There is a great deal of difference, from an economic point of view, between spirit or carburetted air engines which we have just described and motors using ordinary commercial petroleum oil. As a rule such oil is much less dangerous, having a lower flashing-point, while at the same time it is cheaper than the more artificial products such as benzoline, naphtha, etc. We shall presently give a table comparing the cost of power produced by various systems of motors, and which will explain why the petroleum engine is gradually ousting the carburetted air motor from its place.

The first motor using ordinary petroleum was brought out twenty-six years ago by Brayton in 1872, and may be said to be the father of the large family of motors which consume oils varying from naphtha to others which have boiling-points as high as 150° C.

Ready motor (Brayton).—This motor belongs to the third group of the table on page 21 (combustion and compression). There are two cylinders, one motive the other auxiliary, and used for compressing air. The compressed air passes through a space filled with some absorbent material such as felt, and kept saturated with petroleum injected into it by a special pump. The carburation of the air proceeds as follows: the high-pressure jet of air causes the petroleum to froth up, and in passing through it draws along a quantity of oil in the shape of finely-divided particles. This petroleum mist passes on to the cylinder. The advantage of this method of obtaining an explosive mixture is that it matters not what oil is used, in fact, the heavier oils are even to be preferred, as their partial condensation in the cylinder helps to lubricate it. One volume of petroleum will by this process produce 24,000 of explosive mixture.

The motor is constructed in both the horizontal and vertical form, the auxiliary being superposed upon the motor cylinder. The admission of the pulverized petroleum lasts for one-third of the forward stroke, and the return stroke expels the burnt gases. The engine is double-acting, using both sides of the piston.

A reservoir of compressed air is used for starting the engine, and saves the trouble of turning the heavy fly-wheel round by hand to give it the necessary impetus.

The Brayton motors are constructed by an American firm from 1 to 10 horse-power. The cost of maintenance and fuel is moderate, rendering them exceedingly practicable where coal gas or the lighter petroleum are unobtainable. A later type than the one we have just described appeared in 1890 which is slightly more economic, single-acting, and uses the Otto cycle. The process for obtaining a spray of oil is also much improved. Both the auxiliary and the motor cylinders are connected by a beam and connecting rods to the shaft. A red-hot platinum tube ignites the explosive mixture.

SÉcuritÉ petroleum motor.—This engine was patented in 1887 by MM. Belmont Chabout and Diedrichs, and appeared at the Paris Exhibition of 1889. An Otto cycle is used, and ignition is obtained by a platinum capsule heated to incandescence by a jet of carburetted air which passes through a spiral coil warmed by the waste heat of the cylinder. This ignition tube is situated at the back of the cylinder, and is placed in communication with it by a port and valve, which determines the exact instant at which the explosion takes place. The petroleum is vaporized by passing through pipes coiled up in a vessel traversed by the heated products of combustion. The vapour thus formed has a high enough pressure to draw air along with it as it passes across an air space between two nozzles; an explosive mixture is therefore formed. The motor is started by carburating a small quantity of air by means of a reservoir containing a light oil, and this supplies sufficient energy to work the motor until the spiral coils are hot enough to vaporize the petroleum. The whole apparatus is rather complicated, especially as there are two separate vaporizers using different oils. This disadvantage, combined with only a moderate efficiency and a high prime cost, has severely handicapped the engine.

Priestman oil engine.—The first patent of this interesting engine was taken out in 1886 by Messrs. Dent and Priestman. The engine as a whole represents probably the highest point of perfection which it is possible to attain to in this class of motor, and the general arrangement is somewhat similar to that of Dr. Otto’s engines, but in addition there is a very ingenious apparatus for working with heavy petroleum oils. A single-stroke pump, driven by an eccentric keyed to an auxiliary shaft turning half as fast as the main shaft, compresses the air to a standard pressure (depending in value, on the size of the engine) in a reservoir supplied with a governing valve and placed near the front of the engine. This reservoir contains petroleum, and the pressure of the air forces it up into a spray-maker, where it is pulverized. From thence the petroleum spray passes to a vaporizer heated by the waste gases, and becomes mixed with a certain volume of air forming an explosive mixture, which is admitted to the cylinder by an automatic valve. Compression and ignition take place in the cylinder, and the waste gases escape through a valve worked by the same eccentric which drives the air-pump. The number of parts of the mechanism is by this means reduced to a minimum, and it is found that this peculiar vaporizing process completely prevents the cylinder from being fouled, as is usual when petroleum is heated to such a high temperature, often decomposing it. The Priestman engine gave a fresh lease to the life of oil motors, which had at one time been almost abandoned by engineers. It is well adapted for all kinds of industrial operations requiring powers of from 1 to 50 horse-power, and large vertical engines have been constructed for marine purposes up to 100 horse-power, and have always given complete satisfaction to persons using them. A few minutes are required for the apparatus to become sufficiently heated to be able to start, after which the consumption of fuel is about 380 to 500 grammes of petroleum per horse-power hour, or about ·85 lb. The six-horse size weighs about 1½ tons. Ignition can be had either electric or by a flame, according to the wishes of the purchaser.

