The Eight-Cylinder Racing Engine

Figure 9.—4-Cylinder vertical engine: a, magneto side; b, valve port side with intake manifold removed; c, flywheel end of engine at Carillon Park Museum, Dayton, Ohio; d, magneto side with crankcase cover removed. (Photos: a, Smithsonian A-3773; b, d, Pratt & Whitney D-15003, 15007; c, by A. L. Rockwell.)

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. Figure10cshows 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 Figures9,10and11).

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 Figure10f). 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.

Figure 10.—4-Cylinder vertical engine: a, cylinder assembly with valve mechanism parts; b, cylinder disassembled, and parts; c, pistons and connecting rods; d, bottom side of piston; e, crankshaft, flywheel and crankcase end closure; f, crankcase, with compression release parts. (Pratt & Whitney photos D-14996, 15001, 14998, 14994, 14999, 14989, respectively.)

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 ofthe 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 simplerand 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 theNo. 2andNo. 3flat 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 Figure10f). 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 Marré in France. The latter was much more prominent and their engines were used in several early European airplanes.

Figure 11.—4-Cylinder vertical engine assembly, Bariquand et Marré version. (Drawing courtesy Bristol Siddeley Engines, Ltd.)

THE WRIGHT BROTHERS AERO ENGINE

The French manufacturer, without altering the basic design, made anumber 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.

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°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]

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 theBaby 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°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 theBaby 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 accommodatethe 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°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.

Figure 12.—Original 6-cylinder engine: a, push-rod side; b, valve-port side; c, crankcase with sump removed. (Photos: Smithsonian A-3773A, 45598; Pratt & Whitney D-15015, respectively.)

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 theywere 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.

The 4-in. cylinder in combination with a 5-in. stroke would provide in four cylinders about the displacement they wanted. Strokes of 6 in. were not uncommon and cylinders of 6-in. bore had been very successfully utilized in high-output automobile racing engines many years before this, so there was seemingly no reason to doubt that the 5-in. cylinder could be made to operate satisfactorily, but it is not difficult to imagine the Wrights' thoughts concerning the roughness of an engine with cylinders of this diameter. The question of the grade of available fuel may possibly have entered into their decision to some extent, but it seems far more likely that roughness, their perennial concern, was the predominant reason for not staying with the more simple 4-cylinder form (as we have seen, roughness to them meant the effect of the cylinder explosion forces). Actually, of course, they never went larger than a 4-3/8-in. cylinder bore, and later aircraft engine experience would seem generally to confirm their judgment, for with the piston engine it has always been much more difficult to make the larger bores operate satisfactorily at any given specific output.

While the 90°V, 8-cylinder arrangement would have enabled them to utilize a great number of the 4-cylinder-engine parts, it would have given them a somewhat larger engine than was their apparent desire, unless they reduced the cylinder size. And while they had had some limited experience in building and operating this kind of engine, and twice had chosen it when seeking more power, both of these choices were greatly influenced by the desire to obtain quickly an engine of higher power. It is also possible that something in their experience with the V-8 moved them away from it; the unbalanced shaking force inherent in the arrangement may well have become evident to them. What probably also helped them to their final conclusion was the fundamental consideration that the V-8 provided two extra cylinders which were not really needed.

The eventual selection of the 6-cylinder was a slight compromise. In order to get the desired output the cylinder displacement was increased, but this was done by lengthening the stroke—the first time this had been altered since the original design. The increase (from 4 to 4-1/2 in.) was only 1/2 in., and the bore, the more important influence on fuel performance, was kept the same. Overall, the choice was quite logical. They were utilizing the in-line construction upon which almost all of their now considerable experience had been based, and the sizes of and requirements for parts also conformed to this experience. They could, in fact, use manyof the same parts. The natural balance of the 6-cylinder arrangement gave them a very smooth engine, and had they stiffened the shaft and counter-weighted the cranks, they would have produced the smoothest engine that could have been built at that time.

