The Romance of Modern Mechanism / With Interesting Descriptions in Non-technical Language of Wonderful Machinery and Mechanical Devices and Marvellously Delicate Scientific Instruments

by Archibald Williams

OIL ENGINES

These fall into two main classes:—

(a) Those using light, volatile, mineral oils—such as petrol and benzoline—and alcohol, a vegetable product.

(b) Those using heavy oils, such as paraffin oil (kerosene) and the denser constituents of rock-oil left in the stills after the kerosene has been driven off. American petroleum is rich in burning-oil and petrol; Russian in the very heavy residue, called astakti. Given the proper apparatus for vaporisation, mineral oils of any density can be used in the explosion engine.

The first class is so well known as the mover of motor vehicles and boats that we need not linger here on it. It may, however, be remarked that engines using the easily-vaporised oils are not of large powers, since the fuel is too expensive to make them valuable for installations where large units of power are needed. They have been adopted for locomotives on account of their lightness, and the ease with which they can be started. Petrol vaporises at ordinary temperatures, so that air merely passed over the spirit absorbs sufficient vapour to form an explosive mixture. The "jet" carburetter, now generally employed, makes the mixture more positive by atomising the spirit as it passes through a very fine nozzle into the mixing chamber under the suction from the cylinder. On account of their small size spirit engines work at very high speeds as compared with the large oil or gas engine. Thus, while a 2,000 h.p. Körting gas engine develops full power at eighty-five revolutions a minute, the tiny cycle motor must be driven at 2,000 to 3,000 revolutions. Speaking generally, as the size increases the speed decreases.

Of heavy oil engines there are some dozens of well-tried types. They differ in their methods of effecting the following operations.

1. The feeding of the oil fuel to the engine.

2. The conversion of the oil into vapour.

3. The ignition of the charge.

4. The governing of speed.

All these engines have a vaporiser, or chamber wherein the oil is converted into gas by the action of heat. When starting-up the engine, this chamber must be heated by a specially designed lamp, similar in principle to that used by house painters for burning old paint off wood or metal.

Let us now consider the operations enumerated above in some detail.

1. The oil supply. Fuel is transferred from the storage tank to the vaporiser either by the action of gravity through a regulating device to prevent "flooding," or by means of a small pump, or by the suction of the piston, which lifts the liquid. In some engines the air and gas enter the cylinder through a single valve; in others through separate valves.

2. Vaporisation. As already remarked, the vaporising chamber must be heated to start the engine. When work has begun the lamp may be removed if the engine is so designed that the chamber stores up sufficient heat in its walls from each explosion to vaporise the charge for the next power stroke. The Crossley engine has a lamp continuously burning; the Hornsby-Ackroyd depends upon the storage of heat from explosions in a chamber opening into the cylinder. The best designs are fairly equally divided between the two systems.

3. Ignition of the compressed charge is effected in one of four ways: by bringing the charge, at the end of the compression stroke, into contact with a closed tube projecting from the cylinder and heated outside by a continuously burning lamp; by the heat stored in some part of the combustion chamber (i.e. that portion of the cylinder not swept by the piston); by an electric spark; or by the mere heat of compression. The second and third methods are confined to comparatively few makes; and the Diesel Oil Engine (of which more presently) has a monopoly of the fourth.

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.