The Gasoline Motor

by Harold Whiting Slauson

The cam shafts may be driven by a chain, the links of which fit over teeth cut on sprocket wheels, but there must always be a constant relation between the position of the cam shaft and that of the crank shaft. This constant relation is necessary in order that the valves will open and close at the proper points during the travel of the piston. For example, the exhaust valve should open toward the end of the explosion stroke in order to allow the burned gases to be forced out, and the cam for operating this valve should always be in the lifting position at exactly the proper moment.

If the cam shaft is not positively driven, this position may change and the exhaust valve might be opened at the beginning of the ignition of the charge, in which case the force of the explosion would be wasted almost entirely. On the other hand, the inlet valve should open at about the beginning of the suction stroke in order that the fresh charge may be drawn in by the downward travel of the piston; it is evident that this cannot be opened at any other time without a resulting loss in the power developed by the motor.

The proper timing of the action of the valves is consequently one of the most important adjustments of a motor. When the motor is assembled and tested at the factory, the valves are properly timed and there is no possibility that they will require further adjustment in this respect until after the engine is "taken down" for the purpose of cleaning or the renewal of a broken part. If it should ever become necessary to remove one of the cam shafts or any of the gears constituting the forward train, the greatest care should be taken to make certain that all are returned to exactly their original position. A difference of one tooth in the relative meshing of the gears may result in a loss of fifty per cent. of the power developed by the motor.

Absolute rules for the proper timing of the valves cannot be given here, for various motors are designed with slightly different positions at which the exhaust and inlet valves should be opened and closed. A cam shaft should never be removed, however, without first marking the intermeshing teeth of its driving gear and those of its companions. This may best be done by means of a small prick punch which, when tapped lightly with a hammer, will make a permanent mark at the desired point on the surface of the gear. If the motor is of the "T"-head type, having its valves operated by two cam shafts, care should be taken to designate the right and left-hand gears so that their positions will not be reversed if both have been removed at the same time.

A safe method to pursue is to indicate the right-hand gear with one punch mark, while two should be used for the gear at the left. Three teeth should be marked on each pair of intermeshing gears. That is, a tooth on one gear should be marked, and then each of the teeth between which it meshes on the other gear. The second cam shaft gear should be marked before the motor is turned.

As has been stated, the cams on many motors are forged integral with their shafts, and there is consequently no possibility of the removal of one from the other. Those cams which are keyed to their shafts are accurately and rigidly set and the keyways so cut that there is slight chance of a mistake in returning a cam that has been removed. It should seldom be necessary to remove a cam from its shaft, however.

Many motors are provided with timing marks on the flywheel to indicate the positions of the latter at which the valves of the various cylinders should open and close. In connection with these marks a pointer attached to the crank case and indicating the top of the flywheel is used. When the line on the flywheel marked, for example, 4 Ex 0, is under the pointer, it indicates that the exhaust valve on the fourth cylinder should be about to open. If the motor is turned but very little beyond this point, a lifting should be felt at the proper push rod or valve stem.

It is well to test the setting of the valves occasionally by means of these marks, for wear at the rocker arms, the push rods, the valve stem, or the cam travelers will result in unevenly-timed valves. It should be remembered that it is the valve itself that should open after the proper mark on the flywheel has been passed, and that the movement of a long push rod is not sufficient evidence that the valve is beginning to leave its seat. There may be so great an amount of lost motion between the push rod, cam, rocker arm, and valve stem that the flywheel may be turned several degrees beyond the proper point before this "play" will be taken up and the valve itself will begin to move.

Although the timing of a motor may be given in inches of piston travel beyond a certain dead center, at which point an exhaust or inlet valve should open or close, it is generally expressed in degrees of flywheel revolution. Suppose, for example, it is said that the inlet valve should open ten degrees after the beginning of the suction stroke. This would indicate that the flywheel should be turned through an arc of ten degrees from the point at which the piston is at its upper dead center before the inlet valve for that particular cylinder should begin to open. Expressed in terms of flywheel revolution, the total travel of the piston during each stroke is 180 degrees, and as in the proximity of its dead centers the piston moves but a short distance in comparison with the size of the arc through which the flywheel swings, valves may be set very accurately by this method.

Not all cam shafts for operating the valves are located in the crank case. On several designs of motors the cam shaft extends along the top of the cylinders and is driven by a vertical shaft and two sets of bevel gears. On such motors both inlet and exhaust valves are located in the cylinder heads, and owing to the proximity of the cam shaft, but short push rods and valve stems are needed. The valves are sometimes operated by means of a bell crank or rocker arm that acts directly against the cam surface and end of the valve stem.