In the literature are two references to a Wright 6-cylinder engine constructed around the cylinders of the vertical 4. One of these is in Angle'sAirplane Engine Encyclopedia, published in 1921, and the other is inAerosphere 1939, published in 1940. The wording of the latter is essentially identical with that of the former; it seems a reasonable conclusion that it is a copy. Although it is possible that such an engine was built at some time, just as the 8-cylinder racing engine was cobbled up out of parts from the 4-cylinder vertical, no other record, no engines, and no illustrations have been found. It is thus quite certain that no significant quantity was ever manufactured or utilized.

The crankcase was considerably changed from that of the vertical 4, and was now in two pieces, with the split on the crankshaft center line. However, the shaft was not supported by the lower half of the case, as eventually became standard practice, but by bearing caps bolted to the ends of the upper case and, in between, to heavy ribs running across the upper case between the cylinders. The lower half of the case thus received none of the dynamic or explosion loads, and, serving only to support the engine and to provide for its mounting, was lightly ribbed. In it were incorporated integral-boss standpipe oil drains which discharged into a bolted-on sump. The upper half of the case was again left open on one side, giving the desired access to the interior, and, additionally, the design was altered to provide a method of camshaft assembly that was much simpler than that of the vertical 4 (see p.42).

The cylinder was also greatly altered from that of the vertical 4. It was made in three parts, a piece of seamless steel tubing being shrunk on a cast-iron barrel to form the water jacket, with a cast-iron cylinder head shrunk on the upper end of the barrel. This construction compelled the use of long studs running from the cylinder head to the case for fastening down the cylinder (see Figures12a-c). For the first time the cylinder heads were water-cooled, cored passages being provided, and more barrel surface was jacketed than previously, although a considerable area at the bottom was still left uncooled, obviously by direct intent, as the ported exhaust arrangement was no longer employed.

Also for the first time one-piece forged valves were used, but just when these were incorporated is not certain and, surprisingly, they were applied to the inlet only, the exhaust valve being continued in the previous two-piece screwed and riveted construction. The reasoning behind this is not evident. If a satisfactory two-piece exhaust valve had finally been developed itwould be logical to carry it over to the new design; but exhaust valves normally being much more troublesome, it would seem that a good exhaust valve would make an even better inlet valve and, in the quantities utilized, the two-piece design should have been much cheaper. In the original 6-cylinder engine the inlet valves operated automatically as in all previous models, but at the time of a later extensive redesign (1913) this was changed to mechanical actuation, and the succeeding engines incorporated this feature. All the valve-actuating mechanism was similar to that of the vertical 4, and the engine had the usual compression-release mechanism, the detail design being carried over directly from the 4-cylinder.

Design of the piston followed their previous practice, with wide rings above the pin and shallow grooves below the pin on the thrust face, and with the pin fastened in the piston by a set screw. The piston had four ribs underneath the head (see Figure13b) radiating from the center and with the two over the pin bosses incorporating strengthening webs running down and joining the bosses. The piston length was reduced by 1 in., thus giving a much less clumsy appearance and, with other minor alterations, a weight saving of 40 percent (see Figures13bandc). The rods were for the first time made of I-section forgings, a major departure, machined on the sides and hand finished at the ends, with a babbit lining in the big end, the piston pin bearing remaining steel on steel.

Figure 13.—Original 6-cylinder engine: a, cylinder assembly and valve parts; b, bottom side of piston; c, piston, piston pin and connecting rod; d, valve mechanism; e, crankshaft and flywheel. (Pratt & Whitney photos D-15012, 15017, 15013, 15018, respectively.)

