superior to any other fuel tested up to that time. Obviously, to have triptane available in quantity at a reasonable price for experimental use in an extensive engine program, it was necessary to have a better method for making it; and to evaluate it as a specific fuel component, it was also necessary to have it reeasonably pure.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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has proper governmental authorization to receive such infomation for use in the prosecution of the war, and when possible we will be pleased to make eamples available. It was decided to use this m e t h G to make a large-scale sample of triptane, several tank cars in amount. This particular reaction is now definitely out of date, but its temporary use w&s dictated purely by expediency and must be regarded in the same light as making experimental crankshafts from large billets of steel. After progressing through laboratory development and a small-scale pilot plant, a larger-scale pilot plant to make up to 5 or 10 barrels of triptane a day was built. This W M done with the approval and assistance of the War Department and the Army Air Forces. A photograph of this plant is shown on page 1082. It was completed in late 1943, and by January, 1944, had been in operation producing triptane in somewhat better than expected yields and with a purity of about 99%. On this sample it is hoped to he able t o obtain enough data to evaluate, in a comprehensive manner, the possibilities of triptane as a fuel, both pure and blended with other fuels, including work on actual flight tests in airplanes. Such tests are underway a t Wright Field a t Dayton, a t the Aircraft Engine Research Laboratory of the National Advisory Committee for Aeronautics in Cleveland! a t the Allison Engine Division of General Motors Corporation, and at the General 3fotors Research Laboratories.
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Following a long series of researches in General Motors Laboratories, it was found that pure triptane can be made by a relatively simple procedure, the specific character of which cannot be disclosed a t this time because of secrecy orders. However, information about the process may now be obtained by any person who
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Knocking Characteristics of Heptanes
INDUSTRIAL AND ENGINEERING CHEMISTRY
1084 34c
Vol. 36, No. 12
Such utilizations of triptane are primarily related to the power of development of a n engine or to how much fuel and air can be burned in the engine. As mentioned before, a more fundamental advantage of knock-free fuels, such as triptane, lies in using them to produce more work per pound of fuel or higher thermodynamic efficiency-for example, by the use of higher compression rati’os.
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Performance of Fuels in a Single-Cylinder Supercharged Engine
Enslne speed 900 r p.rn.r luel-elr ratio 0.0724 07451 cornprerrlon ratio 6.51 swk advent;, P V l ’ jacket ternperatwe: 900. F.; inlet-air tempenlure, 25bo F.
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A considerable number of engine tests have already been run in a variety of single- as well as multicylinder engines. They demonstrate the remarkable gains that can be made in the fuel-engine combination. The magnitude of such gains depends upon the particular engine and conditions of operation; with triptane containing added tetraethyllead, they have amounted to as much as four times the power and to as much as 25% gain in fuel economy over isc-octane or 100-octane gasoline. The gains, under extremely severe engine conditions, may be much less than those mentioned. This fact again emphasizes the circumstance that the production of power is both a fuel and an engine problem: Figure 10 shows data obtained on a specific and probably reasonably representative single-cylinder engine in General Motors Research Laboratories. A twelve-cylinder Allison airplane engine has been operated on a 60% blend of triptane in 100-wtane gasoline at a n output of well over 2500 horsepower, although the rated takeoff horsepower with 100-octane aviation gasoline is only about 1500 horsepower.
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Figure 10. Knock-Limited Power Curves for Triptane, Tested in a Single-Cylinder Supercharged Engine
Studies are underway to evaluate the commercial possibilities of triptane and related compounds, with some preliminary indication that the value of the fuel may easily justify the present projected cost per gallon, which is relatively high as compared with fuels of lower quality. More important, an engine develop ment program is underway to see what may be done with engines and their applications to various uses, once the barrier of knock has been removed to such an important degree. The objective is to find the best combination of engine and fuel to give the greatest value per total dollar, irrespective of what form i t may take. This phase of the problem is how best to design future engines to take advantage of the greater fuel possibilities.
December, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
1085
ENOINE OF HIGHEST EFFICIENCY
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In a broader view this is only one phase of the problem of the fuel-engine relation. The important point is to get the most from the fuel by suitable treatment before use and then by burning it in an engine made t o fit the prior treatment. As t o what type of engine to build a t any one time, it does not matter whether i t be carburetor, injection, Diesel two-cycle, four-cycle, nonreciprocating, or whatever, so long as the over-all economies are right. Engines are all made from metal and brains, and what we want to do is to put as much of the latter in as possible. When that is done, we get the most and the cheapest power from a pound of fuel. What the engine problem looks like with relation to fuels is a circle, as shown in Figure 11. If we think of a barrel of crude oil entering the circle at some point, we can refine it around one direction to increase the octane number or around the other direction to increase the cetane number. But the two processes meet at the same point in a very high compression engine which has about the same thermal efficiency, since efficiency is largely determined by compression ratio. But one important difference is that our present octane-number (gasdine) engine can be run about a third faster than the cetane number (Diesel) engine. This barrier may be removed soon after the war. The combustion process is the place where the fuel and the engine meet, and that mechanism is what we are trying to utilize to best advantage. The future development of automobile aircraft, and Diesel engines will depend upon how well management, as well as engineers and chemists, understand the fun& mentals of the relation between the fuels and proper engines. We are studying fuels from the standpoint of the engine builder and not as fuel producers. We do similar researches in metallurgy, fabrics, rubber, and many other materials which we use. From this research we will know the practical limitations of the material we use and not ask our suppliers to do things which are outside of the bounds of technical and commercial practicability. It also helps us determine the road which our future researches
Figure 11.
Oil Refining and Engine Design
should take because what may be technically and commercially impractical today may easily be an everyday product in the not too distant tomorrow.
Calingsert, George, IND.ENO.CHSM..35, adv. p. 6 (Oot., 1943). Chavanne, G.,and Lejeune, B., BdL doc. A i m . Belg., 31,98-102 (1922).
Edgar, Graham, Calingaert, George, and Marker, R. E., J . Ana. Chem. SOC.,51, 1483 (1929). Kettering, C. F..J . Soe. Automotive En#re., 4, 263 (1919).
Zbid., 5, 197 (1919). Lovell, W. G., Campbell, $. M., and Boyd, T. A., IND.ENB. C8E%f.,23, 26 (1931). Zbid., 23,666 (1981). Zbid., 25, 1107 (1933). Zbid., 26, 476 (1934). Zbid., 26, 1106 (1934). Midgley, Thomas, Jr., J. SOC.Automotive Engre., 7, 489 (1920). Midgley, Thomas, Jr., and Boyd, T. A., IND.ENB.C ~ M 14, ., 894 (1922).
Blending Aviation Gasoline Components S. STANLEY LUKOFSKY Eastern States Petroleum Company (of Texas), Houston, Texas
A
N IMPORTANT part of aviation gasoline manufacture consists in the proper blending of available gasoline components to meet the three primary specifications of vapor pressure, 1-C octane rating, and a third specification which we call “property C”. For the purposes of this discussion the actual specifications are unimportant provided we realize that, whatever they are they must be met in our final blend. Let us call the 1-Coctane number K I , the adjusted or true initial vapor and the property C number Ka. pressure K2, The 1-C octane number and property C number, as given, are understood to be the indices chosen by Petroleum Administration for War to be volumetrically proportional to the blending effect of the component in mixtures; the laboratory or Reid vapor pressure must be translated into true initial vapor pressure via well known conversion data. It is recognized that vapor pressure,
as so defined, is proportional to molar ratios. Thus we must urive a t the correct values for Kt, KI, and KIin the final blend. K 1 and Ka are determined by the volumetric proportions; KSia determined by the molar proportions in the mixture. The symbols used in the mathematical discussion are as follows:
z y z w
u D
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R
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component 1 in blend, gal. component 2 in blend, gal. component 8 in blend, gal. component 4 in blend, gal. = component 5 in blend, gal. density of subscribed component, lb./gal. molecular weight of subscribed component, lb./mole volumetric proportion of subscribed component, dimensionless mole fraction of subscribed component, dimensionless 1-C octane number of subscribed component, dimensionlass
