Engine Parts and Functions--Why Multiple Cylinder Engines Are
Best--Describing Sequence of Operations--Simple Engines--Four
and Six Cylinder Vertical Tandem Engines--Eight and Twelve
Cylinder V Engines--Radial Cylinder Arrangement--Rotary Cylinder
Forms.
ENGINE PARTS AND FUNCTIONS
The principal elements of a gas engine are not difficult to understand
and their functions are easily defined. In place of the barrel of the
gun one has a smoothly machined cylinder in which a small cylindrical or
barrel-shaped element fitting the bore closely may be likened to a
bullet or cannon ball. It differs in this important respect, however, as
while the shot is discharged from the mouth of the cannon the piston
member sliding inside of the main cylinder cannot leave it, as its
movements back and forth from the open to the closed end and back again
are limited by simple mechanical connection or linkage which comprises
crank and connection rod. It is by this means that the reciprocating
movement of the piston is transformed into a rotary motion of the
crank-shaft.
The fly-wheel is a heavy member attached to the crank-shaft of an
automobile engine which has energy stored in its rim as the member
revolves, and the momentum of this revolving mass tends to equalize the
intermittent pushes on the piston head produced by the explosion of the
gas in the cylinder. In aviation engines, the weight of the propeller or
that of rotating cylinders themselves performs the duty of a fly-wheel,
so no separate member is needed. If some explosive is placed in the
chamber formed by the piston and closed end of the cylinder and
exploded, the piston would be the only part that would yield to the
pressure which would produce a downward movement. As this is forced down
the crank-shaft is turned by the connecting rod, and as this part is
hinged at both ends it is free to oscillate as the crank turns, and thus
the piston may slide back and forth while the crank-shaft is rotating or
describing a curvilinear path.
[Illustration: Fig. 21.--Side Sectional View of Typical Airplane Engine,
Showing Parts and Their Relation to Each Other. This Engine is an
Aeromarine Design and Utilizes a Distinctive Concentric Valve
Construction.]
In addition to the simple elements described it is evident that a
gasoline engine must have other parts. The most important of these are
the valves, of which there are generally two to each cylinder. One
closes the passage connecting to the gas supply and opens during one
stroke of the piston in order to let the explosive gas into the
combustion chamber. The other member, or exhaust valve, serves as a
cover for the opening through which the burned gases can leave the
cylinder after their work is done. The spark plug is a simple device
which may be compared to the fuse or percussion cap of the cannon. It
permits one to produce an electric spark in the cylinder when the piston
is at the best point to utilize the pressure which obtains when the
compressed gas is fired. The valves are open one at a time, the inlet
valve being lifted from its seat while the cylinder is filling and the
exhaust valve is opened when the cylinder is being cleared. They are
normally kept seated by means of compression springs. In the simple
motor shown at Fig. 5, the exhaust valve is operated by means of a
pivoted bell crank rocked by a cam which turns at half the speed of the
crank-shaft. The inlet valve operates automatically, as will be
explained in proper sequence.
In order to obtain a perfectly tight combustion chamber, both intake and
exhaust valves are closed before the gas is ignited, because all of the
pressure produced by the exploding gas is to be directed against the top
of the movable piston. When the piston reaches the bottom of its power
stroke, the exhaust valve is lifted by means of the bell crank which is
rocked because of the point or lift on the cam. The cam-shaft is driven
by positive gearing and revolves at half the engine speed. The exhaust
valve remains open during the whole of the return stroke of the piston,
and as this member moves toward the closed end of the cylinder it
forces out burned gases ahead of it, through the passage controlled by
the exhaust valve. The cam-shaft is revolved at half the engine speed
because the exhaust valve is raised from its seat during only one stroke
out of four, or only once every two revolutions. Obviously, if the cam
was turned at the same speed as the crank-shaft it would remain open
once every revolution, whereas the burned gases are expelled from the
individual cylinders only once in two turns of the crank-shaft.
WHY MULTIPLE CYLINDER FORMS ARE BEST
Owing to the vibration which obtains from the heavy explosion in the
large single-cylinder engines used for stationary power other forms were
evolved in which the cylinder was smaller and power obtained by running
the engine faster, but these are suitable only for very low powers.
