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of as little as 1 molecule of tetraethyl lead in over 200,000 molecules of a combustible mixture of kerosene and air exerts an effect in the supression of detonation that is equivalent to the blending of 25 per cent of benzene by volume with the kerosene. Benzene is a fuel that does not detonate even when burned at compressions in excess of 200 lbs. per sq. in.; and, when it is blended with kerosene, the resulting mixture partakes of the nondetonating property of benzene to the extent to which benzene is present. It has previously been suggested that, in view of the small amounts of some of these materials needed to exert a large effect on the character of combustion, their action bears some analogy to that of catalytic agents.

An adaptation of the Perrin radiation theory has been proposed as an explanation for the behavior of knock-suppressing and knock-inducing materials. The presence of a sinall amount of one of these materials during the combustion of a highly compressed mixture of gases is conceived as presenting a screen which absorbs some radiation and thereby controls the velocity of flame movement. The application of the principles of this theory to the explanation of the behavior of such materials would appear to be profitable field for research on this subject. With the exception of this one suggestion, which is not as yet borne out by any experimental evidence, no adequate explanation of the action of these materials has been advanced.

PHYSICAL CHEMISTRY OF THE DETONATION-INFLUENCING PROPERTY But whatever the mechanism by which these materials influence combustion, quantitative measurements of their effects may be obtained by the application of proper instrumentation to the internal-combustion engine. Thus, in a given motor, when operating on a given kerosene, the constants in the equation W = KVT*T* (see above) were found to have the following values."

K = 3.25 X 10–

0.9

m = 3.22 After 2 cc. of tetraethyl lead per gallon had been added to the kerosene, the values obtained in the same way were

K= 3.075 X 10– n = 0.9

m = 3.22 The following data show how K varies with a number of concentrations of tetraethyl lead in the kerosene used as fuel:

Concentration PbEta
Cc./Gal.

K
0

3.25 X 10– 1

3.17 X 10^? 2

3.075 X 10– 4

2.97 X 10– 8

2.86 x 1016

2.76 x 10-7 From these figures it is obvious that the effect of the presence of tetraethyl lead in a detonating fuel is to change the value of K in the equation. The results of a number of determinations show the relationship between the concentration of tetraethyl lead and the constant K to be as follows:

K= K. – 0.173 X 10-log, (C+1) where K, is the reaction-velocity constant of the original fuel, C is the concentration of tetraethyl lead in cubic centimeters per gallon, and 0.173 x 10-7 is a constant.

The form of this equation is suggestive of a selective radiation absorption, but by itself it must, of course, be considered as inconclusive. Nevertheless, it does make a material addition to the data tending to substantiate the applicability of the Perrin radiation theory to these cases.

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These figures should not be considered as expressing the relationship of all antiknock materials to the constant K, because the values for aromatic amines are known to differ from those of tetraethyl lead. It will be some time before complete data of this character on the different types of antiknock materials can be obtained and the true relationships existing between them definitely ascertained.

The amount of work necessary to determine the values of antiknock materials in a comparative way may be materially reduced by determining simply the concentration of any given compound required to produce an effect on detonation that is equal to some standard. Thus, 1 per cent of aniline by volume may be used as the standard. The values so obtained are only relative, because in some cases the relationship changes with concentration. This should, therefore, be considered as an approximate rather than an absolute method of comparison. However, for comparing the antiknock effect of one compound with that of another this method is a satisfactory one.

DETONATION-INFLUENCING PROPERTY PRIMARILY IN THE ATOM The magnitude of the detonation-influencing effect is primarily a function of one atom in the molecule, but it is modified to a great extent by the radicals or groups attached to that atom. The relative antiknock effects of the ethyl and phenyl compounds of four elements are given in Table I. By comparing the values for the ethyl compounds of iodine, selenium, and tellurium with those for their corresponding phenyl compounds, it is observed that they are of the same order of magnitude for each element, the alkyl compounds being somewhat more effective than the aryl compounds. The reverse is true with oxygen, diethyl ether being a slight inducer of detonation, while diphenyl ether is a weak antiknock material. From the table it may be observed further that the antiknock value changes enormously with the element, while it is merely modified by the change in grouping. Thus, diethyl telluride is about twenty-five times as effective molecularly as ethyl iodide, and the same relation holds for the phenyl compounds of these elements. It seems reasonable to conclude, therefore, that the antiknock property is primarily a function of the element rather than of the groups attached to it.

