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not only discharge purer exhaust gases but would work on very much less petrol than they do at present.
Practically all modern petrol engines arc controlled by throttling the whole charge. In the earlier days several methods of control were attempted: (i) missing impulses as in fig. i of the Daimler engines; (2) altering the timing of spark; (3) throttling petrol supply, and (4) throttling the mixture of petrol and air. The last method has proved to be the best. By maintaining the proportion of explosive mixture, but diminishing the total volume admitted to the cylinder per stroke, graduated impulses are obtained without any, or but few, missed ignitions. The effect of the throttling is to reduce compression by diminishing total charge weight. To a certain extent the proportion of petrol to total charge also varies, because the residual exhaust gases remain constant through a wide range. The thermal efficiency diminishes as the throttling increases; but, down to a third of the brake power, the diminution is not great, because although compression is reduced the expansion remains the same. At low compressions, however, the engine works practically as a non-compression engine, and the point of maximum pressure becomes greatly delayed. The efficiency, therefore, falls markedly, but this is not of much importance at light loads. Experiments by Callendar, Hopkinson, Watson and others have proved that the thermal efficiency obtained from these small engines with the throttle full open is very high indeed; 28% of the whole heat in the petrol is often given as indicated work when the carburettor is properly adjusted. As a large gas engine for the same compression cannot do better than 35%, it appears that the loss of heat due to small dimensions is compensated by the small time of exposure of the gases of explosion due to the high speed of rotation. Throttle control is very effective, and it has the great advantage of diminishing maximum
pressures to which the piston and cylinders are exposed while the engine is running at the lower loads. This is important both for smooth running and good wearing qualities. Theoretically, better results could be obtained from the point of view of economy by retaining a constant compression pressure, constant charge of air, and producing ignition, somewhat in the manner of the Diesel engine. Such a method, however, would have the disadvantage of producing practically the same maximum pressure for all loads, and this would tend to give an engine which would not run smoothly at slow speeds.
As has been said, tube ignition was speedily abandoned for electric ignition by accumulator, induction coil distributor and sparking plug. This in its turn was largely displaced by the low-tension magneto system, in which the spark was formed between contacts which were mechanically separated within the cylinders. The separable contacts gave rise to complications, and at present the most popular system of ignition is undoubtedly that of the high-tension magneto. In this system the ordinary high-tension sparking plugs are used, and the high-tension current is generated in a secondary winding on the armature of the magneto, and reaches the sparking plugs by way of a rotary distributor. In many cases the high-tension magneto system is used for the ordinary running of the engine, combined with an accumulator or battery and induction coil for starting the engine from rest. Such systems are called dual ignition systems. Sometimes the same ignition plugs are adapted to spark from either source, and in other cases separate plugs are used. The magneto systems have the great advantage of generating current without battery, and by their use noise is reduced to a minimum. All electrical systems are now arranged to allow of advancing and retarding the spark from the steering wheel. In modern magneto methods, however, the spark is automatically retarded when the engine slows and advanced when the speed rises, so that less change is required from the wheel than is necessary with battery and coil.
Sir Oliver Lodge has invented a most interesting system of electric ignition, depending upon the production of an extra oscillator)' current of enormous tension produced by the combined use of spark gap and condenser. This extra spark passes freely even under water, and it is impossible to stop it by any ordinary sooting or fouling of the ignition plug.
