[p234]

CHAPTER X
CONSTRUCTION AND TESTS OF THE LARGE ENGINE

The main requirement in an engine for an aerodrome—aside from reliability and smoothness of operation, which are necessary in an engine for any kind of locomotion—is that it shall develop the greatest amount of power for the least weight. It is, therefore, desirable to reduce the weight and number of parts of the engine to the very minimum, so far as this can be done without sacrificing reliability and smoothness of running. Furthermore, since the strongest metal for its weight is steel, and since the greatest strength of steel is utilized when the stress acting on it is one of tension, it is advisable to design the engine so that the parts which sustain the greatest strains shall be of steel and, as far as possible, meet with strains which are purely tensional ones.

In designing the new engine for the large aerodrome it was, therefore, planned to make it entirely of steel, as far as this was possible. The only parts which were not of steel were the bronze bushings for the bearings, the cast-iron pistons, and cast-iron liners of the cylinders. Previous experience had shown that, while it is possible to use a cast-iron piston in a steel cylinder or even a steel piston in a steel cylinder, provided the lubrication be kept exactly adjusted, yet the proper lubrication of the piston and cylinder of a gas engine is difficult even under the most favorable conditions, owing to the fact that excessive lubrication causes trouble from the surplus oil interfering with the sparking apparatus. It was, therefore, determined not to risk serious trouble by attempting to have the pistons bear directly on the steel walls of the cylinders.

While visiting the French engine builders in the summer of 1900 in the attempt to find one willing to undertake the construction of a suitable engine for the aerodrome, it was pointed out to them that the great amount of weight which they claimed to be necessary for the cylinders, and which they stated made it impossible for them to build an engine which would meet the requirements as to power and weight, could be very greatly reduced by making the cylinders in the form of thin steel shells having cast-iron linings. All, however, to whom this suggestion was made declared that it was impossible to build satisfactory cylinders in this way; some of them even stated that they had tried it and found it impossible to keep the thin liners tight in the steel shells. The difficulty which they had encountered is due to the difference in expansion of the steel and the iron when raised to a rather high temperature by the heat of the explosions, if the cylinders are not well jacketed with water; and if the steel [p235] shells are water jacketed they then do not expand as much as the cast-iron liners, and this causes the latter to become “out of round” because of the compression strains produced in them when trying to expand more than the steel shells. As past experience had shown, however, that it was possible to keep the liners tight in small cylinders, it was believed that by taking proper care in the construction there would be no difficulty in this respect with the cylinders of this larger engine.

In carrying out these plans, however, of making the cylinders of steel, numerous constructional difficulties were encountered which could not be foreseen when the design was made. Had they been foreseen, provision for obviating them could easily have been made. As will be seen from the drawing, Plate [78], the engine cylinders consisted primarily of a main outer shell of steel one-sixteenth of an inch thick, near the bottom end of which was screwed and brazed a suitable flange, by which it was bolted to the supporting drum or crank chamber. These shells, which were seamless, with the heads formed integral, were designed to be of sufficient strength to withstand the force of the explosion in them, and, in order to provide a suitable wearing surface for the piston, a cast-iron liner one-sixteenth of an inch thick was carefully shrunk into them. Entering the side of the cylinder near the top, was the combustion chamber, machined out of a solid steel forging, which also formed the port which entered the cylinder and was fastened to it by brazing. The water jackets, which were formed of sheet steel .020 inch thick, were also fastened to the cylinder by brazing, and it was in connection with the brazing of these water jackets that the first serious difficulty was met in the construction of the engine. In the first place, as the jackets were of an irregular shape and of a different thickness of metal from the walls of the cylinder to which they were joined, the expansion and contraction due to the extreme heat necessary for properly brazing the joints caused such serious strains in various and unexpected directions that it was only by exercising the very greatest care and patience that a completely tight joint at all points of the jacket could be secured. In the second place, the size of the cylinders and the consequently large extent of water-jacket surface, complicated the problem. The maintenance over this large surface of the extreme heat necessary for brazing involved discomfort and, indeed, actual suffering to the person engaged in the work, and much care and skill were demanded in so distributing the heat that the temperature of the surface of the jackets would be uniform enough to prevent serious strains from expansion and contraction. As no workman could be found either competent to do the work or willing to undergo the personal discomfort, the writer was obliged to do all this brazing work himself. Besides the difficulties due to the expansion and contraction of the jackets while they were being brazed, the greatest care had to be exercised to avoid heating the cylinders so hot as to weaken the [p236] joint where the explosion chambers were joined to the cylinders, which, of course, had been brazed before the jackets were fitted to them preparatory to brazing them.

Another great difficulty was that the ring which encircled the cylinder near the middle of its length, and which formed the bottom part of the water jacket, expanded very much more than the cylinder itself, so that, if it was brazed to the cylinder before the jacket was brazed to it, the heat of brazing the jacket to the ring would cause the ring to break loose from the cylinder; while if the ring was not previously brazed to the cylinder, but was brazed after the jacket had been brazed to it, the very much greater heat required for brazing the ring to the cylinder caused the spelter to burn out of the joint between the jacket and the ring. Furthermore, it was found very difficult to braze the two joints at the same time, since in brazing the ring to the cylinder it was best to have the cylinder in an inverted vertical position, so that the spelter could be made to flow evenly around the ring and form a fillet against the wall of the cylinder, while in brazing the jackets to the ring it was best to have the cylinder in the reverse vertical position or lying on its side so that the spelter could properly flow into this joint. Finally, however, after what proved to be most exasperating and tedious work, the five cylinders necessary for the engine were completed and a series of tests was immediately made. During the course of these tests the water circulation became obstructed in several instances, and the consequent high temperature to which the cylinders and jackets were raised caused severe strains in the jackets which, in turn, produced breaks in the brazed joints. These breaks had to be rebrazed, and in brazing them it was necessary in almost every case to remove the cast-iron liners and rebraze the entire jackets from start to finish, as the application of the intense heat necessary for brazing at any one point produced such severe strains that before the break which was being repaired could be completed other breaks developed at various points of the jacket. It was, therefore, necessary to get the whole jacket up to a fairly uniform heat and complete the brazing while it was in this condition, and then keep the whole cylinder at a uniform but gradually decreasing temperature until it had sufficiently cooled off.

On account of these troubles with the water jackets and the cylinders, it was decided to build some extra cylinders, not only because past experience had suggested improvements in detail in the construction of the jackets, which would prevent to a large extent the great troubles which had been met with in the brazed joints, but also to insure having sufficient cylinders to enable the engine to be always in working condition, even though several of the cylinders might be out of commission from slight imperfections in the jackets or at other points. While the construction of these new cylinders involved a repetition of the arduous task of brazing, yet the minor improvements which were introduced [p237] proved eminently successful in providing against future troubles from leaky jackets.

5in Cylinder by 512in Stroke Engine. Scale 12 Full Size. PL. 78. ENGINE OF AERODROME A. SECTION THROUGH CYLINDER AND DRUM [◊] [lgr]