THE DESIGN OF MODEL STEAM-ENGINES
INSTEAD of describing the construction of several model engines of different design, the author thinks it advisable to put the reader in possession of the fundamentals of model steam-engine design and construction. In this way the model engineer will be able to design and construct model steam-engines according to his own ideas and in accordance with the raw materials and miscellaneous parts he may find in his workshop. Unless the young mechanic is in possession of a very well equipped workshop, it is quite impossible to construct a steam-engine according to certain specifications. However, if he has in mind the fundamental principles of steam-engine design, he can go ahead and design his engine, for which he will have no trouble in machining or producing the parts that enter into its construction. By this the author means that the workman can design his engine to meet the materials he has on hand.
Notice [Fig. 117]. This is a cylinder into which is fitted a piston. If steam is forced into the cylinder the piston will be forced to the opposite end of the cylinder. If some means is then provided so that the steam can escape and the piston come back, another impulse can be given it by admitting more steam, and thus a continuous motion may be produced. This is how the steam-engine works.
Having learned how motion is imparted to the piston by the expansion of steam under pressure, attention is directed to what is known as the "D" slide-valve. This slide-valve permits steam to enter the cylinder and to exhaust at proper intervals. See [Fig. 118]. Steam enters the steam-chest through the pipe A. The slide-valve is shown at D. When the slide-valve is in the position shown, steam enters the cylinder, and by the time the cylinder has arrived in the position shown by the dotted line C, the slide-valve moves over, closing the passage B. The steam under pressure forces the piston to the opposite end of the cylinder. When the piston reaches the opposite end of the cylinder, steam that has entered through the passage F again forces the piston back to its original position. This is caused by the slide-valve shifting its position, because of the impulse it received at the opposite end of the cylinder. Thus it will be seen that when the piston is at one end of the cylinder the opposite end is exhausting. By carefully studying [Fig. 118] the action of the D valve will be understood. The connecting-rod E is connected to the crankshaft and in this way the engine is caused to revolve.
A cylinder similar to that shown in [Fig. 118] is called a double-acting cylinder. This is because the steam acts on both sides of the piston. Single-acting cylinders are cylinders in which the steam expands on only one side of the piston. In the single-acting engines the D valve is modified.
The "stroke" of a steam-engine depends upon the length of the cylinder; really, the stroke is the distance travelled by the piston. In model engines it ranges from 3/8 of an inch to 11/2 inches. The bore of a cylinder is its internal diameter. The bore is usually a trifle smaller than the stroke. Thus it is common to have a stroke of 7/8 inch and a cylinder-bore of 3/4 inch.