The Wright Brothers' Engines and Their Design

by Leonard S. Hobbs

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's Airplane Engine Encyclopedia, published in 1921, and the other is in Aerosphere 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 Figures [12a]-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 it would 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 Figure [13b]) 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 Figures [13b] and [c]). 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 conventional three-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.