The most popular engines are now of the four and six cylinder types,
Fig. . transver
the cylinder.. _, _. _.. r „
cf the connecting-rods. These scoops take up oil from the crank chamber. Forced lubrication is used. The oil pump M is of the toothed wheel type, and it is driven by skew gearing. An oil sump is arranged at I., and the oil is pumped from this sump by the pump described. The overflow from the main bearings supplies the channels* in the crank case from which the oil scoops take their charge. It will be seen that the two inside pistons are attached to cranks of coincident centres, and this is true of the two outside pistons also. Thi* it the usual arrangement in four-cylinder engines. By this device the primary forces are balanced; but a small secondary unbalanced force remains, due to the difference in motion of the piston? at the up and down portions of their stroke. A six-cylinder engine has the advantage of getting rid of this secondary unbalanced force: but it requires a longer and more rigid crank chamber. lathi* engine the inlet and exhaust valves of each cylinder are placed in the came pocket and are driven from one cam-shaft. This is a »er> favourite arrangement; but many engines are constructed in which the inlet and exhaust valves operate on opposite sides of the cylinder in separate ports and are driven from separate camshafts. Dual ignition is applied to this engine; that is, an ignition composed of high-tension magneto and also battery and coil for starting. \ is the high-tension magneto. Under the figure there is shown a list of parts which sufficiently indicate the nature of the engine.
An interesting and novel form of engine is shown at fig. 6. This is a well-known engine designed by Mr Knight, an American inventor, and now made bv the Daimler and other companies. It will be observed in the figure that the ordinary lift valves are entirely dispensed with, and slide valves arc used of the cylindrical shell type. The engine operates on the ordinary Oito cycle, and all the valve actions necessary to admit charge and discharge exhaust gases arc accomplished by means pf i wo sleeves sliding one within the other.
Fig. 5 shows a modern four-cylinder engine in longitudinal and insversc sections as made by the Wolseley Company. A, A arc •- cylinders; B, B, water jackets; G1, oil scoops on the large ends
The outer sleeve does not require rings at alf. Its function is simply to distribute the gases so that the exhaust port is closed by the outer sleeve when the inlet port is open. The outer sleeve acts really as a distributor; the inner sleeve supplies the pressure tightness required to resist compression and explosion. The idea of working exhaust and intct by two sleeves within which the main piston operates is very daring and ingenious; and for these small engines the sleeve valve system works admirably. There are many advantages; the shape 01 the compression space is a most favourable one for reducing loss by cooling. All the valve ports required in ordinary lift valw engines are entirely dispensed with; that is, the surface exposed to the explosion causing loss of heat is reduced to a minimum. The engines are found in use to be very flexible and economical.
The petrol engines hitherto described, although light compared to the old stationary gas engines, arc heavy when compared with recent motors developed for the purpose of aeroplanes. Many of these motors have been produced, but two only will be noticed here—the Anzani. because Blcriot's great flight across the Channel was accomplished by means of an Anzani engine, and the Gnome engine, because it was used in the aeroplane with which Paulhan flew from London to Manchester.
Fig. 7 shows transverse and longitudinal sections through the Anzani motor. Looking at the longitudinal section it will be observed that the cylinders arc of the air-cooled type; the exhaust valves alone are positively operated, and the inlet valves are of the automatic lift kind. The transverse section shows that three radially arranged cylinders are used and three pistons act upon one crank-pin. The Otto cycle is followed so that three impulses are obtained for
every two revolutions. The cylinders are spaced apart 60° and project from the upper side of the crank chamber. Although not shown in the drawing, the pistons overrun a row of holes at the out end of the stroke and the exhaust first discharges through these holes. This is a very common device in aeroplane engines, and it greatly increases ihe rapidity of the exhaust discharged and reduces the work falling upon the exhaust valve. The pistons and cylinders are of cast iron; the rings arc of cast iron; the ignition is electric, and the petrol is fed by gravity. The engine used by Bttriot in his Cross-Channel flight was 25 H.P..cylinders 105 mm. borcX 130 mm. stroke; revolutions. 1600 per minute; total weight, 145 Ib. The engine, it will be seen, is exceedingly simple, although air-cooling seems somewhat primitive for anything except short flights. The larger Anzani motors are water-cooled.
