I. THE VERTICAL TYPE
The balloon was but a year old when the brothers Robert, in 1784
attempted propulsion of an aerial vehicle by hand-power, and succeeded,
to a certain extent, since they were able to make progress when there
was only a slight wind to counteract their work. But, as may be easily
understood, the manual power provided gave but a very slow speed, and in
any wind it all the would-be airship became an uncontrolled balloon.
Henson and Stringfellow, with their light steam engines, were first to
attempt conquest of the problem of mechanical propulsion in the
air; their work in this direction is so fully linked up with their
constructed models that it has been outlined in the section dealing
with the development of the aeroplane. But, very shortly after these
two began, there came into the field a Monsieur Henri Giffard, who first
achieved success in the propulsion by mechanical means of dirigible
balloons, for his was the first airship to fly against the wind.
He employed a small steam-engine developing about 3 horse-power and
weighing 350 lbs. with boiler, fitting the whole in a car suspended from
the gas-bag of his dirigible. The propeller which this engine worked
was 11 feet in diameter, and the inventor, who made several flights,
obtained a speed of 6 miles an hour against a slight wind. The power
was not sufficient to render the invention practicable, as the dirigible
could only be used in calm weather, but Giffard was sufficiently
encouraged by his results to get out plans for immense dirigibles,
which through lack of funds he was unable to construct. When, later, his
invention of the steam-injector gave him the means he desired, he became
blind, and in 1882 died, having built but the one famous dirigible.
This appears to have been the only instance of a steam engine being
fitted to a dirigible; the inherent disadvantage of this form of motive
power is that a boiler to generate the steam must be carried, and this,
together with the weight of water and fuel, renders the steam engine
uneconomical in relation to the lift either of plane or gas-bag. Again,
even if the weight could be brought down to a reasonable amount, the
attention required by steam plant renders it undesirable as a motive
power for aircraft when compared with the internal combustion engine.
Maxim, in Artificial and Natural Flight, details the engine which he
constructed for use with his giant experimental flying machine, and his
description is worthy of reproduction since it is that of the only steam
engine besides Giffard's, and apart from those used for the propulsion
of models, designed for driving an aeroplane. 'In 1889,' Maxim says,
'I had my attention drawn to some very thin, strong, and comparatively
cheap tubes which were being made in France, and it was only after I had
seen these tubes that I seriously considered the question of making a
flying machine. I obtained a large quantity of them and found that they
were very light, that they would stand enormously high pressures, and
generate a very large quantity of steam. Upon going into a mathematical
calculation of the whole subject, I found that it would be possible to
make a machine on the aeroplane system, driven by a steam engine, which
would be sufficiently strong to lift itself into the air. I first made
drawings of a steam engine, and a pair of these engines was afterwards
made. These engines are constructed, for the most part, of a very high
grade of cast steel, the cylinders being only 3/32 of an inch thick,
the crank shafts hollow, and every part as strong and light as possible.
They are compound, each having a high-pressure piston with an area of
20 square inches, a low-pressure piston of 50.26 square inches, and a
common stroke of 1 foot. When first finished they were found to weigh
300 lbs. each; but after putting on the oil cups, felting, painting, and
making some slight alterations, the weight was brought up to 320 lbs.
each, or a total of 640 lbs. for the two engines, which have since
developed 362 horsepower with a steam pressure of 320 lbs. per square
inch.'
The result is remarkable, being less than 2 lbs. weight per horse-power,
especially when one considers the state of development to which the
steam engine had attained at the time these experiments were made. The
fining down of the internal combustion engine, which has done so much to
solve the problems of power in relation to weight for use with aircraft,
had not then been begun, and Maxim had nothing to guide him, so far
as work on the part of his predecessors was concerned, save the
experimental engines of Stringfellow, which, being constructed on so
small a scale in comparison with his own, afforded little guidance.
Concerning the factor of power, he says: 'When first designing this
engine, I did not know how much power I might require from it. I thought
that in some cases it might be necessary to allow the high-pressure
steam to enter the low-pressure cylinder direct, but as this would
involve a considerable loss, I constructed a species of injector. This
injector may be so adjusted that when the steam in the boiler rises
above a certain predetermined point, say 300 lbs., to the square inch,
it opens a valve and escapes past the high-pressure cylinder instead of
blowing off at the safety valve. In escaping through this valve, a fall
of about 200 lbs. pressure per square inch is made to do work on the
surrounding steam and drive it forward in the pipe, producing a pressure
on the low-pressure piston considerably higher than the back-pressure on
the high-pressure piston. In this way a portion of the work which would
otherwise be lost is utilised, and it is possible, with an unlimited
supply of steam, to cause the engines to develop an enormous amount of
power.'
With regard to boilers, Maxim writes,
'The first boiler which I made was constructed something on the
Herreshof principle, but instead of having one simple pipe in one very
long coil, I used a series of very small and light pipes, connected in
such a manner that there was a rapid circulation through the whole--the
tubes increasing in size and number as the steam was generated. I
intended that there should be a pressure of about 100 lbs. more on the
feed water end of the series than on the steam end, and I believed that
this difference in pressure would be sufficient to ensure direct and
positive circulation through every tube in the series. The first boiler
was exceedingly light, but the workmanship, as far as putting the tubes
together was concerned, was very bad, and it was found impossible to so
adjust the supply of water as to make dry steam without overheating and
destroying the tubes.
'Before making another boiler I obtained a quantity of copper tubes,
about 8 feet long, 3/8 inch external diameter, and 1/50 of an inch
thick. I subjected about 100 of these tubes to an internal pressure of
1 ton per square inch of cold kerosene oil, and as none of them leaked
I did not test any more, but commenced my experiments by placing some
of them in a white-hot petroleum fire. I found that I could evaporate
as much as 26 1/2 lbs. of water per square foot of heating surface per
hour, and that with a forced circulation, although the quantity of water
passing was very small but positive, there was no danger of overheating.
I conducted many experiments with a pressure of over 400 lbs. per square
inch, but none of the tubes failed. I then mounted a single tube in a
white-hot furnace, also with a water circulation, and found that it only
burst under steam at a pressure of 1,650 lbs. per square inch. A large
boiler, having about 800 square feet of heating surface, including the
feed-water heater, was then constructed. This boiler is about 4 1/2 feet
wide at the bottom, 8 feet long and 6 feet high. It weighs, with the
casing, the dome, and the smoke stack and connections, a little less
than 1,000 lbs. The water first passes through a system of small
tubes--1/4 inch in diameter and 1/60 inch thick--which were placed at
the top of the boiler and immediately over the large tubes.... This
feed-water heater is found to be very effective. It utilises the heat
of the products of combustion after they have passed through the boiler
proper and greatly reduces their temperature, while the feed-water
enters the boiler at a temperature of about 250 F. A forced circulation
is maintained in the boiler, the feed-water entering through a spring
valve, the spring valve being adjusted in such a manner that the
pressure on the water is always 30 lbs. per square inch in excess of
the boiler pressure. This fall of 30 lbs. in pressure acts upon the
surrounding hot water which has already passed through the tubes, and
drives it down through a vertical outside tube, thus ensuring a positive
and rapid circulation through all the tubes. This apparatus is found to
act extremely well.'
Thus Maxim, who with this engine as power for his large aeroplane
achieved free flight once, as a matter of experiment, though for what
distance or time the machine was actually off the ground is matter for
debate, since it only got free by tearing up the rails which were to
have held it down in the experiment. Here, however, was a steam engine
which was practicable for use in the air, obviously, and only the rapid
success of the internal combustion engine prevented the steam-producing
type from being developed toward perfection.
The first designers of internal combustion engines, knowing nothing
of the petrol of these days, constructed their examples with a view to
using gas as fuel. As far back as 1872 Herr Paul Haenlein obtained a
speed of about 10 miles an hour with a balloon propelled by an internal
combustion engine, of which the fuel was gas obtained from the balloon
itself. The engine in this case was of the Lenoir type, developing
some 6 horse-power, and, obviously, Haenlein's flights were purely
experimental and of short duration, since he used the gas that sustained
him and decreased the lifting power of his balloon with every stroke of
the piston of his engine. No further progress appears to have been made
with the gas-consuming type of internal combustion engine for work
with aircraft; this type has the disadvantage of requiring either a
gas-producer or a large storage capacity for the gas, either of which
makes the total weight of the power plant much greater than that of
a petrol engine. The latter type also requires less attention when
working, and the fuel is more convenient both for carrying and in the
matter of carburation.
The first airship propelled by the present-day type of internal
combustion engine was constructed by Baumgarten and Wolfert in 1879
at Leipzig, the engine being made by Daimler with a view to working on
benzine--petrol as a fuel had not then come to its own. The construction
of this engine is interesting since it was one of the first of Daimler's
make, and it was the development brought about by the experimental
series of which this engine was one that led to the success of the
motor-car in very few years, incidentally leading to that fining down of
the internal combustion engine which has facilitated the development
of the aeroplane with such remarkable rapidity. Owing to the faulty
construction of the airship no useful information was obtained from
Daimler's pioneer installation, as the vessel got out of control
immediately after it was first launched for flight, and was wrecked.
Subsequent attempts at mechanically-propelled flight by Wolfert ended,
in 1897, in the balloon being set on fire by an explosion of benzine
vapour, resulting in the death of both the aeronauts.
Daimler, from 1882 onward, devoted his attention to the perfecting of
the small, high-speed petrol engine for motor-car work, and owing to
his efforts, together with those of other pioneer engine-builders, the
motorcar was made a success. In a few years the weight of this type of
engine was reduced from near on a hundred pounds per horse-power to less
than a tenth of that weight, but considerable further improvement had to
be made before an engine suitable for use with aircraft was evolved.
