PART V.

GAS, OIL, AND AIR ENGINES.

By BRYAN DONKIN, M. Inst. C. E.

GAS ENGINES.

Internal combustion engines, or engines generating heat and converting it into work in a motor cylinder, have of late (1897) acquired such importance, and increased so greatly in size and number, that they demand treatment in a separate chapter. During the last quarter of a century, the construction of gas engines has passed from the experimental to the practical stage, the H.P. developed has risen from under 10 to 300 and 400, and the purposes to which they are applied are yearly increasing. A great impetus has been given to their manufacture by the introduction of cheap or power gas. The first practical gas engine was brought out by Lenoir in 1861, with a heat efficiency of 4 per cent.; the first 4-cycle compressing engine by Otto in 1876, with a heat efficiency of about 15 per cent. Since then their advance and improvement have been rapid, and a heat efficiency of 28 per cent. has been attained. There are now some 100 firms making gas engines in England and on the continent. In 1878, Mr. Dowson introduced his system of cheap or power gas. In the last ten years, oil engines have also come greatly to the front, but chiefly for small powers. The first oil engine appeared about 1870; the Priestman was brought out in 1888. About 50 firms here and abroad now make them, and they are likely to be much used for motor carriages and portable engines. In the present chapter, the subject of internal combustion engines is divided into-Gas engines, non-compressing, atmospheric, compressing; English and foreign 4-cycle engines; power gas; theory of the gas engine; oil engines; and applications of gas and oil motors.

Principles and Working Method. The principles governing gas and steam engines are the same, namely, the conversion of heat into work by the propulsion of a piston, but they are put into practice differently. Instead of producing motive power by the

expansion of steam, it is obtained by the explosion of a mixture of gas and air, the ratio giving the best results being about 1 of gas to 7 or 8 of air. The charge is introduced directly into the cylinder of an engine, and there exploded, and the pressure thus generated drives out the piston. The products left in the cylinder after explosion are discharged to the atmosphere in the same way as the exhaust steam in a non-condensing engine. There are, however, certain essential differences in the working method of the two.

In the first place, a gas motor has no boiler, the gas being delivered to the cylinder direct from the gas main or gas generator. A fresh charge must be mixed, admitted and fired for each explosion, or each impulse given to the piston. Modern gas engines are also provided with a cooling water jacket. In steam engines, the aim is to keep the cylinder as hot as possible, usually by means of a jacket of hot steam; in a gas motor, the heat due to repeated explosions is so great that the engine will not work, unless the temperature of the cylinder is considerably reduced by a jacket of cold water circulating round it. With very few exceptions, steam engines are made double-acting; in gas motors, on the contrary, the charge is generally admitted on one side of the piston only, the other end being open to the atmosphere. With steam, the pressure is generated in the boiler; in gas engines, the gas and air are compressed before explosion either in the motor cylinder itself, or in an adjoining pump. Successive expansions in two or more cylinders, which are a distinguishing feature in modern steam engines, play no part at present in gas motors. Where several cylinders are used, it is only to increase the power, a fresh charge being admitted, and the same cycle carried out separately in each.

Among the structural differences thus necessitated, the most important is the need of a light of some kind close to the cylinder, to ignite the gas charge when formed. As a rule, gas engines have no stuffing-boxes, the connecting rod often acts directly on the crank, and the parts are perhaps fewer and simpler than in a steam engine. The flywheel is important, and is utilised to store up energy, as there is only one motor stroke in four in most modern gas engines. Governors, usually pendulum, are necessary to regulate the speed when the work varies. With gas motors no time is lost in starting, as in steam engines, as there is no getting up steam.

History. In the earliest heat motors, it was proposed to obtain power by the use of an explosive. Hautefeuille, Papin, and Huyghens used powder, Barber and Street, inflammable gas to procure an explosion; but the first proposal to drive a piston by the combustion of air and gas in a cylinder, according to modern

methods, was made by Lebon, a French engineer, about 1800. For the next sixty years, gas engines remained in an experimental stage, and their development was retarded by two circumstances. The manufacture of lighting gas, the only heat agent at that time available, was still in its infancy, while to obtain sufficient energy to drive a piston smoothly and continuously, without an explosion which shattered the engine, was almost an insuperable difficulty. During this time the construction of the steam engine made rapid progress, the type was fixed, and although many improvements have since been added, the general design has not varied much. Thus, when engineers and inventors at last turned their thoughts to the gas engine, in order to obtain advantages from it that the steam engine could not give, it was natural that the new motor should at first be designed on lines already familiar to them.

