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diff --git a/27286-8.txt b/27286-8.txt new file mode 100644 index 0000000..a5edf76 --- /dev/null +++ b/27286-8.txt @@ -0,0 +1,1969 @@ +The Project Gutenberg EBook of Gas and Oil Engines, Simply Explained, by +Walter C. Runciman + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Gas and Oil Engines, Simply Explained + An Elementary Instruction Book for Amateurs and Engine Attendants + +Author: Walter C. Runciman + +Release Date: November 17, 2008 [EBook #27286] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK GAS AND OIL ENGINES *** + + + + +Produced by Steven Gibbs, Greg Bergquist and the Online +Distributed Proofreading Team at https://www.pgdp.net + + + + + + + + + + + + GAS AND OIL ENGINES + + SIMPLY EXPLAINED + + _An Elementary Instruction Book for Amateurs + and Engine Attendants_ + + + BY + WALTER C. RUNCIMAN + + + _FULLY ILLUSTRATED_ + + + LONDON + + Model Engineer Series. The "Model Engineer" + Series, no. 26. + + 1905 + + + + +CONTENTS + + CHAP. PAGE + + PREFACE 5 + + + I. INTRODUCTORY 7 + + II. THE COMPONENT PARTS OF AN ENGINE 13 + + III. HOW A GAS ENGINE WORKS 22 + + IV. IGNITION DEVICES 33 + + V. MAGNETO IGNITION 47 + + VI. GOVERNING 51 + + VII. CAMS AND VALVE SETTINGS 63 + + VIII. OIL ENGINES 81 + + + + +PREFACE + + +My object in placing this handbook before the reader is to provide him +with a simple and straightforward explanation of how and why a gas +engine, or an oil engine, works. The main features and peculiarities in +the construction of these engines are described, while the methods and +precautions necessary to arrive at desirable results are detailed as +fully as the limited space permits. I have aimed at supplying just that +information which my experience shows is most needed by the user and by +the amateur builder of small power engines. In place of giving a mere +list of common engine troubles and their remedies, I have thought it +better to endeavour to explain thoroughly the fundamental principles and +essentials of good running, so that should any difficulty arise, the +engine attendant will be able to reason out for himself the cause of the +trouble, and will thus know the proper remedy to apply. This will give +him a command over his engine which should render him equal to any +emergency. + + WALTER C. RUNCIMAN. + + LONDON, E.C. + + + + +GAS AND OIL ENGINES + +SIMPLY EXPLAINED + + + + +CHAPTER I + +INTRODUCTORY + + +The history of the gas engine goes back a long way, and the history of +the internal combustion engine proper further still. It will be +interesting to recount the main points in the history of the development +of the class of engine we shall deal with in the following pages, in +order to show what huge strides were made soon after the correct and +most workable theory had been formulated. + +In 1678 Abbé Hautefeuille explained how a machine could be constructed +to work with gunpowder as fuel. His arrangement was to explode the +gunpowder in a closed vessel provided with valves, and cool the products +of combustion, and so cause a partial vacuum to be formed. By the aid of +such a machine, water could be raised. This inventor, however, does not +seem to have carried out any experiments. + +In 1685 Huyghens designed another powder machine; and Papin, in 1688, +described a similar machine, which was provided with regular valves, as +devised by himself, in the _Proceedings of the Leipsic Academy_, 1688. +From this time until 1791, when John Barber took out a patent for the +production of force by the combustion of hydrocarbon in air, practically +no advancement was made. The latter patent, curiously enough, comprised +a very primitive form of rotary engine. Barber proposed to turn coal, +oil, or other combustible stuff into gas by means of external firing, +and then to mix the gases so produced with air in a vessel called the +exploder. This mixture was then ignited as it issued from the vessel, +and the ensuing flash caused a paddle-wheel to rotate. Mention is also +made that it was an object to inject a little water into the exploder, +in order to strengthen the force of the flash. + +Robert Street's patent of 1794 mentions a piston engine, in the cylinder +of which, coal tar, spirit, or turpentine was vaporised, the gases being +ignited by a light burning outside the cylinder. The piston in this +engine was thrown upwards, this in turn forcing a pump piston down which +did work in raising water. This was the first real gas engine, though it +was crude and very imperfectly arranged. + +In 1801 Franzose Lebon described a machine to be driven by means of +coal-gas. Two pumps were used to compress air and gas, and the mixture +was fired, as recommended by the inventor, by an electric spark, and +drove a piston in a double-working cylinder. + +The atmospheric engine of Samuel Brown, 1823, had a piston working in a +cylinder into which gas was introduced, and the latter, being ignited, +expanded the air in cylinder whilst burning like a flame. The fly-wheel +carried the piston up to the top of its stroke, then water was used to +cool the burnt gases, which also escaped through valves, the latter +closing when the piston had reached the top of its stroke. A partial +vacuum was formed, and the atmospheric pressure did work on the piston +on its down stroke. A number of cylinders were required in this engine, +three being shown in the specification all connected to the same +crank-shaft. According to the _Mechanic's Magazine_, such an engine with +a complete gas generating plant was fitted to a boat which ran as an +experiment upon the Thames. + +A two-cylinder engine working on to a beam was built in Paris, but no +useful results were obtained. + +Wright's engine of 1833 used a mixture of combustible gas and air, which +operated like steam in a steam engine. This engine had a water-jacket, +centrifugal governor, and flame ignition. In 1838 Barnett applied the +principle of compression to a single-acting engine. He also employed a +gas and air pump, which were placed respectively on either side of the +engine cylinder, communication being established between the receiver +into which the pumps delivered and the working cylinder as the charge +was fired. The double-acting engines which Barnett devised later were +not so successful. + +From this time to about 1860 very few practical developments are +recorded. A number of French and English patents were taken out, +referring to hydrogen motors, but are not of much practical value. + +Lenoir's patent, dating from 24th January 1860, refers to a form of +engine which received considerable commercial support, and consequently +became very popular. A manufacturer, named Marinoni, built several of +these engines, which were set to work in Paris in a short time. Then, +due to sudden demand, the Lenoir Company was formed to undertake the +manufacture of these engines. It was claimed that a 4-horse-power engine +could be run at a cost of 3ˇ4 shillings per day, or just one half the +cost of a steam engine using 9ˇ9 pounds of coal per horse-power per +hour. Many similar exaggerated accounts of their economy in consumption +were circulated, and the public, on the strength of these figures, +bought. + +It was understood that 17ˇ6 cubic ft. of gas were required per +horse-power per hour, but it was found that as much as 105 cubic ft. +were often consumed. The discrepancy between the stated figures and the +actual performance of the engine was a disappointment to the using +public, and, as a result, the Lenoir engine got a bad name. + +Hugon, director of the Parisian gas-works, who, together with Reithmann, +a watchmaker of Münich, hotly contested Lenoir's priority to this +invention, brought out a modification of this engine. He cooled the +cylinder by injecting water as well as using a water-jacket, and used +flame instead of electric ignition. The consumption was now brought down +to 87ˇ5 cubic ft. + +At the second Parisian International Exhibition, 1867, an atmospheric +engine, invented by Otto & Langen about this time, was shown. In this +engine a free piston was used in a vertical cylinder, the former being +thrown up by the force of the explosion. The only work done on the +up-stroke was that to overcome the weight of the piston and piston rod, +and the latter being made in the form of a rack, engaged with a toothed +wheel on the axle as the piston descended, causing the fly-wheel and +pulley to rotate. + +Barsanti and Matteucci were engaged in devising and experimenting with +an engine very similar to this some years before, but Otto & Langen, no +doubt, worked quite independently. Barsanti's engine never became a +commercial article; while Otto & Langen's firm, it is said, held their +own for ten years, and turned out about 4000 engines. In 1862 the French +engineer, Beau de Rochas, laid down the necessary conditions which must +prevail in order to obtain maximum efficiency. His patent says there are +four conditions for perfectly utilising the force of expansion of gas in +an engine. + +(1) Largest possible cylinder volume contained by a minimum of surface. + +(2) The highest possible speed of working. + +(3) Maximum expansion. + +(4) Maximum pressure at beginning of expansion. + +These are the conditions and principles, briefly stated, that combine to +form the now well-known cycle upon which most gas engines work at the +present time. + +It was not until 1876, fifteen years after these principles had been +enumerated, that Otto carried them into practical effect when he brought +out a new type of engine, with compression before ignition, higher +piston speed, more rapid expansion, and a general reduction of +dimensions for a given power. Due to this achievement, the cycle above +referred to has always been termed the "Otto" cycle. + + + + +CHAPTER II + +THE COMPONENT PARTS OF AN ENGINE + + +Having recounted very briefly the chief points in the development of the +gas engine from its beginning, we may proceed to deal with matters of +perhaps more practical interest to those who we are assuming have had +little or no actual experience in making or working internal combustion +engines. + +The modern gas engine comprises comparatively few parts. Apart from the +two main castings--the bed and cylinder--a small engine, generally +speaking, consists of four fundamental members, viz., the valves and +their operating mechanism, the cams and levers; the ignition device for +firing the charge; and the governing mechanism for regulating the supply +and