set up in the primary winding. This current is conducted by the screw c to the circuit-breaker when it is interrupted once during each revolution of the magneto shaft d. The interruption of the primary current causes a current of high voltage to be produced in the secondary winding. This high-tension current is led to the slip ring e, by a connection from one end of the secondary winding, and is collected by the brush ƒ and delivered to the ignition system through the terminal g. A condenser his connected in the primary circuit to prevent burning of the circuit-breaker contact points and to strengthen the secondary current. The terminal i is for the purpose of connecting the primary circuit to a switch, by means of which the circuit may be opened and the magneto made inoperative.

64. The circuit-breaker in the magneto takes the place of the vibrator in the induction coil. The screw c and the block j are insulated from the interrupter disk k, which is electrically connected to the armature coil. The interrupter contact block j carries a platinum contact screw fitted with locknuts. The interrupter lever I is in the form of a bell-crank and carries on one end a platinum screw that is held in contact with the one in the block j by a spring. The interrupter lever is electrically connected to the armature core and hence to one end of the primary winding, which is thus short-circuited as long as the platinum screws of the contact block and interrupter lever are in contact. The interrupter housing m can be rotated to advance or retard the spark; it carries a steel cam that, through coming in contact with one end of the interrupter lever 1, breaks the contact between the screws of the interrupter lever and contact block j, thereby suddenly breaking the primary circuit.

65. A magneto of this type designed for use with two cylinders contains two interrupter cams located opposite each other, and is provided with two secondary terminals, so that a high-tension current may be obtained twice during each revolution of the magneto shaft. For engines of more than two cylinders, a single high-tension terminal is used and the current is led from this to a distributor, usually a part of the

magneto, by means of which it is delivered to the various cylinders. The speed at which a magneto runs depends on the construction of the engine, although it must always bear a fixed relation to the engine speed. For instance, for a singlecylinder four-cycle engine, the magneto is driven at one-half crank-shaft speed, as only one explosion occurs for every two revolutions of the crank-shaft; while for a single-cylinder twocycle engine, the magneto is driven at crank-shaft speed. On two-cylinder four-cycle engines with cranks set side by side, the explosions in the two cylinders will occur one revolution of the crank shaft apart, and the armature of the magneto is therefore driven at one-half crank-shaft speed, assuming that the magneto is designed for two-cylinder engines and gives two sparks per revolution of the armature. Magneto armatures are usually driven by spur gears or helical gears, although in some cases silent chains are used for this purpose.

IGNITERS AND SPARK PLUGS

66. Igniters.-The sparking device employed with the low-tension, or make-and-break, system of ignition is called an igniter. The make-and-break system uses a low voltage current obtained from a battery or low-tension magneto, and the purpose of the igniter is to break the circuit inside of the cylinder and thus cause a spark to be formed. Igniters are of two general types, mechanical and magnetic. A mechanical igniter is operated from some moving part of the engine by mechanical means, while a magnetic igniter is operated by means of an electromagnet that receives its current from the ignition system.

67. A mechanical igniter with the mechanism for actuating this and similar devices, is shown in Fig. 25, view (a) being a front elevation and view (b) a side elevation. At a is shown a shaft turning at one-half the speed of the engine if it is of the four-cycle type, or at the same speed if it is of the two-cycle type; in this case it may be the engine crank-shaft itself. On this shaft is a cam b, frequently called a snap cam,

that bears against a roller c held in contact with the cam by the spring d on the tappet rod e. The lower end of this rod, or plunger, is threaded, so that by adjusting the nuts at f, and thus increasing or decreasing the distance that the foot of the rod extends into the socket g, the length of the rod and, consequently, the width of the gap when the igniter points separate, can be varied at will.

When the roller c is in its lowest position, the ball on the upper end of the tappet rod e rests in a socket in the lever arm h. This arm is secured to a rocking stem that passes through the wall of the combustion chamber, as d shown by the dotted lines of the side elevation. The inner end,

0000

a

m

which has a ground joint to prevent the gases from blowing past it, is prolonged to the finger i. This finger makes contact with an insulated stem j, to whose outer end one of the wires of the electric circuit is attached. A light spring k holds the finger i against the stem j, except when the two are separated by the pull of the head of the tappet rod e in the socket of the arm h. Because the greater tension of the spring d overcomes that of the spring k, the contact points are normally out of contact except when the tappet rod is pushed up by the cam. The adjustment of the tappet rod is such that after contact has been made it leaves the socket of the arm h and continues its upward motion a short distance, so that, when the roller c drops off from the cam, the

(a)

FIG. 25

(ბ)

head of the tappet rod strikes the arm h a smart blow, thereby causing an abrupt separation of the contact points.

As shown, the roller c is mounted in a rocker-arm 1, one end of which is pin-connected to a second rocker-arm m, which is operated by hand to control the time of ignition. When the arm m is moved toward the right, as indicated by the dotted outline n, the ignition is earlier than when it is in the position m; when m is moved toward the left, to the other dotted position o, the ignition is made later.

68. A magnetic igniter designed to be operated on current from a battery or a low-tension magneto, is illustrated in

Fig. 26, in which (a) is an external view and (b) a longitudinal section. The hexagon-headed plug a is screwed into the engine cylinder and a wire from the battery or magneto is connected to the terminal at the top. The body b of the plug contains a spoolwound magnet coil c and the upper end of the interrupter lever d. The igniting current passes through the

magnet coil, and moves the interrupter lever so as to break the circuit and form a spark at the contact points of the igniter. When the current flows through the coil, the core e is magnetized and a magnet pole is thus formed in the region of the brass insert f. This pole attracts and draws toward it the upper end of the interrupter lever d, thus rocking the lever on its knife-edge support and moving the contact point g away from the point h. This breaks the electric circuit and causes a spark

at the contact points, which extend inside of the combustion chamber. The body a is insulated from g by the steatite cone i and mica plates j.

69. Spark Plugs.-The universal method of obtaining a spark with the high-tension system of ignition, which uses a current of high voltage, is by means of a spark plug. The

spark plug consists of a small wire or rod that passes through some kind of insulating material and to within a short distance of a threaded piece of metal or bushing that surrounds the insulation. The space left between the wire and the outer metal bushing is the gap across which the spark jumps. This gap is usually from inch to inch, depending on the strength

1

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