Chapter 10
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 1-1/2 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.
At this juncture the author would caution the more inexperienced young mechanics not to build double-acting engines. The valve mechanism is somewhat intricate and very difficult to regulate. The construction is also much more complicated, and this also holds true of the designing. On the other hand, single-acting engines, while not so powerful for a given size, will do very nicely in driving model boats, and will deliver sufficient power for all ordinary purposes.
Your attention is directed to Fig. 119. This shows a design for a model single-cylinder, single-acting steam-engine. The reader should carefully study each drawing before continuing to digest the following matter. The cylinder _L_ can be made from a piece of tubing. This can be either brass or copper. Aluminum should not be used, owing to the fact that it is difficult to solder and difficult to work with. The piston is made so that it will fit nicely into the cylinder and move up and down without binding. It will be seen that a groove, _M_, is cut around the piston near the top. String soaked in oil is placed in this groove. This is called packing, and the presence of this packing prevents steam leakage between the piston and the cylinder walls and thereby materially increases the efficiency of the engine.
In this case the connecting-rod _R_ is made in a circular piece. It is attached to the piston by a pin, _F_. The connecting-rod must be free to revolve upon this pin. The engine shown has a stroke of 7/8 inch. Therefore, the crank-pin _K_ on the crank-disk _N_ must be placed 1/2 of 7/8 or 7/16 inch from the center of the disk _N_, so that when this disk makes one revolution, the piston will move 7/8 inch in the cycle. Thus it will be seen that the distance of the crank-pin _K_ from the center of the crank disk _N_ will depend entirely upon the stroke of the engine. It may be well to mention here that the worker should always start designing his engine by first determining the bore and stroke. Everything depends upon these two factors. It is also well to mention here that the piston should never travel completely to the top of the cylinder--a small space must always be left for the steam to expand. One eighth of an inch is plenty of space to leave.
It will be noticed that the valve mechanisms on the particular engine shown bear no resemblance to the _D_ valve previously described. The holes _G_ which are bored around the cylinder are the exhaust ports. It will be seen that when the piston is at the end of its downward stroke it uncovers these exhaust ports and permits the steam to escape. The momentum of the flywheel _A_ pushes the piston upward, closing these holes. As these holes are closed the valve _H_ uncovers the entrance _I_ and permits steam to enter from the boiler through _J_. By the time the piston has reached the upward limit of its stroke a considerable steam pressure has developed on top of the cylinder, and this again forces the piston downward. Thus the same cycle of movement is gone through repeatedly.
The valve on this little engine is extremely simple. It consists of a circular piece of brass drilled out, as shown. A hole (_I_ and _J_) is drilled transversely through this. The little cylinder shown in the insert at _O_ slides in the larger hole, and when it is at its upper limit it cuts off the steam. At the proper intervals the valve is pulled down by the eccentric _C_. It will be seen that the moving parts, i.e., the valve and the piston, must be properly timed. That is, the eccentric _C_ must be mounted on the crank-shaft _B_ so that the valve will close and open at proper intervals. When the engine is made, the eccentric can be shifted about by means of a set-screw, _Q_, until the engine operates satisfactorily. This set-screw is used to hold the eccentric to the crank-shaft. The word eccentric merely means "off center." Thus the eccentric in this case is formed by a little disk of brass with the hole drilled off center. The distances these holes are placed off center will depend entirely upon the motion of the valve. It will be seen that the valve is connected to the eccentric by means of the valve-rod _E_. The valve-rod, in turn, is held to a circular strap which is placed around the eccentric. A groove should be cut in the surface of the eccentric, so that this strap will not slip off. If the strap is not put on too tightly and the eccentric is free to revolve within it, the valve will be forced up and down as the eccentric revolves.
The crank-shaft _B_ revolves in two bearings, _D D_. The flywheel is held to the crank-shaft by means of a set-screw _S_.
Most small engines with a bore under one inch will operate nicely on from 20 to 30 pounds of steam, and this pressure can easily be generated in the boiler that was described in the chapter on model-boat power plants.