Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(93) Timer Construction.

Fig. 97 shows a typical timer and circuit arranged for a four cylinder engine. The device can be arranged for any number of cylinders, however, by changing the number of sectors, the sectors being equal to the number of cylinders. There are timers on the market that differ from the one shown in the diagram but the principle of operation is the same with all. The shaft E is usually connected to the cam shaft and is electrically grounded to the engine frame at L by means of the bearing in which the shaft rotates.

The lever F mounted on the shaft E carries the pivoted arm H which is free to move on the pivot to a limited extent to allow for wear on the walls W-W-W-W. At one extremity of H is the roller I which rotates on the pin J, as the roller runs around W-W-W-W. At the other extremity of H is fastened the spring S, which forces I into contact with the walls. A-B-C-D are metallic contact sectors whose connections lead to the four spark coils.

When the metal roller I comes into contact with one of the sectors as at B, the sector is grounded to the engine frame by the roller, the current traveling through the roller and its pin, through lever H and its pin, through the lever F and shaft E to ground at L, the course of the current being indicated by the arrows.

Fig. 97. Timer Diagram.

As the shaft E rotates and carries with it roller I, the roller makes contact with the sectors in order B-C-D-A, if rotated in the direction shown by arrow, which rotation grounds the primary coils of the spark coils R³-R⁴-R¹-R² in succession; the connection from the timer to the primary being to the primary binding posts P³-P⁴-P¹-P². A high tension spark occurs at each contact of the roller with the sectors, as the contact allows current to flow through the primary of the coils. The high tension binding posts S¹-S²-S³-S⁴ are connected with the spark plugs or spark gaps U¹-U²-U³-U⁴ by means of high tension cables. As soon as the timer grounds a coil, the coil produces a high tension spark in its corresponding spark plug.

It is evident from the foregoing that the timer not only determines the time at which a spark will take place, but it also determines the cylinder in which the spark will be produced, providing of course that a spark coil is provided for each cylinder.

The contact sectors A-B-C-D are insulated from each other by the insulating walls W-W-W-W, the inner surface of which provides a path on which the contact roller I revolves.

The contact sectors and insulating walls are encased by the protective housing Z, to which they are rigidly fastened.

The housing Z can be moved back and forth on the shaft E for advance and retard, by means of the lever K.

The current flows from the battery terminal V (with the roller in the position shown) through the switch M, through coil R³, post P³ to sector B, from which it passes through the roller I, levers H and F to ground. From the ground on the engine frame the current flows back to its source, the battery O, thus completing the circuit. When the roller makes contact with sector C, the coil R⁴ is energized, contact with D energizes R¹, and so on. No two coils can be thrown on simultaneously as only one coil is grounded at a time. The high tension current flows from each coil to its plug as soon as the current passes through the primary of that coil.

In some timers, the current is taken from the revolving arm through a separate connection to ground instead of grounding the shaft through the bearings. With these timers, the connection is not affected by worn bearings or an oil film that tends to insulate the shaft from the bearings.