Early Lenoir engine (1860).—The motor (Fig. 1) resembled in external appearance a horizontal double-acting steam engine. This design was in great favour at that time, being copied from the steam engine, and was to a certain extent suitable for use with an explosive gas instead of steam. The valve chest is cylindrical and the valves themselves flat, and work off two eccentrics; ignition is effected by an electric spark from a Ruhmkorff coil, which passes through the gas in the cylinder when the piston is commencing the second half of the forward stroke. The exploded gases having done their work are driven out through the exhaust in the return stroke, during which work is being done by a similar explosion on the other side of the piston. A water jacket prevents the cylinder walls from becoming overheated. This arrangement is therefore double-acting, but a compression of the explosive gas is not possible without the use of a second cylinder. It has been abandoned because regularity of working is only obtained at the expense of economy, and by using both sides of the piston as explosive chambers it is found that the quantity of gas used is quite out of proportion to the power developed.

The Bisschop gas engine.—This motor (Fig. 2), based on a mixed principle, uses the explosion to do work during the forward stroke, and in the return the atmosphere exerts an excess of pressure on the other side of the piston, as in the Otto and Langen atmospheric engine which we have previously mentioned. In its time the Bisschop gas engine obtained a great measure of success, but it has now almost completely disappeared. It was, however, well thought out and constructed; the cylinder was vertical, and relied on longitudinal corrugations, and the air to keep it cool. Above the cylinder was placed a cylindrical guide; a connecting rod and cross-head formed the attachment between the piston rod and crank. The machine was principally constructed for small workshops requiring small powers of from a quarter to one horse-power, the cost of fuel for the half horse-power size being about one penny per hour. The inventor received a prize of 1000 francs from the SociÉtÉ d’Encouragement for the best small motor applicable to home industries.

FranÇois motor.—This type, which is now quite obsolete, was somewhat similar in character to the last, but rather more complicated and perfect. The crank shaft was not in a line with the cylinder, and was connected by two connecting rods to the cross-head. Two fly-wheels were placed one on each side of the cylinder and connected by toothed wheels. The machine was on the whole too complicated, and although its consumption of fuel was comparatively small and its speed constant, it did not succeed in ousting the Bisschop motor from its position.

BÉnier gas engine.—This motor was the first conceived by the inventors of the combined gas plant and engine which we will describe later, and is extremely simple. This piston rod is connected to the crank in a manner similar to a beam engine (Fig. 3). Both the admission of the gases and their ignition are accomplished by a single spring valve worked by a cam on the crank-shaft. The cylinder, which is vertical and inverted, draws in the gases for half the forward stroke, and then the valve, which has moved still further forward, brings a flame opposite the admission port and ignites the mixture; a small auxiliary gas-jet re-ignites the flame at each stroke. The gases escape from the cylinder by a second port with a special valve and cam. A water jacket for the cylinder is provided to carry off the surplus heat. The consumption is high, being about 1400 litres per horse-power hour, but owing to the extreme simplicity of the working parts this motor met with a certain amount of success about 1880.

Forest gas engine (Fig. 4).—This motor, being of the single-acting type without compression, had at one time a considerable sale, being used where only a small power was required. The rectilinear motion is changed into a rotary one by means of an Oliver Evans beam, and a connecting rod which returned alongside of the cylinder to the crank-shaft and fly-wheel, which are placed at the back. Ignition is obtained by a burner which is re-lit by a smaller one at each stroke, and the cylinder is cooled not by a water jacket but by a helical groove, which increases the surface. This helix is formed by a thin plate cast on the cylinder. The fuel consumed was about 1400 litres of gas per horse-power hour, which may be considered good for such an engine.

Economic motor.—Constructed in New York. This engine is another example of the early attempts to obtain economy without compression. As a rule they were not constructed of more than half horse-power size, and the general arrangement is ingenious, but rather more complicated than those which we have so far spoken of. The piston rod is guided by being attached to one end of a lever, connected with crank by means of a vertical connecting rod. The cylinder is grooved, and cooled by the circulation of the air round it, and constancy of speed is obtained by a centrifugal governor, which cuts off the supply of gas when the speed is too high. The engine seems to have given some very fair results.

Lentz gas engine.—It is difficult to conceive a more simple mechanism than is to be found in this motor. The supply of gas is drawn into the cylinder by an open valve, and a gas flame situated in this admission port ignites the explosive gases. The force of the explosion closes the admission valve, and on the return stroke a cam opens an exhaust port situated underneath the cylinder. There is no water jacket, but the cylinder is formed of two parts connected together by a non-conducting joint. In order to smooth down the jerk of the explosion the head of the connecting rod slides in a groove, and is kept pressed against the crank-pin by a spring, the result being that the connecting rod is longitudinally elastic and deadens the shock of the explosion.

