the exhaust took place close to the operator's ears? It would have
been possible, starting with the original design, to turn the engine
around so that the exhaust was on the other side. This would have
little effect on the location of the center of gravity, and the two
main drive chains would then have been of more equal length. However,
of the many factors involved, probably one of the principal
considerations in arriving at their final decision was the location of
the spark-advance control, which was in effect the only control they
had of engine output, except for complete shutoff. In their design
this was immediately adjacent to the operator; with a turned-around
engine, an extension control mechanism of some sort would have been
required. The noise of the exhaust apparently became of some concern
to them, as Orville's diary in early 1904 contains an entry with a
sketch labeled "Design for Muffler for Engine," but there is no
further comment.
The problem of keeping joints tight, and for that matter the entire
construction itself, were both greatly simplified by their decision to
water-jacket only a part of the cylinder head proper, and the valve
box not at all. This was undoubtedly the correct decision for their
immediate purpose, as again they were effecting savings in time, cost,
complexity, and weight. There is nothing in the record, however, to
show why they continued this practice long after they had advanced to
much greater power outputs and longer flight times. Their own
statements show that they were well aware of the effect of the very
hot cylinder head on power output and they must also have realized its
influence on exhaust-valve temperature.
The cylinder assembly was made somewhat more complicated by their
desire to oil the piston and cylinder by means of holes near the
crankshaft end in what was, with the engine in the horizontal
position, the upper side of the cylinder barrel. This complication was
no doubt taken care of by not drilling the holes until a tight
assembly had been made by screwing the barrel into place, and by
marking the desired location on the barrel. Since this position was
determined by a metal-to-metal jam fit of the crankcase and cylinder
barrel flange, the barrel would reassemble with the holes in very
nearly the same relative position after disassembly.
With the valve box, or housing, cylindrical, the task of locking and
fastening the intake and exhaust valve guides and seats in place was
easy. The guide was made integral with and in the center of one end of
a circular cage, the other end of which contained the valve seat (see
Figure 5). Four sections were cut out of the circular wall of the cage
so that in effect the seat and guide were joined by four narrow legs,
the spaces between which provided passages for the flow of the
cylinder gases. These cages were then dropped into the ends of the
valve boxes until they came up against machined shoulders and were
held in place by internal ring nuts screwed into the valve box. The
intake manifold or passage was placed over the intake valves so that
the intake charge flowed directly into and through the valve cage
around the open valve and into the cylinder. The exhaust gas, after
flowing through the passages in the valve cage, was discharged
directly to the atmosphere through a series of holes machined in one
side of the valve box.
[Illustration: _Figure 5._--First flight engine, 1903, assembly.
(Phantom cutaway by J. H. Clark, with key, courtesy _Aeroplane_.)
KEY
1 and 2. Bearing caps in one piece with plate 3.
3. Plated screwed over hole 4 in crankcase end.
4. Key-shaped hole as hole 5 in intermediate ribs.
6. Inter-bearings cap (white-metal lined) and screwed to inter-rib
halves 7.
8. Splash-drip feed to bearings.
9. Return to pump from each compartment of crankcase base ("sump") via
gallery 10 and pipe to pump 11 underneath jacket.
12. Oil feed from pump via rubber tube 13.
13. Drip feeds to cylinders and pistons.
14. Gear drive to pump.
15. Big-end nuts, lock-strip, and shims.
16. Gudgeon-pin lock.
17. Piston-ring retainer pegs.
18. Cylinder liner screwing 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. Assembly 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: Assuming 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.
[Illustration: _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 principal concern in the design to have been
obtaining maximum valve capacity--that is, a quite rapid opening with
a long dwell. This apparent desire to get rid of the exhaust gas
quickly is manifested again in the alacrity with which they adopted a
piston-controlled exhaust port immediately they had really mastered
flight and were contemplating more powerful and more durable engines.
This maximum-capacity theory of valve operation, with its neglect of
acceleration forces and seating velocities, may well have been at
least partially if not largely the cause of their exhaust-valve
troubles and the seemingly disproportionate amount of development they
devoted to this part, as reported by Chenoweth, although it is also
true that the exhaust valve continued to present a problem in the
aircraft piston engine for a great many years after, even with the
most scientific of cam designs.
The rocker arm (102, Figure 6) is probably the best example of a small
part which met all of their many specific requirements with an extreme
of simplicity. It consisted of two identical side pieces, or walls, of
sheet steel shaped to the desired side contour of the assembly, in
which were drilled three holes, one in each end, to carry the roller
axles, and the third in the approximate middle for the rocker axle
shaft proper. This consisted of a piece of solid rod positioned by
cotter pins in each end outside the side walls (see Figure 5). The
assembly was made by riveting over the ends of the roller axles so
that the walls were held tightly against the shoulders on the axles,
thus providing the correct clearance for the rollers. The construction
was so light and serviceable that it was essentially carried over to
the last engine the Wrights ever built.