At least two different general carburetion and induction systems were utilized, possibly three. One, and most probably the original, consisted of a duplicate of the injection pump of the 4-cylinder fitted to a manifold which ran the length of the engine, with three takeoffs, each of which then split into two, one for each cylinder. Of this arrangement they tried at least two variations involving changes in the location and method of injecting the fuel into the manifold; and there seems to have been an intermediate manifold arrangement, using fuel-pump injection at the middle of the straight side, or gallery, manifold, which was fed additional air at both ends through short auxiliary inlet pipes. This would indicate that with the original arrangement, the end cylinders were receiving too rich a mixture, when the fuel in the manifold was not properly vaporized. Although the exhaust was on the same side of the engine as the inlet system, no attempt was made to heat the incoming charge at any point in its travel. An entirely different system adopted at the time of the complete redesign in 1913 consisted of two float-feed Zenith carburetors each feeding a conventionalthree-outlet manifold. This carburetor was one of the first of the plain-tube type, that is, with the airflow through a straight venturi without any spring-loaded or auxiliary air valves, and was the simplest that could be devised. When properly fitted to the engine, it gave a quite good approximation of the correct fuel and air mixture ratio over the speed-load running range, although it is considerably more than doubtful that this was maintained at altitude, as is stated in one of the best descriptions of the engine published at the time the carburetors were applied.

The compression ratio of this engine was lowered by almost 20 percent from that of the vertical 4. This, in combination with the low bore-to-stroke ratio, the unheated charge, and the later mechanically operated inlet valve, indicates that the Wrights were now attempting for the first time to secure from an engine something approaching the maximum output of which it was capable.

As the engine originally came out, it continued to utilize only one spark plug in each cylinder. The high-tension magneto had a wide range of spark advance adjustment, which again provided the only control of the engine when equipped with the original fuel pump injection.

The location of the valves and pushrods was similar to that in the 4, so that the cams were immediately adjacent to the camshaft bearings, which were carried in the crankcase ends and in the heavy webs. The camshaft was gear-driven and the cam shape was similar to that of the last 4s, with a quite rapid opening and closing and a long dwell, leaving the valve opening accelerations and seating velocities still quite high.

The crankshaft was a continuation of their basic design of rather light construction, particularly in the webs. The cheeks were even thinner (by 1/4 in.) than those of the 4 although the width was increased by 1/8 in. (see Figure13e). For the first time they went to a forging, the rough contour type of the time, and utilized a chrome-nickel alloy steel.

Lubrication was by means of the usual gear pump, and the piston and rod bearings continued to be splash-fed. The rod big-end bearing carried a small sharp undrilled boss at the point where, on the other engines, had been located scuppers whose purpose was apparently still to throw lubricating oil on the cylinder wall carrying the more highly loaded side of the piston. The rod big-end bearing was lubricated by a hole on the top of the big-end boss catching some of the crankcase splash, which was then carried to the bearing by a groove.

When the 6-cylinder engine was completely redesigned in 1913 this led to the introduction in late fall of that year of a new model called the 6-60, the 60 designating the rating in horsepower. There is little in the Wright records to show why such a radical revision was thought necessary, but the general history of the period gives a rather clear indication. The competitionhad caught up to the Wrights in powerplants. Other engines were being installed in Wright airplanes, and Navy log books show these other engines being used interchangeably with those of the Wrights.

Most of the descriptions of the new model published at the time it was introduced concentrate on the addition of the two carburetors and the mechanical operation of the inlet valves, but these were only two of many major changes. The cylinder was completely revised, the intake being moved to the camshaft side of the engine from its position adjacent to the exhaust, so that the two ports were now on opposite sides of the cylinder. By proper positioning of the rocker-arm supports and choice of their length and angles, all valves were made operable from a single camshaft. The shrunk-on steel water jacket cylinder was retained, but the water connections were repositioned so that the water entered at the bottom and came out at the top of the cylinder. Over the life of the 6-cylinder engine several different valve types were used but the published specifications for the model 6-60 called for "cast iron heads"—the old two-piece construction. The piston pins were case hardened and ground and the crankshaft pins and journals were heat treated and ground.