Group II., Class I.—One cycle per revolution.

Dugald-Clerk gas engine.—In the ideal motor we should have at least one explosion per revolution of the fly-wheel, which is not the case in the Otto cycle. For this reason many inventors have tried to construct gas engines with one cycle per revolution, but experience has taught us that though they may be mechanically more simple, they lose in efficiency what they gain in simplicity, and in spite of many eminent inventors attempting to solve the problem. Even the best-designed motors of this type have been unable to hold their own against the Otto cycle because they are not as efficient.

The first attempt was made by Dugald-Clerk in 1881. His engine is simple in the extreme, containing no gear-wheels, and working steadily and noiselessly at a fairly constant speed (Fig. 5).

There are two cylinders of equal diameter placed side by side, and projecting over the end of a cast-iron bed-plate. The first of these is the motive cylinder in which the explosion takes place; the other is used for compressing the explosive mixture, this compression taking place in the Otto cycle in the motive cylinder itself.

This secondary cylinder also serves for another purpose; it draws in a certain volume of air directly after the explosion, which is afterwards driven through the motive cylinder, effectively clearing out the waste gases. The advantage which this arrangement of double cylinders has over the Otto cycle, lies in the fact that one explosion can take place during each revolution of the crank, and consequently such very heavy fly-wheels as are used for the Otto type of engine are not necessary. The great disadvantage which the Dugald-Clerk motor possesses is the extreme suddenness of the explosion, which is practically complete before the piston begins to move. In spite of this defect the machines have a fairly high efficiency. The gases are ignited by a gas-jet situated in a sliding valve. A water jacket is used for cooling the cylinder, and in some of these motors a mechanism is attached for converting the engine from simple to compound. The compression cylinder then becomes double-acting, and the gases are further expanded in what was previously the motor cylinder only.

Owing to the fact that this motor was not as efficient as those of the Otto type it never became a commercial success, and it is doubtful if any are working at the present date.

Early Stockport gas engine.—The working of this motor affords a good example of British ingenuity. The compression cylinder is situated behind the motive cylinder, being a prolongation of it on the same axis; the two are firmly bolted together end on. The active piston is in front and connected to the crank, drawing behind it the piston of the compression chamber. Each cylinder has a separate sliding valve. The rear cylinder having aspirated and compressed a volume of the explosive gases, they are passed into the motive cylinder through a sliding valve which also serves to ignite them. The waste gas escapes into the air by a special valve. The aspiration and compression take place in the auxiliary cylinder once in every revolution of the crank, and besides, the motive piston also compresses the gases a trifle before the explosion takes place. The exhaust valve is made to open slightly before the piston reaches the end of its stroke. The motor was rather inefficient, and since its appearance a new type has been brought out with an Otto cycle which we will describe later on.

Benz motor (Fig. 6).—In this motor the inventor attempts to drive out the whole of the exhaust gases before the second half of the backward stroke of the piston is reached. To do this he injects a certain volume of air under pressure, driving out the burnt gases and substituting itself in their place. Before the end of this return stroke a small auxiliary pump introduces the requisite amount of coal gas, which is then compressed during the rest of the stroke. The ignition is effected by a small magneto machine driven by the engine itself, the sparks being generated between two fine metallic points in the cylinder. The jet of air required to drive out the product of the combustion is furnished from a reservoir, pressure in it being maintained by using the other side of the piston as an air-pump. This arrangement is an advantage, because cold air is being continually drawn into the cylinder which keeps it cool, and enables the lubrification to take place more effectively. There is also an external cooling apparatus in the form of a water jacket, which uses about 40 litres of water per horse-power hour, and keeps the cylinder at about 75° C. The Benz motor is very well known on the Continent, and much of its success is due to the fact that it seems to work as well with gasoline as with coal gas! It is constructed by M. Roger of Paris, and in consequence of the extreme constancy of its speed, it has been successfully applied to driving dynamos, and also to launches, motor cars, etc.

Baldwin gas engine.—The cycle of this engine is somewhat similar to that of the Benz motor, one side of the piston being used for expansion and the other side for compression. Part of the bed-plate casting is arranged so as to form a reservoir for the compressed gases. The coal gas is also admitted into this vessel, so that it contains an explosive mixture. As the vessel is only made of cast-iron this arrangement is rather dangerous. There are three valves, the admission valve being regulated by the governor. The power developed is, therefore, always kept proportional to the demand, and the constancy of speed is sufficient to warrant the use of these engines for running dynamos for electric light. The ignition is by an electric spark, and is generally obtained from some extra apparatus, such as accumulators or batteries, and an induction coil. The engine is constructed by Messrs. Otis Bros. of New York.