The basic intake manifold (see Figure 5) consisted of a very low flat
box of sheet steel which ran across the tops of the valve boxes and
was directly connected to the top of each of them so that the cages,
and thus the valves, were open to the interior of the manifold.
Through an opening in the side toward the engine the manifold was
connected to a flat induction chamber (21, Figure 5) which served to
vaporize the fuel and mix it with the incoming air. This chamber was
formed by screw-fastening a piece of sheet steel to vertical ribs cast
integral with the crankcase, the crankcase wall itself thus forming
the bottom of the chamber. A beaded sheet-steel cylinder resembling a
can (73, Figure 6) but open at both ends was fastened upright to the
top of this chamber. In the absence of anything else, this can could
be called the carburetor, as a fuel supply line entered the cylinder
near the top and discharged the fuel into the incoming air stream,
both the fuel and air then going directly into the mixing chamber. The
can was attached near one corner of the chamber, and vertical baffles,
also cast integral with the case, were so located that the incoming
mixture was forced to circulate over the entire area of exposed
crankcase inside the chamber before it reached the outlet to the
manifold proper, the hot surface vaporizing that part of the fuel
still liquid.
[Illustration: _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 cock
and the other a type whose opening could be regulated. The latter was
adjusted to supply the correct amount of fuel under the desired flight
operating condition; the shutoff cock was used for starting and
stopping. The rate of fuel supply to the engine would decrease as the
level in the fuel tank dropped, but as the head being utilized was a
matter of several feet and the height of the supply tank a matter of
inches, the fuel-air ratio was still maintained well within the range
that would ignite and burn properly in the contemplated one-power
condition of their flight operation.
This arrangement is one of the best of the many illustrations of how
by the use of foresight and ingenuity the Wrights met the challenge of
a complex requirement with a simple device, for while carburetors were
not in the perfected stage later attained, quite good ones that would
both control power output and supply a fairly constant fuel-air
mixture over a range of operating conditions were available, but they
were complex, heavy, and expensive. The arrangement, moreover, secured
at no cost a good vaporizer, or modern "hot spot." In their subsequent
engines they took the control of the fuel metering away from the
regulating valve and gravity tank combination and substituted an
engine-driven fuel pump which provided a fuel supply bearing a fairly
close relationship to engine speed.
The reasons behind selection of the type of ignition used, and the
considerations entering into the decision, are open to speculation, as
are those concerning many other elements that eventually made up the
engine. Both the high-tension spark plug and low-tension
make-and-break systems had been in wide use for many years, with the
latter constituting the majority in 1902. Both were serviceable and
therefore acceptable, and both required a "magneto". The art of the
spark plug was in a sense esoteric (to a certain extent it so remains
to this day), but the spark-plug system did involve a much simpler
combination of parts: in addition to the plug and magneto there would
be needed only a timer, or distributor, together with coils and
points, or some substitute arrangement. The make-and-break system, on
the other hand, required for each cylinder what was physically the
equivalent of a spark plug, that is, a moving arm and contact point
inside the cylinder, a spring-loaded snap mechanism to break the
contact outside the cylinder, and a camshaft and cams to actuate the
breaker mechanism at the proper time. Furthermore, as the Wrights
applied it, the system required dry cells and a coil for starting,
although these did not accompany the engine in flight. And finally
there was the problem of keeping tight the joint where the
oscillating shaft required to operate the moving point in the spark
plug entered the cylinder.
This is one of the few occasions, if not the only one, when the
Wrights chose the more complex solution in connection with a major
part--in this particular case, one with far more bits and pieces.
However, it did carry with it some quite major advantages. The common
spark plug, always subject to fouling or failure to function because
of a decreased gap, was not very reliable over a lengthy period, and
was undoubtedly much more so in those days when control of the amount
of oil inside the cylinder was not at all exact. Make-and-break
points, on the other hand, were unaffected by excess oil in the
cylinder. Because of this resistance to fouling, the system was
particularly suitable for use with the compression-release method of
power control which they later utilized, although they probably could
not have been looking that far ahead at the time they chose it.
High-tension current has always, and rightfully so, been thought of as
a troublemaker in service; in Beaumont's 1900 edition of _Motor
Vehicles and Motors_, which seems to have been technically the best
volume of its time, the editor predicted that low-tension
make-and-break ignition would ultimately supersede all other methods.
And finally, the large number of small parts required for the
make-and-break system could all be made in the Wright Brothers' shop
or easily procured, and in the end this was probably the factor, plus
reliability, that determined the decision which, all things
considered, was the correct one.
