The Silent Otto
The Silent Otto
By LYNWOOD BRYANT
The story of the gas engine that I am calling the Silent Otto—the first engine to operate on the four-stroke cycle, and the first to achieve compression of the charge within the working cylinder—has been told before.! But I should like to go over it again in order to consider a question that still seems significant to a historian interested in technology and culture: Why was this engine suddenly successful after seventy-five years of rather fumbling efforts to develop an internal-combustion engine?
Nicolaus August Otto (1832-91) built the first Silent Otto in 1876 in the Gasmotorenfabrik Deutz near Cologne (Fig. 1). In its original form the engine had one horizontal cylinder, drew in a charge of illuminating gas and air through a slide valve, compressed it within the cylinder with a compression ratio of about 2½:1, and ignited it with a flame. The engine developed about three horsepower at a speed of 180 revolutions per minute, had a thermal efficiency of about 14 per cent (two or three times as good as a comparable steam engine), and weighed about a ton per horsepower.2 They called it “silent” not because you could not hear it when it was running—you certainly could —but because it ran much more smoothly and quietly than its immediate predecessor, the Otto and Langen, is a one-cylinder atmospheric gas Mr. Bryant is associate professor of history at the Massachusetts Institute of Technology. engine that crashed and clanged like a rapid-fire pile driver and must have been a difficult engine to live with. So the great selling point of Otto's engine of 1876 was that it was quiet.
In designing this engine Otto was aiming at the rapidly growing market for small stationary power plants; he needed an engine that was smaller and smoother and more flexible than the noisy and awkward Otto and Langen, and especially one that could be built for larger powers, for the atmospheric engine was limited to about three horsepower. In competition with the steam engine, the gas engine could offer superior efficiency, but it used an expensive fuel. For small powers, though, where fuel economy was not a first consideration, it had the decisive advantage of easy operation and adaptability to intermittent use. A steam engine had a long warm-up period, and its fire had to be kept going whether the engine was working or not; it had a dangerous boiler and required a full-time attendant, a license, and perhaps a separate structure to house the power plant and the fuel supply. The gas engine, on the other hand, could be started and stopped at will, it did not have to be fed when it was not working, and it could be safely installed right in a shop, with no license, no special operator, and no fuel-storage problem if it used city gas. It was advantages like these that led Otto and other heroes of the development of the internal combustion engine to dream of supplanting the steam engine.
The Silent Otto was a spectacular success. It was a sensation at the Paris Exposition of 1878, and within a few years it dominated the market for small stationary power plants. It proved to be a form of engine capable of development into many shapes and sizes, vertical and horizontal, large and small, with single and multiple cylinders, using gas or liquid fuel, and flame or hot-tube or electric ignition. Almost overnight the hot-air engine and the atmospheric gas engine were rendered obsolete, and the two-stroke engine was not a serious threat for some time. At the Paris Exposition of 1889, after the patents had been broken in Germany and France, there were said to have been fifty types of engine operating on Otto’s principle.3 At the end of the century a four-cylinder engine of one-thousand horsepower was delivered,4 and there must have been one hundred thousand engines in service bearing Otto’s name, not to mention the dozens of competing brands using the same principle, providing power for machine shops, breweries, printing presses, pumping installations, and electric-light plants.5
In order to account for the sudden success of this engine let us | go back to the beginning of Otto’s work with engines, follow the course of his thought through the fifteen years of work that culminated in the Silent Otto, and try to see what the breakthrough was that led to an adequate solution to the problem that had been troubling workers on the internal-combustion engine for a generation: the problem of how to get a smooth flow of power out of an explosive fuel burned in the working cylinder.
In the 1870's and the 1880’s it was not easy to say what made the Silent Otto silent. The question was argued in great length in courts of law and in professional journals, and the most eminent experts disagreed with each other and changed their minds. Otto thought that the key to his success was a special way he had of introducing fuel and air into the cylinder that cushioned the shock of the explosion. The prevailing—but not unanimous—opinion of the profession was that he was wrong, and even today engineers do not always agree on the desirability of heterogeneous or stratified charges of the kind that Otto thought he had in his engine.