A diagrammatic transverse section of the Gnome motor is «hown at fig. 8. lii this interesting engine there are seven cylinders disposed
radially round a fixed crank-shaft. The seven pistons are all connected to the same crank-shaft, one piston being rigidly connected to a big end of peculiar construction by a connecting-rod, while the other connecting-rods are linked on to the same big end by pins; that is, a hollow fixed crank-shaft has a single throw to which only one connecting-rod is attached; all the other connecting-rods work on pins let into the big end of that connecting-rod. The cylinders revolve round the fixed crank in the manner of the wellknown engines first introdued to practice by Mr John Rigg. The explosive mixture is led from the carburettor through the hollow crank-shaft into the crank-case, and it is admitted into the cylinders by means of automatic inlet valves placed in the heads of the pistons. The exhaust valves are arranged on the cylinder heads. Dual ignition is provided by high tension magneto and storage battery and coil. The cylinders are ribbed outside like the Anzani, and are very effectively air-cooled by their rotation through the air as well as by the passage of the aeroplane through the atmosphere. The cylinders in the 35 H.P. motor are no mm. bore X 120 mm. stroke. The speed of rotation is usually 1200 revolutions per minute. The total weight of the engine complete is 180 Ib, or just over 5 Ib per brake horse-power. The subject of aeroplane petrol engines is a most interesting one, and rapid progress is being made.
So far, only 4-cycle engines have been described, and they are almost universal for use in motor-cars and aeroplanes. Some motor cars, however, use a-cyclc engines. Several types follow the "Clerk" cycle (see Gas Engine) and others the "Day" cycle. In America the Day cycle is very popular for motor
launches, as the engine is of 'a very simple, easily managed kind. At present, however, the two-cycle engine has made but little way in motor car or aeroplane work. It is capable of great development and the attention given to it is increasing.
So far, petrol has been alluded to as the main liquid fuel for these motors. Other hydrocarbons have also been used; benzol, for example, obtained from gas tar is used to some extent, and alcohol has been applied to a considerable extent both for stationary and locomotive engines. Alcohol, however, has not been entirely successful. The amount of heat obtained for a given monetary expenditure is only about half that obtained by means of petrol. On the continent of Europe, however, alcohol motors have been considerably used for public vehicles.
The majority of petrol motors are provided with water jackets around their cylinders and combustion spaces. As only a small quantity of water can be carried, it is necessary to cool the water as fast as it becomes hot. For this purpose radiators of various constructions are applied. Generally a pump is used to produce a forced circulation, discharging the hot water from the engine jackets through the radiator and returning the cooled water to the jackets at another place. The radiators consist in some casesof fine tubes covered with projecting finsorgilJs; the motion of the car forces air over the exterior of those surfaces and is assisted by the operation of a powerful fan driven from the engine. A favourite form of radiator consists of numerous small tubes set into a casing and arranged somewhat like a steamengine condenser. Water is forced by the pump round these tubes, and air passes from the atmosphere through them. This type of radiator is sometimes known as the "honeycomb '* radiator. A very large cooling surface is provided, so that the same water is used over and over again. In a day's run with a modern petrol engine very little water is lost from the system. Some engines dispense with a pump and depend on what is called the t her mo-syphon. This is the old gas-engine system of circulation, depending on the different density of water when hot and cool. The engine shown at fig. 5 is provided with a water-circulation system of this kind. For the smaller engines the thermo-syphon works extremely well.
Heavy oil engines are those which consume oil having a flashing-point above 75° F.—the minimum at present allowed by act of parliament in Great Britain for oils to be consumed in ordinary illuminating lamps. Such oils are American and Russian petroleums and Scottish paraffins. They vary in specific gravity from -78 to -825, and in flashing-point from 75° to 152" F. Engines burning such oils may be divided into three distinct classes: (i) Engines in which the oil is subjected to a spraying operation before vaporization; (2) Engines in which the oil is injected into the cylinder and vaporized within the cylinder; (3) Engines in which the oil is vaporized in a device external to the cylinder and introduced into the cylinder in the state of vapour.