The fuel and oil pumps were removed from the side of the crankcase and a different ignition system was applied, although still of the high-tension spark-plug type which by this time had become general practice on all so-called high-speed internal-combustion engines. A second threaded spark-plug hole was provided in the cylinder head and despite its more common use for other purposes, it is evident that the intention was to provide two-plug ignition. It is doubtful that at the specific output of this engine any power difference would be found between one-and two-plug operation, so that the objective was clearly to provide a reserve unit in case of plug failure. However, it was also used for the installation of a priming cock for starting and because of the prevalence of single-wire ignition systems on existing and illustrated engines, it seems to have been used mostly in this manner, even though dual-ignition systems later became an unvarying standard for aircraft engines.

Viewed externally, the only part of the engine that appears the same as the original 6 is the small lower portion of the crankcase; but what is more visually striking is the beauty of the new lines and extreme cleanness of the exterior design (see Figures14and15). Many of their individual parts had shown the beauty of the sparse design of pure utility but it was now in evidence in the whole. Despite the proven practical value of their other models, this is the only one that can be called a good-looking engine, instantly appealing to the aesthetic sense, even though the vertical 4 is not an ugly engine. The appearance of their final effort, in a field they were originally reluctant to enter and concerning which they always deprecatedthe results of their own work, was a thing of which a technically trained professional engine designer could be proud.

The 6-60 was continued in production and development until it became the 6-70, and indications are that it eventually approached an output of 80 horsepower.

Figure 14.—6-Cylinder 6-60 and 6-70 engine, right rear intake side. (Pratt & Whitney photo.)

Figure 15.—6-Cylinder 6-70 engine, incorporating flexible flywheel drive, exhaust side. (Smithsonian photo A-54381.)

In the Wright brothers' various models were many minor design items which altogether required a great deal of consideration, but which did not materially affect overall engine performance. The results generally could all be classed as good practice; however, one of these utilized in the 4-cylinder vertical engine was rather unorthodox and consisted of offsetting the cylinders with relation to the crankshaft. This arrangement, which can be seen in the drawing (Figure11) was apparently an attempt to reduce the maximum side load on the piston during the power stroke, but since the peak gas loading usually occurs at about 10 to 15 percent of the power stroke, this probably did not have much effect, and it was not carried over to the 6-cylinder design.

All engine bearings were of the plain sleeve type and, except for the bronze and steel bearings in the connecting rod, were of babbit. The advantages of babbit for bearings were discovered very early in the development of the mechanical arts, and apparently the Wrights never encountered a bearing loading sufficiently high to cause a structural breakdown in this relatively weak material.

Valve openings show no variation through the successive production engines, although the Wrights most probably experimented with different amounts. The 1903 engine and the vertical 4-and 6-cylinder all had lifts of 5/16 in., but the valve-seat angles varied somewhat; the records show included angles of 110° to 90°—not a large difference.

The valve-operating mechanism was the same from the first vertical 4 onward. The high side thrust caused by the cam shape required for the very rapid valve opening they chose was, no doubt, the reason for the use of the hinged cam follower, and since the same general cam design was used in their last engine, the 6-cylinder, the same method of operation which had apparently proved very serviceable was continued. How satisfactory was the considerably simpler substitute used in the Bariquand et Marré version of the 4-cylinder engine is not known. Possibly it was one of the alterations in the Wrights' design that Wilbur Wright objected to,although in principle it more closely conforms to the later fairly standard combination valve tappet and roller construction: The available drawings do indicate, however, that the cam of the Bariquand et Marré engine was also altered to give a considerably less abrupt valve opening than the Wright design, so that there was less side thrust. For the Wright 6-cylinder engine their 4-cylinder cam was slightly altered to provide a rounding off near the top of the lobe, thus providing some reduction in the velocity before maximum opening was reached. All their cam designs indicate a somewhat greater fear of the effect of seating velocities than of opening accelerations.

