Chapter II: Part 2

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.

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.

The control of the spark timing was effected by advancing or retarding the ignition camshaft in relation to the exhaust valve camshaft. The spur gear (37, Figure 5) driving the ignition camshaft had its hub on one side extended out to provide what was in effect a sleeve around the camshaft integral with the gear. The gear and integral sleeve were slidable on the shaft and the sleeve at one place (39, Figure 5) was completely slotted through to the shaft at an angle of 45 deg. to the longitudinal axis of the shaft. The shaft was driven by a pin tightly fitted in it and extending into the slot. The fore-and-aft position of the sleeve on the shaft was determined by a lever-operated cam (40, Figure 5) on one side and a spring on the other. The movement of the sleeve along the shaft would cause the shaft to rotate in relation to it because of the angle of the slot, thus providing the desired variation in timing of the spark. The "magneto" was a purchased item driven by means of a friction wheel contacting the flywheel, and several different makes were used later, but the original is indicated to have been a Miller-Knoblock (see Figure 5).

The connecting rod is another example of how, seemingly without trouble, they were able to meet the basic requirements they had set for themselves. It consisted of a piece of seamless steel tubing with each end fastened into a phosphor-bronze casting, these castings comprising the big and little ends, drilled through to make the bearings (See Figures 5 and 6). It was strong, stiff and light.[13] Forged rods were in rather wide use at the time and at least one existing engine even had a forged I-beam section design that was tapered down from big to little end. The Wrights' rod was obtained in little more time than it took to make the simple patterns for the two ends. The weight was easily controlled, no bearing liners were necessary, and a very minimum of machining was required. Concerning the big-end material, there exists a contradiction in the records: Baker, whose data are generally most accurate, states that these were babbited, but this must be in error, as the existing engine has straight bronze castings without babbiting, and there is no record, or drawing, or other indication of the bearings having been otherwise.

[Footnote 13: A rather thorough stress analysis of the rod shows it to compare very favorably with modern practice. In the absence of an indicator card for the 1903 engine, if a maximum gas pressure of five times the MEP is assumed, the yield-tension factor of safety is measurably higher than that of two designs of piston engines still in wide service, and the column factor of safety only slightly less. The shear stresses in the brazed and threaded joints are so low as to be negligible.]

Different methods of assembling the rod were used. At one time the tube ends were screwed into the bronze castings and pinned, and at another the ends were pinned and soldered. There is an indication that at one time soldering and threads were used in combination. One of the many conflicts between the two primary sets of drawings exists at this point. The Smithsonian drawings show the use at each end of adapters between the rod and end castings, the adapters being first screwed into the castings and pinned and then brazed to the inside of the tube. The Science Museum drawings show the tube section threaded and screwed into the castings. The direct screw assembly method called for accurate machining and hand fitting in order to make the ends of the tubing jam against the bottom of the threaded holes in the castings, and at the same time have the end bearings properly lined up. The weakness of the basic design patently lies in the joints. It is an attempt to utilize what was probably in the beginning a combination five-piece assembly and later three, in a very highly stressed part where the load was reversing. It gave them considerable trouble from time to time, particularly in the 4-cylinder vertical engines, and was abandoned for a forged I-beam section type in their last engine model; but it was nevertheless the ideal solution for their first engine.

The crankshaft was made from a solid block of relatively high carbon steel which, aside from its bulk and the major amount of machining required, presented no special problems. It was heat-treated to a machinable hardness before being worked on, but was not further tempered. The design was an orthodox straight pin and cheek combination and, as previously noted, there were no counterweights to complicate the machining or assembly. A sizable bearing was provided on each side of each crank of the shaft, which helped reduce the stiffness requirement.

Their only serious design consideration was to maintain the desired strength and still keep within weight limitations. A fundamental that every professional designer knows is that it is with this particular sort of part that weight gets out of control; even an additional 1/16 in., if added in a few places, can balloon the weight. With their usual foresight and planning, the Wrights carefully checked and recorded the weight of each part as it was finished, but even this does not quite explain how these two individuals, inexperienced in multicylinder engines--much less in extra-light construction--could, in two months, bring through an engine which was both operable and somewhat lighter than their specification.

