Figure 7.—First flight engine, 1903: cylinder, valve box, and gear mechanism; below, miscellaneous parts. (Photos courtesy Science Museum, London, and Louis P. Christman.)
Fuel was gravity fed to the can through copper and rubber tubing from a tank fastened to a strut, several feet above the engine. Of the two valves placed in the fuel line, one was a simple on-off shutoff cock and the other a type whose opening could be regulated. The latter was adjusted to supply the correct amount of fuel under the desired flight operating condition; the shutoff cock was used for starting and stopping. The rate of fuel supply to the engine would decrease as the level in the fuel tank dropped, but as the head being utilized was a matter of several feet and the height of the supply tank a matter of inches, the fuel-air ratio was still maintained well within the range that would ignite and burn properly in the contemplated one-power condition of their flight operation.
This arrangement is one of the best of the many illustrations of how by the use of foresight and ingenuity the Wrights met the challenge of a complex requirement with a simple device, for while carburetors were not in the perfected stage later attained, quite good ones that would both control power output and supply a fairly constant fuel-air mixture over a range of operating conditions were available, but they were complex, heavy, and expensive. The arrangement, moreover, secured at no cost a good vaporizer, or modern "hot spot." In their subsequent engines they took the control of the fuel metering away from the regulating valve and gravity tank combination and substituted an engine-driven fuel pump which provided a fuel supply bearing a fairly close relationship to engine speed.
The reasons behind selection of the type of ignition used, and the considerations entering into the decision, are open to speculation, as are those concerning many other elements that eventually made up the engine. Both the high-tension spark plug and low-tension make-and-break systems had been in wide use for many years, with the latter constituting the majority in 1902. Both were serviceable and therefore acceptable, and both required a "magneto". The art of the spark plug was in a sense esoteric (to a certain extent it so remains to this day), but the spark-plug system did involve a much simpler combination of parts: in addition to the plug and magneto there would be needed only a timer, or distributor, together with coils and points, or some substitute arrangement. The make-and-break system, on the other hand, required for each cylinder what was physically the equivalent of a spark plug, that is, a moving arm and contact point inside the cylinder, a spring-loaded snap mechanism to break the contact outside the cylinder, and a camshaft and cams to actuate the breaker mechanism at the proper time. Furthermore, as the Wrights applied it, the system required dry cells and a coil for starting, although these did not accompany the engine in flight. And finally there was the problem of keeping tight the joint where the oscillating shaft required to operate the moving point in the spark plug entered the cylinder.
This is one of the few occasions, if not the only one, when the Wrights chose the more complex solution in connection with a major part—in this particular case, one with far more bits and pieces. However, it did carry with it some quite major advantages. The common spark plug, always subject to fouling or failure to function because of a decreased gap, was not very reliable over a lengthy period, and was undoubtedly much more so in those days when control of the amount of oil inside the cylinder was not at all exact. Make-and-break points, on the other hand, were unaffected by excess oil in the cylinder. Because of this resistance to fouling, the system was particularly suitable for use with the compression-release method of power control which they later utilized, although they probably could not have been looking that far ahead at the time they chose it. High-tension current has always, and rightfully so, been thought of as a troublemaker in service; in Beaumont's 1900 edition of Motor Vehicles and Motors, which seems to have been technically the best volume of its time, the editor predicted that low-tension make-and-break ignition would ultimately supersede all other methods. And finally, the large number of small parts required for the make-and-break system could all be made in the Wright Brothers' shop or easily procured, and in the end this was probably the factor, plus reliability, that determined the decision which, all things considered, was the correct one.
There was nothing exceptional about the exact form the Wrights devised. It displayed the usual refined simplicity (the cams were made of a single small piece of strip steel bent to shape and clamped to the ignition camshaft with a simple self-locking screw), and lightness. The ignition camshaft (38, Figure [5]), a piece of small-diameter bar stock, was located on the same side as the exhaust valve camshaft, approximately midway between it and the valve boxes, and was operated by the exhaust camshaft through spur gearing. That the Wrights were thinking of something beyond mere hops or short flights is shown by the fact that the ignition points were platinum-faced, whereas even soft iron would have been satisfactory for the duration of all their flying for many years.