Methods of Cylinder Construction--Block Castings--Influence on
Crank-Shaft Design--Combustion Chamber Design--Bore and Stroke
Ratio--Meaning of Piston Speed--Advantage of Off-Set Cylinders--
Valve Location of Vital Import--Valve Installation Practice--
Valve Design and Construction--Valve Operation--Methods of
Driving Cam-Shaft--Valve Springs--Valve Timing--Blowing Back--
Lead Given Exhaust Valve--Exhaust Closing, Inlet Opening--
Closing the Inlet Valve--Time of Ignition--How an Engine Is
Timed--Gnome "Monosoupape" Valve Timing--Springless Valves--Four
Valves per Cylinder.
The improvements noted in the modern internal combustion motors have
been due to many conditions. The continual experimenting by leading
mechanical minds could have but one ultimate result. The parts of the
engines have been lightened and strengthened, and greater power has been
obtained without increasing piston displacement. A careful study has
been made of the many conditions which make for efficient motor action,
and that the main principles are well recognized by all engineers is
well shown by the standardization of design noted in modern power
plants. There are many different methods of applying the same principle,
and it will be the purpose of this chapter to define the ways in which
the construction may be changed and still achieve the same results. The
various components may exist in many different forms, and all have their
advantages and disadvantages. That all methods are practical is best
shown by the large number of successful engines which use radically
different designs.
METHODS OF CYLINDER CONSTRUCTION
One of the most important parts of the gasoline engine and one that has
material bearing upon its efficiency is the cylinder unit. The cylinders
may be cast individually, or in pairs, and it is possible to make all
cylinders a unit or block casting. Some typical methods of cylinder
construction are shown in accompanying illustrations. The appearance of
individual cylinder castings may be ascertained by examination of the
Hall-Scott airplane engine. Air-cooled engine cylinders are always of
the individual pattern.
Considered from a purely theoretical point of view, the individual
cylinder casting has much in its favor. It is advanced that more uniform
cooling is possible than where the cylinders are cast either in pairs or
three or four in one casting. More uniform cooling insures that the
expansion or change of form due to heating will be more equal. This is
an important condition because the cylinder bore must remain true under
all conditions of operation. If the heating effect is not uniform, which
condition is liable to obtain if metal is not evenly distributed, the
cylinder may become distorted by heat and the bore be out of truth. When
separate cylinders are used it is possible to make a uniform water space
and have the cooling liquid evenly distributed around the cylinder. In
multiple cylinder castings this is not always the rule, as in many
instances, especially in four-cylinder block motors where compactness is
the main feature, there is but little space between the cylinders for
the passage of water. Under such circumstances the cooling effect is not
even, and the stresses which obtain because of unequal expansion may
distort the cylinder to some extent. When steel cylinders are made from
forgings, the water jackets are usually of copper or sheet steel
attached to the forging by autogenous welding; in the case of the latter
and, in some cases, the former may be electro-deposited on the
cylinders.
BLOCK CASTINGS
[Illustration: Fig. 86.--Views of Four-Cylinder Duesenberg Airplane
Engine Cylinder Block.]
The advantage of casting the cylinders in blocks is that a motor may be
much shorter than it would be if individual castings were used. It is
admitted that when the cylinders are cast together a more compact,
rigid, and stronger power plant is obtained than when cast separately.
There is a disadvantage, however, in that if one cylinder becomes
damaged it will be necessary to replace the entire unit, which means
scrapping three good cylinders because one of the four has failed. When
the cylinders are cast separately one need only replace the one that has
become damaged. The casting of four cylinders in one unit is made
possible by improved foundry methods, and when proper provision is made
for holding the cores when the metal is poured and the cylinder casts
are good, the construction is one of distinct merit. It is sometimes the
case that the proportion of sound castings is less when cylinders are
cast in block, but if the proper precautions are observed in molding and
the proper mixtures of cast iron used, the ratio of defective castings
is no more than when cylinders are molded individually. As an example of
the courage of engineers in departing from old-established rules, the
cylinder casting shown at Fig. 86 may be considered typical. This is
used on the Duesenberg four-cylinder sixteen-valve 4-3/4" x 7" engine
which has a piston displacement of 496 cu. in. At a speed of 2,000
r.p.m., corresponding to a piston speed of 2,325 ft. per min., the
engine is guaranteed to develop 125 horse-power. The weight of the model
engine without gear reduction is 436 lbs., but a number of refinements
have been made in the design whereby it is expected to get the weight
down to 390 lbs. The four cylinders are cast from semi-steel in a single
block, with integral heads. The cylinder construction is the same as
that which has always been used by Mr. Duesenberg, inlet and exhaust
valves being arranged horizontally opposite each other in the head.
There are large openings in the water jacket at both sides and at the
ends, which are closed by means of aluminum covers, water-tightness
being secured by the use of gaskets. This results in a saving in weight
because the aluminum covers can be made considerably lighter than it
would be possible to cast the jacket walls, and, besides, it permits of
obtaining a more nearly uniform thickness of cylinder wall, as the cores
can be much better supported. The cooling water passes completely around
each cylinder, and there is a very considerable space between the two
central cylinders, this being made necessary in order to get the large
bearing area desirable for the central bearing.
It is common practice to cast the water jackets integral with the
cylinders, if cast iron or aluminum is used, and this is also the most
economical method of applying it because it gives good results in
practice. An important detail is that the water spaces must be
proportioned so that they are equal around the cylinders whether these
members are cast individually, in pairs, threes or fours. When cylinders
are cast in block form it is good practice to leave a large opening in
the jacket wall which will assist in supporting the core and make for
uniform water space. It will be noticed that the casting shown at Fig.
86 has a large opening in the side of the cylinder block. These openings
are closed after the interior of the casting is thoroughly cleaned of
all sand, core wire, etc., by brass, cast iron or aluminum plates. These
also have particular value in that they may be removed after the motor
has been in use, thus permitting one to clean out the interior of the
water jacket and dispose of the rust, sediment, and incrustation which
are always present after the engine has been in active service for a
time.
Among the advantages claimed for the practice of casting cylinders in
blocks may be mentioned compactness, lightness, rigidity, simplicity of
water piping, as well as permitting the use of simple forms of inlet and
exhaust manifolds. The light weight is not only due to the reduction of
the cylinder mass but because the block construction permits one to
lighten the entire motor. The fact that all cylinders are cast together
decreases vibration, and as the construction is very rigid, disalignment
of working parts is practically eliminated. When inlet and exhaust
manifolds are cored in the block casting, as is sometimes the case, but
one joint is needed on each of these instead of the multiplicity of
joints which obtain when the cylinders are individual castings. The
water piping is also simplified. In the case of a four-cylinder block
motor but two pipes are used; one for the water to enter the cylinder
jacket, the other for the cooling liquid to discharge through.
INFLUENCE ON CRANK-SHAFT DESIGN
[Illustration: Fig. 87.--Twin-Cylinder Block of Sturtevant Airplane
Engine is Cast of Aluminum, and Has Removable Cylinder Head.]
The method of casting the cylinders has a material influence on the
design of the crank-shaft as will be shown in proper sequence. When four
cylinders are combined in one block it is possible to use a two-bearing
crank-shaft. Where cylinders are cast in pairs a three-bearing
crank-shaft is commonly supplied, and when cylinders are cast as
individual units it is thought necessary to supply a five-bearing
crank-shaft, though sometimes shafts having but three journals are used
successfully. Obviously the shafts must be stronger and stiffer to
withstand the stresses imposed if two supporting bearings are used than
if a larger number are employed. In this connection it may be stated
that there is less difficulty in securing alignment with a lesser number
of bearings and there is also less friction. On the other hand, the
greater the number of points of support a crank-shaft has the lighter
the webs can be made and still have requisite strength.