In one matter it would seem that they were quite fortunate. The records are not complete, but with one exception there is no indication of any chronic or even occasional crankshaft failure. This would seem to show that it apparently never happened that any of their designs came out such that the frequency of a vibrating force of any magnitude occurred at the natural frequency of the shaft. Much later, when this type of vibration became understood, it was found virtually impossible, with power outputs of any magnitude, to design an undampened shaft, within the space and weight limitations existing in an ordinary engine, strong enough to withstand the stress generated when the frequency of the imposed vibration approximated the natural frequency of the shaft. The vibratory forces were mostly relatively small in their engines, so that forced vibration probably was not encountered, and the operating speed range of the engines was so limited that the natural frequency always fell outside this range.

The flywheel was about the least complex of any of their engine parts and required little studied consideration, although they did have to balance its weight against the magnitude of the explosion forces which would reach the power transmission chains, with their complete lack of rigidity, a problem about which they were particularly concerned. The flywheel was made of cast iron and was both keyed to and shrunk on the shaft.

Some doubt still exists about the exact method of lubricating the first engine. The unit presently in the airplane has a gear-type oil pump driven by the crankshaft through a worm gear and cross shaft, and the Appendix to the _Papers_ states that it was lubricated by a small pump; nevertheless Baker says, after careful research, that despite this evidence, it was not. Also, the drawings prepared by Christman (they were commenced under the supervision of Orville Wright) do not show the oil pump. In March 1905 Wilbur Wright wrote to Chanute, "However we have added oiling and feeding devices to the engine ..."; but this could possibly have referred to something other than an oil pump. But even if a pump was not included originally, its presence in the present engine is easily explained. Breakage of the crankcase casting caused the retirement of this engine, which was not rebuilt until much later, and the pattern for this part had no doubt long since been altered to incorporate a pump. It was therefore easier in rebuilding to include than to omit the pump, even though this required the addition of a cross shaft and worm gear combination. On later engines, when the pump was used, oil was carried to a small pipe, running along the inside of the case, which had four small drill holes so located as to throw the oil in a jet on the higher, thrust-loaded side of each cylinder. The rods had a sharp scupper on the outside of the big end so placed as also to throw the oil on this same thrust face. Some scuppers were drilled through to carry oil to the rod bearing and some were not.

The first engine was finished and assembled in February 1903 and given its first operating test on 22 February. The Wrights were quite pleased with its operation, and particularly with its smoothness. Their father, Bishop Wright, was the recorder of their satisfaction over its initial performance, but what he noted was probably the afterglow of the ineffable feeling of deep satisfaction that is the reward that comes to every maker of a new engine when it first comes to life and then throbs. They obtained 13 hp originally: later figures went as high as almost 16, but as different engine speeds were utilized it is rather difficult to settle on any single power figure. The most realistic is probably that given in the _Papers_ as having been attained later, after an accurate check had been made of the power required to turn a set of propellers at a given rpm. This came out at approximately 12 hp, the design goal having been 8. Following exactly the procedure that exists to this day, the engine went through an extended development period, and it was the end of September 1903 before it was taken, with the airplane, to Kitty Hawk where the historic flights, which have had such a profound effect on the lives of all men, were made on 17 December 1903.

The Engines With Which They Mastered The Art of Flying

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 deg. 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.

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.

The Wrights again exhibited their engineering mastery of a novel basic situation when, starting out to make flight a practical thing, they provided engine _No. 3_ to be used for experimental purposes. In so doing they initiated a system which continues to be fundamental in the art of providing serviceable aircraft engines to this day--one that is expensive and time consuming, but for which no substitute has yet been found. Their two objectives were: improvement in performance and improvement in reliability, and the engine was operated rather continuously from early 1904 until well into 1906. Unfortunately, again, no complete record exists of the many changes made and the ideas tested, although occasional notes are scattered through the diaries and notebooks.

