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.
[Illustration: _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 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.
[Illustration: _Figure 11._--4-Cylinder vertical engine assembly,
Bariquand et Marre version. (Drawing courtesy Bristol Siddeley
Engines, Ltd.)]
[Illustration: THE WRIGHT BROTHERS AERO ENGINE]
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.
[Illustration: _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.)]
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 deg.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 many of 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'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.
[Illustration: _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.)]