COMBUSTION CHAMBER DESIGN
[Illustration: Fig. 88.--Aluminum Cylinder Pair Casting of Thomas 150
Horse-Power Airplane Engine is of the L Head Type.]
Another point of importance in the design of the cylinder, and one which
has considerable influence upon the power developed, is the shape of the
combustion chamber. The endeavor of designers is to obtain maximum power
from a cylinder of certain proportions, and the greater energy obtained
without increasing piston displacement or fuel consumption the higher
the efficiency of the motor. To prevent troubles due to pre-ignition it
is necessary that the combustion chamber be made so that there will be
no roughness, sharp corners, or edges of metal which may remain
incandescent when heated or which will serve to collect carbon deposits
by providing a point of anchorage. With the object of providing an
absolutely clean combustion chamber some makers use a separable head
unit to their twin cylinder castings, such as shown at Fig. 87 and Fig.
88. These permit one to machine the entire interior of the cylinder and
combustion chamber. The relation of valve location and combustion
chamber design will be considered in proper sequence. These cylinders
are cast of aluminum, instead of cast iron, as is customary, and are
provided with steel or cast iron cylinder liners forced in the soft
metal casting bores.
BORE AND STROKE RATIO
A question that has been a vexed one and which has been the subject of
considerable controversy is the proper proportion of the bore to the
stroke. The early gas engines had a certain well-defined bore to stroke
ratio, as it was usual at that time to make the stroke twice as long as
the bore was wide, but this cannot be done when high speed is desired.
With the development of the present-day motor the stroke or piston
travel has been gradually shortened so that the relative proportions of
bore and stroke have become nearly equal. Of late there seems to be a
tendency among designers to return to the proportions which formerly
obtained, and the stroke is sometimes one and a half or one and
three-quarter times the bore.
Engines designed for high speed should have the stroke not much longer
than the diameter of the bore. The disadvantage of short-stroke engines
is that they will not pull well at low speeds, though they run with
great regularity and smoothness at high velocity. The long-stroke engine
is much superior for slow speed work, and it will pull steadily and with
increasing power at low speed. It was formerly thought that such engines
should never turn more than a moderate number of revolutions, in order
not to exceed the safe piston speed of 1,000 feet per minute. This old
theory or rule of practice has been discarded in designing high
efficiency automobile racing and aviation engines, and piston speeds
from 2,500 to 3,000 feet per minute are sometimes used, though the
average is around 2,000 feet per minute. While both short- and
long-stroke motors have their advantages, it would seem desirable to
average between the two. That is why a proportion of four to five or six
seems to be more general than that of four to seven or eight, which
would be a long-stroke ratio. Careful analysis of a number of foreign
aviation motors shows that the average stroke is about 1.2 times the
bore dimensions, though some instances were noted where it was as high
as 1.7 times the bore.
MEANING OF PISTON SPEED
The factor which limits the stroke and makes the speed of rotation so
dependent upon the travel of the piston is piston speed. Lubrication is
the main factor which determines piston speed, and the higher the rate
of piston travel the greater care must be taken to insure proper oiling.
Let us fully consider what is meant by piston speed.
Assume that a motor has a piston travel or stroke of six inches, for the
sake of illustration. It would take two strokes of the piston to cover
one foot, or twelve inches, and as there are two strokes to a revolution
it will be seen that this permits of a normal speed of 1,000 revolutions
per minute for an engine with a six-inch stroke, if one does not exceed
1,000 feet per minute. If the stroke was only four inches, a normal
speed of 1,500 revolutions per minute would be possible without
exceeding the prescribed limit. The crank-shaft of a small engine,
having three-inch stroke, could turn at a speed of 2,000 revolutions per
minute without danger of exceeding the safe speed limit. It will be seen
that the longer the stroke the slower the speed of the engine, if one
desires to keep the piston speed within the bounds as recommended, but
modern practice allows of greatly exceeding the speeds formerly thought
best.
ADVANTAGES OF OFF-SET CYLINDERS
[Illustration: Fig. 90.--Cross Section of Austro-Daimler Engine, Showing
Offset Cylinder Construction. Note Applied Water Jacket and Peculiar
Valve Action.]
Another point upon which considerable difference of opinion exists
relates to the method of placing the cylinder upon the crank-case--i.e.,
whether its center line should be placed directly over the center of the
crank-shaft, or to one side of center. The motor shown at Fig. 90 is an
off-set type, in that the center line of the cylinder is a little to
one side of the center of the crank-shaft. Diagrams are presented at
Fig. 91 which show the advantages of off-set crank-shaft construction.
The view at A is a section through a simple motor with the conventional
cylinder placing, the center line of both crank-shaft and cylinder
coinciding. The view at B shows the cylinder placed to one side of
center so that its center line is distinct from that of the crank-shaft
and at some distance from it. The amount of off-set allowed is a point
of contention, the usual amount being from fifteen to twenty-five per
cent. of the stroke. The advantages of the off-set are shown at Fig. 91,
C. If the crank turns in direction of the arrow there is a certain
resistance to motion which is proportional to the amount of energy
exerted by the engine and the resistance offered by the load. There are
two thrusts acting against the cylinder wall to be considered, that due
to explosion or expansion of the gas, and that which resists the motion
of the piston. These thrusts may be represented by arrows, one which
acts directly in a vertical direction on the piston top, the other
along a straight line through the center of the connecting rod. Between
these two thrusts one can draw a line representing a resultant force
which serves to bring the piston in forcible contact with one side of
the cylinder wall, this being known as side thrust. As shown at C, the
crank-shaft is at 90 degrees, or about one-half stroke, and the
connecting rod is at 20 degrees angle. The shorter connecting rod would
increase the diagonal resultant and side thrusts, while a longer one
would reduce the angle of the connecting rod and the side thrust of the
piston would be less. With the off-set construction, as shown at D, it
will be noticed that with the same connecting-rod length as shown at C
and with the crank-shaft at 90 degrees of the circle that the
connecting-rod angle is 14 degrees and the side thrust is reduced
proportionately.
[Illustration: Fig. 91.--Diagrams Demonstrating Advantages of Offset
Crank-Shaft Construction.]
Another important advantage is that greater efficiency is obtained from
the explosion with an off-set crank-shaft, because the crank is already
inclined when the piston is at top center, and all the energy imparted
to the piston by the burning mixture can be exerted directly into
producing a useful turning effort. When a cylinder is placed directly on
a line with the crank-shaft, as shown at A, it will be evident that some
of the force produced by the expansion of the gas will be exerted in a
direct line and until the crank moves the crank throw and connecting rod
are practically a solid member. The pressure which might be employed in
obtaining useful turning effort is wasted by causing a direct pressure
upon the lower half of the main bearing and the upper half of the
crank-pin bushing.
Very good and easily understood illustrations showing advantages of the
off-set construction are shown at E and F. This is a bicycle
crank-hanger. It is advanced that the effort of the rider is not as well
applied when the crank is at position E as when it is at position F.
Position E corresponds to the position of the parts when the cylinder is
placed directly over the crank-shaft center. Position F may be compared
to the condition which is present when the off-set cylinder construction
is used.