Campbell gas engine.—Fig. 29 shows a very compact engine, built by the Campbell Gas Engine Co. There are only three valves; the admission valve is regulated by a centrifugal governor. The petroleum is gasified in a vaporizer heated by a lamp. The speed is very constant and the motor easily started.

Grob motor.—This engine is constructed in Leipzig and uses the Otto cycle, and, like the motors already described, works well with ordinary petroleum. The arrangement (Fig. 30) is vertical; the cylinder and valve gear are supported on a hollow cast-iron column, which in turn is bolted to a circular bed-plate. The shaft has on one side a fly-wheel and on the other a pulley. The working valve parts are all situated on the outside so as to be easily got at. The oil-pump marked in the figure drives, the petroleum into a vaporizer, where it is broken up into very minute drops. This oil spray becomes mixed with air, and passes through a vaporizing tube heated by a flame. In the state of vapour the explosive mixture passes into the cylinder, where it is ignited at the right moment by a red-hot tube. In consequence of the compression during a quarter of the cycle, the combustion is very rapid, the power of these engines being very great for their size. Ignition, and therefore the explosion, only takes place when the speed falls below the normal. This regulation is obtained by a pendulum governor, and ensures a very constant speed. The cylinder walls are warm enough to make sure of ignition happening.

All heat engines based on the principle of the explosion of a volume of gas require an arrangement for drawing superfluous heat from the cylinder, otherwise it would soon become red-hot. This cooling is obtained in small engines by the circulation of air about a large surface especially attached to the cylinder walls, but in larger engines this means is insufficient, and it becomes necessary to use water as a cooling agent. If water is laid on there is no difficulty about this, but it occasionally happens that water is too expensive, or that it is unobtainable; in such cases it is usual to erect a large cylindrical reservoir of galvanized iron holding about ten litres of water per horse-power of the engine. This reservoir is connected by two pipes from the top and bottom to the water jacket of the engine cylinder, and circulation takes place by the difference of density of hot and cold water. In the Grob motor a somewhat different device is resorted to. The water arriving from the jacket at a temperature of about 70° C. is divided into a number of fine streams, which pass up through a network inside the reservoir. At the same time a small centrifugal pump driven by the engine forces in addition a stream of air through the water; the effect is to cool the water before it returns to the jacket to a temperature of from 80° to 90° Fahrenheit. The Grob motor is built in the same manner as the Capitaine engine, which we shall describe next, but it does not work either as smoothly or efficiently as the latter engine. It possesses, in addition, other faults which are objectionable. The vaporizing apparatus is particularly troublesome, for the result of gasifying petroleum at such a high temperature is very often to decompose it, and thereby foul the cylinder and valve mechanism.

Capitaine petroleum motor (Figs. 32, 33, 34).—More than seventeen years have elapsed since Emile Capitaine, already well known for his experiments on gas engines, first tried his hand at oil motors. His first patent dates from the year 1879, and since that time he has continuously worked at the subject, expending much patience and money in order to produce a high-speed motor capable of using ordinary petroleum of ·88 specific gravity. The result of his labours is a machine which is excellent in all respects, requiring no igniting or heating apparatus. In the earlier type it was found that the motor would not work at less than three-quarters of its full load, because when it was doing less work the vaporizer became cooled in the intervals which occurred between the rarer explosions. This motor could only work under certain conditions, and after further trials and experiments Capitaine brought out a motor which worked at small loads as well as full loads, and which only consumed fuel in proportion to the power developed. This machine was considered too complicated, and a fresh type was brought out.

This third engine has a vaporizer which is in permanent communication with the cylinder, and is so constructed that even if it became red-hot there would be no risk of pre-ignition or the formation of tarry oils which would foul the cylinder. The vaporizer only draws in sufficient petroleum at a time for one explosion, and then only during the aspirating stroke of the piston. At the end of this stroke the whole of the petroleum in the vaporizer is gasified. The first motor on this plan was built by Capitaine in 1889, and many of them are still constructed at the present date by M. Louis Herlicq of Paris.

