The Origin of the Four-Stroke Cycle

By Lynwood Bryant

Professor BRYANT of the Massachusetts Institute of Technology has done much research on the origins and development of the internal-combustion engine.

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In an earlier paper¹ on the genesis of a gas engine built by Nicolaus August Otto in 1876, I tried to follow the course of Otto’s thought and to explain the success of his engine. Otto attributed his success to what he called the stratified charge, a special way he devised of mixing the fuel and air in his cylinder. His theory was wrong, but guided by this theory he built a remarkably successful and influential engine, the earliest recognizable ancestor of today’s automobile engine.

I should now like to argue that Otto’s engine of 1876 was the first to use the four-stroke cycle. The four-stroke process, which first made it practical for a gas engine to use compression, was a key idea in the evolution of the internal-combustion engine, and the question of priority in its use was once a matter of critical importance: fortunes were at stake, or at least the owners of the patents involved thought so. Now the patents and the inventors are dead, and all that is at stake is credit for the invention. But the historical literature on early engines still reflects the confusion and disagreement of the old patent litigation, and perhaps it would be a service to review and evaluate the competing claims now that no money is at stake.

Four men are said to have used the four-stroke cycle before 1876. All of these claims arose in the course of patent litigation from ten to forty years after the time of the invention, and they all still appear from time to time in the literature. None is supported by substantial contemporary evidence. In 1862, the French engineer Beau de Rochas expressed the idea of the four-stroke cycle, but there is no evidence that he built an engine of this type or had any influence on anybody who did. The Munich watchmaker Reithmann and the famous Viennese inventor Marcus both were followers and not predecessors of Otto with their four-stroke engines. And the claim that Otto himself used the four-stroke process in an experimental engine in 1862 rests on extremely slight contemporary evidence. I conclude that primary credit for the invention of the four-stroke cycle should go to Otto, and that the invention should be dated 1876.

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The earliest known expression of the idea of the four-stroke cycle is in a queer pamphlet published in 1862 by an obscure French engineer named Alphonse-Eugéne Beau de Rochas (1815-93) as an incidental part of some purely theoretical speculations about the possibility of using gas engines in conjunction with steam engines to improve the performance of railroad locomotives. The idea was clearly and precisely expressed (Fig. 1), but the pamphlet was very obscurely published: it did not enter the stream of public knowledge until it was brought to light in the course of litigation over Otto’s patents in 1884.

Not much is known about Beau de Rochas beyond what may be inferred from a dozen publications of his that have been found.² He was apparently a quixotic inventor familiar with the current literature on thermodynamics and given to enthusiastic attacks on problems in the exciting fields of the telegraph and the railroad. Around the middle of the century, he published proposals for a submarine telegraph, a railroad tunnel under the English Channel, a new kind of drive for canal boats consisting of an endless chain resting on the bottom of the canal (the screw propeller he did not regard as practical except on the high seas), the use of steel for high-pressure boilers, and a way of improving the adhesion of locomotives so that the railroad could conquer the Alps. His proposals impress one as perfectly respectable intellectually, but irresponsible in the sense that he seemed unrestricted by the need to finance or manage the devices he proposed. One suspects that his ideas were too extreme to be influential, too outlandish to enter the normal channels of technical communication. They were sometimes published in pamphlets rather than in technical journals, and occasionally they appeared in peculiar forms.

The booklet that we are concerned with here, for example, is not set in type at all, but is a lithographic reproduction of fifty-three pages of fine handwriting, bearing the title Nouvelles Recherches sur les conditions pratiques de plus grande utilisation de la chaleur et en Général de la force motrice. Avec application au chemin de fer et la navigation. The title page identifies the author as “Ingénieur attaché au Service central des Chemins du Midi.” The book is a collection of suggestions for improving the efficiency of steam engines, a favorite topic in the technical literature of the time. The table of contents outlines these suggestions systematically, but without making any special feature of the gas engine and without mentioning the four-stroke cycle, so that even if the pamphlet happened to fall into the hands of a man working on the gas engine, it might very well not communicate this key idea to him.

Section 6 of the book contains various interesting proposals for combinations of gas and steam engines. The exhaust heat of a gas

engine, for example, could be used to generate steam for a steam engine, Beau de Rochas says. Steam and gas could be used in different cylinders, or on opposite sides of the piston in the same cylinder, with the steam used to get the engine started and up to the speed required by a gas engine. This section includes an interesting statement of the conditions for maximum efficiency of a gas engine: the piston speed should be as high as possible, and the expansion should proceed as far as possible. Cylinder space should have as low a surface-to- volume ratio as possible, so that one large cylinder would be better than several small ones. The initial pressure should be as high as possible, but a practical limit is set by the temperature at which the charge will ignite spontaneously, which will be reached, Beau de Rochas guesses, with a compression ratio of about four to one. (It is interesting to note that Beau de Rochas mentions the possibility of spontaneous ignition of a compressed charge. One could stretch a point and argue that he an- ticipated Diesel as well as Otto, but what is being compressed in his hypothetical engine is a mixture of fuel and air. There is no indication that Beau de Rochas thought of reaching high temperatures by compressing air alone, as Diesel did.)

As an incidental part of this theoretical analysis of the gas engine, Beau de Rochas suggests that a four-stroke cycle will make it possible to achieve the compression within the working cylinder and defines the four strokes as follows (Fig. 1):

1. aspiration pendant une course entiére du piston; (suction during an entire stroke of the piston)

2. compression pendant la course suivante; (compression during the next stroke)

3. inflammation au point mort et détente pendant la troisiéme course; (ignition at neutral and relaxation during the third stroke)

4. refoulement des gas brûlés hors du cylindre au quatriéme et dernier retour. (discharge of burnt gases out of the cylinder at the fourth and final return.)

Actually Beau de Rochas seems rather apologetic about the four-stroke cycle. It is a possible way of compressing the charge, he seems to be saying, not necessarily the best way, but the best he can think of at the moment. He recognizes the obvious objection that it reduces the number of power strokes per revolution, but mentions the compensating advantage of the improved efficiency that will come with compression, and he goes on to suggest that by making the engine double-acting one can achieve at least a half-power engine, so to speak (à demi-effet), by which he means one with half as many power strokes per revolution as an ordinary steam engine.

This clear expression of the four-stroke idea, which seems so remarkable in retrospect, had no influence in the 1860’s and 1870’s. Perhaps nobody read it. I can find no evidence that anybody knew about it before a German manufacturer accused of infringement somehow discovered the pamphlet in 1883 and used it to invalidate Otto’s four-stroke claim in Germany the following year.³ The obscurity of the

1. “The Silent Otto,” Technology and Culture, VII (Spring 1966), 184-200.
2. Information about Beau de Rochas is concentrated in two groups of articles— one in Bulletin de la Société d’ Encouragement pour VIndustrie Nationale, CXXXVII (1938), 209-39, especially C. Walckenaer, “L’invention de Beau de Rochas,” pp. 212 25; the other in Documents pour l'histoire des techniques, Cahier No. 2 [1962] pp. 3-42, especially Jacques Payen, “Beau de Rochas. Etude biobibliographique,” pp. 3-24, which contains a useful list of the known publications by and about Beau de Rochas.
3. The earliest reference I have seen is in a letter from C. Wigand dated December 14, 1883, published in Zeitschrift des Vereines deutscher Ingenieure, XXVIII (1884), 45-47.