The increase in power of the engines fitted to airships has made
steady progress from the outset; Haenlein's engine developed about 6
horse-power; the Santos-Dumont airship of 1898 was propelled by a motor
of 4 horse-power; in 1902 the Lebaudy airship was fitted with an engine
of 40 horse-power, while, in 1910, the Lebaudy brothers fitted an
engine of nearly 300 horsepower to the airship they were then
constructing--1,400 horse-power was common in the airships of the War
period, and the later British rigids developed yet more.
Before passing on to consideration of the petrol-driven type of engine,
it is necessary to accord brief mention to the dirigible constructed in
1884 by Gaston and Albert Tissandier, who at Grenelle, France, achieved
a directed flight in a wind of 8 miles an hour, obtaining their power
for the propeller from 1 1/3 horse-power Siemens electric motor, which
weighed 121 lbs. and took its current from a bichromate battery weighing
496 lbs. A two-bladed propeller, 9 feet in diameter, was used, and
the horse-power output was estimated to have run up to 1 1/2 as the
dirigible successfully described a semicircle in a wind of 8 miles an
hour, subsequently making headway transversely to a wind of 7 miles
an hour. The dirigible with which this motor was used was of the
conventional pointed-end type, with a length of 92 feet, diameter of 30
feet, and capacity of 37,440 cubic feet of gas. Commandant Renard, of
the French army balloon corps, followed up Tissandier's attempt in
the next year--1885--making a trip from Chalais-Meudon to Paris and
returning to the point of departure quite successfully. In this case the
motive power was derived from an electric plant of the type used by
the Tissandiers, weighing altogether 1,174 lbs., and developing
9 horsepower. A speed of 14 miles an hour was attained with this
dirigible, which had a length of 165 feet, diameter of 27 feet, and
capacity of 65,836 cubic feet of gas.
Reverting to the petrol-fed type again, it is to be noted that
Santos-Dumont was practically the first to develop the use of the
ordinary automobile engine for air work--his work is of such importance
that it has been considered best to treat of it as one whole, and
details of the power plants are included in the account of his
experiments. Coming to the Lebaudy brothers and their work, their engine
of 1902 was a 40 horse-power Daimler, four-cylindered; it was virtually
a large edition of the Daimler car engine, the arrangement of the
various details being on the lines usually adopted for the standard
Daimler type of that period. The cylinders were fully water-jacketed,
and no special attempt toward securing lightness for air work appears to
have been made.
The fining down of detail that brought weight to such limits as would
fit the engine for work with heavier-than-air craft appears to have
waited for the brothers Wright. Toward the end of 1903 they fitted
to their first practicable flying machine the engine which made the
historic first aeroplane flight; this engine developed 30 horse-power,
and weighed only about 7 lbs. per horse-power developed, its design and
workmanship being far ahead of any previous design in this respect, with
the exception of the remarkable engine, designed by Manly, installed in
Langley's ill-fated aeroplane--or 'aerodrome,' as he preferred to call
it--tried in 1903.
The light weight of the Wright brothers' engine did not necessitate a
high number of revolutions per minute to get the requisite power; the
speed was only 1,300 revolutions per minute, which, with a piston
stroke of 3.94 inches, was quite moderate. Four cylinders were used,
the cylinder diameter being 4.42 inches; the engine was of the
vertical type, arranged to drive two propellers at a rate of about 350
revolutions per minute, gearing being accomplished by means of chain
drive from crank-shaft end to propeller spindle.
The methods adopted by the Wrights for obtaining a light-weight engine
were of considerable interest, in view of the fact that the honour
of first achieving flight by means of the driven plane belongs to
them--unless Ader actually flew as he claimed. The cylinders of this
first Wright engine were separate castings of steel, and only the
barrels were jacketed, this being done by fixing loose, thin aluminium
covers round the outside of each cylinder. The combustion head and valve
pockets were cast together with the cylinder barrel, and were not water
cooled. The inlet valves were of the automatic type, arranged on the
tops of the cylinders, while the exhaust valves were also overhead,
operated by rockers and push-rods. The pistons and piston rings were
of the ordinary type, made of cast-iron, and the connecting rods were
circular in form, with a hole drilled down the middle of each to reduce
the weight.
Necessity for increasing power and ever lighter weight in relation to
the power produced has led to the evolution of a number of different
designs of internal combustion engines. It was quickly realised that
increasing the number of cylinders on an engine was a better way of
getting more power than that of increasing the cylinder diameter, as the
greater number of cylinders gives better torque-even turning effect--as
well as keeping down the weight--this latter because the bigger
cylinders must be more stoutly constructed than the small sizes; this
fact has led to the construction of engines having as many as eighteen
cylinders, arranged in three parallel rows in order to keep the length
of crankshaft within reasonable limits. The aero engine of to-day may,
roughly, be divided into four classes: these are the V type, in which
two rows of cylinders are set parallel at a certain angle to each other;
the radial type, which consists of cylinders arranged radially and
remaining stationary while the crankshaft revolves; the rotary, where
the cylinders are disposed round a common centre and revolve round
a stationary shaft, and the vertical type, of four or six
cylinders--seldom more than this--arranged in one row. A modification of
the V type is the eighteen-cylindered engine--the Sunbeam is one of the
best examples--in which three rows of cylinders are set parallel to each
other, working on a common crankshaft. The development these four types
started with that of the vertical--the simplest of all; the V, radial,
and rotary types came after the vertical, in the order given.
The evolution of the motor-car led to the adoption of the vertical
type of internal combustion engine in preference to any other, and
it followed naturally that vertical engines should be first used for
aeroplane propulsion, as by taking an engine that had been developed to
some extent, and adapting it to its new work, the problem of mechanical
flight was rendered easier than if a totally new type had had to be
evolved. It was quickly realised--by the Wrights, in fact-that the
minimum of weight per horse-power was the prime requirement for the
successful development of heavier-than-air machines, and at the same
time it was equally apparent that the utmost reliability had to be
obtained from the engine, while a third requisite was economy, in order
to reduce the weight of petrol necessary for flight.
Daimler, working steadily toward the improvement of the internal
combustion engine, had made considerable progress by the end of
last century. His two-cylinder engine of 1897 was approaching to
the present-day type, except as regards the method of ignition; the
cylinders had 3.55 inch diameter, with a 4.75 inch piston stroke,
and the engine was rated at 4.5 brake horse-power, though it probably
developed more than this in actual running at its rated speed of 800
revolutions per minute. Power was limited by the inlet and exhaust
passages, which, compared with present-day practice, were very small.
The heavy castings of which the engine was made up are accounted for by
the necessity for considering foundry practice of the time, for in 1897
castings were far below the present-day standard. The crank-case of
this two-cylinder vertical Daimler engine was the only part made of
aluminium, and even with this no attempt was made to attain lightness,
for a circular flange was cast at the bottom to form a stand for the
engine during machining and erection. The general design can be followed
from the sectional views, and these will show, too, that ignition was by
means of a hot tube on the cylinder head, which had to be heated with a
blow-lamp before starting the engine. With all its well known and hated
troubles, at that time tube ignition had an advantage over the magneto,
and the coil and accumulator system, in reliability; sparking plugs,
too, were not so reliable then as they are now. Daimler fitted a very
simple type of carburettor to this engine, consisting only of a float
with a single jet placed in the air passage. It may be said that this
twin-cylindered vertical was the first of the series from which has been
evolved the Mercedes-Daimler car and airship engines, built in sizes up
to and even beyond 240 horse-power.
In 1901 the development of the petrol engine was still so slight that it
did not admit of the construction, by any European maker, of an engine
weighing less than 12 lbs. per horse-power. Manly, working at the
instance of Professor Langley, produced a five-cylindered radial type
engine, in which both the design and workmanship showed a remarkable
advance in construction. At 950 revolutions per minute it developed 52.4
horse-power, weighing only 2.4 pounds per horse-power; it was a very
remarkable achievement in engine design, considering the power developed
in relation to the total weight, and it was, too, an interruption in
the development of the vertical type which showed that there were other
equally great possibilities in design.
In England, the first vertical aero-engine of note was that designed
by Green, the cylinder dimensions being 4.15 inch diameter by 4.75
stroke--a fairly complete idea of this engine can be obtained from the
accompanying diagrams. At a speed of 1,160 revolutions per minute
it developed 35 brake horse-power, and by accelerating up to 1,220
revolutions per minute a maximum of 40 brake horse-power could be
obtained--the first-mentioned was the rated working speed of the engine
for continuous runs. A flywheel, weighing 23.5 lbs., was fitted to the
engine, and this, together with the ignition system, brought the weight
up to 188 lbs., giving 5.4 lbs. per horse-power. In comparison with the
engine fitted to the Wrights' aeroplane a greater power was obtained
from approximately the same cylinder volume, and an appreciable saving
in weight had also been effected. The illustration shows the arrangement
of the vertical valves at the top of the cylinder and the overhead cam
shaft, while the position of the carburettor and inlet pipes can be
also seen. The water jackets were formed by thin copper casings, each
cylinder being separate and having its independent jacket rigidly
fastened to the cylinder at the top only, thus allowing for free
expansion of the casing; the joint at the bottom end was formed by
sliding the jacket over a rubber ring. Each cylinder was bolted to the
crank-case and set out of line with the crankshaft, so that the crank
has passed over the upper dead centre by the time that the piston is at
the top of its stroke when receiving the full force of fuel explosion.
The advantage of this desaxe setting is that the pressure in the
cylinder acts on the crank-pin with a more effective leverage during
that part of the stroke when that pressure is highest, and in addition
the side pressure of the piston on the cylinder wall, due to the thrust
of the connecting rod, is reduced. Possibly the charging of the cylinder
is also more complete by this arrangement, owing to the slower movement
of the piston at the bottom of its stroke allowing time for an increased
charge of mixture to enter the cylinder.