In its present form--it is on exhibition at the Engineers Club in Dayton, Ohio--the _No. 3_ engine embodies one feature which became standard construction on all the Wright 4-cylinder models. This was the addition of a number of holes in a line part way around the circumference of the cylinder barrel so that they were uncovered by the piston at the end of its stroke toward the shaft, thus becoming exhaust ports (see Figure 9). This arrangement, although not entirely novel, was just beginning to come into use, and in its original form the ports exhausted into a separate chamber, which in turn was evacuated by means of a mechanically operated valve, so that two exhaust valves were needed per cylinder. Elimination of this chamber and the valve arrangement is typical of the Wrights' simplifying procedure, and it would seem that they were among the very first to use this form.[14]

[Footnote 14: Rankin Kennedy, _Flying Machines--Practice and Design_, 1909.]

The primary purpose of the scheme was to reduce, by this early release and consequent pressure and temperature drop, the temperature of the exhaust gases passing the exhaust valve, this valve being one of their main sources of mechanical trouble. It is probable that with the automatic intake valves being used there was also a slight effect in the direction of increasing the inlet charge, although with the small area of the ports and the short time of opening, the amount of this was certainly minor. With the original one-piece crankcase and cylinder jacket construction, the incorporation of this auxiliary porting was not easy, but this difficulty was overcome in the development engine by making different castings for the crankcase itself and for the cylinder jacket and separating them by several inches, so that room was provided between the two for the ports.

This engine demonstrated the most power of any of the flat 4s, eventually reaching an output of approximately 25 hp, which was even somewhat more than that developed by the slightly larger 4-1/8-in.-bore flight engine, with which 21 hp was not exceeded. Indicative of the development that had taken place, the performance of the _No. 3_ engine was twice the utilized output of the original engine of the same size, an increase that was accomplished in a period of less than three years.

The Wrights were only twice charged with having plagiarized others' work, a somewhat unusual record in view of their successes, and both times apparently entirely without foundation. A statement was published that the 1903 flight engine was a reworked Pope Toledo automobile unit, and it was repeated in an English lecture on the Wright brothers. This was adequately refuted by McFarland but additionally, it must be noted, there was no Pope Toledo company or car when the Wright engine was built. This company, an outgrowth of another which had previously manufactured one-and two-cylinder automobiles, was formed, or reformed, and a Pope license arrangement entered into during the year 1903.

The other incident was connected with Whitehead's activities and designs. Whitehead was an early experimenter in flying, about the time of the Wrights, whose rather extraordinary claims of successful flight were published in the 1901-1903 period but received little attention until very much later. His first engines were designed by a clever engineer, Anton Pruckner, who left at the end of 1901, after which Whitehead himself became solely responsible for them. It was stated that the Wrights visited the Whitehead plant in Bridgeport, Connecticut, and that Wilbur remained for several days, spending his time in their machine shop. This was not only categorically denied by Orville Wright when he heard of it but it is quite obvious that the 1903 or any other of the Wright engine designs bears little resemblance to Pruckner's work. In fact, its principal design features are just the opposite of Pruckner's, who utilized vertical cylinders, the 2-stroke cycle, and air-cooling, which Whitehead at some point changed to water-cooling.[15]

[Footnote 15: Considerable doubt surrounds Whitehead's actual flight accomplishments, but Pruckner's engines were certainly used, as several were sold to early pioneers, including Charles Wittemann. It is probable that the specific power output was not very great, for the air-cooled art of this time was not very advanced and Pruckner had a rather poor fin design. But the change to water cooling eliminated this trouble, and the engines were most simple, should have been relatively quite light, and with enough development could probably have been made into sufficiently satisfactory flying units for that period.]

The Four-Cylinder Vertical Demonstration Engine and the First Production Engine

In 1906, while still doing general development work on the flat experimental engine, the Wrights started two new engines, and for the first time the brothers engaged in separate efforts. One was "a modification of the old ones" by Wilbur and the other, "an entirely new pattern" by Orville. There is no record of any of the features of Wilbur's project or what was done in connection with it. Two months after the experimental operation of the two designs began, an entry in Wilbur's diary gives some weight and performance figures for the "4" x 4" rebuilt horizontal," and since Orville's design was vertical the data clearly refer to Wilbur's; but since the output is given only in test-fan rpm it does not serve to indicate what had been accomplished and there is no further mention of it.