VALVE LOCATION OF VITAL IMPORT
It has often been said that a chain is no stronger than its weakest
link, and this is as true of the explosive motor as it is of any other
piece of mechanism. Many motors which appeared to be excellently
designed and which were well constructed did not prove satisfactory
because some minor detail or part had not been properly considered by
the designer. A factor having material bearing upon the efficiency of
the internal combustion motor is the location of the valves and the
shape of the combustion chamber which is largely influenced by their
placing. The fundamental consideration of valve design is that the gases
be admitted and discharged from the cylinder as quickly as possible in
order that the speed of gas flow will not be impeded and produce back
pressure. This is imperative in obtaining satisfactory operation in any
form of motor. If the inlet passages are constricted the cylinder will
not fill with explosive mixture promptly, whereas if the exhaust gases
are not fully expelled the parts of the inert products of combustion
retained dilute the fresh charge, making it slow burning and causing
lost power and overheating. When an engine employs water as a cooling
medium this substance will absorb the surplus heat readily, and the
effects of overheating are not noticed as quickly as when air-cooled
cylinders are employed. Valve sizes have a decided bearing upon the
speed of motors and some valve locations permit the use of larger
members than do other positions.
While piston velocity is an important factor in determinations of power
output, it must be considered from the aspect of the wear produced upon
the various parts of the motor. It is evident that engines which run
very fast, especially of high power, must be under a greater strain than
those operating at lower speeds. The valve-operating mechanism is
especially susceptible to the influence of rapid movement, and the
slower the engine the longer the parts will wear and the more reliable
the valve action.
[Illustration: Fig. 92.--Diagram Showing Forms of Cylinder Demanded by
Different Valve Placings. A--T Head Type, Valves on Opposite Sides. B--L
Head Cylinder, Valves Side by Side. C--L Head Cylinder, One Valve in
Head, Other in Pocket. D--Inlet Valve Over Exhaust Member, Both in Side
Pocket. E--Valve-in-the-Head Type with Vertical Valves. F--Inclined
Valves Placed to Open Directly into Combustion Chamber.]
As will be seen by reference to the accompanying illustration, Fig. 92,
there are many ways in which valves may be placed in the cylinder. Each
method outlined possesses some point of advantage, because all of the
types illustrated are used by reputable automobile manufacturers. The
method outlined at Fig. 92, A, is widely used, and because of its shape
the cylinder is known as the "T" form. It is approved for automobile use
for several reasons, the most important being that large valves can be
employed and a well-balanced and symmetrical cylinder casting obtained.
Two independent cam-shafts are needed, one operating the inlet valves,
the other the exhaust members. The valve-operating mechanism can be very
simple in form, consisting of a plunger actuated by the cam which
transmits the cam motion to the valve-stem, raising the valve as the cam
follower rides on the point of the cam. Piping may be placed without
crowding, and larger manifolds can be fitted than in some other
constructions. This has special value, as it permits the use of an
adequate discharge pipe on the exhaust side with its obvious advantages.
This method of cylinder construction is never found on airplane engines
because it does not permit of maximum power output.
On the other hand, if considered from a viewpoint of actual heat
efficiency, it is theoretically the worst form of combustion chamber.
This disadvantage is probably compensated for by uniformity of expansion
of the cylinder because of balanced design. The ignition spark-plug may
be located directly over the inlet valve in the path of the incoming
fresh gases, and both valves may be easily removed and inspected by
unscrewing the valve caps without taking off the manifolds.
The valve installation shown at C is somewhat unusual, though it
provides for the use of valves of large diameter. Easy charging is
insured because of the large inlet valve directly in the top of the
cylinder. Conditions may be reversed if necessary, and the gases
discharged through this large valve. Both methods are used, though it
would seem that the free exhaust provided by allowing the gases to
escape directly from the combustion chamber through the overhead valve
to the exhaust manifold would make for more power. The method outlined
at Fig. 92, F and at Fig. 90 is one that has been widely employed on
large automobile racing motors where extreme power is required, as well
as in engines constructed for aviation service. The inclination of the
valves permits the use of large valves, and these open directly into the
combustion chamber. There are no pockets to retain heat or dead gas, and
free intake and outlet of gas is obtained. This form is quite
satisfactory from a theoretical point of view because of the almost
ideal combustion chamber form. Some difficulty is experienced, however,
in properly water-jacketing the valve chamber which experience has shown
to be necessary if the engine is to have high power.
The motor shown at Fig. 92, B and Fig. 88 employs cylinders of the "L"
type. Both valves are placed in a common extension from the combustion
chamber, and being located side by side both are actuated from a common
cam-shaft. The inlet and exhaust pipes may be placed on the same side of
the engine and a very compact assemblage is obtained, though this is
optional if passages are cored in the cylinder pairs to lead the gases
to opposite sides. The valves may be easily removed if desired, and the
construction is fairly good from the viewpoint of both foundry man and
machinist. The chief disadvantage is the limited area of the valves and
the loss of heat efficiency due to the pocket. This form of combustion
chamber, however, is more efficient than the "T" head construction,
though with the latter the use of larger valves probably compensates for
the greater heat loss. It has been stated as an advantage of this
construction that both manifolds can be placed at the same side of the
engine and a compact assembly secured. On the other hand, the
disadvantage may be cited that in order to put both pipes on the same
side they must be of smaller size than can be used when the valves are
oppositely placed. The "L" form cylinder is sometimes made more
efficient if but one valve is placed in the pocket while the other is
placed over it. This construction is well shown at Fig. 92, D and is
found on Anzani motors.
[Illustration: Fig. 93.--Sectional View of Engine Cylinder Showing Valve
and Cage Installation.]
The method of valve application shown at Fig. 87 is an ingenious method
of overcoming some of the disadvantages inherent with valve-in-the-head
motors. In the first place it is possible to water-jacket the valves
thoroughly, which is difficult to accomplish when they are mounted in
cages. The water circulates directly around the walls of the valve
chambers, which is superior to a construction where separate cages are
used, as there are two thicknesses of metal with the latter, that of the
valve-cage proper and the wall of the cylinder. The cooling medium is in
contact only with the outer wall, and as there is always a loss of heat
conductivity at a joint it is practically impossible to keep the
exhaust valves and their seats at a uniform temperature. The valves may
be of larger size without the use of pockets when seating directly in
the head. In fact, they could be equal in diameter to almost half the
bore of the cylinder, which provides an ideal condition of charge
placement and exhaust. When valve grinding is necessary the entire head
is easily removed by taking off six nuts and loosening inlet manifold
connections, which operation would be necessary even if cages were
employed, as in the engine shown at Fig. 93.
[Illustration: Fig. 94.--Diagrams Showing How Gas Enters Cylinder
Through Overhead Valves and Other Types. A--Tee Head Cylinder. B--L Head
Cylinder. C--Overhead Valve.]
[Illustration: Fig. 95.--Conventional Methods of Operating Internal
Combustion Motor Valves.]
At Fig. 94, A and B, a section through a typical "L"-shaped cylinder is
depicted. It will be evident that where a pocket construction is
employed, in addition to its faculty for absorbing heat, the passage of
gas would be impeded. For example, the inlet gas rushing in through the
open valve would impinge sharply upon the valve-cap or combustion head
directly over the valve and then must turn at a sharp angle to enter the
combustion chamber and then at another sharp angle to fill the
cylinders. The same conditions apply to the exhaust gases, though they
are reversed. When the valve-in-the-head type of cylinder is employed,
as at C, the only resistance offered the gas is in the manifold. As far
as the passage of the gases in and out of the cylinder is concerned,
ideal conditions obtain. It is claimed that valve-in-the-head motors are
more flexible and responsive than other forms, but the construction has
the disadvantage in that the valves must be opened through a rather
complicated system of push rods and rocker arms instead of the simpler
and direct plunger which can be used with either the "T" or "L" head
cylinders. This is clearly outlined in the illustrations at Fig. 95,
where A shows the valve in the head-operating mechanism necessary if the
cam-shaft is carried at the cylinder base, while B shows the most direct
push-rod action obtained with "T" or "L" head cylinder placing.
