(2) The producer-gas engine and the suction-gas engine,
(3) Blast-furnace gas engines,
with reference to the installations used in connection with the last two.
All explosion engines (excepting the very small types employed on motor cycles) have a water-jacket round the cylinders to absorb some of the heat of combustion, which would otherwise render the metal so hot as to make proper lubrication impossible, and also would unduly expand the incoming charge of gas and air before compression. The ideal engine would take in a full charge of cold mixture, which would receive no heat from the walls of the cylinder, and during the explosion would pass no heat through the walls. In other words, the ideal metal for the cylinders would be one absolutely non-receptive of heat. In the absence of this, engineers are obliged to make a compromise, and to keep the cylinder at such a temperature that it can be lubricated fittingly, while not becoming so cold as to absorb too much of the heat of explosion.
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.