Orville's design became the most used of any model they produced. It saw them through the years from 1906 to 1911 or 1912, which included the crucial European and United States Army demonstrations, and more engines of this model were manufactured than any of their others including their later 6-cylinder. Although its ancestry is traceable to the original 1903 engine, the design form, particularly the external configuration, was considerably altered. Along with many individual parts it retained the basic conception of four medium-size cylinders positioned in line and driving the propellers through two sprocket wheels. From the general tenor of the record it would seem, despite there being no specific indication, that from this time on Orville served as the leader in engine design, although this occurred with no effect whatsoever on their finely balanced, exactly equal partnership which endured until Wilbur's death in 1912.

The first major change from the 1903 design, putting the engine in an upright instead of flat position, was probably done primarily to provide for a minimum variation in the location of the center of gravity with and without a passenger. Whether or not it had any influence, the vertical cylinder arrangement was becoming predominant in automobile powerplants by this time, and the Wright engines now began to resemble this prevailing form of the internal combustion engine--a basic form that, in a wide variety of uses, was to endure for a long time.

Over the years, the Wrights seem to have made many changes in the engine: the bore was varied at different times, rod assembly methods were altered, and rod ends were changed from bronze to steel. Chenoweth states that on later engines an oil-control ring was added on the bottom of the piston, necessitating a considerable increase in the length of the cylinder barrel. This arrangement could not have been considered successful, as it apparently was applied to only a limited number of units and was not carried over to the later 6-cylinder engine model. There was much experimentation with cam shapes and most probably variations of these got into production.

With the crankcase, they did not go all the way to the modern two-piece form but instead retained the one-piece construction. Assembly was effected through the ends and a detachable plate was provided on one side for access to the interior. It is clear that they regarded this ability to get at the interior of the case without major disassembly as a valuable characteristic, and later featured it in their sales literature. They were apparently willing to accept the resultant weakening of the case and continued the construction through their last engine model. The integrally cast cylinder water jackets were abandoned and the top of the crankcase was machined flat to provide a mounting deck for individual cylinders. The use of aluminum alloy was continued, and the interior of the case was provided with strengthening webs of considerable thickness, together with supporting ribs. The cam shaft was supported directly in the case.

The individual cylinder design was of extreme simplicity, a single iron casting embodying everything except the water jacket. The valves seated directly on the cast-iron cylinder head and the guides and ports were all contained in an integral boss on top of the head. The exhaust valve location on the side of the engine opposite the pilot was a decided advantage over that of the 1903 design, where the exhaust was toward the pilot. A four-cornered flange near the bottom of the cylinder provided for fastening it to the crankcase, and a threaded hole in the top of the head received a vertical eyebolt which served as the rocker-arm support. The cylinder was machined all over; two flanges, one at the bottom and the other about two-thirds of the way down provided the surfaces against which the water jacket was shrunk. The jacket was an aluminum casting incorporating the necessary bosses and double shrunk on the barrel; that is, the jacket itself was shrunk on the cylinder-barrel flanges and then steel rings were shrunk on the ends of the jacket over the flanges. The jacket thickness was reduced by machining at the ends, making a semigroove into which the steel shrink rings fitted. These rings insured the maintenance of a tight joint despite the tendency of the aluminum jacket to expand away from the cast-iron barrel.

Why the one-piece crankcase and cylinder jacket combination of the 1903 engine was abandoned for the individual cylinder construction can only be surmised. The difference in weight was probably slight, as the inherent weight advantage of the original crankcase casting was largely offset by the relatively heavy valve boxes, and the difference in the total amount of machining required, because of the separate valve boxes, cages, and attaching parts, also was probably slight. Although the crankcase had shown itself to be structurally weak, this could have been cared for by proper strengthening. The 1903 design did have some fundamental disadvantages: it required a fairly complex pattern and expensive casting, plus some difficult machining, part of which had to be very accurate in order to maintain both gas and water joints tight; and the failure of any one cylinder that affected the jacket meant a complete crankcase replacement.