When a single-cylinder engine is employed a very heavy fly-wheel is
needed to carry the moving parts through idle strokes necessary to
obtain a power impulse. For this reason automobile and aircraft
designers must use more than one cylinder, and the tendency is to
produce power by frequently occurring light impulses rather than
by a smaller number of explosions having greater force. When a
single-cylinder motor is employed the construction is heavier than is
needed with a multiple-cylinder form. Using two or more cylinders
conduces to steady power generation and a lessening of vibration. Most
modern motor cars employ four-cylinder engines because a power impulse
may be secured twice every revolution of the crank-shaft, or a total of
four power strokes during two revolutions. The parts are so arranged
that while the charge of gas in one cylinder is exploding, those which
come next in firing order are compressing, discharging the inert gases
and drawing in a fresh charge respectively. When the power stroke is
completed in one cylinder, the piston in that member in which a charge
of gas has just been compressed has reached the top of its stroke and
when the gas is exploded the piston is reciprocated and keeps the
crank-shaft turning. When a multiple-cylinder engine is used the
fly-wheel can be made much lighter than that of the simpler form and
eliminated altogether in some designs. In fact, many modern
multiple-cylinder engines developing 300 horse-power weigh less than the
early single- and double-cylinder forms which developed but one-tenth or
one-twentieth that amount of energy.
DESCRIBING SEQUENCE OF OPERATIONS
Referring to Fig. 22, A, the sequence of operation in a single-cylinder
motor can be easily understood. Assuming that the crank-shaft is turning
in the direction of the arrow, it will be seen that the intake stroke
comes first, then the compression, which is followed by the power
impulse, and lastly the exhaust stroke. If two cylinders are used, it is
possible to balance the explosions in such a way that one will occur
each revolution. This is true with either one of two forms of four-cycle
motors. At B, a two-cylinder vertical engine using a crank-shaft in
which the crank-pins are on the same plane is shown. The two pistons
move up and down simultaneously. Referring to the diagram describing the
strokes, and assuming that the outer circle represents the cycle of
operations in one cylinder while the inner circle represents the
sequence of events in the other cylinder, while cylinder No. 1 is taking
in a fresh charge of gas, cylinder No. 2 is exploding. When cylinder No.
1 is compressing, cylinder No. 2 is exhausting. During the time that the
charge in cylinder No. 1 is exploded, cylinder No. 2 is being filled
with fresh gas. While the exhaust gases are being discharged from
cylinder No. 1, cylinder No. 2 is compressing the gas previously taken.
[Illustration: Fig. 22.--Diagrams Illustrating Sequence of Cycles in
One- and Two-Cylinder Engines Showing More Uniform Turning Effort on
Crank-Shaft with Two-Cylinder Motors.]
The same condition obtains when the crank-pins are arranged at one
hundred and eighty degrees and the cylinders are opposed, as shown at C.
The reason that the two-cylinder opposed motor is more popular than
that having two vertical cylinders is that it is difficult to balance
the construction shown at B, so that the vibration will not be
excessive. The two-cylinder opposed motor has much less vibration than
the other form, and as the explosions occur evenly and the motor is a
simple one to construct, it has been very popular in the past on light
cars and has received limited application on some early, light
airplanes.
To demonstrate very clearly the advantages of multiple-cylinder engines
the diagrams at Fig. 23 have been prepared. At A, a three-cylinder
motor, having crank-pins at one hundred and twenty degrees, which means
that they are spaced at thirds of the circle, we have a form of
construction that gives a more even turning than that possible with a
two-cylinder engine. Instead of one explosion per revolution of the
crank-shaft, one will obtain three explosions in two revolutions. The
manner in which the explosion strokes occur and the manner they overlap
strokes in the other cylinder is shown at A. Assuming that the cylinders
fire in the following order, first No. 1, then No. 2, and last No. 3, we
will see that while cylinder No. 1, represented by the outer circle, is
on the power stroke, cylinder No. 3 has completed the last two-thirds of
its exhaust stroke and has started on its intake stroke. Cylinder No. 2,
represented by the middle circle, during this same period has completed
its intake stroke and two-thirds of its compression stroke. A study of
the diagram will show that there is an appreciable lapse of time between
each explosion.
Three-cylinder engines are not used on aircraft at the present time,
though Bleriot's flight across the British Channel was made with a
three-cylinder Anzani motor. It was not a conventional form, however.