De Ravel motors.—The first motor constructed by M. de Ravel was exhibited in Paris in 1878, and was of the oscillating cylinder type, with a variable centre of gravity. The explosion drove up a heavy piston whose rod was directly connected to the crank-pin. The revolution of the crank-pin caused the whole cylinder to move in the same manner as that of the early oscillating steam engines. This movement was used as a means of opening and closing the ports. The efficiency of the engine was low, using some 600 to 700 litres of gas per horse-power hour; and besides the motor had, owing to faulty mechanism, the unhappy knack of suddenly stopping dead. These defects caused M. de Ravel to abandon this type and to bring out a second motor in 1885, performing one cycle per revolution. This new motor only had one cylinder, whose rear half acted as a compression chamber during the backward stroke of the piston, whilst the explosion took place in the front end of the cylinder. The consumption of gas was slightly more than an Otto engine, and the motor ran exceedingly silently and evenly, but this advantage was not of much service in the struggle against the all-conquering Otto motor.

Midland motor (Taylor).—Constructed in Nottingham, this engine is of the horizontal double-cylinder type. One cylinder compresses the explosive mixture and passes it on to the other, where it is ignited and does work. The cranks connected to the two pistons are placed 65° apart, and a complete cycle in the cylinders is performed every revolution. The makers of this engine claim a consumption of only 600 litres of gas per horse-power hour.

Campbell gas engine.—The mechanism of this engine is very much like that of the Dugald-Clerk motor, two cylinders being placed side by side. The utilization of the heat is, however, far superior, and only about 500 litres of coal gas per horse-power hour are required. A result which the inventor of this type of cycle never succeeded in obtaining, but as far as we know the motor has never had any official trial, and the above figures are only taken from the makers prospectus. However, there is only a hardly perceptible shock from the explosion, and the motor can be safely recommended to anybody requiring a silent gas engine. There still exist gas engines worked on this same cycle per revolution principle, such as the Conelly, Day, De Ravel motors, descriptions of which must be sought elsewhere. The limited space at our disposal prevents us from discussing all those types which have only obtained a very small measure of success. The only remaining engine based on this principle worth mentioning is the BÉnier motor, which is exceedingly instructive, but we shall speak of it later under the head of poor-gas engines. We shall at present pass on to the consideration of engines based in principle on the patent of Beau de Rochas, and first practically realized in 1877 by Dr. Otto.

Otto gas engine (Fig. 7).—The principle on which this engine is based is known as the Otto cycle, named after Dr. Otto, but first suggested by Beau de Rochas. Since the patents have expired numerous copies and imitations have been brought out, but very few surpass or even equal some of the earlier types.

The explanation of the working of the Otto motor, which we are about to give, will save us from returning to it in the descriptions of analogous types brought out after this famous system. The cylinder is continued in a backward direction so as to form a compression chamber, into which the mixture of gas and air is drawn during the forward stroke of the piston. The mixture is compressed during the return stroke in this chamber, the pressure rising at the end of the stroke to about 3 or 4 atmospheres. At this point in the cycle a flame is brought into contact with the compressed gases and they explode. This explosion raises the temperature of the gases to 1500° C., and drives forward the piston under a pressure of about 150 lbs. to the square inch. During the second return stroke, corresponding to the latter half of the second revolution of the crank, the piston drives out the products of the combustion into the air under a pressure of about an atmosphere. The heating of the cylinder is avoided by keeping water automatically circulating through a jacket surrounding it. This is necessary, because if the cylinder walls became heated, the oil upon them would become decomposed, and lose its lubricating properties. Dr. Otto paid special attention to the efficiency of the engine, and in order to increase it, he diluted the air and gas drawn into the cylinder with a portion of the gases already burnt in the previous stroke. Consequently the explosion at the beginning of the stroke is less violent, and the gases continue burning while the piston moves forward. The cause of this slow combustion has been wrongly attributed to stratification of superimposed layers of gas and air, but it is probably due to the action of the cylinder walls.

