The work of M. L. Le Chatelier, noticed in our last, entitled “ Railway Economy,” ar,.d in which the above subject is discussed, is probably the most important work on railway engineering recently published. The improvement in the application of counter-pressure steam which gave rise to the work, we consider the greatest advance made in railway engineering since Stephenson demonstrated that a train could 'be drawn on smooth rails by smooth surfaced wheels. We shall give an illustration and an account of this improvement, extracted from the treatise above alluded to, which will serve to impart a general idea of its nature ; but there are many nice scientific points connected with its operation, which the reader will seek from the work itself, and which will amply repay the research. The author gives the history of the improvement as follows: About the middle of 1865, when I first thought of organizing a system of experiments for removing the difficulties of reversing the steam, I began by trying whether it would be possible to work the engine for [my considerable time by means of the compressed-air apparatus of M. de Bergue. I soon convinced myself that the heating of the cylinders went on >'0 rapidly that this system was inapplicable for any length of run. It was then that I drew up a complete programme of experiments, the sum and substance of which was to establish a communication between the boiler and the lower end of the exhaust pipe, in order to supply there a jet of steam or of water, and to force into the boiler the elastic fluids—steam or gases discharged from the cylinders by the return stroke of the piston. I pointed out three combinations to be experimented on in succession, according to the greater or less difficulty found in completely cooling the cylinders. 1st. Injection of steam mixed with air. 2d. Injection of steam in sufficient excess to prevent the entrance of air. 3d. Injection of water, instead of steam. At first I supposed that the steam would carry along with it a sufficient quantity of water to absorb the heat produced, and that it would be condensed before reaching the cylinders. This idea was incorrect. During the working with steam reversed, the water ceases to be in a state of violent ebullition, and is only carried over in small quantities ; and, besides, when the steam expands in issuing from the boiler, it dries, and the small quantity of water brOught with it is almost entirely converted into steam. The first experiment with a mixture of steam and gases drawn into the cylinders did not give favorable results. With the injection of an excess of steam—a system which I characterized as an inverted steam engine—more satisfactory results were obtained, and it was found possible to work with a moderate - admission of steam with light loads on moderate gradients, without burning the packings, and without injuring the rubbing surfaces. We have in France the example of a railway on which 200 engines have only a cock for the injection of steam, and the substitution of this for the drawn from the smoke-box has proved sufficient to render the counter-pressure steam applicable for stopping and shunting in stations, and for moderating the speed in the descent of goods trains on gradients of 1 in 260. Indeed, the injection of steam alone has been effectually applied to light trains on a short incline of 1 in 22. But experience soon showed that the only general and complete solution of the question is found in the injection of water. To complete the absorption of the heat produced by the compression in the cylinders, to force back the steam into the boiler, and to render the reversal of the steam an ab T- Fin. 2 GOODES' KNIEE GUARD. lutely innocuous operation, water is the only appliance. The engineers in Spain, to whom I intrusted the experiments, never quite understood the effects which the injection of water should produce. The application of it was therefore made with timidity and with doubt. They imagined” that it must be reduced to the minimum quantity. As a consequence, the results obtained in this, the first application, were never complete, despite successive increments of the quantity of water added to the steam. It was in France that engineers first recognized the necessity of giving a great preponderance to water over sterm, and thus succeeded in rendering the new system applicable under every circumstance of the locomotive service. For many months the official reports sent from Spain announced that the results obtained, from a mixture of steam and water, were quite satisfactory, but this was afterwards proved not to be the case. For two years and a half after my first programme was drawn- up, this idea, that the solution of the problem consisted in injecting a mixture of steam and water in the exhaust pipe, prevailed. By successive trials—by rendering the steam and water cocks altogether independent of each other under the hand of the driver, the suitable proportion of water to steam in the various circumstances of admissson, speed, distribution, and dimensions of cylinders, was arrived at. It is by correcting this erroneous notion, which attributes to steam a necessary part in the action, that in France alone the system has been applied to 1,800 engines in ,vork, or being fitted with the necessary apparatus. The practical result has been complete, because of the independence of the injecting cocks, which has allowed of the proportion of water being carried to the necessary limit in each case. At the end of the year 1868, being free from my usual ocu pations, I determined on a consecutive study of the question, and on the verification of the results which had been obtained independently of my control. I soon perceived that my original notion—on which I had often by correspondence in- sisted—was correct in every respect; that the true solution consisted absolutely in the injection of water—that this solution satisfied every condition of the problem, and is probably the only one entirely applicable in cases of full admission and great speed. Steam, in fact, plays only a secondary part, prejudicial when above certain proportions, and, when used, to be applied with great caution, and only within certain limits. When we speak of injecting water issuing from the boiler into the cylinders of a locomotive engine, it must be b0rne in mind that it is not water in the state in which it would flow from a fountain; it is at a high temperature when it issues from the