This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.
CARL H. CLARK
The jump spark ignition is very simple in appearance, the only engine attachment being the commuta-tat«»r, or device for making and breaking the primary circuit whenever a spark is desired; this does away with a con-idenable amount, of gear and simplilies the engines. The engines represented in Figs. 11 and 18 are of this type.
Fig. 31. Fig. 31 represents the simplest type of make and break device. This particular device is u ually located just behind the flywheel. O is the engine shaft: D is a disc of fiber or other insulating material, which is
Fig. 32. fastened to the shaft and revolves with it. This disc, D, has on its circumference a small sector S of brass or other conducting material, which is in electrical connection with the shaft by the wire or clip e. The frame A encircles and is held by the end of the engine bed, but may be revolved around it. The outer end of the arm carries a binding screw B, to which is attached the edge of the disc I). One terminal is fastened to B and the other to the engine. As will be seen, when the spring c rests upon the fiber, there is no circuit, but when the segment s passes under it the circuit is completed through the engine shaft and body of the engine. By turning the arm A around the shaft the rehnive time of sparking may be changed. The engine shown in Fig. II has a commutator, or timer, of this kind; the handle for regulating it may be seen just behind the flywheel.
Fig. 32 shows two views of a similar form of timer; in this case the plunger P makes the contact, being pressed out by the coiled springs. B is the binding post for fastening the wire from the batteries, A handle fastened at 11 is provided for changing the time of ignition.
Another form of timer is shown by Fig. 33; the shaft is sho.\n by 0 as before, the cam C is fixed on the shaft and turns with it. this cam has a projection or "nub" on its face, the casting A encircles the hub of the bearing and has the arm A, at the end of which is the post p, supporting the contact spring 8, which rests normally upon the round face of the cam C. At B is a binding post having the adjustable contact screw e; this post B is insulated from the frame A and the contact screw e is adjusted to leave a slight space between it and the contact springs. The lead wire is attached to II, and in the position shown no current will pass. At the time of ignition, however, the "nub" on the cam C will pass under the spring s and force it up into contact with the point of the screw e and thus complete the electrical circuit through the metal of the engine. 11 is a handle for regulating the time of sparking by revolving the whole around the shaft, thus causing the came to strike the spring earlier or later.
The timers thus described are for single cylinder engines, but may be adapted to two cylinders by duplicating the mechanism in a diametrically opposite position. Timers of this description would hardly he used for more than two cylinders; they areals/best suited to two cycle engines, as they give a contact for each revolution; this class of timer is, in fact, most commonly fitted on two cycle engines of medium and low price.
A more pretentious timer is represented by Fig. 34. It is entirely separate from the mechanism of the engine and is run from a separate shaft, such as the half time shaft of the four cycle engine, or an independent shaft driven by gears from the main shift of a two cy-engine. It is rather similar to that described in Fig. 33, the cam Cpressing the steel spring 8 outward
so as to bring the platinum point on the spring into contact with that on the binding post B, completing the circuit. In this particular timer both posts are insulated from the body of the timer, a wire to each passing through the metal of the engine. The whole is covered with a removable cover, as shown. The engine illustrated in Fig. 11 is fitted with a timer of this kind located on the rear end of the valve shaft. A little consideration will show that the timer of a four cycle engine should be governed by the valve shalt in order to give the proper timing, since the four cycle engine gives an explosion for each two evolutions; the timer must be so arranged as to give a spark in the same interval.
Fig. 35 represents a timer for a four cylinder engine; it is similar in action to that shown in Fig. 31, the steel balls B are pressed out bv the coil spring s; the cam C having the projection P is revolved by the shaft, the wires are attached, as shown. At the proper time the projection P rubs past the ball B, thus completing the circuit. The ball holders must, of course, be insulated from each other and the body of
the timer; it is indeed common to make the entire body of liber or hard rubber. In this form the timer shaft and body of the engine form a part of the circuit. There must, of course, be as many binding posts as there are cylinders. In order to change the time of ignition the entire case is turned slightly by an attachment to the lever //. The case is covered by a removable cap protecting it from dust and moisture.
When a timer of this type is fitted to a two cycle engine it is very convenient to place it on the end of a vertical shaft standing in front of the cylinders and just behind the flywheel, and driven by bevel gears from the engine shaft.
Fig. 36 shows two views of another type of timer on the same principle as that of Fig. 33. The binding posts B are insulated from the metal of the body by the rubber sleeve /, the contact point is forced out by the coil spring .s, but is in electrical contact with />'. The lever /,, pivoted at e, carries in its outer end the roller R, the flat spring s presses the roller in and maintains it in contact with the cam C. The projection on the cam C passes under the roller R and raises the lever L. bringing it into contact with the point P, and completing the circuit through the timer shaft and the metal of engine. The whole is covered by the cap k.
A very common form of spark plug is shown in Fig. 37; it consists of an inner spindle or rod R and an outer sleeves, with an insulating core C of porcelain or mica. At the end of the rod R is a spindle point p, and a similar one P projects from the outer shell s. At T is a thread by which the plug is attached to the engine, a threaded hole pasting through the water picket and allows the sparking points to project inside the cylinder. The core is passed through the shell s and is held in place by the internal but N, two rings of asbestos packing, G G, are inserted to prevent the gases
Figs. 37, 38 and 39.
from blowing out between the core and the sleeve. Nuts B on the end of the rod R, serve as a binding post for the lead wires. The points p, P, are seperated by a small amount 1 32 to 1.16 in. When the plug is screwed into the cylinder there is no electrical connection between them; if one wire from the coil is grounded on the engine and the other is fastened to B the current will jump across the gap between the points when the contact is made.
Fig. 38 shows a type of mica insulated plug. The insulating core consists of a layer of mica wound around the rod ; outside of this layer of mica is a series of mica discs slipped on over It; the head on the end of the rod holds the washers against slipping off and allows them to be pressed tightly together by the nut n. The remainder of the plug is similar to the one above.
In the form shown in Fig. 39 there are two porcelain insulators, the inner one Cas before, and the outer one K; they are held in place by the nuts N, n. The metal shell s is prolonged and the spark takes place between a point on the end of the rod and the edge of the shells.
ft is necessary that the insulation between the two portions of the plung he very complete, as the secondary current used with this form of ignition is of very high voltage and will easily penetrate any of the ordinary forms of insulation. Porcelain, mica, or some kinds of homogeneous stone are the only satisfactory insulating materials for this use. The plug is one of the most sensitive portions of the engine, as well as one of the most important; it must, therefore, be well taken care of and kept in good condition.
The details of coils and wiring are to be considered in the following chapter.