A 60 horse-power engine was also made, having four vertical cylinders,
each with a diameter of 5.5 inches and stroke of 5.75 inches, developing
its rated power at 1,100 revolutions per minute. By accelerating up to
1,200 revolutions per minute 70 brake horsepower could be obtained, and
a maximum of 80 brake horse-power was actually attained with the type.
The flywheel, fitted as with the original 35 horse-power engine, weighed
37 lbs.; with this and with the ignition system the total weight of
the engine was only 250 lbs., or 4.2 lbs. per horse-power at the normal
rating. In this design, however, low weight in relation to power was
not the ruling factor, for Green gave more attention to reliability and
economy of fuel consumption, which latter was approximately 0.6 pint of
petrol per brake horse-power per hour. Both the oil for lubricating
the bearings and the water for cooling the cylinders were circulated by
pumps, and all parts of the valve gear, etc., were completely enclosed
for protection from dust.
A later development of the Green engine was a six-cylindered vertical,
cylinder dimensions being 5.5 inch diameter by 6 inch stroke, developing
120 brake horsepower when running at 1,250 revolutions per minute. The
total weight of the engine with ignition system 398 was 440 lbs., or
3.66 lbs. per horse-power. One of these engines was used on the machine
which, in 1909, won the prize of L1,000 for the first circular mile
flight, and it may be noted, too, that S. F. Cody, making the circuit
of England in 1911, used a four-cylinder Green engine. Again, it was a
Green engine that in 1914 won the L5,000 prize offered for the best aero
engine in the Naval and Military aeroplane engine competition.
Manufacture of the Green engines, in the period of the War, had
standardised to the production of three types. Two of these were
six-cylinder models, giving respectively 100 and 150 brake horse-power,
and the third was a twelve-cylindered model rated at 275 brake
horse-power.
In 1910 J. S. Critchley compiled a list showing the types of engine then
being manufactured; twenty-two out of a total of seventy-six were of the
four-cylindered vertical type, and in addition to these there were two
six-cylindered verticals. The sizes of the four-cylinder types ranged
from 26 up to 118 brake horse-power; fourteen of them developed less
than 50 horse-power, and only two developed over 100 horse-power.
It became apparent, even in the early stages of heavier-than-air flying,
that four-cylinder engines did not produce the even torque that was
required for the rotation of the power shaft, even though a flywheel
was fitted to the engine. With this type of engine the breakage of
air-screws was of frequent occurrence, and an engine having a more
regular rotation was sought, both for this and to avoid the excessive
vibration often experienced with the four-cylinder type. Another, point
that forced itself on engine builders was that the increased power which
was becoming necessary for the propulsion of aircraft made an increase
in the number of cylinders essential, in order to obtain a light engine.
An instance of the weight reduction obtainable in using six cylinders
instead of four is shown in Critchley's list, for one of the
four-cylinder engines developed 118.5 brake horse-power and weighed
1,100 lbs., whereas a six-cylinder engine by the same manufacturer
developed 117.5 brake horse-power with a weight of 880 lbs., the
respective cylinder dimensions being 7.48 diameter by 9.06 stroke
for the four-cylinder engine, and 6.1 diameter by 7.28 stroke for the
six-cylinder type.
A list of aeroplane engines, prepared in 1912 by Graham Clark, showed
that, out of the total number of 112 engines then being manufactured,
forty-two were of the vertical type, and of this number twenty-four had
four-cylinders while sixteen were six-cylindered. The German aeroplane
engine trials were held a year later, and sixty-six engines entered the
competition, fourteen of these being made with air-cooled cylinders.
All of the ten engines that were chosen for the final trials were of the
water-cooled type, and the first place was won by a Benz four-cylinder
vertical engine which developed 102 brake horse-power at 1,288
revolutions per minute. The cylinder dimensions of this engine were 5.1
inch diameter by 7.1 inch stroke, and the weight of the engine worked
out at 3.4 lbs. per brake horse-power. During the trials the full-load
petrol consumption was 0.53 pint per horse-power per hour, and the
amount of lubricating oil used was 0.0385 pint per brake horse-power per
hour. In general construction this Benz engine was somewhat similar to
the Green engine already described; the overhead valves, fitted in the
tops of the cylinders, were similarly arranged, as was the cam-shaft;
two springs were fitted to each of the valves to guard against the
possibility of the engine being put out of action by breakage of one
of the springs, and ignition was obtained by two high-tension magnetos
giving simultaneous sparks in each cylinder by means of two sparking
plugs--this dual ignition reduced the possibility of ignition troubles.
The cylinder jackets were made of welded sheet steel so fitted around
the cylinder that the head was also water-cooled, and the jackets were
corrugated in the middle to admit of independent expansion. Even the
lubrication system was duplicated, two sets of pumps being used, one to
circulate the main supply of lubricating oil, and the other to give a
continuous supply of fresh oil to the bearings, so that if the
supply from one pump failed the other could still maintain effective
lubrication.
Development of the early Daimler type brought about the four-cylinder
vertical Mercedes-Daimler engine of 85 horse-power, with cylinders
of 5.5 diameter with 5.9 inch stroke, the cylinders being cast in two
pairs. The overhead arrangement of valves was adopted, and in later
designs push-rods were eliminated, the overhead cam-shaft being adopted
in their place. By 1914 the four-cylinder Mercedes-Daimler had been
partially displaced from favour by a six-cylindered model, made in two
sizes; the first of these gave a nominal brake horse-power of 80, having
cylinders of 4.1 inches diameter by 5.5 inches stroke; the second type
developed 100 horse-power with cylinders 4.7 inches in diameter and 5.5
inches stroke, both types being run at 1,200 revolutions per minute. The
cylinders of both these types were cast in pairs, and, instead of the
water jackets forming part of the casting, as in the design of the
original four-cylinder Mercedes-Daimler engine, they were made of steel
welded to flanges on the cylinders. Steel pistons, fitted with cast-iron
rings, were used, and the overhead arrangement of valves and cam-shaft
was adopted. About 0.55 pint per brake horse-power per hour was the
usual fuel consumption necessary to full load running, and the engine
was also economical as regards the consumption of lubricating oil,
the lubricating system being 'forced' for all parts, including the
cam-shaft. The shape of these engines was very well suited for work
with aircraft, being narrow enough to admit of a streamline form being
obtained, while all the accessories could be so mounted as to produce
little or no wind resistance, and very little obstruction to the pilot's
view.
The eight-cylinder Mercedes-Daimler engine, used for airship propulsion
during the War, developed 240 brake horse-power at 1,100 revolutions per
minute; the cylinder dimensions were 6.88 diameter by 6.5 stroke--one
of the instances in which the short stroke in relation to bore was very
noticeable.
Other instances of successful vertical design-the types already detailed
are fully sufficient to give particulars of the type generally--are
the Panhard, Chenu, Maybach, N.A.G., Argus, Mulag, and the well-known
Austro-Daimler, which by 1917 was being copied in every combatant
country. There are also the later Wright engines, and in America
the Wisconsin six-cylinder vertical, weighing well under 4 lbs. per
horse-power, is evidence of the progress made with this first type of
aero engine to develop.
II. THE VEE TYPE
An offshoot from the vertical type, doubling the power of this with only
a very slight--if any--increase in the length of crankshaft, the Vee
or diagonal type of aero engine leaped to success through the insistent
demand for greater power. Although the design came after that of the
vertical engine, by 1910, according to Critchley's list of aero engines,
there were more Vee type engines being made than any other type,
twenty-five sizes being given in the list, with an average rating of
57.4 brake horse-power.
The arrangement of the cylinders in Vee form over the crankshaft,
enabling the pistons of each pair of opposite cylinders to act upon the
same crank pin, permits of a very short, compact engine being built, and
also permits of reduction of the weight per horsepower, comparing this
with that of the vertical type of engine, with one row of cylinders.
Further, at the introduction of this type of engine it was seen that
crankshaft vibration, an evil of the early vertical engines, was
practically eliminated, as was the want of longitudinal stiffness that
characterised the higher-powered vertical engines.
Of the Vee type engines shown in Critchley's list in 1910 nineteen
different sizes were constructed with eight cylinders, and with
horse-powers ranging from thirty to just over the hundred; the lightest
of these weighed 2.9 lbs. per horse-power--a considerable advance in
design on the average vertical engine, in this respect of weight per
horse-power. There were also two sixteen-cylinder engines of Vee design,
the larger of which developed 134 horse-power with a weight of only 2
lbs. per brake horse-power. Subsequent developments have indicated that
this type, with the further development from it of the double-Vee, or
engine with three rows of cylinders, is likely to become the standard
design of aero engine where high powers are required. The construction
permits of placing every part so that it is easy of access, and the
form of the engine implies very little head resistance, while it can be
placed on the machine--supposing that machine to be of the single-engine
type--in such a way that the view of the pilot is very little obstructed
while in flight.
An even torque, or great uniformity of rotation, is transmitted to the
air-screw by these engines, while the design also permits of such good
balance of the engine itself that vibration is practically eliminated.
The angle between the two rows of cylinders is varied according to the
number of cylinders, in order to give working impulses at equal angles
of rotation and thus provide even torque; this angle is determined by
dividing the number of degrees in a circle by the number of cylinders
in either row of the engine. In an eight-cylindered Vee type engine, the
angle between the cylinders is 90 degrees; if it is a twelve-cylindered
engine, the angle drops to 60 degrees.