Hence the principles of the steam engine were adhered to in the Lenoir, the first working gas engine, brought out in 1860. In it air and coal gas, in the proportions of about 8 parts by volume of air to 1 of gas, were introduced into a cylinder alternately, on both sides of the piston. The admission was cut off, and the mixture exploded by electricity, the explosion caused a sudden increase of pressure, the gaseous mixture expanded, driving out the piston till the stroke was completed, and was exhausted during the return stroke. The same cycle was repeated on the other face of the piston, the return stroke of the one being the explosive and expansive stroke of the other. To prevent over-heating the cylinder was cooled by water circulating round it. The admission valve chest was on one side, the exhaust on the other, and each carried duplicate sets of ports. The success of this engine was at first phenomenal. Thousands of small motors were made in France and England, and it was confidently predicted that they would drive steam from the field. The engine was handy, easily started, required no boiler, and was said to work with great economy; but its defects were soon apparent. Notwithstanding the water jacket, the cylinder became red-hot, and nearly all the great heat developed by explosion on both sides of the piston was wasted. The heat efficiency of the Lenoir engine, or the proportion of heat in the gas turned into useful work, was only 4 per cent., and the consumption of gas was from 94 to 112 cubic feet per I.H.P. hour. In spite of the great heat and high consumption the pressure on the piston never exceeded 73.5 lbs. per sq. inch. Explosion was often retarded, the piston having sometimes moved through of the stroke before the charge ignited. Hence the expansive force, generated by explosion, had much less time to act, and the gases were discharged at too high a pressure.

In the gas engine brought out about the same time by Hugon,

a small jet of water was injected into the cylinder by a pump, during each return stroke of the piston. This diminished the back pressure, and reduced the quantity of water required to keep the cylinder cool, and the lubricating oil. The explosive mixture was fired by two stationary gas flames, with which it was brought in contact at a given moment, through ports in a slide valve, worked by an eccentric on the crank shaft. The engine carried two slide valves, one communicating with the cylinder, the other with the gas reservoir, from whence the gas was pumped under slight pressure. Another pump supplied the injection water. The Hugon was the first engine to admit the charge through an ordinary slide valve, a method which has now been superseded by lift valves. The motor exhibited some other improvements, such as the ignition of the charge by a flame, and the consumption of gas was rather lower than in the Lenoir, but it had the defect of being double-acting, and never became popular.

Atmospheric Engines. Of these, the chief were the curious Barsanti and Matteucci, the Otto and Langen (brought out in 1867), and the Bisschop, all engines forming a special type. To remedy the great defect of the Lenoir and Hugon-namely, insufficient expansion -- the double-acting type was abandoned. Explosion took place at the lower end of the vertical cylinder and piston, the latter was thrown up violently, without being connected to the crank shaft, and the expansion of the gases alone limited the stroke. The piston descended in the partial vacuum formed by the cooling of the gases, as soon as their energy was exhausted, and acted on the crank only in its descent, making the shaft revolve. In the Barsanti and Matteucci engine (1854), the cylinder was open at the top, to keep it cool; gas and air were introduced through a slide valve at the bottom, fired by electricity, and the explosion forced up the piston. The piston rod carried a rack gearing by means of a pawl and rachet into a wheel on the crank shaft during the descent of the piston; the crank shaft was wedged solid with the rack, and descended with it. The same arrangement appears in the Otto and Langen engine. Here also there was a long vertical cylinder, with piston and rod connected to the main shaft by means of rachet work, acting only during the down stroke. Admission and explosion of the charge were effected by two slide valves at the bottom of the cylinder, worked by eccentrics on an auxiliary shaft. The piston shot up with great speed until it was brought to rest; the gases cooled rapidly, gave out very little heat to the cylinder, and fell at the end of expansion to a pressure below atmosphere. The down stroke was produced by gravity and the atmospheric pressure, and the rapid cooling of the walls assisted the vacuum in which the piston descended. The

rack and clutch gear was ingenious, and more prompt in action than the pawl of the Barsanti engine, but the Otto and Langen motor was noisy and unsteady. The consumption of gas was from 30 to 40 cubic feet per I.H.P. hour. In the Bisschop, brought out in 1870 and still made, the piston works in an upright hollow column, and is connected through a crosshead and the connecting rod to the crank. Explosion takes place below it, drives it up, and carries the crank round through half a revolution. Although the piston is not perfectly free, explosion and expansion are exceedingly rapid; the charge is admitted and ignited through a trunk valve piston.

The great defect of all these early gas engines was the low pressure of the charge after explosion, and the small power generated, as compared with the consumption of gas. The true principles governing the cycle of work in a gas engine were first laid down by M. Beau de Rochas in 1862, who put forward the following conditions as essential to efficiency:—

1. Largest cylinder volume, with smallest circumferential surface. 2. Maximum speed of piston.

3. Greatest possible expansion.

4. Highest pressure at beginning of the stroke.

The last of these contained the essence of the question, the idea first worked out by Otto, and which has since formed the leading feature of every modern gas engine-namely, the compression of the charge of gas and air before ignition. By this method the highest pressure is attained at the instant of explosion, and the volume of the charge increased per stroke. The principle effected a revolution in the gas engine, both from a theoretical and a practical point of view, although fifteen years elapsed before it was put in action. Since its introduction, gas engines may be divided into three classes.

Types.—1. Non-compressing engines, in which the charge is admitted at atmospheric pressure, and its pressure raised by explosion only.

2. Engines in which the charge is compressed before explosion, and ignited and exploded before the piston has moved out, or at constant volume.

3. Engines in which the charge is compressed before ignition, enters the cylinder in a state of flame, and drives the piston out by the pressure of gradual combustion. In this type, which theoretically gives the best heat efficiency, there is no real explosion, combustion taking place at constant pressure.

Of these, class 1, represented by the Lenoir and Hugon engines, and class 3 are no longer made. In type 3, or engines igniting the