The Otto gas engine is a marvel of simplicity from a mechanical point of view, very much more so than a Corliss steam engine for instance. The admission and exhaust valves are worked by cams, and the ignition takes place under pressure. The governor is sometimes of the centrifugal type, and at others of the inertia type, but in both it is a case of all or nothing, the supply being completely shut off if the engine is going too fast. The connecting rod joins the crank to the piston rod by a cross-head running into a bored out-guide. It is necessary to have a heavy fly-wheel, because, as only one explosion takes place per two revolutions, the fly-wheel must store up enough energy during that explosion to carry it through the rest of the cycle. Many different types of Otto gas engines now exist, some having two cylinders and a single crank, and others two fly-wheels, in order to ensure constancy of speed for driving dynamos. Dr. Otto devised a compound gas engine, but it did not succeed, and also a cheaper vertical type (Fig. 8), which is very convenient for small workshops. Since the invention of carburetted air the creator of the Otto cycle has devised another motor for use with gasoline instead of coal gas.

Otto devised the first practical gas engine and opened up the path for others, who, following in his footsteps, have confined their attention to improvement of detail. Some have undoubtedly succeeded, and by avoiding waste of heat, and by raising the initial temperature of the gases, they have considerably reduced the consumption of fuel. We shall now discuss different types of motors which have appeared during the last fifteen years, confining ourselves to the really successful ones.

Second Lenoir motor.—Twenty-five years separated the appearances of the first and second Lenoir motors, and during this time M. Lenoir gained a great deal of practical experience, so that if reference be made to Figs. 1 and 9 they will be seen to have very little in common. In the later type the cylinder projects over the back of the bed-plate, and is provided with deep circular grooves on the outside to increase the cooling surface. Ignition is obtained by the spark from a coil supplied by a battery as in the early form. The consumption of gas is about 800 litres per horse-power hour. Later on we shall discuss a petroleum motor for motor cars, and also a stationary petroleum engine by the same inventor. These engines were originally constructed by Mignon and Rouart, but later by the Compagnie Parisienne du Gaz, and are very well designed and constructed.

Koerting-Lieckfeld motor.—The first of these motors was constructed in 1877, and was based on the Dugald-Clerk cycle. The original type has, however, been abandoned, and the firm of BrÛlÉ et Cie. of Paris now construct these engines on the Otto system. All the moving parts are attached to a vertical frame of cast-iron (Fig. 10), the lower half containing the cylinder. The admission and exhaust valves are situated in front and near the base. The governor is of the centrifugal type and acts directly on the levers of the valves. The flame ignition makes it necessary to occasionally clean out the valves, but otherwise the motor has few drawbacks, and is very neat and compact. The consumption of coal gas for engines of 8 horse-power and upwards is about 800 litres per horse-power hour. The crank-shaft is placed horizontally across the top of the frame, and the cams acting upon the valves are rotated by a bevel gear enclosed in a case, driving a thin supplementary shaft on which they are placed. The motor is self-lubricating, and is also constructed as a horizontal gas engine.

Andrews’ motor.—The governing apparatus in this engine is exceedingly simple and ingenious, consisting of a weight fixed to an oscillating lever which controls the admission valve. The position which the weight takes up depends upon the rapidity with which the lever oscillates, and consequently upon the speed of the engine. If, therefore, the engine is running too fast or too slow the weight takes up a new position, and the effect upon the admission valve is to either slow down or quicken the speed. The gases are ignited by means of a tube kept red-hot by a gas flame. This engine possesses the special advantage of being self-starting, that is to say, it is not necessary, as in many other motors, to start the engine by giving the fly-wheel a few rapid turns by hand. The motor is stopped with the crank in a position slightly in advance of the point corresponding to ignition. The gas is allowed to enter by a small auxiliary valve, which closes after the first explosion. This volume of gas entering the cylinder mixes with the air already in it, forming an explosive mixture. This explosive mixture then begins to escape by similar automatic self-closing valves situated at the top of the red-hot ignition tube. Ignition takes place, and is communicated to the rest of the gas in the cylinder, closing the two small valves by the force of the explosion. The piston is therefore driven forward, and the energy of this combustion is sufficient to start the engine. It must be understood that this operation is only performed once at starting, for immediately afterwards the engine falls into its normal cycle of operations. Two fly-wheels are as a rule provided to ensure constancy of speed. The consumption of gas is as low as 580 litres per horse-power hour in the large units of 100 horse-power.

The Andrews gas engine is also constructed of a special type for consuming poor gas produced by the Dowson process, and gives very good results. As a rule, about 600 to 800 grammes of anthracite are necessary to produce one horse-power hour. In one particular plant generating electricity, the cost has been certified to be as low as one penny per kilowatt hour, including lubrication.