TABLE 1.-COMPARISON OF RELATIVE EFFECTS EXERTED UPON DETONATION BY ETHYL AND PHENYL

COMPOUNDS OF 4 ELEMENTS

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Bryant, Lynwood. “The Origin of the Automobile Engine,"

Scientific American, (March 1967) 102–110, 112
This article reviews the development of the gasoline engine,
particularly with respect to its use in automobiles. The major
portion of the article is devoted to the work of Nicolaus Otto,
whose innovations made the spark ignition engine viable. It is
interesting to note that he also worked on the idea of stratified
charge to improve the combustion process.

THE ORIGIN OF THE AUTOMOBILE ENGINE

(BY LYNWOOD BRYANT)

The first internal-combustion engine to operate successfully on the four-stroke cycle was built in 1876 by Nicolaus August Otto. llis Silent Otto" was a good machine with a poor theory.

The modern automobile is driven by a heat engine whose basic principle was first demonstrated 91 years ago. The principle is the Otto cycle, named after Nicolaus August Otto, a self-taught German engineer who stumbled on a way to burn a compressed mixture of gas and air in the cylinder of an engine without producing destructive explosions. The “Silent Otto," as the engine was somewhat extravagantly called, employed a scheme in which the piston required four strokes to complete one cycle: an inward (toward the crankshaft) fuel-and-air-intake stroke, an outward (away from the crankshaft) compression stroke, an inward power stroke and an outward exhaust stroke.

The term “Otto cycle" is sometimes used loosely to denote this four-stroke mode of operation, but actually an engine of the Otto type can be either two-stroke or four-stroke (or for that matter six-stroke, as some of the early ones were). The four functions of intake, compression, expansion and exhaust must be performed in any Otto engine, but they do not have to be performrd in four distinct strokes. Strictly speaking, what distinguishes the Otto cycle from other cycles that can be used in piston engines is that an engine of the Otto type takes in a controlled mixture of fuel and air, compresses it to a moderate pressure and ignites it by some kind of ignition device, nowadays a spark plug.

The original Otto engine achieved an efficiency three or four times greater than the steam engines of the day, with the result that Otto's factory near Cologne became a world-famous source of stationary power plants. Two of his associates in this enterprise were Gottlieb Daimler, who later became a pioneer in the automobile business, and Wilhelm Maybach, who designed most of the early Daimler automobiles and went on to make excellent engines for aircraft.

Otto's engine would be important if only because it was the ancestor of the automobile power plant. It has a special appeal for the historian of technology because it was a good machine built on a bad theory. In the early days of the gas engine (and it was gas, not gasoline) the central problem was how to get a smooth flow of power out of a series of explosions. Otto solved the problem-or thought he did-by mixing fuel and air in a special way that yielded what he described as a stratified charge. This was supposed to cushion the shock of the explosions. Otto was wrong in attributing the success of his engine to the distribution of the gases in the cylinder, but his error led him to a mode of operation that is still employed in more than 10 million new engines a year.

The Silent Otto looks more like a steam engine than an automobile engine. This is not surprising, because it was the steam engine that provided the theory, the experience and even the hardware used by early workers on gas engines. The cylinder in Otto's single-cylinder engine was in fact a converted steam engine cylinder. It incorporated a slide valve like the valve of a steam engine, except that it had a more complicated system of passages because it had to control three fluids-fuel, air and the ignition flame_rather than steam alone. The Silent Otto developed about three horsepower at 180 revolutions per minute.

