It was stated above that the piston travel was a reciprocating motion while the crank shaft revolution was a rotary motion, and that it was necessary to convert this reciprocating motion into rotary motion, for the reason stated above. This conversion of motion is performed by the connecting rod, as it is hinged to both piston and crank shaft and this conversion of motion may be depicted as follows:

When the piston is at its upper point of travel in the cylinder the crank pin is standing vertical above the crank shaft, but as the piston and connecting rod move downward under the influence of the expanding gases within the cylinders, the crank pin is constrained and revolves downwardly, thus turning the crank shaft. The crank pin passes through its horizontal position and as the motion of the piston and connecting rod continues, finally reaches a position vertically below the crank shaft, the piston at this moment being at its lowest point and the crank shaft has been revolved through one-half revolution. As the piston begins to move upward upon its return stroke the crank shaft is again constrained by the connecting rod to revolve and gradually approaches and passes its other horizontal position and finally, when the piston is all the way up, the crank pin has reached its original vertical position again, having turned the crank pin through one complete revolution.

Thus it can readily be understood that the upper end or piston pin end of the connecting rod reciprocates in harmony with the piston, while its lower end or crank pin end rotates in harmony with the crank pin, converting the reciprocating motion of the piston into rotary motion of the crank shaft. It should be remembered that in multi-cylinder engines the explosion force in one cylinder will move the piston downward in the other cylinder which is paired with it and is on the intake stroke. Thus in Fig. 12 the two end crank pins are vertical and while the right hand one is being forced downward it carries the left-hand pin with it. The left-hand cylinder being on the intake stroke, it also forces the two lower crank pins upward, the corresponding cylinder of one of these being on the compression stroke and the other on an exhaust stroke. In this way the right-hand pin is revolved upward when the lower pin on the compression stroke begins to move downward on the next power stroke which, as stated in the previous article, follows compression. This operation is followed by all crank pins on the various power strokes of the pistons.

In the single-cylinder motors this return motion is obtained by the storage of energy in the flywheel on the power stroke, which liberates itself on the idle strokes of the piston. This was discussed under the heading of multi-cylinder engines.

The operation of the parts in two-cycle motors was depicted previously, also the function of the valves in four-cycle motors, the function of the valves being to permit the entrance and escape of the gases from the cylinders at the proper time. These valves are termed poppet valves and consist of a disc of metal with a stem on one side, which closes a circular opening in the combustion chamber, being held against the seat in the wall by a coiled wire spring. The opening and closing of these valves is by a force imparted from the cam shaft, which will be treated later.

Fig. 11. Section of a Cylinder. A Combination of Valve-in-Head and L-Head Type.

Fig. 12. Connect ing Rod, Piston Pin Crank Shaft. Flywheel and Main Bearings of a Four Cylinder Engine.

Commercial car engines of the poppet-valve type are generally classified by the location of the valves within the cylinder, as this point generally controls the entire construction of the motor, as well as the various factors which enter into its design. The various types of cylinders classified by their valve arrangements are as follows: