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at three-quarters the speed of the crankshaft, while a four cylinder device is driven at the same speed as the crankshaft. Owing to the greater frequency in explosions in a six cylinder motor, it is necessary to turn the magneto armature at 1.5 times the crankshaft speed.

Q. How are distributor contacts spaced?

A. The contact segments in a secondary distributor are spaced just the same as those of a primary timer when used on the same type of engine.

Q. Why must magneto speed and distributor arrangement be changed for the different number of cylinders?

A. As the number of cylinders are increased the number of explosions needed each revolution of the crankshaft augments proportionately, so it is necessary to drive the magneto faster because a spark is obtained but twice each revolution of the magneto armature. On a four cylinder engine, which has two explosions per revolution of the crankshaft, the magneto armature is driven at the same speed as the crankshaft, because it is capable of supplying a spark each half revolution of the armature. On a six cylinder engine where the crankshaft receives three power impulses per revolution it is necessary to drive the magneto at a speed that will enable it to deliver three sparks for each revolution of the crankshaft, which means that the magneto armature must turn half again as fast as the motor shaft. Obviously, the distributor contacts must be changed in number on each size of magneto because one contact segment is used for each cylinder to be fired. Q. How are magnetos driven?

A. Magnetos of the high tension type used for ignition purposes are always positively driven by chain or gear connections with the engine crankshaft or camshaft.

Q. Why are positive driving means essential?

A. The magneto armature must be driven positively because the current generation must be timed to occur at the proper time for ignition in the engine cylinders and this makes it imperative that the drive be by some non-yielding positive means in order to

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Fig. 100.-Double Ignition System, Comprising Two Independent Groups, One Deriving Current From Batteries, the Other From a High Tension Magneto.

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insure that the ignition spark will always occur at the same time relative to the piston position or at top center in the cylinder about to be fired.

Q. How are dynamos driven?

A. Dynamos may be driven by direct gearing, if desired, though this is not necessary because this instrument delivers a low tension current that must be used in connection with suitable timing and intensifying means that are auxiliaries to the dynamo. A dynamo can be driven by flat or V leather belts and in some forms the armature is rotated by contact of a friction pulley against the engine flywheel periphery.

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Fig. 100A.-Outlining Method of "Floating Battery on Line" Generally Used When Current From Dynamo Type Generator is Employed for Ignition, Electric Lighting or Self-Starter Service.

Q. Describe a combination battery-magneto ignition system? A. On some forms of engines two practically separate ignition systems are provided, one being by battery, timer and spark coil, while the other system is by magneto, which is another selfcontained system. When used in the manner outlined at Fig. 100, either magneto or battery outfit may be utilized for ignition pur

poses. The change from one to the other is easily accomplished by shifting a switch lever. When a system of this kind is provided the battery group is intended to be used only for starting and emergency service in event of failure of the mechanical generator. The magneto is used regularly in order to conserve the battery energy. Combination systems of this nature are seldom provided at the present time because the improvements made in magneto design and construction have practically eliminated all liability of trouble at this point. Many cars made in previous years are fitted with combination systems so it is well for the reader to become familiar with the arrangement of the parts and their functions.

Q. What is meant by "floating the battery on the line"?

A. A storage battery is said to be "floated on the line" when it is used in connection with a dynamo which keeps it always fully charged as outlined at Fig. 100-A. The dynamo current is first directed to an automatic cut-out arrangement and from thence to the storage battery, the cut-out being used to interrupt the circuit when the dynamo is not generating energy in order to prevent the storage battery from discharging back through the dynamo and converting that into a motor. The current for ignition is taken from the storage battery instead of from the dynamo so that a constant delivery of current is obtained regardless of fluctuations in engine speeds and the changing amounts of current delivered from the battery charger because of these variations in rotative speed of the dynamo armature. When utilized in this manner a storage battery gives very good service, because it is always kept fully charged by the dynamo and is operating at maximum efficiency. This system can be applied to great advantage in connection with electric car lighting and engine starting systems.

LESSON TEN

LOW TENSION IGNITION SYSTEMS

Q. Name principal parts of low tension ignition system. A. A low tension ignition system consists essentially of the current generator usually of the magneto form and a series of makeand-break contact plates attached to the cylinder wall in such a way that a pair of contact points extend through into the combustion chamber.

Q. How is the spark produced in the cylinder?.

A. The spark is produced in the cylinder by the separation of the contact points at the instant of maximum current generation in the magneto armature.

Q. How is the spark timed?

A. The make-and-break igniter plates are operated by a camshaft, as outlined at Fig. 101, and the points of the igniter plate are only separated by the mechanism at the time a spark is desired in the combustion chamber. The spark time is thus controlled by the position of the operating cam which is usually so timed in relation to the crankshaft travel that it will separate the igniter points in the cylinder about to fire at just the proper time.

Q. Is battery current strong enough to produce low tension spark?

A. Battery current can be utilized in producing a low tension spark though more energy is required than would be needed by & high tension ignition system. The battery current must be intensified to some extent by means of a "kick-coil" which increases the current value at the time contact is broken in the engine cylinder. Dry cells are not suitable for use on low tension igniter plates unless these have been designed especially for use with battery current.

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