admission of the explosive charge. There are innumerable designs of +each one of these parts, and no two makes are precisely alike in detail, +as every maker employs his own method of achieving the same end, namely, +the production of an engine which comprises maximum efficiency with a +minimum of wear and tear and attention. + +Therefore, before dealing with each of these primary parts in an +arbitrary manner, and with the cycle of operations in detail, we propose +to make the reader familiar with the general arrangement and method of +working which usually obtains in the smaller power engines. In the +following illustrations these parts are shown. A (fig. 1) is the +ignition device which carries the ignition tube to fire the charge. H +and I (fig. 2) are the main valves, and GC (fig. 1.) is the gas-cock. +The side or cam shaft N (sometimes called the 2 to 1 shaft), the cams +which move the levers M, the latter in turn operating the valves, and +causing them to open and close at the proper time, are shown in fig. 11. +A bracket bolted up to the side of cylinder forms a bearing for one end +of the side shaft, and also carries a spindle at its lower end on which +the levers oscillate, transmitting the motion imparted to them by the +cams to the valves. The main cylinder casting and the bed need no +description. In some cases the bed is in two portions, though now a +great many makers are discarding the lower portion altogether, having +found that it is cheaper, and quite as satisfactory, to use a built-up +foundation instead, and, if necessary, to cut a trough for the fly-wheel +to run it. This arrangement, however, only obtains where larger engines +are concerned. A half-compression handle by which the exhaust cam is +moved laterally on the side shaft as required is not needed on very +small engines. + +[Illustration: FIG. 1.--General Arrangement of a Gas Engine and +Accessories.] + +Further reference will be made to this in another chapter, and, +although this is not a necessity on a _small_ engine, it is always +employed on engines over 2 B.H.P. In fig. 1, HW is the cooling water +outlet and CW the inlet. A small drain cock is shown at DC, through +which the water in the cylinder water-jacket may be drawn off when +required. The pipes leading to the inlet and outlet of this supply are +connected to the cooling water tank by means of a couple of broad, flat +nuts and lead washers, one inside and the other outside the tank, the +latter, when clamped up well, making a perfectly water-tight joint. The +outlet pipe making an acute angle with the side of tank, the washers +used there should be wedge-shape in section. It is also desirable to fit +a stop-cock SC, so that the pipes can be disconnected from the engine +entirely, or the water-jacket emptied without running the whole of the +water out of the tank. The exhaust pipe EP is made up of gas-barrel. It +should lead from the engine to the silencer or exhaust box (if one is +found to be necessary) as directly as possible, _i.e._, with no more +bends than are needed, and what there are should not be acute. The +silencer can be inside or outside the engine-room, whichever is most +convenient; but both it and the exhaust piping should be kept from all +direct contact with wood-work, and at the same time in a readily +accessible position. + +Beyond the exhaust-pipe and box and the water-tank, the gas bag GB and +gas meter (where small powers are concerned, the ordinary house or +workshop lighting meter may be used without inconvenience) are the only +other accessories which are included in a small installation. + +[Illustration: FIG. 2.--A Section of a Gas Engine.] + +Fig. 2 gives a sectional view, showing the cylinder and liner. The +latter is a very desirable feature in any type of gas engine, but +especially in the larger sizes; for at any future time, should it be +found necessary to re-bore the liner, it can be removed with comparative +ease, and is, moreover, more readily dealt with in the lathe than the +whole cylinder casting would be. + +The liner is virtually a cast-iron tube, with a specially shaped flange +at either end. At the back end the joint between it and the cylinder +casting has to be very carefully made. This is a water _and_ explosion +joint; hence it has not only to prevent water entering the cylinder from +the water-jacket, but also to be sufficiently strong to withstand the +pressure generated in the cylinder when the charge is fired. For this +purpose specially prepared coppered asbestos rings are used, which will +stand both water and intense heat. Sometimes a copper ring alone is +employed to make the joint. At the front end the liner is just a good +fit, and enters the bed easily, and a couple of bolts fitted in +corresponding lugs on the liner, pass through the back end of cylinder +casting, so that by tightening up these the joint at back end is made +secure. A small groove is cut on a flange, and a rubber ring, of about +1/4-in. sectional diameter, is inserted here when the liner is fitted +into the cylinder casting. This makes the water-jacket joint at the +front end. + +[Illustration: FIG. 3.] + +[Illustration: FIG. 5.] + +[Illustration: FIG. 4.] + +Lugs are provided on the bed and cylinder castings, and are bored to +receive steel bolts--three are sufficient, provided the metal in and +around these lugs is not pinched. In some cases a continuous flange is +provided on both bed and cylinder, and a number of bolts inserted all +the way round. This, however, is unnecessary, and has a somewhat clumsy +appearance. When these bolts are tightened up, the cylinder and liner +are clamped firmly to the bed; but the liner being free at the open end, +can expand longitudinally without causing stresses in the cylinder +casting. + +The combustion chamber K is virtually part of the cylinder, and has +approximately equal to one-fourth the total volume of the cylinder. The +shape varies somewhat in different makes of engines; in some it is +rectangular, with all the corners well rounded off; in others it is +practically a continuation of the cylinder, _i.e._, it is circular in +cross-section, with the back end more or less spherical; while, again, +it is made slightly oval in cross-section; but in every case the corners +should be _well_ curved and rounded off, so that there is no one part +which is liable to become heated disproportionately with the rest of the +casting; in fact, in the whole cylinder casting there should be no +sudden change, but a uniformity in the thickness of the metal employed. +This point should be carefully remembered, although it applies more +particularly to those parts of the casting subjected to higher +temperatures than the rest. + +The main bearings are usually of brass or gun-metal, and are adjusted +for running in the same manner as any steam or other engines would be. +The "brasses" are in halves, and are held down by the cast-iron caps, as +shown in fig. 1. + +These bearings require extremely little attention, and do not show the +wear and tear of running nearly so soon as the connecting-rod brasses. +These, too, are usually of brass or gun-metal; but there are various +forms of construction employed in connection with the back end or piston +pin bearings. On very small engines the connecting rod is swollen at the +back end in the forging, and then machined up and drilled, as shown in +fig. 3. In this hole the brasses are inserted after being scraped up to +a good fit on the piston pin. + +A flat is cut on one of the brasses, and a set screw is fitted, as +shown, to prevent any movement of the latter after the final adjustment +has been made. A lock nut should be used in conjunction with this set +screw. Another method, and one more generally used on larger engines, is +shown in fig. 4. In this case the brasses are larger than in the former, +where they are virtually a split bush; here they have holes drilled in +them to take the bolts, the latter usually and preferably being turned +up to the shape shown in fig. 5. + + + + +CHAPTER III + +HOW A GAS ENGINE WORKS + + +The gas engine of the present day, although from a structural point of +view is very different to the early engine, or even that of fifteen +years ago, is, in respect to the principle upon which it works, very +similar. The greater number of smaller power engines in use in this +country work on what is known as the Otto or four-cycle principle; and +it is with this class of engine we propose to deal. + +Reference to the various diagrams in the text will help considerably, +and make it an easy matter for any reader hitherto totally unacquainted +with such engines to see why and how they work. + +Coal-gas consists primarily of five other gases, mixed together in +certain proportions, these proportions varying slightly in different +parts of the country:--Hydrogen (H), 50; marsh gas (CH4), 38; +carbon-monoxide, 4; olefines (C6H4), 4; nitrogen (N), 4. + +Gas _alone_ is not explosive; and before any practical use can be made +of it, a considerable quantity of air has to be added, diluting it down +to approximately ten parts air to one of pure gas. This mixture is _now_ +highly explosive. + +The reader will do well to bear these facts constantly in mind, +especially when he is repairing, adjusting, or experimenting with a gas +engine. We wish to emphasise this at the outset, because a consideration +of these facts will keep cropping up throughout all our dealings with +the gas engine, and if once a fairly clear conception is obtained of how +gas will behave under certain and various conditions, half, or even more +than half, our "troubles" will disappear; the cry that the gas engine +has "gone wrong" will be heard less often, and users would soon learn +that the gas engine is in reality as worthy of their confidence as any +other form of power generator in common use. + +But to revert to the explanation of the cycle of operations. The cycle +is completed in four strokes of the piston, _i.e._, two revolutions of +the crank shaft. + +At the commencement of the first out-stroke (the charging or suction +stroke) gas and air are admitted to the cylinder through the respective +valves (fig. 6), and continue to be drawn in by what may be termed the +sucking action of the piston, until the completion of this stroke (the +_precise_ position of the closing and opening of the valves will be +referred to later on). The next stroke (fig. 7) is the compression +stroke. All the valves are closed whilst the piston moves inwards, +compressing the gases, until at the end of this stroke, and at the +instant of maximum compression, the highly explosive charge is fired by +means of the hot tube or an electric spark, as the case may be. The +ensuing stroke--the second out-stroke of the cycle--is the result of the +explosion, the expanding gases driving the piston rapidly before them; +this, then, is the expansion, or working stroke (fig. 8.) + +[Illustration: FIG. 6.