Fielding gas engine.—The characteristic point in these engines is the extreme simplicity of the valve gear, only one valve being ever subjected to pressure. Even in the small engines of this type all sliding valves are replaced by those of the spring pattern, in fact, the valve mechanism consists simply of two spring-valves, one of which fulfils two functions, controlling the admission and the escape of the gases. The two valves are moved by a double lever actuated by a single cam. The cycle is that of Dr. Otto. When the piston is starting on the return stroke after an explosion has taken place, the lever lifts one of the valves, and the products of the combustion escape into a circular space situated below. In the next forward stroke this valve is still further lifted, opening the admission port, while at the same moment the exhaust port is closed by the second valve; a new charge is therefore drawn into the cylinder. At the end of the admission stroke the valves are released by the lever, and the compression can now take place during the second return stroke. The movement causing the lifting of the valve continuously throughout one whole revolution is effected by means of a cam with two prominences on it, which act in succession on the lever. There is only one valve, therefore, which is subjected to pressure, and even this is well provided against risk of leakage by the second valve being placed behind it, acting as a double seating. A chamber is also provided in which the gases are first mixed, and this mixing is regulated by a second lever and a special valve. The governor is of the inertia type, and is attached to the upper side of the double lever. It consists of a straight rod with a ball at one end and a knife edge at the other, pivoted at its centre to the end of the valve lever. As this lever is thrown forward the knife edge strikes the end of the spring-valve admitting gas to the mixing chamber. If the speed is too great the sudden jerk on the ball at the other end of the knife edge causes it to miss the valve, and no gas is admitted till the speed is reduced to the normal number of revolutions per minute. This arrangement is very sensitive, and keeps the speed exceedingly constant. Mr. Fielding has constructed some large gas engines of 350 horse-power which are started by compressed air, this air having been previously stored up under pressure when the motor was stopping.

Niel motor.—As will be seen from Fig. 11, the valves are actuated by cams rotating on a supplementary shaft placed parallel to the cylinder. A pair of toothed wheels transmit the rotation of the crank to this valve-shaft, and it is arranged so as to give one rotation to every two of the crank. The cycle of operations is somewhat similar to the Otto cycle; gas is only admitted to the cylinder for two-thirds of the forward stroke, so that the compression on the back stroke is somewhat lessened. It is doubtful whether there is much advantage in this method, but the Niel motors have had a large sale, which is a sufficient proof of their good qualities.

Lombart, Martini (Fig. 12), Adam, Le Parisien, and Le Kientzy motors.—All these motors, each of which is constructed by a different maker, are based in principle on the Otto engine, and except for slight modifications of the working parts, they do not call for any particular notice. The small amount of space at our disposal only admits of our mentioning them.

Lablin motor (Fig. 13).—M. Lablin of Nantes set himself to produce a motor which should correspond with the Brotherood and Westinghouse steam engine, that is to say, a motor developing the maximum of power for the minimum of space and weight. This might be termed the dynamic density of the motor, which M. Lablin sought to increase. He has succeeded in constructing gas motors of ½ horse-power weighing about 90 lbs., and of 8 horse-power weighing 7 cwt. Unfortunately they consume a rather large quantity of fuel—about 1000 litres of coal gas, or 500 grammes of gasoline per horse-power hour. There are three cylinders grouped at equal distances round the same shaft, and all attached to one single crank. By this arrangement a more constant torque or twisting movement on the shaft is obtained, and consequently the weight of the fly-wheel can be considerably reduced. Three explosions are produced during every revolution, that is to say, each cylinder performs a complete cycle of operations during every revolution, a fact which accounts for the low efficiency. Ignition is obtained by a platinum tube heated to incandescence when using coal gas, and by an electric spark when carburetted air is used. The governor is centrifugal, and controls the admission of gas.

Crossley Bros. gas engine (Figs. 14, 15).—This engine is from the cycle point of view purely and simply an Otto gas engine. A light shaft runs parallel to the cylinder, being driven by a worm-gear of the crank-shaft. On it are situated the cams which force open the four spring-valves, controlling respectively the admission of air gas and the ignition and exhaust.

The pressure of the gases is raised during compression to about four atmospheres, and immediately after the explosion it rises to about 180 lbs. on the square inch; during the exhaust it averages about 10 lbs. per square inch. Ignition is obtained by a tube heated to a bright red incandescence by a Bunsen flame. At the right moment a valve is opened, placing the explosive mixture in contact with it and causing the explosion. When starting the machine the ignition is retarded or takes place a little after passing the dead point, so that the machine cannot start the wrong way by mistake. Two or three other features call for special notice, especially the device for lubricating the cylinder. In the illustration (Fig. 14) will be seen a small bell-shaped receptacle. This vessel contains oil, and also a small crank inside driven by a belt off the valve-shaft. As this crank rotates it dips into the oil at the bottom of the vessel, and at the top of its path it wipes off the oil which it has gathered on to a cup which allows it to flow into the cylinder.

