These are the conditions and principles, briefly stated, that combine to form the now well-known cycle upon which most gas engines work at the present time.
It was not until 1876, fifteen years after these principles had been enumerated, that Otto carried them into practical effect when he brought out a new type of engine, with compression before ignition, higher piston speed, more rapid expansion, and a general reduction of dimensions for a given power. Due to this achievement, the cycle above referred to has always been termed the "Otto" cycle.
CHAPTER II
THE COMPONENT PARTS OF AN ENGINE
Having recounted very briefly the chief points in the development of the gas engine from its beginning, we may proceed to deal with matters of perhaps more practical interest to those who we are assuming have had little or no actual experience in making or working internal combustion engines.
The modern gas engine comprises comparatively few parts. Apart from the two main castings—the bed and cylinder—a small engine, generally speaking, consists of four fundamental members, viz., the valves and their operating mechanism, the cams and levers; the ignition device for firing the charge; and the governing mechanism for regulating the supply and admission of the explosive charge. There are innumerable designs of each one of these parts, and no two makes are precisely alike in detail, as every maker employs his own method of achieving the same end, namely, the production of an engine which comprises maximum efficiency with a minimum of wear and tear and attention.
Therefore, before dealing with each of these primary parts in an arbitrary manner, and with the cycle of operations in detail, we propose to make the reader familiar with the general arrangement and method of working which usually obtains in the smaller power engines. In the following illustrations these parts are shown. A (fig. 1) is the ignition device which carries the ignition tube to fire the charge. H and I (fig. 2) are the main valves, and GC (fig. 1.) is the gas-cock. The side or cam shaft N (sometimes called the 2 to 1 shaft), the cams which move the levers M, the latter in turn operating the valves, and causing them to open and close at the proper time, are shown in fig. 11. A bracket bolted up to the side of cylinder forms a bearing for one end of the side shaft, and also carries a spindle at its lower end on which the levers oscillate, transmitting the motion imparted to them by the cams to the valves. The main cylinder casting and the bed need no description. In some cases the bed is in two portions, though now a great many makers are discarding the lower portion altogether, having found that it is cheaper, and quite as satisfactory, to use a built-up foundation instead, and, if necessary, to cut a trough for the fly-wheel to run it. This arrangement, however, only obtains where larger engines are concerned. A half-compression handle by which the exhaust cam is moved laterally on the side shaft as required is not needed on very small engines.