The Otto engine, demonstrated in 1876, was the first to use the four-stroke cycle. To observers brought up on the steam engine this must have seemed a wasteful way to run an engine because it yielded only one power stroke for every two revolutions of the crankshaft. The steam engine not only had a twostroke cycle but also was usually double-acting; this meant the piston was pushed by steam in both directions, so that there were two power strokes for each revolution of the crankshaft-four times as many as in the single-acting four-stroke gas engine. Otto seems to have adopted this mode of operation reluctantly and temporarily for lack of a better way to compress the charge. Such a drastic reduction in the frequency of power strokes must have seemed a high price to pay for the advantages of compression. Otto claimed the four-stroke cycle in his patent, but only incidentally. He promptly set to work to improve his engine by developing a two-stroke process, and so did a dozen other inventors. He was

never able to improve on the four-stroke process, nor has anyone else been able to for engines of a size appropriate to automobiles.

Otto's fuel was illuminating gas. Although gasoline was known, it was regarded as being extremely dangerous. The problem of mixing gasoline vapor and air in the exact proportions required by an engine proved to be an intractable one that was not solved until the carburetor was devised in its present form in the 1890's. Gas, on the other hand, was a convenient and reliable fuel that was already in wide use for lighting. Usually produced by heating coal in the absence of air, it consisted chiefly of hydrogen, methane and carbon monoxide. Gas technology has been evolving for some 50 years, and whenever a city installed a gas system someone was likely to get the idea that this new source of energy could be used for power as well as light.

Early inventors therefore envisioned a small gas engine the user could turn on whenever he needed power (as we now plug in an electric motor) and turn off when he was through. They hoped that such an engine might compete with the steam engine, particularly in small sizes, because of its convenience and adaptability to intermittent use. The more enthusiastic promoters dreamed of supplanting the steam engine entirely but to a more realistic goal was to provide stationary power plans for small enterprises-pumping stations, breweries, printing shops and the like--that could not afford a steam engine or did not use power continuously.

Otto and others naturally considered using a gas engine to drive a vehicle, but that was not a practical objective in the 1870's. The Silent Otto and the first generation of its rivals were much too heavy (they weighed about a ton per horsepower) and they were tied to a stationary gas system. Nonetheless, Otto's engine of 1876 embodied the essential concepts that later made the automobile engine possible, after much refinement of detail, chiefly reduction of weight and adaptation to liquid fuel.

The concept of internal combustion—that is, the notion of burning the fuel inside the working cylinder of an engine and dispensing with firebox and boilerwas an attractive one in Otto's time, and scores of inventors were working on it. Practical engineers knew little thermodynamics in those days, but some knew enough to measure the thermal efficiency of the steam engine. They found it scandalously low, usually well under 5 percent. The internal-combustion approach seemed a promising one because it offered an opportunity to avoid the heat losses, not to mention the weight and expense, associated with firebox, smokestack and boiler. The heat would be generated at the face of the piston, so that it could immediately be converted into work without losses in transmission and storage.

At the time engineers also talked about another kind of economy: the sav. ing of latent heat. The trouble with the steam engine, they said, is that much energy has to be spent in converting water into steam before any work is done, and that this investment in “latent" heat is not recovered if the exhaust steam is discharged into the atmosphere, as it usually is. The key advantage they saw in the internal-combustion process was that it utilized the products of combustion to drive the piston directly, without wasting energy in generating an intermediate working fluid such as steam. Actually the essential advantage, as more sophisticated engineers eventually learned, was that the internal-combustion engine was able to operate through a wider range of temperatures than the steam engine could ; the nature of the working fluid made no essential difference.

Otto was not the first to try internal combustion. His most famous predecessor was Étienne Lenoir of France, who like Otto had no technical training. In 1860 Lenoir built a two-stroke engine much like a steam engine. It drew in a mixture of gas and air for the first half of each intake stroke and then ignited it with an electric spark. The second half of the stroke was used for expansion, and there was no compression of charge. The engine was double-acting : it used both sides of the piston like a steam engine.

The Lenoir engine created a furry of excitement in the early 1860's. Scientific American quoted French journals as saying that it marked the end of the age of steam. "Watt and Fulton will soon be forgotten,” the article said. “This is the war ther do such things in France." Then professional engineers who knew some thermodynamics ran tests on the engine and published discouraging reports. It used large quantities of oil and cooling water, it overheated badly and it was hardly better than a steam engine in efficiency. Several hundred Lenoir engines were made, but within a few years they were mostly scrapped or converted to steam.