The water jacket is cast separately from the cylinder, and not, as in many engines, in one piece with it. There is an advantage in this, because there is less likelihood of flaws or blow-holes in the cylinder wall passing unobserved when the engine is leaving the maker’s hands. The governor in some of the sizes is centrifugal, and in others of the inertia type similar to that described in the Fielding gas engine.

PygmÉe motor (Lefebvre).—This motor, shown in Fig. 16, gives one the idea of solidity and compactness. It possesses the peculiar property of working equally well in any position, either horizontal or vertical. This is due to the fact that it is particularly well balanced, and when running does not vibrate at all. Easily started, these engines run at a very constant speed, and their power in relation to their size is truly remarkable, hence the name PygmÉe. They have been especially designed for self-propelled vehicles, and are not affected by the worst running conditions, such as inclement weather or bad roads. In this type they are constructed with two cylinders in order to obtain a more constant torque, and also have an arrangement by which the speed can be changed. For stationary purposes the motor is mounted on a cast-iron stand (Fig. 16). In virtue of their exceedingly small dimensions and reduced weight they are specially suitable where small-power motors are required for home industries or small workshops, and also for driving private electric installations and pumping water. They have economically replaced steam engines in agricultural operations, both on a large and small scale. The working parts being entirely enclosed they stand a good deal of rough usage, and will work in positions in which other motors would be useless. Where it is necessary to bring the motor to the work it is required to perform they are bolted to a carriage instead of a cast-iron base.

The “National” gas engine.—As a gas engine this machine is constructed in all sizes, from one horse-power up to large units requiring a gas plant of their own. As petroleum motors they range from one to ten horse-power. They are all provided with two fly-wheels, which keep them well balanced and steadies their speed. They have besides been especially designed with a view to economy of coal gas or petroleum. An idea of the general arrangement of the parts will be obtained by glancing at Fig. 17. The petroleum motors are provided in addition with a vaporizer and a petroleum lamp placed at the front end of the machine, and the oil reservoir is situated immediately above the cylinder. M. Herckenrath has specially devoted himself to simplifying the mechanism and making it more self-contained and less unsightly. Up to 50 horse-power only one cylinder is found necessary, and for larger powers two are provided. The patent governor is centrifugal and rather novel in construction, and to it is partly due the high efficiency of these engines. The large sizes are as easily started as the smaller ones, and the lubrication is perfectly automatic. Besides these advantages they require a minimum of attention, in fact, a skilled attendant can be dispensed with, a few explanations and instructions being all that is necessary to enable a boy or labourer to take competent charge of them.

Forest motor (Figs. 18, 19, 20).—We have already described the earlier attempts of M. Forest to produce a practical gas engine. The idea embodied in the motor depicted in Fig. 20, if not particularly advantageous, is none the less highly original. The ends of the single cylinder are open, and within it are two pistons, between which the explosive mixture is introduced. The explosion drives these pistons out in different directions, but by means of a suitable mechanism they are each connected to one of two cranks on the same shaft. The whole forms a very neat and compact arrangement. In another type by the same inventor (Figs. 18, 19), also working on the Otto cycle, the cylinders are vertical, there being one, two, or four placed side by side. In the last case two explosions per revolution are obtained, ensuring a very constant torque throughout. M. Forest has, however, particularly applied himself in collaboration with M. Gallice to designing petroleum engines for small boats. These two inventors have conceived a highly ingenious arrangement for reversing the direction of rotation of the engine, which is absolutely necessary in the propulsion of boats. All the cams which actuate the valves are grouped together on a light shaft, and by a simple twisting of this shaft from one end by a handle the engine is reversed. The engines are self-starting, and are so especially suitable for the propulsion of boats that they have been adopted by the French navy, who at the present date have a number of small vessels propelled by this motive power.

Cuinat gas engines.—These engines are constructed in four types. The A type is vertical with the cylinder above and fly-wheel and shaft below. This is a more stable arrangement than placing them in the reversed order, as is more frequently done. For small engines the vertical type is undoubtedly the best, space, or rather lack of space, being very often an important consideration to the purchaser. The B type is similar to the previous one, except that two fly-wheels are provided so as to make it suitable for small electric light installations. The C type is horizontal, having all the valves placed vertically, which plan seems to work better for engines of power greater than 10 horse-power. The D type is also horizontal, but has two fly-wheels, having been specially designed for electric lighting purposes. In order to take up as little space as possible the dynamo is situated underneath the projecting cylinder, both the engine and dynamo being bolted down on to a special bed-plate. This arrangement is more stable than placing them on separate foundations. In the petroleum engines an electric spark is used to ignite the gases, and in the gas engines both electricity and a gas flame.

