THE recent development in the use of the Otto' cycle gas engine is very striking; more than 700,000 actual horse-power being in use in Great Britain at the present time, including sizes ranging from horse-power to 400 horse-power. During the early stages of its use, there was a common opinion-held, too, by many engineers who believed in its future that the gas engine was only suitable for small powers; but single-acting engines, with one cylinder developing 200 effective horse-power, and with two cylinders developing 400 effective horse-power, are now made by many firms, and, whilst not yet in common use, are beyond the experimental stage, and fulfilling all requirements. There cannot be a doubt that the near future will see many manufacturers turning their attention to the larger powers in gas engines. The credit of establishing them on a sound mechanical basis is justly awarded to Messrs. Crossley Brothers, who from the first had great faith in their future as prime movers, or they would not have undertaken the manufacture of the 1866 freepiston type, which now appears so crude when compared with the celebrated 1876 pattern. If Messrs. Crossley Brothers thought it would one day become the serious rival it has proved to be to the steam engine, they must have stood almost alone, and can look with pride at the successful results of their pioneer work. To these pioneers the difficulties must have been very great, inasmuch as they were dealing with a new motor different in principle from the steam engine, and whose working agent was a mixture of gas and air instead of steam. The heat of the steam transformed into useful effort through a process of expansion in the steam engine cylinder needed a separate furnace to generate it, whereas the gas engine dispensed with this outside source of heat, and by burning a mixture of gas and air, generated the heat within itself, and transformed it from the burning and expanding mixture directly into useful energy. Hence the gas engine is an internal combustion engine. Compared with former experience, gas engine builders had to become accustomed to exceedingly high temperatures, and design and work accordingly. Fortunately the power of certain mineral oils to withstand excessive heat, gets over the difficulty of cylinder lubrication. One point, however, to this day is not satisfactorily solved, viz. :-the manner or method of packing glands for piston rods, to allow the use of a double-acting engine. This system of transferring the furnace, wherein the heat energy is primarily developed, into the cylinder itself, and there utilising directly the benefits of combustion, is undoubtedly the greatest step of the present half-century in the direction of high economy and efficiency. The theory of the gas engine has been discussed and rediscussed through a long series of years, and there are even now those who hold views far from consistent with actual facts. Therefore it is advisable that a little should be said now concerning the practical aspect of gas motors in their modern presentment. The advantages of compressing the gas and air before firing were first pointed out about 1860 by Beau de Rochas, who indicated the process of carrying out the four operations-charging, compressing, firing, and exhaust with one piston. This cycle was realised in 1876 by the Otto Silent' gas engine. It is generally accepted that the modern gas engine, as we have it at present, has been developed by experiment, and so far most engineers, after lamentable errors and waste of time, have reluctantly adopted the Beau de Rochas cycle, as being the simplest system on which an engine can be constructed that will give satisfactory results. It is not in the scope of this treatise to deal with any other cycle, but it may safely be taken that engines which give an impulse every revolution when working at full power will ultimately take the place of the simple form of 'Four' cycle engine. CHAPTER II GENERAL DESCRIPTION OF OTTO' CYCLE GAS ENGINE THE present-day form of the commercial gas engine has a single cylinder, and gives an impulse every two revolutions when working at full power, and closely resembles a single-acting steam engine, with the working parts of excessive strength. The cylinder is open-ended, and has a trunk piston having sufficient bearing surface to dispense with the use of crosshead slides, the connecting rod joining directly the piston and the crank. The crank shaft is heavy and flywheels large, as considerable energy has to be stored to take the piston through the negative parts of the cycle (see fig. 1). The cylinder is considerably longer than the piston stroke, leaving a space at the back end, into which the piston does not enter, called the combustion chamber. Below the centre line of the engine is placed a shaft, rotating at half the speed of the crank shaft, from which it receives its motion by worm gearing, actuating at suitable times the air, gas, exhaust, timing valve, governor, and cylinder lubricator. The cylinder serves alternately the purpose of motor and pump, as during the forward stroke of the piston the air and gas valves are opened by means of cams and levers, and the gas and air enters from about the beginning to the end of the stroke, the return stroke being utilised in compressing the mixture into the compression space; and when the piston is full in the pressure has risen to |