[Illustration: Fig. 96.--Examples of Direct Valve Actuation by Overhead
Cam-Shaft. A--Mercedes. B--Hall-Scott. C--Wisconsin.]
[Illustration: Fig. 97.
CENSORED]
[Illustration: Fig. 98.
CENSORED]
The objection can be easily met by carrying the cam-shaft above the
cylinders and driving it by means of gearing. The types of engine
cylinders using this construction are shown at Fig. 96, and it will be
evident that a positive and direct valve action is possible by following
the construction originated by the Mercedes (German) aviation engine
designers and outlined at A. The other forms at B and C are very clearly
adaptations of this design. The Hall-Scott engine at Fig. 97 is depicted
in part section and no trouble will be experienced in understanding the
bevel pinion and gear drive from the crank-shaft to the overhead
cam-shaft through a vertical counter-shaft. A very direct valve action
is used in the Duesenberg engines, one of which is shown in part section
at Fig. 98. The valves are parallel with the piston top and are actuated
by rocker arms, one end of which bears against the valve stem, and the
other rides the cam-shaft.
[Illustration: Fig. 99.--Sectional Views Showing Arrangement of Novel
Concentric Valve Arrangement Devised by Panhard for Aerial Engines.]
The form shown at Fig. 99 shows an ingenious application of the
valve-in-the-head idea which permits one to obtain large valves. It has
been used on some of the Panhard aviation engines and on the American
Aeromarine power plants. The inlet passage is controlled by the sliding
sleeve which is hollow and slotted so as to permit the inlet gases to
enter the cylinder through the regular type poppet valve which seats in
the exhaust sleeve. When the exhaust valve is operated by the tappet rod
and rocker arm the intake valve is also carried down with it. The
intake gas passage is closed, however, and the burned gases are
discharged through the large annular passage surrounding the sleeve.
When the inlet valve leaves its seat in the sleeve the passage of cool
gas around the sleeve keeps the temperature of both valves to a low
point and the danger of warping is minimized. A dome-shaped combustion
chamber may be used, which is an ideal form in conserving heat
efficiency, and as large valves may be installed the flow of both fresh
and exhaust gases may be obtained with minimum resistance. The intake
valve is opened by a small auxiliary rocker arm which is lifted when the
cam follower rides into the depression in the cam by the action of the
strong spring around the push rod. When the cam follower rides on the
high point the exhaust sleeve is depressed from its seat against the
cylinder. By using a cam having both positive and negative profiles, a
single rod suffices for both valves because of its push and pull action.
VALVE DESIGN AND CONSTRUCTION
Valve dimensions are an important detail to be considered and can be
determined by several conditions, among which may be cited method of
installation, operating mechanism, material employed, engine speed
desired, manner of cylinder cooling and degree of lift desired. A review
of various methods of valve location has shown that when the valves are
placed directly in the head we can obtain the ideal cylinder form,
though larger valves may be used if housed in a separate pocket, as
afforded by the "T" head construction. The method of operation has much
to do with the size of the valves. For example, if an automatic inlet
valve is employed it is good practice to limit the lift and obtain the
required area of port opening by augmenting the diameter. Because of
this a valve of the automatic type is usually made twenty per cent.
larger than one mechanically operated. When both are actuated by cam
mechanism, as is now common practice, they are usually made the same
size and are interchangeable, which greatly simplifies manufacture. The
relation of valve diameter to cylinder bore is one that has been
discussed for some time by engineers. The writer's experience would
indicate that they should be at least half the bore, if possible. While
the mushroom type or poppet valve has become standard and is the most
widely used form at the present time, there is some difference of
opinion among designers as to the materials employed and the angle of
the seat. Most valves have a bevel seat, though some have a flat
seating. The flat seat valve has the distinctive advantage of providing
a clear opening with lesser lift, this conducing to free gas flow. It
also has value because it is silent in operation, but the disadvantage
is present that best material and workmanship must be used in their
construction to obtain satisfactory results. As it can be made very
light it is particularly well adapted for use as an automatic inlet
valve. Among other disadvantages cited is the claim that it is more
susceptible to derangement, owing to the particles of foreign matter
getting under the seat. With a bevel seat it is argued that the foreign
matter would be more easily dislodged by the gas flow, and that the
valve would close tighter because it is drawn positively against the
bevel seat.
Several methods of valve construction are the vogue, the most popular
form being the one-piece type; those which are composed of a head of one
material and stem of another are seldom used in airplane engines because
they are not reliable. In the built-up construction the head is usually
of high nickel steel or cast iron, which metals possess good
heat-resisting qualities. Heads made of these materials are not likely
to warp, scale, or pit, as is sometimes the case when ordinary grades of
machinery steel are used. The cast-iron head construction is not popular
because it is often difficult to keep the head tight on the stem. There
is a slight difference in expansion ratio between the head and the stem,
and as the stem is either screwed or riveted to the cast-iron head the
constant hammering of the valve against its seat may loosen the joint.
As soon as the head is loose on the stem the action of the valve becomes
erratic. The best practice is to machine the valves from tungsten steel
forgings. This material has splendid heat-resisting qualities and will
not pit or become scored easily. Even the electrically welded head to
stem types which are used in automobile engines are not looked upon with
favor in the aviation engine. Valve stem guides and valve stems must be
machined very accurately to insure correct action. The usual practice in
automobile engines is shown at Fig. 100.
[Illustration: Fig. 100.--Showing Clearance Allowed Between Valve Stem
and Valve Stem Guide to Secure Free Action.]
VALVE OPERATION
The methods of valve operation commonly used vary according to the type
of cylinder construction employed. In all cases the valves are lifted
from their seats by cam-actuated mechanism. Various forms of
valve-lifting cams are shown at Fig. 101. As will be seen, a cam
consists of a circle to which a raised, approximately triangular member
has been added at one point. When the cam follower rides on the circle,
as shown at Fig. 102, there is no difference in height between the cam
center and its periphery and there is no movement of the plunger. As
soon as the raised portion of the cam strikes the plunger it will lift
it, and this reciprocating movement is transmitted to the valve stem by
suitable mechanical connections.
[Illustration: Fig. 101.--Forms of Valve-Lifting Cams Generally
Employed. A--Cam Profile for Long Dwell and Quick Lift. B--Typical Inlet
Cam Used with Mushroom Type Follower. C--Average Form of Cam.
D--Designed to Give Quick Lift and Gradual Closing.]
The cam forms outlined at Fig. 101 are those commonly used. That at A is
used on engines where it is desired to obtain a quick lift and to keep
the valve fully opened as long as possible. It is a noisy form, however,
and is not very widely employed. That at B is utilized more often as an
inlet cam while the profile shown at C is generally depended on to
operate exhaust valves. The cam shown at D is a composite form which has
some of the features of the other three types. It will give the quick
opening of form A, the gradual closing of form B, and the time of
maximum valve opening provided by cam profile C.
[Illustration: Fig. 102.--Showing Principal Types of Cam Followers which
Have Received General Application.]