There was nothing exceptional about the exact form the Wrights
devised. It displayed the usual refined simplicity (the cams were made
of a single small piece of strip steel bent to shape and clamped to
the ignition camshaft with a simple self-locking screw), and
lightness. The ignition camshaft (38, Figure 5), a piece of
small-diameter bar stock, was located on the same side as the exhaust
valve camshaft, approximately midway between it and the valve boxes,
and was operated by the exhaust camshaft through spur gearing. That
the Wrights were thinking of something beyond mere hops or short
flights is shown by the fact that the ignition points were
platinum-faced, whereas even soft iron would have been satisfactory
for the duration of all their flying for many years.
The control of the spark timing was effected by advancing or retarding
the ignition camshaft in relation to the exhaust valve camshaft. The
spur gear (37, Figure 5) driving the ignition camshaft had its hub on
one side extended out to provide what was in effect a sleeve around
the camshaft integral with the gear. The gear and integral sleeve were
slidable on the shaft and the sleeve at one place (39, Figure 5) was
completely slotted through to the shaft at an angle of 45 deg. to the
longitudinal axis of the shaft. The shaft was driven by a pin tightly
fitted in it and extending into the slot. The fore-and-aft position of
the sleeve on the shaft was determined by a lever-operated cam (40,
Figure 5) on one side and a spring on the other. The movement of the
sleeve along the shaft would cause the shaft to rotate in relation to
it because of the angle of the slot, thus providing the desired
variation in timing of the spark. The "magneto" was a purchased item
driven by means of a friction wheel contacting the flywheel, and
several different makes were used later, but the original is indicated
to have been a Miller-Knoblock (see Figure 5).
The connecting rod is another example of how, seemingly without
trouble, they were able to meet the basic requirements they had set
for themselves. It consisted of a piece of seamless steel tubing with
each end fastened into a phosphor-bronze casting, these castings
comprising the big and little ends, drilled through to make the
bearings (See Figures 5 and 6). It was strong, stiff and light.[13]
Forged rods were in rather wide use at the time and at least one
existing engine even had a forged I-beam section design that was
tapered down from big to little end. The Wrights' rod was obtained in
little more time than it took to make the simple patterns for the two
ends. The weight was easily controlled, no bearing liners were
necessary, and a very minimum of machining was required. Concerning
the big-end material, there exists a contradiction in the records:
Baker, whose data are generally most accurate, states that these were
babbited, but this must be in error, as the existing engine has
straight bronze castings without babbiting, and there is no record, or
drawing, or other indication of the bearings having been otherwise.
[Footnote 13: A rather thorough stress analysis of the rod shows it to
compare very favorably with modern practice. In the absence of an
indicator card for the 1903 engine, if a maximum gas pressure of five
times the MEP is assumed, the yield-tension factor of safety is
measurably higher than that of two designs of piston engines still in
wide service, and the column factor of safety only slightly less. The
shear stresses in the brazed and threaded joints are so low as to be
negligible.]
Different methods of assembling the rod were used. At one time the
tube ends were screwed into the bronze castings and pinned, and at
another the ends were pinned and soldered. There is an indication that
at one time soldering and threads were used in combination. One of the
many conflicts between the two primary sets of drawings exists at this
point. The Smithsonian drawings show the use at each end of adapters
between the rod and end castings, the adapters being first screwed
into the castings and pinned and then brazed to the inside of the
tube. The Science Museum drawings show the tube section threaded and
screwed into the castings. The direct screw assembly method called for
accurate machining and hand fitting in order to make the ends of the
tubing jam against the bottom of the threaded holes in the castings,
and at the same time have the end bearings properly lined up. The
weakness of the basic design patently lies in the joints. It is an
attempt to utilize what was probably in the beginning a combination
five-piece assembly and later three, in a very highly stressed part
where the load was reversing. It gave them considerable trouble from
time to time, particularly in the 4-cylinder vertical engines, and was
abandoned for a forged I-beam section type in their last engine model;
but it was nevertheless the ideal solution for their first engine.
The crankshaft was made from a solid block of relatively high carbon
steel which, aside from its bulk and the major amount of machining
required, presented no special problems. It was heat-treated to a
machinable hardness before being worked on, but was not further
tempered. The design was an orthodox straight pin and cheek
combination and, as previously noted, there were no counterweights to
complicate the machining or assembly. A sizable bearing was provided
on each side of each crank of the shaft, which helped reduce the
stiffness requirement.
Their only serious design consideration was to maintain the desired
strength and still keep within weight limitations. A fundamental that
every professional designer knows is that it is with this particular
sort of part that weight gets out of control; even an additional 1/16
in., if added in a few places, can balloon the weight. With their
usual foresight and planning, the Wrights carefully checked and
recorded the weight of each part as it was finished, but even this
does not quite explain how these two individuals, inexperienced in
multicylinder engines--much less in extra-light construction--could,
in two months, bring through an engine which was both operable and
somewhat lighter than their specification.