--Commencement of first out-stroke suction or +charging stroke. Gas and air valve about to open.] + +[Illustration: FIG. 7.--Compression stroke, during which all valves +remain closed.] + +During the last--the second inward--stroke (fig. 9) the exhaust valve is +opened, and the returning piston sweeps all the burnt gases (the product +of combustion) out into the exhaust pipe and so into the atmosphere. +This completes the cycle, and the piston, crank, and valves are in the +same relative positions as formerly, and the same series of operations +is repeated again and again. Of course, it is not always the case that +both air _and_ gas valve are opened on the charging stroke; that depends +upon the method employed to govern the speed of the engine. Supposing +it were governed on the hit and miss principle (to be explained +hereafter), the gas valve would be allowed to remain closed during the +charging stroke, and air alone would be drawn into the cylinder, then +compressed, but not being explosive would simply expand again on the +working stroke, giving back nearly all the energy which was absorbed in +compressing it, and finally be exhausted in the same manner as the burnt +gases are. + +[Illustration: FIG. 8.--Second out stroke, showing position of valves +during working stroke.] + +[Illustration: FIG. 9.--Second inward stroke, showing position of valves +during the exhaust stroke.] + +[Illustration: FIG. 10.--First out-stroke, showing position of valves +during the charging stroke.] + +Fig. 10 shows diagrammatically the position of crank, piston, and +valves _during_ the charging stroke. + +[Illustration: FIG. 11.--Cross Section of Cylinder.] + +In figs. 1 and 2 we gave drawings of two gas engines, which are typical +examples of modern practice. Huge strides have been made in recent years +in gas-engine work, as regards both workmanship and efficiency, so that +to-day we have in the gas engine a machine whose mechanical efficiency +compares favourably with that of any other power generator, and whose +thermal efficiency is very much greater. + +[Illustration: FIG. 12.--Longitudinal Section of Cylinder.] + +Figs. 11 and 12 show respectively a sectional end and side elevation of +the cylinder, from which it will not be difficult for the reader, +however unacquainted he may be with gas-engine work, to see how the +various requirements and peculiarities of the engine should be +considered and provided for. + +A most important desideratum in any machine or engine is that it shall +be as simple in construction as ever possible; complicated mechanism +should only be introduced when such addition or complication compensates +adequately for what must necessarily be a higher first cost, and +incidentally the greater wear and tear and attention involved. Figs. 11 +and 12 show what has been done to simplify the construction of the gas +engine in recent years. The main feature in this case is the very +get-at-able position of the two main valves--the air valve F and the +exhaust E. These valves, as may be seen from the drawing, are capable of +withdrawal after the cover of the combustion chamber has been removed. +The latter is an iron casting, shaped and faced up to make an absolutely +tight joint; no asbestos or any packing is used to make this joint--and +is held in place by four studs, as shown. Thus, all that is necessary is +to remove the four nuts, lift the cover off, then pull out the pins +which keep the spiral springs in position, and withdraw the valves. The +latter are seated direct on to the metal of the cylinder casting, the +gun-metal bushes A and B acting as guides. Further reference to A (the +mixer), which serves a twofold purpose, will be made later on. + +The gas valve and cock are mounted in a separate casting, which is +carried by a couple of studs, the joint between this and cylinder being +made with a piece of rubber insertion. The gas enters at the gas-cock, +passes through the valve and port G, and round the annular space in the +bush or "mixer" A, previously mentioned, and thence through a number of +small holes in same, immediately below the seat of the air valve F. At +the same time, pure air is drawn in _via_ the air box (as explained +hereafter), through port L (fig. 11), and thence up the centre of bush A +and over the small holes through which the gas is flowing. The two then +thoroughly mix and enter the combustion chamber together as the air +valve F is opened. This device produces a perfectly homogeneous mixture, +which conduces in no small measure to perfect combustion when the +explosion takes place, and upon which, to a very great extent, depends +the efficiency of the engine. Besides possible loss in this direction, +however, there is another source of waste which cannot be eliminated, +and that is the heat taken away by the cooling water which surrounds the +cylinder. As this loss is inevitable, the best thing we can do is to +make it as small as possible. Theoretically, it would be no small +advantage if we could work at very much higher temperatures than we do +at the present time, and it is only certain mechanical difficulties +which bar the way and so effectually prevent the already high thermal +efficiency of the engine being greatly increased. + +It is no easy matter to overcome these difficulties completely, but +improvements in this direction are continually being made, so that +troubles which attended the gas-engine user years ago no longer exist. + +All that we require of the cooling water is that it shall keep certain +working parts of the engine at a reasonable temperature; for instance, +the cylinder must not be so hot as to deprive the lubricating oil of its +property to lubricate, neither must the exhaust valve become so hot as +to cause it to seize in the bush and stick up; but, beyond such +considerations as these, the higher the temperature is at the +commencement of each explosion the more efficient will the engine be. +The object, then, is to do as little cooling as possible, and to apply +the cooling effect at the right parts; hence the passages and chambers +through which the cooling water circulates should be so arranged that +those which require to be kept at a low temperature are in close +proximity to the cooling water. On some of the engines of days gone by, +the exhaust valve was carried in a large iron casting, this in turn +being bolted to the cylinder casting and communicating with the +combustion chamber by means of a port. Such an arrangement was found to +be not only clumsy but inefficient; the water passages were small and +difficult to get at; they readily furred up; and moreover, the joint +between this casting and the cylinder was necessarily a water _and_ +explosion joint, and the fewer we have of these the better. + +The method--if it may be called a method--of overcoming or preventing +the exhaust valve becoming too hot is, in the case of figs. 11 and 12, +simply one of judicious arrangement and design. The cooling water enters +by the inlet K (fig. 11), and circulates round the exhaust valve port X +and valve E immediately, before becoming heated, thus keeping the +hottest of the working parts of the engine at a suitable temperature; +and the valve seat, being in direct metallic communication with the cold +water, does not become burnt or pitted. On the other side of the exhaust +valve we have the air valve and its passages, through which cool air is +continually being drawn; this also helps to keep the exhaust valve cool. + +From this, then, we may conclude that overheating of the cylinder will +not occur under normal conditions, given an engine of good design; but, +if this trouble does arise, we may safely look first of all for some +defect in the cooling water circulation. Some waters contain a greater +amount of impurities than others, and consequently the water space may +furr up more rapidly in one district than in another. But this deposit, +even under the worst conditions, accumulates very slowly, and the +operation of cleaning out the water-jacket is a very infrequent +necessity. The exhaust valve, however, may become overheated if it is +allowed to get into bad condition, _i.e._, leaky. Its seat should be +well looked after, or the hot gases will blow past when it is +presumably shut; and if this defect, slight though it may be to begin +with, is allowed to develop, both the seat, the valve head, and the +spindle will become burnt away and pitted, perhaps badly, due to the +excessive heat. + + + + +CHAPTER IV + +IGNITION DEVICES + + +The ignition devices commonly employed may be divided into three main +classes--the metal tube, the porcelain tube, and the electric ignition. +These again may be subdivided: The first being either iron or nickel +(hecknum as they are sometimes called); the second are of two +kinds--single-ended and double-ended; and the third takes many forms +which many of my readers are possibly well acquainted with, such as the +magneto, the induction coil and trembler, and the high-tension magneto +ignition, the latter device having been used successfully on various +occasions, though not yet universally adopted. + +The first-named have one or two advantages over the nickel tube. They +are very inexpensive, and are easily heated to the required temperature; +moreover, they can be made at home, should occasion demand. On the other +hand, they are not so durable, have a very uncertain life, and +consequently need renewing frequently--their average life being not +more than 60 working hours. Fig. 13 gives an outline drawing of an iron +tube, with its burner and chimney fixed in position. The tube is very +similar to a piece of 1/4-in. gas-barrel, closed up at one end and a +taper thread (1/4-in. gas) cut on the other; in fact, gas-barrel may be +used for making these tubes at home--and measure about 7 or 8 in. over +all It is screwed into a firing block, which in turn is screwed into the +combustion chamber end, so that when right home it is in such position +that the tube stands quite vertical. The section of the tube, fig. 13, +shows the condition it gets into after