The Wright Brothers' Engines and Their Design

by Leonard S. Hobbs

Two more engines of this first general design were built but they differed somewhat from each other as well as from the original. Together with a third 8-cylinder engine these were begun right after the first of the year in 1904, shortly after the Wrights' return from Kitty Hawk. In planning the 8-cylinder engine they were again only being forehanded, but considerably so, in providing more power for increased airplane performance beyond that which might possibly be obtained from the 4-cylinder units. Progress with the 4-cylinder engines was such that they fairly quickly concluded that the 8-cylinder size would not be necessary, and it was abandoned before completion. Exactly how far it was carried is not known. The record contains only a single note covering the final scrapping of the parts that had been completed; and apparently there were no drawings, so that even its intended appearance is not known with any exactness. It was probably a 90° V-type using their original basic cylinder construction.

The changes carried through in the two 4-cylinder engines were not major. The water-cooled area of the cylinder barrel was increased by nearly ten percent but the head remained only partially cooled. In hindsight, this consistent avoidance of complete cylinder-head cooling presents the one most inexplicable of the more important design decisions they made, as it does not appear logical. In the original engine, where the factors of time and simplicity were of paramount importance, this made sense, but now they were contemplating considerably increased power requirements, knowing the effect of temperature on both the cylinder and the weight of cylinder charge, and knowing that valve failure was one of their most troublesome service problems. Nor does it seem that they could have been avoiding complete cylinder cooling through fear of the slightly increased complexity or the difficulty of keeping the water connections and joints tight, for they had faced a much more severe problem in their first engine, where their basic design required that three joints be kept tight with only two sets of threads, and had rather easily mastered it; so there must have been some much more major but not easily discernible factor which governed, for they still continued to use the poorly cooled head, even carrying it over to their next engine series. Very probably they did not know the effect on detonation of a high-temperature fuel-charge.

One of the new engines was intended for use in their future experimental flying and has become known as No. 2. It had a bore of 4-1/8 in., incorporated an oil pump, and at some time shortly after its construction a fuel pump was added. The fuel pump was undoubtedly intended to provide a metering system responsive to engine speed and possibly also to eliminate the small inherent variation in flow of the original gravity system.

This engine incorporated a cylinder compression release device not on the original. The exact reason or reasons for the application of the compression release have not been determined, although the record shows it to have been utilized for several different purposes under different operating conditions. Whatever the motivation for its initial application, it was apparently useful, as it was retained in one form or another in subsequent engine models up to the last 6-cylinder design. Essentially it was a manually controlled mechanism whereby all the exhaust valves could be held open as long as desired, thus preventing any normal charge intake or compression in the cylinder. Its one certain and common use was to facilitate starting, the open exhaust valves easing the task of turning the engine over by hand and making priming easy. In flight, its operation had the effect of completely shutting off the power. The propellers would then "windmill" and keep the engine revolving. One advantage stated for this method of operation was that when power was required and the control released, the engine would be at fairly high speed, so that full power was delivered immediately fuel reached the engine. It is also reported to have been used both in making normal landings and in emergencies, when an instant power shutdown was desired. Although it is not clear whether the fuel shutoff cock was intended to be manipulated when the compression release was used for any of these reasons, over the many years of its availability, undoubtedly at one time or another every conceivable combination of operating conditions of the various elements was tried. Because of the pumping power required with at least one valve open during every stroke, the windmilling speed of the engine was probably less than with any other method of completely stopping power output, but whether this difference was large enough to be noticeable, or was even considered, is doubtful.

Since a simple ignition switch was all that was required to stop the power output, regardless of whether a fuel-control valve or a spark-advance control was used, it must be concluded that the primary function of the compression release was to facilitate starting, and any other useful result was something obtained at no cost. The compression release was later generally abandoned, and until the advent of the mechanical starter during the 1920s, starting an engine by "pulling the propeller through" could be a difficult task. With the Wrights' demonstrated belief that frugality was a first principle of design, it is hardly conceivable that they would have accepted for any other reason the complication of the compression-release mechanism if a simple ignition switch would have sufficed.

The compression-release mechanism was kept relatively simple, considering what it was required to accomplish. A small non-revolving shaft was located directly under the rocker arm rollers that actuated the exhaust valves. Four slidable stops were placed on this shaft, each in the proper location, so that at one extreme of their travel they would be directly underneath the rocker roller and at the other extreme completely in the clear. They were positioned along the shaft by a spring forcing them in one direction against a shoulder integral with the shaft, and the shaft was slidable in its bearings, its position being determined by a manually controlled lever. When the lever was moved in one direction the spring pressure then imposed on the stops would cause each of them to move under the corresponding rocker roller as the exhaust valve opened, thus holding the exhaust valve in the open position. When the shaft was moved in the other direction the collar on the shaft would mechanically move the stop from underneath the roller, allowing the valve to return to normal operation.

Figure 8.—Development engine No. 3, 1904-1906, showing auxiliary exhaust port, separate one-piece water-jacket block. (Photo by author.)

If the 1903 engine is the most significant of all that the Wrights built and flew, then certainly the No. 2 unit was the most useful, for it was their sole power source during all their flying of 1904 and 1905 and, as they affirmed, it was during this period that they perfected the art, progressing from a short straightaway flight of 59 seconds to a flight controllable in all directions with the duration limited only by the fuel supply. It is to be greatly regretted that no complete log or record was kept of this engine.