One of the earliest of the British-built Vee type engines was an
eight-cylinder 50 horse-power by the Wolseley Company, constructed in
1908 with a cylinder bore of 3.75 inches and stroke of 5 inches, running
at a normal speed of 1,350 revolutions per minute. With this engine, a
gearing was introduced to enable the propeller to run at a lower speed
than that of the engine, the slight loss of efficiency caused by the
friction of the gearing being compensated by the slower speed of the
air-screw, which had higher efficiency than would have been the case if
it had been run at the engine speed. The ratio of the gearing--that is,
the speed of the air-screw relatively to that of the engine, could be
chosen so as to suit exactly the requirements of the air-screw, and the
gearing itself, on this engine, was accomplished on the half-speed shaft
actuating the valves.
Very soon after this first design had been tried out, a second Vee type
engine was produced which, at 1,200 revolutions per minute, developed 60
horse-power; the size of this engine was practically identical with that
of its forerunner, the only exception being an increase of half an inch
in the cylinder stroke--a very long stroke of piston in relation to
the bore of the cylinder. In the first of these two engines, which was
designed for airship propulsion, the weight had been about 8 lbs. per
brake horse-power, no special attempt appearing to have been made to
fine down for extreme lightness; in this 60 horse-power design, the
weight was reduced to 6.1 lbs. per horse-power, counting the latter
as normally rated; the engine actually gave a maximum of 75 brake
horse-power, reducing the ratio of weight to power very considerably
below the figure given.
The accompanying diagram illustrates a later Wolseley model, end
elevation, the eight-cylindered 120 horse-power Vee type aero engine
of the early war period. With this engine, each crank pin has two
connecting rods bearing on it, these being placed side by side and
connected to the pistons of opposite cylinders and the two cylinders of
the pair are staggered by an amount equal to the width of the connecting
rod bearing, to afford accommodation for the rods. The crankshaft was a
nickel chrome steel forging, machined hollow, with four crank pins set
at 180 degrees to each other, and carried in three bearings lined with
anti-friction metal. The connecting rods were made of tubular nickel
chrome steel, and the pistons of drawn steel, each being fitted with
four piston rings. Of these the two rings nearest to the piston head
were of the ordinary cast-iron type, while the others were of phosphor
bronze, so arranged as to take the side thrust of the piston. The
cylinders were of steel, arranged in two groups or rows of four, the
angular distance between them being 90 degrees. In the space above the
crankshaft, between the cylinder rows, was placed the valve-operating
mechanism, together with the carburettor and ignition system, thus
rendering this a very compact and accessible engine. The combustion
heads of the cylinders were made of cast-iron, screwed into the steel
cylinder barrels; the water-jacket was of spun aluminium, with one end
fitting over the combustion head and the other free to slide on the
cylinder; the water-joint at the lower end was made tight by a Dermatine
ring carried between small flanges formed on the cylinder barrel.
Overhead valves were adopted, and in order to make these as large as
possible the combustion chamber was made slightly larger in diameter
than the cylinder, and the valves set at an angle. Dual ignition was
fitted in each cylinder, coil and accumulator being used for starting
and as a reserve in case of failure of the high-tension magneto system
fitted for normal running. There was a double set of lubricating pumps,
ensuring continuity of the oil supply to all the bearings of the engine.
The feature most noteworthy in connection with the running of this type
of engine was its flexibility; the normal output of power was
obtained with 1,150 revolutions per minute of the crankshaft, but, by
accelerating up to 1,400 revolutions, a maximum of 147 brake horse-power
could be obtained. The weight was about 5 lbs. per horse-power, the
cylinder dimensions being 5 inches bore by 7 inches stroke. Economy in
running was obtained, the fuel consumption being 0.58 pint per brake
horse-power per hour at full load, with an expenditure of about 0.075
pint of lubricating oil per brake horse-power per hour.
Another Wolseley Vee type that was standardised was a 90 horse-power
eight-cylinder engine running at 1,800 revolutions per minute, with
a reducing gear introduced by fitting the air screw on the half-speed
shaft. First made semi-cooled--the exhaust valve was left air-cooled,
and then entirely water-jacketed--this engine demonstrated the advantage
of full water cooling, for under the latter condition the same power was
developed with cylinders a quarter of an inch less in diameter than in
the semi-cooled pattern; at the same time the weight was brought down to
4 1/2 lbs. per horsepower.
A different but equally efficient type of Vee design was the Dorman
engine, of which an end elevation is shown; this developed 80 brake
horse-power at a speed of 1,300 revolutions per minute, with a cylinder
bore of 5 inches; each cylinder was made in cast-iron in one piece with
the combustion chamber, the barrel only being water-jacketed. Auxiliary
exhaust ports were adopted, the holes through the cylinder wall being
uncovered by the piston at the bottom of its stroke--the piston, 4.75
inches in length, was longer than its stroke, so that these ports were
covered when it was at the top of the cylinder. The exhaust discharged
through the ports into a belt surrounding the cylinder, the belts on the
cylinders being connected so that the exhaust gases were taken through
a single pipe. The air was drawn through the crank case, before reaching
the carburettor, this having the effect of cooling the oil in the crank
case as well as warming the air and thus assisting in vaporising the
petrol for each charge of the cylinders. The inlet and exhaust valves
were of the overhead type, as may be gathered from the diagram, and in
spite of cast-iron cylinders being employed a light design was obtained,
the total weight with radiator, piping, and water being only 5.5 lbs.
per horse-power.
Here was the antithesis of the Wolseley type in the matter of bore in
relation to stroke; from about 1907 up to the beginning of the war, and
even later, there was controversy as to which type--that in which the
bore exceeded the stroke, or vice versa--gave greater efficiency.
The short-stroke enthusiasts pointed to the high piston speed of the
long-stroke type, while those who favoured the latter design contended
that full power could not be obtained from each explosion in the
short-stroke type of cylinder. It is now generally conceded that the
long-stroke engine yields higher efficiency, and in addition to this,
so far as car engines are concerned, the method of rating horse-power
in relation to bore without taking stroke into account has given the
long-stroke engine an advantage, actual horse-power with a long stroke
engine being in excess of the nominal rating. This may have had some
influence on aero engine design, but, however this may have been, the
long-stroke engine has gradually come to favour, and its rival has taken
second place.
For some time pride of place among British Vee type engines was held
by the Sunbeam Company, which, owing to the genius of Louis Coatalen,
together with the very high standard of construction maintained by the
firm, achieved records and fame in the middle and later periods of the
war. Their 225 horse-power twelve-cylinder engine ran at a normal
speed of 2,000 revolutions per minute; the air screw was driven through
gearing at half this speed, its shaft being separate from the timing
gear and carried in ball-bearings on the nose-piece of the engine. The
cylinders were of cast-iron, entirely water-cooled; a thin casing formed
the water-jacket, and a very light design was obtained, the weight being
only 3.2 lbs. per horse-power. The first engine of Sunbeam design had
eight cylinders and developed 150 horse-power at 2,000 revolutions
per minute; the final type of Vee design produced during the war was
twelve-cylindered, and yielded 310 horse-power with cylinders 4.3 inches
bore by 6.4 inches stroke. Evidence in favour of the long-stroke engine
is afforded in this type as regards economy of working; under full load,
working at 2,000 revolutions per minute, the consumption was 0.55 pints
of fuel per brake horse-power per hour, which seems to indicate that the
long stroke permitted of full use being made of the power resulting from
each explosion, in spite of the high rate of speed of the piston.
Developing from the Vee type, the eighteen-cylinder 475 brake
horse-power engine, designed during the war, represented for a time
the limit of power obtainable from a single plant. It was water-cooled
throughout, and the ignition to each cylinder was duplicated; this
engine proved fully efficient, and economical in fuel consumption.
It was largely used for seaplane work, where reliability was fully as
necessary as high power.
The abnormal needs of the war period brought many British firms into the
ranks of Vee-type engine-builders, and, apart from those mentioned,
the most notable types produced are the Rolls-Royce and the Napier.
The first mentioned of these firms, previous to 1914 had concentrated
entirely on car engines, and their very high standard of production in
this department of internal combustion engine work led, once they took
up the making of aero engines, to extreme efficiency both of design and
workmanship. The first experimental aero engine, of what became known
as the 'Eagle' type, was of Vee design--it was completed in March
of 1915--and was so successful that it was standardised for quantity
production. How far the original was from the perfection subsequently
ascertained is shown by the steady increase in developed horse-power
of the type; originally designed to develop 200 horse-power, it was
developed and improved before its first practical trial in October of
1915, when it developed 255 horsepower on a brake test. Research
and experiment produced still further improvements, for, without any
enlargement of the dimensions, or radical alteration in design, the
power of the engine was brought up to 266 horse-power by March of 1916,
the rate of revolutions of 1,800 per minute being maintained throughout.
July, 1916 gave 284 horse-power; by the cud of the year this had been
increased to 322 horse-power; by September of 1917 the increase was to
350 horse-power, and by February of 1918 then 'Eagle' type of engine was
rated at 360 horse-power, at which standard it stayed. But there is no
more remarkable development in engine design than this, a 75 per cent
increase of power in the same engine in a period of less than three
years.
To meet the demand for a smaller type of engine for use on training
machines, the Rolls-Royce firm produced the 'Hawk' Vee-type engine of
100 horsepower, and, intermediately between this and the 'Eagle,' the
'Falcon' engine came to being with an original rated horse-power of 205
at 1,800 revolutions per minute, in April of 1916. Here was another case
of growth of power in the same engine through research, almost similar
to that of the 'Eagle' type, for by July of 1918 the 'Falcon' was
developing 285 horse-power with no radical alteration of design.
Finally, in response to the constant demand for increase of power in a
single plant, the Rolls-Royce company designed and produced the 'Condor'
type of engine, which yielded 600 horse-power on its first test in
August of 1918. The cessation of hostilities and consequent falling off
in the demand for extremely high-powered plants prevented the 'Condor'
being developed to its limit, as had been the 'Falcon' and 'Eagle'
types.
