Impulses in 4-Cylinder Engine.—In such a case the four cylinders are arranged so the impulse will be at four different points of the shaft, and we may assume that the four cylinders in Figs. 63, 64, 65 and 66, show the relative positions of the four pistons in a four cylinder engine.
The Cylinder Case, and Connections.—A cross section of a case and the relative positions of the various parts, is shown in Fig. 67. The cylinder A is provided with a water jacket B, so as to form a space C around the cylinder which has an inlet pipe D at the bottom, and an outlet pipe E at the upper end.
Fig. 67. Automatic Inlet Valve.
The inlet valve F is in the head of the cylinder, and it is held against its seat by a tension spring G. The exhaust valve H is placed in a lateral extension of the cylinder, in such a position that it is directly above the secondary shaft I running through the crank case. The stem J of the valve, is actuated by a cam K on the secondary shaft, and it is, preferably, made in two parts, the upper being so arranged that it has a limited longitudinal movement independently of the lower part, and a spring is arranged so as to provide for longitudinal thrust in either direction.
The crank shaft M has alongside the crank, a gear wheel N, which meshes with a gear O on the secondary shaft I, this latter gear being twice the diameter of the gear N.
Piston and Crank Construction.—The piston is hollow, and the crank is located as close to the head as possible. This has two or more circumferential grooves, to receive packing rings. The rings are made of very hard steel, and are turned up slightly larger than the diameter of the cylinder, and then cut across diagonally, so they may be sprung into place, and when in position they will bear against the inside of the cylinder, and thus serve to prevent the passage of the gases.
Calculating the Efficiency.—The great problem with every beginner is to know something of the power of the engine, and how it is determined. Considering that the boy knows nothing of the terms used to designate the step we shall try to make the following description as free from technicalities as possible.
In Fig. 68 a cylinder is represented, containing a piston A. B C indicate the limits of the stroke, and for convenience this space is provided with eleven marks to represent the pressure of the ignited gases at various portions of the travel of the piston.
Pressure in Explosion.—When the explosion takes place, at B, the pressure will be, approximately, 230 pounds per square inch of the piston. When it moves to the next mark the pressure has decreased to 220 pounds, at the next mark it is 200, and so on, until, at the end of the stroke, opposite C, the pressure is only 40 pounds.