The method of ignition might also be used to divide the engines into those igniting by the electric spark, by an incandescent tube, by compression, or by the heat of the internal surfaces of the combustion space. Spiel's engine was ignited by a flame igniting device similar to that used in Clerk's gas engine, and it was the only one introduced into Great Britain in which this method was adopted, though on the continent flame igniters were not uncommon. Electrically-operated igniters have come into extensive use throughout the world.
The engines first used in Great Britain which fell under the first head were the Pricstman and Samuelson, the oil being ,... sprayed before being jj ^ '.'.'•'s.: •',""•"" vaporized in both. The £=»i^o*£j>>/ .'>>-Y-V-"' principle of the spray pro*"*"<~^* >_"-.:.-•_• • duccr Used is that so well and so widely known in connexion with the atomizers or spray producers used by perfumers. Fig. 9 shows such a spray producer in section. An air blast passing from the small jet A crosses the top of the tube B and creates within it a partial vacuum. The liquid contained in C Bows up the lube B and issuing at the top of the tube through a »mall orince is at once blown into very fine spray by the action of the air jet. If such a scent distributor be filled with petroleum ofl, such as Royal Daylight or Russoline, the oil will be blown into fine spray, which can be ignited by a flame and will burn, if the jets be properly proportioned, with an intense blue nonluminous flame. The earlier inventors often expressed the idea lhat an explosive mixture could be prepared without any vaporization whatever, by simply producing an atmosphere containing inflammable liquid in extremely small particles distributed throughout the air in such proportion as to allow of complete combustion. The familiar explosive combustion of Jycopodium, and the disastrous explosions caused in the exhaustion rooms of flour-mills by the presence of finely divided flour in the air, have also suggested to inventors the idea of producing explosions for power purposes from combustible solids. Although, doubtless, explosions could be produced in that way, yet in oil engines the production of spray is only a preliminary to the vaporization of the oil. If a sample of oil is sprayed in the manner just described, and injected in a hot chamber also filled with hot air, it at once passes into a state of vapour within that chamber, even though the air be at a temperature far below the boiling-point of the oil; the spray producer, in fact, furnishes a rea.dy means of saturating any volume of air with
Fie. 9.—Perfume Spray Producer.
heavy petroleum oil to the full extent possible from the vapour tension of the oil at that particular temperature. The oil engines described below are in reality explosion gas engines of the ordinary Otto type, with special arrangements to enable them to vaporize the oil to be used. Only such parts of them as are necessary for the treatment and ignition will therefore be described.
Fig. to is a vertical section through the cylinder and vaporizer of a Pricstman engine, and fig. 11 is a section on a larger scale, showing the vaporizing jet and the air admission and regulation valve
Fig. 10.—Pricstman Oil Engine (vertical section through cylinder and vaporizer).
leading to the vaporizer. Oil is forced by means of air pressure from a reservoir through a pipe to the spraying nozzle a, and air passes from an air-pump by way of the annular channel b into the sprayer c, and there meets the oil jet issuing] from o. The oil is thus broken up into spray, and the air charged with spray flows into the vaporizer E, which is heated up in the first place on starting the engine by means of a lamp. In the vaporizer the oil spray becomes oil vapour, saturating the air within the hot walls. On the out-charging stroke of the piston the mixture passes by way of the inlet valve H into the cylinder, air flowing into the vaporizer to replace it through the valve / (fig. ii). The cylinder K is thus charged with a mixture of air and hydrocarbon vapour, some of which may exist in the form of very fine spray. The piston L then returns and compresses the mixture, and when the compression is quite complete an electric spark is passed between the points M, and a compression explosion is obtained precisely similar to that obtained in the gas engine. The piston moves out, and on its return stroke the exhaust valve N is opened and the exhaust gases discharged by way of the pipe O, round the jacket P, enclosing the vaporizing chamber. The latter is thus kept hot by the exhaust gases when the engine is at work, and it remains sufficiently hot without the use of the lamp provided for starting. To obtain the electric spark a bichromate battery with an induction coil is used. The spark is timed by contact pieces operated by an eccentric rod, used to actuate the exhaust valve and the air-pump for supplying the oil chamber and the spraying jet. To start the engine a hand pump is worked until the pressure is sufficient to force the oil through the spraying nozzle, and oil spray is formed in the starting lamp; the spray and air mixea produce a blue flame which heats the vaporizer. The fly-wheel is then rotated by hand and the engine moves away. The eccentric shaft is driven from the crank-shaft by means of toothed wheels, which reduce the speed to one-half the revolutions of the crank-shaft. The charging inlet valve is automatic. Governing is effected by throttling the oil and air supply. The governor operates on the butterfly valve T (fig. ii), and on the plug-cock t connected to it, by means of the spindle f'. The air and oil are thus simultaneously reduced, and the attempt is made to maintain the charge entering the cylinder at a constant proportion by weight of oil and air, while reducing the total weight, and therefore volume, of the charge entering. The Pricstman engine thus gives an explosion on every second revolution in all circumstances, whether the engine be running light or loaded.