One of the suggestions offered for improving the efficiency of the Lenoir engine was to try compressing the charge. The idea of compression is a simple and obvious one that came up frequently in the 15 years before the Silent Otto. A commonsense engineer would see compression as a way of increasing the power of an engine by packing in more fuel, or as way of reducing the size of the engine by simply packing the same amount of fuel into a smaller volume. One of the advantages of gas as an engine fuel is that the designer can choose a convenient density.

Otto discovered the value of compression as he was experimenting with a model of the Lenoir engine. While working as a traveling salesman for a wholesale grocer in the Rhineland he read about Lenoir in a newspaper and had a mechanic build the model engine for him according to the newspaper description so that he could try it out. Once in the course of testing various mixtures and sizes of charge and times of ignition he drew in a full cylinder of gas and air and then compressed it on the next stroke of the piston. (Indeed, one could scarcely do anything else with a full charge.) When he set off the compressed charge, he got a surprisingly violent explosion that drove the flywheel through several revolutions. He later said that this was the starting point of the four-stroke cycle.

Otto probably did not follow up this discovery at the time because a violent explosion was the last thing he wanted. His problem was to reduce the violence of the explosions. He was not looking for ways to increase the power of his engine but rather for ways of controlling the power he had. A worker on gas engines in the 1860's bad two models to use in his thinking. One was the steam engine, in which the steam delivered an easy, steady pressure to the cylinder that could be turned on and off at will and also could be easily transferred to the other side of the piston. This was the ideal to be achieved in a gas engine.

The other model was the gun, which provided experience with the behavior of explosions in cylinders. The gun was the first internal-combustion engine, and it delivered energy in spectacular bursts. The problem of developing a practical gas engine was like the problem of getting useful work out of a gun. One objective is to generate a series of explosions-to make it a machine gun. The other, the critical objective for Otto, was somehow to convert the series of explosive impulses into a smooth flow of power. A flywheel would obviously smooth out the impulses, but Otto still had the problem, or thought he did, of moderating or cushioning the explosions so that they would not destroy the engine.

Otto tried three different solutions to this problem. This first was to use an extra free piston in the cylinder that would act as a spring to absorb the shock of the explosion. Otto later said he had tried such an arrangement in 1862. It was incorporated in a four-cylinder engine (very surprising for 1862) that compressed the charge and was said to employ the four-stroke cycle. The drawing was prepared from memory by a mechanic in 1885 in the course of patent litigation to support Otto's doubtful contention that he had used the four-stroke cycle in 1862; its purpose here is only to illustrate Otto's first solution to the shock problem. Each cylinder had a working piston connected to the crankshaft in the usual way, and between this main piston and the end of the cylinder was a free piston mounted on a plunger that was free to move back and forth in a hollow passage in the connecting rod of the main piston. The free piston would act as a shock absorber; when the explosion came, it would be driven back against the main piston, and the air trapped between the two pistons and the air in the hollow passage of the connecting rod would cushion the shock. The main piston would therefore be driven back more smoothly and gradually than the free pisten.

This double-piston arrangement had another function: it helped to drive out the exhaust gases. In an ordinary four-stroke engine the piston does not reach the end of the cylinder because it has to leave room for the compressed charge to be burned. Thus the space occupied by the charge at the end of the compression stroke is still there at the end of the exhaust stroke, and it contains unexpelled gases that remain to contaminate the next charge. At first engineers were concerned about this contamination. Nowadays they do not have to worry so much about it because they work with much higher compression ratios, say 10 to 1 (a tenfold increase in pressure). This means that the space not swept out by the piston is much smaller than it was, for example, in the Silent Otto, which had a compression ratio of 2.5 to 1. The free piston shown in Otto's 1862 arrangement would have been gradually forced back against the main piston by the increasing pressure of the charge being compressed, thereby leaving room for

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