All About Engines

200 All About Engines Again, in engines of small size the water jacket is effective in keeping the piston cool, but not in a large one. The central portion of the end, continu- ally exposed to explosions, is too far removed from the cooling influence of the jacket to be affected. It is not only liable to excessive stresses, but the tem- perature may rise so high that it jams in the cylinder. It is necessary, therefore, to make the head of the piston hollow and to keep it supplied with water by means of jointed pipes which follow the motion of the piston. This all adds to the initial cost and possibility of breakdown. Up to a certain point the rise of the big gas engine was rapid, and then un- expected difficulties such as these held it back, so that it has failed to compete with the steam engine for higher powers. Increased power is not obtained, however, merely by an increase in size, but also by devices which secure that there shall be more than one explosion every two revolutions. In 1881, when the gas engine was yet in its infancy, Mr. Dugald Clerk patented a method of obtaining an explosion for each revolu- tion ; but the Otto cycle had too strong a follow- ing, and no one would take the matter up. Years afterwards Koerting and others in Germany revived the two-stroke engine and met the demand for large powers without a corresponding increase in size. To understand how this is done, recall for a moment the operations in the Otto cycle. In four successive strokes there are (i) charge drawn in, (2) com- pression, (3) explosion, (4) exhaust. In order to pro-