Capitaine came to an agreement with Grob et Cie. in order to exploit his inventions in a wider field. Unfortunately differences arose, and in 1891 the agreement was cancelled. A lawsuit followed, lasting till March 26, 1893, when it was brought to an end by Grob et Cie. agreeing to pay Capitaine the sum of 125,000 francs in consideration for permission to use his name and patents in connection with their motors. After breaking up the partnership with Grob et Cie., Capitaine continued to build engines, and has since then taken out several patents, which have considerably enhanced the value of his previous inventions. In the 1892–93 type a lamp is used for starting which can afterwards be dispensed with, the heat of the compression being sufficient to maintain the temperature of the vaporizer. This is, however, only if the motor is developing more than three-quarters of its full power. When it was found that the same shaped indicator diagram could be obtained from these motors even when they were giving only 75% of their full power, it became obvious that it was only necessary to keep the vaporizer as warm as possible by covering it with non-conducting jackets, in order to get the engine to run at quite a small fraction of its full power. Capitaine has, by carefully observing this condition, obtained a motor which will run at all loads without any external heating apparatus.

The vaporizer forms part of the combustion chamber, and is carefully covered with non-conducting material. Only hot gases are allowed to pass through it. It must be started by heating for a few minutes with a small hand-lamp, and the engine is then ready for work.

Merlin motor.—This motor is of the vertical type, and resembles the Capitaine motor in some respects. The oil is stored in a reservoir in the bed-plate; this receptacle is in communication with an air-pump driven by the motor, and also with an oil-pump. The pressure of air generated by the former forces the petroleum up into the oil-pump, which in its turn passes it drop by drop through a pulverizer, after which the spray of oil enters a vaporizer and becomes gasified. The vaporizer is heated by a special petroleum lamp. Ignition takes place when the explosive mixture comes into contact with the heated walls of the vaporizer, their temperature being always high enough to ensure ignition taking place. The consumption of fuel is regulated by a governor acting on the oil-pump, preventing it from supplying more oil than is absolutely necessary to keep up the speed. The governor regulates the exhaust valve as well, so that the speed, which is pretty high, is kept quite constant.

Hornsby-Akroyd oil engine (Figs. 35, 36).—This interesting machine was invented by Messrs. Akroyd, Stuart and Binney; it works with ordinary petroleum, and without the help of a carburator uses oils varying greatly in specific gravity. The reservoir of fuel is situated in the bed-plate. The ignition is automatic, and an electric spark is unnecessary, in fact, the whole engine is of the simplest construction, in order that there may be no inconvenience due to breakdowns. Referring to Fig. 35, L is a special petroleum lamp situated at the back of the cylinder A. When starting this lamp is supplied with air from a rotary fan turned by hand; in a short space of time, by the help of this stream of high-pressure air, the cartridge C becomes sufficiently heated to ensure the ignition of the explosive mixture. This cartridge C is provided on its interior with radiating ribs, which greatly increase the surface. During the back-stroke of the piston it becomes filled with compressed air from the cylinder, and towards the end of this stroke a quantity of petroleum, exactly sufficient for the explosion, is squirted into it. To do this a small pump is provided, which is actuated by a cam driving the oil into the cartridge and brought back by a spring. The centrifugal governor G acts on the admission valve from this pump. The sudden injection of the oil into the middle of the heated chamber completely vaporizes it, forming with the air already present the necessary explosive charge, which immediately ignites in contact with the heated walls.

When the vaporizing cartridge has been sufficiently heated a rapid turn of the fly-wheel by hand, so as to produce the first explosion, is sufficient to start it, after which the vaporizer keeps itself warm. The pump which keeps up the supply of oil will be seen in the right-hand bottom corner of Fig. 35. It is of the plunger pattern, and at every stroke of the piston draws in the right quantity of oil and injects it into the vaporizer through a valve. The illustration on page 91 shows the other side of the engine with the valve-shaft, which revolves once for every two revolutions of the crank, also driving the centrifugal Porter governor. If the speed should happen to exceed the normal, a steel finger moved by the governor closes the valve admitting oil to the vaporizer, which then returns to the reservoir, having failed to accomplish its object. As the ordinary Otto cycle is used, a heavy fly-wheel is necessary to maintain the speed constant between the explosions. After being once started very little attention is necessary, everything working automatically. A water jacket W cools the cylinder.

The specific gravity of the oil is usually about ·854, and of this the engine consumes about one pint per hour per B.H.-P., at a cost of 1½d. An additional advantage of the high speed is, that there is less chance of the cylinder becoming fouled by tarry products.