It is best not to use animal or vegetable oils for lubricating the inside of the cylinder, because they decompose, forming fatty acids which have a corrosive action. Besides this, when they have fulfilled their function of lubrication they settle down to a thick paste, which has a most injurious effect on the working of the engine. It is best, therefore, to use nothing but perfectly pure mineral oils and to avoid all others. This does not only apply to the type of engine we have just been describing, but to all gas engines. The Cuinat gas and petroleum motors do not mix a portion of their exhausted gases with the fresh charge as is frequently done, but completely sweep away the products of the explosion before admitting a new charge. The result is, that combustion is more complete but at the same time rather more violent. An examination of an indicator diagram taken from one of these engines shows that the combustion takes place at once as an explosion, and that the final expanded pressure is as low as it is possible to get it. These being the conditions necessary for a high efficiency, it is needless to state that the consumption of fuel in these engines is as low as in any other engine.

NoËl motor.—This type, constructed at Provins, has the advantage of being exceedingly simple. The entries to the cylinder are controlled by spring-valves, and the gases are ignited by electricity. From the ¼ up to the 2 horse-power size the cylinder is not provided with a water jacket, and the general arrangement is either vertical or horizontal. These engines work equally well with coal gas or carburetted air; in the latter case the carburator is placed inside the cast-iron frame of the motor. The guaranteed consumption of fuel is about 900 litres of gas or 500 grammes of gasoline per horse-power hour, which is quite satisfactory.

Tenting motor.—This is a horizontal motor with the cylinder cooled atmospherically, and the gases ignited by electricity. The governor acts upon the exhaust valve; the products of the explosion remain in the cylinder if the speed is too great, and then the admission valve, which is automatic, no longer rises to admit a fresh charge until the speed has once more fallen to the normal. This little motor is one of the most practical small-power engines existing, partly because of the great simplicity of the mechanism. It works well with carburetted air, and the vertical type has been successfully applied to the propulsion of small pleasure-boats.

Atkinson motor (Figs. 21, 22).—This apparatus, constructed by the British Gas Engine Company, was until recently the most efficient heat engine in existence, its indicated efficiency being 22·8%. Mr. Atkinson, the inventor, has arrived at this result by making the gases burn gradually and by shortening the compression stroke. The discharge of the residual gases is complete, this being directly opposite to Dr. Otto’s procedure, for he diluted his explosive mixture largely with the exhausted gases. In order, however, to make the piston execute two strokes relative to the cylinder of different lengths, the inventor has had to devise a rather complicated mechanism (Fig. 21). The result, however, quite neutralizes this slight disadvantage.

Charon motor (Fig. 23).—M. Charon has attempted to obtain a prolonged combustion without the complicated devices resorted to by Atkinson. He has obtained the same result by means of a regulator controlling a double cam, one half of which actuates the admission valve and the other half an auxiliary valve, which opens into a tube forming a sort of reservoir, into which part of the explosive mixture passes during the compression. After the explosion this stored-up mixture is gradually allowed to re-enter the cylinder and prolong the combustion. By this means a considerable gain in economy is obtained, so that engines of this type only use about 500 litres of coal gas per horse-power hour. In appearance the motor much resembles the Otto motor, the crank-shaft and valve-shaft and valves being placed in the same relative positions. Although these motors are rather more expensive than others working with the Otto cycle, they are nevertheless widely used. This is due to the fact that if cost of coal gas in a particular district is high it is cheaper in the end to pay a higher price for an efficient engine than to buy a less expensive and at the same time less economical machine.

Roger motor.—This excellent little engine (Fig. 24) was especially designed for small workshops. The extreme simplicity of the working parts in no way lessens the efficiency, for the two horse-power only burns 700 litres of gas per horse-power hour. The simple design also keeps the cost of construction low, and the price averages about £30 per horse-power.

The governor controls the admission valve. Ignition is obtained by means of an incandescent tube; the cylinder is cooled by a water jacket; the mean speed is about 200 revolutions per minute, and this speed is constant enough to allow the engine to drive a dynamo for electric lighting. The motor has been very favourably received abroad, but is not much known in this country.

Motor of the Compagnie Parisienne du Gaz (Fig. 25).—Owing to its very large consumption of gas the engine can only be used if the price of gas is low, but it has several advantages which to a certain extent neutralize this defect. It runs at a high speed of 400 revolutions per minute, and the parts are arranged so as to be easily accessible.