The 'Eagle 'engine was fitted to the two Handley-Page aeroplanes--which
made flights from England to India--it was virtually standard on the
Handley-Page bombers of the later War period, though to a certain extent
the American 'Liberty' engine was also used. Its chief record, however,
is that of being the type fitted to the Vickers-Vimy aeroplane which
made the first Atlantic flight, covering the distance of 1,880 miles at
a speed averaging 117 miles an hour.
The Napier Company specialised on one type of engine from the outset,
a power plant which became known as the 'Lion' engine, giving 450
horse-power with twelve cylinders arranged in three rows of four each.
Considering the engine as 'dry,' or without fuel and accessories, an
abnormally light weight per horse-power--only 1.89 lbs.--was attained
when running at the normal rate of revolution. The cylinders and
water-jackets are of steel, and there is fitted a detachable aluminium
cylinder head containing inlet and exhaust valves and valve actuating
mechanism; pistons are of aluminium alloy, and there are two inlet and
two exhaust valves to each cylinder, the whole of the valve mechanism
being enclosed in an oil-tight aluminium case. Connecting rods and
crankshaft are of steel, the latter being machined from a solid steel
forging and carried in five roller bearings and one plain bearing at the
forward end. The front end of the crank-case encloses reduction gear for
the propeller shaft, together with the shaft and bearings. There are
two suction and one pressure type oil pumps driven through gears at
half-engine speed, and two 12 spark magnetos, giving 2 sparks in each
cylinder.
The cylinders are set with the central row vertical, and the two side
rows at angles of 60 degrees each; cylinder bore is 5 1/2 inches, and
stroke 5 1/8 inches; the normal rate of revolution is 1,350 per minute,
and the reducing gear gives one revolution of the propeller shaft to
1.52 revolutions of crankshaft. Fuel consumption is 0.48lbs. of fuel per
brake horse-power hour at full load, and oil consumption is 0.020 lbs.
per brake horsepower hour. The dry weight of the engine, complete with
propeller boss, carburettors, and induction pipes, is 850 lbs., and the
gross weight in running order, with fuel and oil for six hours working,
is 2,671 lbs., exclusive of cooling water.
To this engine belongs an altitude record of 30,500 feet, made at
Martlesham, near Ipswich, on January 2nd, 1919, by Captain Lang, R.A.F.,
the climb being accomplished in 66 minutes 15 seconds. Previous to this,
the altitude record was held by an Italian pilot, who made 25,800 feet
in an hour and 57 minutes in 1916. Lang's climb was stopped through
the pressure of air, at the altitude he reached, being insufficient for
driving the small propellers on the machine which worked the petrol and
oil pumps, or he might have made the height said to have been attained
by Major Schroeder on February 27th, 1920, at Dayton, Ohio. Schroeder
is said to have reached an altitude of 36,020 feet on a Napier biplane,
and, owing to failure of the oxygen supply, to have lost consciousness,
fallen five miles, righted his machine when 2,000 feet in the air, and
alighted successfully. Major Schroeder is an American.
Turning back a little, and considering other than British design of Vee
and double-Vee or 'Broad arrow' type of engine, the Renault firm from
the earliest days devoted considerable attention to the development of
this type, their air-cooled engines having been notable examples from
the earliest days of heavier-than-air machines. In 1910 they were making
three sizes of eight-cylindered Vee-type engines, and by 1915 they had
increased to the manufacture of five sizes, ranging from 25 to 100 brake
horse-power, the largest of the five sizes having twelve cylinders but
still retaining the air-cooled principle. The De Dion firm, also,
made Vee-type engines in 1914, being represented by an 80 horse-power
eight-cylindered engine, air-cooled, and a 150 horse-power, also
of eight cylinders, water-cooled, running at a normal rate of 1,600
revolutions per minute. Another notable example of French construction
was the Panhard and Levassor 100 horse-power eight-cylinder Vee engine,
developing its rated power at 1,500 revolutions per minute, and having
the--for that time--low weight of 4.4 lbs. per horse-power.
American Vee design has followed the British fairly cclosely; the
Curtiss Company produced originally a 75 horse-power eight-cylinder Vee
type running at 1,200 revolutions per minute, supplementing this with
a 170 horse-power engine running at 1,600 revolutions per minute, and
later with a twelve-cylinder model Vee type, developing 300 horse-power
at 1,500 revolutions per minute, with cylinder bore of 5 inches and
stroke of 7 inches. An exceptional type of American design was the Kemp
Vee engine of 80 horse-power in which the cylinders were cooled by a
current of air obtained from a fan at the forward end of the engine.
With cylinders of 4.25 inches bore and 4.75 inches stroke, the rater
power was developed at 1,150 revolutions per minute, and with the engine
complete the weight was only 4.75 lbs. per horse-power.
III. THE RADIAL TYPE
The very first successful design of internal combustion aero engine made
was that of Charles Manly, who built a five-cylinder radial engine in
1901 for use with Langley's 'aerodrome,' as the latter inventor decided
to call what has since become known as the aeroplane. Manly made a
number of experiments, and finally decided on radial design, in which
the cylinders are so rayed round a central crank-pin that the pistons
act successively upon it; by this arrangement a very short and compact
engine is obtained, with a minimum of weight, and a regular crankshaft
rotation and perfect balance of inertia forces.
When Manly designed his radial engine, high speed internal combustion
engines were in their infancy, and the difficulties in construction
can be partly realised when the lack of manufacturing methods for this
high-class engine work, and the lack of experimental data on the various
materials, are taken into account. During its tests, Manly's engine
developed 52.4 brake horsepower at a speed of 950 revolutions per
minute, with the remarkably low weight of only 2.4 lbs. per horsepower;
this latter was increased to 3.6 lbs. when the engine was completed
by the addition of ignition system, radiator, petrol tank, and all
accessories, together with the cooling water for the cylinders.
In Manly's engine, the cylinders were of steel, machined outside and
inside to 1/16 of an inch thickness; on the side of cylinder, at the top
end, the valve chamber was brazed, being machined from a solid forging,
The casing which formed the water-jacket was of sheet steel, 1/50 of an
inch in thickness, and this also was brazed on the cylinder and to
the valve chamber. Automatic inlet valves were fitted, and the exhaust
valves were operated by a cam which had two points, 180 degrees
apart; the cam was rotated in the opposite direction to the engine at
one-quarter engine speed. Ignition was obtained by using a one-spark
coil and vibrator for all cylinders, with a distributor to select
the right cylinder for each spark--this was before the days of the
high-tension magneto and the almost perfect ignition systems that makers
now employ. The scheme of ignition for this engine was originated by
Manly himself, and he also designed the sparking plugs fitted in the
tops of the cylinders. Through fear of trouble resulting if the steel
pistons worked on the steel cylinders, cast iron liners were introduced
in the latter, 1/16 of an inch thick.
The connecting rods of this engine were of virtually the same type as is
employed on nearly all modern radial engines. The rod for one cylinder
had a bearing along the whole of the crank pin, and its end enclosed the
pin; the other four rods had bearings upon the end of the first rod,
and did not touch the crank pin. The accompanying diagram shows this
construction, together with the means employed for securing the ends of
the four rods--the collars were placed in position after the rods had
been put on. The bearings of these rods did not receive any of the
rubbing effect due to the rotation of the crank pin, the rubbing on them
being only that of the small angular displacement of the rods during
each revolution; thus there was no difficulty experienced with the
lubrication.
Another early example of the radial type of engine was the French
Anzani, of which type one was fitted to the machine with which Bleriot
first crossed the English Channel--this was of 25 horse-power. The
earliest Anzani engines were of the three-cylinder fan type, one
cylinder being vertical, and the other two placed at an angle of 72
degrees on each side, as the possibility of over-lubrication of the
bottom cylinders was feared if a regular radial construction were
adopted. In order to overcome the unequal balance of this type, balance
weights were fitted inside the crank case.
The final development of this three-cylinder radial was the 'Y' type of
engine, in which the cylinders were regularly disposed at 120 degrees
apart, the bore was 4.1, stroke 4.7 inches, and the power developed was
30 brake horse-power at 1,300 revolutions per minute.
Critchley's list of aero engines being constructed in 1910 shows twelve
of the radial type, with powers of between 14 and 100 horse-power, and
with from three to ten cylinder--this last is probably the greatest
number of cylinders that can be successfully arranged in circular form.
Of the twelve types of 1910, only two were water-cooled, and it is to be
noted that these two ran at the slowest speeds and had the lowest weight
per horse-power of any.
The Anzani radial was considerably developed special attention being
paid to this type by its makers and by 1914 the Anzani list comprised
seven different sizes of air-cooled radials. Of these the largest had
twenty cylinders, developing 200 brake horse-power--it was virtually
a double radial--and the smallest was the original 30 horse-power
three-cylinder design. A six-cylinder model was formed by a combination
of two groups of three cylinders each, acting upon a double-throw
crankshaft; the two crank pins were set at 180 degrees to each other,
and the cylinder groups were staggered by an amount equal to the
distance between the centres of the crank pins. Ten-cylinder radial
engines are made with two groups of five cylinders acting upon two
crank pins set at 180 degrees to each other, the largest Anzani 'ten'
developed 125 horsepower at 1,200 revolutions per minute, the ten
cylinders being each 4.5 inches in bore with stroke of 5.9 inches, and
the weight of the engine being 3.7 lbs. per horse-power. In the 200
horse-power Anzani radial the cylinders are arranged in four groups of
five each, acting on two crank pins. The bore of the cylinders in this
engine is the same as in the three-cylinder, but the stroke is increased
to 5.5 inches. The rated power is developed at 1,300 revolutions per
minute, and the engine complete weighs 3.4 lbs. per horse-power.