The three-cylinder engine is practically obsolete at this time for any
purpose except "penguins" or school machines that are incapable of
flight and which are used in some French training schools for aviators.
[Illustration: Fig. 23.--Diagrams Demonstrating Clearly Advantages which
Obtain when Multiple-Cylinder Motors are Used as Power Plants.]
FOUR- AND SIX-CYLINDER ENGINES
In the four-cylinder engine operation which is shown at Fig. 23, B, it
will be seen that the power strokes follow each other without loss of
time, and one cylinder begins to fire and the piston moves down just as
soon as the member ahead of it has completed its power stroke. In a
four-cylinder motor, the crank-pins are placed at one hundred and eighty
degrees, or on the halves of the crank circle. The crank-pins for
cylinders No. 1 and No. 4 are on the same plane, while those for
cylinders No. 2 and No. 3 also move in unison. The diagram describing
sequence of operations in each cylinder is based on a firing order of
one, two, four, three. The outer circle, as in previous instances,
represents the cycle of operations in cylinder one. The next one toward
the center, cylinder No. 2, the third circle represents the sequence of
events in cylinder No. 3, while the inner circle outlines the strokes in
cylinder four. The various cylinders are working as follows:
1. 2. 3. 4.
Explosion Compression Exhaust Intake
Exhaust Explosion Intake Compression
Intake Exhaust Compression Explosion
Compression Intake Explosion Exhaust
It will be obvious that regardless of the method of construction, or the
number of cylinders employed, exactly the same number of parts must be
used in each cylinder assembly and one can conveniently compare any
multiple-cylinder power plant as a series of single-cylinder engines
joined one behind the other and so coupled that one will deliver power
and produce useful energy at the crank-shaft where the other leaves off.
The same fundamental laws governing the action of a single cylinder
obtain when a number are employed, and the sequence of operation is the
same in all members, except that the necessary functions take place at
different times. If, for instance, all the cylinders of a four-cylinder
motor were fired at the same time, one would obtain the same effect as
though a one-piston engine was used, which had a piston displacement
equal to that of the four smaller members. As is the case with a
single-cylinder engine, the motor would be out of correct mechanical
balance because all the connecting rods would be placed on crank-pins
that lie in the same plane. A very large fly-wheel would be necessary to
carry the piston through the idle strokes, and large balance weights
would be fitted to the crank-shaft in an effort to compensate for the
weight of the four pistons, and thus reduce vibratory stresses which
obtain when parts are not in correct balance.
There would be no advantage gained by using four cylinders in this
manner, and there would be more loss of heat and more power consumed in
friction than in a one-piston motor of the same capacity. This is the
reason that when four cylinders are used the arrangement of crank-pins
is always as shown at Fig. 23, B--i.e., two pistons are up, while the
other two are at the bottom of the stroke. With this construction, we
have seen that it is possible to string out the explosions so that there
will always be one cylinder applying power to the crank-shaft. The
explosions are spaced equally. The parts are in correct mechanical
balance because two pistons are on the upstroke while the other two are
descending. Care is taken to have one set of moving members weigh
exactly the same as the other. With a four-cylinder engine one has
correct balance and continuous application of energy. This insures a
smoother running motor which has greater efficiency than the simpler
one-, two-, and three-cylinder forms previously described. Eliminating
the stresses which would obtain if we had an unbalanced mechanism and
irregular power application makes for longer life. Obviously a large
number of relatively light explosions will produce less wear and strain
than would a lesser number of powerful ones. As the parts can be built
lighter if the explosions are not heavy, the engine can be operated at
higher rotative speeds than when large and cumbersome members are
utilized. Four-cylinder engines intended for aviation work have been
built according to the designs shown at Fig. 24, but these forms are
unconventional and seldom if ever used.
[Illustration: Fig. 24.--Showing Three Possible Though Unconventional
Arrangements of Four-Cylinder Engines.]
The six-cylinder type of motor, the action of which is shown at Fig. 23,
C, is superior to the four-cylinder, inasmuch as the power strokes
overlap, and instead of having two explosions each revolution we have
three explosions. The conventional crank-shaft arrangement in a
six-cylinder engine is just the same as though one used two
three-cylinder shafts fastened together, so pistons 1 and 6 are on the
same plane as are pistons 2 and 5. Pistons 3 and 4 also travel together.