Letombe motor.—Constructed by the firm of Mollet-Fontaine of Lille, this engine presents several interesting features which we will briefly enumerate. The cylinder is double-acting, giving one impulse to the piston during every revolution. The speed is therefore maintained fairly constant. The efficiency is high owing to the gases in the cylinder being made to burn slowly as in the Charon motor. The machine on the whole works exceedingly satisfactorily and reflects great credit on its inventors.

Robuste (Levasseur) motor.—The composition, admission, and ignition of explosive mixture are regulated by a sliding valve as in the Otto motor; in this case, however, it is a piston-valve and not a flat one. The valve-chest is at the back of the cylinder. The governor is of the inertia type, and suppresses the admission if the speed becomes higher than the normal. A double water jacket is provided for cooling the cylinder walls. Although not presenting any new features, this motor fully merits its name, and its solid construction enables it to withstand a surprising amount of bad handling.

Richardson and Norris gas engine.—Yet another high-speed engine, running at a speed of 230 revolutions per minute. Roby & Co. construct this machine especially for driving dynamos, and for this purpose two fly-wheels are provided in order to make it run smoothly without variation of speed. The gases are ignited through a valve with a double seating by means of a red-hot tube. The motor is reversible, which is an advantage under certain conditions. Poor gas can also be used instead of coal gas, consuming about 510 grammes of anthracite per horse-power hour for an 86 horse-power engine supplied with Dowson gas. This works out to a thermal efficiency of 21%, a result which places this engine above criticism.

H. C. motor.—This is an enclosed motor for use in mines or dusty places, the fly-wheel alone of the moving parts being visible. It works equally well with coal gas, carburetted air, or petroleum, and is constructed in sizes from ½ to 60 horse-power. In spite of its original features it has not met with much success up to the present time.

Le Marcel and Le Maurice motors (Cadiot) (Fig. 26).—The smallest types of the Marcel motors are of one man-power, and the largest of one horse-power, so that they are only suitable for small operations. One impulse is given to the piston every two revolutions, the cycle being that of Dr. Otto. The gas is compressed in a red-hot tube during half of the backward stroke. There are only two valves, one for the admission and the other for the escape of the gases, actuated by a single cam on the crank-shaft. The cylinder is cooled by a water jacket. The governor works on the all or nothing principle, the supply of gas being completely shut off if the speed rises above the normal of 350 revolutions per minute.

The Maurice motors are somewhat similar in construction, but are designed for operating dynamos. For this purpose two fly-wheels are provided. Many of these little motors are to be seen about the country working fans, lathes, pumps, etc.

Various.—We have described about thirty different sorts of motors, selected from the best-known and most original types. About one hundred other motors exist in Europe, which are similar in one way or another to those already described, such as the engines of DÜrkopp, Forward, Brouhot, Debry de Soissons, Narjot, Archat, Wertenbruch, the “AcmÉ” motor, and many others. But we are obliged to limit our descriptions, and to conclude the chapter by a couple of examples of motors performing one cycle of operations per three revolutions.

Griffin motor.—In this engine we have only two explosions over three revolutions, but as it is double-acting this number is reduced to one explosion per revolution and a half. The different operations are as follows:—(1) gases drawn into the cylinder, (2) compression of gases, (3) ignition and expansion, (4) products of combustion driven out of the cylinder, (5) a volume drawn into it to completely sweep away any residue of the exhaust gases, (6) this volume of air drawn out.

Admission and ignition are obtained by the action of a sliding valve and eccentric. The governor causes the gas admission valve to remain open for a shorter or longer time, so as to ensure constancy of speed. The exhaust gases escape by two valves actuated by a pair of cams, opening them at every turn and a half, so that the gases are alternately discharged from the back and front parts of the cylinder. The consumption of fuel for a 12 horse-power motor was about 792 litres of coal gas per horse-power hour in an official trial. The speed is very constant in spite of the long cycle.

Rollason gas engine.—This is also an engine using a long cycle of operations, the arrangement of the parts being copied off the Otto motor. The governor is electric, and acts on the admission valve, varying the amount of gas admitted to the cylinder in proportion to the demand for power. In the larger size of from 20 to 100 horse-power a self-starting arrangement has been added. This engine, like the Griffin motor, has proved that it is possible, by completely getting rid of the products of combustion, to use a very dilute explosive mixture, which would be impossible in the Otto motor. The Rollason engine is constructed by Messrs. Beck and Co. of Newcastle-on-Tyne.