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fit a double-spark system successfully, one of the plugs must be a double pole member to which the high-tension current is first delivered, while the other may be one of ordinary construction.

A typical double-pole plug is shown in section at Fig. 54, A. In this member two concentric electrodes are used, these being well insulated from each other. One of these is composed of the usual form passing through the center of the insulating bushing, while the other is a metal tube surrounding the tube of insulating material which is wound around the center wire. The current enters the plug through the terminal at the top in the usual manner, but it does not go to the ground because the sparking points are insulated from the steel body of the plug which screws into the cylinder. After the current has jumped the gap between the sparking head and the point, it flows back to the terminal plate at the top, from which it is conducted to the insulated terminal of the usual type plug.

The method of wiring these plugs is shown at Fig. 54, B. The secondary wire from the coil or magneto is attached to the central terminal of the double-pole plug, and another cable is attached to the insulated terminal plate below it and to the terminal of the regular type plug. One is installed over the inlet valve, the other over the exhaust valve, if the system is fitted to a T head cylinder. Before the current can return to the source it must jump the gap between the points of the double-pole plug as well as those of the ordinary plug, which is grounded because it is screwed into the cylinder. When a magneto of the high-tension type furnishes the current a double distributor is sometimes fitted, which will permit one to use two ordinary single-pole' plugs instead of the unconventional double-pole member. Each of the plugs is joined to an individual distributor, and as but one primary contact breaker or timer is used to determine the time of sparking at both plugs, the ignition is properly synchronized and the sparks occur simultaneously.

Sometimes a spark plug of the special form shown at Fig. 53, C, is used in connection with a regular spark plug of the form shown at A, the special plug being placed first in the circuit and joined to the regular plug by a length of wire bridging the free terminal

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of the plug at C with that on top of the insulator of the regular pattern. As the plugs are in series, the current must jump the gap of both plugs and thus two sparks occur, which is said to increase power by accelerating the rate of flame propagation, which of course results in more energetic ignition. The insulator is shaped to form a double V, the sides being slightly concave and

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Fig. 55.--Assembly View of Four Cylinder Battery Ignition Group,

Showing Devices and Methods of Wiring.

larger than the center V, which ends in a sharp point. This construction is said to cause the point to be self-cleaning by the explosion. Two electrodes pass through the insulating member instead of one, these being insulated from each other and the plug body as well. The high tension current enters one terminal and passes down one of the electrodes, jumps the air gap, and can only reach the ground if the terminal connected to the second electrode is in electrical connection with the terminal of an ordi

nary form of spark plug or if it is bridged down to the plug body by the keeper B. When this keeper is in place, as indicated, the plug will act the same as a single electrode sparker. When the plug is to be used for double ignition in connection with one of the regular forms, the keeper B should be removed and a short


Fig. 56.—Method of Employing Single Vibrator Coil to Fire Four Cylin

ders when Secondary Current is Distributed Instead of Battery Energy.

wire used to join the terminal to which the keeper was attached to the terminal of the regular pattern spark plug.

Typical Battery Ignition Systems.—The components of typical battery ignition systems may be easily determined by studying the illustrations given at Figs. 55, 56 and 57. The four-cylinder ignition group shown at Fig. 55 depicts the conventional method of wiring two sets of batteries, a four-point timer or commutator, and a four-unit induction coil together. It will be seen that eight dry cells are wired together in series and are used as an auxiliary

to a six-volt or three-cell storage battery. The negative terminals of the storage battery and dry cell set are coupled together by a short length of wire and are grounded by being attached to the engine base by a suitable conductor. The positive terminals are coupled to the two binding posts under the switch or the coil. The four points of the commutator are attached to the different units of the coil while the secondary wires run from the high

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Fig. 57.—Distributor and Coil Ignition Group for Six Cylinder Motor,

Showing Order of Firing and Wiring Connections.

tension terminals on the bottom of the coil to the spark plugs in the cylinders. If the switch lever is placed on one contact button, the current is obtained from the dry cells. If it is swung over to the other side, electricity from the storage battery is utilized:

A typical high-tension distributor system is shown at Fig. 56. Two sources of primary current are provided, one being a six-cell, dry battery, the other a three-cell, or six-volt storage battery. The battery connections are similar to those previously shown and but a single unit coil is needed to fire all cylinders. A single

primary wire is attached to the commutator section of the distributor. The secondary wire from the induction coil is joined to the distributing terminal on the top of the distributor, from which it is delivered to the collecting terminals spaced on quarters around the outer periphery of the distributor casing by means

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Fig. 58.—Complete Ford Magneto Ignition System, a Distinctive Method

Found Only on This Car.

of a central distributing segment. Suitable conductors connect the distributor with the spark plugs in the cylinders.

The illustration at Fig. 57 is practically the same as that at Fig. 56, except that a distributor capable of firing a six-cylinder engine is used. If individual unit coils were to be employed, as is the case at Fig. 55, the coil box would contain six units and the

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