The Wright Brothers' Engines and Their Design

by Leonard S. Hobbs

The entire valve system, including guides and seats, was of cast iron, a favorite material of the Wrights, except for the valve stems, which were, at different times, of various carbon steels. Ordinary cold-rolled apparently was used in those of the original engine, but in later engines this was changed to a high-carbon steel.

The piston design presented no difficulty. In some measure this was due to the remarkable similarity that seems to have existed among all the different engines of the time in the construction of this particular part, for, although there were some major variations, it was, in fact, almost as if some standard had been adopted. Pistons all were of cast iron and comparatively quite long (it was a number of years before they evolved into the short ones of modern practice); they were almost invariably equipped with three wide piston rings between the piston pin and the head; and, although there were in existence a few pistons with four rings, no oil wiper or other ring seems to have been placed below the piston pin. The Wrights' piston was typical of the time, with the rings pinned in the grooves to prevent turning and the piston pin locked in the piston with a setscrew. In designing this first engine they were, however, apparently somewhat unsure about this latter feature, as they provided the rod with a split little end and a clamping bolt (see Figure [6]), so that the pin could be held in the rod if desired; but no examples of this use have been encountered.

The Wrights' selection of an "automatic" or suction-operated inlet valve was entirely logical. Mechanically operated inlet valves were in use and their history went back many years, but the great majority of the engines of that time still had the automatic type, and with this construction one complete set of valve-operating mechanisms was eliminated. They were well aware of the loss of volumetric efficiency inherent in this valve, and apparently went to some pains to obtain from it the best performance possible. Speaking of the first engine, Orville Wright wrote, "Since putting in heavier springs to actuate the valves on our engine we have increased its power to nearly 16 hp and at the same time reduced the amount of gasoline consumed per hour to about one-half of what it was."[12]

Why they continued with this form on their later engines is a question a little more difficult to answer, as they were then seeking more and more power and were building larger engines. The advantages of simplicity and a reduced number of parts still existed, but there also was a sizable power increase to be had which possibly would have more than balanced off the increased cost and weight. They did not utilize mechanical operation until after a major redesign of their last engine model. Very possibly the answer lies in the phenomenon of fuel detonation. This was only beginning to be understood in the late 1920s, and it is quite evident from their writings that they had little knowledge of what made a good fuel in this respect. It is fairly certain, however, that they did know of the existence of cylinder "knock," or detonation, and particularly that the compression ratio had a major effect on it. The ratios they utilized on their different engines varied considerably, ranging from what, for that time, was medium to what was relatively high. The original flight engine had a compression ratio of 4.4:1. The last of their service engines had a compression ratio about twenty percent under that of the previous series—a clear indication that they considered that they had previously gone too high. Quite possibly they concluded that increasing the amount of the cylinder charge seemed to bring on detonation, and that the complication of the mechanical inlet valve was therefore not warranted.

Figure 6.—First flight engine, 1903, cross section. (Drawing courtesy Science Museum, London.)

The camshaft for the exhaust valves (101, Figure [6]), was chain driven from the crankshaft and was carried along the bottom of the crankcase in three babbit-lined bearings in bearing boxes or lugs cast integral with the case. Both the driving chain and the sprockets were standard bicycle parts, and a number of bicycle thread standards and other items of bicycle practice were incorporated in several places in the engine, easing their construction task. The shaft itself, of mild carbon steel, was hollow and on each side of an end bearing sweated-on washers provided shoulders to locate it longitudinally. Its location adjacent to the valves, with the cam operating directly on the rocker arm, eliminated push rods and attendant parts, a major economy. The cams were machined as separate parts and then sweated onto the shaft. Their shape shows the principal concern in the design to have been obtaining maximum valve capacity—that is, a quite rapid opening with a long dwell. This apparent desire to get rid of the exhaust gas quickly is manifested again in the alacrity with which they adopted a piston-controlled exhaust port immediately they had really mastered flight and were contemplating more powerful and more durable engines. This maximum-capacity theory of valve operation, with its neglect of acceleration forces and seating velocities, may well have been at least partially if not largely the cause of their exhaust-valve troubles and the seemingly disproportionate amount of development they devoted to this part, as reported by Chenoweth, although it is also true that the exhaust valve continued to present a problem in the aircraft piston engine for a great many years after, even with the most scientific of cam designs.

The rocker arm (102, Figure [6]) is probably the best example of a small part which met all of their many specific requirements with an extreme of simplicity. It consisted of two identical side pieces, or walls, of sheet steel shaped to the desired side contour of the assembly, in which were drilled three holes, one in each end, to carry the roller axles, and the third in the approximate middle for the rocker axle shaft proper. This consisted of a piece of solid rod positioned by cotter pins in each end outside the side walls (see Figure [5]). The assembly was made by riveting over the ends of the roller axles so that the walls were held tightly against the shoulders on the axles, thus providing the correct clearance for the rollers. The construction was so light and serviceable that it was essentially carried over to the last engine the Wrights ever built.

The basic intake manifold (see Figure [5]) consisted of a very low flat box of sheet steel which ran across the tops of the valve boxes and was directly connected to the top of each of them so that the cages, and thus the valves, were open to the interior of the manifold. Through an opening in the side toward the engine the manifold was connected to a flat induction chamber (21, Figure [5]) which served to vaporize the fuel and mix it with the incoming air. This chamber was formed by screw-fastening a piece of sheet steel to vertical ribs cast integral with the crankcase, the crankcase wall itself thus forming the bottom of the chamber. A beaded sheet-steel cylinder resembling a can (73, Figure [6]) but open at both ends was fastened upright to the top of this chamber. In the absence of anything else, this can could be called the carburetor, as a fuel supply line entered the cylinder near the top and discharged the fuel into the incoming air stream, both the fuel and air then going directly into the mixing chamber. The can was attached near one corner of the chamber, and vertical baffles, also cast integral with the case, were so located that the incoming mixture was forced to circulate over the entire area of exposed crankcase inside the chamber before it reached the outlet to the manifold proper, the hot surface vaporizing that part of the fuel still liquid.