The various types of valve plungers used are shown at Fig. 102. That
shown at A is the simplest form, consisting of a simple cylindrical
member having a rounded end which follows the cam profile. These are
sometimes made of square stock or kept from rotating by means of a key
or pin. A line contact is possible when the plunger is kept from
turning, whereas but a single point bearing is obtained when the plunger
is cylindrical and free to revolve. The plunger shown at A will follow
only cam profiles which have gradual lifts. The plunger shown at B is
left free to revolve in the guide bushing and is provided with a flat
mushroom head which serves as a cam follower. The type shown at C
carries a roller at its lower end and may follow very irregular cam
profiles if abrupt lifts are desired. While forms A and B are the
simplest, that outlined at C in its various forms is more widely used.
Compound plungers are used on the Curtiss OX-2 motors, one inside the
other. The small or inner one works on a cam of conventional design, the
outer plunger follows a profile having a flat spot to permit of a pull
rod action instead of a push rod action. All the methods in which levers
are used to operate valves are more or less noisy because clearance must
be left between the valve stem and the stop of the plunger. The space
must be taken up before the valve will leave its seat, and when the
engine is operated at high speeds the forcible contact between the
plunger and valve stem produces a rattling sound until the valves become
heated and expand and the stems lengthen out. Clearance must be left
between the valve stems and actuating means. This clearance is clearly
shown in Fig. 103 and should be .020" (twenty thousandths) when engine
is cold. The amount of clearance allowed depends entirely upon the
design of the engine and length of valve stem. On the Curtiss OX-2
engines the clearance is but .010" (ten thousandths) because the valve
stems are shorter. Too little clearance will result in loss of power or
misfiring when engine is hot. Too much clearance will not allow the
valve to open its full amount and will disturb the timing.
[Illustration: Fig. 103.--Diagram Showing Proper Clearance to Allow
Between Adjusting Screw and Valve Stems in Hall-Scott Aviation Engines.]
METHODS OF DRIVING CAM-SHAFT
Two systems of cam-shaft operation are used. The most common of these is
by means of gearing of some form. If the cam-shaft is at right angles to
the crank-shaft it may be driven by worm, spiral, or bevel gearing. If
the cam-shaft is parallel to the crank-shaft, simple spur gear or chain
connection may be used to turn it. A typical cam-shaft for an
eight-cylinder V engine is shown at Fig. 104. It will be seen that the
sixteen cams are forged integrally with the shaft and that it is
spur-gear driven. The cam-shaft drive of the Hall-Scott motor is shown
at Fig. 97.
[Illustration: Fig. 104.--Cam-Shaft of Thomas Airplane Motor Has Cams
Forged Integral. Note Split Cam-Shaft Bearings and Method of Gear
Retention.]
While gearing is more commonly used, considerable attention has been
directed of late to silent chains for cam-shaft operation. The ordinary
forms of block or roller chain have not proven successful in this
application, but the silent chain, which is in reality a link belt
operating over toothed pulleys, has demonstrated its worth. The tendency
to its use is more noted on foreign motors than those of American
design. It first came to public notice when employed on the
Daimler-Knight engine for driving the small auxiliary crank-shafts which
reciprocated the sleeve valves. The advantages cited for the application
of chains are, first, silent operation, which obtains even after the
chains have worn considerably; second, in designing it is not necessary
to figure on maintaining certain absolute center distances between the
crank-shaft and cam-shaft sprockets, as would be the case if
conventional forms of gearing were used. On some forms of motor
employing gears, three and even four members are needed to turn the
cam-shaft. With a chain drive but two sprockets are necessary, the chain
forming a flexible connection which permits the driving and driven
members to be placed at any distance apart that the exigencies of the
design demand. When chains are used it is advised that some means for
compensating chain slack be provided, or the valve timing will lag when
chains are worn. Many combination drives may be worked out with chains
that would not be possible with other forms of gearing. Direct gear
drive is favored at the present time by airplane engine designers
because they are the most certain and positive means, even when a number
of gears must be used as intermediate drive members. With overhead
cam-shafts, bevel gears work out very well in practice, as in the
Hall-Scott motors and others of that type.
VALVE SPRINGS
[Illustration: Fig. 105.--Section Through Cylinder of Knight Motor,
Showing Important Parts of Valve Motion.]
Another consideration of importance is the use of proper valve-springs,
and particular care should be taken with those, of automatic valves. The
spring must be weak enough to allow the valve to open when the suction
is light, and must be of sufficient strength to close it in time at high
speeds. It should be made as large as possible in diameter and with a
large number of convolutions, in order that fatigue of the metal be
obviated, and it is imperative that all springs be of the same strength
when used on a multiple-cylinder engine. Practically all valves used to
control the gas flow in airplane engines are mechanically operated. On
the exhaust valve the spring must be strong enough so that the valve
will not be sucked in on the inlet stroke. It should be borne in mind
that if the spring is too strong a strain will be imposed on the
valve-operating mechanism, and a hammering action produced which may
cause deformation of the valve-seat. Only pressure enough to insure that
the operating mechanism will follow the cam is required. It is common
practice to make the inlet and exhaust valve springs of the same
tension when the valves are of the same size and both mechanically
operated. This is done merely to simplify manufacture and not because it
is necessary for the inlet valve-spring to be as strong as the other.
Valve springs of the helical coil type are generally used, though
torsion or "scissors" springs and laminated or single-leaf springs are
also utilized in special applications. Two springs are used on each
valve in some valve-in-the-head types; a spring of small pitch diameter
inside the regular valve-spring and concentric with it. Its function is
to keep the valve from falling into the cylinder in event of breakage of
the main spring in some cases, and to provide a stronger return action
in others.
[Illustration: Fig. 106.--Diagrams Showing Knight Sleeve Valve Action.]
KNIGHT SLIDE VALVE MOTOR
The sectional view through the cylinder at Fig. 105 shows the Knight
sliding sleeves and their actuating means very clearly. The diagrams at
Fig. 106 show graphically the sleeve movements and their relation to the
crank-shaft and piston travel. The action may be summed up as follows:
The inlet port begins to open when the lower edge of the opening of the
outside sleeve which is moving down passes the top of the slot in the
inner member also moving downwardly. The inlet port is closed when the
lower edge of the slot in the inner sleeve which is moving up passes the
top edge of the port in the outer sleeve which is also moving toward the
top of the cylinder. The inlet opening extends over two hundred degrees
of crank motion. The exhaust port is uncovered slightly when the lower
edge of the port in the inner sleeve which is moving down passes the
lower edge of the portion of the cylinder head which protrudes in the
cylinder. When the top of the port in the outer sleeve traveling toward
the bottom of the cylinder passes the lower edge of the slot in the
cylinder wall the exhaust passage is closed. The exhaust opening extends
over a period corresponding to about two hundred and forty degrees of
crank motion. The Knight motor has not been applied to aircraft to the
writer's knowledge, but an eight-cylinder Vee design that might be
useful in that connection if lightened is shown at Fig. 107. The main
object is to show that the Knight valve action is the only other besides
the mushroom or poppet valve that has been applied successfully to high
speed gasoline engines.
VALVE TIMING
It is in valve timing that the greatest difference of opinion prevails
among engineers, and it is rare that one will see the same formula in
different motors. It is true that the same timing could not be used with
motors of different construction, as there are many factors which
determine the amount of lead to be given to the valves. The most
important of these is the relative size of the valve to the cylinder
bore, the speed of rotation it is desired to obtain, the fuel
efficiency, the location of the valves, and other factors too numerous
to mention.
[Illustration: Fig. 107.--Cross Sectional View of Knight Type Eight
Cylinder V Engine.]