With the cranks arranged as outlined at Fig. 23, C, the firing order is
one, five, three, six, two, four. The manner in which the power strokes
overlap is clearly shown in the diagram. An interesting comparison is
also made in the diagrams at Fig. 25 and in the upper corner of Fig. 23,
C.
[Illustration: Fig. 25.--Diagrams Outlining Advantages of Multiple
Cylinder Motors, and Why They Deliver Power More Evenly Than Single
Cylinder Types.]
A rectangle is divided into four columns; each of these corresponds to
one hundred and eighty degrees, or half a revolution. Thus the first
revolution of the crank-shaft is represented by the first two columns,
while the second revolution is represented by the last two. Taking the
portion of the diagram which shows the power impulse in a one-cylinder
engine, we see that during the first revolution there has been no power
impulse. During the first half of the second revolution, however, an
explosion takes place and a power impulse is obtained. The last portion
of the second revolution is devoted to exhausting the burned gases, so
that there are three idle strokes and but one power stroke. The effect
when two cylinders are employed is shown immediately below.
[Illustration: Fig. 26.--Diagrams Showing Duration of Events for a
Four-Stroke Cycle, Six-Cylinder Engine.]
Here we have one explosion during the first half of the first revolution
in one cylinder and another during the first half of the second
revolution in the other cylinder. With a four-cylinder engine there is
an explosion each half revolution, while in a six-cylinder engine there
is one and one-half explosions during each half revolution. When six
cylinders are used there is no lapse of time between power impulses, as
these overlap and a continuous and smooth-turning movement is imparted
to the crank shaft. The diagram shown at Fig. 26, prepared by E. P.
Pulley, can be studied to advantage in securing an idea of the
coordination of effort that takes place in an engine of the six-cylinder
type.
ACTUAL DURATION OF DIFFERENT STROKES
[Illustration: Fig. 27.--Diagram Showing Actual Duration of Different
Strokes in Degrees.]
In the diagrams previously presented the writer has assumed, for the
sake of simplicity, that each stroke takes place during half of one
revolution of the crank-shaft, which corresponds to a crank-pin travel
of one hundred and eighty degrees. The actual duration of these strokes
is somewhat different. For example, the inlet stroke is usually a trifle
more than a half revolution, and the exhaust is always considerably
more. The diagram showing the comparative duration of the strokes is
shown at Fig. 27. The inlet valve opens ten degrees after the piston
starts to go down and remains open thirty degrees after the piston has
reached the bottom of its stroke. This means that the suction stroke
corresponds to a crank-pin travel of two hundred degrees, while the
compression stroke is measured by a movement of but one hundred and
fifty degrees. It is common practice to open the exhaust valve before
the piston reaches the end of the power stroke so that the actual
duration of the power stroke is about one hundred and forty degrees,
while the exhaust stroke corresponds to a crank-pin travel of two
hundred and twenty-five degrees. In this diagram, which represents
proper time for the valves to open and close, the dimensions in inches
given are measured on the fly-wheel and apply only to a certain
automobile motor. If the fly-wheel were smaller ten degrees would take
up less than the dimensions given, while if the fly-wheel was larger a
greater space on its circumference would represent the same crank-pin
travel. Aviation engines are timed by using a timing disc attached to
the crank-shaft as they are not provided with fly-wheels. Obviously, the
distance measured in inches will depend upon the diameter of the disc,
though the number of degrees interval would not change.
[Illustration: Fig. 28.--Another Diagram to Facilitate Understanding
Sequence of Functions in Six-Cylinder Engine.]
EIGHT- AND TWELVE-CYLINDER V ENGINES
Those who have followed the development of the gasoline engine will
recall the arguments that were made when the six-cylinder motor was
introduced at a time that the four-cylinder type was considered
standard. The arrival of the eight-cylinder has created similar futile
discussion of its practicability as this is so clearly established as to
be accepted without question. It has been a standard power plant for
aeroplanes for many years, early exponents having been the Antoinette,
the Woolsley, the Renault, the E. N. V. in Europe and the Curtiss in the
United States.
[Illustration: Fig. 29.--Types of Eight-Cylinder Engines Showing the
Advantage of the V Method of Cylinder Placing.]