With this 200 horse-power Anzani, a petrol consumption of as low as 0.49
lbs. of fuel per brake horse-power per hour has been obtained, but
the consumption of lubricating oil is compensatingly high, being up to
one-fifth of the fuel used. The cylinders are set desaxe with the
crank shaft, and are of cast-iron, provided with radiating ribs for
air-cooling; they are attached to the crank case by long bolts passing
through bosses at the top of the cylinders, and connected to other bolts
at right angles through the crank case. The tops of the cylinders are
formed flat, and seats for the inlet and exhaust valves are formed on
them. The pistons are cast-iron, fitted with ordinary cast-iron spring
rings. An aluminium crank case is used, being made in two halves
connected together by bolts, which latter also attach the engine to the
frame of the machine. The crankshaft is of nickel steel, made hollow,
and mounted on ball-bearings in such a manner that practically a
combination of ball and plain bearings is obtained; the central web
of the shaft is bent to bring the centres of the crank pins as close
together as possible, leaving only room for the connecting rods, and
the pins are 180 degrees apart. Nickel steel valves of the cone-seated,
poppet type are fitted, the inlet valves being automatic, and those for
the exhaust cam-operated by means of push-rods. With an engine having
such a number of cylinders a very uniform rotation of the crankshaft is
obtained, and in actual running there are always five of the cylinders
giving impulses to the crankshaft at the same time.
An interesting type of pioneer radial engine was the Farcot, in which
the cylinders were arranged in a horizontal plane, with a vertical
crankshaft which operated the air-screw through bevel gearing. This was
an eight-cylinder engine, developing 64 horse-power at 1,200 revolutions
per minute. The R.E.P. type,in the early days, was a 'fan' engine,
but the designer, M. Robert Pelterie, turned from this design to a
seven-cylinder radial, which at 1,100 revolutions per minute gave 95
horse-power. Several makers entered into radial engine development in
the years immediately preceding the War, and in 1914 there were
some twenty-two different sizes and types, ranging from 30 to 600
horse-power, being made, according to report; the actual construction of
the latter size at this time, however, is doubtful.
Probably the best example of radial construction up to the outbreak of
War was the Salmson (Canton-Unne) water-cooled, of which in 1914
six sizes were listed as available. Of these the smallest was a
seven-cylinder 90 horse-power engine, and the largest, rated at 600
horse-power, had eighteen cylinders. These engines, during the War, were
made under license by the Dudbridge Ironworks in Great Britain.
The accompanying diagram shows the construction of the cylinders in the
200 horse-power size, showing the method of cooling, and the arrangement
of the connecting rods. A patent planetary gear, also shown in the
diagram, gives exactly the same stroke to all the pistons. The complete
engine has fourteen cylinders, of forged steel machined all over, and
so secured to the crank case that any one can be removed without parting
the crank case. The water-jackets are of spun copper, brazed on to the
cylinder, and corrugated so as to admit of free expansion; the water is
circulated by means of a centrifugal pump. The pistons are of cast-iron,
each fitted with three rings, and the connecting rods are of high grade
steel, machined all over and fitted with bushes of phosphor bronze;
these rods are connected to a central collar, carried on the crank pin
by two ball-bearings. The crankshaft has a single throw, and is made
in two parts to allow the cage for carrying the big end-pins of the
connecting rods to be placed in position.
The casing is in two parts, on one of which the brackets for fixing the
engine are carried, while the other part carries the valve-gear. Bolts
secure the two parts together. The mechanically-operated steel valves
on the cylinders are each fitted with double springs and the valves are
operated by rods and levers. Two Zenith carburettors are fitted on the
rear half of the crank case, and short induction pipes are led to each
cylinder; each of the carburettors is heated by the exhaust gases.
Ignition is by two high-tension magnetos, and a compressed air
self-starting arrangement is provided. Two oil pumps are fitted for
lubricating purposes, one of which forces oil to the crankshaft and
connecting-rod bearings, while the second forces oil to the valve gear,
the cylinders being so arranged that the oil which flows along the walls
cannot flood the lower cylinders. This engine operates upon a six-stroke
cycle, a rather rare arrangement for internal combustion engines of the
electrical ignition type; this is done in order to obtain equal angular
intervals for the working impulses imparted to the rotating crankshaft,
as the cylinders are arranged in groups of seven, and all act upon the
one crankshaft. The angle, therefore, between the impulses is 77 1/7
degrees. A diagram is inset giving a side view of the engine, in order
to show the grouping of the cylinders.
The 600 horse-power Salmson engine was designed with a view to fitting
to airships, and was in reality two nine-cylindered engines, with a
gear-box connecting them; double air-screws were fitted, and these were
so arranged that either or both of them might be driven by either or
both engines; in addition to this, the two engines were complete and
separate engines as regards carburation and ignition, etc., so that
they could be run independently of each other. The cylinders were
exceptionally 'long stroke,' being 5.9 inches bore to 8.27 inches
stroke, and the rated power was developed at 1,200 revolutions per
minute, the weight of the complete engine being only 4.1 lbs. per
horse-power at the normal rating.
A type of engine specially devised for airship propulsion is that in
which the cylinders are arranged horizontally instead of vertically, the
main advantages of this form being the reduction of head resistance and
less obstruction to the view of the pilot. A casing, mounted on the top
of the engine, supports the air-screw, which is driven through bevel
gearing from the upper end of the crankshaft. With this type of engine
a better rate of air-screw efficiency is obtained by gearing the screw
down to half the rate of revolution of the engine, this giving a more
even torque. The petrol consumption of the type is very low, being only
0.48 lbs. per horse-power per hour, and equal economy is claimed as
regards lubricating oil, a consumption of as little as 0.04 lbs. per
horse-power per hour being claimed.
Certain American radial engines were made previous to 1914, the
principal being the Albatross six-cylinder engines of 50 and 100
horse-powers. Of these the smaller size was air-cooled, with cylinders
of 4.5 inches bore and 5 inches stroke, developing the rated power
at 1,230 revolutions per minute, with a weight of about 5 lbs. per
horse-power. The 100 horse-power size had cylinders of 5.5 inches bore,
developing its rated power at 1,230 revolutions per minute, and weighing
only 2.75 lbs. per horse-power. This engine was markedly similar to the
six-cylindered Anzani, having all the valves mechanically operated, and
with auxiliary exhaust ports at the bottoms of the cylinders, overrun
by long pistons. These Albatross engines had their cylinders arranged in
two groups of three, with each group of three pistons operating on one
of two crank pins, each 180 degrees apart.
The radial type of engine, thanks to Charles Manly, had the honour of
being first in the field as regards aero work. Its many advantages,
among which may be specially noted the very short crankshaft as compared
with vertical, Vee, or 'broad arrow' type of engine, and consequent
greater rigidity, ensure it consideration by designers of to-day, and
render it certain that the type will endure. Enthusiasts claim that the
'broad arrow' type, or Vee with a third row of cylinders inset between
the original two, is just as much a development from the radial engine
as from the vertical and resulting Vee; however this may be, there is
a place for the radial type in air-work for as long as the internal
combustion engine remains as a power plant.
IV. THE ROTARY TYPE
M. Laurent Seguin, the inventor of the Gnome rotary aero engine,
provided as great a stimulus to aviation as any that was given anterior
to the war period, and brought about a great advance in mechanical
flight, since these well-made engines gave a high-power output for their
weight, and were extremely smooth in running. In the rotary design the
crankshaft of the engine is stationary, and the cylinders, crank
case, and all their adherent parts rotate; the working is thus exactly
opposite in principle to that of the radial type of aero engine, and
the advantage of the rotary lies in the considerable flywheel effect
produced by the revolving cylinders, with consequent evenness of torque.
Another advantage is that air-cooling, adopted in all the Gnome engines,
is rendered much more effective by the rotation of the cylinders, though
there is a tendency to distortion through the leading side of each
cylinder being more efficiently cooled than the opposite side; advocates
of other types are prone to claim that the air resistance to the
revolving cylinders absorbs some 10 per cent of the power developed by
the rotary engine, but that has not prevented the rotary from attaining
to great popularity as a prime mover.
There were, in the list of aero engines compiled in 1910, five rotary
engines included, all air-cooled. Three of these were Gnome engines, and
two of the make known as 'International.' They ranged from 21.5 to 123
horse-power, the latter being rated at only 1.8 lbs. weight per brake
horse-power, and having fourteen cylinders, 4.33 inches in diameter
by 4.7 inches stroke. By 1914 forty-three different sizes and types
of rotary engine were being constructed, and in 1913 five rotary type
engines were entered for the series of aeroplane engine trials held
in Germany. Minor defects ruled out four of these, and only the
German Bayerischer Motoren Flugzeugwerke completed the seven-hour test
prescribed for competing engines. Its large fuel consumption barred this
engine from the final trials, the consumption being some 0.95 pints
per horse-power per hour. The consumption of lubricating oil, also was
excessive, standing at 0.123 pint per horse-power per hour. The engine
gave 37.5 effective horse-power during its trial, and the loss due to
air resistance was 4.6 horse-power, about 11 per cent. The accompanying
drawing shows the construction of the engine, in which the seven
cylinders are arranged radially on the crank case; the method of
connecting the pistons to the crank pins can be seen. The mixture is
drawn through the crank chamber, and to enter the cylinder it passes
through the two automatic valves in the crown of the piston; the exhaust
valves are situated in the tops of the cylinders, and are actuated by
cams and push-rods. Cooling of the cylinder is assisted by the radial
rings, and the diameter of these rings is increased round the hottest
part of the cylinder. When long flights are undertaken the advantage of
the light weight of this engine is more than counterbalanced by its high
fuel and lubricating oil consumption, but there are other makes which
are much better than this seven-cylinder German in respect of this.
Rotation of the cylinders in engines of this type is produced by the
side pressure of the pistons on the cylinder walls, and in order to
prevent this pressure from becoming abnormally large it is necessary
to keep the weight of the piston as low as possible, as the pressure is
produced by the tangential acceleration and retardation of the piston.