Most of the readers should be familiar with the cycle of operation of
the internal combustion motor of the four-stroke type, and it seems
unnecessary to go into detail except to present a review. The first
stroke of the piston is one in which a charge of gas is taken into the
motor; the second stroke, which is in reverse direction to the first, is
a compression stroke, at the end of which the spark takes place,
exploding the charge and driving the piston down on the third or
expansion stroke, which is in the same direction as the intake stroke,
and finally, after the piston has nearly reached the end of this stroke,
another valve opens to allow the burned gases to escape, and remains
open until the piston has reached the end of the fourth stroke and is in
a position to begin the series over again. The ends of the strokes are
reached when the piston comes to a stop at either top or bottom of the
cylinder and reverses its motion. That point is known as a center, and
there are two for each cylinder, top and bottom centers, respectively.
All circles may be divided into 360 parts, each of which is known as a
degree, and, in turn, each of these degrees may be again divided into
minutes and seconds, though we need not concern ourselves with anything
less than the degree. Each stroke of the piston represents 180 degrees
travel of the crank, because two strokes represent one complete
revolution of three hundred and sixty degrees. The top and bottom
centers are therefore separated by 180 degrees. Theoretically each phase
of a four-cycle engine begins and ends at a center, though in actual
practice the inertia or movement of the gases makes it necessary to
allow a lead or lag to the valve, as the case may be. If a valve opens
before a center, the distance is called "lead"; if it closes after a
center, this distance is known as "lag." The profile of the cams
ordinarily used to open or close the valves represents a considerable
time in relation to the 180 degrees of the crank-shaft travel, and the
area of the passages through which the gases are admitted or exhausted
is quite small owing to the necessity of having to open or close the
valves at stated times; therefore, to open an adequately large passage
for the gases it is necessary to open the valves earlier and close them
later than at centers.
That advancing the opening of the exhaust valve was of value was
discovered on the early motors and is explained by the necessity of
releasing a large amount of gas, the volume of which has been greatly
raised by the heat of combustion. When the inlet valves were
mechanically operated it was found that allowing them to lag at closing
enabled the inspiration of a greater volume of gas. Disregarding the
inertia or flow of the gases, opening the exhaust at center would enable
one to obtain full value of the expanding gases the entire length of the
piston stroke, and it would not be necessary to keep the valve open
after the top center, as the reverse stroke would produce a suction
effect which might draw some of the inert charge back into the cylinder.
On the other hand, giving full consideration to the inertia of the gas,
opening the valve before center is reached will provide for quick
expulsion of the gases, which have sufficient velocity at the end of the
stroke, so that if the valve is allowed to remain open a little longer,
the amount of lag varying with the opinions of the designer, the
cylinder is cleared in a more thorough manner.
BLOWING BACK
When the factor of retarded opening is considered without reckoning the
inertia of the gases, it would appear that if the valve were allowed to
remain open after center had passed, say, on the closing of the inlet,
the piston, having reversed its motion, would have the effect of
expelling part of the fresh charge through the still open valve as it
passed inward at its compression stroke. This effect is called blowing
back, and is often noted with motors where the valve settings are not
absolutely correct, or where the valve-springs or seats are defective
and prevent proper closing.
This factor is not of as much import as might appear, as on closer
consideration it will be seen that the movement of the piston as the
crank reaches either end of the stroke is less per degree of angular
movement than it is when the angle of the connecting rod is greater.
Then, again, a certain length of time is required for the reversal of
motion of the piston, during which time the crank is in motion but the
piston practically at a standstill. If the valves are allowed to remain
open during this period, the passage of the gas in or out of the
cylinder will be by its own momentum.
LEAD GIVEN EXHAUST VALVE
The faster a motor turns, all other things being equal, the greater the
amount of lead or advance it is necessary to give the opening of the
exhaust valve. It is self-evident truth that if the speed of a motor is
doubled it travels twice as many degrees in the time necessary to lower
the pressure. As most designers are cognizant of this fact, the valves
are proportioned accordingly. It is well to consider in this respect
that the cam profile has much to do with the manner in which the valve
is opened; that is, the lift may be abrupt and the gas allowed to escape
in a body, or the opening may be gradual, the gas issuing from the
cylinder in thin streams. An analogy may be made with the opening of any
bottle which contains liquid highly carbonated. If the cork is removed
suddenly the gas escapes with a loud pop, but, on the other hand, if the
bottle is uncorked gradually, the gas escapes from the receptacle in
thin streams around the cork, and passage of the gases to the air is
accomplished without noise. While the second plan is not harsh, it is
slower than the former, as must be evident.
EXHAUST CLOSING, INLET OPENING
A point which has been much discussed by engineers is the proper
relation of the closing of the exhaust valve and the opening of the
inlet. Theoretically they should succeed each other, the exhaust closing
at upper dead center and the inlet opening immediately afterward. The
reason why a certain amount of lag is given the exhaust closing in
practice is that the piston cannot drive the gases out of the cylinder
unless they are compressed to a degree in excess of that existing in the
manifold or passages, and while toward the end of the stroke this
pressure may be feeble, it is nevertheless indispensable. At the end of
the piston's stroke, as marked by the upper dead center, this
compression still exists, no matter how little it may be, so that if the
exhaust valve is closed and the inlet opened immediately afterward, the
pressure which exists in the cylinder may retard the entrance of the
fresh gas and a certain portion of the inert gas may penetrate into the
manifold. As the piston immediately begins to aspirate, this may not be
serious, but as these gases are drawn back into the cylinder the fresh
charge will be diluted and weakened in value. If the spark-plug is in a
pocket, the points may be surrounded by this weak gas, and the explosion
will not be nearly as energetic as when the ignition spark takes place
in pure mixture.
It is a well-known fact that the exhaust valve should close after dead
center and that a certain amount of lag should be given to opening of
the inlet. The lag given the closing of the exhaust valve should not be
as great as that given the closing of the inlet valve. Assuming that the
excess pressure of the exhaust will equal the depression during
aspiration, the time necessary to complete the emptying of the cylinder
will be proportional to the volume of the gas within it. At the end of
the suction stroke the volume of gas contained in the cylinder is equal
to the cylindrical volume plus the space of the combustion chamber. At
the end of the exhaust stroke the volume is but that of the dead space,
and from one-third to one-fifth its volume before compression. While it
is natural to assume that this excess of burned gas will escape faster
than the fresh gas will enter the cylinder, it will be seen that if the
inlet valve were allowed to lag twenty degrees, the exhaust valve lag
need not be more than five degrees, providing that the capacity of the
combustion chamber was such that the gases occupied one-quarter of their
former volume.
It is evident that no absolute rule can be given, as back pressure will
vary with the design of the valve passages, the manifolds, and the
construction of the muffler. The more direct the opening, the sooner the
valve can be closed and the better the cylinder cleared. Ten degrees
represent an appreciable angle of the crank, and the time required for
the crank to cover this angular motion is not inconsiderable and an
important quantity of the exhaust may escape, but the piston is very
close to the dead center after the distance has been covered.
Before the inlet valve opens there should be a certain depression in the
cylinder, and considerable lag may be allowed before the depression is
appreciable. So far as the volume of fresh gas introduced during the
admission stroke is concerned, this is determined by the displacement of
the piston between the point where the inlet valve opens and the point
of closing, assuming that sufficient gas has been inspired so that an
equilibrium of pressure has been established between the interior of the
cylinder and the outer air. The point of inlet opening varies with
different motors. It would appear that a fair amount of lag would be
fifteen degrees past top center for the inlet opening, as a certain
depression will exist in the cylinder, assuming that the exhaust valve
has closed five or ten degrees after center, and at the same time the
piston has not gone down far enough on its stroke to materially decrease
the amount of gas which will be taken into the cylinder.