The reason the V type shown at Fig. 29, A is favored is that the
"all-in-line form" which is shown at Fig. 29, B is not practical for
aircraft because of its length. Compared to the standard four-cylinder
engine it is nearly twice as long and it required a much stronger and
longer crank-shaft. It will be evident that it could not be located to
advantage in the airplane fuselage. These undesirable factors are
eliminated in the V type eight-cylinder motor, as it consists of two
blocks of four cylinders each, so arranged that one set or block is at
an angle of forty-five degrees from the vertical center line of the
motor, or at an angle of ninety degrees with the other set. This
arrangement of cylinders produces a motor that is no longer than a
four-cylinder engine of half the power would be.
[Illustration: Fig. 30.--Curves Showing Torque of Various Engine Types
Demonstrate Graphically Marked Advantage of the Eight-Cylinder Type.]
Apparently there is considerable misconception as to the advantage of
the two extra cylinders of the eight as compared with the six-cylinder.
It should be borne in mind that the multiplication in the number of
cylinders noticed since the early days of automobile development has not
been for solely increasing the power of the engine, but to secure a more
even turning movement, greater flexibility and to eliminate destructive
vibration. The ideal internal combustion motor, is the one having the
most uniform turning movement with the least mechanical friction loss.
Study of the torque outlines or plotted graphics shown at Figs. 25 and
30 will show how multiplication of cylinders will produce steady power
delivery due to overlapping impulses. The most practical form would be
that which more nearly conforms to the steady running produced by a
steam turbine or electric motor. The advocates of the eight-cylinder
engine bring up the item of uniform torque as one of the most important
advantages of the eight-cylinder design. A number of torque diagrams are
shown at Fig. 30. While these appear to be deeply technical, they may be
very easily followed when their purpose is explained. At the top is
shown the torque diagram of a single-cylinder motor of the four-cycle
type. The high point in the line represents the period of greatest
torque or power generation, and it will be evident that this occurs
early in the first revolution of the crank-shaft. Below this diagram is
shown a similar curve except that it is produced by a four-cylinder
engine. Inspection will show that the turning-moment is much more
uniform than in the single cylinder; similarly, the six-cylinder
diagram is an improvement over the four, and the eight-cylinder diagram
is an improvement over the six-cylinder.
[Illustration: Fig. 31.--Diagrams Showing How Increasing Number of
Cylinders Makes for More Uniform Power Application.]
The reason that practically continuous torque is obtained in an
eight-cylinder engine is that one cylinder fires every ninety degrees of
crank-shaft rotation, and as each impulse lasts nearly seventy-five per
cent. of the stroke, one can easily appreciate that an engine that will
give four explosions per revolution of the crank-shaft will run more
uniformly than one that gives but three explosions per revolution, as
the six-cylinder does, and will be twice as smooth running as a
four-cylinder, in which but two explosions occur per revolution of the
crank-shaft. The comparison is so clearly shown in graphical diagrams
and in Fig. 31 that further description is unnecessary.
Any eight-cylinder engine may be considered a "twin-four,"
twelve-cylinder engines may be considered "twin sixes."
[Illustration: Fig. 32.--How the Angle Between the Cylinders of an
Eight- and Twelve-Cylinder V Motor Varies.]
The only points in which an eight-cylinder motor differs from a
four-cylinder is in the arrangement of the connecting rod, as in many
designs it is necessary to have two rods working from the same
crank-pin. This difficulty is easily overcome in some designs by
staggering the cylinders and having the two connecting rod big ends of
conventional form side by side on a common crank-pin. In other designs
one rod is a forked form and works on the outside of a rod of the
regular pattern. Still another method is to have a boss just above the
main bearing on one connecting rod to which the lower portion of the
connecting rod in the opposite cylinder is hinged. As the eight-cylinder
engine may actually be made lighter than the six-cylinder of equal
power, it is possible to use smaller reciprocating parts, such as
pistons, connecting rods and valve gear, and obtain higher engine speed
with practically no vibration. The firing order in nearly every case is
the same as in a four-cylinder except that the explosions occur
alternately in each set of cylinders. The firing order of an
eight-cylinder motor is apt to be confusing to the motorist,
especially if one considers that there are eight possible sequences. The
majority of engineers favor the alternate firing from side to side.