On the upward stroke the circumferential velocity of the piston is
rapidly increased, which causes it to exert a considerable tangential
pressure on the side of the cylinder, and on the return stroke there
is a corresponding retarding effect due to the reduction of the
circumferential velocity of the piston. These side pressures cause an
appreciable increase in the temperatures of the cylinders and pistons,
which makes it necessary to keep the power rating of the engines fairly
low.
Seguin designed his first Gnome rotary as a 34 horse-power engine when
run at a speed of 1,300 revolutions per minute. It had five cylinders,
and the weight was 3.9 lbs. per horse-power. A seven-cylinder model soon
displaced this first engine, and this latter, with a total weight of 165
lbs., gave 61.5 horse-power. The cylinders were machined out of solid
nickel chrome-steel ingots, and the machining was carried out so that
the cylinder walls were under 1/6 of an inch in thickness. The pistons
were cast-iron, fitted each with two rings, and the automatic inlet
valve to the cylinder was placed in the crown of the piston. The
connecting rods, of 'H' section, were of nickel chrome-steel, and the
large end of one rod, known as the 'master-rod' embraced the crank pin;
on the end of this rod six hollow steel pins were carried, and to these
the remaining six connecting-rods were attached. The crankshaft of the
engine was made of nickel chrome-steel, and was in two parts connected
together at the crank pin; these two parts, after the master-rod had
been placed in position and the other connecting rods had been attached
to it, were firmly secured. The steel crank case was made in five parts,
the two central ones holding the cylinders in place, and on one side
another of the five castings formed a cam-box, to the outside of which
was secured the extension to which the air-screw was attached. On the
other side of the crank case another casting carried the thrust-box, and
the whole crank case, with its cylinders and gear, was carried on the
fixed crank shaft by means of four ball-bearings, one of which also took
the axial thrust of the air-screw.
For these engines, castor oil is the lubricant usually adopted, and it
is pumped to the crankshaft by means of a gear-driven oil pump; from
this shaft the other parts of the engine are lubricated by means of
centrifugal force, and in actual practice sufficient unburnt oil passes
through the cylinders to lubricate the exhaust valve, which partly
accounts for the high rate of consumption of lubricating oil. A very
simple carburettor of the float less, single-spray type was used, and
the mixture was passed along the hollow crankshaft to the interior of
the crank case, thence through the automatic inlet valves in the tops of
the pistons to the combustion chambers of the cylinders. Ignition was
by means of a high-tension magneto specially geared to give the correct
timing, and the working impulses occurred at equal angular intervals of
102.85 degrees. The ignition was timed so that the firing spark occurred
when the cylinder was 26 degrees before the position in which the piston
was at the outer end of its stroke, and this timing gave a maximum
pressure in the cylinder just after the piston had passed this position.
By 1913, eight different sizes of the Gnome engine were being
constructed, ranging from 45 to 180 brake horse-power; four of these
were single-crank engines one having nine and the other three having
seven cylinders. The remaining four were constructed with two cranks;
three of them had fourteen cylinders apiece, ranged in groups of seven,
acting on the cranks, and the one other had eighteen cylinders ranged in
two groups of nine, acting on its two cranks. Cylinders of the two-crank
engines are so arranged (in the fourteen-cylinder type) that fourteen
equal angular impulses occur during each cycle; these engines are
supported on bearings on both sides of the engine, the air-screw being
placed outside the front support. In the eighteen-cylinder model the
impulses occur at each 40 degrees of angular rotation of the cylinders,
securing an extremely even rotation of the air-screw.
In 1913 the Gnome Monosoupape engine was introduced, a model in which
the inlet valve to the cylinder was omitted, while the piston was of the
ordinary cast-iron type. A single exhaust valve in the cylinder head was
operated in a manner similar to that on the previous Gnome engines, and
the fact of this being the only valve on the cylinder gave the engine
its name. Each cylinder contained ports at the bottom which communicated
with the crank chamber, and were overrun by the piston when this was
approaching the bottom end of its stroke. During the working cycle of
the engine the exhaust valve was opened early to allow the exhaust gases
to escape from the cylinder, so that by the time the piston overran the
ports at the bottom the pressure within the cylinder was approximately
equal to that in the crank case, and practically no flow of gas took
place in either direction through the ports. The exhaust valve remained
open as usual during the succeeding up-stroke of the piston, and
the valve was held open until the piston had returned through about
one-third of its downward stroke, thus permitting fresh air to enter the
cylinder. The exhaust valve then closed, and the downward motion of the
piston, continuing, caused a partial vacuum inside the cylinder; when
the piston overran the ports, the rich mixture from the crank case
immediately entered. The cylinder was then full of the mixture, and the
next upward stroke of the piston compressed the charge; upon ignition
the working cycle was repeated. The speed variation of this engine
was obtained by varying the extent and duration of the opening of the
exhaust valves, and was controlled by the pilot by hand-operated levers
acting on the valve tappet rollers. The weight per horsepower of these
engines was slightly less than that of the two-valve type, while the
lubrication of the gudgeon pin and piston showed an improvement, so that
a lower lubricating oil consumption was obtained. The 100 horse-power
Gnome Monosoupape was built with nine cylinders, each 4.33 inches
bore by 5.9 inches stroke, and it developed its rated power at 1,200
revolutions per minute.
An engine of the rotary type, almost as well known as the Gnome, is the
Clerget, in which both cylinders and crank case are made of steel, the
former having the usual radial fins for cooling. In this type the
inlet and exhaust valves are both located in the cylinder head, and
mechanically operated by push-rods and rockers. Pipes are carried from
the crank case to the inlet valve casings to convey the mixture to the
cylinders, a carburettor of the central needle type being used. The
carburetted mixture is taken into the crank case chamber in a manner
similar to that of the Gnome engine. Pistons of aluminium alloy, with
three cast-iron rings, are fitted, the top ring being of the obturator
type. The large end of one of the nine connecting rods embraces the
crank pin and the pressure is taken on two ball-bearings housed in the
end of the rod. This carries eight pins, to which the other rods are
attached, and the main rod being rigid between the crank pin and piston
pin determines the position of the pistons. Hollow connecting-rods
are used, and the lubricating oil for the piston pins passes from the
crankshaft through the centres of the rods. Inlet and exhaust valves
can be set quite independently of one another--a useful point, since
the correct timing of the opening of these valves is of importance. The
inlet valve opens 4 degrees from top centre and closes after the bottom
dead centre of the piston; the exhaust valve opens 68 degrees before
the bottom centre and closes 4 degrees after the top dead centre of the
piston. The magnetos are set to give the spark in the cylinder at 25
degrees before the end of the compression stroke--two high-tension
magnetos are used: if desired, the second one can be adjusted to give
a later spark for assisting the starting of the engine. The lubricating
oil pump is of the valveless two-plunger type, so geared that it runs
at seven revolutions to 100 revolutions of the engine; by counting
the pulsations the speed of the engine can be quickly calculated by
multiplying the pulsations by 100 and dividing by seven. In the 115
horse-power nine-cylinder Clerget the cylinders are 4.7 bore with a 6.3
inches stroke, and the rated power of the engine is obtained at
1,200 revolutions per minute. The petrol consumption is 0.75 pint per
horse-power per hour.
A third rotary aero engine, equally well known with the foregoing two,
is the Le Rhone, made in four different sizes with power outputs of from
50 to 160 horse-power; the two smaller sizes are single crank engines
with seven and nine cylinders respectively, and the larger sizes are
of double-crank design, being merely the two smaller sizes
doubled--fourteen and eighteen-cylinder engines. The inlet and
exhaust valves are located in the cylinder head, and both valves are
mechanically operated by one push-rod and rocker, radial pipes from
crank case to inlet valve casing taking the mixture to the cylinders.
The exhaust valves are placed on the leading, or air-screw side, of the
engine, in order to get the fullest possible cooling effect. The rated
power of each type of engine is obtained at 1,200 revolutions per
minute, and for all four sizes the cylinder bore is 4.13 inches, with
a 5.5 inches piston stroke. Thin cast-iron liners are shrunk into
the steel cylinders in order to reduce the amount of piston friction.
Although the Le Rhone engines are constructed practically throughout
of steel, the weight is only 2.9 lbs. per horse-power in the
eighteen-cylinder type.
American enterprise in the construction of the rotary type is perhaps
best illustrated in the 'Gyro 'engine; this was first constructed with
inlet valves in the heads of the pistons, after the Gnome pattern, the
exhaust valves being in the heads of the cylinders. The inlet valve in
the crown of each piston was mechanically operated in a very ingenious
manner by the oscillation of the connecting-rod. The Gyro-Duplex engine
superseded this original design, and a small cross-section illustration
of this is appended. It is constructed in seven and nine-cylinder sizes,
with a power range of from 50 to 100 horse-power; with the largest size
the low weight of 2.5 lbs.. per horse-power is reached. The design is
of considerable interest to the internal combustion engineer, for it
embodies a piston valve for controlling auxiliary exhaust ports, which
also acts as the inlet valve to the cylinder. The piston uncovers the
auxiliary ports when it reaches the bottom of its stroke, and at the end
of the power stroke the piston is in such a position that the exhaust
can escape over the top of it. The exhaust valve in the cylinder head is
then opened by means of the push-rod and rocker, and is held open until
the piston has completed its upward stroke and returned through more
than half its subsequent return stroke. When the exhaust valve closes,
the cylinder has a charge of fresh air, drawn in through the exhaust
valve, and the further motion of the piston causes a partial vacuum;
by the time the piston reaches bottom dead centre the piston-valve has
moved up to give communication between the cylinder and the crank case,
therefore the mixture is drawn into the cylinder. Both the piston valve
and exhaust valve are operated by cams formed on the one casting, which
rotates at seven-eighths engine speed for the seven-cylinder type, and
nine-tenths engine speed for the nine-cylinder engines. Each of these
cams has four or five points respectively, to suit the number of
cylinders.