having been in use some time. The +bore, it will be seen, has become almost completely closed up, so that +there is practically no communication between the hot part of the tube +and the combustion chamber. This closing up of the bore is very gradual, +and it is in the early stages of this process that erratic firing is +likely to occur; sometimes the charge will be successfully fired and +sometimes not. It may be as well to mention here that the length of the +tube, although to a certain extent immaterial, should neither be +excessively long nor abnormally short, the precise length varying with +the size of the engine. A 1/4-in. tube, 8 ins. long, may be used +successfully on engines ranging from 1/2 to 6 horse-power, provided a +suitable burner is fitted enabling the tube to be heated at any required +spot. After the first charge has been fired, and the exhaust takes +place, practically all the burnt gases are cleared out of the cylinder, +but a small amount of these will generally remain in the tube and the +bore of the firing block. On the ensuing compression stroke these inert +gases are compressed to the far end of the tube, thus making way for the +explosive mixture to reach the hot portion, and explode, thus sending a +jet of flame into the main volume of the mixture which is immediately +ignited. Hence there is no advantage in having a tube too long, while, +on the other hand, it _must_ not be too short. + +[Illustration: FIG. 13.] + +[Illustration: FIG. 14.] + +[Illustration: FIG. 15.] + +The asbestos lining, shown in fig. 13, may be of various thicknesses, +according to the size of the chimney and the tube; the reason for this +will be apparent to many; but being a most important factor in the +heating of the tube, and consequently the working of the engine, it will +be advisable to deal with this point more fully. + +Due mainly to the peculiar behaviour of iron tubes under heat and +internal pressure, it is always advisable to look to them first of all +when the engine shows signs of missing fire; and to always examine the +bore of a fresh one, and ascertain that it is perfectly clear before +putting it in. The adjustment of the ignition tube, although one of the +most important and necessary to be made on the whole engine, is in +itself a perfectly simple matter. It must be understood that the +ignition tube cannot, with the ordinary means at our disposal, be kept +at too high a temperature; but it must not be assumed that either the +_size_ of the flame, or the _time_ the flame has been alight, is +conclusive evidence that the tube is, or ought to be, sufficiently hot +to fire the charge successfully. It is an uncommon thing to hear a man +exclaim--after it has been pointed out that his tube is practically +cold--"Why, it's been alight for hours!" + +If such is the case with you, reader, you may very rightly assume that +the burner is not properly adjusted, and so does not give the _right +kind of flame_. + +In order to get the hottest possible flame, the quantity of gas and air +must be mixed in the right proportions. A common fault is that there is +too much gas allowed to flow through the nipple, compared with the +amount of air being drawn in at the air aperture, fig. 13. The result +is, we get a flame of great length, but one which is not at all suited +to our requirements; and instead of giving up its heat to the tube and +the asbestos lining of the chimney, a large amount of gas we are +presumably burning _in_ the chimney is not being burnt there at all, +for, on applying a light just above the chimney top, a quantity of this +gas we are wasting will be seen to burn with a flickering blue flame. + +To put matters right, it is necessary to do one of two things--either +cut down the supply of gas or increase the air-supply. Providing the air +aperture is normal, _i.e._, the same size as it was originally, it is +better to adjust the _gas_, which may be done by tapping up the nipple +N, as indicated in the enlarged sketch, fig. 14, until just the right +amount of gas can flow. + +As a rule, if there is too much air, the flame will burn with a loud +roaring noise, and is liable to fire back. The nipple should then be +opened out with a small reamer--the tang of a small file, ground to a +long taper point, makes an admirable tool for this purpose. Whether the +burner is of the ordinary bunsen type, or the ring or stove type, the +above remarks apply, as in every case the flow of gas is governed by the +size of the orifice through which it flows. + +There is no need to use anything beyond a touch of oil when putting in a +new tube, in order to make a perfectly tight joint; white or red lead +are quite unnecessary, and are liable to make it a troublesome matter to +remove the tube on future occasions. Neither should undue force be +applied when putting in new tubes; it is liable to wear the thread in +the firing block, which results in a partial stoppage of the ignition +hole, as indicated in fig. 15. This is especially the case if we happen +to get hold of a tube with its screwed part slightly smaller than usual. + +The asbestos with which the chimney is lined should be about 1/8 in. +thick, and, when renewing, the same thickness should be used as +originally. A thicker board will reduce the annular space round the +tube, and will have a choking effect on the flame--much the same as +referred to above, when there is too much gas and not enough air. A +simple method of lining the chimney is to cut a block of wood to the +inside dimensions of the chimney, less 1/4 in. in width and thickness, +then soften the asbestos cardboard by immersing in water, and bend it +round the wood, cutting off to the required size, _i.e._, till the two +edges form a neat butt joint. It can be allowed to remain on the mould +until dry--when it will retain its shape--or can be put into the chimney +straight away, if it is wanted for use immediately. In the latter case, +however, it will be some fifteen minutes or so before the tube will +attain its working temperature. Asbestos linings gradually become worn +and ragged, and small flakes are apt to detach themselves and fall down +into the burner, which, of course, prevents the flame playing as it +should around the tube. In such cases it is not always necessary to fit +a new lining; if the chimney is removed, the loose flakes shaken out and +the asbestos well damped and patted down with a wooden or steel +foot-rule or other suitably shaped tool, it will be fit for another long +spell of work. + +The nickel or hecknum tubes are treated in the same manner as the iron, +but, as we mentioned before, are more durable, but require more heating +to get them up to a workable temperature. Their greater first cost is +compensated to some extent by makers in some cases guaranteeing them for +six months. + +Of the porcelain ignition devices, we will deal with the double-ended +tube first, it being the more commonly used of the two in this country. +This form of tube is usually about 3 in. long, 1/2 in. diameter, and +open at both ends. It may be mounted in a metal casting, in form not +unlike the small gas stoves for heating soldering irons. It is heated +the greater part of its length by a couple of rows of gas jets, and is +frequently surrounded by an asbestos lining. The whole arrangement is +in reality a tiny furnace. When in position for working, one end of the +tube is open to the ignition passage leading and communicating with the +combustion chamber, while the other end is sealed, through butting up +against a metal cap or plate. An asbestos washer is interposed between +the tube at each end and the metal it bears against, thus making a more +or less flexible joint. A thumb screw is arranged at the outside end of +the tube, by means of which pressure can be applied to clamp it up +between the washers to the desired extent. Some care has to be exercised +in adjusting this form of tube for running. When heated to the working +temperature it, of course, expands, so that, if tightened up too much +when cold, it is under a fairly high compression; and when the engine is +started, and the explosion takes place, it not infrequently bursts, if +there is not sufficient "give" in the washers to allow for the +expansion. On the other hand, if not clamped up sufficiently tight to +start with, when the explosion occurs, the washer at one or each end is +blown out. This adjustment has to be made to a nicety, and, although a +somewhat difficult matter, success may be attained after one or two +trials. It is advisable, after a new tube has been put in, to start up +the engine gently, _i.e._, with less than the normal supply of gas, and +increase to the full amount gradually whilst running. This may be done +by simply opening the gas-cock on engine partially in the first place. + +The single-ended porcelain tube is not so well known here as on the +continent; why, we cannot say; certainly it is preferable in every way. +We give a few illustrations, showing the method of using this tube. + +Figs. 16 and 17 show the general arrangement of tube and chimney and the +manner in which they are fixed to the cylinder. The device consists +primarily of three parts--the body or chimney B, the cover C, and the +tube itself T. The body is a light iron casting, carried by a couple of +studs SS, which are either screwed into the firing block F, or direct +into the metal of the cylinder casting if no firing-block is used; the +latter may very well be dispensed with in the smaller-sized engines. + +The tube is made of thin porcelain, slightly bell-mouthed at its open +end, and is mounted in a thick metal washer W, as shown in fig. 18 in +section, the joint being made with a little asbestos paper, moistened. + +The block F and the face of the body B (fig. 16) are recessed to take +the washer W easily, but the depth of both recesses taken together must +be about 1/16 in. less than the thickness of the washer W; thus, when +the tube is placed in position between the body B and the block F, and +the former screwed up by means of the two nuts, as shown in the figure +16, the effect is to clamp the _washer_ which carries the tube, but _not +the porcelain tube itself_. + +[Illustration: FIG. 16.] + +[Illustration: FIG. 17.] + +[Illustration: FIG. 18.] + +The latter is left perfectly free to expand; and yet, owing to its +particular shape, the pressure in the cylinder during the compression +and explosion stroke only tends to make the joint between the tube and +washer more secure. The