CLOSING THE INLET VALVE
As in the case with the other points of opening and closing, there is a
wide diversity of practice as relates to closing the inlet valve. Some
of the designers close this exactly at bottom center, but this practice
cannot be commended, as there is a considerable portion of time, at
least ten or fifteen degrees angular motion of the crank, before the
piston will commence to travel to any extent on its compression stroke.
The gases rushing into the cylinder have considerable velocity, and
unless an equilibrium is obtained between the pressure inside and that
of the atmosphere outside, they will continue to rush into the cylinder
even after the piston ceases to exert any suction effect.
For this reason, if the valve is closed exactly on center, a full charge
may not be inspired into the cylinder, though if the time of closing is
delayed, this momentum or inertia of the gas will be enough to insure
that a maximum charge is taken into the cylinder. The writer considers
that nothing will be gained if the valve is allowed to remain open
longer than twenty degrees, and an analysis of practice in this respect
would seem to confirm this opinion. From that point in the crank
movement the piston travel increases and the compressive effect is
appreciable, and it would appear that a considerable proportion of the
charge might be exhausted into the manifold and carburetor if the valve
were allowed to remain open beyond a point corresponding to twenty
degrees angular movement of the crank.
TIME OF IGNITION
In this country engineers unite in providing a variable time of
ignition, though abroad some difference of opinion is noted on this
point. The practice of advancing the time of ignition, when affected
electrically, was severely condemned by early makers, these maintaining
that it was necessary because of insufficient heat and volume of the
spark, and it was thought that advancing ignition was injurious. The
engineers of to-day appreciate the fact that the heat of the electric
spark, especially when from a mechanical generator of electrical energy,
is the only means by which we can obtain practically instantaneous
explosion, as required by the operation of motors at high speeds, and
for the combustion of large volumes of gas.
[Illustration: Fig. 108.--Diagrams Explaining Valve and Ignition Timing
of Hall-Scott Aviation Engine.]
It is apparent that a motor with a fixed point of ignition is not as
desirable, in every way, as one in which the ignition can be advanced to
best meet different requirements, and the writer does not readily
perceive any advantage outside of simplicity of control in establishing
a fixed point of ignition. In fact, there seems to be some difference of
opinion among those designers who favor fixed ignition, and in one case
this is located forty-three degrees ahead of center, and in another
motor the point is fixed at twenty degrees, so that it may be said that
this will vary as much as one hundred per cent. in various forms. This
point will vary with different methods of ignition, as well as the
location of the spark-plug or igniter. For the sake of simplicity, most
airplane engines use set spark; if an advancing and retarding mechanism
is fitted, it is only to facilitate starting, as the spark is kept
advanced while in flight, and control is by throttle alone.
[Illustration: Fig. 109.--Timing Diagram of Typical Six-Cylinder
Engine.]
It is obvious by consideration of the foregoing that there can be no
arbitrary rules established for timing, because of the many conditions
which determine the best times for opening and closing the valves. It is
customary to try various settings when a new motor is designed until the
most satisfactory points are determined, and the setting which will be
very suitable for one motor is not always right for one of different
design. The timing diagram shown at Fig. 108 applies to the Hall-Scott
engine, and may be considered typical. It should be easily followed in
view of the very complete explanation given in preceding pages. Another
six-cylinder engine diagram is shown at Fig. 109, and an eight-cylinder
timing diagram is shown at Fig. 110. In timing automobile engines no
trouble is experienced, because timing marks are always indicated on
the engine fly-wheel register with an indicating trammel on the
crank-case. To time an airplane engine accurately, as is necessary to
test for a suspected cam-shaft defect, a timing disc of aluminum is
attached to the crank-shaft which has the timing marks indicated
thereon. If the disc is made 10 or 12 inches in diameter, it may be
divided into degrees without difficulty.
[Illustration: Fig. 110.--Timing Diagram of Typical Eight-Cylinder V
Engine.]
HOW AN ENGINE IS TIMED
In timing a motor from the marks on the timing disc rim it is necessary
to regulate the valves of but one cylinder at a time. Assuming that the
disc is revolving in the direction of engine rotation, and that the
firing order of the cylinders is 1-3-4-2, the operation of timing would
be carried on as follows: The crank-shaft would be revolved until the
line marked "Exhaust opens 1 and 4" registered with the trammel on the
motor bed. At this point the exhaust-valve of either cylinder No. 1 or
No. 4 should begin to open. This can be easily determined by noting
which of these cylinders holds the compressed charge ready for ignition.
Assuming that the spark has occurred in cylinder No. 1, then when the
fly-wheel is turned from the position to that in which the line marked
"Exhaust opens 1 and 4" coincides with the trammel point, the
valve-plunger under the exhaust-valve of cylinder No. 1 should be
adjusted in such a way that there is no clearance between it and the
valve stem. Further movement of the wheel in the same direction should
produce a lift of the exhaust valve. The disc is turned about two
hundred and twenty-five degrees, or a little less than three-quarters of
a revolution; then the line marked "Exhaust closes 1 and 4" will
register with the trammel point. At this period the valve-plunger and
the valve-stem should separate and a certain amount of clearance obtain
between them. The next cylinder to time would be No. 3. The crank-shaft
is rotated until mark "Exhaust opens 2 and 3" comes in line with the
trammel. At this point the exhaust valve of cylinder No. 3 should be
just about opening. The closing is determined by rotating the shaft
until the line "Exhaust closes 2 and 3" comes under the trammel.
This operation is carried on with all the cylinders, it being well to
remember that but one cylinder is working at a time and that a
half-revolution of the fly-wheel corresponds to a full working stroke of
all the cylinders, and that while one is exhausting the others are
respectively taking in a new charge, compressing and exploding. For
instance, if cylinder No. 1 has just completed its power-stroke, the
piston in cylinder No. 3 has reached the point where the gas may be
ignited to advantage. The piston of cylinder No. 4, which is next to
fire, is at the bottom of its stroke and will have inspired a charge,
while cylinder No. 2, which is the last to fire, will have just finished
expelling a charge of burned gas, and will be starting the intake
stroke. This timing relates to a four-cylinder engine in order to
simplify the explanation. The timing instructions given apply only to
the conventional motor types. Rotary cylinder engines, especially the
Gnome "monosoupape," have a distinctive valve timing on account of the
peculiarities of design.
GNOME "MONOSOUPAPE" VALVE TIMING
In the present design of the Gnome motor, a cycle of operations somewhat
different from that employed in the ordinary four-cycle engine is made
use of, says a writer in "The Automobile," in describing the action of
this power-plant. This cycle does away with the need for the usual inlet
valve and makes the engine operable with only a single valve, hence the
name _monosoupape_, or "single-valve." The cycle is as follows: A charge
being compressed in the outer end of the cylinder or combustion chamber,
it is ignited by a spark produced by the spark-plug located in the side
of this chamber, and the burning charge expands as the piston moves down
in the cylinder while the latter revolves around the crank-shaft. When
the piston is about half-way down on the power stroke, the exhaust
valve, which is located in the center of the cylinder-head, is
mechanically opened, and during the following upstroke of the piston the
burnt gases are expelled from the cylinder through the exhaust valve
directly into the atmosphere.
Instead of closing at the end of the exhaust stroke, or a few degrees
thereafter, the exhaust valve is held open for about two-thirds of the
following inlet stroke of the piston, with the result that fresh air is
drawn through the exhaust valve into the cylinder. When the cylinder is
still 65 degrees from the end of the inlet half-revolution, the exhaust
valve closes. As no more air can get into the cylinder, and as the
piston continues to move inwardly, it is obvious that a partial vacuum
is formed.
