All About Engines

How a Modern Engine Works 11 to the centre of the shaft is called the “throw,” and corresponds to the “ throw ” of a crank. The eccentric, in fact, acts like a small crank. If the “ throw ” is 2 inches the travel of the valve is 4 inches, but the sheave is fixed on the shaft so that the motion of the valve is opposite to that of the piston, so that when the piston is moving forwards the valve is moving backwards and vice versa.1 An engine supplied with these parts will run quite well so long as the work it is required to do, or the “ load,” is constant. But if the load varies the speed of the engine will vary, and if it is taken off altogether the engine will “ race.” Now racing is undesirable, not only on account of possible damage to the engine, but also because of the unnecessary waste of steam. Steam requires coal to produce it, and coal costs money. So that if money is not to be wasted, steam must not be wasted, and some means has to be em- ployed whereby the engine takes only as much steam as it requires to keep up the speed. Suppose a heavy weight is tied to a string. Let the string be held in the hand with the weight hang- ing down. Then let a gentle but gradually increas- ing circular motion be given to the suspended weight. As the speed increases the weight tends to fly farther outwards, but in doing so it also rises. This is the principle upon which the governor acts. Two iron balls are attached to rods which are held by pin joints to the top of the main rod in Fig. 8, in such 1 This is not quite true. As will be explained later, the valve begins its stroke in a given direction just before tbe piston.