Since the range of cylinder diameters utilized did not vary greatly, the valve sizes were correspondingly fairly uniform. The diameter of the valves for the original 4-in.-bore cylinder was 2 in., while that for the 4-3/8-in. bore used in the 6-cylinder engine was actually slightly smaller, 1-7/8 in. Possibly the Wrights clung too long to the automatic inlet valve, although it did serve them well; but possibly, as has been previously noted, there were valid reasons for continuing its use despite the inherently low volumetric efficiency this entailed.

The inherent weakness in the joints of the three-piece connecting rod has been pointed out, but aside from this, the design was excellent, for all the materials and manufacturing methods required were readily available, and structurally it was very sound. Tubular rods were still in use in aircraft engines in the 1920s.

The Wrights had a surprisingly thorough grasp of the metallurgy of the time, and their choice of materials could hardly have been improved upon. Generally they relied upon the more simple and commonly used metals even though more sophisticated and technically better alloys and combinations were available.[17]Case hardening was in widespread use in this period but their only utilization of it was in some parts of the drive chains purchased completely assembled and in the piston pins of their last engine. The treatment of the crankshafts of all their engines except the final 6-cylinder was typical of their uncomplicated procedure: the particular material was chosen on the basis of many years of experience with it, hardening was a very simple process, and the expedient of carrying this to apoint just below the non-machinable range gave them bearing surfaces that were sufficiently hard, yet at the same time it eliminated the possibility—present in a heat-treating operation—of warping the finished piece.

In the entire 1903 engine only five basic materials—excepting those in the purchased "magneto" and the platinum facing on the ignition-system firing points—were used: steel, cast iron, aluminum, phosphor bronze, and babbit. The steels were all plain carbon types with the exception of the sheet manifold, which contained manganese, and no doubt this was used because the sheet available came in a standard alloy of the time.

Overall, the Wright engines performed well, and in every case met or exceeded the existing requirements. Even though aircraft engines then were simpler than they became later and the design-development time much shorter, their performance stands as remarkable. As a result, the Wrights never lacked for a suitable powerplant despite the rapid growth in airplane size and performance, and the continual demand for increased power and endurance.

Few service records dating from before 1911, when the military services started keeping log books, have been found. Some of those for the period toward the end of their active era have been preserved, but for that momentous period spanning the first few years when the Wrights had the only engines in actual continuous flight operation, there seems to be essentially nothing—perhaps because there were no standard development methods or routines to follow, no requirements to be met with respect to pre-flight demonstrations or the keeping of service records. Beginning in 1904, however, and continuing as long as they were actively in business, they apparently had in progress work on one or more developmental or experimental engines. This policy, in combination with the basic simplicity of design of these engines, accounted in large measure for their ability to conduct both demonstrations and routine flying essentially whenever they chose.

Time between engine overhauls obviously varied. In mid 1906 an engine was "rebuilt after running about 12 hours." This is comparatively quite a good performance, particularly when it is remembered that essentially all the "running" was at full power output. It was considerably after 1920 before the Liberty engine was redesigned and developed to the stage where it was capable of operating 100 hours between overhauls, even though it was being used at cruising, or less than full, power for most of this time.

The Wrights of course met with troubles and failures, but it is difficult, from the limited information available, to evaluate these and judge their relative severity. Lubrication seems to have been a rather constant problem, particularly in the early years. Although some bearing lubricationtroubles were encountered from time to time, this was not of major proportions, and they never had to resort to force-feed lubrication of the main or rod big-end bearings. The piston and cylinder-barrel bearing surfaces seem to have given them the most trouble by far, and examination of almost any used early Wright engine will usually show one or more pistons with evidence of scuffing in varying degrees, and this is also apparent in the photographs in the record. This is a little difficult to understand inasmuch as most of the time they had the very favorable operating condition of cast iron on cast iron. Many references to piston seizure or incipient seizure, indicated by a loss of power, occur, and this trouble may have been aggravated by the very small piston clearances utilized. Why these small clearances were continued is also not readily explainable, except that with no combination of true oil-scraper rings, which was the basic reason why the final form of aviation piston engine was able to reach its unbelievably low oil consumptions, their large and rather weak compression rings were probably not doing an adequate job of oil control, and they were attempting to overcome this with a quite tight piston fit.[18]In any event, they did encounter scuffing or seizing pistons and cylinder over-oiling at the same time. As late as 4 May 1908 in the WrightPapersthere appears the notation: "The only important change has been in the oiling. The engine now feeds entirely by splash...."