It seems probable that a change was initially made mandatory by their intention to utilize the ported exhaust feature, the value of which they had proved in the experimental engine. The separate one-piece water jacket construction they had arrived at in this engine was available, but once the decision to change was made, the individual cylinder with its shrunk-on jacket had much to commend it--simplicity, cost, ease of manufacture and assembly and attachment, and serviceability. The advantages of the auxiliary, or ported, exhaust were not obtained without cost, however, as the water jacket around the barrel could not very easily be extended below the ports. Thus, even though the water was carried as high as possible on the upper end, a large portion of the barrel was left uncooled, and the lack of cooling at the lower end, in conjunction with the uncooled portion of the head, meant that only approximately half the entire cylinder surface was cooled directly.

The piston was generally the same as in the 1903 engine, except that six radial ribs were added on the under side of the head, tapering from maximum thickness at the center to nothing near the wall. They were probably incorporated as an added path for heat to flow from the center of the piston toward the outside, as their shape was not the best use of material for strength. The piston pin was locked in the piston by the usual set screw, but here no provision was made for the alternate practice of clamping the rod on the pin. This piston-pin setscrew construction had become a standard arrangement in automobile practice. The piston rings were the normal wide design of that time, with what would now be considered a low unit pressure.

Quite early in the life of this engine model the practice was initiated of incorporating shallow grooves in the surface of the more highly loaded thrust face of the piston below the piston pin to provide additional lubrication. This development apparently proceeded haphazardly. Figure 10c shows three of the pistons from an engine of low serial number--the first of this model to be delivered to the U.S. Navy--and it will be noted that one has no grooves, another has one, and the other has three. The eventual standardized arrangement provided three of these grooves, approximately 1/16 in. wide, extending halfway around the piston, and, although the depth was only a few thousandths of an inch, the amount of oil carried in them was apparently sufficient to assist in the lubrication of the face, as they were used in both the 4-and 6-cylinder engines.

Each cylinder was fastened to the crankcase by four nuts on studs driven into the aluminum case. Valves and rocker arms were similar to those of the early engines, the automatic inlet valve being retained. The continued use of the two-piece valve is not notable, even though one-piece forgings were available and in use at this time; the automobile continued for many years to use this construction. The camshaft was placed at the bottom of the engine, inside the crankcase, and the rocker arms were actuated by pushrods which were operated by hinged cam followers. The pushrod was fastened in the rocker by a pin, about which it operated, through its upper end and was positioned near the bottom by a guide in the crankcase deck. The lower end of the rod bore directly on the flat upper surface of the cam follower, and valve clearance adjustment was obtained by grinding this end. The camshaft and magneto were driven by the crankshaft through a three-member train of spur gears (see Figures 9, 10 and 11).

The built-up construction of the connecting rod was carried over from the first engine, and in the beginning apparently the same materials were used, except that the big end was babbited. Later the rod ends were changed from bronze to steel. The big end incorporated a small pointed scupper on one side for lubrication, as with the original, and this was sometimes drilled to feed a groove which carried oil to the rod bearing, but where the drilling was omitted, the only function the scupper then could perform was, as in the original engine, to throw a small amount of oil on the cylinder wall.

The crankshaft and flywheel were similar in design to those on the 1903 engine, except that the sharp corners at the top and bottom of the crank cheeks were machined off to save weight (see Figure 10f). An oil pump and a fuel pump were mounted side by side in bosses cast on the valve side of the crankcase; they were driven from the camshaft by worm gears and small shafts crossing the case.

The camshaft construction was considerably altered from the 1903 design. Although the reason is not entirely clear, one indication suggests that breakage or distortion of the shaft may have been encountered: whereas in the 1903 engine there had been no relationship between the location of the cams and the camshaft bearings, in this engine the exhaust valves were carefully positioned so that all cams were located very close to the supporting bearings in the crankcase. Also, the camshaft was solid, although it would seem that the original hollow shaft construction could have provided equal stiffness with less weight. The final decision was possibly determined by the practicality that there existed no standard tubing even approximating the size and wall thickness desired.