action of this ignition device depends upon the +tube heater H, which is merely a small bunsen burner, the flame of which +impinges on the tube at one particular spot, raising it to a very high +temperature--almost white heat. Most of my readers will know the +formation of the bunsen flame. It in composed of two distinct zones. The +inner one, marked A in fig. 18, is a perfectly cold part of the flame, +and appears to be a pale-blue coloured cone. + +It is the outer zone which is the hot portion of the flame, hence this +part _only_ must be allowed to play on the tube. The tip of the blue +cone A must be kept about 1/4 in. below the tube, in order to ensure the +hottest part of the flame impinging precisely where the heat is +required. + +The total length of the whole flame is, to a certain extent, immaterial; +but, generally speaking, it should be adjusted so that the length of the +inner cone A is about 1 in. or 1-1/4 in. The same methods which we +described in the early part of this chapter can be employed in the +adjustment of this burner, but some care should be exercised to get the +correct flame length. + +The result of allowing the cold part of the flame to impinge on the tube +is observable in fig. 18. The black spot indicated on the drawing +actually appears as a black or sooty spot when looking at the tube under +these conditions; but in reality no discoloration whatever takes place, +the spot disappearing immediately the cone A is made shorter, or the +burner H lowered in the chimney B, so that the tip of A is just below, +and does not touch the tube at all. + +The adjustment of the length of cone A may be accomplished in two +ways--(1) by keeping the supply of gas constant, and varying the amount +of air admitted at aperture K, fig. 18; (2) by keeping the supply of air +constant, and varying the amount of gas admitted through nipple N. The +first method is to be preferred when it is necessary to make any slight +adjustment due to the variation of gas pressure during the day, and may +be accomplished by fitting a small sliding shield G, as shown in the +figs. 16 and 17, and moving it round so that it covers, more or less, +the aperture K. Thus the length of cone A may be adjusted to a nicety in +a very few seconds. This shield keeps all draughts and puffs of wind +from the fly-wheel away from the aperture, and helps the flame to burn +very steadily. In the first place, of course, the flame will be +regulated by opening out or tapping up the nipple N (an enlarged sketch +of which is given in fig. 14), so that cone A is just about 1-1/4 in. +long when air aperture is full open; but once this is done, any future +adjustment can be made by throttling the air-supply, or raising or +lowering the burner bodily, the set screw keeping it in any desired +position (see fig. 17). + +From the foregoing remarks it will be seen that the most noteworthy +features of this form of ignition are the ease and certainty with which +the tube can be fixed in a few moments; that when the two nuts on the +studs SS have been tightened up there is no likelihood of the joints +being "blown," for, as we said before, only the metal washer is clamped +up, the porcelain tube itself being as free to expand as it was before. +It is also at once obvious when any adjustment of the flame is +necessary; there need be no uncertainty as to whether the tube is hot +enough or not. + + + + +CHAPTER V + +MAGNETO IGNITION + + +The third form of ignition we have to deal with is the electric. + +There are a great number of different types made and used, but for +gas-engine use perhaps that known as the magneto ignition is the most +satisfactory. With this form, neither accumulators, dry batteries, or +spark coils are required, and consequently a greater simplicity is +arrived at than would otherwise be the case. + +In fig. 19 we show diagrammatically the ordinary form of magneto +machine. Virtually it is a small dynamo which is fixed to the side of +cylinder casting, and is operated in the manner shortly to be described. +As we do not propose to enter into more than a brief explanation of why +and how this apparatus generates current to produce the required spark, +perhaps a simple analogy will make matters most intelligible to any +reader not well acquainted with electrical phenomena. We know that when +a current of electricity is flowing in a wire, and the wire be suddenly +broken, a spark will occur at the point of breakage. This fact may be +observed in an ordinary electric bell when ringing; at the tip of the +contact breaker a number of tiny sparks may be seen to occur, due to the +rapid make and break of the current flowing in the circuit. Precisely +the same action takes place in our magneto-igniter, but, instead of a +multitude of tiny sparks, we produce one at a time, at definite +intervals, viz., at the commencement of each explosion stroke. + +[Illustration: FIG. 19.] + +In the later form of magneto machines there is a soft iron sleeve +between the magnet poles and the armature. The former is connected to a +system of levers by which a reciprocating motion is imparted to it by +means of a suitably arranged cam on the side shaft. It has been found +that better results are obtained by causing the magnetic field to move +relative to the armature winding than to move the latter through a +stationary field. Reference to the diagrams, figs. 20 and 21, will make +this clear. + +In fig. 19 the cam C is shown just on the point of allowing the lever L +to fly back into its normal position, due to the action of the springs +comprising a dashpot S. As the cam rotates, it pushes the lever L to the +left, the sleeve (or virtually the armature A) is also rotated through a +portion of a revolution comparatively slowly; but as soon as L is +released, the sleeve (or armature) flies back again almost +instantaneously and for the moment is generating a current in the same +manner as would any ordinary continuous current dynamo. + +[Illustration: FIG. 20.] + +[Illustration: FIG. 21.] + +At the instant the maximum current is being generated, the circuit is +broken by means of the contact breaker D, fig. 19, which we show in +detail in fig. 22. The latter is mounted on the end of the combustion +chamber, and consists of two parts, D and P. + +[Illustration: FIG. 22.] + +D is an easy fit in the hole bored to receive it, and has a mushroom +valve head and seating, as shown, so that it moves readily when struck +by the projection E on the rod R (fig. 19); but yet, acting in the +manner of a non-return valve, it allows no gas to escape when the +explosion takes place in the cylinder. D is therefore in direct metallic +communication with the engine frame and earth. + +P is a fixed metal pin, carefully insulated from all contact with the +engine frame and earth. To this pin one end of the armature winding is +connected, whilst the other end is connected to the engine frame. + +Thus a closed circuit is formed, and when the current is generated it +flows from one terminal of magneto through wire to pin P, on to D, +through D to earth (_i.e._, engine frame), and so back to other terminal +on magneto. + +And as the circuit is broken between D and P, we obtain a spark, as +previously explained, which may be timed to take place by adjusting the +position of cam C on side shaft relatively to the position of piston. + +It may be said that the position of the magneto-igniter is immaterial; +it will be fixed in different positions on different types of engines, +and so long as the operating mechanism is simple and effective, _i.e._, +as direct as is practicable, it works well, and requires little +attention. The timing of the spark will be dealt with in the chapter on +Cams and Valve Settings. + + + + +CHAPTER VI + +GOVERNING + + +The devices for governing the speed of the engine may be divided, +broadly speaking, into two classes--the inertia or hit and miss +governor, and the centrifugal. Of the latter type we will give an +instance first. In figs. 23 and 24 the governor gear is shown +diagrammatically, consisting of a couple of weights WW suspended from a +vertical spindle. These fly apart when caused to revolve by the bevel +wheel gearing BB, and raise the sleeve S to a greater or lesser extent. +A recess in the latter engages a lever arm L, through which the vertical +movement of the sleeve S is converted into a horizontal movement of the +sleeve T. The latter is carried by the valve lever P, and is virtually a +roller which engages with one or other of the steps of the cam C, +according to the speed of the engine. The object of this arrangement is +to keep the ratio of air to gas uniform throughout all variations of +load. The gas and air valve are shown as both being operated by the same +lever P, the accurate timing of the latter being obtained by means of +set screws. + +[Illustration: FIG. 23.] + +[Illustration: FIG. 24.] + +Messrs Dougill & Co.'s engines are fitted with a step down cam and +governor such as this. The centrifugal governor is often arranged so +that instead of the charge being merely reduced in volume, the whole +charge is cut out, and no explosion whatever takes place. (In this +respect the same results are obtained as when a hit and miss governor is +used, and the latter form therefore is to be preferred, especially on +small engines, where the difference between the indicated power and the +brake power is always, even under the best conditions, fairly great.) + +In this case the governor lever only operates the gas valve; the air +valve being opened on every charging or suction stroke, whether gas is +admitted or not. + +Another application of the centrifugal governor is to suspend a distance +piece on the end of the governor lever, so that at normal speed this +distance piece is interposed between the gas valve spindle and the lever +operating it. In that case the gas valve will be opened. But if the +speed is above the normal, the distance piece will be raised clear of +the valve spindle, and the opening mechanism (driven by a cam on the +side shaft) will simply move forward and recede again without ever +touching the gas valve. + +There are any number of movements which have been, and there are many +more which could be, devised to give the same result; and it depends +principally upon the form of engine in question which device we adopt. + +The simplest and most direct action is, however, always the best; +complicated mechanism is to be deprecated, especially on small engines. +For this reason is the inertia governor