Vulcan motor.—This engine is in many respects a counterpart of the Akroyd motor. The cylinder is horizontal, and projects over the end of the bed-plate. Two fly-wheels are provided if very steady running is required. The lamp which vaporizes the petroleum at starting is removed when once the normal speed is attained. A water jacket prevents the cylinder from getting too hot. The most noticeable feature is the rather curious arrangement for governing by means of bent oscillating levers, which control both the admission and escape valves. The governor itself is situated inside the transmission pulley and keeps the speed within 1% of the normal; 600 grammes of petroleum per hour per horse-power is the usual consumption. The motor is rather too complicated, and can scarcely be said to have justified the expectations formed of it.

Ragot petroleum motor (Figs. 37, 38).—This engine, like the preceding ones, uses heavy petroleum and particularly schistic oil, which has a flashing-point above 77° Fahrenheit, and which may be bought very much more cheaply than naphtha, benzoline, or other artificial products of petroleum. Several different patterns of the engines are on the market, of which certain ones are of great value for electric lighting purposes, not only because of their steady running, but also because, by burning cheap oil, they allow the cost per horse-power to be reduced to a minimum. One high-speed pattern has been especially designed for coupling direct to dynamos, forming a very neat and compact plant. The latest developments in the construction of these engines tend to the simplification of the parts, and they are now so free from unnecessary mechanism, that persons with little or no knowledge of mechanics are quite capable of looking after them, and even taking them to pieces and putting them together again if a breakdown should occur, which is unlikely.

By a special arrangement in the carburator all constituents of the petroleum which might foul the cylinder are removed before the oil is allowed to enter the cylinder, and therefore it seldom requires cleaning out. The carburator (Fig. 39) is very ingeniously arranged; it consists of a cast-iron cylinder smooth on the inside, but provided with a spiral rib on its outer surface, and heated by an oil lamp. The spiral rib is itself enclosed by a covering, so that it forms a spiral tube. The petroleum is allowed to enter at the top, and gradually winds its way down, passing over a continually warmer and warmer surface as it approaches the lamp. The more volatile portions of the oil are therefore vaporized first, and the heavier oil passes on and is not vaporized till near the base, where the temperature is high enough to gasify the heavy residue. The air is heated in a jacket surrounding the base of the spiral, and is afterwards mixed with the oil vapour.

Root motor.—These engines work with the ordinary four-cycle, and are interesting because of the double-ignition apparatus. A lateral chamber is attached to the cylinder, communicating with it by an orifice situated at the back end, which is covered when the piston is at the beginning of its stroke. During the compression stroke a portion of the mixture is enclosed in this chamber and separated from the rest of the charge, which is compressed and ignited in the ordinary way. The explosion drives forward the piston, and when it has moved forward some distance it unmasks the above-mentioned orifice, igniting the gas contained in the lateral chamber, and helping to maintain the pressure behind the piston. This addition to the pressure is noticeable in a diagram taken from the cylinder, by sudden rise of pressure. We don’t quite see the utility of this arrangement, which seems to merely complicate matters. The first ignition is effected by a red-hot tube. The Root vaporizer utilizes the heat of the exhaust gases; the air is heated in a spiral coil wound round the tube through which the vaporized oil passes. The two become mixed in a small vessel, and the vaporizer is supplied by a chamber placed directly round the exit of the exhaust gases. Strictly speaking, there is no oil pump, but a plunger in a small auxiliary cylinder serves to direct the flow of oil into the right channels. Admission takes place through an automatic valve. The escape valve is actuated by the same mechanism which drives the piston governing the flow of oil.

Koerting-Boulet motor.—In this four-cycle engine we shall only describe the vaporizer and spray-maker. The oil is stored in a reservoir placed about six feet from the ground, and a pipe conducts it into a space between two discs with a valve at the top. This circular space, about half a millimetre in thickness, is traversed by the air which pulverizes the oil; the spray thus formed is vaporized by the high temperature developed by a lamp situated underneath the tube which conducts the carburetted air to the cylinder. In addition, this lamp is used for igniting the gases through a porcelain tube kept at red heat by it. An auxiliary spirit-lamp is used for starting the engine, which takes about fifteen minutes to acquire a sufficiently high temperature.