When the cylinder approaches within 20 degrees of the end of the inlet
half-revolution a series of small inlet ports all around the
circumference of the cylinder wall is uncovered by the top edge of the
piston, whereby the combustion chamber is placed in communication with
the crank chamber. As the pressure in the crank chamber is substantially
atmospheric and that in the combustion chamber is below atmospheric,
there results a suction effect which causes the air from the crank
chamber to flow into the combustion chamber. The air in the crank
chamber is heavily charged with gasoline vapor, which is due to the fact
that a spray nozzle connected with the gasoline supply tank is located
inside the chamber. The proportion of gasoline vapor in the air in the
crank chamber is several times as great as in the ordinary combustible
mixture drawn from a carburetor into the cylinder. This extra-rich
mixture is diluted in the combustion chamber with the air which entered
it through the exhaust valve during the first part of the inlet stroke,
thus forming a mixture of the proper proportion for complete combustion.
The inlet ports in the cylinder wall remain open until 20 degrees of
the compression half-revolution has been completed, and from that moment
to near the end of the compression stroke the gases are compressed in
the cylinder. Near the end of the stroke ignition takes place and this
completes the cycle.
The exact timing of the different phases of the cycle is shown in the
diagram at Fig. 111. It will be seen that ignition occurs substantially
20 degrees ahead of the outer dead center, and expansion of the burning
gases continues until 85 degrees past the outer dead center, when the
piston is a little past half-stroke. Then the exhaust-valve opens and
remains open for somewhat more than a complete revolution of the
cylinders, or, to be exact, for 390 degrees of cylinder travel, until
115 degrees past the top dead center on the second revolution. Then for
45 degrees of travel the charge within the cylinder is expanded,
whereupon the inlet ports are uncovered and remain open for 40 degrees
of cylinder travel, 20 degrees on each side of the inward dead center
position.
SPRINGLESS VALVES
Springless valves are the latest development on French racing car
engines, and it is possible that the positively-operated types will be
introduced on aviation engines also. Two makes of positively-actuated
valves are shown at Fig. 112. The positive-valve motor differs from the
conventional form by having no necessity for valve-springs, as a cam not
only assures the opening of the valve, but also causes it to return to
the valve-seat. In this respect it is much like the sleeve-valve motor,
where the uncovering of the ports is absolutely positive. The cars
equipped with these valves were a success in long-distance auto races.
Claims made for this type of valve mechanism include the possibility of
a higher number of revolutions and consequently greater engine power.
With the spring-controlled, single-cam operated valve a point is reached
where the spring is not capable of returning the valve to its seat
before the cam has again begun its opening movement. It is possible to
extend the limits considerably by using a light valve on a strong
spring, but the valve still remains a limiting factor in the speed of
the motor.
[Illustration: Fig. 111.--Timing Diagram Showing Peculiar Valve Timing
of Gnome "Monosoupape" Rotary Motor.]
A part sectional view through a cylinder of an engine designed by G.
Michaux is shown at Fig. 112, A. There are two valves per cylinder,
inclined at about ten degrees from the vertical. The valve-stems are of
large diameter, as owing to positive control, there is no necessity of
lightening this part in an unusual degree. A single overhead cam-shaft
has eight pairs of cams, which are shown in detail at B. For each valve
there is a three-armed rocker, one arm of which is connected to the stem
of the valve and the two others are in contact respectively with the
opening and closing cams. The connection to the end of the valve-stem is
made by a short connecting link, which is screwed on to the end of the
valve-stem and locked in position. This allows some adjustment to be
made between the valves and the actuating rocker. It will be evident
that one cam and one rocker arm produce the opening of the valve and
that the corresponding rocker arm and cam result in the closing of the
valve. If the opening cam has the usual convex profile, the closing cam
has a correspondingly concave profile. It will be noticed that a light
valve-spring is shown in drawing. This is provided to give a final
seating to its valve after it has been closed by the cam. This is not
absolutely necessary, as an engine has been run successfully without
these springs. The whole mechanism is contained within an overhead
aluminum cover.
[Illustration: Fig. 112.--Two Methods of Operating Valves by Positive
Cam Mechanism Which Closes as Well as Opens Them.]
The positive-valve system used on the De Lage motor is shown at D. In
this the valves are actuated as shown in sectional views D and E. The
valve system is unique in that four valves are provided per cylinder,
two for exhaust and two for intake. The valves are mounted side by side,
as shown at E, so the double actuator member may be operated by a single
set of cams. The valve-operating member consists of a yoke having guide
bars at the top and bottom. The actuating cam works inside of this yoke.
The usual form of cam acts on the lower portion of the yoke to open the
valve, while the concave cam acts on the upper part to close the valves.
In this design provision is made for expansion of the valve-stems due to
heat, and these are not positively connected to the actuating member. As
shown at E, the valves are held against the seat by short coil springs
at the upper end of the stem. These are very stiff and are only intended
to provide for expansion. A slight space is left between the top of the
valve-stem and the portion of the operating member that bears against
them when the regular profile cam exerts its pressure on the bottom of
the valve-operating mechanism. Another novelty in this motor design is
that the cam-shafts and the valve-operating members are carried in
casing attached above the motor by housing supports in the form of small
steel pillars. The overhead cam-shafts are operated by means of bevel
gearing.
FOUR VALVES PER CYLINDER
[Illustration: Fig. 113.--Diagram Comparing Two Large Valves and Four
Small Ones of Practically the Same Area. Note How Easily Small Valves
are Installed to Open Directly Into the Cylinder.]
Mention has been previously made of the sixteen-valve four-cylinder
Duesenberg motor and its great power output for the piston displacement.
This is made possible by the superior volumetric efficiency of a motor
provided with four valves in each cylinder instead of but two. This
principle was thoroughly tried out in racing automobile motors, and is
especially valuable in permitting of greater speed and power output from
simple four- and six-cylinder engines. On eight- and twelve-cylinder
types, it is doubtful if the resulting complication due to using a very
large number of valves would be worth while. When extremely large valves
are used, as shown in diagram at Fig. 113, it is difficult to have them
open directly into the cylinder, and pockets are sometimes necessary. A
large valve would weigh more than two smaller valves having an area
slightly larger in the aggregate; it would require a stiffer valve
spring on account of its greater weight. A certain amount of metal in
the valve-head is necessary to prevent warping; therefore, the inertia
forces will be greater in the large valve than in the two smaller
valves. As a greater port area is obtained by the use of two valves,
the gases will be drawn into the cylinder or expelled faster than with a
lesser area. Even if the areas are practically the same as in the
diagram at Fig. 113, the smaller valves may have a greater lift without
imposing greater stresses on the valve-operating mechanism and quicker
gas intake and exhaust obtained. The smaller valves are not affected by
heat as much as larger ones are. The quicker gas movements made
possible, as well as reduction of inertia forces, permits of higher
rotative speed, and, consequently, greater power output for a given
piston displacement. The drawings at Fig. 114 show a sixteen-valve motor
of the four-cylinder type that has been designed for automobile racing
purposes, and it is apparent that very slight modifications would make
it suitable for aviation purposes. Part of the efficiency is due to the
reduction of bearing friction by the use of ball bearings, but the
multiple-valve feature is primarily responsible for the excellent
performance.
[Illustration: Fig. 114.--Sectional Views of Sixteen-Valve Four-Cylinder
Automobile Racing Engine That May Have Possibilities for Aviation
Service.]
[Illustration: Fig. 115.--Front View of Curtiss OX-3 Aviation Motor,
Showing Unconventional Valve Action by Concentric Push Rod and Pull
Tube.]