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end of its compression stroke. Some engines have the spark set 45 degrees advance. With the spark advance lever set about half way of its travel the spark may be made to occur just when the piston reaches the end of its compression stroke, or on top center. It is necessary to provide a wider range of spark advance on a battery and coil ignition system than when a magneto is used, as it is said that a range of advance of 60 degrees is sufficient for four-cylinder motors and 27 degrees for six-cylinder motors with magneto ignition.
In timing a strange car it is easy to tell whether the movement of the spark lever advances or retards the timer case by noting the direction of movement of that member. If the spark advance lever is pushed in a certain direction, say from the point on the sector nearest the driver to the other extreme, and the segments on the timer move to meet the advancing contact roller, it is evi. dent that a movement of the spark advance lever from front to rear advances the ignition. If the timer case oscillates so the segment moves away from the advancing contact roller, that move. ment of the spark lever retards the ignition. In most timers the rotating contact member is fastened to the shaft in such a way that it may be moved independent of engine rotation, if desired, by releasing the fastening. Sometimes it is held on a tapered shaft by a clamping nut, in other constructions it is driven by a hollow shaft which is set screwed to the timer driving shaft the position of which can be changed as desired. In every case the roller should be set in contact with the segments joined to coil unit No. 1, the remaining terminals being wired according to the firing order and the direction of rotation of the timer brush. In the diagram now under discussion after the roller leaves unit No. 1 segment it will go to that in connection with unit No. 2, then to the one joined to unit No. 4, and finally to the terminal conveying the electrical current to unit No. 3. This means that the plug in cylinder No. 1 fires first, followed by those in cylinders 2, 4, 3, in the order named. With the switch lever in the position shown or between the two contact buttons, the ignition is interrupted and battery current cannot flow to the coil unit. If the switch lever is moved to the button on the right marked “storage bat
the secondary current producer will furnish ignition. If moved to the button on the left, the dry cells will be brought into action. The same method is employed in timing a two, three or six-cylinder motor, the only precaution to be observed being to run the wires from the timer to the coils so the cylinders will fire in proper order.
At one time secondary distributor systems using a single unit vibrator coil for firing a multiple cylinder engine were very popular, but at the present time few cars use the long contact timer and distributor combination. The modern cars that employ battery ignition use a short contact timer and a non-vibrator coil unit, Popular systems of this nature are the Atwater-Kent and the Delco, both of which have been previously described. Practically the same method of timing is employed with these systems except that there is but one primary terminal on the contact breaker portion of the distributor which is joined to the corresponding terminal of the spark coil. A proper distribution of current to the cylinders is made by connecting the distributing terminals to the plugs in proper firing order.
MAGNETO IGNITION SYSTEMS
Magneto Generator Construction—Low Tension Magnetos—Typical American
Magneto Forms—Magnetos for Eight- and Twelve-Cylinder MotorsSimple Magneto Ignition System-Double System—Transformer Coil Method—Dual Ignition-Duplex Ignition—Two-Spark Magneto-Magnetic Plug System-Impulse Starter-Automatic Spark Advance—Low Tension Magneto Troubles—High Tension Magneto Troubles—Recharging Magnets -Adjusting Parts—Application to Typical Engines—Timing Magneto Ignition System-Firing Orders of Typical Engines.
Magneto Generator Construction.—The magneto is a simple form of dynamo and a mechanical generator of electricity in which permanent magnets are used to produce the magnetic field and between which the armature revolves. The permanent magnets are called "field magnets" and at their ends are provided cast-iron shoes which form the walls of the armature tunnel and which are known as pole pieces. A typical magneto adapted for single-cylinder ignition is shown in section at Fig. 92. It consists of two compound horseshoe magnets attached to the pole pieces which collect and concentrate the magnetism upon the armature. The armature is shuttle-shaped and carries a double winding of wire which consists of two coils, one of coarse, the other of fine conductor. The armature is attached to end pieces which carry shafts and the whole assembly revolves on annular ball bearings. An ebonite or hard rubber spool is carried at one end while the condenser is housed at the other. The make-and-break mechanism is partly carried by an oscillating casing and the revolving member is turned from the armature shaft.
The current generated in the coil is delivered to a metal ring on the ebonite spool from which it is taken by a carbon brush and delivered directly to the spark plug. Every time the contact points
in the make-and-break devices become separated, a current of high potential passes through the wires attached to the spark plug and produces a spark between the points. The magneto is the simplest and most practical form of ignition appliance as it is self-contained and includes the current generator and the timing device in one unit. In the one-cylinder form shown all connections are made inside of the device and but one wire leading to the spark plug is necessary to form the external circuit.
Fig. 92.—Simple High Tension Magneto for One Cylinder Ignition, a
Complete Apparatus Comprising Source of Current and Timing De-
A magneto employed for multiple-cylinder ignition is not much more complicated than that used for single-cylinder service, the only difference being that a different form of cam is provided in the breaker box and that a secondary distributor is added to commutate the current to the plugs in the various cylinders. The distributor consists of a block of insulating material fixed to the magnets which carries as many segments as there are cylinders to be fired. A central distributing arm or segment is driven from the armature shaft by means of gearing, and is employed to distribute the high-tension current to the spark plugs. The spacing
of the distributor segments does not differ materially from that of the battery timers previously described.
Various distributor forms used on magnetos are shown at Fig. 93. That at A is employed for a double opposed cylinder motor and the contacts are separated by a space of 180 degrees. When
Fig. 93.—How Distributor Contacts are Spaced on Two, Three, Four and
Six Cylinder Magneto.
a three-cylinder engine is used, as is sometimes the case in the two-cycle forms, the distributor segments are separated by distances of 120 degrees. If the distributor is used on a four-cylinder motor the segments are spaced 90 degrees apart, as shown at C. To fire a six-cylinder motor, six segments must be used and they