Firing orders will be considered in proper sequence.
[Illustration: Fig. 33.--The Hall-Scott Four-Cylinder 100 Horse-Power
Aviation Motor.]
[Illustration: Fig. 34.--Two Views of the Duesenberg Sixteen Valve
Four-Cylinder Aviation Motor.]
The demand of aircraft designers for more power has stimulated designers
to work out twelve-cylinder motors. These are high-speed motors
incorporating all recent features of design in securing light
reciprocating parts, large valve openings, etc. The twelve-cylinder
motor incorporates the best features of high-speed motor design and
there is no need at this time to discuss further the pros and cons of
the twelve-cylinder versus the eight or six, because it is conceded by
all that there is the same degree of steady power application in the
twelve over the eight as there would be in the eight over the six. The
question resolves itself into having a motor of high power that will
run with minimum vibration and that produces smooth action. This is well
shown by diagrams at Fig. 31. It should be remembered that if an
eight-cylinder engine will give four explosions per revolution of the
fly-wheel, a twelve-cylinder type will give six explosions per
revolution, and instead of the impulses coming 90 degrees crank travel
apart, as in the case of the eight-cylinder, these will come but 60
degrees of crank travel apart in the case of the twelve-cylinder. For
this reason, the cylinders of a twelve are usually separated by 60
degrees while the eight has the blocks spaced 90 degrees apart. The
comparison can be easily made by comparing the sectional views of Vee
engines at Fig. 32. When one realizes that the actual duration of the
power stroke is considerably greater than 120 degrees crank travel, it
will be apparent that the overlapping of explosions must deliver a very
uniform application of power. Vee engines have been devised having the
cylinders spaced but 45 degrees apart, but the explosions cannot be
timed at equal intervals as when 90 degrees separate the cylinder center
lines.
[Illustration: Fig. 35.--The Hall-Scott Six-Cylinder Aviation Engine.]
RADIAL CYLINDER ARRANGEMENTS
[Illustration: Fig. 36.--The Curtiss Eight-Cylinder, 200 Horse-Power
Aviation Engine.]
While the fixed cylinder forms of engines, having the cylinders in
tandem in the four- and six-cylinder models as shown at Figs. 33 to 35
inclusive and the eight-cylinder V types as outlined at Figs. 36 and 37
have been generally used and are most in favor at the present time,
other forms of motors having unconventional cylinder arrangements have
been devised, though most of these are practically obsolete. While many
methods of decreasing weight and increasing mechanical efficiency of a
motor are known to designers, one of the first to be applied to the
construction of aeronautical power plants was an endeavor to group the
components, which in themselves were not extremely light, into a form
that would be considerably lighter than the conventional design. As an
example, we may consider those multiple-cylinder forms in which the
cylinders are disposed around a short crank-case, either radiating from
a common center as at Fig. 38 or of the fan shape shown at Fig. 39. This
makes it possible to use a crank-case but slightly larger than that
needed for one or two cylinders and it also permits of a corresponding
decrease in length of the crank-shaft. The weight of the engine is
lessened because of the reduction in crank-shaft and crank-case weight
and the elimination of a number of intermediate bearings and their
supporting webs which would be necessary with the usual tandem
construction. While there are six power impulses to every two
revolutions of the crank-shaft, in the six-cylinder engine, they are
not evenly spaced as is possible with the conventional arrangement.
[Illustration: Fig. 37.--The Sturtevant Eight-Cylinder, High Speed
Aviation Motor.]
[Illustration: Fig. 38.--Anzani 40-50 Horse-Power Five-Cylinder Air
Cooled Engine.]
In the Anzani form, which is shown at Fig. 38, the crank-case is
stationary and a revolving crank-shaft is employed as in conventional
construction. The cylinders are five in number and the engine develops
40 to 50 H.P. with a weight of 72 kilograms or 158.4 lbs. The cylinders
are of the usual air-cooled form having cooling flanges only part of the
way down the cylinder. By using five cylinders it is possible to have
the power impulses come regularly, they coming 145 deg. crank-shaft
travel apart, the crank-shaft making two turns to every five explosions.
The balance is good and power output regular. The valves are placed
directly in the cylinder head and are operated by a common pushrod.
