Above all, the gas engine is more efficient, or, if you like to express the same thing in other words, more economical. If you use only one horse-power for one hour a day, it doesn't much matter whether that horse-power-hour costs 4d. or 5d. But in a factory where a thousand horse-power is required all day long, the extra pence make a big total. If, therefore, the proprietor finds that a shilling's-worth of gas or oil does a quarter as much work again as a shilling's-worth of coal, and that either form of fuel is easily obtained, you may be sure that, so far as economy is concerned, he will make up his mind without difficulty as to the class of engine to be employed. A pound of coal burnt under the best type of steam-engine gives but 10 per cent. of its heating value in useful work. A good oil-engine gives 20-25 per cent., and in special types the figures are said to rise to 35-40 per cent. We may notice another point, viz. that, while a steam-engine must be kept as hot as possible to be efficient, an internal combustion engine must be cooled. In the former case no advantage, beyond increased efficiency, results. But in the latter the water passed round the cylinders to take up the surplus heat has a value for warming the building or for manufacturing processes.
Putting one thing with another, experts agree that the explosion engine is the prime mover of the future. Steam has apparently been developed almost to its limit. Its rival is but half-grown, though already a giant.
Some internal combustion engines use petroleum as their fuel, converting it into gas before it is mixed with air to form the charge; others use coal-gas drawn from the lighting mains; "poor gas" made in special plants for power purposes; or natural gas issuing from the ground. Natural gas occurs in very large quantities in the United States, where it is conveyed through pipes under pressure for hundreds of miles, and distributed among factories and houses for driving machinery, heating, and cooking. In England and Europe the petroleum engine and coal-gas engine have been most utilised; but of late the employment of smelting-furnace gases--formerly blown into the air and wasted--and of "producer" gas has come into great favour with manufacturers. The latest development is the "suction" gas engine, which makes its own gas by drawing steam and air through glowing fuel during the suction stroke.