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.