Knight gas engine.—In his endeavour to completely drive out the products of combustion from the cylinder, Mr. Knight has had recourse to the Griffin cycle and to a double-acting cylinder: we have already discussed the relative merits of this system, and think it is sufficient to point out the greater constancy of speed obtained by it. The vaporizer is placed behind the motor-cylinder, and is separated from it by a steel plate, so that it is maintained at a high temperature; this vaporizer is made of bronze, and is provided with radiating ribs. A pump injects oil into it, and it is also supplied with compressed air by a special pump, which furnishes in addition a jet for producing a blow-pipe flame. This flame white-heats a platinum wire, which ignites the gases. The wire is situated in an opening in the ignition valve; the valve moves forward, and the white-hot wire is brought into contact with the explosive mixture at the right moment. The admission valve is automatic, and situated in the steel plate separating the vaporizer from the cylinder. The other organs of the motor are not of any special interest.

Crossley-Holt oil engine (Fig. 40).—This is practically the same engine which we described in the chapter on gas engines; a vaporizer is added in the oil motor to enable it to work with petroleum.

It consists of a chamber divided into four channels by vertical partitions, and heated by a lamp placed underneath. The flame traverses these channels before reaching the chimney placed at the top. The petroleum is converted into a spray, and drawn off by a jet of air warmed by passing it through a spiral coil placed round the lamp chimney. This jet of air is supplied by a small pump worked by a lever. The oil lamp has no wick, which is rather a novel arrangement: it is supplied by a small pump similar to that which provides the air. Since its first appearance this engine has undergone certain changes which have considerably improved it. Messrs. Crossley Bros. have especially devoted themselves to shortening the length of time necessary to heat and start the engine. Besides this they have paid great attention to simplicity of the working parts.

Trusty oil engine.—This motor was devised by Mr. Knight, and is constructed by Messrs. Weyman and Hitchcock. There is no spray-maker, the liquid oil being directly converted into vapour. The vaporizer is heated by the waste gas from the cylinder, and the oil falls drop by drop on to the hot metal walls and becomes vaporized. The engine is of the ordinary four-cycle type, the most noticeable feature being the governing arrangement; a governor of the inertia pattern regulating the admission valve and the oil-pump. If the speed becomes too great the action of the governor on the valves ceases, and oil is no longer supplied to the vaporizer from the pump and the admission valve remains closed. Ignition is obtained by a red-hot tube heated by an oil lamp; a layer of asbestos on the outside prevents the tube from cooling, and keeps the tube at a high temperature. It takes twenty minutes before the vaporizer becomes sufficiently heated to enable a start to be made, and the cylinder is very liable to become fouled by tarry products from the oil. These serious disadvantages are scarcely neutralized by the variety and small quantity of oils which the engine will burn. An engine of this type with two cylinders consumed in certain trials only half-a-pint of oil, which speaks for itself.

Griffin oil engine (Figs. 41, 42).—Yet another Otto cycle engine provided with a pump reservoir and vaporizer. The latter is hidden in the bed-plate, and is heated by the gases from the exhaust. The pump is driven by an eccentric on the main shaft, and compresses the air to about 8 lbs. on the square inch in a reservoir also situated in the bed-plate. The compressed air passes from this chamber into a needle pulverizer, along with a quantity of oil which it has gathered in a small ante-chamber. This pulverized oil arrives in the vaporizer at the same moment as the air for the explosive mixture. A portion of the vaporized oil and air is used for the lamp, which red-heats the ignition tube. At starting, a small quantity of air is compressed in a hand pump, and in ten minutes the vaporizer is hot enough to gasify the oil. The Griffin engine is very economical, and only uses about a pint of petroleum per horse-power hour.

Niel petroleum engine.—In these motors, the general arrangement of whose parts is similar to that of the gas engines constructed by the same firm, the principal part is the vaporizer. It consists of a small cast-iron chamber, provided with radiating ribs in its interior so as to increase the surface; the petroleum, coming from a reservoir placed at a height of about six feet, passes through a regulating valve, and is dispersed in fine drops over the surface of the radiating ribs; the vaporization is consequently very steady. An automatic valve allows air to pass into this chamber from the outside, forming an explosive mixture with the vaporized oil. A special tube coming from the reservoir supplies a lamp with oil for heating the ignition tube and also the vaporizer at the same time. A methylated spirit lamp is used for warming them up to the right temperature before starting the engine. The governor acts on both the admission and the exhaust, and the supply of oil is indirectly regulated by the closing of the exhaust valve. The whole mechanism is ingenious, giving excellent results; the speed is very constant, and the consumption of oil is within wide limits proportional to the demand for power. About a pound of oil is consumed per horse-power hour, including that necessary for the lamp.

Two patterns of this engine are built, one vertical and the other horizontal. In the latter, called the Atlas engine, the consumption per hour is slightly greater. The governor is also different. The compression in the cylinder rises to about two atmospheres, and the quantity of water necessary for the water jacket varies from seven to ten gallons per horse-power hour, according to the size of the motor.