Otto was a traveling salesman without technical training in 1860 when he read in the paper an enthusiastic account of a gas engine built by a Frenchman named Etienne Lenoir (1822-1900). He arranged to have a similar small engine built by a mechanic, started tinkering with it in his spare time, and promptly ran up against the key problem that baffled everyone at that time: how to control an explosive fuel. The Lenoir engine, which was very widely discussed in the popular and the technical press in the period 1860-63, was a two-stroke, double-acting affair that looked very much like a steam engine and operated without compression. It sucked in a mixture of illuminating gas and air, ignited it by an electric spark halfway through the stroke, and used the remaining half as the power stroke. On the way back the piston was driven by a similar impulse on the other side. The publicity | represented this engine as quiet and smooth running, and apparently it |
| was when it was exhibited idling; but clearly Lenoir and his customers |
| had trouble with rough running as soon as the engine took on a load, |
| for Lenoir took out supplementary patents on two devices designed to |
| cushion the shock, some sort of spring between the piston and the load, |
| and some sort of auxiliary shock-absorbing cylinder.” He also tried |
| injecting water into the cylinder in the hope of retarding the combus- |
| tion and smoothing out the flow of power by generating steam.® An- |
| other promising approach at this time was to use some sort of burner |
| outside the cylinder and let the burning gases flow into the cylinder |
| like steam.? |
| Sooner or later Otto tried all of these approaches to the problem of |
| avoiding the destructive shocks caused by an explosive fuel. In the |
| early sixties he worked on two experimental engines, first the one- |
| cylinder model of the Lenoir engine and then a small four-cylinder |
| affair with pairs of cylinders on opposite sides of a crankshaft (like |
| today’s Volkswagen engine). Otto began work on the Lenoir model by |
| trying all possible variations of timing, fuel-air ratio, and size of charge |
| in an effort to find a combination that would give him a smooth- |
| running engine. Rich mixtures ignited easily but gave heavy shocks; |
| lean mixtures gave gentler impulses but difficulties with ignition. His |
| problem for the next fifteen years was how to achieve both smooth |
| burning and dependable ignition. He tried drawing in charges of vari- |
| ous sizes before ignition—a quarter, a half, three-quarters of a cylinder |
| full-and found, as Lenoir had, that it was best to ignite the charge half- |
| way through the stroke. Otto later said that he also tried drawing in a |
| full cylinder load and then compressing it on the back stroke (what else |
| could you do with a full charge?) and found that he got a terrific ex- |
| plosion that drove the flywheel through several revolutions. This, he |
| later said, was the starting point of the four-stroke cycle. |
| I see no reason to doubt that Otto tried compressing the charge like |
| this—it seems a perfectly natural thing to do—and undoubtedly such an |
| experience would make a man notice the surprising increase in available |
| energy that comes with compression. Otto also could have learned of |
| the advantages of compression from the contemporary patents or pro- |
| fessional journals. Schemes for compressing fuel or air or both, or for |
| pumping them under pressure to a working cylinder, had been pro- |
| posed in a number of gas-engine patents before this time. The strict |
| Prussian patent office had in fact rejected an application involving com- |
| pression on the ground that the process was already well known.! So |
| Otto could easily have been aware of the theoretical advantages of |
| compressing the charge before ignition, and undoubtedly he ran into |
| the phenomenon in his experimental work in the early 1860’s. Neverthe- |
| less, it seems to me very doubtful if he had at that time any real appre- |
| ciation of the value of compression as a practical working principle |
| for an engine. In 1861 he was looking in the opposite direction, seeking |
| ways of mitigating the severity of the shock of an explosive fuel. He |
| had trouble enough at atmospheric pressure; it would not have made |
| sense at that time to compress the charge if he could help it, except out |
| of pure curiosity. |
| One of Otto’s ideas in the early sixties was to cushion the shock by |
| having an extra piston in the cylinder to act as a spring between the |
| explosion and the main piston. Such an auxiliary piston he used (accord- |
| ing to later recollections) in the small four-cylinder engine that he had |
| built to replace the Lenoir model (Fig. 2), and a similar piston appears |
| in his first patent application for an atmospheric engine in 1863. The |
| extra piston had its rod fitted inside the hollow stem of the main piston, |
| so that when the explosion occurred it would be driven against the air |
| trapped within the main piston rod or between the two pistons.!2 |
| Later Otto said that this piston also had the function of driving out
| the exhaust gases. In those days it was regarded as essential to get rid |
| of the spent gases completely before starting a new cycle, so that the |
| fresh charge would not be weakened by being mixed with inert gases. |
| This requirement was very troublesome to designers, especially later |
| when they were trying to compress the mixture within the cylinder, |
| so troublesome that it may have been an important force in delaying |
| the success of the four-stroke cycle. For in a four-stroke compression |
| engine the space occupied by the compressed mixture at the end of the |
| compression stroke, which was a rather large space in the early engines |
| with low compression ratios, is still there at the end of the exhaust |
| stroke, when it is occupied by unexpelled exhaust gases. One or two |
| early compression engines drove out the exhaust by using a complicated |
| system of rods and cranks that made the exhaust stroke longer than |
| the compression stroke.!® Otto’s extra free piston performed this func- |
| tion, he thought, because on the compression stroke it was forced back |
| into the main piston by the increasing pressure of the charge being |
| compressed, but on the exhaust stroke it was free to move right up to |
| the end of the cylinder and achieve complete expulsion. Anyway, |
| Otto’s four-cylinder experimental engine was a failure, and no engineer |
| who sees that arrangement with the four free pistons is surprised.'* |
| At this point, Otto, baffled by the shock problem, turned off in a |
| new direction which could have been suggested by the atmospheric |
| steam engine or by his own experience. In his first experimental engine |
| Otto had found that when he used a charge too small to drive the piston |
| to the end of the cylinder, the piston would return by itself, because the |
| gas cooled and contracted surprisingly quickly and atmospheric pres- |
| sure drove the piston back. Otto turned to the atmospheric principle |
| in 1863, and the solution he finally adopted was to use the explosion |
| to drive a heavy piston up in a vertical cylinder without restriction and |
| then let the gentler atmospheric pressure and the weight of the piston |
| do the work on the way down. Otto is said to have worked out this |
| engine by himself, but the first form of it is so close to an engine of |
| Barsanti and Mateucci—they both even have one of those extra free |
Reference
- Recently, e.g. by Gustav Goldbeck in “Entwicklungsstufen des Verbrennungsmotors,” Motortechnische Zeitschrift, XXIII (1962), 76-80, and in several other articles; by Friedrich Sass in Geschichte des deutschen Verbrennungsmotorenbaues von 1860 bis 1918 (Berlin, 1962), pp. 19-74, 147-67; and by Arnold Langen in Nicolaus August Otto, der Schopfer des Verbrennungsmotors (Stuttgart, 1949). I rely heavily on Professor Sass for information about engines. I also acknowledge very helpful conversations with Dr. Goldbeck, archivist of Klockner-Humboldt-Deutz, and with Professors C. Fayette Taylor and Augustus R. Rogowski of the Massachusetts Institute of Technology.