Their troubles tended to concentrate in the cylinder-piston combination, as has been true of almost all piston engines. References to broken cylinders are frequent. These were quite obviously cylinder barrels, as replacement was common, and this again is not readily explainable. The material itself, according to Orville Wright, had a very high tensile strength, and in the 1903 engine more than ample material was provided, as the barrel all the way down to well below the attachment to the case was 7/32 in. thick. The exact location of the point of failure was never recorded, but in its design are many square corners serving as points of stress concentration. Also, of course, no method was then available for determining a faulty casting, except by visual observation of imperfections on the surface, and this was probably the more common cause. It is interesting, however, that the engine finally assembled in 1928 for installation in the 1903 airplane sent to England has a cracked cylinder barrel, the crack originating at asharp corner in the slot provided at the bottom of the barrel for screwing it in place.

Valve failures were also a continuing problem, and Chenoweth reports that a large proportion of the operating time of the 1904-1906 development engine was concentrated on attempts to remedy this trouble. None of their cams, including those of the 6-cylinder engine, evidence any attempt to effect a major reduction in seating velocities. United States Navy log books of 1912 and 1913 record many instances of inlet valves "broken at the weld," indicating that some of the earlier 6-cylinder engines were fitted with valves of welded construction.

For the engineer particularly, the fascination of the Wrights' engine story lies in its delineation of the essentially perfect engineering achievement by the classic definition of engineering—to utilize the available art and science to accomplish the desired end with a minimum expenditure of time, energy, and material. Light weight and operability were the guiding considerations; these could be obtained only through constant striving for the utmost simplicity. Always modest, the Wrights seem to have been even more so in connection with their engine accomplishments. Although the analogy is somewhat inexact, the situation is reminiscent of the truism often heard in the aircraft propulsion business—few people know the name of Paul Revere's horse. Yet, as McFarland has pointed out, "The engine was in fact far from their meanest achievement." With hardly any experience in this field and only a meagerly equipped machine shop, they designed and assembled an internal combustion engine that exceeded the specifications they had laid down as necessary for flight and had it operating in a period of about two months elapsed time. The basic form they evolved during this unequalled performance carried them through two years of such successful evolutionary flight development that their flying progressed from a hop to mastery of the art. And the overall record of their powerplants shows them to have been remarkably reliable in view of the state of the internal combustion engine at that time.

It is not possible to state the exact quantities of each engine that the Wrights produced up to the time that their factory ceased operation in 1915. Chenoweth gives an estimate, based on the recollection of their test foreman, of 100 vertical 4s and 50 6s. My estimate (see page2) places the total of all engines at close to 200. Original Wright-built engines of all four of these basic designs are in existence, although they are rather widely scattered. The Smithsonian's National Air and Space Museum has examples of them all, including, of course, the unique first-flight engine. Their condition varies, but many are operable, or could easily be made so. Among the best are the first-flight engine and the last vertical 6, at the Smithsonian, the first vertical 6, at the United States Air Force Museum, and the vertical 4, at the Carillon Park Museum.

The Wrights were constantly experimenting and altering, and this in connection with the lack of complete records makes it almost impossible to state with any certainty specific performances of individual engines at given times. Weights sometimes included accessories and at others did not. Often they were of the complete powerplant unit, including radiator and water and fuel, with no clarification. In the table, performance is given in ranges which are thought to be the most representative of those actually utilized. Occasionally performances were attained even beyond the ranges given. For example, the 4×4-in. flat development engine eventually demonstrated 25 hp at an MEP of approximately 65 psi.