FlG. II.—Priestman OH Engine (section on a larger scale).
The compression pressure of the mixture before admission is, however, steadily reduced as the load is reduced, and at very light loads the engine is running practically as a non-compression engine.
A test by Profcs&or Unwin of a 4! nominal horse-power Priestman engine, cylinder 8-5 in. diameter, 12 in. stroke, normal speed 180 revolutions per minute, showed the consumption of oil per indicated horse-power hour to be 1-066 Ib and per brake horse-power hour 1-243 ">* The oil used was that known as Broxburn Lighthouse, a Scottish paraffin oil produced by the destructive distillation of shale, having a density of -81 and a flashing-point about 152° F. With a 5 H.P. engine of the same dimensions, the volume swept by the piston per stroke being -395 cub. ft. and the clearance space in the cylinder at the end of the stroke -210 cub. ft., the principal results were:—
With daylight oil the explosion pressure was 151-4 In per square inch above atmosphere, and with Russoline 134-3 R>- The terminal pressure at the moment of opening the exhaust valve with daylight oil was 35-4 tb and with Russoline 33-7 per square inch. The compression pressure with daylight oil was 35 Ib, and with Russoline 27-6 tb pressure above atmosphere. Professor Unwin calculated the amount of heat accounted for by the indicator as 18-8% in the case of daylight oil and 15-2 in the case of Russoline oil.
The Hornsby-Ackroyd engine is an example of the class in which the oil is injected into the cylinder and there vaporized. Fig. 12
Fig. 12.—Hornsby-Ackroyd Fig. 13.—Hornsby - Ackroyd Engine (section through Engine (section through valves, vaporizer and cylinder). vaporizer and cylinder).
is a section through the vaporizer and cylinder of this engine, and fig. 13 shows the inlet and exhaust valves also in section placed in front of the vaporizer and cylinder section. Vaporizing is conducted in the interior of the combustion cliambcr, which is so arranged that the heat of each explosion maintains it .it a temperature sufficiently high to enable the oil to be vaporized by mere injection upon the hot surfaces. The vaporizer A is heated up by a separate lamp, the oil is injected at the oil inlet 8, and the engine is rotated by hand. The piston
thn takes in a charge of air by the air- R
inlet valve into the cylinder, the air passing by the port directly into the cylinder without passing through the vaporizer chamber. While the piston is moving forward, taking in the charge of air, the oil thrown into the vaporizer is vaporizing and diffusing itself through the vaporizer chamber, mixing, however, only with the hot products of combustion left by the preceding explosion. During the charging stroke the air enters through the cylinder, and the vapour formed from the oil is almost entirely confined to the combustion chamber. On the return stroke of the piston air is forced through the somewhat narrow neck a into the|combust ion chamber, and is there mixed with the vapour contained in it. At first, however, the mixture is too rich in inflammable vapour to be capable of ignition. As the compression
proceeds, however, more and more air is forced into the vaporizer chamber, and just as compression is completed the mixture attains proper explosive proportions. The sides of the chamber are sufficiently hot to cause explosion, under the pressure of which the piston moves forward. As the vaporizer A is not water-jacketed, and is connected to the metal of the back cover only by the small section or area of cast-iron forming the metal neck a, the heat given to the