Attention is directed to the novel method of installing the carburetor
which supplies the mixture to the engine base from which inlet pipes
radiate to the various cylinders. This engine is used on French school
machines.
[Illustration: Fig. 39.--Unconventional Six-Cylinder Aircraft Motor of
Masson Design.]
In the form shown at Fig. 39 six cylinders are used, all being placed
above the crank-shaft center line. This engine is also of the air-cooled
form and develops 50 H. P. and weighs 105 kilograms, or 231 lbs. The
carburetor is connected to a manifold casting attached to the engine
base from which the induction pipes radiate to the various cylinders.
The propeller design and size relative to the engine is clearly shown in
this view. While flights have been made with both of the engines
described, this method of construction is not generally followed and has
been almost entirely displaced abroad by the revolving motors or by the
more conventional eight-cylinder V engines. Both of the engines shown
were designed about eight years ago and would be entirely too small and
weak for use in modern airplanes intended for active duty.
ROTARY ENGINES
[Illustration: Fig. 40.--The Gnome Fourteen-Cylinder Revolving Motor.]
Rotary engines such as shown at Fig. 40 are generally associated with
the idea of light construction and it is rather an interesting point
that is often overlooked in connection with the application of this idea
to flight motors, that the reason why rotary engines are popularly
supposed to be lighter than the others is because they form their own
fly-wheel, yet on aeroplanes, engines are seldom fitted with a fly-wheel
at all. As a matter of fact the Gnome engine is not so light because it
is a rotary motor, and it is a rotary motor because the design that has
been adopted as that most conducive to lightness is also most suited to
an engine working in this way. The cylinders could be fixed and
crank-shaft revolve without increasing the weight to any extent. There
are two prime factors governing the lightness of an engine, one being
the initial design, and the other the quality of the materials employed.
The consideration of reducing weight by cutting away metal is a
subsidiary method that ought not to play a part in standard practice,
however useful it may be in special cases. In the Gnome rotary engine
the lightness is entirely due to the initial design and to the materials
employed in manufacture. Thus, in the first case, the engine is a radial
engine, and has its seven or nine cylinders spaced equally around a
crank-chamber that is no wider or rather longer than would be required
for any one of the cylinders. This shortening of the crank-chamber not
only effects a considerable saving of weight on its own account, but
there is a corresponding saving in the shafts and other members, the
dimensions of which are governed by the size of the crank-chamber. With
regard to materials, nothing but steel is used throughout, and most of
the metal is forged chrome nickel steel. The beautifully steady running
of the engine is largely due to the fact that there are literally no
reciprocating parts in the absolute sense, the apparent reciprocation
between the pistons and cylinders being solely a relative reciprocation
since both travel in circular paths, that of the pistons, however, being
electric by one-half of the stroke length to that of the cylinder.
While the Gnome engine has many advantages, on the other hand the head
resistance offered by a motor of this type is considerable; there is a
large waste of lubricating oil due to the centrifugal force which tends
to throw the oil away from the cylinders; the gyroscopic effect of the
rotary motor is detrimental to the best working of the aeroplane, and
moreover it requires about seven per cent. of the total power developed
by the motor to drive the revolving cylinders around the shaft. Of
necessity, the compression of this type of motor is rather low, and an
additional disadvantage manifests itself in the fact that there is as
yet no satisfactory way of muffling the rotary type of motor. The modern
Gnome engine has been widely copied in various European countries, but
its design was originated in America, the early Adams-Farwell engine
being the pioneer form. It has been made in seven- and nine-cylinder
types and forms of double these numbers. The engine illustrated at Fig.
40 is a fourteen-cylinder form. The simple engines have an odd number of
cylinders in order to secure evenly spaced explosions. In the
seven-cylinder, the impulses come 102.8 deg. apart. In the nine-cylinder
form, the power strokes are spaced 80 deg. apart. The fourteen-cylinder
engine is virtually two seven-cylinder types mounted together, the
cranks being just the same as in a double cylinder opposed motor, the
explosions coming 51.4 deg. apart; while in the eighteen-cylinder model the
power impulses come every 40 deg. cylinder travel. Other rotary motors have
been devised, such as the Le Rhone and the Clerget in France and several
German copies of these various types. The mechanical features of these
motors will be fully considered later.