more generally fitted to such +engines. + +[Illustration: FIG. 25.] + +A simple form of this governor is shown in fig. 25. The gas valve V is +shown on its seating. It is screwed into a pecker block B, and pinned as +shown. The latter should be of cast steel, tempered to a straw colour; +or if mild steel or iron is used, it must be well case-hardened, in +order to resist wear. The pecker P (also tempered hard) is mounted on +the cast-iron weight W, which in turn is pivoted on the valve lever L. +It will be seen that the weight W (which is only held in the position +shown by the spring S) will tend to lag behind when a sudden upward +motion is imparted to the lever L. Thus it depends upon the degree of +suddenness with which L moves whether the pecker P remains in the same +relative position to the lever as the latter travels upwards and engages +with the pecker block B, or whether it misses it and simply slides over +the face of the block. The adjustment of the spring S is effected by +screwing up or slacking out the milled nuts T; and on the degree to +which this spring is compressed depends the sensitiveness of the +governor, and consequently the speed of the engine. To obtain accurate +and steady governing with this type of mechanism it is essential that +the weight be perfectly free on its spindle, and that nothing but the +spring S holds, or tends to hold, it in the position shown. On this +account it is advisable to provide a "lip" on the pecker block, as +shown, to keep the area of contact as small as possible. This +effectually prevents any sticking, should a superfluity of oil happen to +get on either block or pecker. For similar reasons there should be some +clearance between A and the pecker, _i.e._, the latter should only bear +at one point and not bed flat against A. + +Another form of inertia governor is shown in fig. 26 of the hit and miss +type, which is employed by Messrs Capel & Co. on many of their engines. + +It consists of three main parts--the brass arm L carried on a stud D, on +which it is free to move; the weight W, which carries the pecker P +pivoted at the upper end of L; and the pecker block B, which engages the +pecker when the engine requires a charge of gas. + +The governing action is dependent upon the shape of the operating cam +from X to Y. (In the case already dealt with, the lever L serves to +operate both air and gas valves, and so one cam only is necessary; but +in this instance the gas valve is operated by a separate cam, and a +greater nicety of adjustment is obtainable.) + +[Illustration: FIG. 26.] + +If the speed of the engine is sufficiently high, the arm L is thrust +forward at such a rate that the weight W tends to lag behind, with the +result that P is raised above the notch in B, as shown by the dotted +lines in drawing. On the other hand, when the speed is too low, the arm +L will not be thrust forward with so great a degree of suddenness, the +weight W will have time to move with L, and the relative position of W +and P to L will remain the same. Hence, in the first case, when a +_further_ forward movement is given to L by the cam, the pecker P is +clear of B, and omits to open the gas valve V; in the second case, P +engages with B, and the gas valve is held open during the time the +portion of cam Y to Z is passing over the roller R on arm L. + +The great drawback to some forms of governors is not that they fail to +govern well when new, but that no provision is made to ensure them +working steadily when a bit worn. The shape of the cam has everything to +do with the regular working of this form of governor. + +Supposing our cam was of the shape shown in fig. 27, _i.e._, the +governing and opening portion all in one curve, it would cause the +pecker to move both _forward_ and in an _upward_ direction at the _same +time_, so that at the moment of engaging B, P might still be moving in +an upward direction, which would cause uncertainty of action, especially +if the tips of the engaging members were at all blunt through wear; and, +in all probability, P would fly off B after partially opening the gas +valve. + +This behaviour is very undesirable, as the small quantity of gas so +admitted to the cylinder is quite useless, and a sheer waste is +incurred. With the governing arrangement shown in fig. 26, this trouble +does not exist. The cam is so designed that the first rise from X to A +determines whether or not the valve is to be opened; the curve from A to +Y is struck from the centre of the side shaft; thus, during that +portion of the revolution the arm L is stationary, and the pecker at the +same instant takes up a definite position either in the notch in B or on +top of it, and is ready to open the valve if the speed of the engine is +such as to require an explosion, or simply to slide over the top of B, +allowing the valve to remain closed. It is most interesting to observe +the action of this governor; when an engine fitted with one is running +very slowly, the three distinct movements of the pecker P may be clearly +discerned as the respective portions of the cam pass over the small +roller R. + +[Illustration: FIG. 27.] + + + + +CHAPTER VII + +CAMS AND VALVE SETTINGS + + +With the gas, as with any other kind of engine, the valve settings are +of primary importance. On very small engines it is often the case that +only the exhaust valve is operated mechanically. + +Again, there are several well-known makes which operate the gas and +exhaust mechanically while the air valve is opened by suction alone. +Though opinions differ as to which is the best course to take, there can +be little doubt that, with all three valves mechanically operated, a +greater nicety of adjustment is obtainable than would be otherwise +possible. And provided the working parts are neatly made and finished, +they will take but little power to drive them; and such loss would be +compensated by the additional power and efficiency obtained from the +engine, due to satisfactory and correct adjustment. + +In fig. 28 we give a diagram showing the exact positions of the crank +when the gas, air, and exhaust valves open and close respectively, under +normal conditions of working. The solid circle represents the first +revolution of the crank shaft, starting from the commencement of the +suction stroke, and the dotted circle the second revolution, during +which the explosion and exhaust strokes take place; the dotted +horizontal line shows the position of crank at the back and front dead +centres. + +As a clear conception of why certain things happen under certain +conditions is most desirable, we will first describe the operation of +marking off the cams which operate the respective valve levers, and then +discuss the effect of various "settings" of the valves on the running of +the engine. + +[Illustration: FIG. 28.] + +Assuming that we are still dealing with the Otto cycle engine, the cam +or side shaft will revolve at precisely half the speed of the crank +shaft. This 2 to 1 motion is obtained by means of toothed wheels, or a +screw gear. In the former case, where plain or bevel cog-wheels are +employed, the one fixed on the crank shaft must be exactly half the +diameter of the one on the side shaft, _i.e._, it must have one half the +number of teeth. On the other hand, if a screw gear is used, the +relative diameters of the two wheels may vary, but the pitch of the +teeth on the one must be twice that of the other. These wheels sometimes +have the teeth or thread formed in the casting, and sometimes they are +cut after a plain casting has been made. The latter kind are, needless +to say, better than the former, which often require filing up in order +to make every tooth alike, and ensure sweet running. + +We know already in what positions our crank has to be at the opening and +closing of the three valves, and with the aid of the diagram, fig. 28, +we can determine the size of the cams. In fig. 29, S is the side shaft +to which the cams have to be keyed, R the roller on valve lever, the +latter being represented by the centre lines LL, as all we require to +find is the motion this lever will transmit to the valve, the spindle of +which is shown at V. + +Fig. 30 shows diagrammatically the position of crank at the opening and +closing of the air valve. From this we see that the angle through which +the crank travels during the time the air valve is open is equal to the +obtuse angle ABC. Now, as the side shaft S revolves at half the speed of +crank, it is obvious that the former will travel through only half that +angle in the same space of time, _i.e._, through an angle equal to ABD. +We can now transfer this angle on to S, fig. 29, and draw two lines SE, +SF, cutting a circle GHJ, representing the back of the cam, which latter +passes in front of the roller R without causing any movement of the +lever L. + +[Illustration: FIG. 29.] + +[Illustration: FIG. 30.] + +[Illustration: FIG. 31.] + +It will be seen that by drawing a line forming a tangent to the circle +GHJ at F and another at E, and producing these, they will meet at point +K. Consequently, as the side shaft rotates in the direction indicated, +the lever L will _begin_ to open the valve V when the cam is in the +position shown in fig. 29, reach a maximum opening at K, and finally +close when the cam has moved so that point E is now where F was. With a +cam of this shape, however, a considerable portion of the stroke would +have passed before the valve was raised any _appreciable_ distance off +its seat; it would only be fully open for an instant, viz., when K was +passing over R, and would begin to close again directly. + +Moreover, if the engine were running at even a slow speed, the motion +imparted to lever L would be indefinite; and this, especially if the +governor is fitted to the air valve lever, as in fig. 25, is very +undesirable. Therefore, to obtain a definite opening we must set out the +cam, as shown in fig. 31. In this diagram the roller is shown standing +clear of the back of cam by about 1/16 in. A line MN is then drawn, +forming a tangent to both roller R and circle GHJ at points F and O +respectively. This gives us the opening portion of cam. Then from the +centre S with radius SF describe the arc FE (shown dotted in fig. 31), +and set off the angle required (ABD, fig. 30), as previously explained. +Through point E draw a line forming a tangent to circle GHJ, and produce +it towards P. This line gives us the closing portion of cam. The +distance W is of course variable, according to