surface by each explosion is sufficient to keep its temperature at about 700-800° C. Oil vapour mixed with air will explode by contact with a metal surface at a comparatively low temperature; this accounts for the explosion of the compressed mixture in the combustion chamber A, which is never really raised to a red heat. It has long been known that under certain conditions of internal surface a gas engine may be made to run with very Rreat regularity, without incandescent tube or any other form of igniter, if some portion of the interior surfaces of the cylinder or combustion space be so arranged that the temperature can rise moderately; then, although the temperature may be too low to ignite the mixture at
atmospheric temperature, yet when compression is completed the mixture will often ignite in a perfectly regular manner. It is a curious fact that with heavy oils ignition is more easily accomplished at a low temperature than with light oils. The explanation seems to be that, while in the case of light oils the hydrocarbon vapours formed are tolerably stable from a chemical point of view, the heavy oils very easily decompose by heat, and separate out their carbons, liberating the combined hydrogen, and at the moment of liberation the hydrotren, being in what chemists know as the nascent state, very readily enters into combination with the oxygen beside it. To start the engine the vaporizer is heated by a separate heating lamp, which is supplied with an air blast by meansof a hand-operated fan. This operation should take about nine minutes. The engine is then moved round by hand, and starts in the usual manner. The oil tank is placed in the bed plate of the engine. The air and exhaust valves are driven by cams on a valve shaft. The governing is effected by a centrifugal governor which operates a by-pass valve, opening it when the speed is too high, and causes the oil pump to return the oil to the oil tank. At a test of one of these engines, which weighed 40 cwt. and was given as of 8 brake horse-power, with cylinder 10 in. in diameter and 15 in. stroke, according to Professor Capper's report, the revolutions were very constant, and the power developed did not vary one quarter of a brake horse-power from day to day. The oil consumed, reckoned on the average of the three days over which the trial extended, was -919.Ib per brake horse-power per hour, the mean power exerted being 8-35 brake horse. At another full-power trial of the same engine a brake horse-power of 8-57 was obtained, the mean speed being 239-66 revolutions per minute and the test lasting for two hours; the indicated power was 10-3 horse, the explosions per minute 119-83, the mean effective pressure 28-9 per sq. in., the oil used per indicated horse-power per hour was -oi Ib, and per brake horse-power per hour —-977 Ib. In a test at half power, the brake horse-power developed was 4-57 at 335-9 revolutions per minute, and the oil used per brake horse-power was 1-48 Ib. On a four hours' test, without a load, at 240 revolutions per minute, the consumption of oil was 4-23 Ib per hour. Engines of this class are those manufactured by Messrs Crossley Bros., Ltd., and the National Gas Engine Co., Ltd.
Figs. 14 and 15 show a longitudinal section and detail views of the operative parts of the Crossley oil engine. On the suction stroke, air is drawn into the cylinder by the piston A through the automatic inlet valve D, and oil is then pumped into the heated vaporizer C through the oil sprayer G, as seen in section at fig. 15. The vaporizer C is bolted to the water-jacketed part B; and, like the Hornsby, this vaporizer is first heated by lamp and then the heat of the explosions keeps up its temperature to a. sufficiently high point to vaporize the oil when sprayed against it. 'On the compression stroke
Fig. 14.—Crossley Oil Engine.
of the piston A the charge of air is forced into the combustion chamber B and the vaporizer chamber C, where it mixes with the oil vapour, and the mixture is ignited at the termination of the stroke by the ignition tube H. This tube is isolated to some extent from the vaporizer chamber C. and so it becomes hotter than the chamber C and is relied upon to ignite the mixture when formed at times when C would be too cold for the purpose. E is the exhaust valve, which