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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 a 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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Fig. 101.-Outlining Operation of Low Tension or Make and Break Spark Plates for Four-Cylinder
Ignition Systems.

A storage battery is more suitable because it will maintain a steadier flow of current and is not exhausted so soon as primary cells, especially if the igniter plate construction is based on the use of electrical current supplied by a magneto. An igniter plate intended for use solely with batteries is devised in such a way that contact is established and broken between the points of the igniter plate only when a spark is neces

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sary, whereas on igniter plates that operate with magneto current the points are kept permanently short-circuited or in contact together until the spark is needed in the cylinder. Where the contact points are practically always together it will be seen that the battery current would be permanently short circuited. Therefore, on most forms of low tension ignition systems

on Locomobile Cars.

the battery is supplied Fig. 102.-Low Tension Igniter Plate Used only for an emergency

use, as it will soon become exhausted if employed for regular ignition service.

Q. How is spark coil made, and what is the difference between low tension "kick coil" and induction coil used in high tension systems?

A. The kick coil consists of a core of soft iron wire around which is wrapped eight or ten windings of coarse magnet wire No. 16 or 18 gauge. Unlike the induction coil the "kick coil" has only one winding of coarse wire and does not employ a vibrator or condenser. It is much simpler than the induction coil, because the secondary winding, the condenser and the vibrator parts are eliminated. It is connected in series with the battery current, i. e. the positive

wire which leads to the bus bar or to the insulated terminals of the igniter plate is connected to one of the terminals of the coil so the winding of the coil really forms an extension of the conductor leading to the igniter plates. The other battery terminal is connected to the ground through a simple form of switch.

Q. Describe construction of igniter plate.

A. A typical igniter plate which has been used successfully on many Locomobile cars is shown at Fig. 102. The view at 1 shows the exterior of the plate which is approximately triangular in form and which has three projecting lugs by which it is bolted to the combustion chamber. The interior view is shown at 2, this showing the movable hammer member, which is rocked by the simple lever extending to the tappet rod operated by the camshaft. The combination of hammer and operating lever forms a bell-crank provided with a suitable journal in the igniter plate so the hammer member can rock back and forth and establish contact with the fixed insulated member which is called the "anvil" and which is shown detached from the igniter plate at 3. The hammer is provided with a platinum contact point which normally rests against a similar contact member on the stationary anvil.

The wire conveying current from the magneto or battery is attached to the fixed terminal or anvil insulated from the main body portion of the igniter plate. The plate shown is provided with a taper joint, so that it makes a gas tight connection when inserted into the opening made to receive it in the combustion chamber wall.

The method of operation of the igniter plate is clearly outlined at Fig. 103-A. As will be seen the cam lifts the roller on the tappet rod plunger and establishes the circuit which is broken as soon as the roller rides off of the point of the cam and the contact points at the gniter plate are separated.

In the diagram at Fig. 103-B the various parts of the igniter plate operating mechanism are clearly outlined. The main current conveyer or bus bar 1 is brought into electrical connection with the insulated terminal 6 and the anvil carried in plate 3 by means of the switch lever 2. The grounded hammer member 4 is operated by the external lever 5 which establishes contact between the hammer 4 and the anvil 6 when it is raised by the rod 9 which applies

its pressure through the medium of the spring 8 as the roll carrying plunger 11 is raised by the cam member in the guide bushing 10. When the roller carrier 11 drops off the point of the cam, the rod 9 is pulled down and the hammer 4 is separated from the anvil 6 by means of the nut 7 at the end of the rod 9. The spark time is advanced and retarded by sliding the igniter operating camshaft along so that the roller on the member 11 is raised sooner or later depending upon its position with reference to the inclined cam point. Q. Is coil needed with magneto on low tension ignition system?

A. As the magneto armature may be wound with sufficient

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Fig. 103. Showing Action of Locomobile Low Tension Igniter Plate.

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