The steel cylinders are machined from solid forgings and provided with
webs for air-cooling as shown. Cast-iron pistons are used, and are
connected to the crankshaft in the same manner as with the Gnome and Le
Rhone engines. Petrol is sprayed into the crank case by a small geared
pump and the mixture is taken from there to the piston valves by radial
pipes. Two separate pumps are used for lubrication, one forcing oil to
the crank-pin bearing and the other spraying the cylinders.
Among other designs of rotary aero engines the E.J.C. is noteworthy,
in that the cylinders and crank case of this engine rotate in opposite
directions, and two air-screws are used, one being attached to the end
of the crankshaft, and the other to the crank case. Another interesting
type is the Burlat rotary, in which both the cylinders and crankshaft
rotate in the same direction, the rotation of the crankshaft being twice
that of the cylinders as regards speed. This engine is arranged to
work on the four-stroke cycle with the crankshaft making four, and the
cylinders two, revolutions per cycle.
It would appear that the rotary type of engine is capable of but little
more improvement--save for such devices as these of the last two engines
mentioned, there is little that Laurent Seguin has not already done in
the Gnome type. The limitation of the rotary lies in its high fuel and
lubricating oil consumption, which renders it unsuited for long-distance
aero work; it was, in the war period, an admirable engine for such
short runs as might be involved in patrol work 'over the lines,' and
for similar purposes, but the watercooled Vee or even vertical, with
its much lower fuel consumption, was and is to be preferred for distance
work. The rotary air-cooled type has its uses, and for them it will
probably remain among the range of current types for some time to come.
Experience of matters aeronautical is sufficient to show, however, that
prophecy in any direction is most unsafe.
V. THE HORIZONTALLY-OPPOSED ENGINE
Among the first internal combustion engines to be taken into use with
aircraft were those of the horizontally-opposed four-stroke cycle type,
and, in every case in which these engines were used, their excellent
balance and extremely even torque rendered them ideal-until the
tremendous increase in power requirements rendered the type too long and
bulky for placing in the fuselage of an aeroplane. As power increased,
there came a tendency toward placing cylinders radially round a central
crankshaft, and, as in the case of the early Anzani, it may be said that
the radial engine grew out of the horizontal opposed piston type. There
were, in 1910--that is, in the early days of small power units,
ten different sizes of the horizontally opposed engine listed for
manufacture, but increase in power requirements practically ruled out
the type for air work.
The Darracq firm were the leading makers of these engines in 1910; their
smallest size was a 24 horsepower engine, with two cylinders each of 5.1
inches bore by 4.7 inches stroke. This engine developed its rated power
at 1,500 revolutions per minute, and worked out at a weight of 5 lbs.
per horse-power. With these engines the cranks are so placed that two
regular impulses are given to the crankshaft for each cycle of working,
an arrangement which permits of very even balancing of the inertia
forces of the engine. The Darracq firm also made a four-cylindered
horizontal opposed piston engine, in which two revolutions were given to
the crankshaft per revolution, at equal angular intervals.
The Dutheil-Chambers was another engine of this type, and had
the distinction of being the second largest constructed. At 1,000
revolutions per minute it developed 97 horse-power; its four cylinders
were each of 4.93 inches bore by 11.8 inches stroke--an abnormally long
stroke in comparison with the bore. The weight--which owing to the build
of the engine and its length of stroke was bound to be rather high,
actually amounted to 8.2 lbs. per horse-power. Water cooling was
adopted, and the engine was, like the Darracq four-cylinder type,
so arranged as to give two impulses per revolution at equal angular
intervals of crankshaft rotation.
One of the first engines of this type to be constructed in England was
the Alvaston, a water-cooled model which was made in 20, 30, and 50
brake horse-power sizes, the largest being a four-cylinder engine. All
three sizes were constructed to run at 1,200 revolutions per minute. In
this make the cylinders were secured to the crank case by means of
four long tie bolts passing through bridge pieces arranged across the
cylinder heads, thus relieving the cylinder walls of all longitudinal
explosion stresses. These bridge pieces were formed from chrome
vanadium steel and milled to an 'H' section, and the bearings for the
valve-tappet were forged solid with them. Special attention was given
to the machining of the interiors of the cylinders and the combustion
heads, with the result that the exceptionally high compression of 95
lbs. per square inch was obtained, giving a very flexible engine. The
cylinder heads were completely water-jacketed, and copper water-jackets
were also fitted round the cylinders. The mechanically operated valves
were actuated by specially shaped cams, and were so arranged that only
two cams were required for the set of eight valves. The inlet valves at
both ends of the engine were connected by a single feed-pipe to which
the carburettor was attached, the induction piping being arranged above
the engine in an easily accessible position. Auxiliary air ports were
provided in the cylinder walls so that the pistons overran them at the
end of their stroke. A single vertical shaft running in ball-bearings
operated the valves and water circulating pump, being driven by spiral
gearing from the crankshaft at half speed. In addition to the excellent
balance obtained with this engine, the makers claimed with justice that
the number of working parts was reduced to an absolute minimum.
In the two-cylinder Darracq, the steel cylinders were machined from
solid, and auxiliary exhaust ports, overrun by the piston at the inner
end of its stroke, were provided in the cylinder walls, consisting of a
circular row of drilled holes--this arrangement was subsequently adopted
on some of the Darracq racing car engines. The water jackets were of
copper, soldered to the cylinder walls; both the inlet and exhaust
valves were located in the cylinder heads, being operated by rockers and
push-rods actuated by cams on the halftime shaft driven from one end
of the crankshaft. Ignition was by means of a high-tension magneto,
and long induction pipes connected the-ends of the cylinders to the
carburettor, the latter being placed underneath the engine. Lubrication
was effected by spraying oil into the crank case by means of a pump, and
a second pump circulated the cooling water.
Another good example of this type of engine was the Eole, which had
eight opposed pistons, each pair of which was actuated by a common
combustion chamber at the centre of the engine, two crankshafts being
placed at the outer ends of the engine. This reversal of the ordinary
arrangement had two advantages; it simplified induction, and further
obviated the need for cylinder heads, since the explosion drove at two
piston heads instead of at one piston head and the top of the cylinder;
against this, however, the engine had to be constructed strongly enough
to withstand the longitudinal stresses due to the explosions, as the
cranks are placed on the outer ends and the cylinders and crank-cases
take the full force of each explosion. Each crankshaft drove a separate
air-screw.
This pattern of engine was taken up by the Dutheil-Chambers firm in
the pioneer days of aircraft, when the firm in question produced seven
different sizes of horizontal engines. The Demoiselle monoplane used
by Santos-Dumont in 1909 was fitted with a two-cylinder,
horizontally-opposed Dutheil-Chambers engine, which developed 25 brake
horse-power at a speed of 1,100 revolutions per minute, the cylinders
being of 5 inches bore by 5.1 inches stroke, and the total weight of the
engine being some 120 lbs. The crankshafts of these engines were usually
fitted with steel flywheels in order to give a very even torque,
the wheels being specially constructed with wire spokes. In all the
Dutheil-Chambers engines water cooling was adopted, and the cylinders
were attached to the crank cases by means of long bolts passing through
the combustion heads.
For their earliest machines, the Clement-Bayard firm constructed
horizontal engines of the opposed piston type. The best known of these
was the 30 horse-power size, which had cylinders of 4.7 inches diameter
by 5.1 inches stroke, and gave its rated power at 1,200 revolutions per
minute. In this engine the steel cylinders were secured to the crank
case by flanges, and radiating ribs were formed around the barrel
to assist the air-cooling. Inlet and exhaust valves were actuated by
push-rods and rockers actuated from the second motion shaft mounted
above the crank case; this shaft also drove the high-tension magneto
with which the engine was fitted. A ring of holes drilled round each
cylinder constituted auxiliary ports which the piston uncovered at the
inner end of its stroke, and these were of considerable assistance not
only in expelling exhaust gases, but also in moderating the temperature
of the cylinder and of the main exhaust valve fitted in the cylinder
head. A water-cooled Clement-Bayard horizontal engine was also made, and
in this the auxiliary exhaust ports were not embodied; except in this
particular, the engine was very similar to the water-cooled Darracq.
The American Ashmusen horizontal engine, developing 100 horse-power, is
probably the largest example of this type constructed. It was made with
six cylinders arranged on each side of a common crank case, with long
bolts passing through the cylinder heads to assist in holding them down.
The induction piping and valve-operating gear were arranged below the
engine, and the half-speed shaft carried the air-screw.
Messrs Palons and Beuse, Germans, constructed a light-weight,
air-cooled, horizontally-opposed engine, two-cylindered. In this the
cast-iron cylinders were made very thin, and were secured to the
crank case by bolts passing through lugs cast on the outer ends of
the cylinders; the crankshaft was made hollow, and holes were drilled
through the webs of the connecting-rods in order to reduce the weight.
The valves were fitted to the cylinder heads, the inlet valves being of
the automatic type, while the exhaust valves were mechanically operated
from the cam-shaft by means of rockers and push-rods. Two carburettors
were fitted, to reduce the induction piping to a minimum; one was
attached to each combustion chamber, and ignition was by the normal
high-tension magneto driven from the halftime shaft.
There was also a Nieuport two-cylinder air-cooled horizontal engine,
developing 35 horse-power when running at 1,300 revolutions per minute,
and being built at a weight of 5.1 lbs. per horse-power. The cylinders
were of 5.3 inches diameter by 5.9 inches stroke; the engine followed
the lines of the Darracq and Dutheil-Chambers pretty closely, and thus
calls for no special description.