There still was no carburetor, a gear pump metering the fuel in the same manner as on the 1904-1905 engine. Basically, the intake charge was fed to the cylinders by a round gallery manifold running alongside the engine. This was split internally by a baffle extending almost from end to end, so that the fuel mixture entering the manifold on one side of the baffle was compelled to travel to the two ends before it could return to the inside cylinder, this feature being a copy of their 1903 general intake arrangement. Apparently various shapes and positions of entrance pipes with which to spray the fuel into the manifold were used; and the injection arrangement seems also to have been varied at different times. The fuel pump was not necessarily always used, as the engine in some of the illustrations did not incorporate one, the fuel apparently being fed by gravity, as on the original engine. Chenoweth describes an arrangement in which exhaust heat was applied to the inlet manifold to assist the fuel vaporization process, but it is believed that this was one of the many changes made in the engine during its lifetime and not necessarily a standard feature.

A water circulation pump was provided, driven directly by the crankshaft through a two-arm universal joint intended to care for any misalignment between the shaft and the pump. The water was piped to a horizontal manifold running along the cylinders just below the intake manifold, and a similar manifold on the other side of the engine collected it for delivery to the radiator. It is a little difficult to understand why it was not introduced at the bottom of the water jackets.

The crankcase was a relatively strong and well proportioned structure with three heavy strengthening ribs running from side to side, its only weakness being the one open side. A sheet-iron sump was fastened to the bottom by screws and it would appear from its design, method of attachment, and location of the engine mounting pads that this was added some time after the crankcase had been designed; but if so it was apparently retrofitted, as engines with quite low serial numbers have this part.

The ignition was by high-tension magneto and spark plug and this decision to change from the make-and-break system was undoubtedly the correct one, just as adoption of the other form originally was logical under the circumstances that existed then. The high-tension system was simpler and had now collected more service experience. The magneto was driven through the camshaft gear, and a shelf, or bracket, cast as an integral part of the case, was provided for mounting it. The spark advance control was in the magneto and, since spark timing was the only means of regulating the engine power and speed, a wide range of adjustment was provided.

The engine had the controllable compression release which had been added to the _No. 2_ and _No. 3_ flat engines, although mechanically it was considerably altered from the original design. Instead of the movable stop operating directly on the rocker roller to hold the exhaust valve open, it was located underneath a collar on the pushrod. This stop was hinged to the crankcase and actuated by a small rod running along and supported by the crankcase deck. Longitudinal movement of this rod in one direction would, by spring pressure on each stop, push them underneath the collars as the exhaust valves were successively opened. A reverse movement of the rod would release them (see Figure 10f). Why they retained the method of manually operating the compression release, which was the same as had been used in the 1904-1905 engine, is not quite clear. That is, the mechanism was put into operation by pulling a wire running from the pilot to a lever actuating the cam which moved the control rod. When normal valve operation was subsequently desired, the pilot was compelled to reach with his hand and operate the lever manually, whereas a second wire or push-pull mechanism would have obviated the necessity for both the awkward manual operation of the lever and the gear guard which was added to protect the pilot's hand, the lever being located close to the camshaft gear.

The 4-cylinder vertical engine was a considerable improvement over the previous designs. They had obtained a power increase of about 40 percent, with a weight decrease of 10 percent, and now had an engine whose design was almost standard form for good internal combustion engines for years to come. In fact, had they split the crankcase at the crankshaft center line and operated the inlet valves mechanically, they would have had what could be termed a truly modern design. They needed more cylinder cooling, both barrel and head, particularly the latter, and an opened-up induction system for maximum power output, but this was not what they were yet striving for. They had directly stated that they were much more interested in reliability than light weight.

This engine model was the only one of the Wright designs to be licensed and produced abroad, being manufactured in Germany by the Neue Automobil-Gesellschaft and by Bariquand et Marre in France. The latter was much more prominent and their engines were used in several early European airplanes.

