All About Engines

i86 All About Engines turely—that is, before the piston has reached the end of its stroke. Generally speaking, it must not reach 125 lb. on the square inch. At this point the mixture is ignited, and an explosion occurs which drives the piston towards the open end of the cylinder, and communicates energy to the flywheel. Finally, on the next return stroke, the spent gases are swept out through the exhaust valve, which is opened just before the former stroke is completed. In this sequence of operations, which is called a cycle, the crank shaft receives an impulse every two revolutions or every four strokes. For that reason an engine working on this plan is often called a four- stroke engine. It is very simple in construction, as will be seen from Fig. in on Plate 17, which repre- sents the type made by Messrs. Crossley Bros., who were the first firm in this country to manufacture engines under Dr. Otto’s patent. The piston, it will be noticed, is a cylinder open at one end, the type being known as a trunk piston. This form renders a piston rod unnecessary, and reduces the length, and therefore the weight, of the bed. Moreover, it serves also as a cross-head in taking up the thrust in the connecting rod, and the pressure produced in this way is distributed over a large area of the piston and cylinder, so that the wear is uniform. The valves of gas engines are invariably of the mushroom type, held down on their seats by springs, and opened at the right moment and for the right period by cams and levers. A cam is like a small