the amount of lift we +give the valve. By comparing these two diagrams it will be seen that in +both cases the valve will be opened the same length of time, but in +first case the motion will be indefinite and uncertain. In practice the +corners are rounded off somewhat, in order to obtain a steady motion; +and when the air cam is also the governing cam, it is advisable to round +off the opening face, as indicated in fig. 32. Upon the shape of this +face both the sensitiveness and the life of the governor gear depends. +If it is nicely rounded off, giving a gradual rise, very little tension +(or compression, as the case may be) of the controlling spring will be +necessary to give the required speed to engine; whereas, if the rise is +sudden, the spring will have to be screwed up tighter, and, if uneven +and lumpy (_i.e._, not a fair curve), the result will, of course, be +erratic governing. + +[Illustration: FIG. 32.] + +A certain amount of clearance should always be provided between the +roller and the back of cam (compare figs. 29 and 31), that is, the +roller should not bear against the cam, except during that portion of +the stroke in which it is actually operating the valve, viz., from F to +E (fig. 31). A small stop interposed between the lever and some +convenient part of the engine, such as the side-shaft bracket bearing, +answers this purpose. + +[Illustration: FIG. 33.] + +[Illustration: FIG. 34.] + +The size and shape of the exhaust cam is found in the same manner as +above described; the angle through which it operates is greater than +that of the air cam, and is shown in fig. 33. A fair margin should be +allowed for filing or machining these castings up; the shape and sizes +arrived at by the above described method being finished measurements. +Fig. 34 gives the outline of an exhaust cam worked out from the setting +diagram, fig. 33. + +[Illustration: FIG. 35.] + +[Illustration: FIG. 37.] + +[Illustration: FIG. 41.] + +[Illustration: FIG. 42.] + +We may now consider the relative positions these two cams will occupy +when keyed up on the side shaft. Assuming that we have both cams +finished to the proper shape and size, and the keyway cut in the side +shaft, we can commence to mark off the position of keyway in the air +cam. With the crank in the position shown in fig. 35, the air cam is +slipped on to the side shaft and brought to the position shown in fig. +32. The keyway being already cut in the side shaft, the position for +that in the cam may be scribed off, as shown by dotted lines (fig. 32), +the cam removed, and the keyway cut. It is as well, however, to check +this mark by turning the crank round to position shown in fig. 37, +_i.e._, the closing of air valve. The side shaft will also turn through +exactly half this angle, so that when the cam is again slipped on the +latter, the scriber marks and keyway in shaft should be exactly in line, +as they were in fig. 32, and the fall of the cam--the closing +portion--should just be touching roller R, but not sufficient to keep +the valve open (see fig. 38). The slightest movement of the crank from +this point in a forward direction should result in a little play being +felt in the lever L, assuming that the cam is also moved just enough to +keep the scriber marks in line with the existing keyway. + +[Illustration: FIG. 36.] + +[Illustration: FIG. 38.] + +[Illustration: FIG. 39.] + +By these operations it will be at once evident whether the cam is too +large or too small. Supposing it is too small, we will obtain two sets +of marks indicating the position of keyway, as shown in fig. 39, and it +is obvious that we must give the lever less play by screwing up the set +screws shown in fig. 11. The effect of this is to cause the valve to +open earlier and close later than it would if the play were greater; as +it would were the operating portion of cam larger. A minimum amount of +play must always be allowed, however. When two sets of marks are +obtained, the mean must be taken and the keyway cut as shown by the +thick lines in fig. 39. The exhaust cam in larger engines is usually +made with a swelling on the opening portion, as shown in fig. 40, so +that the valve is _very slightly_ opened some time before the crank has +reached the position shown in fig. 41. Fig. 42 shows position of crank +at the close of exhaust valve, and the two last-mentioned diagrams +correspond with the two positions in which the exhaust cam is shown in +fig. 34. The small lump on the back of exhaust cam, fig. 40, is only +required on engines above 3 B.H.P. to relieve the compression on the +compression stroke when starting up. By moving the roller R on valve +lever longitudinally, so that it engages both parts of cam as they pass +in front of it, the exhaust valve is held open during a small portion of +the compression stroke, usually closing when the crank has reached the +bottom centre. + +Referring again to fig. 26, this gas or governor cam may be set out, and +the keyway marked on the same principle as already described for the air +and exhaust valves. An end view of the three cams keyed up on the side +shaft is given in fig. 40A. In small engines it is convenient to have +the air and exhaust cams made in one casting, when one key only will be +required. On some engines, instead of employing a movable roller or +valve lever, the exhaust cam is fitted on side shaft with a +"feather"--_i.e._, a headless key--and the cam being capable of +longitudinal movement, such movement being controlled by a small lever +or handle, called the half-compression lever. + +[Illustration: FIG. 40.] + +[Illustration: FIG. 40A.] + +Having once thoroughly grasped the important part the cams play in the +working of the engine, it will be an easy matter to adjust the valve +settings, and to keep them adjusted correctly. The effect of a wrong +setting will then be strikingly apparent. On small engines a separate +cam to operate the gas valve is not a necessity; and the practice of +fitting the gas valve spindle (or the pecker, the effect would be the +same) with a device for increasing or diminishing its length, is also +unnecessary and unsound. + +The wear on a well-designed gas valve operating mechanism is practically +nil; and even if there was wear, the effect would be to cause the valve +to open a trifle later and close sooner than it would otherwise, _i.e._, +it would remain open a shorter time during each charging stroke. This in +turn (other conditions remaining the same) would give us a weaker +mixture; and although too weak a mixture is preferable to a too rich +one, we should have to adopt some means of increasing the richness of +the mixture; otherwise the maximum power of the engine would soon be +seen to diminish. + +To get the mixture normal again we must either enlarge the gas inlet or +cut down the air-supply somewhat, and so keep the proportions the same. +That is to say, the quality of the mixture is dependent upon the +relative dimension of the gas and air inlets. We know by actual trial +that if at the completion of the charging stroke the pressure in the +cylinder is approximately that of the atmosphere, better results are +obtained than when the pressure is considerably below that of the +atmosphere. Thus, the larger we make the inlet ports (but still +retaining correct relative dimensions) the more readily will the mixture +be drawn into the cylinder as the piston moves forward, tending to +create a vacuum. Of the two courses open to us to retain a good mixture +it is preferable to open out the gas-supply, for by cutting down the +air-supply, and sucking the gas in, due to the partial vacuum being +formed, we should be keeping the proportions correct at the expense of +reducing the total volume of the explosive mixture (more strictly +speaking, the density of the charge) admitted to the cylinder. + +Under normal conditions it is not necessary to create a high vacuum to +suck the gas into the cylinder, but it is as well to understand what +results we would tend to produce, did we work on these lines. Of course, +with small high-speed engines fitted with suction air valve, the vacuum +is higher than it would be in slow-speed engines with mechanically +operated valves. If we take an extreme case as an example, where, to get +any gas to speak of into the cylinder the air-supply would have to be +cut down or throttled to an abnormal extent, we will realise at once +that such a small quantity of both air and gas would have been drawn in, +and consequently the mixture would be so rarefied that on the +compression stroke the pressure would possibly be extremely low and +totally inadequate to produce efficient working. Moreover, working at +such a high vacuum as this would not only prevent us obtaining a normal +explosion in the cylinder, but would upset the working of the exhaust +valve. The latter being held down on its seat during the suction stroke +by means of a spiral spring would be lifted off its seat by suction (the +partial vacuum in the cylinder), and any burnt gases which happened to +be hanging about in the exhaust port or pipe would be drawn into the +cylinder again, and tend to damp the ensuing explosion. Too early +closing of the exhaust should be avoided almost as rigorously as too +late. The latter will affect the working in a similar way to the exhaust +being lifted on the charging stroke by suction; on the other hand, if it +closes too soon, the entire volume of burnt gases will not have been +swept out of the cylinder, and the effect will again be to damp the +following explosion. + +The gas valve opens just after the crank is above the back centre and +closes just before the front centre is reached, that is, opening a +little after the air valve and closing a shade before it, thus every +particle of gas is used in the cylinder, due to a draught of air being +drawn in after the gas valve has been closed. + +The settings of the valve being of primary importance, no matter what +size engine we are dealing with, and being also the most confusing +matter for anyone unacquainted with gas engines to grasp, it will not be +out of place to suggest a simple method of checking these settings. + +Let us begin by pulling the fly-wheel round backwards until we feel the +piston is on the compression stroke, then from this point--the crank +being about 45° above the front centre--pull the wheel round until the +crank is in the position for the exhaust opening (see fig. 28). In this +position there should be but the _slightest_ play in the exhaust