The French manufacturer, without altering the basic design, made a number of changes of detail which seem to have greatly annoyed Wilbur Wright, although some of them could probably be listed as improvements, based on several features of later standard design. One consisted of an alteration in the position of the fuel and oil pumps, the latter being lowered to the level of the sump. The crankcase was drilled to provide forced-feed lubrication to the connecting rod big end and crankshaft main bearings. Strengthening ribs were added to the pistons running from the upper side of the pin bosses to the piston wall, and the crankcase studs holding down the cylinders were replaced with bolts having their heads inside the case. The hinged cam follower was omitted and the pushrod bore directly on the cam through a roller in its end. The magneto was moved toward the rear of the engine a considerable distance and an ignition timing control device was introduced between it and its driving gear. Instead of the magneto being mounted directly on the special bracket integral with the crankcase, a wooden board running from front to rear of the engine was used and this was fastened to the two engine support pads, the magneto bracket being omitted entirely.

Despite his criticism of the French motor and the quality of its manufacture, Wilbur was compelled to install one in his own exhibition airplane during his early French demonstrations at Le Mans after rod failure had broken his spare crankcase, and much of his subsequent demonstration flying was made with the French product.

The Eight-Cylinder Racing Engine

By 1909 regular and special air meets and races were being held and various competitions for trophies conducted. Among these the Gordon Bennett Cup Race for many years was considered a major event. For the 1910 competition it was decided to enter a Wright machine and, since this was a race with speed the sole objective, the available 4-cylinder engine, even in a version pushed to its maximum output, was deemed too small. They built for it a special 8-cylinder unit in a 90 deg.V form. They were thus resorting to one of their 1904 concepts--modifying and enlarging a version known and proved in use--as the proper method of most quickly increasing output. Unfortunately again, there are essentially no detailed drawings available, so that the design cannot be studied.[16]

[Footnote 16: A drawing of the camshaft is held by The Franklin Institute.]

Only one engine is historically recorded as having been built, although in view of the Wrights' record of foresight and preparation it is almost certain that at least one spare unit, assembled or in parts, was provided. In any case, the airplane--it was called the _Baby Grand Racer_--and engine were wrecked just before the race, and no physical parts were retained, so that the sole descriptions come from external photographs, memory, and hearsay. McFarland thinks that possibly Orville Wright, particularly, was somewhat discomfited over the accident that eliminated the machine, as he had previously flown it quite successfully at a speed substantially higher than that of the ultimate winner, and he wanted to get it out of sight and mind as quickly as possible. The Air Force Museum at Wright Field, Dayton, Ohio, has an incomplete set of drawings of a 90 deg.V, 8-cylinder Wright engine, but it is quite obvious from the basic design and individual features, as well as from at least one date on the drawings, that this conception is of a considerably later vintage than that of the _Baby Grand Racer_.

The racing engine was in essence a combination of two of the standard 4s on a redesigned crankcase utilizing as many of the 4-cylinder engine parts as possible. The rods were reported to have been placed side by side, and the regular 4-cylinder crankshaft, with alterations to accommodate the rods, was utilized. A single cam operated all the exhaust valves. It was compact and light, its only fundamental disadvantage being the inherent unbalance of the 90 deg.V-8. The arrangement provided a much higher powered unit in the cheapest and quickest manner, and one that could be expected to operate satisfactorily with the least development.

The Six-Cylinder Vertical Engines

Shortly after the construction of the 8-cylinder engine the Wrights were again faced with the ever-recurrent problem of providing a higher powered standard production engine for their airplanes, which were now being produced in some numbers. By this time, 1911, there had been a relatively tremendous growth in both flying and automotive use of the internal combustion engine and as a result many kinds and sizes had been produced and utilized, so that numerous choices were presented to them. But if they were both to make use of their past experience and retain the simplicity they had always striven for, the more practical possibilities narrowed down to three: they could increase the cylinder size in the 4-cylinder combination, or they could go either to 6 or 8 cylinders in the approximate size they had previously used.