boiler, and rushes into space at atmospheric pressure. It.enters at once into ebullition, and becomes steam at 1OO ° C in quantity corresponding to the heat employed, The new system of reversing steam has been, until recently, limited to the use of a mixture of steam and water. The engineers to whom I had intrusted the task of making the first trials, followed my instructions with some apprehension, endeavoring as much as possible to avoid the injection of water into “ the cylinders. The result has been that, even now, in Spain, where these first trials were made, the use of counter-pressure steam has not had the success which it has had elsewhere. In France, the part played by the water was better understood; it has been abundantly injected and the results have been most satisfactory ; but up to the moment when I had an opportunity of personally experimenting, in order to verify the correctness of my first conceptions, steam was universally considered as a necessary agent, and was used in a greater or less proportion. It was supposed that its function was to fill the cylinders during the period of aspiration, and that it served as the vehicle for the water which was shut in with it, behind the piston, at the moment the period of cushioning and forcing back commenced. It was supposed that the water led from the boiler was applied directly to the absorption of heat. I have shown that the water is . converted into steam from the moment that it enters the cylinder, even during the period of aspiration, and the conclusion is that not only is it not required to take steam directly from the boiler, but that the addition of steam to the water, beyond a certain limit, might become prejudicial. In every case the substitution of steam for, or the addition of steam to water, results in a discharge of a less moist steam from the cylinders into the boiler, and it is the same with the steam in the exhaust-pipe used for aspiration. The rubbing surfaces are therefore drier, and the friction greater. The more the proportion of steam is increased, the more these effects become sensible. At last the steam actually diverts the water indispensable for the absorption of the heat although large quantities of steam escape by the funnel, and, although no gases from the smoke-box get into the .cylinders. The intervention of steam during the working with inverse admission, unless required for some particular purpose, which I shall point out presently, is always more or less prejudicial. The rule, in fact, should be, to add the least possible quantity of steam to the water. The wet steam, on the water issuing from the boiler, gives this minimum proportion. The apparatus to be fitted to the locomotive to admit of working counter-pressure steam as a brake, is as simple as the. principle itself. It consists of a tube of an inch to an inch and a quarter in diameter—one inch diameter is very convenient—which communicates between the boiler and the exhaust pipe, and a distributing cock by which the driver regulates the supply. If, as I advise, although it is not indispensable, it is desired to have the power of injecting water and steam alternately or simultaneously, a second cock is placed, with a short tube as' a branch from the first, at a short distance from its origin. The one tube enters the boiler below the - lowest level of the water, the other above the highest, so that steam only shall pass through the latter. When the engines have external cylinders, the exhaust- pipe divides into two branches. The injection tube must therefore have also two branches ; one going to the under side of each branch of the exhaust pipe. The bifurcation should be perfectly symmetrical, so that the water held in suspension in the steam may not take the line of steepest descent, and that the distribution to each cylinder may be equal. The engraving shows how the injection tube is joined to the exhaust pipes at two distinct places; but various other arrangements may be adopted. The pipes,D,leading from the boiler to the exhaust, discharge into the exhaust at the point A, or B, the engraving representing at one view two different arrangements in this respect, showing two distinct ways in which the wet vapor may reach the cylinders. The branch piece C, should be of brass, and should be joined on to a straight length as long as possible. The drops of water in suspension in the steam tend to continue to move in a straight line, by virtue of their inertia and of their quantity of motion. If the bifurcation be not symmetrical, the distribution is unequal. Again, it is essential that beyond the point of bifurcation the two branches of the tube should have the same length, the same form, and the same section. Thus it will be found convenient to carry the tube under the center of the boiler ; or, if room can be found for it, along the back of the boiler, in order to place the bifurcation at equal distances from the two cylinders. Want of symmetry might, of course, be compensated by difference of section in the tubes ; but it is better to use a greater length of main tube in order to reach a point which allows of a perfectly symmetrical arrangement. The injection of water might be used with the ordinary lever arrangement of reversing gear, where the consequences of a sudden spontaneous return of the handle would be unimportant. But the application is only quite satisfactory when the screw motion is used, as adopted by M. Marie, after Mr. Kitson's model. This apparatus, as a complement to tlie counter-pressure steam, has rendered most important service. Without it there must have been a long struggle against the natural repugnance of the engineers to reversing the steam. With it, the continual changes of the degree of admission, in order to maintain a uniform speed on lines with many changes of gradient, or for stopping trains at the right point in sta, tions are made without fatigue or anxiety to the engineer, There are no longer sudden jumps from one notch to another ; the regulator remains open, and consequently all the manipulations are more quickly effected, even when thee steam has to be rapidly reversed . It is to this happy combination that the rapidity is to be ascribed with which tIle Paris, Lyons, and Mediterranean Company hay') already adopted (May 1869) the counter-pressure-steam apparatus for not less than 1,400 engines.
This article was originally published with the title "The Use of Counter-Pressure Steam Tee Locomotive Engine as a Brake"