lever, +showing that the valve is _just_ on point of opening; and by keeping +one's hand on the lever whilst the fly-wheel is pulled round _very +slowly_ (it is a good plan to get some one else to do the pulling +round), it is possible to ascertain the precise point at which the valve +opens. Next pull round till the crank is in the position for the air +valve opening, and observe that it is set correctly. Then go on to a +trifle above the back centre, where the exhaust valve should close, and +so on till the opening and closing of each valve has been checked. It +will be noticed that the air, and sometimes the gas, valve opens before +the exhaust closes. This overlap is necessary; and it will be found that +the smaller the engine and the higher the speed the greater this overlap +will be to obtain good results, although a good deal of individual +judgment must be used in settling the exact amount of overlap, as the +requisite amount may, to get the best results, vary in different engines +of precisely the same dimensions and type. + +When dealing with engines which have no separate gas valve--the gas +being admitted with the air, which is sometimes the case with very small +engines--the above notes referring to the gas setting independently, +will, of course, not hold good. + +It may be mentioned with regard to the lump on the opening side of the +exhaust cam, that this if overdone is found to be detrimental on large +engines, and even on small ones. If it is too large, it will cause both +exhaust valve and seat to become burnt and pitted, due to the surface +being exposed to the exceedingly high temperature of the expanding +gases. If it is too large, it is equivalent to opening the exhaust valve +too early, and the effect is the same, viz., a waste of power and damage +to the valve and its seat. + +[Illustration: FIG. 43.] + +[Illustration: FIG. 44.--Brake Testing.] + +The method of grinding in the valves to their seats with emery powder +and oil is so well known that no further description is needed here. We +give, however, in fig. 43 a sketch showing a very expeditious way of +dealing with very badly worn or burnt seats. The sketch explains itself. +Such a tool is readily made; even the cutter could be turned and filed +up to shape and then hardened at home. By lightly tapping in the taper +cotter pin little by little, sufficient pressure is put on the cutter to +make it an easy matter to completely re-face an old seat or form a new +one. A T-wrench or "tommy" can be used to work the cutter spindle. The +lower part of the latter must be the same diameter as the existing valve +spindle; the bush acts as a guide; and as the bevel of the cutter +should be the same as that of the valve, a very little grinding in with +emery powder is required to finish the job off. + +In fig. 44 we give a diagram showing the method of testing for Brake +H.P. of engine, as it is frequently interesting to make such a simple +test after any alterations or adjustments have been made. + +Two spring balances and a rope or cord (according to the size of the +engine), fitted with a few wood blocks as shown in section, fig. 44, to +keep the rope on the rim of fly-wheel, is all that is required for this +test. The following formula may be used for arriving at the B.H.P.:-- + + B.H.P. = (S1 - S2) 3.14 x D x R / 33000 + + S1 = Reading in lbs. of spring balance No. 1. + S2 = Reading in lbs. of spring balance No. 2. + D = Diameter of fly-wheel and diameter of brake + rope in feet. + R = Revolutions of fly-wheel per minute. + + As 3.14 x D / 33000 will always remain the same for any given engine and + gear, we may call that expression C; then the B.H.P. may be written-- + + B.H.P. = (S1 - S2) C R + + + + +CHAPTER VIII + +OIL ENGINES + + +The small oil engine is practically the same as the gas engine, with the +addition of a vaporiser for converting the oil into gas, or vapour, to +be exploded in the cylinder; consequently the one may be converted into +the other in many cases without much trouble. The difficulty of +producing an efficient oil engine lies principally in devising a +satisfactory and reliable vapouriser--one which will work equally well +under all loads. The heat supplied to the chamber must be sufficient to +vaporise the oil, but not great enough to decompose it. There are +various methods of vaporising the oil, and many types of vaporisers are +employed to attain the same end. There are some in which a charge of oil +is drawn by suction into a hot chamber in which it is converted into +vapour and at the same time mixed with a small quantity of hot air; this +rich mixture is then passed into the combustion chamber of the engine, +in the same manner as coal-gas would be, where it is further diluted +with more air drawn in through the air valve. Other arrangements cause +a jet of oil to be injected into a chamber containing hot air, in the +form of spray, which immediately converts the oil into vapour, and is +then passed into the cylinder, compressed, and fired. Then, again, we +can pump oil through a spraying nipple into the vapouriser (which is +kept at a suitable temperature) whilst the cylinder is being filled with +air on the suction stroke. On the following compression stroke the air +is driven into the vapouriser, which communicates with the cylinder +through a narrow neck, and mixes intimately with the oil vapour. +Gradually, as the pressure rises, due to compression, the charge becomes +more and more explosive, until at the completion of this stroke it has +attained the proper proportions of air and oil vapour, and is fired by +the temperature of the vapouriser and that caused by a high compression; +that is, the charge is fired automatically; and once the engine is +running, no heating lamp is required to keep the vapouriser at the +correct temperature. It is necessary, however, to raise it to the +workable temperature at starting. This is known as the Hornsby-Akroyd +method. + +Capel's arrangement is also simple and efficient, and has the additional +advantage of being capable of being fitted to their existing gas +engines, the conversion being made in a very short time. This vapouriser +consists primarily of a tubular casting A, on the outside of which are +formed a series of vertical ribs, shown in plan, fig. 46, running to +within a short distance of the flange at one end, as shown in the +section, fig. 45, thus providing an annular space C between the upper +ends of the ribs and the flange. This casting is enclosed by an outer +casing B, which fits well over the inner tube. It has also a number of +small holes drilled near the lower end communicating with the channels +between the ribs. Thus it will be seen that when the gas valve is opened +and suction takes place, air is drawn in through these holes, passes up +into the annular space C below the top flange, from there travels to the +opposite side of vapouriser, and mixes with the oil which is also being +drawn in through a small nipper at N, fig. 45. Both then pass between a +series of pegs, where they become thoroughly mixed, and finally pass on +to the inlet valve V, fig. 47, and so into the cylinder, where the +complete charge is mixed up and compressed and fired in the usual +manner. Iron ignition tubes may be used, and one heating lamp serves a +double purpose in keeping the tube and vapouriser hot at the same time. +This lamp is fed by means of a pump actuated from the side shaft. The +plunger of the pump is loaded with a spiral spring, which may be +adjusted to give any desired pressure, and is kept constant and steady +by means of an air vessel. This pump is shown in fig. 48. It is actuated +by means of a rod and lever from the side shaft of engine. The plunger P +works in a barrel B, which is carried by a small reservoir R, the latter +being in communication with the main oil tank by means of the pipe H. + +[Illustration: FIG. 46.] + +[Illustration: FIG. 45.] + +[Illustration: FIG. 47.] + +[Illustration: FIG. 48.] + +[Illustration: FIG. 49.] + +[Illustration: FIG. 50.] + +[Illustration: FIG. 51.] + +The plunger is loaded with a spiral spring, and has a ball valve, as +shown. Intermediate between this small reservoir and the main oil tank +is another set of valves, shown in fig. 49. It will be seen that the +suction of the pump will draw the oil up, the small and lower ball +valve, of course, allowing it to pass freely. On the down stroke the +lower valve will be automatically closed, and the oil will be put under +pressure, this being determined by the load on the plunger valve, which +is adjustable by means of the screw S, fig. 48. When the required +pressure in the pipe P, figs. 45 and 49, has been attained, the plunger +valve lifts on each stroke and the surplus oil flows through the plunger +into the small reservoir R. The latter is at about the same level as +another still smaller reservoir M (shown in figs. 47 and 50), a flow of +oil being established between the two by means of a pipe Q (see figs. 48 +and 50). In the reservoir R is fitted an overflow pipe, so that the oil +cannot rise beyond a certain level; hence the head of oil in the smaller +one M is always constant. On the suction stroke a partial vacuum is +formed in the engine cylinder, consequently the pressure in the +vapouriser drops somewhat below that of the atmosphere, and this small +difference in pressure is enough to cause the oil to rise in the small +passage X, fig. 45, beyond its normal level, and overflow into the +vapourising chamber, as previously described. The valve or nipper N is +shown open in the diagram, fig. 45, and all that is required to stop the +engine when running is to drop the small handle L, fig. 45, when the +valve will close, due to the spring S. The air vessel shown in fig. 49 +is in communication with the pipe leading to the blow lamp. A pressure +gauge can also be fitted, although it is not in any way a necessity. + +The ratchet wheel and pawl shown in fig. 48 are part of the lubricator. +The wheel drives a brass or gun-metal plug, producing an intermittent +rotary motion. The plug has a small hole in its periphery, which becomes +filled with oil when it is at the upper part of its travel, and empties +the oil out into a discharge pipe T, when it is inverted, and is then +led away and applied to the piston at the required spot. Fig. 51 shows +this arrangement in section. + + + + + +End of the Project Gutenberg EBook of Gas and Oil Engines, Simply Explained, by +Walter C. 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