All About Engines

The Locomotive 269 same size, each taking steam directly from the boiler ; but a few of the most powerful types have four cylinders. Compound engines, which are popular on the Continent and in the United States, have been tried in this country and discarded. One disadvan- tage lies in the fact that steam enters the cylinders in succession, so that when starting from rest the whole of the duty falls upon the high-pressure cylinder ; and as this cylinder has a smaller bore than the cylinders of an ordinary double-cylinder engine of the same power a heavy train is only set in motion with difficulty. To obviate this a “ by- pass ” is arranged so that, on starting, steam may be admitted to both cylinders at once, and com- pound working reverted to when the train has acquired a fair speed. But this adds to the com- plication of the mechanism, increasing the first cost and the expense of up-keep, and must be set off against the gain from economy of fuel. In countries such as Great Britain, where coal is cheap, the saving in the cost of fuel by compound working is said to be insufficient to compensate for the increased cost of construction and maintenance. Let us now consider the different types of engines and see how they are distinguished from one another. The terms “ express,” “ goods,” etc., do not convey much information, and a more exact classification is based upon the arrangement of the wheels. Thus if the connecting rod acts upon only one pair of wheels •—that is, if there are no coupling rods—the engine is called a “ single ” ; if two pairs of wheels are