One important figure—the horsepower actually utilized during the first flight—is quite accurately known. In 1904 the 1904-1905 flight engine, after having been calibrated by their prony-brake test-fan method, was used to turn the 1903 flight propellers, and Orville Wright calculated this power to be 12.05 bhp by comparing the calibrated engine results with those obtained with the flight engine at Kitty Hawk when tested under similar conditions. However, since the tests were conducted in still air with the engine stationary, this did not exactly represent the flight condition. No doubt the rotational speed of the engine and propellers increased somewhat with the forward velocity of the airplane so that unless the power-rpm curve of the engine was flat, the actual horsepower utilized was probably a small amount greater than Orville's figures. The lowest power figure shown for this engine is that of its first operation.

No fuel consumption figures are given, primarily because no comprehensive data have been found. This is most probably because in the early flight years, when the Wrights were so meticulously measuring and recording technical information on the important factors affecting their work, the flights were of such short duration that fuel economy was of very minor importance. After success had been achieved, they ceased to keep detailed records on very much except their first interest—the flyingmachine itself—and when the time of longer flights arrived, the fuel consumption that resulted from their best engine design efforts was simply accepted. The range obtained became mostly a matter of aerodynamic design and weight carried. Orville Wright quotes an early figure of brake thermal efficiency for the 1903 engine that gives a specific fuel consumption of .580 lb of fuel per bhp/hr based on an estimate of the heating value of the fuel they had. This seems low, considering the compression ratio and probable leakage past their rather weak piston rings, but it is possible. In an undated entry, presumably in 1905, Orville Wright's notebook covered fuel consumption in terms of miles of flight; one of the stated assumptions in the entry is, "One horsepower consumes .60 pounds per horsepower hour"—still quite good for the existing conditions. Published figures for the 6-60 engine centered around .67 lb/hp hr for combined fuel and oil consumption.

Despite the fact that the Wright shop engine was not a flight unit, it is interesting both because it was a well designed stationary powerplant with several exceedingly ingenious features, and because its complete success was doubtless a major factor in the Wrights' decision to design and build their own first flight engine. Put in service in their small shop in the fall of 1901, it was utilized in the construction of engine and airframe parts during the vital years from 1902 through 1908 and, in addition, it provided the sole means of determining the power output of all of their early flight engines. By means of a prony brake, its power output was carefully measured and from this the amount of power required for it to turn certain fans or test clubs was determined. These were then fitted to the flight engines and the power developed calculated from the speed at which the engines under test would turn the calibrated clubs. Although a somewhat complex method of using power per explosion of the shop engine was made necessary by the basic governor control of the engine, the final figures calculated by means of the propeller cube law seem to have been surprisingly accurate.[19]Restored under the personal direction of Charles Taylor, it is in the Henry Ford Museum in Dearborn, Michigan, together with the shop machinery it operated.

The engine was a single cylinder, 4-stroke-cycle "hot-tube" ignition type. The cylinder, of cast iron quite finely and completely finned for itsday, was air-cooled, or rather, air-radiated, as there was no forced circulation of air over it, the atmosphere surrounding the engine simply soaking up the dissipated heat. Although this was possibly a desirable adjunct in winter, inside the small shop in Dayton, the temperature there in summer must have been quite high at times. The operating fuel was city illuminating gas, which was also utilized to heat, by means of a burner, the ignition tube. This part was of copper, with one completely closed end positioned directly in the burner flame; the other end was open and connected the interior of the tube to the combustion chamber. The inlet valve was of the usual automatic type while the exhaust valve was mechanically operated. The fuel gas flow was controlled by a separate valve mechanically connected to the inlet valve so that the opening of the inlet valve also opened the gas valve, and gas and air were carried into the cylinder together.

Back to Index Next