The Wright Brothers' Engines and Their Design Part 2

18. Cylinder liner s.c.r.e.w.i.n.g into jacket.

19. Open-ended "can" admits air.

20. Fuel supply.

21. (Hot) side of water jacket makes surface carburetter.

22. Sparking plug (comprising positive electrode 23 and spark-producing make-and-break 24).

25. Lever attached to lever 26 via bearing 27 screwed into chamber neck 28.

26. Levers with mainspring 29 and inter-spring 30, and rocked by "cam"

31.

31. Cam with another alongside (for adjacent cylinder).

32. Positive busbar feed to all four cylinders.

33. a.s.sembly retaining-rings.

34. Sealing disc.

35. Exhaust outlet ports.

36. Camshaft right along on underside of jacket and also driving oil pump 11 via 14.

37. Spring-loaded sliding pinion drives make-and-break shaft 38 through peg in inclined slot 39.

40. Cam to push pinion 37 along and so alter its angular relation with shaft 38 (to vary timing).

41. Exhaust-valve cams bear on rollers 42 mounted in end of rocker-arms 43.

44. Generator floating coils.

45. Friction-drive off flywheel.

46. Sight-feed lubricator (on stationary sleeve).

47. Hardwood chain tensioner.]

The intake and exhaust valves were identical and of two-piece construction, with the stems screwed tightly into and through the heads and the protruding ends then peened over. This construction was not novel, having had much usage behind it, and it continued for a long time in both automobile and aircraft practice. One-piece cast and forged valves were available but here again it was a choice of the quick, cheap, and proven answer.

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]

[Footnote 12: a.s.suming a rich mixture, consumption of all the air, and an airbrake thermal efficiency of 24.50% for the original engine, the approximate volumetric efficiency of the cylinder is calculated to have been just under 40%.]

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.

[Ill.u.s.tration: _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 princ.i.p.al 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 a.s.sembly, 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 a.s.sembly 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.

[Ill.u.s.tration: _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 c.o.c.k 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 c.o.c.k 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 ill.u.s.trations 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 subst.i.tuted 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 const.i.tuting 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 subst.i.tute 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 r.e.t.a.r.ding 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 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-k.n.o.block (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 a.n.a.lysis 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 a.s.sumed, 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 a.s.sembling 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 a.s.sembly 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 a.s.sembly 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 a.s.sembly. 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 s.p.a.ce 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 a.s.sembled 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.