- Sass, p. 44; Paul Gille, “Beau de Rochas. Les trois mémoires de 1862 sur différentes conditions d’application de Iénergie,” Documents pour Pbistoire des techniques, No. 2, p. 37; Conrad Matschoss, Geschichte der Gasmotorenfabrik Deutz (Berlin, 1921), p. 55.
- Gustav Goldbeck, “Nicolas-Auguste Otto (1832-1881),” Documents pour Ibistoire des techniques, No. 1, p. 38. The year of Otto’s death is misprinted in this title. It should be 1891.
- Sass, p. 319.
- Matschoss, pp. 107-8.
- For an early popular account see Alfred Darcel, “Un nouveau Moteur,” L’Dlustration, XXXV (1860), 342-43. For a technical study see H. Tresca, “Procés-Verbal des expériences faites sur les moteurs a gaz de M. Lenoir,” Annales du Conservatoire Impérial des Arts et Métiers, 1 (1861), 849-79. Articles by engineers appearing in German journals that Otto could have seen are conveniently summarized under the heading “Gaskraftmaschine” in Jabresbericht iiber die Fortschritte der mechanischen Technik und Technologie, 1 (1863), 111-31. “According to Cosmos, and other French papers,” said the Scientific American (III [1860], 193), “the age of steam is ended—Watt and Fulton will soon be forgotten. This is the way they do such things in France.”
- Goldbeck, “Entwicklungsstufen des Verbrennungsmotors,” p. 78.
- Henry Henderson, “The Use of Water and Steam in Internal-Combustion Engines,” Cassier’s Magazine, XXXIII (1907), 381-89.
- This was the principle of the Brayton engine patented in the United States in 1872, described in Bryan Donkin, 4 Text-Book on Gas, Oil, and Air Engines (London, 1894), pp. 51, 289-92. George B. Selden imitated this engine in his famous automobile patent of 1879.
- Eugen Langen, Vortrag des Herrn Kommmerzienrat Eugen Langen gebalten in der Sitzung des Kilner Bezirksvereins deutscher Ingemieure am 2. Mirz 1886 uber das Urteil des Reichgerichtes vom 30. Januar 1886 betreffend die Patente der Gasmotorenfabrik Deutz (Cologne, 1886) ; Arnold Langen, pp. 25-29; Kurt Schnauffer, “Nicolaus August Ottos Vierzylinder-Viertakt Motor von 1861. Zum 100 jahrigen | Jubilium des Viertaktverfahrens,” Motortechnische Zeitschrift XXIII (1962), 1-4. Otto's experimental work of the early 1860’s is referred to in a few contemporary letters but is described only in his testimony in patent litigation about twenty years later and in his unpublished reminiscences written in 1889, parts of which are quoted in Sass and in the biographical work of Arnold Langen and Goldbeck already cited. The engines themselves do not survive, and there are no contemporary drawings.
- Gustav Goldbeck, “Nicolas-Auguste Otto,” p. 38. An example of a technical article that Otto could have seen specifically recommending compression is Gustav Schmidt, “Theorie der Lenoir’ schen Gasmachine,” [Dingler’s] Polytechnisches Journal, CLX (1861), 321-37. The Million engine using compression patented in England in 1861 is fully described and illustrated in Polytechnisches Journal, CLXXIX (1866), 329-40.
- Sass, pp. 23-24; Eugen Langen, pp. 2-3. A drawing of this engine made in 1885 for use in patent litigation is reproduced in Sass, p. 23. Schnauffer uses the same drawing in the article quoted in n. 10, with minor changes to support the view that this engine used the four-stroke cycle.
- E.g., the Atkinson engine described in Donkin, pp. 104-5, or in the Scientific American Supplement, XXVII (1889), 10929.
- I note that Frank Duryea’s first engine in America, also a failure, had a similar free-piston arrangement to drive out exhaust and that the free piston apparently caused loud knocks (Don H. Berkebile, “The 1893 Duryea Automobile,” United States National Museum Bulletin 240 [Washington, D.C., 1964], pp. 10-15).