4. Governing. All engines which turn machinery doing intermittent work—such as that of a sawmill, or electric generating plant connected with a number of motors—must be very carefully guarded from overrunning. Imagine the effect on an engine which is putting out its whole strength and getting full charges of fuel, if the belt suddenly slipped off and it were "allowed its head." A burst fly-wheel would be only one of the results. The steam-engine is easily controlled by the centrifugal action of a ball-governor, which, as the speed increases, gradually spreads its balls and lifts a lever connected with a valve in the steam supply pipe. Owing to its elastic nature, steam will do useful work if admitted in small quantities to the cylinder. But a difficulty arises with the internal combustion engine if the supply of mixture is similarly throttled, because a loss of quantity means loss of compression and bad ignition. Many oil engines are therefore governed by apparatus which, when the speed exceeds a certain limit, cuts off the supply altogether, either by throwing the oil-pump temporarily out of action, or by lifting the exhaust valve so that the movement of the piston causes no suction—the "hit-and-miss" method, as it is called.
The means adopted depends on the design of the engine; and it must be said that, though all the devices commonly used effect their purpose, none are perfect; this being due rather to the nature of an internal explosion engine than to any lack of ingenuity on the part of inventors. The steadiest running is probably given with the throttle control, which diminishes the supply. On motor cars this method has practically ousted the "hit-and-miss" governed exhaust valve; but in stationary engines we more commonly find the speed controlled by robbing the mixture of the explosive gas in inverse proportion to the amount of the work required from the engine.
THE DIESEL OIL ENGINE,
on account of some features peculiar to it, is treated separately. In 1901 an expert wrote of it that "the engine has not attained any commercial position." Herr Rudolph Diesel, the inventor, has, however, won a high place for his prime-mover among those which consume liquid fuel, on account of its extraordinary economy. The makers claim—as the result of many tests—that with the crude rock-oil (costing in bulk about 2d. a gallon) which it uses, a horse-power can be developed for one hour by this engine for one-tenth of a penny. The daily fuel bill for a 100 h.p. engine running ten hours per day would therefore be 8s. 4d. To compete with the Diesel engine a steam installation would have to be of the very highest class of triple-expansion type, of not less than 400 h.p., and using every hour per horse-power only 1 3 / 4 lbs. of coal at 9s. per ton. Very few large steam-engines work under conditions so favourable, and with small sizes 3-4 lbs. of coal would be burnt for every "horse-power-hour."
The Diesel differs from other internal combustion engines in the following respects:—
1. It works with very much higher compression.
2. The ignition is spontaneous, resulting from the high compression of the charge alone.
3. The fuel is not admitted into the cylinder until the power-stroke begins, and enters in the form of a fine spray.
4. The combustion of the fuel is much slower, and therefore gives a more continuous and elastic push to the piston.